Waste and Biomass Valorization

, Volume 7, Issue 2, pp 201–235 | Cite as

Biochar as an Exceptional Bioresource for Energy, Agronomy, Carbon Sequestration, Activated Carbon and Specialty Materials

  • Sonil Nanda
  • Ajay K. Dalai
  • Franco Berruti
  • Janusz A. Kozinski
Review

Abstract

Biofuels and biomaterials are gaining increased attention because of their ecofriendly nature and renewable precursors. Biochar is a recalcitrant carbonaceous product obtained from pyrolysis of biomass and other biogenic wastes. Biochar has found many notable applications in diverse areas because of its versatile physicochemical properties. Some of the promising biochar applications discussed in this paper include char gasification and combustion for energy production, soil remediation, carbon sequestration, catalysis, as well as development of activated carbon and specialty materials with biomedical and industrial uses. The pyrolysis temperature and heating rates are the limiting factors that determine the biochar properties such as fixed carbon, volatile matter, mineral phases, surface area, porosity and pore size distribution, alkalinity, electrical conductivity, cation-exchange capacity, etc. A broad investigation of these properties determining biochar application is rare in literature. With this objective, this paper comprehensively reviews the evolution of biochar from several lignocellulosic biomasses influenced by pyrolysis temperature and heating rate. Lower pyrolysis temperatures produce biochar with higher yields, and greater levels of volatiles, electrical conductivity and cation-exchange capacity. Conversely, higher temperatures generate biochar with a greater extent of aromatic carbon, alkalinity and surface area with microporosity. Nevertheless, this coherent review summarizes the valorization potentials of biochar for various environmental, industrial and biomedical applications.

Graphical Abstract

Keywords

Biochar Pyrolysis Gasification Soil amendment Carbon sequestration Activated carbon 

Introduction

Energy is a fundamental element in the quality of current livelihood and an essential ingredient in all sectors of the modern economy. The worldwide energy demand by 2050 is expected to be at least twice than that of today’s consumption level. With the increasing global population and economic development, the energy consumption is estimated to increase by 1.1 % per year, i.e. from 5.3 × 1020 joules in 2006 to 7.4 × 1020 joules in 2030 [1]. It is predictable that, by 2030, most of this energy (~84 %) will be from fossil fuels including 6 % from nuclear sources and about 8 % from renewable resources [2, 3].

Fossil-based solid and liquid fuels, particularly gasoline, coal and natural gas have long been the desired energy sources for the human civilization. Although, fossil fuels have augmented the global industrialization in the past years, yet their increased consumption has led to both environmental and geopolitical issues. A few adverse impacts of elevating fossil fuel consumption include rising fuel prices, increasing greenhouse gas (GHG) emissions especially CO2, climate change, air pollution, and water pollution by incidental oil spills via long-distance transportation. These concerns have raised awareness for alternative energy supply, especially from renewable resources. Although most of the sustainable resources such as wind, solar, geothermal and nuclear generate heat and electricity, they are incapable of producing liquid and gaseous fuels. Conversely, renewable carbon in the form of waste or residual lignocellulosic biomasses can potentially be transformed into hydrocarbon-based fuels.

Lignocellulosic biomasses are non-edible plant materials that contain cellulose, hemicellulose and lignin. Lignocellulosic feedstocks broadly include plant residues from agriculture (e.g. corn stover, corn cob, bagasse, straw, stalk, husk, etc.), forestry (e.g. wood chips, sawdust, insect-infested wood, etc.) and grasslands (e.g. switchgrass, timothy grass, elephant grass, etc.). These biomasses are primarily composed of cellulose (38–50 wt%), hemicellulose (23–32 wt%) and lignin (10–25 wt%) [4]. Cellulose, hemicellulose and lignin constitute nearly 90 % of the dry matter in lignocellulosic biomass [5]. This makes lignocellulosic materials a potential resource for the production of liquid (e.g. ethanol, butanol and bio-oils) and gaseous (e.g. synthesis gas, methane and biohydrogen) biofuels. The biomass conversion technologies available today include thermochemical (e.g. torrefaction, pyrolysis, gasification, combustion, liquefaction and supercritical fluid techniques) and biochemical (e.g. anaerobic digestion, ethanol and butanol fermentations, and syngas fermentation) pathways [6]. The biofuels produced from lignocellulosic biomass are considered carbon-neutral because the CO2 emitted from their combustion is used by the plants for photosynthesis. In simple words, biofuel has its energy derived from biological carbon fixation [7]. Nonetheless, biochar as a product of biomass pyrolysis has potential to capture the carbon in the environment for a longer timespan making the biorefining process carbon-negative [8]. Biochar application can draw CO2 from the atmosphere and alleviate global warming.

Biochar, a solid product from biomass pyrolysis, is obtained along with liquids (e.g. bio-oil, water and volatiles) and gas components (e.g. H2, CO, CO2 and CH4). Biochar comprises of mostly stable aromatic forms of carbon and thus cannot be readily returned to the atmosphere as CO2 even under favorable environmental conditions [9]. Hence, it prevails in the soil for a longer time holding the recalcitrant carbon. It is this process of preserving carbon in soil for hundreds to thousands of years that makes biochar a carbon-negative resource. Pyrolysis under controlled operating conditions stabilizes some of the biomass’ carbon in solid form but also produces energy-dense liquids and gases that can be used to generate heat and power. Although the chemical energy from biomass is retained in the biochar, the amount of energy released from the biomass in pyrolysis may be higher than that in combustion. In contrast to combustion, pyrolysis is more efficient in terms of carbon emissions as production of biochar has better abatement potential to store the carbon.

In conventional perspectives, maximizing bio-oil yields at the expense of minimizing biochar production has long been considered profitable for a biorefinery [10]. While the estimated cost of bio-oil being US $740/toe [11], biochar is priced less than US $500/ton [12]. Today, the current upgrading technologies to transform bio-oil into advanced fuels are too expensive in relation to the fossil fuel prices. As a result, the only potential market for bio-oils is for production of high-value chemicals. In such a scenario, biochar may be more valuable than bio-oil. Bio-oils could be catalytically upgraded to transportation fuels or serve as a feedstock for biochemical production. On the other hand, biochar utilization is targeted more towards solid fuels, adsorbents, specialty materials or land applications.

Biochar is a porous carbonaceous material largely containing carbon along with the inorganic components of the biomass utilized, such as alkali (e.g. Li, Na and K) and alkaline earth (e.g. Ca, Mg and Ba) metals [13, 14, 15, 16]. Due to the metal content in biochar, it can also be used in metallurgical processes by substituting coke that is conventionally derived from fossil fuels [8]. Biochar can be mesoporous or microporous depending on the operating conditions employed for its production and the feedstock type. Biochar has also been found to be an effective catalyst for thermochemical cracking of bio-oil [17], and tars during pyrolysis or gasification [18]. Additionally, the high carbon content and thermal stability of biochar result in carbon sequestration, which makes the process potentially eligible for carbon credits [19]. Biochar has versatile properties relating to its structure, composition and reactivity leading to their many applications in chemical and pharmaceutical industries as well as agronomy.

Broadly, biochar from waste or residual resources has found multifarious applications in bioenergy (e.g. co-gasification co-firing and combustion); biomaterials (e.g. activated carbon, adsorbents and specialty materials) production; chemical reactions (e.g. catalyst and catalyst support); agronomy (e.g. water retention and plant nutrition); environmental remediation (e.g. carbon sequestration and sorption of pollutants); and pharmacy (e.g. adsorption of drugs and toxins). The efficiency of biochar in most of these applications significantly depends on its carbon content, surface area, pore size distribution, alkalinity, ion-exchange capacity and elemental composition. These properties are subject to variations depending on the biochar feedstock, pyrolysis temperature, heating rate, residence time, oxidation medium and potential post-processing treatments.

Although ample amount of literature is available for biomass pyrolysis, yet it appears to be scattered and discounted for biochar characterization. Biochar has attracted interest only recently, as early technologies were directed to maximizing liquid products in an attempt to generate crudes that could be easily and economically refined. Biochar is a salient bioresource for energy production, advanced agronomic practices and land applications, biomaterial production and climate change mitigation. In addition, integrating pyrolysis with biochar application to soil for carbon sequestration could also lead to a noteworthy strategy for extenuating climate change. This paper highlights the chemical properties of biochar generated from a broad range of lignocellulosic materials and residues at different pyrolysis conditions. The environmental, industrial and biomedical applications of biochar, especially in the areas of energy production, agronomy, carbon sequestration, activated carbon and other specialty materials, and waste remediation are broadly and cohesively discussed.

Thermochemical Production of Biochar

Biochar is a co-product of thermochemical conversion of organic materials where temperature plays a vital role. Several processes involving the application of high temperatures in waste and residual biomass conversion to liberate heat, liquids or gas also result in the production of solid char. Some of such processes include combustion, pyrolysis, gasification, torrefaction and carbonization where there is a control over the products’ yields. The occurrence of forest fire is a broad scale thermochemical process that generates enormous amounts of biochar. Wild forest fires could occur either naturally by the increase in diurnal temperature or intentionally through prescribed forest fires to clear forest cover [20]. More than 130 million tons of waste woody biomass is produced every year by harvesting forest residues and thinning of forest canopy to reduce wildfire hazards [21]. From this amount, nearly 36 million dry tons of biomass is accessible to biorefineries for energy production [22].

The prescribed thinning of managed forest resources to prevent wildfire poses certain economic and environmental concerns associated with their utilization in biofuel production. Some of such concerns include: (1) logistics cost associated with biomass transportation to biorefineries, (2) nutrient removal from forest soil, and (3) compromise in forest carbon sequestration. However, these challenges could be addressed through appropriate planning or, preferably, by the development of mobile pyrolysis units that could not only transform forest biomass to biofuels near the harvesting sites but also generate biochar for sequestering forest carbon. Mobile pyrolysis systems have the potential to reduce more than half of the cost associated with biomass transportation to refineries by generating bio-oil on-site with nearly 6–7 times higher energy density than that of raw biomass [23]. Furthermore, pyrolysis-derived biochar can be used for long-term carbon sequestration, unlike forest fires that cause enormous quantity of CO2 released into the atmosphere [24].

Biomass pyrolysis is a thermochemical conversion technology operating in the absence of oxygen that results in bio-oil, biochar and gases. The yields of biochar, bio-oil and gas products from pyrolysis depends on the operating conditions such as temperature, heating rate, residence time, reactor configuration and feedstock type [25]. There are various modes of pyrolysis available today such as slow, fast, flash, vacuum, intermediate and pressurized ultra-pyrolysis [26, 27]. High bio-oil yields are typically obtained at intermediate temperatures, faster heating rates and short residence times, which are characteristic of fast and flash pyrolysis. On the other hand, biochar production is considerably favored at moderate temperatures, slower heating rates and longer residence time, representative of slow or conventional pyrolysis.

Slow pyrolysis or conventional carbonization operates at 300–700 °C with a low heating rate of 0.1–1 °C/s and residence time of 10–100 min [28]. Fast pyrolysis prefers reaction temperatures of 400–500 °C with a high heating rate of 10–200 °C/s and vapor residence time of 30–1500 ms [29]. Flash pyrolysis is favored at temperature ranging from 400 to 600 °C with a heating rate greater than 1000 °C/s and vapor residence time lower than 100 ms [30, 31]. Nonetheless, intermediate pyrolysis operates at temperatures around 500 °C with moderate residence times of 10–20 s [32]. While the bio-oil yield is nearly 50 % in the intermediate pyrolysis, biochar and gas yields are around 25 % each.

Pyrolysis of biomass leads to both primary and secondary reactions during the vapor release process [33]. The two basic reactions that occur during biomass pyrolysis are primary and secondary cracking. The primary cracking results in the evolution of gases and volatile components including water vapor from biomass that are rapidly quenched during the condensation process. Some components such as H2, CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8, C4H8 and C4H10 remain non-condensable and exit as the producer gas [34]. A temperature around 500 °C is preferable for maximizing bio-oil yield that promotes biomass pyrolysis by restraining secondary cracking reactions [35]. On the other hand, temperatures beyond 500 °C with a longer residence time enhance gas production by accelerating secondary cracking of biomass components [29]. Due to longer vapor residence time in the reactor with lower condensation rates and intermediate temperatures, secondary reactions also cause the formation of heavy molecular weight compound such as tars and char.

