The main condition for gas combustion is the presence of oxygen (and therefore air). Without the presence of air, gas combustion is impossible. In the process of gas combustion, a chemical reaction of the combination of oxygen in the air with carbon and hydrogen in the fuel takes place. The reaction occurs with the release of heat, light, as well as carbon dioxide and water vapor.
Depending on the amount of air involved in the process of combustion of gas, its complete or incomplete combustion occurs.
With sufficient air supply, complete combustion of the gas occurs, as a result of which its combustion products contain non-combustible gases: carbon dioxide CO2, nitrogen N2, water vapor H20. Most of all (by volume) in the combustion products of nitrogen - 69.3-74%.
For complete combustion of gas, it is also necessary that it mixes with air in certain (for each gas) quantities. The higher the calorific value of the gas, the more air is required. So, for burning 1 m3 of natural gas, about 10 m3 of air is required, artificial - about 5 m3, mixed - about 8.5 m3.
In case of insufficient air supply, incomplete combustion of gas or chemical underburning of combustible components occurs; combustible gases appear in the combustion products - carbon monoxide CO, methane CH4 and hydrogen H2
With incomplete combustion of gas, a long, smoky, luminous, opaque, yellow torch is observed.
Thus, a lack of air leads to incomplete combustion of the gas, and an excess of air leads to excessive cooling of the flame temperature. The ignition temperature of natural gas is 530 °C, coke - 640 °C, mixed - 600 °C. In addition, with a significant excess of air, incomplete combustion of the gas also occurs. In this case, the end of the torch is yellowish, not completely transparent, with a blurry bluish-green core; the flame is unstable and breaks away from the burner.
Rice. 1. Gas flame i - without preliminary mixing of gas with air; b -with partial prev. fiduciary mixing of gas with air; c - with preliminary complete mixing of gas with air; 1 - inner dark zone; 2 - smoky luminous cone; 3 - burning layer; 4 - combustion products
In the first case (Fig. 1a), the torch is long and consists of three zones. Pure gas burns in atmospheric air. In the first inner dark zone, the gas does not burn: it is not mixed with atmospheric oxygen and is not heated to the ignition temperature. In the second zone, the air enters in insufficient quantities: it is delayed by the burning layer, and therefore it cannot mix well with the gas. This is evidenced by the brightly luminous, light yellow smoky color of the flame. In the third zone, air enters in sufficient quantities, the oxygen of which mixes well with the gas, the gas burns in a bluish color.
With this method, gas and air are fed into the furnace separately. In the furnace, not only the combustion of the gas-air mixture takes place, but also the process of preparing the mixture. This method of gas combustion is widely used in industrial plants.
In the second case (Fig. 1.6), gas combustion is much better. As a result of partial preliminary mixing of gas with air, the prepared gas-air mixture enters the combustion zone. The flame becomes shorter, non-luminous, has two zones - internal and external.
The gas-air mixture in the inner zone does not burn, since it was not heated to the ignition temperature. In the outer zone, the gas-air mixture burns, while the temperature rises sharply in the upper part of the zone.
With partial mixing of gas with air, in this case, complete combustion of the gas occurs only with an additional supply of air to the torch. In the process of gas combustion, air is supplied twice: the first time - before entering the furnace (primary air), the second time - directly into the furnace (secondary air). This method of gas combustion is the basis for the construction of gas burners for household appliances and heating boilers.
In the third case, the torch is significantly shortened and the gas burns more completely, since the gas-air mixture was previously prepared. The completeness of gas combustion is evidenced by a short transparent blue torch (flameless combustion), which is used in infrared radiation devices for gas heating.
- Gas combustion process
General information. Another important source of internal pollution, a strong sensitizing factor for humans, is natural gas and its combustion products. Gas is a multicomponent system consisting of dozens of different compounds, including specially added ones (Table 1).
There is direct evidence that the use of appliances that burn natural gas (gas stoves and boilers) has an adverse effect on human health. In addition, individuals with increased sensitivity to environmental factors react inadequately to natural gas components and products of its combustion.
