The mechanisms of water uptake by trees suggest several methods of drying terrestrial biomass. The most obvious method is to expose biomass to circulat­ing, low-humidity air that is heated. Open-air solar drying meets these require­ments and has been used for hundreds of years to season or cure woods and grasses. The final moisture content of the air-dried biomass is usually in the 35-wt % range or less. The advantage of this partial drying method is that it is low in cost. The disadvantages are several. The process is slow and depends on the local climate. Some labor is required to arrange the freshly harvested biomass in suitable piles or windrows to facilitate exposure to sunlight and air circulation. Periodic turning of the windrows may be necessary to allow drying of plant parts in direct contact with the soil and to prevent fungal infection of wet biomass. Natural precipitation may require excessive drying times. Forage crops have traditionally been partially dried in open air to this moisture level so they can be removed from the field and stored without significant deterioration and loss of nutrient value. Solar drying also facilitates densification of hay by baling.

In a field study in Florida of the tall grasses elephantgrass (Pennisetum purpureum) and energycane (Saccharum spontaneum L.), which are good candi­dates as biomass energy crops, the air drying in windrows of mature crops of 2- to 4-cm stem diameters required about 7 to 10 days without rainfall to reach moisture levels of 15 to 20 wt % (Mislevy and Fluck, 1993). The seasoning of freshly harvested mature trees by air drying requires longer time periods to reduce the moisture level to about 25 to 35 wt % because of the larger diameter trunks and pieces. Decay fungi that may be present progress rarely, if ever, at moisture contents below 25 wt %. Green wood chips can be air dried in less time because of their smaller size. In a study of the use of hybrid willow harvested at З-year rotations as fuel for a direct wood-fired, gas turbine power plant, it was projected that air-dried willow bundles would reach 30

to 35 wt % moisture at the same cost as green wood chips at 50 wt % moisture content (Ismail and Quick, 1991). The cost is the same because what is saved in not chipping the wood is spent on bundling and storage for 6 months to air-dry the bundles.

Kiln drying under controlled conditions is commonly employed to improve the stability and physical characteristics of lumber products used as materials of construction or for manufacturing furniture, whereas open-air drying is traditionally employed for the curing or seasoning of tree parts and roundwoods to be used as fuel. Kiln drying promotes the removal of moisture by circulating heated air by natural draft or with fans or blowers through the wood, which is carefully piled in the kiln to promote the drying process. Heat is transferred from hot air heated by steam coils supplied by a boiler, or from hot stack gases heated by the burning of waste biomass or other fuels through manifolds. In the batch-drying of large volumes of wood, the temperature of the air can be gradually increased; the final temperatures and humidities are usually near 90°C and 15%. Kiln drying is rapid compared to the rate of open-air solar drying, but it is too slow for some continuous, thermochemical conversion processes unless the dryers and storage facilities are sized to handle the demand for predried feedstock. The continuous drying of wood chips, wood chunks, and hog fuel with industrial dryers or in drying ducts installed prior to the conversion unit is the approach that is often used when predrying is judged to be sufficiently beneficial. Continuous, direct-heat drying, in which hot air or stack gas contacts the biomass as it is fed to the conversion reactor, and indirect-heat drying, in which heat is transferred by convection and radiation from conducting surfaces to the biomass, can be utilized. Many commercial drying ovens and dryers such as rotary drum dryers, which have been effectively used for many years for drying wood and other biomass, are available. The use of superheated steam for drying rather than burning some of the feedstock as a heat source may allow further improvements in efficiency (cf. Wiltsee, McGowin, and Hughes, 1993). The direct-heat systems are generally lower in cost than the indirect-heat systems if commercial drying units are used. Thermochemical conversion reactors can also be designed so that incoming fresh feed is dried to the desired level by heat transfer from the hot reaction products. The simple addition of enclosed drying tunnels for passage of hot air or stack gases over and through incoming fresh feed can sometimes suffice to reduce moisture to the desired level and preheat the feed without the need to install industrial driers.

Note, however, that stack gases from biomass-fired boilers contain about 15 wt % moisture, and that at temperatures below 250°C, only a small amount of additional moisture can be absorbed before the gas becomes fully saturated. This is evident from the following equation (Routly, 1991):

WG = (2940 M)/T, — T0 where WG = drying gas weight, kg/h M = water evaporated, kg/h T, = temperature of drying gas entering, °С T0 = temperature of drying gas leaving, °С

This equation indicates that large fans and motors are required for circulation of the drying gases when low-temperature gas is used as the drying medium. To obtain sufficient heat for drying purposes, some of the stack gas may have to be extracted upstream of the boiler heat recovery equipment, which can have an adverse effect on steam generation. Stack gas drying should therefore be evaluated for each application to determine whether it is technically and economically feasible. For most thermochemical conversion systems that pro­cess green biomass, a balance is usually struck among the optimum moisture range needed for conversion, the feedstock demand rate, the drying require­ments, the size of the feedstock storage facility, feedstock stability on storage, and the cost of supplying predried feedstock.

The transpirational drying in open air of whole trees felled in the forest has been evaluated, but has not been widely adopted (McMinn, 1986). How­ever, the drying of whole trees has been incorporated as part of the whole — tree-burning concept for power production (Chapters 7 and 14; also see Ostlie and Drennan, 1989). Whole trees including branches are dried in large build­ings equipped with heat exchangers supplied with warm water at temperatures up to 50°C. Additional higher temperature waste heat is available from the power plant for peaking. Fans along the base of the drying buildings draw outside air over the heat exchangers and circulate it through piles of whole trees. The resulting warm, moist air is drawn out of the buildings through vents. For optimal drying conditions, the relative humidity levels are kept below 35%. After approximately 30 days of storage in the drying buildings, the moisture content of the whole trees is reduced to 25 wt % or less. Experi­mental testing of whole tree drying provided several interesting and perhaps unexpected results. The two tree species tested, aspen and eastern cottonwood, dried significantly faster with the leaves intact than without the leaves. It was also found that logs do not appear to dry more quickly than whole trees, and that the branches of the trees tested were drier than the corresponding trunks.

Whenever it is necessary to remove moisture from virgin or waste biomass feedstocks, air drying, mechanical dewatering, and drying with waste heat or stack gases should be evaluated first. The lower costs of these methods com­pared to the costs of thermal drying in which external fuel or a portion of the feedstock supplies heat may justify their use.