Table 1 summarizes the product yields from a broad range of biomass samples pyrolyzed at varying temperatures and heating rates. With an increase in temperature, the average liquid product yield from pyrolysis is found to escalate in the order: 300–400 °C (30 wt%) < 400–500 °C (42 wt%) < 500–600 °C (47 wt%). In contrast, the solid product yield (mostly biochar as well as dehydrated and decarboxylated biomass) decreases with increasing temperature as: 200–300 °C (77 wt%) > 300–400 °C (49 wt%) > 400–500 °C (34 wt%) > 500–600 °C (27 wt%). This indicates that pyrolysis at higher temperatures leads to greater cracking of organic components thereby reducing biochar production [55].
Table 1

Product yields from the pyrolysis of various feedstocks at different temperatures

Biomass

Temperature (°C)

Heating rate (°C/min)

Weight percent

References

Biochar

Biocrudea

Gas

Amazon tucuma seed

400

31

40

19

[19]

450

19.5

50.1

30

500

10

60

32

550

9

54

37

600

8

51

40

Apa wood

400

39.4

[36]

500

35.6

600

33.3

700

31.6

800

27

Apricot kernel shell

400

10

35.2

20.6

38

[37]

450

32

21

39

500

30

22.6

41

550

29.4

21.4

43

400

50

28.7

23

30.8

450

29

23.5

31

500

27

25

32

550

26.7

24

33.6

Aspen poplar

425

30.6

59.7

5.9

[38]

465

18.9

72.7

8.5

500

11.2

77.7

12.1

541

9

71

21.2

Canola meal

300

7

50

11

24

[15]

400

36

19

29

500

34

24

27

600

32

20

38

700

31

18

45

Cottonseed hull

200

83.4

[39]

350

36.8

500

28.9

650

25.4

800

24.2

Fescue grass

100

99.9

[40]

200

96.9

300

75.8

400

37.2

500

31.4

600

29.8

700

28.8

Grape skin

300

63.2

22.7

10.3

[41]

400

55.9

31.8

11.3

450

47.1

33.2

15.7

500

38.8

36.4

21.6

550

33.8

38.1

26.7

600

27.7

33.7

36.5

Grape seed and skin

300

54.4

26

18.4

[41]

400

44.1

32.8

19.4

450

42.1

37.4

19.5

500

37.4

38.7

20.1

550

35.3

35.2

26.6

600

26.5

30.1

40.4

Iroko wood

400

34.1

[36]

500

33.5

600

30.9

700

29.5

800

27.8

Maple wood

482

20.9

66.9

6.3

[38]

500

12.6

72.9

8.2

532

9

64

7.8

Mixed wood shavings

400

24.1

65.5

10.2

[42]

450

21.4

65.7

11.1

500

18.9

66

14.6

550

17

67

15.3

Newspaper

816

30.1

44.1

25.8

[43]

Olive husk

350

1620

36.2

39.6

24.2

[28]

400

31.5

39

29.4

500

28.2

32.7

39.5

550

22.3

26.6

48.2

Orange peel

150

82.4

[44]

200

61.6

250

48.3

300

37.2

350

33

400

30

500

26.9

600

26.7

700

22.2

Oriental white oak sawdust

400

29

48

24

[45]

450

22

51

27

500

19

47

34

550

17

41

43

Palm kernel shell

400

38.8

[36]

500

35.4

600

33.8

700

32.1

800

30.9

Peanut hull

400

59

[46]

500

56

Peanut shell

300

7

36.9

[47]

700

21.9

Pecan shell

350

62

[46]

700

53

Pine needle

100

91.2

[48]

200

75.3

250

56.1

300

48.6

400

30

500

26.1

600

20.4

700

14

Pinewood

450

2

44

32

24

[16]

450

450

24

59

17

Pinewood shavings

100

99.8

[40]

200

95.9

300

62.2

400

35.3

500

28.4

600

23.9

700

22

Poultry litter

300

20

60.1

[49]

350

56.2

400

51.5

450

48.7

500

47.6

550

46.6

600

45.7

Rapeseed straw and stalk

400

5

39.4

54.3

6.3

[50]

500

35.6

53.5

10.9

600

32.2

56.3

11.4

700

29.6

55.7

14.7

800

28.2

56.1

15.7

900

27.9

55.3

16.9

800

10

28.1

55.6

16.3

800

15

27.1

52.2

20.7

Refuse derived fuel pellet

400

10

49.8

30

18.6

[51]

500

40

39

19

600

35

48

20

700

32.3

50

20.1

Rice straw

600

10

31.2

[52]

700

30

800

28.7

900

27.4

1000

26.2

900

18,000

29.3

Safflower seed

400

100

28

37.5

27

[53]

500

22.5

48.5

25

550

19

49.5

25.5

600

17.5

51

26

700

15.5

48

30.5

400

300

25

38.5

26

500

20

51

23

550

17

54

24

600

16.5

54.5

24.5

700

15.5

52

27.8

400

800

24.5

39

26.5

500

18

51.5

24.5

550

16.5

53.5

25

600

15

54

25.5

700

14

52

29.5

Soybean stover

300

7

37

[47]

700

21.6

Sugarcane bagasse

320

43.8

33.8

15.6

[54]

400

28.1

31.5

16.7

450

27.3

48.3

19

500

23

51.9

20.3

550

20.2

53.2

19.8

600

20.7

58.8

21.6

Switchgrass

250

89

[46]

500

51

Timothy grass

450

2

42.6

30.9

26.5

[16]

450

450

22

56

22

Wood

350

1620

29

45.5

25.5

[28]

400

26.1

44.9

28.7

500

22.9

43.4

32.7

550

21.1

41.1

36.4

Wheat straw

450

2

41.5

33.4

25.1

[16]

450

450

21

55

24

Some of the data presented in this table were extracted from the figures of respective references where the exact yield values are not mentioned

aBiocrude includes bio-oil and water fraction in the pyrolysis liquid

With about 77 wt% (average basis) of solid product yield (including biochar and torrefied residues) at 200–300 °C, there is nearly 20–30 % loss in biomass weight (Table 1). The yield of biochar from pyrolysis at lower temperature (>300 °C) and low heating rate (~15 °C/min) is comparable with torrefaction. However, torrefaction occurs between 250 and 300 °C at low heating rates to produce torrefied biomass with mass yield of 59–78 % and energy yield of 67–88 % [56]. At temperatures below 220 °C, cellulose in biomass dehydrates releasing water but as the temperature rises above 250 °C it denatures to CO2 and CO [57]. The average increase in gas evolution is also noticed with increase in temperature in the sequence: 300–400 °C (20 wt%) < 400–500 °C (22 wt%) < 500–600 °C (26 wt%).

The liquid products obtained through pyrolysis are typically composed of an aqueous and an organic phase. The aqueous phase chiefly contains water, acids and small concentrations of aldehydes, ketones, alcohols, ethers and esters [58]. In contrast, the organic phase mainly contains phenolics, carbonyls, tar and heavy oil (biocrude). Bio-oils obtained through fast pyrolysis of biomass usually contain methanol, acetone, furfural, phenols, acetic acid, formic acid, cyclopentanone, methoxyphenol, levoglucosan, guaiacol and alkylated phenol derivatives [59, 60]. Moreover, the type of biomass along with its elemental and organic (e.g. cellulose, hemicellulose and lignin) contents significantly influence the yields and compositions of bio-oil, biochar and gas products.

Effects of Temperature on Biochar Properties

Biochar yield depends on the rate limiting factors such as temperature and heating rate. Temperature plays a prominent role in determining biochar quantity and quality. A few properties of biochar that are widely influenced by pyrolysis temperature are: (1) ash, volatile matter and fixed carbon content; (2) elemental composition i.e. carbon–hydrogen–nitrogen–sulfur–oxygen: CHNSO; (3) particle size, surface area, total pore volume and pore size distribution; (4) heating value; as well as (5) pH, electrical conductivity and cation-exchange capacity. Figure 1 shows an artistic graphical representation of biochar and the evolution of its properties with pyrolysis temperature.
Fig. 1

Graphical representation for the effects of temperature on biochar properties. Note: This is an original sketch made by Dr. Sonil Nanda, and not adapted from any source

Table 2 highlights the elemental composition (CHNSO) of various biochars produced at variable temperatures and heating rates. An increase in pyrolysis temperature leads to three distinctive traits in biochar in terms of elemental composition such as: (1) increase in carbon content, (2) decrease in hydrogen content, and (3) decrease in oxygen content. The trend for increase in average biochar carbon levels is found to be: 300–400 °C (59 wt%) < 400–500 °C (72 wt%) < 500–600 °C (74 wt%) < 600–700 °C (82 wt%). This indicates that biochar produced at higher temperatures (>500 °C) has a stronger occurrence of C–C bonds compared to C–H and C–O bonds. This trend also decreases the atomic H/C and O/C ratios.
Table 2

Elemental composition of biochars produced from various feedstocks at different pyrolysis temperatures

Biochar

Temperature (°C)

Heating rate (°C/min)

Weight percent

Reference

   

Carbon

Hydrogen

Nitrogen

Sulfur

Oxygen

 

Apa wood char

400

82.7

3.3

0.6

0.2

11.8

[36]

500

86.5

0.6

0.8

0.2

10.4

600

88.3

0.5

0.8

0.1

8.3

700

94.5

0.4

1

0.1

1.9

800

96.1

0.2

1.2

0.04

0.04

Blackbutt wood char

450

72

3.5

0.1

0.01

24.4

[61]

550

85

1

0.3

0.02

13.7

Broiler litter char

350

45.6

4

4.5

0.7

[39]

700

46

1.4

2.8

1

Canola meal char

300

7

66.9

4.9

9.4

0.4

18.4

[15]

400

72.5

3.9

8.4

0.1

15.1

500

75.8

2.6

8.2

0.1

13.3

600

77.5

2.3

7.6

0.3

12.3

700

81.8

1.6

7.6

0.3

8.7

Cherry seed char

300

5

72.6

5.2

3.7

0.03

18.4

[62]

400

76.2

3.5

2.8

0.02

17.4

500

77.8

2.9

1.9

0.02

17.3

600

80.1

2.3

2.1

0.02

15.4

Cherry seed shell char

300

5

71.7

4.5

1.7

0.04

22.1

[62]

400

72.3

3.6

2.6

0.04

21.4

500

78.9

3

2.4

0.05

15.6

600

83.7

2.4

1.5

0.02

12.4

Corn stover char

450

33.5

1.4

0.8

<0.3

8.6

[63]

500

57.3

2.9

1.5

0.2

5.5

[64]

Cottonseed hull char

200

51.9

6

0.6

1

40.5

[39]

350

77

4.5

1.9

0.8

15.7

500

87.5

2.8

1.5

0.5

7.6

650

91

1.3

1.6

0.3

5.9

800

90

0.6

1.9

0.2

7

Dairy manure char

100

25

36.8

3.1

[65]

200

31.1

3

350

25.2

2.2

500

1.7

0.04

350

2.5

46.5

5.5

2.3

0.3

33.2

[66]

700

8.3

56.7

0.9

1.5

0.2

4.1

Fescue grass char

100

48.6

7.2

0.6

44.1

[40]

200

47.2

7.1

0.6

45.1

300

59.7

6.6

1

32.7

400

77.3

4.7

1.24

16.7

500

82.2

3.3

1.1

13.4

600

89

2.5

1

7.6

700

94.2

1.5

0.7

3.6

Flax straw char

400

71.1

5

1.2

0.1

22.6

[67]

475

76.6

4.6

1

18

550

82.6

3.7

1

0.02

12.7

Hazelnut shell char

197

53.3

6.1

40.6

[68]

277

75

5.5

19.5

377

82.3

3.6

14.1

477

88.4

2.4

8.6

577

92.5

1.9

6

677

94.3

1.5

4.2

777

95.6

1.3

3.1

Iroko wood char

400

80.2

0.1

1.2

0.3

13.3

[36]

500

82

0.1

1.2

0.2

9.7

600

83

0.1

1.3

0.1

7.9

700

90.5

0.05

1.4

0.2

0.5

800

91.1

0.1

1.5

0.02

0.04

Mixed wood shavings char

400

68.1

3.2

28.2

[42]

450

71.9

3.2

24.2

500

73

3.2

22.9

550

71.6

2.6

24.4

Orange peel char

150

50.6

6.2

1.8

41

[44]

200

57.9

5.5

1.9

34.4

250

65.1

5.1

2.2

26.5

300

69.3

4.5

2.4

22.2

350

73.2

4.2

2.3

18.3

400

71.7

3.5

1.9

20.8

500

71.4

2.3

1.8

20.3

600

77.8

2

1.8

14.4

700

71.6

1.8

1.7

22.2

Palm kernel shell char

400

81.9

0.9

0.8

0.4

11.6

[36]

