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Proximate and Ultimate Analysis of Wood Wood is usually converted into energy by burning. Combustion commences by evaporating the water present in the wood structure. Then combustible and noncombustible components are driven off at temperatures from 100˚ to 600˚ C. Table 91 gives the proximate analysis of wood and bark, showingthat about 75 to 85% of the wood can be volatilized. Carbon is oxidized in the final stage of combustion. Combustion of wood involves two reactions:
Table 91 also gives the chemical analysis (termed ultimate analysis) of wood and bark, showing average percentage composition of carbon, hydrogen, and oxygen. Neither ultimate nor proximate analyses vary greatly between species. Part of the oxygen in these reactions comes from the oxygen already present in the wood, and the rest that is needed comes from the air. The minimum total amount of oxygen and hence air needed from these two sources can be theoretically calculated, but in practice more air than the theoretical amount is admitted to ensure complete combustion. This additional air is called excess air and the amount is carefully controlled in modern combustion systems. Higher heating value (HHV) is a laboratory measurement of the stored chemical energy in a fuel and is expressed in British thermal units per pound (Btu/lb), joules per kilogram (J/kg), or megajoules per kilogram (MJ/kg). Calculations in this chapter are in Imperial units. See Appendix 1 for conversions between energy measures. Table 92 presents average higher heating values for northwestern species. Wilson et al. (1987) have provided an extensive compilation of ultimate and proximate analysis and higher heating values for northwestern species. Note that HHV is measured for ovendry wood (zero percent moisture content). Differences between species, between wood and bark, and variations within a species reflect differences in chemical composition, especially extractives. In engineering calculations, it is common to use an average HHV of 9,000 Btu/dry lb for the more resinous conifers and 8,300 Btu/dry lb for the less resinous hardwoods. Since a pound of fuelwood is generally not received in the ovendry condition, HHV does not correctly represent the available potential heat. For example, a pound of wood fuel may contain 0.6 pound of ovendry wood and 0.4 pound of water. In the wood fuel literature, moisture content is usually reported on a wet basis (Chapter 1), hence the moisture content (MC_{w}) in this example is 40%. Since HHV assumes that this pound is entirely ovendry wood, HHV must be reduced to reflect the actual amount of ovendry wood present (Wd). The following equation represents the wood and water components in percent form: Wd + MC_{w = 100.} Therefore, the gross heating value of a pound of wet wood is obtained by adjusting HHV for the wood moisture content as follows: GHV = HHV (1  MC_{w}/100). Thus GHV represents the available potential
heat at a given moisture content and is equal to HHV only GHV = 9,000 (1  40/100) = 5,400 Btu/lb of ovendry wood in the fuel. Table 91. Typical proximate and ultimate analysis of wood.
Source: Arola (1976). Note: Ash percentages may differ since different data sources were averaged. Table 92. Higher heating value of wood and bark of some northwestern species (Btu/ovendry lb).
Note: Values are averaged from multiple sources and rounded to the nearest 100 Btu/lb. For additional details on these and other species, see Ince (1979) and Wilson et al. (1987). GHV does not represent the amount of heat available for use because it includes all the heat that can be extracted from the products of combustion, and there are several sources of loss that must be accounted for in determining the recoverable heat. Recoverable heat (RH) is important in comparing costs of fuels and fuel systems and represents the heat energy that, for example, produces steam for industrial purposes. The literature on these losses can sometimes be confusing because authors differ on which losses to consider and how some of them are calculated. Losses due to the moisture content of wood are often referred to as the stack heat loss. A pound of wet fuel has a certain fraction that is ovendry wood plus another fraction that is its water content. In the example above, there was 0.6 pound of ovendry wood (Wd) and 0.4 pound of water (MC_{w} = 40%). Formulas for eight forms of heat loss, numbered H1 through H8, are discussed below (Bethel 1977; Ince 1979). H1. Heat used to raise the temperature of water in the wood to the boiling point. In Btus this is H1 = (212  T1) _{*} MC_{w} / 100 where T1 = ambient temperature. H2. Heat required to vaporize
