The Government of the Republic of Indonesia states that the thermal energy for hot-mixed asphalt production shall be supplied by the direct combustion of fossil fuels in the form of diesel oil, natural gas, or fuel gas from coal gasification which may generate GHG emission. Biomasses are able to substitute the fossil fuels through gasification technology. Gasification converts the biomass using limited air into gaseous fuel containing mainly CO and H2 that are subsequently combusted to produce heat, carbon dioxide, and water. It is obvious that the CO2 is then absorbed by the plants for photosynthesis, maintaining a balanced closed cycle. This study examines the level of global warming potential of this system for supplying heat based on the openLCA v1.9 software. The analysis used a gate-to-gate approach to evaluate scenarios of shell gasification to produce 1 metric tonne of hot-mixed asphalt. The scope covers raw material supply and transportation, palm kernel shell gasification, and products. The evaluation concludes that gasification could potentially reduce CO2 emissions. Environmental impact analysis and interpretation of the results using the openLCA database of Traci 2.1 recommend that greater CO2 emission reduction is possible using palm kernel shell gasification, not only for supplying heat but also for electricity generation to operate all electrical equipments.
The economic development of a nation is closely related to the development of infrastructure: a good infrastructure is one of the driving forces for economic growth in the region. The role of infrastructure in the development of regions and cities contributes significantly to economic, social and environmental aspects. For example, the construction of road infrastructure is an important activity in promoting the economic growth of such regions. This public work encourages an increase in productivity of certain production processes. As the road infrastructure development programme in Indonesia continues to be boosted, the demand for hot-mixed asphalt (HMA) as supporting material for road construction is increasing [1,2,3]. This is proportional to the energy requirements as examined by Thives and Ghisi  that an asphalt mixing plant (AMP) requires energy for aggregate heating. Unfortunately, the government regulation based on the Ministry of Public Works Policy No.10/SE/M/2011 only covers the use of diesel, natural gas and coal gasification for supplying this heating energy . Every ton of HMA requires 7.6–11.4 L of diesel fuel , equivalent to energy supply from direct combustion of 10.49–12.45 Nm3 natural gas or 14.46–18.15 kg coal through coal gasification .
Provision of energy using diesel fuel through direct combustion in engines or burners results in combustion emissions that have the potential for global warming [7,8,9]. According to the report of Renewables 2012 Global Status Report, the supply of energy through direct combustion of fossil fuels still dominates energy supply to industry. Combustion reactions of liquid fuel and gaseous fuel mixture such as aviation kerosene and natural gas at various compositions emit CO2, CO and H2O . With about 300–350 asphalt mixing plants in Indonesia that operate with the direct combustion of fossil fuels, CO2 generation potentially increases. One of the efforts to reduce this emission involves replacing fossil fuels with abundantly available renewable energy sources such as palm kernel shell, which is sustainable and CO2-neutral. The substitution of fuel sources by renewable energy is essential to reduce the need for fossil fuels, which are running low, and to reduce greenhouse gas emissions.
Biomass is a carbon-neutral renewable resource [11, 12]. In Indonesia, one of the biomasses that is likely to be an attractive energy source is palm kernel shell (PKS). This is a waste product of the palm oil production process, wherein typically every ton of fresh palm oil fruit bunches generates 6.73% PKS waste, 23.83% empty fruit bunches (EFBs), 13.47% palm mesocarp fibre (PMF), 4.14% wet decanter solid, and 51.81% liquid waste [13, 14]. According to previous research, the calorific values of PKS, EFB and PMF are 16.14–19.45 MJ/kg [15,16,17], 16.46–18.31 MJ/kg and 17.1–18.09 MJ/kg, respectively [18, 19]. PMF, together with some PKS, is already utilised by palm oil producers to generate steam and electricity, while the EFBs are generally converted to a potassium source for fertiliser. It is, however, the abundant quantity of PKS available as a renewable energy source. This is supported by the productivity of the Indonesian palm oil industry, which supplies almost 62% of the world’s demand for palm oil .
Besides its relatively high calorific value, PKS has suitable physical properties, such as shape, thickness, bulk density and specific gravity  that are appropriate for its conversion into fuel gas through gasification. Biomass gasification with air as gasifying agent is a flexible and reliable energy conversion to convert solid biomass into gaseous fuel, also known as producer gas, the major components of which are CO, H2, CH4, CO2, and N2 [22,23,24,25]. This producer gas can be used as a substitute for fossil fuel for aggregate heating energy in an AMP. Biomass gasification technology offers a free of solid particulate combustible gas such as soot and ash which contaminate the quality of heated aggregates compared with direct biomass combustion.
