2023 Volume 11 Pages 54-75
The effort of electrifying Sarawak also comes with challenges mainly caused by geographic and demographic factors. Sarawak’s population scatters over a wide spatial area, where families inhabit small villages located in areas of challenging terrains and thick jungles. As a result, electrification through grid connection becomes infeasible and uneconomic. Biogas has immense potential to contribute to energy supply, especially in rural areas. It not only reduces waste but can also be used in generating electricity and subsequently reduces the dependency on fossil fuels. Approximately 993,000 hectares of Sarawak land were planted with oil palm in 2019. The predicted biogas generation from palm oil mill effluent (POME) could create enough electricity to power nearly 2 million rural Sarawak households, in which the Sarawak population in 2020 was 2.9 million. The lagoon system and continuous stirred tank reactor are common technologies used in biogas production. Other technologies used in biogas production are the fixed dome reactor from the Chinese model and the floating dome reactor from the Indian model. The standard technology involves the combustion of biogas in a heat engine called an internal combustion engine to produce heat to generate steam that drives a turbine for electricity generation. This work studied a new biogas utilisation method, fuel cell technology. Solid oxide fuel cell (SOFC) has high efficiency of up to 60% and is generally more prominent than conventional combustion of biogas in a gas engine to generate electricity. With the continual development of biogas fuel cells, a great prospect is predicted for rural areas of Sarawak in biogas production and utilisation. Thus, biogas could contribute a larger role in contributing to a higher renewable energy mix and rural electrification in Sarawak.
Sarawak as the largest state in Malaysia aims to achieve more economical development. However, with the challenging geography and large land size, and population scattered in rural areas, supplying electricity in rural areas becomes very costly compared to areas with denser populations. With the electrification predicament in rural areas, economic development becomes impeded. Nevertheless, the large land area provides exceptional opportunities for Sarawak to involve in agriculture. Agricultural activities in Sarawak provide possibilities for the utilization of the waste-to-energy strategy. Therefore, it is important for Sarawak to devise a plan in consolidating available options to overcome obstacles in economic development. This paper aims to study the potential of using biogas in fulfilling energy needs in the rural areas in Sarawak. In addition, the study is also extended to the current and future biogas technology to provide insight into the sustainable development of biogas as part of the renewable energy mix in Sarawak.
1.1 Energy Demand and Supply in SarawakMalaysia depends on fossil fuels such as natural gas, coal, and oil to generate electricity to fulfill the country’s energy demand. With the current population of 32 million, it was projected that an energy supply of 7571 kWh/person by 2030 will be required to cater to the increasing population’s needs [1]. Another forecast indicated that Malaysian energy demand could drastically rise from 96 TWh in 2019 to 206 TWh by 2035 [2]. Under this increasing stringent condition, Malaysia plans to increase its renewable energy share to 31% by 2025 and 40% by 2035 [3]. This is also to achieve Malaysian’s pledge in Paris Agreement to reduce 45% of her greenhouse gas emission by 2030 [4].
Sarawak is the largest state in Malaysia, with a population of 2.8 million across its vast land size of 124,450 km2 [5]. Sarawak is undergoing significant urbanisation, and energy consumption will dramatically increase from 4,100 MW in 2020 to 6,500 MW in 2025 [6]. It was reported that Sarawak consumed 23,921,112 kWh in 2016, a substantial increase from the previous year of 17,067,840 kWh. This leads to a significant annual increase in electricity demand in Sarawak. Currently, 75% of Sarawak’s electricity supply is hydro-powered, while the remaining 25% is generated in thermal coal-powered plants. Electricity is generated in three hydroelectric plants: Bakun, Batang Ai, and Murum, which generate total hydropower of 3,452MW. However, 4% of rural areas in Sarawak still do not have access to electricity [7]. In comparison, there were 840 million people in the world that still did not have electricity in 2020 when the total world population was 7.884 billion [8].
1.2 Powering the Rural Areas of SarawakIn consensus, it is challenging to define rural accurately. Thus, several definitions are developed based on various disciplines’ strengths and weaknesses [9]. According to the Organisation for Economic Co-operation and Development (OECD), a rural area was defined as an area with a population density of fewer than 150 people per square kilometer [10]. Table 1 shows the population statistics of divisions in Sarawak. Analysis shows that all the divisions in Sarawak have a population density lower than 150, except the Kuching division. This means 96.6% of the area in Sarawak is considered rural. This number could be even larger considering the number of big cities in Sarawak is small.
| Divisions | Population (2020 census) [85] |
Area (km2) [93] |
Population Density (People/km2) |
|---|---|---|---|
| Kuching | 812,900 | 4,180 | 194.5 |
| Sri Aman | 111,400 | 6,212 | 17.9 |
| Sibu | 349,700 | 4,706 | 74.3 |
| Miri | 433,800 | 26,605 | 16.3 |
| Limbang | 103,100 | 7,728 | 13.3 |
| Sarikei | 139,400 | 6,526 | 21.4 |
| Kapit | 134,800 | 39,352 | 3.4 |
| Samarahan | 293,300 | 2,852 | 102.9 |
| Bintulu | 266,200 | 12,024 | 22.1 |
| Betong | 128,000 | 3,494 | 36.6 |
| Mukah | 134,900 | 8,190 | 16.5 |
| Serian | 105,800 | 2,493 | 42.4 |
Electricity in Sarawak is supplied by the state-owned comprehensive energy provider, Sarawak Energy Berhad (SEB). SEB is responsible for producing, transmitting, distributing, and selling electricity in Sarawak. However, the effort of electrifying Sarawak also comes with sets of challenges that are mainly caused by geographic and demographic factors. Being the largest state in Malaysia with a land area of 124,450 km2, almost half of Sarawak’s population is scattered over a wide spatial area, where families inhabit small villages located in difficult terrains and thick jungles. As a result, electrification through grid connection becomes unfeasible and uneconomic [11, 12].
