Sponsored by the U.S. Department of Energy, the National Carbon Capture Center provides first-class facilities to test carbon capture technologies.
With coal gasification now in modern commercial-scale applications, the U.S. Department of Energy's (DOE) Office of Fossil Energy has turned its attention to future gasification concepts that offer significant improvements in efficiency, fuel flexibility, economics and environmental sustainability.
Fuel flexibility is especially important. Tomorrow's gasification plants conceivably could process a wide variety of low-cost feedstocks, handling not only coal but also biomass, municipal and other solid wastes, or perhaps combinations of these feedstocks. The Department of Energy is investigating new gasifier configurations that can adapt to variances in fuel composition, heating values, ash content, and other factors.
DOE's Power Systems Development Facility
Economical construction and operation will also be important if coal gasification is to reach its full market potential. At the Power Systems Development Facility (PSDF) in Wilsonville, Alabama, (a DOE/industry partnership project to develop clean, economical and reliable processes for power and chemical production from coal and other feedstocks) the DOE is developing a new, potentially low-cost configuration for a future gasifier. Called the "transport reactor," the gasifier is an advanced circulating fluidized-bed reactor ("circulating fluidized bed" describes the fluid-like way coal particles act in the gasifier with currents of air or oxygen). Inside the gasifier, a chemical sorbent can be added to capture sulfur impurities. The PSDF is co-located with the National Carbon Capture Center (NCCC).
Another way to make coal gasification more economical is to reduce the cost of oxygen used in the gasification process. Unlike air, pure oxygen isn't diluted by large quantities of nitrogen. Therefore, oxygen-blown coal gasifiers can be more efficient and produce a much more concentrated stream of carbon dioxide (a greenhouse gas) that can be more easily captured and sequestered. Making oxygen today, however, typically involves a complex, energy-intensive super-cooling (cryogenic) process. A much lower cost alternative being explored by the Department of Energy is to use new innovations in ceramic membranes to separate oxygen from the air at elevated temperatures.
Membranes may also become an important new technology for separating gases produced by coal gasifiers. An especially important goal of the Department of Energy's coal gasification program is to develop inexpensive membranes that can selectively remove hydrogen from syngas so that it can be used as a fuel for turbines, future fuel cells or refineries, or perhaps one day as a substitute for gasoline in a hydrogen-powered automobile. Other gas separation research is focused on removing carbon dioxide from syngas streams so that it can be kept from entering the atmosphere and contributing to increases in CO2 concentrations.
Removing impurities from the gases produced by coal gasifiers will also be important for the Department of Energy to achieve its goal of developing a virtually pollution-free power plant. The Office of Fossil Energy is supporting research into new types of pollutant-capturing sorbents that work at elevated temperatures and do not degrade under the harsh conditions of a gasification system. Also, new types of gas filters and novel cleaning approaches are being examined.
Gasification produces less solid wastes than other coal-based power generation options. Additionally, the byproducts of coal gasifiers can have commercial value. Gasification plants operating today are converting sulfur extracted from coal into commercial-grade sulfuric acid or elemental sulfur. Slag (the solid, inert glassy mineral matter left behind when coal is gasified) is being used for road construction. If the Department of Energy's research proves successful, gasifier ash and slag could find other commercial markets. The Department is also investigating ways to use the solid material produced when coal and other feedstocks (e.g., biomass, municipal waste, etc.) are mixed in the gasification process.
Finally, gasification power plants typically consume about one-third less water than conventional coal plants and can be designed for zero liquid discharge.