See also: Ringelmann Smoke Chart
This page has technical information about smoke from domestic solid fuel use. There is information about which fuels smoke and which don't on the Solid Fuels page, about how to burn wood with low smoke under Wood Fuel and solutions to some smoke problems with the Fireplace Doctor.
What smoke is, and how to spot it
Smoke is a suspension of tiny solid particles in a gas. In the context of solid fuel heating it is mainly particles of fuel which have not been completely burned, which are usually carried upwards by the The Chimney Effect and emitted to the atmosphere. It is important to differentiate between 'white' and 'dark' smoke.
White 'smoke' is not strictly smoke at all but mostly harmless tiny water droplets, generated when steam released from the fuel condenses in cool air. White smoke is visible against a dark background, or in darkness when illuminated, but will be difficult to see against a light sky background.
Dark smoke is mainly black carbon, the result of incomplete combustion. Generally, dark smoke is easily visible against a light sky but difficult to see at night or against a dark background such as a slate roof.
How smoke is generated, and how it dissapears
When solid fuels become hot, volatile matter inside the fuel element boils, giving rise to a myriad of sudden tiny explosions - occasionally visible to the naked eye - which shatter the solid material around them, throwing off tiny particles of fuel. These particles have significant velocity which often prevents them becoming either hot enough to fully burn, givig rise to smoke. Fuels with more volatile matter available to boil - notably bituminous coal and damp wood - will emit very, very significantly more smoke.
Nonetheless, smoke particles are fuel and if can be arranged to keep them at a high TEMPERATURE for enough TIME and engender sufficient TURBULENCE to get them to mix with sufficuient oxygen it is possible to get the smoke to burn away to, largely harmless, gases and water vapour. While the design of small domestic appliances can help a great deal with this, smoke emission is ultimately equally dependant on the method of operation. Depending on the choice and placement of fuel, the setting of controls etc, pretty much any small hand-fired stove can be made to operate either smokelessley or to emit smoke.
How dangerous is smoke?
Dark smoke can be very dangerous because small particles are absorbed into the lungs, especially the very tiny ones below 10 microns in size (known as 'pm10s'). Long-term exposure can contribute to wide range of ailments including heart disease and cancers. Smoke is thought by the World Health Organisation to be one of the main causes of premature death in developing countries (see: www.who.int) where indoor smoke, largely from poorly performing solid fuel stoves may contribute to around 1.6 million deaths each year, mostly of children under five. In advanced countries with good appliances and effective chimneys, smoke is rarely encountered indoors, yet even outdoor smoke can still sometimes be a real health problem (see: www.imj.ie).
A domestic kitchen in rural Nepal, 2013
Image: Glyn Hughes/Soliftec
The variability of smoke
There is no agreed definition of smoke, so there is no definitive method of measuring it. There is no consenus on how small or large a particle must be before it ceases to 'smoke', nor is there any agreement as to whether soluble matter should be included. There is no agreed method of generating smoke, and no standardised form of smoke to measure others against.
The smoke emission of small hand-fired appliances appears to be wildly variable, afftected by even tiny alterations in, say, the position of logs, and hugely by stove settings, flue draught, fuel moisture, fuel sizes and shape. Prof. O. Skreiberg's 1996 'Round robin test' of smoke measurements on the same stove at 9 different laboratories concluded that emission testing "gives a random outcome." The Comparison of Emissions from Wood Combustion by Jürgen Orasche and others of 2013 found that, on the same stove, particle emission could vary by a factor of 8, and total toxins by 80. Our Modern Stove Matrix table shows that, even directly sucessive firings of the same stove under precisely the same conditions and settings, by the same technician using seemingly identical fuel from the same batch can still show a variation in smoke emission by a factor of 4. Measurement of smoke from appliances is never more than a 'good guess', at best.
Measurement of Smoke
Smoke from heating appliances is commonly measured in one of the following ways:
In-line capture. A smoke sample is simply sucked through a filter which is weighed before and after the test and the mass of smoke found. This is the simplest and probably the most accurate method, but can only be used where the smoke concentration is slight, as the filter can quickly become blocked.
The ASTM smoke pump is a simple and widely used method of in-line capture where a measured volume of smoke is pulled through a filter paper and the dark spot so formed is compared with a standard.
Filter/dilution tunnel. A smoke sample is drawn through a tube where it is diluted with air, the resulting smoke/air mixture is then pulled through a filter and weighed. This is the internationally recognized method of measuring smoke from combustion.
