Fluidized Bed Combustion

Fluidized bed combustion (FBC) is burning of various solid fuels in the fluidized state—a condition where a gas-solid mixture behaves like a free-flowing fluid.

Mass burning (MB) and traveling grate (TG) stokers burn solid fuel in static mode with the fuel resting on a grate, whereas pulverized fuel (PF) firing burns the fuel in suspension mode in transport condition. Between the two extremes of static and transport modes lies the fluidized mode. With the right proportioning of air pressure and the proper sizing of fuel, the air-solid mixture behaves like a fluid. The fluid bed experiences progressively more turbulence, as air velocity is increased. FBC is the combustion in this state—bubbling FBC (BFBC) at the lower end and circulating FBC (CFBC) at the higher end.

FBC development dates back to the 1970s as an answer to the then-emerging pollution limits for emissions, specifically SOx, and NOx. A cleaner and cooler combustion was the only way to meet stricter levels of SOx and NOx, respectively, without the need of costly secondary gas cleaning measures. By limiting the bed temperatures to —850°C (optimum temperature for sulfur-calcium coal reaction) these twin objectives could be met by

• Limestone addition

• Low NOX due to no thermal NOX generation

In the classical three Ts of combustion—time, turbulence, and temperature—the negative effect of cooler combustion was overcome by increasing the other factors. The combustion efficiency was, in fact, raised drastically because of high turbulence in the bed and longer residence time, despite a lower combustion temperature. Bed tubes for bubbling beds and ash recirculation for circulating beds were employed for bed temperature control. This way there emerged two distinct types of FBC.

1. Bubbling fluidized bed combustion boilers with lesser residence time, com­paratively lower turbulence, and reduced combustion efficiency and lesser desulfurization.

2. Circulating fluidized bed combustion boilers with much longer residence time (due to ash recirculation) and higher turbulence and hence higher combustion efficiency.

By the 1990s, BFBC boilers practically ended the reign of the stoker-fired boilers world­wide, at least for prime fuels such as coal and lignite. Very different factors were respon­sible for this dramatic growth of FBC:

• Environmental compliance in the developed markets

• Efficient combustion of poor and ashy coals in the developing markets

These factors dislodged the stokers from their premium position worldwide, confining them to areas such as biofuels because

• BFBC poses no SOx or NOx issues as both S and N are low.

• The fuels are too light or fluffy to be fluidized.

• Consistency of fuel sizing is a problem.

• The fan power is less in grates.

• No efficiency is gained from BFBC.

Likewise, in the industry requiring up to —100 MW captive power, CFBC boilers have edged out PF firing for captive and cogeneration power with their

• Efficiencies comparable to those of PF

• Much better fuel flexibility

• Superior environmental friendliness

• Lower O&M costs

However, for large utility boilers with no multifuel requirement, PF boilers offer a slightly better efficiency of —1% (as the fan power in fluidization exceeds milling power) at practi­cally the same cost. The sharp reduction of deSOx and deNOx helped in keeping the overall costs of PF boiler plants at levels competitive to those of CFBC, despite larger furnaces with low NOX burners increasing the basic cost of the boiler. Also, many newer CFBC designs are available in large sizes, leaving the field only for hot cyclone and compact designs, both of which are not as trouble-free as conventional PF. In most cases large CFBCs are planned for fuels that are difficult to burn in PF, such as pet coke, anthracite, and peat. This is slowly changing as CFBC boiler costs are coming down and their availability is increasing.

This chapter covers both types of boilers, the BFBC and the CFBC, as they share the same principle of fluidization. Within these two main technologies, several variations of designs appeared and also exited in the process of consolidation. The surviving technologies are proprietary technologies under constant development, and the information available tends to be fragmented and limited. If this point is taken into account, what is presented here can certainly provide an adequate and accurate picture of various designs and their progress.

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