The assessment of flue gas particulate abatement in wood burning boilers

Transcription

1 The assessment of flue gas particulate abatement in wood burning boilers Report for Forestry Commission Scotland Restricted Commercial ED56285 Issue Number 3 December 2010

2 The assessment of flue gas particulate abatement in wood burning boilers Customer: Forestry Commission Scotland Customer reference: Confidentiality, copyright & reproduction: Copyright AEA Technology plc This report is the Copyright of Forestry Commission Scotland and has been prepared by AEA Technology plc under contract to Forestry Commission Scotland. The contents of this report may not be reproduced in whole or in part, nor passed to any organisation or person without the specific prior written permission of Forestry Commission Scotland. AEA Technology plc accepts no liability whatsoever to any third party for any loss or damage arising from any interpretation or use of the information contained in this report, or reliance on any views expressed therein. Contact: AEA Glengarnock Technology Centre Caledonian Rd Lochshore Business Park Glengarnock Ayrshire KA14 3DD t: f: e: AEA is a business name of AEA Technology plc AEA is certificated to ISO9001 and ISO14001 Author: Dr Scott Hamilton, Stephen Fleming, Robert Stewart Approved By: Dr Scott Hamilton Date: Signed: AEA reference: ED Issue Number 3 This report was prepared for Forestry Commission Scotland under the Regional Biomass Advice Network (RBAN) project, in partnership with Scottish Government, Scottish Enterprise and Forest Research. RBAN is part-funded by the European Regional Development Fund.. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 ii

3 The assessment of flue gas particulate abatement in wood burning boilers Executive summary The project aims to investigate the technologies available and coming onto the market which abate particulate emissions from wood-burning boilers, whilst considering engineering, operational and cost issues. Phase 1 is intended to outline the available technologies to reduce particulate matter (PM) emissions to a level which will limit local air quality impacts, while considering issues such as cost and applicability to small biomass boiler installations ( kw output). This report is the first phase of the study and will be carried forward to phase 2 where measurement techniques will be discussed and therefore is not conclusive to the full study. Biomass boilers currently in use in Scotland are typically either log wood boilers, overfeed boilers running on pellets, underfeed boilers burning chip or pellets, or moving grate boilers. In many cases these boiler technologies have basic particle abatement fitted, most likely a cyclone or multicyclone system. Particle emission can also be avoided in the first instance to a large extent by the selection of the correct appliances that offer both complete high efficiency combustion across its design load and limits the number of stop start operations. Avoiding particle emissions through a combination of system design and basic abatement is probably the current norm in the Scottish market and there are still not many instances of small installations having secondary abatement fitted. Emissions from non-domestic small boilers (up to 2MW) in Scotland are largely regulated through the Clean Air Act which has provisions such as prohibiting emission of black smoke, or requiring chimney height authorisations from the Local Authority where the system burns more than 45.4kg/hr of fuel. The Act also makes provision for declaration of Smoke Control Areas which allows control of small boilers (including domestic boilers) through a type approval mechanism for appliances that can be used in such areas ( Exempt appliances). In addition, the Local Air Quality Management system requires Local Authorities to review and assess air quality within their boundaries following a prescribed timetable. The Authority has a duty to assess air pollution sources and check for potential exceedences of air quality objectives so biomass installations are typically considered during this process, mainly in terms of NO x or PM 10 emissions. Difficulties arise due to the Clean Air Act being outdated because it does not consider the pollutants that are currently of concern in the same way that the Local Air Quality Management (LAQM) regime does. For example it does not consider current background concentrations of pollution which can be important when specifying chimney heights or other abatement methods and, there are no controls on domestic appliances outside smoke control areas. Current abatement technologies can be categorised thus: Inertial- such as cyclones or multicyclones Filtration- such as bag or ceramic filters Electrostatic- such as electrostatic precipitators It is currently unusual for abatement other than inertial separators to be fitted to small boilers in Scotland though where the installations are proposed for areas of existing poor air quality, more advanced secondary measures may be required. Inertial separators do abate some of the particle load and relatively low overall emission rates can be achieved, though they are limited at removing the smallest size fractions (PM 10 and below). There are many small Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 ii

