Incomplete combustion of methane. Burning gas

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

Burning can be complete or incomplete. Full burning occurs when there is sufficient oxygen. Lack of it causes incomplete combustion, in which less heat is released than with complete combustion, and carbon monoxide (CO), which is toxic to the operating personnel, forms soot that settles on the heating surface of the boiler and increases heat loss, which leads to excessive fuel consumption and a decrease in boiler efficiency, atmospheric 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 natural gas air is supplied to the furnace with a slight excess. The ratio of the actual volume of air consumed V d to the theoretically necessary V t is called the excess air coefficient \u003d V d / V t. This indicator depends on the design gas burner and furnaces: the more perfect they are, the smaller. It is necessary to ensure that the excess air coefficient is not less than 1, as this leads to incomplete combustion of the gas. Increasing the excess air ratio reduces the efficiency of the boiler.

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.

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.

Gaseous fuels are increasingly used in various industries every year. National economy. In agricultural production, gaseous fuel is widely used for technological (for heating greenhouses, greenhouses, dryers, livestock and poultry complexes) and domestic purposes. AT recent times it is increasingly used for engines internal combustion.

Compared to other types of gaseous fuel, it has the following advantages:

burns in the theoretical amount of air, which ensures high thermal efficiency and combustion temperature;

when burned, it does not form undesirable products of dry distillation and sulfur compounds, soot and smoke;

it is relatively easy to be supplied through gas pipelines to remote objects of consumption and can be stored centrally;

easily ignites at any ambient temperature;

requires relatively low costs for extraction, which means that it is a cheaper type of fuel compared to other types of fuel;

can be used in compressed or liquefied form for internal combustion engines;

has high anti-knock properties;

does not form condensate during combustion, which provides a significant reduction in wear of engine parts, etc.

At the same time, gaseous fuel also has certain negative properties, which include: a toxic effect, the formation of explosive mixtures when mixed with air, easy flow through leaky joints, etc. Therefore, when working with gaseous fuel, careful observance of the relevant safety regulations is required.

The use of gaseous fuels is determined by their composition and properties of the hydrocarbon part. The most widely used are natural or associated gas from oil or gas fields, as well as factory gases from oil refineries and other plants. The main constituents of these gases are hydrocarbons with the number of carbon atoms in the molecule from one to four (methane, ethane, propane, butane and their derivatives).

Natural gases from gas fields consist almost entirely of methane (82...98%), with little use of gaseous fuel for internal combustion engines A constantly growing fleet of vehicles requires an increasing amount of fuel. It is possible to solve the most important national economic problems of stable provision of automobile engines with efficient energy carriers and reduction in the consumption of liquid fuels of petroleum origin through the use of gaseous fuels - liquefied petroleum and natural gases.

For cars, only high-calorie or medium-calorie gases are used. When operating on low-calorie gas, the engine does not develop the necessary power, and the driving range of the car is also reduced, which is economically unprofitable. Pa). Release the following types compressed gases: natural, mechanized coke and enriched coke

The main combustible component of these gases is methane. As well as for liquid fuel, the presence of hydrogen sulfide in gaseous fuel is undesirable because of its corrosive effect on gas equipment and engine parts. The octane number of gases allows boosting car engines in terms of compression ratio (up to 10 ... 12).

The presence of cyanide CN is highly undesirable in car gas. When combined with water, it forms hydrocyanic acid, under the action of which the smallest cracks are formed in the walls of the cylinders. The presence of tarry substances and mechanical impurities in the gas leads to the formation of deposits and pollution on gas equipment devices and on engine parts.

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 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 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 corresponding measurements are carried out 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.

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

Composition of air

Anthropotoxins;

Destruction products of polymeric materials;

Substances entering the room with polluted atmospheric air;

Chemicals 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 has been established air environment from the total saturation of the premises 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 assumed that the concentrations of volatile substances released from polymers in residential and public buildings should not exceed their MPC 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 of polymeric materials and products made from them, it is proposed to limit their 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 indoor air made it possible to identify a number of toxic substances in them, the distribution of which according to hazard classes is in the following way: dimethylamine, hydrogen sulfide, nitrogen dioxide, ethylene oxide, benzene (second hazard class - highly hazardous substances); acetic acid, phenol, methylstyrene, toluene, methanol, vinyl acetate (the third hazard class - 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 shutdown 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 tightness 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 airborne infections and colds, irritation of the mucous membranes of the eyes, nose, throat, 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 the manifestation of symptoms is possibly 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 supply air and his 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 lingers in the respiratory tract of a person for much more than neutral).

