Biomass is a promising option for providing locally produced, renewable energy in Pennsylvania. While it is not unusual for homes in the state to be heated with firewood, other forms of biomass fuel are not as common and commercial-scale use of biomass fuel is very limited. A person who plans to use biomass for fuel or design equipment for biomass heat needs to understand the performance characteristics of biomass in order to avoid possible problems and utilize the biomass effectively. Biomass can be a source of liquid fuel (e.g., biodiesel) or gaseous fuel (e.g., "wood gas"), but the most common use is as a solid fuel (e.g., wood, biomass pellets). This fact sheet presents some of the more important characteristics of solid biomass fuel and explains their significance.
Biomass Fuel Performance
The heat value, or amount of heat available in a fuel (kJ/kg), is one of the most important characteristics of a fuel because it indicates the total amount of energy that is available in the fuel. The heat value in a given fuel type is mostly a function of the fuel's chemical composition.
The heat value can be expressed in one of two ways: the higher heating value or the lower heating value. The higher heating value (HHV) is the total amount of heat energy that is available in the fuel, including the energy contained in the water vapor in the exhaust gases. The lower heating value (LHV) does not include the energy embodied in the water vapor. Generally, the HHV is the appropriate value to use for biomass combustors, although some manufacturers may utilize the LHV instead, which can lead to confusion.
Some species of biomass tend to have more energy per unit of mass than others. However, the variation between species is often no greater than the natural variations found within one species or another. The heat content of a fuel type can vary significantly depending on the climate and soil in which the fuel is grown, as well as other conditions. As a result, the energy content of a biomass fuel should be thought of as a range rather than a fixed value. Figure 1 shows the typical range of some common fuels. The most important noticeable trend in these data is that wood (which has a lower ash content) tends to have a slightly higher heat value than field crops.
Figure 1. Heat content of various fuels (oven dry). One MJ (megajoule) is enough energy to brew about 10 cups of coffee if your pot is 100 percent efficient. It is also equal to about 1,000 BTUs.
Fresh, "green" wood is often about half water, and many leafy crops are primarily water. A low moisture level in the fuel is usually preferable because high-moisture fuels burn less readily and provide less useful heat per unit mass (much of the energy in wet fuel is used to heat and vaporize the water). Extremely dry fuel, however, can cause problems such as dust that fouls equipment or can even be an explosion hazard.
The moisture content in a fuel can be calculated by one of two methods: wet basis or dry basis. In the case of wet-basis calculations, the moisture content is equal to the mass of water in the fuel divided by the total mass of the fuel. In the case of dry-basis calculations, the moisture content is equal to the mass of water in the fuel divided by the mass of the dry portion of the fuel. It is important to know which type of calculation is being used, as the two values can be quite different in magnitude. For example, a 50 percent wet-basis moisture level is the same as a 100 percent dry-basis moisture level.
The practical maximum moisture level for combusting fuel is about 60 percent (wet basis), although most commercial equipment operates tolerably well with fuels that only have up to about 40 percent moisture. The HHV and LHV of wood fuel is shown in Figure 2 as a function of fuel moisture content.
Figure 2. Typical biomass higher heating value and lower heating value versus moisture content.
In addition to heat content, other differences in fuel performance are related to composition of the various biofuels. The three most significant compositional properties are (1) ash content, (2) susceptibility to slagging and fouling, and (3) percent volatiles.
Ash content (the mass fraction of uncombustible material) is an important parameter, with grasses, bark, and field crop residues typically having much higher amounts of ash than wood. Systems that are designed to combust wood can be overwhelmed by the volume of ash if other biofuels are used, which can reduce the combustion efficiency or clog the ash handling mechanisms.
Slagging and fouling are problems that occur when the ash begins to melt, causing deposits inside the combustion equipment. Ash ideally remains in a powdery form at all times. However, under some conditions, the combustion ash can partially melt, forming deposits on the combustor surfaces (fouling) or hard chunks of material in the base of the combustion chamber (slagging/clinkering). Certain mineral components in biomass fuels, primarily silica, potassium, and chlorine, can cause these problems to occur at lower temperatures than might be expected.
