How a PFBC System Works

How a PFBC System Works 2018-06-06T21:14:34+00:00

A. In pressurized fluidized bed combustion, jets of air support a bed of fuel, mixed with sorbent (dolomite or limestone), as it burns. Water flowing through the tube bundle that is immersed in the hot, turbulent bed turns to steam, which drives the steam turbine. Hot flue gases are cleaned, then routed to the gas turbine. The steam turbine supplies about 80 percent of the system’s electrical output, while the gas turbine provides the remaining 20 percent.

B. The power plant operator can use either dry coal or slurry. With dry fuel, high-pressure air moves crushed coal and sorbent into the combuster. With slurry, a high-pressure pump supplies the combuster with wet coal and sorbent. Calcium in the sorbent captures sulfur released by oxidation, minimizing SO formation.

C. Combustion air enters the low-pressure compressor, passes through the intercooler, then flows to the high-pressure compressor, which sends it into the pressure vessel. The cyclone ash cooler preheats the air before it fluidizes the bed.

D. The fuel in the fluidized bed travels at about 3 ft/sec. Keeping the oxidation temperature at 1562°F ensures high carbon conversion and optimal sulfur capture.

E. Exhaust gas and suspended ash leave the bed through the freeboard. Multi-stream two-stage cyclones remove ash particles before the gas enters the gas turbine, which drives the generator and compressors.

F. Gas-turbine exhaust travels through the economizer and cools to about 285°F. A baghouse filter catches residual fine dust before the exhaust goes up the stack.

G. This design separates fly ash at the bag house and withdraws bed ash and cyclone ash through a lock-hopper system in the bed bottom.

H. Preheated by the economizer, feedwater enters the pressure vessel, is further heated as it passes through the combuster walls, and then travels through the tube bundle, which includes the evaporator and superheaters. The resulting steam is sent to a conventional steam turbine.

The gas cycle
The P-200 combined-cycle generator is based on a specially designed GT35P turbine. More than 160 reliable GT35s have gone into service since they were introduced in the 1950s. The twin-spool design incorporates an intercooler, an intercept valve, start-up equipment, and instrumentation, all designed to withstand erosion, corrosion, and fouling from coal exhaust gas. The unique profile of the coated turbine blades is optimized for PFBC applications.

Operating at loads of 40 to 100 percent, the low-pressure side of the turbine is equipped with a variable-inlet guide vane to control the low-pressure shaft’s speed, enabling air flow regulation and maintaining high part-load efficiency. An intercooler in the duct between the low-and high-pressure compressors keeps the air entering the pressure vessel below 600°F. Integrating the cross-flow intercooler with the feedwater pre-heating system and cooling it with condensate enables the plant to operate at maximum output without compromising efficiency.

During start-up, the gas turbine generator acts as a variable-speed motor, driving the high-pressure compressor and supplying air to pressurize the pressure vessel and turbine.

The steam cycle
The steam cycle is the workhorse of the PFBC plant, producing roughly 80 percent of the system’s power. An economizer recovers residual energy from turbine outlet gasses, operating like the heat-recovery steam generator in a combined-cycle gas turbine plant. Rather than generating steam, the economizer preheats feedwater and condensate. A PFBC plant can accommodate any steam power plant design: reheat, non-reheat, subcritical, supercritical, straightforward condensing, or any combination of power production with steam extraction or back pressure.