What substances are released when natural gas is burned. Products of gas combustion and control of the combustion process. Physical and chemical properties of natural gas

Section content

When burning organic fuels in boiler furnaces, various combustion products are formed, such as carbon oxides CO x \u003d CO + CO 2, water vapor H 2 O, sulfur oxides SO x \u003d SO 2 + SO 3, nitrogen oxides NO x \u003d NO + NO 2 , polycyclic aromatic hydrocarbons (PAHs), fluorides, vanadium compounds V 2 O 5 , particulate matter, etc. (see Table 7.1.1). In the case of incomplete combustion of fuel in furnaces, exhaust gases may also contain hydrocarbons CH4, C2H4, etc. All products of incomplete combustion are harmful, but their formation can be minimized with modern fuel combustion technology [1].

Table 7.1.1. Specific emissions from flaring of organic fuels in power boilers [3]

Symbols: A p, S p – respectively, the content of ash and sulfur per working mass of fuel, %.

The criterion for the sanitary assessment of the environment is the maximum permissible concentration (MPC) of a harmful substance in the atmospheric air at ground level. MPC should be understood as such a concentration of various substances and chemical compounds, which, with daily exposure for a long time to the human body, does not cause any pathological changes or diseases.

Maximum allowable concentrations (MPC) of harmful substances in the atmospheric air of populated areas are given in Table. 7.1.2 [4]. The maximum one-time concentration of harmful substances is determined by samples taken within 20 minutes, the average daily - per day.

Table 7.1.2. Maximum permissible concentrations of harmful substances in the atmospheric air of populated areas

Calculations are carried out for each harmful substance separately, so that the concentration of each of them does not exceed the values ​​given in Table. 7.1.2. For boiler houses, these conditions are tightened by the introduction of additional requirements on the need to sum up the effects of sulfur and nitrogen oxides, which is determined by the expression

At the same time, due to local air deficiencies or unfavorable thermal and aerodynamic conditions, incomplete combustion products are formed in the furnaces and combustion chambers, consisting mainly of carbon monoxide CO (carbon monoxide), hydrogen H 2 and various hydrocarbons, which characterize heat losses in boiler unit from chemical incompleteness of combustion (chemical underburning).

In addition, during the combustion process, a number of chemical compounds are obtained, which are formed as a result of the oxidation of various components of the fuel and nitrogen in the air N 2. The most significant part of them is nitrogen oxides NO x and sulfur SO x .

Nitrogen oxides are formed due to the oxidation of both molecular nitrogen in the air and nitrogen contained in the fuel. Experimental studies have shown that the main share of NO x formed in the furnaces of boilers, namely 96÷100%, falls on nitrogen monoxide (oxide) NO. Nitrogen dioxide NO 2 and nitrogen hemioxide N 2 O are formed in much smaller quantities, and their share is approximately: for NO 2 - up to 4%, and for N 2 O - hundredths of a percent of the total NO x emission. Under typical conditions of flaring of fuels in boilers, the concentrations of nitrogen dioxide NO 2 are, as a rule, negligible compared to the content of NO and usually range from 0÷7 ppm up to 20÷30 ppm. At the same time, the rapid mixing of hot and cold regions in a turbulent flame can lead to relatively large concentrations of nitrogen dioxide in the cold zones of the flow. In addition, partial emission of NO 2 occurs in the upper part of the furnace and in the horizontal flue (at T> 900÷1000 K) and under certain conditions can also reach noticeable sizes.

Nitrogen hemoxide N 2 O, formed during the combustion of fuels, is, apparently, a short-lived intermediate. N 2 O is practically absent in the combustion products behind the boilers.

The sulfur contained in the fuel is a source of formation of sulfur oxides SO x: sulfurous SO 2 (sulfur dioxide) and sulfuric SO 3 (sulfur trioxide) anhydrides. The total mass emission of SO x depends only on the sulfur content in the fuel S p , and their concentration in flue gases also depends on the air flow coefficient α. As a rule, the share of SO 2 is 97÷99%, and the share of SO 3 is 1÷3% of the total output of SO x . The actual content of SO 2 in the gases leaving the boilers ranges from 0.08 to 0.6%, and the concentration of SO 3 - from 0.0001 to 0.008%.

