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More energy more efficiently from wood
From Vermont Woodlands, Autumn, 1998
By Nancy Bazilchuk
People have been burning wood for heat and energy ever since humankind discovered fire. But at the McNeil Generating Station in the Burlington Intervale, this oldest of energy sources is being married with cutting-edge technology in a process that engineers say could be the cleanest, most efficient way to burn wood yet.
The 14-year-old McNeil station already uses wood to make kilowatts, yet it does so in the most traditional of ways: by boiling water to make steam, which is then used to turn turbines. The $18 million Vermont Biomass Gasification Project, which is sited at McNeil and started operating this year, converts wood into a high-energy gas that ultimately can be used to drive more efficient gas turbines.
The project is partially funded by a $9.2 million Department of Energy grant awarded in 1994. Workers finished building the gasifier in late 1997 and McNeil's giant boilers first burned the gas from the gasifier in Februrary.
It's an experiment that is being watched by engineers around the world because of its potential to bring efficient energy to developing countries without relying on fossil fuel, says Ralph Overend, principal scientist at the United States National Renewable Energy Laboratory.
"Burlington, Vermont is at the forefront of the world competition to modernize biomass," he says. "The modernizing world wants energy, but the only way we can get there is to convert biomass into advanced fuel forms," like the gas being made at McNeil.
Turning wood chips into gas is a more efficient way to generat electricity than McNeil's existing plant because it cuts a step out of the whole process. With McNeil's wood-fired boiler, wood is used to boil water and make steam. The steam turns turbines to generate electricity, and then is condensed back into water and returned to the boiler to be turned back into steam. The whole process has an efficiency of about 25 percent, says Ronald Belval, assistant general manager of the Burlington Electric Department, which owns half of the McNeil plant.
Turning the steam back to water takes energy, reducing the efficiency of the system, Belval says. Using gas to directly fire a turbine and create electricity has an efficiency of about 35 percent, he said. And as turbine technologies advance to allow hotter operating temperatures, efficiencies could top 45 percent, he said.
The gasifier works like this: Chipped wood enters the gasifier -- a long cylinder several stories tall that is lined with heat resistant bricks -- and is mixed with sand heated to 1,800 degrees F. The hot sand liberates the carbon, hydrogen and oxygen in the wood as a combustible gas. This gas, along with the sand that has been cooled in the process and the char -- the remaining unburned wood -- are transferred to a giant cyclone separator, which is like a giant centrifuge. The gas in then cleaned in a scrubber, after which it can be burned in the McNeil boilers or in a new gas turbine.
The cooled sand and char go to a second cylindrical vessel called a combustor, where air is added to burn the char, which in turn heats the sand agaoin to 1,800 degrees F. The sand and ash go to a second cyclone separator, where the sand is then returned to the gasifier and the ash is collected for disposal. Even the hot flue gases from the combustor can be used in the process after they have been run through a scrubber.
"It's essentially a zero discharge system, because all the flue gas and the product gas (from the gasifier) go to the boiler. The only discharge is ash, which is the same as what McNeil is doing anyway," says Stuart Hand, the project manager for Future Energy Resources Company (FERCO) of Georgia, the gasifier's chief developer.
McHeil works with a private contractor who mixes the ash with limestone so that it can be used as a soil conditioner.
The future of Vermont's energy?
Vermont's own Department of Public Service has also kept a close watch on the test. If the experiment works as expected, Public Service Commissioner Richard Sedano sees a time when wood gasifiers could produce as much as 20 percent of Vermont's annual electricity demand of about 1,000 megawatts.
"We need to have technologies like wood-gasification available as existing power plants are retired, so we can have energy in a sustainable way," Sedano said. Vermont Yankee, Vermont's largest in-state source of electricity, generates 540 megawatts, of which about 300 megawatts are used in state. Vermont Yankee's operating license is due to expire in 2012. The next largest in-state producers of electricity are Vermont's hydroelectric power plants -- utility owned and independently owned -- which account for 11 percent of the state's power supply.
The gasifier at McNeil "is the first step toward high-efficiency biomass energy production at a large size," Sedano says. "It is terrific that it is happening, and it is even more terrific that it is happening in Vermont."
The state's 1997 Comprehensive Energy Plan and Greenhouse Gas Action plan specifically indentifies wood-fired power plants as a way to curb emissions of gases scientists say are warming the Earth.
While burning wood produces carbon dioxide, one of the main greenhouse gases, growing trees consume carbon dioxide from the atmosphere -- it's an essential part of photosynthesis. Thus, as long as new growth exceeds the volume of the trees burned in a wood-fired plant, the technology's net contribution to the global warming problem is zero. That's in contrast to burning non-renewable fossil fuels like coal or oil. The carbon in fossil fuels was stored by the plants millions of years ago while the plants were growing. That carbon, when burned today, combines with oxygen to produce carbon dioxide.
Because of its size and ready wood supply, McNeil is one of the few places in the country where a large-scale test of the wood gasifier could be built. By conventional standards, McNeil's existing 53-megawatt plant is tiny -- the Vermont Yankee nuclear power plant is 540 megawatts, in contrast -- but it is one of the largest such wood-fired generators in North America. That meant the plant already had all the infrastructure the gasifier would need, says John Irving, plant manager. In 1989, McNeil's boilers were fitted to burn natural gas as well as wood, which makes it relatively easy to burn gas from the gasifier, Irving says.
