Natural gas is the most commonly used fuel today. Natural gas is called natural gas because it is extracted from the very depths of the Earth.
Gas combustion is a chemical reaction in which natural gas interacts with oxygen in the air.
The gaseous fuel contains a combustible and 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 5 to 15%. These qualities made it possible to use natural gas as one of the main types of fuel. The concentration of methane more than 10% is life-threatening, so asphyxiation may occur due to lack of oxygen.
To detect a gas leak, 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 can contain flammable gases - propane, butane and ethane.
To ensure high-quality combustion of gas, it is necessary to supply air in sufficient quantities to the combustion zone and achieve good mixing of gas with air. The optimal ratio is 1: 10. That is, one part of the gas accounts for ten parts of air. In addition, it is necessary to create the desired temperature regime. In order for the gas to ignite, it is necessary to heat it up 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 emitted 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 = CO 2 + 2H 2 O
In addition to these gases, nitrogen and the remaining oxygen are released into the atmosphere with combustible gases. N 2 + O 2
If the combustion of the gas does not occur completely, then flammable substances are emitted into the atmosphere - carbon monoxide, hydrogen, soot.
Incomplete combustion of gas occurs due to insufficient air. At the same time, soot tongues appear in the flame.
The danger of incomplete combustion of the gas is that carbon monoxide can poison the boiler room personnel. The CO content in the air of 0.01-0.02% can cause mild poisoning. 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 reduces the efficiency of the boiler room. Soot conducts heat 200 times worse than methane.
Theoretically, 1m3 of gas requires 9m3 of air to burn. 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 theoretically necessary.
The alpha coefficient depends on the type of a specific burner and is usually prescribed in the burner passport or in accordance with the recommendations of the organization of 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, flame separation may occur, creating an emergency. If the amount of air is less than recommended, the combustion will be incomplete, thereby creating a threat of poisoning for the boiler room personnel.
For a more accurate control of the quality of fuel combustion, there are instruments - gas analyzers that measure the content of certain substances in the composition of flue gases.
Gas analyzers can be supplied with boilers. If they are not there, the appropriate 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.
- discharge in the firebox.
- Boiler efficiency.
In this case, the coefficient of efficiency of the boiler means the ratio of useful heat to the amount of all consumed heat.
Air composition
| 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% |
The combustion products of natural gas are carbon dioxide, water vapor, some excess oxygen and nitrogen. The 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" has been 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 data in the table 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, with the help of a gas analyzer, it is necessary to determine the amount of CO 2 in the gas combustion products and divide the CO 2max value taken from the table by the obtained value. So, for example, if during gas combustion, the combustion products contain 10.2% of carbon dioxide, then the excess air ratio in the furnace is
α = CO 2max / CO 2 analysis = 11.8 / 10.2 = 1.15.
The most perfect way to control the flow of air into the furnace and the completeness of its combustion is to analyze the combustion products using automatic gas analyzers. Gas analyzers periodically take a sample of off-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 arrow readings of the gas analyzer on the scale (CO 2 + H 2) are equal to zero, this means that the combustion is complete, and there is no carbon monoxide and 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 arrow of the gas analyzer should show the maximum content of CO 2max 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.
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), fluorides, vanadium compounds V 2 O 5, solid particles, etc. (see Table 7.1.1). In case of incomplete combustion of fuel in furnaces, exhaust gases may also contain hydrocarbons CH 4, C 2 H 4, etc. All products of incomplete combustion are harmful, however, with modern technology of fuel combustion, 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 ash and sulfur content per working mass of fuel,%.
The criterion for the sanitary assessment of the environment is the maximum permissible concentration (MPC) of a harmful substance in the ambient air at ground level. MPC should be understood as such a concentration of various substances and chemical compounds, which does not cause any pathological changes or diseases with daily exposure for a long time on the human body.
The maximum permissible concentration (MPC) of harmful substances in the atmospheric air of populated areas are given in table. 7.1.2 [4]. The maximum one-time concentration of harmful substances is determined by samples taken within 20 minutes, the average daily - per day.
Table 7.1.2. Maximum permissible concentration of harmful substances in the atmospheric air of populated areas
| Pollutant | Maximum permissible concentration, mg / m 3 | |
| Maximum one-time | Average daily | |
| Dust non-toxic | 0,5 | 0,15 |
| sulphur dioxide | 0,5 | 0,05 |
| Carbon monoxide | 3,0 | 1,0 |
| Carbon monoxide | 3,0 | 1,0 |
| Nitrogen dioxide | 0,085 | 0,04 |
| Nitric oxide | 0,6 | 0,06 |
| Soot (soot) | 0,15 | 0,05 |
| Hydrogen sulfide | 0,008 | 0,008 |
| Benz (a) pyrene | - | 0.1 μg / 100 m 3 |
| Vanadium pentaxide | - | 0,002 |
| Fluoride compounds (by fluorine) | 0,02 | 0,005 |
| Chlorine | 0,1 | 0,03 |
Calculations are carried out for each hazardous substance separately, so that the concentration of each of them does not exceed the values given in table. 7.1.2. For boiler houses, these conditions are toughened by the introduction of additional requirements on the need to add up the effect of sulfur and nitrogen oxides, which is determined by the expression
At the same time, due to local air shortages or unfavorable thermal and aerodynamic conditions, incomplete combustion products are formed in 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 incompleteness of combustion (chemical incomplete combustion).
