Products of gas combustion and control of the combustion process. Complete and incomplete combustion of gas

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 interactions occur natural gas with oxygen 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 necessary 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 number excess air higher than recommended, 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

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%

Anthropotoxins;

Destruction products of polymeric materials;

Substances entering the room with polluted atmospheric air;

Chemical substances released from polymeric materials, even in small quantities, can cause significant disturbances in the state of a living organism, for example, in the case of allergic exposure to polymeric materials.

The intensity of the release of volatile substances depends on the operating conditions of polymeric materials - temperature, humidity, air exchange rate, operating time.

A direct dependence of the level of chemical pollution of the air environment on the total saturation of the premises has been established. polymeric materials.

A growing organism is more sensitive to the effects of volatile components from polymeric materials. An increased sensitivity of patients to the effects of chemical substances released from plastics compared to healthy ones. Studies have shown that in rooms with a high saturation of polymers, the susceptibility of the population to allergic, colds, neurasthenia, vegetative dystonia, and hypertension was higher than in rooms where polymer materials were used in smaller quantities.

To ensure the safety of the use of polymeric materials, it is accepted that the concentrations of volatile substances released from polymers in residential and public buildings should not exceed their MPCs established for atmospheric air, and the total ratio of the detected concentrations of several substances to their MPC should not exceed one. For the purpose of preventive sanitary supervision for polymeric materials and products made from them, it was proposed to limit the release of harmful substances in environment or at the stage of manufacture, or shortly after their release by manufacturers. Permissible levels of about 100 chemicals released from polymeric materials have now been substantiated.

AT modern construction there is a growing trend towards chemization technological processes and use as mixtures of various substances, primarily concrete and reinforced concrete. From a hygienic point of view, it is important to take into account the adverse effects of chemical additives in building materials due to the release of toxic substances.

No less powerful internal source of pollution of the indoor environment are human waste products anthropotoxins. It has been established that in the process of life a person releases approximately 400 chemical compounds.

Studies have shown that the air environment of unventilated rooms deteriorates in proportion to the number of people and the time they spend in the room. Chemical analysis of indoor air made it possible to identify a number of toxic substances in them, the distribution of which according to hazard classes is as follows: dimethylamine, hydrogen sulfide, nitrogen dioxide, ethylene oxide, benzene (the second hazard class is highly hazardous substances); acetic acid, phenol, methylstyrene, toluene, methanol, vinyl acetate (the third hazard class is low-hazard substances). One fifth of the identified anthropotoxins are classified as highly hazardous substances. At the same time, it was found that in an unventilated room, the concentrations of dimethylamine and hydrogen sulfide exceeded the MPC for atmospheric air. The concentrations of substances such as carbon dioxide, carbon monoxide, and ammonia also exceeded the MPC or were at their level. The remaining substances, although they amounted to tenths and smaller fractions of the MPC, taken together testified to the unfavorable air environment, since even a two-four-hour stay in these conditions had a negative effect on the mental performance of the subjects.

The study of the air environment of gasified premises showed that during the hourly combustion of gas in the indoor air, the concentration of substances was (mg / m 3): carbon monoxide - an average of 15, formaldehyde - 0.037, nitrogen oxide - 0.62, nitrogen dioxide - 0.44, benzene - 0.07. The air temperature in the room during the combustion of gas increased by 3-6 ° C, the humidity increased by 10-15%. Moreover, high concentrations of chemical compounds were observed not only in the kitchen, but also in the living quarters of the apartment. After turning off the gas appliances, the content of carbon monoxide and other chemicals in the air decreased, but sometimes did not return to the initial values ​​even after 1.5-2.5 hours.

The study of the effect of household gas combustion products on human external respiration revealed an increase in the load on the respiratory system and a change in the functional state of the central nervous system.

One of the most common sources of indoor air pollution is smoking. Spectrometric analysis of air polluted with tobacco smoke revealed 186 chemical compounds. In insufficiently ventilated rooms, air pollution by smoking products can reach 60-90%.

