A more efficient way to extract energy from coal

A process that produces hydrogen from coal with close to 80% energy conversion efficiency, plus coproduction of a carbon-dioxide-rich stream for sequestration, is being developed at Ohio State University (Columbus, Ohio; www.osu.edu). This compares with around 60% conversion efficiency for traditional coal gasification processes, says Fanxing Li, a research associate and co-inventor of the process along with chemical engineering professor Liang-Shih Fan.

In the two-step process (flowsheet), called chemical looping conversion, pulverized coal, iron oxide pellets (a patented composite of iron and such materials as alumina and silica) and oxygen are fed into a moving-bed reactor (a reduction reactor). The carbon in the coal reacts with the iron oxide at about 850°C and 450 psi to produce iron and CO2. The Fe passes to the second (oxidation) reactor, where it reacts with steam at around 800°C and 450 psi to yield H2. The Fe is reoxidized and recycled to the first reactor.

The advantage of the process is that it produces H2 without the traditional water-gas shift reaction and without the energy intensive step of separating the CO2 from the resultant gas mixture, says Li. The oxygen requirement is only about 40% that of conventional coal gasification. Alternatively, the use of O2 can be avoided by burning part of the iron oxide feed and using the sensible heat from those particles to drive the first reaction.

So far, the gasification process is being tested at a scale of 25 kWth, or about 10 lb/h of coal. Li adds that a variation of the process can be used for "indirect" coal combustion, with CO2 sequestration. In this configuration, the Fe is burned with air in the oxidation reactor and the hot gases are used to drive a steam turbine.