Westfield poultry waste plant comes on line

21 September 2001

The Westfield biomass plant in Fife, Scotland, is the first power station in the world to use a fluidised bed system to burn poultry litter. It has been designated a reference site by the EU to demonstrate the viability of alternative energy.

The 10 MW Westfield Power Station, near Glenrothes, in Fife, Scotland, can convert 120 000 tonne per year of poultry litter, or chicken waste, into electricity and high-grade agricultural fertiliser. It is the first FBC plant in the world burning any kind of poultry litter, and the first biomass electrical power plant in Scotland. It has been designated a reference site by the EU which will monitor the site’s performance and conduct combustion trials using other biomass fuels, and is strongly supported by the Scottish Environmental Protection Agency. Long-term contracts with Scottish utility companies are already in place for the sale of the electricity.
The owners, Energy Power Resources Scotland, have seen performance tests completed at what is regarded as the world's most technologically advanced power station to be fuelled by poultry litter. It was engineered by McLellan & Partners, built by Abengoa of Spain and financed by the Bank of Scotland. Mitsui Babcock have a long term operation and maintenance contract. Full commercial production started in June this year.
The plant is currently producing 11.8 MW of power, of which 1.5 MW is parasitic, and performing better than its guaranteed efficiency, output and emissions levels. The generated electricity is fed into the utility grid and EPR is paid an SRO green price (approx. 5.6p/kWh currently). The plant, which cost £22 million to build and employs a full-time staff of 20, is a design tailor made for its fuel and actually burns all such waste produced by food chicken farmers in the whole of Scotland.
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 supply the balance of litter that the plant needs.
The driving forces behind an increasing demand for reliable biomass fired power plant are Kyoto-inspired state sponsorship for green electricity, increasing landfill costs and multi-fuel concepts. Landfilling can lead to environmental problems via the leaching of pollutants into watercourses and in future is going to be increasingly expensive. The practice of spreading poultry litter on the land, raising phosphate levels, has put the industry under pressure to dispose of its waste in a more environmentally responsible manner.
Poultry litter has a number of advantages over conventional waste, when it is used as a fuel source. The ash produced during combustion has the appropriate levels of potash and phosphate to render if suitable for making agricultural fertilisers, and it adds to the income and therefore the efficiency of the plant. From 100 units of litter the plant is producing 10 units of fertiliser.

Combustion and steam generation
The fuel is a mixture of chicken droppings with soft wood shavings and sometimes straw, with a moisture content around 30 per cent and a calorific value about half that of coal.
The properties of the fuel led to the decision to use a fluidised bubbling bed combustor with a staged air supply. Low grade fuel with the possibility of a variable moisture content demands a flexible and tolerant system.
The staged air supply with a substoichiometric combustion system in the bed was developed by Austrian Energy to fire a wide range of heating values. The bubbling bed is a high efficiency process when firing difficult fuels and gives excellent emissions results.
In 1998 Austrian Energy (now part of Babcock Borsig Power), which specialises in ‘advanced solutions’ relating to biomass fired power plants, was awarded the contract for the fluidised bed boiler including related fuel supply, combustion air and start up equipment and flue gas cleaning. The condensing turbine and generator contracts were also awarded to AE.
The fluidised bed, the air distributor and the post combustion chamber form an integral part of the boiler unit. The bubbling bed is characterised by a substoichiometric air supply via the air distributor to control the bed temperature.
A secondary air system feeds in the balance of the required combustion air above the fluidised bed, which results in good burnout in the post-combustion chamber. In addition a flue gas recirculation system is installed, mixing flue gas into the post combustion chamber as well as into the primary air. This allows a high degree of temperature control and leads to low emissions.
The requirements regarding combustion, (a minimum of 850 °C for two seconds and a minimum O2 content of 6 per cent (by volume) in the flue gas), as well as the ash properties in terms of agglomeration tendencies, necessitates tight temperature control. This is realised with substoichiometric operation of the fluidised bed, where temperatures between 700 and 850°C can be controlled with only small variations. Moreover, the reduced supply of primary air at high pressure reduces the parasitic power consumption of the plant. Secondary air, which is the greater part of the combustion air, is taken from the fuel storage house. This avoids creating unpleasant odours in the surrounding area.
Two burners firing fuel oil No 2 are used to start up the bed. They are switched off when the FBC has reached its operating temperature.

Boiler design
The steam generator is a bottom supported 4-pass type with natural circulation, with 3 passes integrated in the evaporator waterwalls. The bubbling fluidised bed and the post-combustion chamber are the first pass. The waterwalls in the fluidised bed and part of the post-combustion chamber are coated with refractory material for erosion protection and to increase conductivity. The second pass is a radiation chamber to cool down the flue gas before it contacts the convection surfaces, which avoids fouling problems in the superheater section. The two superheater sections are situated in the third pass. The fourth pass contains the economiser, which further cools the flue gases to 160°C.

