I've recently been reading Neal Stephenson's Quicksilver, which is partly concerned with the 17th century popularity of the pseudo-science of alchemy. The goal of alchemy was turning a base material such as lead into something else that was prized (e.g. gold.) The closest thing to this today may just be the gasification process, which has been highlighted recently as part of the clean coal "integrated gasification combined cycle" power plant (IGCC) concept. The New York Times covered this in some detail in Sunday's business section, but without really explaining how gasification works. I'm surprised to find that in nearly 18 months of blogging, I've never covered that explanation, either.
As the name suggests, gasification turns a solid or liquid carbon-based fuel into a gas that can either be used as a chemical feed or burned. That is particularly useful in a power plant, because gasification eliminates most of the impurities that would otherwise become exhaust gas pollutants, by converting them into forms that can easily be separated from the resulting "synthesis gas" (syngas). Let's look at how this works in the case of coal.
Instead of being fed into a combustion chamber, pulverized coal is mixed with water to form a slurry, which is then injected into a reactor along with pure oxygen, in proportions less than those required for complete combustion. (This is why the process is often referred to as "partial oxidation".) The output of this reactor is the syngas stream, containing hydrogen, carbon monoxide, and hydrogen sulfide gas. The latter is easily stripped out and safely converted to elemental sulfur, which can be used in fertilizer production. The ash and metals content of the coal drops out of the reactor as solid slag, rather than the particles found in a typical coal power plant's exhaust stack.
We're used to thinking of carbon monoxide as a poison, but it turns out to be a handy industrial product. It can either be burned with the hydrogen in a combined-cycle gas turbine--the "CC" in IGCC--or it can be used in a variety of chemical reactions. The most interesting of these from an energy perspective is the so-called "water-gas shift reaction", in which the CO is reacted with steam over a catalyst to produce hydrogen, resulting in both pure hydrogen and nearly-pure carbon dioxide, ideal for use in the food industry or for sequestration. The latter is what makes gasification so appealing relative to climate change. And unlike traditional processes for making hydrogen from fossil fuels, it's not a problem if the feed has a low hydrogen content to start with. That means that in addition to coal, you can utilize feeds such as petroleum coke or various wastes without generating lots of pollution.
If I sound like a shill for this approach, it's no accident. I had a lot of contact with it at Texaco, where one version of this process was invented--the same technology used at the Tampa Electric plant cited in the Times article and that was ultimately sold to GE. I worked briefly on a coal gasification pilot plant in Germany and a liquids gasification hydrogen plant at Texaco's Los Angeles refinery. I was always impressed by the elegance and great versatility of the process, but without its applicability to greenhouse gas emissions control, it would probably have remained an interesting, but always just-a-bit-too-expensive alternative energy process. Given growing concerns about climate change and the need for alternatives to oil and natural gas, though, it may finally find its ideal milieu.
The chief drawback is cost. As you might guess, it's a lot cheaper to grind up coal and shove it into a furnace than to do all this pre-processing. That's especially true if you aren't too concerned about what's going to come out the exhaust stack, in terms of sulfur dioxide, nitrogen oxides, particulates and heavy metals, let alone greenhouse gases. It still costs about 10-15% more to build an IGCC power plant than a conventional coal power plant with full emission controls, and so it takes a while for the IGCC's higher thermal efficiency and environmental benefits to pay for the difference. As optimistic as I am about this process, I remain skeptical that this initial cost premium will ever disappear entirely, as the Times suggests. But as we move towards a world in which we will have to pay to emit carbon dioxide to the atmosphere, the payoff for that premium will become increasingly attractive. We should see this is Europe within the next few years. Look for gasification to play an increasing role there, as the impact of the EU's emissions trading system begins to shift the direction of the energy economy.
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