[Biomass Case Studies]
Westfield Biomass Plant, ARBRE Plant; Eggborough, Straw-Fired Power Station; Ely, Greengairs; Scotland, EFW Plant; Dundee
The Westfield Biomass Plant is located at the Westfield Development Centre, Cardenden, near to Glenrothes, Fife, Scotland and has been operational since October 2000. It is the first power plant in the world to use a fluidised bed combustion system to burn poultry litter and turn it into two useful products – electricity and fertiliser. The plant, which has a net electricity output of 10MW produces electricity by raising steam to drive a steam turbine, runs in a base load operating regime generating around 87,000MWh/year, which is sold to the Scottish Electricity Companies. Energy Power Resources Ltd (EPR) - an energy company specialising in the biomass to energy, waste to energy and renewable energy sectors, owns the project. Poultry litter was previously spread in large quantities on farmland but this is no longer considered an acceptable environmental solution. EPR therefore identified poultry litter disposal in an environmentally acceptable manner as a business opportunity.
Scotland was identified as a key catchment area for poultry litter in the UK and Westfield was chosen because of its central location in the poultry farming industry. The site has excellent road links and is in close proximity to the electricity grid system. In addition, the site is in an area of industrial activity and planning permission was straightforward, with broad support from the local community.
The fluidised bed combustor is of the bubbling bed type and incorporates flue gas recycling for combustion temperature control. The poultry litter combustion temperature is maintained at 850°C. The hot combustion gases are used to raise steam, which supplies an 11.8MW condensing steam turbine generator unit generating at 11kV. The plant's electricity output is sold to Scottish Power and Scottish and Southern Energy under an SRO-1 contract.
Grampian Foods, the largest poultry farmer in Scotland, has a long-term contract with EPR to supply poultry litter from its meat production division. Additional top-up contracts provide the balance of litter that the plant needs. A key feature at Westfield is the totally enclosed fuel store which incorporates an automated mixing system and can hold 3500 tonnes of poultry litter; this ensures that the plant can accept deliveries in accordance with the production cycles of the poultry farming industry. The plant incorporates a 46 tonnes/hour fluidised bed boiler unit and cost £22 million to develop and implement, it has been financed by the Bank of Scotland.
The poultry litter fuel is delivered to the site in covered tipper lorries during daytime, with each load being weighed on arrival. The fuel is stored in a reception bunker before being fed - 14 tonnes per hour - by a semi-automatic crane to a push floor feeder, which feeds the combustor. Poultry litter requires no pre-treatment.
Plant Data
Net Electrical Output – 10MW
Fuel - Poultry litter with moisture content between 20-45%
Fuel Consumption - 14 t/hr
Steam Conditions - 62 bar/500°C
Poultry litter consists of a mixture of wood shavings, straw and poultry droppings and has a calorific value around 50% that of coal. The Westfield plant converts 115,000 tonnes/year of the litter into electricity and ash (which is sold as a fertiliser) without producing any waste. Utilising fluidised bed technology; the plant has been selected by the EU as a demonstration plant in the field of waste to energy.
The Scottish Environmental Protection Agency (SEPA) strongly supported the project, seeing it as an example of "best practice" in dealing with the disposal of poultry litter. The flue gas emissions at the plant are typically less than half the limits set by SEPA. Alan Blair, Project Director, notes: "The low flue gas emissions are a very good guide to the efficiency of the combustion process and the appropriateness of using fluidised bed technology for burning poultry litter".
The ARBRE plant is the UK's first wood fuelled electricity generating plant at Eggborough in North Yorkshire. It generates 10 MW of electricity. 80% of this is exported to the grid while 20% is retained for use in the running of the plant.
As trees grow they store energy from the sun in their biomass. At ARBRE's power plant the energy stored in the biomass is converted to electricity. In a conventional coal fired power plant the coal is burned to produce heat to raise steam. The steam drives a turbine to generate electricity - a simple steam cycle.
The plant requires 43,000 oven-dry tonnes per year of wood fuel. A minimum of 1500 hectares of short-rotation coppices (to meet 60% of the plant's supply) is being established in a programme that commenced spring 1996. The short-rotation coppices are grown in a radius of 40 miles on arable set-aside land, with some on reclaimed land.
ARBRE has adopted a new and more advanced technology for converting wood fuel that offers higher efficiency. The wood will first be converted to a gas, known as syngas, which has a calorific value of around 5.4 MJ per cubic metre. The syngas will be combusted in a combined cycle plant that contains a gas turbine and a steam turbine to maximise efficiency. This system is termed Biomass Integrated Gasification Combined Cycle (BIG-CC).
Developing energy crops have considerable environmental benefits due to their low requirement for fertiliser inputs and by providing increased habitat bio-diversity in the countryside. Once short-rotation coppices are established the need for weed and pest control is low, thereby reducing potential polluting effects of pesticides.
The fuel sources are:
• Short rotation coppice consists primarily of densely planted willow shrubs harvested on a three-year cycle. The rootstock is left in the ground and after each harvest new shoots emerge, as in a traditional coppice
• Forestry Sources - materials derived from forest and woodland management.
