Humans have been using the warmth and lightfrom wood fires for hundreds of thousands ofyears. Even now up to half the people in the

world cook their food on open fires. Wood fuel isnews again now because it is a renewable source ofenergy. At present almost all our energy comes fromfossil fuels and in the long term these will run out.Burning fossil fuels also releases carbon dioxide andcontributes to global climate change. We need toreplace them with renewable and more sustainablesources of power.

The government has set a target for 10% of our elec-tricity to be generated from renewable sources by2010. At present, renewables supply only 1.5% of UKelectricity.

BiomassTrees are made of biomass. This term covers materialfrom both plants and animals. Wood, straw, animaldung and domestic refuse are all biomass fuels.

Biomass stores energy from the Sun. Chemicalreactions harness the energy in sunlight to synthesise

energy-rich compounds. This is the process of photo-synthesis. Carbon dioxide from the air and waterfrom the ground are combined in this process tomake the carbohydrates that are the main buildingblocks of biomass. When biomass is burnt the carbo-hydrates react with oxygen from the air to make

Photosynthesis

CO2

H2O

H2O

O2

Combustion

Useful energy

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Janet Taylor

Wood is a renewable source of energy. Could weuse wood to generate electricity in the UK, andwhat are the advantages and disadvantages?

GCSE key wordsBiomass

Energy resourcesPhotosynthesis

RenewablesSustainability Figure 1 When it burns, wood fuel gives off only the

carbon dioxide it took from the atmosphere when it grew

Box 1 Renewable and sustainableenergy sourcesWind, wave, tidal, solar voltaics (PV), hydro,geothermal and biomass are all renewable energysources. They are called renewable because they donot get used up. However they have another veryimportant advantage over fossil fuels. They do notincrease the amount of the greenhouse gas, carbondioxide, in the atmosphere.

Nowadays the word sustainable is often used inconnection with renewable energy sources. To besustainable, an energy source must not run out,damage the environment, create health hazardsor cause wars or social injustice. A sustainableenergy source is one whose use now will not makeit more difficult for future generations to meettheir needs.

l A tree weighs severaltonnes. About half thismass is carbon. Wherehave the carbon atomscome from?

l A useful website is:http://www.woodfuelresource.org.uk

Tree power

carbon dioxide and water again. The biomass gives offonly the carbon dioxide it took from the atmospherewhen it was growing (Figure 1). If the forests thatsupply the fuel wood are replanted at the same ratethey are cut down then the wood is a renewableenergy source.

Have we got wood to burn?About 9% of the total land area of Great Britain iswoodland. This could give a sustainable yield of nearly700 000 tonnes of air-dried wood per year. In thepast, wood was harvested as the raw material formaking paper but the paper industry has shiftedoverseas. We also import timber for the building

industry, so today we only harvest half the yearlygrowth of wood from our forests.

Table 1 shows that wood is a less concentratedstore of energy than any of the fossil fuels. Because itis bulky the use of wood increases lorry traff ic.However, a wood-fuel industry could help to createjobs in the countryside.

Converting wood energy into heat andelectricityDomestic and community heating projects are alreadyusing the thinnings and trimmings from existingwoods and forests. Landowners and farmers areincreasing the supply of wood fuel by growing ‘energycrops’ like willow or poplar. The wood-fired heatingsystem at Weobley School in Herefordshire uses150–300 tonnes of wood chips every year.

There is debate aboutwhether we haveenough land to growsufficient energy crops.

