Seeing the wood for the trees

As biomass becomes a key element in many sustainability strategies it’s important to consider the practicalities of burners as well as the potential impact of new air quality legislation. Ian Dagley explains

It is very clear that the use of renewable biomass fuels is to play a vital in the government’s strategy to meet its emissions targets. Consequently we are already seeing a major uptake in biomass across a wide range of both large and small projects in both the private and public sectors. In ensuring these projects are successful and address the requirements of all stakeholders, it’s important to plan such projects in detail, taking account of all factors.

Firstly, air quality must be considered. EC Directive 2008/50/EC, Ambient Air Quality and Cleaner Air for Europe, creates a requirement for tighter control of the smaller particles that are not filtered by traditional mechanisms such as multi-cyclones. Consequently, we will soon be faced with a need for more efficient, retrofittable filtration of particulate emissions from biomass boilers.

It’s worth reviewing the background to this situation. Directive 2008/50/EC came into force on 11 June 2008 and must be transposed into national legislation no later than June 2010. Of particular significance in these regulations is the size of particles that will be controlled. In the past, emphasis has been on particles with a diameter of 10 microns or above (PM10). However, the new directive will seek to introduce a new control framework for particles down to 2.5 microns in diameter (PM2.5).

Size matters
As noted above, the majority of these particles in the UK come from vehicle emissions but all biomass boilers also make a small contribution to particulate emissions. The level of emissions will vary with the quality of the fuel and combustion efficiency of the boiler, so using boilers that comply with EN 303-5 Class 3, in conjunction with high quality fuel, is the first step in minimising particulate emissions.

Of course, many biomass installations already use a cyclone or multi-cyclone to remove particles from flue gases. However, cyclones are totally dependent on the mass of the particles for removal, so while they will remove around 50 percent of the coarser particles they do not remove particles below PM10. This is why the new Directive and its emphasis on PM2.5 has such significance for biomass installations.

One alternative to cyclones and multi-cyclones is electrostatic precipitation, whereby particles are charged and removed from the flue gases in an electrostatic field to a collector. This is very effective for smaller particles but electrostatic filters tend to be both very expensive and very large – often too large for typical UK plant rooms.

More recently, a ceramic filter originally developed for industrial applications has been optimised for biomass use without making the overall cost of a biomass installation prohibitive. Capable of removing up to 96 percent of PM2.5 and PM10 particles, Hoval CF ceramic filters can be used with any type of biomass boiler and can be retrofitted to existing installations, so they have the potential to address many of concerns (real or perceived) about particulate emissions from biomass.

Ceramic filters are connected to the back of the boiler, in the same way as a cyclone. Each unit contains a matrix of porous ceramic tubes which are closed at the lower end. The number of tubes in each matrix is aligned with flue gas volumes for each boiler.

As flue gases are drawn through the filter by an inline fan, the gases are able to pass through the walls of the ceramic tubes, while particles are trapped. At regular intervals (timed and/or in response to a pressure drop across the filter) a pulse of air is used to dislodge the particles, which fall into a collection bin.

Some local authorities have already expressed concern about biomass emissions, so there is a chance that not addressing the emissions issue could curtail the use of biomass. Hoval’s ceramic filters have the potential to meet the challenge of particulate emissions and ensure that biomass remains as a viable element in the UK’s renewable heating strategy without compromising on air quality.

Living with biomass
As mentioned at the beginning of this article there are also practical considerations that will impinge on the everyday running of a biomass installation. These range from the most appropriate choice of biomass fuel to the design of the fuel storage, access for delivery vehicles and the duties of maintenance staff. Hoval’s extensive experience in this field shows that addressing all of these issues at the beginning of the project is key to ensuring success.

In this respect, biomass differs from most other building services projects because the specifier or installer needs to look beyond the basic engineering design and consider day-to-day practicalities. Nevertheless, the engineering is still important and the first stage is to understand the range of heat loads through the year and to design as much as possible for continual load with minimum cycling.
In terms of fuel, the choice in the UK usually comes down to wood chips or wood pellets, both of which have their place but offer different characteristics. Wood chips, for instance, undergo less processing than pellets and tend to show greater variation in combustion efficiency and physical size. Crucially, the calorific value and the bulk density of chips is lower than pellets so will require higher volumes of storage and/or more frequent deliveries than pellets to deliver the same heating capacity. In addition, the variable moisture content and size of chips may necessitate more complex, and therefore more expensive, controls to maintain even combustion.

The irregular shape and size of chips means they need more robust handling equipment with larger augers and motors, compared to pellets. In addition, the distance from storage to boiler should be as short as possible to minimise the number of augers required, the chances of fuel blockages and the cost.

As the choice of fuel influences storage volumes and frequency of deliveries, it also influences the type of delivery vehicle – the size of which will dictate the required dimensions of access routes and turning circles.

