Thermochemical methods of biomass conversion | Gasification, Pyrolysis, Liquefaction, Combustion and Torrefaction

What is thermochemical conversion of biomass ?

Thermochemical conversion of biomass is a process that transforms organic materials, like wood, crop residues, or even waste, into useful energy or products through heat and chemical reactions. This method uses high temperatures to break down the biomass, releasing gases, liquids, and solids. These components can then be used to produce heat, electricity, biofuels, or chemicals. In simpler terms, it's like turning natural materials into energy by heating them up and using the resulting substances for various purposes.

Thermochemical methods of biomass conversion

Why is thermochemical conversion of biomass important ?

The conversion of biomass through thermochemical processes offers a significant advantage by reducing our reliance on fossil fuels. This approach not only helps lower greenhouse gas emissions but also contributes to a cleaner environment. Furthermore, it addresses waste management concerns by utilising organic materials that would otherwise end up in landfills. Overall, this technique holds immense value as it transforms biomass into energy and valuable products, effectively meeting both our energy needs and environmental goals.

What are the different types of thermochemical conversion processes ?

Biomass serves as a valuable resource for energy production, undergoing a range of conversion methods. Among these, gasification and pyrolysis stand out as the predominant techniques, engaging in the thermochemical alteration of biomass. Furthermore, alternative strategies like combustion, liquefaction, and torrefaction find application in the conversion process, collectively contributing to the generation of energy from biomass sources.

Latest renewable energy news articles

Biomass gasification

Biomass gasification is a transformative procedure that turns biomass materials into a valuable gas known as syngas. This gas is composed of several components, including hydrogen, carbon monoxide, carbon dioxide, and methane. The resulting syngas has diverse applications, such as producing heat and electricity, as well as serving as a raw material for various industrial operations. These operations encompass the creation of hydrogen and liquid fuels, exemplifying the versatile potential of biomass gasification.

Energy Science and technology books

How does biomass gasification work ?

Biomass gasification is a thermochemical process that unfolds within a gasifier, a specialised reactor operating at high temperatures (usually between 700 and 900 degrees Celsius). In this transformative journey, biomass materials are introduced into the gasifier, undergoing a sequence of reactions that culminate in the creation of syngas.

The process kicks off with the biomass being dried by subjecting it to temperatures of approximately 100 degrees Celsius. Once the biomass is suitably dried, it enters the pyrolysis phase, where thermal decomposition takes place, breaking down the biomass into smaller molecular components. The outcome of pyrolysis encompasses gases, liquids, and solids.

The gases and liquids produced through pyrolysis subsequently undergo a crucial transformation inside the gasifier, where they're converted into syngas. This conversion process involves a reaction between the gases, liquids, oxygen, and steam. The end result of these intricate reactions is the formation of syngas, along with the release of heat and the creation of ash byproducts.

Types of biomass gasifiers

There are three main types of biomass gasifiers: fixed bed gasifiers, fluidized bed gasifiers and entrained flow gasifiers.

Fixed bed gasifier

The fixed-bed gasifier represents a specific type of gasification system. In this arrangement, the fuel is situated as an immobile layer comprising solid particles. Unlike fluidized bed gasifiers, where the fuel particles move around, in a fixed bed gasifier, they remain stationary. To start the gasification process, a gasifying agent such as air, steam, or oxygen is introduced. This agent heats up the bed of fuel, prompting gasification reactions to occur. These reactions give rise to a blend of gases called syngas. This syngas holds diverse utility, ranging from generating electrical power to supplying heat and even fueling vehicles.

Types of fixed bed gasifiers : Updraft Gasifiers, Downdraft Gasifiers, and Crossdraft Gasifiers.

Updraft gasifiers represent the simplest form of gasifier. In this setup, Biomass is introduced from the top while air is forced in from the bottom. The gasification reactions happen as the biomass travels downward within the gasifier. These gasifiers are straightforward to construct and operate, yet they generate a substantial amount of tars and pollutants in the gas output.

