Introduction, Current Technology, Feedstock, Conventional Forestry, Short Rotational Coppicing (SRC), Forestry Residues, Agricultural residues, Oil Bearing Plants, Energy from Waste, Municipal Solid Waste (MSW), Industrial Waste, Physical Processing – initial treatment, Refuse Derived Fuel (RDF), d-RDF, Conversion, Thermo-chemical Processing, Combustion, Fluidised Bed Combustion (FBC), Advanced Thermal Conversion Techniques, Gasification, Pyrolysis, Fast Pyrolysis, Bio-chemical processing, Anaerobic Digestion, Biogas, Landfill Gas, Fermentation, Products, Economics, Social Implications, Environmental Aspects, Conclusions



Biomass; all earths living matter, is a very abundant source of energy. From the very beginning of human history the predominant sources for heat and power were wood for heat and cooking, charcoal (from wood) wind and water for the power industry and crops. Wood and charcoal are forms of biomass. In an energy context it can be defined as “all non-fossil organic materials that have intrinsic chemical energy content”[1]. This energy content is known as Bioenergy. Bioenergy can be produced from various different biomass resources (or feedstocks) through various processes making biomass one of the most versatile energy sources. In addition biomass is a CO2 emissions neutral source of energy. This is due to the fact that CO2 is taken from the atmosphere and used by plants to grow. Planting trees and crops does soak up an amount of carbon dioxide but it’s limited, if these are then harvested for biomass fuels fossil fuel use is offset thus further reducing the emission of CO2. Although some CO2 is emitted when manufacturing and burning biomass fuels, it is ultimately equal to the carbon dioxide absorbed by the plants used to produce this fuel (if the crops and trees are sustainably managed). For example the use of short rotation coppice (SRC) of willow and poplar, as a substitute for fossil fuels, produces no net CO2 emissions and low emissions of nitrogen and sulphur pollutants from its combustion. In this way fuel produced by energy crops could help ‘phase out’ fossil fuel generation as it would be a carbon dioxide (CO2) neutral energy, providing the  rate of consumption is equal to the rate of re-planting. This neutrality is another reason biomass is a very viable source of energy. The chart below indicates the versatility of bioenergy.



Current Technology


Conventional Forestry  

Contrary to what some might believe this type of forestry does not involve cutting down rainforests and ancient woodlands. It is a technique initially used in Sweden. Coniferous trees are planted at high density and after a period of growth selective cutting reduces the density of trees. The thinning produces wood chip that is later used for various purposes. 


Short Rotational Coppicing (SRC) 

Short rotational arable coppicing, is currently viewed by some as a potentially important source of fuel for electricity generation in Scotland and the UK - the UK Governments Department of Trade and Industry estimates (Energy Paper 62) that the maximum total realistic UK resource potential by 2025 could be up to 150TWh/yr - half the current UK electricity demand! Some farmers already burn straw in special plants used on their farms for power. Over the last couple of years, there has been great debate over the future of energy crops in Scotland: whether or not it makes sense to utilise agricultural land, especially ‘set aside’ land to grow an energy source for the future. Currently farmers get paid for ‘set aside’ land as part of government subsidies and it appears to make far more sense to grow energy crops rather than pay farmers for doing nothing.

Crops tend to be established by planting cuttings in cultivated ground. Coppicing occurs every 2-4 years when stems are harvested by cutting 5-10cms above ground level (cutting cycle). Cutting cycles can vary depending on the objectives of management and available land - research indicates that a yield increase of up to 70% can be achieved for one 4-year rotation compared with two 2-year rotations grown over the same periods.