In addition, the combustion process produces a number of chemical compounds formed as a result of the oxidation of various components of the fuel and nitrogen in the air N 2. The most significant part of them are nitrogen oxides NO x and sulfur SO x.
Nitrogen oxides are formed due to the oxidation of both molecular nitrogen in the air and nitrogen contained in the fuel. Experimental studies have shown that the main share of NO x formed in the furnaces of boilers, namely 96 ÷ 100%, falls on nitrogen monoxide (oxide) NO. Dioxide NO 2 and nitrogen hemioxide N 2 O are formed in much smaller quantities, and their share is approximately: for NO 2 - up to 4%, and for N 2 O - hundredths of a percent of the total NO x emission. Under typical conditions of flare combustion of fuels in boilers, the concentration of nitrogen dioxide NO 2, as a rule, is negligible in comparison with the content of NO and usually ranges 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 relatively high concentrations of nitrogen dioxide in the cold regions of the flow. In addition, partial emission of NO 2 occurs in the upper part of the furnace and in the horizontal gas duct (at T> 900 ÷ 1000 K) and under certain conditions can also reach noticeable sizes.
Nitrogen hemioxide N 2 O, formed during fuel combustion, is, most likely, a short-lived intermediate. N 2 O is practically absent in combustion products behind boilers.
The sulfur contained in the fuel is the source of the formation of sulfur oxides SO x: sulfurous SO 2 (sulfur dioxide) and sulfuric SO 3 (sulfur trioxide) anhydrides. The total mass emission of SO x depends only on the sulfur content in the fuel S p, and their concentration in the flue gases also depends on the air flow coefficient α. As a rule, the share of SO 2 is 97 ÷ 99%, and the proportion of SO 3 is 1 ÷ 3% of the total SO x output. The actual SO 2 content in the gases leaving the boilers ranges from 0.08 to 0.6%, and the SO 3 concentration - from 0.0001 to 0.008%.
Among the harmful components of flue gases, a large group of polycyclic aromatic hydrocarbons (PAHs) occupies a special place. Many PAHs have high carcinogenic and (or) mutagenic activity, activate photochemical smogs in cities, which requires strict control and limitation of their emission. 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 the 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 in furnaces (combustion chambers) of local oxidizing zones with a low temperature or zones with excess fuel.
Due to the large amount of different PAHs in flue gases and the difficulty of measuring their concentrations, it is customary to assess the level of carcinogenic pollution of combustion products and atmospheric air by the concentration of the most powerful and stable carcinogen - benzo (a) pyrene (B (a) P) C 20 H 12.
Due to the high toxicity, such products of fuel oil combustion as vanadium oxides should be especially noted. Vanadium is contained in the mineral part of fuel oil and, when burned, forms vanadium oxides VO, VO 2. However, during the formation of deposits on convective surfaces, vanadium oxides are present mainly in the form of V 2 O 5. Vanadium pentoxide V 2 O 5 is the most toxic form of vanadium oxides; therefore, their emissions are accounted for in terms of V 2 O 5.
Table 7.1.3. Approximate concentration of harmful substances in combustion products during flare combustion of organic fuels in power boilers
| Emissions = | Concentration, mg / m 3 | ||
| Natural gas | Fuel oil | Coal | |
| Nitrogen oxides NO x (in terms of NO 2) | 200 ÷ 1200 | 300 ÷ 1000 | 350 ÷ 1500 |
| Sulfurous anhydride SO 2 | - | 2000 ÷ 6000 | 1000 ÷ 5000 |
| Sulfuric anhydride SO 3 | - | 4 ÷ 250 | 2 ÷ 100 |
| Carbon monoxide CO | 10 ÷ 125 | 10 ÷ 150 | 15 ÷ 150 |
| Benz (a) pyrene С 20 Н 12 | (0.1 ÷ 1.0) 10 -3 | (0.2 ÷ 4.0) · 10 -3 | (0.3 ÷ 14) · 10 -3 |
| Solid particles | - | <100 | 150 ÷ 300 |
When fuel oil and solid fuels are burned, the emissions also contain particulate matter, consisting of fly ash, soot particles, PAHs and unburned fuel as a result of mechanical underburning.
The ranges of concentrations of harmful substances in flue gases during the combustion of various types of fuels are given in table. 7.1.3.
