Coal-mining operations are major generators of methane gases, and one company has set up an entire study project to capture it.

By Dan Rafter

The engineers with Pittsburgh-based Consol Energy knew the gas that its Bailey Mine released daily was a wasted resource. The mine, located in West Finley, PA, releases an average of 39,500 cubic feet of gas daily, gas that contains from 43% to 55% methane. In 2004, the mine’s degas vent released a total of 9.5 million cubic feet of methane into the atmosphere. In 2005, it released 6.9 million cubic feet of the gas.

In early 2006, Consol’s engineers, assisted by Davidson, NC–based Ingersoll Rand Industrial Technologies, began planning a way to do something about this wasted resource.

Consol, which is the largest producer of high-Btu bituminous coal in the United States, decided to install a coal-mine methane-fueled low-emission 70-kW microturbine generator, provided by Ingersoll Rand, on a degas vent at the underground Bailey Mine. The microturbine would then eliminate methane emissions by capturing them and converting them into usable electricity.

Consol is running the microturbine as a test, keeping the equipment operating for at least a year to see how well the unit performs and how much coal-mine methane gas it captures. The company’s goal is to prove that it can transform the mine’s methane emissions into a usable fuel. It’s a worthy task: The company operates 17 bituminous coal-mining complexes in six states. These mines release a lot of methane into the atmosphere. This represents not only a wasted source of potential energy but also a high concentration of greenhouse gases.

“We recognize that we vent the methane from the coal mines and that it is a resource,” says Deborah Kosmack of Consol Energy Inc. Research & Development, a separate arm of the company. “When it is vented into the atmosphere, we are losing a potential resource. We are trying to find alternative uses for that resource. That’s what this experiment is about.”

Consol already has a long history of retaining and using the methane the company generates during its pre-mining operations. Consol typically captures 90%–95% of this methane for alternative uses.

The quality of the methane generated from actual mining operations, though, is not as high as that emitted during pre-mining work and, therefore, is not as useful. Consol engineers, then, have wrestled with the question of how to use this wasted gas. The hope is that the microturbine experiment at Bailey will provide a blueprint of how Consol can capture the released methane from all of its mines and put it to use.

So far, the microturbine has been performing as advertised. It began operating at Bailey Mine in July 2006 and will continue running for at least one year from that time. During this period, Consol engineers will study the data to determine how efficiently the microturbine has been capturing the methane gas and whether the experiment is working well enough to expand it to the company’s other
mining operations.

Engineers entered the project with the hope that the microturbine would allow them to convert the low and variable concentrations of methane contained in coal methane gas into a useful form of energy. They also hoped they could provide the electric power generated by the microturbine to an existing power grid and, as a charitable enterprise, make a monetary contribution to a local school district equal to the value of the electricity generated during the project’s first year.

Most important, engineers wanted to determine the amount of useful energy the microturbine could economically produce when processing methane from a working coal mine.

So far, the results from the company’s experiment have all been positive. The microturbine is meeting the goals Consol had set for it.

“When we make electricity from this otherwise wasted resource, we are also doing something useful,” Kosmack says. “We are eliminating greenhouse gases. We are using this wasted resource. We can benefit from the electricity itself.”

Benefiting the Environment
Coal-mine methane is methane that, as its name suggests, is liberated during coal mining, either naturally or intentionally through drainage or degasification systems. Mining operations must remove this methane from their mines to provide a safe working environment for their miners.

Consol officials chose a 70-kW microturbine by Ingersoll Rand for the Bailey Mine project.

The gas is also one of the biggest culprits when it comes to global warming.

Scientists rank coal-mine methane as the second most important nonwater greenhouse gas, with a global-warming potential 21 times as great as that of carbon dioxide on a mass basis. The gas also accounts for about 10% of anthropogenic methane emissions in the United States.

Coal-mining operations are major generators of methane gases. Officials with Consol, for example, say that degasification methane accounts for about 8% of the company’s methane emissions, which were 27 billion cubic feet in 2004. This means a potential resource of more than 2 trillion Btu of methane was vented rather than captured from Consol’s degas operations in 2004 alone.

This helps explain why Consol went to such efforts—setting up an entire study project—to decipher ways to capture this wasted gas.

“Consol wanted to demonstrate the viability of this technology for potential future development,” explains Joe Catina, manager of projects for energy systems with Ingersoll Rand Industrial Technologies. “It has a lot of methane opportunities it would like to tap into.”

