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FAIR-CT95-0682
Abatement of emissions from small-scale combustion of biofuels |
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Proposal No: | FAIR-CT95-0682 |
| Date Prepared: | July 2001, September 1999 | |
| Source: | Final Report Abstract and Executive Summary |
Source: Final Report December 1998
Consortium: The project was co-ordinated by TPS Termiska Processer AS, Nyköping (Sweden) in partnership with the Royal Institute of Technology, Stockholm (Sweden), Aabo Akademi University, Abo (Finland), Graz University of Technology (Austria), CRE Group Ltd, Cheltenham (UK), ENSSPICAM, Marseille (France) and Perstorp AB, Perstorp (Sweden).
Summary
Introduction In Europe, and especially in the Nordic countries and the Alps region, biofuels are widely available from agriculture and forestry. Biofuels are renewable and therefore do not affect the global atmospheric carbon dioxide balance. By tradition and due to the cost of transportation of the bulky fuels large amount of biofuels are used in local small-scale heating appliances. In Sweden the small scale combustion of biofuels, primarily wood, for house heating is expected to be 11-12 TWh, which is approximately 25% of the total energy utilisation for heating single family houses. The share of biofuels in Finland's energy consumption is 18 % and the use is expected to increase in the future. Also in Austria there is a significant use of biofuels in small-scale appliances for house-heating.
The consumption of wood in the United Kingdom as a domestic heating fuel has been estimated at 900,000 tonnes per year. This indicates that wood consumption represents only 0.3% of domestic energy use, with gas providing 64%, electricity 20%, mineral solid fuels 8% and liquid fuels 7%. The use of fuels is believed to follow a similar pattern in the other continental EU-countries but with regional variations.
Although C02-neutral there are other harmful emissions from, in particular, small-scale combustion of biofuels. These harmful emissions mainly consist of volatile organic compounds (VOC), tars including polyaromatic hydrocarbons (PAH), carbon monoxide and particulates. Under certain conditions there is also a significant emission of NOx due to high concentrations of nitrogen in the fuel and high excess air levels. The emission of VOC is especially important since the VOC, when released in the atmosphere, will in combination with NOx participate in the ground level atmospheric formation of ozone, which has a destructive effect on growing plants.
Tar is formed by condensation of the high boiling fractions of the volatile obtained in the pyrolysis of biofuels. Tar is a complex mixture of organic compounds, among which are the carcinogenic polyaromatic hydrocarbons (PAH). The formation of soot and tar depends strongly both on the fuel and its moisture content and the conditions of combustion. The combustion temperature in small-scale appliances is often too low to cause efficient bum out of these hydrocarbons and soot.
Since there are no common international standards for emission control from small-scale biofuel burning, national or even regional emission standards are used. In certain regions in Austria all new appliances burning biofuels have to conform to a low emission standard. In the UK there are urban regions that can be designated as Smoke Control Areas (SCAS) by the local authority, who generally regulate the air quality standards in the area in question. Once designated as an SCA, it becomes illegal to use fuels that are not authorised as smokeless unless that fuel is burned in an exempted low emission appliance. In other countries there is only recommendations or quality markings such as the Green Swan in Sweden for low emitting combustors.
The Comite Europeen de Normalisation (CEN) has suggested an emission standard of 2500, 100 and 150 mg/Nm3 respectively for carbon monoxide, hydrocarbons and particulates from boilers and wood stoves with a fuel supply of <150 kW. The proposal is under consideration by the standardisation organisations of the EU member states. Although it is a slow process all EU countries are expected to join this common standard which will be a strong driving force on the development of low emission equipment. These levels are however, still rather high-compared with the levels that can be achieved if the latest technology is applied in the development of new boilers.
There are a few general ways of reducing the emissions of unburned products from combustion, combustion modification, non-catalytic flue gas cleaning and catalytic flue gas treatment. Briefly combustion modification includes optimised air supply and proper mixing of reacting species at a controlled, high temperature over a sufficiently long time to complete the combustion reactions. In traditional wood combustion these design modifications includes downdraft and co-current flow combustion and thermally insulated secondary combustion volumes with proper residence time for burn out of the gases. However most actions taken to reduce the emission of unburned products will be counter productive as far as the emission of NOx is concerned.
