Production of Biochar from Oilseed Residue (Deoiled Cakes): State-of-the-Art

Abstract

Even today, the generation of chemicals and energy is still reliant on fossil-based resources in industrialized countries. Biomass could be a valuable renewable energy source that could reduce dependence on fossil fuels as well as provide a significant reduction of carbon dioxide and greenhouse gas emissions. In this scenario, residue from natural oil extraction units is uplifted to produce biofuels as replacement of fossil fuels. In the process of bio-refinery, well established technologies were presented. Those are thermochemical treatment (pyrolysis, liquefaction, gasification, etc.), anaerobic digestion, catalysis, etc. Especially, importance is given to pyrolysis as it is the feasible technique to utilize residue and to produce wealthy products. The role of intrinsic bio-polymers in quantity of final pyrolytic products was discussed. Major process parameters were critically elucidated, however, the investigation of advanced pyrolysis technologies requires further research.

1. Introduction

The future energy demand is projected to rise further in accordance with ever-growing population. In addition to this, global energy consumption relies on non-renewable sources (like coal, petroleum, and natural gas) and those depleting day by day [1]. Moreover, the growth in greenhouse gas emissions from fossil fuel usage generates major menace to the climate. Therefore, it is imperative to search for environmentally and sustainable resources and utilize them to their full potential in light of biofuels. Biomass, as a renewable energy source, can be widely viewed as a promising alternative, because of its numerous applications, especially biofuels and bio-fertilizers [2]. There are several bio-refinery platforms that will be capable of processing biomass feed-stocks, which are the key intermediates between biomass feed-stocks and their value-added products. The bio-refineries usually add value to biomass supply chains through the production of various bio-based products. Selection of biomass has numerous advantages such as (a) abundant availability of precursor material; (b) environmentally friendliness; and (c) biodegradability and sustainability. In a typical bio-refinery, by using appropriate technologies, biomass by-products can be processed to generate various high-valued fuels. The bioenergy sector has many major platforms, for example, syngas from gasification, fuel and fertilizer from anaerobic digestion, sugars from starch, extraction of cellulose, hemicellulose and lignin from biomass, solid carbon and bio-oil from pyrolysis, etc. A comparative ideology of thermochemical conversion technologies is illustrated in Figure 1.

Figure 1.

Comparison of common thermochemical conversion techniques.

Much research has been conducted on converting industrial and agricultural waste into valuable products [3, 4]. Still, this research topic is emerging because of its abundant scope for future energy demands. However, agricultural residue is prominent in energy conversion policies. Primarily, rice straw, rice husk, wheat straw, sugar cane bagasse, bamboo, areca nut shells, corn cob, coconut shells, etc., were used to produce energy fuels [5, 6, 7, 8, 9]. Apart from that, a vast range of oilseeds can be grown in India due to its diverse agro-ecological conditions and these all are coming under two major categories, such as edible oil seeds and non-edible oil seeds. The oil seeds are used for extraction of either edible oils or other purposes. In general, extraction of kg oil seeds can produce 250–350 g of oil [7], however, 650 g of residue seed cake is also obtained in the process of oil extraction [8].

Consequently, almost 65% of residue will available at the end of the extraction process. Some portion of extracted residue from edible seeds such as soybean, ground nut, flax seeds, sunflower, and etc., can be used as feed material for animal and the remaining unused. But, the residue from non-edible seeds contains toxic compounds, which limit their utilization as edible material. Hence, the oilseed residue (OSR) especially from non-edible seeds can be a potential material to produce biofuels [9], instead of unused. To convert oil seed residue (OSR) into valuable biofuels, thermochemical or bio-chemical conversion techniques can be used [10].

Thermochemical conversion involves thermal cracking of chemical organic bonds of biomass (OSR) into solids, gases, or liquid form [10]. Possible thermochemical conversion processes are torrefaction, pyrolysis, gasification, and liquefaction. The interest has been increasing day by day toward thermochemical conversion, because it is user friendly and sustainable too. However, in bio-chemical conversion, specified bacteria (yeast) may be used to digest or convert biomass into different biofuels. Bio-chemical conversion includes anaerobic digestion, alcoholic fermentation, transesterification, and so on. Compared to bio-chemical conversion methods, thermochemical process has superior qualities. The final product obtained from thermochemical process has higher carbon content, lower volatile matter, and reduced contaminants. Also, it is more energy-efficient method and can mitigate the release of harmful emissions and pollutants. Hence, the subsequent categories in thermochemical conversion technique are discussed as follows.

