Biogas and Landfill Gas Fact Sheet 05

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R E N E W A B L E E N E R G Y

I M P R O V I N G E N E R G Y C H O I C E SImage courtesy of URS New Zealand Ltd

Biogas is a gas produced during the biological breakdown of organicmatter which can be used to provide energy. The gas is producedfrom the decay of vegetation and other organic materials such asanimal manures, sewage treatment sludge or food processing waste. This can occur in places where there is little or no oxygensuch as in a landfill (where it is known as landfill gas) or under controlled conditions such as an engineered waste digester.

Biogas emitted from landfill sites or engineered waste digesters can be used to provideheat or to generate electricity.

Why use biogas for energy?Utilising biogas makes sense because it occurs as a result of a number of existing naturalprocesses and the gas would otherwise be emitted into the atmosphere as a harmfulgreenhouse gas. Biogas contains methane, which has twenty times more greenhouse effectthan carbon dioxide. The process of burning biogas for electricity generation or heat convertsthe methane into carbon dioxide, therefore significantly reducing the environmental impact.Using biogas technology also means that the polluting potential of organic material isreduced. For example a farmer may choose to collect the effluent from a shed and convertit into gas in an engineered waste digester. By doing so, the farmer avoids having to disposeof the material in the local river which can harm the environment, incur a disposal chargeand possibly contaminate the area if any disease was present in the original waste. Thefarmer can then use the sludge from the digester as a fertiliser as any harmful bacteria willhave been killed during the gasification process.

Biogas and landfill gas productionBiogas production processes. Biogas is commonly produced by anaerobic digestion of wetorganic waste. In warm, wet and airless conditions, like in decaying vegetation at the bottomof wetlands and ponds, bacteria digests the organic matter. This process can also occur insituations created by human activities, such as in concentrations of sewage and animalmanure, in rubbish buried in landfill sites and in engineered waste digesters.The bacterial process itself produces some heat, which helps keep the temperature at an ideal35C. However, if the engineered waste disposal system is in a cooler climate some of the gasproduced may need to be fed back into the process to maintain the optimal temperature.The amount of biogas produced from each dry tonne of feedstock, and the actual useful

methane content (which varies from 50 to 90%), both depend on the characteristics of thefeedstock. Biogas has a heat value of 19-26 megajoules per cubic meter (MJ/m 3 ).


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Landfill gas. In landfills, biogas is produced relatively slowly,as the conditions are not ideal for bacteria. The amount of biogas produced will depend on the management and designof the landfill. Depending on how the waste has been laiddown and how gas is collected, landfill gas may still beproduced for 10 to 50 years after the landfill is sealed.

In a landfill, the gas is collected by an array of perforatedpipes that are buried as the landfill is filled, or alternativelyvertical wells can be drilled to bring the gas to the surfaceusing a vacuum collection system.A typical landfill gas mix will consist of between 40 and 60%methane, with most of the balance being carbon dioxide.Whereas natural gas typically has a heat value of 40MJ/m 3,landfill gas is usually around 19-22MJ/m 3. Of course the amountof gas produced varies with the proportion of biological materialcontained in the domestic rubbish, which is typically up to50%, and in practice most existing landfills have not beendesigned or managed to achieve optimal gas production.Engineered biomass waste digester. Under controlled andoptimised conditions, an engineered biomass waste digester

can convert feedstock into biogas in a few days. The anaerobic

digestion process works on biomass with high moisturecontent (sometimes up to 95% water), and uses a mixed

population of bacteria types to slowly break down the organicmaterials into sugars and then into various acids. These acidssubsequently decompose into a mixture of gases, consisting mainly of methane, carbon dioxide and a little hydrogensulphide. An inert residue is left behind and often containsnutrients and organic matter which can make it a useful soilconditioner. However if materials in the feedstock are toxicto the processing bacteria, the gas production system canstop completely.

There are many digester designs and sizes, ranging from 1m 3

in volume for a single household unit to 15,000m 3 for a largecommercial installation.

The potential for energy productionBiogas from sewage and landfills is already used for energyin some areas of New Zealand and has the potential to provide

even more. In 2003 it provided enough electricity forapproximately 19,000 houses; 85GWh of electricity wasproduced from three landfill gas plants in the Aucklandand Wellington regions alone, and 70GWh of electricitywas produced from sewage works in Auckland, Christchurchand Hamilton.