Downdraft gasifiers, although more intricate than updraft versions, produce cleaner gas. In this design, biomass is inserted from the bottom, and air is supplied from the top. The gasification reactions occur as the fuel moves upwards within the gasifier. Downdraft gasifiers offer better efficiency compared to updraft ones, but they entail higher costs for construction and operation.

Crossdraft gasifiers combine attributes from both updraft and downdraft models. Biomass is introduced from the side, while air is blown in from both the top and bottom. Gasification reactions take place as the fuel progresses through the gasifier. Crossdraft gasifiers strike a balance between efficiency and cleanliness of gas production, but their construction is more intricate and costly than either updraft or downdraft gasifiers.

Energy Science and technology books

Here is a table summarising the key features of Updraft Gasifiers, Downdraft Gasifiers, and Crossdraft Gasifiers


Updraft Gasifier

Downdraft Gasifier

Crossdraft Gasifier



Wood, agricultural waste, municipal solid waste

Wood, sawdust, high-carbon biomass

Wood, agricultural waste, low-carbon biomass

Fuel feed direction

Top to bottom

Bottom to top

Side to side

Air feed direction

Bottom to top

Top to bottom

Top or bottom to side

Gas flow direction




Syngas purity




Tar content












Cost of construction




Cost of operation




Complexity of operation





Power generation, heat production, cooking

Power generation, heat production

Power generation, heat production, industrial applications

Fluidized bed gasifiers 

Fluidized bed gasifiers are a specific kind of gasifier. In these gasifiers, an inert material like sand holds up the biomass fuel. By introducing a flow of air or steam, the biomass heats up and reacts, creating a mixture called syngas. This syngas is made up of various gases such as hydrogen, carbon monoxide, and methane. The resulting syngas has versatile uses, like producing electricity, providing heat, or acting as fuel for vehicles.

Types of fluidized bed gasifiers

Bubbling fluidized bed gasifiers represent one of the most prevalent methods for gasifying a range of fuels, encompassing coal, biomass, and waste materials. Their appeal lies in their operational simplicity. The procedure involves introducing fuel at the base of the reactor, followed by the controlled injection of air or oxygen just above the critical fluidization point. This results in a rhythmic up-and-down movement of the fuel particles, reminiscent of a liquid's behaviour. Within this dynamic environment, the particles are subjected to the heat of the gas, prompting a sequence of transformations: first drying, then pyrolysis (a heat-induced chemical breakdown), and ultimately gasification. The end products encompass a blend of gases and tars, which are extracted from the upper section of the reactor.

Circulating fluidized bed gasifiers are more complex in comparison to bubbling fluidized bed gasifiers. In the circulating type, the gasifying agent swiftly traverses the bed material, causing it to become fluid and circulate out of the reactor. Subsequently, a cyclone separator is employed to separate solids from the gas, which are then returned to the reactor.

Circulating fluidized bed gasifiers boast enhanced gasification efficiency and the ability to accommodate a wider array of fuel types. However, they are burdened with higher construction and operational costs when contrasted with bubbling fluidized bed gasifiers.


Bubbling fluidized bed gasifier

Circulating fluidized bed gasifier

Fluidization method

Air or gas injected through a distributor plate

Air or gas injected through a number of nozzles

Bed material circulation



Syngas quality



Mixing and heat transfer



Fuel flexibility

Wide range of fuels can be used

More limited range of fuels can be used




Capital cost

Lower than CFBG

Higher than BFBG

Operating cost



Gas velocity

1 - 2 m/s

5 - 10 m/s

Solids circulation



Tar and particulates production










Power generation, heat production, syngas production

Power generation, heat production, syngas production, waste treatment

Entrained flow gasifiers 

An entrained flow gasifier is a sophisticated type of gasifier that employs a high-pressure, high-temperature gas stream to efficiently convert solid fuels into syngas. While pulverised coal is a common fuel, entrained flow gasifiers can also utilise biomass, petroleum coke, and municipal solid waste.

