New gasification technology offers promise for biomass plants
A new gasification process burns biomass to power a gas turbine with no turbine modifications
By Dr. Richard L. Bain and Dr. Ralph P. Overend, National Renewable Energy Laboratory, Department of Energy
Biomass moved a key step toward competitiveness with fossil fuels in a landmark event last summer, the groundbreaking for adding a facility to the McNeil Generating Station, a 12-year-old biomass power plant in Burlington, Vt. When completed, this facility will house the world`s first gasifier capable of burning biomass to fuel an unmodified natural gas turbine-generator for electric power generation.
Biomass gasification has the potential to marry the environmental advantages of renewable biomass fuels with power generation efficiency levels matching those of coal and oil power plants. Gasification promotion is part of a major initiative by the U.S. Department of Energy (DOE) to expand the use of biomass. DOE`s Biomass Power Program supports demonstration of new methods to produce feedstocks and development of equipment to convert feedstocks into electric power or process heat. For the McNeil project, Battelle Columbus Laboratory of Columbus, Ohio, took the lead in conceptualizing and developing a pilot-scale version of the gasifier. DOE provided technical expertise to Battelle and is cost-sharing the commercial scaleup with the gasifier`s licensee, Future Energy Resources Co. (FERCO) of Atlanta, Ga.
From steam to gas
Biomass fuel can be viewed as solar energy stored by photosynthesis in living plants. It includes any harvested plant matter used directly as fuel or converted into fluid fuels or electricity. In the U.S. today, more than 350 biomass-fired power plants are connected to power grids, with a combined rated capacity of about 7,000 MW.
Common feedstocks are residues from paper mills, sawmills, wood products manufacturing, urban wood surplus (such as tree trimmings or used pallets), biomass resulting from good forest management practices and orchard pruning, and agricultural residues (such as nut shells and straw). Power plant size is limited by feedstock availability within 50 to 75 miles of the plant. As a result, plants are of modest size–typically less than 25 MW. In the future, dedicated feedstock “plantations” will fuel biomass plants.
At present, nearly all biomass plants use steam turbines. Boiler systems include manually fed “dutch ovens,” in which biomass burns in a pile; automated, stoker-fed systems in which biomass burns on a stationary or moving grate; or fluidized-bed units in which biomass burns as it is fluidized from below by a continuous jet of combustion air. Fluidized-bed units offer the advantages of low NOx and CO emissions and high carbon burnout. Overall biomass steam-turbine system efficiency is limited to about 25 percent, due to the feedstock`s inherently high moisture content (about 50 percent) and the practical limits of power plant construction materials. This efficiency level compares to about 35 percent for a modern coal plant.
Biomass plants boast environmental advantages over coal- and oil-fired plants because the fuel is cleaner. For example, biomass contains minimal sulfur (< 0.1 percent by weight).1 As a result, one application may be to retrofit existing plants for co-firing coal and wood, reducing overall emissions and meeting Clean Air Act Amendments requirements. An even greater long-term environmental biomass advantage may be its potential for diminishing net U.S. CO2 emissions. CO2 is released during biomass combustion but taken up during photosynthesis of trees and plants used as feedstocks. Thus, when grown and used as a fuel on a sustainable basis, biomass is at least CO2 neutral.
However, if biomass resources are to become widely economical for power generation, generation technologies must be used that offer higher efficiency and lower unit capital cost at the modest sizes of most biomass plants. One solution would involve gas-turbine technologies, which achieve efficiencies that exceed those of conventional steam plants. To make these technologies effective, biomass must provide fuel to power generation systems in a form that does not require significant system modification. Biomass gasification could make this conversion possible.
In biomass gasification, heat applied to solid biomass transforms it into a gas, fueling a turbine for electricity generation. For even higher efficiency, the gas-turbine cycle can be combined with a steam cycle in an integrated gasifier combined-cycle (IGCC) system or used with a steam-injected gas turbine.
The Battelle/FERCO gasifier
The Battelle/FERCO gasification process for power generation incorporates a gasifier/combustor system, gas conditioning to remove condensables from the gas and a combustion turbine. When operating as part of an IGCC system, the system also includes heat recovery and a condensing steam turbine (Figure 1).
At the heart of the system lies Battelle`s patented biomass gasifier. Other biomass gasification systems derive from, or are influenced by, coal gasification. The current design grew from investigations, beginning in the late 1970s, led by Battelle`s Mark Paisley. “These investigations showed that wood is more chemically reactive than coal,” said Paisley. “Battelle`s innovation was to exploit this high reactivity and to heat the biomass fuel indirectly.”
