Enhancing sustainable energy production through biomass gasification gas technology: a review

2.1 Biomass composition

Figure 4 demonstrates how various components of the same biomass have different physical and chemical properties. This diversity is even more pronounced when considering different biomass sources (Figure 4: Illustration of the various properties of biomass components). The inherent heterogeneity of biomass poses a challenge in accurately predicting the optimal operating conditions and final product qualities, representing a notable drawback in biomass gasification (Mostafa et al., 2019). Fundamental analyses of biomass characteristics typically encompass the measurement of heating value, proximate analysis, and ultimate analysis. Key components of biomass composition, such as moisture content, volatile matter, fixed carbon, and ash concentrations, are identified using proximate analysis. Importantly, the moisture content notably affects the progression and phases of the process (Gilvari, de Jong and Schott, 2019; Alabi, Adeyi and Ekun, 2023a). Fixed carbon represents the carbonaceous solid material, whereas volatile matter comprises the gases released during this phase. It’s worth noting that the presence of tar from volatile biomass materials can pose challenges during the gasification process. The final examination makes it possible to identify the biomass’s components for carbon, oxygen, hydrogen, nitrogen, and sulfur. Both fixed and volatile carbon contribute to the carbon content in char and volatile substances (Siyal et al., 2021). Biomass further comprises components like lignin, hemicellulose, and cellulose, each decomposing differently during gasification. Higher lignin content slows down the rate of biomass decomposition, necessitating elevated gasification temperatures at the expense of reduced gas output (Du et al., 2019).

2.2 Gasification processes and reactors

Gasification is a thermochemical conversion process that transforms carbon-containing feedstock, such as biomass, coal, or waste materials, into a gaseous mixture known as syngas (synthesis gas). This versatile and energy-efficient technology is employed in various applications, including power generation, heat production, and the synthesis of valuable chemicals and fuels. Gasification processes and reactors play a crucial role in the efficient conversion of solid, liquid, or gaseous fuels into syngas. Several gasification technologies and reactor designs are available, each with its own characteristics and advantages. Gasifying agents play a crucial role in various thermochemical processes, such as gasification and combustion, where solid, liquid, or gaseous fuels are converted into valuable products like syngas (synthesis gas), heat, or electricity. The choice of a gasifying agent has a significant impact on the efficiency, product composition, and environmental performance of these processes.

The selection of gasifying agents plays a central role in every gasification reaction, influencing reactivity and gas composition. Different gasifying agents, whether individually or in combination, lead to diverse outcomes in terms of the intended results (Frodeson, Henriksson and Berghel, 2019; Singh et al., 2014). Because of its affordability and widespread availability, air stands as the most commonly employed gasifying agent among these options. For the production of syngas with an elevated heating value, it is recommended to utilize pure oxygen or oxygen-enriched air. This is because the high nitrogen concentration in regular air can diminish the heating value of the resulting syngas. It’s essential to keep in mind that this choice may lead to increased operational expenses since extracting oxygen and nitrogen from the air requires energy. The air-to-fuel ratio, also known as the equivalency ratio (ER), employed in the gasification process governs the extent of partial oxidation within the gasifier when air is used as the gasifying agent. To prevent full combustion, the ER value is usually kept below 1. The ER ratio is also thought to impact the ultimate heating value of the syngas, typically falling within the range of 0.2 to 0.4 for optimal formation. Utilizing pure oxygen from the air is a cost-effective approach compared to partial oxidation, offering a feasible means to attain higher gas content and reduce undesirable byproducts. However, this choice necessitates additional energy due to the endothermic reactions involved in primary and secondary steam processes, as well as the water gas shift reaction (Gilvari, de Jong and Schott, 2019; Singh et al., 2014). Moreover, steam is frequently employed to expedite the sluggish reaction rate, obtaining energy for endothermic processes from exothermic partial oxidations. It has been proposed that for gasifiers, a combination of air and 40–70 mol% steam is advisable (Frodeson, Henriksson and Berghel, 2019; Lee et al., 2020a).