In 2009, the Rural Electrification Scheme (RES) and Sarawak Alternative Rural Electrification Scheme (SARES) were introduced as a stop-gap economic initiative to power up the foremost inaccessible families. Presently, SEB reported that around 18,000 rural family units have been connected to a solid power supply within the Limbang division through the concurrent execution of both schemes. In 2021, about 200 more family units within the division were planned to be connected to the grid. There are still about 866 family units that need dependable access to 24-hour power to attain full electrification in the Limbang division [13].
Rural areas located far from the grid need at least 1095 kWh per year of renewable electricity to power up the basic needs of a typical rural family unit, such as lighting, fans, television, small refrigerator, and rice cooker [14]. Having adequate electricity supply, rural families no longer require frequent trips to the town as before. Before having a 24-hour dependable electricity source, they needed to buy provisions and diesel for their generator sets regularly. Each generator set could cost them about RM300 to RM600 monthly when the price of diesel is RM0.07/L. This fuel consumption could easily use up more than 50% of income because the minimum wage in Sarawak was RM1200, and the diesel price was RM2.15 in 2022. If there is no stable electricity supply, families in rural areas like Long Pinasat will light up typical kerosene lanterns for their children to do school work. A kerosene lantern is expensive because its price ranges between RM7.24 to RM15.26 per kilo-lux hour (klxh). Compared to a light-emitting diode (LED) powered by electricity, the cost is only RM0.20 per klxh [15]. Insufficient education exposure due to no electricity supply would continue to trap them in poverty.
2.2 Economical AdvantagesElectrification of rural areas could increase the productivity of local villagers and utilise their skills to gain extra income. For instance, electricity allows villagers to turn on the machines to mill rice and dry crops [16]. It was reported that a 30% increment in total income could be obtained with the electricity supply [17]. In another example, if there is enough electricity to light up the rural houses, families do not have to rely solely on the income from agriculture. The women in Long Pasia could produce handicrafts and sew clothes at night to help generate income and could noticeably increase household income [15]. Homestay and chalet businesses could be conducted in rural areas like Kabong in Betong District if there is a reliable electricity supply. These accommodation provision services require electricity for electrical power appliances such as air conditioners, refrigerators, fans, and lights in the homestay or chalet for tourist comfort and security. As a result, a satisfied tourist would be most likely to return for more visits. This helps promote tourism activities in rural areas and subsequently improves the income among rural villagers. Without a reliable electricity supply, choices of economic activities are limited.
The utilisation of renewable energy could be the solution to some environmental issues. For example, greenhouse gas (GHG) emissions from the burning of fossil fuels can be reduced if renewable energy is widely employed [18]. Reducing GHG emissions is a crucial action to mitigate climate change. Therefore, converting biomass to generate renewable energy like biogas could be one of the solutions to replace Sarawak’s coal-powered energy plant and further improve Sarawak’s renewable energy supply mix and reduce GHG emissions. In addition, it is imperative to help rural families by supplying stable and 24h electricity to them. To achieve 100% electrification in Sarawak, SEB considered using renewable energy such as solar energy via a decentralised system to power up rural areas. Apart from solar energy, biogas has the potential to be utilised to power up rural areas due to the abundance of organic resources in rural areas. Therefore, a rough estimation of potential energy generated from biomass will be calculated to see if biomass could fulfill Sarawak’s energy demand that keeps increasing annually. Thus, this paper reviews the potential of using biogas to increase Sarawak’s renewable energy mix and 100% rural area electrification.
Biogas is a type of renewable energy generated when organic matter, such as food waste and animal manure, undergoes anaerobic digestion with the help of various microorganisms, broken down into simple molecules such as carbon dioxide and methane. Anaerobic digestion could occur without using any strong oxidising agent and does not require oxygen. It is considered a simple waste to renewable energy technology because no pre-treatment is necessary, and only a favorable environment is needed for microorganisms to decompose the organic waste [2]. The produced biogas contains 55–65% of methane (CH4), 35–45% of carbon dioxide (CO2), and impurities; 0–1% hydrogen sulfide (H2S), 0–3% nitrogen (N2) [19, 20, 21, 22]. Apart from that, biogas could also oxygen (O2) at concentrations of 0–1%, which may come from the influent substrate or leakages [23, 24]. The properties of biogas are shown in Table 2 [21]. One of biogas’s fundamental components is methane, an important fuel. Thus, biogas has a variety of applications, including car fuel, heating, and electricity generation. Biogas can burn with a calorific value of approximately 5340 kcal/m3 at 15°C compared to 9000 kcal/m3 of pure methane. Besides that, biogas can be further upgraded to increase methane purity up to 98% [22]. Methane has a higher global warming potential of 25 times higher than carbon dioxide in 100 years [25]. Thus, its emission must be regulated with strict energy policies, especially in the palm oil industries, to avoid its negative impact on climate change. This restriction is not merely a burden to palm oil industries as biogas can be captured for electricity generation, which can be sold to the national grid or used in the plant. However, additional capital investment is required for the existing plant. Biogas production from organic wastes was reported to be economically feasible [26, 27]. Typically, 29 m3 of biogas can be produced per tonne of POME [28]. When 1 m3 of biogas that contains 6 kWh of energy [29] is converted to electricity in a biogas-powered electric generator such as an internal combustion engine (ICE) with a thermal efficiency of around 37% [30], 2 kWh of electricity is generated, while the remainder is converted to heat. The heat is generally wasted unless it is utilised in a heating process.
