A. Hardware

The purposes of solid fuel-burning equipment are to proportion and mix the fuel and air, to initiate and maintain ignition, to volatilize the fuel, to position the flames in areas of useful heat release, and to supply fuel and air at the proper rates and pressures to facilitate each of these functions (Reed, 1983). The specific equipment appropriate for most biomass combustion and energy recovery systems depends on the types, amounts, and characteristics of the biomass fuel; the ultimate energy form desired (heat, steam, electric or cogener­ated power); the relationship of the system to other systems in the plant (independent, integrated); whether recycling or co-combustion is practiced; the disposal methods needed for residues; and environmental factors. The design of efficient, large-scale biomass combustion systems requires detailed analysis of many parameters and hardware components. Among them are the numerical values and variability of moisture, volatile matter content, ash content, composition, and energy content of the biomass fuel; biomass han­dling, drying, and grinding equipment; the furnace design and associated heat transfer requirements and materials of construction; combustion and emissions controls; the amounts, composition, fusion temperature, agglomerating charac­teristics, and disposal of ash; and flue gas compositions and treatment that may be needed to meet emissions limitations.

In conventional biomass combustion equipment, combustion of the solid fuel takes place on horizontal or inclined steel grates or in shallow suspension above the grate. The grate is a stationary, vibrating, reciprocating, or traveling platform, and the fuel is supplied in the batch, semicontinuous, or continuous mode. Many furnace designs have been used such as pile-burning systems (Dutch ovens), fixed — and moving-bed furnaces, multiple hearth furnaces, stationary and rotating horizontal and inclined kilns, overfeed, underfeed, and spreader stokers, and pulverized fuel burners. The principal difference between conventional solid fuel-burning equipment and liquid-fuel — or gas-burning equipment is that furnaces for solid fuels must allow for additional fuel resi­dence time for the slower burning chars to combust after all gases and volatile liquids have been driven off. One of the principal methods of expediting this process is by burning smaller fuel particles. Advanced combustion designs such as fluid-bed and cyclonic combustors further improve biomass combustion and are discussed in Part E.

The differences in the furnaces suitable for biomass combustion reside mainly in the design of the combustion chambers, the operating temperatures, and the heat transfer mechanisms. Refractory-lined furnaces operating at about 1000°C were standard until the introduction a few years ago of water-wall incinerators. Ash buildup can occur rapidly in refractory-lined furnaces, and excess air must be introduced to limit the wall temperature. The water-wall incinerator has combustion chamber walls containing banks of tubes through which water is circulated, thereby reducing the amount of cooling air needed. Heat is transferred directly to the tubes to produce steam. There are numerous configurations, but the basic concept has not changed for many years, apart from operating conditions and materials improvements to improve heat transfer and thermal efficiencies. Considerable advancements have been made, how­ever, in ancillary hardware designs to control the combustion process and reduce emissions, to remove ash, and to remove flyash and emissions from the stack gas. Improvements have also been made in the methods used to recover sensible heat from the stack gases and heat from the condensate and boiler blowdown. Other overall efficiency improvements have resulted from advances in predrying hardware for moisture reduction in the incoming bio­mass fuel.