Many studies have observed that the high mineral content in grasses and field crops can contribute to fouling and clinkering--a potentially expensive problem for a combustion system. The timing of harvest can affect this property, with late harvested crops having noticeably lower ash content (Adler et al., 2006). Dirt in the fuel also adds to the mineral content and associated clinkering and fouling problems; therefore, fuel should be kept free of soil and other contaminants.
Slagging and fouling can be minimized by keeping the combustion temperature low enough to prevent the ash from fusing. Alternately, some biomass combustion equipment uses an opposite approach--it is designed to encourage the formation of clinkers but is able to dispose of the hardened ash in an effective manner. Table 1 shows a "slagging index" and a "fouling index" for several fuels, which are two measures that give some indication of the tendency of a fuel to form slag or foul a boiler. Values lower than 0.6 are preferable. These indices were developed for use with coal, however, and their significance for use with biomass fuels is questionable. Treat these values with caution.
The "percent volatiles" in a fuel is a less commonly known property that refers to the fraction of the fuel that will readily volatilize (turn to gas) when heated to a high temperature. Fuels with "high volatiles" will tend to vaporize before combusting ("flaming combustion"), whereas fuels with low volatiles will burn primarily as glowing "char." This property affects the performance of the combustion chamber and should be taken into account when designing a combustor.
Table 1. Examples of ash, slagging, fouling, and volatiles.
|Fuel||Percent ash content||Slagging Index||Fouling Index||Percent volatiles||Reference|
|Wood, clean and dry||0.3||0.05||7||82||20|
These values are representative only and can be expected to vary depending on cultivar, soil, weather, and cultural practice. While some variations in composition do exist between tree species, the properties of wood fuel, on a per-kilogram basis, are surprisingly similar for common species in Pennsylvania.
Fuel Size and Density
The size and density of the biomass fuel particles is also important. They affect the burning characteristics of the fuel by affecting the rate of heating and drying during the combustion process. Fuel size also dictates the type of handling equipment that is used. The wrong size fuel will have an impact on the efficiency of the combustion process and may cause jamming or damage to the handling equipment. Smaller-sized fuel is more common for commercial-scale systems because smaller fuel is easier to use in automatic feed systems and also allows for finer control of the burn rate by controlling the rate at which fuel is added to the combustion chamber. Fuel size and density are probably the most over-looked factors affecting fuel performance and should be given careful consideration when selecting a fuel type.
Table 2. Typical size and density of biomass fuels.
|Fuel||Length (m)||Bulk density (kg/m3)|
|Green wood chips||0.025-0.075||500|
Several characteristics affect the performance of biomass fuel, including the heat value, moisture level, chemical composition, and size and density of the fuel. These characteristics can vary noticeably from fuel to fuel. In addition, natural variations of a given fuel type can be significant. Combustion equipment can and should be designed to handle this range of properties. For further information on biomass heating, see the other related Renewable and Alternative Energy Fact Sheets An Introduction to Biomass Heatand Commercial Scale Biomass Combustors.
Adler, P., M. Sanderson, A. A. Boateng, P. J. Weimer, and H. J. G. Jung. "Biomass Yield and Biofuel Quality of Switchgrass Harvested in Fall or Spring." Agron. J. 98 (2006): 1518-25.
Bain, R., W. Amos, M. Downing, and R. Perlack. Biopower Tech nical Assessment: State of the Industry and Technology. National Renewable Energy Laboratory Report # NREL/TP-510-33123. Golden, Colo.: National Renewable Energy Laboratory, 2003.
Bergman, R., and T. M. Maker. Fuels for Schools: Case Study in Darby, Montana. General Technical Report FPL-GTR-173. Madison, Wis.: USDA Forest Service, Forest Products Laboratory, 2007.
Demirbas, A. "Calculation of Higher Heating Values of Biomass Fuels." Fuel 76, no. 5 (1997): 431-34.
Dinkelbach, L. Thermochemical Conversion of Willow from Short Rotation Forestry. European Union Project Report ECN-C-00-028, 2000.
Dobie, J., and D. Wright. Conversion Factors for the Forest Products Industry in Western Canada. West. For. Prod. Lab. Inf. Rep. VP-X-97. Canadian Forest Service, 1972.
