What is biomass?
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 can be classified according to the supply sector, as shown in the table below.
|Forestry||Dedicated forestry||Short rotation plantations (e.g. willow, poplar, eucalyptus)|
|Forestry by-products||Wood blocks, wood chips from thinnings|
|Agriculture||Dry lignocellulosic energy crops||Herbaceous crops (e.g. miscanthus, reed canarygrass, giant reed)|
|Oil, sugar and starch energy crops||Oil seeds for methylesters (e.g. rape seed, sunflower)|
|Sugar crops for ethanol (e.g. sugar cane, sweet sorghum)|
|Starch crops for ethanol (e.g. maize, wheat)|
|Agricultural residues||Straw, prunings from vineyards and fruit trees|
|Livestock waste||Wet and dry manure|
|Industry||Industrial residues||Industrial waste wood, sawdust from sawmills|
|Fibrous vegetable waste from paper industries|
|Waste||Dry lignocellulosic||Residues from parks and gardens (e.g. prunings, grass)|
|Contaminated waste||Demolition wood|
|Organic fraction of municipal solid waste|
|Biodegradable landfilled waste, landfill gas|
|Chipping of forest residues||Straw bales||A rapeseed field|
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)|
Biomass resources include a wide variety of materials diverse in both physical and chemical properties. Depending on the application, these variations may be critical for the final performance of the system. In particular, some advanced applications require fairly narrow specifications for moisture, ash content, ash composition. Both the physical and chemical characteristics vary significantly within and between the different biomass raw materials.
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.
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/