Wood energy content

The chemical characteristics of biomass vary over a broad range because of the many different types of species. Table 8 compares the typical analyses and energy contents of land- and water-based biomass, ie, wood, grass, kelp, and water hyacinth, and waste biomass, ie, manure, urban refuse, and primary sewage sludge, with those of cellulose, peat, and bituminous coal. Pure cellulose, a representative primary photosynthetic product, has a carbon content of... 

Chemical Composition. Chemical compositional data iaclude proximate and ultimate analyses, measures of aromaticity and reactivity, elemental composition of ash, and trace metal compositions of fuel and ash. All of these characteristics impact the combustion processes associated with wastes as fuels. Table 4 presents an analysis of a variety of wood-waste fuels these energy sources have modest energy contents. 

Mills that process rice may also generate process heat, that can be used for direct heating, steam generation, mechanical power or electrical power. For every five tons of rice milled, one ton of husks with an energy content equivalent to one ton of wood is left as residue. A rice mill in Louisiana has satisfied all its power needs since 1984 from an on-site rice-husk power plant. The plant sells surplus energy to the local utility. 

The replacement of timber products by nonrenewable materials is an unfortunate development, since it has been repeatedly shown that the use of timber does have associated environmental benefits compared with the use of nonrenewables (e.g. Marcea and Lau, 1992 Hillier and Murphy, 2000 Bowyer etal., 2003 Lippke etal., 2004). Timber has a lower embodied energy content (and hence a more favourable carbon emission profile) compared to most other building materials and can provide other benefits, such as improved thermal properties. It and the products made from it (in common with other renewable materials) can be used as a repository for atmospheric carbon dioxide. Wood is derived from a renewable resource, albeit potentially an exhaustible one unless it is managed correctly. Disposal of wood can be readily achieved with little environmental impact (subject to how the wood has been treated prior to disposal). 

Extractives and lignin have a higher energy content than cellulose and hemicellu-loses due to the difference in oxygen content of these fractions. An example of an estimated energy distribution by fraction in two wood species is given in Figure 2.3.3. 

Almost every aspect of human life involves carbohydrates in one form or another. Like other animals, we use the energy content of carbohydrates in our food to produce and store energy in our cells. Clothing is made from cotton and linen, two forms of cellulose. Other fabrics are made by manipulating cellulose to convert it to the semisynthetic fibers rayon and cellulose acetate. In the form of wood, we use cellulose to construct our houses and as a fuel to heat them. Even this page is made from cellulose fibers. 

The average stored energy content of wood is 8600 BTU/lb. Variations in heat values for wood as a function of species occur within a relatively narrow range of 8000-10,000 BTU/lb. Bark values are slightly higher than wood values, about 10,000 BTU/lb. 

Charcoal yields and the volatile matter content of the charcoal produced decrease with increasing temperature (see figure 2). For willow wood the required diarcoal yield of 0.38 kg/kg dry feed can be obtained at a temperature of 350-400 C, giving a product with a volatile matter content of 30-50 wt,%. In this temperature range 46-60% (LHV-basis) of the energy content of the feedstock is retained in the charcoal. 

Two different techniques of NMR have been applied to measure wood moisture content based on the presence of the hydrogen nuclei in water. In one of these, designated as a steady-state method, the wood is subjected to an alternating magnetic field of constant frequency, with Ho varied slowly so as to resonate 7 with respect to the applied frequency. At resonance a strong absorption of energy occurs, and the width and intensity of this absorption curve give information on the moisture content of the wood (16). 

The curve for the energy level of bound water shown in Figure 21 indicates that Qi and increase with decreasing wood moisture content below fiber saturation Mf, This increase means that more... 

Figure 24 shows curves of Qi, AG, and TAS plotted against wood moisture content. All energy terms are negative (heat is given off) when wood takes up water from the liquid state. The decrease in entropy indicates that bound water is more ordered than liquid water, in analogy to the greater order of water in ice compared with the liquid state. As the moisture content approaches fiber saturation the distinction between liquid water and water in wood decreases toward zero. However, even above fiber saturation the water in cell cavities may be different from ordinary liquid water because of capillary forces and/or dissolved materials. 

Operation with Emphasis on Material Recovery Figure 2 shows this type of operation. The input material consists primarily of substances with relatively high energy contents, such as used tyres, plastics, rugs, wood and special organic industrial wastes. 

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