2. Value addition technologies

2.1 Pyrolysis

In pyrolysis, the organic material and volatiles present in the biomass undergo an irreversible thermochemical decomposition reaction to produce biofuels. Pyrolysis technologies can be classified as slow, flash, and fast [1]. The fast heating rate and short vapor residence times result in rapid exit of the volatile hydrocarbon vapors from the reactor and condensation into bio-oil, whereas slow pyrolysis results in greater biochar production due to increasing of carbonization of biomass due to slow heating rate and longer vapor residence time [11]. However, slow pyrolysis is the prominent technique than others to produce biochar. The remaining pyrolysis techniques enable the production of bio-oil and gases. A flow chart of pyrolysis is present in Figure 2.

Figure 2.

A simple flow chart of pyrolysis process.

2.2 Torrefaction

Torrefaction is the one of the clean thermal treatments of biomass. Further, torrefaction actively removes the moisture content from biomass, however, similarly improves the efficiency of subsequent thermochemical conversion methods [12]. Biomass that has been torrefied has a number of superior quality over raw biomass, including higher grindability, lower hydrophobicity, higher heating value, and pelletization properties [13]. In the process of torrefaction, several factors influence their performance, such as, torrefaction temperature, holding time, heating rate, environment, moisture content, particle size [14]. Figure 3 illustrates general flow chart of torrefaction process.

Figure 3.

Illustration of torrefaction process.

2.3 Gasification

Gasification is another of the thermochemical conversion method, used to produce gaseous material from OSR [11]. Gasification flow diagram can be observed in Figure 4. Though, the produced gaseous material can be used for heat, electricity, methane and hydrogen generation [15]. However, gasification not only produces gaseous products but also produces bio-chemical liquids. In general, gasification occurs at high temperature value, tar content in the gas is very low, and the cleaning process is very simple [16].

Figure 4.

Process flow diagram of gasification.

2.4 Liquefaction

Other than torrefaction, pyrolysis, gasification, an alternative thermochemical process for converting biomass to biofuel is liquefaction [17]. In order to form bio-crude oil, biomass must be hydrogenated and thermally disintegrated at high pressures. In the process of hydrothermal liquefaction, solid waste contains lots of moisture and it is transformed into bio-crude oil in the presence of some catalysts [18]. The temperature and pressure ranges for liquefaction generally fall within the ranges of 240–380°C and 5–30 MPa, respectively [11]. In Figure 5, a view of liquefaction process is reported.

Figure 5.

Simple flow diagram of liquefaction.

3. Role of bio-polymeric compounds in characteristics of the oil seed formed

Lignocellulosic materials (oilseed residue) generally consist of several intrinsic bio-polymeric components [19]. Among others, lignin, cellulose, and hemicellulose play a vital role in composition of pyrolytic products. When the lignocellulosic material undergoes heat treatment, the degradation trends of each bio-polymer decompose differently [20]. Moreover, the percentage of total carbon also depends on the degradation of these bio-polymers. Briefly, hemicellulose is susceptible to lower temperatures of torrefaction or pyrolysis, average range of the temperature for hemicellulose is 150–300°C. However, cellulose also gets volatized at slighter high temperatures than the hemicellulose. Lignin is a complex long chain bio-polymer and it starts to decompose partially at the temperature of cellulose and hemicellulose, however, it gets decomposed at higher temperature i.e., > 500°C [21].

