Energy-Tech Magazine

In the CFB process, there is no distinct furnace bed, but the solids concentration varies from roughly 40 lb/ft³ (640 kg/m³) in the bottom of the furnace to 0.6 lb/ft³ (10 kg/m³) or less, in the upper furnace – about 2/3 of the solids are within the first 5′ of the furnace and 1/3 in the upper furnace. There is a considerable ‘thermal flywheel,’ consisting of the solids circulating from the bed to the upper furnace, cyclone and via a sealing device, commonly called a loopseal or siphon, back to bed. This flywheel of solids evens out any temperature differences. At full load, the bed and cyclone temperature difference is typically 50°F (30°C) or less. Further, the solids in the upper furnace transfer heat to the boiler walls and any in-furnace heat surfaces (such as wingwall superheaters). In cases where limestone is used, the limestone particles also react in the upper furnace with SO₂. When efficient hot cyclone designs are used, more than 99.9 percent of the solids are separated.

The fluidizing

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grid area of a CFB furnace is less than that of a BFB. Thus, even in a large scale biomass application, fuel feeding is easy and high combustion efficiency can be maintained. As shown in Figure 4, a wide range of fuel heating values can be accommodated in a CFB boiler. Additionally, as compared to a BFB, the fuel particle size distribution is less important in a CFB and large fractions of fine fuels, such as saw dust, can be readily combusted.

Sample projects
551,000 lb/hr CFB Boiler for Klabin S.A., Brazil
Figure 5 shows a 60 MW CFB boiler island. This unit is located in Monte Alegre, Brazil, and burns eucalyptus and pine wood and bark. TDF can be used, providing up to 30 percent heat input, and backup fuel is heavy fuel oil, providing up to 85 percent heat input. The steam parameters are 551,000 lb/hr/1,450 psig/932°F (69 kg/s/100 barg/500°C). The unit has been in commercial operation for more than a year.

230,000 lb/hr BFB for Pratt Industries, Georgia
Pratt Industries is America’s 6th largest paper and packaging company. One of its plants is located in Conyers, Ga. The steaming capacity of the boiler installed at this facility is 230,000 lb/hr at 930 psig and 860°F (29 kg/s/64 barg/460°C). The photo of the plant in Figure 6 shows it in the early commissioning phase in August 2009.

The Pratt design fuel consists of:

• Construction wood waste
• Paper sludge
• Paper rejects
• Carpet waste
• Tire Derived Fuel (TDF)
• Plastic chips

The guarantee mixture is shown in Table 1. It is based on 68 percent-wt. construction wood waste, 28 percent-wt. paper sludge and 4 percent-wt. rejects. The specified heating value range is from 4,983 to 6,601 Btu/lb as received (12-15 MJ/kg HHV).

The emission guarantees are listed in Table 2. The emission limits are aggressive and thus, the flue gas cleaning train contains several pieces of equipment, including a circulating dry scrubber integrated with a baghouse, reheat of flue gas with steam and a low temperature tail-end SCR.

The NO_X emission guarantee is 0.029 lb/mmBtu, requiring an SCR. Some of the design fuels contain heavy metals such as lead. Thus, a high dust SCR was out of question and a low temperature tail-end SCR was selected. The other very aggressive emission limit is HCl, requiring 97 percent capture. To minimize sorbent consumption and make sure that the guarantee emission can be achieved at all conditions, a proprietary circulating dry scrubber reactor was selected.