| Properties | Values |
|---|---|
| Energy content | 6.0–6.5 kWh/m3 |
| Wobbe index | 19.5 MJ/m3 |
| Fuel equivalent | 0.60–0.65 1 oil/m3 biogas |
| Ignition temperature | 650–750°C |
| Heat of vaporisation | 0.5 MJ/kg |
| Methane number | 124–150 |
| Lower heating value | 17 MJ/kg |
Biogas utilisation is considered renewable because carbon recycles and remains in the biosphere when biogas is burnt to produce carbon dioxide that plants use. Biogas can be repeatedly produced as long as organic materials decompose via anaerobic digestion. Nowadays, an increased amount of organic waste is produced by the increasing population. In general, several types of organic waste, such as food waste, agricultural waste, and animal waste, can be utilised to generate biogas. Unfortunately, these wastes were not converted into energy but produced 29% of the country’s greenhouse gas emissions [31]. These organic wastes are difficult to dispose of and easily treated as burdens. However, with a proper system, conversion of these wastes to energy is achievable [32]. Biogas technology using an anaerobic digestion system is considered uncomplicated as it can self-create an environment for microorganisms to decompose the organic matter in waste. Globally, there has been a significant surge in the implementation and use of anaerobic digestion (AD) systems, responding to the need for additional renewable energy and sustainable waste reduction options. The urgent threat of climate change and the necessity to enhance non-fossil fuel sources to meet global residential, industrial, and commercial energy requirements has fuelled the growth and development of AD systems [23].
From controlled anaerobic digestion in a bioreactor, the amount and composition of CH4 and CO2 present in biogas are determined mainly by the pH and temperature as well as the type of substrates like animal manure, municipal solid wastes, and agro-industrial wastes [33, 34]. However, most biogas plants nowadays use co-digestion of agricultural waste such as manure of cows, pigs, and chickens from the farm, food waste, and municipal solid waste from households and crops such as palm oil mill effluent [20]. Currently, a sum of the global capacity of biogas projects managed to generate up to 3.2 × 106 kW and was utilised heavily in the US and Europe landfills [35]. However, the time required for biogas production greatly depends on the local climate condition, decay rate, and the type and amount of waste. On the other hand, biogas can also be produced in an ideal anoxic environment where microorganisms can thrive and intensify production. Studies on AD have proven that this technology is used widely globally [36, 37, 38] due to its technical simplicity and flexibility toward a variety of scales as well as the types of waste streams. It is also feasible to be used in large or small-scale operations.
Biogas can be produced from various feedstock such as food waste (FW), municipal solid waste (MSW), palm oil mill effluent (POME), garden waste (GW), sewage sludge (SS), and animal manure (AM) [39].
4.1 Animal WasteSolid and liquid animal manure, used bedding, and spilled feed are examples of animal waste. For instance, production of 0.23–0.26 m3 of biogas from 1kg of cow manure was reported [2]. An analysis of the potential of using biogas from animal waste was conducted in Indonesia [40]. The result of the study predicted a possibility of 9597.4 Mm3 per year of biogas could be obtained and would generate an electric supply of up to 1.7 × 106 kWh yearly. In another study, farm animals and slaughterhouses in Malaysia are potentially found to produce 4589.49 million m3 of biogas annually. This biogas could generate about 8.27 × 109 kWh of electricity [41].
4.2 Food Waste / Municipal WasteThe case study of a mini biogas power plant installed in a remote area found that the electric power generated was 600 kW, and 182 m3 of methane gas was produced daily with food waste input of 1000 kg per day. This energy system could provide 24-hour electrification if the waste supply is continuous and the generator does not run out of biogas [42]. Another Australian case study estimated biogas yields and assessed the feasibility of utilising biogas, total conversion of food waste can reduce greenhouse gas emissions (GHG) by up to 5,07,434 tonnes per year and generate $52.38 million. The biogas generated from food waste can replace up to 52.36 GW of fossil fuel-based electricity generation and 554.4 TJ of heat annually. This finding has proven that converting food waste into biogas through anaerobic digestion has a pivotal role in generating renewable electricity.