Natural gas in the home is a source of many different pollutants. These include compounds that are directly present in the gas (odorants, gaseous hydrocarbons, toxic organometallic complexes and radioactive gas radon), products of incomplete combustion (carbon monoxide, nitrogen dioxide, aerosol organic particles, polycyclic aromatic hydrocarbons and small amounts of volatile organic compounds). All of these components can affect the human body both by themselves and in combination with each other (synergistic effect).
Table 12.3
Composition of gaseous fuel
Odorants. Odorants are sulfur-containing organic aromatic compounds (mercaptans, thioethers and thio-aromatic compounds). They are added to natural gas in order to detect it in case of leaks. Although these compounds are present in very low, sub-threshold concentrations that are not considered toxic to most individuals, their odor can cause nausea and headaches in otherwise healthy individuals.
Clinical experience and epidemiological data indicate that chemically sensitive individuals react inappropriately to chemicals present even at subthreshold concentrations. Individuals with asthma often identify odor as a promoter (trigger) of asthmatic attacks.
Odorants include, for example, methanethiol. Methanethiol, also known as methylmercaptan (mercaptomethane, thiomethylalcohol), is a gaseous compound commonly used as an aromatic additive to natural gas. The malodor is experienced by most people at a concentration of 1 part per 140 million, but this compound can be detected at much lower concentrations by highly sensitive individuals.
Toxicological studies in animals have shown that 0.16% methanethiol, 3.3% ethanethiol, or 9.6% dimethyl sulfide can induce comatose states in 50% of rats exposed to these compounds for 15 minutes.
Another mercaptan, also used as an aromatic additive to natural gas, is mercaptoethanol (C2H6OS) also known as 2-thioethanol, ethyl mercaptan. Severe irritant to eyes and skin, capable of exerting a toxic effect through the skin. It is flammable and decomposes when heated to form highly toxic SOx fumes.
Mercaptans, being indoor air pollutants, contain sulfur and can capture elemental mercury. In high concentrations, mercaptans can cause impaired peripheral circulation and increased heart rate, can stimulate loss of consciousness, the development of cyanosis, or even death.
Aerosols. Combustion of natural gas results in the formation of fine organic particles (aerosols), including carcinogenic aromatic hydrocarbons, as well as some volatile organic compounds. DOS are suspected sensitizing agents that are capable of inducing, together with other components, the "sick building" syndrome, as well as multiple chemical sensitivity (MCS).
DOS also includes formaldehyde, which is formed in small quantities during the combustion of gas. The use of gas appliances in a home where sensitive individuals live increases exposure to these irritants, subsequently exacerbating the signs of illness and also promoting further sensitization.
Aerosols formed during the combustion of natural gas can become adsorption centers for a variety of chemical compounds present in the air. Thus, air pollutants can be concentrated in microvolumes, react with each other, especially when metals act as catalysts for reactions. The smaller the particle, the higher the concentration activity of such a process.
Moreover, water vapor generated during the combustion of natural gas is a transport link for aerosol particles and pollutants when they are transferred to the pulmonary alveoli.
During the combustion of natural gas, aerosols containing polycyclic aromatic hydrocarbons are also formed. They have adverse effects on the respiratory system and are known carcinogens. In addition, hydrocarbons can lead to chronic intoxication in susceptible people.
The formation of benzene, toluene, ethylbenzene and xylene when burning natural gas is also unfavorable to human health. Benzene is known to be carcinogenic at doses well below the threshold. Exposure to benzene has been correlated with an increased risk of cancer, especially leukemia. The sensitizing effects of benzene are not known.
organometallic compounds. Some natural gas components may contain high concentrations of toxic heavy metals, including lead, copper, mercury, silver, and arsenic. In all likelihood, these metals are present in natural gas in the form of organometallic complexes of the trimethylarsenite (CH3)3As type. The association with the organic matrix of these toxic metals makes them lipid soluble. This leads to a high level of absorption and a tendency to bioaccumulate in human adipose tissue. The high toxicity of tetramethylplumbite (CH3)4Pb and dimethylmercury (CH3)2Hg suggests an impact on human health, as the methylated compounds of these metals are more toxic than the metals themselves. Of particular danger are these compounds during lactation in women, since in this case there is a migration of lipids from the fat depots of the body.
