Built in Värnamo, Sweden, in 1993, the first plant to demonstrate the biomass integrated gasification combined cycle had a total efficiency of 82 to 83 percent and an electrical efficiency of 33 percent

Biomass is obtained from numerous sources, including by-products from the timber industry, agricultural crops, raw material from the forest, major parts of household waste, and demolition wood. Several factors make it an attractive option for power generation. Biomass, if properly grown and managed, is a renewable resource. It does not add carbon dioxide to the atmosphere because it absorbs the same amount of carbon in growing as it releases when consumed as fuel. Its low sulfur content means biomass combustion is much less acidifying than with coal, for example. Also, the ashes from biomass consumption, which are very low in heavy metals, can be recycled.

The use of biomass for power generation has increased over the last decade. In the United States, electricity generation from biomass grew by 7 percent each year between 1990 and 1994, reaching 59,000 gigawatt-hours in 1994. Such growth could result in an industry with a capacity of approximately 30 gigawatts, producing 150,000 to 200,000 gigawatt-hours of electricity by 2020. In Europe, biomass energy currently accounts for about 2 percent of total consumption, and the European Commission predicts that figure will reach 15 percent in the European Union over the next 15 years.

Unlike renewable-based systems that require costly advanced technology (such as solar photovoltaics), biomass can generate electricity with the same type of equipment and power plants that now burn fossil fuels. Many innovations in power generation with other fossil fuels may also be adaptable to the use of biomass fuels.

Various factors have hindered the growth of the renewable energy resource, however. Most biomass power plants operating today are characterized by low boiler and thermal-plant efficiencies; both the fuel's characteristics and the small size of most facilities contribute to these efficiencies. In addition, such plants are costly to build. Today's best biomass-based power plants cost approximately $2,000 per kilowatt of electricity to build, with a thermal efficiency of about 40 percent, while large coal-fired stations cost about $1,500 per kilowatt, with a thermal efficiency of approximately 45 percent. The main challenge to using biomass for power generation, therefore, is to develop more-efficient, lower-cost systems.

Advanced biomass-based systems for power generation require fuel upgrading, combustion and cycle improvement, and better flue-gas treatment. Future biomass-based power-generation technologies have to provide superior environmental protection at lower cost by combining sophisticated biomass preparation, combustion, and conversion processes with postcombustion cleanup. Such systems include fluidized-bed combustion, biomass-integrated gasification, and biomass externally fired gas turbines.

The development of fluidized-bed-combustion technology over the past 20 years has significantly increased the use of various biomass and waste products in power and heat generation. In Finland, for example, the installed capacity of fluidized-bed boilers has already reached 1,200 megawatts.

The technology has been applied to a very wide range of fuels including wood-based fuels and residues such as wood chips, sludge from paper mills and deinking plants, and refuse-derived fuel. Two different combustion methods can be used: the bubbling fluidized bed (BFB), in which all the solid material is stationary in the bed; and the circulating fluidized bed (CFB), in which the solid material circulates through the bed to a cyclone, then back to the bed. Fluidized-bed boilers are now available with capacities up to 100 megawatts with BFB and 250 megawatts with CFB.

Biomass residues from the timber industry will help produce up to 200,000 gigawatt-hours of electricity in the United States by 2020

The choice between BFB and CFB technology is largely linked to the choice of fuels. BFB, the simpler and cheaper technology, has been favored for plants exclusively fueled with biomass or similar low-grade fuels containing highly volatile substances. Enhanced CFB designs, on the other hand, may be competitive even in smaller biomass-fired plants. In either case, the low operating temperature of fluidized-bed boilers means that effectively no thermal nitrogen oxides are formed. Also, because of the low sulfur content of biomass, sulfur emissions control is not required.

The benefits from fluidized-bed-combustion technology have attracted considerable interest in biomass heat and power generation. In January, Brista Kraft, a new 122-megawatt biomass CFB plant (with a power output of 44 megawatts and heat output of 75 megawatts) was activated in the area around Stockholm, Sweden. The boiler is a compact second-generation CFB from Foster Wheeler Energia in Oy, Finland, and the steam turbine is from ABB in Stal, Sweden. Wood chips are used as the main fuel, and peat is the support fuel. The plant is designed to burn 250 million to 300 million kilograms of wood waste annually.

One promising technology is the gasification of biomass. Gasification involves transforming solid biomass into a gas that consists mainly of nitrogen (50 percent), carbon monoxide (20 percent), and hydrogen (15 percent). The process results in a combustible gas that can be used in a gas turbine, for example, to generate electricity. Also, the gas can be separated from unwanted constituents prior to combustion. Specific enabling areas of research and development include improving hot-gas cleanup to remove alkaline compounds, identifying the source of turbine-blade deposits, and finding methods to remove particulates. In the United States, gasifier projects in Vermont and Hawaii are part of a major Department of Energy initiative to demonstrate biomass gasification for electricity production.

Several processes have been proposed over the past 10 years for advanced biomass gasification. The first plant to demonstrate the biomass integrated gasification combined cycle, built in 1993 at VŠrnamo, Sweden, produced 6 megawatts of power and 9 megawatts of heat. The system includes a pressurized circulating fluidized-bed gasifier, a gas turbine, and a steam turbine. The total efficiency of the Värnamo plant is 82 to 83 percent, and the electrical efficiency is 33 percent. The project is a joint venture of Foster Wheeler and Sweden's largest privately owned electric utility, Sydkraft in Malmö.