Energy crops include

· Willows

· Poplars

· Hemp

· Miscanthus: a temperate climate grass adapted to moist soils (sewage sludge can be used as fertiliser for this plant)

· Maize

· Sorghum (a grass like grain crop that produces sugars)

Willow has been the main tree species highlighted for SRC - poplar may also be used. This is because willow is an ideal species to grow in the Northern hemisphere, mainly in cold and wet areas. It produces large amounts of biomass in relatively short periods due to its fast growing nature. It is reasonably simple to establish and requires low input of agricultural chemicals during its growth. Willow will thrive in a wide range of soil types and can be grown from cuttings provided there is an adequate water supply (and as far as “sunny” Scotland is concerned this does not pose a problem). Care needs to be taken to control weeds and protect new plantations from rabbits and deer. During growth the use of fertilisers and chemicals is lower than for most agricultural crops. The willow can be cut back at the end of the first year as this encourages growth of multiple shoots, alternatively plantations could be removed and the land returned to agricultural crops if things are not going as planned. Yields from coppicing can be stored until needed and then used to generate heat and electricity.

So far SRC does not appear to carry much wait in terms of economic viability - this is improving. Prices will have to fall in order to become a real option for the future or the level of subsidies will have to increase in progress is to be made.


Forestry Residues 

Forestry residues are produced when controlled thinning of plantations and trimming of felled trees is undertaken to reduce forest fire risk and to accelerate the forest growing rate that can sometimes be prevented if the area is overpopulated. This waste is usually just left to rot on the forest ground; extracting and collecting it clears up the forest making it easily accessible and manageable. In this category waste from public gardens and woods can also be included. The waste can be collected, dried and used as fuel.


Agricultural residues 

These include straw, manure, vegetables, fruit and general garden waste. Until recently the excess straw produced in the UK was burned in the fields or ploughed back into the land. As of the end of 1992 environmental legislation put in place has restricted field burning and thus straw has been seen as a potential source of energy. Other residues include potatoes and sugar beet tops as well as damaged fruit and around 5 million tonnes of nursery wastes. Using agricultural residues as a source for energy tackles another problem apart for the need to find alternative energy sources. Agricultural residues include animal wastes. Use of these wastes reduces the possibilities for odour and water pollution by manures. Manures from cattle, chickens and pigs are the most common ‘wet wastes’; in the UK about 7 million tonnes of such wastes are produced in a year!


Oil Bearing Plants 

Various plants have seeds that can be crushed on the farm to produce a range of vegetable oils and although they are not good enough for human consumption they can be used to power motors and onsite generators like those used for combined heat and power plants (CHP).


Energy from Waste 

One of the main sustainable development issues in Scotland is the effective management of waste. Waste disposal sites are a source of pollution, in terms of emissions to the atmosphere and water and are associated with health and environmental effects. Ultimately, the goal is to reduce waste production and to maximise recycling and reuse. But the waste that is produced needs to be dealt with effectively and sometimes energy generation is the optimum choice due to technical, geographic or market barriers to recycling.

Energy can be generated from the vast amounts of municipal and industrial waste that society produces.  


Municipal Solid Waste (MSW) 

This is waste that relates to a city or town, therefore it is wide ranging in composition, for example:

·         Paper and paper products

·         Plastic

·         Rubber and leather

·         Textiles

·         Wood

·         Food wastes

·         Yard wastes

·         Glass and ceramics

·         Metals

·         Miscellaneous (including even fridges!!)

This can cause problems when the waste is incinerated as different elements burn at different temperatures and speeds, leading to an uneven mixture and some waste not being completely burned. This means that energy is not extracted as efficiently as possible. Scotland creates 3 million tonnes per year of MSW, 90% of which goes to landfill, 5% to recycling and reuse, and 5% to incineration (DETR, SEPA). Incineration is therefore a small part of waste management for Scotland when dealing with MSW. In the long term, this picture will change and Scotland will follow the European example of countries like Denmark, where at present 60% is recycled, 35% to EFW, and 4% to landfill. The EU Landfill directive aims to divert wastes from landfills and will also influence the increase in the number of incinerators. These will all be part of a grand picture where recycling, reuse and EFW all work together to deliver an integrated, sustainable waste strategy. 