Making gas out of wood is not in itself a new technology; the concept has been around at least 50 years. But wood's drawbacks -- chiefly the amount of space needed to store fuel -- has limited the technology's development, especially with the widespread availablity of oil, natural gas and coal as cheap alternatives.
Maine and New Hampshire also have wood-fired boilers to generate electricity, but when it was first built in 1984, McNeil was the largest wood-fired boiler in the world. The large size of the plant and the correspondingly large size of the gasifier are part of what make the McNeil project unique.
The project is being conducted in two steps. First, plant managers have been making gas out of wood in the gasifier since late February. After they've tweaked the system and assured engineers that the process works, FERCO will buy a gas turbine for installation and testing by June of 1999.
Depending upon the size of the turbine purchased, the McNeil plant could churn out as much as an additional 20 megawatts of power. But gasifier, combined with the turbine, will be twice as efficient as the present conventional McNeil plant, says Hand.
One of the big selling points of the McNeil Station, and also the gasifer, is that the wood it burns can be relatively low quality. Seventy percent of the wood chips that are burned at McNeil come from low-quality trees and tops from timber harvests. McNeil employs four foresters led by chief forester Bill Kropelin to purchase the chips because the plant's operating permit requires owners to certify the wood has been harvested properly and won't damage wildlife habitat. The remainder comes from sawmill residues, such as sawdust, chips and bark. Another advantage of the plant is that is can burn homeowners' yard waste or construction debris that is free of paint, stains or other contaminants.
Demonstrating to the world at large that the gasifier can work is more than just proving the technology. McNeil's experience and ability to handle large supplies of wood chips is a critical part of making the overall project work.
Juggling supply and demand can be a logistical nightmare as the McNeil foresters found out when the existing McNeil plant began operating in June, 1984. Like a homeowner with a new woodstove, managers didn't know exactly how often they would operate the plant and how much wood they would need from day to day to meet demand. One of the uncertainties comes from the New England Power Pool, a consortium of all the region's power companies that decides when power stations in New England should run, based on the cost of the plant and power demand. So at first foresters kept a large supply of wood on hand.
By September 1985, about 140,000 tons of wood chips were piled around the plant like debris from a giant's sawmill. One pile covered seven acres and was more than 90 feet high, says plant manager Irving, who at the time was working for another firm.
"Then it started smoldering -- spontaneous combustion," Irving said. Engineers decided to split the pile in two, but they were in for an ugly suprise. The wood chips at the very bottom of the pile were as much as a year old and had fermented, giving off a pungent, sickly sweet smell that wafted through Burlington like swamp gas and angered neighbors and Burlington residents.
"It was pretty stinky,"Irving said.
Based on that experience and after considerable experimentation, foresters now make sure their wood chips are no more than five months old. The storage piles are small and shaped in a way that dissipates heat. "It's not high tech, but it's all part of making wood energy a viable energy resource," says chief forester Kropelin.
The existing plant at full capacity has the ability to burn 2200 tons of wood a day, and the gasifier will consume another 360 tons or more a day. However, the plant doesn't run every day. At 3.5 cents per kilowatthour, it's more expensive on average than more traditional oil-fired power plants, which typically produce power for between 2 and 3 cents per kilowatthour. That cost difference means the New England Power Pool asks McNeil to run only about one-third of the year. Typically, the plant runs during the winter and the summer, when power demands are greatest, said Ronald Belval. IN the past year, McNeil operated about 45 percent of the time, Belval said, because it was helping to pulg holes left by a number of power plants in southern New England that were out of commission.
In addition, foresters have to plan for any number of annual or unusual circumstances, from the spring mud season, when logging trucks can't travel the dirt roads that typically flank Vermont's forested lands, to the January 1998 ice storm, the worst since Europeans reached Vermont.
After the ice storm, wood supply boomed and the plant took truckloads of chipped storm debris, so much so that by April the plant was awash in wood chips -- more than 55,000 tons stored both at McNeil and at a railhead in Swanton. The timing was fortuitous, coming at a time of peak demand and providing a good stockpile in anticipation of the annual mud season slowdown.
In addition to McNeil's current sources, they are conducting a test of different quick growing trees that could be planted by farmers as energy crops. (See sidebar on short-rotation crop trees.)
The state's energy plan suggests that in the next 15 years, new wood-fired power gasifiers providing 154 megawatts could be built, which would require as much as 590,000 cords, or about 1.5 million tons of wood annually. Assuming, as the state's plan does, that Vermonters can sustainably harvest as much as 6 million tons of wood each year, gasifiers could become an increasingly important purchaser. Under that circumstance, farmers planting energy crops can both provide a good income for the farmer and provide fuel for Vermont's energy needs, Kropelin says.
"We hope we can find a system for providing an alternative crop for idle land," he said. "There's no thought of taking forest land out of production for this.. But by planting these trees to hold the soil and take up nutrients, we think this could be a viable crop for farmers."