Although decreasing, the fuel mix in thermal power stations in Europe is still dominated by solid fuels, contributing about 55% of the total energy consumed in 1997. As an example in Germany in 1998 520, 000 million kWh of electricity was generated in 982 power plants. The share of coal was 52%, with 27% produced from hard coal and 25% from brown coal. For power production about 45.8 million tons of hard coal, equivalent to 1363PJ, and 151 million tons of brown coal, 1345PJ, were fired in German power plants. These numbers appear to have stabilised at this level over the past few years, after changes following the German reunification in 1990.
The co-combustion of biomass, wastes or other residual matter in existing coal-fuelled power plants renders a range of advantages:
- The high capacity installed in existing power stations offers a great capacity regarding biomass or waste utilisation, even with small shares of the additional fuel. It involves, however, that the site has to be tested for suitability.
- The levels of efficiency of power production in industrial-scale combustion chambers are high.
- In case of non-availability of the biomass, the output can be balanced by coal.
- Previous studies have shown that there could be an economic advantage because of lower investment costs. For instance, the new construction of a decentralised biomass combustion plant requires investments up to 400 to 750 euros per kW of installed thermal capacity. In the case of retrofitting an existing coal-fuelled power plant for co-combustion with biomass, the additional investments are estimated to only 75 and 150 euros per kW of thermal capacity, the major part of the costs being required for fuel preparation.
The quantities of supplemental fuels that could be co-fired in e.g. German power plants can be calculated on the basis of primary energy used. If 10% of the 2708PJ are replaced by a fuel with a lower heating value of 10MJ/kg, about 27 million metric tons could be co-fired in power stations.
Though the potential for co-firing in the stoker and fluidised bed firings should not be neglected, the focus of this paper will be on pulverised fuel systems. However, most of the findings can be transferred to the other firing systems.
Co-combustion of limited shares of supplemental fuel is technically feasible, even though detailed questions, like the changes on the composition of the residues and operational problems (e.g. slagging, fouling, corrosion), need further clarification. Sewage sludge co-firing in power stations can be considered as a functioning technique. Still, possibly higher emissions of volatile trace metals, e.g. mercury, have to be examined. There is a rather large base of knowledge in co-combustion of bio-fuels (straw, wood) already established through the last years. However, the potential of the large co-combustion capacity within the European power industry has yet to be recognised. For this, fuel qualities for the use in power plants have to be defined and fuel markets need to be established, while careful cost/benefit considerations are required under new boundary conditions as a consequence of the liberated electricity marked.