Hence, biomass could significantly reduce fossil-fuel dependency in Sarawak and turn Sarawak to 100% use of green energy aside from hydropower. In the case of Sarawak, municipal waste could be one of the options to produce biogas. Table 3 shows biogas potential from waste commonly found in Sarawak. Sarawak’s highest daily municipal waste generation was in Kuching with 690 tonnes, Miri with 281 tonnes, Serian with 176.7 tonnes, and Betong with 70 tonnes. From the municipal solid waste collected from Sarawak’s central city, 40–45% of the waste was food residues, 3–4% is garden waste, and the remainder was other materials that were not biomass [43]. This indicated that the food wastes from these cities were more than 510 tonnes per day. At the approximation of 90% of food waste is volatile solid, average biogas (caloric value 6 kWh/m3 [29]) production rate of 450 mL/g VS was achieved [44]. If converted to biogas, there is a potential to generate approximately 183,600,000 kWh/year at 40% efficiency from these food wastes. If this could happen, Sarawak could satisfy part of her energy demand and reduce fossil fuel dependency [39] because this amount of energy could supply up to 19,500 households according to average energy consumption per household at 25.8 kWh/day [45]. Besides that, with Sarawak’s average MSW production rate per capita as high as 0.92 kg/person/day [43], MSW alone contains enough energy to comply with Sarawak’s energy demand. Thus, it is quite inadequate that these resources were not properly exploited, but mainly disposed of in landfills [46].
| Waste type | Biogas potential (m3/kg DM) | CH4 in biogas by volume (%) |
|---|---|---|
| Banana | 0.41 | 60 |
| Apple | 0.52 | 52 |
| Cocoa beans | 0.41 | 55 |
| Palm oil mill effluent | 0.997 | 60 |
| Rice husk | 0.05 | 60 |
| Eggs | 0.975 | 60 |
| Milk | 0.9 | 60 |
| Tomatoes | 0.50 | 55 |
| Vegetable waste | 0.50 | 56 |
Apart from municipal or food waste, Sarawak generated a lot of agro-industrial waste too. Agro-industrial waste is made up of by-products from the food processing industry. Examples of these by-products are molasses and bagasse from the sugar industry, rice husk, vegetable, fruit skin and pomace, de-oiled cakes of linseed, cottonseed, palm, or soybean, from which the oil has been extracted, starch residue, eggshells, poultry, and farm animal skin, as well as meat from the respective industries [49]. Table 4 shows the agro-industrial wastes and their types. Agricultural residues can be further divided into field residues and process residues. Field residues are remnants from the crop harvesting process in the field. Leaves, stalks, seed pods, and stems are among the field residues, whereas process residues are those that remain after the crop has been processed into a different valuable resource. In addition, industrial waste is any material that is considered useless in a production process in factories, mills, and mining operations are known as industrial waste. Examples of industrial residues include fruit peels, groundnut, coconut, and soybean oil cakes.
| Agricultural residues | Industrial residues | |
|---|---|---|
| Field residues | Process residues | |
| Stems Stalks Leaves Seed pods |
Husks Seeds Roots Bagasse Molasses |
Fruit peels Ground nut oil cake Coconut oil cake Soybean oil cake |
There is no problem with the supply of these agro-industrial wastes because they can be found abundantly in Sarawak. This is especially true for POME because Sarawak is the state with the largest palm oil plantation area. Biogas can be produced through the anaerobic digestion of waste from the ever-growing palm oil sector, palm oil mill effluent (POME). Currently, the release of methane or biogas into the environment by palm oil mills is overlooked, despite its enormous potential to be used as a renewable energy source.
Palm oil mill effluent (POME) is a thick brownish liquid waste produced in the oil extraction of fresh fruit bunches in palm oil mills. It is estimated that 5–7.5 tonnes of water are used in the production of every tonne of crude palm oil (CPO), with more than half of the water ending up as POME [50]. This indicates that for every tonne of CPO produced, around 2.5–3.75 tonnes of POME will be produced [51]. Without proper waste management, the high amount of POME generated could cause severe water pollution [52] because of its characteristics, as shown in Table 5 [53, 54]. The raw POME has a high organic content, a valuable source to produce methane via anaerobic digestion. POME also contains biodegradable elements with a BOD/COD ratio of 0.5, suggesting that it can be processed biologically into an energy source easily [55]. Thus, POME must be treated before it can be discharged to prevent water pollution. Currently, anaerobic digestion is a common method in palm oil mills because of its simplicity and lower cost. However, the by-product of anaerobic digestion, biogas, is generally emitted into the atmosphere without being utilised. This is because additional capital investment for gas engines is required to convert biogas into heat and electricity. However, government regulation has enforced that all palm oil mills eventually must comply with greenhouse gas emission reduction goals by fully utilising and converting methane generated in the anaerobic digestion process. This is because methane is a greenhouse gas with a much more significant global warming potential (25 times larger) than carbon dioxide. Therefore, it is reasonable to retrieve the chemical energy of methane and avoid methane emission into the atmosphere.
