Making the switch to biomass boilers

Not only does Hoval make biomass boilers in the UK, but it has also installed them to heat its factory and for training and demonstration purposes. Martin Murrish explains how Hoval went about this retrofit project. When retrofitting biomass boilers there are a number of factors to consider.

Of the various renewable heat sources that are available in the UK, wood-fuel biomass is proving to be particularly popular with specifiers and building operators. This is partly because the idea of burning a fuel to heat water is a very familiar and well understood procedure — and one that is very effective. It also delivers a fast return on investment, particularly when the installation qualifies for the Renewable Heat Incentive (RHI).

In retrofit projects where a fossil-fuel boiler is being replaced or supplemented by a biomass boiler, a further advantage is that the biomass boiler will use the same flow and return temperatures as the previous boiler. This means there is no need to change the heat emitters, as might be the case with other renewable heat sources. Existing pumps, pressurisation sets and other ancillary items can also be retained.

The nearer of these two wood-pellet boilers heats Hoval’s factory. The smaller boiler is for training and demonstration purposes.

Nevertheless, there are some important factors to consider when switching to biomass — as illustrated through the procedures we followed in installing biomass heating at our factory in Newark-on-Trent.

This is the factory where our latest generation STU wood-pellet biomass boilers are manufactured, and we have two STUs installed at the factory. One 600 kW unit replaces the previous gas-fired boiler to deliver hot water (80°C flow/60°C return) to fan-coil units mounted in each of the three factory spaces. This boiler is sized to meet all space-heating requirements throughout the year and is coupled to a heat meter for the RHI.

In addition, a 195 kW STU boiler has been installed for training and demonstration purposes and is connected to a small, separate heating system so as not to interfere with metering for the RHI.

Finding a space for the boilers was relatively straightforward as the latest STUs use a standardised skid-mounted design that includes pumps, fans and fuel delivery augers, so that footprint is greatly reduced. However, we also needed to find space for storing fuel as close to the boilers as feasible to keep the fuel-delivery system simple.

The most appropriate solution was to opt for a 35 m3 GRP (glass-reinforced plastic) silo sited just outside the building, connecting it to both boilers with flexible augers. It was important that this decision was made early on because siting such structures outside requires planning permission.

Wood-pellets for the two biomass boilers at Hoval’s factory are stored in a 35 m3 GRP silo with load cells to track fuel reserves.

The silo was delivered to site in one piece, so the only preparatory work was to construct a concrete base for it. The silo holds 20 to 25 t of wood pellets and incorporates load cells to track fuel reserves. The system can be set up to send an SMS message to the fuel supplier when reserves reach a predetermined level. Fuel is delivered in a special vehicle and blown directly into the silo, so there was no need to make special provision for delivery access.

Flueing is another consideration, as biomass boilers have slightly different flue specifications to gas- and oil-fired boilers. In the event the existing flue for the gas boiler also met the specification for biomass, so we were able to use that for the 600 kW STU.

For the 195 kW STU we installed a new flue, which had to be routed around an overhead crane, so the sizing of the flue had to take account of this configuration.

To optimise performance we have also installed a 6000 l thermal store, which serves several purposes.

It helps to spread the residual heat when the boiler switches off at night, helping with heat dissipation. Also, when the water has reached design temperature the boiler will not fire again until the temperature falls to 75°C. So the thermal store acts as a buffer to maintain flow temperatures while reducing the firing of the boiler. This improves efficiency by preventing excessive cycling.

In addition, the thermal store retains much of its heat through the night, which accelerates the warm-up times in the mornings. This is a useful feature as biomass boilers take a little longer to reach full temperature than fossil-fuel boilers.

Another major benefit of this exercise, which would not apply to most projects, is that it has enabled us to use the system to further refine the FlameTronix control software that was launched earlier this year. Using historical data, we have been able to adjust the control of stokers to optimise performance and apply this upgrade to the software used by new STU boilers. This includes remote web-based monitoring of various boiler operating conditions.

Given the significant benefits of using biomass heating in terms of reduced carbon emissions, cost savings and potential additional revenue through the RHI, it is certainly worth considering. It also makes sense to work with a supplier that has the technical expertise and practical knowledge to ensure the system delivers maximum performance at every level.

Martin Murrish is technical manager with Hoval

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