Review paperEnergy production from biomass (part 1): overview of biomass
Section snippets
Background
Biomass is a term for all organic material that stems from plants (including algae, trees and crops). Biomass is produced by green plants converting sunlight into plant material through photosynthesis and includes all land- and water-based vegetation, as well as all organic wastes. The biomass resource can be considered as organic matter, in which the energy of sunlight is stored in chemical bonds. When the bonds between adjacent carbon, hydrogen and oxygen molecules are broken by digestion,
Drivers for biomass
In the past 10 years, there has been renewed interest, world-wide, in biomass as an energy source. There are several reasons for this situation:
- •
Firstly, technological developments relating to the conversion, crop production, etc. promise the application of biomass at lower cost and with higher conversion efficiency than was possible previously. For example, when low cost biomass residues are used for fuel, the cost of electricity is already now often competitive with fossil fuel-based power
Biomass types
Researchers characterise the various types of biomass in different ways but one simple method is to define four main types, namely;
- •
woody plants,
- •
herbaceous plants/grasses,
- •
aquatic plants,
- •
manures.
Within this categorisation, herbaceous plants can be further subdivided into those with high- and low-moisture contents. Apart from specific applications or needs, most commercial activity has been directed towards the lower moisture-content types, woody plants and herbaceous species and these will be the
Plant characteristics
Biomass contains varying amounts of cellulose, hemicellulose, lignin and a small amount of other extractives. Woody plant species are typically characterised by slow growth and are composed of tightly bound fibres, giving a hard external surface, while herbaceous plants are usually perennial, with more loosely bound fibres, indicating a lower proportion of lignin, which binds together the cellulosic fibres: both materials are examples of polysaccharides; long-chain natural polymers. The
Photosynthesis
Photosynthesis is the process by which chlorophyll-containing organisms – green plants, algae, and some bacteria – capture energy in the form of light and convert it to chemical energy. Virtually all the energy available for life in the Earth's biosphere, the zone in which life can exist, is made available through photosynthesis.
A generalised, unbalanced, chemical equation for photosynthesis is
The formula H2A represents a compound that can be oxidised, i.e.
Plant species
The choice of plant species depends upon the end-use, bio-conversion option of interest e.g. combustion, gasification, pyrolysis, fermentation or mechanical extraction of oils. Some plant species are amenable to nearly all of the potential conversion technologies: e.g. oil seed rape can be processed via combustion, gasification, pyrolysis or mechanical extraction, while others such as wood and cereal crops, are suitable for combustion, gasification, pyrolysis and fermentation.
It is important to
Biomass properties
It is the inherent properties of the biomass source that determines both the choice of conversion process and any subsequent processing difficulties that may arise. Equally, the choice of biomass source is influenced by the form in which the energy is required and it is the interplay between these two aspects that enables flexibility to be introduced into the use of biomass as an energy source.
As indicated above, the categories of biomass considered in this study are woody and herbaceous
Harvesting
Establishment costs for SRC willow are currently estimated to be about , including cuttings, planting, weed control and rabbit/deer fencing (Anon, 1999). It is expected that as more schemes come to fruition, establishment costs will fall, perhaps by as much as 50%.
Harvesting biomass represents one of the significant cost factors in the production of biomass energy crops. The harvesting process is both energy-intensive – due primarily to transport fuel costs – and can introduce
Yields
The quantity of dry matter produced by a biomass species per unit area of production, determines the potential energy production capacity, or yield, of the available land area. Production is measured in dmt/ha and combined with the HHV of the biomass, the energy yield of the cultivated crop can be calculated. Table 7 indicates the range of energy yields for a number of types of biomass.
There is intensive research and development into increasing biomass yields using hybrid plants. Experimental
Energy production
It is acknowledged that each species of biomass has a specific yield/output, dependent on climate, soil, etc. However, to provide data for outline process designs, it is useful to assume some general biomass properties.
In the case of wood derived from SRC, it is assumed that the average LHV is 18 MJ/kg. At full generation rate, 1 kg of woodchips converts to 1 kWh(e) via use in a gasifier/gas engine generator, giving an overall efficiency of conversion to electricity of about 20%: this takes no
Conclusions
- •
The use of biomass, as a traditional energy source for the third world, can play a pivotal role in helping the developed world reduce the environmental impact of burning fossil fuels to produce energy but only if significant areas of replanting are actioned immediately.
- •
Biomass is an accepted form of renewable energy and is seen as a means of helping to reduce global warming, by displacing the use of fossil fuels: up to 10% of the UK's electricity needs is targeted to be generated from renewable
References (11)
- et al.
Converting biomass to electricity on a farm-sized scale using downdraft gasification and a spark-ignition engine
Bioresource Technology
(1995) - ADAS, 1992. The potential of miscanthus as a fuel crop. ETSU Report ETSU B...
- Anon, 1999. Willows in the Wind. Country Landowner, November, pp....
- Coombs, J., 1996. Bioconversion Assessment Study. CPL Scientific Ltd., UK,...
- Culshaw, F., Butler, C., 1992. A Review of the Potential of Bio-Diesel as a Transport Fuel....
Cited by (0)
- 1
Present address. MSE Ltd, Arle Crt, Hatherley Lane, Cheltenham GL51 6PN, UK. Tel.: +01242 269685.
- 2
Correspondence address. Green Acre, Dark Lane, Bristol BS40 8QD, UK. Tel.: +44-1242-269685.