Co-firing Coal & Biomass

Ecoal, March 2010, Volume 70

The IEA Clean Coal Centre (IEA CCC) recently released a report on "Co-gasification and Indirect Co-firing of Coal and Biomass". In this article, Rohan Fernando, IEA CCC, provides a summary of the report, which looks into the technical issues around co-firing.

Many countries have initiated incentives in recent years to encourage the utilisation of biomass for energy production, as one method of reducing CO₂ emissions from coal-fired power plants is to substitute part of the coal with a renewable fuel. Biomass can be considered to be renewable as, if it is grown in a regenerative manner, its combustion does not produce any net CO₂ emissions. However, there are drawbacks in using biomass for power generation relating to its production, transportation and composition.

Co-firing

Biomass can be co-fired directly in coal-fired boilers, where the biomass and coal are fired in the same boiler, or indirectly, where the combustion or gasification of the biomass occurs in a separate unit.

Direct co-firing is the simplest and the most widely applied technology for co-firing biomass. However, several technical issues arise because all the components in the biomass enter the coal boiler. These issues do not arise with indirect co-firing as the biomass ash and the coal ash are kept separate. However, indirect co-firing is more costly than direct co-firing. Hence, though there have been over a hundred coal-fired power plants which have directly fired biomass, only a handful have incorporated indirect co-firing.These include:

• Enstedværket power plant in Aabenraa, - Denmark
• Amergas power station in - Geertruidenberg, The Netherlands
• Kymijärvi coal-fired power plant in Lahti, - Finland
• Electrabel Ruien power plant in Belgium
• Mälarnergi CHP plant in Västerås City, - Sweden

Co-gasification

Recent years have also witnessed an increased interest in the gasification of coal. Part of the reason for this has been the considerable fluctuations in the price of crude oil and natural gas and concerns about their security of supply, which have focussed attention on whether fuel for the power industry and feedstocks for the chemical industry could be obtained from coal gasification. The need to reduce greenhouse gas emissions has enhanced the prospects of power generation from Integrated Gasification Combined Cycle (IGCC) power plants. Co-gasifying biomass with the coal further reduces CO₂ emissions. However, while there are many dozens of plants which have gasified coal, relatively few have co-gasified coal and biomass.

There are some similarities between the gasification of biomass and coal, including the fact that the temperature required for the complete thermal gasification of biomass is similar to coal. However, there are a number of differences which can be attributed to their different fuel properties:

• The quality of biomass ash, which has a comparatively low melting point and in the molten state is very aggressive.
• Biomass is a more reactive fuel than coal.
• Some biomass fuels have a fibrous characteristic.
• At low gasification temperatures, biomass can produce high tar levels.

The main advantages of co-gasifying coal and biomass are similar to those for co-firing the two fuels. For the biomass fuel, gasifying in a larger coal gasifier achieves higher efficiencies through economies of scale. For the coal plant, partly gasifying biomass enables the plant to obtain credits for the use of a renewable fuel and it may be possible to take advantage of a lower cost feedstock. However, there are several technical considerations such as those regarding fuel delivery, storage and preparation which need to be addressed.

Plants

Moving bed gasifiers can accept solid biomass or waste if it is briquetted or pelletised with the coal. Fluidised bed gasifiers can accept biomass and waste fuels of certain types as either partial or total feedstock.

Dry and fine biomass and waste solids such as sewage sludge solids and wood dust can be co-gasified with coal in dry fed, entrained flow gasifiers. Biomass has been co-gasified at several large-scale coal gasifiers including IGCC plants:

• The Polk IGCC plant in Florida co-gasified eucalyptus and bahia grass between 2001 and 2004.
• A wide range of secondary fuels including sewage sludge, waste wood and chicken litter have been co-gasified at the Buggenum IGCC plant in The Netherlands since 2001.
• The Puertollano IGCC plant in Spain has co-gasified meat and bone meal and more recently olive oil waste.
• The Schwarze Pumpe plant in Germany has co-gasified several types of solid waste between 1995 and 2002.
• Sasol in South Africa have investigated the - feasibility of co-gasifying biomass and coal in their fixed bed coal-to liquids gasifier.
• The US Department of Defense has investigated the possibility of producing synthetic fuels from coal and biomass. One route to achieve this is by coal-to-liquids production utilising coal gasification followed by Fischer-Tropsch synthesis.

Summary

The co-gasification of coal and biomass has been demonstrated in the three main types of gasifiers in several countries. The types of biomass and waste that have been co-gasified have included agricultural crops and wastes, wood and wood waste, sewage sludge and municipal wastes. Some of the projects such as in the Polk, Elcogas and Berrenrath plants have co-gasified small proportions of biomass only for a few days. Though these trials show that co-gasification is technically feasible, they would not identify any long term issues that could arise. The demonstrations at Buggenum and Schwarze Pumpe have co-gasified much larger proportions of biomass for several years.