The direct firing of biomass involves burning the fuel in an excess of air inside a furnace to generate heat. Aside from heat the primary products of the combustion reaction are carbon dioxide and a small quantity of ash. The heat is absorbed by a boiler placed above the main furnace chamber and water in tubes within the boiler is heated and eventually boiled, producing steam which is used to drive a steam turbine.
The simplest type of direct-firing system has a fixed grate onto which the fuel is piled and burned in air which enters the furnace chamber from beneath the grate. This type of direct-firing system, called a pile burner, can burn wet and dirty fuel but its overall efficiency is only around 20% at the best. The fixed grate makes it impossible to remove ash except when the furnace is shut down, so this type of plant cannot be operated continuously.
An improvement over the pile burner is the stoker combustor which has a moving grate or stoker. The moving grate allows ash to be removed continuously and fuel can be spread more evenly than in a pile burner, encouraging more efficient combustion. Air still enters the furnace from beneath the grate and this airflow cools the grate. Thus the airflow determines that maximum temperature at which the grate and hence the furnace can operate and this in turn determines the maximum moisture content of the wood that can be burnt, since the dampest wood will require the highest
Figure 15.1 Layout of a direct-fired biomass combustion system
Figure 15.1 Layout of a direct-fired biomass combustion system temperature if spontaneous combustion is to be maintained. There are a number of refinements to the stoker combustor such as an inclined and a water-cooled grate. Even so, maximum overall efficiency is only 25%.
Most modern coal-fired power plants burn finely ground coal which is fed into the power plant furnace through a burner and then ignites in midair inside the furnace chamber, a process called suspended combustion. It is possible to burn biomass in this way but particle size must be carefully controlled and moisture content of the fuel should be below 15%. Suspended combustion, while it can provide a higher efficiency, is not widely used in dedicated biomass power plants. However it does form the basis for co-firing which is discussed at greater length below.
As an alternative to the traditional pile burner of stoker combustor, many new biomass-fired power plants utilise a fluidised-bed furnace. The fluidised bed contains a layer of a finely sized refractory material such as sand which is agitated by passing air through it under pressure so that it becomes entrained and behaves much like a fluid. When the bed becomes hot enough, fuel mixed with the refractory bed will burn in the same way as in a conventional furnace. Fuel content within the bed in usually maintained at around 5%. Fluidised beds can burn a wide range of biomass fuels with moisture content as high as 55%. However overall efficiency is again only 25% at the best.
Direct-fired biomass power plants typically have a generating capacity of around 25-50 MW. This small size, combined with the relatively low-combustion temperature in the furnace (biomass is more reactive than coal and so tends to burn at a lower temperature) are the two main reasons for these plants' low efficiencies compared to coal plants where overall efficiencies above 40% are now common in new facilities.
Improvements are possible. Increasing the size of the typical plant to 100-200 MW will allow larger, more efficient, steam turbines to be used. New small steam turbines which incorporate advanced design features currently found only in large coal-plant turbines will also improve efficiency. Adding the ability to dry the biomass fuel prior to combustion will result in a significant increase in performance. With these changes, direct-fired biomass plants should be able to achieve 34% efficiency by the end of the first decade of this century.
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