Combustion is a reaction in which the chemical energy of the fuel is converted into heat.
Burning is complete and incomplete. Complete combustion takes place with sufficient oxygen. Lack of it causes incomplete combustion, in which less heat is released than with complete combustion, and carbon monoxide (CO), poisoning the operating personnel, forms soot that settles on the heating surface of the boiler and increases heat loss, which leads to excessive consumption of fuel 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 the complete combustion of natural gas, air is supplied to the furnace with a slight excess. The ratio of the actually consumed volume of air V d to the theoretically required V t is called the excess air ratio = V d / V t. This indicator depends on the design of the gas burner and the furnace: the more perfect they are, the less. It is necessary to ensure that the excess air factor is not less than 1, as this leads to incomplete combustion of the gas. An increase in 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 regulated by increasing the air supply to the boiler furnace or decreasing the gas supply. This process uses primary (mixed with gas in the burner - before combustion) and secondary (combined 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 through injection and is regulated by an adjusting washer, and secondary air through the blower doors.
In boilers with mixing burners, primary and secondary air is supplied to the burner by a fan and regulated 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 the 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 - breakthrough.
If the flame breaks off and breaks through, the operating personnel must extinguish the boiler, ventilate the furnace and gas ducts and re-ignite the boiler.
Every year gaseous fuel finds more and more widespread use in various sectors of the national economy. In agricultural production, gaseous fuel is widely used for technological (heating greenhouses, hotbeds, dryers, livestock and poultry complexes) and domestic purposes. Recently, it has been increasingly used for internal combustion engines.
Compared to other types of fuels, gaseous fuels have the following advantages:
burns in a theoretical amount of air, which ensures high thermal efficiency and combustion temperature;
during combustion does not form undesirable products of dry distillation and sulfur compounds, soot and smoke;
it is relatively easy to supply via gas pipelines to remote consumption objects and can be stored centrally;
easily ignites at any ambient temperature;
requires relatively low production costs, which means that it is a cheaper type of fuel in comparison with other;
can be used in compressed or liquefied form for internal combustion engines;
has high anti-knock properties;
does not form condensation 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 poisonous effect, the formation of explosive mixtures when mixed with air, easy flow through leaks, etc. Therefore, when working with gaseous fuel, careful observance of the relevant safety rules is required.
The use of gaseous fuels is determined by their composition and properties of the hydrocarbon part. The most widely used natural or associated gas of oil or gas fields, as well as plant gases of oil refineries and other plants. The main components of these gases are hydrocarbons with the number of carbon atoms in a molecule from one to four (methane, ethane, propane, butane and their derivatives).
Natural gases from gas fields almost entirely consist of methane (82 ... 98%), with a small Use of gaseous fuels for internal combustion engines The continuously increasing fleet of cars requires more and more fuel. It is possible to solve the most important national economic problems of a stable supply of automobile engines with efficient energy carriers and a 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 running on low-calorific gas, the engine does not develop the required power, and the vehicle's range is reduced, which is economically unprofitable. Pa). The following types of compressed gases are produced: natural, mechanized coke oven and enriched coke oven
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 due to its corrosive effect on gas equipment and engine parts. The octane number of gases makes it possible to boost automobile engines in terms of compression ratio (up to 10 ... 12).
The presence of cyanogen CN in automobile gas is highly undesirable. Combining with water, it forms hydrocyanic acid, under the action of which tiny cracks are formed in the walls of the cylinders. The presence of resinous substances and mechanical impurities in the gas leads to the formation of deposits and contamination on the instruments of gas equipment and on engine parts.
Composition and properties of natural gas. Natural gas (combustible natural gas; GGP) - Gaseous mixture consisting of methane and heavier hydrocarbons, nitrogen, carbon dioxide, water vapor, sulfur-containing compounds, inert gases . Methane is the main component of GHP. HGP usually also contains trace amounts of other components (Figure 1).
1. Combustible components include hydrocarbons:
a) methane (CH 4) - the main component of natural gas, up to 98% by volume (other components are present in small amounts or absent). Colorless, odorless and tasteless, non-toxic, explosive, lighter than air;
b) heavy (saturated) hydrocarbons [ethane (C 2 H 6), propane (C 3 H 8), butane (C 4 H 10), etc.] - colorless, odorless and tasteless, non-toxic, explosive, heavier than air.
2. Non-combustible components (ballast) :
a) nitrogen (N 2) - an integral part of air, colorless, odorless and tasteless; inert gas, since it does not interact with oxygen;
b) oxygen (O 2) - an integral part of air; colorless, odorless and tasteless; an oxidizing agent.
c) carbon dioxide (carbon dioxide CO 2) - colorless with a slightly sour taste. When the content in the air is more than 10%, it is toxic, heavier than air;
Air . Dry atmospheric air is a multicomponent gas mixture consisting of (vol.%): Nitrogen N 2 - 78%, oxygen O 2 - 21%, inert gases (argon, neon, krypton, etc.) - 0.94% and carbon dioxide - 0.03%.
