Briquettes from maize cobs and Ceiba pentandra at room temperature and low compacting pressure without a binder

Abstract

This paper reports the findings of densifying ground maize cobs and Ceibapentandra sawdust at room temperature using low compacting pressurewithout a binder. The maize cobs were crushed using a hammer mill. Particlesizes of maize cobs and C. pentandra used for the study were≤1 mm. The two materials were combined at mixing percentages of90:10, 70:30 and 50:50 (C. pentandra/maize cobs). Briquettes wereproduced using a laboratory hydraulic press. Compacting pressure was varied from20 to 50 MPa at an interval of 10 MPa. The results indicated that therelaxed density of briquettes produced from particles of maize cobs only rangedfrom 541 to 659 kg/m³ whilst that made from a mixture of maizecobs and C. pentandra ranged from 565 to 774 kg/m³.Compressive strength in cleft of briquettes produced from maize cobs only rangedfrom 0.12 to 0.54 N/mm whilst that produced from a combination of maizecobs and C. pentandra ranged from 7.72 to 59.22 N/mm.Additionally, at all compacting pressure levels, briquettes made from maize cobsonly had an impact resistance index of 0%. Whilst those made from a combinationof maize cobs and C. pentandra ranged from 115% to 500%. Thus,briquettes with adequate physical and mechanical characteristics could beproduced from maize cobs at room temperature using low compacting pressure whenmaize cobs are combined with sawdust of C. pentandra. These findingscould enhance the existing technology for densifying agricultural residues, forexample, maize cobs, especially in rural communities.

Background

The solution to the current and future energy needs of Ghana requires diversifyingthe energy sources. Previous studies suggested that energy production methods shouldbe best matched with the available natural resources in the region [1]. Ghana is an agricultural country and has an estimated workforce of over60% employed by the agricultural sector [2]. This culminates into the cultivation of various crops such as millet,rice, maize, sugar cane and cocoa by farmers. This leads to the generation of largevolumes of agricultural crop residues such as maize cobs, rice husk, millet stalks,baggasse and cocoa bean shells as presented in Table 1.These agricultural residues normally obtained from field and processing sites areoften left to rot or burnt inefficiently in their loose form causing air pollution [3].

Table 1 Production of selected agricultural crops and estimated potentialresidues in Ghana in the year 2010

As shown in Table 1, by the end of the year 2010, about1,872,000 tonnes of maize residue was generated in Ghana. This constitutes about 41%of the total agricultural crop residues generated. Maize cob, a residue of the maizecrop, is a lignocellulosic biomass material which contains high amounts of organicconstituents and energy. Therefore, it is recognised as a potential source ofrenewable energy [5]. However, its use as a domestic fuel presents its own challenges. Itsstructure is porous and has a low bulk density. The bulk density of a crushed maizecob is 227 kg/m³ and it is more than double the bulk density ofuncrushed cobs which is 104 kg/m³[6]. This low bulk density makes it difficult to handle, store and transportand also results in low energy density. Densification which employs high pressure tocompress biomass raw materials in order to increase their density could be used tomitigate these challenges. Densification of maize cobs will result in improvedenergy density, lower volume (higher bulk density), and better mechanical handling [7, 8]. However, maize cobs, is noted to have low lignin content(5.6%), low water soluble carbohydrates (1.1%) and low protein (2.5%). Thesechemicals are largely responsible for forming solid bridge bonds duringdensification or briquetting [9, 10]. Therefore, densification of maize cobs would require a high compactingpressure and/or an external binder. A considerable amount of research on briquettingtechnology on maize cobs has been conducted. Previous studies showed that pressingmaize cob particles at a temperature of 25°C, using a compacting pressure of150 MPa, resulted in the production of briquettes having a durability of 0% anda relaxed density of 940 kg/m³[9]. Further studies indicated that when maize cob particles were preheatedto 85°C before pressing at 150 MPa, the durability and density were raisedto 90% and above 1,100 kg/m³, respectively [9]. Even though improved binding of maize cob particles due to preheating to85°C is feasible, this could invariably increase production cost as a result ofincreased energy input therefore limiting its utilisation. Thus, it is imperativethat one selects the most economic technology for densification or briquetting ofmaize cob particles to reduce the cost of the finished product. Ceibapentandra is a low-density species (409.22 kg/m³) withacid-insoluble lignin and alpha-cellulose content of 24.34% and 41.24%, respectively [11]. Briquettes produced from sawdust of C. pentandra at roomtemperature (28°C) and low compacting pressure (20 to 50 MPa) without abinder had adequate relaxed density, high compressive strength in cleft and highimpact resistance index (IRI) [11, 12]. Studies on densified fuels derived from blends of two biomass materialsindicate that the mechanical strength of briquettes produced from only one type ofbiomass can be improved by blending that biomass with another biomass material [13]. Therefore, this study seeks to investigate the effect of combining maizecob particles and sawdust of C. pentandra on the relaxed density,compressive strength in cleft and impact resistance index of briquettes produced atroom temperature using low compacting pressure.