When studying the effects of components tobacco smoke on non-smokers (passive smoking), the subjects experienced irritation of the mucous membranes of the eyes, an increase in the content of carboxyhemoglobin in the blood, an increase in heart rate, an increase in the level blood pressure. Thus, main sources of pollution The air environment of the premises can be conditionally divided into four groups:

The significance of internal sources of pollution in different types of buildings is not the same. AT administrative buildings the level of total pollution most closely correlates with the saturation of the premises with polymeric materials (R = 0.75), in indoor sports facilities the level of chemical pollution correlates most well with the number of people in them (R = 0.75). For residential buildings the closeness of the correlation between the level of chemical pollution both with the saturation of the premises with polymeric materials and with the number of people in the premises is approximately the same.

Chemical pollution of the air environment of residential and public buildings under certain conditions (poor ventilation, excessive saturation of the premises with polymeric materials, large crowds of people, etc.) can reach a level that Negative influence on the general condition of the human body.

AT last years According to WHO, the number of reports of the so-called sick building syndrome has increased significantly. The described symptoms of deterioration in the health of people living or working in such buildings are very diverse, but they also have a number common features namely: headaches, mental fatigue, increased frequency of airborne infections and colds, irritation of the mucous membranes of the eyes, nose, pharynx, a feeling of dryness of the mucous membranes and skin, nausea, dizziness.

The first category - temporarily "sick" buildings- includes newly built or recently renovated buildings in which the intensity of the manifestation of these symptoms weakens over time and in most cases they disappear completely after about six months. The decrease in the severity of symptoms may be associated with the patterns of emission of volatile components contained in building materials, paints, etc.

In buildings of the second category - constantly "sick" the described symptoms are observed for many years, and even large-scale recreational activities may not have an effect. As a rule, it is difficult to find an explanation for this situation, despite a thorough study of the composition of air, work ventilation system and building design features.

It should be noted that it is not always possible to detect a direct relationship between the state of the indoor air environment and the state of public health.

However, providing an optimal air environment for residential and public buildings is an important hygienic and engineering problem. The leading link in solving this problem is the air exchange of the premises, which provides the required parameters of the air environment. When designing air conditioning systems in residential and public buildings, the required air supply rate is calculated in an amount sufficient to assimilate human heat and moisture emissions, exhaled carbon dioxide, and in rooms intended for smoking, the need to remove tobacco smoke is also taken into account.

In addition to regulating the amount of supply air and its chemical composition known value to ensure air comfort indoors, it has an electrical characteristic of the air environment. The latter is determined by the ionic regime of the premises, i.e., the level of positive and negative air ionization. Negative impact both insufficient and excessive air ionization has an effect on the body.

Living in areas with a content of negative air ions of the order of 1000-2000 in 1 ml of air has a positive effect on the health of the population.

The presence of people in the premises causes a decrease in the content of light air ions. At the same time, the ionization of air changes more intensively, the more people in the room and the smaller its area.

A decrease in the number of light ions is associated with the loss of air refreshing properties, with its lower physiological and chemical activity, which adversely affects the human body and causes complaints of stuffiness and "lack of oxygen". Therefore, of particular interest are the processes of deionization and artificial ionization of indoor air, which, of course, must have hygienic regulation.

It should be emphasized that artificial ionization of indoor air without sufficient air supply under conditions high humidity and dustiness of the air leads to an inevitable increase in the number of heavy ions. In addition, in the case of ionization of dusty air, the percentage of dust retention in the respiratory tract increases sharply (dust carrying electrical charges is retained in the respiratory tract of a person in much larger quantities than neutral dust).

Consequently, artificial air ionization is not a universal panacea for improving indoor air. Without improving all the hygienic parameters of the air environment, artificial ionization not only does not improve human living conditions, but, on the contrary, can have a negative effect.

The optimal total concentrations of light ions are levels of the order of 3 x 10, and the minimum required is 5 x 10 in 1 cm 3. These recommendations formed the basis of the current Russian Federation sanitary and hygienic standards of permissible levels of air ionization in industrial and public premises (Table 6.1).

Combustion of natural gas is a complex physical and chemical process of interaction of its combustible components with an oxidizing agent, while the chemical energy of the fuel is converted into heat. Burning can be complete or incomplete. When gas is mixed with air, the temperature in the furnace is high enough for combustion, the fuel and air are continuously supplied, complete combustion of the fuel is carried out. Incomplete combustion of fuel occurs when these rules are not observed, which leads to less heat release, (CO), hydrogen (H2), methane (CH4), and as a result, to soot deposition on heating surfaces, worsening heat transfer and increasing heat loss, which in turn, leads to excessive fuel consumption and a decrease in the efficiency of the boiler and, accordingly, to air pollution.