500

82.2

0.6

0.9

0.3

8.8

600

87.6

0.4

1

0.1

3.2

700

92.5

0.1

1

0.1

0.3

800

93

1.1

0.3

0.03

Paved feedlot char

350

2.5

53.3

4.1

3.6

0.5

15.7

[66]

700

8.3

52.4

0.9

1.7

0.4

7.2

Peanut hull char

400

74.8

4.5

2.7

0.1

9.7

[46]

500

81.8

2.9

2.7

0.1

3.3

Peanut shell char

300

7

68.3

3.9

1.9

0.1

25.9

[47]

700

83.8

1.8

1.1

13.3

350

64.5

5.3

0.3

0.01

27.6

[46]

700

91.2

1.5

0.5

0.01

1.6

Pine needles char

100

50.9

6.2

0.7

42.3

[48]

200

57.1

5.7

0.9

36.3

250

61.2

5.5

0.9

32.4

300

68.9

4.3

1.1

25.7

400

77.9

3

1.2

18

500

81.7

2.3

1.1

15

600

85.4

1.9

1

11.8

700

86.5

1.3

1.1

11.1

Pinewood shavings char

100

50.6

6.7

0.05

42.7

[40]

200

50.9

6.9

0.04

42.2

300

54.8

6.5

0.05

38.7

400

74.1

4.9

0.1

20.9

500

81.9

3.5

0.1

14.5

600

89

3

0.1

8

700

92.3

1.6

0.1

6

Poplar wood char

400

8

67.3

4.4

0.8

[69]

460

70

3.5

1

525

77.9

2.7

1.1

Poultry litter char

400

69.2

5.8

10

2

13

[67]

475

77.1

4.7

9.1

2.8

6.3

550

83

4.2

8.4

2.9

1.5

350

2.5

51.1

3.8

4.5

0.6

15.6

[66]

700

8.3

45.9

2

2.1

0.6

10.5

350

46.1

3.7

4.9

0.8

8.6

[46]

700

44

0.3

2.8

1

<0.01

Rapeseed straw and stalk char

400

5

71.3

3.9

0.2

1.4

10.8

[50]

500

75

2.6

0.2

1.4

7.8

600

78.5

1.9

0.3

1.5

3.9

700

79.5

1.2

0.3

1.4

3.3

800

79.5

0.7

0.4

1.5

2.6

900

79.9

0.4

0.4

1.6

1.7

800

10

79.2

0.8

1.4

0.3

3

800

15

78.8

0.8

1.3

0.3

3

Refuse derived fuel pellet char

400

10

55.1

4.3

1

0.6

[51]

500

53

4

1.1

0.5

600

52.4

1.9

1

0.2

700

49.9

0.8

0.9

0.2

Rice straw char

500

200

49.3

2.8

1.3

15.1

[70]

600

10

82.7

0.6

1.3

15.4

[52]

700

83.9

0.5

1.3

14.3

800

87.7

0.5

1.4

10.4

900

92

0.5

1.4

6.1

1000

95.8

0.5

1.5

2.2

900

18,000

88.9

0.5

1.4

9.1

Safflower seed char

400

100

78.1

4.9

2.8

14.2

[53]

600

81.7

3.4

2.5

12.4

700

84.8

1.7

1.9

11.7

400

300

78

4.8

3.2

14

600

82.4

3.8

2.7

11.1

700

85.3

2.1

1.7

10.8

400

800

78.8

5.2

3.1

12.9

600

82.6

3.6

3

10.5

700

86.1

2.3

2.8

8.8

Sawdust char

400

71.5

4.7

0.05

0.2

23.6

[67]

475

80.4

31.8

0.1

15.7

550

81.3

3.6

0.2

0.01

14.9

Soybean stover char

300

7

68.8

4.3

1.9

0.04

25

[47]

700

82

1.3

1.3

15.5

Spruce wood char

400

8

63.5

5.5

1.02

[69]

460

79.6

3.3

1.2

525

78.3

3

1.2

Swine solid waste char

350

2.5

51.5

4.9

3.5

0.8

11.1

[66]

700

8.3

44.1

0.7

2.6

0.9

4

Switchgrass char

250

55.3

6

0.4

0.1

35.6

[46]

500

84.4

2.4

1.1

0.1

4.3

Timothy grass char

450

2

67.5

2.3

1.9

0.1

28.2

[16]

450

450

63.7

3.6

1.9

0.04

30.8

Turkey litter char

350

2.5

49.3

3.6

4.1

0.6

15.4

[66]

700

8.3

44.8

0.9

1.9

0.4

5.8

Wheat straw char

400

71.3

4.8

0.6

0.5

22.7

[67]

475

74.9

3.9

0.5

0.1

20.5

550

83.1

3.2

0.6

0.2

12.9

450

2

65.2

2.3

0.9

0.1

31.5

[16]

450

450

64.8

3.1

0.8

0.1

31.2

A van Krevelen diagram was plotted to compare atomic H/C and O/C ratios of biochars produced at temperatures ranging from 100 to 1000 °C with reference to raw biomass (Fig. 2). The van Krevelen diagram provides a qualitative estimation of the degree of aromaticity and carbonation in biomass and biochar where atomic H/C and O/C ratios serve as the indices. Atomic H/C and O/C ratios in biochar tend to lower with increasing pyrolysis temperature due to dehydration, demethanation, decarboxylation and decarbonylation of oligosaccharides [71]. Atomic H/C ratios ≤0.2 for biochar derived at 800 °C suggest partial continuous transformation from aromatic to graphitic structures, particular to black carbon [72]. A lower atomic H/C and O/C values indicate higher calorific value of the fuel. In addition, biochar produced at temperatures above 500 °C becomes decarboxylated due to the loss of O- and H-containing functional groups. The aromatization of char is typically due to H2 and CH4 generation at high temperatures from organic materials. This results in the development of their less polar surface that may positively affect organic contaminant adsorption [73].
Fig. 2

The van Krevelen plot for biochar produced at different pyrolysis temperatures

Moisture is also reported to have certain impacts on biochar yield. High moisture containing biomasses are found to improve the yield of biochar at higher pressures [74]. Agricultural residues such as straw, husk and bagasse contain a considerable amount of moisture thus being favorable for biochar production. However, studies by Mohanty et al. [16] on woody and agricultural biomass have reported high biochar yields from woody biomass due to the higher amount of lignin content. Pyrolysis of biomass with high lignin content has a strong tendency for high biochar yields [75]. Hence, the inherent composition of biomass (i.e. cellulose, hemicellulose, lignin, extractives and minerals) should also be considered for determining biochar yields.

High lignin-containing biomass such as woody biomass usually leads to higher aromatic carbon content in the resulting biochar and bio-oil. Biochar derived from woody biomass e.g. Apa wood, blackbutt wood sawdust, Iroko wood, Jarrah wood, Leucaena wood, oak wood and bark, pinewood and bark show relatively higher carbon content (Table 2). Lignin has higher carbon content (63 wt%) compared to that of cellulose and hemicellulose (42 wt%) [76]. In addition, lignin also has lower oxygen (28 wt%) levels than that of cellulose (51 wt%) and hemicellulose (46 wt%). The higher carbon and lower oxygen concentrations together render a high calorific value to lignin (23.3–25.6 MJ/kg), which is about 30 % greater than that of cellulose (18.6 MJ/kg) and hemicellulose (13.6 MJ/kg) [77].

Table 3 evaluates the impacts of temperature on biochar properties such as ash, volatile matter, fixed carbon, higher heating value, pH, surface area and pore volume. An increasing temperature reduces the volatile matter of biochar mainly by dehydration and devolatilization. The oligosaccharides in biomass are preserved with the loss of moisture below 250 °C [79]. Apart from the moisture released through dehydration of hydroxyl groups in biomass, the volatiles in the form of condensable and non-condensable gases are liberated via degradation of lignocellulosic network. The volatile matter mostly comprises of light hydrocarbons, moisture, tars, CO, CO2 and H2 [80]. Carrier et al. [81] suggested that the thermal degradation of hemicellulose, cellulose and lignin occurs at 200–300 °C, 250–350 °C and 200–500 °C, respectively.
Table 3

Variation in physico-chemical properties of biochar produced from various feedstocks at different pyrolysis temperatures

Biochar

Temperature (°C)

Heating rate (°C/min)

Weight percent

Higher heating value (MJ/kg)

pH

Surface area (m2/g)

Pore volume (cm3/g)

Reference

Ash

Volatile matter

Fixed carbon

Apa wood chara

400

1.3

42.3

56.4

30.6

[36]

500

1.6

35

63.4

28.2

600

1.9

10.7

87.3

29

700

2.2

3.8

94

31.9

800

2.3

2.1

95.4

32.5

Blackbutt wood chara

450

1.1

40.1

57.2

25.3

5.9

[61]

550

2.8

13.6

82.6

27.8

12.1

Canola meal char

300

7

13.2

49.9

36.8

25.9

8.5

<2

<0.001

[15]

400

16.7

25

58.2

29.8

9.2

500

18.6

15.4

65.7

27.4

9.4

600

20.2

13.2

66.3

28.1

9.4

700

20.5

8

71.4

27.2

10.1

Cottonseed hull chara

200

3.1

69.3

22.3

19.7

3.7

[39]

350

5.7

34.9

52.6

29.8

6.9

4.7

500

7.9

18.6

67

32.1

8.5

650

8.3

13.3

70.3

31.4

8.6

34

800

9.2

11.4

69.5

29.9

7.7

322

Dairy manure char

350

2.5

24.2

80.7

23.2

20.9

9.2

1.6

[66]

700

8.3

39.5

53.5

34.7

19

9.9

186.5

Fescue grass chara

100

6.9

69.6

23.5

19.7

1.8

[40]

200

5.7

70.7

23.6

19

3.3

300

9.4

54.4

36.2

24.4

4.5

400

16.3

26.8

56.9

30

8.7

500

15.4

20.3

64.3

30.2

50

600

18.9

13.5

67.6

32.2

75

700

19.3

9.1

71.6

33.1

139

Flax straw char

400

8.9

38.1

48.4

21.8

6

<5

<0.005

[67]

475

9.4

31.3

55.9

26.3

7.2

550

10.6

19.4

64.3

30.1

8

Hazelnut shell char

277

2.3

20.6

77.1

28.7

[68]

377

2.3

18.2

79.4

29.1

477

2.4

16.7

81.9

30

577

2.7

8.2

89.2

31

677

2.7

5.5

91.8

31.4

777

2.8

3.9

93.3

32

Iroko wood chara

400

4.9

20.5

74.7

25

[36]

500

6.4

16.5

77.4

26.1

600

7.5

11

81.5

26.7

700

8

7.6

84.4

30.3

800

8.2

6.7

85.1

30.7

Orange peel chara

150

0.4

19.5

22.8

0.023

[44]

200

0.3

21.9

7.8

0.01

250

1.1

25

33.3

0.02

300

1.6

26.2

32.3

0.031

350

2

27.7

51

0.01

400

2.2

25.8

34

0.01

500

2.1

24.1

42.4

0.019

600

0.1

26.7

7.8

0.008

700

0.2

21.1

201

0.035

Palm kernel shell chara

400

4.1

22.3

73.7

26.9

[36]

500

5.2

16.3

78.5

27

600

6.2

10.4

83.4

29.4

700

6.3

2.1

91.6

31.1

800

6.4

1.3

92.3

31.2

Paved feedlot char

350

2.5

28.7

47.9

23.5

20.4

9.1

1.3

[66]

700

8.3

44

19.8

36.3

17.2

10.3

145.2

Peanut hull chara

400

8.2

38.4

59

30

7.9

0.5

[46]

500

9.3

18.1

56

31

8.6

1.2

Peanut shell chara

300

7

1.2

60.5

37

24.4

7.8

3.1

[47]

700

8.9

32.7

58.1

28.6

10.6

448.2

0.2

Pecan shell chara

350

2.4

61.6

62

24.9

5.9

1

[46]

700

5.2

9.7

53

32.4

7.2

222

Pine needle chara

100

1.1

19.4

0.7

[48]

200

0.9

21.6

6.2

250

1.2

23.3

9.5

300

1.9

25.2

19.9

400

2.3

27.6

112.4

0.044

500

2.8

28.3

236.4

0.095

600

2.8

29.5

206.7

0.076

700

2.2

29.1

490.8

0.186

Pinewood char

450

2

4.6

8.2

86.4

28.6

166

0.167

[16]

