Biomass conversion plants tend to be relatively small, in the order of 10–30 MWe, due to limitations of biomass availability and transport logistics. Consequently, investments are higher and efficiencies lower than for large fossil fuel power plants. These disadvantages can be overcome by integrating biomass conversion with existing large-scale power plants. Examples in the Netherlands are: co-firing of biomass in a conventional coal-fired power station in Nijmegen; co-firing of gas from a biomass gasification plant in a coal-fired power plant at Geertruidenberg; and integration of a municipal solid waste incinerator with a natural gas-fired combined cycle power station at Moerdijk.
The technique of parallel combustion has yet to be demonstrated in the Netherlands. However, it has been demonstrated at existing coal-fired power plants in Denmark. This article summarises the results of a feasibility study of parallel combustion of biomass at an existing 600 MWe coal-fired power plant carried out by Stork Engineering & Consultancy.
Parallel combustion involves burning biomass in a separate boiler to raise steam, which is then integrated into the steam system of a large-scale coal-fired power plant. As biomass usually has a low ash-softening point, the steam produced is of low quality. Higher steam temperatures would result in ash-softening and fouling of the superheaters. In parallel combustion, the boiler of a conventionally-fired power plant improves the quality of the steam by heating it to higher temperatures, giving higher biomass electricity generation efficiencies. Utilisation of other equipment at the existing power plant also contributes to a high overall efficiency.
The advantage of separate biomass combustion is that there is no risk of corrosion, fouling or ash contamination of the expensive coal-fired boilers. Dedicated boilers are constructed specifically for biomass such as straw, grass and wood chips. Parallel combustion offers the advantages of reducing the fuel consumption of the existing power plant and giving a high efficiency of biomass conversion, with low investment costs.
The following conversion technologies were compared:
A conventional coal-fired power plant of 600 MWe with a net electrical efficiency of 41.7% was used as the reference. Integration of biomass conversion affected neither net power production nor coal conversion efficiency. All the effects of biomass integration were, therefore, attributed to the biomass combustion.
The study found that the availability and reliability of combustion methods were higher than for gasification, due to combustion being a more established technology.
Fuel flexibility is an important advantage of parallel combustion. The separate boiler is dedicated to combust a variety of biomass. Parallel combustion of biomass uses well-established technology that has been tried and tested extensively on a wide variety of fuels.
Co-firing and parallel gasification methods had the disadvantage of possible non-scheduled maintenance, and fouling and slagging problems, which decreased the availability of the complete power plant. Parallel combustion intervenes only in the steam cycle of the existing plant, so the availability and reliability of the existing plant are unaffected by any biomass boiler problems.
A disadvantage of parallel combustion, where both the fuels and flue gases remain separated, is that emission control is more difficult. Both parallel gasification and co-firing methods benefit from flue gas integration with the coal-fired power plant, reducing treatment costs.
Emission level requirements depend on governmental restrictions. Emissions from clean wood combustion present no problems; those from contaminated wood can be controlled by state-of-the-art equipment.
Table 2 compares the investment, annual and electricity costs of several biomass combustion techniques. All the plants produce 20 MW net power and have the necessary flue gas treatment to incinerate a variety of biomass forms, including contaminated wood. The plants with parallel combustion, parallel gasification and direct co-firing are combined with coal-based power plants. The cost of coal is 50 EURO/tonne. The biomass is given at no cost, ie 0 EURO/tonne. Figure 1 (page 11) shows the sensitivity of the electricity cost to the price paid for the biomass fuel.
This evaluation highlighted the following advantages of parallel combustion:
Usually, the preferred option for integrating biomass conversion with conventional power plants would be direct co-firing, due to lower costs. The investment (18 million EURO) is significantly lower than for parallel combustion (27 million EURO). If, however, the risks of slagging, fouling and corrosion cannot be eliminated, parallel combustion is preferable.
Parallel combustion can be used with existing power plants where integration with the steam cycle is possible. Integration with existing CHP plants is more complicated because of the more complex steam cycle.
For more information contact the CADDET Dutch National Team in Sittard.
The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.
The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.
Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.