The term biomass can be taken to mean various things ranging from animal dung to grass through to wood and can be read about at the Biomass Energy Centre. In terms of the domestic and commercial boilers that are currently being installed in UK buildings the biomass fuels are typically wood based and this guide will concentrate on such wood fuelled boilers. This wood is normally in the form of chips or pellets but also, for mainly manually loaded boilers, log sand briquettes. There are boilers, and dual fuel boilers, that can burn waste materials as well as other biomass materials. Woody material can also be gasified to produce gas that is then used as a fuel in its own right – this is outside the scope of this guide but more information can the principles can be seen at How Stuff Works and an example is described in this downloadable report BedZed (look at page 3).

The use of biomass boiler installations is becoming more popular due to one or more of the following...

Planning - Some planning requirements include a certain proportion of renewable energy in the design to meet carbon targets. Biomass is renewable as it can be re-grown at the same rate as it is burnt (unlike coal or natural gas that take thousands of years to replenish).These requirements have developed from the Merton Rule.

Building Regulations - In the calculation necessary for planning compliance (known as a Building Emission Rate, BER) the use of biomass boilers can reduce the overall carbon dioxide(CO2) emissions. For example Approved Document Part L2, Conservation of fuel and power of the 2010 England/Wales Building Regulations specifically include biomass as a means of providing Low- and zero-carbon (LZC) energy.

Environmental respectability - Building owners/operators increasingly ask for their project to employ technology that has a low carbon footprint. The use of biomass, with a properly organised supply of fuel, provides low overall CO2 emissions since growing wood absorbs CO2 to balance that given off when the wood is burnt.

Low fuel prices - The potential to have lower basic fuel prices than oil or electricity (and potentially cheaper than gas).

Cash incentives - The Renewable Heat Incentive (RHI) launching in 2011 will provide cash payments for using renewable sources of heating. There are also grants and tax incentives for businesses installing recognised biomass equipment.

There are particular requirements for biomass boilers in the relevant supporting documents for the Building Regulations.

England and Wales - All the Approved Documents are available from the Planning Portal of particular relevance are Building Regulation AD Part J – Combustion Appliances and Fuel Storage Systems and Building Regulation AD Part L – Combustion Appliances and Fuel Storage Systems

To determine whether particular applances are allowable under the England and Wales Building Regulations see section 2 of  - Non-Domestic Building Services Compliance Guide 2010 Edition

Scotland - The Technical Guidance for the Scottish Building Regulations are available and of particular relevance are Section 3 - Environment (includes chimneys) and Section 6 (Energy)

Ireland - The Technical Booklets supporting the regs (make sure you download the relevant amendments booklets). Particularly relevant are Techical Booklet F2:2006 - Conservation of fuel and power in buildings other than dwellings and Technical Booklet L: 2006 - Combustion appliances and fuel storage systems

There are a many British Standards that affect the installation of Biomass Boilers. B&ES's "TR38-Installation of Biofuel Heating" (available from the bookshop) has an extensive list of the relevant standards.

Commercial biomass boilers fall under the Clean Air Act 1993 (in Northern Ireland the 1981 Clean Air Order) regulated by the local authority. If the local authority has designated a Smoke Control Area the biomass boiler must be an approved 'exempt appliance'. Where an appliance is designed to burn fuel at a rate greater than 45.4 kg per hour (about 160kW for wood chips) the local authority will need to approve the chimney height. Smaller plant is subject to fewer controls - principally avoiding dark smoke, nuisance odour and soot.

It is important to contact the Environmental Health Department (in addition to planning) to ensure compliance with local requirements and if the fuel is based on waste materials it is essential.

The development will need to have been assessed under the planning process to determine its impact on air quality and if the site is in an Air Quality Management Area it is likely to have to comply with more stringent requirements. The installed boiler must meet the performance levels of that proposed during the planning process. DEFRA maintains a resource on smoke control for details of Smoke Control Areas and Air Quality Management Areas. This web page has a good overview of relevant legislation.

