Pellet heating

from Wikipedia, the free encyclopedia
Pellet heating of a house with fuel supply via a screw (metal pipe on the left)
the different screw diameters

A pellet heating system (or pellet heating system ) is a heating system in whose boiler mostly wood pellets (small pellets made from wood chips and sawdust ) are gasified. Other pelletized fuels such as straw pellets , straw pellets , peat pellets, pellets made from olive pits and pressed olive residue , coconut shells or other biogenic residues are also used.

Central heating systems with wood pellets as a renewable energy source are comparable to oil and gas heating systems and achieve a combustion efficiency of over 90%. Due to the nature of the raw materials and the principle involved, the emission of fine dust is higher than with oil and gas heating systems.


Model of a pellet heating system with pellet store (left), screw conveyor (silver, below), boiler (right), buffer storage (silver, rear) and chimney

Basically, pellet heating systems as systems for the operation of central house heating systems including control and regulation technology (i.e. pellet central heating systems ) are to be distinguished from individual pellet stoves with direct heat output into the living space.

Single ovens

Individual pellet stoves are systems with a performance range of max. 6–8 kW and less that are installed directly in the living space. They have a small fuel storage container that can hold one to a few daily rations. Fuel supply and ignition and the regulation of the combustion are controlled automatically, the ash removal is done by hand. The heat is usually given off directly to the room air. Water-bearing stoves use part of the energy to heat water that is in water pockets around the combustion chamber. This means that the performance of an existing central heating system can be supported or replaced in individual cases. The range of individual pellet stoves is as diverse as that of log stoves; versions with a viewing window on the fire or combined log-pellet table stoves are also available.

Central heating

Pellet central heating systems come into consideration for nominal heat outputs or the heat demand (= so-called heating load , calculation according to EN 12 831) from 3.9 kW upwards. Pellet heating systems are not only suitable for use in single and two-family houses (up to approx. 30 kW), but also for larger residential or business units; in the latter case, larger systems or several pellet heating systems connected in series ( cascade systems ) with a few hundred kW are used. Hybrid or combination systems can also be loaded with other firewood (such as split logs or wood chips ).

Pellet burner systems primarily run optimally under full load and can be regulated up to around 30% of the nominal output. Short combustion phases have a negative effect on the fuel efficiency, since with pellet heating the energetically less efficient warm-up phase is longer than with oil or gas heating; The pollutant emissions are correspondingly higher. This disadvantage is mitigated by combining pellet heating systems with heat buffer systems , generally via hot water storage tanks .

Attachment pellet burner

Separate pellet burners, which are mounted on an existing oil or wood boiler, are offered as an inexpensive alternative to a complete heating changeover. The efficiency in the combustion is reduced with these solutions, however. In contrast to heating or stoves tailored to pellets, the investment costs are not funded by public funds.


Sectional view of a pellet heating system with drop shaft firing

Similar to wood chip heating systems , the fuel is delivered periodically and automatically brought into the combustion chamber as required by means of feed devices from the pellet store (heating systems) or the day container (individual stoves). Standard combustion chambers are used in wood heating systems for combustion. With pellet central heating systems, the heat generated is used to heat water from the heating circuit in the boiler of the pellet heating system. As with other central heating systems, the heat is distributed through the heated water. In contrast to oil or gas heating systems, the integration of a hot water storage tank into the heating system makes sense or is even necessary, which stores the heat generated in the combustion process with little loss until the heat is requested by the heating system or ensures heat dissipation until the boiler has cooled down.


Combustion chamber in cross section

The furnace, designed as a fixed bed reactor , is automatically supplied with material to be fired. The control and regulation technology of the plant gradually supplies the fuel in the amount that corresponds to the required heat output. Depending on the design, the supplied wood pellets are either ignited automatically with hot air blowers, or a bed of embers is kept permanently in the combustion chamber.

Wood pellet heating systems work with different charging techniques: Today, the drop chute or pellet firing specially developed for pellet combustion, the underfeed firing, the cross-infeed firing or the use of a roller grate system are in use.

