Sources - Biomass

The Intergovernmental Panel on Climate Change (IPCC) has endorsed the large scale use of carbon-neutral or low-carbon biomass derived energy across the globe. The IPCC envisions as much as 400 million hectares devoted to biomass energy production. The idea of using carbon-neutral or low-carbon fuels is fundamental to maintain renewable production.

(The process of photosynthesis)

All biomass is produced by plants converting sunlight into glucose for respiration through photosynthesis.

The carbon dioxide absorbed by the plants during photosynthesis is broken down into oxygen (O2) and carbon with the O2 released back into the environment. The carbon is stored inside the plant and which forms part of the “Carbon Cycle”. The “Carbon Cycle” is the cycle of carbon or carbon dioxide as it is absorbed from the atmosphere by plants and then released back into the atmosphere when the plant decomposes or is burnt. The breakdown of vegetation and animal wastes, by a variety of processes, releases heat which can then be used directly or to produce electricity.

Unlike fossil fuels, which have developed over millions of years and are rich in carbon, biomass fuels do not add to the global carbon budget. This is because the carbon released by burning the fuel is equal to the amount the organism consumed in its development. The reason this is a benefit is that the system of biomass fuels are perfectly carbon neutral, as illustrated in the diagram and therefore do not contribute to the greenhouse effect through the imbalance of the carbon cycle.

Dedicated energy crops and trees

Aquatic plants

Agricultural food and feed crops

Animal wastes

Agricultural crop wastes and residues

Organic municipal wastes

Wood wastes and residues

Other organic waste materials

In the UK the main developments have been in the production of wood as a fuel. Trees which grow fast and produce high yields in this country such as willow and poplar can be grown and coppiced on a three year cycle. The wood which is harvested can then be used as a fuel and the tree grows back over another three year period until it is ready for harvesting again. On a more individual scale, it is possible to heat your home using wood burning stoves; this system is a renewable energy source provided you purchase the fuel from sustainable forests and the stoves operate at a reasonable efficiency.

Growing crops for energy is not a new concept. Before the advent of fossil fuels wood was a commonly used fuel for heating and agricultural crops were grown to fuel the horse, a major means of transport and source of power at the time. The modern production of crops for energy is looking at the same sources of fuel but growing and utilising them in a more efficient way. Fast growing trees are being developed to produce a cheap and reliable energy source for conversion to electricity and heat. Agricultural crops such as wheat and oil seed rape are being processed to produce liquid transport fuels such as ethanol and biodiesel.

Short rotation coppice (SRC) is a system of producing wood fuel for use in energy production through the cultivation of species of willow or poplar. It is generally grown over a 3-year cycle, starting with planting in year one and a harvest in year three or four.

SRC currently attracts establishment grant payments from DEFRA under the Energy Crops Scheme. The establishment grants are £1000 or £1600 per hectare depending on land type, for establishing SRC of either willow or poplar and £920 per hectare for establishing miscanthus (a type of fast growing grass). Set aside payments can also be claimed where appropriate.

Perennial grasses such as Miscanthus are a possible alternative to wood for high yielding dry matter production. Miscanthus is still at the R&D stage with trials of this perennial grass native to Asia and Africa being undertaken across the UK to see how it performs in our climate. In theory it should perform better than native plants as it has a more efficient photosynthetic process.

However, its success as an energy crop will depend on a number of factors such as its costs of production and the quality of the biomass produced.

It is difficult to determine the overall costs of installing the necessary infrastructure and producing energy crops. However, generally, it will cost between £500 and £60,000 to install the necessary equipment to generate heat from wood based fuels on a small scale.

Liquid bio-fuels include oil derived from vegetable products and its derivatives and alcohols distilled from fermentation processes involving plant materials. The advantages of liquid bio-fuels over solid bio-fuels include:

High energy density (36GJ/t).

Easy to handle by pumping and piping.

Cheaply transported and distributed.

Can be used to fuel vehicles – not just stationary plant.

Can be used directly in engines to generate electricity easily and cheaply.

Plant used to convert oil to heat is relatively cheap and efficient.

The production of bioethanol from sugary or starchy biomass feedstocks such as sugar cane, sugar beet and even potatoes or grain is similar to the production of alcohol but on an industrial scale. A “beer” is made from the feedstock in a yeast fermentation process and is then distilled to separate the alcohol (ethanol) from the water. In most situations the ethanol has been mixed with petroleum and distributed through conventional garage forecourts. At low concentrations changes to engine components are negligible, the higher the concentration the greater the changes needed, although still relatively trivial.

Although there is relatively little direct experience of bioethanol in the UK, the expertise to grow the crop and knowledge of the process is well established in the parts of the country where sugar beet is grown to produce sugar for feed.

In order to use bio-oil at a significant scale in transport applications, the oil needs to be processed so it can be used in conventional combustion engines without modification as a substitute for processed fossil fuels.

The process of transesterification converts oil into a diesel substitute known as biodiesel. Pure biodiesel can be used in unmodified, conventional compression-ignition (diesel) engines. Transesterification is a relatively simple process and can be achieved economically at a fairly small industrial scale. The basic process is to mix vegetable oil with methanol and heat it at around 50°C in the presence of a catalyst (usually sodium or potassium hydroxide). The products of this process are biodiesel and glycerine which is a saleable product.

The production of biodiesel from virgin rape oil results in the biodiesel costing around twice as much as conventional petroleum diesel. As yet the UK government has not provided sufficient tax credits to allow a viable production of biodiesel from virgin rape oil possible.

Anaerobic digestion is the process to produce a fuel gas from a biomass waste arising in the form of slurry such as, sewage, animal wastes and waste products from the food processing industry.

The organic matter in the slurry is digested by microbes in an anaerobic environment (without oxygen) to produce gas products. The main gaseous products of the digestion process are methane 60-65% and carbon dioxide 35-40%. The gas can then be used as a fuel in an engine for electricity production or it can be used for heating purposes by burning in a boiler. The byproduct of digested slurry can be used as a peat substitute or applied directly onto agricultural land as a fertilizer.

• Norbury Park Sawmill woodfuel boiler, Surrey
• The Living Rainforest woodfuel boiler, W Berks 
• Tanner's Hatch YH wood stove, Surrey 
• Environment Centre woodfuel boiler, Wycombe 
• Shortenills woodfuel boiler, Chalfont St Peter 
• St Peter & Paul Scout Hut wood stove, Godalming