Industrial Waste 

Waste from timber processing is a great source of biomass feedstock. Dry sawdust and offcuts usually thrown away after the processing o cut timber make exceptionally good fuel. The furniture industry in the UK is estimated to produce 35,000 tonnes of such residues a year!  


Systems and Supply 

To date, one of the key characteristics of fossil fuels are that they can be easily acquired, transported and stored for use without their intrinsic energy content being compromised. This means that in order for biomass to be a competitive rival to fossil fuels it should be transportable and readily available for use. Unfortunately this is not as easy as it may sound; biomass is wholly organic and thus has a short shelf life by nature. For example the water content of biomass does not contribute to its stored energy. Water contents can be as high as 95% for fresh plants! This means that only 5% of the plant has energy to be tapped into. Further more if the matter is not dried then decomposition sets in quickly and renders the feedstock unusable. This means that in order to use biomass it has to be dried to reach water content of about 20%.  In addition transporting biomass resources poses a problem i.e. they have to be processed in such a manner that aids transportation. For example if plant matter is dried and then chipped not only will burning it be made easier but transporting will also be simple. 


Physical Processing – initial treatment 

As stated above biomass is a mixture of organic compounds and whatever form it comes in it must be used within a short period of time on site (otherwise it must be processed so that it’s shelf life is extended).

This physical processing involves:

·         Removing the moisture (This can be achieved by in-situ drying facilities)

·         Chipping or creating fuel pellets (Chippers can be placed next to dryers to prepare the feedstock for transportation, storage or immediate use).

An example of physical processing is:

Refuse Derived Fuel (RDF) 

This is a result of processing MSW, facilitating recycling, re-use and ensuring the homogenous nature of the waste. The non-combustible elements of the waste are left over after processing, giving the waste a higher calorific value. The new improved composition of the waste allows increased efficiency from incineration and reduced emissions and products as the waste that creates the harmful effects can be removed during processing. Said processing involves

·         Separating unwanted components (for example by magnetically extracting ferrous metals)

·         Shredding

·         Drying

·         Compacting the material into pellets

This type of conversion allows for and otherwise very cumbersome fuel resources to become easily transported and more hygienically handled, it also allows incineration to play a part in waste management where reduction, reuse and recycling can be maximised.




Further processing has also been developed; the end product is called densified refuse derived fuel (d-RDF). This is a process by which the combustible part of the waste is separated, pulverised, compressed and dried to produce solid fuel pellets about 5cm long.



In order to produce bioenergy various processes can be used to convert the intrinsic chemical energy of biomass directly to heat or electricity or to the intermediate biofuel. Biofuels include Methane Gas, Liquid Ethanol and Methanol or Solid Char or Charcoal. 


Thermo-chemical Processing 


All fuels contain two combustible constituents; the volatile matter and char. As the temperature of the fuel rises the volatile matter is released in the form of vapours or vaporised tars and oils. The spurts of flame, for example as wood burns, are an indication of the combustion of these products.

After this process (which is known as thermal degradation) ends the solid remnants comprise of char and inert matter. The char, which is mainly carbon, can be further combusted to produce heat and CO2. The inert matter then becomes clinker, slag or ashes.

Most of the bioenergy is within the initial volatile matter. This means that any furnace designed to burn biomass fuels should be designed in such a way to ensure complete combustion of these vapours. In addition, air must reach all of the char; this could be accomplished if small pieces of the matter are burnt (another reason behind the need to physically process biomass before it’s use)

The following diagram illustrates how combustion can be used to produce energy:


Mass Burn Combustion (MBC) can use municipal Solid Waste (MSW) to generate electricity. This is one of the main methods of incineration and has been around for years. It is commercially available and has been optimised. The plant operates by feeding waste onto a moving grate where it is burned; the heat generated by this is used to generate steam that drives a generator to produce electricity. The burning of the waste produces two types of ash. Incinerator Bottom ash falls through the grate for collection and is either landfilled or used in the construction industry. Fly Ash, which escapes with the flue gases that are emitted to the atmosphere, can contain sufficient dioxins and metals that require cleaning. As it involves the mass burning of MSW; this has many concerns attached to it.

