“New Developments could improve Cost /Efficiency Ratio for Power Plant Air Pollution Control” - Hot Topic on July 12  

The future of coal-fired power generation is threatened by its negative environmental impact and the assumed cost of reducing that impact. However, coal could be cleaner than wind or solar under the following scenario. An ultrasupercritical coal-fired power plant burning 80 percent coal and 20 percent biomass with 90 percent CO₂ capture and sequestration (CCS) would actually be removing CO₂ from the atmosphere. It is sequestering more CO₂ than is generated by the coal- firing. The various CO₂ capture technologies also remove any residual pollutants in the flue gas.

McIlvaine has developed a common metric to rate harm from each pollutant. A ton of SO_x causes as much harm as 100 tons of CO₂.  But if you eliminate all the SO₂ emissions from coal and capture excess CO₂ then the total harm to the environment is less than solar or wind. So any absolute ban on coal makes no sense. The prohibition of new coal-fired power plants in favor of life extension of ancient inefficient coal-fired power plants is completely illogical from an environmental perspective.

What we need is a meeting of the minds between power generators and environmentalists for a harm reduction program that is not dependent on power plant age or on one specific pollutant. 

As can be seen from the chart below, U.S. coal-fired power plants emit pollutants with harm equivalent to 4.4 billion tons/yr of CO₂

A new fleet of coal-fired power plants using ultrasupercritical technology and highly efficient air pollution control technologies could reduce this burden by 3 billion tons of SO₂ equivalent at almost no net cost increase in electricity. These power plants would not include CO₂ capture.  If CCS is added later the net burden would be reduced another 75 percent. But the decision to do this or to switch to wind or solar could be made in 2040.

Whether it is heavy reliance on new power plants or retrofitting existing plants, the future of coal will rest on the ability to economically reduce the environmental burden. Fortunately there are a number of promising technologies which have the potential to do so. The two hour session yesterday showed substantial potential for eight different technologies.

Ceramic media to remove both NO_x and particulate at 850°F and higher is available. Richard Lydon of Clear Edge provided details on success of Cerafil and TopKat.  Cerafil consists of ceramic fibers which are formed into a rigid filter media. Very low emissions are being obtained on some tough applications such as gasified waste. TopKat is a variation which incorporates catalyst into the filter elements. High NO_x removal can be achieved. TopKat could replace an existing precipitator and eliminate the need for SCR. So it could be retrofitted into older plants which are considering shutting down because of lack of room. Equally important is the ability to operate at 850°F. Topkat would be ahead of the heat exchanger. Once the dust is removed a more efficient heat exchange technology can be utilized. Recovering this heat could improve boiler efficiency significantly.

Developments using bromine and other halogens to capture mercury were chronicled by Jon Lehmkuhler of Chemtura. The various bromine compounds made by the company can be injected with the coal, as impregnations of activated carbon, or ahead of the scrubber. Very high mercury oxidation and subsequent capture can be achieved. Chemtura does have worldwide reach and supply to assure adequate quantities of the compounds despite substantial increases in demand.

Other novel mercury approaches were discussed. One is the absorption media which is supplied by W. L. Gore.  A new arrangement with URS provides a commercial route for this technology

Fine mesh and condensation phenomena to capture sub-micron aerosols has substantial benefits. These were explained by Chris Pedersen of Kimre. Mercury and sulfuric acid mist can be captured by first condensing water on the particles and then using mesh for further particle growth. The last stage only has to capture the large particles formed in the process. Kimre has extensive experience in waste incineration and in the chemical industry. This technology should be easily adapted to compete with Wet ESPs in the power industry. However, Kimre says they will need a partner to pursue this market.

New approaches to SO₂ capture including co-production of hydrochloric acid were briefly discussed. When high chlorine coals are burned, a two-step scrubbing process can create 30 percent hydrochloric acid and gypsum. Another approach is to start with salt and make chlorine and sodium hydroxide. The Skyonics process then reacts the sodium with the CO₂ to make sodium carbonate. Substantial new funding for this approach was recently announced.

Ozone can be utilized to oxidize mercury and NO_x for capture in the downstream scrubber.  Naresh Suchak of Linde displayed a number of successful installations of Lotox. NO_x removal above 90 percent can be achieved while mercury capture above 50 percent is also achieved. There are a number of recent installations in China on fluid catalytic cracking processes. Some years ago there was an extensive demonstration program on a 25 MW coal-fired boiler in the U.S. There seems to be no reason that the success in the refinery industry cannot be duplicated in coal-fired power plants. One attractive aspect for older coal-fired power plants is the relatively low capital cost and footprint. If a scrubber has already been installed, then the ozone generator can be furnished on a skid, so installation costs for equipment would be minimal.

There is even promise for many larger newer power plants in states which are going to force increased removal of NO_x to reduce ambient particulate nitrate levels. If a power plant has an SCR meeting 90 percent NO_x removal followed by an FGD scrubber but now needs to increase NO_x removal to 95 percent, what is the best route? Should you add two more catalyst beds and increase the fan horsepower and change lots of ductwork or should you inject ozone?  The capital cost of the expanded SCR will be substantial, so ozone could be very attractive.

Finely ground limestone injection has lots of advantages according to Jerry Radway of Enerchem.  When you inject limestone particles which are less than 1 micron in diameter into your boiler, there is a tremendous surface area exposed to the SO₂. High removal efficiencies can be achieved. When compared to trona the cost will be less with the Enerchem approach. There are a number of ways to achieve the small particle size. The flyash in highly alkaline coals is one source of raw material. Powdered limestone is a byproduct of some operations. In some cases it will be necessary to provide grinding facilities. High removal of SO₃ has been achieved. Therefore the approach could be applied to systems which already have scrubbers. If they are burning high sulfur coal and have a sulfuric acid plume, the Enerchem approach would likely be more cost effective than the installation of a wet electrostatic precipitator.