4 The assessment of flue gas particulate abatement in wood burning boilers boilers currently on the market that meet the Renewable Heat Incentive (RHI) consultation emission criteria 1 for PM without the need for bolt-on abatement. Other particle abatement techniques such as ceramic or fabric filters are typically more expensive to implement and each has specific engineering and operational issues that have to be overcome. These technologies offer very high levels of particle abatement (up to >99%) though are currently expensive and represent a high percentage of the overall capital cost of any biomass project. Typically these would only be implemented where a Local Authority has a specific concern about air quality impacts, perhaps relating to the existence of an Air Quality Management Area or specific sensitive human receptors. Electrostatic precipitators are not currently in use in Scotland for small boilers though similar operational and cost issues will probably arise should the technology be implemented in future. Information in the literature suggests that cyclones can have a collection efficiency <1% for particles smaller than about 2.5 µm and perhaps 70% for 10 µm particles. Higher abatement efficiencies are also reported for cyclones, but efficiencies rapidly diminish for particle sizes below about 5 µm and, at best, abatement efficiencies of 50% might be expected for.2.5 µm particles. Note that these figures indicate that cyclones are not effective abatement devices for PM 2.5 (i.e. particles smaller than nominally 2.5 µm). Generally fabric and ceramic filters have a very high efficiency across the range of particle sizes. However, initial efficiency will depend on the time taken to develop a layer of collected material. Research on 20 fabric filter materials indicated PM 2.5 and PM concentrations on the clean side of the filter less than 0.09 and 0.12 mg/m 3 respectively (the challenge concentration was about 18 g/m 3 ). An Electrostatic Precipitator (ESP) has an unusual efficiency curve with a slight drop in efficiency between about PM 0.5 and PM 1, however, collection efficiency for the coarse and finest particles is high. For a small biomass boiler, however, it should be noted that efficiency levels may be difficult to achieve. The efficiencies of the various abatement techniques are relatively well understood though the issue of cost is important. Cyclone abatement systems are the cheapest though the least efficient, with the more advanced filtration techniques being several times more expensive. In theory it is possible to remove virtually all of the particles from an exhaust stream though clearly the decision on which abatement technique to choose must be based on the benefits balanced against the cost of implementation. In general the relative costs of more basic abatement such as cyclones or multicyclones will be much lower although this is balanced against their reduced effectiveness. The issue of locational sensitivity to particle emissions may be the dominant factor in the decision to implement the more costly solutions. In carrying out Phase 1 of this work we have identified what we perceive as gaps in the knowledge base around biomass particulate emissions and their abatement. To summarise these are: Integration of equipment- there are sometimes operational issues with bolt-on solutions which can affect warranties and performance guarantees Capital and operational costs- there is little information in the public domain on cost, especially operational costs. The data supplied in this report is largely sourced from personal communications with suppliers. Boiler performance and operational costs- integrating emission abatement equipment is likely to require additional energy which does not factor into efficiency calculations for example under EN303 Part 5. Life of abatement equipment- this is reasonably well understood for large plant but not for small boilers making it difficult to fully understand life cycle costs for the systems. 1 The RHI specifies 30 g/gj for particulate matter and 150 g/gj for NOx (grammes pollutant per GigaJoule net heat input). Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 iii

5 The assessment of flue gas particulate abatement in wood burning boilers Effectiveness of abatement in service- there is little information on maintenance for abatement technologies on small boilers, ideally the operation should minimise user intervention. Residues- there is little information on ash quantities likely to be generated by small boilers in continuous operation. Applicability of emission test data- typically all particulate is assumed to be in the smaller size fractions and speciation is possible but complex, and there is no EN Standard for the testing. Quality of test data- different countries use different standards for PM measurement making assessment of data quality difficult. Abatement performance standards- there are no CEN or ISO standards for assessing performance of abatement equipment. However, CEN recently produced a Workshop Agreement which covers environmental technology verification and identifies relevant emission measurement standards. Combustion efficiency on modern boilers is relatively high and primary control measures (to avoid products of incomplete combustion) have limited scope to improve particulate emissions. Revisions to EN303 Part 5 should also remove lower efficiency appliances <500 kw output from the market. More sophisticated control systems offer some scope for improvements in combustion efficiency but they are more likely to provide more effective use of air supplies to reduce or manage air requirements better. Better control of air, particularly on larger appliances, could reduce fuel and ash entrainment issues, however, high combustion efficiency requires mixing of fuel and air. Condensing technology is being considered by manufacturers and may offer some co-benefit for particulate capture. Regulatory development is focussing research on small-scale abatement measures. Most effort appears to be add-on technologies to reduce particulate emissions from stoves and similar appliances. There are research and near market ESP devices which claim varying levels of PM emission reduction. These devices are aimed at appliances which are very much smaller than 50 kw as well as residential boilers and with intermittent use but it is likely that they will provide a driver for lowering the size range of application for ESP technology. One boiler manufacturer has recently introduced a ceramic filter product which can be applied to boilers >50 kw. This is based on existing technology but extends the traditional range of application to small scale biomass. Fabric filters are not commonly applied to small-scale biomass combustion at present in the UK but are commonly applied in similar sizes for local exhaust ventilation as well as part of Best Available Techniques (BAT) in larger combustion activities. A key issue for fabric filters is the potential damage (and loss) from hot particles, damage from high flue gas temperatures and blinding of filters due to condensing moisture at start-up or from intermittent operation. Gasification and pyrolysis offer the potential to convert solid fuels into a more uniform gaseous (or liquid) fuel. Many modern boilers aim to create conditions where volatile components are gasified and combusted in a secondary combustion zone. For example, the downdraught wood log boilers are commonly referred to as gasification boilers. However, full gasification modifies both volatile material and potentially char material to a fuel gas that can then be cleaned and burned in a boiler or more commonly an engine with potential for low or negligible PM emission. Small-scale gasification has a long history but current technology is largely based on technologies larger than 2 MW. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 iv

6 The assessment of flue gas particulate abatement in wood burning boilers Table of contents 1 Introduction Project aims Phase Report structure Combustion techniques Particulate emissions from biomass combustion Fuel choice and particulate emissions Emission limits applicable to biomass boilers <2 MW output in Scotland Local Air Quality Management Current particle abatement technologies and how they work Technologies Inertial separators Electrostatic Precipitators (ESP) Filtration techniques Current abatement technologies for small biomass plant Available boiler emissions data Current particle abatement useage patterns in Scotland Typical abatement performance Overview Grit arrestor Cyclones Electro-static precipitator Fabric and ceramic filters Indicative costs of abatement Market prices Gaps in the knowledge base Boilers and abatement Residues Emission data Abatement Performance Standards Emerging technologies Combustion and boiler efficiency improvement Abatement measures Advanced combustion technologies Abatement comparison matrix...34 Appendices Appendix 1 Bibliography Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 v

