A. Fundamentals

Dewatering refers to the removal of all or part of the contained moisture from biomass as a liquid. Drying is a similar process, except that the moisture is removed as vapor. In some cases where a waste or virgin biomass feedstock is thermally processed directly for energy recovery, it may be necessary to partially dry the raw feed before conversion. Otherwise, more energy might be consumed by the conversion process than would be produced in the form of energy or fuel. Open-air solar drying is usually the lowest cost drying method, if it can be used. Raw materials that are not sufficiently stable to be dried by solar methods can be dried more rapidly using industrial dryers such as spray dryers, drum dryers, and convection ovens if cost permits. For large — scale drying applications, forced-air furnaces and drying systems designed to use hot stack gases are sometimes just as efficient.

Since a large portion of a feedstock’s equivalent energy content can be expended for drying, there is a balance between the cost of moisture removal, the incremental improvement in efficiency on conversion, and the advantages of handling drier feedstock. The key biomass property that should obviously be examined, in addition to conversion process requirements, is the moisture content of the fresh biomass, the methods available for its partial or total removal, and the effects, if any, on the properties of the remaining biomass. The moisture content of biomass is as variable as the multitude of biomass species available as potential feedstocks.

Aquatic biomass is one category of feedstock that can be classified as high — water content biomass. Freshwater, marine, and microalgal biomass, such as water hyacinth, giant brown kelp, and Chlorella, respectively, contain large amounts of intracellular moisture. The water content is usually over 95 wt % of the fresh biomass (see Appendix A, Part A.8, for the definitions of wet and dry weight percentages). Several types of municipal, industrial, and farm animal wastes are also produced in association with water and are potential feedstocks. One example is untreated municipal biosolids. Because of the nature of the collection systems (i. e., dilution with water to facilitate localized disposal and transport in municipal lines to wastewater treatment plants), raw municipal biosolids contain over 95 wt % water. They are stabilized at the wastewater treatment plant by subjecting them to various primary and secondary treat­ments which reduce volatile solids, BOD, and COD, and are dewatered for disposal. Examples of high-water-content industrial wastes are pulp and paper mill sludges and alcoholic beverage industry sludges. They are often subjected to biological stabilization and dewatering processes similar to those used for municipal biosolids. The major farm animal wastes that are potential feedstocks for conversion processes are cattle, hog, and poultry manures. The moisture contents of most of these wastes range up to 80 wt %.

Terrestrial biomass, as freshly harvested green biomass, generally contains 40 to 60 wt % moisture. These potential feedstocks include most herbaceous species, softwoods, and hardwoods. Agricultural crop residues that have been exposed to open-air solar drying contain less moisture, often in the 15-wt % range or less. Straws are good examples. Other examples consist of by-products such as corn cobs, rice hulls, and nut shells from the processing of agricultural and orchard crops. Many of these residuals have even lower moisture levels. As-received municipal solid waste (MSW) usually contains 10 to 30 wt % moisture depending upon the season of the year and geographic location where it is collected. All of these terrestrial biomass species and residuals are suitable feedstocks for one or more different thermochemical conversion processes. Microbial conversion is also feasible in many cases after suitable pretreatment. Unlike many high-water-content biomass species, most terrestrial virgin bio­mass and residuals are sufficiently stable to undergo solar or thermal drying.

Woody biomass is the largest source of standing biomass and is the preferred feedstock for large-scale, integrated, biomass production-conversion systems. So it is worthwhile to consider how moisture accumulates in these materials internally during growth and from humid air. Water is contained in all tissues of a tree, both dead and alive (Mirov, 1949). Young leaves and roots contain up to 90 wt % moisture; tree trunks contain as much as 50 wt %. In the transpiration process, water is absorbed by the roots from the soil, pushed into the sapwood, and then pulled up to the leaves above ground where it is given off to the atmosphere by evaporation through the stomata, the same openings that admit C02 (Chapter 4). Various osmotic and diffusional forces drive the water from the soil to the leaves through semipermeable membranes and capillary passages. About one-half of the solar energy falling on the leaves supplies the energy to facilitate transpiration, which is necessary for photosyn­thesis to occur. The stoichiometric photosynthesis of 100 kg of cellulose requires about 55.6 kg of water, but during the process, the tree transpires about 100,000 kg of water to the atmosphere.

Wood also absorbs moisture from humid air and is the equivalent of an elastic gel that exhibits limited swelling as water vapor is taken up from the air. Two different mechanisms are operative: adsorption and absorption (Nikitin et al, 1962). In adsorption, moisture is transferred from air to the wood surfaces and results from the attraction between polar water molecules and the negatively charged surfaces of the wood. The negative charges involve functional groups on the surface that can carry full or partial negative charges or organic molecules that can exist as dipoles with the negative ends clustered on the surface. The amount of moisture adsorbed on wood surfaces is relatively small; it ranges up to about 5 to 6 wt % of the wood at 20°C and 100% relative humidity. In absorption, water molecules are drawn into the permeable pores of the wood by spongelike processes due to diffusional and osmotic forces followed by capillary condensation. A large number of fine capillaries in the wood fibers facilitate this process. The amount of moisture absorbed within the woody structures depends upon the pore diameters and distribution of the capillaries. In spruce wood pulps, for example, the amount of water vapor absorbed at 20°C and 100% relative humidity is about 25 wt %. The maximum total amount of water taken up from air at ambient conditions by adsorption and absorption is about 30 wt % of the wood, but can reach 200 wt % or more if the wood is soaked in liquid water (Nikitin et al, 1962). Exposure to precipitation is the third mechanism that raises the moisture content of green wood to an average of about 50 wt %. It is evident from this explanation of the various mechanisms of water uptake by wood that transpiration of soil moisture is essential for tree growth to occur, that the temperature and humid­ity of the surrounding air affects the moisture content of the wood if sufficient time is allowed to reach equilibrium moisture levels, and that wood is hygro­scopic. The same principles are applicable to most terrestrial biomass because of the vascular structure typical of many species.