Among the harmful components of flue gases, a large group of polycyclic aromatic hydrocarbons (PAHs) occupies a special place. Many PAHs have high carcinogenic and (or) mutagenic activity, activate photochemical smog in cities, which requires strict control and limitation of their emissions. At the same time, some PAHs, such as phenanthrene, fluoranthene, pyrene, and a number of others, are almost physiologically inert and are not carcinogenic.

PAHs are formed as a result of incomplete combustion of any hydrocarbon fuels. The latter occurs due to the inhibition of the reactions of oxidation of fuel hydrocarbons by the cold walls of the combustion devices, and can also be caused by an unsatisfactory mixture of fuel and air. This leads to the formation in the furnaces (combustion chambers) of local oxidizing zones with a low temperature or zones with excess fuel.

Due to the large number of different PAHs in flue gases and the difficulty of measuring their concentrations, it is customary to estimate the level of carcinogenic contamination of combustion products and atmospheric air by the concentration of the strongest and most stable carcinogen, benzo(a)pyrene (B(a)P) C 20 H 12 .

Due to the high toxicity, special mention should be made of such fuel oil combustion products as vanadium oxides. Vanadium is contained in the mineral part of fuel oil and, when burned, forms vanadium oxides VO, VO 2 . However, during the formation of deposits on convective surfaces, vanadium oxides are present mainly in the form of V 2 O 5 . Vanadium pentoxide V 2 O 5 is the most toxic form of vanadium oxides, therefore their emissions are accounted for in terms of V 2 O 5 .

Table 7.1.3. Approximate concentration of harmful substances in combustion products during flaring of organic fuels in power boilers

During the combustion of fuel oil and solid fuels, emissions also contain particulate matter, consisting of fly ash, soot particles, PAHs and unburned fuel as a result of mechanical underburning.

The ranges of concentrations of harmful substances in flue gases during the combustion of various types of fuels are given in Table. 7.1.3.

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 concentrate 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, the 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 impact from several compounds can 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.

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.

Physical and chemical properties of natural gas

Natural gas is colorless, odorless and tasteless, non-toxic.

Density of gases at t = 0°C, Р = 760 mm Hg. Art.: methane - 0.72 kg / m 3, air -1.29 kg / m 3.

The autoignition temperature of methane is 545 - 650°C. This means that any mixture of natural gas and air heated to this temperature will ignite without an ignition source and will burn.

The combustion temperature of methane is 2100°C in furnaces of 1800°C.

Calorific value of methane: Q n \u003d 8500 kcal / m 3, Q in \u003d 9500 kcal / m 3.

Explosiveness. Distinguish:

- the lower explosive limit is the lowest gas content in the air at which an explosion occurs, it is 5% for methane.

With a lower content of gas in the air, there will be no explosion due to a lack of gas. When introducing a third-party energy source - pops.

- the upper explosive limit is the highest gas content in the air at which an explosion occurs, it is 15% for methane.

With a higher content of gas in the air, there will be no explosion due to lack of air. When a third-party source of energy is introduced - fire, fire.

For a gas explosion, in addition to keeping it in the air within the limits of its explosibility, an external source of energy (spark, flame, etc.) is needed.

During a gas explosion in a closed volume (a room, a firebox, a tank, etc.), there is more destruction than in the open air.

When burning gas with underburning, i.e., with a lack of oxygen, carbon monoxide (CO), or carbon monoxide, is formed in the combustion products, which is a highly toxic gas.

Flame propagation speed is the speed at which the flame front moves relative to the fresh mixture jet.

Estimated flame propagation speed methane - 0.67 m / s. It depends on the composition, temperature, pressure of the mixture, the ratio of gas and air in the mixture, the diameter of the flame front, the nature of the movement of the mixture (laminar or turbulent) and determines the stability of combustion.