 Fig. 2. Air composition. |
The air also contains water vapor and random impurities - ammonia, sulfur dioxide, dust, microorganisms, etc. ( rice. 2). The gases that are part of the air are evenly distributed in it and each of them retains its properties in the mixture.
3. Harmful components :
a) hydrogen sulfide (H 2 S) - colorless, with the smell of rotten eggs, toxic, burns, heavier than air.
b) hydrocyanic (hydrocyanic) acid (HCN) is a colorless light liquid, in a gas it has a gaseous state. Poisonous, corrosive to metal.
4. Mechanical impurities (the content depends on the conditions of gas transportation):
a) tar and dust - mixing can form blockages in gas pipelines;
b) water - freezes at low temperatures, forming ice plugs, which leads to freezing of the reducing devices.
GGPon toxicological characterization refer to substances of the ΙV-th hazard class in accordance with GOST 12.1.007. These are gaseous, low-toxic, fire-and-explosive products.
Density: atmospheric air density under normal conditions - 1.29 kg / m 3, and methane - 0.72 kg / m 3 hence methane is lighter than air.
GOST 5542-2014 requirements for GGP indicators:
1) mass concentration of hydrogen sulfide- no more than 0.02 g / m 3;
2) mass concentration of mercaptan sulfur- no more than 0.036 g / m 3;
3) oxygen mole fraction- no more than 0.050%;
4) permissible content of mechanical impurities- no more than 0.001 g / m 3;
5) mole fraction of carbon dioxide in natural gas, not more than 2.5%.
6) Net calorific value GGP under standard combustion conditions according to GOST 5542-14 - 7600 kcal / m 3 ;
8) gas odor intensity for for household purposes with a volume fraction of 1% in the air - not less than 3 points, and for for industrial gas, this indicator is set by agreement with the consumer.
Unit of commercial expenditure GGP - 1 m 3 of gas at a pressure of 760 mm Hg. Art. and a temperature of 20 о С;
Autoignition temperature- the lowest temperature of the heated surface, which, under specified conditions, ignites flammable substances in the form of a gas or vapor-air mixture. For methane it is 537 ° C. Combustion temperature (maximum temperature in the combustion zone): methane - 2043 ° C.
Specific heat of combustion of methane: the lowest - Q H = 8500 kcal / m 3, the highest - Qw - 9500 kcal / m 3. For the purposes of comparing fuel types, the concept was introduced equivalent fuel (standard fuel) , in RF for its unit the heat of combustion of 1 kg of coal was taken, equal to 29.3 MJ or 7000 kcal / kg.
Gas flow measurement conditions are:
· normal conditions(n. at): standard physical conditions with which the properties of substances are usually correlated. Reference conditions are defined by IUPAC (International Union of Practical and Applied Chemistry) as follows: Atmosphere pressure 101325 Pa = 760 mmHg st..Air temperature 273.15 K = 0 ° C .Density of methane at Well.- 0.72 kg / m 3,
· standard conditions(With. at) volume at mutual ( commercial) settlements with consumers - GOST 2939-63: temperature 20 ° С, pressure 760 mm Hg. (101325 N / m), humidity is zero. (By GOST 8.615-2013 normal conditions are referred to as "standard conditions"). Density of methane at s.u.- 0.717 kg / m 3.
Flame propagation speed (burning speed)- the speed of movement of the flame front relative to the fresh jet of the combustible mixture in a given direction... Approximate flame propagation speed: propane - 0.83 m / s, butane - 0.82 m / s, methane - 0.67 m / s, hydrogen - 4.83 m / s., 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.
To the disadvantages (dangerous properties) GGP include: explosiveness (flammability); intense burning; rapid spread in space; impossibility of determining the location; suffocating action, with a lack of oxygen for breathing .
Explosiveness (flammability) . Distinguish:
a) lower flammability limit ( NPV) - the lowest gas content in the air at which the gas ignites (methane - 4.4%) ... With a lower gas content in the air, ignition will not occur due to a lack of gas; (fig. 3)
b) upper flammability limit ( ERW) - the highest gas content in the air, at which the ignition process occurs ( methane - 17%) ... With a higher gas content in the air, there will be no ignition due to lack of air. (fig. 3)
V FNP NPV and ERW are called lower and upper concentration limits and flame propagation ( NKPRP and VKPRP) .
At increasing gas pressure the range between the upper and lower limits of gas pressure - decreases (Fig. 4).
For a gas explosion (methane) Besides its content in the air within the flammability range is necessary third-party energy source (spark, flame, etc.) ... Gas explosion in a closed space (room, firebox, reservoir, etc.), more destruction than an explosion in the open air (rice. 5).
