Typical proximate analyses and higher heating values (product water in liquid state, HHV) of representative biomass types and species illustrate the wide range of some parameters such as moisture and ash contents and the relatively limited range of heating values (Table 3.3). The physical moisture contents of biomass are generally determined experimentally by drying a sample at 100 to 105°C at atmospheric pressure or at lower temperature and reduced pressure. In a few cases, some organic compounds may be lost by these procedures because of volatilization and/or steam distillation, but generally the results are suitable for biomass characterization. The moisture contents listed in Table 3.3 range from a low of 2 to 3 wt % for the biomass derivatives char and paper to a high of 98 wt % for primary biosolids (primary sewage sludge). Green wood in the field before drying usually contains about 50 wt % moisture, whereas primary biosolids contain only a few percent suspended and dissolved solids in water. Similarly, the marine biomass giant brown kelp (Macrocystis pyrifera) and most other aquatic biomass contain only a few percent organic matter when first harvested; the main component is intracellular water.

Total organic matter is estimated by difference between 100 and the ash percentage that is experimentally determined by ashing the biomass samples at elevated temperature using standard methods (cf. annual volumes of ASTM Standards, American Society for Testing and Materials; Methods for the Examina­tion of Water and Wastewater, American Public Health Association). The chemi­cal reactions that occur during ashing result in the uptake of oxygen and the formation of metal oxides, so the experimental ash content is not identical to the inorganic mineral matter in the original sample. Ideally, all carbon in the original sample is eliminated on ashing, the metals are not oxidized, and none of the metals is lost. Such is not the case for some ashing procedures, particu­larly when samples contain high alkali metal concentrations. The loss of mate­rial due to the volatility of some alkali metal oxides at the ashing temperature causes errors in the analysis. Adjustments are sometimes made to the experi­mental ash determinations so that they correspond more closely to the inor­ganic matter present in the unashed samples. Nevertheless, subtraction of the experimental ash values in percent dry weight of the biomass from 100 to obtain the percent organic matter is adequate for most purposes.

Detailed chemical analyses of the components in the ash from two woody and one herbaceous biomass samples (Table 3.4) show that many metal oxides

’The ash, organic matter, and heating values were obtained from Boley and Landers (1969), Bowerman (1969),Chow et al. (1995), Hodgman (1949), Jerger et al. (1982), Klass (1980, 1984), Monk et al. (1984), Paisley et al. (1993), Pober and Bauer (1977), Tillman (1978), Wen et al. (1974). The moisture content ranges of biomass in the field were measured, estimated, or obtained from various literature sources.

are present, but that the distribution of the metallic elements is quite different in each sample analyzed. The oxides of calcium and potassium are dominant in hybrid poplar ash; the oxides of calcium and silica are dominant in pine ash; and the oxides of potassium and silica are dominant in switchgrass ash. As will be shown in later chapters, the distribution of the metals in biomass and the compositions of the ash are important in the development of certain types of biomass conversion processes because they can affect process perfor­mance. Also, some biomass species that have an unusually large amount of a specific metal have been harvested and used as a commercial source of that material during times of shortages.

It is evident from the data in Table 3.3 that the organic matter content and the HHV are affected by the ash, which in almost all cases has no energy value. The higher the ash value, the lower the organic matter and the HHV, as expected.

Intuitively, it might also be expected that the composition of biomass would vary over a broad range because there are so many different types and species.

The elemental compositions summarized in Table 3.5 support this hypothesis. In this table, typical ultimate and proximate analyses and the HHVs of land — and water-based biomass (pine wood, Kentucky bluegrass, giant brown kelp) and waste biomass (cattle feedlot manure, municipal solid waste, primary biosolids) are compared with those of cellulose, peat, and bituminous coal. On a dry basis, the ash values for these particular samples range from 0.5 wt % for pine wood to about 39 wt % for giant brown kelp. Also, on a dry basis, the total organic matter and the elemental analyses for carbon and hydrogen do not vary quite as much as the moisture and ash contents. Pure cellulose, a representative primary photosynthetic product, has a carbon content of 44.4%. Most of the renewable carbon sources listed in Table 3.5 have carbon contents near this value. When adjusted for moisture and ash contents, it is seen that with the exception of the biosolids sample, the carbon contents are slightly higher than that of cellulose, but span a relatively narrow range. It is also evident from the data in Table 3.5 that the HHVs per unit mass of carbon are quite close. Even those for reed sedge peat and Illinois bituminous coal are close to those calculated per unit mass of biomass carbon. As will be shown below, a reasonably good correlation exists between the carbon content of biomass and its energy content.

One of the analyses not included in the compositional information presented here on biomass is the percentage of so-called fixed carbon. This subject will be discussed in Chapter 8 under pyrolysis because there is no fixed carbon as such in biomass.