Micro-Scale Power: Recycling"Waste" as Fuel

Gasification, probably the wave of the future for big coal-using power plants, can play other roles as well. Bradley Paul, an associate professor of mining and mineral resources engineering, has been working closely with a new Illinois company called Coaltec Energy USA to optimize and commercialize modular gasifiers for energy use by factories, mines, ethanol plants, and communities.

These small, simple-to-operate gasifiers have two modules. The first gasifies the fuel, and a second burns the gas for heat. If you want to generate electricity with the system, it's a simple matter to add an off-the-shelf heat recovery boiler and turbine.

Modular gasifiers are nothing if not versatile. They can use a whole range of carbon-based feedstocks, gasified separately or with coal, that are typically treated as waste, not fuel. Many of these feedstocks are biomass in some form or another.

Most modular gasifiers in the United States are being used to gasify wood waste, for example. But Paul has chosen to focus on "fuels that other gasification enterprises have steered clear of or ignored," he says.

"They're usually high in moisture or high in ash [unburnable mineral content], and they're found all over. You wouldn't want to transport them very far--you'd be spending more on transportation than they're worth as fuel. To be able to use them economically, you've got to be able to use them for something nearby." Hence the idea of microgeneration: generating heat or electricity for local users.

For example, from 5 to 10 percent of Illinois coal ends up as waste fines at preparation plants, which clean the coal for burning. These tiny particles, below recovery size, are removed from the plant's water stream so the water can be reused. The damp fines are typically disposed of in slurry ponds. "Most of our power plants simply aren't designed to handle that kind of moisture," Paul says.

But modular gasifiers are a different story.

"Suppose a coal mine fed the fines back into a small modular gasification plant to run the mine," says Paul. "We've done studies showing that, for most mines, 70 percent of the energy from that modular plant would fill all of the mine's electrical needs. So the mines are literally throwing away enough energy to run the entire operation, and the 30 percent left over could be sold to communities in the area."

A couple of research grants Paul had in the 1990s brought together the industry partners who later formed Coaltec Energy, which began operation in 2000. (Paul has a small ownership interest and serves on the company's board of directors.) Funding from the Illinois Clean Coal Review Board enabled Coaltec to build a commercial-scale gasifier system for testing and demonstration at the Illinois Coal Development Park in Carterville, which is co-administered by SIUC and the state.

Besides coal fines, Paul has modified the gasifier system to work with corn stubble and with ethanol mash, the partially decomposed grain left over from ethanol manufacture. Other fuels could include other crop residues, poultry litter, and even manure.

Air emissions data from the system--measuring nitrogen oxides, carbon monoxide, sulfur dioxide, and particulate matter--have been excellent with both coal and biomass fuels, Paul says. The first test the team ran was with extremely-high-sulfur coal fines from a processing plant. "I kept asking them to run the test over again," he says, "because the sulfur dioxide was coming in lower than sulfur emissions at scrubbed power plants." What he discovered was that the ash in the fuel acts to "grab" the sulfur during gasification, binding most of it up in solid form.

This technology "is not going to replace power plants," says Paul, "but there are niche markets. We can reduce waste volumes considerably"--and get some pretty clean power in return.

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