Challenge
In order to reduce CO2-emissions the Dutch government has set a target to generate more than 17 per cent of the total electricity production from sustainable sources in 2020. This target, combined with new legislation with respect to waste disposal, is a driving force to increase the production of electricity from biomass and waste streams in Dutch electricity generation. Co-firing of these secondary fuels in a pulverized coal-fired power plant is an attractive and cost effective option. Up till now co-firing experience in Dutch pulverized coal-fired power plants and similar plants abroad is confined to less than 5 per cent of coal replacement, which must be increased by a factor 3. For these higher percentages thermodynamic model calculations are able to reveal possible operational limitations.

Solution
KEMA has investigated the technical constraints on the flue gas- and steam/water cycle in coal-fired power plants if co-firing is increased to 10 per cent and even more of the heat input of the coal. Different kinds of wood with different amounts of water content are considered varying from 10 to 40 per cent. Also the secondary fuels RDF, char, pyrolysis oil, chicken litter and syngas from the gasification of wood are taken into account. This study considers only co-firing of secondary fuel or syngas produced by gasification of secondary fuel.

The thermodynamic calculations are carried out with very detailed models of the power plants Amer 8 and Hemweg 8. To analyze the influence of the location of the feed of the secondary fuels on heat transfer in the boiler and finally on the conversion efficiency, special models for multi-level burners are built and integrated in the models of the two power plants considered.

Advantages
The constraints mentioned can be predicted with detailed thermal dynamical SPENCE® models of the coal-fired power plant. Based on the results of the two power plants considered  the constraints for other Dutch coal-fired power plants can be derived. The constraints as determined may partially be overcome by increasing the capacity of the flue gas fans or considering pre-treatment like drying and/or pre-conversion of the secondary fuel through pyrolysis, gasification or pre-combustion.

Costs and benefits
The survey shows that even the substitution of a relatively high percentage of the heat input of the coal by secondary fuels is feasible from a thermodynamical point of view. Only secondary fuels with a high content of water can cause capacity limitations of flue gas fans and steam temperature controls.

The general conclusions are also valid for the other coal-fired power plants in the Netherlands. However, the limitation of the substitution of coal will be more restrictive for slag and ash quality.

The relative efficiency of the conversion process of secondary fuels on LHV is 39.5 to 45 per cent in the case of co-firing. This depends on the composition of the fuel and in particular on the water content and efficiency of the power plant where the co-firing is realized. Energy consumption for preparing and feeding the secondary fuel is not included in the efficiency figures. In general, it is concluded that co-firing contributes significantly to the reduction of CO2 emissions.

Sponsors
Project was subsidized by NOVEM (EWAB-program) and the Dutch Power Generating Companies’ Technical Service Agreement.