Although renewable energy sources such as biomass are considered carbon-neutral, GHG emissions continue to be generated. Utilisation of bioenergy through gasification of PKS emits GHGs at a lower level than palm oil mill processes, shell handling and transportation, and electricity generation for operating the gasification system. Several experiments related to GHG emissions of biomass gasification have already been performed using life cycle analysis (LCA). A previous research reported the CO2 equivalence of the combustion of producer gas from Canadian pine wood gasification in a fluidised bed gasifier and an entrained bed gasifier for producing hydrogen. The entrained bed gasifier showed better performance than the fluidised bed gasifier . It was also observed that generating 1 kWh electricity from on a mixture of 80% (w/w) woody biomass and 20% (w/w) sewage sludge by gasification in a fixed-bed gasifier emitted lower levels of GHGs than incinerating sewage sludge . This is in good agreement with another study on municipal solid waste gasification technology that showed better performance than pyrolysis and incineration for generating electricity. The results of CO2 equivalence from municipal solid waste gasification, incinerating and pyrolysis technology are 2.0, 5.5, and 10.0 kg CO2equivalent/kWh, respectively .
This study evaluates the equivalent carbon dioxide emission of PKS gasification for heat supply to an AMP using LCA. The scope of the study includes emissions from supplying raw materials, emissions from electricity generation for the gasification process, and the emissions in the flue gas. Figure 1 shows the model design of PKS gasification for aggregate heating in an AMP.
For evaluating the CO2 emission by the palm kernel gasification system to supply heat to an AMP, the process was grouped into four parts as follows: (1) PKS production in a Crude Palm Oil (CPO) mill, (2) PKS transportation and handling, (3) electricity generation for gasification operations, and (4) producer gas combustion. The CO2 equivalents emitted in parts 1–3 were predicted with openLCA v1.9 software using Traci and Traci 2.1 databases, while that from part 4 was calculated assuming excess air combustion. PKS was produced from a CPO mill for which the CO2 emission was calculated using the available palm kernel expeller model. It was assumed that the shell is transported by freight transport by road, cut-off local area trucking, and 3.5–7.5 ton lorry operation. The PKS was moved from storage yard to hopper feeder base on a 1.5-ton truck loader. A 50 kVA diesel generator supplies electricity demand for gasification operation. This consumes about 0.40 L diesel fuel/kWh. The CO2 emission as a result of producer gas combustion in the burner was calculated with 10% excess combustion air using combustion of CO as the gasification product. In addition, CO2 as one of the PKS gasification products contributes to the total emissions.
The LCA method determines the objectives, scope, inventory analysis, environmental impact assessment and interpretation of this case according to ISO-14040 procedures. The GHG emission prediction refers to the IPCC method . The analysis was developed using a gate-to-gate approach  including determination of the environmental impacts of application of PKS gasification for the supply of energy to produce 1 ton of HMA. As a comparison, a diesel fuel direct combustion system for the same supply was evaluated. Figure 2 explains the evaluation process diagrammatically.
Results and discussion
Palm kernel gasification
Biomass gasification converts PKS into producer gas using air as reactant in a fixed-bed reactor, leaving PKS char as a solid residue. The heat content of the gas is utilised to increase the temperature of the aggregate to produce HMA at a certain capacity. PKS is appropriate as raw material for this technology as the chemical composition and calorific value are suitable and the physical properties meet the needs of the reactor. The chemical composition and calorific value of PKS were analysed in a certified laboratory, SUCOFINDO (Table 1). Using mass-energy balance calculations, every ton of HMA produced by PKS gasification requires 32.5 kg of the shells to increase the aggregate temperature to 150–200 °C. This gasification produces 81.43 Nm3 of producer gas. In addition to producer gas, this gasification process results solid residues which contain more ashes and carbon with rather than the PKS feed. This is a promising raw material for soil conditioner in a palm plantation and planting media as its pore characteristics support water sequestration and nutrient storage.
The producer gas chemical composition was analysed by gas chromatography method (Shimadzu GC-2014 with TCD-14 sensor) and is presented in Table 2. It is observed that the gasification produces not only CO, H2, and CH4 as combustible gases, but also non-combustible CO2. When using air as gasifying agent, the N2 from the air dominates the producer gas composition. To calculate the GHG emissions of the gasification system, CO2 content in flue gas was calculated as the result of combustion of the producer gas with 10% excess combustion air and the CO2 content of the producer gas.
This analysis investigates all aspects involved and quantities supporting the heat supply to an AMP that are likely to contribute to CO2 equivalent in the system by using direct combustion of diesel fuel and a palm kernel gasification system as described in Fig. 2. A direct combustion system requires only diesel fuel to generate heat and electricity, while a gasification system needs PKS as raw material and diesel fuel to generate electricity and fuel the raw material handling system. The diesel fuel is transported by fuel tanker from the nearest fuel station and the PKS is supplied by dump truck from the nearest CPO mill. They are stored in a diesel fuel tank and a closed yard, respectively. A pump and piping system transport the diesel fuel from the tank to the burner. Moving the shells from the yard to the PKS hopper uses a wheeled loader. Both systems demand electricity for lighting and driving equipment as described in Fig. 3. It is obvious that the electricity demand of the gasification system is greater than the direct combustion system as it requires more equipment. Tables 3 and 4 present the inventory analysis results of both systems.
