Since Al Gore's ad on the topic came out recently, there has been a lively discussion regarding "clean coal," what that really means and whether it actually exists. Most people posting on this site have answered the latter negatively, although the opinions are certainly not uniform. I, for one, have been imploring many of you to distinguish between carbon capture and sequestration (CCS) and integrated gasification combined cycle (IGCC) in your arguments. I think IGCC is a promising technology that could help us bridge the gap between our current situation and a renewable energy economy. This diary is an explanation of my position, and I hope you will read it with an open mind.
Follow me below the jump for what I hope is an easy-to-follow technical explanation of the benefits of IGCC (for those of you not already familiar with it) and a brief discussion of the economics and politics.
Let me say off the bat that I am an applied scientist working in solid state ionics, spending a good portion of my time on questions related to solid-oxide fuel cells (SOFCs). SOFCs are fuel cells that operate on either hydrogen, carbon monoxide, or a hydrocarbon-based fuel stream (such as natural gas). One of the potential applications of SOFCs is in conjunction with IGCC power plants. U.S. government programs pursuing this possibility have funded my work in the past, and I expect that they will do so in the future. My name is on a provisional patent related to SOFC technology. So there's your disclosure, though of course I stand by the facts appearing in what follows as absolutely accurate to the best of my knowledge.
Most of us can agree that coal and other fossil fuels must eventually be abandoned in favor of an all-renewable energy portfolio. Having said this, there are potential problems with dumping coal entirely within 10 or even 20 years, as some have suggested. Electricity in the United States is, at present, in the neighborhood of 50% coal-generated. Even if we were able to build that much renewable generating capacity within that time, there no good renewable solution yet for the problem of intermittency in wind and solar, and all such power stations must, at present, be backed up with fossil (or biomass, forseeably) generation in order to be viable. While this problem is solved -- through either improved battery technology, biomass, an intermediate carrier like hydrogen or something else -- we will be forced to rely on at least some fossil power to pick up the slack.
If fossil fuels are going to be with us for some time, what's the best option from the standpoint of the environment? The answer is undoubtedly natural gas (for reasons I'll detail below), and I gather that it was, not too long ago, a widespread belief that gas would completely supplant coal in fossil power generation. But then the price of gas went through the roof, and coal came roaring back.
So if coal power is not going away overnight, it seems eminently sensible that we should try to improve our use of it, making it more efficient and less polluting. Coal gasification has the potential to accomplish both of these, and unlike carbon sequestration, it has been demonstrated on a large scale.
Technical description
Let's start with a discussion of conventional coal power generation versus gasification from a standpoint of thermal efficiency.
The basic process in combustion-based power generation is the simple steam cycle. It entails the conversion of chemical energy (in the form of the fuel) first into heat, then into mechanical energy, then into electricity. The thermal efficiency is the percentage of chemical energy converted into electrical energy. To look at it another way, the thermal efficiency is the amount of electricity that can be extracted from a certain amount of coal.
This schematic depicts a simple steam cycle found in a typical coal-fired generating station. (This actually purports to be a schematic of a nuclear plant, but there is no difference, at this schematic level, between a nuclear plant and a traditional coal plant. Just realize that where the schematic says "reactor," would be labeled "boiler," and in this boiler, instead of nuclear fission there is the burning of fuel.)
Coal is burned at the boiler, converting water into steam. The steam is much less dense than the water, producing a high pressure inside the boiler, just like you get in a pressure-cooker. The steam is released from the boiler in such a way that it spins a turbine, producing electricity. There are various ways of getting the most out of this process, by burning the coal more thoroughly, or reheating the steam. Typical thermal efficiencies in steam cycle power generation are on the order of 35%. To look at it another way, 65% of the energy in the coal is essentially wasted.
One of the reasons that number is so low is the fact that much of the energy released from the coal when it burns goes into the volumetric expansion of the combustion product (CO2). In other words, the fact that CO2, itself, has a much higher volume (if the pressure is fixed) or pressure (if the volume is fixed) than the coal it came from is a manifestation of the energy contained in the coal. In a simple steam process, however, this energy is totally lost, since the burning of a solid needs to be a relatively diffuse process (the coal is spread out in little chunks, the smaller the better) in order to ensure thorough combustion.
With a gaseous fuel, however, some of that gas-expansion energy can be captured as electricity. This is what's done in combined-cycle.
Here's a schematic of a combined-cycle plant. The fuel, which must be in a gaseous form, is burned. The fact that it's a gas instead of a solid that's burning is already an advantage from an efficiency and environmental standpoint, since the combustion process is much more thorough and much cleaner. But in addition we can take advantage of the volume change that the gas undergoes during the combustion process. When we fill a small, closed container with natural gas and air in the right proportions and light a match, the products of the combustion (steam and CO2) try to expand out against the walls of the container, producing a high pressure. This pressure can be directly utilized in producing electricity, using what's called a gas turbine. Afterwards, the heat produced in the combustion can also be used in a simple steam process. The result is a higher conversion efficiency, potentially on the order of 55%.
So let's stop and think about this. By switching from a simple steam cycle to combined-cycle, the total amount of electricity generated per unit of fuel has increased by 50-60%. In other words, the amount of CO2 going into the atmosphere per kWh of electricity generated has been cut by something like 1/3. As a bonus, since burning a gas is much cleaner, the pollution is much reduced: particulates, which are the worst pollutant, are down to practically zero, as are NOx and SOx, which are the precursors of smog and acid rain, respectively.
