Preaching to the Converted

preaching to the converted

A sylvan source—200 tons of wood a day—is helping to fuel a Green Mountain gasifier.

By Michael Valenti, Senior Editor

One of the largest American users of biomass energy is helping utility, government, and private industry participants refine gasification technologies. The gasifier operating at the Joseph C. McNeil Generating Station in Burlington, Vt., converts up to 200 tons of wood per day into a gas that is used to co-fuel a wood-fired boiler to generate electricity. The lessons learned at McNeil were presented at ASME International's Turbo Expo in New Orleans last June.

Biomass gasification involves converting organic fuels to create a relatively clean combustible gas. The advantages of biomass gasification are its ability to convert relatively cheap stocks, such as sawdust, switch grass, bagasse, agricultural wastes, or specifically grown energy plantation crops like willow trees, into fuel that will not produce as many emissions, especially of alleged greenhouse gases, as will the direct burning of organic solids. These advantages are augmented when biomass gas fuels energy-efficient gas turbines.

The process used in the McNeil gasifier is called SilvaGas and was invented by Battelle Laboratories of Columbus, Ohio. It was purchased and commercialized by the Future Energy Resources Corp. of Norcross, Ga., in 1992. From 1980 to 2000, Battelle tested the process on a pilot scale, coupled to a 200-kilowatt Solar Spartan gas turbine at its Columbus labs.

Two hundred tons of locally produced wood are converted to a gaseous fuel each day in the FERCO gasifier operating at the Joseph C. McNeil Generating Station in Burlington, Vt.

The pilot unit was a stainless steel vessel with an internal diameter of 10 inches. The Battelle researchers gasified up to 10 dry tons, or 15 to 20 wet tons, per day of a wide variety of biomass products fed into the vessel, including switch grass, municipal waste, wood chips, backyard waste, and energy plantation crops.

Screw conveyors transported the biomass from lock hoppers into the gasifier. At the same time, sand heated to 1,500°F was delivered into the gasifier, to form a fluidized bed of material that distributes heat throughout the biomass. This caused the biomass to break down into a medium heating value gas, 500 Btu per cubic foot, and a small amount of char that mixed with the sand.

A cyclone separated the char, sand, and product gas. The char was sent to a combustor. The heat from the combustor was used to reheat the sand, which circu-lated in a closed loop. The ash from burning char was deemed harmless for interment in a landfill, or for use in soil treatment.

INDIRECT HEATING IMPROVES BTU CONTENT

"Typically, gasification processes use steam to directly heat, or air to partially combust, the raw feedstock in the gasifier," said Mark Paisley, a chemical engineer who was one of the inventors and testers of the process at Battelle. "The nitrogen in the air and the carbon dioxide produced by partial combustion dilute the product gas to a low heating value, about 150 Btu per cubic foot. By indirectly heating the feedstock with sand, we eliminate the dilution to produce a much more energy-rich product gas." Paisley is now vice president of technology at Future Energy Resources Corp., which also calls itself FERCO.

Plant operators improved the materials handling process of the McNeil gasifier, raising the unit's capacity to 350 dry tons per day.

The relatively high energy value of the biomass gas meant it could be combined with natural gas or distillate oil, which the company believes is a necessity to commercialize the process.

The researchers used a variety of cleaning techniques to prepare the gas for burning in the turbine. "We developed a proprietary catalyst we called DN 34 to convert the condensable organics, basically aromatic tars, into hydrogen and carbon monoxide," Paisley said. "We removed solid particulates with standard wet scrubbing and sent the gas to a compressor before directing it to the Solar turbine."

The gasifier was tested for more than 20,000 hours at Columbus. Its designers learned to evaluate the process of gasifying a range of feedstocks. With a high degree of confidence, they also could identify the conversion rates of biomass into gas, biomass gas characteristics, and its cleanup requirements. This data would enable engineers to adapt the FERCO process to commercial scale when the opportunity presented itself.