Operating within the temperature range of 1,400 to 1,600 degrees Celsius (2,550 to 2,910 degrees Fahrenheit) and pressure levels of 20 to 70 bar, these gasifiers facilitate rapid and effective gasification reactions. This process results in the conversion of tar and other impurities into a clean syngas mixture comprising hydrogen, carbon monoxide, and carbon dioxide.

The strengths of entrained flow gasifiers include their exceptional carbon conversion efficiencies (ranging from 98% to 99.5%) and the production of tar-free syngas. They can adeptly accommodate various fuel types and find applications across power generation, chemical production, and the iron and steel industry. Furthermore, these gasifiers are being explored for their potential role in generating hydrogen for fuel cell vehicles.

Yet, it's important to note that entrained flow gasifiers also present challenges. Their intricate and expensive operational requirements, coupled with high initial investment costs, can make them financially demanding to manage. Additionally, the gasifiers necessitate high-quality fuel inputs to function optimally.

In essence, entrained flow gasifiers are an advanced technology offering impressive benefits such as efficient carbon conversion and versatile fuel compatibility. Nevertheless, their intricacy, costliness, and fuel quality demands pose noteworthy considerations in their utilisation.

Energy Science and technology latest books on Discount Price

Biomass gasifiers use different types of materials for various purposes :

1. Refractory Materials : These are used to protect the gasifier from high temperatures and corrosive chemicals. They're made from materials like firebrick, silicon carbide, or alumina.

2. Metal Materials : These are used to build the gasifier and its parts, like the reactor, heat exchanger, and ash removal system. Common materials are steel, stainless steel, or cast iron.

3. Catalysts : These speed up gasification reactions and enhance gas quality. They're typically composed of nickel, cobalt, or copper.

Advantages of Biomass Gasification :

1) Clean and Renewable Energy Source

2) Produces Power, Heat, Fuel, and Chemicals

3) Reduces Greenhouse Gas Emissions

4) Boosts Economy and Creates Jobs

Disadvantages of Biomass Gasification :

1) High Setup and Operating Costs

2) Requires Consistent Biomass Supply

3) Gas Produced Requires Cleaning

4) Produces Pollutants like Nitrogen Oxides and Sulphur Dioxide

Applications of Biomass Gasification :

1) Power Generation : Gas powered turbines or fuel cells for electricity.

2) Heat Production : Heats homes, businesses, and industries.

3) Vehicle Fuel : Powers buses, trucks, and cars.

4) Chemicals Production : Creates fertilisers, plastics, pharmaceuticals.

Challenges of Biomass Gasification :

1) High Costs Limit Adoption

2) Biomass Supply Availability Varies

3) Pollutant Control is Necessary

4) Research Needed for Better Efficiency and Performance

Biomass Pyrolysis

Biomass pyrolysis is a sophisticated thermochemical method that transforms biomass into valuable resources like bio-oil, biochar, and syngas. This process involves heating biomass without oxygen at temperatures ranging from 300 to 900 degrees Celsius.

How does biomass pyrolysis work ?

This intricate process can be broken down into three main stages:

1. Dehydration: Moisture within the biomass is removed.

2. Pyrolysis proper: Chemical bonds in the biomass molecules break down and reconfigure, resulting in new compounds.

3. Char formation: The remaining carbon-based material is transformed into biochar.

The outcome of biomass pyrolysis varies based on factors such as the biomass type, temperature, and heating rate. The primary products usually include:

1) Bio-oil: A liquid fuel that serves as an eco-friendly alternative to traditional diesel or gasoline.

2) Biochar: A solid, carbon-rich substance that has applications in improving soil quality and as a fuel.

3) Syngas: A mixture of gases that can be employed for electricity generation or heating.

Types of biomass pyrolysis

There are three primary methods of biomass pyrolysis:

1) Slow Pyrolysis: This common method is executed at lower temperatures (300–500 °C) and extended residence times (1-2 hours). Biomass is heated in the absence of oxygen, causing chemical bonds to break down. This leads to the formation of bio-oil, syngas, and char.

