Combustion of gases. Gas burning

Physicochemical characteristics natural gas

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

Gas density at t = 0°C, P = 760 mm Hg. Art.: methane - 0.72 kg/m 3, air -1.29 kg/m 3.

The auto-ignition 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.

Methane combustion temperature is 2100°C in furnaces 1800°C.

Heat of combustion of methane: Qn = 8500 kcal/m3, Qv = 9500 kcal/m3.

Explosiveness. There are:

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

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

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

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

For a gas explosion, in addition to keeping it in the air within the limits of its explosiveness, a third-party source of energy (spark, flame, etc.) is required.

When a gas explodes in a closed volume (room, furnace, tank, etc.), there is more destruction than in the open air.

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

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

The approximate speed of methane flame propagation is 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 gas to give the gas an odor before delivery to consumers.

Requirements for odorants:

– pungent specific odor;

– must not interfere with combustion;

– must 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, the rate doubles in winter.

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 flammable components (hydrogen and carbon) with oxygen contained in the air. Occurs with the release of heat and light.

When carbon is burned, it is formed carbon dioxide(C0 2), and hydrogen is water vapor (H 2 0).

Stages of combustion: 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 is burned and everything required amount air takes part in combustion, combustion reaction of 1 m 3 of gas:

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

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

Almost all of the combustion air supplied will take part in combustion.

Therefore, in addition to carbon dioxide (C0 2) and water vapor (H 2 0), combustion products will contain:

– carbon monoxide, or carbon monoxide (CO), if released into a room can cause poisoning service personnel;

– atomic carbon, or soot (C), deposited in flues and furnaces, impairs draft, and heat transfer on heating surfaces.

– unburned gas and hydrogen accumulate in fireboxes and flues and form an explosive mixture.

When there is a lack of air, no complete combustion fuel – the combustion process occurs with underburning. Underburning also occurs when the gas is poorly mixed with air and the temperature in the combustion zone is low.

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 greater quantities than theoretically required, i.e. in excess, not all of the air will take part in combustion. Part of the heat will be used to heat this excess air and will be released into the atmosphere.

Excess air coefficient α is a number showing how many times the actual combustion consumption is greater than it is theoretically required:

α = V d / V t

where Vd is the actual air flow, m 3;

V t - theoretically required air, m 3.

α = 1.05 – 1.2.

Gas combustion methods

Combustion air can be:

– primary – 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 and requires a large combustion space.

2. Mixed method - part of the air is supplied inside 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 and a small combustion 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.

Burner equipped 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 complete combustion of gas under all operating conditions with minimal excess air and minimal emissions harmful substances in atmosphere;

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

4) must have simple design, be available for repairs and revisions;

5) must operate stably within the operating regulation limits, if necessary, have stabilizers to prevent flame separation and breakthrough;

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

Options gas burners

1) thermal power of the burner N g - the amount of heat released during gas combustion 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 flame separation or flashover;

3) minimum power N min – power of the lowest limit, increased by 10%;

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

5) maximum power N max – upper limit power, reduced by 10%;

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

7) range of operating regulation – power values ​​from N min to N nom;

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

Classification of gas burners:

1) according to the method of supplying combustion air:

– blowless – air enters the furnace due to rarefaction in it;

– injection – air is sucked into the burner due to the energy of the gas stream;

– blowing – air is supplied to the burner or 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 combustion products flow (low – up to 20 m/s, medium – 20-70 m/s, high – more than 70 m/s);

4) by 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 - manually controlled, semi-automatic, automatic.

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

1) Diffusion. All air comes to the torch from the surrounding space. Gas is supplied to the burner without primary air and, leaving the manifold, is mixed with air outside it.

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

A variety is a hearth burner. Consists of a gas manifold made of steel pipe, plugged at one end. Holes are drilled in the pipe in two rows. The collector is installed in the slots, made of refractory bricks resting on the grate. Gas exits through holes in the manifold into the gap. Air enters the same slot through the grate due to vacuum in the firebox or with the help of a fan. During operation, the refractory lining of the slot heats up, ensuring stabilization of the flame in all operating modes.

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

Flaws: low power, uneconomical, high flame.