Methods

Materials and material preparation

Sawdust of C. pentandra and maize cobs were used for the study. Both themaize cobs and sawdust were sun-dried at an average relative humidity andtemperature of 75% and 28°C, respectively, for 5 to 7 days. The maizecobs were crushed using a hammer mill. Particle sizes of maize cobs and sawdustused for the study were 1 mm or less. The two materials were combined atmixing percentages of 90:10, 70:30 and 50:50 (C. pentandra/maizecobs).

Moisture content

The moisture content, on oven-dry basis, of the crushed maize cobs and sawdustwas determined in accordance with [14]. Five samples of sawdust, maize cobs particles and their combination,each weighing 2 g were weighed and placed in a laboratory oven at a temperatureof (103°C ± 2°C). Each sample was dried until thedifference in mass between two successive weighings separated by an interval of2 h was 0.01 g or less. The moisture content of the specimen was thencomputed as follows:

$$\mathrm{Moisture}\mathrm{content}\left(\mathrm{\%}\right)\mathit{db}=\frac{M_1-M_o}{M_o}\times 100,$$

(1)

where M₁ and M_o were masses (g) of test samples before drying and after ovendrying, respectively. On the average, the moisture content of the maize cobparticles, C. pentandra and the mixed samples were 9.00%, 13.27% and9.21%, respectively.

Briquetting process

A 55.3-mm ID × 52.5-cm height cylindrical mould was used toproduce the briquettes. Ninety grammes of each biomass material was weighed andfilled into the mould. A manual hydraulic press with a gauge and piston was usedto compress the biomass raw material without a binder to form the briquettes. Aclearance of about 0.1 mm was provided between the piston and the innerwall of the mould to allow for air escape. The samples were then pressed usingthe following predetermined compacting pressure levels: 20, 30, 40 and50 MPa. The dwelling time for each press was maintained at 10 s. Thisprocess was repeated for all the biomass materials.

Physical and mechanical properties of briquettes

The relaxed density, compressive strength in cleft and impact resistance index ofthe briquettes produced were investigated using standard testing methods.

Relaxed density

Relaxed density (RD) of the briquettes was determined 30 days after removalfrom the press in accordance with [15]. The mass of briquettes were determined using a laboratory electronicbalance with an accuracy of 0.01 g. The diameter and length of a briquettewere measured at three points with a digital vernier calliper. Relaxed densitywas then computed as follows:

$$\begin{array}{l}\mathrm{RD}\left(\raisebox{1ex}{$\mathrm{g}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{cm}}^3$}\right.\right)\\ =\frac{108000\times M\left(\mathrm{g}\right)}{\pi {\left[d_1\left(\mathrm{mm}\right)+d_2\left(\mathrm{mm}\right)+d_3\left(\mathrm{mm}\right)\right]}^2\times \left[l_1\left(\mathrm{mm}\right)+l_2\left(\mathrm{mm}\right)+l_3\left(\mathrm{mm}\right)\right]},\end{array}$$

(2)

where d₁, d₂ and d₃ were diameters (mm) measured at three different points on thebriquettes. L₁, L₂ and L₃ were lengths (mm) measured at three different points on thebriquettes. M (g) is the mass of briquette.