7 1 Introduction 1.1 Project aims Phase 1 The project aims to investigate the technologies available and coming onto the market which abate particulate emissions from wood-burning boilers. Phase 1 is intended to outline the available technologies to reduce particulate matter (PM) emissions to a level which will limit local air quality impacts, while considering issues such as cost and applicability to small biomass boiler installations ( kw output). 1.2 Report structure The report introduction summarises general information on combustion technologies and fuels which can influence the amount and characteristics of PM emitted. This information has been gathered primarily from web searches of current major manufacturers who have larger numbers of boilers operating within the UK Note that only automatic fired boilers whose primary role is to provide heat for heating or industrial process uses are considered; manually-fed boilers and appliances such as stoves and cookers which provide direct heating as well as hot water are not considered. Particulate emissions from manually-stoked appliances without mechanical air supply (forced or induced draught fans) can be higher due to comparatively low combustion efficiency and subsequent emissions of semi-volatile products of incomplete combustion which contribute to the particulate emission. An overview of emission regulation is also provided. Subsequent sections provide more detailed information on the abatement technologies, operating and cost factors. 1.3 Combustion techniques Log Wood Boilers These boilers are generally fairly small (<100 kw output); log storage and transport within the boiler becomes difficult for larger outputs. These are classified by the way the primary combustion air is introduced to the combustible material. Examples are updraught, over draught and side draught. Considerable developments have been made on the combustion control of downdraught designs which give superior performance for efficiency and emissions compared to the other designs. A typical modern downdraught log wood boiler is shown in Figure 1-1. A downdraught boiler typically has no emission abatement equipment but can achieve low emissions of particulate matter by good control of the combustion process. However, these boilers generally cannot modulate to different outputs as easily as other types and effectively operate at a fixed output, cycling on and off to respond to heat demand. When the boiler cycles off, emissions of smoke can develop from smouldering fuel in the combustion chamber. Correct sizing of the appliance is critical and many log boiler manufacturers recommend use of an accumulator tank to reduce cycling operation (in other countries use of an accumulator tank with wood log boilers is a mandatory requirement). Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 6

8 Figure 1-1 Downdraught log wood boiler Overfeed Boilers (OFB) These boilers cover a similar range as the log wood units but are more commonly up to 50 kw in size. They are designed to operate in a continuous manner but need a uniform fuel both in size and consistency and therefore are generally fired only on wood pellets. Some boilers have integral fuel hoppers that hold fuel for up to two days operation but these can be increased for larger sizes. As the name suggests the wood pellets are dropped on to a fixed grate (often into a burner cup or bowl) and primary air is blown through the grate and secondary air introduced into a mixing area ensuring good combustion. An example of an overfeed boiler is shown in Figure 1-2. Figure 1-2 Overfeed pellet boiler An OFB typically has no emission abatement equipment and achieves low emissions of particulate matter by good control of the combustion process. These boilers can modulate to Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 7

9 different outputs with an appropriate control system and tend to have only a small inventory of fuel in the combustion chamber. However, appropriate sizing of the appliance is important to achieve optimum efficiency and avoid cycling. Some larger output appliances can incorporate cyclones for particulate abatement, but the use of cyclones is unlikely at smaller boiler outputs Underfeed boilers (UFB) Underfeed boilers are the most common design for small commercial and industrial heating applications. They have a considerable size range depending on the manufacturers and make up the majority of boilers currently operating within Scotland and typically range from 50 to 3000 kw. UFBs can fire either wood chip or wood pellets and in both cases work in the same principle of feeding the wood by means of an Auger below and up onto the grate. This has the advantage over the log wood boiler of varying the amount of fuel feed, and through Programmable Logic Controller (PLC) controls, the correct amount of air is supplied which gives better combustion control and reduced particulate emissions. An example of an UFB is shown in Figure 1-3. Figure 1-3 Underfeed boiler The smallest UFB do not have abatement and achieve low emissions of particulate matter by good control of the combustion process. Depending on the individual boiler design, size and fuel there can be potential for entrainment of fuel and ash material in the flue gases. Larger output appliances often incorporate a cyclone for particulate abatement; the largest appliances can be fitted with a multicyclone. These boilers can modulate to different outputs with an appropriate control system. However, appropriate sizing of the appliance is important to achieve optimum efficiency and avoid cycling Moving Grate Boilers (MGB) Moving grate boilers are generally used for larger commercial or industrial heating applications ranging from 500 to kw and come in a number of different moving grate designs. The grates can generally be distinguished by the way that they move, such as inclined, step or travelling grate. In all these designs the fuel is fed to the front section of the grate and the primary air is then mixed with the fuel. As the fuel is burned the grate moves the partially burned material along by the use of hydraulic rams or belts until combustion is complete at the end of the grate. The main ash and deposits fall off the end and are then collected automatically. An example of a MGB is shown in Figure 1-4. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 8