Dimethylmercury (CH3)2Hg is a particularly dangerous organometallic compound due to its high lipophilicity. Methylmercury can be incorporated into the body through inhalation as well as through the skin. The absorption of this compound in the gastrointestinal tract is almost 100%. Mercury has a pronounced neurotoxic effect and the ability to influence the human reproductive function. Toxicology does not have data on safe levels of mercury for living organisms.
Organic arsenic compounds are also very toxic, especially when they are metabolically destroyed (metabolic activation), resulting in the formation of highly toxic inorganic forms.
Combustion products of natural gas. Nitrogen dioxide is able to act on the pulmonary system, which facilitates the development of allergic reactions to other substances, reduces lung function, susceptibility to infectious diseases of the lungs, potentiates bronchial asthma and other respiratory diseases. This is especially pronounced in children.
There is evidence that N02 produced by burning natural gas can induce:
- inflammation of the pulmonary system and a decrease in the vital function of the lungs;
- increased risk of asthma-like symptoms, including wheezing, shortness of breath and asthma attacks. This is especially common in women cooking on gas stoves, as well as in children;
- a decrease in resistance to bacterial lung diseases due to a decrease in the immunological mechanisms of lung protection;
- providing adverse effects in general on the immune system of humans and animals;
- impact as an adjuvant on the development of allergic reactions to other components;
- increased sensitivity and increased allergic response to side allergens.
The combustion products of natural gas contain a rather high concentration of hydrogen sulfide (H2S), which pollutes the environment. It is poisonous at concentrations lower than 50.ppm, and at concentrations of 0.1-0.2% it is fatal even with short exposure. Since the body has a mechanism to detoxify this compound, the toxicity of hydrogen sulfide is related more to the exposure concentration than to the duration of exposure.
Although hydrogen sulfide has a strong odor, continuous exposure to low concentrations leads to a loss of the sense of smell. This makes a toxic effect possible for people who may unknowingly be exposed to dangerous levels of this gas. Insignificant concentrations of it in the air of residential premises lead to irritation of the eyes, nasopharynx. Moderate levels cause headache, dizziness, as well as coughing and difficulty breathing. High levels lead to shock, convulsions, coma, which ends in death. Survivors of acute toxic exposure to hydrogen sulfide experience neurological dysfunctions such as amnesia, tremors, imbalance, and sometimes more severe brain damage.
The acute toxicity at relatively high concentrations of hydrogen sulfide is well known, however, unfortunately little information is available on chronic LOW-DOSE effects of this component.
Radon. Radon (222Rn) is also present in natural gas and can be transported through pipelines to gas stoves, which become sources of pollution. Since radon decays to lead (210Pb has a half-life of 3.8 days), this results in a thin layer of radioactive lead (on average 0.01 cm thick) that coats the interior surfaces of pipes and equipment. The formation of a layer of radioactive lead increases the background value of radioactivity by several thousand disintegrations per minute (over an area of 100 cm2). Removing it is very difficult and requires the replacement of pipes.
It should be borne in mind that simply turning off the gas equipment is not enough to remove the toxic effects and bring relief to chemically sensitive patients. Gas equipment must be completely removed from the premises, as even a non-working gas stove continues to release aromatic compounds that it has absorbed over the years of use.
The cumulative effects of natural gas, aromatic compounds, and combustion products on human health are not exactly known. It is assumed that the effects from several compounds may be multiplied, while the response from exposure to several pollutants may be greater than the sum of the individual effects.
Thus, the characteristics of natural gas that are of concern to human and animal health are:
- flammability and explosive nature;
- asphyxic properties;
- pollution by products of combustion of the indoor air;
- the presence of radioactive elements (radon);
- the content of highly toxic compounds in the combustion products;
- the presence of trace amounts of toxic metals;
- the content of toxic aromatic compounds added to natural gas (especially for people with multiple chemical sensitivities);
- the ability of gas components to sensitize.