Table 1 reports that the liquid production from safflower seed cake (residue) is more, when the average percentages of cellulose and hemicellulose are 40% and 16%, respectively [30]. However, in another study, the authors suggested that if the mustard oilseed residue contains an average percentage of cellulose and hemicellulose at 52.21% and 10.85%, respectively, the residue is potentially suitable for bio-fuel production [8]. In the same way, a comparative study of African star apple seed (ASA) and silk cotton seed residue (SCS), bio-oil production is more in ASA because of its high lignin content at optimum temperature of 400°C [23]. Kinetic studies of Jatropha de-oiled cake residue were conducted to examine the utilization of Jatropha residue for bio-fuel production [25]. Withal, bio-polymeric compositions in OSR affect the yield of outcome of pyrolysis. The further studies of variations with heat treatment could have great scope in future [31, 32]. In addition, proximate and ultimate analysis also plays a key role in pyrolysis. Here, elucidation of their role and importance is not mentioned, as they are mentioned in many literatures [30, 33].

Residue	Lignin (%)	Cellulose (%)	Hemicellulose (%)	Reference
Safflower seed press cake	26.7	40	16	[22]
Mustard oil residue	15.27	52.21	10.85	[8]
African star apple seed cake	10.0	16.21	14.0	[23]
Silk cotton seed cake	4.0	19.14	19.33
Raspberry seed cake	19.3	42.1	16.9	[24]
Jatropha de-oiled cake	24.9	53.5	16.6	[25]
Pongammia pinnata de-oiled cake	21.87	21.34	49.66	[26]
Black cumin seed cake	26.73	37.14	10.44	[27]
Raspberry Seed Cake	19.3	42.1	16.9	[28]
Radish seed cake	5.62	3.89	4.15	[29]

Table 1.

Bio-polymeric composition of oilseed residues.

4. Effect of process parameters on pyrolytic products

Specifically, pyrolysis reactor selection is the principal factor to decide the quality and quantity of final products [34]. From the studies, it is observed that fixed bed reactor (slow pyrolysis) is mostly used to produce biofuels as it is simple in operation and maintenance as well easy in construction. However, advanced pyrolytic techniques such as plasma pyrolysis, microwave pyrolysis, ultrasonic pyrolysis, solar-based pyrolysis are being explored and they will likely emerge as significant concepts in the future.

From tabulated studies, it is observed that biochar yield increases at slow pyrolysis rate, i.e., approximately 350–400°C at 5°C/min for 30 minutes of holding time. Whereas, bio-oil and bio-gas occur at little higher temperatures and higher heating rates (>500°C for 20°C/min). The above phenomenon clarifies that slower heating rate provides enough time to settlement of volatiles during thermal treatment, but not in higher heating rates [35]. Similarly, pyrolysis of safflower seed cake occurs at 400–600°C at 50°C interval and at 10, 30, and 50°C/min. The authors claimed that with increasing heating rates, bio-oil yield increases, but with increase in temperature from 400 to 600°C, yield of bio-oil decreases. Because, at higher temperatures, secondary reactions occur and lead to an increase in the content of gases rather than bio-oil. However, biochar yield decreases with increment in both heating rate and temperature [22]. In Table 2, the effect of different thermal process parameters and their influence on pyrolytic output are listed.

OSR	Reactor	Temperature (°C)/Heating rate (°C/min)	Time (min)	Pyrolysis products			References
				Solid (%)	Liquid (%)	Gas (%)
Flaxseed cake	Fixed bed	350–650/5, 20	30	44.6	43.3	12.1	[35]
Safflower seed cake	Fixed bed	400–600/10, 30, 50	30	34.2	27.5	18.9	[22]
Sesame seed cake	Semi-Batch type	350–700/25	—	29	58.5	12.4	[36]
Neem seed cake	--do--	350–700/25	—	51.1	40.2	8.5
Mustard seed cake	--do--	350–700/25	—	29.9	53.2	16.7
Mahua seed cake	--do--	350–600/25		33	41.36	25.64	[37]
Sunflower seed cake	Heniz retort	550/7	—	—	34.8	—	[38]
Mahua seed cake	Furnace	400–600	—	26.43	30.9	—	[39]
Karanja seed cake	—	800/5, 10, 20	—	18.11	—	—	[40]

Table 2.

Process parameters of pyrolysis process.

5. Challenges and limitations

The conversion and utilization of oilseed residue could provide better scope for future economy. However, it encounters with several challenges and difficulties while processing the residue and those are discussed below.