Gas odorization- this is the addition of a strong-smelling substance (odorant) to the gas to give the gas an odor before delivery to consumers.

Requirements for odorants:

- a sharp specific smell;

- must not prevent combustion;

- should not dissolve in water;

– must be harmless to humans and equipment.

Ethyl mercaptan (C 2 H 5 SH) is used as an odorant, it is added to methane - 16 g per 1000 m 3, in winter the rate doubles.

A person should smell the odorant in the air when the gas content in the air is 20% of the lower explosive limit for methane - 1% by volume.

This is a chemical process of combining combustible components (hydrogen and carbon) with oxygen contained in the air. Occurs with the release of heat and light.

When carbon is burned, carbon dioxide (CO 2) is formed, and hydrogen is formed into water vapor (H 2 0).

Combustion stages: supply of gas and air, formation of a gas-air mixture, ignition of the mixture, its combustion, removal of combustion products.

Theoretically, when all the gas burns out and all the necessary amount of air takes part in combustion, the combustion reaction of 1 m 3 of gas:

CH 4 + 20 2 \u003d CO 2 + 2H 2 O + 8500 kcal / m 3.

To burn 1 m 3 of methane, 9.52 m 3 of air is needed.

Practically not all the air supplied to combustion will take part in combustion.

Therefore, in addition to carbon dioxide (CO 2) and water vapor (H 2 0), the following will appear in the combustion products:

- carbon monoxide, or carbon monoxide (CO), if it enters the room, can cause poisoning of the attendants;

- atomic carbon, or soot (C), being deposited in gas ducts and furnaces, worsens traction, and heat transfer on heating surfaces.

- unburned gas and hydrogen - accumulating in furnaces and gas ducts, form an explosive mixture.

With a lack of air, incomplete combustion of the fuel occurs - the combustion process occurs with underburning. Underburning also occurs with poor mixing of gas with air and low temperature in the combustion zone.

For complete combustion of gas, combustion air is supplied in sufficient quantity, air and gas must be well mixed, and a high temperature is required in the combustion zone.

For complete combustion of gas, air is supplied in a larger quantity than theoretically required, i.e., with excess, not all air will take part in combustion. Part of the heat will be spent on heating this excess air and will be released into the atmosphere.

The excess air coefficient α is a number showing how many times the actual combustion flow is greater than it is theoretically required:

α = V d / V t

where V d - actual air consumption, m 3;

V t - theoretically necessary air, m 3.

α = 1.05 - 1.2.

Gas flaring methods

Combustion air can be:

- primary - is fed into the burner, mixed with gas, and the gas-air mixture is used for combustion;

- secondary - enters the combustion zone.

Gas combustion methods:

1. Diffusion method - gas and combustion air are supplied separately and mixed in the combustion zone, all air is secondary. The flame is long, a large furnace space is required.

2. Mixed method - part of the air is supplied into the burner, mixed with gas (primary air), part of the air is supplied to the combustion zone (secondary). The flame is shorter than with the diffusion method.

3. Kinetic method - all air is mixed with gas inside the burner, i.e. all air is primary. The flame is short, a small furnace space is required.

Gas burner devices

Gas burners are devices that supply gas and air to the combustion front, form a gas-air mixture, stabilize the combustion front, and ensure the required intensity of the combustion process.

A burner equipped with an additional device (tunnel, air distribution device, etc.) is called a gas burner device.

Burner requirements:

1) must be factory-made and pass state tests;

2) must ensure the completeness of gas combustion in all operating modes with a minimum excess of air and a minimum emission of harmful substances into the atmosphere;

3) be able to use automatic control and safety, as well as measuring the parameters of gas and air in front of the burner;

4) must have a simple design, be accessible for repair and revision;

5) must work stably within the operating regulation, if necessary, have stabilizers to prevent separation and flashback of the flame;

6) for working burners, the noise level should not exceed 85 dB, and the surface temperature should not exceed 45 ° C.