Natural gas is the most widely used fuel today. Natural gas is called natural gas because it is extracted from the very bowels of the Earth.
The process of gas combustion is a chemical reaction in which natural gas interacts with oxygen contained in the air.
In gaseous fuel there is a combustible part and a non-combustible part.
The main combustible component of natural gas is methane - CH4. Its content in natural gas reaches 98%. Methane is odorless, tasteless and non-toxic. Its flammability limit is from 5 to 15%. It is these qualities that made it possible to use natural gas as one of the main types of fuel. The concentration of methane is more than 10% dangerous for life, so suffocation can occur due to lack of oxygen.
To detect a gas leak, the gas is subjected to odorization, in other words, a strong-smelling substance (ethyl mercaptan) is added. In this case, the gas can be detected already at a concentration of 1%.
In addition to methane, combustible gases such as propane, butane and ethane may be present in natural gas.
To ensure high-quality gas combustion, it is necessary to bring air into the combustion zone in sufficient quantities and achieve good mixing of gas with air. The ratio of 1: 10 is considered optimal. That is, ten parts of air fall on one part of the gas. In addition, it is necessary to create the desired temperature regime. In order for the gas to ignite, it must be heated to its ignition temperature and in the future the temperature should not fall below the ignition temperature.
It is necessary to organize the removal of combustion products into the atmosphere.
Complete combustion is achieved if there are no combustible substances in the combustion products released into the atmosphere. In this case, carbon and hydrogen combine together and form carbon dioxide and water vapor.
Visually, with complete combustion, the flame is light blue or bluish-violet.
In addition to these gases, nitrogen and the remaining oxygen enter the atmosphere with combustible gases. N 2 + O 2
If the combustion of gas is not complete, then combustible substances are emitted into the atmosphere - carbon monoxide, hydrogen, soot.
Incomplete combustion of gas occurs due to insufficient air. At the same time, tongues of soot appear visually in the flame.
The danger of incomplete combustion of gas is that carbon monoxide can cause poisoning of boiler room personnel. The content of CO in the air 0.01-0.02% can cause mild poisoning. Higher concentrations can lead to severe poisoning and death.
The resulting soot settles on the walls of the boilers, thereby worsening the transfer of heat to the coolant, which reduces the efficiency of the boiler house. Soot conducts heat 200 times worse than methane.
Theoretically, 9m3 of air is needed to burn 1m3 of gas. In real conditions, more air is needed.
That is, an excess amount of air is needed. This value, denoted alpha, shows how many times more air is consumed than theoretically necessary.
The alpha coefficient depends on the type of a particular burner and is usually prescribed in the burner passport or in accordance with the recommendations of the commissioning organization.
With an increase in the amount of excess air above the recommended one, heat losses increase. With a significant increase in the amount of air, flame separation can occur, creating an emergency. If the amount of air is less than recommended, then combustion will be incomplete, thereby creating a risk of poisoning the boiler room personnel.
For more accurate control of the quality of fuel combustion, there are devices - gas analyzers that measure the content of certain substances in the composition of exhaust gases.
Gas analyzers can be supplied with boilers. If they are not available, the relevant measurements are carried out by the commissioning organization using portable gas analyzers. A regime map is compiled in which the necessary control parameters are prescribed. By adhering to them, you can ensure the normal complete combustion of the fuel.
The main parameters for fuel combustion control are:
- the ratio of gas and air supplied to the burners.
- excess air ratio.
- crack in the furnace.
- Boiler efficiency factor.
At the same time, the efficiency of the boiler means the ratio of useful heat to the value of the total heat expended.