  • When Fallschachtfeuerung the pellets slide down a chute into a burner pot. By using a burner pot, the combustion area is precisely defined and the combustion can therefore be precisely controlled.
  • When Underfeed the pellets by means of a feed screw pressed from below of a burner plate, burned there and the remaining ash falls outside the box into the underlying ash container.
  • The cross- feed firing works in a similar way to the underfeeding firing, except that the fuel is pushed onto the burner plate from the side via a screw conveyor. Both the burner plate and the air supply can be specially shaped to adapt to partial outputs.
  • With the roller grate system , the pellets fall from above onto several slowly rotating steel disks with little space between them. A scraper comb cleans the gaps with every rotation, so that the ashes can fall down unhindered and combustion air can be fed upwards.
Image: Partition burner from SOLARvent
Cross-insertion burner with special design of the air control (partition burner BJ 2006)
  • In contrast , with the fall fire technology the pellets fall from above onto a grate in a combustion chamber. The flames are drawn down through the grate with the help of an induced draft fan. This system produces the least amount of ash.

In order to optimize the efficiency and pollutant content of the exhaust air, modern pellet burners control the combustion either via a temperature or flame chamber sensor in connection with a combustion air supply which is continuously adjustable via an induced draft fan or a lambda probe . The hot combustion gases are fed into the chimney via a heat exchanger with manual or automatic cleaning of the after-heating surfaces or turbulators (also called turbulators) .

The ashes fall into an ash box. In order to reduce the intervals in which it is necessary to remove the ash, the ash is partially compressed in the ash box. Ash removal systems are also occasionally used, in which the combustion residues are transported to collecting containers by means of screw conveyors.

Heat transfer and storage

Just as with the use of other fuels, the combustion of the energy carrier in the boiler heats the water, which serves as a heat exchanger for a heating and / or hot water system and transports the heat energy to the place of consumption via pumps and pipes. Since a largely complete combustion of the wood pellets is only possible in regular operation and greater losses and higher emissions occur during the warm-up and burn-out phases, the heated water in heating systems is usually first fed into a buffer storage unit , as is the case with log heating systems , from where it is Consumers as needed. This guarantees sufficiently long uninterrupted firing periods.

Measurement, control and regulation

The measuring, control and regulation technology of the pellet heating is usually more complex than the comparable heating systems with fossil fuels. On the one hand, the integration of one or more heat accumulators requires regulation of the hot water storage, delivery and subsequent delivery; on the other hand, regulation of the fuel supply, combustion air supply and firing is more complex.

Safety devices

Because of the special features of the fuel, pellet heating systems have different safety devices than oil or gas burners. All modern wood pellet heating systems are equipped with a burn-back protection, which prevents burn-back in the feed / storage area of ​​the pellets. Underpressure controls in the furnace prevent toxic or flammable gases from escaping into the boiler room, overheating protection in some systems> 25 kW or combination boilers is made possible by safety heat exchangers, which automatically feed cold water through a heat exchanger in the event of overheating.

Power range and efficiency

Pellet heating systems are available in all power ranges from approx. 3.9 kW, as individual stoves between approx. 4 and 20 kW. Most of the systems available today have a capacity control for the fuel and combustion air supply, so that they can be operated at full load as well as at part load. Pellet boilers currently achieve a combustion efficiency of around 85–95% in full load operation (nominal heat output) in calorific value operation. With pellet boilers in condensing technology, boiler efficiencies of up to approx. 106% can be achieved. In this case, the evaporation energy is (at least partially) recovered through the condensation of the water vapor in the exhaust gases. This achieves an exhaust gas temperature of only 30 ° C-40 ° C. Corrosion-resistant materials such as stainless steel or graphite are used as the material for the heat exchanger required for this. Special measures are necessary in the chimney and the drainage of the condensed water (350 liters per ton of pellet).

With a few exceptions, the efficiency drops when the pellet boiler works in the partial load range. The firing efficiencies described here can differ greatly from the actual plant efficiencies , which is why the plant concept plays a major role. It makes sense to use a sufficiently large buffer memory .

Degree of automation, support and maintenance

Opened pellet boiler. Circularly arranged flue gas trains (here, 12) in the heat exchanger area. In order to remove the combustion residues that settle on the pipe walls, helical springs are inserted, which are moved by an electric motor over the shaft at regular intervals. A large part of the ash residue falls back into the combustion chamber.

