The Pros and Cons of Coal Gasification

As we have learned in class, of the natural fossil fuels we have at our disposal, coal is pretty much the left handed, red headed step child of the group. It's the dirtiest, the hardest to harvest, and has less energy per kg than our other options, but it does have one thing going for it: it's really plentiful. Compared to the skyrocketing prices of natural gas and petroleum, coal is looking pretty good right now and as many proponents tout, it's a step towards energy independence. 

Now, we have talked in class about how much of the coal we have is laden with sulfur, which quite frankly, is not so hot for the environment and the process for cleaning coal can be expensive. Thankfully we have a "new" process called coal gasification that may just be the answer to our dirty coal needs. At least for now. Here is a summary of the coal gasification process, as well as the pros and cons of utilizing it.  (from popularmechanics.com)

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.

The great thing is, once coal is converted to a gas, it's fairly straightforward to remove pollutants. Mercury, sulfur, and particulates can be stripped out and sold commercially. This "clean" gas can then be used for a variety of fuels such as oil for heating homes or fuel for cars. "The gas can also sub for natural gas, fueling "integrated gasification combined-cycle" (IGCC) power plants to create electricity. Pennsylvania hopes to replace many of its filthy coal-fired power plants with IGCC plants."

""You could put a million scrubbers on an old coal-fired power plant and never even approach the environmental performance of a coal gasification plant," says Kathleen McGinty, secretary of Pennsylvania's Department of Environmental Protection."

So the benefits work out as a reduction of air pollutants, a solution to the waste-coal problem, and a boost to economy.

So what are the drawbacks?

"There are three drawbacks, and they are substantial:

• If gasification takes off, there isn't enough waste coal in the country to feed the beast. Thus, you're back to coal mining, which is a nightmare.
• Gasification is largely untested and unproven, at least in the U.S. And IGCC plants are more expensive than old-fashioned dirty coal-fired plants. Thus, gasification relies heavily on subsidies. State and U.S. Dept. of Energy tax incentives for the Penn. plant, for instance, add up to over $140 million. More big industries getting chummy with gov't; more semi-permanent corporate welfare recipients.
• Carbon dioxide. Global warming. That whole thing. IGCC plants are certainly an improvement over dirty coal-fired plants -- they use less coal to create more energy -- but they still produce plenty of CO2. They do make the CO2 fairly easy to capture, which is nice, but the question is what to do with it once it's captured. The big idea is to sequester it: pump it underground or into plant tissues and soil. However:

"The effectiveness of CO2 storage in those systems is completely unknown," says Anne Hedges, program director for the Montana Environmental Information Center. "It's a nice theory, and I sure hope it works. But there's absolutely no evidence it does on a long-term basis." "

To sum it up, as an alternative energy solution, don't waste your time with coal gasification. However, as an improvement to dirty coal, gasification is just that: an improvement. We need to look to other technologies, but as it is, this is a good step in the right direction.

sources:

http://www.popularmechanics.com/science/earth/4218251.html

http://www.grist.org/article/coal-gasification-clean-coal-or-subsidy-hungry-boondoggle