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bioweb.sungrant.org » Technical » Biopower » Technologies » Combustion » Direct-Firing
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Biopower is the use of biomass resources to produce, either singly or in combination, electricity, heat/steam, and cooling. Most electricity produced in the U.S. uses coal with natural gas use increasing (19% of electricity; 27% of natural gas use). Only 3% of the electricity is from petroleum (~3% of total petroleum use). Other major sources of electricity generation include nuclear and non-biomass renewable. About 1.5% of the total electricity generated is the U.S. is from biomass (DOE, 2006b), and in 2005, used 1.8 Quads (1 Quad = 1015 Btu) of wood and waste to generate 0.87 Quads of electricity and useful thermal output.
Industrial settings are well-suited to using biomass to produce biopower because generally their requirements are relatively consistent and the combustion process can be operated at a relatively steady condition. In 2002, the industrial sector utilized 1.56 quads of biomass, of which 0.48 quads were used to generate electricity and 1.08 quads generated useful thermal output (DOE, 2005).
Currently, the paper and pulp industry is the largest producer of biomass electricity and heat/steam, generating more than 50% of its energy needs internally, from black liquor and waste wood. The wood products industry also generates more than 50% of its energy needs internally.
With the exception of biopower produced from black liquor, most electricity biopower production is on a small scale (<50 MWe, average 20e MWe) compared to fossil-fuel fired power plants (typically 100 to 1300 MWe) due to the dispersed supply of biomass, its relatively low energy density, and its high moisture content.
Direct Firing Technologies. Currently, most direct fire applications occur in industrial settings where the biomass resource is a by-product of the manufacturing process. They involve the production of electricity alone, or produce electricity and also capture the heat that is generated for use in the manufacturing process or for heating buildings (combined heat and power—CHP). The installed capacity for combined heat and power generation for industrial and commercial operations in 2000 was 49,000 MW, and the potential capacity was 163,000 MW (CBO, 2003). Home and commercial heating applications have also been growing.
Pile burners and stoker combustors are the most commonly used direct fire technologies. Pile burners using wood have traditionally been used in industrial applications. They consist of a two-stage combustion chamber with a lower (primary) combustion area and an upper (secondary) combustion area. A separate furnace and boiler are located above the secondary combustion chamber. Fuel is introduced onto a grate in the primary section and air is fed upward through the fuel and inward from the walls. Combustion is completed in the secondary chamber section using over-fire air. Fuel may be introduced either on top of the pile or from an underfeed system. The underfeed system provides better combustion control, but increases system complexity and lowers reliability. Ash is manually removed by dumping from the grate. Pile burners typically have low combustion efficiencies, can be erratic and difficult to control, and operate cyclically due to the need to manually remove the ash.
Stoker combustors utilize a moving grate which allows continuous ash collection, eliminating down time and providing for continuous operation. Fuel is introduced in a thin layer over the grate using either a pneumatic or mechanical stoker. Stoker fired boilers were first used in the 1920’s and originally designed for coal. In the late 1940’s the Detroit Stoker Company introduced the traveling grate spreader stoker boiler for wood. The bottom of the grate is cooled using under-fire air which defines the maximum grate temperature. Newer designs include the Kabliz grate which uses a sloping, reciprocating grate that is water-cooled. Its construction is simple and yields ash carryover in the flue. |
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| Use of Biomass in Direct-Fire Applications |
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Historically, wood and wood industry waste products (such as black liquor produced in the pulp and paper industry) have been the major sources of biomass feedstocks. More recently, other biomass resources have been used including those from agricultural processing (e.g., oat hulls; corn stover; wheat, rice, and grass seed straw; sugar cane bagasse, corn-ethanol byproducts; nut shells; and fruit pits among others) and urban wastes products (waste paper; cardboard; combustible organic matter from municipal solid waste). Dedicated herbaceous energy crops (e.g., switchgrass and Miscanthus) and short rotation wood crops (e.g., hybrid poplar and willow) are also being evaluated as feedstock sources for biopower production.
Increasing the use of biomass resources to produce biopower raises several technical and feedstock supply logistics issues. Technical issues involve adapting the biomass (fuel) feed system and the burn technology to the biomass feedstock(s). Biomass feedstocks typically have lower energy and weight densities than fossil fuels currently used, and the feeding system may need to be altered to accommodate these characteristics and maintain the efficiency of the unit. For example, the density of wood chips is about 10 dry lb/ft3 with an energy density around 17 MMBtu/dry ton (8500 Btu/ft3). By contrast, coal has an energy density of 25 MMBtu/ton and natural gas has an energy density of 1.03 MMBtu/ft3.