10 Figure 1-4 Moving grate boiler MGB achieve low emissions of particulate matter by good control of the combustion process. However, the size of the combustion chamber and volume of air required mean that fuel and ash can be entrained in the combustion air and flue gases. Appliances often incorporate a cyclone for particulate abatement and often appliances can be fitted with multicyclone systems. These boilers can also modulate to different outputs with an appropriate control system. Appropriate sizing of the appliance is important to achieve optimum efficiency and avoid cycling. 1.4 Particulate emissions from biomass combustion The reduction of particulates from biomass combustion systems can be best achieved by avoiding the production of the particulates in the first case, often referred to as primary abatement. These measures are often incorporated in the boiler design but also include correct boiler sizing, appropriate fuel selection, fuel quality and other combustion control techniques. Secondary measures are abatement technologies put in place to remove a proportion of the remaining emissions and are installed after the combustion process. The emissions from any combustion process originate from either complete or incomplete combustion, and this has an impact on the amount and type of particulates produced. During a combustion process the particulates are made up from either larger coarse fly ashes which are produced from carry over of ash and fuel particles from the combustion bed, or fine particulates which are sometimes referred to as aerosols. The fine particulates are caused either by the incomplete reaction of the combustible components of the fuel (products of incomplete combustion) or from inorganic salts released during the combustion process. In the case where complete combustion is achieved the production of particulates may to some degree be limited by optimal design of the boiler combustion zones that ensures that they do not leave the boiler but are collected internally within the ash collection system. Where incomplete combustion occurs this is generally caused by a number of factors which should be considered: Combustion temperature too low usually as a consequence of wet fuel; this can also be during start up or where the boiler is in slumber or cycling between on/off modes of operation. Poor mixing of air and fuel giving fuel rich combustion zones. Short residence time that does not allow the fuel to burn out correctly; this can be caused by over firing the boiler or inadequate combustion chamber volume. Inadequate air supply; this can be due to fan-sizing problems or poor boiler house ventilation. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 9

11 The production of primary particulate emissions from biomass boilers can be substantially reduced where careful consideration has been given to ensure that the system as a whole is appropriately designed for the intended fuel, combustion equipment, and combustion controls. 1.5 Fuel choice and particulate emissions CEN/TC 335 is a technical committee currently developing a draft European standard to categorise and define all types of solid bio fuels, including wood chips and wood pellets. The standard will provide essential information of the fuel such as source, size, moisture content and chemical composition. The amount and type of emissions from biomass equipment is highly dependent on the fuel selection and the type of combustion and boiler appliance chosen to convert the energy. Biomass materials come in a very wide variety of compositions and moisture content each of which are suitable for different technologies and therefore give different particulate emission performances. This causes major problems when trying to assess emission levels for a single biomass source when so many combustion methods and boiler designs are available to combust the material. The two main issues to consider in fuel selection are: Material source- most biomass materials currently used are sourced from virgin wood supplies and therefore the fuel is not altered or added to in such a way as to affect the emission performance. Provided the source of the fuel can be identified then certain levels of performance can be expected. When fuels of this type are considered it is important to recognise that fuels with the highest bark content will generally produce the highest percentage of ash within the materials. This is likely to be carried over as fly ash and collected in the abatement equipment. If pelleted fuel is being used then the source of the material is also important, as for example the ash content of the material used to produce the pellet will effect the combustion and particulate emissions. Care should be taken to ensure that pellets comply with current standards which require labelling to advise source material, ash content, moisture, use of additives. Moisture content- to ensure the correct combustion environment is achieved it is important that a high enough flame temperature is achieved; normally >850 o C. This can be difficult to achieve with fuel that has moisture content above 50% and normally require specialist burners and boilers such as fluidised bed. Wetter fuel also cause poorer emission performance during start up or cycling operations when correct temperatures are not achieved. 1.6 Emission limits applicable to biomass boilers <2 MW output in Scotland Clean Air Act 1993 The Clean Air Act 1993 includes several provisions relevant to biomass boilers 2. Emissions of black smoke are prohibited (with some derogations in separate regulations for start-up periods). 2 The Act can be accessed here Note that there are several additional regulations arising from the Act. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 10

