Energy density, often termed “heating value”, refers to the amount of energy released per unit fuel combusted, usually measured in terms of energy content per unit mass for solids (e. g. MJ/kg) and per unit volume for liquids (e. g. MJ/l). Energy density can be expressed in two forms, higher heating value (HHV) or lower heating value (LHV). HHV represents the total energy released when the fuel is combusted in air, including the latent heat contained in the resulting water vapor product — the maximum potentially recoverable energy from a given feedstock. The latent heat contained in the water vapor, however, typically cannot be used effectively. LHV, therefore, is the appropriate value to use when quantifying the energy available for subsequent use. As noted above, moisture content significantly affects biomass feedstock energy density. Freshly cut wood, for example, might have as much as 60% moisture and a relatively low energy content (e. g., 6 MJ/kg). In contrast, oven-dried wood with little moisture might have up to 18 MJ/kg. Representative LHV values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.