Heating biomass in a restricted supply of air produces an energy-rich gas that can be used to supply heat or to run engines and generators, providing clean, reliable electricity for villages or factories. That means families can access electricity for the first time, helping replace dangerous and polluting kerosene lamps, and businesses can thrive and grow.
- Hundreds of biomass gasifiers are in use for generating electricity, mainly in India and China.
- Small scale gasification systems usually cost US$1,500 per kW and lower.
- Opportunity to use agricultural and forest residues to replace fossil fuels.
How biomass gasifiers work
Gasifiers have been used to make fuel gas from biomass and coal since the middle of the 19th century. Town gas, made from coal, was supplied commercially in many countries in the early 20th century until it was replaced with natural gas.
The gasification of biomass takes place in four stages:
- Drying: water-vapour is driven off the biomass.
- Pyrolysis: as the temperature increases the dry biomass decomposes into organic.vapours, gases, carbon (char) and tars.
- Reduction: water-vapour reacts with carbon, producing hydrogen, carbon monoxide and methane. Carbon dioxide reacts with carbon to produce more carbon monoxide.
- Combustion: some of the char and tars burn with oxygen from air to give heat and carbon dioxide. This heat enables the other stages of the gasification process to take place.
The producer gas (sometime called ‘wood gas’) from a gasifier contains carbon monoxide, hydrogen and methane all of which can be burned to release heat, as well as nitrogen and carbon dioxide, which are inert. The heat released by producer gas is quite low (about 4 MJ per kg compared to 50 MJ per kg for pure methane).
In small-scale gasifiers, the reactions take place in a stationary or fixed ‘bed’ of biomass. In an updraft gasifier, biomass is loaded at the top of the gasifier and air is blown in at the bottom. This type of gasifier produces gas that is contaminated by tar and is therefore too dirty to be used in an internal combustion engine.
In a downdraft gasifier, air is drawn downwards through the biomass. The main reactions occur in a constriction or ‘throat’, where the tars and volatile gases break down into carbon monoxide and hydrogen at a much higher temperature than in an updraft gasifier. The throat is usually made from ceramic to withstand this temperature. Downdraft gasifiers produce cleaner gas.
The producer gas leaves at a temperature of over 600°C, and contains fine particles of char and ash. The gas must be filtered to remove these particles and also cooled to below 100°C to condense tars, before it can be used in an engine. Some ash falls out from the base of the plant.
The power output from fixed bed gasification/engine systems ranges from about 10 kW to 1,000 kW (electrical). Systems such as fluidised beds, two-stage processes and plasma technologies have been used for larger-scale biomass gasification. Another approach is to heat the biomass externally, using by-products from the gasification process, such as char. These approaches have met with varying success.
How biomass gasification systems are used
Clean producer gas can fuel either a compression-ignition (diesel) engine or a spark ignition (gasoline) engine. Compression ignition engines are more efficient, but many can’t run on pure producer gas, because they need a small amount of diesel for ignition. The engine produces mechanical power which can be used directly, or else to drive an alternator to generate electricity.
Husk Power Systems has developed and commercialised a 35 kW compression-ignition engine that is able to run on pure producer gas. Each downdraft gasifier and engine provides power to about 500 households and small businesses through a local electricity grid, usually in places that have never been reached by the mains grid. Rice husk is the main fuel for these gasifiers, because there is a plentiful local supply. However, it is a tricky material to gasify, and successful operation requires a rigorous cleaning and maintenance programme.
Producer gas can also be burned to supply heat. Ashden winner Sustainable Green Fuel Enterprise uses optimised gasifiers to supply both char and heat from waste coconut shells. The char is used to make charcoal briquettes and the heat is used to dry them.
Producer gas is not great for use in homes, because carbon monoxide is poisonous and hydrogen is explosive. However, cooking stoves have been developed which gasify biomass and burn the gas within the same appliance, so there is less risk of these gases reaching dangerous concentrations. Gasifier stoves, although expensive, are more efficient and produce less air pollution than any other type of wood-burning stove. There is currently great interest in developing them, because of the huge impact of air pollution on people’s health.
What are the benefits of gasification?
Gasified biomass replaces fossil fuels for generating electricity or heating. This makes use of locally available resources and provides local jobs, while also cutting greenhouse gas emissions.
Small-scale generation enables off-grid villages to access electricity for the first time. Families can replace smoky kerosene lamps with brighter electric light, and use phone chargers, radio and TV at home. New businesses that need electric power for machinery can be started, and existing businesses can extend their working hours with better light.
It is easier to regulate the heat production from gas than from wood, so producer gas is useful in rural industries where careful temperature control is required – like briquette production (above), cardamom drying and silk reeling. Used in this way, producer gas does not require the amount of cleaning that is needed to burn it in an engine..
The Indian Institute of Science estimated the cost of generating electricity via biomass gasification (2007). They suggested a capital cost of about US$1,500 per kW (electrical) for plants up to 100 kW, and US$1,200 per kW for plants between 100 kW and 1,000 kW. Running costs were estimated at about US$0.05 per kWh generated. Similar costs were estimated for China. However, with growing commercial interest in biomass gasification, costs are coming down. The cost of the 35 kW Husk Power systems is below US$1,000 per kW.
The main successes in producing and using biomass gasifiers are in Asia. An Indian government programme encourages their use for off-grid electrification through subsidies and training. According to government statistics, around 130 MW (electric) of biomass gasifiers were in use in 2011. The government of China has also supported the technology, and has a target of 30 GW (electric) of biomass gasifiers by 2020.
In Cambodia, many industries (like garment factories) use wood gasifiers to generate electricity to meet their own needs and for sale, as grid power is chronically unreliable.
1.3 billion people in the world don’t have access to electricity, most of them in low-income countries. Even in India, where about three quarters of the population have some access to the mains grid, many parts of the country have frequent and lengthy power cuts.
Biomass gasification has a continuing role in providing access to electricity, and an increasing number of companies in Asia and elsewhere are producing gasifiers for this purpose. Governments also recognise the value of gasification in using local agricultural residues to replace high-carbon and increasingly expensive fossil fuels.
- View our biomass gasification and pellets photo collection on flickr
- Peter Quaak, Harrie Knoef, Hubert Stassen (1999) ‘Energy from biomass: a review of combustion and gasification technologies’ The World Bank
- S Chopra and A Jain (2007) ‘A review of fixed bed gasification systems for biomass’ Agricultural Engineering International: the CIGR Ejournal invited overview 5, volume 9
Lead authors: Dr David Fulford and Dr Anne Wheldon