12 There is a requirement for new non-domestic furnaces to be authorised by the Local Authority and to operate as far as practicable without emitting smoke when burning the design fuel. There are grit and dust emission limits for furnaces >240 kw output 3. If a furnace burns more than 45.4 kg/hr of solid fuel (more than about 160 kw output for a fuel calorific value of 15 MJ/kg) then the chimney height needs approval from the Local Authority. In addition, the system needs grit and dust abatement equipment (or an exemption from the Local Authority or Secretary of State). Within a designated Smoke Control Area, no smoke can be emitted from a boiler chimney unless due to use of an Authorised fuel or if the boiler is an appliance exempted by the Secretary of State. Wood is not an Authorised fuel 4 so wood burning appliances need to be Exempted 5 for use in designated Smoke Control Areas. The current criteria applied to boilers are provided in Table 1-1 and are based on testing requirements for smoke reducing residential appliances 6 published in Table 1-1 Summary of Clean Air Act exemption particulate emission limits Parameter Limit Comment Particulate emission rate 5+ ((Output, kw) 3) g/hour Extrapolation of BS PD 6434:1969 Particulate concentration <150 mg/m 3 dry gas, stack O 2 and STP (0 C, kpa) Concentration value used as indication of smokeless operation Pollution Prevention and Control (Scotland) Regulations 2000 These regulations are not applicable to biomass boilers <2 MW output unless: The boilers are burning a waste or waste-derived fuel They are directly associated with a process regulated under the regulations They are part of a larger combustion installation (>50 MW thermal input) In many instances, the air quality impact of emissions arising from a PPC process would be assessed as part of the permit application process. For the purposes of this study, only biomass boilers which fall outside of the PPC regulations are considered Product Standard The European Standard EN303 Part 5 covers solid fuel central heating hot water boilers up to 300 kw output 7 and has classes for efficiency and particulate emission (at rated output). The Standard is not a harmonised Standard but is commonly applied for performance testing of biomass hot water boilers. However, achievement of the emission criteria is not a legal requirement. The Standard is under review which includes a proposal to extend the output limit to 500 kw and revise PM emission classes. The current and proposed total particulate emission classes for biogenic fuels are provided in Table The Clean Air Act (emission of grit and dust from furnaces) regulations An Authorised fuel is one which has been tested and meets the criteria for smokeless operation an emission rate of <5 grammes/hour when burned on a standard open fire to BS3841- Part 1:1994. Wood fuels tend to emit too much smoke to be authorised. 5 An Exempted appliance which has been tested and demonstrated to be capable of operating without producing any smoke or a substantial quantity of smoke. The emission requirements for appliances <45 kw output are laid down in BS PD 6434:1969 and BS 3841 Part 2 : Requirements for non-residential appliances are based on limits extrapolated from BS PD 6434 and other criteria. 6 BS PD (published document) 6434:1969 Recommendations for the design and testing of smoke reducing solid fuel burning domestic appliances 7 BS EN 303-5:1999 Central heating boilers. Heating boilers for solid fuels, hand and automatically fired, nominal heat output of up to 300 kw. Terminology, requirements, testing and marking. Note this Standard is under revision. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 11

13 Table 1-2 : Summary of EN303-5 Emission classes Stoking EN 303 Class Current Proposed Concentrations, mg/m 3 at 10% O 2, dry and STP (0 C, kpa) Manual Automatic Derived emission factors, g/gj (net thermal input) Manual Automatic Note: Conversions assume wood combustion and are based on US Environmental Protection Agency stoichiometric flue gas volumes and Digest of UK Energy Statistics calorific values. Proposed classes taken from August 2010 pren Local Air Quality Management In addition to their responsibilities under the Clean Air Act, Local Authorities have a duty to review and assess air quality within their boundaries following a 3-year cycle mandated by the Environment Act Biomass sources are considered during this process, both as single installations and as cumulative sources and are initially screened during Updating and Screening Assessments. These are carried out in the first year of the three year review and assessment cycle and where potential issues are noted, more detailed dispersion modelling assessments are normally carried out. As such, screening tools have been developed as part of the most recent technical guidance 8. These take the form of nomographs which allow the user to assess existing installations to check for potential exceedences of the PM 10 annual and daily objectives. It is however difficult to apply these tools to prospective biomass installations as they are not strictly intended to support decision making for planning purposes. Indeed many Local Authorities are interested in protecting existing good air quality and are uncomfortable with the idea that the air quality objectives represent a form of headroom that can be used as a sink for particle emissions. To aid decision making a planning tool 9 has been developed by Local Government Regulation (formerly LACORS) and Environmental Protection UK which does allow prospective installations to be assessed. This allows the user to enter simple parameters such as emission rate, planned stack height and width, and relevant building dimensions. The tool then predicts the maximum impact expected from the installation and can therefore be used to feed into decisions on abatement requirements such as stack height or the need for secondary measures. This in turn allows the locational sensitivity (for example high background concentrations of particles) of the development to also feed into decision making. The tool does not, however, provide the location of the maximum concentration, so it is not possible to use it for estimating the impact at specific receptor locations. In practice it is likely that well designed and operated plant with the appropriate stack height and cyclone or multicyclone abatement are unlikely to cause exceedences of particle objectives provided the background is not already high. That said, where an authority is seeking additional protection of air quality, and will for example only accept a local contribution from an installation to the PM 10 annual mean of say 1µg/m 3 then additional evidence may be required from the developer to support the decision not to include secondary abatement. In theory it would be reasonably straightforward to use the EPUK tool 8 LAQM.TG(09), Defra and the Devolved Administrations 9 Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 12

14 for this purpose as the sensitivity of ambient concentrations to changes in boiler, stack height, emission rates and other parameters can be easily assessed. Some authorities have published supplementary planning guidance for air quality which outlines what evidence is required of developers and what assessment methodologies are acceptable in such cases 10, Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 13

