The potential of biochar-slurry fuel from agricultural wastes in Indonesia

Abstract

Biomass derived from agricultural waste in Indonesia is abundant and has potential as a source of renewable energy feedstock. However, analysis is needed to determine the appropriate technology to convert potential biomass into consumable energy forms. This review summarises various data on Indonesia’s potential biomass that can be used as a renewable energy feedstock. Biomass and biochar characteristics are systematically discussed, including proximate and ultimate analysis. We then summarise various biochar production technologies based on the biomass used and their yields. The characteristics of biochar produced in both proximate and ultimate analysis and the conversion of biochar into slurry fuel as one of the consumable forms of renewable energy were discussed. Slurry fuel is a potential use of biochar as a liquid fuel. The challenges of developing potential biomass-based bio-slurry production in Indonesia and the future work that needs to be done are presented.

Keywords:

• Biochar
• biomass
• pyrolysis
• slurry fuel

REVIEWING EDITOR:

• Sanjay Kumar Shukla, Edith Cowan University, Australia

SUBJECTS:

• Power & Energy; Technology; Chemical Engineering

1. Introduction

Modern civilisation requires a supply of energy that is both environmentally friendly and easily accessible. However, the use of fossil fuels poses numerous challenges, such as the depletion of their sources, environmental problems, geopolitical conflicts, and rising prices (Susilawati et al., 2020). Thus, finding and producing alternative sources of energy is becoming increasingly crucial. Biomass is a promising renewable energy source that can be developed in an environmentally friendly process, as depicted in Figure 1. As a tropical country, Indonesia has access to various types of biomass that can be utilised as an energy source. The estimated biomass production in Indonesia is approximately 200 million tons per year, which theoretically can generate 49,810 MW, but currently, only 302.4 MW has been realised (Pranoto et al., 2013). The available agricultural waste from main commodities in Indonesia, such as rice husk (54.25%), corncob (9.74%), cassava waste (6.45%), oil palm waste (2.29%), coconut waste (2.3%), and forest waste (24.69%), can be utilised for energy production (Pranoto et al., 2013). The rice husk has a calorific value of 21.34 MJ/kg (Chen et al., 2024), corncob 10.79 MJ/kg (Wojcieszak et al., 2022), cassava waste 27.5 MJ/kg (Chen et al., 2024), oil palm waste 15.49 MJ/kg, coconut waste 4,128.9 calories/ton of biomass, and forest waste 3,992.6 calories/ton of biomass (Budhijanto et al., 2019). The waste is scattered in various locations, with the largest resource in western Indonesia (Mahidin et al., 2020). Despite their widespread availability, agricultural residues still need to be fully utilised for energy production, even though they have the most potential as a biomass source (Kabir Ahmad et al., 2022). Developing countries, whose economies rely heavily on agriculture and forestry, demand biomass energy applications highly. If employed as an energy source, this method could tackle the issue of biomass waste disposal.

Figure 1. A framework of biomass utilisation into heat and power via slurry fuel.

Nevertheless, utilising unprocessed solid biomass as fuel presents specific difficulties, including high moisture levels and low volumetric energy density, which may result in problems during the conversion process. Fast pyrolysis can overcome these issues and transform biomass into biochar and bio-oil (Abdullah et al., 2010). Bio-oil has the potential to replace fossil fuels as a fuel source (Setyawan et al., 2020a, 2020b). However, corrosion, poor stability, and a low heating value limit the continued development of bio-oil. In addition to spontaneously igniting when exposed to oxygen, biochar as a fuel can catch fire while being moved or stored (Laird et al., 2009).

The consumable form of renewable fuel is bio-slurry, which has been developed and is suitable for various stationary combustion equipment. Reviewing previous studies on bio-slurry production techniques can inform the appropriate technology for large-scale production in Indonesia. Finally, some suggestions for further research on biochar slurry fuel applications were put forward. However, this review has limitations, such as the need to specifically calculate the economic benefit of making slurry fuel from biochar.

2. Characteristics of biomass source

Biomass is a term used to refer to all organic materials derived from cultivated plants, algae, and organic waste. One biomass that exists in Indonesia in very abundant quantities is lignocellulosic. Lignocellulosic biomass consists of three main components, namely 35–55% cellulose, 40% hemicellulose, and 15–25% lignin (Anjali et al., 2022). The lignocellulose content varies based on several factors, namely plant type, growing conditions, plant parts, and harvesting age (Sharma et al., 2019). Its availability as agricultural, plantation, and forestry waste makes this material a potential energy source through physical, chemical, and biological conversion processes. For example, wood waste contains higher carbon and hydrogen levels than agricultural waste. Agricultural waste generally includes waste from cultivation and post-harvest operations. Waste from cultivation is the remaining materials, such as stems, stalks, leaves, and seed pods. Waste generated in the post-harvest process is the material that remains after processing the crop into usable resources. The waste includes husk, seeds, roots, bagasse, and molasses and is used for animal feed, soil improvement, fertilisers, manufacturing, and various other processes (Anjali et al., 2022; Kumar et al., 2022). The complexity and variability of feedstocks pose many challenges in the waste-to-energy process. Technically, the amount of moisture in the feedstock limits the conversion pathways for producing biofuels, bioproducts, and biopower. Wastes from agriculture, plantations, and forestry are summarised in Table 1. Anjali et al. (2022) characterised a value-added product, biochar, from a variety of lignocellulosic biomass such as sawdust, sugarcane bagasse, and soap pith fibres that had been pyrolysed at 300 °C and 600 °C for 30 minutes. Sawdust is the processed residue of hardwood biomass obtained from the woodworking industry, which has a combustion capacity and is a source of fuel in thermal processes (Anjali et al., 2022; Domingues et al., 2017). The elemental composition of sawdust includes carbon (44.13 wt%), hydrogen (4.13 wt%), and nitrogen (0.42 wt%).

