Calorimetry and Chemical Energy in Biomass

The chemical energy stored in dry biomass can be estimated using calorimetry. A bomb calorimeter gives the most accurate results.Method: A known mass of dry biomass is placed in a sealed chamber (the bomb) filled with pure oxygen. The chamber sits in a known mass of water inside an insulated container. The sample is ignited electrically and burns completely. The energy released heats the water, which is stirred so that it heats evenly, and the temperature rise is measured.The energy released is calculated from:energy = m × c × ΔTwhere m is the mass of water (g), c is the specific heat capacity of water (4.18 J g⁻¹ °C⁻¹) and ΔT is the temperature rise (°C). Dividing by the mass of biomass burned gives the energy per gram.Worked example: Burning 1.0 g of dry plant material raises the temperature of 1000 g of water by 4.0 °C.
Energy = 1000 × 4.18 × 4.0 = 16 720 J = 16.7 kJ, so the biomass stores 16.7 kJ g⁻¹.Dry biomass is used because water in the sample would absorb energy. Pure oxygen ensures complete combustion, and the insulation reduces energy lost to the surroundings. A simple calorimeter, where a sample burns in air under a tube of water, gives lower values because much energy is lost and combustion may be incomplete.Multiplying the energy per gram by the biomass per unit area gives the chemical energy store per unit area, in kJ m⁻².