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Biomass gasification

The main feedstock for the Saran gasifier plant is a woody native plant called Dhaincha, shown here stacked up to dry

Wood and other types of biomass can be heated in a restricted supply of air to produce an energy-rich gas. This ‘producer gas’ can be used to run an engine and generator, providing electricity from biomass at village level, or at larger scale. It can also be used as a low-carbon alternative to fossil fuels for heating.

  • Hundreds of biomass gasifiers in use, mainly in India and China.
  • Cost of small scale gasification systems for electricity generation is US$1,500 per kW and lower.
  • Opportunity to use agricultural and forest residues to replace fossil fuels.

Read on for more information about biomass gasification systems, or follow the links to short films and detailed case studies of Ashden Award winners that have used biomass gasification.

Resource
Early designs of gasifier used wood blocks as the main fuel. These had to be of a fairly uniform size, because the air had to flow uniformly around the fuel to get good gasification. Hoppers often got blocked by finer biomass, such as sawdust.

Gasifiers have been developed in Europe and Asia that can use wood chips, but care is needed to keep them flowing. Pellets and briquettes made from biomass residues work well in gasifiers, because they are of uniform size and flow more easily than chips. Recent work has developed gasifiers that can use loose biomass, such as rice husk or chopped agricultural residues, as a fuel.

Dhaincha grows rapidly in marshy land, and during the monsoon season

 

How biomass gasifiers work

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 combustible carbon monoxide, hydrogen and methane, as well as inert nitrogen and carbon dioxide. The energy content of 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 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.

Adding a load of chopped wood to a hopper

The water used to cool the gas is recycled

 

How gasification systems are used

Clean producer gas can be used in either a compression-ignition engine (diesel engine) or in a spark ignition engine (gasoline engine). The engines produce mechanical power which can be used directly, or else to drive an alternator to generate electricity.

The compression ignition engine has higher efficiency but most designs need between 10 and 20% diesel for ignition, and cannot run on pure producer gas. Ashden Award-winner Saran Renewable Energy uses producer gas from a downdraft gasifier to run a 128 kW dual-fuel engine and generator, providing an independent electricity source in a village in Bihar where the grid supply is very unreliable. The feedstock for the gasifier is wood and a fast-growing woody native plant called dhaincha. Producing feedstock generates income for local farmers.

Ashden finalist Husk Power Systems has recently commercialised a 35 kW compression-ignition engine that runs on pure producer gas. Each downdraft gasifier and engine provides power to about 500 households and small businesses through a local grid, usually in places which have never been reached by the mains grid. The main fuel used for these gasifiers is readily-available rice husk. This is a tricky material to gasify, and successful operation is achieved by a rigorous cleaning and maintenance programme.

Producer gas can also be burned to provide heat for rural industries, such as cardamom drying and silk reeling, where a high degree of temperature control is required. Used in the way the gas does not require the amount of cleaning that is needed to burn it in an engine.

Rice-husk gasifier and filters for a 35 kW Husk Power plant

35 kW engine for a Husk Power plant

Gasifier stoves
Producer gas is not really suitable as a fuel in domestic applications, such as cooking, because carbon monoxide is poisonous, and hydrogen is explosive. However, a recent development is the gasifier stove, in which gas is generated from biomass and then burnt in the same stove, so there is less danger of a build up of dangerous gases. Although efficient and clean-burning, gasifier stoves are larger and currently much more costly than other biomass stoves.

 

What are the benefits of gasification?

Gasified biomass replaces fossil fuels for generating electricity or heating, and therefore cuts greenhouse gas emissions. But why not just use the biomass directly, rather than going to the trouble and expense of gasifying it?

The main reason is that through gasification biomass can be used on a much smaller scale for electricity generation, down to about 10 kW. Gasification enables a range of different types of biomass to be used as fuel, and internal combustion engines and alternators to be used for power generation. Small-scale generation enables off-grid villages to get access to electricity for the first time. Families can replace smoky kerosene lamps with electric light, and use radio, TV and phone chargers at home. Small businesses can extend their working hours, and new businesses can start up.

Producer gas can supply heat more efficiently than if biomass is just burnt in a furnace. Gas burners can be designed to direct heat to where it is needed, or to give uniform heating over a region. The gas flow can be regulated to maintain the required temperature.

At a larger scale (above1 MW), the use of gasified biomass with a gas turbine can be more efficient than using direct combustion and a steam turbine, because the gas turbine operates at a higher temperature and thus higher efficiency.

The ash which is left over after biomass is gasified can be used for making incense sticks

Cold stores use electricity from the Saran plant. Here potatoes are being taken to a store

Small welding businesses have a reliable supply of electricity from a gasification plant

 

Cost

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.

 

Numbers

Although companies in Europe, New Zealand and the USA have tried to commercialise small-scale and large-scale gasification systems, the main successes have been in Asia. The government of India has had a programme to encourage the use of biomass gasification for rural electrification since 1999. In 2008, a survey identified 16 Indian manufacturers who had supplied 428 gasifier plants in India, of which 140 were used to generate electricity.

China has been encouraging the development of biomass gasification for over 30 years. In 2002, there were150 gasifiers generating electricity. Demonstration plants, both small-scale (fixed bed) and large-scale (fluidised bed) have also been built.

 

The future

The main growth of the use of biomass gasification continues to be in Asia, where many companies in India and China have added gasifiers to their product list. Governments recognise that gasification provides a way to use local agricultural residues to replace high-carbon and increasingly expensive fossil fuels, for the supply of electricity and process heat.

The government of India continues to provide subsidies and training to encourage the construction of small off-grid systems (up to 250 kW) and grid-connected systems (up to 2 MW). The Chinese government has a target of 30 GW of electricity from biomass by 2020.

 

Useful links

 

Lead authors: Dr David Fulford and Dr Anne Wheldon