2.3 Gasification byproducts and applications

The outcome of the gasification process is directly influenced by the size of biomass particles. Smaller particles offer a larger specific surface area for gasification reactions, typically resulting in higher gas production and lower energy consumption. They also enable faster heat transfer and reaction rates (Roncancio and Gore, 2021). Consequently, gasifying smaller biomass particles yields increased syngas production and reduced tar and char formation. In contrast, larger biomass particles, due to their greater heat resistance and incomplete decomposition, tend to generate more char (Skorupskaitė et al., 2015).

While smaller biomass particle sizes offer advantages, it’s important to consider that the energy required for size reduction may lead to a decrease in overall energy efficiency. Various gasifier types have been developed to accommodate a range of biomass particle sizes. For example, gasification requires biomass particles with a diameter less than 0.15 mm. Thus, pre-processing of biomass before it enters the gasifier remains an important consideration, despite some studies suggesting otherwise (Singh et al., 2014).

2.4 Operating conditions

In gasification, the two main working conditions are temperature and pressure. Among them, temperature is crucial to the gasification process since it has a direct impact on a number of factors, such as gas yield, composition, heating value, formation of tar and char, carbon conversion, and cold gas efficiency. Higher gasification temperatures result in gas products with elevated heating values, characterized by increased CO and H₂ content and reduced tar. Moreover, as temperature increases, both carbon conversion and cold gas efficiency improve (Yao et al., 2023). However, it’s important to keep in mind that other elements, such as the biomass mixture’s composition and particle size, can affect the necessary combustion rate. For instance, higher temperatures are required to produce more gas from waste with a higher fiber content. In addition, the breakdown of the biomass structure requires greater temperatures for larger catalyst particles (Sekar et al., 2021). Normally, gasification is kept going at temperatures above 700°C since this is the point at which the spontaneous steam reforming reaction starts to happen. In the case of steam gasification, temperatures should exceed 750°C.

Gasification primarily takes place under atmospheric conditions, although pressurized approaches are now being extensively researched and employed. Due to its low initial investment costs, atmospheric gasification is more prevalent for small-scale gasifiers. Pressurized gasification, on the other hand, is a more effective technique that results in enhanced gasification efficiency and reduced tar production. Conversely, pressurized gasification is a more efficient method, leading to improved gasification efficiency and reduced tar production (Gilvari, de Jong and Schott, 2019). It’s important to bear in mind that gasification typically operates at low pressure and high temperature, in the context of chemical equilibrium (Skorupskaitė et al., 2015).

The schematic procedure for this particular type of gasifier is shown in Figure 5(a). The updraft gasifier stands out as an efficient gasification method with a high thermal efficiency. It achieves this by utilizing the sensible heat of the hot gas to preheat and dehumidify the internal components of the system before the lower-temperature gas exits from the top of the gasifier, typically falling within the range of 200°C to 400°C. Remarkably, it demonstrates an impressive performance, capable of accommodating biomass with moisture content as high as it also effectively processes particles ranging from 5 to 100 mm in size (Sarker et al., 2023). Despite its efficiency, the air gasification process tends to produce a greater amount of tar compared to other methods. The tar generated in the pyrolysis zone flows upward to the cooler region, preventing it from reaching the high-temperature zone where it could be cracked (Zhang and Zhang, 2019; Razi and Dincer, 2022).

Figure 5. (a) Updraft gasifier and (b) Downdraft gasifier designs.

Source: Roncancio and Gore (2021).

The gasifier in Figure 5(b) uses a unique method for introducing the gasifying agent. Biomass is introduced from the top or sides of the reactor, while the gasifying agent is introduced from the top or sides. This results in a co-current flow between the biomass and the gasifying agent (Roncancio and Gore, 2021). This suggests that the biomass and gasifying agent come into touch during the gasification operations, improving the quality of the generated gas that leaves the reactor’s bottom. Due to the ability of devolatilization products to enter the high-temperature oxidation zone, despite their brief residence duration in that zone, the tar production in this setup is minimal (Lee, Balu and Chung, 2013). However, there is relatively limited heat transfer between the hot and cold zones of the reactor in this design, which reduces its tolerance for moisture content. Therefore, only biomass with a moisture level below 30% is suitable for processing. Additionally, due to the alignment of the drag force with gravity, biomass travels downward more swiftly during its residence time in the reactor. Consequently, the carbon conversion efficiency is lower in comparison to an updraft gasifier. This type of gasifier is also susceptible to issues like blocking, channeling, and bridging, requiring consistent particle size to stop these occurrences. Typically, the permissible particle size range is between 40 and 100 mm. With a typical capacity ranging from 10 kW to 1 MW, the downdraft gasifier is well-suited for small-scale power plant applications (Pavkov et al., 2022).

