PFBC unit

FIG. 17-31 71 MWe PFBC unit. (From Steam, 40th ed., 29-9, Bahcock [Pg.1576]

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

Particulate Emissions To meet environmental regulations, AFBC boilers, and some PFBC boilers, use a back-end particulate collector, such as a baghouse or an electrostatic precipitator (ESP). Compared to PC units, the ash from FBCs has higher resistivity and is finer Decause the flue-gas path contains cyclones. Both factors result in reduced ESP collection efficiency with AFBC units, but good performance has been achieved with PFBC units, where the SO3 present in the flue gas lowers the ash resistivity. In general, however, bag-houses are the preferred collection devices for both AFBC and PFBC apphcations. 

In the 1970s commercial fluidized-bed combustors were limited to the atmospheric, bubbling-bed system, called the atmospheric fluidized-bed combustor (AFBC). In the late 1970s the circulating fluidized combustor (CFG) was introduced commercially, and in the 1980s the new commercial unit was the pressurized fluidized-bed combustor (PFBC). 

There are two types of FBC unit distinguished by their operating flow characteristics bubbling and circulating. These two types operate at atmospheric pressure, AFBC, or at elevated pressure, PFBC. Pressures for PFBC are in the range 0.6 to 1.6 MPa (90 to 240 psia). Typical superficial fluidizing velocities are tabulated as follows. 

A PFBC boiler is visually similar to an AFBC boiler. The combustor is made of water-wall tubing, which contains the high-temperature environment, but the whole assembly is placed within a pressure vessel. Unlike an AFBC unit, there is no convection pass, as the flue-gas temperature must be maintained at boiler temperature to maximize energy recovery by the expansion turbine. There is an economizer after the turbine for final heat recoveiy. A simplified schematic is presented in Fig. 27-49. An 80-MWe demonstration plant, operating at 1.2 MPa (180 psia), began operation in 1989 with a power producdion intensity of 3 MWe/m (1 MWe/3.5 fU). By 1996, five units of this size had been construcded, and a 320-MWe unit is planned to commence operation in 1998. 

In second-generation PFBC, a topping combustor is used to raise the turbine rotor inlet temperature to state-of-the-art levels. Pulverized coal is fed to a partial-gasifier unit that operates about 870° to 925°C (1,600° to 1,700°F) to produce a low heating value fuel gas and combustible char. The char is burned in the PFBC. The fuel gas, after filtration, is piped back to the gas turbine, along with the PFBC exhaust. 

The capital investment for any FBC plant depends upon several factors, including the cost of capital, size of unit, geographic location, and coal type. EPRI has completed several economic evaluations and projects the following costs, in 1994 US dollars, for plants located in Kenosha, Wisconsin, burning Illinois No. 6 bituminous coal contain-ing 4 percent sulfur 200-MWe circulating AFBC, 1520/kW 350-MWe bubbling PFBC, 1220/kW 350-MWe circulating PFBC, 1040/kW 320-MWe advanced PFBC, 1110/kW. The advanced PFBC has the most potential for cost reduction, and capital investment could be reduced to below 1000/kW. 

Figure 6 Schematics of two alternative routes for treating the flue gases of a PFBC coal boiler, (a) Process based on conventional technologies (b) process employing catalytic filters. Legend for scheme (a) I. PFBC boiler 2. air preheater 3. fabric filter 4. air compressor 5. turbine 6. SO2 wet scrubber 7. economizer 8. postheater 9. DeNOx catalytic unit. Legend for scheme (b) 1. PFBC boiler 2. air preheater 3. SOi dry-scrubber 4. catalytic filter unit 5. turbine 6. air compressor 7. air preheater.

Advanced Hot Gas Cleanup Systems. Advanced hot gas cleanup systems must achieve effective final particulate removal without cooling the gas. Extensive development of the technology is under way for advanced coal gasification and pressurized fluidized-bed combustion (PFBC) applications. However, per unit of coal, hot gas final particulate removal for coal gasification has only one-fifth the gas flow of PFBC. This fact is significant because the gas flow rate determines the size and cost of advanced hot gas particulate removal devices. 

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