Problem is, it’s not easy to capture and use coal-mine methane. Supplying it as commercial-pipeline natural gas can be challenging because of the low methane concentrations in it, concentrations that are sometimes only 30%–50%. Any given degas vent also emits a relatively low amount of gas, making the capture and use of it often not worth the effort.

Consol officials had three options to choose from when trying to overcome these hurdles and capture the coal-mine methane. This gas is often naturally diluted with nitrogen, oxygen, and carbon dioxide as degas vents emit it. When the methane concentrations in these gases drop from 30% to 50%, it is classified as a medium-Btu fuel. This medium-energy-rated fuel can produce electricity with several different generation technologies, including reciprocating engines, fuel cells, or microturbines.

For several reasons, microturbines became Consol’s obvious choice.

Reciprocating engines offer low costs and high efficiency, but they generate high emissions and require significant maintenance, something Consol wanted to avoid. Also, these types of engine do not always handle fluctuating methane-concentration levels well.

Fuel cells offered another alternative, and several varieties of fuel cells are now under development. But most of these are still not economically competitive with alternative sources of energy. These cells do have high efficiency ratings, but they are quite costly. Also, many of these fuel-cell systems now under development are far too large to be used at the small degas vent that Consol had targeted for the experiment. Smaller fuel-cell units that do exist are even more costly than their larger cousins.

Microturbines, then, became the choice for several reasons. First, the microturbine’s simple design—a small gas turbine engine that drives a generator—produces a high power output and operates quietly. Second, the small size of a microturbine means Consol officials can request that it be installed at a remote site. And third, because the microturbines are modular, Consol can also request multiple-unit systems designed to tackle a variety of jobs at several different sites.

Consol officials also chose this option because microturbines, though having higher initial capital costs, boast far lower maintenance expenses than do reciprocating engines.

Going Micro
For the Bailey Mine project, Consol officials chose a 70-kW Ingersoll Rand microturbine engine rated at 13,500 Btu per kilowatt-hour higher heating value. The system includes a gas compressor, a recuperator, a combustor, a turbine, and an induction-type electric generator. Air enters the unit and is compressed to about 35 pounds per square inch in the gas generator compressor. The microturbine then heats the air to about 1,000°F in the recuperator.

The fuel, coal-mine methane, enters the unit and is compressed with a screw type of compressor/fuel booster. The compressed air and fuel are mixed and then burned in the combustor under
constant pressure.

The resulting hot gases expand through the two turbine wheels to perform work. The first stage drives the compressor, providing air to the cycle. The second stage powers a generator that produces electricity. The exhaust gas then exits the turbine and enters the recuperator. This piece of equipment captures a portion of the thermal energy and uses it to preheat the air entering the combustor, improving the overall efficiency of the system. The process ends when the exhaust gas exits the recuperator and the unit.

“This is an interesting project. The people at Consol are trying to prove that a basic technology works on a small scale. Then they plan on deploying it on a larger scale,” Catina says. “That’s why they decided to go with the one single 70-kilowatt turbine. They wanted to do something relatively small and keep their overall project assessment under control as opposed to working with multiple turbines and larger turbines.”

So far, the results have been positive. Consol officials have been able to capture the coal-mine methane gas and are using it to generate electricity that they are sending to an existing electric power grid.

These positive results are no surprise to Catina.

“We were confident the process would work,” he says. “We were confident that the microturbine would generate power. We have a lot of experience with lower-Btu fuels, such as the fuel we were going to be taking out at Bailey. That wasn’t our concern. Our technical concerns were related more to the actual operability and practicality of it. The fuel there has quite a bit of variability in the heat content. It is also a lot less processed than most of the low-Btu fuels we are accustomed to. That was our only concern.”

Officials with Consol, too, have been impressed with the results.

“We’ve been able to demonstrate that this unit is an innovative way to harvest and use a greenhouse gas that would normally be vented into the atmosphere,” says Nick Deluliis, president and chief executive officer of CNX Gas Corp.—which is majority-owned by Consol—in a written statement. “If the economic evaluation is positive, this technology can be applied in coalfields either in isolated areas that may lack access to electricity transmission lines or at individual commercial power-generation sites,” Deluliis adds.

