Getting to grips with biomass

On Nov 6, 2012

With the increasing use of biomass for heating, many specifiers and installers find themselves in the unfamiliar territory of dealing with solid fuel. Martin Murrish, Technical Manager with Hoval, examines some of the key issues.

Without a doubt, biomass is proving to be the UK’s most popular renewable heat source. However, there are a number of factors that need to be taken into account with biomass boilers that do not apply to gas or oil fired boilers. Being aware of these factors is the key to ensuring the project goes smoothly and end clients get the solution they need.

Biomass has a number of advantages over competing renewable technologies. For example, biomass boilers typically produce hot water temperatures between 60°C and 85°C which suits many existing heating systems and means biomass boilers can be retrofitted with no need to alter heat emitters. Biomass can also meet a wide range of requirements, from 10kW to over 5MW, and is easily integrated into systems that also use gas or oil fired boilers.

Furthermore, the elements of a biomass installation typically consist of well-established and generally quite simple mechanical equipment that most specifiers and installers are familiar with. Consequently, many of the skills necessary for the design, installation, servicing and operation of conventional fossil fuel boilers are readily transferrable to a biomass installation.

In addition, there is a growing national biomass infrastructure so that earlier concerns about security of fuel supply have now been allayed. And, of course, biomass heating is eligible for payments through the Renewable Heat Incentive (RHI), so this can have a significant impact on the return on investment calculations for the project.

Choosing the right fuel

The vast majority of UK commercial biomass installations will utilise either wood chip or wood pellet fuel. Some are initially planned to be `either/or’ but in practise the system design will favour one or the other.

As noted earlier, what makes biomass heating slightly unfamiliar is the use of solid fuel and the implications this has for various aspects of the system. A good biomass boiler installation design, therefore, starts with a firm understanding of the properties of the intended fuel, as this impacts on combustion performance, fuel storage and delivery requirements, fuel handling equipment and levels of maintenance.

Wood chip

Wood chips are produced by mechanically shredding any source of untreated wood and will vary in size. Moisture content ranges from 20% to 60% (<35% = dry fuel, >35% = wet fuel – BS EN 303-5). Ash content is low, varying from 0.5% to 2% (by weight) depending primarily on bark content.

Wood pellet

Wood pellets are compressed wood dust usually manufactured from a clean or virgin wood source. They are typically 6mm in diameter and 15-30mm long with a moisture content never exceeding 10%. Ash content is generally <1% and often <0.5%.

Chip or pellet?

As wood chip requires less processing its unit or p/kWh price is generally lower than pellets but the physical properties mean wood pellets deliver far higher energy density than chip. In fact there is almost four times as much energy available from a cubic metre of pellets compared to good quality chip (see Table 1).

Typically wood pellets also offer a more consistent homogenous energy source, arguably leading to more reliable boiler operation.

There are a number of national standards to define the fuel to be used in a biomass installation based on these properties along with the European Standard BS EN 14962-1 that summarises all common biomass fuels including wood chip. There are also quality assurance schemes, such as EN+ A1 for wood pellets.

Storage and delivery

Unlike gas fired heating, biomass installations require particular attention to fuel storage. For instance, the lower calorific value of wood chips, compared to pellets, means that higher storage volumes or more frequent deliveries will be required for a given heating capacity.

Also, because of the volumes involved, wood chips are normally delivered by tipper lorry and this will determine the dimensions for access routes and turning circles. Below-ground storage will also be required to receive fuel from the tipper, along with 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.

The irregular shape and size of chips means they need more robust handling equipment with larger augers and motors, compared to pellets.

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.

In all cases, the distance from storage to boiler should be as short as possible to minimise the number of augers required, thereby minimising cost.

By bringing together modern PLC control and improved combustion sensors, with well-established biomass firing techniques, the latest generation of biomass boilers has been shown in independent testing to deliver operating efficiencies above 92% (nCV basis). This is achieved through an intelligent combination of inverter controlled fuel feed augers and an extremely responsive induced air flow, regulated by a high quality, robust and strategically located lambda probe. The touchscreen control system optimises combustion and ensures the boiler matches demand with maximum efficiency.

Control, efficiency and emissions

Careful and close modulation of the fuel feed in relation to the boiler load coupled with efficient combustion also helps to minimise the level of particulate emissions. The latest biomass boilers are able to reduce particulate emissions to less than 10mg/MJ without the use of cyclones or filters. To put this into perspective, the maximum proposed for the RHI is 30mg/MJ.

The key requirements for biomass heating systems, therefore, are to achieve a close marriage between controls, hardware and fuel to get as close to complete combustion as possible. In parallel, specifiers need to select the best fuel for the project and ensure that delivery, storage and fuel handling requirements are all taken into account in the early design stages. From a sustainability point of view there are clear benefits to sourcing UK manufactured biomass boilers to minimise the embedded carbon associated with transportation.

Another practical consideration is that it is also vital to get the buy-in of the people who will use the boilers on a day to day basis. They need to understand how these boilers differ from more familiar gas or oil, particularly in terms of any maintenance aspects.

Biomass heating is central to the UK’s carbon reduction programme and building services specifiers have a vital role to play in ensuring that biomass heating systems deliver maximum benefits. Taking account of the factors discussed here will go a long way to ensuring that happens.

Without a doubt, biomass is proving to be the UK’s most popular renewable heat source. However, there are a number of factors that need to be t
aken into account with biomass boilers that do not apply to gas or oil fired boilers. Being aware of these factors is the key to ensuring the project goes smoothly and end clients get the solution they need.