The availability of fuel and having appropriate fuel storage determines the viability and long term success of using a biomass installation .Freshly cut wood would typically have between 35 and 60% moisture – too wet to burn in many instances although moving grate boilers can accept high moisture content materials. Wood chips and wood pellets are the two biomass fuels that are most widely used to fuel domestic and commercial installations as they can fed automatically to the boiler, logs and briquettes require manual handling.

The specification for these fuels is given in standard BS EN14961 - download from the Biomass Energy Centre - this is likely to provide the basis of the regulatory UK requirements.

Wood chips are made from wood waste (left when timber is cut from trees) or more frequently purpose grown trees and are created using a chipping machine. To be useful as a fuel the chips would normally have less than 30% (by weight) of water - dryer chips will produce more useful heat. They are classified by a number of parameters including size, moisture content and ash content. Chips produced from industrial waste products may be contaminated (for example with paints and oils) and should be carefully assessed or avoided.

Wood Pellets are made from sawdust that is forced, under very high pressure, through small holes where the natural lignum in the wood 'glues' the particles together to form small (less than 6 to 12mm diameter) pellets. They have a moisture content of 10%or less. Some pellets have additives to reduce dust. A comprehensive booklet created through an EU project covering all aspects of wood pellets is available here

Logs should be 'seasoned' (dependent of wood left for at least a year to dry out) before being used as a fuel. Their dimensions will naturally vary but a 'normal' log would be approximately 100mm diameter and 300mm in length. These would require manual handling to feed the boiler. See the Logpile site for information.

Briquettes are made of wood chips and sawdust (being a brick form of the wood pellet), the yare used in a similar way to logs but have twice the energy as an equal volume of logs.

For sustained use the quality and certainty of the fuel supply is critical and a fuel supplier needs to be not only found but integrated at the design stage of the project.

For commercial installations the predominant fuel type is wood chips and pellets – pellets take up about a third of the storage volume compared to an equivalent energy store of wood chips.

Wood Fuel Form	Wood chips	Wood pellets	Seasoned Logs	Briquettes
Range of individual boiler outputs (kW)	30 - 10,000	8 - 500	10 - 80	10 - 100
Fuel moisture content % (by mass)	20 - 30	8 - 10	18 - 15	8 - 10
Calorific value kWh per kg	3.6	4.7	4.0	4.7
Calorific value MWh per m3	0.8 - 1.0	3.0 - 3.1	1.4 - 1.9	3.1 - 3.8
Ash % (by mass)	1.0	0.5	1.0+	0.5-1.0

The price of wood chips is likely be far less than pellets and so can offset the additional cost of handling and maintenance needed to ensure that they feed reliably into the boiler. The cheaper price of chips is particularly beneficial on larger systems (above 50kW).

The appliance is designed to operate at a temperature that ensures full combustion of the material and gases. The combustion of biomass is a chemical process and takes place in these phases

Heating and Drying - This requires energy to bring the wood up to temperature and evaporate the water and particular boilers are designed to accept fuel at specific ranges of moisture content.

Pyrolysis - The heat releases the flammable vapours (hydrocarbons, tars and other gases) from the wood and leaves behind 'char' (charcoal)

Gas ignition - if oxygen is available and the temperature is somewhere above 300°C the gases will ignite producing heat

Char burning - The char will burn long after the initial flammable gases are burnt off – as long as air is present.

The feed rate of both fuel and air is an important balance to produce effective combustion as well as controlling the boiler output. Primary air is fed under the fuel for the combustion of solids and secondary air above for combustion of gases. Without proper ventilation air and proper flue designthere is potential for carbon monoxide to accumulate – this can be lethal.

Boilers may be manually batch fed for smaller installations(less than 50kW), or continuous automatic. There are 3 principal types of automatic fuel feed.

A Stoker burner is suitable for wood chips preferably dryer than 25% mc or pellets and receives its fuel at a controlled rate directly into the burner head - the ash being pushed away by the incoming fuel. The output can be closely controlled as the amount of burning material is small – using wood pellets this can go down to below 10% of peak output. Most boilers below 100kW would be based on this type.

An auger fed underfed hearth pushes the wood chips or pellets fuel in from under the fire bed and the fuel displaces the ash and char above it. There is not much opportunity for the incoming fuel to be dried by the radiant heat so this is generally most suitable where the mc is below 30%. Ash removal can be manual or automatic.

Auger or Ram fed moving grate systems used up to about 1000kW, have a moving set of fire bars that shift the fuel from its entry point gradually along horizontal or inclined firebed. (Ram feed is used for largest boilers) As the fuel moves along it will follow the four stages of combustion

heating/drying -> pyrolysis -> gas ignition -> char burning

The ash comes off the end of the process. Moving grates allow the use of a much wetter fuel that can have moisture contents beyond 50% (but at a reduced overall efficiency).

The heat will be transferred from the combustion chamber to the water through heat exchangers surrounding the chamber and then from the flue gas as it passes through vertical or horizontal tubular pathways snaking through the water jacket towards the chimney. The combustion air will be supplied by natural draught, induced or forced fan depending on the type and size of boiler.

Whatever the type of boiler it will run most efficiently when operating at, or near, full load. This can mean that a biomass boiler is frequently sized to meet a part of the building load and traditional gas or oil boilers (that can be controlled quickly across a wide range) will be used to provide variable top-up heating. A thermal store is frequently used with commercial biomass boiler systems to allow boilers to be used at high load for longer periods.

To allow biomass boilers to consume fuel efficiently the systems will frequently include a large water thermal storage tank (also called an accumulator tank) that is heated up when the building does not directly require the heat from the boiler – this is quite separate from the domestic hot water storage. The stored heat is then used during times of peak load for heating or to heat domestic hot water and allows for the operation of building heating management system in a similar way as for a traditional gas or oil fired plant. The size of the insulated thermal store will depend on the pattern of load variation in the building compared with the base load but is typically between 10 and 20 litres per kW peak biomass boiler load. An example layout may be seen here.

The controls integrated into a boiler will control both the air supply as well as the mechanisms feeding and (in the case of wood chip boilers) agitating the fuel. The thermal inertia of the bulk of the burning material will mean that biomass boilers may not react as quickly as gas or oil boilers (although some wood pellet boilers can provide similar levels of control to a gas boiler). So to maintain the efficiency of combustion (to minimise pollution) the boilers must be operated over a smaller range – in some boilers it is possible control down to 30% of maximum output whilst still maintaining relatively high efficiencies.

'Lambda' control is often used - this senses oxygen in the flue which varies the fuel supply and/or the inlet air flow to ensure that optimum combustion is maintained and internal controls protect the boiler operating at extreme temperatures. Externally there would be the normal time clocks,compensators and schedulers.

For proper operation of biomass plant it is important to establish a reliable source of fuel prior to the installation of equipment, as there may be particular requirements for access - pellets can be blown (typically up to 25m) into a hopper, whereas wood chips (logs and briquettes) need to be tipped from a delivery vehicle. Wood chips 'bunkers' are normally below ground to ease delivery and where space is limited pellets may also be stored in underground tanks. Storage volumes range from hoppers adjacent to the boiler that are manually filled daily from sacks of pellets or logs, through to stores that hold many months fuel supply that automatically feed chips or pellets to the boiler plant. The storage must: -

• Be dry – both to ensure combustion efficiency and to prevent deterioration of fuel.
• Provide safe access to remove blockages, for cleaning, and maintenance.
• Maintain appropriate ventilation to prevent mould growth and remove dust from pellets
• Control the risk of fire and include appropriate fire detection(protected from dust and mechanical damage)

The reliability of appropriate fuel supply and storage is essential for long term operational success.

Bulk storage of wood chips must include mechanisms to ensure that voids (or 'bridging') do not form that would prevent the chips moving into the feed mechanism. 'Walking floors' can be used to shift wood chips across the store into the final feed auger.

The store must be oversized so that it is able to receive full delivery of fuel whilst it still has some remaining.

Information is available for general storage considerations, for pellet storage and wood chip storage including some general sketches of stores.