So you can see why many environmentalists like natural gas power plants, which are combined-cycle plants, so much better than they like coal plants. As mentioned above, natural gas would be a good option for improving our use of fossil fuels, for this reason. But the relative scarcity of natural gas makes it highly uncertain that this could be a workable replacement for coal.
So, now to the topic of this post: coal gasification. Hopefully by now you can see where this is heading. Coal gasification is a process in which the solid coal is turned into a gas before being burned. Here's the schematic.
The interesting stuff takes place in the gasifier. A gasifier is basically just a boiler, where coal is burned, but the combustion is starved of oxygen, with water (steam) provided instead. This causes most of the carbon in the coal to react with the water, forming carbon monoxide and hydrogen. This mix of CO and H2 can then be used as the fuel in a combined-cycle generating process. What is really nice about this -- having CO and H2 as the product of the gasification -- is that this is chemically a very flexible combination of gases. (Chemical engineers call it "synthesis gas," or "syn gas," because once you have it, you can make basically anything out of it.) In particular, it is especially well suited for use as a fuel in SOFCs. Fuel cells are not subject to the built-in efficiency limitations of all heat-mechanical-electrical plants, since they convert chemical energy directly into electricity. This means the efficiency of the process could be increased even further.
So that's the idea. The potential is this: coal-generated electricity with two-thirds or less of the CO2 production of the traditional coal process per kWh, with virtually no particulates, NOx or SOx.
So if that's the potential, what's the reality? The current reality on the efficiency side is better than traditional coal, but not at its full potential. Efficiencies reached by the Wabash IGCC station in Indiana are on the order of 42% -- about 20% better than traditional coal. There are reports that the Japanese have managed slightly better, but still under 50%. Why the lag? The gasification process is considerably different from its traditional counterparts, and it will take some time operating plants at very large scales before the full potential can be realized. Nobody really knows where the efficiency numbers will top out, but the potential to use fuel cells is, to me, encouraging. As far as the cleanliness of the process -- the reductions in particulates, NOx, SOx and mercury relative to traditional coal -- that has already been achieved on a large scale.
Economics
So that's what I have to say about the technology. What about the economics? The cost of building an IGCC power plant is always going to be higher than that of traditional plants, because there is an extra component: the gasifier. Current cost premiums over the most advanced traditional coal generating plants are anywhere from 20-30%, but a GE-Bechtel partnership has just announced a new system whose costs will only be about 10% higher, at least from a standpoint of the basic equipment. The cost of operating an IGCC plant compared to a traditional plant per unit of electricity produced are about the same. Some have said that the construction costs of IGCC are spiraling upward -- but I haven't found any good evidence for this. The costs of the newest planned IGCC plant in Minnesota -- the Masaba generating station -- have spiraled upward. Much of this seems to be due to issues associated with this particular project, which has become very politically contentious, rather than a general rule.
Politics
The actors in the electric power industry, as I'm sure I hardly need to remind anyone, are not generally good citizens. Their lobbying operations rival almost anybody's in size and bought-and-paid-for access, and they typically aren't lobbying to try to increase air quality and lower carbon emissions. The Bush administration has been a boon to the dirtiest generators in the industry, such as Duke Energy, American Electric Power and Southern Company, who have essentially received a get-out-of-jail-free card regarding compliance with provisions of the Clean Air Act designed to reduce the amount of particulate emissions from coal plants. Particulates are responsible for an obscene amount of illness and premature death among the general population, and the industry knows this. How can we trust these people, who are now telling us that "clean coal" is the way to go?
I don't think we can trust them. But we still have to work with them, unfortunately. So at the end of the day, it would seem that the best option is this: enforce the Clean Air Act. Tax carbon emissions. Set a rigid cap. Do something to penalize the bad behavior, and let the utilities figure out the best way to go about changing. It can surely be done in a reasonable way, that doesn't stress their operations too much while adequately protecting the air and the climate. If gasification or even sequestration works, great. If not, they'll have to figure out something else.
What does not help is when certain environmental groups decide that, of the 100+ coal-fired power plants in planning stages around this country, they are going to target the IGCC plants. This has the truly perverse effect of delaying the potentially cleanest of these coal plants and hindering the environmentally friendliest coal technology while allowing the traditional, dirtier plants to proceed. I have a feeling this has something to do with antipathy for the "clean coal" designation, but note to these environmentalists: if you can't stop them all, please stop the dirtiest ones and leave IGCC alone.
Finally, I know many of you are passionate about the abuses of the coal mining industry. I know much less about this from a technical, economic or regulatory standpoint, but on a personal level I can say that I share your concerns. My favorite places on the earth are in the Appalachian mountains, and pictures of mountaintop removals are like pictures of a crime scene to me. As far as I am aware, Appalachian coal is less valuable than the western variety, because of its relatively higher sulfur content. In any event, as someone opposed to this horrible practice, I am hopeful that coal mining could be practiced in a responsible way.
Sources
Let me share a few sources. First and foremost are the National Energy Technology Laboratory's gasification web pages. An interesting article about global warming and energy in general, with an interesting perspective on coal use appeared a couple of years ago in Harvard Magazine. And there is an industry website www.clean-energy.us, which actually contains a lot of information, useful so long as you realize where it's coming from. Also, by "industry" here I mean a coalition of companies involved with gasification, which includes not only utilities and chemical concerns but technology companies like GE and Black & Veatch. (Not all utilities are taking part in this project -- Southern Co. is notably absent.)
Thanks for reading. I'd be happy to answer your questions or learn from your experience.