This 850 pupil school is a 1960s flat roofed building with large windows and little potential for insulation. It is situated in an exposed position with a high heat load in winter. There is an all-year-round heat demand for the swimming pool and school kitchens. The boilerhouse is a separate building housing three heating boilers and a separate boiler system for domestic hot water. Two of the boilers have been converted to gas.

How it works

The SCF 200 boiler is an efficient underfed boiler with a system of heat exchange tubes around the upper half of the boiler. In the combustion chamber, the fire burns in a rectangular retort surrounded by tuyeres which direct the forced draught primary air to the centre of the fire. Secondary air is supplied over the fire and through new holes drilled in the door.

Wood pellets are stored in a bunker adjacent to the boilers. They are conveyed by a feedscrew to the transfer box where they drop onto the stoker. This carries the pellets through the feed pipe to the centre of the combustion chamber. An anti burn back device was added to the feed pipe to prevent fire spreading back along the stoker into the bunker. If fire is detected in the feed pipe, a solenoid opens a valve in a pipe carrying water to the stoker, which dowses the fire. Controls ensure that the pellet boiler acts as lead boiler, topped up by the gas boilers as needed.

Maintenance

Half a dustbin of ash is removed manually twice a week. Once a month the tubes are cleaned out and the whole system inspected. The stoker system has been refurbished and is not expected to require any maintenance for several years. Occasionally, if a foreign body should get into the pellets, a blockage may occur. In this case a shear pin will break causing a stoppage, but can be quickly replaced.

Fuel

Wood pellets are supplied by Premier, part of Durham County Waste Management Group. They are manufactured near Coxhoe at the Joint Stocks Recycling Centre where wood and other waste materials are processed. Clean wood is crushed and metals removed by magnets. After manual checking the wood passes through a hammer mill and a screening process. The wood chips thus produced are supplied to a chipboard manufacturer. The screened fines are pelletised, in a CPM pellet mill. The pellets are subject to a rigorous quality control system and comply with the British BioGen code of good practice. They are marketed under the trade name Pelletheat.

Pellets are delivered to the school pellet bunker by a pneumatic animal feed wagon and blown in through the coal feed pipes, They flow by gravity onto the feedscrew and are augered into the boiler. The 20sq m fuel store holds three weeks supply of pellets.

Economics

The net calorific value of pellets is about half that of coal (17.5GJ/tonne compared to 29GJ/tonne). The cost of burning pellets at Ferryhill School is expected to be similar to that of burning coal, but is cheaper than gas at April 2002 prices. As fossil fuel prices continue to rise and pellets become cheaper through bulk manufacture, they should become even more competitive for school use. For smaller users who pay higher fuel prices pellets could already be economic. Wood fuel is exempt from Climate Change Levy making the price more competitive for non-domestic users.

The cost of fuel research, the boiler conversion and experimental work was assisted by County Durham Environmental Trust, ETSU for the DTI, Durham County Council and Business Link. Conversion and refurbishment of similar boilers in future is expected to be done at modest cost, including modifications to the fuel store. DCC have launched a rolling programme of conversions under their capital works budget.

Owners Comments

The school and Durham County Council are pleased with the performance of the pellet boiler. They are glad to be able to save CO² emissions and reduce landfill. The caretaker's work is lessened. The boiler provides a good teaching tool for science, technology and citizenship and the school are able to host visits for demonstration purposes.