Composition of air
| Gas name | Chemical element | Content in the air |
| Nitrogen | N2 | 78 % |
| Oxygen | O2 | 21 % |
| Argon | Ar | 1 % |
| Carbon dioxide | CO2 | 0.03 % |
| Helium | He | less than 0.001% |
| Hydrogen | H2 | less than 0.001% |
| Neon | Ne | less than 0.001% |
| Methane | CH4 | less than 0.001% |
| Krypton | kr | less than 0.001% |
| Xenon | Xe | less than 0.001% |
Odorization
Combustible gases have no smell. For timely determination of their presence in the air, quick and accurate detection of leaks, gas is odorized (give a smell). Ethyl mercaptan (C 2 H 5 SH) is used for odorization. The odorization rate is 16 g of ethyl mercaptan per 1000 m 3 of gas, 8 g of ethyl mercaptan sulfur per 1000 m³. Odorization is carried out at gas distribution stations (GDS). If there is 1% natural gas in the air, its smell should be felt.
20% gas in a room causes asphyxiation
5-15% burst
0.15% carbon monoxide SO- poisoning; 0.5% CO = 30 min. breathe death; 1% carbon monoxide fatal.
Methane and other hydrocarbon gases are not poisonous, but inhaling them causes dizziness, and a significant amount in the air leads to suffocation due to lack of oxygen.
Combustion of fuel is complete and incomplete:
It takes 10m³ of air to burn 1m³ of gas.
Combustion of natural gas is a reaction in which the chemical energy of the fuel is converted into heat.
Burning can be complete or incomplete. Complete combustion occurs with sufficient oxygen.
With the complete combustion of gas, CO 2 (carbon dioxide), H 2 O is formed
(water). Incomplete combustion of gas results in loss of heat. Lack of oxygen O 2 oxidant.
Products of incomplete combustion of CO - carbon monoxide, toxic effects, C carbon, soot.
Incomplete combustion is an unsatisfactory mixing of gas with air, excessive cooling of the flame until the combustion reaction is completed.
The combustion reaction of the main components of natural gas:
1:10 methane CH 4 + 20 2 \u003d CO 2 + 2H 2 O \u003d carbon dioxide + water
incomplete combustion of gas CH 4 + 1.5O 2 \u003d 2H 2 O + CO - carbon monoxide
Advantages and disadvantages of natural gas over other fuels.
Advantages:
The cost of extracting gas is much lower than that of coal and oil;
High heat of combustion;
The completeness of combustion and the relief of the conditions of the service personnel are ensured;
The absence of carbon monoxide and hydrogen sulfide in natural gases prevents poisoning in case of gas leaks;
When burning gas, a minimum residual air in the furnace is required and there are no costs as a result of mechanical afterburning;
When burning gaseous fuel, more precise temperature control is provided;
When burning gas, the burners can be placed in an accessible place in the furnace, which allows for better heat transfer and the need for temperature control;
The ability to change the shape of the flame for heating in a specific place.
Flaws:
Explosive and fire hazardous;
The process of gas combustion is possible only when oxygen is displaced;
The effect of an explosion during spontaneous combustion;
The possibility of detonation of a mixture of gas and air.
A similar defect is associated with a malfunction of the boiler automation system. Note that it is strictly forbidden to operate the boiler with the automation turned off (for example, if the start button is forcibly jammed in the pressed state). This can lead to tragic consequences, since if the gas supply is interrupted for a short time or if the flame is extinguished by a strong air flow, the gas will begin to flow into the room. To understand the causes of such a defect, let us consider in more detail the operation of the automation system. On fig. 5 shows a simplified diagram of this system. The circuit consists of an electromagnet, a valve, a draft sensor and a thermocouple. To turn on the igniter, press the start button. The rod connected to the button presses on the valve membrane, and the gas begins to flow to the igniter. After that, the igniter is lit. The igniter flame touches the body of the temperature sensor (thermocouple). After some time (30 ... 40 s), the thermocouple heats up and an EMF appears on its terminals, which is enough to trigger the electromagnet. The latter, in turn, fixes the rod in the lower (as in Fig. 5) position. Now the start button can be released. The draft sensor consists of a bimetallic plate and a contact (Fig. 6). The sensor is located in the upper part of the boiler, near the pipe for the removal of combustion products into the atmosphere. In the event of a clogged pipe, its temperature rises sharply. The bimetallic plate heats up and breaks the voltage supply circuit to the electromagnet - the rod is no longer held by the electromagnet, the valve closes, and the gas supply stops.