Many processes in modern pellet heating systems run automatically. Necessary regular work on the heater is filling the store, removing the ashes and, in the case of simpler models, cleaning the flues. For regular cleaning and maintenance work, shorter intervals are usually necessary compared to oil or gas heating . Intervals of several weeks (ash disposal) or a few months (cleaning of the combustion chamber) are usual; for individual pellet central heating systems, due to further developments, annual maintenance intervals are now sufficient. One of the oil or gas heating systems comparable ease of use is an important development goal of manufacturers.

Breathing gas from the side

Fuel purchase

On-board weighing system of a silo vehicle for wood pellets

The fuel is offered in bags (15–20 kg) for manual filling, in 1–2 m³ large plastic bags ( big bags ) or loose. While bagged goods are particularly suitable for single ovens or small systems, the use of big bags requires appropriate suspension systems and lifting technology.

The delivery of loose wood pellets is mostly carried out by silo vehicles similar to the delivery of feed pellets . The pellets are blown in, except in the case of dust-tight bag silos, and in the case of storage rooms, air is usually extracted at the same time in order to avoid dust pollution. Typical delivery quantities for end consumers are 3–10 tons.

Storage and discharge

Daily storage container

The wood pellets are stored in bulk in a tank or storage room and fed to the burner by means of a conveyor system. The storage room must be dry, as the pellets react very hygroscopically to wall humidity or excessively high air humidity by crumbling during storage. In damp rooms, airtight tank systems must be used to ensure the pellet quality.

Compared to oil, wood pellets require about three times the storage volume. The demands on the storage room are technically lower because, unlike heating oil, pellets are not water-polluting substances.

The floor is built in the form of a funnel - usually made of wood - at the lower end of which the inlet of a screw or sampling probes for a fan can be found. Tapping points can be designed redundantly to prevent malfunctions. As an alternative to the storage room with a funnel bottom, there are prefabricated tanks made of fabric or sheet steel. If there is not enough space in the building, buried underground tanks or free-standing silos can be used.

Trickle , suction fan or screw systems can be used for charging . The choice depends primarily on the distance between the store and the boiler room; for distances over 2 m, multi-stage or flexible screw conveyors are usually necessary. Blower systems can be used flexibly and convey up to over 20 m. The discharge from the storage room or container is supported by a sloping container base or funnel outlet.

Burner systems with active discharge control supply themselves with the required fuel dose. Otherwise, a small buffer is also required, from which the burner uses.

Incorrect ventilation can cause carbon monoxide poisoning in the storage room .


Stock of pellet boilers in Austria
year number
Small combustion systems <100 kW, end of year

Pellets as pressed sawdust for energy use and pellet stoves were developed in the USA in the 1970s. Since the end of the 1970s, European boiler manufacturers, especially in Scandinavia and Austria, began developing pellet heating. The market for wood pellet heating in Germany did not develop until later, after the use of wood pellets in Germany was approved in 1997. Today Germany is the sales market for wood pellet heating with the highest turnover.

In Austria, according to industry reports, the proportion of pellet heating systems in new buildings is 35%, the number of pellet heating systems is increasing annually by more than 10%, currently (January 2013) it is just over 100,000 systems. With 12.6 pellet heating systems per 1,000 inhabitants, Austria has the highest density of pellet heating systems in Europe. Between 1999 and 2008, around 100,000 pellet systems were installed in Germany.

Economy and operating costs

The acquisition costs of a pellet system are higher than comparable gas and oil heating systems, but the operating costs - depending on the fuel and the current fuel price - are often cheaper in Central Europe than with fossil fuels . In the case of larger systems, the share of the investment costs compared to the operating costs is lower, so that cost savings start there after fewer years of operation than with small systems. Once again, there are other regenerative alternatives available for supplying heat to larger properties, with even lower operating costs than pellet heating systems, including wood chip heating systems or waste heat from biogas plants . The available financial subsidies are currently in favor of pellet heating.