2) Fast Pyrolysis: More recent than slow pyrolysis, fast pyrolysis occurs at higher temperatures (500–700 °C) and shorter residence times (1–10 minutes). The absence of oxygen breaks down chemical bonds, resulting in the creation of bio-oil, syngas, and char.

3. Flash Pyrolysis: The quick technique involves very high temperatures (700–900 °C) and extremely brief residence times (seconds). Biomass is rapidly heated without oxygen, leading to the breakdown of chemical bonds and the production of syngas and char.


Slow Pyrolysis

Fast Pyrolysis

Flash Pyrolysis


300-500 °C

500-700 °C

700-900 °C

Residence time

1-2 hours

1-10 minutes


Bio-oil yield



Very low

Syngas yield



Very high

Char yield




Energy efficiency



Very high

Capital cost



Very high

Operating cost



Very low

Suitability for bio-oil production




Suitability for syngas production




Materials used in biomass pyrolysis

The components utilized in biomass pyrolysis are contingent on the specific biomass and desired outputs. Common components encompass:

1) Reactor vessels: These vessels are where biomass is heated and can be fashioned from materials like steel, stainless steel, and refractory bricks.

2) Heating elements: These elements generate the necessary heat for the pyrolysis process and can be powered by electricity, gas, or oil.

3) Catalysts: Certain catalysts can expedite the pyrolysis process and enhance the yield of specific products. Metal oxides like zinc oxide and cobalt oxide are often used as catalysts.

Pros and Cons of Biomass Pyrolysis

Advantages of biomass pyrolysis :

1) It's a renewable energy source with minimal environmental impact.

2) It yields a range of valuable products like bio-oil, biochar, and syngas.

3) It aids in decreasing greenhouse gas emissions.

Disadvantages of biomass pyrolysis :

1) The process requires a significant amount of energy input.

2) The resulting products can be corrosive in nature.

3) Emission of pollutants, such as nitrogen oxides and volatile organic compounds, is possible.

Applications of Biomass Pyrolysis

1) Biochar Production: Biomass pyrolysis yields biochar, a solid, carbon-rich material. Biochar serves as a soil enhancer, improving soil fertility, water retention, and microbial activity. Additionally, it aids in reducing greenhouse gas emissions and enhancing carbon sequestration in the soil.

2) Bio-oil production: Through biomass pyrolysis, bio-oil is produced, which serves as a versatile liquid fuel. Bio-oil can be a substitute for traditional petroleum-based fuels like diesel and gasoline. It also finds application in the production of chemicals and other high-value products.

3) Synthetic Gas Generation: Biomass pyrolysis generates synthetic gas (syngas), a mixture of gases rich in hydrogen, carbon monoxide, and other components. Syngas can be used for power generation, heating, and as a feedstock for chemical production processes.

Challenges of biomass pyrolysis

Challenges include:

1) The process demands substantial energy input.

2) Potential corrosiveness of the resultant products.

3) Emission of pollutants.

4) The technology is in the developmental phase, necessitating further research to enhance efficiency and economic viability.

Liquefaction of biomass : Turning Biomass into Bio-Oil

Biomass liquefaction is a thermochemical conversion process that transforms biomass into bio-oil under high-temperature and high-pressure conditions in the presence of a solvent. The process involves grinding the biomass into particles, mixing it with a solvent, and subjecting it to heat and pressure in a reactor. This breaks down the biomass molecules into smaller components, resulting in the formation of bio-oil.

The Advantages of liquefaction of biomass:

1) Offers a renewable process, reducing reliance on fossil fuels.

2) Bio-oil can serve as a fuel for transportation, heating, and power generation.

3) Bio-oil can be utilised as a feedstock for chemical and product manufacturing.

The Disdvantages of liquefaction of biomass:

1) High setup and operational costs.

2) Handling and storage complexities due to the intricate chemical composition of bio-oil

3) Potential corrosiveness to certain materials

Liquefaction of biomass : Application

1) Bio-oil finds use as a versatile fuel source for transportation, heating, and power generation.

2) Bio-oil serves as a valuable feedstock for the production of various products, including plastics, fertilisers, and pharmaceuticals.

