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  Biomass conversion processes
 

Introduction l

Benefits l

Conversion  l

   

 

Biomass can be burned directly or converted to intermediate solid, liquid or gaseous fuels to generate heat and electricity. Main biomass conversion processes include:

  • Thermo-chemical conversion: combustion; gasification; pyrolysis.
  • Physico-chemical conversion: pressing/extraction and esterification.
  • Biological conversion: hydrolysis and fermentation; anaerobic digestion.

The Table below summarises the biomass conversion processes options and corresponding end-uses, together with an indication of the status of the respective technologies.

 

Conversion technology

Resource type

Examples of fuels

Product

End-use

Technology status

Thermo-chemical

Combustion

Mainly solid biomass

  • wood logs, chips and pellets;
  • agricultural residues;
  • chicken litter

Heat

  • heat;
  • electricity (steam turbine, Stirling engine, reciprocating steam engine)

Commercial (boilers and steam turbines)

Gasification

Mainly solid biomass

  • wood chips and pellets;
  • agricultural residues

Product gas

  • heat(boiler);
  • electricity (engine, gas turbine, fuel cell, combined cycles);
  • transport fuels (methanol, hydrogen)

Demonstration/

Early commercial

Pyrolysis

Mainly solid biomass

  • wood chips and pellets;
  • agricultural residues

Pyrolysis oil + by-products (product gas, char)

  • heat (boiler);
  • electricity (engine, turbine)

Demonstration

Physico-chemical

Esterification

Mainly solid and liquid biomass

  • rape seeds
  • soybeans
  • oil or fat waste

Liquid fuels (biodiesel)

  • heat;
  • electricity
  • transport fuel

Demonstration/Early commercial

Biological

Hydrolysis fermentation

Mainly solid biomass

  • wheat;
  • maize;
  • sugar beet;
  • potatoes;
  • colza;
  • miscanthus

Liquid fuels (bioethanol)

  • transport fuel

Commercial

Anaerobic digestion

Wet biomass

  • manure;
  • sewage sludge

Biogas + by-products

  • heat (boiler);
  • electricity (engine, gas turbine, fuel cell);
  • transport fuel

Commercial

Combustion

Biomass is burned in boilers to generate heat, electricity or combined heat and power (CHP). Electricity from biomass is generated mostly from steam turbines, based on the Rankine thermodynamic cycle. Biomass combustion systems are in commercial use around the world, using different boiler technologies that can burn a wide range of biomass fuels. The most common boiler types are: pile burners, stoker fired boilers, suspension fired boilers and fluidised bed boilers.

Gasification

Gasification is the conversion, by partial oxidation at high temperature, of a carbonaceous feedstock into a gaseous fuel; the heating value of the resulting gas generally ranges from about one-tenth to half that of natural gas, depending on the gasification process used. The product gas can be used to generate heat and electricity by direct firing in engines, turbines and boilers after suitable clean up. Alternatively, the product gas can be reformed to produce fuels such as methanol and hydrogen, which could then be used in fuel cells, for example. Gasification-based systems may present advantages compared to combustion in terms of economies of scale and clean and efficient operation.

Pyrolysis

Pyrolysis is the thermal degradation of biomass in the absence of oxygen, in which the volatile components of a solid carbonaceous feedstock are vaporised by the high temperatures, leaving a residue composed of char and ash. Biomass pyrolysis always produces a product gas, vapour that can be collected as a liquid and a solid residue. The liquid fuel can be transported and stored, and allows for de-coupling of the fuel production and energy generation stages. It can be used to generate heat and electricity by combustion in boilers, engines and turbines. Products other than liquid fuels can be obtained from pyrolysis, such as charcoal, product gas and speciality chemicals.

Esterification

Fats and oils are combined with an alcohol, commonly methanol, and a catalyst, commonly sodium or potassium hydroxide, to produce a chemical reaction giving off fatty acid methyl esters and a glycerol co-product. Many esterification technologies can be used to produce biodiesel; all of them involve basic fats, an alcohol, and a catalyst.

The fats and oils required for the process can by obtained by extraction or pressing of seeds.

Hydrolysis and Fermentation

Liquid fuels (mostly alcohols) can be produced from biomass through hydrolysis and sugar fermentation processes.

In the hydrolysis process, the complex carbohydrate polymers (mainly cellulose, hemicellulose and lignin) contained in the biomass material are converted into sugars: the polymers are broken down (hydrolysed) by enzymes or dilute acids into sucrose sugar. There are three main types of hydrolysis: concentrated acid hydrolysis, diluted acid hydrolysis and enzymatic hydrolysis.

Finally during fermentation, sugar is then transformed into an alcohol (usually ethanol) by the action fermenting micro organisms.

Anaerobic digestion

Anaerobic digestion is a biological process that converts solid or liquid biomass to a gas in the absence of oxygen. The product gas is usually referred to as biogas and consists mainly of methane and carbon dioxide and contains various trace elements.

Anaerobic digestion is used in the treatment of wet wastes of industrial, agricultural and domestic origin. The derived gas is increasingly used for the production of heat and electricity; in addition, demonstrations are in place where biogas resulting from anaerobic digestion is used as a transport fuel in bus fleets, replacing methane. The solid and liquid residues from the anaerobic digestion process can be used as compost and fertilisers.

For further information contact Ausilio Bauen (email: firstname.lastname@e4tech.com) from E4tech

 
 
 
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