| Parameter | Unit | Range |
|---|---|---|
| pH | - | 3.8–5.1 |
| Biological Oxygen Demand (BOD) | mg/L | 10,250–43,750 |
| Chemical Oxygen Demand (COD) | mg/L | 15,000–100,000 |
| Total Solids (TS) | mg/L | 11,500–79,000 |
| Suspended Solids (SS) | mg/L | 5,000–54,000 |
| Total Volatile Solids (TVS) | mg/L | 27,300–34,000 |
| Oil and Grease (O and G) | mg/L | 17410 |
| Total nitrogen (TN) | mg/L | 180–1400 |
Large-scale palm oil planting in Sarawak began in the early 1990s and was less than 60 years [56]. As of 2020, the KAO Corporation recorded about 57 Palm Oil Mills located in the rural areas of Sarawak [57]. Another local company, Sarawak Oil Palms Berhad, with a total plantation size of nearly 90,000 hectares in a rural area (as shown in Table 6 [92]), produced 390,481 tonnes of crude palm oil in 2020 [58]. As shown in Figure 1 [59], approximately 993,000 hectares of Sarawak land area was planted with oil palm in 2019 [60]. As a result, the crude oil production would result in an estimate of 925,000–1,387,000 tonnes of POME annually.
| No | Estates | Land Area (Hectare) | Location |
|---|---|---|---|
| 1 | Batu Lintang | 2,317.57 | P.O Box 810, 95000 Sri Aman |
| 2 | Galasah Estate | 1,811.44 | KM 120, Miri – Bintulu Road, Miri Sarawak |
| 3 | Karabungan Estate | 1,971.86 | KM 48, Miri – Bintulu Road, Miri |
| 4 | Kuala Tatau Estate | 3,829.77 | KM 40, Bintulu – Sibu Road, Off KM 13, Tatau |
| 5 | Lamaus Estate | 1,208.61 | KM 3, Jalan Ulu Niah, Off KM 110, Miri – Bintulu Road, Miri |
| 6 | Lambir Estate | 4,273.99 | KM 41, Miri – Bintulu Road, Miri Sarawak |
| 7 | Maleh Estate | 329.41 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 8 | Manong Estate | 440.38 | KM 3, Jalan Ulu Niah, Off KM 110, Miri – Bintulu Road, Miri |
| 9 | Menawan Estate | 6,663.36 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 10 | Metanik Estate | 5,345.32 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 11 | Penyuan Estate | 6,385.65 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 12 | Pinang Estate | 1,268.30 | KM 120, Miri – Bintulu Road, Miri Sarawak |
| 13 | Sabaju Estate | 8,043.36 | KM 53, Bintulu – Miri, KM 31 Bakun Road |
| 14 | Sebungan Estate | 1,639.69 | KM 25, Bintulu – Miri Road, KM 19 To Sebauh |
| 15 | Sehat Estate | 2,613.48 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 16 | Sepakau Estate | 4,695.71 | KM 81, Jalan Bakun off KM 53, Jalan Bintulu – Miri |
| 17 | Seping Estate | 2,787.40 | Sungai Maleh, Belaga Blk 89 & Blk 90 Murum LD 96950 Murum Sarawak |
| 18 | Sg. Balim Estate | 2,793.64 | KM 3, Kubur Road, Off Sepupok, Niah |
| 19 | Sg. Liuk Estate | 1,945.06 | KM 8, Balingian – Mukah Road, Balingian |
| 20 | Sg. Meris Estate | 1,912.77 | KM 8, Balingian – Mukah Road, Balingian |
| 21 | Sg. Trus Estate | 1,951.45 | KM 3, Kubur Road, Off Sepupok, Niah |
| 22 | Taniku Estate | 5,026.13 | Jalan Pujut 7, Permyjaya KM 9 East of Miri |
| 23 | Telabit Estate | 2,615.15 | KM 115, Miri – Bintulu Road, plus further 9KM |
| 24 | Telong Estate | 1,472.83 | Lot 162 & 163, Suai, Off KM 101 Miri Bintulu Road, Miri |
| 25 | Tibus Estate | 832.78 | Lot 157, Suai (Behind Telabit) |
| 26 | Tinbarap Estate | 13,395.89 | KM 22, Miri – Bintulu Road, Off KM 45, From BLD Junction |
Figure 1: Sarawak oil palm concessions [59]
In an estimation shown in Table 7 [49, 56, 61], more than 570,000 tonnes of methane could be generated based on Sarawak Oil Palms Berhad’s CPO production in 2020 if all POME from this company were digested anaerobically [51]. In terms of calorific value, this amount of methane gas generated was comparable to about 800 million litters of diesel, or 3.2 million MWh of electricity, or 400 MW of a gas engine power plant with 40% efficiency. Rural areas far from the grid require at least 1,095 kWh of renewable electricity per year to power the basic needs of a typical rural family unit [14].
| Parameter | Unit | Value |
|---|---|---|
| CPO production | Tonnes | 369921.10 |
| POME generateda | m3 | 1109763.30 |
| COD level in POMEb | mg/L | 51,000 |
| COD Convertedc | Tonnes | 2,314,770 |
| CH4 Producedd | Tonnes | 578,692.50 |
| Energy rate | MJ | 28,934,625,000 |
| MWh | 8,037,395.83 | |
| Diesel equivalentf | L | 823,316,213.30 |
| Electricity generatedg | MWh | 3,214,958.33 |
| Power plant capacity (gas engine) g | MW | 401.87 |
Note:
a. Assume that each tonne of CPO produced generates 3 m3 POME.
b. POME COD is based on the Malaysia Palm Oil Board’s mean value (MPOB)
c. Assume that the digester’s efficiency is 80%.
d. 0.25 kg CH4 per kg COD is the theoretical methane conversion factor.
e. CH4 has a calorific value of 50 MJ/kg.
f. Diesel has a calorific value of 35.144 MJ/L.
g. Assume the gas engine runs 8000 hours per year and has 40% efficiency.