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).

The combustion of gaseous fuel is a combination of the following physical and chemical processes: mixing combustible gas with air, heating the mixture, thermal decomposition of combustible components, ignition and chemical combination of combustible elements with atmospheric oxygen.

Stable combustion of a gas-air mixture is possible with a continuous supply of the necessary amounts of combustible gas and air to the combustion front, their thorough mixing and heating to the ignition or self-ignition temperature (Table 5).

The ignition of the gas-air mixture can be carried out:

• heating the entire volume of the gas-air mixture to the auto-ignition temperature. This method is used in internal combustion engines, where the gas-air mixture is heated by rapid compression to a certain pressure;
• the use of foreign sources of ignition (igniters, etc.). In this case, not the entire gas-air mixture is heated to the ignition temperature, but part of it. This method it is used when burning gases in burners of gas appliances;
• existing torch continuously in the combustion process.

To start the combustion reaction of gaseous fuel, it is necessary to spend a certain amount of the energy needed to break molecular bonds and create new ones.

Chemical formula for combustion gas fuel with an indication of the entire reaction mechanism associated with the appearance and disappearance a large number free atoms, radicals and other active particles is complex. Therefore, for simplification, equations are used that express the initial and final states of gas combustion reactions.

If hydrocarbon gases are denoted C m H n, then the equation chemical reaction combustion of these gases in oxygen takes the form

C m H n + (m + n/4)O 2 = mCO 2 + (n/2)H 2 O,

where m is the number of carbon atoms in the hydrocarbon gas; n is the number of hydrogen atoms in the gas; (m + n/4) - the amount of oxygen required for complete combustion of the gas.

In accordance with the formula, the equations for the combustion of gases are derived:

• methane CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
• ethane C 2 H 6 + 3.5O 2 \u003d 2CO 2 + ZH 2 O
• butane C 4 H 10 + 6.5O 2 \u003d 4CO 2 + 5H 2 0
• propane C 3 H 8 + 5O 3 \u003d ZSO 2 + 4H 2 O.

In practical conditions of gas combustion, oxygen is not taken in its pure form, but is part of the air. Since air consists of 79% nitrogen and 21% oxygen by volume, 100:21 = 4.76 volumes of air or 79:21 = 3.76 volumes of nitrogen is required for each volume of oxygen. Then the combustion reaction of methane in air can be written as follows:

CH 4 + 2O 2 + 2 * 3.76N 2 \u003d CO 2 + 2H 2 O + 7.52N 2.

The equation shows that for the combustion of 1 m 3 of methane, 1 m 3 of oxygen and 7.52 m 3 of nitrogen or 2 + 7.52 = 9.52 m 3 of air are required.

As a result of the combustion of 1 m 3 of methane, 1 m 3 of carbon dioxide, 2 m 3 of water vapor and 7.52 m 3 of nitrogen are obtained. The table below shows these data for the most common combustible gases.

For the process of combustion of a gas-air mixture, it is necessary that the amount of gas and air in the gas-air mixture be within certain limits. These limits are called flammability limits or explosive limits. There are lower and upper flammability limits. The minimum gas content in the gas-air mixture, expressed as a percentage by volume, at which ignition occurs, is called the lower flammability limit. The maximum gas content in the gas-air mixture, above which the mixture does not ignite without the supply of additional heat, is called the upper flammability limit.

The amount of oxygen and air during the combustion of certain gases

If the gas-air mixture contains less gas lower limit flammable, it will not burn. If there is not enough air in the gas-air mixture, then combustion does not proceed completely.

Inert impurities in gases have a great influence on the magnitude of the explosive limits. An increase in the ballast content (N 2 and CO 2) in the gas narrows the flammability limits, and when the ballast content increases above certain limits, the gas-air mixture does not ignite at any ratio of gas and air (table below).

