What is the use of soot

soot

Lexicon> Letter R> Soot

Definition: a black, mostly powdery material, which mainly consists of carbon, often arises from combustion processes

English: soot, grime

Categories: Basic Concepts, Ecology and Environmental Technology

Author: Dr. Rüdiger Paschotta

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Original creation: December 29, 2014; last change: 07/30/2020

URL: https://www.energie-lexikon.info/russ.html

Soot is a black, powdery, sometimes hard or greasy material, which is often produced when carbon-containing materials are burned, especially when there is a lack of oxygen and when the combustion temperatures are too low. From a chemical point of view, it is mostly elemental carbon in the form of graphite. However, depending on the formation of the soot, various other substances can also be included, for example various hydrocarbons (e.g. unburned or chemically slightly modified components of a fuel) and mineral substances such as sulfates.

In contrast to coal, for example, soot consists of relatively small particles, the size of which can be in the micrometer range or even in the range of a few nanometers, depending on the formation conditions. The surface is correspondingly large.

Depending on the distribution of the particle sizes and the contamination with other substances, a distinction is made between different types of soot, for example dust or flake soot, smear soot and glossy soot (hard soot). In other cases, types of soot are differentiated according to their origin, for example diesel soot or wood soot. Various properties of soot, for example density, hardness, inflammable cold and toxicity, can vary greatly depending on the starting materials and the conditions during formation.

Small soot particles are often contained in exhaust gases. With a correspondingly high concentration, the exhaust gas then forms a gray or even deep black smoke. The soot particles (especially the larger ones) can be deposited in the area and form black traces there.

Unwanted soot formation in combustion processes

In combustion processes that generate heat, the formation of soot is usually very undesirable. It not only leads to a loss of heat due to incomplete combustion, but above all to relatively stubborn soiling of system parts, which then hinders the transfer of heat from the combustion gases to the heat transfer medium (e.g. the boiler water). As a result, the exhaust gas temperature of the system can increase, so that the exhaust gas losses are also increased accordingly.

The formation of soot is particularly strongly promoted under the following two circumstances:

Lack of oxygen is a common cause of soot formation.
  • Wherever carbon-containing fuels are burned in a lack of oxygen, soot is often produced in addition to the toxic carbon monoxide. However, this soot can still burn in the flame a little later if it gets into oxygen-rich regions that are still sufficiently hot for a sufficiently high reaction rate. However, if the total amount of combustion air is insufficient (that is, the combustion air ratio becomes very small), soot formation is often practically unavoidable.
A flame should never be exposed directly to relatively cool parts e.g. B. come into contact with a boiler!
  • Combustion temperatures that are too low are also an important factor. These occur, for example, when a flame comes too close to the relatively cool wall of a boiler. The flame gases are then rapidly cooled by contact with this wall, so that the further oxidation (combustion) of the soot particles that have formed up to that point is very much slowed down or stopped entirely. Even if a fuel (e.g. wood) contains too much moisture (water) or has a very low specific calorific value for other reasons, the combustion temperature can be lowered too far.

In detail, however, the microscopic processes involved in the formation of soot are very complex. It is therefore not trivial to design a burner, for example, in such a way that the formation of soot is minimized.

The deposition of thick layers of soot in a chimney can be very dangerous!

When a thick layer of soot is deposited in a chimney (often in the form of Black carbon), this creates a serious fire hazard. This soot can be ignited under certain conditions, and this fire in the chimney can develop extremely high temperatures, which can occasionally cause entire buildings to burn down. If you try to extinguish such a chimney fire with water, the chimney can burst. For these reasons, chimneys must be inspected regularly, and if there is a lot of soot (e.g. as a result of using damp firewood) it is essential to remove most of the soot. Sometimes this is only possible through a controlled burning off of the soot layer under the supervision of the fire brigade.

Some examples of systems that often have a certain amount of soot build-up:

  • Boilers that are operated with heating oil usually produce a significant amount of soot in the course of a year of operation, although the design of the oil burner used and its adaptation to the boiler are very important. The quality of the heating oil also plays a certain role. In addition, frequent burner starts z. B. unfavorable due to cycle operation with inadequate dimensioning. The resulting soot can significantly increase exhaust gas losses and therefore requires regular removal. It is also possible that the chimney will become sooty over time.
  • Significant amounts of soot can also occur in wood boilers, which are also strongly dependent on the combustion conditions. Well-dried firewood produces far less soot than freshly felled firewood, which still contains a lot of moisture.
Full throttle at very low speeds causes the diesel engine to generate a lot of soot, so that a particle filter has to be regenerated quickly.
  • In diesel engines, after the fuel injection, a substantial amount of small soot particles are always formed within the flame, which ideally burn completely (i.e. are oxidized) before the temperature drops too much. However, under unfavorable operating conditions (for example full throttle at very low speeds or during the warm-up phase), considerable amounts of soot can occur in the exhaust gas. The tendency to form soot also depends on the fuel quality; it is increased if the diesel fuel contains a high amount of aromatics or even polycyclic hydrocarbons (PAH). It should be noted that particle emissions are not only caused by unburned carbon, but also, for example, by sulphate particles from the sulfur content of diesel fuel. Such particles can also form fine dust, but are not to be referred to as soot. Their formation has nothing to do with a lack of oxygen. The majority of the resulting particles - both soot particles and others - can be removed from the exhaust gas with a modern soot particle filter. Since the filter becomes clogged over time, it must be regenerated regularly, for example by briefly increasing the exhaust gas temperature so that the soot can be oxidized.
Direct injection gasoline engines produce certain soot emissions - unlike conventional gasoline engines.
  • While gasoline engines, which are mostly powered by gasoline, generally emit practically no soot particles at all, certain amounts of very small soot particles (with diameters in the nanometer range) can occur, especially in modern engines with direct injection. In contrast to intake manifold injection, the combustion takes place here in an area that still contains droplets of liquid fuel, and oxygen deficiency can occur in the immediate vicinity of these droplets. Although the resulting amounts of soot are very small in terms of mass, the number of tiny particles can be considerable, and precisely this aspect is of great importance for the health effect (see below).