Compressive strength

Compressive strength in cleft of briquettes was determined in accordance with [16] using an Instron universal strength testing machine (Norwood, MA,USA) with a load cell capacity of 100 kN. The crosshead speed was0.305 mm/min. A sample of briquette to be tested was placed horizontally inthe compression test fixture and a load was applied at a constant rate of0.305 mm/min until the briquette failed by cracking. The compressivestrength in cleft was then computed as follows:

$$\begin{array}{ll}\mathrm{Compressive}& \mathrm{strength}\mathrm{in}\mathrm{cleft}\left(\raisebox{1ex}{$\mathrm{N}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{mm}$}\right.\right)\\ =\frac{3\times \mathrm{The}\mathrm{load}\mathrm{at}\mathrm{fracture}\mathrm{point}\left(\mathrm{N}\right)}{\left[l_1\left(\mathrm{mm}\right)+l_2\left(\mathrm{mm}\right)+l_3\left(\mathrm{mm}\right)\right]}\end{array}$$

(3)

Impact resistance index

IRI of the briquettes produced was determined in accordance with [17], using the drop shatter test for coal. Five drops were set as thestandard. Briquettes were released from a vertical height of 2 m andallowed to freely fall onto a concrete floor. After five drops, the brokenpieces of briquettes as a result of the impact were collected and weighed usingan electronic balance with an accuracy of 0.01 g. Only the number of pieceswhich weighed 5% or more of the initial weight was recorded for the purpose ofcalculating the impact resistance index. The impact resistance index wascomputed as follows:

$$\mathrm{IRI}\left(\%\right)=\frac{N}{n}\times 100,$$

(4)

where N is the number of drops and n is the number of piecesthat weighed 5% or more of the initial weight of briquette after Ndrops.

Results and discussions

Relaxed density of briquettes produced from maize cob particles, Ceibapentandra and their mixture

Relaxed density of briquettes produced from maize cobs, C. pentandra andtheir combined materials are presented in Table 2.Relaxed density of briquettes produced from only C. pentandra rangedfrom 523 to 716 kg/m³ while that produced from maize cobparticles only ranged from 541 to 659 kg/m³.

Table 2 Relaxed density (kg/m ³ ) of briquettes produced from maize cobs, C. pentandra and their combination

Additionally, the relaxed density of briquettes obtained from a mixture of maizecob particles and C. pentandra ranged from 565 to774 kg/m³. The relaxed density of briquettes obtained fromall the biomass materials used for the study could be considered adequate sincethey fall within the recommended values of relaxed density for briquette madeusing the hydraulic press. It has been previously reported that briquettes madefrom hydraulic piston press are usually less than 1,000 kg/m³and fall between 300 to 600 kg/m³[18, 19]. It could be further inferred from this study that the relaxeddensity of all the briquettes produced from the combination of maize cobs andC. pentandra particles were higher than that of their correspondingvalues for the pure biomass materials. This may be due to the variations in thecomposition and structure of these two materials (C. pentandra andmaize cob) which resulted in better rearrangement and densification when mixedtogether and pressed.

Correlation analysis between proportion of maize cob particles in the mixture andrelaxed density of briquettes produced indicated that there was no significantcorrelation between the two parameters (Pearson’sr = 0.166, p value = 0.102;N = 60, α = 0.05; one-tailed).However, the compacting pressure was found to have a high significant positivecorrelation with the relaxed density of the briquettes produced from the mixtureof C. pentandra and maize cob particles (Pearson’sr = 0.969, p value = 0.000;N = 60, α = 0.05; one-tailed).Increased compacting pressure level resulted in a reduction in intermoleculardistance as well as increased crushed cell walls, therefore resulting in theformation of denser briquettes.

Analysis of variance (Table 3) on the effect ofbiomass raw material (maize cobs, C. pentandra, and their combination)and compacting pressure on relaxed density of briquettes produced indicated thatat 5% level of significance, the biomass raw material, compacting pressure andtheir interactions have significant effect on the relaxed density of thebriquettes produced (p value < 0.05). The multiplecoefficient of determination (R²) and root means square error of the ANOVA model were 0.9730 and12.93, respectively. The R² value of 0.9730 indicates that the biomass raw material andcompacting pressure could explain about 97.30% of the variance in the relaxeddensity of briquettes produced.