4.1 Technology economic viability

Biomass technology, which harnesses energy from organic materials like wood, agricultural residues, and waste, has garnered significant attention due to its potential as a renewable and sustainable energy source. The economic viability of biomass technology is a critical aspect that influences its adoption and sustainability. Large- and medium-scale gasification systems of over 1 MW capacity are becoming increasingly popular worldwide, particularly in developed nations, for generating electricity (Situmorang et al., 2020). These systems are not only cost-effective in terms of per unit of electricity produced, but also have a significant impact on the electricity distribution landscape within a country or region. To fully utilize the potential, however, biomass gasification system development and acceptance are essential, particularly in developing and emerging nations (Sarker et al., 2023). At present, most commercially available gasifiers exhibit relatively low gasification efficiency, including updraft and downdraft gasifiers. There is a critical need to create next-generation gasifiers that offer both high efficiency and cost-effectiveness in order to increase the application of small-scale biomass gasification systems in the power generation sector.

4.1.1 Development of new gasifier

Larger-scale power generation systems often employ fluidized-bed gasifiers, especially circulating fluidized bed gasifiers like dual-bed gasifiers known for their high efficiency. However, scaling down and effectively adapting these systems for small-scale use is a challenging task. Notably, small-scale biomass gasification systems need to be versatile and capable of handling various types of biomass (Carter et al., 2018). The small gasifiers currently on the market are more suitable for high-quality biomass sources, like woody biomass. When lower-quality feedstocks are used, these gasifiers tend to produce notable quantities of tar and ash, which can present challenges for their operation (Perera et al., 2021). As a result, the development of these systems must address challenges related to their production. One approach to mitigating tar issues is to implement a two-stage gasification system that includes an initial pyrolysis process, which can reduce tar production. However, this design typically results in a larger gasifier size, making operation more complex and costly. Alternatively, tar in the gas can be post-treated using syngas cleaning systems or catalysts. Furthermore, notable enhancements in gasification efficiency can be attained through the design and incorporation of a distinct setup. As depicted in Figure 6, this biomass gasification system operates similarly to a large-scale triple-bed gasification system, performing biomass pyrolysis, charcoal gasification, and tar reforming as separate, independent processes (Situmorang et al., 2020; Evaristo et al., 2022).

Figure 6. A three-bed gasification system.

Source: Situmorang et al. (2020).

4.1.2 Economic evaluation and policy

The cost of building the power generating system and the revenue from selling the energy produced are two crucial considerations in the economic assessment of a biomass gasification system used for power generation (Arslan et al., 2022). The investment costs for the project are an accurate representation of the costs paid, and a crucial metric is the minimum power price based on the levelized cost of electricity (LCOE). According to this split (Arslan et al., 2022), the gasification unit receives around 24% of the investment, the gas engine and electrical supporting systems receive 54%, and the remaining expenses, such as the purchase of raw materials and transportation, are covered by the remaining funds. Another study on small-scale biomass gasification power plants in Brazil found that the power plant’s capital cost was approximately 1267 USD/kW, resulting in an LCOE of 0.53 USD/kWh (Bacenetti et al., 2013). According to Carrara’s theory, the investment cost for a gasification system coupled with a 100kW internal gas engine combustion would be between 1200-3300 €/kW (or 906-2491 USD/kW based on 2010 exchange rates) (Bacenetti et al., 2013). In comparison, 1 MW and 5 MW systems were estimated to have costs of 900-1800 €/kW and 700-1300 €/kW, respectively. According to the International Renewable Energy Agency (IRENA), the typical LCOE for small-scale fixed-bed biomass gasification power plants (up to 600 kW) with internal combustion engines falls within the range of 0.065 to 0.24 USD/kWh. It’s worth noting that costs can vary depending on the country, and despite such variations, large-scale deployments of biomass gasification power systems continue to involve substantial expenses (Asadullah, 2014).

