It is normally not necessary to reduce the water content of high-moisture — content or wet biomass feedstocks for microbial conversion processes. This contrasts with thermal conversion processes such as combustion. Dry biomass burns at higher temperatures and thermal efficiencies than wet biomass. For example, the flame temperatures of green wood containing 50 wt % moisture and dry wood in conventional combustors that supply boiler heat are about 980°C and 1260 to 1370°C, respectively (cf. FBT, Inc., 1994). Flame tempera­ture is directly related to the amount of heat necessary to evaporate the moisture contained in the wood—the lower the moisture content, the lower the amount of energy needed to remove the water and the higher the boiler efficiency. Although flame temperature is not the actual bed temperature in advanced — design, fluid-bed combustors, the effects on temperature and efficiency are the same. The maximum amounts of acceptable moisture in wood fuels for conventional furnace systems are illustrated in Table 6.1, and the typical moisture contents and heating values of several biomass fuels for combustion in commercial fluid-bed, grate, and suspension firing units are presented in Table 6.2. With the exception of suspension firing units, for which the moisture content of the fuel is usually in the 20-wt % range, the maximum moisture content range is 55 to 65 wt %. Indeed, combustion of biomass containing 65 wt % moisture in conventional grate-type systems can result in lowering of the adiabatic flame temperature to the point where self-sustained combustion does not occur.

Many of the large-scale biomass combustion systems for producing heat, hot water, or steam accept biomass fuels containing relatively large amounts of moisture and are operated without much apparent concern for the effects of moisture content of the fuel on the combustion process itself. One of the

largest biomass-fueled power plants equipped with traveling grates operates very well with wood chips containing an average of 50 wt % moisture, although a few initial handling and storage problems caused by high-moisture fuel supplies had to be solved (Tewksbury, 1987). Another power plant equipped with traveling grates operates very well on fuel containing about the same amount of moisture and consisting of a mixture of about 80% hogged mill wastes and 20% wood chips (Ganotis, 1988). Some air drying of stringy bark fuels is needed in the spring to eliminate fuel handling problems. Still another power plant equipped with a fluidized, bubbling-bed combustor operates well with a mixture of about 40% whole tree chips, 20% sawmill residues, and 40% agricultural residue from almond orchards. The fluid-bed combustors are designed to operate with fuels having a variable moisture content up to about 50 wt % (Normoyle and Gershengoren, 1989).

Thus, biomass fuels containing up to about 50 to 55 wt % moisture do not require pre-drying for acceptable performance in combustion systems that are designed for such fuels. However, a moisture content of 15 wt % is generally recognized as optimum for efficient thermochemical gasification of biomass (Miles, 1984), although several thermochemical gasification processes satisfac­torily convert feedstocks containing up to about 30 to 35 wt % moisture. Also, in certain types of thermochemical gasification processes such as steam gasification, water is a reactant and the contained water in the feedstock can be beneficial (Chapter 8).

To illustrate more quantitatively the effect of moisture content on the performance of a thermochemical process, consider the direct combustion of sugarcane bagasse in a conventional boiler to raise steam and the effects on boiler efficiency of bagasse moisture content relative to the other sources of efficiency losses. The results of a complex series of calculations to examine boiler losses and efficiencies are shown in Table 6.3 (Institute for Energy Studies, 1977). A typical amount of excess air used in bagasse-fired boilers was chosen as 30%, the moisture content of the bagasse ranged from 0 to 60 wt % in 10-wt % increments, and the stack gas temperatures ranged from 177°C (450 K) to 260°C (533 K) in approximately 28°C increments. Boiler efficiency is 100 minus the sum of the boiler losses in percentage units. The boiler losses, other than those caused by the moisture content of the bagasse, include those due to dry gases of combustion, which refers to nonuseful heat losses; those due to moisture in air; those due to the moisture formed on bagasse combustion; and other losses. Plots of bagasse moisture content against boiler efficiency and the losses due to bagasse moisture content alone at stack gas temperatures of 450 К are shown in Fig. 6.1. The analysis shows that when the moisture content of the bagasse is more than about 35 wt %, it has a greater impact on boiler efficiency losses than the other moisture sources. Note that the incremental improvement in boiler efficiency for drying the bagasse from about 35 to 0.0 wt % moisture increases boiler efficiencies by only about 7%. It was concluded by the analysts who performed the study that sugar mills can increase boiler efficiency about 5% by drying the bagasse from its typical level of 48 wt % moisture to about 35 wt % moisture. Bagasse moisture reductions by solar drying and drying with stack gases were suggested as low-cost approaches to increasing boiler efficiencies. It was also concluded that the higher temperature generated using dryer bagasse can increase heat transfer efficiencies in addition to reducing stack gas losses. This efficiency increase results from both higher temperature differentials and lower furnace gas velocities. Historically, the drying of bagasse to improve boiler efficiencies was proposed in the early 1900s. The same principles apply generally to improving the efficiencies of biomass combustion processes.