5.1 Logistical challenges

Collecting oilseed waste from diverse sources presents a logistical challenge. Establishing an efficient collection system requires collaboration with local bodies and cooperatives. Optimizing biomass logistics, including transportation from collection points to processing facilities, is crucial to reduce costs and energy inputs.

5.2 Oilseed variability

OSR from different plant species exhibits significant variability in oil content due to various genetic, environmental, and agricultural factors. Understanding and managing this variability are crucial for ensuring a consistent and reliable feedstock supply. Plant breeding and selection, monitoring of crop growth, proper soil management, and suitable irrigation strategies can help mitigate the impact of environmental factors on oil yield [11].

5.3 Non-edible seed residue

The development of non-edible crops as potential feedstock for biodiesel presents unique challenges toward achieving self-reliance in energy security. These challenges include the forest origin of non-edible crops, making harvesting, collection, and transportation difficult. Additionally, lower fuel economy, seasonal availability, improper marketing channels, and high FFAs and moisture content in non-edible oils require pre-treatment before the transesterification process. The lack of post-harvest technologies further affects the oil quality of non-edible crops [41].

5.4 Seasonal nature and annual variability

The seasonal nature of oilseed cultivation results in fluctuations in feedstock supply, which can affect continuous production processes. Addressing these fluctuations requires effective planning and coordination of agricultural activities. Despite the challenges, non-edible crops offer opportunities for cultivating harsh and unused lands in developing countries, such as seashores, riverbanks, deserts, and wastelands. These crops could maximize land utilization for crop production. Implementation of advanced technology, particularly waterless extraction technology like supercritical fluids, offers potential solutions to avoid FFAs and moisture content in non-edible oil [42].

5.5 Efficient storage and self-decomposition

Proper storage of oilseed waste is essential to maintain its quality and energy potential. Prolonged on-site storage, however, can lead to self-decomposition and the release of harmful gases, posing environmental and safety risks. Developing appropriate storage techniques is critical to preserve the feedstock’s quality [43].

5.6 Environmental impact and carbon cycle

A life cycle assessment (LCA) of the entire bioenergy production process is crucial to understand the environmental implications and carbon cycle. This assessment helps in identifying potential areas for improvement in terms of sustainability [44].

5.7 Acceptance and demand

Market and community acceptance of biochar and biofuels derived from oilseed waste are vital for sustainable production. Promoting sustainable agriculture practices and establishing standards can enhance demand and acceptability in the local market.

Overcoming the challenges in producing biochar and biofuels from oilseed waste requires collaborative efforts from researchers, policymakers, and industry stakeholders. By addressing these obstacles, we can unlock the full potential of oilseed waste as a valuable feedstock for bioenergy production, contributing to a more sustainable and environmentally friendly future. However, careful consideration is necessary to balance the use of agricultural crops and edible oils for biodiesel production to avoid potential conflicts between “food vs. fuel” [41, 45].

6. Conclusion

Different technologies such as pyrolysis, liquefaction, gasification, torrefaction are reported to convert oilseed residues as high-valued products. Withal, in terms of “waste-to-energy” technology, pyrolysis is considered one of the most cost-effective and environmentally benign techniques. In order to reduce GHG emissions, pollution, global warming, and fossil fuel dependence, bio-fuel products can be used as drop-in alternatives to fossil fuels or blended with fossil fuels. An idea on importance of bio-polymer compositions in preparation of pyrolytic products is reported. The different process parameters and their role in production process are reported. However, this article highlighted the selection of pyrolysis method and bio-polymer compositions in preparation of pyrolytic products. It critically explained the difference between process parameters depending on final product requirement. Yet, the research needs to establish toward the adaption of new methods of pyrolysis such as plasma, ultrasonic, microwave-assisted pyrolysis etc.

Acknowledgments

The authors are sincerely thankful to Director (ICAR-CIAE, Bhopal); Dean (IARI, New Delhi) for their support and encouragement.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contribution

All authors contributed to the study conception and design. Material preparation, initial draft preparation, conceptualization were performed by Mattaparthi Lakshmi Durga, Aseeya Wahid. The first draft of the manuscript was written by Mattaparthi Lakshmi Durga, Aseeya Wahid, Lalita.