Based on Sarawak Oil Palms Berhad’s CPO production in 2020 [58], the predicted biogas generation from POME could create enough electricity to power nearly 2 million rural Sarawak households in which Sarawak population in 2020 was 2.9 million. In short, there is abundant feedstock for the production of biogas. Biogas has a significant potential to contribute to energy supply, especially in rural areas. It not only reduces waste but also can be used in generating electricity and subsequently reduces the dependency on fossil fuels. In Malaysia, biogas can be primarily produced in palm oil mills because of POME treatment in anaerobic ponds, commonly known as the open-ponding system. Unfortunately, biogas produced in these ponds is often left to be emitted into the atmosphere since there was previously no direct policy on biogas capture.
Besides the feedstock for biogas production, biogas production technology is also gaining a lot of attention recently because of the necessity to improve waste to energy conversion efficiency. In Malaysia, two common technologies are used in biogas production: the lagoon system and the continuously stirred tank reactor. Currently, 58% of biogas industries for POME treatment are using continuous stirred tank reactors, also known as closed anaerobic digester tanks. The other 42% use using lagoon system [39]. Such technologies of producing biogas for energy should be deployed in Sarawak to kickstart the use of biogas as renewable energy. Another type of technology used in biogas production is the fixed dome reactor from the Chinese model or the floating dome reactor from the Indian model. The schematic designs for both models are presented in Figure 2 [40, 87]. A comparison of these technologies is shown in Table 8 [47, 62]. A floating dome offers more flexibility in the amount of biogas that could be stored compared to a fixed dome.
| Type of technology | Advantages | Disadvantages |
|---|---|---|
| Covered Lagoon | Low operating cost Consume low energy during operation |
Require a large area of land to build |
| Continuous Stirred Tank Reactor | Easy to construct Low cost to build |
The process is time-consuming |
| Fixed Dome Reactor | Low maintenance cost Long life span |
Need to be heavily supervised |
| Floating Dome Reactor | Simple operation and construction | Low lifespan |
Carbon dioxide, CO2, is a non-combustible compound produced during anaerobic digestion where CO2 acts as an electron acceptor for methanogens. The percentage depends on the digester’s temperature, pressure, and liquid content. CO2 is a greenhouse gas that can cause global warming. Greenhouse gases will absorb the sunlight that bounces off the Earth’s surface in the form of infrared irradiation. This trapped heat will cause the Earth’s average temperature to rise and lead to climate change.
Hydrogen sulfide is produced during sulfur compounds’ degradation and sulfates’ desulphurisation in anaerobic digestion. Acidic hydrogen sulfide poses severe risks to human health and the environment because it is highly toxic. It is a colorless gas with a substantial rotten egg-like smell and is detectable by humans even at very low concentrations from 0.05 ppm [21]. To protect industrial workers, the Occupational Safety and Health Administration (OSHA) standard limit and peak limit for hydrogen sulfide are merely 20 and 50 ppm, respectively [63]. Furthermore, its acidic characteristic would result in corroded gas engine components and shortened lifespan. If emitted freely into the atmosphere, it will cause acid rain to precipitate, which can be harmful to the environment.
Biogas can be upgraded by removing carbon dioxide (CO2) and cleaned by removing hydrogen sulfide (H2S). This involves a few processes that can eventually upgrade biogas to 95% of methane. The first process is called Pressure swing adsorption (PSA) by using porous solids as adsorbents. The cleaned biogas is compressed to high pressure before entering an adsorption column. There, carbon dioxide will be retained by the adsorbent. The permeated gas will contain a high amount of methane that is not retained. When the adsorbent is saturated with CO2, the pressure is reduced to enable the desorption of carbon dioxide and release it into the off-gas stream [39]. The process flow diagram of the common set-up, which consists of two adsorption columns, is presented in Figure 3 [88].
Figure 3: Process flow diagram of Pressure Swing Adsorption (PSA) [88]
The other step for biogas upgrade is water scrubbing. Water scrubbing is a separation process where carbon dioxide and hydrogen sulfide are physically absorbed in a liquid solvent like water. The process equipment layout typically contains the absorber, the scrubber responsible for gas stream cleaning, and the stripper, which is responsible for solvent regeneration and the recovery or removal of acid gases from the process presented in Figure 4 [64]. Methane purity then is based on the remaining volume % of non-condensable gases like nitrogen, where the purity can go up to 80% to 99%. Water scrubbing can be used to separate CO2 because of its high solubility in water compared to methane (CH4) and is usable for separating H2S. Still, the H2S concentration must not exceed 2500 ppmv because pH (water) will be decreased by H2S and cause a decrease in CO2 solubility in the solvent/water [65].
Figure 4: Process flow diagram for Water Scrubbing process [89]
Amine scrubbing is another process that can be done to upgrade biogas. Amine scrubbing involves both physical and chemical adsorption of gases based on the action between CO2 and amines. Commonly used amines in biogas industries are Diethanolamine (DEA), Methyl diethanolamine (MDEA), and Monoethanolamine (MEA) [66]. The typical equipment used in the process is an absorber, a stripper, a heat exchanger, a reboiler, and a condenser. The process flow diagram for the equipment is presented in Figure 5.
