Why is carbon monoxide so flammable

Carbon monoxide

Lexicon> Letter K> Carbon Monoxide

Definition: a poisonous gas that can be produced during combustion processes

Alternative terms: carbon monoxide, carbon monoxide, carbon monoxide

More general term: air pollutant

Molecular formula: CO

English: carbon monoxide

Categories: energy sources, ecology and environmental technology

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: October 24, 2010; last change: 01/09/2021

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

Carbon monoxide (also Carbon monoxide or Carbon monoxide or Carbon monoxide) (CO) is a flammable and very poisonous gas that can be produced when fuels and fuels containing carbon are not completely burned - especially when they are burned in a lack of oxygen (i.e. if the combustion air ratio is too small). In contrast to carbon dioxide (CO2) in the carbon monoxide molecule, the carbon atom is connected to only one oxygen atom (instead of two).

With the addition of oxygen, carbon monoxide can be burned further to form carbon dioxide, which is not toxic in moderate concentrations. The reaction equation is 2 CO + O2 → 2 CO2. Since this creates heat, carbon monoxide (in concentrated form or as a proportion of synthesis gas) can also be used as fuel (energy carrier). The calorific value of CO (11.5 MJ / m3) is low - for example, over three times smaller than that of methane.

Carbon monoxide molecules are about the same weight as nitrogen molecules (N.2), so that the density of this gas corresponds almost exactly to that of air. Because of this, carbon monoxide released into the atmosphere neither rises (like natural gas), nor does it collect on the ground.

Creation, avoidance and use of carbon monoxide

Carbon monoxide is often produced in a combustion in the absence of oxygen.

Most carbonaceous fuels and fuels end up burning mainly to carbon dioxide (CO2) and only traces of carbon monoxide (CO) if sufficient oxygen is supplied during combustion. (Substantial amounts of carbon monoxide are often produced in intermediate stages of the combustion process, but these are oxidized to carbon dioxide in later phases.) However, if there is a lack of oxygen, considerable amounts of carbon monoxide can remain in the exhaust gas. Even if there was enough oxygen in total, z. B. in wood fires there are places with a lack of oxygen where carbon monoxide is produced. However, this can lead to CO2 burned if it is caught in another part of the flame where additional oxygen is present and the temperature is not too low.

Combustion engines generate significantly more CO than boilers - especially when idling.

In general, combustion in internal combustion engines (especially in Otto engines) is much more delicate than even combustion, e.g. B. in boilers. The highest CO emissions (as well as emissions of unburned hydrocarbons) occur in gasoline engines when idling directly after a cold start. (In the past, a so-called choke was often used in the warm-up phase of gasoline engines, which could significantly increase the CO emissions by enriching the mixture.) B. for heating oil or natural gas, on the other hand, almost no CO emissions can occur, unless the air supply is set too low. With boilers for solid fuels such as coal or wood, it is much more difficult to minimize CO emissions, especially if at the same time a high excess of air is to be avoided for energy reasons. Modern constructions z. B. wood boilers, in which the combustion air is supplied well dosed with a fan, are in this regard much better than non-optimized boilers.

Especially in boilers with a modulating burner, the residual oxygen content in the flue gas is often monitored using a lambda probe in order to always set the combustion air ratio correctly. This reliably keeps the CO formation at a very low level without using an unnecessarily high excess of air. The control electronics used should also be able to reliably detect defects in components that can lead to massive increases in CO emissions, so that the system can then be switched off automatically. Older devices such as gas boilers often do not yet have this kind of technology and, in the event of defects, can generate very high CO emissions, which under certain circumstances can also lead to life-threatening CO levels in rooms.

A catalytic converter can greatly reduce CO emissions - but only if the temperature is high enough and residual oxygen is present.

Unwanted carbon monoxide emissions are often reduced with the help of catalytic converters, especially in internal combustion engines, in which CO is combined with additional oxygen to form CO2 is oxidized. However, this process only takes place if, on the one hand, there is enough residual oxygen in the exhaust gas (i.e. not in the case of full load enrichment) and, on the other hand, the catalytic converter is sufficiently hot. For this reason, auto catalytic converters are unfortunately still ineffective immediately after a cold start - when the engine CO emissions are particularly high - and this often also applies when engines are operated at full throttle for other reasons. Despite these restrictions, the CO emissions of today's cars are much lower than what was allowed in 1990 and only occurs in classic cars today due to the increasingly stricter emission standards.

A particularly large amount of carbon monoxide can also be formed in the embers, which can arise, for example, in a grill or after extensive burn-up in a wood-burning oven. It is therefore very dangerous to operate a charcoal grill in a garage with the door closed, for example. The fact that such devices heated with charcoal are sometimes even advertised as “indoor grills” is incomprehensible in view of the massive risk situation; occasionally there are even deaths.

Small motorized devices can have extremely high CO emissions!

Extremely high CO emissions often occur with small motorized devices (e.g. lawn mowers, chainsaws and leaf blowers). In particular for hand-held tools such as leaf blowers, where the use of four-stroke engines and catalytic converters is difficult, very high CO emission limits have so far applied, for example of 805 g / kWh for engines with a displacement of less than 50 cm3. With such values, a car engine could by far not comply with today's exhaust emission limits (e.g. Euro 5: 1 g / km): For every kilometer driven, such an engine generates roughly 0.1 kWh of drive energy, which corresponds to emissions of 805 g / kWh would correspond to an amount of CO of approx. 80 g / km; this would be 80 times above the Euro 5 limit. Small appliances generate much lower amounts of exhaust gas than cars, but locally lead to severe impairment of the breathing air quality, for example for workers who have to stand in the exhaust plumes of such devices.

CO also turns into CO in the atmosphere2 oxidized. Although this process is slow, it still prevents CO from accumulating in the atmosphere over the long term. Typical atmospheric CO concentrations (away from emitters) are in the order of 0.1 ppm, which is completely harmless to health.

Carbon monoxide is specifically produced in certain industrial processes, for example in coal gasification. The town gas produced with the help of the water gas reaction, which contains hydrogen and various other gases in addition to the combustible carbon monoxide, is hardly used any more because of its high toxicity. Also Generator gas, which is produced by burning coke in a lack of oxygen, is usually not used as town gas. Usually, carbon monoxide is processed directly into other substances, for example methanol, formic acid or acetic acid, or it is burned into carbon dioxide for the purpose of generating heat. For example, synthesis gas containing CO from coal gasification can be converted into electricity in a combined gas and steam process, which enables more efficient and cleaner use of coal compared to conventional coal-fired power plants.

Research is being carried out into processes with which carbon monoxide could be produced from carbon dioxide using electrical energy in the future. This could look like a plasma in carbon dioxide is generated by an electrical gas discharge by converting part of the gas into carbon monoxide and oxygen; Efficiencies of up to approx. 60% have already been demonstrated and could become even higher in the future. Ultimately, from the carbon monoxide z. B. produce climate-neutral chemical raw materials or fuels. This would be a type of power to gas that is not based on electrolysis and therefore avoids its disadvantages, which is why it may also be possible to implement more cheaply.

Toxicity and harmfulness to health

Like carbon dioxide, carbon monoxide is colorless and odorless, but in contrast to carbon monoxide it is very toxic. Its toxic effect is due to the fact that it is much stronger than oxygen (O2) binds to the hemoglobin of the red blood cells; as these can then no longer transport oxygen, there is a lack of oxygen. Concentrations of carbon monoxide in the breathing air from 100 ppm (0.01 percent by volume) are already considered to be hazardous to health in the event of long-term exposure. Such values ​​are often reached in congested urban areas. An acutely fatal effect through suffocation within a few minutes occurs at concentrations in the order of magnitude of 20,000 ppm (2 percent by volume). (An inhaled amount of CO in the order of magnitude of one gram is sufficient to kill a person.) Such concentrations can arise quickly, for example, when a car engine is operated in a small closed space (such as a garage) (especially when the catalytic converter is still cold) ). Significant CO concentrations also occur in tobacco smoke, which is why the oxygen supply is significantly reduced in heavy smokers.

If limited amounts of CO are inhaled (e.g. in polluted city air), it then takes a few hours in fresh air until the CO is largely excreted again. During this time, the physical and mental performance can be somewhat reduced. Chronic exposure can also result in various chronic health problems, such as heart damage and depression.

For example, where there is a risk of CO poisoning in the event of defects, a CO alarm should be used that warns of dangers in good time.

In critical environments such as B. In underground garages, the CO concentration in the air should be automatically monitored with gas sensors (CO detectors) in order to switch on a ventilation system (or to increase the air volume) or to trigger an alarm if a certain limit value is exceeded. CO alarms should also be used in households or basements in which gas boilers (especially older types), chimney stoves or open fireplaces are operated or where wood pellets are stored in order to detect dangerous situations in good time.

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See also: carbon dioxide, exhaust gas, town gas, synthesis gas, fuel, energy carriers, combustion, measurement methods for fuel consumption and exhaust gas values
as well as other articles in the categories of energy sources, ecology and environmental technology