Biomass designers face quality challenges
Biomass boilers are becoming increasingly popular, but heating engineers must address a range of design challenges to make sure each installation reaches its full potential, says Ian Halford of Baxi Commercial.
Biomass has several advantages over other types of renewable and low-carbon heating. In particular, it is far more predictable than weather-dependent wind, tidal and solar. This makes it easier for heating engineers to calculate the likely performance and capacity of a biomass installation, as the calorific value of the fuel source and the running hours for the plant should be consistent.
This consistency also has advantages when it comes to sizing the plant for optimum performance, making it ideal for packaged and prefabricated systems.
Consistency of performance should also make it relatively easy to calculate the payments due to plant owners under the Renewable Heat Incentive (RHI), which is largely why biomass accounts for up to 95% of all applications to the commercial RHI.
Rural areas, in particular, present a huge opportunity for biomass because many buildings there are off the gas grid and are currently operating on relatively expensive oil, LPG and solid fuel.
The initial installation cost for biomass is relatively high, but, as with many renewable solutions, should be regarded as a long-term investment rather than a quick fix. Switching to biomass is not your standard heating ‘distress purchase’, but a decision to take a new approach with the prospect of long-term sustainability and running cost savings. The payback can also be considerably shortened by payments from the RHI.
However, the source and quality of the fuel to be used are critical factors. Before doing anything else, the design team should examine the long-term viability of the fuel supply and ease of delivery to the site. Not only does this have implications for running costs, but it is also an important part of assessing whether biomass is the most sustainable solution in the widest sense — transporting bio-fuel from far-flung destinations is not a recipe for minimising carbon emissions.
Biomass installations are subject to the UK’s Clean Air Act, and wood used must be legally supplied from a sustainable source in line with timber procurement policy and standards. This should ensure that a fuel in pellet form will have moisture content typically lower than 10% or, if in wood-chip form, around 30%.
Freshly cut wood would typically have a moisture content of 35 to 60%, which is far too wet to burn. Wood chips and wood pellets are the most popular biomass fuels, as they can be fed automatically into the boiler.
The storage options can range from purpose-built rooms, pre-fabricated silos and containerised stores — so the available space on site is an important aspect to consider. There are also associated safety issues that will need to be addressed — particularly fire safety.
A poorly designed and/or maintained biomass plant will release damaging emissions such as particulates, oxides of nitrogen (NOx) and carbon — so the plant will need more maintenance than gas-fired equivalents to ensure efficient operation, clean flue-gas emissions and to prevent damage to internal components.
A prefabricated containerised plant room is a good way of neatly addressing a number of these issues and can also save up to 30% on initial construction and long-term running costs.
Since 24 September 2013, any biomass-boiler owner hoping to qualify for RHI payments must have an emission certificate or an environmental permit. This is to ensure that emissions do not exceed 30 g/GJ net heat input for particulate matter and 150 g/GJ for NOx (expressed as NO2).
Most biomass, including wood, is composed of roughly 50% carbon by weight, 40% oxygen and 5% hydrogen. Under ideal combustion conditions these components are completely converted to carbon dioxide and water vapour. It is, therefore, an important consideration for consulting engineers to look at how effectively the plant will be used as part of the overall building-services project design to make sure it does burn cleanly. The specification for biomass fuel is outlined in standard BS EN14961.
For the plant owner to receive the appropriate RHI payment, the usable heat produced by the boilers must also be accurately measured. This means anyone designing and installing a system must take into account position and quality of heat meters. If the meter is missing or incorrectly installed it becomes the Achilles heel of the whole system because it undermines the economic viability and gives the plant operator a major headache when trying to work out how well the boiler is working.
Ofgem, the regulator overseeing the RHI, has simplified meter arrangements, but the onus remains on installers to make sure the meters themselves meet the quality criteria and are fitted correctly so they measure the right things at the right time.
Ultimately, the success or failure of a biomass project depends on how well it is integrated into an existing building and how well it works in tandem with other carbon-saving measures such as fabric improvements and energy efficiency upgrades. Any replacement plant should be carefully designed into the existing infrastructure to ensure it does not conflict with other measures for the smooth operation of the facility. This makes the controls and commissioning strategy crucial.
A biomass boiler, like any other renewable technology, will do little for the sustainability and efficient running of a building if it is simply ‘bolted on’.
Ian Halford is prefabricated-systems manager with Baxi Commercial.