Table 1. Characteristics of biomass sources.

Feedstock	Sawdust	Sugarcane bagasse	Soapnut pith	Sugarcane straw	Coconut shell	Coconut husk	Disposable bamboo chopsticks	Corncob	Cassava rhizome	Rice husk	Cassava Stem	Peanut shell	Palm shell
Proximate analysis (wt %)
Moisture	1.12	7.9	–	13	10.1	7.5	–	–	–	5.7	–	–	3
Volatile matter	–	78.4	–	80	75.5	85.3	–	–	–	80.9	–	–	74.1
Ash	1.2	2.5	–	10.7	3.2	5.3	–	–	–	19.6	–	–	2
Fixed carbon*	–	11.2	–	9.3	11.1	14.7	–	–	–	19.1	–	–	25.9
Ultimate analysis (wt%)
Carbon	44.13	40.4	43.17	44.2	64.23	44.7	45.37	41.66 ± 1.12	37.60 ± 2.22	42.5	41.55 ± 0.92	28 ± 2.33	53.1
Hydrogen	4.13	6.17	10.65	5.4	6.89	7.5	6.18	6.84 ± 0.36	6.15 ± 0.36	6.5	6.04 ± 0.13	5.6 ± 0.08	7.1
Nitrogen	0.42	0.89	1.93	0.6	0.77	0.8	0.34	0.74 ± 0.02	0.88 ± 0.04	1.3	1.27 ± 0.14	6 ± 0.65	0.7
Sulfur	0.17	nd	0.01	0.06						0	–	–	0
Oxygen*	–		–	38.7	27.73	61.8	48.11	50.76 ± 0.89	55.37 ± 1.64	46	51.14 ± 1.18	36 ± 2.87	46.8
Cl	–		–	0.32	–	–	–	–	–	–	–	–	–
H/C	0.093	0.152	0.246	–	–	–	1.63	1.97 ± 0.17	1.96 ± 0.27	1.8	1.74 ± 0.06	0.19 ± 0.03	1.6
C/N	105.07	45.39	22.36	–	–	–	154.52	65.63 ± 1.89	50.16 ± 3.86	–	39.30 ± 3.53	5 ± 0.08	–
O/C				–	–	–	0.8	0.91 ± 0.05	1.10 ± 0.09	0.8	0.92 ± 0.05	–	0.7
Fiber analysis
Cellulose	45.1	29.68	–	–	–	20	44.12	60.61 ± 2.37	68.71 ± 1.62	61.18 ± 0.59	–	–	30
Hemicellulose	28.1	35.2	–	–	–	49	25.65	4.50 ± 0.30	3.01 ± 0.72	0.45 ± 0.18	–	–	18
Lignin	2.5	21.7	–	–	–	30	19.2	12.81 ± 0.62	10.00 ± 1.53	3.33 ± 0.19	–	–	53
Reference	Anjali et al. (2022)	Anjali et al. (2022)	Anjali et al. (2022)	Rey et al. (2021)	Khuenkaeo and Tippayawong (2020)	Windeatt et al. (2014)	Wijitkosum (2023)	Wijitkosum (2022)	Wijitkosum (2022)	Windeatt et al. (2014)	Wijitkosum and Jiwnok (2019)	Behera et al. (2020)	Windeatt et al. (2014)
	Sadh et al. (2018)	Guida and Hannioui (2017)				Ajien et al. (2023)		Wijitkosum and Jiwnok (2019)	Wijitkosum and Jiwnok (2019)	Wijitkosum (2022)

Sugarcane bagasse is the fibrous agricultural residue after the stalks are crushed and extracted for sugar production. Sugarcane bagasse contains hemicellulose (29.68%), cellulose (35.20%), and lignin (21.70%) (Guida & Hannioui, 2017). The elemental composition of sugarcane bagasse is carbon (40.40 wt%), hydrogen (6.17 wt%), and nitrogen (0.89 wt%). Soapnut pith fibres are one of the wastes obtained from soap fruit at the end of saponin extraction. The fruit manufactures hair tonics, cosmetics, skin creams, and anti-spermatic activity. The elemental composition of soapnut pith is carbon (43.17 wt%), hydrogen (10.65 wt%), and nitrogen (1.93 wt%). The weight percentage of carbon in sawdust biomass is higher than in sugarcane bagasse and soapnut pith fibres. Due to the high carbon and low nitrogen content in sawdust, the carbon-to-nitrogen (C/N) ratio of 105.07 wt% was the highest. In contrast, soapnut pith fibres’ high hydrogen and low carbon content showed that the hydrogen-to-carbon ratio was maximum (0.246 wt%).

Sugarcane straw is a waste of green leaves, dry leaves, and sugarcane kites that are usually left in the field. An average sugarcane yield of 100 tons/ha produces 12 kg of sugarcane straw. It has the potential to increase energy sources in the sugarcane sector. Sugarcane straw’s composition, consistency, and calorific value vary with climate, soil, crop variety, and harvesting method. The elemental analysis shows that sugarcane straw contains carbon (44.20 wt%), hydrogen (5.4 wt%), and nitrogen (0.60 wt%) (Rey et al., 2021). In addition, the ash concentration is relatively high, limiting its use as a fuel for the gasification process. Sugarcane straw is more suitable for combustion in steam boilers. Mixed biomass sources are also of interest, such as diesel and co-pyrolytic oil blends from sugarcane (Mohapatra et al., 2021).

Feedstocks similar to sugarcane straw are rice straw and wheat straw, which are commonly used in combustion systems for energy production, and coconut shells (Carvalho et al., 2017). The main composition, including proximate, final, and thermal analysis of the coconut shell, can be seen in Table 1. The water content of coconut shells is 10.1 wt%, which qualifies <11% for pyrolysis processing.