Meeting the Challenges
The coal-mine methane gas that fuels the Bailey Mine microturbine freely vents from the mine’s degas vent. There is certainly enough gas available.

Other than methane, the primary component of the gas is nitrogen. The concentration of nitrogen in the gas ranges from 35% to 47%. The gas is also made up, though all at much smaller levels, of oxygen, carbon dioxide, ethane, and water.

Ingersoll Rand’s microturbine is designed to operate with medium-Btu gas, meaning gas with a rating of 350–575 Btus per standard cubic feet, with a minimum methane concentration of 35%. The microturbine’s methane conversion rate is extremely efficient, converting more than 99% of the gas.

Ingersoll officials estimate that each year the microturbine is operating, it will prevent emissions of 166 short tons of methane while producing 583 MWh of electricity. The 70 kW of electricity that the microturbine generates is used by the Bailey Mine itself for operations. Consol also plans to donate the monetary value of the electricity generated during the microturbine’s first full year of operation to the West Greene School District in Waynesburg, PA.

Though the experiment has proved a success so far, both Consol and Ingersoll Rand officials had to overcome challenges. The biggest? How would the microturbine handle the high amount of moisture in what is a relatively unprocessed gas?

In the winter, the gas coming from the Bailey Mine is extremely saturated with moisture. In January, when the temperatures dipped, this moisture froze, freezing the microturbine’s fuel line at the same time.

“We did have to go through a first-time learning curve,” Kosmack recalls. “So we had to make some adjustments to the equipment to improve the operation of it. That was to be expected because we were all learning. Getting it through the winter was the biggest challenge. We had freeze-ups in the fuel line. The gas that comes through that line is saturated with water. In the winter that is problematic. Any place that didn’t have freeze protection on it froze.”

Ingersoll solved the problem by adding a regenerative blower to the microturbine. The blower, which acts as a small radiator, controlled the temperature of the gas moving through the microturbine, allowing it to stay warm—and not freeze up—in the winter. Once Ingersoll added the blower, the microturbine ran without any major problems.

“Coal-mine methane, it turns out, has a lot more water in it than does average landfill gas. We had some issues with that,” Catina says. “We were prepared to take water out; we just didn’t expect it to be showing up in certain places. It all didn’t come out in the knockout phase. The thermostat changes we made with the blower took care of that.”

The moisture problem, surprisingly, was one of the few large issues the microturbine project faced. This, Catina says, is fairly unusual, especially for an experimental project that had never been attempted.

“The neat part of this project was that there weren’t any major issues,” he says. “There were little issues that happen in any project. But to the best of my knowledge, I don’t believe anyone on the team thinks there were any major problems.”

Because the microturbine project is being run by Consol’s research-and-development team, the experiment has its own Web-based data interface. This means officials with both Consol and Ingersoll Rand are obtaining data on the microturbine’s operating hours, efficiency, run times, starts, and stops. This information will prove useful if Consol officials decide to install microturbines in the company’s other mines.

“Now that this microturbine is up, I haven’t heard any squeaks from anybody at Consol,” Catina says. “Usually if there is a problem, someone is calling me up to let me know. Until we got rid of the freeze-up problems, I was hearing from Consol on a regular basis. Since then, it’s been quiet.”

Future Plans
Once Consol officials run an economic analysis of the microturbine’s performance at Bailey Mine, they will decide whether to install additional microturbines in some of the other mines the company operates.

If the microturbines can put coal-mine methane to use, it would certainly be a boost to the environment.

Methane vents are part of many operating and abandoned underground coal mines. Because the gas is low quality—and the vents are located in areas that are not easily accessible—it usually isn’t feasible to supply it to a commercial pipeline.

Many mine operators, then, cap the vents or let them vent their gases into the atmosphere. Microturbines, though, provide a modular solution to generating energy close to the source gas.

With their small size, microturbines can be tied together to generate as much energy as needed, based on the volume and quality of the gas being vented. The coal-mine methane gas released from a single vent emitting 20,000–100,000 cubic feet per day could produce 154–2,303 MWh of electricity a year with a microturbine operating at 28% efficiency.

It would be a shame to waste this potential source of energy, Kosmack says. “We are using this wasted resource. We can benefit from this source of power,” she notes. “We think this is an important experiment because of this.”

Based in Chesterton, IN, Dan Rafter is a frequent contributor to Forester publications.

DE - September/October 2007