Research on commercial gasifier feed systems is occurring in two primary areas of fuel (i.e. coal, biomass, etc.) feed and advanced air separation technologies. Fuel feed focuses on innovative technology to allow increased use of lower cost, abundant low-rank coals in dry feeding of high-pressure gasifiers, and co-feeding of coal with significant fractions of advantageous fuels such as woody biomass. Advanced air separation technologies are being developed to significantly lower cost of oxygen production and allow efficient integration of air separation processes with gasification-based power and co-production plants. Advances in these technologies will have significant effect on reducing cost and increasing overall gasifier system efficiency.
Coal Feed Technology
Commercially available coal feeding methods for gasification consist of injection of coal-water slurry, or dry feed lock-hoppers. Coal-water slurry can be injected at very high pressure to efficient, high-pressure gasifiers; however, the debit of additional water required for slurry feeding tends to limit the types of coal used in these slurry-fed gasifiers to higher-rank, drier, and more expensive coals (primarily bituminous coal). In other words, if low-rank coals were to be slurried, their inherent high moisture levels combined with addition of slurry water results in excessive water content, causing inefficiencies in gasification. Dry feed lock-hoppers can handle dry coal feeds, but cannot be utilized except at lower operating pressures, at which gasifier operation is less efficient. Therefore, utilizing the Nation's large reserves of low-cost, low-rank coals in more efficient IGCC systems is currently limited by these constraints in available coal feeding technology.
High-pressure solid feed systems are being developed to enable use of low-rank coals in dry feeding of high-pressure gasifiers, to allow co-feeding of coal with other advantageous fuels (such as biomass), and to encourage higher pressure (and therefore more efficient) operation of dry-feed gasifiers.
One promising high-pressure dry feed technology is the Posimetric® pump currently under development by GE.
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Another new high-pressure solids feed pump is being developed by Pratt Whitney Rocketdyne (PWR). This pump is similarly intended to improve the feeding of dry coal/petcoke/biomass into high-pressure gasifiers, thereby increasing the efficiency of the gasifier and reducing plant capital, maintenance, and operating costs. A 600-tpd PWR pump prototype has been built, semi-scale tests completed, and commissioning begun.
Electric Power Research Institute (EPRI) is investigating a concept to utilize high-pressure CO2 as a carrier fluid (essentially forming a liquid CO2/coal slurry) to feed low-rank coal into gasifiers. This would require recovery of CO2 from syngas, and therefore dovetails with carbon capture and storage/sequestration possibilities.
National Carbon Capture Center
The Power Systems Development Facility (PSDF) has designed and fabricated the Pressure Decoupled Advanced Coal feeder. The feeder is a non-mechanical feed control device operating at 500 psig or greater, with no moving parts. It combines some of the successful concepts developed with the PSDF continuous ash depressurization systems with traditional designs for flow rate control. It uses conveying gas flow to control the solids feed rate.
National Carbon Capture Center at the Power Systems Development Facility
Advanced Air Separation Technology
The cryogenic air separation unit (ASU) in a conventional IGCC plant typically accounts for 12 to 15 percent of the overall capital cost of the plant, and requires a large parasitic power load primarily to operate gas compressors. These high costs are the impetus for development of advanced air separation technology to produce commercial-scale quantities of oxygen at significantly lower cost than the conventional cryogenic systems currently used. The technology being developed (Ion Transport Membrane or ITM) is projected to cost one-quarter to one-third less than an equivalent-sized state-of-the-art cryogenic ASU in terms of capital cost. ITM systems afford the opportunity of integrated operation with turbines, and operate at elevated temperatures, thereby increasing efficiency/reducing parasitic energy penalty compared to conventional cryogenic oxygen production systems.
ITMs are nonporous ceramic membranes that are permeable only to oxygen ions and are therefore 100% selective. At temperatures of 1450-1650°F, oxygen from the air feed adsorbs on the membrane and dissociates to form oxygen ions by electron transfer. The oxygen anions enter and migrate through the ceramic lattice counter-currently with electrons, and are driven toward the permeate side by the oxygen partial pressure differential.
Air Products and Chemicals, Inc. is performing a major R&D effort including developing, scaling-up, and demonstrating ITM-based air separation technology. Membranes have been fabricated that meet or exceed commercial flux targets. Full-scale modules have been fabricated and tested in a 5-ton-per-day (TPD) test system; construction is well underway on a 100-TPD oxygen production test facility.
Process analyses continue to show significant cost and efficiency advantages with the application of high-temperature membranes for oxygen production compared to conventional cryogenic technology.
As part of the support for the Feed Systems key technology, systems studies are being conducted to provide unbiased comparisons of competing technologies, determine the best way to integrate process technology steps, and predict the economic and environmental impacts of successful development.
Other key technologies within Gasification Systems include the following: