Biomass

Biomass

The term ‘biomass’ has become synonymous recently with the burning of wood to produce heat such as in biomass boilers or similar.  In actual fact the true meaning is much wider, as the article below explains.  The article describes the main sources of biomass and why utilizing biomass to produce energy is much more efficient and much less onerous on the planet than burning fossil fuels.  It also explains the processes through which biomass is converted to energy and the advantages and practicalities of generating energy from biomass.

What is biomass then, really?

  

In broad terms, biomass is any naturally derived organic material found predominately in plant matter and is made solely from compounds composed of carbon, hydrogen and oxygen.

The main source of biomass for generating energy is wood, but it also comes in the form of:

  • agricultural residues and energy crops
  • food, usually food waste from food supply chains,
  • wood and non-wood wastes (including sewage) from industrial processes

These headings are discussed in more detail under ‘What are the main sources of biomass?’

Municipal Solid Waste can sometimes be classed as biomass if it is made up solely of organically derived matter, such as food, paper and wood fuel products.  More commonly though it is made up of any household waste items, such as furniture, clothing and household appliances.  For this reason it will not be discussed under ‘biomass’ in this article

How is biomass formed?
  

The out-lining process is linked entirely to the Earths Carbon Cycle, a chain of events that plays a pivotal role in regulating atmospheric CO2 levels.  Plant life capture’s energy from the sun, converting carbon dioxide from the air, and water from the ground into carbohydrates.  This biogeochemical cycle by which carbon is exchanged with the Earth’s spheres is the key sequence in making the planet capable of sustaining life; a process that has happened ever since plant matter was formed.

During its life the plant material tends to follow three general trends; firstly it can be broken down by micro-organisms and then released back to the atmosphere as carbon dioxide or methane; secondly it is burned and released solely as carbon dioxide or thirdly it can be eaten by animals and therefore converted into animal biomass.

How is burning biomass better for the planet than burning fossil fuels?

Biomass production follows a similar pattern to fossil fuels such as coal, oil and gas (fuels also derived from biological material).  The critical difference that exists is in terms of CO2 concentrations released back into the atmosphere. Fossil fuels when burned release CO2 absorbed by the earth  over millions of years, contributing to vastly  increased levels of CO2 in the atmosphere and subsequently affecting the overall atmospheric balance.

Biomass on the other hand forms a ‘closed loop’ cycle. Quantities of CO2 released into the atmosphere during the decomposition process exactly match the carbon that the plant has absorbed during its life-span, hence the overall process is found to be carbon lean.(Not completely ‘carbon neutral’ due to minor carbon emissions associated with processing the fuel).

How is energy generated from biomass?
   

As a renewable technology, biomass must be converted into energy in order to be utilised.

Traditionally the most common way of achieving this is via direct combustion of wood to generate heat.  The energy produced can then be used directly for heating, cooking and industrial processes or indirectly to generate electricity.   Energy systems used to burn the fuel range from small simple wood burning stoves, large fully automated boilers similar to those utilising ‘natural gas’ as a means of combustion, to large scale dedicated biomass plants where demand is met via industrial size boilers and/or CHP plant.

Generating heat via combustion is not the only method available: gasification, pyrolysis and anaerobic digestion all offer practical alternatives incorporating advanced technical systems and plant found commonly in the differing UK industries.

These methods of energy generation are covered in more depth in ‘What are the means of generating energy from biomass?”

What are the main sources of biomass?
   

There are broad range of potential materials found locally in the UK that can be classed as biomass and considered for renewable, low-carbon heating technologies.

Traditionally the main fuel source for biomass heating is wood, supplied in a varying number of commercial forms.  Wood fuels are commonly derived from conventional forestry practices however can also incorporate wood waste taken from second generation processes.  Alternative sources are also available, each having equal importance in their own right: agricultural processes, food supply chain, industrial operations and Municipal Solid Waste also contribute to the overall market for potential biomass.

a Wood Fuel

 

As an initial explanation, the term ‘wood’ refers to a range of products such as round wood, sawdust and bark which have not been finished 
or previously treated.  In terms of purchasing, wood can mainly be sourced from the forestry, timber merchants or from sawmills producing residues and bi-products from internal processes.  Logs, wood pellets, briquettes and wood chips are the common forms available to buy.

The use of wood for fuel is not primarily resigned to the aforementioned ‘standard’ methods of supply and purchasing.  Wood can be sourced from other areas such as recovered materials from municipal waste streams, from the construction industry, from arboricultural arising’s, forest residues (above ground biomass left on the ground after tree harvesting) and from other forestry means such as short rotation coppice and unmanaged woodlands.

In terms of wood’s use for combustion purposes, it is recommended that materials have a moisture content of 35% or below in order to assess the calorific value of the fuel and to achieve the correct/efficient operation of boilers and stoves.

The main types of wood fuel currently available on the biomass market can therefore be categorised as:

  • Logs
  • Chips
  • Pellets/Briquettes

i  Logs are normally purchased straight from a supplier after they have been left for a period of time to dry, reducing the overall moisture content.  Logs are primarily ‘hand-fed’ into a boiler or stove and are therefore best suited for small projects with smaller budgets, lower technology requirements and local access to wood supplies.  This type of fuel is not suitable for larger systems whereby automatic feed systems are utilised.

ii  Wood chips are a much more uniform fuel normally used for commercial and industrial boilers where an automatic feed is achieved via a hopper and heavy duty auger system.  This provides a steady rate of flow in order to meet the requirements of the boiler’s burner to offset required heating demands.  Chips are sold in relation to size and moisture content, dependant on the wood they are sourced from and are prepared from logs and waste wood being run through a piece of machinery known as a ‘chipper’.

iii  Pellets/Briquettes tend to be made from dry feed-stock such as saw dust, chips or wood dust.  The common method of production of such a fuel is via extrusion, forcing the biomass through a narrow passage known as a ‘die’ to compact.  Pellets or briquettes are then cut to the required length and packaged for whole-sale.

This type of fuel production is more energy intensive than other processes, dependent on the moisture content of the initial dry feed-stock.  They do however have a higher calorific value than chips; hence more energy can be obtained per delivery, improving the efficiency of energy consumption by transport.  The fundamental issue is in terms of suitability to the type of system being fuelled and whether or not the amount of fuel required is cost effective in off-setting other energy costs.  Perhaps best suited therefore to domestic type systems where less fuel is required in terms of storage capacity.  

b  Fuel from Agricultural 

Agricultural processes can be divided into two specific sections, both which have a means of being utilised for biomass energy applications.  These are:

  • Agricultural residues
  • Energy crops

i  Agricultural Residues

Residues from agriculture come in the form of both wet and drycrops and wastes.

Dry residues incorporate part of the crop not used for producing food, feed/fibre and animal bedding; with the main forms being straw, corn stover and poultry litter.  Utilising straw for biomass as an example represents a highly  resource efficient method of its disposal, pre-empting the need for it to be ploughed back into the land.  Commonly straw is burned in a purpose made boiler similar in characteristics to a wood fuelled boiler system.

Wet residues are common waste streams found in farming processes such as slurry, manure and silage.  Such outputs must be firstly dried out in order to make the process of combustion or gasification efficient in terms of both energy output and financial suitability.

Waste is converted into a gas by heating it under pressure and without oxygen in a “gasifier” or “anaerobic digester” (discussed under ‘Food Waste’). The gas can then be burned to produce heat, steam, or electricity.

ii  Energy Crops

Energy crops describe the method of growing plant matter specifically for use as fuel, offering higher output per hectare with low inputs.  The process of burning to make energy is similar to that used in wood fuelled scenarios.  The main forms of this type of potential energy source are:

  • Short rotation forests
  • Reed canary grass 
  • Switch grass  
  • Wheat grass
  • Miscanthus (elephant grass)
  • Oil plants

c  Food Waste 

The “total food supply chain” offers a number of different opportunities to primarily recover energy from waste processes.  This is in relation to the processing and manufacturing of food matter that subsequently produce organic waste, and, waste outputs from final consumers such as homes, restaurants and hotels.

i  Anaerobic digestion is possibly the best example of converting wet food waste into fuel suitable for energy.  Micro-organisms break down the organic matter in the waste by heating without oxygen to produce biogas.  Biogas can then be burned either in a boiler or a CHP unit to produce heat and electricity.  The fuel can also be utilised to produce bio-methane an alternative to natural gas or fuel for motor vehicles.

ii  Fermentation is another potential method for re-using wet food waste.  Wastes with high levels of starch and sugar can be fermented to produce bio-ethanol fuel. 

d  Industrial Waste 

Industrial waste can be categorised as the bi or co products of traditional operations within the process or manufacturing industries.  Classification splits these outputs into ‘wood wastes’ and ‘non wood wastes’.

i  Wood Wastes

Wood wastes come in the forms of both treated and untreated wood, while also including composite materials.  Untreated wood comes in the form of offcuts and sawdust.  Treated wood is primarily that from the construction industry incorporating wood not used as part of any perceived works or in the form of wooden pallets used in the transportation of goods and materials.  Composite materials are wood made up from two or more components, each having different physical and chemical properties.

ii  Non-Wood Wastes 

Paper, pulp and sewage are examples of ‘non wood’ wastes found to be suitable for use as efficient biomass energy.  Cardboard and waste paper material rich in biomass can generate quantities of energy through methods such as combustion and gasification, similar to that used for wood fuel.

Pulp products are also found to be energy useful, and can be utilised in the provision of biogas through the process of anaerobic digestion.

iii  Sewage sludge, a bi-product of waste water treatment is another form of waste suitable for energy production.  It can either be dried and used in combustion or gasification, or, anaerobically digested due to its high moisture content.  In reality though, the treatment of sewage sludge into a form suitable for energy production is costly in terms of energy and finance and so is rarely practiced.Back to top

What are the means of generating energy from biomass?
  

a  Direct Combustion Systems

The direct combustion of wood based fuel is the most common method of biomass energy transfer and is likely to be the most practical and efficient use of biomass in building applications.

Systems can generally be classified into the following categories:

  • Small scale (up to 50kW)
  • Medium scale (50kW to 3MW)
  • Large scale (3MW or greater)

i  Small scale can be categorised as a heating technology for domestic and small commercial purposes.  These particular biomass systems are capable of heating domestic buildings through to small commercial installations, also potentially incorporating small district heating schemes.  Wood in the form of chips, logs or pellets are the most common method of fuel utilised.  Systems therefore range from small wood burning stoves (normally manually fed) that provide radiant space heating only, to smaller end boilers complete with an automatic hopper feed that can produce hot water for both heating and domestic hot water purposes.

ii  Medium scale systems are primarily log, wood chip or pellet boilers of similar characteristics to the smaller end boilers yet with advancements in size and technological aspects, to suit larger commercial situations.

The operation of these boilers is slightly different from traditional ‘natural gas fired’ models commonly found.  Wood fuelled systems take longer to initially fire compared to gas systems and retain/produce heat for a continued period, even when the demand for this heat is met and the system has completely shut-down.  Wood systems also cannot modulate down to lower levels and subsequently struggle with lower temperature operation.  To make the process more energy efficient, it is common practice to install thermal buffer tanks to prevent boiler cycling and to store any excess boiler heat that can be used at different times; even after the boiler has ceased firing.

Biomass boilers to current design standards have full/part load efficiencies of ≥90% and incorporate the following common design features:

  • High operating reliability
  • Auto ignition
  • Automatic feed
  • Quiet speed controlled draught fan
  • Automatic heat exchanger cleaning system
  • Automatic ash collection system
  • Complete controls system inc a lambda probe and rapid heating facility
  • Patented glow zone combustion chamber
  • Buffer tanks/thermal stores for heating and hot water purposes
  • Potential to be linked to solar panels for summer operation

iii  At grid scale there are now a substantial number of biomass power plants in operation throughout the UK.  This has either been through the production of dedicated biomass facilities or previously through retro-fitting fossil fuel plants to enable co-firing.  None the less, UK Government is currently reviewing its existing credit agreements which look like to be phased out by 2027.  Producing energy from biomass at this level has always been seen as a transitional technology, to be replaced by other forms of renewable energy with lower carbon footprints in the medium to long term, not least of all because electricity production from biomass is far less efficient than heat production. 

b  Anaerobic Digestion 

Organic material with high methane content found primarily in agricultural, household, commercial and industrial residues can be utilized efficiently by converting it into a fuel suitable for energy.  Anaerobic digestion is a process that produces both biogas and a solid residue (digestate) similar to compost by heating without oxygen; producing a fuel that can be utilised to power a gas engine to generate heat and electricity.

Taking the farm manure process as an example; manure gathered is collected in a tank or vessel and then pumped to the main digester where it is heated and starved of oxygen in order to produce the fuel.  The gas produced will be stored and refined further before being utilised to power an engine or burned in a gas boiler.  The digestate is also stored, separated into solid and liquid forms; liquids used for fertiliser, solids used for composting.

c  Gasification 

When wood is converted into a combustible gas through a thermo chemical process, it is commonly referred to as ‘gasification’.  Methane, carbon monoxide, carbon dioxide and hydrogen are all produced by the partial oxidation of the wood; normal oxidisers being air, steam or oxygen.

The process is dependent on the gasifier being supplied with the correct type of fuel.  Different systems require specific fuels to operate efficiently; fuels with unique traits such as moisture content and surface composition.  Different gasification systems have different make ups i.e. the oxidiser used, the method of supplying heat, the reactor type and the direction of flow (counter-current or co-current gasification).   Use for the fuel is exactly as that produced by an anaerobic digestion system.Back to top

What are the benefits of biomass?
  • It’s a “carbon lean” process producing a small amount of carbon emissions compared to burning fossil fuels.
  • Can deliver emissions reductions of 90% in comparison to fossil fuels
  • Fuel can be sourced locally, therefore supporting local businesses and the rural economy.
  • Most regions or counties in the UK can produce biomass.
  • Can be stored and used on demand (key advantage over other renewable sources).
  • Fuel does not mean simply wood products used for combustion purposes; agricultural, food and industrial wastes and residues can also be utilised.
  • The expanding market for biomass encourages biodiversity by providing an economic incentive for managed woodland.
  • Technological advances mean that biomass can now be utilised in a more efficient way – for both space and water heating.
  • Inherently flexible, offers potential supply of heat, electricity and transport fuels.
  • Biomass systems are highly efficient, therefore contributing to lower energy bills in comparison to traditionally used fuels.
  • Biomass systems are a good option for rural areas when a natural gas supply is not available and other traditional fuels such as oil and LPG are normally used.
  • Biomass qualifies for the Renewable Heat Incentive (RHI); helping businesses meet the cost of installing renewable heat technologies
Time for biomass?
  

Biomass (predominately wood fuel) as an energy producer is certainly a resource efficient technology that is growing rapidly across Scotland; with the demand for timber increasing year on year. Currently we have a significant amount of well managed forestry that can provide an indefinite supply of heat, therefore helping towards delivering the 11% renewable heat vision set by Scottish Government.  Furthermore there are actually large areas of untapped forestry that could also potentially be used as a wood fuel source.

In terms of implementation as a renewable heating technology; wood as a fuel is not the answer for every area of Scotland; but due to the significant amount of rural areas that exist, it certainly is a more than viable option at specific local levels.  The Scottish Government would prefer to see biomass deployed in heat only or combined heat and power schemes, off gas-grid, at a scale appropriate to make best use of both the available heat, and of local supply.

What should be taken into consideration at the design stage?
  • Investigate the varying methods available and have discussions with a competent surveyor, design engineer or manufacturer.
  • New build or existing retro-fit project?
  • Determine if space is available for a boiler and fuel store.
  • Is there a fuel supply available?
  • Will there be suitable access for vehicles to deliver fuel?
  • Current Legislation: The Clean Air Act, Building Regulations, Local Planning, The Environmental Permitting Regulations 2011, Waste Incineration Directive.
  • Always use an MCS certificated installer as they have the necessary training and expertise.  The Energy Saving Trust has useful guidance on finding an installer.  

This will ensure compliance with all current Building Regulations, British Standards and Approved Codes of Practice and Guidance documents.

It is critical to get the correct storage sorted out at the design stage.

With all biomass installations there will be the requirement for an on-site storage facility.  Such a facility will normally be sized in relation to the volume of fuel to be stored, the capacity of the biomass boiler itself and the subsequent rate of use.

As moisture content of the fuel is critical for the efficiency of the biomass burning process, the storage facility must be suitably water tight, keeping contents dry and protected from both rain and groundwater.  Consideration should also be given to protecting fuel when deliveries and maintenance are taking place. Furthermore the facility should be suitably ventilated to avoid the risk of condensation while also preventing the growth of mould and the potential for the biomass to compost.  Drainage should also be provided in order to allow the removal of water, either from unintentional ingress or for cleaning purposes.

Delivery of the fuel to the storage facility is normally achieved by manually tipping straight from the lorry or walking floor trailer, or, by pumping the wood pellets/chips through pipework or a connecting tube.Back to top

What government funding is there for biomass?

The Renewable Heat Incentive (RHI) is a UK government scheme that aims to reduce carbon emissions by supporting renewable heat installations such as biomass.  Biomass heating is currently eligible for both the domestic and non domestic RHI schemes.

For full details on the Renewable Heat Incentive please click here.

Case studies on biomass installations

Further Information
Please visit the DECC website for updates on the Renewable Heat Incentive (RHI).

For more information on how to become Microgeneration Certification Scheme accredited please click here.

 

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