450

450

4.1

8.9

86.3

27.1

 

185

0.178

 

450

1.4

44.7

52.2

5.1

[71]

600

2.1

19.7

77.3

6.5

800

5.2

2.6

91.6

10.4

Pinewood shavings chara

100

1.2

77.1

21.7

19.9

1.6

[40]

200

1.5

77.1

21.4

20.4

2.3

300

1.5

70.3

28.2

21.6

3

400

1.4

36.4

62.2

28.6

28.7

500

2.1

25.2

72.7

30.2

196

600

3.7

11.1

85.2

32.9

392

700

1.7

6.3

92

32.3

347

Poultry litter char

300

20

47.9

9.5

2.7

[49]

350

51.3

10.2

3.4

400

56.6

10.3

3.9

450

58.7

10.4

4.4

500

60.6

10.7

4.8

550

60.7

11

5.1

600

60.8

11.5

5.8

400

33.3

37

25.6

23.9

7.8

<5

<0.005

[67]

475

40.5

29.2

29.2

18.9

9.4

550

46.3

20.4

31.6

18.7

9.5

350

2.5

30.7

42.3

27

19

8.7

3.9

[66]

700

8.3

46.2

18.3

35.5

14.8

10.3

50.9

350a

35.9

36.7

72

35.9

8.7

1.1

[46]

700a

52.4

14.1

44

52.4

10.3

9

Rapeseed straw and stalk chara

400

5

12.2

27.1

60.7

27.3

16

1.24

[50]

500

12.9

17.5

69.6

27.7

15.7

1.15

600

13.9

11.5

74.7

27.6

17.6

1.26

700

14.4

9

76.7

26.7

19.3

1.25

800

15.3

6.1

79.7

27

19

1.15

900

16.1

3.6

27

140.4

1.32

800

10

15.3

6.5

78.2

26.6

32.3

1.28

800

15

15.7

5

79.3

27

119.8

1.17

Refuse derived fuel pellet char

400

10

26.8

42

26.2

5.3

[51]

500

37.6

22.1

34.4

10.4

600

39.4

17.4

38

72.2

700

41.4

11.3

41.6

82.1

Rice straw char

600

10

26

4.8

0.022

[52]

700

 

26.5

11.5

0.031

800

 

28.3

16.1

0.035

900

 

30.4

30.9

0.045

1000

 

32.3

26.2

0.044

900

18,000

28.9

44.6

0.052

Sawdust char

400

1.8

42.3

52.9

24.4

4.4

<5

<0.005

[67]

475

3.1

27.7

65.2

28.6

4.8

550

4

20.9

72.4

26

5.1

Soybean stover char

300

7

10.4

46.3

38.8

25.3

7.3

5.61

0

[47]

700

17.2

14.7

67.7

26.9

11.3

420.3

0.19

Swine solid waste char

350

2.5

32.5

49.8

17.7

21.1

8.4

0.9

[66]

700

8.3

52.9

13.4

33.8

15.1

9.5

4.1

620

13

44.7

14.1

41.2

18.3

[78]

Switchgrass char

250

2.6

74.4

89

21.6

5.4

0.4

[46]

500

7.8

13.4

51

31

8

62.2

  

450

13.4

26.3

58.4

9.1

[71]

600

19.4

11.2

68.5

10.6

800

21.5

3.3

74.7

11.2

Timothy grass char

450

2

3.5

7.5

88.4

20.8

179

0.188

[16]

450

450

3.1

8.3

87.7

21.1

203

0.198

Turkey litter char

350

2.5

34.8

42.1

23.1

17.3

8

2.6

[66]

700

8.3

49.9

20.8

29.2

14.5

9.9

66.7

Wheat straw char

400

5.2

36.3

56.6

21.9

7.8

<5

<0.005

[67]

475

7.4

22.5

64

25.2

6.5

550

8.3

18.6

69

25.8

9.6

450

2

3.9

88.1

7.2

20.5

178

0.184

[16]

450

450

3.6

87.9

7.7

20.8

184

0.179

aHigher heating values (HHV) for these samples were calculated using modified Dulong’s formula: \( {\text{HHV}}\left( {{\text{MJ}}/{\text{kg}}} \right) = \left( {\frac{{33.5 \times {\text{C}}\,{\text{wt}}\% }}{100}} \right) + \left( {\frac{{142.3 \times {\text{H}}\,{\text{wt}}\% }}{100}} \right) - \left( {\frac{{15.4 \times {\text{O}}\,{\text{wt}}\% }}{100}} \right) \)

With maximum devolatilization of biomass components occurring between 200 and 400 °C, and formation of biochar residues, the phase is considered as primary pyrolysis [82]. However, with a constant increase in temperature above 400 °C, the organics continue to crack with slower or lesser volatile matter release as biochar residues undergo significant chemical and physical transformations. Due to these secondary reactions, there is a delay for residual volatiles to escape the lignocellulosic matrix. With this delay, the volatile vapors react with degrading-carbohydrate and increase the level of fixed carbon [57] (see Table 3). The considerable loss of volatile matter from biochar at higher temperatures may also lead to the development of porosity.

Ash content in biochar usually shows an increasing tendency with rising temperature. Ash includes the several inorganic elements of the original biomass as well as impurities and typically contains diverse species of silicates, carbonates, sulfates and phosphates [83]. The inorganic composition of ash is critical during pyrolysis since it can lead to operational issues such as slag formation at higher temperatures, thus reducing the process efficiency. Biomass is a heterogeneous mixture of organic and inorganic matter. While the organic matters include cellulose, hemicellulose, lignin and extractives, the inorganic components include both major elements (e.g. Na, Mg, K, Ca and Si) and minor elements (e.g. Al, Fe, Mn, P and S). Among all the elements, biomass contains a large proportion of alkali and alkaline earth metals that aid in the plant’s metabolism during its life-cycle [84]. These elements, especially Na, Mg, K and Ca often catalyze the thermochemical decomposition of biomass and various char-forming reactions [85].

Various studies have shown the amplifying levels of alkali and alkaline earth metals relative to increasing temperatures. Kim et al. [71] reported pinewood biochar produced at 800 °C contained higher alkali metals compared to biochar at 450 °C. In another study, Titiladunayo et al. [36] reported high alkali metal content in palm kernel shell biochar produced at 800 °C than at 400 °C. Moreover, the heating rate also influences the elemental composition of biochar at a particular pyrolysis temperature. Pinewood biochar generated at low heating rate i.e. 2 °C/min resulted in higher concentration of alkaline elements compared to biochar produced at high heating rate i.e. 450 °C/min [16].

With the rise in temperature, the pH scale for biochar inclines towards alkalinity. This is because of the higher concentrations of alkali and alkaline earth metals at higher pyrolysis temperature that impart alkalinity to respective biochar. Table 3 shows the increase in pH values for biochar produced at elevating temperatures. Cao and Harris [65] suggested that at lower temperatures (200–300 °C), cellulose and hemicellulose degrade to produce organic acids and phenolics that lower the pH value of biochar. With an increase in ash content at elevated temperatures, the concentration of alkali metals intensifies, thus raising the biochar pH above 10. The augmenting levels of inorganic elements and declining acidic functionalities of biochars produced at higher temperatures suggest their possible amendment for neutralization of acidic soil.

The pH of biochar also determines its ionic strength, thus influencing the sorption of organic particles onto its surface. With an increase in pH from 7.7 to 8.7, the sorption capacity of biochar for methyl violet increased noticeably [86]. The rise in pH caused dissociation of phenolic –OH groups in biochar to increase its net negative surface charge, thus enhancing the electrostatic attraction towards methyl violet. The application of alkaline biochar to soil causes immobilization of metals and mobilization of oxyanions [32]. In a study by Moon et al. [87], biochar amendment to lead-contaminated soil increased the soil pH thereby inducing negative charge on biochar surface and favoring the sorption of lead. Furthermore, several O-containing functional groups (e.g. hydroxyl, carboxyl, carbonyl, ether and lactone) are developed in biochar during pyrolysis, which determine its sorption ability for ionic solutes [49].

Electrical conductivity of biochar is another parameter influenced by pyrolysis temperature. Biochar surfaces are usually charged negative which facilitates the electrostatic attraction towards positively charged cationic organic compounds [32]. Unlike pH, electrical conductivity of biochar decreases with increasing temperature. Azargohar et al. [15] reported that the electrical conductivity was higher for canola meal biochar derived at 300 °C (2370 µS/cm) compared to that derived at 700 °C (957 µS/cm). In contrast, the pH of biochar generated at 300 °C (8.5) was lower than that of 700 °C (10.1).

Electrical conductivity of biochar is necessary to determine the potential for any unwanted salt effects in soils with high biochar application rates, especially 3–30 tons/ha [88]. Similar to electrical conductivity, cation-exchange capacity of biochar also decreases with increasing temperature. Cation-exchange capacity determines the tendency of biochar to adsorb cationic nutrients. In a study by Song and Guo [49], poultry litter derived biochar generated at 300 °C (51.1 cmol/kg) showed higher cation-exchange capacity compared to that of 600 °C (29.2 cmol/kg).

Temperature also imparts morphological alterations to biochar such as changes in surface area, pore volume and particle size distribution. Surface area and pore volume are chief parameters to evaluate the absorption by biochar, particularly for organic molecules. Table 3 shows the changes in surface area for biochar produced at different temperatures. Pyrolysis of biomass at temperatures near to 500 °C produces biochar with lower surface area. However, pyrolysis at temperatures above 700 °C produces biochar with higher surface area retaining better adsorptive properties at the expense of their yield [74]. Cantrell et al. [66] determined the surface area for biochars produced at 350 °C from dairy manure, paved feedlot, poultry litter, swine solids and turkey litter as 1.6, 1.3, 3.9, 0.9 and 2.6 m2/g, respectively. However, the surface area increased up to 186.5, 145.2, 50.9, 4.1 and 66.7 m2/g for the biochars produced at 750 °C from dairy manure, paved feedlot, poultry litter, swine solids and turkey litter, respectively.

In general, the surface area increases with a rise in the pyrolysis temperature. The degradation of aliphatic alkyl and ester groups causes the exposure of aromatic lignin matrix at elevated temperatures thus increasing biochar’s surface area. Song and Guo [49] reported an increase in surface area for poultry litter biochar from 2.7 m2/g at 300 °C to 5.8 m2/g at 600 °C. The decrease in surface area is caused by the clogging of biochar micropores with tar deposits [89]. However, the surface area of biochar is found to decrease with an increase in gas pressure (~3 MPa) inside the pyrolysis reactor [90]. Conversely, pyrolysis at elevated pressures (1–3 MPa) enhances biochar yield due to the increase in vapor residence time [74].

Biochar produced at higher temperature and low heating rate results in lower surface area. Mohanty et al. [16] determined the surface area for biochars produced at 450 °C with high heating rate (450 °C/min) from wheat straw, timothy grass and pinewood as 184, 203, 185 m2/g, respectively. However, at 450 °C with low heating rate (2 °C/min), biochars from wheat straw, timothy grass and pinewood showed a relatively lower surface area of 178, 179 and 166 m2/g, respectively. The development of deformations, cracking or obstruction of micropores in biochar produced at low heating rate is responsible for its reduced surface area [32]. This effect is regardless of the volatilization of solid hydrocarbons from biochar surface at higher temperatures. Biochar produced above 400 °C are more effective for contaminant adsorption because of its larger surface area and better micropore development [73]. Ahmad et al. [32] proposed that biochar derived at higher temperatures are more efficient for adsorption of organic contaminants due to the higher surface area and pore volume, whereas biochar generated at lower temperatures is effective for adsorption of inorganic contaminants due to greater intensity of O-containing functionalities and higher incidence of cationic complexes in the soil.

There is a positive correlation between total pore volume and surface area of biochar suggesting the pore and pore size distribution as key factors responsible for the increase in surface area. Chen et al. [48] showed a dramatic increase in the surface area of biochar produced at 100–700 °C from pine needles as: 100 °C (0.7 m2/g) < 700 °C (490.8 m2/g) (see Table 3). In the same study, total pore volume of biochar increased as: 400 °C (0.044 mL/g) < 700 °C (0.186 mL/g). In addition, the temperature also affects the particle size of biochar due to the loss of volatiles and increase in porosity. Azargohar et al. [15] reported a decrease in the particle size distribution of canola meal biochar with the increase in temperature from 300 °C (477 µm) to 700 °C (286 µm). Higher pyrolysis temperature results in biochar with relatively smaller particle size that is proportionate to its larger surface area and better adsorptive properties.