The excess air ratio depends on the design of the gas burner and furnace. The excess air coefficient must be at least 1, otherwise it may lead to incomplete combustion of the gas. And also an increase in the excess air coefficient reduces the efficiency of the heat-using installation due to large heat losses with the exhaust gases.

The completeness of combustion is determined using a gas analyzer and by color and smell.

Complete combustion of gas. methane + oxygen \u003d carbon dioxide + water CH4 + 2O2 \u003d CO2 + 2H2O In addition to these gases, nitrogen and the remaining oxygen enter the atmosphere with combustible gases. N2 + O2 If the combustion of gas is incomplete, then combustible substances are emitted into the atmosphere - carbon monoxide, hydrogen, soot.CO + H + C

Incomplete combustion of gas occurs due to insufficient air. At the same time, soot tongues 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. A higher concentration can lead to severe poisoning and death. The resulting soot settles on the walls of the boilers, 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, 9 m3 of air is needed to burn 1 m3 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's 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. Incomplete combustion is determined by:

Combustion is a reaction in which the chemical energy of a fuel is converted into heat.

Burning can be complete or incomplete. Complete combustion occurs with sufficient oxygen. Its lack causes incomplete combustion, in which less heat is released than with complete combustion, and carbon monoxide (CO), which is poisonous to service staff, soot is formed, which settles on the heating surface of the boiler and increases heat loss, which leads to excessive fuel consumption and a decrease in efficiency. boiler, air pollution.

For the combustion of 1 m 3 of methane, 10 m 3 of air is needed, in which there is 2 m 3 of oxygen. For complete combustion of natural gas, air is supplied to the furnace with a slight excess. The ratio of the actually consumed air volume V d to the theoretically necessary V t is called the excess air coefficient a = V d / V t. This indicator depends on the design of the gas burner and furnace: the more perfect they are, the less a. It is necessary to ensure that the excess air coefficient is not less than 1, as this leads to incomplete combustion of the gas. An increase in the excess air ratio reduces the efficiency. boiler unit.

The completeness of fuel combustion can be determined using a gas analyzer and visually - by the color and nature of the flame: transparent bluish - complete combustion;

red or yellow - incomplete combustion.

The speed at which the combustion zone advances in a direction perpendicular to the zone itself is called the flame propagation velocity. The flame propagation speed characterizes the speed of heating the gas-air mixture to the ignition temperature. The flame of hydrogen, water gas (3 m / sec) has the highest propagation speed, the flame of natural gas and propane-butane mixture has the lowest. A high speed of flame propagation favorably affects the completeness of gas combustion, and a small one, on the contrary, is one of the reasons for incomplete combustion of gas. The speed of flame propagation increases when using a gas-oxygen mixture instead of a gas-air one.

Combustion is controlled by increasing the air supply to the boiler furnace or by decreasing the gas supply. This process uses primary (mixes with gas in the burner - before combustion) and secondary (combines with gas or gas-air mixture in the boiler furnace during combustion) air.

In boilers equipped with diffusion burners (without forced air supply), the secondary air, under the action of vacuum, enters the furnace through the blower doors.

In boilers equipped with injection burners: primary air enters the burner due to injection and is regulated by an adjusting washer, and secondary air enters the burner through the blower doors.

In boilers with mixing burners, primary and secondary air is supplied to the burner by a fan and controlled by air dampers.

Violation of the ratio between the speed of the gas-air mixture at the outlet of the burner and the speed of flame propagation leads to separation or overshoot of the flame on the burners.

If the speed of the gas-air mixture at the outlet of the burner is greater than the speed of flame propagation - separation, and if less - slip.

In the event of a flame breaking off and flashing through, the operating personnel must extinguish the boiler, ventilate the furnace and gas ducts, and re-ignite the boiler.

K category: Gas supply

Gas combustion process

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 large quantity air. 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 combustibles occurs. constituent parts; 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 color torch.

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. AT atmospheric air pure gas burns. 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 of the device 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 indicated by a short transparent blue torch (flameless combustion), which is used in appliances infrared radiation with gas heating.

- Gas combustion process