In conventional systems, heat is directly applied to the biomass in the presence of a gasifying agent such as air or pure oxygen. Gasification with air produces a low-Btu gas, with a heating value about one-fifth that of natural gas. Gasification with pure oxygen produces a medium-Btu gas, with a heating value of about 300 Btu/standard cubic foot (scf), or about one-third that of natural gas. The costs of pure oxygen, however, are high.
The Battelle/FERCO gasifier uses two physically separate, circulating fluidized-bed reactors: 1) a gasification reactor in which the biomass is converted into gas and residual char and 2) a combustion reactor that burns the residual char to provide heat for gasification. The heat is applied to the biomass indirectly by a “carrier”–in this case, sand–that circulates between the two reactors. The combustor heats the sand to between 1,800 and 1,900 F. The sand is then fed to the gasification reactor, where it literally surrounds every biomass particle, quickly turning it into gas before returning to the combustor to be reheated.
“Because the hot sand method does not (directly) use air or oxygen, it avoids their expense and the generation of by-products such as CO2 or nitrogen, which have no heating value,” explained Paisley. “The result is a gas with the highest possible heating value (about 500 Btu/scf).”
The new design reduces capital costs compared to conventional systems because the hot sand method is quicker and achieves extremely high throughputs (pounds of biomass per unit cross section of the reactor). This opens the door to plant designs with smaller and less costly reactors than with other gasification processes and simplifies integrating the process into existing industrial sites. Throughputs will be 3,000 lb/hr-ft2–far higher than the 200 lb/hr-ft2 of conventional gasification systems.2 In the pilot-scale demonstrations to date, developers have fed 1,000 pounds of biomass per hour into the pilot plant`s 10-inch diameter gasifier. The scaled-up commercial version at the Burlington plant, which will have a gasifier 3 1/2 feet in diameter, will feed 200 tons per day of dry wood.
Capital costs are also lower because the medium-Btu gas produced by the gasifier is interchangeable with natural gas, which permits using existing gas turbines without modifying the combustors.
The new design also holds down operating costs because the feedstock requires only minimal preparation. In addition, the gasifier produces a gas with a consistent heating value, regardless of the feedstock`s moisture or ash content. This capability ensures reliable plant operation while allowing plant operators to choose among feedstocks with different moisture and ash levels to obtain least-cost supplies.
When biomass is gasified, the resulting gas contains trace amounts of condensable materials, or “tars.” In the Battelle/FERCO system, the high heatup rates made possible with the circulating sand, along with short residence times in the gasification reactor, effectively reduce the tendency to form these tars. However, while they constitute only a small component of biogas, tars can coat equipment surfaces downstream, fouling and damaging turbines or combustors.
The Battelle/FERCO pilot plant tested two gas conditioning systems for tar removal. A conventional two-stage scrubbing system, utilizing a venturi scrubber followed by a spray tower scrubber, removed all but a fraction of the tars. The other system–a fluidized-bed catalyst chamber employing a new catalyst called DN-34–essentially eliminated all tars from the gas and provided an acceptable feed gas for power generation (see Sidebar).
Once the system has conditioned the biogas and pressurized it to 100 psig, it is ready to fire a gas turbine. In pilot tests, the new gasifier fed biogas to a Solar Turbines 200 kW conventional gas turbine-generator. These tests marked the first time an unmodified gas turbine generator was run with biogas. The only modification to the turbine was the addition of a control valve to allow switching from natural gas to biomass-derived fuel.3
Projected economic performance
The Battelle/FERCO gasifier gets more net energy from the wood than burning alone and captures waste heat for recycling back into the process. In a plant performance projection, the system`s estimated net efficiency is approximately 36 percent (Table 1).
As shown in Table 2, the total capital investment for a hypothetical 56 MW plant would be $1,037/kW, according to FERCO`s study of the technology`s cost when it`s mature, making it as economical as a conventional coal-fired plant.
In August 1995, developers broke ground at McNeil to install the scaled-up gasifier. McNeil, a 50 MW plant, was built in 1984 and is the nation`s largest wood-fired power plant. The New England Power Pool operates the facility as an intermediate cycling plant, jointly owned by the Burlington Electric Department, Central Vermont Public Service Corp., Green Mountain Power Corp. and the Vermont Public Power Supply Authority. At full load the plant burns 85 tons per hour of wood chips made from residue portions of harvested trees or trees removed for thinning operations.