SCALING UP FOR BURLINGTON

In 1994, the U.S. Department of Energy and FERCO jointly funded the construction of a commercial-scale biomass gasification demonstration at the McNeil station in Burlington under the auspices of the DOE's Biomass Power Program. McNeil is a conventional wood-fired plant about a mile from Lake Champlain. It has been operating commercially since June 1984.

The plant is jointly owned by the Burlington Electric Department, Central Vermont Public Service, Vermont Public Power Supply Authority, and Green Mountain Power. At the time of its construction, it was the largest wood-burning power plant in the country, and one of the largest in the world. The plant has a capacity of 50 megawatts net, which is comparable to the average electric load for Burlington, Vermont's largest city. McNeil can also use natural gas or fuel oil.

According to Burlington Electric, 70 percent of the wood that fuels McNeil comes from low-quality trees that cannot be used for manufacturing or pulping, and from forest residues. About 25 percent of the wood fuel is the byproduct from local sawmills—sawdust, chips, and bark—and 5 percent is recycled urban wood waste that is free of contaminants. When the McNeil plant operates at full load, it consumes up to 76 tons of wood chips or 550,000 cubic feet of natural gas per hour. Using gas as a backup fuel improves the flexibility of McNeil. The thermal energy is used to heat a boiler to create steam that is sent to a Brown Boveri turbine generator manufactured in Oerlikon, Switzerland. The turbine was specifically designed to accommodate McNeil's cycling service, and can provide a gross output of 59.4 megawatts.

The fuel gas produced at the McNeil power plant can be burned in conjunction with wood, natural gas, oil, or any combination of these fuels.

FERCO finished constructing the McNeil gasifier in late 1997. It has an internal diameter of 40 inches, and converts 200 tons per day of wood wastes into a fuel gas that is burned in the plant's boilers, supplying 10 to 15 percent of McNeil's fuel needs. The fuel gas can be burned in conjunction with wood, natural gas, oil, or any combination of these fuels.

Plant operators started up the unit and made process improvements, including materials handling, solids separation, and product gas scrubbing. "The original feed system in Vermont had been supplied by a company that was not experienced with handling biomass, which is very heterogeneous," recalled Paisley. "We contacted biomass handling consultants who recommended internal changes in the screw conveyors and lock hoppers to remediate the problem." This improvement raised the gasifier's capacity to 350 dry tons, or 500 wet tons, of biomass per day without any other modification of the system.

The feed system screens wood particles larger than 3 inches. As was the case at Columbus, FERCO's DN 34 catalyst cracks tars in the gas stream, and char produced is used to reheat the sand. Flue gas is sent to an economizer to enhance the process's energy efficiency.

FERCO uses a personal computer-based data acquisition and control system equipped with WonderWare software for the McNeil gasifier.

Although the biomass gas is usually mixed with other fuels at McNeil, it was burned alone during an interruption in the plant's wood supply.

By indirectly heating biomass feedstocks with sand, the SilvaGas process generates a more energy-rich product gas than the steam heating or partial combustion gasification processes that are commonly used.

The improved gasification process helped reduce the time required to start up a cold system from 24 hours to 12 hours.
The next step in developing the FERCO process at McNeil is sending the biogas directly to a turbine, and harnessing the turbine's exhaust heat in a combined cycle. "We will use our proven catalytic system to clean up the gas, compress it with a standard natural gas compressor, such as Dresser or Caterpillar, then send it directly to the turbine," Paisley said.

In the integrated gasification combined-cycle system, the biomass process will have 40 percent energy efficiency, compared with 25 percent for conventional single-cycle conventional biomass schemes.

Currently, FERCO has two development agreements to install its biomass gasification process. One installation would gasify the wood waste from a pulp and paper plant, and the other would gasify urban wood waste materials. Both are to generate power on-site.

"Both of these facilities are in the United States, and we have discussed our process with European, South American, and Australian firms as well," Paisley said.