Maximum allowable concentration ( MPC) HGP hazardous substances in the air of the working area are set in GOST 12.1.005.
Maximum one-time MPC in the air of the working area (in terms of carbon) is 300 mg / m 3.
Dangerous concentration GGP (volume fraction of gas in air) Is concentration equal to 20% of the lower flammability limit of the gas.
Toxicity - the ability to poison the human body. Hydrocarbon gases do not have a strong toxicological effect on the human body, but their inhalation makes a person dizzy, and their significant content is in the inhaled air. When oxygen decreases to 16% or less, can lead to suffocate.
At burning gas with a lack of oxygen, i.e., with underburning, carbon monoxide (CO), or carbon monoxide, which is a highly toxic gas.
Gas odorization - adding a strong-smelling substance (odorant) to the gas to impart a scent GGP before delivery to consumers in urban networks. At use for odorization of ethyl mercaptan (C 2 H 5 SH - by the degree of impact on the body belongs to the ΙΙ-th class of toxicological hazard according to GOST 12.1.007-76 ), it is added 16 g per 1000m 3 . The odor intensity of odorized HGP with a volume fraction of 1% in the air must be at least 3 points in accordance with GOST 22387.5.
Non-odorized gas can be supplied to industrial plants, because the intensity of the smell of natural gas for industrial enterprises consuming gas from main gas pipelines is established by agreement with the consumer.
Combustion of gases. The firebox of a boiler (furnace), in which gaseous (liquid) fuel is burned in a torch, corresponds to the concept of "chamber furnace of a stationary boiler".
Combustion of hydrocarbon gases - chemical combination of combustible gas components (carbon C and hydrogen H) with atmospheric oxygen O 2 (oxidation) with the release of heat and light: CH 4 + 2O 2 = CO 2 + 2H 2 O .
With complete combustion carbon produced carbon dioxide (CO 2), and water kind - water vapor (H 2 O) .
In theory for combustion of 1 m 3 of methane, 2 m 3 of oxygen are required, which are contained in 9.52 m 3 of air (Fig. 6). If insufficient combustion air , then for some of the molecules of the combustible components there will not be enough oxygen molecules and in combustion products, in addition to carbon dioxide (CO 2), nitrogen (N 2) and water vapor (H 2 O), will appear products incomplete combustion of gas :
-carbon monoxide (CO), which, if it enters the room, can cause poisoning of the service personnel;
- soot (C) , which, being deposited on heating surfaces impairs heat transfer;
- unburned methane and hydrogen , which can accumulate in furnaces and gas ducts (chimneys), forming an explosive mixture. When there is a lack of air, incomplete combustion of fuel or, as they say, the combustion process occurs with underburning... Underburning can also occur when poor mixing of gas with air and low temperature in the combustion zone.
For the complete combustion of gas, it is necessary: the presence of air in the place of combustion in sufficient quantity and mixing it well with gas; high temperature in the combustion zone.
To ensure complete combustion of the gas, air is supplied in a larger than theoretically required quantity, i.e., in excess, while not all of the air will take part in the combustion. Part of the heat will be spent on heating this excess air and will be released into the atmosphere along with the flue gas.
The completeness of combustion is determined visually (there should be a bluish-bluish flame with violet ends) or by analyzing the composition of flue gases.
Theoretical (stoichiometric) combustion air volume Is the amount of air required to completely burn a unit of volume ( 1 m 3 dry gas or fuel mass, calculated by the chemical composition of the fuel ).
Valid (actual, required) combustion air volume is the amount of air actually consumed to burn a unit volume or mass of fuel.
Excess combustion air ratio α is the ratio of the actual volume of combustion air to the theoretical one: α = V f / V t >1,
where: V f - the actual volume of the supplied air, m 3;
V t - theoretical volume of air, m 3.
Coefficient excess shows how many times actual air consumption for gas combustion exceeds theoretical and depends on the design of the gas burner and furnace: the more perfect they are, the α less. If the excess air ratio for boilers is less than 1, it leads to incomplete combustion of the gas. An increase in the excess air ratio decreases the efficiency. gas-powered installation. For a number of furnaces where metal melting takes place, in order to avoid oxygen corrosion - α < 1 and a afterburning chamber for unburned combustible components is installed behind the firebox.
For thrust control, guide vanes, gate valves, butterfly valves and electromechanical couplings are used.
Advantages of gaseous fuels versus solid and liquid fuels- low cost, easier labor of personnel, low amount of harmful impurities in combustion products, improved environmental protection conditions, no need for road and rail transport, good mixing with air (less α), full automation, high efficiency.
Gas combustion methods. Combustion air can be:
1) primary, is supplied to the inside of the burner, where it is mixed with gas (gas-air mixture is used for combustion).