2) Injection burners:

A) low pressure or atmospheric (refer to burners with partial premixing). The gas stream comes out of the nozzle at high speed and, due to its energy, captures air into the confuser, dragging it inside the burner. The mixing of gas with air occurs in a mixer consisting of a neck, a diffuser and a fire nozzle. The vacuum created by the injector increases with increasing gas pressure, and the amount of primary air sucked in changes. 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 adjusted.

To ensure complete combustion of the fuel, part of the air is supplied due to rarefaction in the firebox (secondary air). Its flow rate is regulated by changing the vacuum.

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

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

b) Medium pressure. By increasing the gas pressure, 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. Refer to burners for complete pre-mixing of gas with air. As a result of good mixing of gas and air, they operate with a low excess air ratio (1.05-1.1). Kazantsev burner. Consists of a primary air regulator, nozzle, mixer, nozzle and plate stabilizer. Coming out of the nozzle, the gas has enough energy to inject all the air needed for combustion. In the mixer, gas and air are completely mixed. The primary air regulator simultaneously suppresses noise caused by high speed 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);

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

Flaws:

– large dimensions of 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. Gas and air are supplied 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 movement using swirlers with a constant or adjustable blade angle.

Gas vortex 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 swirled using a blade swirler. The blades are welded at an angle of 45 0 to the outer surface of the gas manifold. Inside the gas manifold there is a pipe to monitor the combustion process. When working with fuel oil, a steam-mechanical nozzle is installed in it.

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

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

Disadvantages of burners: sufficient complexity of design; flame separation and breakthrough are possible, which makes it 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 influencing 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. As the amount of air in the mixture increases, the speed of flame propagation increases, reaching its greatest value when the air content is about 90% of its theoretical amount required for complete combustion of the gas. As the air flow to the burner increases, a mixture that is leaner in gas is created, which can burn faster and cause the flame to leak into the burner. Therefore, if it is necessary to increase the load, first increase the gas supply and then the air. If it is necessary to reduce the load, do the opposite - first reduce the air supply, and then the gas. At the moment of starting the burners, no air should enter them and the gas is ignited in a diffusion mode due to the air entering the firebox, followed by the 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 firebox.

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

2. Flame breakthrough - movement of the torch zone towards the combustible mixture. Happens only in burners with pre-mixing 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 deformation of the burner due to overheating. If there is a breakthrough, there may be a small pop, the flame will go out, and gas contamination of the firebox and flue ducts will occur through the inoperative burner.

Surge occurs when: the gas pressure in front of the burner decreases below the permissible level; ignition of the burner when supplying primary air; large gas supply at low air pressure, reduction in burner productivity by pre-mixing gas and air below the values ​​​​specified in the passport. Not possible with the diffusion method of gas combustion.

Actions of personnel in the event of a burner accident:

– turn off the burner,

– ventilate the firebox,

- to figure out cause of the accident,

- make a journal entry,

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 those specially added (Table

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 the components of natural gas and its combustion products.

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, aerosolized organic particles, polycyclic aromatic hydrocarbons and small amounts of volatile organic compounds). All of these components can affect the human body either on their own or in combination with each other (synergy effect).

Table 12.3

Composition of gaseous fuel

Odorants. Odorants are sulfur-containing organic aromatic compounds (mercaptans, thioethers and thio-aromatic compounds). Added to natural gas to detect leaks. Although these compounds are present in very small, subthreshold concentrations that are not considered toxic to most individuals, their odor can cause nausea and headaches in healthy people.

Clinical experience and epidemiological data indicate that chemically sensitive people react inappropriately to chemical compounds 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 methyl mercaptan (mercaptomethane, thiomethyl alcohol), is a gaseous compound that is commonly used as an aromatic additive to natural gas. Unpleasant smell is experienced by most people at a concentration of 1 part in 140 ppm, however this compound can be detected at significantly lower concentrations by highly sensitive individuals.

Toxicological studies in animals have shown that 0.16% methanethiol, 3.3% ethanethiol, or 9.6% dimethyl sulfide are capable of inducing coma 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. Strong irritant to eyes and skin, capable of causing toxic effects through the skin. It is flammable and decomposes when heated to form highly toxic SOx vapors.

