Power Plant Emissions Get Cleaner
MOUNDSVILLE — Ohio Valley residents can breathe easier, according to American Electric Power’s Wayne Irons, since emissions controls were installed at the company’s Mitchell Power Plant a year ago.
The Mitchell plant installed its first sulfur dioxide-catching scrubber about one year ago and added a second last April, plant manager Irons said. And he is pleased with their performance.
“We are making better than designed capture,” he said. “We are at about 98 percent capture of SO2.”
That success, coupled with the installation of Selective Catalytic Reduction Systems that remove nitrogen oxides produced in the coal-burning process, has made the air cleaner for area residents. It also is reducing acid rain conditions for residents of the northeast part of the country, he claimed.
According to ecologist and Wheeling Jesuit University professor of biology Ben Stout, having this clean air technology in place “will have a positive effect, especially when we have an atmospheric inversion.” He said the SO2 and nitrogen oxides are both strong acids. Removing them in the coal-burning process is a good thing.
“Every little bit helps,” he said.
Irons agreed. “If we were the problem in the past, we should be seeing a major reduction in acid rain in the Northeast. Cars and steel mills also contribute to the problem, but our reductions do have effects locally as well as regionally.”
Irons said during the first year of scrubber operation, there have been no forced outages due to the scrubbers or the SCR systems, which remove about 90 percent of nitrogen oxides. The SCRs use ammonia gas to create a chemical reaction that converts nitrogen oxide into nitrogen and water.
The nitrogen oxides, he said, are a contributing factor to ozone depletion. Because SCRs are temperature-dependent, they are not running year-round at this time, but they are expected to be shortly. At the present time, SCR technology is the best available technology for making significant reductions in nitrogen oxide emissions, according to Irons.
“We have had no major problems with either system — just a few minor things which you could expect. Nothing major. No show stoppers. They have been good equipment,” he added.
The Mitchell Power Plant is a 1,600-megawatt generating facility. It has twin 800-megawatt generating units and started producing electricity in 1971.
It is jointly managed with the neighboring Kammer facility, which began operation in the early 1950s and has no scrubbers. No plans are being made to install them at that plant because of its age. It is required to use exclusively low-sulfur coal.
Electricity from both plants is put into an electric grid. Where the power goes depends on where it is needed. Much of it goes to Ohio. Since the installation of the emissions-cleaning scrubbers at the Mitchell Plant, Irons said, there has been about a 4-percent reduction in the plant’s electric output.
According to Irons, no plans are being made at this time to upgrade the plant with higher-efficiency turbines to make up for the lost output. He noted one of the two units at the plant was upgraded about two and a half years ago — before the scrubbers were installed.
AEP invested $900 million to put the scrubbers and SCRs in place at the Mitchell facility. The cost included more than 3 miles of new conveyor belts and structures to transport limestone from the Ohio River where it is unloaded, coal for blending and to carry gypsum. Gypsum is a by-product of the scrubbing process, and it is sent to the new CertainTeed Plant across W.Va. 2, where it is now being stockpiled for future use in the production of building products such as drywall.
During the “scrubbing,” chemical and mechanical processes remove sulfur dioxide from gas produced by burning coal. Exhaust gas from a coal-fired unit’s steam generator is routed through absorber vessels, where chemical reactions take place and the sulfur dioxide is removed. In the absorbers, flue gas passes through a mixture, or slurry, of pulverized limestone and water, which is sprayed into the flue gas stream.
The sulfur dioxide then reacts with the slurry and forms the gypsum. A dewatering system, consisting of a vacuum belt filter, literally sucks most of the water from the mixture leaving just the gypsum, which is able to be used without additional treatment.
“Our gypsum quality has remained in line with their (CertainTeed’s) specifications. It’s real good. We have very little waste. It’s been very encouraging.” Irons said. “We’ve totally met their specifications. They are taking 100 percent of it, which is great. It means we don’t have to landfill it and it’s a good deal.”
A railroad spur also was installed across W.Va. 2 to connect the two facilities.
“Carbon dioxide is also an emerging issue, as well as how to reduce it effectively,” Irons added. “It’s a lot bigger issue than most people realize. We can’t reduce carbon dioxide output. We can capture it, but it is not efficient or cost-effective. The only process is to pump it underground, and it’s very expensive.”
The Mitchell plant uses on average more than11,000 tons of a medium sulfur mixed blend of locally mined high- and low-sulfur coal daily. Irons said the mix is used to generate the greatest efficiency from the scrubbers. The average annual coal use at the plant is more than 4 million tons. The Kammer Plant uses an average 6,120 tons of low-sulfur coal on a daily basis.
Irons also noted limestone used in the scrubbers is brought in from the Oglebay Norton Co. in Michigan by river.
“I don’t think people fully realize just how big of a project this was,” he concluded.