The number of volumes of inert gas per 1 volume of combustible gas at which the gas-air mixture ceases to be explosive

The smallest amount of air required for complete combustion of gas is called the theoretical air flow and is denoted by Lt, that is, if the net calorific value of gas fuel is 33520 kJ / m 3 , then theoretically required amount combustion air 1 m 3 gas

L T\u003d (33 520/4190) / 1.1 \u003d 8.8 m 3.

However, the actual air flow always exceeds the theoretical one. This is explained by the fact that it is very difficult to achieve complete combustion of gas at theoretical air flow rates. Therefore, any gas installation for gas combustion works with some excess air.

So, practical air flow

L n = αL T,

where L n- practical air consumption; α - coefficient of excess air; L T- theoretical air consumption.

The excess air coefficient is always greater than one. For natural gas it is α = 1.05 - 1.2. Coefficient α shows how many times the actual air flow exceeds the theoretical one, taken as a unit. If a α = 1, then the gas-air mixture is called stoichiometric.

At α = 1.2 gas combustion is carried out with an excess of air by 20%. As a rule, combustion of gases should take place with minimum value a, since with a decrease in excess air, heat losses with exhaust gases are reduced. The air involved in combustion is primary and secondary. Primary called the air entering the burner for mixing with gas in it; secondary- air entering the combustion zone is not mixed with gas, but separately.

Gas combustion is a combination following processes:

Mixing combustible gas with air

heating the mixture

thermal decomposition of combustible components,

Ignition and chemical combination of combustible components with atmospheric oxygen, accompanied by the formation of a torch and intense heat release.

The combustion of methane occurs according to the reaction:

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

Conditions required for gas combustion:

Ensuring the required ratio of combustible gas and air,

heating up to ignition temperature.

If the gas-air mixture of gas is less than the lower flammable limit, then it will not burn.

If there is more gas in the gas-air mixture than the upper flammable limit, then it will not burn completely.

The composition of the products of complete combustion of gas:

CO 2 - carbon dioxide

H 2 O - water vapor

* N 2 - nitrogen (it does not react with oxygen during combustion)

Composition of products of incomplete combustion of gas:

CO - carbon monoxide

C - soot.

Combustion of 1 m 3 of natural gas requires 9.5 m 3 of air. In practice, air consumption is always higher.

Attitude actual consumption air to theoretically required flow is called the excess air coefficient: α = L/L t .,

Where: L- actual expense;

L t - theoretically required flow.

The excess air coefficient is always greater than one. For natural gas, it is 1.05 - 1.2.

2. Purpose, device and main characteristics of instantaneous water heaters.

Flowing gas water heaters. Designed to heat water to a certain temperature during drawdown. Flowing water heaters are divided according to the load of thermal power: 33600, 75600, 105000 kJ, according to the degree of automation - into the highest and first classes. efficiency water heaters 80%, oxide content is not more than 0.05%, the temperature of the combustion products behind the draft interrupter is not less than 180 0 C. The principle is based on heating water during the drawdown period.

The main units of instantaneous water heaters are: a gas burner, a heat exchanger, an automation system and a gas outlet. Gas low pressure fed into the injection burner. The combustion products pass through the heat exchanger and are discharged into the chimney. The heat of combustion is transferred to the water flowing through the heat exchanger. To cool the fire chamber, a coil is used, through which water circulates, passing through the heater. Gas instantaneous water heaters are equipped with gas exhaust devices and draft breakers, which, in the event of a short-term violation of draft, prevent the flame of the gas burner from extinguishing. There is a flue pipe for connection to the chimney.

Gas instantaneous water heater– HSV. On the front wall of the casing there are: a gas cock control knob, a button for turning on the solenoid valve and a viewing window for observing the flame of the pilot and main burners. At the top of the device there is a smoke exhaust device, at the bottom there are branch pipes for connecting the device to the gas and water systems. The gas enters solenoid valve, the gas shut-off valve of the water and gas burner block sequentially switches on the pilot burner and supplies gas to the main burner.

Blocking the flow of gas to the main burner, with the obligatory operation of the igniter, is carried out by an electromagnetic valve operating from a thermocouple. Blocking the gas supply to the main burner, depending on the presence of water intake, is carried out by a valve driven through the stem from the membrane of the water block valve.