Termiska Processer AB (TPS) in Nyköping, Sweden, has proposed a system that is based on low-pressure gasification. The TPS scheme involves gasification at about 1.8 bar, followed by a series of gas-conditioning steps prior to the gas turbine: cracking of tars to noncondensable gases, cooling, baghouse filtration, scrubbing, compression, and reheating. The TPS proprietary tar-cracking technology is fundamental to the goals of generating cool, clean fuel gas for the compressor while avoiding the production of significant quantities of noxious wastes.

Components of TPS technology have been demonstrated on a pilot or small commercial scale. For example, the TPS gasifier is in operation in a 12-megawatt plant using refuse-derived fuel at Greve-in-Chianti, Italy. TPS has also participated in the Brazilian 30-megawatt biomass-power demonstration project funded by the Global Environment Facility. a U.N. Environment Program/World Bank program. In addition, Yorkshire Environmental in Yorkshire, England, is planning a 10-megawatt wood-fuel power plant that uses TPS gasification technology.

The biomass externally fired gas-turbine system provides another effective way of adapting solid fuel such as biomass and coal in a gas turbine, thereby increasing overall plant efficiency while lowering emissions. The main features of this turbine are an atmospheric combustor and a high-temperature heat exchanger, replacing the conventional combustion system in an open gas-turbine cycle. In this system, the biomass is directly fired in a biomass boiler. The discharge air from the compressor is heated in a high-temperature heat exchanger in the boiler. Thus, only clean air is sent to the expander, so the turbine's gas path is never exposed to the corrosive elements in the fuel. The high-temperature heat exchanger is the key piece of equipment that needs to be developed to achieve a high turbine-inlet temperature.

By adding a bottoming cycle to the gas turbine, an externally fired combined cycle can enhance overall cycle performance. Heat recovered from gas-turbine exhaust can be used to generate power and heat in the bottoming cycle. Efficiency can also be improved by adding water into the compressor discharge air of this gas turbine to form an externally fired evaporative gas turbine (EVGT) or humid air turbine.

In a program initiated by the Swedish National Board for Industrial and Technical Development in Stockholm, several Swedish universities, companies, and utilities are collaborating to accelerate the demonstration of the advanced EVGT for natural-gas firing, especially in small-scale units. A natural-gas-fired EVGT pilot plant (0.6 megawatts of power output for a simple gas-turbine cycle) should start operation in Lund, Sweden, in 1998.

An important consideration for the future use of biomass-fired power plants is the treatment of biomass flue gases. Biomass-combustion flue gases have high moisture content. When the flue gas is cooled to a temperature below the dew point, water vapor starts to condense. By using flue-gas condensation, sensible and latent heat can be recovered for district heating or other heat-consuming processes; this increases the heat generation from a cogeneration plant by more than 30 percent.

Flue-gas condensation not only recovers heat but also captures dust and hazardous pollutants from flue gases at the same time. Most dioxins, chlorine, mercury, and dust are removed, and sulfur oxides are separated out to some extent. Another feature of flue gas condensation is water recovery, which helps solve the problem of water consumption in evaporative gas turbines.

This technology can be integrated into the various biomass-based systems already discussed. In Sweden, for example, flue-gas condensation is in use in more than 60 units for heat generation or for both heat and power generation. At least 40 of these units are biomass-fueled plants mainly for district-heating purposes, with a fuel input of 5 to 10 megawatts, corresponding to about 2,000 gigawatt-hours of additional heat produced per year.

Even if these new technologies prove workable in the long term, other barriers exist to the growth of biomass power: the high cost of delivered fuel compared with fossil fuels, fuel-supply reliability, and a lack of understanding of the environmental impacts of the technologies and fuel-supply systems. The complexity of biomass-power infrastructure systems is also challenging for utilities more familiar with well-established coal and natural-gas markets. The unique economics of biomass for power generation must be understood and strategically developed so the fuel can become competitive with cheap and abundant fossil fuels.

Energy production based on biomass may be hampered by limitations in the supply and/or quality of biomass. Cofiring can be used to attain maximum fuel flexibility and fuel-supply security. Biomass cofiring with coal may provide a cost-effective, near-term opportunity in some countries with a tradition of coal-fired generation. This strategy will reduce risk and investment costs.

Several other factors are perhaps outside the control of the biomass industry, such as prices and markets for competing fuels. In addition, environmental issues may cause some governments to strengthen policies toward renewable energy, which could make the economics of biomass-based power generation more favorable for investors or prompt consumers to be more willing to pay a few cents more per kilowatt-hour for biomass-derived electricity. New regulations and taxes for fossil fuels and bioenergy also play a significant role.

Interest in biomass may go beyond simply using it as fuel. Biomass could offer near-term business advantages and more strategic, long-term value. The benefits derived from biomass power generation, such as emissions offsets, waste reduction, and local economic growth, can enhance the technology's overall appeal to utilities.

Advanced biomass-based systems have the potential to contribute significantly to the energy supply in the future. The central goal for development is to create an economically and environmentally sustainable system, based on a fuel-supply infrastructure that will enable investment in modern high-efficiency power-production cycles.

Jinyue Yan is an associate professor, Per Alvfors is an associate professor, Lars Eidensten is a project leader, and Gunnar Svedberg is a professor and vice president of the Royal Institute of Technology in Stockholm, Sweden.

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