See also: Building heating - to calculate the profitability and dimensioning of heating systems in general


  • In Germany , the Federal Ministry for Economic Affairs and Energy supports the installation of pellet heating systems as part of a market incentive program (MAP). The subsidy is 1400 € for pellet stoves with a water pocket (water-bearing stoves), 2400 € for pellet boilers without buffer storage (boiler output 5 to 66.6 kW) and 2900 € for pellet boilers with newly built buffer storage with at least 30 liters of content per kW max. Boiler output (here 5 to 80.5 kW). Additional bonuses are granted if simultaneously the installation of a recoverable thermal solar system ( bivalent heating ) and / or a plant for solar hot water takes place preparation or energy efficiency ( heat insulation ) of the heated object is brought to a specific standard. The installation of an exhaust gas heat exchanger to increase the efficiency and / or a particle filter to separate the fine dust contained in the exhaust gas is supported by additional innovation funding.
  • In Austria there are subsidies from the federal government, the federal states and individual municipalities for new central heating systems or the conversion to pellet heating.
  • In Switzerland wood pellet are also encouraged. This is regulated differently by canton.
  • In Belgium , the Walloon Region pays the following premiums per system in 2008–2009: € 1,750 up to 50 kW (+ € 35 per kW up to 100 kW), € 3,500 for 100 kW (+ € 18 per kW up to 500 kW), € 10,700 for 500 kW (+ € 8 per kW up to a maximum of € 15,000).


Wood pellets

Wood pellets (DIN plus) have a calorific value of 5 kWh / kg and an oil equivalent of 2.16 kg / l or 3.33 l / l OE. The energy content of one kilogram of pellets is equivalent to that of half a liter of heating oil , and in terms of volume (in bulk cubic meters ) a third of a liter.

Price development

The pellet market has seen strong increases in supply and demand in recent years. After an initially quite high price after the market launch at the end of the 1990s, a phase of relatively low prices around 3.5 cents / kWh in Germany from 2002 to 2005 and several months of high pellet prices of more than 5 cents / kWh in winter 2006/07 Manufacturers have significantly expanded their capacities due to supply bottlenecks, so that the retail value has fallen since 2007 to a level between approx. The minimum was reached in 2008 and the pellet price has been rising again since then. In 2018, prices between around 4.8 and 6.4 cents / kWh were achieved.

In comparison to natural gas, the price development in Austria can be objectively observed using the Austrian price indices for the products: In January 2013, the pellet price index was 139.91 compared to the base value in January 2006, i.e. the pellet price only rose by 1 40 times that of the Austrian gas price index of 143.75, i.e. H. the gas price rose in these 7 years by 1.44 times (to clarify the usual price fluctuations see gas price development ).

For the delivery of loose goods, a flat-rate blow-in charge of approx. € 30 net can be charged; If the delivery quantity is less than 3 tons, surcharges for small quantities are often due. The 15 kg bagged goods cost between 7 and 20% more than loose goods and surcharges are also charged for big bags (750 to 1000 kg).

Origin of raw materials and fuel alternatives

In addition to the sawdust originally used, wood assortments are increasingly being used for pellet production, which are also in demand by the paper and wood processing industries. In addition to wood chips, this also includes residual forest wood and trunk wood . The growth rates and total consumption of wood pellets strengthen the demand in the area of ​​lower wood qualities. The strategies for increasing the supply of raw materials include the use of whole trees , the increased use of fast-growing types of wood and the creation of short rotation plantations on agricultural land. While energy wood production on arable land can be beneficial from an environmental point of view, the intensification of the raw material yield in the forest can lead to conflicting goals with nature , environmental and / or soil protection . For the origin of the raw material, see also Holzpellet # review .

Alternative fuels for operating pellet heating systems are currently being developed and tested. In addition to straw pellets, residual value pellets (e.g. mill residues) and other pelleted biomass such as B. giant Chinese reed or rapeseed cake are possible raw materials. Non-woody biomass is used in pellet form for biomass thermal power stations , but is currently used for pellet heating i. d. Usually unsuitable, as these fuels require more effort both in terms of combustion technology (e.g. due to high silicon content and sintering ) and in terms of exhaust gas cleaning. Grain as fuel requires suitable systems for this ( grain incineration ) and is restricted in use according to the regulation on small and medium-sized combustion systems.

Environmental sustainability

Central aspects of the environmental compatibility of pellet heating are the origin of the raw materials, the pollutant emissions resulting from combustion and the climate balance .