ARBRE is bringing the benefits that can be gained from the sustainable use of wood fuels one step closer.
The world's largest straw-fired power station, generating 38 megawatts of electricity has recently become operational and is helping the United Kingdom to fulfil its environmental conservation obligation of using more renewable energy sources. The power plant is the first of its kind in the UK, located in Ely at the Elean Business Park, Cambridgeshire, England and was constructed by Energy Power Resources Ltd (EPR). It has the capacity to generate 38MW of electricity. The electricity will be sold under a Non-Fossil Fuel Obligation (NFFO) contract from start of operations until 28th August 2013, after which it will be sold at the available market rate.
To ensure uninterrupted fuel deliveries, a company called Anglian Straw owned by EPR manages the logistics with a dedicated fleet of 10 covered heavy-duty vehicles. The straw is unloaded, weighed and tested by four semi-automatic cranes, then stored in two enclosed barns with a total storage capacity of 2,100 tonnes, which is sufficient for up to 76 hours of operation. The power station consumes around 200,000 tonnes of straw per year
The plant, which cost £60 million, will also be capable of burning a range of other bio-fuels and up to 10% natural gas. Long-term straw production contracts are in place with major farmers and contractors, ensuring that local economy is also improved, within a 43-mile radius of the plant, to supply baled straw - with moisture content greater than 25%.
The Ely power scheme also offers a unique opportunity for the development of baled energy crops such as Miscanthus and other energy crops. It is also intended to promote a gradual diversification of fuel into the project as a way of encouraging UK agriculture towards alternative crops.
The plant consists of 18m high straw barns flanking a 25m high central boiler
Greengairs is Scotland's largest landfill site. It handles around 750,000 tonnes/year of waste. The power plant at Greengairs uses landfill gas to generate almost 8MW of power. This was Scotland's first landfill gas-to-electricity plant. Shanks, a UK waste specialist is contracted to supply gas to the engine plant and Finning is responsible for supplying the electricity produced by the to Scottish Power (75%) and Scottish Hydro (25%).
The Greengairs landfill site covers 40 hectares and serves the domestic waste needs of Central Scotland including Glasgow and Edinburgh. The operation includes a number of commercial and trade waste contracts as well as the collection of domestic waste from transfer stations. The site currently has 90 gas wells feeding 6km of pipes. All of the gas produced at the site is filtered, dewatered, regulated for pressure and methane content before being supplied to the engine plant.
Demonstrating further commitment to environmental concerns, the engines are housed in a purpose built facility designed by Shanks to reduce noise attenuation. Some heat is also recovered and utilised in the leachate treatment process at the site.
Construction of Dundee's new £35 million energy from waste (EFW) facility on the Baldovie Industrial Estate, commissioned to meet European Union (EU) emission limits, came on-line in autumn 1999. The new plant processes 120,000 tonnes/year of municipal and commercial waste plus small amounts of non-hazardous clinical waste and liquid wastes. The plant, which is the first in the UK to use bubbling fluidised bed technology for waste treatment, uses heat released during combustion to produce steam that drives turbines. Around 10.5 MW of electricity is generated, of which 79% is sold to the grid under a Scottish Renewables Obligation (SRO) contract.
The new facility encompasses two separate stages - a fuel processing stage and a combustion stage
To produce the fuel, incoming waste is tipped onto a flat floor in the waste reception area. It is then fed into one of two hammer mills, where it is shredded. (The hammer mills can handle almost double the waste that is currently shredded by then per hour, this over-rating is to allow for unplanned downtime of the machines.) Magnets then remove ferrous materials and the floc is then fed by conveyor belts through the roof of the fuel storage building. The coarse RDF (refuse-derived fuel) is then automatically fed by conveyor belt into the two boilers. The RDF is burned in the bubbling fluidised bed boilers in a bed of hot sand, which is "fluidised" by the upward flow of pressurised air through it. Non-combustible material is continuously removed from the bed, which has dolomite added to it to reduce boiler tube fouling and control emissions of sulphur dioxide. Heat released during combustion is used to produce steam, which in turn drives a turbine.
The incineration process carried out by DERL will burn mainly municipal waste in two 17 MW thermal input bubbling fluidised bed boilers, the first of their kind in the UK. The steam raised is used to generate electricity in a single condensing turbine generator with up to 8.3 MW being exported to the national electricity network, each boiler is capable of burning waste at a nominal design rate of 7.6 tonnes/hr per boiler.
The EFW plant operates for around 88% of the year, producing around 80.9 GWh of electricity.
Combustion of waste material at the plant diverts some 97,000 tonnes/year of waste away from landfill. In addition, fuel processing for the plant contributes to the regions ferrous and non-ferrous materials waste diversion. 5,000 tonnes/year of ferrous metals and 450 tonnes/year of non-ferrous metals will be recovered.
Dundee City Council has established an ecologically sound, energy saving disposal route for its waste. The plant is aiming to provide a cost-effective long-term solution for the disposal of wastes.
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