By Steve Blankinship, Associate Editor
The story of converting the coal feeder system at Dominion Generation’s Mt. Storm plant in West Virginia has more dramatic plot twists than some Hollywood movies. They include a seemingly hopeless challenge, uncertainty, conflict, hope, disappointment and finally an ending that seemingly exceeded everyone’s most optimistic expectations.
Dave Welch, the project’s manager, described the events like this: “There were times when I thought I was going to lose my job over this. And I ended up getting a bonus.”
Mt. Storm’s new hoppers are larger than those they replaced. Photo courtesy of Dominion Generation.
Mt. Storm consists of three 550 MW coal-fired units. Units 1 and 2 were built in the 1960s and Unit 3 went into service in 1973. The plant was built as a mine mouth facility, with power sent hundreds of miles to serve customers in Virginia and the Washington, D.C. area. The mine-mouth fuel was eastern bituminous lump coal with low fines. The bunkers, feeders and piping were designed to handle lump coal. Eastern bituminous is a low volatile fuel, meaning stagnant coal left in hoppers or along the handling system does not spontaneously ignite.
Welch said that because scrubbers and other air pollution control equipment had not evolved to their current level of sophistication, raw coal could be burned as long as the precipitators worked well and the coal contained enough sulfur. Mt. Storm’s units were not designed to handle processed coal.
But environmental regulations, economics and availability eventually forced Mt. Storm to use processed coal. That meant using coal washed to lower sulfur content and ash. “Coal companies don’t want to waste a bit of fuel as the result of the washing,” said Welch. “They wanted to recover everything possible, including all the fines they could.” That meant the consistency of the coal became completely different from what the plant was designed for. And that’s when the trouble started.
Under dry conditions, the fine-textured fuel flowed well. But moisture can become a problem for fine coal. Even a little too much moisture can impede the coal flow through the slopes and openings of the coal handling equipment. Mt. Storm was at the constant mercy of changing weather and atmospheric conditions.
Such dusty, fine coal had become a problem at plants all over the east by the 1990s. Plants had entered an era of handling problems, and not just in the coal yard. Once the fuel reached the bunker still more flow problems emerged, Welch said.
“We might have 70 feeder pluggages in a shift” involving multiple feeder losses at the same time, he said.
Plant operators couldn’t clear feeder blockages fast enough to prevent the units from becoming unstable. Operators began to burn stabilizing oil, which can be expensive. Load swings occurred and megawatts were being lost.
“You can’t meet load when steam temperatures and pressures are swinging above the boiler design points and creating wear and tear on the boiler tubes,” said Welch. Plant personnel spent much of their time fighting feeder losses and system wear and tear.
The first attempt to remedy the situation was to install flow-enhancing devices like vibrators, air cannons and rappers on the bunker hoppers. Those devices helped some, but “it still took a lot of baby-sitting to keep up with all of it,” said Welch. “It was not a good solution.”
In 2001, Dominion hired Jenike & Johansson (J&J), an engineering firm that specializes in resolving solids flow problems. J&J’s recommendation was to convert the bunkers and feeder systems to mass flow. In a mass flow system, coal travels down the bunker and hopper walls, through the feeder and into the mill instead of rat holing. The arrangement means that little stagnant coal can be trapped in the system. One of J&J’s suggestions was to change the geometry and material surfaces of the bunker and bunker hoppers. The firm also recommended a different style of coal feeder.
Welch visited a plant in Pennsylvania where PennCrusher’s Posimetric feeders had already been installed. During his tour the operators told him the feeders worked and how their steam chartwhich had been “all over the place”were now steady. Outside operators said that if you can get the coal to the feeders, the feeders will put it in the mill.
“And the coal they were handling at the time was so wet you could make a ball out of it,” Welch said. “The feeders were handling a product that was wetter and more tenacious than what we were handling.”
Changing the feeders was a small part of the project. The cost of changing the geometry of the bunkers turned out to be far more than anticipated. One of the major changes was that the original feeders had a two-foot diameter pipe connecting it with the bunker hopper. Lab testing on a comprehensive range of Dominion fuels determined that to achieve the needed bridging distance, the pipe would have to change to a 72-inch by 22-inch stainless steel rectangular conduit to assure the right flow characteristics. Carbon steel could not be used because coal would fail to release from it.
That meant the hopper above it would have to be changed to a stainless steel design. The upper hopper had to have partitions installed and plating that ran all the way up into the bunker.
“When we put the (cost) figures together, the number scared everybody,” said Welch. We almost stopped the project.”
Dominion went back to J&J and suggested a partial rather than a complete mass flow system. That would mean changing the feeders and the bottom hopper, but not the upper hopper, bunker or bunker box itself. J&J said that approach would work.
“They told us the upper hopper would still have a large rat hole in itabout 9 or 10 feet across,” Welch said. But the hole would remain too large to impede flow. And because the coal had a low volatility, stagnant coal in the upper hopper would not ignite on its own (although Powder River Basin coal would.) As a result, the partial solution was a fit for low volatile coal.
Dominion’s upper management was skeptical, but decided to allow the conversion of one unit to proceed. Unit 2 was the first to be refitted. Despite the fact that the feeders were just a small piece of the total cost, it was the largest feeder order PennCrusher had ever taken up to that time. Replacement of the bunker required more than 100 tons of steel.
But the story was about to take another turn. As work was set to start, a contractor load analysis determined that the existing feeder deck flooring would not support the hoppers and reinforcement steel. Not only would the feeder deck not support the new feeders, it was already overstressed. Adding the feeders would make the situation worse. Since reinforcement beams could not be fitted under the feeder deck due to numerous cables installed there already, additional beams were placed above the feeder deck to add suspension support for the feeders.
Once the crisis passed, the Unit 2 mass flow conversion was completed ahead of schedule. Once the unit came back up all went well, at least at first.
“Our goal was to reduce the feeder pluggages by 90 percent and we ran for a couple of weeks with no problems at all,” said Welch. Then the unit began to experience pluggages. But they were not above the feeder or in it; rather they were below the feeder.
“That was a problem we had never had before,” Welch said.
The source of the problem was the fact that the feeder discharge was 74 inches wide and narrowed to 11 inches as it entered the coal mill. Moist coal was sticking at the transition. Plant personnel went to work changing the way the tube was attached to the chute. Because the transition had to be smooth, pipe all along that part of the route was replaced and transition points were improved. “That solved the problem,” Welch said.
A few issues remained that were related to the need to control the feeders. These problems were quickly resolved. As Unit 2 ran smoothly for several months, benefits began to emerge that no one had anticipated. Operators said Units 1 and 3 needed to be shut down so the same improvements could be added as soon as possible. As scheduled outages occurred, that’s what happened. The mass flow conversion to units 1 and 3 went smoothly and improvements to chutes below the hoppers were enhanced over the first project.
“We eliminated load losses and swingseven eliminated temperature swings we used to have even when we didn’t have feeder pluggages,” said Welch. “Before the conversions, we were happy to just get a plus or minus 15 degree swing. After this, we were able to control to plus or minus five degrees.”
Another unexpected benefit has been completely eliminating the need for stabilizing oil. The project team predicted the mass flow conversion would let the plant use less oil. Instead, operators totally eliminated stabilizing oil on each unit.
“The fuel department asked us what happened because we were not buying tankers of oil like we had been,” Welch said. “We’re talking about more than a million dollars a year.”
He also said that parasitic power consumption was reduced as amperage swings caused by the uneven fuel flows were eliminated.
“The amperage doesn’t swing much any more and the power needed to bring the coal to the furnace is less,” Welch said.
Seems like fair weather is in the forecast once again at Mt. Storm.