15 2 Current particle abatement technologies and how they work 2.1 Technologies Current available abatement technologies come in a number of different designs but primarily serve the purpose of separating solid particles from the products of combustion or flue gases. Each technology uses different methods of separation and collection and they have a wide ranging level of effectiveness depending on operational constraints, particulate size and velocities. The following section outlines the key design, operation and effectiveness of each technology. Technologies are classified according their separation techniques: Inertial essentially relies on the inability of denser, heavier particles to follow a change in the direction of gas flow. Particle separation effectiveness increases as the change in direction increases. Electrostatic charging particles and then attracting and collecting them on suitable charged collection areas. Filtration passing flue gases through a filter or other collection media. The applied techniques for particulate abatement are classified into these categories and described below. Note that while most technologies are available from third party vendors, currently, it is very unusual for such equipment to be fitted to boilers. Boiler manufacturers generally do not offer additional abatement equipment beyond the cyclone or multicyclone which may be an option to the specification. Developers wishing to install biomass boilers in Air Quality Management Areas (AQMAs) or locations where there is little headroom for increased levels of particulates in the ambient air are considering additional measures to achieve lower emissions. 2.2 Inertial separators Integral boiler separators (grit arrestor) Inertial separators are generally incorporated within the design of the vast array of packaged biomass boilers and are the simplest form of separation which can act as the only form of separation on smaller boilers or as a form of pre-separation on larger boiler plant. They have no moving parts and use both inertia and gravity to separate dust particles by slowing down or redirecting the air flow which causes the heavier particles to be separated by gravity and collected within the boiler or transferred to internal hoppers for disposal. These types of separation are the least effective but will collect larger particles / sparks before being carried over to the main abatement equipment. The advantages of grit arrestors are provided in Table 2-1. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 14

16 Table 2-1 Advantages and disadvantages of grit arrestors Advantages Low gas side pressure drop important for keeping main fan motor size low Simple no moving part design Low cost; usually built into package boiler Handles high capacity particulate burden Can act as flame extinguisher Disadvantages Needs large amount of space to achieve gas velocity drop Low collection efficiency for fine particulates Integral and external Single Cell Cyclones All cyclones (see Figure 2-1) create a cyclonic or centrifugal force, which separates the particulates from the flue gases. The centrifugal force is created when the gas enters the top of the cylindrical collector at an angle and is spun rapidly downward in a vortex. As the air flow moves in a circular fashion downward, heavier particles can t stay in the gas flow and are thrown against the walls of the collector and slide down into a hopper. Efficiencies depend on: Particle size (particles with larger mass being subjected to greater force) Force exerted on the dust particles Time that the force is exerted on the particles Figure 2-1 Cyclone separator A number of appliances incorporate cyclones (some as an option) whether as an integral part of the boiler or as a bolt-on control device. The advantages and disadvantages of cyclone separators are show in Table 2-2. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 15

17 Table 2-2 Advantages and disadvantages of cyclone separators Advantages Low to moderate gas side pressure drop Simple, robust well-proven design Handles high capacity particulate burden Can act as flame extinguisher Small space demand/footprint Low cost Works at wide range of temperatures Disadvantages Can need amount of head room Efficiency drops at part-load conditions Low collection efficiency for smaller particles Sticky deposits or tars may condense within the unit Integral and external Multi cell Cyclone Separators (Multicyclones) Multi cell cyclone separators operate in the same way as single cell and consist of a number of small diameter cyclones normally mm diameter placed parallel to one another with vane spinners. They have a common inlet and outlet for air. The smaller diameter of the barrels and longer length makes them more efficient than regular cyclones. Particulates are retained inside for a greater amount of time and the smaller diameter of the barrel provides more efficient separation of particles. As with single cell devices, a multicyclone can be incorporated in appliance design as an integral part of the boiler design or as a bolt-on external control device. The advantages and disadvantages of multicyclones are shown in Table 2-3. Table 2-3 Advantages and disadvantages of multicyclones Advantages Low to moderate gas side pressure drop (a multicyclone has a slightly higher pressure drop than cyclones) Simple robust well-proven design Handles high capacity particulate burden Can act as flame extinguisher Little floor space required Low cost Works at wide range of temperatures Disadvantages Needs large amount of head room as fan normally mounted above unit Efficiency drops at part-load conditions Low collection efficiency for smaller particles Sticky deposits or tars may condense within the unit Other inertial devices In addition to dry systems there are a range of wet abatement technologies which attempt to incorporate particulates into a liquid phase and usually incorporate cyclones to separate the particulate material. The more effective techniques require a high degree of energy input to assure turbulent mixing of the liquid (usually recirculated water) and the particulate-laden gas stream. A common technology is the venturi scrubber but other similar technologies are also employed. In a venturi scrubber, water is sprayed into the gas stream upstream of a constriction (venturi) and the resulting aerosol is then separated in a cyclone. However, such systems are rarely applied to combustion plant for particulate control and are no longer commonly applied to particulate control on other industrial plant (except where there are cobenefits from removal of soluble gas components). Whilst they offer more effective Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 16

