Biomass and coal co-combustion in utility scale - operating experience of Alholmens Kraft

摘要

CFB technology has been scaled-up and demonstrated in the size range of around 250 MW_e. One of the biggest advantages of CFB is it's capability to burn different kind of fuels either at the same time or separately. The utilization of biomass alone or in co-combustion with coal has been demonstrated in this size scale in Alholmens Kraft CFB for three years now. Alholmens Kraft produces electricity for owners in Finland and in Sweden and also supplies process steam to the nearby UPM-Kymmene paper mill, and for district heating in town of Pietarsaari, Finland. The CFB boiler capacity is maximum 240 MW_e. The Alholmens Kraft CFB boiler is a multi-fuel boiler, whose main fuels are bark, wood residue and peat, with coal as a support and back-up fuel. Bark and wood residue are low calorific, high moisture fuels with varying energy content and having also very heterogeneous physical properties. Peat is also low calorific fuel with high moisture content but with much more stable quality. Fuel consumption utilizing these fuel components is about 20 000 m~3/d (700 000 ft~3/d), which sets huge challenges to the whole logistic chain of fuel procurement. The maximization of CO_2 neutral fuels is favoured, but some higher heating value and always available fuel, coal, is needed to ensure the high plant availability. Fuel mixture is variating all the time and it is optimized to fulfil operational, environmental and economical requirements. This paper describes the experience of using several fuel components in different operational situations during first three years of operation. The paper introduces also experiences of auxiliary system needed for reliable operation in co-combustion. The paper will also introduce the experience of the boiler controllability - changing load or fuel mixture - with this heterogeneous fuel combination. The co-combustion of biomass and coal is a real opportunity also in a large scale when including certain additional features in boiler and auxiliary equipment design. Interest towards co-combustion seems to be continuous - design considerations of a new boiler project of the same size is introduced in this paper.