Biomass, a renewable fuel source for generating energy, is available in large quantities in the U.S. Typical biomass consists of wood chips, construction and demolition wood, bark, residual logging debris, saw dust, paper rejects, and paper and sewage sludge.
Depending on the heating value of the fuel, emission requirements and boiler size, combustion technologies ranging from stokers to fluidized beds and pulverized coal boilers can be used. Stoker boilers are sometimes used in small scale combustion of biomass, provided emission limits are relaxed and combustion efficiency of the fuel is not very important. In pulverized coal-fired units, biomass co-combustion is usually limited to no more than 20 percent of the fuel heat input. Where this limitation is compatible with the available fuel supply, these boilers can effectively use biomass as a fuel. Since these boilers are able to burn a wide variety of biomass fuels while achieving high combustion efficiency and low emissions, fluidized bed boilers are an attractive option. Experience has shown that bubbling fluidized bed
Fluidized bed boilers and reactors
Figures 1–3 illustrate common designs of BFB and CFB boilers used for biofuels. Figure 1 shows a BFB design developed for challenging fuels, such as sludges and construction or demolition wood.
The design and steam outlet conditions are selected to enhance superheater life and unit availability. The finishing superheater is located in the third pass, after an empty, waterwall-cooled, radiative second pass. The flue gas enters the third pass and is further cooled by two SH banks before passing over the final superheater. With this configuration, the fouling and corrosion characteristics of the cooled flue gases are greatly reduced and a satisfactory superheater life can be achieved, even with these challenging fuels. Because of the deliberate cooling of the flue gas before entering the finishing SH bank, the required superheating and membrane wall surface is relatively large.
Figure 2 illustrates a design for clean biofuels such as bark, wood chips and forestry residues. With clean biofuels, the finishing superheater can be located in a higher gas temperature zone, resulting in a more effective use of heating surfaces.
A special refractory-lined BFB reactor was commercially proven to be suitable for incinerating very high moisture (typically 70-80 percent) biomass fuels. Sewage sludge and ethanol by-products (distiller’s wet grain and syrup) are two common examples of high moisture biomass fuels. Natural gas or other suitable fuels might be needed to stabilize the combustion. The BFB reactor can be coupled with a waste-heat boiler, or at a minimum a combustion air preheater, for heat recovery.
For large scale power production from biofuels (above 50 MW), the CFB boiler has proven to be an attractive option. In this size class, the fuel flexibility inherent to CFB boiler design is often valuable for the customer. For the CFB boiler shown in Figure 3, combustion reactions continue through the full height of the combustor and into the cyclone, where oxygen in the flue gas mixes with CO and VOC, further reducing emissions. Selective Non-Catalytic Reduction (SNCR) for NOX emission control can be achieved in the cyclone, where ammonia injection at the inlet of the cyclone is readily mixed and reacts with NOX without the use of a catalyst.
Figure 4 illustrates different fuels that can be effectively and efficiently combusted in the various fluidized bed boilers and reactors described above. As noted, with corrosive and fouling fuels, special designs have to be used to ensure reliable steam generation and acceptable boiler life. The graph is a simplification and is intended to be a general indication of the heating values and degree of fouling and corrosion potential of various fossil, biomass and other opportunity fuels that are burned in fluidized bed boilers and combustors. Each fuel needs to be carefully evaluated, since there is a wide variation of quality in each fuel category. Most plants burn several different types of fuels. Co-firing a small percentage of difficult fuels with easy fuels is often a safe approach, even if the boiler design does not allow burning the difficult fuel alone.