Parallel combustion of biomass
by W van Zanten, Novem, the Netherlands

Parallel combustion is a process that integrates the conversion of biomass into energy with large-scale power production plants, thus reducing investment and increasing efficiency. Specifically, it involves burning biomass in a separate boiler to produce steam, which is then integrated into the steam system of a large-scale, coal-fired power plant. In this article, the process is compared with rival technologies.

Introduction

Biomass conversion plants tend to be relatively small, in the order of 1030 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.

The process

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.

Technical evaluation

The following conversion technologies were compared:

  • parallel combustion of biomass and integration with a coal-fired boiler;
  • direct co-firing of biomass in a coal-fired boiler;
  • gasification of biomass and gas combustion in a coal-fired boiler (ie parallel gasification);
  • stand-alone combustion of biomass;
  • stand-alone gasification of biomass.

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.

Table 1: Relative efficiencies of biomass conversion

Conversion Technology

Electircal efficiency of biomass conversion

Parallel combustion

40%

Parallel gasification

36%

Direct co-firing

37.5%

Stand-alone gasification

38%

Stand-alone combustion

26%

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.

Economics

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.

Table 2: Investment, annual and electricity costs for a 20Mwe biomass plant (zero biomas cost)

Installation type

Specific investment (EURO/kWe)

Total (million EURO)

Annual Costs (million EURO/year)

Electricity Costs (EURO/kWh)

Parallel combustion

1,360

27

1.8

0.030

Parallel gasification

1,270

25

1.7

0.029

Direct co-firing

680

14

0.45

0.021

Stand-alone combustion (FBC1)

1,890

38

5.8

0.039

Stand-alone gasification(IGC C2)

2,950

59

8.7

0.058

1FBC = Fluidised Bed Combustion 2IGCC = Integrated Gasification Combined Cycle

Conclusions

This evaluation highlighted the following advantages of parallel combustion:

  • high electrical efficiency (higher than for direct co-firing and parallel gasification);
  • low investment cost (lower than for stand-alone technologies and equal to parallel gasification, although higher than for direct co-firing);
  • fuel flexibility (low temperatures in a dedicated boiler);
  • standard technology (combustion is a widely applied technology);
  • minimal operating risk to existing plant.

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.

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