Harnessing the Power of Biomass
Biomass power producers aren't pausing while current U.S. federal policies favor renewable fuels development in an effort to reduce the nation's dependence on foreign oil. Renewable power will play a vital role in the world's attempts to reduce greenhouse gases. Now that the first-generation technology has matured, work continues on developing new technologies based on lessons learned.
Fuel issues topped the list in the report, titled "Lessons Learned from Existing Biomass Power Plants." The fuel issues included acquiring a low-cost fuel, while paying attention to where the biomass is piled and how it's fed into the plant and planning for feedstock flexibility. The lessons are valid today, says Bain, who is with the National Bioenergy Center within the National Renewable Energy Laboratory (NREL). Even though Bain's work is now focused on biofuels, managing the biorefinery analysis group at the NREL, he has kept an eye on the development of the biopower industry.
Some of the lessons learned were from the McNeil Generating Station in Burlington, Vt., which has a quarter century of experience turning wood chips into power. It was one of the first and biggest public utility biomass generators when it started up in 1984. About 5 percent of its feedstock is waste wood-shipping pallets, yard waste and Christmas trees-supplied by local residents and businesses. Another 25 percent comes from area manufacturers-saw mills, furniture factories and a veneer manufacturer. About 30 contractors supply the remaining 70 percent from forest residues-byproducts of the harvest for timber, pulp or firewood. The contractors use a mobile chipper on-site and truck the wood chips to McNeil's railhead storage yard. Because the McNeil station is within the Burlington city limits and next to a residential district, the plant is required to receive 75 percent of its feedstock by rail, which adds 20 percent to the cost of wood, says John Irving, manager of the McNeil station.
The McNeil plant is configured to burn wood chips, natural gas or fuel oil, and switches fuels based upon cost and availability. The plant's wood cost was stable at $19 per ton until four years ago when the price increased to $29 per ton. However, Irving figures the McNeil station's cost to produce electricity from wood is $50.45 for one megawatt hour (or 5 cents per kilowatt hour), which is still a bargain when compared with other fuels. At current rates, the electrical cost would be $151 per megawatt hour from fuel oil and $98 from natural gas. McNeil's generating load is determined by the New England power pool on a daily basis. The pool directs it to generate specific levels of power at specific times of the day based on the needs of the entire power pool.
Cofiring and Pyrolysis
As it relies on experience, the biomass power industry continues to develop new technologies focused on efficient fuel utilization. Now that the first generation of direct combustion technologies have matured, cofiring and pyrolysis are two of the technologies that are being studied further.
Switchgrass gets a lot a press as the future feedstock for cellulosic ethanol, however, in Ottumwa, Iowa, Alliant Energy Corp. is ready to go commercial cofiring switchgrass with coal to generate electricity. The Iowa project illustrates many of the lessons highlighted in the 2000 report.
Nearly 15 years ago, the Chariton Valley Resource Conservation and Development group began investigating switchgrass as a potential new crop for area farmers. Alliant Energy partnered with the U.S. DOE to study the potential for cofiring switchgrass in its 725 megawatt coal-fired Ottumwa Generating Station. A test in 2000, burned 1,300 tons of switchgrass to gather preliminary results and establish the next steps in evaluating the feedstock's potential. In December 2003, another test used 1,500 tons of locally grown switchgrass, and the final three-month continuous firing test completed in May 2006, burned 15,000 tons. Based on the series of tests, several issues had to be addressed, says Bill Johnson, manager of biomass markets for Alliant. "Handling hay is different than handling coal," he says. "There are dust hazards, mechanical issues plus the combustion characteristics." They also had to test the suitability of the fly ash byproduct for use in concrete to build roads. Those standards are based on coal fly ash. "That's a pretty important economic resource for us," Johnson says. "What can't be sold for use in a cement mix has to go to the landfill." The standards now accept the mixed ash.
Using agricultural biomass for power can present challenges, Bain admits. "In California, orchard prunings are an agricultural waste that presents no problems," he says. However, high levels of potassium content in biomass can cause slagging problems. Slagging occurs when minerals change to liquids in a high-temperature boiler, which can foul heat transfer surfaces, reduce efficiency and even cause shutdowns. To prevent these problems from occurring, each potential feedstock has to be evaluated for performance. Even the timing of the biomass harvest has an effect. Switchgrass, for example, is better suited for cofiring when it's harvested in the fall because the nutrients get stored in the roots, Bain says. If producers try to get two cuttings, it could create problems in the power plant because summer switchgrass has higher potassium content.
With the testing completed and the plant modifications made, the Ottumwa plant is ready to supply 5 percent of its energy needs with switchgrass. That will require as much as 200,000 tons of grass annually from 50,000 acres of land and involve as many as 500 farmers. "Right now a local group is developing a business structure for aggregating switchgrass," Johnson says. "We don't want to serve as the aggregator."
Building on what it has learned, Alliant is planning for additional generation capacity at two existing power plants near Cassville, Wis., and Marshalltown, Iowa, to burn 10 percent to 20 percent biomass. The company sees a benefit from reducing its carbon footprint, Johnson says. "We see benefits for the community and environment in providing new markets for cover crops that can be grown on marginal lands and by providing new business opportunities for the people involved in the aggregation, transportation and processing of the biomass," he says.
