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:
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.
The table below shows possible ranges in moisture content for selected biomass resources.
|Biomass resource||Moisture content|
|Industrial fresh wood chips and sawdust||40-60 wt. % (wb)|
|Industrial dry wood chips and sawdust||10-20 wt. % (wb)|
|Fresh forest wood chips||40-60 wt. % (wb)|
|Chips from wood stored and air-dried several months||30-40 wt. % (wb)|
|Waste wood||10-30 wt. % (wb)|
|Dry straw||15 wt. % (wb)|
Some typical characteristics of biomass fuels compared to oil and coal.
|Typical characteristics||GJ/t||toe/t||kg/m³||GJ/m³||Volume oil
|Pellets 8% moist.||17,5||0,42||650||11,4||3,5|
|Pile wood (stacked, 50%)||9,5||0,23||600||5,7||7,0|
|Industrial softwood chips 50% moist.||9,5||0,23||320||3,0||13,1|
|Industrial softwood chips 20% moist.||15,2||0,36||210||3,2||12,5|
|Forest softwood chips 30% moist.||13,3||0,32||250||3,3||12,0|
|Forest hardwood chips 30% moist.||13,3||0,32||320||4,3||9,3|
|Straw chopped 15% moist.||14,5||0,35||60||0,9||45,9|
|Straw big bales 15% moist.||14,5||0,35||140||2,0||19,7|
Databases on biomass characteristics
Detailed information on energy and chemical characteristics for a wide range of biomass fuels can be found in the following databases:
- IEA Task 32 biomass database www.ieabcc.nl
- Phyllis biomass database www.ecn.nl/phyllis/
- University of technology of Vienna biomass database www.vt.tuwien.ac.at/biobib
In the framework of the IEA Bio-energy task 29, the following educational website was developed http://www.task29.net/