18 abatement for smaller sizes of particulates than a dry cyclone, disadvantages include a prominent water vapour plume, need for a water supply and recirculation/settling equipment, disposal of the sludge waste and a higher energy consumption than cyclones (particularly for venturi systems). Consequently wet systems are not considered a practical technology for biomass boilers <2 MW. 2.3 Electrostatic Precipitators (ESP) These use electrostatic charges to separate particulates from the flue gas. They incorporate a number of high voltage direct current electrodes (carrying negative charge) which are placed between grounded electrodes (carrying positive charge). The dust borne air stream is passed through the passage between the discharging (negative) electrodes and collecting (positive) electrodes. Dust particles receive a negative charge from the discharging electrodes (ionizing section) and are attracted to the positively charged grounded electrode (collection plates) and attach to it. Cleaning is done by rapping or vibrating the collecting electrode wherein dust particles fall away. Cleaning can be done without interrupting the flow. A schematic showing the principles of operation is shown in Figure 2-2. Figure 2-2 Principle of operation of ESP In larger applications, an ESP comprises several zones which allows application of different electric fields and cleaning frequencies to the initial (dirty gas), intermediate and final (clean gas) zones. The use of multiple zones also mitigates re-entrainment of particulate into the gas flow during cleaning. Most ESP s are used dry but there are wet ESP (WESP) which can be applied where flue gases have a high moisture content (or are saturated). They are also used where sticky particles are encountered. In a WESP cleaning is generally by in-situ washing of the plates. For more thorough cleaning, the collection cell can be removed and washed by hand or in a parts washer with an aluminium safe detergent. Some ESP air cleaners have automatic self washing mechanisms. The efficiency of electrostatic precipitators can be increased by incorporating larger collection surface areas and by lowering air flow rates to give more time and area for dust particles to collect or by forcing particulates towards the collection electrodes The basic components of an electrostatic precipitator are (i) a power supply unit (to impart high voltage, uni-directional current), (ii) an ionizing section where charge is imparted to the dust filled air stream (see Figure 2-3), (iii) a cleaning system to remove dust particles and (iv) a housing for the precipitator. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 17

19 Figure 2-3 ESP Collection Plates The advantages and disadvantages of ESP are shown in Table 2.4. Table 2-4 Advantages and disadvantages of ESP Advantages Very high efficiency (>99 % in larger applications) Collects the very small particles Will collect both wet and dry particles Can be used in high gas flows and temperatures up to 480 C Low gas side pressure drop. Maintenance is low unless corrosive materials are being collected Disadvantages High capital cost Sensitive to varying flow rates and particle burdens High voltage used and precautions required Collection efficiencies fall during use 2.4 Filtration techniques Fabric filters Bag filters generally consist of a large woven specialist fabric bag which is suspended within a solid framework. The particle laden flue gas passes directly through the woven fabric which collects the particles. As the particles build up on the fabric they can improve the overall efficiency as this causes an initial barrier layer, but with time as the layers are built up the resistance increases dramatically and the filter needs to be cleaned. Cleaning is normally carried out using compressed air or vibration. Bag filters (see Figure 2-4) generally have an upper temperature limit of 250 o C which is adequate for heating applications but specialist material may be required for industrial high pressure steam systems that operate at a much higher temperature. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 18

20 Figure 2-4 Fabric filter The advantages and disadvantages of fabric filters are shown in Table 2-5. Table 2-5 Advantages and disadvantages of fabric filters Advantages Disadvantages Very high efficiency (>99 % in larger Sensitive to velocity through the filter applications) Temperature limited to 250 C unless Collects the very small particles special materials are used Small size and can be floor mounted Replacement filters required after 3-5 Monitors resistance and automatically years operation cleans when required Can be damaged by embers or hot Applied across a wide range of particles industries and processes Needs additional fan to cope with pressure loss across filter Additional maintenance cost Requires compressed air supply Ceramic Filters Ceramic filters are used to filter out particles from the flue gas by means of introducing a filtration barrier between the upstream and downstream of the gas flow. For a biomass system these filters normally consist of a number of vertically mounted specially coated porous ceramic tubes. The dust particles stopped in the filter form a dust cake on the incoming surface and as more dust particles collect on the filter media the thickness of the dust cake increases. Manufacturers claim the technology can remove up to 96% of particulates from the gas stream. However, the efficiency of ceramic filters in other uses (such as incinerator filtration and synthetic fuel gas cleaning) is generally claimed to be similar to that of fabric filters (>99%). The filtration units are generally housed above the collection tray and cleaned automatically at periods normally determined by the increased pressure drop over the units. The cleaning can be by a burst of clean air in the reverse direction to the air stream with a velocity greater than the air stream. The greater force of the cleaning air burst dislodges the dust cake from the filter media and drops it into the hopper Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 19

21 area. For greater efficiency, a fan is used to push or pull air through the filter media openings. During start up the ceramic elements are subjected to high levels of moisture. This has no detrimental effect on the ceramic media due to the component material s ability to withstand both temperature and moisture which is caused during boiler warm up periods. The main advantages and disadvantages of ceramic filters are provided in Table 2-6. The two main functions of ceramic filters are: to stop dust particles on its surface while allowing flue gas molecules to pass through and to provide easy release of the dust cake during cleaning. Table 2-6 Advantages and disadvantages of ceramic filters Advantages Very high efficiency up to 99 % Collects very fine particles Resistance to moisture Small size and can be floor mounted Monitors resistance and automatically cleans when required Can withstand fires Disadvantages Sensitive to velocity through the filter Filter cartridges can be subject to mechanical damage and chemical attack Needs additional fan to overcome back pressure. Replacement filters required after 3-5 years operation Requires compressed air supply Additional maintenance cost. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 20