Figure 5: Process flow diagram for Amine scrubbing process [90]
The process starts with the biogas entering the absorber column from the side, and the amine solution will be fed from the top. This causes the solution to flow down while the biogas flows up the column. Column packing or plate in the column is essential to provide a large contact surface area for the two materials. This leaves CO2 and H2S to concentrate at the bottom of the column while high purity methane is discharged at the top of the absorber column. Then, the rich amine solution is transferred to the stripper column, acting as a regeneration column to separate CO2 from the amine solutions before reuse in the absorption column for the following process [67].
A comparison between PSA, water scrubbing, and amine scrubbing in terms of investment is shown in Table 9 [68]. The comparison between the biogas upgrading technologies is shown in Table 10 [39, 69].
| Technology | Thermal Energy kWh/Nm3 | Electric Energy kWh/Nm3 | Equipment Investment Cost €/(Nm3/h) |
|---|---|---|---|
| Pressure swing adsorption | - | 0.20–0.30 | 3800 |
| Water scrubbing | - | 0.25–0.30 | 3300 |
| Amine scrubbing | 0.40–0.75 | 0.10–0.15 | 4200 |
| Parameter | Water Scrubbing | Pressure swing adsorption | Amine scrubbing |
|---|---|---|---|
| Methane purity (vol %) | 95.0–99.0 | 95.0–99.0 | >99.0 |
| Methane slip (%) | 1.0–3.0 | 1.0–3.5 | 0.04–0.1 |
| Methane recovery (%) | 92.0–99.5 | 60–98.5 | 99.9 |
| H2S (ppm) | <2 | <4 | <8 |
| Operating pressure (barG) | 6–8 | 4–7 | 0 |
| Electric energy demand (kWh/m3 biomethane) | 0.27 | 0.24 | 0.20 |
| Heating demand and temperature | - | - | High 120–160°C |
| Consumables | Antifouling agent, drying agent | Activated carbon | Amine solution |
| Typical investment (RM/m3 biogas) | 0.71 | 1.39 | 0.95 |
Biogas has enormous potential as an alternative renewable energy source and can become one of the solutions for waste management and global warming mitigation. However, several challenges are faced in implementing small, and large-scale biogas production. They are economical, knowledge, and technical [40]. The economic difficulties involve capital expenditure (as shown in Table 11), operational cost, and segregation of organic and inorganic parts of MSW. The technical challenges include the workforce needed to do maintenance and daily inspection for plant malfunction. Most biogas technologies will dehumidify and purify the biogas before combustion to avoid damaging the gas engine [70]. Moisture must be removed from the biogas to prevent condensate accumulation in the gas line, which will prevent pipes from clogging due to the corrosive acid formation. The main impurity that must be removed is hydrogen sulfide because its acidic property could corrode and damage the engine.
| Upgrading Technique | Capacity (m3/h) |
Total Cost (RM) |
|---|---|---|
| Pressure Swing Adsorption | 100 | 6,104,306 |
| 130 | 3,991,277 | |
| 250 | 7,923,858 | |
| 500 | 10,858,624 | |
| 1000 | 16,441,878 | |
| 500 | 11,920,326 | |
| Chemical Scrubbing (Amine scrubbing) |
100 | 5,576,050 |
| 250 | 7,336,901 | |
| 500 | 10,271,666 | |
| 100 | 5,411,599 | |
| 500 | 13,564,548 | |
| 1800 | 23,676,306 |
According to the regulatory and technical standards, raw biogas must be purified to remove toxic and dangerous ingredients such as hydrogen sulfide, ammonia, VOCs, halides, moisture, siloxane, and particles, especially if it is transported elsewhere instead of utilised locally. Examples of methods used to purify biogas include adsorption, biofiltration, water scrubbing (an absorption process), and refrigeration [70].
The electricity generation system can be categorised into two systems: decentralised and centralised. In a life cycle analysis (LCA) that studied fuel cell-based electricity generation’s environmental and economic aspects, decentralised and centralised systems were analysed [84]. A decentralised system is not interconnected with the national grid, whereas a centralised system is the opposite. The most critical processes reported in the LCA are storage and transportation, as they are costly and significantly impact the environment. The decentralised system involves in situ production of biogas and electricity. It does not include storage and transportation of the produced biogas; therefore, loss in transmission and distribution are avoided.
On the other hand, the centralised system requires a storage tank and transportation for produced biogas, and thus much more expensive. The reported costs of decentralised and centralised systems are RM2.56/kWh and RM 7.04/kWh, respectively [84]. This indicated that the decentralised system is much more cost-effective than the centralized system. And this is especially significant for electricity generation in rural areas.
Extensive utilisation of biogas in palm oil mills cannot be achieved due to inadequate funding for capital investment and palm oil mills being far from the interconnection point at the distribution system, which will cause power loss [51]. Without proper incentives, the investment in the biogas capture and utilisation facilities is unprofitable, especially in the Sarawak region, because the electricity selling price is only RM0.32/kWh, which results in the return of investment (ROI) becomes unattractive [51].
