Biomass Energy

Thetford, largest Biomass plant in Europe
Thetford, largest Biomass plant in Europe

What is Biomass? In Renewable Energy terms Biomass refers to a fuel produced or processed by organic means. This falls into two main camps a) fuels which are grown such as energy crops or forestry products and b) organic waste mostly from Agricultural sources.
Biomass in the form of organic waste can be incinerated or has to be processed further to produce a fuel and this is normally achieved through a number of methods including anaerobic digestion, pyrolysis and gasification. These processes produce a biogas, generally methane, which is then burnt to produce energy. In many cases energy crops are also put through a gasification process and the subsequent biogas is then burnt in gas fired boilers to produce steam which in turn is used to produce electricity (steam turbines).

Westfield, Fife - Biomass plant
Westfield, Fife - Biomass plant

Incineration
Scotland’s first Biomass fuelled power plant was the 9.8MW EPR Westfield installation in Fife. This £18m plant burns 110,000 tonnes of chicken litter per annum and was the first in the world to do so using an advanced fluidised bed combustion method. The end product of this combustion method is rich in phosphates and potassium and is used as a high quality fertiliser. EPR (Energy Power Resources Ltd) are also involved in a number of other chicken litter plants in the UK and in June 2004 took over the running of the largest straw burning electricity plant in the world. The 38MW Elean Power Plant at Ely in Cambridgeshire burns 200,000 tonnes of cereal straw per annum and has recently begun using oil seed rape and miscanthus as part of a wider fuel mix. This now makes EPR the largest Biomass electricity generator in the UK and they are currently involved in exporting their technology to the US and the Netherlands.
The incineration method of disposing of chicken litter has attracted opposition from some environmental groups in the US where EPR’s sister company, Fibrowatt are involved in building a 50MW plant in Benson, Minnesota. The issue revolves around the flue gases produced and their release into the atmosphere but does not take into account where the same contaminants end up if they are not incinerated i.e. in the soil and rivers at greater concentrations due to farmers spreading the chicken litter as fertiliser. In the US the poultry industry has long used arsenic based compounds in chicken feed to control intestinal parasites and these arsenic compounds, particularly Roxarsone, are excreted by the chickens in a chemical form practically unchanged from the original compound.
This type of Biomass incineration should not be confused with the municipal waste incineration which takes place at the only two Scottish plants at Dundee and Lerwick in Shetland were all types of combustable waste are used to produce 'waste to heat' solutions. The Lerwick plant produces heat for over 1,000 homes surrounding the plant as part of the largest District Heating System in the UK.      
Incineration also covers woodfuel systems were the chips or pellets are burnt to produce heat and in some cases heat and power (CHP). Due to the importance of this particular technology for Argyll we have covered this in detail in other sections of our web site. Please see the links in the right or left hand panels on Woodfuel and Energy Crops (Short Rotational Coppice).

Anaerobic Digestion (AD)
The process entails the organic matter being broken down by bacteria in the absence of air to produce a biogas and a residue. The biogas, normally methane, can then be used as a fuel in gas engine or boiler. The residue, called the digestate, can be returned to the land in the form of a fertiliser which is high in organic nutrients.
The Biomass, cattle, chicken and pig waste, is placed in a digester (a warm, sealed and airless container) for roughly 10–25 days. The bacterium produces a fermentation process that results in the biogas which is then drawn off and used as the fuel. This can be on an individual farm basis or on an industrial scale with slurry being brought to a central location for processing similar to the 2.1MW Holsworthy Biogas Plant in Devon. AD is capable of reducing the solid/liquid mass by up to 40-60% and this could be reduced further by addition processing methods.
AD, along with Pyrolysis, is coming to the fore as a solution for organic waste from agricultural and domestic sources due to the pressure to move away from landfill solutions. Greengairs near Airdrie is the largest landfill site in the UK and while the operator, Waste Recycling Group (WRG), uses methane from AD to produce electricity they are continually at odds with the local communities surrounding them over gas emission levels. The site was not designed to be an AD plant and hence the retro engineering co-exists along side traditional landfill methods producing a half way house solution.

Pyrolysis
A method of heating waste in the absence of oxygen to produce gases and a carbon char. The gases are then further treated to cleanse them and can then be used as a fuel. The old process for producing charcoal from wood was an example of pyrolysis.

Gasification
This process involves the breakdown of hydrocarbons to a usable fuel gas by carefully controlling a heat reaction with critical amounts of oxygen. The conversion of coal into town gas is an example of gasification. The ARBRE Project in Yorkshire back in 1997 was a mixture of energy crops, the Short Rotational Coppice (SRC) model and electricity production through the gasification method. Energy Power Resources bought over the project’s power plant in 2002.

More and more pyrolysis is used in conjunction with gasification and combustion as a method of producing Combined Heat and Power solutions. This combination results in a very environmentally friendly, if costly, solution to waste management issues. The Argyll and Bute Area Waste Plan looks to gasification and pyrolysis for energy recovery of specific wastes in stage 2 of the plan (2011-2020) but may have to introduce such a solution earlier if their stated aim of diverting 50% of waste away from landfill is to be achieved by 2010.
   
The MESS* project on Mull recently looked at this type of Biomass AD/Gasification process as a solution for the organic waste from the community owned Mull Slaughterhouse plus other agriculturally produced waste on the island. At a cost of £1.2 Million this may have represented a viable project if it could have be shown that there was enough agricultural slurry waste on the island to keep this type of power plant supplied. Such power plant units represent a solution, in conjunction with recycling, for many of the waste management problems experienced by our island communities, currently the Bute Working Group are looking at an Anaerobic Digester as part of their future developemnt plans and, if built, will be the first such plant in Scotland.

Community groups or companies wishing further information can contact Paul O’Brien of our Infrastructure team, 01546 605423, based in our Lochgilphead office

*Mull and Iona Environmentally Sensitive Solutions