Non catalytic flue gas cleaners can be used for dust and smoke removal but are rarely found on small-scale appliances. The use of a catalyst can speed up the oxidation reactions in the flue gas at lower temperatures and residence times, below those needed for homogeneous oxidation. Catalysts intended for abatement of emissions from wood combustion are found among those currently used for total oxidation of VOC, hydrocarbons and carbon monoxide. Platinum and palladium catalysts are those most commonly used.
The practical application of a particular catalyst depends not only on its activity but also on its thermal stability and resistance to poisons. High temperature and rapid temperature fluctuations cause the thermal degrading of supported catalysts. The sintering of catalysts could be accelerated as a result of the oxidising and steam-containing atmosphere. Metal oxides are an alternative to noble metals as catalysts for total oxidation. They may have sufficient activity, although they are generally less active than noble metals at lower temperatures. Some combinations of oxides may have better thermal stability and be less susceptible to poisoning than noble metals.
For small-scale application combustion control and catalytic flue gas treatment is believed to be the most feasible methods. This R&D project was initiated to solve some of the problems associated with optimising small-scale combustors with catalytic gas cleaning,
Objectives The overall objective of this project was to reduce the emission of unburned products from small-scale combustion of biofuels. Methods to be utilised included both optimisation of combustor design for reducing emissions and catalytic flue gas cleaning, with the overall objective subdivided into a number of specific tasks within the project. as follows:
To develop catalysts useful for oxidation of unburned gases suited for use in small-scale combustion units. To optimise the integration of catalysts in small-scale boilers and design of catalyst package for durability and efficiency. To evaluate catalysts developed within the project for efficiency, under realistic wood- burning conditions. To develop and optimise a low emission combustor design suitable for application in small-scale wood burning stoves.
Activities The general problem with small-scale combustion equipment is insufficient temperature and incomplete mixing of combustibles with the proper amount of air. The emission problem can be handled by combustion control and by removal of the pollutants from the flue gas. In this project combustion modification and catalytic flue gas cleaning has been investigated and new methods developed.
By passing the flames from the burning logs through the glowing charcoal the combustion temperature can be improved while the residence time is increased. By adding further air to the combustible gases coming from the furnace while they still are hot, very good burn out of both soot and hydrocarbons can be achieved. This method has been demonstrated in a biofuel-fired room-heater where the design is based on R&D results that have been achieved within the project.
In this example, both the furnace and the passage where the combustible gases are burnt out are isolated with a ceramic material preventing the gases cooling down. After-burn air is supplied in a way that promotes the mixing and rapid burn out. The emission from this heater is below the new, stringent limits in the UK and the equipment can be used even in the strictest smokeless zones.
Catalysts, utilising new combinations of active material, for use in bio-fuel fired combustors have been developed with the aim of reducing the lifetime cost and improving the efficiency of the catalyst. These catalysts have been evaluated under realistic conditions in a batch-fired commercial boiler. The reduction of the emission of carbon monoxide is better than 80% over a combustion cycle. The reduction of hydrocarbons and in particular the harmful heavy hydrocarbons are in the same range. Very low ignition temperature, below 200oC, has been reached for some of the catalyst developed. To fully utilise the capacity of the catalyst it should be integrated in the combustion equipment in such a way that it is rapidly heated to the temperature where it starts to be effective. At the same time the catalyst has to be protected from excess temperatures which will reduce its lifetime. Like in all combustors the combustibles and the air must be well mixed before the reaction is taking place.
Conclusions By good control of both the air supplied to the combustion and the combustion temperature a significant reduction of the emission of unburned products, such as, soot can be achieve for bio-fuel fired equipment. To reach the very low emission levels, catalyst can be used. There are some special design considerations that have to be considered in the combustor design to fully utilise the potential benefit of the catalyst enabling simplification of a boiler or a room-heater while still conforming to strict emission limitations.
Exploitation The project also produced a leaflet describing the end result, on which the following material is based:
Biofuels are commonly used for house-heating, particularly in the northern parts of Europe and in the Alps region. Despite the modern day convenience of electric and gas-fired fireplaces, wood is still used In fireplaces in most parts of Europe. Relaxing In front of a wood log-burning fireplace is still a pleasure for many people.