The fuel may be moved from the store into the boiler manually, by gravity, by suction or by mechanical augers (wood chips or pellets) or for larger wood chip boilers ram mechanisms. Historically the reliable feed of fuel to biomass systems has provided challenges due to poor design, equipment, installation fuel quality and maintenance leading to blockages and failures. It is important that the flow path is as smooth and straight as possible.

The quality of the fuel – particularly its moisture content and its regularity of size – together with the design of the feed device will determine how effectively and reliably fuel will flow.

It is essential to remove the products of combustion - NOx,SOx, CO2 and other gases as well as the very small particles known as fly ash.

It is important that the height and position of the outlet of the chimney minimises its effect on the building itself and its neighbours. It must satisfy relevant Building Regulations (the England/Wales Approved Document J applies specifically to those boilers under 50 kW, ) and the recommendations of the British Standards. These cover the sizing and allowable materials that will deliver an appropriate minimum performance. Masonry chimney products whilst usually offering long life and high resistance to risk of corrosion, tend to involve more installation work than metal liners and insulated metal chimneys that offer fast installation. However, metal chimneys are less resistant to damage by corrosion particularly if subjected to abuse or inadequate cleaning.

The requirements for a chimney are summarised as part of the HETAS Guide and the Biomass and Air Quality, Developers' Information Leaflet gives simple detailed information on the associated planning process.

Bottom ash removal may be manual on smaller units (normally for small stoker units) or through automated systems that pass the ash in to large, typically wheeled, storage bins. The wood ash that is removed from the boiler is frequently used as a horticultural fertiliser however it is important that an organised method of removal and disposal is planned. Fly ash may need removing from the flue gases by cyclone and/or filter removal and, due to its composition, it is not recommended as a fertiliser.

The testing methods for thermal insulation, system control and hydraulic integrity are similar to those of other boilers. Biomass boiler testing would also include :-

• Fuel feed mechanisms
• Ash removal systems
• Chimney and flue pipe integrity and performance
• Flue gas analysis to assess combustion efficiency

As with all systems biomass systems must be commissioned bay competent person and the building operator should be handed operation manuals, schematics and appropriate certificates as part of their training. Information on the required fuels, expected performance and service requirements must also be provided.

Books to buy

B&ES's 'Guide to Good Practice - Installation of Bisulphate' TR/38 – An excellent manual providing essential knowledge for designers and installers

CIBSE Knowledge Series 10 – 'Biomass Heating' –This provides a great introduction to the basics of Biomass Heating in a concise 23 page document.

'The Handbook of Biomass Combustion and Co-firing' by Sjaak Van Loo,Jaap Koppejan covers the technology comprehensively

BSRIA's Illustrated Guide to Renewable Technologies BG1/2008, including useful tables of fuel energy content and photos of fuel storage and feed facilities.

Freely downloadable

Excellent Guide - Biomass for London: Wood Fuel Guide

Comprehensive and well illustrated - Carbon Trust Biomass heating - An practical guide for potential users

Standards and Regulations

European Standard EN 14961: 2005: Solid biofuels, fuel specifications and classes

England and Wales - All the Approved Documents are available from the Planning Portal of particular relevance are Building Regulation AD Part J – Combustion Appliances and Fuel Storage Systems and Building Regulation AD Part L – Combustion Appliances and Fuel Storage Systems

For allowable appliances under the England and Wales Building Regulations see section 2 of  - Non-Domestic Building Services Compliance Guide 2010 Edition

Scotland - The Technical Guidance for the Scottish Building Regulations are available and of particular relevance are Section 3 - Environment (includes chimneys) and Section 6 (Energy)

Ireland - The Technical Booklets supporting the regs (make sure you download the relevant amendments booklets). Particularly relevant are Techical Booklet F2:2006 - Conservation of fuel and power in buildings other than dwellings and Technical Booklet L: 2006 - Combustion appliances and fuel storage systems

UK/Wales/Scotland Clean air act 1993

The Biomass and Air Quality Guidance for Local Authorities gives some excellent guidance that is useful to both developers and installers