Advanced FBC Technology Demonstrations
Two Clean Coal Technology Demonstration Program projects are providing valuable information:  one at Jacksonville, Florida, which is demonstrating circulating atmospheric FBC by 2000; and the other expected to be sited soon, which will demonstrate commercial-scale advanced GFBCC technology.

Pressurized Fluidized-Bed Combustion System
First-generation PFBCs are operated with 100 percent of the solid fuel conversion happening in the fluidized bed. Since PFBCs have a maximum operating temperature around 870ºC (1600ºF), the gas turbine operates at a relatively inefficient temperature rating.

Depending on the manufacturer and/or site-specific conditions, the fluidized bed could be either the circulating- or bubbling-bed type. First-generation PFBC systems now undergoing commercial demonstration are capable of achieving efficiencies up to 42 percent.

Pressurized circulating fluidized bed (PCFB) partial gasifiers used in CHIPPS and GFBCC have been tested in pilot scale.

Topping Combustor
Second-generation APFBC systems require the development and demonstration of a commercially viable topping combustor with suitable fuel flexibility, flame stability, and NOx emissions.  These need to accept hot APFBC syngas, and hot vitiated air.  Tests of a multi-annular swirl burner (MASB) have demonstrated good flame stability and NOx performance. Systems testing of the MASB was performed at the Wilsonville Power Systems Development Facility (PSDF) during 1998.  With the integration of building-block technologies under development --hot gas cleanup, advanced gasifier technology, and turbine systems-- efficiencies for PFBC systems will eventually exceed 50 percent.

CHIPPS and GFBCC systems use moderate-temperature syngas, and ordinary gas turbine combustion air. Any gas turbine already developed for syngas operations should work.

Combustion By-Products Utilization
FBC economics improve as combustion by-products are reduced or high-value uses are found. The goal is to reduce solid by-products from FBC systems without compromising sulfur capture or producing in-bed sintering.  Variability of limestone will be assessed as a factor in the volume of solid by-products from FBC systems without compromising sulfur capture or producing in-bed sintering. Variability of limestone will be assessed as a factor in the volume of solid by-products, and a limestone utilization model will be developed to optimize sulfur capture and minimize the volume of solid by-products. Expanding markets for FBC by-products will reduce net operating costs and landfill requirements.  FBC ash will be characterized for conventional applications, such as agriculture, mine remediation, and structural fill, and high-value uses of solid by-products from FBC systems will be developed.

Hot Gas Filtration
In APFBC systems, ceramic filters are used that operate in the 1400^oF to 1550^oF temperature range to filter both syngas and vitiated air. Ceramic filter element durability, filter-ash bridging, and system costs are critical development issues being addressed.  The challenge of producing candle-filter elements able to operate for more than three years is being met by enhancing monolithic filter elements made of various materials, such as clay-bonded silicon-carbide, porous-sintered metal, and alumina-mullite oxide. A number of composite-type ceramic and iron aluminide-type filter elements are also undergoing development. Filter cost can be reduced by 25 percent through optimized design of the system; filter vessel cost is about 75 percent of the total system cost.

CHIPPS and GFBCC use moderate temperature metallic syngas filters which have been successfully demonstrated. 

Solids Transfer
Improved handling of hot-solids material --feed and withdrawal, flow control, and fines removal-- can achieve cost reduction and reliability improvement. A feasibility study of a rotary high-pressure dry-solids feeder will evaluate the system's potential for reducing capital and operating costs. An advanced system for simpler and more reliable transfer of hot char from the carbonizer to the fluid-bed combustor will be tested for its ability to decrease materials flow and handling-related downtime by at least 50 percent.

Sulfur/Alkali Removal
Alkali in hot-gas streams can limit gas turbine life and reliability. The severity of the alkali problem must be determined. Gas turbine tolerance to alkali, the amount of alkali released, the effect of filter-cake characteristics, and the ability to control alkali will be assessed. Also, experiments to determine sulfur removal and trace-contaminant levels in the gas stream during integrated demonstration will be conducted.

Cofiring of Biomass and Industrial By-Products
Existing fluidized beds are suitable for cofiring, but to date, only 8 of the 100 units in the U.S. cofire material. Cofiring of biomass and industrial by-products could evolve into a standard practice as a near-term means to reduce CO₂ emissions. R&D data on heavy metals are needed so that environmental approval and permits for cofiring projects are not any more difficult to obtain than for single-fuel solid-combustion units.

In a second-generation PFBC system, the coal is partially gasified in a pressurized fluidized-bed carbonizer. The carbonizer produces a low-Btu gas and a char. The char is burned in a PFBC. Both gases are cleaned by hot-gas filtration, and the carbonizer's syngas is burned in a topping combustor to heat the PFBC flue gas.  This hot flue gas drives a gas turbine to generate power. The flue gas leaving the gas turbine then generates steam in a heat recovery steam generator, which is used to generate additional power. At the Wilsonville Power Systems Development Facility (PSDF), an advanced second-generation PFBC now demonstrates high efficiency at pilot scale.

Second-generation PFBC is also called "advanced circulating pressurized fluidized bed combined cycle," or "APFBC."