Predrying of biomass has sometimes been justified in the past only for large — scale operations, or where low-cost energy is available as waste heat. It is important to realize, however, that the absence of any capability to predry feedstock for thermochemical conversion has sometimes caused severe operat­ing problems, particularly for gasification processes. In one of the early fluid — bed gasification plants fueled with wood chips and sawdust to produce low — energy gas as an on-site boiler fuel, it was very difficult to control combustion. The industrial gas burners installed in the plant did not function satisfactorily

TABLE 6.3 Boiler Efficiency and Losses Due to Moisture on Bagasse Combustion” Sources and amount of boiler losses Bagasse moisture (wt %) Stack gas temperature (°С [°F]) Combustion dry gases (%) Moisture in air (%) Moisture in bagasse (%) Moisture of combustion (%) Other losses (%) Boiler efficiency (%) 0 177 [350] 5.86 0.15 0.00 8.07 5.00 80.92 204 [400] 6.95 0.18 0.00 8.23 5.00 79.64 232 [450] 8.04 0.21 0.00 8.39 5.00 78.36 260 [5001 9.12 0.24 0.00 8.55 5.00 77.09 10 177 [350] 5.86 0.15 1.64 8.19 5.00 79.16 204 [400] 6.94 0.18 1.67 8.35 5.00 77.86 232 [450] 8.03 0.21 1.70 8.51 5.00 76.55 260 [500] 9.11 0.24 1.73 8.67 5.00 75.25 20 177 [350] 5.88 0.15 3.51 8.30 5.00 77.16 204 [400] 6.96 0.18 3.58 8.46 5.00 75.82 232 [450] 8.05 0.21 3.65 8.62 5.00 74.47 260 [500] 9.14 0.24 3.72 8.78 5.00 73.12 30 177 [350] 5.86 0.15 6.55 8.42 5.00 74.02 204 [400] 6.94 0.18 6.68 8.59 5.00 72.61 232 [450] 8.03 0.21 6.81 8.76 5.00 71.19 260 [500] 9.11 0.24 6.94 8.92 5.00 69.79

“Adapted from Institute for Energy Studies (1977). Various assumptions are used for this analysis. The stack gases exhibit perfect gas behavior with constant specific heat equal to 0.558 kj/kg. Superheated steam has a specific heat of 1.093 kj/kg and is constant with temperature. The fuel inlet air temperature is 26.7°C (80°F) and the relative humidity of the air is 60% at 101.3 kPa. The combustion efficiency is 98%. The unburned bagasse fraction is 0.03. The ash sediment fraction is 0.015 in bone-dry bagasse. The psychrometry of the stack gases is not significantly different than that of air because the partial pressure of C02 in the stack gases is small. The calculated value is 15.86 kPa (2.3 psia) and is approximately constant for constant excess air, independent of the bagasse moisture content. “Other losses” include losses due to unburned combustibles (about 3%), losses due to radiation (about 0.5%), and unaccounted — for losses (about 1.5%).

EFFICIENCY (%) LOSSES (%)

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with the product gas (Bircher, 1982). These problems were attributed to large variations in the quality of the gas caused by accepting wood feedstock at any moisture content up to 50 wt %, which in turn resulted in large swings in gas heating values from about 3 to 8 MJ/m3 (about 80 to 200 Btu/ft3). Drying of the feedstock has been found to be extremely important in wood gasification because it is only through the availability of a uniform feedstock that consistent gas quality can be assured (Miller, 1987).