Gaur, S., and T. Reed. Thermal Data for Natural and Synthetic Fuels. New York: Marcel Dekker, 1998.
Hinckley, J. Stack Test Review and Summary. Memo from Re source Systems Group Inc. to USDA Bitter Root RC&D, 2008.
Hoskinson, R., D. Karlen, S. Birrell, C. Radtke, and W. Wilhelm. "Engineering, Nutrient Removal, and Feedstock Conversion Evaluations of Four Corn Stover Harvest Scenarios." Biomass and Bioenergy 31 (2007): 126-36.
Jenkins, B. M., and Ebeling, J. M. Correlation of Physical and Chemical Properties of Terrestrial Biomass with Conversion. Energy from Biomass and Waste Symposium IX, Institute of Gas Technology, Chicago, Illlinois, 1985.
Johansson, L. S., C. Tullin, B. Leckner, and P. Sjovall. "Particle Emissions from Biomass Combustion in Small Combustors." Biomass and Bioenergy 25 (2003): 435-46.
Johnson, J. A., and G. H. Auth. Fuels and Combustion Handbook. New York: McGraw-Hill, 1951.
Kitani, O., and C. Hall, eds. Biomass Handbook. New York: Gordon and Breach, 1989.
Maker, T. Wood Chip Heating Systems: A Guide for Institutional and Commercial Biomass Installations. Montpelier, Vt.: Biomass Energy Resource Center, 2004. Miles, T. R., T. R.
Miles Jr., L. L. Baxter, R. W. Bryers, B. M. Jenkins, and L. L. Oden. Alkali Deposits Found in Biomass Power Plants; A Preliminary Investigation of Their Extent and Nature. NREL/TP-433-8142. Golden, Colo.: National Renewable Energy Laboratory, 1996.
Millikin, D. "Determination of Bark Volumes and Fuel Properties." Pulp and Paper Magazine of Canada (December 1955): 106-8.
Nussbaumer, T. "Combustion and Co-combustion of Biomass: Fundamentals, Technologies, and Primary Measures for Emission Reduction." Energy and Fuels 17 (2003): 1510-21.
Parsons, V. 1980. The Micro-Level Land Use Impacts of Bioconversion. IASTD Energy Symposia, 1980.
Pennsylvania Department of Environmental Protection. Air Quality Permit Exemptions. Document 275-2101-003. Harrisburg: Pennsylvania Department of Environmental Protection, 2003.
Prinzing, D., K. Hellem, and W. McVey. Characterization of Coal and Biomass Fuel Blends. Presented at the spring meeting of the American Chemical Society Division of Fuel Chemistry, Anaheim, Calif. 1995.
Quaak, P., H. Knoef, and H. Stassen. Energy from Biomass: A Review of combustion and Gasification Technologies. World Bank Technical Paper # 422. Washington, D.C.: The World Bank, 1999.
Ragland, K., and D. Aerts. "Properties of Wood for Combustion Analysis." Bioresource Technology 37 (1991): 161-68.
Risser, P. G. "Agricultural and Forestry Residues." In Biomass Conversion Processes for Energy and Fuels, edited by S. S. Soffer and O. R. Zaborsky. New York: Plenum Press, 1981.
Theander, O. "Cellulose, Hemicellulose, and Extractives." In Fundamentals of Thermochemical Biomass Conversion, edited by R. P. Overend. New York: Elsevier, 1982.
Tillman, D. A. Wood as an Energy Resource. New York: Academic Press, 1978.
Tortosa-Marisa, A., R. Buhre, R. Gupta, and T. Wall. "Characterizing Ash of Biomass and Waste." Fuel Processing Technology 88 (2007): 1071-81.
Van den Broek, R., A. Faaij, and A van Wijk. "Biomass Combustion for Power Generation." Biomass and Bioenergy 11, no. 4 (1996): 270-81.
Prepared by Daniel Ciolkosz, extension associate. Reviewed by Bruce Miller, EMS Energy Institute, and Robert Wallace, formerly of the Penn State Bioenergy Bridge.