Coconut shell has a volatile content of 75.5 wt% (Khuenkaeo & Tippayawong, 2020), so it is easy to evaporate. The carbon content in coconut shells (64.3 wt%) is the highest when compared to other feedstocks, resulting in a higher calorific value. Coconut husk, rice husk, and palm shell are residues from the processing stream of agricultural commodities. Some of these wastes are still used as fuel sources in industry. Windeatt et al. (2014) discussed that 50% and 70% of rice husk is currently used as a fuel source by the processing industry, and the remaining 30–50% is currently an untapped waste product. About 30% of coconut husk is also used as a fuel source, and about 10% of palm shell residue is used for purposes such as road construction. The carbon content of coconut husk (44.7 wt%), rice husk (42.5 wt%), and palm shell (53.1 wt%). Palm shell has a high carbon content compared to coconut husk and rice husk due to its low ash and high lignin content.

Disposable bamboo chopsticks are mostly made of bamboo with specific properties such as hardness and resistance to water. Disposable bamboo chopsticks are for one-time use only, thus potentially generating a large amount of waste. One method that recycles the waste of disposable bamboo chopsticks is promising, both from the point of view of environmental protection and practical application, by converting it into biochar and slurry fuel (Chen et al., 2017). Disposable bamboo chopsticks have a carbon content of 45.37 wt%, hydrogen (6.18 wt%), and nitrogen (0.34 wt%) (Wijitkosum, 2023). The corncob waste, cassava rhizome (Wijitkosum, 2022), and cassava stem were processed into biochar using pyrolysis. The results of the study stated that the carbon content of corncob (41.66 wt%) was higher when compared to cassava stem (41.55 wt%) and cassava rhizome (37.60 wt%). In addition, peanut shell waste has been studied by Behera et al. (2020), where the biomass contains carbon (28 wt%), hydrogen (5.6 wt%), and nitrogen (6 wt%).

3. Biomass potential in Indonesia

The top ten commodities in Indonesia have been annually produced by agriculture on average over the last few decades, totaling about 136.22 million tonnes. Rice, corn, and palm oil have emerged as the top three agricultural products, accounting for 75.97, 54.98, and 15.24 million tonnes of dry biomass residue (Hambali & Rivai, 2017). The potential for bio-energy conversion through densification techniques is particularly encouraging for rice straw and husk, corn cob, and stalk, which are staple foods grown abundantly across many regions of Indonesia. On the other hand, the by-products from palm oil production, primarily cultivated as a plantation crop in specific regions of Sumatra, Borneo, Celebes, and the Papua Islands, are used for biodiesel production.

More than 50% of the energy potential within food crops can be derived from the rice straw and husk, which constitute half of the total residue from all food crops (Rhofita et al., 2022). In contrast, the plantation crop palm oil generates 82% of the total residue, equivalent to about 60% of its energy potential. It’s important to note that the characteristics of the biomass were examined within the range of minimum and maximum values.

In Indonesia, around 32,655 MWe of significant biomass capacity are available. Palm oil, with its by-products, empty fruit bunch (EFB), palm kernel shell (PKS), and palm oil mill effluent (POME), is the main source of this potential. A total of 66 biomass power plants, including those that use municipal solid waste, have been installed, with a 1,896.5 megawatt capacity. The majority of biomass energy is produced outside of the centralised electrical grid in industries, including the production of palm oil, sugar, pulp, and paper. Biomass residues also come from a variety of plants, such as oil palm (which produces empty fruit bunches, shells, fibres, and liquid waste), sugar cane (which includes bagasse, sugarcane leaves, and shoots), rubber (in the form of rubberwood), coconut (which produces shells and fibre), wood (which comes from tree waste), rice (which comes from rice husk), cassava (which comes from cassava waste), corncob, and corn leaves. Biomass has a 49.81 gigawatt (GW) potential as a renewable energy source. The available installed power in Indonesia, however, was 445 megawatts (MW) (Yana et al., 2022).

4. Biochar conversion techniques

The conversion of biomass into renewable fuels can be achieved through either biochemical or thermochemical techniques. The degradation of biomass into smaller molecules through the use of bacteria or enzymes, such as in anaerobic/aerobic digestion, fermentation, and enzymatic or acid hydrolysis, constitutes biochemical techniques (Sadh et al., 2018). Although biochemical processes are slower than thermochemical methods, they require less external energy. Anaerobic digestion results in biogas and biosolids, while aerobic digestion or composting generates CO₂, CH_4, heat, and solid residues (Jones et al., 2018). The process of fermentation involves the conversion of starch into sucrose through either acids or enzymes, followed by the transformation of sucrose into ethanol or other compounds with the aid of yeast. Pre-treatment, such as hydrolysis, is required for lignocellulosic materials to convert cellulose and hemicellulose into simple sucrose. Lignin, on the other hand, is synthesised thermochemically and is not oxidised.

A range of thermochemical methods is employed to transform biomass into solid, liquid, or gaseous products (Chiappero et al., 2020). Biochar production commonly involves thermal decomposition (Mukherjee & Kumar, 2021), which can be achieved through thermochemical methods such as pyrolysis, gasification, and self-sustained carbonisation (Ajien et al., 2023) (Table 2). The process of utilising extreme heat to decompose organic matter and generate biochar is discussed in this text. Biochar output is heavily impacted by the type of pyrolysis method implemented, producing a better quality of bio adsorbent (Kumar et al., 2021).

Table 2. Biochar production technology.