Table 3 ANOVA of effect of biomass raw material and compacting pressure onthe relaxed density of briquettes

Compressive strength in cleft of briquettes produced from maize cobs,Ceiba pentandra and their combination

Table 4 shows the results of compressive strength incleft of briquettes produced from sawdust of maize cob particles, C.pentandra and their combination. The result indicates that at allcompacting pressure levels, the compressive strength in cleft of briquettesproduced from maize cob particles only was very low, ranging from 0.12 to0.54 N/mm. Compressive strength in cleft of briquettes produced from C.pentandra only, was very high and ranged from 29.23 to 44.58 N/mm.Furthermore, the compressive strength in cleft of briquettes produced fromcombination of maize cob particles and C. pentandra ranged from 7.72 to59.22 N/mm.

Table 4 Compressive strength in cleft (N/mm) of briquettes produced usingcompacting pressure levels from 20 to 50 MPa

This result suggest that briquettes produced only from maize cob particles at lowcompacting pressure and room temperature will not have adequate compressivestrength in cleft for handling (compressive strength in cleft<19.6 N/mm). However, the compressive strength in cleft of briquettesproduced from maize cob particles could be improved significantly when it iscombined with sawdust from C. pentandra. According to Rahman et al. [20], the briquettes surface compressive strength (i.e. compressivestrength in cleft) of 19.6 N/mm is reasonably adequate for handling or canbe used as fuel for domestic purposes. With the exception of briquettes producedfrom the 70:30 mixing ratio and pressed at a compacting pressure of 20 MPa,all the briquettes produced from 90:10 and 70:30 mixing ratios had adequatecompressive strength in cleft (compressive strength in cleft>19.6 N/mm). Additionally, briquettes produced from the 90:10 (C.pentandra/maize cobs) mixing ratio and pressed at 40 and 50 MPahad compressive strength in cleft higher than their corresponding values forC. pentandra only. Thus, the addition of 10% maize cob particles tosawdust of C. pentandra and pressed at compacting pressures of 40 and50 MPa will significantly improve the compressive strength in cleft ofbriquettes produced from C. pentandra. This result confirms theresearch finding of other researchers who undertook a study on densified fuelsderived from a blend of two biomass materials. Previous studies [13, 21, 22] indicated that the durability and mechanical strength of briquettesproduced from only one type of biomass materials could be improved by blendingthat biomass with another biomass material. For instance, the durability ofwheat straw briquette was enhanced by blending the straw with wood waste [13]. Correlation analysis between proportions of maize cob particles inthe mixing ratio and compressive strength in cleft of briquettes produced,indicated a strong negative correlation between the two parameters(Pearson’s r = -0.770, pvalue = 0.000; N = 60,α = 0.05; one-tailed). This implies that for the mixedbiomass briquettes, an increase in proportion of maize cob particles in themixture resulted in a decrease in compressive strength in cleft of briquettesproduced. Compacting pressure was also found to have a high significant positivecorrelation with the compressive strength in cleft of briquettes produced fromthe combination of maize cob and C. pentandra particles(Pearson’s r = 0.582, pvalue = 0.000; N = 60,α = 0.05; one-tailed).

Two-way analysis of variance (Table 5) on the effectof biomass raw material and compacting pressure on the compressive strength incleft of briquettes produced indicated that at 5% level of significance thebiomass raw material, compacting pressure and their interaction had significanteffect on compressive strength in cleft of briquettes produced (pvalue < 0.05).

Table 5 ANOVA of effect of biomass raw material and compacting pressure oncompressive strength in cleft of briquettes produced

The multiple coefficient of determination (R²) and root mean square error of the ANOVA model were 0.9813 and2.5939, respectively. Thus, about 98.13% of the variability in compressivestrength in cleft of briquettes produced could be explained by the biomass rawmaterial and compacting pressure.