Fuel characteristics
The operators are still discovering some of the variables. For example, the bubbling bed air pressure can be altered, but the distribution pattern is set by the manufacturer. And proper management of the fuel store is important. Fuel density turns out to be a key issue, discovered when it was noticed that fuel from the top of the hopper exhibited different combustion behaviour compared with fuel from the top. Efficiency is slightly higher if the fuel is denser. Operational experience is determining the shape of that curve.
10 MW is the economic break even for this plant, which is already sucking in all the available fuel. To reduce exposure, there is a tentative plan to include feathers from the plucking process; the existing SEPA approval, which cites only ‘poultry litter’, would cover that with only minor amendments.
Litter from egg producing poultry is also a possibility, but it is mostly droppings, so the operators would need to look carefully at the fuel mix. Such litter will become more available because producers who want to expand have great difficulty getting a permit to dump more waste.
The question of burning grain has come up, and is still on the agenda, but has been sidelined because of the foot and mouth crisis.

Fuel handling
A key feature at Westfield is the totally enclosed fuel store incorporating an automated mixing system and capable of holding 3 500 tonnes of litter. This ensures that the plant can accept deliveries in accordance witht eh production cycles of the poultry industry. Fuel arrives at the power plant in 25 tonne loads direct from chicken farms. The trucks dump the litter into a reception bay, an integrated part of the storage building, from where it is lifted by a clam shell overhead travelling crane and transported into the main storage area. The crane system also feeds the fuel bunkers which form the first part of the totally enclosed fuel transfer system. The fuel store is kept under negative pressure at all times to contain any odour, and the secondary air supply for the FBC unit is drawn from this area. The conveyor, screw, and blown feed system have been designed with 100 per cent redundancy – one of the two systems can supply the plant on its own. Eight hours supply is held in the buffer hopper above the plant. Control of the two fuel handling cranes (overhead travelling types) is by automatic computer control using very powerful software. Working in a co-ordinated programme the two cranes convey fuel from the lorry discharge bay to the storage hoppers, feed the day hopper, and carry out housekeeping tasks – keeping the levels uniform, turning the fuel over on a regular basis at night (to prevent composting, methane production and irregular moisture levels). The computer keeps track of storage levels through sensors and of dwell times from loading/unloading records. The store can hold 4000 t, a single lorry, 25 t.

Bed material and ash handling
The level of the fluidised bed is maintained at a fixed level by the extraction of ash from underneath the bed. The ash from the second and third passes is collected with a screw and conveyed to the ash silo. The fly ash collected in the bag filter is also conveyed pneumatically to the ash silo.The fly ash with its high content of potassium and phosphor (20 per cent phosphate, 15 per cent potash) is an excellent fertiliser and is sold under a long term agreement. For this reason, "impurities" like sand and Ca(OH)2 from the flue gas have to be minimised.

Flue gas cleaning
A bag filter removes the dust from the flue gas. The table right shows emissions standards, but actually the plant is running well inside this. The emissions monitoring station is situated half way up the stack. Every three months its readings are sent to SEPA.
A lime silo with delivery system has been built and installed for possible future use – if for instance there was a tightening up of emissions regulations.

Control system
The Siemens PCS including a Profibus-DP remote I/O architecture was selected. Automation specialists Siemens Moore Process Automation (SMPA) designed and developed the PCS, which provides top level plant control and communicates to the remote I/O via the Profibus network. As well as the innovative fuel use demonstrated at Westfield, the control architecture itself incorporates many advanced features. In place of conventional cabling, normally used to transfer information between a power station’s control room and the site itself, the PCS uses fibre optics for communications. These transfer data between the programmable controllers used to manage the boiler, combustion feeder and fluidised bed incinerator, and the human machine interfaces (HMI) which provide operators with access to operating information. The combination of fibre optic links and HMI panels significantly reduced the cabling and hardware costs normally associated with this type of installation.
Access to information relating to the optimisation of plant efficiency was an important consideration for EPR. Management, engineering and maintenance staff all require access to data, while the system also needs to be able to control the plant automatically.
Safety of the installation and personnel was of course a priority for the system builders. Automatic shut down of plant has to be carried out in such a way that the integrity of the power station is not compromised. Safe control of critical areas of the process, including unscheduled shutdowns due to faults or problems caused during power generation, are handled by the PCS. Alarm states are automatically reported to operators in the control room and on the generating floor.

Operational results
Hot commissioning of the combustion system started in March 2000 and the plant was handed over after a successful trial run in October 2000. Problems occurred with the fouling in the post combustion chamber and unexpected fouling of the economiser heating surfaces in low temperature regions. These problems were solved with wall soot-blowers in the combustion chamber and additional soot-blowers in the economiser pass. The success of the FBC design was demonstrated when the operational parameters were achieved. Most of the emission values are far below the guaranteed limits and the ash quality necessary for its sale as a fertiliser could be attained.
At least some of the problems that have arisen at the similar Fibrowatt plant, Thetford, UK, are unlikely to be shared at Westfield. At Westfield potential odour problems during down time, or methane build up as a result of composting have been avoided by the fitting of air burners for use when the plant is off line. Nor is Westfield likely to experience the combustion problems that have occurred at Thetford – slagging of the fuel at high temperatures – because it employs a purpose-built bubbling fluid bed combustor rather than a blown-fuel spreader grate combustor.


Key operational parameters
The steam generator – design basis
Design emissions limits

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