Circulating fluid bed and moving bed technologies, which utilise a dry sorbent to reduce SO2 emissions in a flue gas stream in a dedicated reaction chamber are categorised as dry scrubbers.
In the circulating fluid bed (CFB) dry scrubber process, hydrated lime is injected directly in the CFB reactor. Water is also injected into the bed to obtain an operation close to the adiabatic saturation temperature. The process achieves SO2 removal efficiency of 93-97% at a Ca/S molar ratio of 1.2-1.5. Flue gas enters the CFB reactor at the bottom, then flows vertically upwards through a venturi section and enters an upper cylindrical vessel. The height of the vessel is designed to accommodate the mass of bed-material required to achieve the desired residence time of about three seconds. All external inputs of recirculating material, fresh sorbent and gas humidifying water are introduced to the gas on the diverging wall of the ventur. The process is easy to maintain and operate because it does not require high-maintenance mechanical equipment such as abrasion resistant slurry pumps, water atomisers or sludge dewatering devices. The process can achieve >95% SO2 removal efficiency.
In the moving bed dry scrubber, a dry absorbent made of coal ash and lime is injected into the absorber. There is currently one plant using this technology and achieving 90% SO2 removal efficiency.
The first advanced CFB dry scrubbing process for semi-dry FGD with slaked lime slurry feed to the fluid bed has been operated commercially with good results. It is reported that the process can achieve high SO2 removal efficiency at a substantially reduced lime cost compared with scrubbing by conventional CFB dry scrubbers or spray dry scrubbers (Graf and others, 1995).
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