Clean Coal: How to Make Rock into Biofuel

Despite a Senate battle leaving out important funding for liquid coal research in the new energy bill, gasification remains an important engineering process to our green future

By Tyghe Trimble
Illustration by Dogo [link]
June 21, 2007
Popular Mechanics

Raw coal is dirty: It's a sulfur-filled, mercury laden, sooty, black rock. And before it can even be used, it must be scrubbed clean-or, with new technologies, converted to a liquid or gas. Liquid coal has been getting a lot of attention in the Senate of late, but bipartisan arguments were cut short this week when the promise of $10 billion in "clean coal" funding was cut from the new energy bill.

The big idea is to make coal into a liquid biofuel that could fill our cars. Many argue that this fuel would reduce our dependence on foreign oil, although others point to it as an unacceptable replacement for diesel due to its high output level of greenhouse gases. One thing's for sure: the rock-to-gas transformation remains in the preliminary research stages, with little funding and not much public understanding. Here's how that process, known as the Integrated Gasification Combined Cycle (IGCC), converts coal into synthetic gas and energy-a 20-percent more efficient makeover of the dirty ore you may soon find only in a naughty kid's stocking:

1. The heart of gasification lies in (shocker) the gasifier, which takes coal, water and air and applies heat under high pressure to make "syngas"-a mixture of carbon monoxide and hydrogen. Minerals in the fuel (i.e., the rocks, dirt and other non-carbon-based material) separate, leaving the bottom of the gasifier either literally in ashes or as an inert, glass-like slag-materials that can be reused for materials such as concrete and road fill.

2. The crude syngas leaves the gasifier piping hot and full of contaminants (hydrogen sulfide, ammonia, mercury and nasty particulates, to name a few). A combination of heat exchangers, particulate filters and quench chambers cool the syngas to room temperature and remove most of the solids.

3. Syngas then passes through a small bed of charcoal to capture mercury, removing over 90 percent of this toxic metal (click here to learn more). Used charcoal containing captured mercury leftover is sent to a hazardous landfill for disposal.

4. The final step for cleaning in gasification is the removal of sulfur impurities in acid gas removal units, where the impurities are converted into sulfuric acid or elemental sulfur-both valuable byproducts.

5. A combustion turbine then reheats the clean syngas, dilutes it with nitrogen for control of NOx (the greenhouse gas that makes smog) and burns it, driving a generator to make electricity.

6. Leftover heat from combustion is recovered in a Heat Recovery Steam Generator (HRSG), which generates steam to power the internal turbine. Some of that air is compressed and can be channeled back to the air separation unit for oxygen, which is then reused within the gasifier.

7. The steam generated in the HRSG and the steam made in Step 1 combine to drive a steam turbine for even more power production. The steam then cools and condenses into water, which pumps back into the steam generation cycle. In an IGCC plant, two-thirds of the total electricity produced comes from the gas turbine and one-third from the steam turbine.