Biochar Gasification for Syngas Production and Utilization

Gasification is a thermochemical conversion process in which solid (e.g. biomass, coal, char, etc.) or liquid (e.g. oil, tar, etc.) carbonaceous materials are transformed into gas products including H2, CO, CO2 and CH4. Gasification of biomass and other biogenic carbonaceous materials is usually performed at higher temperatures (800–900 °C) with steam-to-carbon ratios of 0.8–1.5:1 [91]. Compared to gasification of biomass, biochar gasification is relatively slower. This is due to the fact that during gasification of biomass, there is a rapid mass loss of about 80 % of the total biomass due to loss of volatile components and some permanent gases, e.g. CO and CO2 [92]. CO2 generated during gasification also acts as a gasifying reagent. The rate of oxidation of char carbon by O2 is about three orders of magnitude faster than that of CO2 and water vapor [93]. It is essential to separate any trace amount of tars from biochar during pyrolysis that can negatively affect biochar gasification efficiency. Tars have a tendency to crack on the biochar surface and deposit carbon, thus inhibiting the access of gasifying reagents.

The higher levels of alkali and alkaline earth metals accelerate the char reactivity during gasification under steam and/or CO2 conditions [94]. Some of these metallic species such as Na, K, Mg and Ca act as inherent catalysts in biochar gasification that have properties of accelerating and/or retarding the reaction rates. The behavior of these catalysts remain unclear; however it is suggested that their activity depends on the structure and surface properties of biochar as well as the gasification environment [95]. The metal catalysts enhance the adsorption of gasifying agents to create more active sites on the biochar surface or to change the actual gasification pathway. Biochar has the potential to replace coal as a gasifying agent in power generating stations. Salleh et al. [96] performed co-gasification of biochar generated from empty fruit bunches and found an increase in H2 yield from 5.5 to 28 % with the rise in temperature from 500 to 850 °C. In addition, the carbon conversion also increased from 76 to 84 % within the same temperature range.

Li et al. [94] studied the structural features of Victorian brown coal chars produced through gasification in air at 400 °C. The presence of Na and Ca in brown coal char was found to change the reaction pathways between char and oxygen. In the absence of these metal catalysts, the O-containing functional groups formed in the char were closely associated with the aromatic carbon framework and led to a loosened char structure with the progress of gasification. Such fragmented structures allow an increased access of oxygen into the char’s internal framework with higher chances of oxidation and lower environmental stability in non-catalyzed gasification. However, gasification in presence of the metal catalysts produced chars in which the O-containing functional groups were not closely related to the aromatic structures. The main findings of this study were that the non-catalytic gasification was slower and took place on some specific (e.g. sp3 or sp2–sp3) sites in the char. In addition, non-catalytic gasification produced loosened char structure, while catalytic gasification produced chars with cross-linking structures.

Nevertheless, the syngas produced from char gasification has various uses such as in the production of hydrogen, methanol, alkanes and higher alcohols. Hydrogen generation through water–gas shift (WGS) reaction is one of the chief uses of biochar gasification and syngas utilization. The WGS reaction is represented by Eq. 1. The process of H2 production via WGS reaction is realized in two phases. In the first phase, the WGS reaction is performed at high temperatures (350–500 °C) with an iron oxide-based catalyst; while in the second phase occurring at lower temperatures (200 °C), the WGS reaction is catalyzed with Cu-based catalyst [27]. There is a gradual decrease in CO levels from 2 to 3 % in the first phase to 0.2 % in the second phase. The generated H2 can be used as a high-energy fuel in ammonia synthesis and auxiliary petrochemical and biochemical reactions. The purification of H2 is usually done via pressure-swing adsorption, preferential air oxidation, or by using palladium-membranes [97].

Water–gas shift reaction:
$$ {\text{CO}} + {\text{H}}_{2} {\text{O}} \to {\text{CO}}_{2} + {\text{H}}_{2} \quad \left[ {{\text{Enthalpy}} = - 41\,{\text{KJ}}/{\text{gmol at }}27\,{{^\circ }} {\text{C}}} \right] $$
(1)

The WGS reaction can also be catalyzed by certain photoheterotrophic bacteria such as Rhodocyclus gelatinosus, Rhodospirillum rubrum and Rhodopseudomonas gelatinosa at ambient temperatures and pressures [98]. These bacteria can grow in the dark using CO as the sole carbon source to generate adenosine triphosphate (ATP) and release H2 as well as CO2 [99]. The oxidation of CO to CO2 and subsequent release of H2 occurs through the WGS reaction. The purple non-sulfur bacterium Rubrivivax gelatinosus CBS uses CO as the sole source of carbon and has tremendous WGS capabilities. It not only performs the WGS reaction in the dark converting 100 % CO into stoichiometric amount of H2, but also integrates CO in new bacterial cells via CO2 fixation in the presence of light [100]. However, the rate of H2 production with biological methods is around 96 mmol of H2/L/h—much lower than the chemical methods [99].

Synthesis of methanol is another application for syngas obtained from biochar gasification [101]. The recent techno-economic assessment by Shabangu et al. [101] suggests that depending on the marketability of biochar and methanol, the biochar selling prices, although projected to rise, are currently in the range of $220–$280 per ton. Methanol can be used as: (1) transportation fuel in internal combustion engines; (2) feed for methanol fuel cells; and (3) starting material for many fuels and chemicals including H2, gasoline, acetic acid, olefins, dimethyl ether, methyl tert-butyl ether and formaldehyde. Methanol is a widely used industrial solvent that can be synthesized using a combination of WGS reaction and CO2 hydrogenation.

Hydrogenation of CO2:
$$ {\text{CO}}_{2} + 3{\text{H}}_{2} \to {\text{CH}}_{3} {\text{OH}} + {\text{H}}_{2} {\text{O}} $$
(2)
Methanol synthesis:
$$ {\text{CO}} + 2{\text{H}}_{2} \to {\text{CH}}_{3} {\text{OH}} + {\text{H}}_{2} {\text{O}} $$
(3)

The methanol synthesis reaction from syngas is usually done at lower temperatures (220–300 °C) and higher pressures (5–10 MPa) with Cu/ZnO-based catalysts [27]. Some of the byproducts of this process include methane, acetone, dimethyl ether, methyl formate and higher alcohols. Syngas components in varying combinations and proportions can be used for methanol production such as (H2 + CO), (H2 + CO2) and (H2 + CO + CO2). The rate of methanol production is nearly seven times higher for (H2 + CO + CO2) than other mixtures [102]. Higher selectivity for methanol is achieved at (H2 + CO2)/(CO + CO2) ratio slightly above two [27].

The Fischer–Tropsch (FT) process is a series of chemical reactions that converts a mixture of H2 and CO to liquid hydrocarbons. Initially developed by Franz Fischer and Hans Tropsch in 1925, the FT process has become a critical route of gas-to-liquids (GTL) conversion technology for production of alcohols, synthetic fuels and lubrication oils. Various cobalt, iron and ruthenium-based catalysts are employed in the FT process to produce a broad range of hydrocarbons from syngas. The WGS reaction and its reverse reaction occur during the process by adjusting the CO/H2 ratio. The products from the FT process are mostly straight chain alkanes ranging from C1 to C50; however the alkane products are dependent on the chain growth. Various studies have concentrated on the production of heavy waxes through FT process followed by hydrocracking to gasoline and diesel [27].

A variety of syngas compositions can be used for the FT process. The optimal H2/CO ratio is around 1.8–2.1 for most cobalt-based catalysts. The iron-based catalysts enhance the WGS reaction and can, therefore, tolerate lower H2/CO ratios. Several reactions are used to adjust the H2/CO ratio in syngas. One of such reactions is the WGS reaction that provides a source of H2 at the expense of CO. Steam reforming is another primary reaction for the FT process that use CH4 in syngas as the feed gas and converts it to CO and H2. The overall reaction for the FT process is given by Eq. 5.

Steam reforming reaction:
$$ {\text{CH}}_{4} + {\text{H}}_{2} {\text{O}} \to {\text{CO}} + 3{\text{H}}_{2} $$
(4)
Fischer–Tropsch process:
$$ n{\text{CO}} + \left( {2n + 1} \right) {\text{H}}_{2} \to {\text{C}}_{n} {\text{H}}_{{\left( {2n + 2} \right)}} + n{\text{H}}_{2} {\text{O}} $$
(5)

In general, the FT process is carried out at 150–300 °C as higher temperatures are favorable for faster reactions and higher conversion rates. An increase in pressure leads to higher conversion rates and favors the formation of long-chained alkanes. Moreover, modifying the properties of catalysts allows tuning the product selectivity. However, syngas desulfurization is required as any sulfur components in the feed gas can poison the catalyst. Currently in North America, the GTL conversion processes seem to be economically viable as the syngas price is relatively cheap on an energy equivalent basis to fossil-based oil. It has been estimated that the FT process has a carbon efficiency ranging between 25 and 50 %, and a thermal efficiency of about 50 % [103]. With the price of biochar being low, it could serve as a potential precursor for syngas production.

Utilization of Activated Carbon from Char

Development of Activated Carbon

Activated carbon, also called as activated charcoal, is a form of carbon processed to have increased surface area by creating small low-volume pores that are available for adsorption or chemical reactions. Activated carbon is usually derived from charcoal or highly porous biochar. About one gram of activated carbon has a surface area of nearly 1500 m2, which is almost 50 times higher than that of the biochar itself [104]. The larger surface area in activated carbons is due to their greater degree of microporosity as determined by gas adsorption. Activated carbon can be produced from carbonaceous materials including waste biomass, biochar, coal, lignite and petroleum pitch; however biochar seems to be a suitable precursor due to its porous properties. In addition, activated carbon produced from biochar is highly microporous which makes it apposite to use as catalyst [104, 105] or catalyst support [106].

It should be noted that char is a product of pyrolysis and carbonization of biomass or coal, while activated carbon is an activation product of the char itself. Hence, the discussion on activated carbon in this section refers to the significant utilization of its precursor char. Momentous development in activated carbon has been made over the past few years. The source of activated carbon ranges from lignocellulosic biomass (e.g. wood, straw, husk, shell, pith, seed, bagasse, etc.) and their derived char [104, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118] to wastes materials such as scrap tires [119, 120]. Activated carbon has found numerous applications today, a few of which include but not limited to catalytic supports, battery electrodes, capacitors, sewage treatment, water and gas purification, gas storage, air filters in gas masks and respirators, decaffeination, metal extraction, gold purification and medicinal applications. However, the applications are determined by the functionalities of carbon powders such as surface area, pore structure, pore volume and chemical composition. The research on development and applications of activated carbons is gaining rapid attention because of their aplenty and inexpensive precursors (i.e. biomass and biochar).

Activated carbon is produced either by physical or chemical activation processes. In physical activation, the feedstock is carbonized under an inert atmosphere followed by activation of char at high temperatures using either steam [112] or CO2 [113] as the activating reagents. This is achieved by two methods, namely carbonization and/or activation (or oxidation). In carbonization, the raw material is pyrolyzed under inert atmosphere (i.e. Ar or N2) at temperatures in the range of 600–900 °C. While in activation or oxidation, the precursor or carbonized material is exposed to oxidizing atmospheres (i.e. O2 or steam) at higher temperatures (600–1200 °C). The chemical activation process involves the use of alkali and metal salts such as KOH, K2CO3, NaOH, Na2CO3, AlCl3, ZnCl2, MgCl2 and H3PO4 [118]. In chemical activation, the raw material is impregnated with an acid, strong base or salt to aid in the initial hydration and further carbonized at temperatures of 450–900 °C [108].

The physical activation requires extensive removal of the precursor’s internal carbon mass to obtain a well-developed porous carbon structure, while the chemical activation process requires dehydration of the precursor to facilitate pyrolytic decomposition and yield of carbon along with the inhibition of tar formation [118]. With this prerequisite, biochar produced at higher temperature seem to be attractive due to its higher fixed carbon content with larger surface area and pore volume. However, chemical activation is the most preferred method because it operates at lower temperatures and in shorter times compared to physical activation. Moreover, chemical activation is also cost-effective and produces activated carbon with greater yield, larger surface area and higher porosity.

Environmental Pollution Control

Due to a broad spectrum of pore structures and surface chemistry, activated carbons are used for the adsorption of gases such as natural gas and hydrogen [116]. This application also helps in designing practical pressure-swing and thermal-swing adsorption processes for separation and purification of various gas mixtures [121]. The gas is attracted to activated carbon through Van der Waals forces with bonding energy between 5 and 10 kJ/mol, which is relatively greater than that of zeolite-based adsorbents. In addition, the gases can be stored in activated carbon at lower pressures, less mass and low volume microenvironment that makes the gas storage more appealing than bulky on-board compression tanks.