Alternatively, the density of the feedstock itself can be altered. Technologies to increase the density of biomass feedstocks include processing them into pellets, cubes or briquettes. A recent Iowa study found that cubing of corn stover did not significantly increase the weight/ft3 density (~25 lb/ft3) relative to large round bales, but did allow the material to more easily flow into a stoker combustor. However, the energy density (Btu/ft3) was insufficient to allow peak performance. Switchgrass could not be successfully cubed if harvested in fall or early to mid winter, but could be if harvested in late winter or early spring. The bulk density was similar to corn stover. Oat hulls could not be cubed under any circumstances, and were pelletized into ¼ inch diameter pellets with a bulk density of 40 lb/ft3, which was sufficiently high to permit the boiler to operate at peak levels. Densification, if performed at the feedstock production/collection site can reduce the cost of transporting the feedstocks. The estimated costs of densification range from $18-$30/ft3 depending on the process selected.
Biomass fuels vary in their ash content, but typically have higher ash contents than coal, burn at different rates than coal, and may have lower ash fusion temperatures which must be considered when using biomass resources. The moisture content of the feedstock is an additional issue when using biomass resources, as it affects conversion efficiency and feedstock storage and handling needs. Moisture content can vary substantially by feedstock type ranging from about 15% for switchgrass harvested after dormancy, to 16 to 35% moisture for corn stover, and up to 50% moisture (wet basis) for freshly harvested wood.
Currently, most biopower production involves the in-house use of processing wastes, with feedstock supplies generated on site and used in an integrated manner with the manufacturing process that generated the waste. Utilizing the waste product generally lowers overall manufacturing costs for the company. The current market situation for an industrial or commercial facility favors meeting its own heat/steam and possibly cooling needs, and generating electricity up to its own needs using a combined heat and power (CHP) process. Expanding biopower production to other types of facilities will require obtaining biomass feedstocks from off-site locations, establishing appropriate feedstock handling and storage systems, and generally learning how to work with an unfamiliar feedstock. |
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| Economics of Biopower Production |
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Most biopower facilities are less than 25 MWe in size compared to fossil fuel facilities (100 to 1300 MWe). Incorporation of energy efficient features are generally more economic in large facilities relative to small, and as a result, most biomass electric facilities have conversion efficiencies of around 20% (high heat value basis) while coal facilities typically have conversion efficiencies in the mid-30% range, and some technologies (e.g., combined cycle fired with natural gas) can be nearly 50% efficient. To justify the more advanced conversion technologies, a sufficiently large scale is needed. However, this situation may be changing as efficiency features are being transferred to smaller capacity facilities. The addition of dryers and the incorporation of more-rigorous steam cycles can increase the conversion efficiency of biomass direct fired facilities to about 30%, which could lower the estimated capital cost from the current cost of about $2000/kW to $1275/kW (Bain, 2003). The estimated capital and operating costs for larger scaled direct fired biomass CHP facilities are shown in table 1 (Bain, 2003).
Other studies estimate the cost of wood-fired boiler-steam turbine systems ranging from $700-1900/kW with maintenance costs ranging from $0.008-0.015/kWh (Lemar, 2004). Estimated boiler/steam turbine costs for wood compared to coal are shown in table 2.
Preliminary evaluations of a variety of small modular biopower systems have been conducted. Capital costs are estimated to be around $1700/kW for a 5 MW system and between $3000 and $4000/kW for a 1 MW system (Bain, 2000).
In addition to cost, small-scale commercial and industrial biopower producers also face additional institutional considerations. Assume that a small industrial or commercial facility can purchase electricity for $0.08/kWh. If the facility generates its own electricity, the value of that electricity is its avoided cost (i.e., the cost of not having to buy electricity) and is worth $0.08/kWh to the facility. If the facility generates more electricity than it needs, and tries to sell the excess to a utility, the utility will purchase the electricity for its avoided cost, which may be in the range of $0.03/kWh. The main economic incentive for an industrial or commercial facility is to produce electricity primarily for its own use, rather than to sell to an electric utility.
