THE SEARCH FOR CLEAN COAL TECHNOLOGY
Anti-coal environmentalist and politicians may want to eliminate coal as an energy resource, but that is impossible without pushing the world back into another Dark Age. Worldwide, 23% of our primary energy needs are met by coal and 39% of our electricity is generated from coal. About 70% of the world's steel production depends on coal.
That's why everyone is talking about the clean coal technology. The problem is that we haven't agreed yet on what "clean coal" is. In fact, clean coal means different things to different people. The coal industry points to the advances in coal prep and scrubbing devices in coal powered generating plants over the last several years and say that today's product is clean. Prominent environmentalists believe there will never be "clean coal." Nuclear power advocates say clean coal must be able to provide the same power output with the same "near zero" emissions.
One clean coal technology that has recently moved from the drawing board to reality is the Oxy-fuel combustion process. Oxy-fuel combustion is the process of burning a fuel using pure oxygen instead of air as the primary oxidant. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible.
In Oxy-fuel combustion, almost all of the nitrogen is removed from input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. The recycled flue gas can also be used to carry fuel into the boiler. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion and produces exhaust consisting primarily of carbon dioxide and water.
A power plant using Oxy-fuel combustion went on-line in September 2008 in Spremberg, Germany. The plant is owned by the Swedish company Vattenfall and was built by the German firm Siemens. The plant is called Schwarze Pumpe Power Station. The plant doesn't produce electricity, but steam for local industries. The facility captures carbon dioxide and sulfides, separates them, and compresses the CO2 into a liquid.
The problem with this technology is that about 15% of the energy is needed to produce the pure oxygen. That has caused some scientists to look at other methods to remove the CO2. One such technology is called Chemical Looping, which causes pulverized coal to react with metal oxides like rust. The reaction converts the rust into iron and produces carbon dioxide. The CO2 can then be compressed and stored. The iron is then exposed to steam, which produces, heat, rust, and hydrogen. The hydrogen is then used to power turbines or fuel cells.
There is also quite a bit of interest in using coal in fuel cells without the chemical looping process. One advantage of coal based fuel cells is their practicality in smaller applications. Coal plants have great economies of scale and it's not economic to make a small one. But there are no economies of scale to make a fuel cell. As a result, they are ideal for smaller-scale systems to power generators and other off-the-grid needs.
These fuel cells convert carbon sources to electricity in a single reaction step, just as current fuel cells do with hydrogen. Under various schemes that pulverize coal and feed it into the fuel cell in a slurry of molten salt or other superheated material. The carbon in the coal reacts in the cell to produce a moving stream of electrons (electricity). Such a fuel cell could approach up to 70 percent efficiency - the highest of any fuel cell class. However, coal prep will be a major challenge for the coal powered fuel cells.
If coal impurities are not removed beforehand, these can contaminate the cell and reduce its lifetime. This brings us to one of the biggest problems with clean coal technology - the wide variety of coal available. Depending on their source, they can differ in energy output and the amount of sulfur, water, ash, and carbon in the fuel. This problem can be seen in the clean coal front runner, the government supported Integrated Gasification Combined Cycle (IGCC) program. It uses coal and steam to produce hydrogen and carbon monoxide.These are then burned in a gas turbine to produce power.
The IGCC plant, however, can cost considerably more if it uses lower quality coal like lignite (37% more). The resulting plant is also 24% less efficient. Compare that to the efficiency of a low quality coal powered conventional power plant that only costs 24% more to build, but only experiences a 10% drop in efficiency. Consequently, it makes more sense to build a conventional power plant and use conventional carbon reduction methods than new technologies.
So, what is the future of clean coal? Short of new, draconian regulations, it will probably be a mix of old and new technologies. The current clean coal technologies used by the industry will remain in use for many more years. And, as new technologies become proven and economical, they will be accepted. However, the era of coal powered fuel cells in everyone's basement is still far away.