Challenges of liquefaction process of biomass

1) Expensive to establish and operate.

2) Handling and storage difficulties for bio-oil.

3) Continued research is needed to enhance process efficiency and economics.

Biomass Combustion: Harnessing Heat and Electricity

Biomass combustion involves burning biomass in a furnace or boiler to generate heat or electricity. The process relies on using the heat produced by combustion to create steam, which in turn drives a turbine to generate electricity. Alternatively, the heat can be directly used for heating or industrial processes.

Advantages of biomass combustion process:

1) Provides a renewable energy source, reducing dependency on fossil fuels.

2) A well-established technology with proven effectiveness.

3) Suitable for generating heat or electricity on a large scale.

Disadvantages of biomass combustion process:

1) Emissions can include pollutants like nitrogen oxides and particulate matter.

2) Transportation of biomass to the combustion plant has environmental impacts.

3) Costs can vary based on biomass type.

Applications of Biomass combustion process:

1) Used for large-scale heat and electricity generation.

2) Provides heating for residential and commercial purposes.

3) Powers various industrial processes.

Challenges of biomass combustion process:

1) Addressing emissions and pollution concerns.

2) Managing transportation impacts.

3) Ensuring cost-effectiveness for different biomass sources.

Biomass Torrefaction: Enhancing Biomass Properties

Biomass torrefaction is a thermochemical process that transforms biomass into torrefied biomass, a solid material with improved fuel properties. The process involves heating ground biomass in an inert atmosphere, releasing moisture and volatiles to create a solid material with a higher energy density.

Advantages of torrefaction:

1) simple and efficient process.

2) Torrefied biomass boasts a higher energy density, facilitating transport and storage.

3) Improved fuel characteristics, such as a higher heating value and reduced moisture content.

Disadvantages of torrefaction:

1) Processes can be energy-intensive.

2) Torrefied biomass may be more expensive than raw biomass.

3) Ongoing research is required to optimise efficiency and economics.

Applications of torrefaction:

1) Torrefied biomass serves as a fuel for combustion, gasification, and pelletization.

2) Utilised as a feedstock for chemical and product manufacturing.

Challenges of Torrefaction:

1) Addressing energy consumption in the process.

2) Balancing costs with benefits

3) Continued research for process enhancements

Materials and catalyst used in thermochemical conversion of biomass

the types of materials and catalysts used in thermochemical conversion of biomass, along with their uses:

Material or Catalyst


Metal oxides

Promote the gasification reaction by providing active sites for the gasification of biomass.

Sulfated metal oxides

Improve the gasification reaction by increasing the reactivity of the biomass.


Act as catalysts for the steam reforming reaction, which converts biomass into syngas.


Promote the gasification reaction by providing active sites for the gasification of biomass and by increasing the reactivity of the biomass.

Activated carbon

Improves the gasification reaction by increasing the surface area of the biomass and by providing active sites for the gasification of biomass.

Mesoporous carbon

Improves the gasification reaction by increasing the surface area of the biomass and by providing active sites for the gasification of biomass.

Environmental Benefits of Thermochemical Conversion of Biomass

1) Reduces greenhouse gas emissions by replacing fossil fuels

2) Produces renewable energy and fuels

Improves air quality by reducing pollutants.

3) Can help conserve water and land resources.


Economic Benefits of Thermochemical Conversion of Biomass 

1) Can create jobs and boost the economy.

2) Can help reduce dependence on imported energy.

3) Can be a source of revenue for farmers and other biomass producers.


Challenges of Scaling Up Thermochemical Conversion of Biomass

1) High upfront costs

2) Technical challenges in converting biomass into energy and fuels

3) Lack of infrastructure to support large-scale production

4) Public acceptance of biomass conversion technologies


Future Trends in Thermochemical Conversion of Biomass

1) Continued research and development of new thermochemical conversion technologies

2) Increased investment in biomass conversion projects

3) Development of new markets for biomass-based energy and fuels

Increased public awareness of the environmental benefits of biomass conversion.