However, the increasing price of natural gas, the most common fuel used to produce heat and steam, creates more incentives for commercial and industrial facilities to use biomass for heat/steam requirements than in the past. Even at $75/dry ton, biomass is only $4.50 to $5/million Btu, which is significantly less than recent natural gas spot market prices of $6 to $14/MMBtu. If only 10% of the CHP potential of 163,000 MW was supplied by biomass, approximately 39 million dry tons of biomass would be required annually. It should be noted however, that biomass availability and price, as well as natural gas prices and electricity prices vary substantially by region which significantly affects the economic potential to use biomass for electricity or CHP.
RETScreen International (2005) offers a computer tool that can analyze costs of biopower production for a number of technologies. An overview of biopower was published by Research Reports International (2006). |
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| Environmental Impacts of Biopower Production |
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Emissions from several existing biopower facilities are summarized in table 3.
The biomass stoker emissions include 23 plants built in California prior to the institution of more restrictive emission standards. Similarly, the biomass fluidized bed emissions include 11 California plants built prior to the new standards. For comparison purposes, emissions for the Pine Tree biomass gasification facility and coal fired and natural gas fired systems are included. The Pine Tree biomass power plant in Westminster, MA utilizes gasification, rather than direct combustion technology (fluidized bed with a low CO and VOC emissions; mechanical collector and baghouse filter to control particulates; selective noncatalytic reduction (SNCR) system for NOx reduction) and is permitted to burn clean construction and demolition wood.
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| References |
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R.J. Briggs, and J.M. Adams, Biomass Combustion Options for Steam Generation, presented at Power-Gen 97, Dallas, TX, December 9-11, 1997
Amanda W. Renshaw, Integrated Assessment Briefs, ORNL/M-u8227, Publication Date 1995
Bio-fuels Industries, info@cogeneration.net
Types of Gasifiers, FAO Corporate Document Repository
Energy Products of Idaho, Coeur d’Alene, Idaho, Epi2@energyproducts.com
BME Consulting, LLC, Alan Teel ateel9835@msn.com
Global Greenlife Institute, Ted Atwood, Director.
Bain, R. 2000. Small modular biopower initiative phase I feasibility studies executive summaries. NREL/TP-510-33502. National Renewable Energy Laboratory, Golden, CO.
Bain, R.L., W.A. Amos, M. Downing, and R.L. Perlack. 2003. Highlights of biopower technical assessment: state of the industry and the technology. NREL/TP-510-33502. National Renewable Energy Laboratory, Golden, CO.
CBO. 2003. Prospects for distributed energy generation. Congressional Budget Office, Washington, DC. Accessed at: http://www.cbo.gov/showdoc.cfm?index=4552&sequence=0.
DOE/EIA. 2005. Renewable Energy Trends 2004. U.S. Department of Energy, Energy Information Administration. Washington, D.C. Accessed at: http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/trends.pdf
DOE/EIA. 2006a. Assumptions to the Annual Energy Outlook 2006. U.S. Department of Energy, Energy Information Administration. Washington, D.C. Accessed at: http://www.eia.doe.gov/oiaf/aeo/assumption/pdf/0554(2006).pdf
DOE/EIA. 2006b. Monthly energy review, November 2006. U.S. Department of Energy, Energy Information Administration. Washington, D.C. Accessed at: http://www.eia.doe.gov/emeu/mer/pdf/mer.pdf.
DOE/EIA. 2006c. Natural gas prices. Accessed at:
http://tonto.eia.doe.gov/dnav/ng/ng_pri_sum_a_EPG0_PRS_DMcf_a.htm
http://tonto.eia.doe.gov/dnav/ng/ng_pri_sum_a_EPG0_PCS_DMcf_a.htm
http://tonto.eia.doe.gov/dnav/ng/ng_pri_sum_a_EPG0_PIN_DMcf_a.htm
Lemar, Paul Jr. and David Jones. 2004. Opportunity fuels for CHP. DOE Central Region webcast, December 14, 2004. Accessed at: http://www.intermountainchp.org/initiative/support/fuelswebcast.pdf
Research Reports International. 2006. The use of biomass for power generation in the U.S.
RETScreen International. 2005. Biomass heating project analysis chapter. Natural Resources Canada. Accessed at: http://www.retscreen.net/ang/t_software.php.
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