22 3 Current abatement technologies for small biomass plant 3.1 Available boiler emissions data Most boilers that are currently installed use primary measures but a number incorporate traditional cyclone abatement. The following table shows the results of various tests carried out on the boilers to measure performance in terms of PM 10 and NO x emissions. The test results provided in Table 3-1 are from test programmes on boilers between kW. Such tests are typically required to assess against various regional performance criteria. Table 3-1 Test data 12 for boilers between 50 and 200 kw Boiler make/type Year of test Boiler size kw Fuel type PM emissions g/gj NOx emissions g/gj Set 1 (boilers meeting RHI consultation emission criteria) 13 30g/GJ PM and 150g/GJ for NOx Buderus Logano SH Pellet BIOTECH HZ Chip Ecotherm HS Chip HMS HP Pellet HEIZOMAT RHK AK Chip 8 88 Rennergy HSV Chip Pyrogrande PMT Pellet Classic 60 Lambda Pellet Turbomatic Chip Turbomatic Pellet UTSS Chip Pelletstar biocontrol Pellet SOLARFOCUS therminator 60 kw Pellet PELLEMATIC PE Pellet 9 97 Type PV Pellet SL 80T Chip RennergyHSV 80S Chip Information from selected test reports published by manufacturer or Austrian and Danish biomass boiler approval authorities based on EN303-5 tests and available here and Note that EN303-5 does not specify NOx measurement method and PM methods are national methods which may differ. 13 Renewable Heat Incentive consultation available here : Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 21

23 Boiler make/type Year of test Boiler size kw Fuel type PM emissions g/gj NOx emissions g/gj Set 1 (boilers meeting RHI consultation emission criteria) 13 30g/GJ PM and 150g/GJ for NOx KWB Multifire Chip KWB Multifire Pellet HSV80S WTH Pellet Buderus Logano SH Pellet Buderus Logano SH Chip Buderus Logano SP Pellet ETA PE K Pellet Kapelbi PE K Pellet SL 99 T Chip UTSS Chip Rennergy HSV 100S Chip KWB Multifire Pellet KWB Multifire Chip Ecotherm HS 100 ECO Chip EVOTHERM P 100 ECO Pellet 7 71 HSV100S WTH Pellet Pyrogrande PMT Pellet 3 46 Turbomatic Chip SL 110T Chip ETA HACK Pellet SL150T Chip The test results provided in Table 3-2 are from test programmes on boilers between kW. Table 3-2 Test data 14 for boilers between 200 and 1000 kw Boiler make/type Year of test Boiler size kw Fuel type PM emissions g/gj NOx emissions g/gj Parameter Set 1 30g/GJ PM and 150g/GJ for NOx Ecotherm HS 200 ECO Chip Binder RRK Mixed From published test reports (See Table 3-1) or reports provided by manufacturer. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 22

24 Boiler make/type Year of test Boiler size kw Fuel type PM emissions g/gj NOx emissions g/gj Parameter Set 1 30g/GJ PM and 150g/GJ for NOx Kolhbach Metnitz Chip Hoval STU 800 Wood pellet boiler Pellet Binder RRK Chip Gilles HPK UTSK Chip Gilles HPK UTSK Pellet The test results provided in Table 3-3 are from test programmes on boilers between 1,000-20,000kW. Table 3-3 Test data 15 for boilers between 1000 and 20,000 kw Boiler make/type Year of test Boiler size kw Fuel type Parameter Set 1 30g/GJ PM and 150g/GJ for NOx PM emissions g/gj NOx emissions g/gj Kohlbach K Chip Gilles HPK UTSK Chip Binder RRK Chip Kohlbach K Chip Current particle abatement usage patterns in Scotland Most small (<2 MW output) biomass boilers installed in Scotland either adopt primary design measures only or primary design measures and inertial separators typically a cyclone. In common with the rest of the UK, additional abatement measures in Scotland have to date been very rare. In one case a small biomass boiler in Angus has been fitted with a ceramic filter by the fuel supplier primarily to investigate the practicality of such equipment. The Scottish Biomass Support Scheme and the current Scottish Biomass Heat Scheme have supported a number of installations on which ceramic filters will be fitted. Following this, Angus council has undertaken to procure and install flue gas particulate filters retrospectively to four of the council s biomass boiler systems 16. Where additional abatement has been provided for small biomass installations it has tended to be to satisfy quite localised air quality issues but also for research purposes. 15 From published test reports (See Table 3-1) or reports provided by manufacturer 16 Angus Council Corporate Services Committee Report No 226/09 of March 2009 on flue gas particulate filtering for biomass boiler systems. Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 23

25 4 Typical abatement performance 4.1 Overview The following information is drawn largely from engineering and other texts and applies to a wide range of equipment sizes. Whilst much of the information is for larger applications, the abatement performance of plant for specific particle size ranges should be similar. Differences arise in the practical applications in smaller facilities. In particular, larger facilities often incorporate redundancy to assure continued performance during maintenance or failure. The management of collected material will also differ as the quantity of particulate material collected at small biomass boilers is comparatively small on the smallest boilers (50 kw) removal of abated material may be achieved by regular manual cleaning by vacuum cleaning of collected material. However, on the largest (2 MW) appliances automatic transfer and storage of collected material will be required. Figure 4-1, Table 4-1 and Table 4-2 summarise abatement performance of various particulate abatement technologies (see Section 2). The figure includes wet scrubber systems but note that these are not generally appropriate for small biomass boiler particulate abatement as they add additional complexity, require water and produce a sludge for disposal and treatment. Figure 4-1 : Summary of abatement technology collection efficiencies From Nussbaumer 2010, report for Swiss Federal Office for the Environment Restricted - Commercial Ref: AEA/ED56285/Issue Number 1 24