Parameters of gas burners

1) thermal power of the burner N g - the amount of heat released during the combustion of gas in 1 hour;

2) the lowest limit of stable operation of the burner N n. .P. . - the lowest power at which the burner operates stably without separation and flashover of the flame;

3) minimum power N min - the power of the lower limit, increased by 10%;

4) the upper limit of stable operation of the burner N in. .P. . - the highest power at which the burner operates stably without separation and flashover of the flame;

5) maximum power N max - power of the upper limit, reduced by 10%;

6) rated power N nom - the highest power with which the burner operates for a long time with the highest efficiency;

7) operating control range - power values ​​from N min to N nom;

8) coefficient of working regulation - the ratio of the rated power to the minimum.

Classification of gas burners:

1) according to the method of supplying air for combustion:

- blastless - air enters the furnace due to rarefaction in it;

- injection - air is sucked into the burner due to the energy of the gas jet;

- blast - air is supplied to the burner or to the furnace using a fan;

2) according to the degree of preparation of the combustible mixture:

– without preliminary mixing of gas with air;

- with complete pre-mixing;

- with incomplete or partial pre-mixing;

3) by the speed of the outflow of combustion products (low - up to 20 m / s, medium - 20-70 m / s, high - more than 70 m / s);

4) according to the gas pressure in front of the burners:

- low up to 0.005 MPa (up to 500 mm water column);

- average from 0.005 MPa to 0.3 MPa (from 500 mm water column to 3 kgf / cm 2);

- high more than 0.3 MPa (more than 3 kgf / cm 2);

5) according to the degree of automation of burner control - with manual control, semi-automatic, automatic.

According to the method of air supply, burners can be:

1) Diffusion. All air enters the torch from the surrounding space. The gas is supplied to the burner without primary air and, leaving the collector, mixes with the air outside it.

The simplest burner in design, usually a pipe with holes drilled in one or two rows.

Variety - hearth burner. It consists of a gas collector made of a steel pipe, plugged at one end. Holes are drilled in two rows in the pipe. The collector is installed in a slot, made of refractory bricks, based on a grate. Gas through the holes in the collector exits into the gap. Air enters the same slot through the grate due to rarefaction in the furnace or with the help of a fan. During operation, the refractory lining of the slot heats up, ensuring flame stabilization in all operating modes.

Advantages of the burner: simple design, reliable operation (flame flashback is impossible), noiselessness, good regulation.

Disadvantages: low power, uneconomical, high flame.

2) Injection burners:

a) low pressure or atmospheric (applies to burners with partial premixing). The gas jet exits the nozzle at high speed and, due to its energy, captures air into the confuser, dragging it inside the burner. Mixing of gas with air takes place in a mixer consisting of a neck, a diffuser and a fire nozzle. The vacuum created by the injector increases with increasing gas pressure, while changing the amount of primary air drawn in. The amount of primary air can be changed using an adjusting washer. By changing the distance between the washer and the confuser, the air supply is regulated.

To ensure complete combustion of the fuel, part of the air enters due to rarefaction in the furnace (secondary air). The regulation of its consumption is carried out by changing the vacuum.

They have the property of self-regulation: with increasing load, the pressure of the gas increases, which injects an increased amount of air into the burner. As the load decreases, the amount of air decreases.

Burners are limitedly used on high-capacity equipment (more than 100 kW). This is due to the fact that the burner collector is located directly in the furnace. During operation, it heats up to high temperatures and quickly fails. They have a high excess air ratio, which leads to uneconomical gas combustion.

b) Medium pressure. When the gas pressure is increased, all the air required for complete combustion of the gas is injected. All air is primary. They operate at gas pressure from 0.005 MPa to 0.3 MPa. Relate to burners of complete pre-mixing of gas with air. As a result of good mixing of gas and air, they work with a small excess air ratio (1.05-1.1). Burner Kazantsev. Consists of primary air regulator, nozzle, mixer, nozzle and plate stabilizer. When leaving the nozzle, the gas has enough energy to inject all the air needed for combustion. In the mixer, the gas is completely mixed with air. The primary air regulator at the same time dampens the noise that occurs due to the high speed of the gas-air mixture. Advantages:

- simplicity of design;

- stable operation when the load changes;

- lack of air supply under pressure (no fan, electric motor, air ducts);

– the possibility of self-regulation (maintaining a constant gas-air ratio).

