Fluidized Bed Technologies for Biomass Combustion

Biomass, a renewable fuel source for generating energy, is available in large quantities in the USA. Typical biomass consists of wood chips, construction and demolition wood, bark, residual logging debris, saw dust, paper rejects, and paper and sewage sludge. Composition and moisture content of biomass vary greatly and affect its heating value. There are several combustion technologies available to generate power from biomass. Fluidized bed boilers are preferred, because of their ability to burn a wide variety of biomass fuels while achieving high combustion efficiency and low emissions. This paper discusses basic design and operation features of bubbling (BFB) and circulating fluidized bed (CFB) boilers, both offering high fuel flexibility. In fluidized bed combustion, reactive biomass fuels are almost completely burned out because of close contact between the hot bed material and the fuel. In advanced BFB and CFB boilers, an open bottom design is used for ash and coarse material removal through the fluidizing air distribution system. This allows combustion of fuels containing large inert particles, such as rocks and metal pieces. If limestone is added to the bed, SO₂ emissions are reduced. By using ammonia or urea in high temperature areas, NOx emissions are reduced. In order to achieve very low emissions, back-end flue gas treatment for SO₂, NOx, HCl, HF, and Hg is required. To treat flue gases, several technologies can be used — such as activated carbon and sodium bicarbonate or Trona injection, Turbosorp® circulating dry scrubber, and SCR. Normally the preferred particulate matter cleaning device is a baghouse since the filter cake allows further reactions between pollutants and sorbents. Different fluidized bed designs are shown and recommended for various biomass fuels. This paper describes design, fuels, and emissions for an advanced BFB boiler producing steam at a rate of 230,000 lb/hr/930 psig/860°F (29.0 kg/s/64 barg/460°C).