The formation of soot particles in flames is not always completely undesirable. For example, a candle flame glows mainly because tiny, hot soot particles emit light from inside it. On the other hand, a flame that is very poor in soot particles (for example when burning methanol) hardly generates any light. Ideally, however, the soot particles burn before they leave the flame.

In the case of an oil burner, the formation of soot can be significantly reduced by using a significant excess of air (i.e. a high combustion air ratio); the flame then burns blue instead of yellow. Unfortunately, this increases exhaust gas losses and the formation of nitrogen oxides can also increase. It is therefore not advisable to operate an oil burner developed as a “yellow burner” in this way. Please note that the soot particles that glow in the flame will be burned as much as possible later if the setting is correct.

Production of carbon black for industrial applications

Carbon black is produced on an industrial scale for a variety of applications - for example, the manufacture of rubber for car tires and many other products. Mostly petroleum products are used for this, namely the relatively heavy residues (coal tar) that arise in petroleum refineries. In addition, heating oil or natural gas is used as fuel. Depending on the desired properties of the carbon black, the details of the manufacturing process must be adapted.

Health effects of soot

Health effects occur in particular with soot that gets into the air. Here, harmful effects can occur in two very different ways:

  • Depending on the conditions in which it is formed, soot can be loaded with various very toxic and sometimes carcinogenic substances, such as polycyclic aromatics (PAH).
  • If the soot particles are very small (with a diameter of less than 10 micrometers (10 μm) or sometimes well below a micrometer), we speak of fine dust. In this case additional effects can occur which have more to do with the particle size than with the exact chemical composition. Fine dust can consist of very different substances, but soot is often an essential component of fine dust in exhaust gases.

Today it must be assumed that diesel soot, for example, is extremely hazardous to health and can even cause lung cancer. The World Health Organization (WHO) classified diesel soot as definitely carcinogenic in 2012. Smaller particles are particularly dangerous in this regard, as they are not filtered out in the nose and can sometimes penetrate very deeply into the lungs. Soot nanoparticles can also get into the bloodstream - possibly even when they come into contact with the skin.

The mass of the soot can be determined relatively easily in terms of measurement technology. However, it is now clear that the mass of the soot is by no means a good criterion for assessing the health risk. (The exhaust gas limit values ​​related to this must be revised in the next few years.) The number of particles, which would of course be much more difficult to determine, seems to be much more informative. Even if a perfect measuring method could determine the exact distribution of the soot particles in different size classes, their degree of danger could not yet be reliably assessed, since corresponding scientific studies are very difficult. Hopefully further research will improve this knowledge significantly.

Climate harm of soot

It has been known for some years that very small soot particles that remain in the atmosphere for a long time are to a considerable extent harmful to the climate, while larger particles are not relevant in this regard because they sink quickly. The harmfulness to the climate per kilogram can be hundreds of thousands of times higher than that of carbon dioxide, although this factor depends crucially on the particle size. Because of their soot emissions, vehicles with diesel engines can be far more damaging to the climate than petrol-powered vehicles, as long as these emissions are not suppressed as far as possible by highly effective soot particle filters.

Questions and comments from readers

02.08.2017

I notice that people only write about the combustion states (in cars) in the engine. I mean, for example, that such conditions occur when accelerating, or that too little oxygen is involved in the combustion. Isn't it also a possibility that diesel fuel ages over time after it leaves the refinery? And then tends to burn worse?

When I haven't used my chainsaw for a long time, it started badly; but with fresh fuel it starts up again quickly. Linking creates fuel molecules that are too large and burn uncleanly. You have to be able to do something about it, or can't you prevent it?

Answer from the author:

First of all, the chainsaw will probably not run on diesel; normally you have a two-stroke gasoline engine with mixture lubrication in it. I rather see the problem that gasoline and lubricating oil could separate.

The fuel molecules cannot simply chain together at room temperature to form longer molecules. This would require so-called activation energy, for which there is not enough at normal storage temperatures - at least not within the few weeks or at most months between fuel production and consumption. Some of the fuels also contain additives that improve shelf life.

In short, I don't think aged fuels are a significant problem.

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See also: fine dust, soot particle filter, diesel engine, exhaust gas, exhaust gas loss
as well as other articles in the categories Basic Concepts, Ecology and Environmental Technology