DEFINITION: Organic matter, such as timber and crops, that can be converted to fuel and is generally regarded as a renewable fuel source. Biomass is sustainable and generally carbon-neutral because the carbon released in the combustion process is offset by the carbon trapped in the organic matter by photosynthesis during its growth. In biomass power plants or boilers, wood waste or other waste is burned to produce stream that runs a turbine to make electricity.
Biomass is considered by many companies and governments to be a renewable and sustainable source of energy, but its use is somewhat dividing the sustainability sector.
Biomass as a sustainable energy source
Biomass has long divided the sustainability sector, with one camp stating it is a renewable source of energy that prevents waste ending up in landfill, while its opponents suggest it is little better than fossil fuel use.
Proponents of biomass say it is a renewable source of energy, because:
• Waste residues will always exist – for instance scrap wood, mill residuals, forest resources – and to burn these prevents them ending up in landfill sites.
• Managed forests will always have more trees, and we will always grow crops, offering an endless source of residual biological matter for biomass.
However, opponents of biomass as a sustainable energy source point out that:
• CO2 is released into the atmosphere when wood or gas biomass is burnt in a power station, just as it is with fossil fuels.
• Biomass contributes to climate change, so it is argued that should not be included in sustainability targets, and renewable energy companies and governments should focus on “true” renewables like solar and wind.
Types of biomass
Wood and agricultural products
Most biomass used today is plant-based. Wood - logs, chips, bark, and sawdust - accounts for about 44% of biomass energy, while other biomass sources include agricultural waste products like fruit stones and corncobs. Currently, a great deal of the electricity and heat generated is used onsite by the industries creating the waste.
Solid biomass for use in power plants is sometimes dried and pelletised for easier transport and cleaner burning, and sometimes co-fired with other fuels. Alternatively, they may be used for heat or combined heat and power (CHP) production.
Solid waste (waste-to-energy)
Burning our rubbish turns waste into a usable form of energy, with 2,000 pounds of average household refuse containing about as much heat energy as 500 pounds of coal.
However not all refuse is biomass, with perhaps half of its energy content coming from plastics. In current refuse disposal systems, separating this plastic would be extremely costly. This, coupled with more general concerns over fuel-based power generators, means the use of solid waste causes a great deal of controversy in the sustainability sector.
Gas biomass (methane)
Methane can be collected from landfill sites, plant matter (silage), or animal manure. Gas biomass is increasingly being produced in specially designed airtight digesters, and the gas enriched to provide a cleaner burn to produce electricity, or for cooking and lighting.
There is also increasing usage of specially-grown energy crops such as elephant grass, which are often mixed with other waste products with a low energy content such as some manures and spoiled grain to boost gas yields.
Ethanol gasoline and biodiesel
As all brewers, wine makers and distillers know, virtually any organic materials containing cellulose, starch, or sugar can be fermented, producing ethanol. Most of the ethanol produced in the US comes from corn, and nearly all the gasoline sold contains 10%-15% ethanol, while fuel containing 85% ethanol is available for adapted vehicles.
Biodiesel is mostly made from soybean oil, and mixes of up to 20% can be used in unmodified vehicles. Biodiesel has become especially popular in South America in countries looking to reduce their dependence on oil-based gasoline.
Co-firing coal power plants with biomass
One of the biomass uses currently attracting a lot of interest is the use of biomass fuel – usually dry wood pellets – in co-firing coal power stations. This offers two key benefits in reducing carbon emissions while lowering reliance on fossil fuels.
However, experts are divided on the net effect of co-firing. One study has suggested that if 5% of coal burned were replaced with biomass pellets, it would reduce CO2 releases by 16%. Others say co-firing has the potential to make coal power stations carbon-neutral, but this would necessitate much greater ratio of biomass to coal.
A key concern of co-firing is that biomass does not have the same ‘heat content’ as coal, meaning it takes a lot more energy to operate the systems and to produce a viable fuel. Therefore, carbon emissions may actually be greater than from burning pure coal.
But co-firing is being developed in countries that traditionally have a high dependence on coal power.
So, is biomass green?
The pros of biomass
Supporters of biomass as a sustainable energy source say it is carbon neutral, since it is generated from renewable organic waste that would otherwise be dumped in landfills, openly burned or left as fodder for forest fires.
New technologies such as pollution controls and combustion engineering have advanced so biomass emissions now generally produce less emissions than comparable fossil fuel use.
Unlike other renewable power sources – such as solar and wind – a biomass power plant can operate 24/7, at any time of the year. Combined with new advancements such as battery storage, supply management and demand response, biomass can offer a more consistent and reliable renewable stream of energy.
Environmental groups including Friends of the Earth challenge the biomass “carbon neutral” claim, indicating that when wood and crops are harvested and burnt, they emit CO2 into the atmosphere and create a “carbon debt”. This debt may be repaid by recapture of carbon from growth and regrowth of crops and forests, but it could take many decades for the emissions to be neutralised.
Wood biomass is also highly dependent on proper management of forest resources, and there is always a risk that burned virgin forests will be replaced by faster-growing trees or even crops (such as palm oil), reducing the carbon capture potential and irrevocably damaging ecosystems.
There are also other factors to consider in the biomass supply chain. All biomass fuels need to be acquired, prepared, and transported. Preparation and transportation can add costs to an otherwise negative value fuel of biomass or other wastes.
The biomass subsidy question
In the UK, the government has recently been attacked for subsidising biomass development, with experts suggesting that solar, wind and energy efficiency upgrades to buildings could be boosted by re-aiming the subsidy streams.
Government bodies are currently at odds over the labelling of biomass as carbon neutral. The European Environment Agency (EEA) says biomass is not always carbon neutral and must follow EU resource efficiency principles.
But there is no question that ‘true’ renewable sources like solar and wind will not be able to replace coal, oil and natural gas use, at least in the short- to medium-term. More research is needed, but biomass has a role to play in reducing dependency on fossil fuels.