In particular the waste is a mixture of different materials if these are combusted they may have a detrimental affect on the environment. The main pollutants that result from MSW incineration are as follows:

·         Gases

·         Metals

·         Organic Substances

·         Particulate Matter

The main area of concern is the contents of the array of gases that are emitted from the plants. The gases contain dioxins, which are suspected of causing many health problems including cancer. The particulate matter is also an area of concern, focusing particularly on ultra fine particles less than ten millionths of a metre. These are often inorganic materials with metals and organic compounds on their surface. In Scotland, all releases to the environment are regulated by SEPA, the Scottish Environmental Protection Agency. 


Fluidised Bed Combustion (FBC)  

This method of incineration uses RDF and is an alternative to the mass burn system. The pellets are fed onto a bed consisting of a mixture of sand and dolomite mineral. Air is then pumped through the whole mixture to create a bubbling liquid. The waste in this new liquid form has an improved combustion efficiency that reduces pollution and increases generation per ton of waste.

A downfall of this technology is that it is slower than MBC and there is limited experience. This form of incineration though has not yet been proven on a commercial scale and requires further investigation.


Advanced Thermal Conversion Techniques 

Two techniques that are very promising for the future of waste incineration are Gasification and Pyrolysis. These technologies are not as developed as MBC but promise many benefits. The two techniques have very similar economic characteristics, there is an option of pressurising the gas, which increases the capital costs but is compensated by cost savings at generation. The savings are created as compression of the gas is no longer required and there is higher system efficiency.



The gasification process in general involves the reaction of a solid fuel with hot steam and air (or oxygen) and the subsequent production of a gaseous fuel by partial oxidation. The diagram bellow illustrates the process. Gasifiers, depending on their type can operate with temperatures varying from a few hundred to over a thousand degrees Celsius and from pressures from around atmospheric (1 atmosphere) up to 30 atmospheres.

·         The gas resulting from this process mainly consists of:

·         Carbon monoxide (CO)

·         Hydrogen (H)

·         Methane (CH4)

·         Carbon dioxide (CO2)

·         Nitrogen (N) The proportion of the gases in the mixture depends on the processing conditions and whether air (78% Nitrogen, 20% Oxygen and 2% of others) or oxygen was used.

The simplest of gasification processes result in gases containing up to 50% by volume of CO2 and N. This means that the fuel has a low energy value so transporting is not economical viable but on site use can prove to be beneficial.  There are benefits to using such a complicated process to produce energy from biomass. The resulting gas is cleaner and more versatile than the original biomass; any unwanted pollutants can be removed during processing.


Synthetic Biofuels 

Gasification using oxygen instead of air produces a mixture of gases containing Hydrogen, Carbon Monoxide and Carbon Dioxide. Removing the CO2 produces a mixture called Synthesis Gas, this gas can then be used to produce almost any hydrocarbon. The most common products are methane and methanol. Methane is a combustible gas that can be used to drive generators although it is a very dangerous and harmful greenhouse gas. Methanol is a liquid fuel that is a direct substitute for gasoline



This age old process, otherwise called destructive distillation, involves the heating of the original biomass in the near absence of air, the temperatures at which this occurs range from 300 to 500 degrees Celsius. These high temperatures drive the volatile matter out of the original material, what is left is the char (charcoal). The usual biomass material used for pyrolysis is wood but nutshells and MSW can be used as well.


Fast Pyrolysis 

Further technological advancement in this sector has lead to a process called fast pyrolysis. This involves the collection of the volatile matter and depending on the temperature of the process these materials can be combusted. This liquid product has the potential to be used as fuel oil. Temperatures range from 800 to 900 degrees Celsius. Fast pyrolysis can leave as little as 10% char and can convert as much as 60% into a gas.   