	Feedstock	Technology	Reactor	Pyrolisis temperature (°C)	Yield (wt %)	Reference
1	Sawdust	Pyrolysis (slow pyrolysis)	Muffle furnace	300	21.36 ± 1.7 (300)	Anjali et al. (2022)
			Muffle furnace	600	5.78 ± 0.6 (600)
		Pyrolysis (microwave-assisted)	Microwave reactor	500	15.18	Wang et al. (2008)
		Pyrolysis (slow pyrolysis)	–	500	19 ± 0.7	Blanco-Vargas et al. (2022)
2	Sugarcane bagasse	Pyrolysis (slow pyrolysis)	Muffle furnace	300	27.71 ± 1.8(300)	Anjali et al. (2022)
			Muffle furnace	600	15.51 ± 2.5(600)
		Pyrolysis (slow pyrolysis)	Tube furnace	550	32.8	Guida and Hannioui (2017)
3	Soapnut pith	Pyrolysis (slow pyrolysis)	Muffle furnace	300	11.94 ± 0.7 (300)
			Muffle furnace	600	1.13 ± 0.5 (600)
4	Sugarcane straw	Gasification	fixed and fluidized bed	–	–	Rey et al. (2021)
		Pyrolysis (slow pyrolysis)	Reactor furnace	350	–	Mielke et al. (2022)
				550	–
				700	–
5	Coconut shell	Pyrolysis (slow pyrolysis)	Tube furnace	550	–	Adorna et al. (2020)
			Muffle furnace	700	–	Baharum et al. (2020)
				300	56.35	Behera et al. (2020)
				400	44.87
				600	25.8
		Pyrolysis (ablative system)	Ablative system	500	28	Khuenkaeo and Tippayawong (2020)
			Fixed-bed reactor	600	–	Windeatt et al. (2014)
		Pyrolysis (microwave-assisted)	Microwave reactor	126–205.40	91.31	Nuryana et al. (2020)
		Pyrolysis (unknown)	Pyrolysis reactor with tar scrubber	–	28	Sari et al. (2020)
		Gasification	Top-lit updraft gasifier	–	–	Atienza et al. (2020)
			Fluidized bed gasifier	850	13	Romero Millán et al. (2019)
		Self-sustained carbonization	Pilot-scale brick reactor	300–500	30–32	Samsudin et al. (2019)
		Pyrolysis	Electrical furnace	400–600	25.4–32	Rout et al. (2016)
6	Coconut husk	Pyrolysis (slow pyrolysis)	Top-lit updraft retort unit	500	45	Ajien et al. (2023); Gonzaga et al. (2018)
			Muffle furnace	400	43.33	Suman and Gautam (2017)
				600	36
				800	34
				1000	31
			Fixed bed reactor	600	30.8	Windeatt et al. (2014)
7	Disposable bamboo chopsticks	Pyrolysis (slow pyrolysis)	Carbolite furnace	400	24.08 ± 1.219	Wijitkosum (2023)
				450	16.54 ± 0.301
				500	15.82 ± 0.673
8	Corn cob	Pyrolysis (slow pyrolysis)	Muffle furnace	300	37.46	Behera et al. (2020)
				400	29.25
				600	19.5
		Pyrolysis (slow pyrolysis)	Controlled Temperature Biochar Retort for Slow pyrolysis Process	350–550	58–60	Wijitkosum and Jiwnok (2019)
		Pyrolysis (slow pyrolysis)	Controlled Temperature Biochar Retort for Slow pyrolysis Process	450–500	60	Wijitkosum (2022)
9	Cassava rhizome	Pyrolysis (slow pyrolysis)	Controlled Temperature Biochar Retort for Slow pyrolysis Process	450–500	60	Wijitkosum (2022)
10	Rice husk	Pyrolysis (slow pyrolysis)	Dual-chamber reactor	380–1000	49	Illankoon et al. (2023)
		Pyrolysis (slow pyrolysis)	Controlled Temperature Rice Husk Biochar Retort for Slow Pyrolysis Process	450–500	45	Wijitkosum (2022)
		Pyrolysis (microwave-assisted)	Microwave reactor	400–700	28–55	Sahoo and Remya (2022)
11	Cassava stem	Pyrolysis (slow pyrolysis)	Controlled Temperature Biochar Retort for Slow pyrolysis Process	350–550	58–60	Wijitkosum and Jiwnok (2019)
12	Peanut shell	Pyrolysis (slow pyrolysis)	Muffle furnace	300	55.67	Behera et al. (2020)
				400	52.05
				600	25.22
		Pyrolysis (slow pyrolysis)	–	250	41	Nazir et al. (2021)
			–	400	36
			–	550	33
13	Palm shell	Pyrolysis (slow pyrolysis)	Fixed bed reactor	600	30.8	Windeatt et al. (2014)
		Pyrolysis (microwave-assisted)	Microwave reactor	500	30.5	Wang et al. (2008)

4.1. Pyrolysis

Pyrolysis is a thermal conversion process that occurs between 300 and 600 °C under a non-oxidizing atmosphere, producing three main products: pyrolysis gas, biochar, and biooil (Ajien et al., 2023). The process can operate in various reactors, including tube furnaces (Adorna et al., 2020; Guida & Hannioui, 2017), muffle furnaces (Anjali et al., 2022; Baharum et al., 2020; Behera et al., 2020; Suman & Gautam, 2017), fixed and fluidized bed (Rey et al., 2021; Romero Millán et al., 2019; Windeatt et al., 2014), ablative systems (Khuenkaeo & Tippayawong, 2020), electrical furnaces (Rout et al., 2016), carbolite furnaces (Wijitkosum, 2023), and dual chamber reactors (Illankoon et al., 2023) at different temperatures, heating rates, retention times and inert gas flow rates. Pyrolysis consists of two stages; in the first stage, the complex molecular bonds of lignin, cellulose, and hemicellulose in biomass are broken to form carboxyl, carbonyl, and hydroxyl deciduous on the surface of biochar (Ajien et al., 2023), which then undergo decarboxylation, dehydration, and dehydrogenation. In the second stage, by applying heat energy continuously, the larger molecules or heavy compounds of biomass undergo several chemical reactions to produce biochar, bio-oil, and syngas (Ajien et al., 2023; Lee et al., 2018). Pyrolysis is the only process producing the most valuable chemicals and industrial raw materials among all thermochemical conversion processes. Fast pyrolysis provides liquid fuels that can replace fuel oil or diesel in static heating, including boilers, furnaces, and power generation applications. The liquid can also be used to produce various specialty chemicals.

