The energy content of biomass is obviously a very important parameter from the standpoint of conversion of biomass to energy and synfuels. The different components in biomass would be expected to have different heats of combus­tion because of the different chemical structures and carbon content. This is illustrated by the HHVs listed in Table 3.6 for each of the main classes of organic compounds in biomass. The more reduced the state of carbon in each class, the higher the energy content. Monosaccharides have the lowest carbon content, highest degree of oxygenation, and lowest heating value. As the carbon content increases and the degree of oxygenation is reduced, the structures become more hydrocarbon-like and the heating value increases. The terpene hydrocarbon components thus have the highest heating values of the compo­nents shown in Table 3.6; the lipids have the next highest heating values. The dominant component in most biomass is cellulose. It has a HHV of 17 51 MJ/ kg (7533 Btu/lb).

“All analyses and HHVs were determined by the Institute of Gas Technology.

^Harvested from a residential site in the Midwest. cMacrocystis pyrifera harvested from kelp beds off the California coast. dEichomia crassipes harvested from a biosolids-fed lagoon in Mississippi.

‘From a commercial cattle feedlot.

^Refuse-derived fuel; i. e., the combustible fraction of municipal solid waste, from a Chicago facility. 8From a Chicago Metropolitan Sanitary District facility.

’’From Minnesota.

‘From Illinois.

JAs received with metals.

Typical lower heating values (LHV, product water in vapor state) of selected biomass species are shown in Table 3.7. Woody and fibrous materials appear to have energy contents between about 19 and 21 MJ/ kg, whereas the water — based algae Chlorella has a higher value, undoubtedly because of its higher lipid or protein contents. Oils derived from plant seeds are much higher in energy content and approach the heating value of paraffinic hydrocarbons. High concentrations of inorganic components in a given biomass species can greatly affect its energy content because inorganic materials generally do not contribute to the heat of combustion, This is illustrated by the HHV for giant brown kelp, which leaves an ash residue equivalent to about 46 wt % of the dry weight, as shown in Table 3.3. On a dry basis, the HHV is about 10 MJ/kg, while on a dry, ash-free basis, the heating value is about 16 MJ/kg.

When the heating values ot the waste and virgin biomass samples and even the peat and coal samples listed in Table 3.5 are converted to energy content per mass unit of carbon, it is apparent that they fall within a narrow range. This is usually characteristic of most biomass. The energy value of the total material can be estimated from the carbon analysis and moisture determinations without actual measurement of the heating values in a calorimeter. Manipula­tion of the data in Table 3.5 leads to a simple equation for calculating the HHV of biomass and also coal and peat with reasonably good accuracy:

HHV in MJ/dry kg = 0.4571(% C on dry basis) — 2.70.

A comparison of the experimental HHVs with the calculated HHVs for the biomass, coal, and peat using the carbon analyses listed in Table 3.5 is shown in Table 3.8. With the exception of the primary biosolids sample, the percentage error of the calculated HHV is relatively small.