Biomass as a Fuel Source
This section gives a background of the different forms that biomass fuels
Biomass is considered to be carbon neutral because the quantity of CO2 released during combustion is the same as that absorbed by the plant during photosynthesis. This differs from fossil fuels in that, although both originating from organic matter, the carbon in fossil fuels has been locked away for millions of years, and when released during combustion, disrupts carbon dioxide levels in the atmosphere.
In reality, biomass is not completely carbon neutral as there are carbon emissions associated with processing the fuel and other possible emissions of green house gases from, for example, fertiliser use. There may also be carbon costs with transportation of biomass and the technologies used. Biomass is also considered environmentally sustainable since in many cases it is derived from the waste products of other industries such as agriculture and forestry management.
Biomass has several potential advantages when compared to other renewable energy resources. One of the main advantages is it can be used for heat, power and combined heat and power (CHP). As well as being able to produce continuous energy, and therefore not have intermittency problems, like those associated with wind and solar etc, it is also controllable so can be adjusted to meet demand. Biomass can be applied to a wide range of scales from 50kW domestic systems, medium 500kW plant to mega watt scale power stations.
Some biomass sources require fuel processing activities for example harvesting,
drying and chipping in the case of woodchips. Therefore when compared
to other renewable energy sources there is greater potential for creating
jobs and stimulating rural development.
Municipal Solid Wastes (MSW)
Municipal Solid Waste refers to a combination of domestic, light industrial and demolition solid wastes, generated within a community. At present there are three main ways of treating MSW; disposal in landfills, combustion and disposal in anaerobic digesters.
Incineration of waste can be problematic as the composition of the waste
is varied and pollutants such as dioxins, heavy metals, acidic gases,
nitrogen oxides and particulates. These can have an effect on health and
impact upon the environment when emitted during incineration. At present
public opinion surrounding incineration of MSW in Energy from Waste plants
is negative due to the fears regarding these pollutants. However, waste
incineration is heavily regulated at many levels by SEPA in Scotland,
and The Environment Agency in England and Wales. The EU Waste Incineration
Directive introduced tight emission standards for waste incinerators,
thus reducing the amount of pollutants entering the atmosphere, making
it more plausible that there may be more energy from waste plants in the
near future. Because of these fears and heavy regulations, it would be
advised that biomass from MSW is only economically viable for large commercial
The wastes produced by commerce and industry can be split into two categories, woody materials and non-woody materials.
In the UK forestry is primarily directed towards the production of timber and paper pulp. This has not always been the case as historically wood cuttings fuelled domestic and industrial stoves, and provided the raw material for charcoal.
Forestry is facing changes because the demand for paper pulp is decreasing as recycling is increasing and more correspondence is conducted through the internet while the demand for construction timber is also on the decline. As a result of this and the fact that large amounts of UK forestry, planted in the 1960’s and 1970’s, will be reaching maturity soon, means that there will be an increased amount of forestry products available to use as wood fuel.
During the literature review conflicting views of supply and demand were found for wood production. One explanation for this could be because wood as a biomass fuel source on a large scale is still in its infancy. Because of this there have been many reports commissioned to analyse the wood fuel potential in the UK.
Graph 1, taken from the Royal Commission on Environmental Pollution, shows that supply is predicted to increase until it reaches a peak, of just under 18 million m3, in the year 2018. The demand for wood has not been predicted though it may increase over this period due to an increase in building houses, which is foreseen in Government strategies for housing and planning2.
Graph 1: Supply and demand of GB Wood2
Graph 2 is taken from the Scottish Executive, which also shows that supply is expected to exceed demand within 15 years.
Graph 2: GB long term forecast-Supply and Demand Scenarios3
The above two sources are very positive about wood production in the UK. However, the ‘Forecast Wood Fibre- Part 14’ report has a different forecast of wood production with supply being able to meet projected demand. It states “forecast demand for coniferous roundwood and sawmill co-products from planned developments by the forest and wood energy industries will exceed the potential availability of these two fibre sources from 2007 to 2016”.
Forestry residues arise from forestry operations such as thinning plantations and trimming felled trees to prevent forest fires and accelerate growth. These residues and wastes offer a large potential for an energy supply.
Although differing facts and figures about supply and demand of wood
production was found during the literature review it should not deter
people from considering wood as a fuel source. These reports all have
slightly differing figures but most reports agree that supply, especially
in Scotland, will be able to support further demand growth especially
at this early stage in the biomass industry. This coupled with the Governments
commitment to renewable energy makes wood a promising biomass fuel.
With an increase in pellet production, some European countries, especially Austria have seen a dramatic increase in small scale domestic wood fuelled heaters.
In Britain, it is estimated that sawmill co-products total around 859 K odt/y, 20% of which is sawdust2. Although 98%2 already has a market, it is estimated by the Forestry Commission that around half of the sawdust could be made available for fuel without serious disruption to existing industries. At the moment, a significant increase in pellet production in the short term will have to come from other sources of wood, such as forest materials or short rotation coppice. In the future, increases in availability of sawdust for pelletisation will come from either an increase in sawmill activity or a decrease in other markets for saw mill co-products. This trend can be explained due to the fact that wood pellets have a much higher economic value to the sawmill than producing chipboard.
Wood: Wood is the most common form of biomass and has
been burned for thousands of years for heating and cooking. The term ‘wood’
includes products such as wood, sawdust and bark that has not been chemically
treated or finished. This ‘wood’ can be obtained from a number
of sources such as forestry, saw mills and timber merchants.