Chemical energy in biogas must be converted into electrical energy before being transported to the consumer. This requires biogas utilisation technology. The standard technology involves the combustion of biogas to produce heat used to generate steam that drives a turbine. In this case, the internal combustion engine (ICE) is a reliable technology for biogas combustion. It is a heat engine where a spark plug ignites the air-fuel mixture in the combustion chamber. Internal combustion engines require the ignition of the mixture by either spark ignition (SI) or compression ignition (CI). SI engines can efficiently utilise biogas when mixed with gasoline after which it can be fed into the cylinder, whereas the CI engines need to use dual-fuel mode and mix with air. A spark-ignition engine requires special attention to biogas’s Wobbe index and calorific values. Wobbe index is the primary indicator of the interchangeability of gases which signifies gaseous fuels that can be interchanged with similar energy output. On the other hand, the calorific value is directly proportional to the methane content. Thus, these factors are essential to ensure the efficiency of power generation in the biogas plant. Upgraded biogas contains more than 95% of methane, has calorific values similar to natural gas, and can be used in the same applications [73].
Biogas in diesel engines (CI engines) operates in a dual-fuel mode that uses biogas as a primary fuel and diesel as the pilot fuel. The standard diesel fuel injection will still supply a certain amount of diesel fuel, and the engine will induce and compress the mixture of air and biogas before the infusion [74]. The mixture will then be ignited by the released energy from the combustion of the diesel fuel. The amount of diesel fuel required for sufficient ignition is between 10–30% of the amount needed for operation on diesel alone at normal working loads [75]. The biogas composition will also affect the operating parameters, combustion, and performance characteristics of both SI and CI engines. The capacities of these engines vary from 45 kW on small farms to a few MW on large-scale landfill sites [74]. Therefore, the choice of ICE type depends on the biogas production rate in the digester.
However, recent development in fuel cell technology allows the direct conversion of chemical energy to electricity, which could vastly increase energy conversion efficiency. A typical fuel cell is shown in Figure 6. It has an anode and cathode, separated by a nonconductive solid or liquid electrolyte. Fuel will flow through the anode, and ions are produced. Oxygen will flow through the cathode. Electrons must pass through an external circuit and generate electricity [21]. Using fuel cells fuelled with biogas can be challenging as some types require upgraded and purified biogas. Besides that, the fuel cell operates at high temperatures. A few types of fuel cells can be found, but only Solid Oxide Fuel Cells (SOFC) will be discussed because of their relevance to the use of biogas. Moreover, biogas is more preferred and suitable for SOFC than hydrogen due to the carbon degradation [76]. Carbon degradation is the breaking down of carbon into smaller molecules by biotic, which is biodegradability or abiotic means such as oxidation.
Figure 6: A simple schematic diagram of a power-generating fuel cell [91]
Solid Oxide Fuel Cells (SOFC) are commonly made of solid oxide or ceramic materials [21]. Nickel-based materials are generally used as the anode due to their low cost. It is made porous by forming a cermet from the mixture of NiO and yttrium stabilised zirconia (YSZ) powder to facilitate higher electronic conductivity. The Ni-ScSZ cermet anode could achieve a cell voltage of 0.9V operated under 1000 °C using cleaned biogas. However, the performance and efficiency of the fuel cell are closely related to the impurities present in biogas. By adding air to biogas, the contaminants can be decreased quickly without affecting the output voltage [77]. SOFC can also work under either intermediate, 600 to 800 °C [71, 78], or high temperature, which is 800 to 1000 °C [79]. Temperature is one of the technical barriers because electrochemical reactions are fast above 600 °C [21, 80]. In theory, the energy conversion efficiency of the fuel cell is up to 80% [81]. But this is limited by the technical barriers such as the temperature. When transformed from chemical to electrical energy, SOFCs are given higher efficiency of up to 60% [82]. Regardless, SOFCs efficiency is generally larger than conventional combustion of biogas in gas engines to generate electricity. Therefore, SOFCs should be used to generate electricity. In addition, the removal of CO2 and H2O is not necessary for SOFC operation [83]. This enhances the feasibility of the SOFC utilisation for biogas conversion. Shafie et al. [22] reported that biogas-based solid oxide fuel cells have the potential to generate approximately 2006.9 MW of electricity from the 877.53 million m3/year of POME. Conclusively, biogas fuel cell technology has the potential to replace conventional combustion in converting biogas to electricity.
Electrifying Sarawak comes with sets of challenges that are mainly caused by geographic and demographic factors. This study shows that biogas has significant potential to contribute to energy supply, especially in rural areas. Common technologies used in biogas production include lagoon system and continuous stirred tank reactor, fixed dome, and floating dome reactor. This study studied a new biogas utilisation method, fuel cell technology. Solid oxide fuel cell (SOFC) has high efficiency of up to 60% and is generally more prominent than conventional combustion of biogas in a gas engine to generate electricity. With the continual development of biogas fuel cells, a great prospect is predicted for rural areas of Sarawak in biogas production and utilisation. Thus, biogas could contribute a larger role in contributing to a higher renewable energy mix and rural electrification in Sarawak.
The authors gratefully acknowledge the financial support from Universiti Teknologi MARA, Sarawak Branch via Dana Kecemerlangan DKCM2020 (Grant No: 600–UITMKS (RMU.5/2) (06/2020/KCMT)).
