cashing in the chips

A portable system aims to make it practical to take the electricity out of small waste heaps.

From a distance, small businesses seem to have small problems. But for the men and women running them, small businesses are no less challenging than large corporations, and they present opportunities for innovative thinking.

One such opportunity is in handling waste biomass. Waste-to-energy projects have proven to be cost efficient for large-scale operations. But small companies don't produce enough volume of waste to make dedicated plants worthwhile. Too often, waste such as wood chips from a lumber mill or cornhusks from a canning plant will simply take up space in a landfill.

One way to get around the problem of small volume is to make the waste processing plant portable. If one plant could service a number of small producers over a regional area, the fixed costs could be spread around sufficiently to make such an operation profitable. Also, as waste accumulates slowly at small-scale manufacturing sites, a mobile waste-to-energy plant could avoid downtime by making the rounds, as it were, and visiting sites where enough waste has piled up to provide several days' worth of continuous operation.

That's the theory that my colleagues and I at the Energy & Environmental Research Center at the University of North Dakota in Grand Forks wanted to test. We have built a system that marries a small-scale gasifier to a set of 30-kilowatt microturbines. The result, we think, is a portable, modular power system that could not only convert solid agricultural and forest waste into electricity, but also provide a return on investment on the order of 20 percent per year.

There isn't anything particularly new about the concept of using portable gasifiers to produce electricity. As far back as 1994, I worked on a project with the Environmental Protection Agency at the Marine Corps Base in Camp Lejeune, N.C. We built a 1 MW gasification power plant and ran connected to the grid for 100 hours. The process worked, but we encountered prohibitively large maintenance difficulties. The key stumbling block until now has been getting the power plant to work with a minimum of human intervention.

Researchers hope to fit this gasifier-microturbine combo onto a tractor-trailer.

In our current project, my group is partnering with FlexEnergy of Mission Viejo, Calif., to develop a portable system. FlexEnergy is working in cooperation with Capstone Turbine Corp. to develop a 30 kW machine capable of handling very-low-Btu gas at atmospheric pressure. In addition to our research center, the California Energy Commission, the National Renewable Energy Laboratory, the U.S. Department of Energy, the State of Arizona, and the National Rural Electric Cooperative Association support the work being conducted by FlexEnergy.

To show that such a system has the potential to work on a small scale, FlexEnergy's microturbine was coupled to a number of different biomass gasifiers. These systems produce a fuel gas from various biological materials, such as wood chips, straw, and even pecan shells.

The system we finally settled on is a downdraft biomass gasification technology employing venturi scrubbing and filtering of the gas. Downdraft gasification was chosen for its ability to reduce tar content of the product gas. Expected total gas contaminant concentration prior to gas cleanup is 1,000 ppm, versus 100,000 ppm seen in updraft and fluid-bed gasification.

Fuel is automatically conveyed to the top of the reactor using an agricultural platform feeder. The material is gasified in the reactor and cleaned with a venturi scrubber, which can remove particulates of less than a micrometer. The gas is then passed through a series of four filters: a coarse filter to coalesce residual water, oil, and heavy tar; a rejuvenating active sawdust filter; a similar passive filter, and, finally, a paper cartridge filter.

These filters can reduce total gas contaminants to less than 10 ppm. The resultant gas is so clean, in fact, that the flame produced by burning it is all but invisible. In order to prove to one skeptic that there was a flame, we used it to roast a hot dog.

Because of the low heating value—just 130 Btu per cubic foot—the output of a conventional piston engine using this gas must be derated by 50 percent, and operation without a stabilizing fossil fuel is difficult. The FlexEnergy microturbine shouldn't run into this problem, as it is designed specifically to run on fuels that are far too weak for any internal combustion engine.

What's more, after more than 60 hours of operation (running on wood chips from a sawmill), maintenance has proved to be minimal, requiring just 15 minutes over a typical eight-hour shift.

A full-scale test is slated for early next year. A log home manufacturer on the Navajo Reservation in Arizona will run a mobile system that will gasify wood waste to power three microturbines. This project should establish the economic baseline for the technology and provide the proof of concept needed for commercialization.

With capital costs expected to be in the range of $1,000 to $1,500 per kilowatt, the technology should be affordable for rural manufacturers, who produce the bulk of biomass waste. Indeed, such a system might be a practical solution to the question of what to do with underbrush left behind by logging operations. Rather than leaving it as potential fuel for forest fires, this tinder can be used to feed electricity to the grid.

Darren D. Schmidt is a researcher at the Energy & Environmental Research Center at the University of North Dakota in Grand Forks.



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