4.2 Environmental benefits of biomass gasification

There are two main environmental issues with the use of a biomass gasification plant: the impact of biomass cultivation and the consequences of biomass gasification itself. Perennial energy crops, as indicated by numerous life cycle assessments (LCAs), have been found to reduce greenhouse gas (GHG) emissions by approximately 5 mg/ha of fossil carbon and decrease NO₂ emissions by 40–50% compared to burning fossil fuels. One specific LCA study suggests that cultivating perennial grasses on marginal land for electricity generation can lead to saving approximately 13% of CO₂ equivalent per hectare per year (Shamsul, Kamarudin and Rahman, 2020). In the context of small-scale biomass gasification systems, their relatively modest biomass resource requirements imply a minimal impact on land use. Moreover, the release of hazardous compounds and GHG emissions is more manageable in such systems, allowing them to efficiently utilize locally generated biomass or agricultural residues.

While biomass gasification has the potential to be a more sustainable alternative to fossil fuels, its environmental impacts must be considered in their entirety. A holistic approach, taking into account the entire life cycle of the technology, is necessary to fully understand the environmental implications. This includes considering the impacts of feedstock procurement, such as the use of resources and land use change, the energy and emissions associated with processing the biomass, the effectiveness of emissions control technologies, and the disposal of any waste products (Tripathi, Sahu and Ganesan, 2016). Only by considering all of these factors can the true environmental impact of biomass gasification be assessed. Also, there are some negative environmental impacts associated with biomass gasification, the technology also offers some significant environmental benefits that can be optimized through the use of best practices and advanced technologies. For example, by using low-impact feedstock sources, such as agricultural waste or dedicated energy crops, and implementing efficient processing techniques, the environmental impacts can be minimized. In addition, the use of advanced emissions control technologies can help to reduce the release of pollutants into the atmosphere. Finally, the utilization of byproducts from biomass gasification, such as biochar, can further offset the environmental impact of the technology. In the end, the particular feedstock used, the technology used, and the operational procedures all have a significant impact on the environmental impact of biomass gasification. Biomass gasification has the potential to significantly contribute to the mitigation of climate change and the shift to a more sustainable energy future when it is developed and used responsibly (Alper et al., 2020).

Biomass gasification can have both positive and negative environmental impacts. On the positive side, biomass gasification is a renewable energy source that can reduce the need for fossil fuels. It can also help reduce greenhouse gas emissions by converting biomass into usable energy, and it can help reduce waste by using biomass that would otherwise be discarded (Güleç et al., 2022; Alper et al., 2020). However, there are also some potential negative environmental impacts of biomass gasification. For example, it can produce air pollutants like nitrogen oxides and volatile organic compounds. It can also release carbon dioxide and methane, which are greenhouse gases. In addition, the process of harvesting biomass for gasification can have a negative impact. The environmental impacts of biomass gasification can vary depending on the specific biomass source and the technology used. For example, using agricultural residues as a biomass source can have a positive environmental impact by reducing the amount of agricultural waste that would otherwise be burned or left to decompose. On the other hand, harvesting biomass from forests can have negative impacts on biodiversity and habitat loss (Abdulyekeen et al., 2021; Bacenetti et al., 2013). In addition, the type of technology used for gasification can have an impact on the overall environmental impact. For example, gasification systems that use inefficient combustion or have a high energy input can have a higher carbon footprint than other technologies (Bacenetti et al., 2013; Asadullah, 2014).

4.3 Practical implication

While there is a plethora of technology vendors capable of providing suitable solutions for small-scale biomass gasification technology, and gasification technologies are now well-established, the number of actual implementations remains limited (Ong et al., 2020; Chyuan et al., 2020; Roncancio and Gore, 2021). The substantial construction expenses remain a notable obstacle, particularly for developing and less economically developed nations. Therefore, research and development efforts aimed at producing high-efficiency technology are anticipated to involve foreign capital in the creation of a few prototype projects. Local governments in developing and less economically developed nations are likely to lend support by instituting appropriate feed-in tariffs. Collaborative efforts between technology and equipment providers can enhance the reliability of commercializing and industrializing biomass gasification power generation systems.