the water in the wood. H2 = 970 _{*} MC_{w} / 100. H3. Heat required to separate
the bound water (water below fiber saturation point) from the
cell walls. H3 = 136 _{*} MC_{b} / 100 where MC_{b} = fiber saturation point (see Chapter 1) if MC_{w } ≥ fsp or MC_{b} = MC_{w} if MC_{w} < fsp. Usually it is assumed that fsp = 30% MC_{od }or 23% MC_{w}. H4. Heat required to raise the temperature of the vaporized water to the temperature of the exhaust gases. In Btus this is H4 = 0.46 _{* }(T2  212) _{*} MC_{w} / 100 where T2 = temperature of exhaust gases. The average specific heat of water over the range from boiling point to exhaust temperature is about 0.46. Losses H1, H2, and H4 are often combined and simplified into a single equation (Ince 1979): MC_{w} / 100 _{*} [970 + (212  T1) + 0.46 _{* }(T2  212)]. H5. Heat required to evaporate water that forms when the hydrogen component of wood is combusted. Since wood is approximately 6% hydrogen (Table 91), a pound of dry wood contains about 0.06 pound of hydrogen. During combustion, one of the chemical reactions is 2H_{2} + O_{2} ®2 H_{2}O + 61,100 Btu/lb of H_{2}. By weight, water is one part hydrogen and eight parts oxygen. Therefore, 0.06 pound of hydrogen combines with 0.48 pound of oxygen to yield 0.54 pound of water, which also escapes via the stack gases. This water incurs losses involved with raising its temperature to the boiling point, evaporating it, and then raising its temperature to that of the exhaust gases. Therefore, H5 = 0.54 Wd (212  T1) + 0.54 Wd (970) + 0.54 Wd [0.46 (T2  212)] = 0.54 _{* }(1  MC_{w}/ 100 )[970 + (212  T1) + 0.46 _{*} (T2  212)]. H6. Heat from combustion other than water vapor: dry gases. In addition to the minimum air needed for combustion reactions, there is generally some "excess" air that enters the furnace, is heated, and exits with the stack gases. Table 91 shows that ovendry wood is about 52% carbon. The reaction to combust this is C + O_{2} ®CO_{2} + 14,100 Btu/lb of C. The atomic weights of carbon and oxygen are 12 and 16 respectively, hence burning 12 pounds of carbon requires 32 pounds of O_{2}, which yields 44 pounds of CO_{2}. Therefore, this process requires 32/12 or 2.7 pounds of O_{2} per pound of carbon. Since one pound of ovendry wood contains 0.52 pound of carbon, the O_{2} demand is 0.52 times 2.7, or 1.4 pound per pound of ovendry wood. It was previously calculated that 0.48 pound of O_{2} is required to burn the 0.06 pound of hydrogen in a pound of ovendry wood, hence the total O_{2} demand is 1.88 pounds per pound of ovendry wood. Since a pound of wood contains 0.40 pound of oxygen (Table 91), the net oxygen required from air is 1.48 pounds. Because air is 23.2% oxygen, it takes 4.3 pounds of air to provide one pound of oxygen. Therefore the minimum air required to burn one pound of ovendry wood is 1.48 times 4.3, or 6.4 pounds. In addition to this theoretical minimum air supply to sustain combustion, some excess air is always admitted. Excess air is usually expressed as a percentage of the theoretical minimum air. Thus 20% excess air means that the total air admitted to burn one pound of ovendry wood is 6.4 times 1.20, or 7.7 pounds. The quantity of heat that escapes in stack gases due to dry gases and excess air (EA) is calculated by H6 = (T2  T1)(1  MC_{w} / 100) [1.44 EA /100 + 1.56]. H7. Heat required to raise the temperature of wood to the combustion temperature. In Btus this is H7 = Wd (T3  T1)[0.226 + 0.000322 (T1 + T3  64)] where T3 = combustion temperature. H8. Other heat losses. Otherlosses occur from radiation, conduction, and convection of heat, incomplete combustion and so forth, and have been estimated as 3 to 4 % (Corder et al. 1970; Miller and Hansen 1951). This can be estimated as H8 = 0.04 GHV. This percentage of loss may be appropriate for a welloperated system burning the type of fuel for which it was designed but could be higher with an improperly operated system or fuels that have improper size particles or excessive moisture. Recoverable Heat and Combustion Efficiency The recoverable heat (RH) is obtained by subtracting the sum of these eight losses from the gross heating value. RH = GHV  (H1 + H2 + H3 + H4 + H5 + H6 + H7 + H8). Combustion efficiency (CE) is the ratio of recoverable heat to available potential heat: CE (%) = (RH / GHV) _{*} 100. With current technology, combustion efficiency of wood fuels ranges from about 80% for dry fuels to about 60% for wet fuels. Unfortunately, many wood fuels are received in a relatively wet condition. See Example 1. The terms lower heating value and net heating value are used in combustion engineering and reflect the net heat available in a fuel after the various losses associated with wood moisture content and water formed during combustion are subtracted from HHV (Harris et al. 1986).Heating Values per Unit Volume Calculating Btus per Cubic Foot of Solid Wood Suppose it is desired to estimate the heat value per cubic foot of a conifer under the following conditions: HHV = 9,000 Btu/ovendry lb MC_{od} = 67% (MC_{w} = 40%) SG_{g} = 0.40. Chapter 1 presents the definitions of moisture content and specific gravity. Table 12 indicates that a cubic foot of this wood weighs nearly 41.9 pounds. Of this, 40% is water, hence there are 16.8 pounds of water and 25.1 pounds of ovendry wood in this cubic foot. The weight of ovendry wood per cubic foot can also be obtained by looking up the value in Table 12 at the intersection of SG_{g} = 0.40 and MC_{od} = 0. The difference between the 25.0 in the table and the above calculation is rounding. Example 1
Recoverable heat (RH) is 3,733 Btu/lb ovendry wood in the fuel. Combustion efficiency (CE) is 69%. Using the data from Example 1: HHV/ft^{3} = 25.1 _{* }9,000 = 225,900 Btu GHV/ft^{3} = 25.1 _{* }5,400 = 135,540 Btu RH/ft^{3} = 25.1 _{* }3,949 = 99,120 Btu. Select the appropriate cubic feet per cord from Table 31 or 32 and multiply by HHV, GHV, or RH per cubic foot calculated above. If a standard cord of the above species contained 80 cubic feet of wood, then HHV/cord = 18,072,000 Btu GHV/cord = 10,843,200 Btu RH/cord = 7,929,600 Btu. Calculating Btus of Chips and Hog Fuel When the chips or hog fuel are in weight units, first deduct weight that corresponds to the moisture content and then apply HHV, GHV, or RH per pound of dry wood in the fuel. When the chips or hog fuel are in volumetricunits, use the residue to solid wood expansion factors in Table 71 or 72 to convert the chip or hog fuel volume to solid wood equivalent. Then apply HHV, GHV, or RH per cubic foot calculated above. For example, if a unit (200 cubic feet) of hog fuel is obtained for the species conditions outlined earlier, first convert the 200 cubic feet of hog fuel to a solid equivalent. Assuming an expansion factor of 2.5 cubic feet of hog fuel per cubic foot of solid wood, a unit becomes 200 / 2.5 = 80 ft^{3} of solid wood. Multiply this by either of the three heating values per cubic foot as desired. Working with Mixtures of Wood and Bark This involves obtaining a weighted average higher heating value, a weighted average moisture content, and a weighted average specific gravity. See Example 2. Example 2 Suppose western redcedar hog fuel has 40% bark and that wood and bark have an MC_{od} of 60 and 100% respectively. Also, Tables 11 and 75 indicate that cedar wood and bark have an SGg of 0.31 and 0.37 respectively. Weighted averages are HHV: 0.4 _{*} 8,700 + (1  0.4) _{*} 9,700 = 9,300 Btu / ovendry lb. MC_{od}: 0.4 _{*} 100 + (1  0.4) _{*} 60 = 76%. SG_{g}: 0.4 _{*} 0.37 + (1  0.4) _{*} 31 = 0.34. Use these values to calculate GHV, HV, and other measures in the preceding sections. Heating Values of Conventional Fuels Table 93 presents heating values of fossil fuels that often compete with wood. Table 93. Fossil fuel heating values.


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