Gasification and Pyrolysis have very similar costs but these are again hard to measure for the sake of generalisation or comparison as they are site specific and there are no large plants to use as an example. They do promise improved efficiencies and lower environmental mitigation costs with relevance to MBC. Furthermore if pressurised gas is used to drive a turbine then this increases capital costs but leads to a saving in terms of generation due to higher system efficiency. They allow improved combustion due to the intermediate fuel that is produced and have lower emissions due to lower gas flows. The production of this fuel means that it can be transported for generation at a different site. To further the technology and to make it economically viable, more funding for research and development is necessary, as well as the financing of the first large-scale commercial plant. 


Bio-chemical processing 

Anaerobic Digestion 

Anaerobic digestion occurs in the absence of air, the decomposition in this case is caused not by heat but by bacterial action. Any organic substance can become subject to anaerobic digestion so long as there are warm, wet and airless conditions. For example ‘marsh gas’ is a product of the anaerobic digestion of vegetation at the bottom of ponds, this gas rises to the surface and bubbles, it is also combustible. With the aid of human intervention there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are very complex. The figure below shows the generalised process.



Biogas is generated from concentrations of sewage or manure. These are usually in the form of slurry comprised mostly of water (almost 95%). The slurry is fed into a digester, this input can be continuous (usually the case with sewage) or in batches. The digestion continues from about ten days up to weeks. The temperature in the digester should be kept at 35°C and although the digestion itself produces heat, in colder climates some top up heat should be provided. In order for the process to remain sustainable the excess heat should be provided by the biogas itself.  In the very extreme cases all the produced biogas has to be used for this heating. In these cases the process is still beneficial as it offsets the need to use fossil fuels in order to process the wastes.


Landfill Gas 

As already stated a huge proportion of the waste produced in Scotland goes to landfill sites. As this waste sits under the ground in these sites, the biodegradable organic matter within the waste goes through anaerobic decomposition and produces a gas that is roughly an even mixture of Carbon Dioxide and Methane. This is an explosive mixture and has been known to cause explosions under ground. This gas used to be flared off or released to the atmosphere. The combustion of this gas reduces net emissions of carbon dioxide if used to offset generation from fossil plants as less is produced. This represents a small resource, but it is economically competitive with other forms of generation and it can provide base load electricity output.

The length of time and amount of gas that is available from a landfill site is very specific to the type of waste, moisture content, temperature, acidity and the design of the site. As the diagram above shows, gas is drawn up from vertical or horizontal wells through a system of pipes. At this stage the gas is usually warm and saturated with moisture. The extraction pipes contain condensate traps and are laid at an angle so that as the gas cools, the moisture does not hinder the flow of the gas. The condition of the gas is specific to what use it will be put to. The plant is constructed so that there is no leak of gas to the surrounding land or air. This methane and CO2 mixture can be used in the same combustion process discussed previously. The generation equipment is usually contained within the same area as the extraction plant. This site is usually away from urban sites due to safety reasons and amenity.



Fermentation is also an anaerobic process. With this process Sugars with the use of micro organisms (usually yeast) are converted into ethanol. Ethanol can be used as the fuel in the combustion processes. This can either be achieved through mixing the ethanol with gasoline or by using it directly in some modified combustion engines. Sugar cane undergoes fermentation most efficiently. Other feedstock’s can be used such as potatoes and corn, but these require processing so that the starch can be converted to sugar.  



The best way to sum up the various uses of Biomass and the processes involved in its harnessing is to sum up the products that can be created from the feedstock and to illustrate there uses (see figure below). So from all of the available biomass feedstock that can be harnessed and through the various processes the products include:

·         Heat (that can be used to heat water for use or central heating and to produce steam for use with steam turbines)

·         Gas (to be used to drive gas turbines)

·         Char that can be burned to produce heat again

·         Oil (for burning)

·         Straw (again for burning)

·         Ethanol (to be used as is or mixed with gasoline)

·         Methane

·         Methanol (to be used as fuel for especially converted engines)





This technology is still in its infancy, therefore there are many unanswered questions regarding its economic viability. The only way for this to be investigated is through demonstration plants but these are expensive and require large investment. Companies are not willing to invest in something that has a long or limited pay back; this is where governmental support is key to the growth of the industry.

