the 'other' Green City Times blog:
Poplars AD plant (England)
Anaerobic digestion (AD) can used for farms, businesses and municipalities as a productive solution to a growing waste problem throughout the world. Instead of waste simply ending up in landfills, or being incinerated, waste (and purpose grown crops) could be turned into energy. AD is the process of turning agricultural waste (such as livestock manure), or municipal, commercial and industrial waste streams (such as food processing waste), into energy using micro-organisms to transform waste into a productive material used to create biogas and digestate.
An anaerobic digester generates biogas (or biomethane) which is burned on-site to generate heat, power or both (so, combined heat and power – CHP) or to generate biogas for use as an energy source for the grid (or biomethane used for heat or transportation). Also produced in the process is digestate, which is a source of nutrients that can be used as a fertilizer. Organic waste finds a purpose as it is put in a digester, such as a biomass plant, along with various types of micro-organisms to produce methane, the useful part of biogas. The anaerobic process also occurs naturally, in addition to the man-made construct in a biomass plant.
AD in a biomass plant is a cost-effective way to produce renewable energy. AD also leads to less landfill waste and is a constructive way for farms, businesses and municipalities to dispose of waste. When used for heat or transportation, as biomethane (biomethane can be used in place of diesel, given modifications to the vehicles in question), there are tremendous greenhouse gas reductions.
The entire bus fleet in Oslo, Norway, is run on biomethane from sewage treatment and organic waste, and they see a dramatic (around 70%) reduction in GHG emissions compared to fossil fuel burning vehicles. Food waste and other waste processed through AD also reduces GHG emissions substantially. Energy produced by AD has a very low carbon footprint.
The AD plant at Cannock, Staffordshire, England (called the Poplars AD plant) is an example of a successful, large-scale AD plant. The £24 million project treats commercial and industrial food and waste to create around 6MW of renewable energy for the national grid. The Poplars plant shows that large-scale AD can be successful.
Ethanol is traditionally made from food crops like corn and sugarcane, but it can also be made from cellulosic feedstocks, non-food crops or inedible waste products. Examples of sources for cellulosic biofuel are crop residues, Miscanthus, switch grass, paper pulp, packaging, cardboard, sawdust, wood chips, rice hulls, corn stover and the byproducts of lawn and tree maintenance.
Technically, almost all plants have the lingocelluloses needed to produce ethanol from cellulosic material. Once glucose is freed from the cellulose using enzymes, fermentation produces ethanol, similar to how ethanol is traditionally produced from 1st generation biofuel sources. Lignin is also produced in the process, which can be burned as a carbon-neutral fuel for local processing plants, businesses and perhaps even homes.
There are tons of cellulose containing raw materials that could be used to produce ethanol that are simply thrown away each year in the U.S. alone. Examples of this are over 100 million dry tons of urban wood wastes and forest residues and over 150 million dry tons of corn stover and wheat straw. That material plus just a fraction of the other paper, wood and plant products that could be used to create ethanol instead of garbage would be enough to make the U.S. independent of foreign oil. This theme is true in other parts of the world as well.
Financial concerns stop cellulosic biofuel from really taking off and providing a consistent source of fuel. This type of ethanol production involves an additional step, the breakdown of the raw material into glucose with enzymes, which translates into a higher cost. However, the raw material is abundant, and the reduction of greenhouse gas emissions from cellulosic biofuel can be up to 90% compared to fossil fuel petroleum, significantly greater than those obtained from traditional 1st generation biofuels. Cellulosic raw material can be easily grown in land marginal for actual agriculture or simply be diverted from landfills, in order to make the production of cellulosic biofuel more cost-effective. Cost-effective processes, such as using inexpensive enzymes to break down the cellulose, are being researched and developed as well.