By A. Robertson, Foster Wheeler Development Corp.
Foster Wheeler Development Corporation is working under a U.S. Department of Energy (DOE) contract to develop a partial gasification module (PGM) that represents a critical element of several potential coal-fired Vision 21 plants. (Vision 21 is a cost-shared partnership between the U.S. government and industry. One of its objectives is to develop the design basis for coal-fueled electric generating plants that operate with efficiencies greater than 60 percent while producing near zero emissions of traditional stack gas pollutants. The cost of electricity generated by these new plants should be comparable to, if not less than, that of present plants and ideally should be amenable to sequestration of stack carbon dioxide gases.)
To avoid the complexity and high costs of a high temperature gasification system, Foster Wheeler (FW) proposes a combined cycle plant utilizing a partial gasifier module (see figure). The PGM operates at lower temperatures that minimize the release of coal contaminants and enable the syngas to be fired hot. The PGM can operate with air, enriched air, or pure oxygen. The admission of pure oxygen requires that the flue gas be recycled or water added to control PGM temperature. The utilization of pure oxygen eliminates all nitrogen from the flue gas and provides a suitable exhaust stream for direct CO2 sequestration. Because of the lower temperature, a char is produced along with the syngas; the char is combusted in an Atmospheric or Pressurized Circulating Fluid Bed Combustor (ACFBC or PCFBC) or a High-Temperature Air Furnace (HITAF) with the heat release being used to support the most advanced steam cycle temperatures (1300 F).
In addition to generating electric power, a portion of the syngas could be used to co-produce valuable by-products such as liquid fuels and/or chemicals. Since FW’s proposed plant utilizes char combustion to achieve steam temperatures as high as 1300 F, PGM carbon conversion efficiencies need only be in the 80 percent range or even lower. These conversion levels can be easily achieved with a fluidized bed reactor operating at about 1800 F. Hence the plant will not be restricted to reactive coals; it can handle less reactive coals as well as alternative fuels such as petroleum coke.
The PGM consists of a pressurized circulating fluidized bed (PCFB) reactor together with a recycle cyclone and a particulate removing barrier filter. Coal, air, steam, and possibly sand are fed to the bottom of the PCFB reactor and establish a relatively dense bed of coal/char in the bottom section. As these constituents react, a hot syngas is produced which conveys the solids residue vertically up through the reactor and into the recycle cyclone. Solids elutriated from the dense bed and contained in the syngas are collected in the cyclone and drain via a dipleg back to the bottom of the PCFB reactor. This recycle loop of hot solids acts as a thermal flywheel and promotes efficient solid-gas chemical reaction.
Left untreated the syngas will contain tar/oil vapors, alkali vapors, and hydrogen sulfide at levels dependent on PGM operating conditions and fuels. The downstream users of the syngas will dictate a tolerance level for each of these gas constituents. If the users can tolerate tar vapors and hydrogen sulfide, the syngas can be cooled to an intermediate level that only condenses the alkali vapors on the particulate being removed by the barrier filter. Although this is a simple solution to an alkali problem, syngas cooling typically lowers the plant efficiency. If efficiency is to be maximized, the clean up can be done hot/without syngas cooling. In this case, lime based sorbents can be fed to the PCFB reactor along with the coal to catalytically enhance tar cracking and react with the hydrogen sulfide to capture the sulfur as calcium sulfide.
Foster Wheeler possesses a coal-fired PCFB pilot plant at its John Blizard Research Center in Livingston, NJ. The pilot plant has a maximum heat input rating of 12 million Btu per hour, and its PCFB reactor can be operated either as a partial gasifier or a combustor at pressures up to 200 psig. The plant typically operates with coal and either limestone or dolomite can be injected with the coal to serve as sulfur capturing sorbents. Coal, sorbent, and sand can be fed to the process via dry, lock hopper type pneumatic transport feed systems or mixed with water and pumped in as a nitrogen atomized paste.
The reactor is a 30 in. OD (outer diameter) x 39.5 ft. tall pressure vessel refractory lined to a 7 in. ID (inner diameter). Air, coal, and lime-based sorbent are injected at the bottom of the unit and a low BTU syngas leaves through a 4 in. ID radial nozzle at the top. Spent bed material and char are drained from the bottom, pass through a cooler, and are depressured in a lock hopper provided under the PCFB reactor.
The exiting syngas passes through a primary stage cyclone located within an adjoining refractory lined pressure vessel. Particulate (hot char, sorbent, and sand) captured by the primary cyclone drain through a vertical dipleg to the base of the PCFB reactor to permit the continuous circulation of hot material through the unit. As the char is consumed and ground by its continuous recirculation through the system, the finer fraction eventually escapes the primary cyclone. This fine material together with the syngas are cooled to 650EF by a tubular heat exchanger and ultimately passed through a barrier type candle filter for the removal of all remaining particulate. The filter utilizes porous metal candles made of iron aluminide. Particulate collect on the outside of the candles and upon reaching a predetermined pressure differential, the dust cake is removed/blown off the candles by a back pulse of nitrogen. The dust cake falls to the bottom and drains by gravity to surge and lock hoppers provided under the filter for depressuring and removal. The higher the syngas dust loading and the higher the gas velocity through the candles, the more frequently it has to be pulse cleaned. To reduce the size and hence cost of the filter, a precleaning cyclone has been provided upstream of the filter. After passing through the filter, the syngas is depressured, cooled to 300 F, flared, and discharged to ambient.
Robertson, senior research associate with Foster Wheeler Power Group Inc., has over 30 years of engineering experience conducting and supervising R&D work in the fields of combustion and fuel technology, thermal/ hydraulic analysis and design, process evaluations, and new product development. He served as project director for the preliminary design and cost estimate of a nominal 250 MWe second generation PFB demonstration plant. He also served as program manager for a team of five companies conducting R&D for the development of second generation PFB technology under a DOE contract. Robertson can be reached at firstname.lastname@example.org.