Particulates are an inescapable downside of any combustion process, and despite its low carbon credentials, biomass is no different from fossil fuels in that respect. Even in modern, high efficiency biomass boilers, particulate emissions can be elevated beyond acceptable levels as a result of poor system design, an inadequate maintenance regime, out-of-specification fuel or incorrect commisioning.
Following an RHI emissions test by market leader Socotec in spring 2017, the 1.2MW woodchip boiler at the Breadalbane Community Campus was identified has having particulate emissions above those that are acceptable for biomass boilers. Despite a thorough servicing and maintenance regime and first class woodchip, the 10 year old system was unable to meet the strict air quality criteria set by the Renewable Heat Incentive legislation, meaning post-combustion cleanup options needed to be explored. The hard working boiler heats not only the large secondary school that forms the core of the site, but also the library, various meeting rooms, cafe, climbing wall, community swimming pool, and a wide range of other community sports facilities that occupy the extensive campus. Being without their biomass boiler isn’t an option for Breadalbane, given the huge contribution it makes to reducing the carbon footprint of Perth and Kinross Councils’ building estate, and the substantial local market it provides for woodchip – a valuable benefit to the rural economy.
Using our knowledge of biomass and emissions clean up technologies, we identified an electrostatic precipitator (ESP) as the most appropriate technology to meet the needs of the site – an ESP would deliver low running costs and lower particulate emissions. However, retrofitting 6 tonnes of advanced emissions reduction technology into a space which had never been intended to accomodate something the size of a large van presented a significant engineering challenge. After exploring equipment and pricing from a number of manufacturers, we eventually settled on an ESP from Austrian manufacturer, Scheuch, which offered the right combination of price, performance and physical dimensions to fit into the space above the biomass boiler.
An extensive detailed design process then followed, using local architects for the building warrant process and engineering consultants Wardell Armstrong to design the load-bearing steelwork that would be needed to ‘hover’ the ESP 5m above the boiler in the plant room. The resulting steel sub-frame was installed piece by piece by feeding the beams through the wall of the adjacent fuel store, assembling it on a temporary scaffolding deck and tying it in with the existing beams and concrete walls. The ESP itself was lifted in by removing a 1m x 2m air vent 11m up the face of the plant room, and bring the ESP in in sections through the access door prior to being lifted up and positioned on the steel frame.
The sections of the ESP went together perfectly, and after a week of long days changing the flue route, electrical installation, fitting the de-ashing mechanism and comissioning the completed unit, the boiler was ready for another emissions test. Creating the full load conditions needed for a compliant emissions test was relatively easy when you have a full size swimming pool to play with and dozens of showers and other hot water demands, and we very quickly created the ideal operating load for the boiler to be tested for particulate emissions.
There was a slight disappointment when the ESP didn’t crackle or glow purple when it was switched on, but this was soon forgotten when the test results came back from Socotec – a 98.6% reduction in particulate emissions compared to the original emissions results, which at 1.8g/GJ is just a fraction of the 30g/GJ limit set by the RHI.
A clean bill of health for the Breadalbane Academy boiler, and cleaner air for Aberfeldy – it’s amazing what #modernbioenergy can deliver!