2) secondary, goes directly to the combustion zone.
There are the following gas combustion methods:
1. Diffusion method- gas and air are supplied separately for combustion and are mixed in the combustion zone, i.e. all air is secondary. The flame is long, a large combustion chamber is required. (Fig.7a).
2. Kinetic method - all the air is mixed with the gas inside the burner, i.e. all air is primary. The flame is short, a small combustion chamber is required (Fig.7c).
3. Mixed method - part of the air is supplied to the inside of the burner, where it is mixed with gas (this is primary air), and part of the air is supplied to the combustion zone (secondary). The flame is shorter than with the diffusion method (Fig. 7b).
Removal of combustion products. The vacuum in the furnace and the removal of combustion products are produced by the traction force, which overcomes the resistance of the smoke path and arises from the pressure difference of equal height columns of external cold air and a lighter hot flue gas. In this case, flue gases move from the furnace to the chimney, and cold air enters the furnace instead of them (Fig. 8).
Traction force depends on: temperature of air and flue gases, height, diameter and wall thickness of the chimney, barometric (atmospheric) pressure, state of gas ducts (chimneys), air suction, vacuum in the furnace .
Natural traction force - created by the height of the chimney, and artificial, which is a smoke exhauster with insufficient natural draft. The thrust force is regulated by dampers, smoke exhauster guide vanes and other devices.
Excess air ratio (α ) depends on the design of the gas burner and the furnace: the more perfect they are, the lower the coefficient and shows: how many times the actual air consumption for gas combustion exceeds the theoretical one.
Pressurization - removal of fuel combustion products due to the operation of blowing fans .When working “under supercharged”, a strong, dense combustion chamber (firebox) is required, capable of withstanding the excess pressure created by the fan.
Gas burner devices.Gas-burners- provide the supply of the required amount of gas and air, their mixing and regulation of the combustion process, and equipped with a tunnel, air distribution device, etc., is called a gas burner device.
Burner requirements:
1) burners must meet the requirements of the relevant technical regulations (have a certificate or a declaration of conformity) or undergo an industrial safety examination;
2) ensure the completeness of gas combustion in all operating modes with a minimum excess of air (except for some burners of gas furnaces) and a minimum emission of harmful substances;
3) be able to use automatic control and safety, as well as measure the parameters of gas and air in front of the burner;
4) must have a simple structure, be accessible for repair and revision;
5) work stably within the limits of the working regulation, if necessary, have stabilizers to prevent the separation and breakthrough of the flame;
Gas burner parameters(fig. 9). According to GOST 17356-89 (Gas, oil and combined burners. Terms and definitions. Rev. N 1) :Burner stability limit at which do not arise yet extinction, stall, separation, flame breakthrough and unacceptable vibrations.
Note. Exists top and bottom the limits of sustainable performance.
1) Heat output of the burner N g... - the amount of heat generated as a result of combustion of fuel supplied to the burner per unit of time, N g = V. Q kcal / h, where V is the hourly gas consumption, m 3 / h; Q n. - heat of combustion of gas, kcal / m 3.
2) Burner stability limits at which do not arise yet extinction, stall, separation, flame breakthrough and unacceptable vibrations . Note. Exists upper - N c.p. . and lower -N n.p the limits of sustainable performance.
3) minimum power N min. - thermal power of the burner, which is 1.1 power, corresponding to the lower limit of its stable operation, i.e. power of the lower limit increased by 10%, N min. = 1.1N n.p.
4) upper limit of stable operation of the burner N c.p. - the highest stable power, work without separation and flame breakthrough.
5) maximum burner power N max - burner thermal power, which is 0.9 power corresponding to the upper limit of its stable operation, i.e. upper limit power reduced by 10%, N max. = 0.9 N c.p.
6) rated power N nom - the highest thermal power of the burner, when the performance indicators correspond to the established standards, i.e. the highest power with which the burner operates for a long time with high efficiency.
7) operating regulation range (thermal power of the burner) - a regulated range in which the thermal power of the burner can change during operation, i.e. power values from N min to N nom. ...