17September 2004

Fuel Energy content (kJ/g)Wood (air-dried) 15Paper (stacked newspapers) 17Dung (dried) 16Straw 14Domestic refuse 9Oil 42Coal 28Natural gas 55

Table 1 Energy content of fuels

Below: The 350 kWboiler at WeobleySchool inHerefordshire wasinstalled in 1998 toheat the primaryschool, the adjacentsecondary school andthe schools’swimming pool

Above: Coppiceplantations like thiscan provide newhabitats andencourage bio-diversity. They needsmaller amounts ofagricultural chemicalsthan arable crops

Wood is a source of energy for many people aroundthe world, including this family in Guatemala

Box 2 Growing energyThe best energy crop for northern Europeanconditions is coppiced willow. Saplings are plantedand, after 1 year, cut back close to the ground. Thisis called coppicing, and it stimulates several newwoody stems to grow. The crop is allowed to growfor 2–4 years and then the stems are cut. The plantwill grow more stems so the cycle is repeated. Onehectare of land can produce 10 tonnes of air-driedwood per year.

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Purpose-built biomass power stations that generateelectricity from wood burning are being planned andbuilt. Wood can also be burnt in combination withcoal (co-firing) in existing power stations.

Gasification and turbinesToday the furnaces that burn wood for space heatingare clean and eff icient. But when it comes togenerating electricity the favoured method is more hi-tech. Biomass is converted to power in an IGCC(integrated gasification combined cycle) power plant.

Gasification is a combustion process in which thereaction is ‘starved’ of oxygen so that it is notcompletely oxidised. Wood is mainly composed ofcarbon, hydrogen and oxygen. If it is oxidised

completely the carbon and the hydrogen atoms formcarbon dioxide and water:

C + O2 Ý CO22H2 + O2 Ý 2H2O

During gasification wood fuel is heated with a mixtureof gases and air in a sealed container. The mixturedoesn’t contain enough oxygen for all the carbon andhydrogen atoms to be completely oxidised. The overallreaction with the carbon atoms is:

2C + O2 Ý 2CO

The gas produced is mainly carbon monoxide withsome hydrogen, hydrocarbon gases and nitrogen. Tarand ash, formed as by-products, are washed out ofthe mixture. The gas can still burn and is now the fuelfor a combined cycle gas turbine (CCGT) plant.

In these power plants the hot gases produced byburning the carbon monoxide turn the first turbine.They then also heat a boiler to make steam, whichdrives a second turbine. Both turbines turn generators,which produce electricity. Combining them makes thesystem very energy efficient.

Carbon monoxide is a deadly poison but good engi-neering and good practice at the plant make the risksnegligible. IGCC is a clean technology.

The ARBRE projectARBRE stands for ARable Biomass Renewable Energy.The project was set up in Yorkshire. It was a prototypefor a 10 MW IGCC plant, using forest residues andenergy crops. The hope was that it would be the firstof many for Britain.

The plan was to fuel the plant with forest residues atf irst and then switch to coppiced willow. By May2000 some 500 hectares had been planted, with afurther 625 hectares to be established within the year.There were many difficulties. Farmers were reluctantto grow willow, harvesting was difficult and expensive.At the end of May 2003, Yorkshire Water, the ownersand instigators of the ARBRE project, sold thecompany, and it was shut down. The plant hadproduced electricity for just 8 days.

The futureWind power provides the largest share of renewableelectricity. But there are days when there is hardly anywind across the whole of the UK (this happened for afew days during the cold spell in January 2003). It isessential to have other types of power station to backup windfarms on windless days.

Power stations fuelled by wood and other types ofbiomass have the great advantage of being able to gen-erate electricity on demand. The failure of the ARBREproject was a setback, but new projects must beencouraged. Energy crops are both green and flexible.

Janet Taylor is a science teacher and has contributed tomany websites and textbooks.

18 Catalyst

100 km

Below: During theSecond World Warthere was a shortageof petrol and 1 millionvehicles ran on gasfrom wood orcharcoal. Heremechanics are fitting a gas-producing unitto a car

Box 3How much electricitycould energy cropsproduce? Energy crops can be grown on farmland that is not neededfor food. The area could be 1 million hectares by 2010. This is 10 000 km2, about half the area of Wales (see map).

By 2010 energy crops couldgenerate 1000–2000 MW. This is less than 2% of our electricity.In the long term, if 10% of thefarmed area of the UK were tobe planted with willows forcoppicing, 9000 MW could beproduced, enough for two largecities.

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