4.2. Slow pyrolysis

The slow pyrolysis technique without/low oxygen yields 30% charcoal, in contrast to fast pyrolysis, which generates only 12%, or gasification, which produces merely 10% (Enaime et al., 2020). Among the combustion procedures, pyrolysis is one of the most straightforward since the biomass is heated in the absence of oxygen. Chemically, this process is an exothermic oxidation reaction that occurs at high temperatures in the absence of oxygen to produce hot flue gas, which consists of CO₂ and H₂O. The by-products of this combustion include biochar, which contains specific carbon black, and ash, which generally contains inorganic oxides and carbonates (Mamaní et al., 2019).

In slow pyrolysis, biomass is thermally degraded at 300–800 °C with a heating rate of 5–10 °C per minute (Lee et al., 2018). Slow pyrolysis takes a long time (usually more than 1 hour) to thermally degrade, resulting in 30–40 wt% biochar, 25–30 wt% bio-oil, and 25–35 wt% synthesis gas (Ajien et al., 2023). During pyrolysis, the biomass is heated to temperatures above 300 °C without oxygen. The organic components thermally decompose releasing the vapor phase, while the remaining solid phase of the biochar remains. The vapour phase is then cooled to produce bio-oil, where polar and high molecular weight compounds are condensed while low molecular weight volatile compounds (e.g., CO, H₂, CH₄, and C₂H₂) remain in the gas phase (Laird et al., 2009). This method is most preferred as it produces the highest biochar yield when compared to bio-oil and biogas yields. The downside of this method is that it is more expensive and requires a relatively long retention time. In addition, it also requires higher energy to produce higher biochar.

4.3. Microwave-assisted pyrolysis

Microwave pyrolysis has been applied to various lignocellulosic feedstocks, such as coconut shell (Nuryana et al., 2020), sawdust (Wang et al., 2008), peanut shell (Behera et al., 2020; Wang et al., 2008), and rice husk (Sahoo & Remya, 2022). Microwave heating can be better than conventional heating because of various advantages. Hot spots, which form under microwave irradiation, would have a significant influence on the yield and characteristics of microwave processing products. The solid products of microwave pyrolysis at proper microwave power levels can have high heating values and specific surface areas with higher gas and solid yields but lower liquid yields than conventional pyrolysis (Ajien et al., 2023; Huang et al., 2016).

In conventional heating processes, heat transfer between the solid biomass feedstock and its surroundings occurs through conduction, convection, and radiation, which limits the flexibility of the heating process to control the operating temperature, whereas microwave pyrolysis utilizes microwave radiation which involves selective and volumetric heating for thermal degradation of biomass (Ajien et al., 2023; Huang et al., 2016). Microwave-assisted pyrolysis is carried out at 400–800 °C (Sahoo & Remya, 2022).

4.4. Ablative pyrolysis

Ablative pyrolysis degrades biomass by using heat energy when biomass particles are in close contact with hot solids or surfaces. Currently, ablative pyrolysis is used to determine the yield of biochar and bio-oil by using an ablative reactor. In ablative pyrolysis, the use of inert gas is not required, but a reactor is needed, which is expensive, medium temperature, and has a low reaction rate during the pyrolysis process. Ablative pyrolysis is one of the rarest types of pyrolysis to be applied in biochar production. Ablative pyrolysis has been applied to various lignocellulosic feedstocks, such as coconut shells (yield biochar 28%) (Ajien et al., 2023). Ablative pyrolysis occurs at 500 °C (Khuenkaeo & Tippayawong, 2020).

4.5. Gasification

Gasification is a thermochemical process by which a carbon substrate (e.g., biomass) is converted into a combustible material. The gas through a series of reactions takes place at high temperatures in the presence of a gasification agent (air, oxygen, water vapour, carbon dioxide, or a mixture thereof) (Rey et al., 2021). Biomass is partially burned at high temperatures of 600–1200 °C with a retention time of 10–20 s (Ajien et al., 2023). Dry biomass undergoes a gasification process using air, CO₂, or steam (H₂O) as gasification material. The biomass undergoes four stages of gasification: (1) drying, (2) devolatilization, (3) partial oxidation, and (4) reduction (Richardson et al., 2015). In the first stage, the biomass is dried to reduce its MC content. In the second stage, the dried biomass undergoes devolatilization to produce tar, water, synthesis gas, and biochar. In the third stage, synthesis gases such as CO, CO₂, H₂, and CH₄ and solid biochar undergo partial oxidation. In the fourth stage, the biochar undergoes reduction or gasification, producing ash and many synthesis gases such as CO, CO₂, CH₄, N₂, H₂, H₂O, and various VM (CxHyOz). The main objective of gasification is to convert biomass into gaseous products.

4.6. Self-sustained carbonisation

In self-sustained carbonisation, the carbonization temperature is maintained by itself while the combusting biomass is changed into biochar inside the reactor. Dry biomass is fed into a self-sustained carbonization reactor, which is mainly made of bricks and withstands high temperatures (300–500°) (Samsudin et al., 2019). The biomass undergoes a combustion process inside the reactor, and the brick reactor is closed to produce an oxygen-free environment. Biomass combustion uses heat energy to break down cellulose, hemicellulose, and lignin in biomass to form a complex biochar network (Lee et al., 2018).

5. Characteristics of biochar product

Biochar, also known as charcoal or char, is a solid material produced from non-fossil-based carbonaceous organic materials like biomass. This process, defined by the International Biochar Initiative (IBI), involves the thermochemical conversion of biomass in an oxygen-limited environment (Setyawan et al., 2023). It’s crucial to differentiate biochar from charcoal, the latter being a carbon-rich solid used mainly as solid fuel for energy production. In contrast, biochar is a product of thermal decomposition, recognized for its ability to mitigate climate change and enhance soil fertility.