Mercaptans, being indoor air pollutants, contain sulfur and are capable of capturing elemental mercury. In high concentrations, mercaptans can cause impaired peripheral circulation and increased heart rate, and can stimulate loss of consciousness, the development of cyanosis, or even death.

Aerosols. The combustion of natural gas produces small organic particles (aerosols), including carcinogenic aromatic hydrocarbons, as well as some volatile organic compounds. DOS are suspected sensitizing agents that, together with other components, can induce the “sick building” syndrome, as well as multiple chemical sensitivity (MCS).

DOS also includes formaldehyde, which is formed in small quantities during gas combustion. Usage gas appliances in a home where sensitive individuals live increases exposure to these irritants, subsequently increasing symptoms of illness and also promoting further sensitization.

Aerosols generated during the combustion of natural gas can become adsorption sites for a variety of chemical compounds present in the air. Thus, air pollutants can concentrate in microvolumes and react with each other, especially when metals act as reaction catalysts. The smaller the particle, the higher the concentration activity of this process.

Moreover, water vapor generated during the combustion of natural gas is a transport link for aerosol particles and pollutants as they are transferred to the pulmonary alveoli.

The combustion of natural gas also produces aerosols containing polycyclic aromatic hydrocarbons. 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 during the combustion of natural gas is also unfavorable for human health. Benzene is known to be carcinogenic at doses well below threshold levels. Exposure to benzene is correlated with an increased risk of cancer, especially leukemia. The sensitizing effects of benzene are not known.

Organometallic compounds. Some components of natural gas 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 such as trimethylarsenite (CH3)3As. The association of these toxic metals with the organic matrix makes them lipid soluble. This leads to high levels 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, since the methylated compounds of these metals are more toxic than the metals themselves. These compounds pose a particular danger during lactation in women, since in this case lipids migrate from the body’s fat depots.

Dimethylmercury (CH3)2Hg is a particularly dangerous organometallic compound due to its high lipophilicity. Methylmercury can be incorporated into the body through inhalation and also 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 human reproductive function. Toxicology does not have data on safe levels mercury for living organisms.

Organic arsenic compounds are also very toxic, especially when they are destroyed metabolically (metabolic activation), resulting in the formation of highly toxic inorganic forms.

Natural gas combustion products. 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 lungs, potentiates bronchial asthma and other respiratory diseases. This is especially pronounced in children.

There is evidence that NO2 produced by burning natural gas can induce:

• inflammation of the pulmonary system and decreased vital function of the lungs;
• increased risk of asthma-like symptoms, including wheezing, shortness of breath and attacks. This is especially common in women who cook on gas stoves, as well as in children;
• decreased resistance to bacterial lung diseases due to a decrease in the immunological mechanisms of lung defense;
• causing adverse effects in general on immune system humans and animals;
• influence as an adjuvant on the development of allergic reactions to other components;
• increased sensitivity and increased allergic response to adverse allergens.

Natural gas combustion products contain a fairly high concentration of hydrogen sulfide (H2S), which pollutes environment. It is poisonous in concentrations lower than 50.ppm, and in concentrations of 0.1-0.2% 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 its exposure concentration than to the duration of exposure.

Although hydrogen sulfide has a strong odor, continuous low concentration exposure leads to loss of the sense of smell. This makes it possible for toxic effects to occur in people who may be unknowingly exposed to dangerous levels of this gas. Minor concentrations of it in the air of residential premises lead to irritation of the eyes and nasopharynx. Moderate levels cause headache, dizziness, as well as coughing and difficulty breathing. High levels lead to shock, convulsions, comatose state that end in death. Survivors of acute hydrogen sulfide toxicity experience neurological dysfunction such as amnesia, tremors, imbalance, and sometimes more severe brain damage.

The acute toxicity of relatively high concentrations of hydrogen sulfide is well known, but unfortunately little information is available on chronic LOW-DOSE exposure to this component.

