Burning any fuel produces a range of emissions in the flue gases. Carbon dioxide and water vapour are inevitable, but using modern, efficient equipment, clean fuel and best practice others can be kept to a minimum.
Burning biomass, or any other fuel, in inappropriate, or badly maintained equipment, or under poor operating conditions can give rise to a number of potential emissions. These can include particulate matter, NOx, CO and other carbon containing compounds.
Most biomass, including wood, is composed of roughly 50% carbon by weight, 40% oxygen and 5% hydrogen. Under ideal combustion conditions these are completely converted to carbon dioxide (CO2) and water vapour (H2O). In addition there can be about 0.3% nitrogen, 0.1% sulphur, 0.1% chlorine, and trace quantities of various minerals such as calcium, potassium, silicon, phosphorus and sodium. The levels of these, and other, elements depend on many factors, including the environment the material was grown in, the species, any contaminants in the soil, water or air, etc. The chemical composition of different parts of a plant also varies; for example higher levels of minerals in bark, lead to increased ash production, while there is more nitrogen and sulphur in green waste and brash. This underlines the importance of obtaining high quality woodfuel, with a high proportion of clean, white stem wood.
The principal concerns about emissions and the impact of combustion systems on air quality are in relation to carbon dioxide and carbon monoxide (CO2 and CO), small particulates (PM10 and PM2.5); particles smaller than 10 microns and 2.5 microns respectively), oxides of nitrogen (NOx) and sulphur dioxide (SO2).
CO2 is a greenhouse gas and an inevitable consequence of burning any organic (carbon containing) material. Under conditions of insufficient oxygen supply, or incomplete combustion carbon monoxide (CO) can also be formed as well as fine particles of unburned carbon, or soot. In a well designed combustion system, which allows sufficient time and turbulence within the combustion chamber for complete combustion, levels of CO within the flue gases can be kept to a minimum. The incorporation of a Lambda oxygen sensor in the flue of many modern systems helps ensure optimum operating parameters.
NOx formation arises as a result of several mechanisms. Nitrogen in the fuel is oxidized, leading to fuel NOx. In addition, in any high temperature combustion process in the presence of air, atmospheric nitrogen can also be oxidized, leading to thermal NOx. This process is temperature dependent and rises rapidly above about 1,100ºC.
The level of sulphur in virgin wood is generally very low, typically around 0.1%, or an order of magnitude lower than in fossil fuels like coal and fuel oil. Much of the sulphur condenses onto the fly ash particles as sulphates, although a significant proportion can be emitted as SO2 and SO3.
As any gardener knows, plants need a number of minerals to thrive, including potassium (K) and phosphorus (P). These are present at trace concentrations in wood, but at higher levels in the bark and leaves. Other minerals can be taken up into growing biomass if present in the soil, water or atmosphere. Most of these will contribute to the ash in the form of various salts. Larger particles fall through the grate as bottom ash, while smaller particles can be carried up with the flue gases, together with any particulate unburned carbon, as fly ash. There are a range of filters available to catch such particles, include cyclones, electrostatic precipitators, and high efficiency fabric filters. It must be noted, however, that even a small amount of soil inclusion in the fuel as a result of poor storage or handling practice, will lead to greatly increased levels of ash, once again underlining the importance of obtaining quality fuel.
There can also be chlorine present in the wood, typically at levels of around 0.1%. However in some environments this can be significantly higher. Wood grown or stored near to the sea, for example, can pick up chlorine from salt spray. This chlorine can form salts of some of the minerals, such as sodium chloride (NaCl) or potassium chloride (KCl) or hydrogen chloride (HCl) and can also enhance the production of a number of harmful halogenated compounds.
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.
Under certain conditions it is possible to generate other, potentially more harmful emissions. It is important this is avoided by appropriate choice of fuel, combustion or abatement equipment.
Although the levels of heavy metals in most virgin, untreated wood are extremely low, wood that has been treated with preservatives or coatings may potentially have high levels of heavy metals. The combustion of treated wood, therefore, is regulated under the Waste Incineration Directive. Wood used for outdoor applications, for example, might have been treated with CCA (Chromated Copper Arsenate) which if burned in unsuitable equipment, could allow the emission of arsenic into the atmosphere, as well as copper and chromium. CCA has now been replaced with less toxic alternatives within the EU.
Significant levels of heavy metals can, exceptionally, build up in some kinds of virgin, untreated timber if it has grown on land in which they are present in high concentrations. This could potentially be on land remediation sites, or where sewage sludge or other effluent has been spread on the land.