8) coefficient of working regulation К рр. - the ratio of the nominal thermal power of the burner to its minimum operating thermal power, i.e. shows how many times the rated power exceeds the minimum: K pp. = N nom./N min
Regime card.According to the "Rules for the use of gas ...", approved by the RF Resolution No. 317 of 17.05.2002(revised 19.06.2017) , upon completion of construction and installation work on the constructed, reconstructed or modernized gas-using equipment and equipment converted to gas from other types of fuel, commissioning and adjustment works are carried out. Gas start-up on constructed, reconstructed or modernized gas-using equipment and equipment converted to gas from other types of fuel, for carrying out commissioning (comprehensive testing) and acceptance of equipment into operation is carried out on the basis of an act on the readiness of gas consumption networks and gas-using equipment of the capital construction facility for connection (technological connection). The rules established that:
· gas-using equipment - boilers, industrial furnaces, technological lines, heat recovery units and other installations using gas as fuel in order to generate thermal energy for centralized heating, hot water supply, in the technological processes of various industries, as well as other devices, apparatus, units, technological equipment and installations that use gas as a raw material;
· commissioning works- complex of works, including preparation for start-up and start-up of gas-using equipment with communications and fittings, bringing the load of gas-using equipment up to the level agreed with the organization - the owner of the equipment, a also adjustment of the combustion mode of gas-using equipment without optimization of the efficiency;
· operating and commissioning works- complex of works, including adjustment of gas-using equipment in order to achieve the design (passport) efficiency in the range of working loads, adjustment of means for automatic regulation of fuel combustion processes, heat recovery plants and auxiliary equipment, including water treatment equipment for boiler houses.
According to GOST R 54961-2012 (Gas distribution systems. Gas consumption networks) it is recommended:Modes of operation gas-using equipment at enterprises and in boiler houses must match the mode cards approved by the technical manager of the enterprise and P produced at least once every three years with correction (if necessary) of regime cards .
An unscheduled adjustment of gas-using equipment should be carried out in the following cases: after a major overhaul of gas-using equipment or making design changes that affect the efficiency of gas use, as well as in case of systematic deviations of the controlled parameters of the operation of gas-using equipment from the operating maps.
Classification of gas burners According to GOST gas burners are classified according to: the method of feeding the component; the degree of preparation of the combustible mixture; the rate of expiration of combustion products; the nature of the mixture flow; nominal gas pressure; degree of automation; the possibility of regulating the excess air ratio and characteristics of the torch; localization of the combustion zone; the possibility of using the heat of combustion products.
V chamber furnace of a gas-powered installation gaseous fuel is flared.
By the method of air supply, the burners can be:
1) Atmospheric burners -air enters the combustion zone directly from the atmosphere:
a. Diffusion – this is the simplest burner in design, which, as a rule, is a pipe with holes drilled in one or two rows. Gas enters the combustion zone from the pipe through the holes, and air - due diffusion and gas jet energy (rice. 10 ), all air is secondary .
Burner advantages : simplicity of design, reliability of operation ( no flame breakthrough possible ), quiet operation, good regulation.
Flaws: low power, uneconomical, high (long) flame, flame stabilizers are required to prevent the burner flame from going out at separation .
b. Injection - air injected, i.e. sucked into the inside of the burner due to the energy of the gas jet leaving the nozzle ... The gas jet creates a vacuum in the nozzle area, where air is sucked in through the gap between the air washer and the burner body. Inside the burner, gas and air are mixed, and the gas-air mixture enters the combustion zone, and the rest of the air necessary for gas combustion (secondary) enters the combustion zone due to diffusion (Fig. 11, 12, 13 ).
Depending on the amount of injected air, a distinction is made between injection burners: with incomplete and complete premixing of gas and air.
Into the burners medium and high pressure gas all the necessary air is sucked in, i.e. all the air is primary, there is a complete preliminary mixing of gas with air. A completely prepared gas-air mixture enters the combustion zone and there is no need for secondary air.
Into the burners low pressure part of the air required for combustion is sucked in (incomplete air injection occurs, this air is primary), and the rest of the air (secondary) enters directly into the combustion zone.
The gas-to-air ratio in these burners is controlled by the position of the air disc relative to the burner body. Burners are single-flare and multi-flare with central and peripheral gas supply (BIG and BIGm) consisting of a set of tubes - mixers 1 with a diameter of 48x3, united by a common gas manifold 2 (Fig. 13 ).
The advantages of burners: simplicity of design and power regulation.
Disadvantages of burners: high noise level, the possibility of flame breakthrough, a small range of operating regulation.
2) Forced air burners - These are burners in which combustion air is supplied from a fan. Gas from the gas pipeline enters the internal combustion chamber (Fig. 14 ).
The air blown by the fan is supplied to the air chamber 2 , passes through the air swirler 4 , twisted and mixed in the mixer 5 with gas that enters the combustion zone from the gas channel 1 through the gas outlets 3 .Burning takes place in a ceramic tunnel 7 .
 Rice. 14. Burner with forced air supply: 1 - gas channel; 2 - air channel; 3 - gas outlet openings; 4 - swirler; 5 - mixer; 6 - ceramic tunnel (combustion stabilizer). |  Rice. 15. Combined single-flow burner: 1 - gas inlet; 2 - fuel oil inlet; 3 - steam inlet, gas outlet openings; 4 - primary air inlet; 5 - secondary air inlet mixer; 6 - steam-oil nozzle; 7 - mounting plate; 8 - primary air swirler; 9 - secondary air swirler; 10 - ceramic tunnel (combustion stabilizer); 11 - gas channel; 12 - secondary air channel. |
Burners advantages: high thermal power, a 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 the design; separation and breakthrough of the flame is possible, in connection with which it becomes necessary to use combustion stabilizers (ceramic tunnel).