Radon. Radon (222Rn) is also present in natural gas and can be carried through pipelines to gas stoves, which become sources of pollution. As radon decays to lead (210Pb has a half-life of 3.8 days), it creates a thin layer of radioactive lead (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 decays per minute (over an area of ​​100 cm2). Removing it is very difficult and requires replacing the 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, since even non-working gas stove continues to release aromatic compounds it has absorbed over years of use.

The cumulative effects of natural gas, the influence of aromatic compounds, and combustion products on human health are not precisely known. It is hypothesized that effects from multiple compounds may be multiplying, and the response from exposure to multiple pollutants may be greater than the sum of the individual effects.

In summary, the characteristics of natural gas that cause concern for human and animal health are:

• flammable and explosive nature;
• asphyxial properties;
• pollution of indoor air by combustion products;
• presence of radioactive elements (radon);
• content of highly toxic compounds in combustion products;
• the presence of trace amounts of toxic metals;
• 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. During the combustion of gas, a chemical reaction occurs when oxygen in the air combines with carbon and hydrogen in the fuel. 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 gas combustion process, 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 C02, nitrogen N2, water vapor H20. Most of all (by volume) nitrogen is in the combustion products - 69.3-74%.

For complete combustion of gas it is also necessary that it be mixed with air in certain (for each gas) quantities. The higher the calorific value of the gas, the greater the amount of air required. Thus, to burn 1 m3 of natural gas, about 10 m3 of air is required, artificial - about 5 m3, mixed - about 8.5 m3.

If there is insufficient air supply, incomplete combustion of gas occurs or chemical underburning flammable components; Combustible gases appear in combustion products: carbon monoxide CO, methane CH4 and hydrogen H2

With incomplete combustion of gas, a long, smoky, luminous, opaque, yellow color torch.

Thus, a lack of air leads to incomplete combustion of the gas, and an excess leads to excessive cooling of the flame temperature. The ignition temperature of natural gas is 530 °C, coke gas - 640 °C, mixed gas - 600 °C. In addition, with a significant excess of air, incomplete combustion of gas also occurs. In this case, the end of the torch is yellowish in color, not completely transparent, with a vague bluish-green core; the flame is unstable and comes off the burner.

Rice. 1. Gas flame - without preliminary mixing of gas with air; b -c partial prev. verifiable 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 longer and consists of three zones. IN atmospheric air pure gas burns. In the first inner dark zone, the gas does not burn: it is not mixed with oxygen in the air and is not heated to the ignition temperature. Air enters the second zone in insufficient quantities: it is retained by the burning layer, and therefore it cannot mix well with the gas. This is evidenced by the brightly glowing, light yellow, smoky color of the flame. Air enters the third zone in sufficient quantities, the oxygen of which mixes well with the gas, the gas burns bluish.

With this method, gas and air are supplied to the furnace separately. In the firebox, not only the combustion of the gas-air mixture occurs, but also the process of preparing the mixture. This method of gas combustion is widely used in industrial installations.

In the second case (Fig. 1.6), gas combustion occurs 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, and has two zones - internal and external.

The gas-air mixture in the inner zone does not burn, since it has not been heated to the ignition temperature. In the outer zone, the gas-air mixture burns, while in the upper part of the zone the temperature rises sharply.

With partial mixing of gas with air, in this case, complete combustion of the gas occurs only with additional air supply to the torch. During 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 design of gas burners for household appliances and heating boiler houses.

In the third case, the torch is significantly shortened and the gas burns more completely, since the gas-air mixture has been previously prepared. A short transparent flame indicates the completeness of gas combustion blue color(flameless combustion), which is used in devices infrared radiation with gas heating.

- Gas combustion process

CH 4+ 2 × O 2 +7.52 × N 2 = CO 2 +2× H 2 O + 7.5× N 2 +8500 Kcal

Air:

, hence the conclusion:

per 1 m 3 O 2 there are 3.76 m 3N 2

When burning 1 m 3 of gas, 9.52 m 3 of air must be consumed (since 2 + 7.52). Upon complete combustion of gas, the following is released:

· Carbon dioxide CO 2;

· Water vapor;

· Nitrogen (air ballast);

· Heat is released.