After adsorption onto the activated carbon (absorbent), the gas can be desorbed at higher temperatures for combustion (aimed at natural gas storage) or used in hydrogen fuel cells (aimed at H2 storage). Several technologies have been developed for desorption of organic pollutants and regeneration of the activated carbon for reuse. Some of such techniques include thermal regeneration, chemical regeneration, bioregeneration, solvent regeneration, electrochemical regeneration, supercritical fluid regeneration, wet oxidation, ultrasound and microwave irradiation [122, 123, 124]. However, the high cost associated with activated carbon regeneration is impeding their extensive implementation.

The role of activated carbon in maintaining the air quality is immensely appealing. Various types of filters with activated carbon are used in air purification systems to remove vapors, odors, oil particles and other hydrocarbons from the air. Due to the ability of activated carbon to adsorb gases, it has been successfully used to treat nitrogen oxide and toluene in indoor air systems [125]. Volatile organic compounds (VOC) are also a group of similar pollutants that have been efficiently removed from the effluent gases using activated carbons with high micropore volume and rapid adsorption abilities [126]. Activated carbon beds are used to filter the contaminated gases and retain particulates and volatile pollutants. The pollutants are trapped in the internal micro-channels of char particles, while the filtered air passes through the bed. Furthermore, another leading approach to controlling mercury emissions from combustion processes such as coal-fired power plants involves the removal of elemental mercury from flue gas by injection of activated carbon [127].

The wastewater treatment and potable water purification using O2 or air to oxidize the organic compounds (i.e. pollutants) to CO2 is a well-established procedure [117]. However, this conventional wet-air oxidation of wastewater without a catalyst requires high temperature and pressure with subsequently more energy and cost input. Metal catalysts such as copper and iron-supported on activated carbon could enhance the oxidation and conversion rates of organic pollutants in wastewater with reduction in reaction temperature and time [128, 129]. The performance of metal catalysts could be improved by their uniform distribution on a porous support. This is attributed to their dispersion which creates an unstable state because of the surface tension favoring a smaller interfacial area for a given mass of wastewater or pollutant. Phenolic pollutants occurring in industrial effluents such as those from coal conversion, petroleum refining, automobiles, resin and plastic have been successfully adsorbed and desorbed using activated carbons from char [123, 124].

In a study by Hu et al. [128], a new heterogeneous copper-catalyst was developed using highly porous activated carbon as the catalyst support to treat dyeing and printing wastewater from a textile industry. The catalyst enhanced the conversion of organic pollutants at lower temperature and pressure range along with shortening the reaction time. Activated carbon as a catalyst support is also advantageous in the way that the organic compounds can be adsorbed onto the support to enhance their concentration, which results in lowering of activation energy for subsequent oxidation reactions. In a similar study by Zazo et al. [129], iron-supported on activated carbon was used to oxidize phenol in the presence of H2O2 with removal rates reaching as high as 85 %.

Biomedical Applications

The rapid adsorption tendencies of activated carbon have also extended the biochar applications to biomedical sciences. Activated carbons or activated charcoal have shown their functionalities in preventing gastrointestinal absorption of certain toxins and drugs, thereby enhancing their elimination even after systemic absorption [130]. It is also believed that activated charcoal is more effective than gastric emptying and/or emesis. Oral administration of activated carbon has been found to eliminate many toxic agents such as aspirin, carbamazepine, cardiac glycosides, dapsone, dextropropoxyphene, meprobamate, phenobarbitone, phenytoin and theophylline [130]. Some of the known exceptions are alcohols [131], cyanide [132], and metals such as iron and lithium [133]. Non-dissociating salts such as iodine and mercuric chloride are well adsorbed by activated carbons, while actively dissociating salts such as NaCl or KNO3 lag behind on being efficiently adsorbed [134]. In addition, administration of activated carbon to patients poisoned with caustic alkalis or acids may cause local tissue damages [130].

In the case of acute intoxications, 50–100 g of activated carbon is recommended for immediate administration to adult patients [130]. Repeated doses of activated carbon also decrease the risk of toxin desorption from the activated carbon-toxin complex as it passes through the gastrointestinal tract. The adsorption of drug on activated carbon results from the weak intermolecular (i.e. Van der Waals) forces, although desorption of the solute (drug) can occur if sufficient amount of activated carbon is not administered [133]. The adsorption of toxins by activated carbon prevents it from being absorbed in the stomach and intestinal tract to cause inflammation. The binding of activated carbon with the toxin also interrupts enterohepatic and enteroenteric circulation of the toxins and its metabolites.

Activated carbon in the form of tablets or capsules is prescribed in many countries as an over-the-counter drug to cure diarrhea, indigestion and flatulence. The maximal binding capacity of activated carbon towards drugs and toxins varies from 35 mg/g activated carbon for potassium cyanide to 1800 mg/g of activated carbon for mercuric chloride, in vitro [134]. The optimal ratio for oral administration of activated carbon to target drug is about 10:1 based on conventional studies [135]. However, a recent study suggests that the optimal ratio of activated carbon to drug should be close to 40:1 [136]. Nonetheless, the dose of administration depends on the type and amount of drug or toxin injected into the system. Ionization and polarity are some of the important factors that also determine the adsorption efficiency of activated carbon from biochar [133].

The typical average surface area for activated carbons is between 800 and 1200 m2/g, while super-activated charcoals have a surface area of 2800–3500 m2/g that can adsorb relatively higher quantities of a drug [133]. This appealing behavior of activated carbon as an antidote to toxins and drug overdose is often considered as an emergency medicine and is recommended for inclusion in the first aid kit for domestic residences and industrial facilities.

Agronomy Applications: Soil Amendment and Fertilization

The occurrence of biochar or charcoal in soil is well known historically from the discovery of the fertile terra preta in the central Amazon region of South America. Terra preta soil is of pre-Columbian origin between 450 BC and 950 AD [137]. Terra preta or “black earth” is a type of dark and fertile anthropogenic soil which is very rich in char content attributed to charcoal, bone and manure. The depth of terra preta soil is estimated to reach about 2 m with a regeneration rate of 1 cm per year. Terra preta soil is formed by high amounts of the low-temperature char from plant residues, animal feces and bones. The source of char in Terra preta is considered to result from incomplete combustion of these residues by natural fires or controlled burning; however the deposits suggests the burnings were deliberate as a management strategy to address low soil fertility [10].

Chemically, terra preta soil contains significant levels of C, N, P, K, Ca, Zn and Mn. The soil also shows high microbial activities with lower frequency of nutrient leaching, unlike most rain forests. More recently, char-rich soil has become locally popular for the production of cash crops such as papaya and mango, which demonstrate three times faster growth compared to the surrounding non-terra preta soil [10]. Due to these remarkable properties, biochar is now used in agronomy to enhance soil fertility and agricultural productivity, as well as land reclamation. Biochar produced through forest fires is found to enhance nitrogen mineralization and nitrification in temperate ecosystems by creating favorable niche for microbial colonization [138, 139]. Novtny et al. [140] reported that humic acid isolated from terra preta soil have greater levels of aryl- and ionisable O-containing functional groups. These functionalities of humic acid retain their increased resistance to oxidation thereby contributing to recurrent soil fertility and cation-exchange capacity. The fire-derived organic matter (includes biochar) comprises up to 40 % of the total soil organic matter and is more stable than other organic matter because of its fused aromatic ring structures [141].

Seifritz [142] proposed three main reasons for applying biochar to agricultural soils as follows: (1) soils seem to be the only storehouse to accommodate biochar at a large-scale for long-term mitigation of climate change, (2) biochar has the ability to enhance soil quality for crop productivity thus reducing the costs associated with its residual energy value, and (3) biochar has tendencies to suppress the emission of methane and nitrous oxide from soil as another measure to offset agricultural GHG emissions. Moreover, biochar application positively impacts the soil carbon content making the soil amended with biochar a large carbon sink.

The heterogeneous chemical composition of biochar suggests that its surface can demonstrate hydrophobic, hydrophilic, acidic or basic properties [143]. An amalgamation of all these properties contributes to the biochar’s ability to react to soil characteristics and alter its quality. Two main features of biochar that make its valuable for soil applications are: (1) high stability against decay, and (2) superior ability to retain essential plant nutrients compared to any other forms of soil organic matter.

The stability of biochar, evidenced from the Amazonian terra preta, is attributed to its resistance to the rapid rates of mineralization compared to any other organic matter. However, the recalcitrance of biochar depends on the type of precursor, process temperature, heating rate, soil properties and climate [144]. The relationship between stability and heterogeneity of biochar could be explained by the presence of recalcitrant aromatic ring structures and readily degradable aliphatic and oxidized carbon structures [72]. The aromatic structures in biochar can also occur in different forms, such as amorphous carbon (found in low-temperature biochar) and turbostratic carbon (found in high-temperature biochar) [40, 145]. The spectroscopic observations on biochars obtained from different feedstocks at variable temperature and heating rates suggest that the recalcitrant aromatic rings arise from the lignin content of the biomass, while the aliphatics and oxidizable carbon structures derive from the cellulose and hemicellulose [13, 14, 15, 16]. Although it is evident that these carbon structures play a significant role in conferring stability to biochar, yet their mechanism is still unclear.

Biochar has positive amendment effects on the bulk density of soil by enhancing the available water content, plants’ rooting pattern and soil fauna [146]. This is because of the micropores of biochar that allow the penetration of air and water, thus reducing the overall bulk density of the soil–biochar mixture. Biochar mixes with water in the soil to open its pre-existing closed pores, thus providing more internal surface for adsorption of soil particles and humus. Biochar retains various mineral and organic macro- and micronutrients in the soil to make them available to plants by adsorption. Lehmann [9] suggested two main routes via which biochar can reduce soil or groundwater pollution, such as: (1) by retaining nutrients and minerals in the soil thereby lowering their chances of leaching into groundwater or being eroded into surface waters, and (2) by improving nutrient retention in the top-soil, thus reducing the use of supplementary chemical fertilizers.

As mentioned previously, cation-exchange capacity is a measure of soil fertility, nutrient retention capacity and ability to protect groundwater contamination from cations. The ability of soils to retain cations in a plant-available form generally increases with an increase in soil organic matter. However, biochar has a greater ability, i.e. higher cation-exchange capacity than other soil organic matter to adsorb cations per unit carbon [147]. This feature is attributed to biochar’s higher surface area, more negative surface charge and greater charge density than the typical soil organic matter [148].

Freshly-prepared biochar can either have net positive or negative surface charge; however its cation-exchange capacities is initially lower compared to soil organic matter on a mass basis [149]. Moreover, pH, surface area and carbon yield of biochar increase at higher pyrolysis temperatures as discussed previously (see Table 3). The cation-exchange capacity for soil organic matter and biochar is less at lower pH and more at higher pH. Biochar is often alkaline in nature due to the presence of considerable amounts of alkali and alkaline earth metals and their application to neutral or acidic soil tends to increase the pH. As a result of the increase in soil pH, the cation-exchange capacity also increases with the enhanced exchangeable minerals available to plants.

Biochar is rich in inorganic elements such as Na, Mg, K and Ca along with substantial amounts of Zn, Cu and Fe [13]. Biochar aplenty with phosphorous is also able to absorb phosphate efficiently than any other soil organic matter [9]. Nitrogen, potassium and phosphorous (collectively represented as NPK) are the three most essential nutrients for improved crop productivity, and biochar acts as a promising soil amendment agent for this purpose. In addition to chemical recalcitrance, biochar particulates are also biologically robust, as they are resistant to microbial decomposition [72]. In other words, biological resistance of biochar is a function of its chemical stability.

Microorganisms in soil need carbon and nitrogen sources for their metabolic growth. Although biochar is a potential source of carbon and nitrogen, its chemical stability makes these elements occur in non-mineralizable forms to microorganisms. However, the internal pores of biochar are accessible to microorganisms to colonize (and their enzymes to adhere) and gain protection against the predators or harsh environment [149]. Moreover, biochar preserves essential nutrients and moisture, which are also beneficial factors for microbial colonization in its periphery.