When complete, the scaled-up gasifier will be 20 times the size of the pilot-scale version and will account for 30 percent of the plant`s load. Construction will take place in two phases. The first phase, scheduled for completion in early 1997, will involve installing and testing a nominal 200 ton/day gasifier. Developers will fire the gasifier output as a supplementary fuel in the existing McNeil boiler.
The second phase will begin after developers gain operating experience with the gasifier and determine specific emissions from gas combustion. Under this phase, scheduled for late 1997 or early 1998, a new gas compression turbine will accept product gas from the gasifier, forming an integral part of a combined-cycle system.
Zurn NEPCO of Portland, Maine, and Redmond, Wash., will perform design and construction of the McNeil gasification plant. “We believe this technology shows great promise to be an efficient and economic process for utilizing renewable biomass resources,” exclaimed Zurn`s Bill Slack. “So we want to be on the forefront of its application through our expertise in designing and building power plants.”
For the Battelle/FERCO gasifier, the unfolding demonstration at McNeil will provide valuable experience with a promising new technology. Other companies and agencies already evaluating the technology and investigating future applications are Weyerhaeuser, General Electric, International Paper, Centerior Energy, the State of Iowa, New York State Energy Research and Development Authority, and the U.S. Environmental Protection Agency. Also, while the demonstration progresses, Battelle is continuing research to improve its gasification process and is testing the capability of the gasifier to burn shredded municipal solid waste.
DOE`s Biomass Power Program supports a variety of efforts to expand future use of the fuel for power generation. These efforts involve developing and demonstrating other gasification technologies, gas cleaning processes, technologies for cofiring wood and coal, and pyrolysis processes that heat biomass to produce oils (“biocrude”) that burn like petroleum.
In addition, for more than 15 years, DOE has been testing the feasibility of growing dedicated feedstocks close to power facilities. These feedstock plantations will address a key barrier to expansion of today`s biomass power plants–access to abundant, low-cost fuel. To date, researchers have made successful trial plantings of short-rotation, high-yield trees, grasses and grains.
With demonstration of advanced process technologies and feedstock establishment, DOE estimates that as much as 10,000 MW of biomass capacity will be in place by the year 2010. z
Dr. Richard L. Bain manages research in biomass power technologies at the National Renewable Energy Laboratory (NREL) in Golden, Colo. He is responsible for directing NREL`s activities in support of the DOE`s Biomass Power Program. Dr. Bain received a doctorate in chemical and petroleum refining engineering from the Colorado School of Mines in 1973 and has more than 20 years of experience developing new energy processes.
Dr. Ralph P. Overend is a principal scientist at NREL, specializing in biomass conversion systems.
1 “Biomass Power,” Program Overview Fiscal Years 1993-1994, National Renewable Energy Laboratory, Department of Energy, March 1995, p. 6.
2 Mark A. Paisley and Farris, Glenn, “Development and Commercialization of a Biomass Gasification/Power Generation System,” presented at the Second Biomass Conference of the Americas, Portland, Ore., Aug. 21-24, 1995, p. 4.
3 Ibid., p. 6.
“Biomass Power,” p. 3.
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The Battelle/ FERCO biomass gasification pilot plant
Key to the success of the Battelle/FERCO gasifier is the development, jointly by Battelle and DOE`s National Renewable Energy Laboratory (NREL), of a technique to remove the trace amounts of condensable materials, or “tars,” produced during biomass gasification.
The tars are primarily polynuclear aromatic hydrocarbons that have proved difficult to destroy by any process other than combustion. The quantity and chemical characteristics of tars produced in a gasification stream vary, depending on the specific technology used for gasification. In the Battelle/FERCO process, approximately 0.5 to 1 percent of the dry wood weight exits the gasification reactor as condensable tar.
Conventional scrubbing systems are the technology of choice for removing these tars from the product gas. However, such scrubbing cools the gas and generates a waste-water stream that must be disposed. In addition, wet scrubbing can leave a small fraction (10 to 30 percent by weight) of the tar in the gas as a fine mist. The mist is hard to remove and can create problems in downstream equipment, such as compressors and turbines. Because many of the chemical compounds in the tar are not very volatile, even small amounts will adhere to downstream surfaces, gradually fouling the equipment.
Since 1990, NREL has sponsored several research efforts to develop alternate hot gas cleanup and conditioning systems to eliminate unwanted biogas by-products. One effort involved identifying a chemical catalyst that would remove biogas tars. The catalyst (DN-34) resulting from this research destroys the tars by “cracking” the hydrocarbons and chemically converting them into additional fuel gas for the turbine. DN-34 is low cost, has chemical characteristics appropriate to the gasifier`s operating conditions and can be easily disposed.