Climate protection

As pellets are made from the renewable and therefore CO 2 -neutral raw material wood, their carbon footprint can be more favorable than that of fossil fuels. The amount of CO 2 that is released during combustion corresponds exactly to the amount of CO 2 that was incorporated into the wood as it grew. The manufacture, in particular the drying, should be carried out using as little fossil fuel as possible.

The CO 2 emissions of wood pellets are around 42 g / kWh, with heating oil the value is around 303 g / kWh. According to the Global Emission Model of Integrated Systems (GEMIS), the life cycle of wood pellets (including transport and material input) as a by- product of higher-quality wood use shows an expenditure of non-renewable energies of around 13% of the useful energy. Due to the subsidies, the demand for pellets has increased significantly. This results in side effects such as price increases and an increased use of forest wood instead of wood waste.

Energy security and regional value creation

The use of the biogenic energy carrier wood pellets reduces the dependency on fossil energy carriers. In addition, in contrast to fossil fuels, when using wood pellets, regionally produced fuels can be used. The expenditures for the fuel then remain largely in the own region and increase the added value there.


Sulfur dioxide (SO 2 )

Wood pellets according to DIN plus or ÖNORM M 7135 have a maximum sulfur content of 0.04  % by weight , which is between natural gas according to DVGW specification (max. 30 mg / m³ or 8 mg / kWh plus sulfur content from average odorization ) and light heating oil ( maximum 0.1 wt% according to § 10 of the 10th BImSchV ). According to the Global Emission Model of Integrated Systems (GEMIS), the release of SO 2 over the entire life cycle of wood pellets from waste wood recycling is around 0.53 g / kWh. Heating oil (condensing technology) and natural gas account for 0.73 g / kWh and 0.18 g / kWh, respectively.

Ozone pollution

The ozone pollution caused by the release of ozone precursors ( nitrogen oxides , carbon monoxide , methane and volatile organic compounds ) is shown in GEMIS to be 0.88 g / kWh for the combustion of wood pellets from waste wood recycling, roughly two times more than when burning Heating oil with condensing technology (0.41 g / kWh) or natural gas (0.35 g / kWh). Since the increased formation of photo-oxidants due to the intensive solar radiation required for this is a problem mainly in the summer months (" summer smog "), while space heating naturally predominantly works in winter, comparatively little problem potential is ascribed to this emission.

Fine dust

The fine dust emissions of modern pellet boilers are significantly better than those from chimneys, tiled stoves or log wood boilers, but due to the principle of the principle, they exceed the emissions of comparable oil and gas burners many times over, especially when you include annual and daily heating changes.

In normal operation, the fine dust emissions are around 8 mg per MJ of heat, corresponding to 29 mg / kWh. In the meantime, there are pellet heating systems which, thanks to optimized combustion, fall below the fine dust emission values ​​and can also be installed in areas where stricter regulations apply. These systems work with condensing technology and have a relatively low-dust and soot-free exhaust gas (approx. 4 mg fine dust per MJ = 14.5 mg / kWh). Otherwise, the so-called partition burner (cross-insert burner with a special design of the air control) also achieves this low particulate matter value, although it works without condensing technology. The comparative value for fine dust emissions is around 150 mg / MJ (= 544 mg / kWh) for single stoves ( open fireplace , tiled stove ), around 90 mg / MJ (= 326 mg / kWh) for old log boilers, and around around modern wood gasification boilers 36 mg / Nm³ and with oil heating at 3 mg / MJ (= 11 mg / kWh).

The particulate matter emission values ​​mentioned relate to heating systems in continuous operation, an operating mode which leads to optimal values ​​with regard to emissions from pellet heating systems. Different behavior at partial load or in intermittent operation is not taken into account. A study by Struschka et al. from 2010 included these dynamic operating states. In the typical winter daily profile, a pellet boiler emitted 34 mg / MJHi fine dust, in contrast to the measured 21 mg / MJHi in continuous operation. The comparative values ​​for the oil burner were 0.026 to 0.046 mg / MJHi in the experiment (there were hardly any differences between continuous and intermittent operation). The study was commissioned by the Institute for Economic Oil Heating (IWO).


