Biomass conversion techniques refer to the processes that transform biomass into energy or other valuable products. These technologies can be broadly categorized into physicochemical, thermochemical, and biochemical conversion processes.
Physicochemical conversion combines physical and chemical techniques to transform biomass, including torrefaction, hydrothermal conversion, Hydrothermal Carbonization, and supercritical fluid extraction.
Thermochemical conversion involves utilizing heat and chemical reactions to convert biomass into energy-rich products, such as combustion, pyrolysis, and gasification. Conversely, biochemical conversion employs biological processes and microorganisms to break down biomass and produce valuable products, such as biofuels, through fermentation and anaerobic digestion.
These biomass conversion technologies provide avenues for sustainable and renewable energy generation, reducing dependence on fossil fuels and mitigating environmental impact. Ongoing research and development focus on improving conversion efficiencies, optimizing product yields, and enhancing biomass conversion processes’ economic viability and environmental sustainability. Let’s explore these technologies in depth.
( Image source: Kumari, K., Singh, A., Marathe, D. and Pariyar, P., 2021. Agricultural biomass as value chain developers in different sectors. In Advanced Technology for the Conversion of Waste into Fuels and Chemicals (pp. 467-509). Woodhead Publishing).
Thermochemical Conversion
It involves the use of heat to convert biomass into energy and valuable products. It is mainly used to convert low-moisture herbaceous and woody biomass energy. There are three primary thermochemical conversion processes.
- Combustion– it is one of the most straightforward and widely used processes. It is a process in which fuel is burnt with oxygen to produce heat and generate steam. It is the direct process of converting biomass into energy. It can be used in various applications like cooking, heating, power generation, and co-generation. Combustion is a process where carbon and hydrogen in the fuel react with oxygen to form carbon dioxide and water, resulting in the liberation of heat. The combustion reaction is as follows
Fuel+ O2→ CO2+ H2O
The completeness of the process usually determines the efficiency of the combustion process. The flame’s temperature, the biomass moisture content, and the air’s amount play an essential role in the efficiency of the combustion process.
2 . Pyrolysis – it is a process which converts biomass into solid char, liquid bio-oil and combustible gasses by heating biomass in the absence of air. Pyrolysis can be classified as slow pyrolysis and fast pyrolysis, depending upon the rate of heating. Slow pyrolysis occurs over hours when temperatures range from 400 to 600 °C. Here the heating rate is 0.1 to 1 C/s. It leads to the production of solid carbonaceous char. As a result, liquid and gaseous products forms but in low quantities. Whereas in fast pyrolysis, temperature ranges between 400-800 °C. Here the heating rate ranges from 10-200 °C for 1-10 seconds. Fast pyrolysis aims to produce liquid fuel (bio-oil) from biomass. Therefore, the features of fast pyrolysis include high heating and a high heat transfer rate.
3. Gasification converts biomass into syngas by heating it in a controlled environment with a limited supply of oxygen. Gasification is the thermal process which results in large quantities of gaseous products with low char and ash formation. The process is carried out at an elevated temperature of 500-1400°C. Pyrolysis is the only step in the conversion process. The other steps are combustion with the air and reduction of the products of the combustion ( water vapour and carbon dioxide) into combustible gases ( carbon monoxide, hydrogen, methane). The formation of tar and solid char during the gasification hamper the engine’s efficiency. Hence, it is usually advised to clean up the producer gas.
Biochemical Conversion
It is the process where microorganisms break down the biomass to produce useful end products. The biochemical conversion consists of the following processes.
- Anaerobic digestion– It is the process of breaking down biomass in the absence of air. This process leads to the production of biogas, which consists of a mixture of methane (55-65%) and carbon dioxide (35-45%) and some impurities like hydrogen sulfide in traces. About 1 Kg of dry organic material will produce about 0.036 m3 of methane or 36 m3/1000 Kg biomass. In India, anaerobic digestion plants are gobar gas plants or biogas plants. In such a process, cow dung slurry is fed into the digester and the water. It is then allowed to ferment for a few weeks. The biogas thus produced is released and utilized as fuel for furnaces, cooking, running diesel engines etc.
- Fermentation is the decomposition process by microorganisms, mainly bacteria and yeast. Examples of fermentation include the decomposition of grains, sugar to form ethyl alcohol and carbon dioxide by yeast (in winemaking) and ethyl alcohol for forming acetic acid (in vinegar making). Various processes have been developed to produce fuel from fermentation. Ethanol fermentation of biomass usually takes place at 20- 30 °C. So, this process results in 90% of liquid and 10-2- % of alcohol, depending upon the tolerance of yeast to alcohol.
Physicochemical conversion
It is a process which involves physical and chemical processes to convert biomass into energy. It is a process that encompasses various technologies like temperature utilization, pressure, solvents and other physical and chemical agents to modify the structure of biomass. Let’s understand various types of physicochemical processes-
- Torrefaction is a process involving biomass heating in the absence of oxygen. Here in the process, temperature ranges between 200- 300 C. During the process; biomass undergoes various transformations like moisture removal and volatile compounds. The resulting biomass is thus known as biochar or bio-coal. It is used in power generation as a solid fuel or as co-firing with coal.
- Hydrothermal conversion is a process that utilizes high temperature and pressure in the presence of moisture to convert biomass into energy. Hence, it mainly consists of two common types of hydrothermal conversion.
a) Hydrothermal liquefaction (HTL)- is a process which breaks down biomass into bio-oil, water and solid residue called hydrochar. It is a process in which biomass is heated in water under high temperatures (250-400 °C). The bio-oil produced during the process is refined to obtain various liquid fuels and chemicals.
b) Hydrothermal gasification (HTG)- biomass is converted into gaseous fuel called syngas in this process. The produced syngas is mainly composed of carbon monoxide and hydrogen. The produced syngas can be utilized for heat and power generation or can be utilized as feedstock for the production of various gases.
- Supercritical fluid extraction– it is a process which involves the utilization of supercritical fluids like carbon dioxide to extract valuable components from biomass. When CO2 is brought to a specific temperature and pressure, it reaches its supercritical state, exhibiting the properties of both a gas and a liquid. One of the advantages of supercritical fluid extraction is that the CO2 can be recycle and reused in the process, making it an environmentally friendly technique. Biomass is treated with supercritical fluids to extract bioactive compounds, essential oils, flavours and other high-value products.
- Hydrothermal Carbonization (HTC): HTC is a process that converts biomass into hydrochar or biochar by subjecting it to high temperature (180°C to 250°C) and pressure in the presence of water. HTC can convert a wide range of biomass feedstocks into a carbon-rich material for energy generation or as a soil amendment. Hydrothermal carbonization offers several advantages as a biomass conversion process. Thus, it can accept many feedstocks, including wet or high-moisture materials.