Which biomass technology is the most economically viable depends on site-specific circumstances. This depicts the type of feedstock that is available and therefore which method of generation is best suited. The transportation of the feedstock has the possibility to incur costs so obviously it makes sense to position plants where minimum transportation is necessary. 


Social Implications 

There are many beneficial factors to consider. Biomass feedstock production, handling and processing due to the nature of all the materials are practiced in rural areas and so these benefits would be for those areas. These include:

·         Rural development through

·         Regional economic gain

·         Return of investments

·         Employment opportunities

·         Job creation

·         Sustainability

But on the other hand, there are many disadvantages to be considered. There will be increased traffic flow in the area and the various plants will represent a visual intrusion. Furthermore the running of the plants will create noise that may be unacceptable for nearby residents. The plants may even affect local ecology and any by-products and wastes must be removed thus adding to the traffic. Therefore reducing the distance the products travel to their point of use is an important part of this sustainable strategy. Only products requiring a specific form of management should be transported and where possible this should be by rail as residents are not keen on masses of lorries passing in front of their otherwise peaceful landscape.

The emissions from plants that use combustion are always a concern for local residents or anyone who is affected by them. The gases and particulates are often carried by the wind to another area other than that where generation takes place. This poses problems and political issues which need to be addressed. The possible detrimental effect on human health has the potential to rule out the use of such plants, especially when there are other alternatives for generation.


Environmental Aspects 

Biomass is by no means the only solution to global warming and other problems and the environmental effects of its use should be examined very closely.

For instance the combustion of wastes solves a disposal problem and may offset the use of fossil fuels but the incombustible materials such as ash have to be removed from the site. This incurs a cost to the environment due to pollution from transportation. In addition if care is not taken and a conventional combustion chamber is used (as opposed to a newly designed one that is capable of filtering out harmful emissions) by-products such as particulates and poly-aromatic hydrocarbons (PAHs) can escape to the atmosphere.

On the other hand tree planting on a very large scale (such as the one needed for arable coppicing) helps in the absorption of CO2. If care is taken in the overall processing (harvesting, chipping, transporting, drying and so on) of this biomass feedstock then the net CO2 emissions after it has been used for fuel will be less then the absorbed CO2 thus benefiting the environment.

In defence of biomass use comes methane (!). Methane is a very harmful greenhouse gas as well as causing accidental explosions due to its migration from landfill sites to nearby buildings. Moreover it has 30 times the damaging effect on the environment than that of CO2. The extraction, use of and combustion of methane actually protects against global warming. Even though a by-product of the process is carbon dioxide the net effect to the environment is much smaller.

Finally another issue concerning the environment is the use of ‘set aside’ land for energy crops. SRC has potential as there are only a few stumbling points that may cause some objections; for example the potentially sensitive nature of sites has to be considered - proximity to settlements and roads- as views could be impeded due to the growth rates causing a 3-dimensional woodland type mass in the field. But integrating the area with the surrounding landscape - trees and other features- when establishing new plantations can reduce visual impact. Also changes in the landscape can be quick during growth and also during harvesting. Additionally the scale of SRC has to be considered to avoid saturation of the landscape by monotonous planting. If farming conforms with regulations about adjacent plots of different tree species and so on most of the above concerns can turn out to be unfounded.



Biomass has the potential to become an important part of electricity generation in Scotland. It can be predicted and planned, therefore making it suitable for base load generation. The technology, however, is not yet far enough advanced but as more demonstration plants begin operation the industry will learn. At present, the main focus for this technology is small scale plants for generating electricity and producing heat. The economic support of the government will be key to the realisation of these plants on a commercial scale.


[1] www.bera1.org/about.html


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