The production method of biochar significantly affects its physicochemical properties (Fuertes et al., 2010; Wiedner et al., 2013). Table 3 shows different properties of biochar derived from various agricultural residues, such as sawdust, sugarcane bagasse, and soapnut pith fibres. Key factors influencing biochar characteristics include pyrolysis temperature and feedstock type, which determine yield, ash content, volatile matter, and fixed carbon content. Higher temperatures yield biochar with increased ash, fixed carbon, and volatile matter (Anjali et al., 2022).

Table 3. Properties of biochar product.

	Sawdust	Sugarcane bagasse	Soapnut pith	Sugarcane straw	Coconut shell	Coconut husk	Disposable bamboo chopsticks-400	Corncob	Cassava rhizome	Rice husk	Cassava Stem	Peanut shell	Palm shell
Proximate analysis (wt %)
Moisture	1.17 ± 0.11 (300) 3.12 ± 0.10 (600)	7.22 ± 0.08 (300) 2.25 ± 0.10 (600)	28.98 ± 0.70 (300) 5.42 ± 0.21 (600)	–	6.86	10.4	–	–	–	5.7	–	4.8	2.2
Volatile matter	76.45 ± 12.25 (300) 24.05 ± 3.5 (600)	81.71 ± 6.6 (300) 66.56 ± 4.3 (600)	86.94 ± 4.2 (300) 64.86 ± 4.0(600)	–	27.19	25.1	41.87 ± 0.047 (400) 36.25 ± 0.391 (450) 35.99 ± 0.613 (500)	–	–	13.9	–	45.3	11.5
Ash	10.80 ± 0.04 (300) 50.84 ± 0.55(600)	6.49 ± 0.55 (300) 11.44 ± 0.81 (600)	6.96 ± 1.25 (300) 27.44 ± 1.66 (600)	5.0 (350) 10.3 (550) 11.6 (750)	1.66	13.5	3.38 ± 0.216 (400) 3.54 ± 0.567 (450) 3.80 ± 0.013 (500)	0.96 ± 0.45	5.02 ± 1.90	47	–	4.02	6.7
Fixed carbon*	12.18 ± 1.6 (300) 21.87 ± 1.5 (600)	5.86 ± 1.4(300) 17.41 ± 1.5(600)	2.16 ± 0.68 (300) 4.17 ± 0.97 (600)	–	69.47	74.9	–	–	–	86.1	–	45.8	88.5
Ultimate analysis (wt%)
Carbon	53.3 (300) 75.0 (600)	57.0 (300) 69.1 (600)	57.1 (300) 59.1 (600)	48.7 (350) 49.1 (550) 59.0 (750)	77.19	82.6	47.99 ± 0.268 (400) 51.48 ± 0.350 (450) 52.73 ± 0.752 (500)	81.35 ± 1.375	64.25 ± 0.169	54.5	62.95 ± 3.71	–	90.6
Hydrogen	0.54 (300) 1.26 (600)	0.51 (300) 3.89 (600)	0.54 (300) 8.33 (600)	–	3.4	2.7	3.22 ± 0.035 (400) 2.96 ± 0.068 (450) 2.83 ± 0.038 (500)	2.87 ± 0.39	2.73 ± 0.040	2.1	2.24 ± 0.12	–	2.8
Nitrogen	1.00 (300) 0.45 (600)	0.91 (300) 0.52 (600)	2.48 (300) 1.81 (600)	0.832 (350) 0.647 (550) 0.403 (750)	0.1	2.4	0.57 ± 0.026 (400) 0.64 ± 0.032 (450) 0.65 ± 0.030 (500)	1.25 ± 1.135	1.65 ± 0.010	1.1	1.37 ± 0.06	–	0.9
Sulfur	0.0 (300) ND	0.0 (300) ND (600)	0.0 (300) ND (600)	–	–	–	–	–	–	–	–	–	–
Oxygen*	–	–	–	–	18.06	12.8	32.73 ± 1.009 (400) 32.30 ± 1.649 (450) 29.11 ± 2.377 (500)	13.58 ± 0.377	26.35 ± 0.138	5.4	33.44 ± 3.81	–	7.9
Cl	–	–	–	–	–	–	–	–	–	–	–	–
H/C	0.01 (300) 0.016 (600)	0.008 (300) 0.056 (600)	0.009 (300) 0.140 (600)		–	–	0.64 ± 0.019 (400) 0.56 ± 0.007 (450) 0.54 ± 0.009 (500)	0.04	0.04	0.5	0.42 ± 0.01	–	0.4
C/N	53.5 (300) 166.6 (600)	62.6 (300) 132.8 (600)	23.0 (300) 32.6 (600)	58.51 (350) 75.83 (550) 146.36 (750)	–	–		65.08	38.94		54.08 ± 4.51	–
O/C	–	–	–	–	–	–	0.41 ± 0.021 (400) 0.40 ± 0.027 (450) 0.35 ± 0.040 (500)	0.17	0.41	0.1	0.39 ± 0.08	–	0.1
Reference	Anjali et al. (2022)	Anjali et al. (2022)	Anjali et al. (2022)	Rey et al. (2021)	Khuenkaeo and Tippayawong (2020)	Windeatt et al. (2014)	Wijitkosum (2023)	Wijitkosum (2022)	Wijitkosum (2022)	Windeatt et al. (2014)	Wijitkosum and Jiwnok (2019)	Behera et al. (2020)	Windeatt et al. (2014)
				Mielke et al. (2022)		Ajien et al. (2023)

Sawdust biochar, specifically, shows the highest ash content at elevated temperatures when compared to those made from sugarcane bagasse and soapnut pith fibres. This ash content escalates progressively with rising pyrolysis temperatures (Anjali et al., 2022). Concurrently, fixed carbon content also increases from pyrolysis at 300 °C to 600 °C, with a notable rise in sawdust biochar due to the loss of volatile matter (Anjali et al., 2022; Crombie et al., 2013). As a general trend, volatile matter percentage in biochar samples diminishes with increasing pyrolysis temperature. Significant variances are observed in sawdust biochar produced at different temperatures, with carbon content varying from 76.45 ± 12.25 wt% at 300 °C to 24.05 ± 3.5 wt% at 600 °C. The higher volatile fraction in sawdust biomass is removed during combustion, and biomass with greater lignin content yields higher energy (Novaes et al., 2010).