The microbial population in soil amended with biochar also increases to an enormous extent. The chief microbial groups that are benefited by biochar amendment are plant growth-promoting rhizobacteria and mycorrhizal fungi [150]. Arbuscular mycorrhiza and ectomycorrhiza are the two forms of mycorrhizal fungi that are positively affected by biochar application to agricultural soils. Mycorrhizal fungi enhance plant growth by symbiotically colonizing plant roots either internally (by arbuscular mycorrhiza) or externally (by ectomycorrhiza). These fungal groups proliferate in soil by forming interconnected hyphal networks. However, the internal pores of biochar serve as a suitable platform for developing such networks and facilitating the exchange of nutrients and essential metabolites between soil–biochar mixture and plant roots [151]. In addition, the fungal hyphae together with plant roots aid in holding the biochar–soil particles in lumps in the rhizosphere (i.e. rooting zone) for the continual supply of moisture and essential nutrients to the plant.

Biochar produced via hydrothermal carbonization has shown to stimulate spore germination of arbuscular mycorrhiza leading to their increased populations in soil [152]. The applications of activated carbon and biochar to soil have shown to increase the nitrification by proliferating free-living nitrogen-fixing soil bacterial population e.g. Azospirillum and Azotobacter [138, 153]. In addition, application of biochar to biodigester can lead to an increase in anaerobic- and cellulolytic bacterial abundance resulting in higher biogas production [154]. With all the aforementioned stimulating properties, it is evident that biochar have many beneficial effects on the infertile, marginal and degraded soils, leading to their efficient remediation.

An increase in the water holding capacity of the soil amended with biochar has many advantages in sandy soils where soil moisture is critical in establishing initial vegetation. Irrigation requirements in agricultural farms are also likely to decline with biochar amendment. An optimal increase in water holding capacity of the soil, especially in the rhizosphere can enhance nutrient movement and leaching [143]. Although, biochar application can improve soil–water permeability, yet it is challenging in soils with high clay content [155]. Because of high surface area and porosity, biochar can maintain soil moisture, organic carbon, NPK and other essential plant nutrients for a longer time in the applied soil.

Sohi et al. [10] suggested a scheme for biochar applications as the soil amendment called “closed-loop biochar system,” in which the produced biochar could be returned to the same land that supplied the feedstock. This method is expected to offer multiple benefits, provided the same plant genus (as initial feedstock) is being cropped in the same land. Some of such advantages include: (1) retaining the basic essential elements of the soil for the fresh plants, (2) lowering the frequency of additional fertilizer application to the soil, and (3) potential reduction of crop rotation practices. Plants accumulate some of the mineralizable elements from the soil (or chemical fertilizers) in their tissues which eventually appear in their biochar. If this biochar is returned to the same soil, it could supplement those essential elements (as in the fertilizer) and reduce the requirement for future chemigation (i.e. fertigation by chemical fertilizers). It should also be noted that the typical concentration of nutrients in chemical fertilizers is less than that in the biochar on a dry weight basis.

Carbon Sequestration

There have been many efforts internationally aiming at offsetting the GHG emissions through sequestration of carbon in the environment. Many strategies have been proposed to sequester carbon, a few of which include extensive afforestation and reforestation, and pumping CO2 deep into oceans and geological layers [144]. In terrestrial ecosystems, carbon can be sequestered by increasing soil carbon content because more than 80 % of organic carbon is preserved in the soil [156].

In addition to soil amelioration, energy and industrial benefits, biochar also demonstrates properties to reduce potential environmental changes resulting from the amplifying atmospheric CO2 levels. Thermochemical conversion technologies, particularly pyrolysis and gasification, offer opportunities to mitigate GHG emissions. In other words, integration of pyrolysis and gasification with biochar application to soil leads to a net withdrawal of CO2 from the atmosphere. Biochar can be used to scrub CO2 through carbon sequestration and exclude NOx and SOx from the flue gas through air purification. Biochar applications to soil also appear to decrease emissions of N2O and CH4 that are 300 and 23 times more potent GHGs than CO2, respectively [157].

Photosynthesis results in the conversion of solar energy to chemical energy in the form of carbohydrates in plants. These carbohydrates in plants can be converted to biofuels such as alcohols, bio-oils, syngas and biogas through various thermochemical or biochemical conversion methods. The combustion of these biofuels results in the emission of CO2, which is again assimilated by plants through photosynthesis leading to the carbon cycle in nature. Although the liquid and gaseous biofuels could be burnt to produce heat energy and CO2 as the main products, yet biochar could retain carbon if applied to the soil. However, if plants fix more CO2, the buried biochar can act as a net storehouse of carbon.

As a result of pyrolysis, a part of biomass’ total carbon (~50 %) is released as bio-oils and gases, while the remaining 50 % of biomass carbon remains as the stable biochar residue [144]. The carbon in bio-oil is used for energy production to reduce fossil fuel usage, and the amount of CO2 liberated from their combustion is taken up by the plants during photosynthesis. In contrast, non-charred carbonaceous materials such as non-living plant or animal matter decompose in the soil to release carbon slowly over time. The release of carbon from natural decomposition of dead matter continues until most of it is lost within a decade from the soil. However, with its stable aromatic carbon, biochar can preserve it for centennial to millennial timescales. In addition, the carbon storage ability of biochar is dissimilar to biomass sequestration by afforestation, conversion to prairies or no-tillage agriculture. While agricultural lands converted to no-tillage may lose efficiency to capture additional carbon within two decades, forestlands eventually mature over decadal periods and start releasing CO2 [158]. These divergences make biochar a long-term sink in reducing CO2 emissions.

Pyrolysis generates about 3–9 times more energy than is invested with possibilities of sequestering the half of the carbon in soil through biochar [158]. Such an integrated system to withdraw CO2, while producing energy could transform pyrolysis from being a carbon-neutral to a carbon-negative technology (Fig. 3). In Brazil, carbonization processing of sugarcane bagasse has shown to optimize the pyrolysis process by producing biochar for use in household and industrial applications [54]. The study demonstrated the candidacy of sugarcane bagasse as an alternative to wood for char production, thereby preventing deforestation.
Fig. 3

Concept of next-generation biorefining system with carbon sequestration

Lee and Li [159] have suggested a strategy of integrated coal-firing that could convert CO2, NOx and SOx into valuable fertilizers, such as sodium bicarbonate and urea. In addition, the process can enhance CO2 sequestration, reduce groundwater contamination by NO3, and stimulate photosynthetic fixation of CO2. It is estimated that about 283 million tons of CO2 per year can be sequestered using this technology, which is equivalent to nearly 5 % of the total CO2 emitted from coal-fired plants globally. In some of the assessment studies to determine the longevity of biochar, it was found that biochar carbon did not show major weight loss over time. While biochar carbon indicated no loss, manure carbon showed 80 % loss from 500 days of an incubation study by Laird and Brown [160]. Biogenic organic residues act as temporary carbon source in soil with the half-life measured in weeks or months, whereas biochar acts as a source of carbon for millennia.

In order to better understand the carbon budget and carbon credit of an integrated bioenergy system, a life-cycle assessment approach is required. Life-cycle assessment is an approach to assess the environmental impacts associated with numerous stages of a product’s existence from “cradle-to-grave” including feedstock collection, processing, pretreatment, conversion, product recovery, upgrading, distribution, utilization and recycling [161]. For example, a few crucial factors to be considered in establishing a carbon budget program should include: (1) GHG emissions associated with the production of biomass; (2) CO2 release and energy consumption during logistics operations involving biomass production, harvest and transportation to refineries; (3) energy consumption during biomass pretreatment and conversion; and (4) energy consumption in final product processing e.g. bio-oil upgrading, product gas purification, biochar handling, etc. These factors could address a better knowledge about the amount of GHG emissions in the entire bioenergy system with the carbon balance in each step along with global warming abatement potential associated with carbon sequestration by biochar.

Carbon credits are set in the proportion of 1:1 in relation to the tons of carbon stored or removed from the atmosphere through sequestration. Approximately 8 Gt per annum of carbon is being added to the atmosphere through burning of fossil fuels [162]. This represents around 8–10 billion carbon credits being created every year. For instance, with a price tag of US $10 per carbon credit, 10 billion carbon credits represent a total amount of US $100 billion. These rates are anticipated to rise in the near future to US $85 per ton, which would globally represent an around $850 billion making it challenging to accept the social costs of climate change. At present, CO2 trading price by Chicago Climate Exchange is set at US $4 per ton [9]. At this point, the carbon credit economy would be comparable in size to the current fossil fuel economy. Hence, in this scenario the earth’s capacity for storing biochar is almost limitless.

Specialty Materials Manufacturing

Besides energy storage, carbon sequestration, water treatment, gas purification, and drug delivery; biochar has emerged as a functional resource for manufacturing several novel specialty materials. Amongst such novel specialty materials, supercapacitors, carbon nanotubes, graphenes as well as fillers and coloring agents for composites are gaining broad interest. Carbon materials mostly produced at temperatures above 1200 °C tend to have promising electrochemical properties [163]. One of the advanced uses of activated carbon from biomass and biochar is found in electrochemical energy and fuel cell catalysis as electrode materials.

A wide variety of waste residues that have been used to prepare activated carbon for supercapacitors include apple pulp, banana fiber, cassava peel, cherry stone, coconut shell, coffee shells, corn grains, egg shell, pistachio shell, potato starch, rice husk, rubber wood sawdust, seaweed, sorghum pith, sugarcane bagasse, sunflower seed shell, tea dust waste, waste newspaper, walnut shell and wheat straw [107]. With the surface area of these wastes range up to 3420 m2/g, the specific capacitance of the developed superconductors can be up to 368 F/g as summarized by Kalyani and Anitha [107]. A wide variety of electrolytes (e.g. NaNO3, KOH, KCl, HNO3, H2SO4, Na2SO4, ionic liquids, etc.) and activating agents (e.g. steam, CO2, ZnCl2, KOH, NaOH, HNO3, H3BO3, etc.) are used to produce activated carbons for electrochemical applications [107].

Beside lithium-ion batteries, supercapacitors are one of the most promising electrochemical energy storage systems available today and applied to wide-ranging devices from cell phones to hybrid-electric vehicles. With their remarkable power density, supercapacitors exhibit rapid charge and discharge abilities that make them ideal for the slender form-factor of today’s electronic gadgets. The electrodes used in capacitors are made up of three main materials, which are carbon-based, transition metal oxides and conducting polymers [164]. The new generation of supercapacitors or ultracapacitors have two oppositely charged electrodes made up of carbon-based materials such as graphite [165], activated carbon [166], carbon aerogels [167], carbon nanotubes [168], carbon nanofibers [169], carbon nanohorns [170], templated porous carbon and nitrogen-containing carbon [171]. The accumulation of ionic charge in the superconductors occurs on the double-layer at the electrode/electrolyte interface, and the storage of electrical charge is mostly non-Faradaic [172].

The carbon-based materials are gaining extensive consideration as electrodes for superconductors because of their abundant and inexpensive raw materials, relatively flexible preparation methods, non-toxicity, biodegradability, high chemical stability, and broad-ranging operating temperatures. Among all, activated carbon is widely used in capacitors because of its high porosity and large surface area that favor better charge accumulation at the interface with the electrolyte, therefore accumulating high capacitance [107]. However, there is a strong correlation between the pore structures of the carbon in electrode to supercapacitor’s performance [171]. As already discussed previously, the pore structure of activated carbon can be engineered by selecting appropriate biomass precursors and tuning the pyrolysis and activation conditions.

Biochars produced from forestry biomass, compressed milling wood residues and municipal wood wastes have been used as microbial fuel cell electrodes for reducing cost and carbon footprint [173, 174]. Supercapacitor electrodes have been generated from highly ordered macroporous biochar with ultra-high carbon content through pyrolysis and carbonization of red cedar wood at 750 °C without any post-treatment [175]. Mini-chunk and thin-film electrodes generated from maple wood biochar have shown good capacitive performance of ~30 F/g in supercapacitors with high stability and no decay even after 2600 cyclic voltammograms cycles [176]. Gu et al. [177] have also demonstrated the use of microporous bamboo biochar as cathodes for lithium-sulfur batteries.

Hydrothermal carbonization or wet torrefaction process is operated at high (300–800 °C) and low (180–260 °C) temperatures for 5–240 min at high (~100 MPa) and low (5 MPa) pressures [178]. The flexibility of the process helps in synthesizing a wide variety of carbon-based materials such as carbon nanotubes, graphite tubes, nanocells, olivary carbon particles, ellipsoidal carbon microparticles and mono-dispersed carbon microspheres from biomass and organic wastes [179]. These architectured carbon structures are gaining broad attention in pollutant sorption, catalysis, electrochemistry, drug delivery and bioimaging (for the detection of damaged cells and toxins).

