More efficient conversion of biomass into electricity can be achieved quite simply and on a relatively large scale in another way, by the use of co-firing. Co-firing involves burning a proportion of biomass instead of coal in a coal-fired power plant. Since most coal stations operate at a much higher efficiencies than traditional direct-fired biomass plants, co-firing can take advantage of this to achieve 35-40% conversion efficiency, possibly higher in a modern high-performance coal-fired facility.

Up to 2% of biomass can be added to the coal in a coal-fired power station without any modification to the plant. The fuel is simply mixed with the coal, prepared with the coal in the plant fuel processing system and then burned in the furnace.

Above 2%, modifications are necessary. In a pulverised-coal plant these will normally include a dedicated biomass fuel processing system and changes to one or more furnace burners so that they can burn the biomass once it has been reduced to fine particles. With these changes, which are still very cheap compared to the cost of a new biomass power plant, 5-15% biomass can be burned in the furnace alongside the coal. Tests have suggested that in fact up to 40% biomass co-firing is possible but such high levels are likely to be more difficult to manage, so 15% probably represents the optimum.

Most forms of biomass, including biomass wastes and energy crops, are suitable for co-firing. Coal plants have typical unit sizes up to 600 MW, where 15% co-firing would provide 90 MW of biomass capacity. This will, in most cases, be considered 90 MW of green generating capacity. Further, biomass has a much lower sulphur content than most coals so co-firing can also reduce sulphur emissions.

Many biomass fuels, but particularly straw, have a high alkali content and this can cause problems of fouling in coal-plant boilers. Additionally, while the ash from coal-fired power plants is often used in the building industry, when biomass ash is added, the resulting residue may not have the required permit for such use. Both problems should be simply soluble.

As an alternative it is possible to combine a biomass gasifier (see below) with a coal-fired power plant. Biomass is first converted into a combustible gas and this gas is burned in the coal-plant furnace alongside the normal coal fuel. This avoids both ash and fouling problems, but at a significantly higher cost.

From an environmental point of view the primary criticism of co-firing is that the technique is so simple and cheap that it could become the principal method of achieving green energy targets where these become mandatory. This would then divert investment from other renewable technologies, damaging their development. There is no evidence yet of this happening.

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