Treated wood can also contain halogenated compounds, such as those containing chlorine, and this too, must be burned in a WID compliant installation. Under certain conditions, with the simultaneous presence of carbon, oxygen and chlorine, it is possible to generate PCDD and PCDF (polychlorinated dibenzodioxins and polychlorinated dibenzofurans), known as dioxins and furans respectively. Significant generation of these, highly toxic, compounds requires very specific conditions, however, and good combustion conditions and equipment design can ensure very low emission levels. The most common mechanism requires carbon in a specific, particulate form (microcrystalline, not amorphous or highly ordered graphitic), molecular oxygen, and chlorine, within a very narrow temperature window of about 250-500ºC. Outside this range there is extremely low formation, and above about 600ºC the reverse destruction reaction increases extremely rapidly. The formation reaction is catalysed by the presence of some metal ions, in particular copper and also, to a lesser extent, iron. Because of the very specific reaction conditions necessary for formation, careful design can ensure very low levels. Such measures include maintaining low levels of particulates in the flue gases, in itself a product of good combustion and ensuring that any surfaces on which particulates can build up are maintained outside the critical temperature window. Rapid quenching of flue gas temperature also contributes to good design and practice, ensuring that emission levels are of the order of those from conventional oil boilers, and an order of magnitude below those of coal combustion.
The final class of emissions that can be encountered under certain conditions of fuel and equipment, are those known collectively as PAH (polycyclic aromatic hydrocarbons). Aromatic organic compounds are those containing rings of typically five or six carbon atoms, and PAHs are compounds of three or more of these rings. The range extends, though, from these small molecules up to the larger molecules that are found as part of the mixtures of chemicals known as creosote and tars. The presence of these, or any other such organic compounds in the flue gases, is indicative of incomplete combustion. The initial stage of combustion of organic material, pyrolysis, is one of thermal decomposition giving rise to the emission of a wide range of complex organic molecules which, when condensed, are known as pyrolysis oil. Provided the temperature, oxygen concentration and residence time are all sufficient, this stage is followed by combustion (oxidation) to carbon dioxide and water. If, however, one of these parameters is not sufficient to allow efficient combustion, PAHs and other unburned hydrocarbons can be emitted in the flue gases. This can arise because of poor quality fuel, especially with a moisture content above the design maximum for the particular combustion equipment, or because the controls for the boiler or stove are incorrectly set, such as insufficient secondary (or tertiary) air. As these gases cool within the flue sequential condensation of progressively smaller molecules takes place, leading to tar deposits, while smaller molecules remain as vapour until emitted into the atmosphere. These are all highly undesirable as a number of the PAHs are carcinogenic, and tar deposits within the flue can lead to corrosion and potentially a chimney fire. Once again, use of good quality fuel, of the specification appropriate for the combustion equipment, and well designed equipment operated according to the manufacturers’ instructions, can keep such emissions down to very low levels, similar to those of conventional liquid fuels, and an order of magnitude below those of coal.
Ash is an inevitable product of the combustion of solid fuel. It can be viewed as a waste to be disposed of, however it also contains valuable minerals and can be used as a fertilizer.
Ash from combustion of biomass in high efficiency modern combustion equipment such as boilers or stoves primarily consists of the non-combustible, mineral constituents of the fuel as oxides or salts. As mentioned above, it falls into two components, those, primarily larger, particles that fall through the grate during combustion and are collected as bottom ash, and the very fine particles that are carried in the flue gases and are known as fly ash.
As it contains the minerals taken up by the biomass during growth, such as potassium and phosphorus, unless it has been contaminated in some way, bottom ash can form a valuable fertilizer, although it does not contain nitrogen. It can be used as a part of sustainable forestry operations. Owing to the highly concentrated nature of this ash, and the presence of oxides of alkali metals such as calcium and potassium, such ash tends to have a very high pH, and consequently it is neither necessary nor desirable, to use too much in one place, but it can be very valuable if distributed thinly. In Sweden the maximum allowed dose of wood ash is 3 tonnes per hectare. [See “Wood ash use in forestry – a review of the environmental impacts”, Rona Pitman, Forestry (2006), which gives guidelines for the UK]. Bottom ash can also be added to a compost heap, at a rate of up to about 15% of the total (by weight). It is recommended that in large scale ash application, such as might be associated with forestry, the ash is not applied in the loose form, but in a ‘granulated’ form, developed in Scandinavia, having been mixed with water and rolled into small balls. This makes it easier to handle and apply, slows nutrient release, and also reduces damage to ground vegetation, particularly mosses, on application.
Bottom ash can also be used for the manufacture of lightweight aggregate blocks, however the quantities required for economic operation make this feasible only for large, power generation scale operations.
Fly ash, collected from flue gas filters, however, should be treated with more caution. Any heavy metals present tend to be concentrated in the fly ash and as very little is generally produced compared with bottom ash it is should not be used as a fertilizer.