Impact resistance index of briquettes produced from maize cobs, Ceibapentandra and their combination

The impact resistance index of briquettes produced from maize cobs, C.pentandra and their combination is presented in Table 6. The results show that at all compacting pressure levels,briquettes produced from only maize cob particles had a very weak or no impactresistance index (%). This may be due to the low lignin content, low watersoluble carbohydrates and low protein in the maize cobs. The impact resistanceindex of briquettes produced from the combination of maize cob and C.pentandra particles ranged from 115% to 500%. All the briquettesproduced from this combination of biomass materials had adequate impactresistance index, that is, the impact resistance index greater than 100%, theminimum value set for this study.

Table 6 Impact resistance index (%) of briquettes pressed using compactingpressure levels from 20–50 MPa

According to the Italian standard for briquettes/pellets (CTI-R04/5) as cited in [23], briquette durability ≥97.7% is adequate. Additionally, thedurability characteristics of briquettes is classified as high when impactresistance index is greater than 80% [24, 25]. From the results obtained in this study, it was observed that whenmaize cob particles is combined with the sawdust of C. pentandra, theimpact resistance index of the briquettes produced was significantly improved.Briquettes produced from the combination of C. pentandra and maize cobparticles at 90:10 mixing ratio showed a very high impact resistance index thanthat produced from just the C. pentandra alone. The impact resistanceindex ranged from 200% to 500%. Furthermore, at the same mixing ratio of 90:10and compacting pressure of 50 MPa, the briquettes produced did not show anydisintegration after being dropped five times from a height of 2 m onto theground. Correlation analysis between proportions of maize cobs in the mixingratio and impact resistance index of briquettes produced indicated a strongnegative correlation between the two parameters (Pearson’sr = -0.577, p value = 0.000;N = 60, α = 0.05;one-tailed). Thus, for a mixture of C. pentandra and maize cobparticles, increases in the proportions of maize cob particles in the mixturecould result in a decrease in the impact resistance index of the briquettesproduced. Additionally, compacting pressure was found to have a high significantpositive correlation with the impact resistance index of briquettes producedfrom the combination of maize cob and C. pentandra particles(Pearson’s r = 0.614, pvalue = 0.000; N = 60,α = 0.05; one-tailed).

A two-way analysis of variance (Table 7) to determinethe effect of biomass raw material and compacting pressure on impact resistanceindex of briquettes produced indicated that at 5% level of significance, thebiomass raw material, compacting pressure and their interactions had significanteffect on the impact resistance index of briquettes produced (pvalue < 0.05). The multiple coefficient of determination and rootmean square error of the ANOVA model were 0.8152 and 77.42, respectively. Thus,about 81.52% of the variance in the impact resistance index of briquettesproduced could be explained by the biomass raw material and compacting pressure.This result confirms that characteristics of the raw material used significantlyhave an effect on the quality of briquettes produced.

Table 7 ANOVA of effect of biomass raw material and compacting pressure onimpact resistance index of briquettes

Conclusion

This study examined the characteristics of briquettes produced from a combination ofmaize cob particles and C. pentandra sawdust at room temperature and lowcompacting pressure without a binder. From the study, it can be concluded that

• 1. Briquettes produced from maize cob particles at room temperature using low compacting pressure does not have adequate compressive strength in cleft (compressive strength in cleft <19.6 N/mm) and impact resistance index (0%) for handling, storage and transporting. However, it has adequate relaxed density.

• 2. Combining C. pentandra and maize cob particles at mixing proportions 90:10 and 70:30 and pressed with compacting pressure 30 MPa or more at room temperature will produce briquettes with adequate compressive strength in cleft (compressive strength in cleft ≥19.6 N/mm).

• 3. Combining C. pentandra and maize cob particles at mixing proportions 90:10, 70:30 and 50:50 and pressed with compacting pressure 30 MPa or more at room temperature will produce briquettes with adequate impact resistance index (impact resistance index >100%).

• 4. The proportion of maize cob particles in a mixture of maize cobs and C. pentandra sawdust significantly affects the compressive strength in cleft and impact resistance index of briquettes produced.

• 5. The type of biomass material and compacting pressure has a significant effect on the relaxed density, compressive strength in cleft and impact resistance index of briquettes produced from maize cobs and sawdust of C. pentandra.

This study therefore reveals that it is possible to produce briquettes with adequatephysical and mechanical properties from maize cobs when it is combined with sawdustof C. pentandra.