The location of the elements of the automation device is shown in fig. 7. It shows that the electromagnet is closed with a protective cap. The wires from the sensors are located inside thin-walled tubes. The tubes are attached to the electromagnet using cap nuts. The body leads of the sensors are connected to the electromagnet through the body of the tubes themselves.
And now consider the method of finding the above fault.
The check begins with the “weakest link” of the automation device - the thrust sensor. The sensor is not protected by a casing, therefore, after 6 ... 12 months of operation, it “overgrows” with a thick layer of dust. The bimetallic plate (see Fig. 6) quickly oxidizes, which leads to poor contact.
The dust coat is removed with a soft brush. Then the plate is pulled away from contact and cleaned with fine sandpaper. We should not forget that it is necessary to clean the contact itself. Good results are obtained by cleaning these elements with a special spray "Contact". It contains substances that actively destroy the oxide film. After cleaning, a thin layer of liquid lubricant is applied to the plate and contact.
The next step is to check the health of the thermocouple. It works in heavy thermal conditions, as it is constantly in the igniter flame, naturally, its service life is much less than the rest of the boiler elements.
The main defect of the thermocouple is burnout (destruction) of its body. In this case, the transition resistance at the welding site (junction) sharply increases. As a result, the current in the circuit Thermocouple - Electromagnet
- The bimetallic plate will be lower than the nominal value, which leads to the fact that the electromagnet will no longer be able to fix the stem (Fig. 5).
To check the thermocouple, unscrew the union nut (Fig. 7), located on the left 
side of the electromagnet. Then the igniter is turned on and the constant voltage (thermo-EMF) at the thermocouple contacts is measured with a voltmeter (Fig. 8). A heated serviceable thermocouple generates an EMF of about 25 ... 30 mV. If this value is less, the thermocouple is faulty. For its final check, the tube is undocked from the casing of the electromagnet and the resistance of the thermocouple is measured. The resistance of the heated thermocouple is less than 1 ohm. If the resistance of the thermocouple is hundreds of ohms or more, it must be replaced. The low value of thermo-EMF generated by a thermocouple can be caused by the following reasons: - clogging of the igniter nozzle (as a result, the heating temperature of the thermocouple may be lower than the nominal one). A similar defect is “treated” by cleaning the igniter hole with any soft wire of a suitable diameter;
- by shifting the position of the thermocouple (naturally, it can also not heat up enough). Eliminate the defect in the following way - loosen the screw fastening the eyeliner near the igniter and adjust the position of the thermocouple (Fig. 10);
- low gas pressure at the boiler inlet.
If the EMF at the thermocouple leads is normal (while maintaining the symptoms of the malfunction indicated above), then the following elements are checked:
- the integrity of the contacts at the connection points of the thermocouple and the draft sensor.
Oxidized contacts must be cleaned. Union nuts are tightened, as they say, "by hand". In this case, it is undesirable to use a wrench, since it is easy to break the wires suitable for the contacts;
- the integrity of the electromagnet winding and, if necessary, solder its conclusions.
The performance of the electromagnet can be checked as follows. Disconnect 
thermocouple lead. Press and hold the start button, then ignite the igniter. From a separate source of direct voltage to the released contact of the electromagnet (from the thermocouple), a voltage of about 1 V is applied relative to the housing (at a current of up to 2 A). To do this, you can use a regular battery (1.5 V), as long as it provides the necessary operating current. Now the button can be released. If the igniter does not go out, the electromagnet and draft sensor are working;
- thrust sensor. First, the force of pressing the contact to the bimetallic plate is checked (with the indicated signs of a malfunction, it is often insufficient). To increase the clamping force, loosen the lock nut and move the contact closer to the plate, then tighten the nut. In this case, no additional adjustments are required - the clamping force does not affect the sensor response temperature. The sensor has a large margin for the angle of deflection of the plate, ensuring reliable breaking of the electrical circuit in the event of an accident.