According to a study by the Austrian Society for Environment and Technology , which compares the cost of capital and running costs of heating systems under different heat consumption and energy price scenarios , “pellet heating systems do not ' pay off ' in comparison to log boilers. With lower or constant energy prices compared to fossil heating systems "only for significantly above average heat consumers", the more energy (especially in low-energy houses ) is saved through thermal insulation , the more the high system construction costs affect the total price over the service life and pellet heating systems then " assuming constant low energy prices etc. U. even represents the most expensive heating system ”. Pellet boilers would bring the lowest total costs in addition to log heating if the heating costs of fossil fuels continue to rise.

The European Environment Agency warns that increased combustion of biomass in private heating systems could worsen air quality, as wood smoke contains fine dust and soot and can contain toxic substances such as dioxins . From around 2000 to 2005, reductions in fine dust by means of lower-emission forms of wood combustion were nullified by an increase in wood combustion systems. According to an investigation by the Federal Environment Agency, the fine dust emissions from wood combustion systems exceeded the emissions from road traffic (incineration only) by 22,700 tons.

(Further criticism can be found in the article wood pellets .)

See also


Web links

Individual evidence

  1. Fuel, especially in the form of pellets, from olive residues and combustible organic and / or inorganic substances , patent specification
  3. Manufacturer of pellets made from tropical wood, coconut shells and oil palm kernels
  4. ^ A b Karl Furtner, Herbert Haneder: Heating oil boiler - market shares are dwindling . Austria is the European champion in heating with pellets. In: Chamber of Agriculture of Lower Austria (ed.): Ökoenergie . 122000th edition. No. 90 . Vienna February 2013, p. 2-3 .
  5. FNR : Market overview of pellet central heating systems and pellet stoves. P. 7
  6. Renewable Energies - Federal Office of Economics and Export Control,
  7. German Energy Wood and Pellet Association DEPV: energy prices in Germany. Retrieved January 24, 2010.
  8. Central Agricultural Raw Material Marketing and Energy Network. Retrieved October 11, 2019.
  9. The pellet price index PPI 06 ( Memento of the original from December 31, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. and associated PDF file  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / 2Template: Dead Link /  
  10. Austian Energy Agency: Austrian Gas Price Index ÖGPI and associated PDF file  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link /  
  11. See graphic gas prices in Germany in cents / kWh from 1991 ( memento of the original from January 18, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. ( Flash ; 369 kB) @1@ 2Template: Webachiv / IABot /
  12. NABU, 2008: ENERGY WOOD PRODUCTION IN AGRICULTURE. Opportunities and risks from the point of view of nature and environmental protection. (pdf; 1.3 MB)
  13. Environmentally friendly heating with wood , Der BUND  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link /  
  14. a b c Global Emissions Model of Integrated Systems, result data from GEMIS 4.2 ( Memento of the original from February 13, 2007 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , November 2004 @1@ 2Template: Webachiv / IABot /
  15. Consequences of the pellet boom: "The forest floor is bleeding from" Spiegel , November 2009
  16. to: Natural gas - the environmentally friendly fossil fuel . Federal Association of the German Gas and Water Management (BGW). o. date. Image 4, p. 17. ( PDF 1.5 MB)
  18. Fine dust, what do our pellet heating systems do? ( Memento from September 28, 2007 in the Internet Archive ) (B.energie AG, Switzerland)
  19. Michael Struschka, Maike Springmann, Julia Goy, Christian Schäfer: Fine dust emissions from oil, gas and pellet boilers ,
  20. Michael Cerveny, Thomas Sturm: Full cost comparison of heating systems for single-family houses - comparison of the life cycle costs of heating oil, natural gas, pellet and log heating for old single-family houses in nine scenarios. Austrian Society for Environment and Technology, Vienna 2011, p. 9. Accessed June 6, 2019 (PDF).
  21. Air quality in Europe - 2017 report (PDF); European Environment Society; EEA Report No 13/2017, ISSN  1977-8449 .
  22. Timothy Spence: Doubts cast on biofuels' air quality claims ; at
  23. The side effects of comfort: fine dust from the fireplace and wood stove. Background paper of the Federal Environment Agency, March 2006.