In total, the direct combustion of diesel fuel and palm kernel gasification systems generate 41.76 kg CO2 equivalent and 63.60 kg CO2 equivalent, respectively (Fig. 4). Using 10% excess air, flue gas contributes a CO2 emission as high as 38.72 kg CO2 equivalent or 60.85% when using gasification. This is relatively higher than the contribution of flue gas from diesel fuel combustion using the same excess air, which emits 25.67 kg CO2 equivalent, or 61.46%. The combustion of producer gas, the components of which are CO, H2 and CH4, is expressed in Eq. (1) . The chemical composition of diesel fuel is assumed to be hexadecane and the combustion reaction follows Eq. (2) .
Using Traci and Traci 2.1 databases, the CO2 equivalents resulting from electricity generation to operate all equipment of the gasification system and the direct combustion of diesel fuel are 8.66 and 3.30 kg CO2 equivalent, respectively. Based on the amount of PKS needed to produce 1 ton of HMA, producing this PKS in a CPO mill emits 10.36 kg CO2 equivalent. Producing diesel fuel for direct combustion to produce the same amount of HMA releases 9.15 kg CO2 equivalent. The CO2 equivalent also arises from transporting the PKS from the CPO mill and the diesel fuel from the fuel station to the AMP. It is calculated that 4.16 kg CO2 equivalent is produced by transporting the PKS, and 3.64 kg CO2 equivalent by transporting diesel fuel. In addition, a wheeled loader to transport the shells from storage to the hopper also emits 1.72 kg CO2 equivalent. Figure 4 describes the calculation method. The total CO2 emissions from both systems are presented in Fig. 5. It is found that CO2 emission from flue gas contributes the most compared with the other sources from both systems.
Interpretation and improvement analysis
Based on LCA calculation, it was found that total emissions of diesel fuel direct combustion were lower, at about 65%, than those form producer gas combustion. However, the emission generated from biomass combustion is considered to be carbon-neutral. The emissions are absorbed by the related plantations in the photosynthesis process in a short carbon cycle. This is a sustainable process, 1 ha of palm plantation during first 10–15 years of its life will probably store around 60.4 tons carbon which is equivalent to 2.44 tons of carbon/ha/year or 8.95 tons CO2/ha/year [33,34,35,36,37].
The PKS gasification system uses diesel fuel to generate electricity, and for transportation and operating vehicles, which contributes 24.90 kg CO2 equivalent. Thus, this system decreases GHG emissions significantly by 40.38% for every ton of HMA produced. This is compared to diesel fuel combustion, where the GHG emissions are from carbon sources formed over a long duration.
To reduce GHG emissions from the PKS gasification process further, three scenarios are proposed, which are related to electricity generation, handling and transportation. The first scenario uses electricity generation based on biodiesel fuel or producer gas. Biodiesel is the product of palm oil transesterification and is used in a diesel generator to substitute for fossil fuel to a certain degree. After cooling and cleaning, producer gas from biomass gasification can also partially replace diesel fuel. The second scenario recommends biodiesel for fuelling the wheeled loader feeders, which are conventionally powered with diesel fuel. In the final scenario, it is also possible to fuel the vehicle transporting the PKS from the palm oil mill to the AMP with biodiesel. It is also suggested to use an electric conveyor rather than a wheeled loader for handling to minimise fossil fuel use.
The study examined the net GHG emissions of processes for heating aggregate in an asphalt mixing plant. The emissions are reduced significantly when PKS gasification replaces diesel fuel direct combustion because PKS is considered as a carbon-neutral resource. This study also found that further emission reduction is possible by substitution of the diesel fuel used for electricity generation in the PKS gasification with biodiesel from palm oil transesterification or producer gas from PKS gasification. In addition, it is also possible to fuel the wheeled loader used for PKS handling in the gasification system with biodiesel.
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The authors express their gratitude to Universitas Sebelas Maret and The Indonesian Oil Palm Plantation Fund Management Agency for funding this study based on contract number PRJ-89/DPKS/2018.
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Pranolo, S.H., Setyono, P. & Fauzi, M.A. Life cycle analysis of palm kernel shell gasification for supplying heat to an asphalt mixing plant. Waste Dispos. Sustain. Energy (2020) doi:10.1007/s42768-019-00023-x
- Global warming
- Palm kernel shell
- Life cycle analysis
- Hot-mixed asphalt