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The actual levels of emissions in the flue gases depend on the equipment itself, the fuel used and, to a very great extent on how they are operated. Levels of NOX emissions tend to vary from about 60 mg/MJ for small pellet boilers, at full output, to 170 mg/MJ for larger chip boilers, however when Phase 2 of the RHI is introduced later in 2012 there will be a cap on all equipment to be supported of 150 mg/MJ. Recent figures from the Austrian testing agency (BLT) showed the vast majority of those tested (representing 95% of Austrian boilers x up, while heartwood will help keep it down, and careful control of combustion temperature, especially if using dry fuel, such as waste wood, can help to keep thermal NOx down.
Since a large proportion of biomass boilers will be installed in areas that are off gas Grid, and hence the alternative will be oil, the NOx impact of using biomass will be very small. These are areas where there are relatively low background levels as a result of relatively low density of housing and especially traffic, where natural dispersion will ensure that NOx levels at ground level will be largely unaffected.
To put these emission values into context, data from the National Atmospheric Emissions Inventory give figures for NOx emissions from diesel cars of 440-530 mg/km (700-850 mg/mile) when hot. This is an average figure, so while some cars will be better, others will be much worse. In addition to this, the cold start emissions will be an additional 270 mg per trip (400 mg per trip for petrol cars). A short 10 mile trip to the local supermarket would result in 14,350 mg NOx emissions. This would be the equivalent of running a 10 kW biomass boiler with NOx emissions of 150 mg/MJ (the maximum allowed under the RHI from October 2012) for over 2.5 hours. Many cars, such as 4x4s could be much worse, and many boilers much better (remember this is the maximum allowed under RHI). These emissions from vehicles will of course be at ground level, while those from a boiler will be emitted from the flue at much higher level, allowing considerable dispersion and dilution, considerably reducing further the actual concentration at head height.
Particulate emissions from burning natural gas tend to be extremely low, typically less than 1 mg/MJ. Boilers burning light fuel oil might have emissions around 5 mg/MJ, while those burning heavy fuel oil might be around 50 mg/MJ and coal might be 120 mg/MJ upwards, and significantly higher for larger and older equipment. Modern, high efficiency biomass boilers operating at full output might produce total particulate emissions (predominantly PM2.5) in the range 10-70 mg/MJ range. The BLT results showed that well over half of those tested achieved 20 mg/MJ or less, and from Phase 2 of the RHI there will be a limit of 30 mg/MJ for all biomass boilers seeking RHI support. Incorrect operation however, can increase these figures significantly. Open fires and some old or poor quality log stoves will produce massively more. However the use of high efficiency modern filters, such as ceramic filters can ensure that particulate emissions are kept extremely low (3 in the flue gas) at all times. As the optimum scale for wood boilers is that of district heating and large, multi occupancy sites, these abatement technologies are economically viable. Usage of best available technology in terms of both combustion equipment and abatement technology can ensure that emissions are no worse than typical oil technology.
The increase in PM emissions from using solid biomass to replace oil, or particularly natural gas, is a factor that should be considered when contemplating their widespread installation within urban areas. In these situations air quality may already by low, substantially as a result of emissions from road traffic, and even the very small relative increas from the use of solid biomass in place of natural gas may be unacceptable. In this case it may be appropriate to adopt suitable abatement technology, which can, if necessary, reduce particulate emissions to negligible levels. In more rural areas, where background levels from traffic, industry and high density housing are not a significant issue, a well designed flue should ensure that sufficient dispersion of combustion products takes place to ensure that the negative contribution to air quality at ground level is negligible. It should be remembered that in rural areas open fires, garden bonfires and log stoves are common and the particulate emissions from these will be considerably higher than from a well maintained, correctly operated modern biomass boiler.
To put these values into context, data from the National Atmospheric Emissions Inventory give figures for PM10 emissions (almost all PM2.5) from diesel cars of 25-30 mg/km (40-50 mg/mile) when hot. This is an average figure, so while some cars will be better, others will be much worse. In addition to this, the cold start emissions will be an additional 100 mg per trip. So a short 10 mile trip to the local supermarket would result in about 900 mg PM10 emissions. This would be the equivalent of running a 9 kW biomass boiler with particulate emissions of 30 mg/MJ (the maximum allowed under the RHI from October 2012) for an hour. Again, these emissions from vehicles will be at ground level, while those from a boiler will be emitted from the flue at much higher level, allowing considerable dispersion and dilution, considerably reducing further the actual concentration at head height.
The low levels of sulphur in most biomass leads to emissions of SO2, at typically 20 mg/MJ, which is considerably lower that those of oil (140 mg/MJ) or coal (900 mg/MJ). However, natural gas generates very low levels of SO2, typically less than 1 mg/MJ.
CO2 emissions from burning wood fuels are actually relatively high, calculated per kWh of energy, compared to most fossil fuels, owing to the relatively low calorific value of wood. However, the carbon released by burning biomass was taken out of the atmosphere recently, is part of the current carbon cycle and, if the fuel was obtained from a sustainably managed source, will be taken up again by subsequent growth. This is why biomass is frequently described as "carbon neutral" although, as explained on the page "Why use biomass" this is not strictly true owing to the energy used in production.
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