Complete and incomplete combustion of gas. Characteristics of combustion products emitted by boiler houses into the atmosphere Gas combustion

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

Full 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.

There is no danger complete combustion 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 being carried out.

As the amount of excess air increases above the recommended level, heat loss increases. With a significant increase in the amount of air, flame rupture may occur, creating 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

The combustion products of natural gas are carbon dioxide, water vapor, some excess oxygen and nitrogen. Products of incomplete combustion of gas can be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, and soot.

The more carbon dioxide CO 2 in the combustion products, the less carbon monoxide CO will be in them and the more complete the combustion will be. The concept of “maximum CO 2 content in combustion products” was introduced into practice. The amount of carbon dioxide in the combustion products of some gases is shown in the table below.

The amount of carbon dioxide in gas combustion products

Using the table data and knowing the percentage of CO 2 in the combustion products, you can easily determine the quality of gas combustion and the excess air coefficient a. To do this, using a gas analyzer, you should determine the amount of CO 2 in the gas combustion products and divide the CO 2max value taken from the table by the resulting value. So, for example, if when burning gas the products of its combustion contain 10.2% carbon dioxide, then the coefficient of excess air in the furnace

α = CO 2max /CO 2 analysis = 11.8/10.2 = 1.15.

The most advanced way to control the flow of air into the furnace and the completeness of its combustion is to analyze combustion products using automatic gas analyzers. Gas analyzers periodically take a sample of exhaust gases and determine the content of carbon dioxide in them, as well as the amount of carbon monoxide and unburned hydrogen (CO + H 2) in volume percent.

If the gas analyzer needle reading on the scale (CO 2 + H 2) is zero, this means that combustion is complete and there is no carbon monoxide or unburned hydrogen in the combustion products. If the arrow deviates from zero to the right, then the combustion products contain carbon monoxide and unburned hydrogen, that is, incomplete combustion occurs. On another scale, the gas analyzer needle should show the maximum CO 2max content in the combustion products. Complete combustion occurs at the maximum percentage of carbon dioxide, when the pointer of the CO + H 2 scale is at zero.

Odorization

Combustible gases have no odor. To timely determine their presence in the air, quickly and accurately detect leakage points, the gas is odorized (gives a smell). For odorization, ethyl mercaptan (C 2 H 5 SH) is used. The odorization rate is 16 g of ethyl mercaptan per 1000 m3 of gas, 8 g of ethyl mercaptan sulfur per 1000 m³. Odorization is carried out at gas distribution stations (GDS). If there is 1% natural gas in the air, you should smell it.

20% of gas indoors causes suffocation

5-15% explosion

0.15% carbon monoxide CO- poisoning; 0.5% CO = 30 min. breathing is fatal; 1% carbon monoxide is lethal.

Methane and other hydrocarbon gases are not poisonous, but inhaling them causes dizziness, and high levels in the air lead to suffocation due to lack of oxygen.

Complete and incomplete combustion of fuel:

To burn 1m³ of gas you need 10m³ of air.

The combustion of natural gas is a reaction that converts the chemical energy of the fuel into heat.

Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen.

With complete combustion of gas, CO 2 (carbon dioxide), H 2 O is formed

(water). When gas is incompletely burned, heat loss occurs. Lack of oxygen O 2 oxidizing agent.

Products incomplete combustion CO - carbon monoxide, poisonous, C carbon, soot.

Incomplete combustion is an unsatisfactory mixture of gas with air, excessive cooling of the flame before the combustion reaction is completed.

Combustion reaction of the main components of natural gas:

1:10 methane CH 4 + 20 2 = CO 2 + 2H 2 O = carbon dioxide + water

incomplete combustion of gas CH 4 + 1.5O 2 = 2H 2 O + CO - carbon monoxide

Advantages and disadvantages of natural gas over other types of fuel.

Advantages:

The cost of gas production is significantly lower than coal and oil;

High calorific value;

Complete combustion and easier conditions for operating personnel are ensured;

The absence of carbon monoxide and hydrogen sulfide in natural gases prevents poisoning from gas leaks;

When burning gas, a minimum air residue in the furnace is required and there are no costs due to mechanical afterburning;

When burning gas fuel provides more precise temperature control;

When burning gas, the burners can be placed in an accessible place in the furnace, which allows for better heat transfer and the required temperature conditions;

The ability to change the shape of the flame to heat in a specific place.

Flaws:

Explosion and fire hazard;

The gas combustion process is only possible when oxygen is displaced;

Explosion effect during spontaneous combustion;

Possibility of detonation of a mixture of gas and air.

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

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

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 external ignition sources (igniters, etc.). In this case, not the entire gas-air mixture, but part of it, is heated to the ignition temperature. This method used when burning gases in burners of gas appliances;
• existing torch continuously during the combustion process.

To start the combustion reaction of gaseous fuel, a certain amount of energy must be expended to break molecular bonds and create new ones.

Chemical formula for the combustion of gas fuel indicating the entire reaction mechanism associated with the appearance and disappearance large quantity 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 the combustion of these gases in oxygen will take 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, gas combustion equations are derived:

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

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

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

From the equation it is clear that to burn 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 flammable gases.

For the combustion process 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 a gas-air mixture, expressed in volume percent, at which ignition occurs is called the lower flammability limit. The maximum gas content in a 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 when burning certain gases

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

Inert impurities in gases have a great influence on the explosion limits. Increasing 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 gas-to-air ratio (table below).

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

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

L T= (33,520/4190)/1.1 = 8.8 m3.

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 To burn gas it works with some excess air.

So, the practical air flow

L n = αL T,

Where Ln- practical air flow; α - excess air coefficient; L T- theoretical air flow.

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 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 a minimum value of a, since with a decrease in excess air, heat losses from the flue gases are reduced. The air that takes part in combustion is primary and secondary. Primary called the air entering the burner to be mixed with gas; secondary- air entering the combustion zone not mixed with gas, but separately.

Section Contents

When organic fuels are burned in boiler furnaces, various combustion products are formed, such as carbon oxides CO x = CO + CO 2, water vapor H 2 O, sulfur oxides SO x = SO 2 + SO 3, nitrogen oxides NO x = NO + NO 2 , polycyclic aromatic hydrocarbons (PAHs), fluoride compounds, vanadium compounds V 2 O 5, solid particles, etc. (see Table 7.1.1). In case of incomplete combustion of fuel in furnaces, the exhaust gases may also contain hydrocarbons CH 4, C 2 H 4, etc. All products of incomplete combustion are harmful, but modern technology By burning fuel, their formation can be minimized [1].

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

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

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

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

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

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

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

In addition, the combustion process produces a number of chemical compounds formed due to the oxidation of various components of the fuel and air nitrogen N2. The most significant part of them consists of nitrogen oxides NO x and sulfur oxides SO x .

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

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

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

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

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

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

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

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

When burning fuel oil and solid fuel, emissions also contain solid particles consisting of fly ash, soot particles, PAHs and unburned fuel as a result of mechanical underburning.

The ranges of concentrations of harmful substances in flue gases when burning various types of fuels are given in table. 7.1.3.