Johnson is evaluating potential feedstocks for the new projects to find out whether the feedstocks can be sustainable for the life of the power plant. "The boilers will have to be designed for the fuel source that is most prevalent," he explains. The benefits may be impressive. At Cassville, initial projections indicate that the combination of new control systems and the addition of biomass cofired with coal will reduce emissions as much as 70 percent, Johnson says. "That's going from a 200 megawatt to a 500 megawatt plant."
While cofiring biomass is being developed to "green" coal power, gasification technology is being researched to maximize the potential for utilizing a wider range of fuels with greater efficiency. Based on the chemistry of pyrolysis, gasifiers are used to heat biomass to high temperatures to create a biogas that can be directly used for cofiring. The challenge has been to devise systems to purify the gas for wider and more efficient applications.
The McNeil Generating Station hosted a demonstration project funded by the DOE to add 12 megawatts of power from gasification to the 50 megawatts it generates from conventionally fired woody biomass. "It worked well in cofiring," Irving says. The biogas was cofired with wood chips to create steam to power the turbines. The research project focused on improving the pyrolysis gas for use in a combined-cycle gas turbine. "That implies you can take the gasification process and clean it suitably to use [the biogas] in a gas turbine, and that's the hard part. That's what we were working on when the plug was pulled," Irving says. When the DOE and other cooperators ended the project in 2001, McNeil mothballed the demonstration's gasifier. Part of the reason it didn't continue with the gasification project at Burlington is that there wasn't a lot of agricultural residue to utilize as a feedstock in Vermont. To make matters worse, there is a negative public perception toward using municipal solid waste so that is ruled out as a potential feedstock, he adds.
Whether to choose cofiring, gasification or direct combustion becomes a site specific decision, Irving says. He visited a 200-megawatt coal-fired plant in Lahti, Finland, that added a 40-megawatt thermal fluidized bed gasifier, which allowed them to burn peat, wood, tires and trash. "They take low Btu (British thermal unit) gas from the gasifier and blow that into their coal-fired boiler and turn it into electricity that way," he says.
There are a half a dozen gasifiers in different configurations continuing the work of creating a better system, according to Glenn Farris, president and CEO of Biomass Gas and Electric LLC. "We believe it's the future of the biomass business," says Farris who worked on the pyrolysis project at McNeil. Atlanta, Ga.-based Biomass Gas and Electric took the first concrete steps in developing two pyrolysis-based power plants when it signed power agreements for plants in Tallahassee, Fla., and Forsyth County, Ga.
Using advanced pyrolysis systems with combined-cycle turbines can generate electricity with 40 percent efficiency, he says, compared with direct combustion of biomass that generates power with efficiencies in the mid-20 percent. Florida State University in Tallahassee is starting a sustainable energy program to integrate hydrogen gas and fuel cell technologies with gasification. "These configurations might take the efficiency up to the 60 percent range," he says.
Biomass Gas and Electric is developing projects choosing whichever gasification technology works best for the situation, Farris says. The company is planning its first commercial application based on advanced pyrolysis gasification and steam reformation, to supply the city of Tallahassee with 38 megawatts of electricity for its municipal power system and 60 decatherms of methanated biomass process gas for its natural gas pipeline. In Forsyth County, Biomass Gas and Electric will use an updraft gasifier to deliver 28 megawatts to the grid in a power contract with Georgia Power Co. The plant will be located next to a construction and demolition landfill which will supply clean woody waste to the gasifier. Farris points out that Georgia has more commercially managed forests than any other state. Because of the loss of pulp and paper production overseas, he says, "we have a surplus of that type of woody material."
Biomass Power Grows
Nationwide, renewable electrical power from sources other than hydroelectric dams is slowly but steadily growing. The latest annual statistics analyzed by the Energy Information Administration (EIA) show renewable power other than hydro grew 5 percent in 2005, compared with the previous year. Biomass from all sources-wood, agricultural residues, municipal solid waste-contributed the biggest portion of nonhydroelectric renewables, and wind was growing the fastest with a 25 percent increase over the previous year. Geothermal and solar were also added to the renewable power column along with wind, hydro and biomass. When industrial combined heat and power generation is added into the mix, biomass takes on a much bigger role. The wood products industry uses wood residues to generate nearly half its total energy needs. Viewed from a different perspective, the EIA reports that of all the sectors consuming renewable energy, electric power consumes as much as the transportation, industrial, commercial and residential sectors combined.
At 2.3 percent, nonhydroelectric renewables are just a tiny portion of the total U.S. electrical power sector. Should public policy place a higher priority on reducing greenhouse gases, the displacement of coal with biomass will become a favored strategy. Coal power comprises 32 percent of the nation's electrical generating capacity, and in 2006 coal generated half the nation's electricity. The interest in renewable energy is growing on the state level, with nearly half the states adopting Renewable Portfolio Standards. The standards require public utilities to generate an increasing percentage of power from renewable energy. (See Industry News story Renewable portfolio standards spread.)
Bain expects biopower to become a priority once again at the DOE where he is now working on biofuels. "If the primary objective is the reduction of foreign oil imports, then transportation fuels are more important. If our primary object were the reduction of carbon for global warming, then substituting biomass for coal in power would be the best thing you could do," Bain says. "I've been here long enough that I never throw my old files away. It's going to come back some day."
Susanne Retka Schill is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 746-8385.