Potential resourcein New ZealandThe potential for energy from biogas using existing sources of organic waste is in the order of 1.4PJ or 0.3%of New Zealands energy demand.Approximately 80 to 200m 3 of landfill gas is produced pertonne of municipal solid waste, and in addition New Zealandhas a relatively large primary agricultural industry with manyopportunities for producing biogas from animal manure.Potential methane resource generated from piggery wasteat farms could provide up to 0.05PJ (14GWh) a year, whichis equivalent to the amount of electricity used by 1,700houses. There is also further potential for biogas from the

poultry and dairy industry.

With a change in agricultural and horticultural practices,crop residues could provide significant additional sources

of biomass for biogas production, without competing withfood production. In many cases, the costs could well proveto be economic because of reduced waste disposal costs.For example, the straw and trash from cereal productioncould be used in an engineered waste digestion systeminstead of burning it, as could the grass clippings frommowing orchards. In both cases, energy would be recoveredfrom the biogas, and the digester residues could be returnedto the land as fertiliser.

Users of biogas for energyOrganisations such as councils and electricity or gasretailers are typical users of biogas as an energy resource.Some individual users, such as farms, will use the gas

directly to provide heat for their own operations. Biogascould be used directly for refrigeration or to generateelectricity, but this is not as efficient or cost effective asusing the gas for heat.


I M P R O V I N G E N E R G Y C H O I C E S

4PJ The potential for energy from biogas using existing sources of organic waste is in the order of 1.4PJ,enough for more than 30,000 homes for a year.


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This fact sheet was produced by the EnergyEfficiency and Conservation Authority (EECA).EECA is a Crown entity implementing theNational Energy Efficiency and ConservationStrategy through improving energy choices.


For more information about EECA, therenewable energy target and links to otherinformation sources visit:www.eeca.govt.nz

For more information on biogas visit:www.epa.govt/landfill

AUCKLANDPO Box 37-444, Parnell, Auckland.Phone (09) 377 5328, Fax (09) 374 3809

WELLINGTONPO Box 388, Wellington.Phone (04) 470 2200, Fax (04) 499 5330

CHRISTCHURCHPO Box 8562, Riccarton, Christchurch.Phone (03) 353 9283, Fax (03) 377 4511

I M P R O V I N G E N E R G Y C H O I C E S

June 2005

ISSN 1176-8584

the high labour requirements and dilute nature of theeffluent being treated. However as environmental standardsin areas such as dairy farming are increased, engineeredwaste digesters will become often the most economic formof waste disposal.Life cycle cost analyses of anaerobic digester plantsand ancillary equipment show that electricity can begenerated from biogas for around 2-5c/kWh if the wastefeed material is free. However the opportunity value of feedstock for other uses, such as animal feed, and thehigh labour requirements may increase this cost estimate

substantially, particularly for small-scale plants. If greencrops are grown for feedstock, the generating costs riseto around 11-18c/kWh.

Images courtesy of EnergyInfo

In late 2001, Hamilton City Council signed an agreementwith its local electricity distribution network company,WEL Networks, and a landfill gas developer, Green Energy,to develop a landfill gas to energy project at Horotiu, nearHamilton. WEL Networks and Green Energy formed a joint

venture, WEL Green Energy (WGE). WGE installed a gasengine and generator at the site to generate electricityfrom the gas, which has been operating since late 2004.The generator is rated at 920kW. In early 2005 it wasrunning at around 750kW (enough to power about 820homes). When the final stage is completed in 2007, thegenerator will operate at full capacity.This project is one of a number that have arisenfrom the Councils energy management programmestartedfive years ago. Over time, annual cost savingshave steadily increased, and are presently in excess of $500,000 per annum. The landfill generator will supplyaround 8 million kWh per year of electricity from arenewable energy source and will displace the equivalentamount from national generators.The Council supplies the landfill gas to WGE and purchasesthe electricity for use in its facilities at a fixed price.

WGE is responsible for maintaining and operating the gasengine and supplying electricity to the Council.As part of covering the landfill, a gas collection systemis installed to extract gas and reduce odour and fire risk.The landfill supplying the generator contains more than600,000 tonnes of waste. Around 650 to 750m

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of biogasis collected every hour for energy conversion from thevertical collectors in the landfill. Once the last stage of the landfill is completed total gas generation is anticipatedto be more than 1,000m 3/hr.The benefits of the project are significant. In addition toconverting methane into less harmful carbon dioxide, ithelps displace generation requirements in New Zealandand has allowed the Council to provide its own hedgeagainst volatile electricity market prices.The project has an expected life of twenty years andover that time will deliver significant cost savings that willbenefit the community. Electricity supplied from the landfillis not subject to the carbon tax charge and this alone could

generate significant savings. National electricity costs areexpected to increase from current levels, and this will furtherimprove the cost effectiveness of the project.

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Biogas and Landfill Gas Fact Sheet 05

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