Much 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 in place of some of the coal in a coal-fired power plant. Since most coal stations operate at much higher efficiencies than traditional direct-fired biomass plants, co-firing can take advantage of this to achieve conversion efficiency of 40% or more in a modern high-performance coal-fired facility.

There is another form of co-firing in which a predominantly biomass combustion plant uses a fossil fuel, normally natural gas, to both stabilize and supplement the biomass fuel. This technique will normally be used in a dedicated biomass plant to increase performance and flexibility.

Co-firing of the first type is attractive to coal plant operators because it allows them to burn biomass and therefore reduce their net carbon dioxide emissions with very little plant modification. Biomass co-firing can also reduce sul- fur emissions because biomass contains virtually no sulfur. Since coal-fired plants can burn large quantities of biomass this also offers a means of establish- ing the biomass infrastructure needed to enable biomass power generation to develop into a large-scale industry.

The most efficient and common type of coal-fired power plant in operation is the pulverized coal (PC) plant that burns coal that has been ground to a fine powder. Plants of this design can burn up to 10% biomass with little modification to their plant. Biomass is simply mixed with the coal before it is delivered to the coal mills where the mixture is ground prior to injection into the combustion chamber. For a 1000 MW power plant this would be equivalent to a 100 MW biomass plant, but with much higher efficiency than a dedicated plant.

Simple co-firing of this type is limited by the type of biomass fuel that can be used and by the proportion of co-firing possible. To overcome both these limits another approach is to have a dedicated biomass fuel delivery line. Fuel from this line can either be mixed with the powdered coal before combustion or delivered to dedicated biomass burners in the furnace. The latter are usually located lower down the combustion chamber allowing a longer transit time to completely burn the biomass fuel. This will allow a plant to burn up to 15% biomass by heat content. Higher proportions are possible but generally require greater plant adaptation and consequently more expense.

Co-firing has become common in the United Kingdom where most plants burn 5–10% biomass and is becoming so in the United States with plants generally using 10% biomass. At this level, boiler efficiency can fall by up to 2%. Such plants will typically burn wood wastes and some will use wood pellets.

An alternative to conventional co-firing involves gasification of the biomass in a dedicated biomass gasifier attached to the coal plant. The combustible gas is then burned in the coal-fired furnace. This approach is more expensive than the simple co-mixing of fuels but it avoids some of the problems that can be associated with conventional co-firing and allows a greater proportion of biomass to be burned.

A third approach is called parallel co-firing. This involves having a separate biomass furnace, with the hot gases generated by combustion of biomass being mixed with the hot coal-flue gases. In a variation of this approach, the biomass has its own steam-raising boiler too with the steam flows blended before entering the steam turbine. Again this is more costly than conventional co-firing.

Conventional co-firing remains the favored approach but it does have its problems. Some wastes such as sawdust can block fuel feed systems and need to be avoided. Other biomass fuels such as grasses have a high alkali content that can cause problems in coal-fired boilers. Biomass is also more volatile than coal when burned so much of the combustion takes place higher in the combustion chamber and more overfire air may be necessary to ensure complete combustion. Additionally, the ash from a co-fired boiler has a different composition to that from a plant that burns only coal. Coal plant ash is often used in various ways by the building industry but the reuse of the ash from a plant that burns biomass has led to regulatory difficulties in the past.

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