Graphene is an allotrope of carbon arranged in a two-dimensional atomic-scale hexagonal lattice where one atom forms each vertex. It has strong mechanical properties (i.e. 207 times stronger than steel by weight), high electrical and thermal conductivities and mass-less transportation properties. These unique features of graphene have been applied in diverse areas such as ultrafast electronic devices, molecular resolution sensors, biochips, biopolymers, biocomposites, liquid–crystal devices, microfluidics and magneto-resistive or Quantum Hall devices [180, 181]. Advanced materials such as carbon aerogels, carbon nanotubes, carbon nanohorns and templated porous carbons are generated from graphene. However, the synergy between biochar application and the emerging graphene technology have led to the next generation synthesis of these advanced materials.

The engineered graphene-coated biochar from cotton wood has shown excellent adsorption ability of polycyclic aromatic hydrocarbons with a maximum methylene blue adsorption capacity of 174 mg/g that was 20 times higher than that of the unaltered cotton wood biochar [181]. In a study by Inyang et al. [182], hybrid multi-walled carbon nanotube-coated biochar was synthesized from slow pyrolysis of hickory and bagasse. The carbon nanotube-biochar composites exhibited the greatest thermal stabilities, surface areas (351–390 m2/g), pore volume (0.14–0.22 cm3/g) and sorption capacities for methylene blue (6.2 mg/g).

Nanostructured carbon containing graphene nanosheets with high capacitance (>200 F/g) have been successfully prepared from biochar obtained from the pyrolysis of dried distillers grains with solubles [183]. The resulting biochar with high specific surface area (2959 m2/g) and high pore volume (1.65 cm3/g) were used in supercapacitors with improvement of capacitance by HNO3 surface oxidation. Interconnected graphitic carbon nanosheets (10–30 nm in thickness) with high specific surface area (~2287 m2/g) and mesoporosity up to 58 % were derived from hemp bast fiber biochar [184]. These biochar-derived carbon nanosheets had good electrical conductivity (211–226 S/m) suggesting their applications towards ultrafast supercapacitors with high energy output.

Carbon aerogels are prepared by the pyrolysis of aerogels of resorcinol and formaldehyde that have intense mesopore structures [185]. Carbon aerogels measuring about 4–9 nm are interconnected with each other to form a network of inter-particle mesopore structure. CO2 activation technique has proven to converting micropores to mesopores in carbon aerogels and doubling the capacitance [186]. Like other specialty materials, carbon nanotubes are attractive electrode materials because of their large surface area allowing greater storage spaces for electrolyte ions and high electrical conductivity [171]. The single-walled nanotubes and multi-walled nanotubes typically have diameters of about 0.8–2 and 5–20 nm, respectively [187]. Carbon nanotubes are unified cylinders of single or multilayers of graphene with open or closed ends.

Attributing to their outstanding durability and light-weightiness, carbon nanotubes have found resourceful applications in designing wind turbine blades, maritime security boat hulls, anti-corrosion coatings, self-cleaning textiles, superconducting wires, transparent conductors for touch-screen devices, thin-film transistors, and biosensors for real-time detection of toxic gases or biohazard [187]. Another breakthrough in carbon nanotube research is the incorporation of graphene in electrodes for the development of thin and highly flexible electrochemical capacitors that could help design power dressing [188]. The power dressing is a new smart concept of self-powering garments with piezoelectric spots that could harvest and store energy from body movements.

Other nanocarbons such as fullerenes, carbon nanohorns and graphene have made their way into photovoltaic and optoelectronic applications. Single-walled carbon nanohorns are graphene tubules with horn-shaped tips of 30–50 nm in length laser-induced ablation and 2–5 nm in diameter that assemble to form a spherical aggregates of 80–100 nm in diameter [189, 190]. The production and application of carbon nanohorns are appealing because of their metal-free synthesis via CO2 laser-induced ablation of graphitic moieties leading to high yields with purities [191]. These carbon nanohorns have been applied to supercapacitor electrodes, dye-sensitized solar cells, gas storage and biosensing [190]. As carbon nanotubes do not contain any toxic metals or contaminants, they act as nanocontainers with large internal volume for loading drugs, contrast agents and sensitizers [189]. Being the thinnest yet flexible material known today, graphene is about hundred times stronger than steel and conducts heat much better than diamond. Graphene’s optical and electronic properties along with its robustness and flexibility is creating breakthroughs in innovating foldable, touch-sensitive and transparent smartphones [192]; inexpensive solar cells; and sensors for detecting single gas molecules or individual DNA (deoxyribonucleic acid) bases [193].

Templated porous carbons are produced by template method can produce both microporous and mesoporous carbons depending on the template used and precursor [171]. Recently, soft-templating method to synthesize mesoporous carbon (pore size: 3–30 nm and surface area: 280–1580 m2/g) is attracting attention due to the ease in approach and flexibility in controlling the pore structure. The carbon precursors are small clusters of resorcinol–formaldehyde (RF) resin and phloroglucinol-formaldehyde (PF) resin derived from acid- or base-catalyzed polymerization of phenol and formaldehyde [194]. The materials used as templates in developing templated porous carbons are mesostructured silica, silica sol, silica gel, opal, zeolites and clay [195]. The porous carbons are chemically inert having hydrophobic surfaces with high surface area, large pore volumes and good mechanical and thermal stability that make them an ideal industrial adsorbent.

Nonetheless, heteroatom doped carbon materials represent a class of emerging specialty materials for use in fuel cells and batteries, electrocatalysis, hydrogen storage and supercapacitors. One of such resources is nitrogen-containing carbon materials or N-doped carbons that have pseudo-capacitance despite lacking high surface area. N-doped carbons are made of pyridine and quinoline using mica templates, carbonized polyaniline and carbonized melamine foam [171]. The significant gain in capacitance by nitrogen doping is due to Faradaic reaction of nitrogen-containing functional groups [172]. Moreover, boron-doped carbon, sulfur-doped carbon and phosphorous-doped carbon are also being available as advanced doping concepts for carbonaceous materials. Boron alters the electronic structures of carbon but using boron and nitrogen together as dopants can yield synergistic effects in supercapacitors and lithium-ion batteries [196]. Sulfur and phosphorus doping to carbon leads to the formation of defect sites and favorable spin densities throughout the carbon structure due to the similar electronegativities of carbon and the dopants. Similar to other nanocarbons, these heterocarbons can be used for hydrogen storage and in supercapacitors [171].

Another use of biochar could be potentially in developing biocomposites. Several natural composites from polypropylene resin and lignocellulosic residues have been used predominantly to manufacture automotive components such as door panels, seat backs, package trays and trunk liners. Wood-plastic composites have also been employed in different sectors such as household, construction, packaging, automotive, domestic and aerospace. The primary drivers of such composites are low-cost raw materials, light-weight properties, less production cost and lower carbon footprint [197]. Although manufactured both as thermoplastics and thermosetting bioplastics, these natural fibers do not excel in strength and robustness when compared to glass and carbon fiber [198]. The tendency of wood biomass to attract moisture decreases the mechanical properties of the wood-plastic composite causing swelling and dimensional instability, thus compromising with the product’s esthetics [199].

In contrast to the drawbacks of wood-plastic composite (e.g. hydrophilic product, poor mechanical and thermal stability, weak molecular interaction of wood and synthetic polymer), use of biochar as a filler in manufacturing advanced biocomposites cannot be counteracted [200]. Although this particular biochar application is in its infancy, a few studies have demonstrated the use of biochar as a filler material in biocomposite development [201, 202]. Ahmetli et al. [201] used polyethylene terephthalate waste char in combination with pine cone char to develop a biocomposite that was found to have thermal stability, surface hardness, tensile strength and Young’s modulus relatively higher than the pure epoxy polymer matrix. Corn starch, corn flour and corn stover biochar and wood-derived biochar have been evaluated as alternative substitutes for carbon black (as filler) in rubber composites using carboxylated styrene-butadiene (as the polymer matrix). The biochar-polymer composite had better reinforcement characteristics, higher tensile strength and toughness compared to the carbon black-filled composite [202]. These promising characteristics of biochar-polymer composites open vast opportunities in engineering resilient biocomposites that will be more resistant to weather impacts with improved durability and thermal stability.

Conclusions and Perspectives

Biochar is a potential bioresource for energy and biomaterial production, soil amelioration, pharmaceuticals and carbon sequestration. However, these applications depend on the physicochemical and electrochemical properties of biochar. Pyrolysis temperature, heating rate and residence time affect biochar yield and properties. Biochar produced at higher pyrolysis temperatures are effective in adsorption of organic contaminants due to the increase in surface area, microporosity and hydrophobicity. On the contrary, biochar obtained at lower temperatures have better electrostatic attraction towards cationic nutrients in the soil. More precisely, high temperature leads to the increase in fixed carbon content, ash, inorganic elements, pH, surface area and porosity. Conversely, lower temperatures result in an increase in biochar yield, volatile matter, electrical conductivity and cation-exchange capacity.

Biochar enhances plant growth by increasing the bioavailability of water and essential plant nutrients, providing microenvironments from proliferation of essential soil microorganisms. The amendment of biochar to soil neutralizes or optimizes the pH, increases total nitrogen and phosphorus, promotes greater plant root development, hosts beneficial soil fungi and bacteria, and improves cation exchange capacity. Biochar acts as a soil conditioner and fertilizer by improving cation-exchange capacity, water retention ability and sequestering toxic heavy metals. The alkaline nature of biochar also reduces the acidity of soils caused by excessive chemical fertilizer application. Most significantly, the recalcitrant carbon in biochar, inherited from the waste biomass, can remain in the soil for centuries making it less-labile to wider soil temperatures and mineralization. Since biochar retains considerably higher amounts of stable carbon in the soil than any uncharred (decomposing) organic matter, it serves as a potential carbon sequestering agent.

Activated carbon and other porous carbon materials have about fifty times higher surface area than its precursor, making it an attractive biomaterial for applications in chemical reactions, contaminated air and water treatment, biomedicine and several other commercial appliances. Activated carbon acts an antidote to toxins and drug overdose that makes it suitable as an emergency medicine. Last but not the least, biochar-derived specialty materials such as graphene, carbon nanosheets, carbon nanotubes, carbon nanofibers, carbon nanohorns, carbon aerogels, templated porous carbon and doped-carbon have found profound applications in photovoltaics, supercapacitors, fuel cell catalysis, drug delivery systems and biocomposite fillers.

Understanding the physicochemical nature of biochar is critical in determining its assorted uses in bioenergy, carbon sequestration, soil amendment, contaminant immobilization, biomedical studies and specialty materials engineering. Despite these benefits, some trade-offs have to been identified. One of such trade-offs is predictable between maximizing biofuel (i.e. bio-oil and syngas) production and deploying biochar recovery simultaneously. Furthermore, biochar has lower abrasive resistance than any commercial catalyst in Fischer–Tropsch process. Biochar with high ash content also exhibits low voltage and power outputs in fuel cell applications. Moreover, for hydrogen storage, biochar necessitates certain surface treatments thereby escalating the process expenditure. In terms of environmental waste remediation, the long-term effectiveness of char-derived activated carbon in organic and inorganic contaminant immobilization is also uncertain.

Although, biochar is effective in enhancing the soil fertility and plant growth by proliferating beneficial soil microorganisms, it is still unclear to understand the ecological interactions between biochar, soil, plant roots and microorganisms. Conversely, life-cycle assessment can help identify strategies to balance the multifarious biochar benefits over these few trade-offs. Such analysis can motivate biorefineries to practise carbon sequestration scheme, i.e. utilizing the main product of interest (bio-oil) while recycling the byproduct (biochar). Besides, long-term environmental experiments and assessment studies are vital to interpret the implications of biochar in pollutant remediation, soil improvement and carbon sequestration. Nonetheless, advanced research on graphene, supercapacitors, fuel cells, biocomposites and other specialty materials, which are of contemporary interest, could help widen the candidacy of biochar in a commercial niche.

Notes

Acknowledgments

The authors would like to thank Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chair (CRC) program for funding this bioenergy research.

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Copyright information

© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  • Sonil Nanda
    • 1
  • Ajay K. Dalai
    • 2
  • Franco Berruti
    • 3
  • Janusz A. Kozinski
    • 1
  1. 1.Department of Earth and Space Science and Engineering, Lassonde School of EngineeringYork UniversityTorontoCanada
  2. 2.Department of Chemical and Biological EngineeringUniversity of SaskatchewanSaskatoonCanada
  3. 3.Institute for Chemicals and Fuels from Alternative Resources, Department of Chemical and Biochemical EngineeringUniversity of Western OntarioLondonCanada

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