FBC Process: Pros and Cons

Characteristics of FBC Boiler

• Combustibles form only —3% of bed material, and the remainder consists of fuel ash, inerts, and limestone (if used).

• In high-ash applications, usually with ash >15% in the fuel, no inert material addi­tion may be required to stabilize the bed, as the fuel ash is sufficient to replenish the bed material.

• Limestone addition occurs only if desulfurization is required, and if so, the bed has to be operated at exactly the controlled temperature of —850°C (1560°F).

• With fuels such as lignite or biomass, where fuel ash contains low-melting alkaline compounds and ash-softening temperatures are likely to be lower, the bed has to be at a lower temperature such as 800°C (1472°F) to avoid agglomeration. Desulfur — ization efficiency decreases in this case.

• For low-volatile fuels such as anthracite and high-ash coals, the bed has to be oper­ated at higher temperature of up to 900°C (—1740°F) to achieve better burnout of such fuels. The exit temperature from the combustor should be —100°C lower than the fusion temperature of fuel ash to optimize the boiler size and minimize the risk of logging.

• With a bed of solids at 800-980°C, there is a huge thermal flywheel in the form of a hot bed to provide an instant ignition and combustion to any type of fuel—even liquid and gas.

• Intense turbulence of the bed and good residence time further aid in the burning of difficult fuels such as anthracite, coke breeze, petroleum coke, and washery rejects efficiently.

• Carbon burn-up efficiency of 90-99% is achieved, depending on the fuel character­istics and ash recirculation.

• Desulfurization of 85-95% is normally achieved, depending on the Ca/S ratio and other practical limits.

• With low temperature of bed and proper air staging, there is no generation of thermal NOx. Even with high N2 fuels, NOx can be limited to 700 mg/N m3, and the normal range is <400 mg/N m3 in BFBC and half of it in CFBC.

• As the bed is always maintained lower than the ash fusion temperature, slagging and fouling of tubes does not take place.

Advantages of FBC Boilers

• Capability to burn almost any fuel

• Excellent multifuel flexibility

• High combustion efficiency

• In situ and very convenient desulfurization

• Very low NOx generation

• No slagging and fouling of tubes

• Good to excellent load response

Limitations of FBC Boilers

• For most biofuels, stoker firing is better. Biofuels have low S and the burning tem­peratures are low due to high moisture in the fuel. Thus, they are nonpolluting, and SOx and NOx are always under control.

• High fan power for fluidization reduces net output per unit fuel by 1% compared to PF if deNOx and deSOx units are not large enough or are absent.

• Tube and refractory erosion issues are not fully resolved.

• Stepless load variation is not possible with BFBC without sacrificing efficiency.

• Single unit sizes of 1000 MWe and above are proven in PF, whereas in CFBC, units >300 MWe are still under initial operation.

The marginal shortfall in efficiency and power consumption together with refractory and tube wear problems of CFBC boilers stands in sharp contrast to the improved milling

Plant and cheaper deNOX and deSOX plants in the contemporary PF boilers. Fuel flexibility of CFBC boilers is not perceived to be of great use by the utilities, at least for now. These factors have seriously impaired rapid growth of CFBC, and the expectations of gaining good ground in utility markets have also been partly belied.

FBC boilers exhibit a steaming range of 10-400 tph for the industry and power applica­tions, cannibalizing stoker, and PF territory, on the merits of

• Better efficiency

• Compliance with new environmental norms

• Burning difficult and/or new fuels

• Multifuel capability

Large CFBC boilers, up to —450 MWe, are going on stream for niche or difficult-to-burn fuels or where high S in coal demands an FGD plant in PF, making it more expensive and power-consuming. But for single-prime fuel for utility application, PF firing is still the choice in sizes >500 MWe.

For —100 to 150 tph and above, the CFBC boiler is economical, whereas for <100-150 tph, the choice is a BFBC boiler, depending on fuel and pollution norms. Sometimes certain special environmental strictness may demand CFBs even for lower sizes. Likewise, BFBC boilers of >150 tph are sometimes built for special applications such as paper mill sludge burning.

Thermal Efficiency: Conventional versus FBC Boilers

Chapter 1 deals with efficiency calculations and performance testing of boilers. In contrast to conventional boilers, the FBC boilers must handle differences in arriving at the correct thermal efficiency, mainly arising from desulfurization, sensible heat loss due to inerts, higher fan power, and higher radiation with cyclones.

• Desulfurization. The calcination and sulfation reactions in the bed, which are endo — thermic and exothermic, respectively, alter the heat marginally. With a Ca/S ratio >2, there is a net loss, whereas at <2, there is a net gain in heat.

• Sensible heat loss. With coals having high ash and employing desulfurization, the bed ash discharge at —850°C can represent loss as high as 5%. At the same time, even with high-ash coals of sub-bituminous variety, which break down and pro­duce mostly fly ash, if there is no desulfurization, the sensible heat loss may not be higher. It is therefore advisable to evaluate the sensible heat loss separately and not make it part of unaccountable loss.

• Fan credits. Forced draft (FD)/primary air (PA) fans consume a lot of power in fluidizing the combustion air. The churning of air in the fan casing to produce such a high pressure heats the air, the power for which is provided by the fan. Suitable credit should be taken by correcting the fan inlet air temperature in calculations.

• Cyclone radiation loss. Radiation loss taken from the standard American Boiler Man­ufacturers Association (ABMA) chart does not account for the losses of cyclones. These are large bodies, and the refractory lining from inside may not be able to cool them to acceptable temperatures such as 50°C, particularly with hot cyclones. Suitable correction should be built into the calculations.

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