Generally, chips would be delivered by a tipper lorry, necessitating below-ground storage and a mechanism (e.g. push/pull floor) for moving the chips from the store to the fuel transport system. A ceiling mounted fuel dispenser can be used to spread the chips throughout the store. If a fuel dispenser is not used multiple fuel delivery hatches will be required to enable the full capacity of the store to be useable.

Pellets are usually delivered by a smaller ‘blower’ vehicle with less demanding access requirements and the best results are achieved by using a vertical storage silo as this maximises fuel recovery. The silo may be manufactured from GRP (glass reinforced polyester) for outdoor use, or a fabric silo can be assembled in-situ indoors. Because of their height, outdoor silos may require planning permission.

All of these factors need to be balanced against the fact that chips cost less than pellets so life cycle costing will also play a role in deciding on the most appropriate choice.

Another contributor to life cycle costs is maintenance – for example, cheaper wood chips tend to produce more ash than pellets or higher quality chips, so this will increase maintenance requirements.

In addition, the people carrying out the maintenance should be made aware of how their role differs compared to working with other boilers. Once the installation is up and running the acceptance of the people who are dealing with the boilers will be critical to the end user’s perception of the system.

For all of these reasons there is no such thing as a standard biomass project and each needs to be carefully evaluated in the light of the factors that have been discussed here. Equally, for those who are less experienced in biomass installations, it makes sense to call on companies with specialist expertise that are able to offer a turnkey biomass service, from design through to ongoing maintenance.

To find out more about biomass visit: hoval.co.uk

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17/02/2009

Seeing the wood for the trees

Biomass is a very popular renewable energy source that can reduce running costs as well as lowering carbon footprint – but matching the biomass fuel and equipment to the needs of the project is vital, Ian Dagley of Hoval explains

Of the various renewable and low carbon technologies that are in use for heating and hot water, biomass boilers are proving to be among the most popular; either used as the sole heat source or in conjunction with other renewables and conventional boilers.

One reason for this is that boilers are a familiar technology, albeit with different combustion characteristics compared to gas or oil. In the case of biomass boilers, these are technically similar to solid fuel boilers such as coal so Hoval, with over 100 years experience in this field, is able to combine its expertise with the latest combustion technologies to deliver high performance biomass solutions.

In addition, biomass provides a carbon-neutral heat source that is reliable and consistent – another important factor when providing vital services to a building.

However, when choosing biomass it’s important to select the most appropriate fuel for the project. To that end, there are two main types of biomass fuel in wide use for larger projects – wood pellets and wood chips. Not only do these have different combustion characteristics, they are also physically dissimilar, which impacts on other aspects of the design of the project, so it’s important to understand these differences.

For example, in a project where there are large forestry operations nearby, or farms growing sustainable short rotation coppice plants such as poplar or willow, wood chips may present the most cost-effective option.

Wood chips are produced by shredding the harvested trees into small pieces, which are then fed to the boiler after varying amounts of drying. This means there is relatively little processing so wood chips tend to cost less. However, they also tend to give out less heat than pellets, so this has implications for storage space and frequency of deliveries required.

Because of this variation the combustion of wood chip also needs to be finely controlled to obtain maximum performance. Hoval’s Forester wood chip boiler has been specially designed to achieve optimum combustion with chips containing as much as 35 percent moisture.

Wood pellets are produced from sawdust, shavings and finely reduced wood waste to international standards. This ensures that they are consistent in size, have a lower moisture content and therefore higher energy content, and leads to more predictable, controlled combustion. This enables a smaller volume of pellets to provide the necessary heat output, compared to chips, so storage and fuel handling requirements are typically less demanding and deliveries less frequent. As a result, pellets are the biomass fuel of choice for many projects where the fuel will need to be transported further – or the project is located in an urban or suburban location.

Wood pellet biomass boilers, such as Hoval’s STU range, are available in a wide range of sizes so they are suitable for many larger projects.

Staying in control
In all cases, a key criterion for choosing the boiler should be the ease of use for the building operator. Consequently, features such as automatic fuel feeding, modulation of heat output and de-ashing are incorporated into all Hoval biomass boilers, ensuring they are as easy to use as a gas or oil fired boiler.

Just as importantly, Hoval is able to provide a comprehensive package from initial project design advice through to commissioning and ongoing maintenance. We can also provide fully integrated solutions that bring biomass together with other renewable energies as well as high efficiency gas or oil fired boilers, making optimum use of each heat source.

Contact details
Ian Dagley
Hoval Ltd
Northgate
Newark
Notts
NG24 1JN
Tel: 01636 672711
Fax: 01636 673532
E-mail: hoval@hoval.co.uk
Web: www.hoval.co.uk

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