The carbon content in biochar ranges widely, from 47.99 wt% in disposable bamboo chopsticks to 90.6 wt% in palm shell biochar. The balance of carbon stored in biochar is a significant factor, with volatile content ranging from 8% to 30%. Generally, a higher ash content correlates with higher carbon content in biochar; the opposite is also true (Windeatt et al., 2014). Biomass raw materials with high ash content tend to produce biochar with lower fixed carbon content due to the inhibition of aromatic carbon formation by the ash. The relationship between lignocellulose composition and fixed carbon content is also notable; raw materials with the highest lignin content, like palm shell, yield high carbon biochar, whereas those with high cellulose content, such as cassava rhizome, result in biochar with the lowest carbon content (Wijitkosum, 2022).

6. Biochar as energy sources

Biomass has become a potential energy source, especially since the fossil fuels crisis and people require more energy (Saad Naggar et al., 2018). The agricultural and plantation industries can supply biomass for renewable energy when this happens. Although agricultural waste is an abundant source of biomass with consistent features, it lacks the necessary density for cost-effective transportation to energy production sites. As a result, agricultural biomass is an excellent candidate for densification before use (Bajwa et al., 2018). Before processing using physical, biological, and thermochemical conversion techniques, the physicochemical qualities of biomass should be examined (Kabir Ahmad et al., 2022).

Biomass consists of three main constituents: cellulose, hemicellulose, and lignin. The majority of biomass feedstocks typically contain around 40–50% cellulose by weight, followed by hemicellulose, which makes up 20–40%, and the remaining portion is mostly lignin. The cellulose-to-lignin ratio is critical in determining whether a plant species is suitable for energy crop processing (McKendry, 2002). In addition to this, other essential considerations include moisture content, calorific value, fixed carbon/volatile, ash content, and alkali metal content (Bajwa et al., 2018).

Biomass is a viable option due to its abundance as an alternative to traditional fossil fuels and its minimal effect on carbon emissions (Liu et al., 2015). As plants take in CO₂ from the air, they generate a significant quantity of biomass that can be altered into bio-oil and biochar. After upgrading, bio-oil can be modified into various biofuels and utilised instead of fossil fuels. It’s important to note that plants can reabsorb any CO₂ released by biofuels. Additionally, biochar is a long-lasting form of carbon that can be used as a carbon storage method for the long term (Huber et al., 2006).

Different methods, including thermochemical and biochemical conversions, are available to transform biomass into energy sources such as heat or electricity and energy carriers such as oil, gas, or charcoal. One commonly used technique is pyrolysis, which produces bio-oil, gas, and biochar. Pyrolysis yields three primary products: pyrolysis liquid (bio-oil), gas, and biochar. One of the products of pyrolysis, biochar, is a high-calorie substance abundant in carbon with a calorific value of approximately 30 MJkg⁻¹ (Wu et al., 2022), and it also has a minimal ash content (Saad Naggar et al., 2018). These characteristics make biochar often used as a slurry fuel (Khanmohammadi et al., 2015; Liu et al., 2017; Rasi et al., 2019). Slurry fuel is a solid suspension that is finely ground and suspended in water, oil, or other organic solvents. Coal-based slurry fuel is a mature technology practised over the last few decades because coal slurry can be handled the same way as oil fuels, thus saving transportation, storage and drying costs. However, the high CO emission of flue gases produced in the manufacturing process causes the use of coal slurry fuel to be limited. To support the Indonesian government’s commitment to reducing greenhouse gas emissions and achieving net-zero emissions, the Ministry of Energy and Mineral Resources has developed strategies, including replacing coal with biomass for steam power plants. Therefore, using biochar in the form of biochar slurry fuel has the potential to meet the world’s energy needs while reducing greenhouse gas emissions.

The use of biochar has the potential to affect agriculture (Mukherjee et al., 2016), particularly in the energy industry. Biochar can be applied to coal cofiring, which could be a cost-effective solution to reduce CO₂ emissions in existing power stations. A techno-economic model has been created based on a pilot plant’s experience to evaluate biochar use in co-firing (De & Assadi, 2009). The analysis showed that the use of biochar can decrease CO₂ emissions. On a smaller scale, using biomass in co-firing has proven to be effective in reducing CO₂ emissions in power plants with capacities ranging from 15 to 50 MW despite the normal increase in cost. However, for large-scale plants (250 MW), a higher percentage of biomass must be burned with coal to reduce CO₂ emissions, resulting in higher costs (Alhashimi & Aktas, 2017).