Electrostatic precipitation. The smoke is passed through an array of metal tubes which contain suspended wires. A (huge) electrical potential is applied across the tubes and wires so that the smoke particles become charged and are attracted to the sides of the tubes. This method can over-read by capturing harmless condensates, or under-read due to the insulating effect of the smoke. However, it is the necessary method for assessing volumes of smoke too great to be forced through a filter, i.e., from bituminous coal.
Ringelmann scale. A measure of smoke color. Invented by Professor Maximilian Ringelmann in Paris in 1888, it is essentially a card with squares of black, white and shades of gray which is held up and the comparative grayness of the smoke judged. Highly dependent on light conditions and the skill of the observer it allocates a grayness number from 0 (white) to 5 (black) which has only a passing relationship to the actual quantity of smoke. Nonetheless, the simplicity of the Ringelmann scale means that it has been adopted as a standard in many countries.
Optical scattering. A light beam is passed through the smoke. A light detector is situated at an angle to the light source, typically at 90°, so that it receives only light reflected from passing particles. A measurement is made of the light received which will be higher as the concentration of smoke particles becomes higher.
Optical obscuration. A light beam is passed through the smoke and a detector opposite measures the light. The more smoke particles are present between the two, the less light will be measured.
Combined optical methods. There are various proprietary optical smoke measurement devices such as the 'nephelometer' or the 'aethalometer' which use several different optical methods, including more than one wavelength of light, inside a single instrument and apply an algorithm to give a good estimate of smoke. It has been claimed that these devices can differentiate types of smoke and so their probable source can be inferred, though this is disputed. See http://www.sciencedirect.com/science/article/pii/S1352231013006353
Inference from carbon monoxide. Smoke is incompletely burned fuel, carbon monoxide is incompletely burned carbon, therefore it has long been assumed that measurement of CO in flue gas (a cheap, simple and very accurate procedure) will provide a good indication of the levels of smoke. Indeed, several jurisdictions use CO measurement as the basis of smoke control. However it is far from clear how accurate the correspondence is.
How much smoke is there?
The variability of smoke emission and the imprecision of its measurement mean that it is very difficult to be certain how much smoke is emitted where and from what. For the same reasons it is often easy to manipulate smoke data in order to present a particular view and it is necessary to be very cautious about generalised smoke data, even from apparently reputable sources.
Assumes: Heaters, 5kW output, Vehicles, Urban cycle, 30km/hr emitting exhaust + other PMs at 0.06mg per km per kg vehicle mass
Sources of particles in the UK, DEFRA, https://uk-air.defra.gov.uk/assets/documents/reports/aqeg/ch4.pdf
Non-exhaust PM emissions from electric vehicles, Timmers & Achten 2016, http://www.sciencedirect.com/science/article/pii/S135223101630187X
In the United Kingdom the Clean Air Act gives local authorities the power to designate 'Smoke Control Areas', known as 'smokeless zones' where the emission of dark smoke from chimneys is forbidden, and gives the Secretary of State (of DEFRA) the power to exempt certain fuels and appliances from the need to conform to smoke control.
General information: http://smokecontrol.defra.gov.uk/
Historical background: http://en.wikipedia.org/wiki/Clean_Air_Act_1956
UK Air Quality Archive http://www.airquality.co.uk/archive/index.php
National Atmospheric Emissions Database http://www.naei.org.uk/
UK Smoke Control Areas http://www.uksmokecontrolareas.co.uk/locations.php
UK Smoke Control Areas - Fuels http://www.uksmokecontrolareas.co.uk/fuels.php?country=a
UK Smoke Control Areas - Exempt Fireplaces http://www.uksmokecontrolareas.co.uk/appliances.php?country=a
Clean Air Act 1993 http://www.opsi.gov.uk/ACTS/acts1993/plain/ukpga_19930011_en
Republic of Ireland
Air Pollution Act, 1987 http://www.irishstatutebook.ie/1987/en/act/pub/0006/index.html
Bituminous ‘smoky’ coal ban and information on regulations and ban areas: http://www.environ.ie/en/Environment/Atmosphere/AirQuality/SmokyCoalBan/
The Chimney Effect and emitted to the atmosphere.
EXAMPLE DATA: Graph shows av. smoke; the blue line is 'domestic' (levanoglucosan) smoke from wood, coal and cooking
Equivalent black carbon concentrations apportioned between fossil fuel and wood burning at Kensington. Concentrations were averaged by hour, day of week, and month using Openair (Carslaw and Ropkins, 2012
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