Burners designed to burn several types of fuel (gaseous, liquid, solid) are called combined (rice. 15 ). They can be single-threaded and double-threaded, i.e. with one or more gas inlets to the burner.
3) Block burner Is an automatic burner with forced sub-air (rice. 16 )combined with a fan into a single unit... The burner is equipped with an automatic regulation system.
The combustion process in block burners is controlled by an electronic device called a combustion manager.
For oil burners, this block includes a fuel pump or a fuel pump and a fuel preheater.
The control unit (combustion manager) controls and monitors the operation of the burner, receiving commands from the thermostat (temperature controller), flame control electrode and gas and air pressure sensors.
Gas flow is regulated by a butterfly valve located outside the burner housing.
The backing washer is responsible for mixing the gas with air in the conical part of the flame tube and is used to regulate the supply air (regulation from the pressure side). Another possibility to change the amount of supplied air is to change the position of the air butterfly valve in the air regulator housing (adjustment from the suction side).
The regulation of the gas-air ratio (control of gas and air butterfly valves) can be:
Connected, from one actuator:
· Frequency regulation of the air flow rate, by changing the rotation frequency of the fan motor using an inverter, which consists of a frequency converter and a pulse sensor.
The burner is ignited automatically by the ignition device using the ignition electrode. The presence of a flame is monitored by a flame monitoring electrode.
Burner start sequence:
· Request for heat generation (from the thermostat);
· Switching on the fan electric motor and preliminary ventilation of the furnace;
· Inclusion of electronic ignition;
· Opening of the solenoid valve, gas supply and ignition of the burner;
· Signal from the flame control sensor about the presence of flame.
Accidents (incidents) on burners. Flame separation - moving the torch root zone from the outlets of the burner in the direction of flow of fuel or combustible mixture... It occurs when the speed of the gas-air mixture or gas becomes greater than the speed of propagation of the flame. The flame moves away from the burner, becomes unstable and may go out. Gas continues to flow through the extinguished burner and an explosive mixture may form in the furnace.
The separation occurs when: the gas pressure rises above the permissible, a sharp increase in the supply of primary air, an increase in the vacuum in the furnace. For tear-off protection apply combustion stabilizers (rice. 17): brick slides and posts; ceramic tunnels of various types and brick crevices; bluff bodies that glow during burner operation (when the flame goes out, a fresh stream will light up from the stabilizer), as well as special pilot burners.
Flame breakthrough - moving the torch zone towards the combustible mixture, in which the flame penetrates into the burner ... This phenomenon occurs only in burners with premixing of gas and air and occurs when the speed of the gas-air mixture becomes less than the speed of propagation of the flame. The flame slips into the inside of the burner, where it continues to burn, causing deformation of the burner from overheating.
A breakthrough occurs when: the gas pressure in front of the burner drops below the permissible value; ignition of the burner when the primary air is supplied; large gas supply at low air pressure. In the event of a breakthrough, a small pop may occur, as a result of which the flame will go out, while gas may continue to flow through the inoperative burner and an explosive mixture may form in the firebox and gas ducts of the gas-using installation. To protect against breakthrough, plate or mesh stabilizers are used., since through narrow slots and small openings there is no flame breakthrough.
Personnel actions in case of burner accidents
In the event of an accident on the burner (separation, breakthrough or extinguishing of the flame) during ignition or in the process of regulation, it is necessary: Immediately stop the gas supply to this burner (burners) and the ignition device; ventilate the furnace and gas ducts for at least 10 minutes; find out the cause of the malfunction; report to the person in charge; after eliminating the causes of malfunctions and checking the tightness of the shut-off valve in front of the burner, at the direction of the person in charge, according to the instructions, reignite.
Change in burner load.
There are burners with different methods of changing the heat output:
Burner with multi-stage heat output regulation Is a burner, during which the fuel consumption regulator can be set in several positions between the maximum and minimum operating positions.
Burner with three-stage heat output regulation is a burner, during which the fuel flow regulator can be set in the positions "maximum flow" - "minimum flow" - "closed".
Burner with two-stage heat output control- burner operating in open-closed positions.
Continuously adjustable burner is a burner, during which the fuel flow regulator can be installed in any position between the maximum and minimum operating positions.
The thermal power of the installation can be regulated by the number of operating burners, if provided by the manufacturer and the regime card.
Changing the heat output manually, in order to avoid the separation of the flame, it is performed:
When increasing: first increase the gas supply and then the air supply.
When decreasing: first reduce the air supply and then the gas supply;
To prevent accidents on burners, the change in their power must be done smoothly (in several steps) according to the regime map.