The European Commission actively supports an increased utilisation of biofuels. The aim is to attain an increased usage of renewable energy, thereby avoiding an escalation of the CO2 concentration in the atmosphere. However, even though biofuels are of natural origin, the pollutants produced can still be harmful if they are not burnt in a proper way. The flue-gases can contain unburnts such as soot, nasty-smelling and hazardous tars and other hydrocarbons, carbon monoxide and NOx. This problem is recognised by the authorities in the EU-countries and new and more stringent regulations limiting the emissions is expected in the near future.
In general, there are two types of combustion equipment used in house heating systems. The room heater (stove) will mainly heat the room in which it is placed. It is developed from the open fireplace and the heat is distributed by radiation and hot air convection. In a central heating system the fuel is burned in a central fireplace and the heat produced is used to heat up water in a boiler. The hot water is then distributed to radiators placed in individual rooms throughout the house to heat it up.
The main problem with small-scale combustion equipment is low combustion temperature and incomplete mixing of combustibles with the correct amount of air. Therefore the combustion of wood in small-scale stoves and boilers give rise to high emissions, particularly of unburned species. In general the emissions from state of the art boilers are lower than for stoves but there is room for further improvements in both types of equipment.
Improved combustion control reduces pollutant emissions Passing the flames from the burning wood logs through the glowing charcoal increases both the combustion temperature and the residence time. By adding more air to the combustible gases coming from the combustor whilst they are still hot, very good burn out of both soot and hydrocarbons can be achieved. This procedure has been demonstrated in a biofuel-fired room heater, the design of which is based on R&D results achieved in the project. In this room heater, both the combustor and the exit passage in which the combustibles are burnt out are insulated with a ceramic material to prevent the gases from cooling down. Burn-out air is supplied to promote good mixing and rapid burn-out in the back end of the combustor.
One of the aims of the new design was to keep a visible flame, although behind glass. Being environmentally friendly does not mean that the pleasure of sitting in front of a living flame fire is lost. The emissions of pollutants from this heater are below the new and stringent UK limits and the equipment can be used even in the strictest smokeless areas.
Catalyst will further reduce the emission of pollutants
A catalyst can be employed to reduce the emissions from combustion. This method of control - similar to that used in modern cars - has some advantages over that of combustion control. One major advantage is that a catalyst will be effective at lower temperatures. The reaction in the catalyst will start at around 200 ºC and will reach its full efficiency for most of the unburned species at near 500 ºC In much of the existing combustion equipment on the market the temperature in the flue gas duct will reach 350 ºC and therefore the use of a catalyst will be beneficial.
Within the R&D project new catalytic materials have been developed for use in biofuel-fired combustors, the aim being to both reduce the lifetime cost and to improve the efficiency of the catalyst. These new catalysts have been evaluated under realistic operational conditions in a batch-fired commercial boiler. Comparison over combustion cycles between the commercial boiler and the same boiler equipped with a catalyst shows a reduction in the emission of carbon monoxide of more than 8o%. The reduction of hydrocarbons, and in particular the harmful heavy hydrocarbons, was in the same range.
To fully utilise the capacity of the catalyst it should be integrated in the combustion equipment in such a way that it is heated rapidly to the temperature at which it starts to become effective. At the same time, the catalyst has to be protected from excessive temperatures which will reduce its life-time. As in all combustors, the combustibles and the air must be well mixed before reaction takes place. In this regard, methods similar to those used in the room heater can be used successfully.
Conclusion The results from the project clearly show ways substantially reduce the emissions from state of the art stoves and boilers. By good control of both the combustion of both the combustion temperature and the air supplied to the combustor of biofuel-fired equipment a significant reduction of the emission of unburned products such as soot can be achieved. To reach very low emission levels, a catalyst can be used. To utilise fully the capability of the catalyst, there are some special considerations that have to be included in the combustor design.
The above procedures can be used to simplify the design and reduce the cost of a boiler or a room heater, which will meet the environmental restrictions.
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