When 1 m 3 of gas is burned, 2 m 3 of water is released. If the temperature of the exhaust flue gases in the chimney is less than 120 ° C and the pipe is high and uninsulated, then these water vapors condense along the walls of the chimney into its bottom part, from where they enter a drainage tank or line through a hole.

To prevent the formation of condensation in the chimney, it is necessary to insulate the chimney or reduce the height of the chimney, having previously calculated the draft in the chimney (i.e., reducing the height of the chimney is dangerous).

Products of complete combustion of gas.

· Carbon dioxide;

· Water vapor.

Products of incomplete combustion of gas.

· Carbon monoxide CO;

· Hydrogen H 2;

· Carbon C.

In real conditions, for gas combustion, the air supply is slightly greater than calculated by the formula. The ratio of the actual volume of air supplied for combustion to the theoretically calculated volume is called the excess air coefficient (a). It should not be more than 1.05...1.2:

Excessive excess air reduces efficiency. boiler

Around town:

175 kg of standard fuel is spent to generate 1 Gcal of heat.

By trade:

162 kg of standard fuel is spent to generate 1 Gcal of heat.

Excess air is determined by flue gas analysis with a device.

Coefficientathe length of the combustion space is not the same. At the beginning of the firebox at the burner, and when the flue gases exit into chimney it is greater than calculated due to air leaks through the leaky lining (casing) of the boiler.

This information refers to boilers operating under vacuum, when the pressure in the firebox is less than atmospheric.

Boilers operating under overpressure gases in the boiler furnace are called pressurized boilers. In such boilers, the lining must be very tight to prevent flue gases from entering the boiler room and poisoning people.

Natural gas is the most common fuel today. Natural gas is called natural gas because it is extracted from the very depths of the Earth.

The gas combustion process is 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 flammable 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 have made it possible to use natural gas as one of the main types of fuel. A methane concentration of more than 10% is life-threatening; suffocation can occur due to lack of oxygen.

To detect gas leaks, the gas is odorized, 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, natural gas may contain flammable gases - propane, butane and ethane.

To ensure high-quality combustion of gas, it is necessary to supply sufficient air to the combustion zone and ensure good mixing of gas with air. The optimal ratio is 1: 10. That is, for one part of gas there are ten parts of air. In addition, it is necessary to create the necessary temperature regime. In order for a 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 flammable 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 remaining oxygen are released into the atmosphere with flammable gases. N2+O2

If gas combustion does not occur completely, then flammable substances are released into the atmosphere - carbon monoxide, hydrogen, soot.

Incomplete combustion of gas occurs due to insufficient air. At the same time, tongues of soot visually appear in the flame.

The danger of incomplete combustion of gas is that carbon monoxide can cause poisoning of boiler room personnel. A CO content in the air of 0.01-0.02% can cause mild poisoning. Higher concentrations can cause severe poisoning and death.

The resulting soot settles on the walls of the boiler, thereby impairing the transfer of heat to the coolant and reducing the efficiency of the boiler room. 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 required.

That is, an excess amount of air is needed. This value, designated alpha, shows how many times more air is consumed than is theoretically necessary.

The alpha coefficient depends on the type of specific burner and is usually specified in the burner passport or in accordance with the recommendations for organizing the commissioning work performed.

As the amount of excess air increases above the recommended amount, heat loss increases. With a significant increase in the amount of air, a flame may break off, creating an emergency situation. If the amount of air is less than recommended, combustion will be incomplete, thereby creating a risk of poisoning for boiler room personnel.

For more accurate control of the quality of fuel combustion, there are devices - gas analyzers, which measure the content of certain substances in the composition of exhaust gases.

Gas analyzers can be supplied complete with boilers. If they are not available, the corresponding measurements are carried out by the commissioning organization using portable gas analyzers. A regime map is drawn up in which the necessary control parameters are prescribed. By adhering to them, you can ensure normal complete combustion of the fuel.

The main parameters for regulating fuel combustion are:

• the ratio of gas and air supplied to the burners.

• excess air coefficient.

• vacuum in the furnace.

• coefficient useful action boiler

In this case, the efficiency of the boiler means the ratio of useful heat to the amount of total heat expended.

Air composition

Gas name	Chemical element	Contents 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%