7. Biochar slurry fuel

Slurry fuel is the homogenisation of fine-sized solid materials with liquid and emulsifiers. Slurry fuel is used as fuel for boiler engines and diesel engines. The type of slurry fuel is divided into various groups based on the liquid used, namely coal-water slurry, coal-oil slurry, coal-oil-water slurry, coal-methanol slurry, and coal-methanol-water slurry (Nunes, 2020). Coal-Water Slurry (CWS): A coal-water slurry is a mixture of finely ground coal particles and water (Manfred, 1986). The coal particles are typically suspended in water, forming a slurry that can be transported through pipelines. This mixture is often used as a fuel for power generation and industrial heating, as it provides better handling and combustion properties compared to dry coal. Coal-Oil Slurry (COS): A coal-oil slurry is a blend of coal and oil. In this mixture, finely ground coal particles are mixed with oil, usually a liquid fuel like diesel or heavy fuel oil. The purpose is similar to that of coal-water slurry, offering improved combustion characteristics and ease of transportation. Coal-Oil-Water Slurry (COWS): A coal-oil-water slurry is a combination of coal, oil, and water (Belonogov et al., 2023). This mixture can include finely ground coal particles suspended in a mix of oil and water. The addition of water might be done for various reasons, such as modifying the viscosity or improving combustion properties. Coal-Methanol Slurry: A coal-methanol slurry is formed by mixing finely ground coal particles with methanol, which is a type of alcohol (Sakai et al., 1985). Similar to other coal slurry mixtures, this is often intended for more efficient and cleaner combustion, especially when using methanol as a fuel. Coal-Methanol-Water Slurry: A coal-methanol-water slurry involves a combination of coal, methanol, and water. The presence of water and methanol, along with the coal particles, can create a mixture with specific characteristics that suit combustion or other applications.

In general, the purpose of creating these mixtures is to improve the handling, transportation, and combustion properties of coal, which is often challenging to use in its raw form due to its bulkiness, low energy density, and environmental concerns related to emissions. These slurry mixtures allow for more efficient coal utilisation while potentially reducing environmental impact and providing flexibility in fuel sources for various applications. The choice between these mixtures depends on factors like intended use, availability of resources, combustion efficiency, and environmental considerations. The composition of slurry fuel consists of 50–75% solid material with a maximum size of 300 μm (coal), 25–50% liquid (water), and 1% surfactant additives to create a homogeneous slurry (Nunes, 2020).

Slurry fuel from renewable biomass can be classified as a better energy product than coal. Biochar slurry fuel creates a homogeneous mixture by mixing finely ground biochar with water and other potential additives (Soloiu et al., 2011). The production process includes numerous steps, including the pyrolysis of biomass to produce biochar, the reduction of particle size through grinding, and blending biochar with water to make the slurry. The biochar size reduction is affected by pyrolysis temperature and the milling process (Setyawan et al., 2023). The benefits of using biochar slurry fuel are numerous. When compared to traditional fossil fuels, it has the potential to cut glasshouse gas emissions drastically (Anand et al., 2022). It simultaneously addresses waste management issues by employing biomass waste as a raw material. Notably, the synthesis and use of biochar have the potential to be carbon-negative, which can help to reduce atmospheric carbon dioxide (Goswami et al., 2021).

The range of uses for biochar slurry fuel is extensive. The versatility of biochar slurry fuel as a renewable energy source for heating, and producing electricity highlights its potential to change how energy is produced. It can be used in specialised burners or gasifiers for energy generation to generate heat and electricity (Liu et al., 2017). Combustion study of biochar slurry fuel with a composition of 60% to 70% water, 1% surfactant, and 30% to 40% biochar with a particle size of 10 μm to 30 μm shows the ability to produce bright sooty flames (Liu et al., 2017). The ideal particle size and slurry consistency are essential for effective application and efficient combustion. Additional materials, such as methanol, need to be added to modify the combustion characteristics of slurry fuel (Zepeda et al., 2023). The characteristics of slurry fuel have been the subject of investigation. Rheological properties of the slurry fuel are one of the important characteristics that are affected by particle size, solvent selection, and additives (Feng & Wu, 2018).

8. Future work

Future research on increasing the caloric value of biochar-slurry fuel, the burner design and biochar-slurry fuel on industrial applications are exciting research endeavours. The characteristics of the resulting slurry fuel will be affected by the solid particle size, particle distribution, concentration of each composition, temperature, and stirring speed. A multidisciplinary strategy incorporating science, engineering, environmental concerns, and socioeconomic considerations will characterise future research on biochar slurry fuel. Future research on biochar slurry fuel will likely concentrate on improving and expanding production methods, exploring its potential applications in diverse fields, overcoming obstacles to its use, and comprehending its effects on the environment and society. The key research area is (1) Production Optimisation: Investigations will focus on increasing the biochar slurry fuel’s production effectiveness. This involves developing strategies to effectively grind biochar particles to the ideal size for slurry formation, exploring novel approaches to combine biochar with water and additives to create a stable and homogenous slurry, and optimising the biomass pyrolysis process to produce biochar with desired properties. (2) Combustion Efficiency: Work must be done to improve the biochar slurry fuel’s ability to burn. Examination of combustion kinetics, temperature profiles, and emissions to maximise energy output while minimising pollutant release is necessary. (3) Application Diversification: Upcoming research needs to examine the biochar slurry fuel’s adaptability to several uses. 4. Environmental Impact Assessment: In-depth life cycle analyses of biochar slurry fuel will be carried out to examine how it will affect the environment overall, from production to by-product disposal. As part of this, the potential for carbon sequestration will be quantified, the emissions decrease compared to alternative fuels will be evaluated, and any potential secondary environmental consequences will be researched.

9. Conclusion

The diverse range of biomass sources available throughout the country, coupled with advancements in technology and research, presents a promising avenue for developing this alternative fuel. The utilisation of agricultural, plantation, and forest residues and various organic waste materials such as wood waste, coconut shells, rice husks, cocoa pods, and corn cobs as slurry fuel offers improved combustion characteristics and ease of transportation. The slurry fuel can have a significant economic impact, especially in small-scale co-firing of biomass and coal. Using agro-industrial waste to make biochar can also minimise the expenditure needed to reduce CO₂ emissions. However, controlling rheological properties and adding other materials to increase combustion characteristics are necessary for slurry fuel combustion. Future work needs to be done on product optimisation, combustion efficiency, application diversification, and environmental impact assessment.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research received financial support from the Ministry of Research, Technology/National Research and Innovation Agency of the Republic of Indonesia through the PDKN program (grant number: 1071.24/UN10.C10/TU/2022), which is administered by the Directorate of Research and Community Service and the Deputy for Strengthening Research and Development.