Flaws:

- large dimensions of the burners along the length, especially burners with increased productivity;

– high noise level.

3) Burners with forced air supply. The formation of the gas-air mixture begins in the burner and ends in the furnace. Air is supplied by a fan. The supply of gas and air is carried out through separate pipes. They operate on low and medium pressure gas. For better mixing, the gas flow is directed through the holes at an angle to the air flow.

To improve mixing, the air flow is given a rotational motion using swirlers with a constant or adjustable blade angle.

Swirl gas burner (GGV) - gas from the distribution manifold exits through holes drilled in one row, and at an angle of 90 0 enters the air flow swirling with a bladed swirler. The blades are welded at an angle of 45 0 to the outer surface of the gas manifold. Inside the gas collector there is a pipe for monitoring the combustion process. When working on fuel oil, a steam-mechanical nozzle is installed in it.

Burners designed to burn several types of fuel are called combined.

Advantages of the burners: high thermal power, wide range of operating regulation, the ability to control the excess air ratio, the possibility of preheating gas and air.

Disadvantages of burners: sufficient design complexity; detachment and breakthrough of the flame are possible, in connection with which it becomes necessary to use combustion stabilizers (ceramic tunnel, pilot torch, etc.).

Burner accidents

The amount of air in the gas-air mixture is the most important factor affecting the speed of flame propagation. In mixtures in which the gas content exceeds the upper limit of its ignition, the flame does not spread at all. With an increase in the amount of air in the mixture, the flame propagation speed increases, reaching the highest value when the air content is about 90% of its theoretical amount necessary for complete combustion of the gas. Increasing the air flow to the burner creates a mixture that is poorer in gas, capable of burning faster and causing a flash of flame into the burner. Therefore, if it is required to increase the load, first increase the gas supply, and then the air. If it is necessary to reduce the load, they do the opposite - first reduce the air supply, and then the gas. At the time of starting the burners, air should not enter them and the gas is ignited in a diffusion mode due to the air entering the furnace, followed by a transition to air supply to the burner

1. Flame separation - movement of the torch zone from the burner outlets in the direction of fuel combustion. Occurs when the speed of the gas-air mixture becomes greater than the speed of flame propagation. The flame becomes unstable and may go out. Gas continues to flow through the extinguished burner, which leads to the formation of an explosive mixture in the furnace.

Separation occurs when: an increase in gas pressure above the permissible one, a sharp increase in the supply of primary air, an increase in vacuum in the furnace, the operation of the burner in transcendental modes relative to those indicated in the passport.

2. Flashback - moving the flame zone towards the combustible mixture. It happens only in burners with a preliminary mixture of gas and air. Occurs when the speed of the gas-air mixture becomes less than the speed of flame propagation. The flame jumps inside the burner, where it continues to burn, causing the burner to deform from overheating. When a slip is possible, a small pop is possible, the flame will go out, gassing of the furnace and gas ducts will occur through the idle burner.

The breakthrough occurs when: the gas pressure in front of the burner drops below the permissible value; ignition of the burner when the primary air is supplied; large gas supply at low air pressure, reducing the performance of burners by pre-mixing gas and air below the values ​​\u200b\u200bspecified in the passport. Not possible with the diffusion method of gas combustion.

Actions of personnel in case of an accident at the burner:

- turn off the burner,

- ventilate the furnace,

- find out the cause of the accident,

- make a journal entry

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.

Complete combustion of gas.

methane + oxygen = carbon dioxide + water

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O

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.

To more accurately control 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