Larger biomass systems can take larger chip sizes, known as hog fuel and measuring 100mm in length but long thin wood chips, known as slivers, should be avoided as they can cause blockages and bridging in a chip feed system.
The characteristics of wood chips depend on the original source, from which they are made, and on the chipper. Forest chips include; log chips made from de-limbed stem wood; whole tree chips made from all the above ground biomass of a tree; logging residue chips made from branches, brash etc. and stump chips made from stumps. Wood residue chips are made from untreated wood residues, recycled wood and cut offs. Sawing residues produce sawing residue chips and other chips are made from short rotation coppice or short rotation forestry5.
Role of Storage: When considering wood chips for a biomass system it is important to consider the storage arrangements as storage facilities for wood chips can be very large. There can also be potential problems with composting in large piles of chips, this can lead to loss of biomass and spontaneous ignition from the rising temperatures, and it is therefore advised not to have piles of woodchips higher than 10 meters so as to prevent excessive build up of heat. However, if the heat from composting is managed properly it can help reduce the moisture content of the chips. If an automatic feed is to be considered the positioning of the boiler and storage unit must be carefully thought out.
Wood Pellets and Briquettes: Wood pellets and briquettes
need to be made from a fine dry feedstock so tend to be made from sawdust
or other by products of the wood processing industry. They can be made
from freshly felled timber but the grinding and drying processes involved
are less energy efficient and could significantly add to costs.
The production of wood pellets is more energy intensive than that of other wood fuel production, with 10% of the pellets own energy content being used in production when using wet, fresh sawdust and 2% of the pellets own energy content when using dry sawdust1. However wood pellets do have a higher calorific value than woodchips meaning than more energy can be obtained per delivery, reducing transport energy impacts. Currently wood pellets are produced by large scale industrial pellet manufacturers with links to large saw mills and/or pellet clients thereby making the production costs more economically viable.
Wood pellets, at present, are more popular with domestic users because they are less bulky to store, easier to handle and more suited to automatic handling systems because they are of a consistent size, when compared to other wood fuel sources. They also have a low moisture content 8%-10%, and a consistent density and heat content1. These characteristics would benefit larger sized boilers as well, however, sourcing large amounts of pellets and the cost of purchasing pellets may negate the environmental and economic benefits of owning a biomass boiler. Also, larger biomass boilers tend to be able to take wood chips as they do not have as delicate a feed system as the smaller domestic boilers.
Agricultural residues and wastes are defined as the by-products of the
agricultural system and can be split into two types, dry and wet.
Below is a table describing types of agricultural residues/ wastes, their production in the UK and their uses.
Table 1: Agricultural residues/ wastes and their uses5
Wet residues typically consist of animal slurry and farmyard manure and grass silage. They are primarily used as fertilisers. They have a high moisture content and are more suited to anaerobic digestion for the production of biogas. This is best done close to the source as the energy and financial costs of transporting wet residues is very high. Below is a table showing the amount of animals in the UK and their corresponding slurry production per year.
Table 2: Amount of animals and corresponding slurry production5
Food Waste: Residues and wastes occur at all points in the food supply chain. A majority of food waste has a high moisture content and can be used in anaerobic digestion. Wastes that have high starch or sugar levels are potentially suitable for fermentation into bioethanol and it is worth considering oil wastes for the production of biodiesel.
Energy crops, in the widest sense, encompass plants/crops that are purposefully
planted to be used as fuel or converted to biofuel. The primary objective,
when selecting energy crops, is to obtain the highest possible amount
of energy given the climatic conditions.
Methods for growing energy crops include short rotation forestry (SRF)
and short rotation coppicing (SRC).
Table 3: Biomass fuel crops, Advantages and Disadvantages2
Another advantage of planting energy crops, in particular short rotation coppice is the cover provided attracts small mammals, invertebrates and insects, which in turn attract small birds2. Larger mammals are also attracted to this type of plantation. These combined can offer biodiversity to the land. However, care must be taken in the planning stages so that biodiversity is enhanced and not reduced, as could be the case if wetlands are replaced. Despite the disadvantages, short rotation coppice provides a promising solution to bio fuel diversity and it should be encouraged as much as possible.
There are many potential biomass sources available, with varying advantages and disadvantages. Although municipal solid waste is heavily regulated and faces opposition from the public it does provide a feasible solution to waste production and can prove economically viable for large commercial power plants. Commercial and industrial wastes can provide a large amount of biomass material and is promising for use on the site where it is produced. Forestry production and residues are seen as a very promising biomass fuel source in the UK, particularly Scotland and the conversion process into fuel is comparatively simple, as are the regulations surrounding it. Agricultural residues and wastes have a large potential for use as a biomass fuel. However, some residues, such as slurry, are particularly wet so are more suited to anaerobic digestion onsite, and therefore not widely available. Energy crops are in their infancy at the moment but hopefully with time they will become more popular and the technology and information can be improved for greater yields, thus providing a reliable biomass fuel source for the future.
Forecast Wood Fibre Availability & Demand in Scotland & Northern England to 2016. 2006. Part 1 Report- Availability and Demand for Coniferous Roundwood and Sawmill Co-products. Prepared for The Wood Fibre Processing and Supply Industry. John Clegg Consulting Ltd with Cawdor Forestry Ltd.
Scottish Executive, Forum for Renewable Energy Development Scotland, 2005. Promoting and Accelerating the Market Penetration of Biomass Technology in Scotland. Crown Copyright.