Biomass generally refers to the organic matter deriving from plants and that is generated through the photosynthesis. Biomass not only provides food but also construction materials, fibers, medicines and energy. In particular, biomass can be referred to as solar energy stored in the chemical bonds of the organic material.
Where does biomass come from?
Carbon dioxide (CO2) from the atmosphere and water absorbed by the plants roots are combined in the photosynthetic process to produce carbohydrates (or sugars) that form the biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the biomass structural components. During biomass combustion, oxygen from the atmosphere combines with the carbon in biomass to produce CO2 and water. The process is therefore cyclic because the carbon dioxide is then available to produce new biomass. This is also the reason why bio-energy is potentially considered as carbon-neutral, although some CO2 emissions occur due to the use of fossil fuels during the production and transport of biofuels.
The figure below shows the global carbon reservoirs in gigatonnes of carbon (1GtC = 1012 kg) and the annual fluxes and accumulation rates in GtC/year, calculated over the period 1990 to 1999. The values shown are approximate and considerable uncertainties exist as to some of the flow values.
Representation of the global carbon cycle
Biomass resources
Biomass resources can be classified according to the supply sector, as shown in the table below.
Plant biomass composition
The chemical composition of plant biomass varies among species. Yet, in general terms, plants are made of approximately 25% lignin and 75% carbohydrates or sugars. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two categories are distinguished: cellulose and hemi-cellulose. The lignin fraction consists of non-sugar type molecules that act as a glue holding together the cellulose fibers.
Typical values for the composition of straw, softwoods and hardwoods
Cellulose Hemi-cellulose Lignin
Softwood 45 25 30
Hardwood 42 38 20
Straw stalks 40 45 15
The energy content of biomass
The calorific value of a fuel is usually expressed as Higher Heating Value (HHV) and/or Lower Heating Value (LHV). The difference is caused by the heat of evaporation of the water formed from the hydrogen in the material and the moisture. Note that the difference between the two heating values depends on the chemical composition of the fuel. The HHV correspond to the maximum potential energy released during complete oxidation of a unit of fuel. It includes the thermal energy recaptured by condensing and cooling all products of combustion. The LHV was created in the late 1800s when it became obvious that condensation of water vapour or sulfur oxide in smoke stacks lead to corrosion and destruction of exhaust systems. As it was technically impossible to condense flue gases of sulfur-rich coal, the heat below 150°C was considered of no practical use and therefore excluded from energy considerations.
The most important property of biomass feedstocks with regard to combustion – and to the other thermo-chemical processes - is the moisture content, which influences the energy content of the fuel. The figure below shows the evolution of the lower heating value (LHV, in MJ/kg) of wood as a function of the moisture content.
However, biomass feedstocks are more uniform for some of their properties compared with competing feedstocks such as coal or petroleum. For example, coals show gross heating value ranges from 20 to 30 GJ/tonne. However, nearly all kinds of biomass feedstocks destined for combustion fall in the range 15-19 GJ/tonne for their LHV. The values for most woody materials are 18-19 GJ/tonne, while for most agricultural residues, the heating values are in the region of 15-17 GJ/tonne.
