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 combustion of gas, 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 desired 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.
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 amount of excess air above the recommended one, heat losses increase. With a significant increase in the amount of air, flame separation can occur, creating an emergency. If the amount of air is less than recommended, then combustion will be incomplete, thereby creating a risk of poisoning the boiler room personnel.
For more accurate control of the quality of fuel combustion, there are devices - gas analyzers that measure the content of certain substances in the composition of exhaust gases.
Gas analyzers can be supplied with boilers. If they are not available, the relevant measurements are carried out by the commissioning organization using portable gas analyzers. A regime map is compiled in which the necessary control parameters are prescribed. By adhering to them, you can ensure the normal complete combustion of the fuel.
The main parameters for fuel combustion control are:
- the ratio of gas and air supplied to the burners.
- excess air ratio.
- crack in the furnace.
- Boiler efficiency factor.
At the same time, the efficiency of the boiler means the ratio of useful heat to the value of the total heat expended.
Composition of air
| Gas name | Chemical element | Content in the air |
| Nitrogen | N2 | 78 % |
| Oxygen | O2 | 21 % |
| Argon | Ar | 1 % |
| Carbon dioxide | CO2 | 0.03 % |
| Helium | He | less than 0.001% |
| Hydrogen | H2 | less than 0.001% |
| Neon | Ne | less than 0.001% |
| Methane | CH4 | less than 0.001% |
| Krypton | kr | less than 0.001% |
| Xenon | Xe | less than 0.001% |
Composition and properties of natural gas. Natural gas (combustible natural gas; GGP) - A gaseous mixture consisting of methane and heavier hydrocarbons, nitrogen, carbon dioxide, water vapor, sulfur-containing compounds, inert gases . Methane is the main component of GGP. HGP usually also contains trace amounts of other components (Fig. 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 quantities or absent). Colorless, odorless and tasteless, non-toxic, explosive, lighter than air;
b) heavy (limiting) hydrocarbons [ethane (C 2 H 6), propane (C h 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) - a component of air, without color, smell and taste; inert gas, because it does not interact with oxygen;
b) oxygen (O 2) - an integral part of the air; colorless, odorless and tasteless; oxidant.
c) carbon dioxide (carbon dioxide CO 2) - no color with a slightly sour taste. When the content in the air is more than 10% 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 make up the air are distributed evenly 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, burning, heavier than air.
b) hydrocyanic (hydrocyanic) acid (HCN) - a colorless light liquid, in a gas it has a gaseous state. Poisonous, causes metal corrosion.
4. Mechanical impurities (content depends on gas transportation conditions):
a) resins and dust - when mixed, they can form blockages in gas pipelines;
b) water - freezes at low temperatures, forming ice plugs, which leads to freezing of reducing devices.
GGPon toxicological characterization belong to substances of the ΙV-th hazard class according to GOST 12.1.007. These are gaseous, low-toxic, fire-explosive products.
Density: atmospheric air density under normal conditions - 1.29 kg / m 3, and methane - 0.72 kg / m 3 Therefore, 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) mole fraction of oxygen- 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 household purposes with a volume fraction of 1% in the air - at least 3 points, and for gas for industrial use, this indicator is set in agreement with the consumer.
Sales expense unit GGP - 1 m 3 gas at a pressure of 760 mm Hg. Art. and temperature 20 o C;
Auto ignition temperature- the lowest temperature of the heated surface, which, under given conditions, ignites combustible 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 \u003d 8500 kcal / m 3, the highest - Qv - 9500 kcal / m 3. For the purpose of comparing types of fuel, the concept equivalent fuel (c.f.) , in RF per unit the calorific value of 1 kg of hard coal was taken equal to 29.3 MJ or 7000 kcal/kg.
Conditions for measuring gas flow 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 273.15K= 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 spread rate (burning rate)- the speed of the flame front relative to the fresh jet of combustible mixture in a given direction. Estimated 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 disadvantages (dangerous properties) GGP include: explosiveness (flammability); intense burning; rapid spread in space; the impossibility of determining the location; suffocating effect, with a lack of oxygen for breathing .
Explosiveness (flammability) . Distinguish:
a) lower flammability limit ( NPS) - the smallest amount of gas in the air at which the gas ignites (methane - 4.4%) . With a lower content of gas in the air, there will be no ignition 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 content of gas in the air, ignition will not occur due to lack of air. (Fig. 3)
AT FNP NPS and ERW called lower and upper concentration limits of flame propagation ( NKPRP and VKPRP) .
At increase in gas pressure the range between the upper and lower limits of gas pressure decreases (Fig. 4).
For gas explosion (methane) Besides its content in the air within the flammable range needed external source of energy (spark, flame, etc.) . With a gas explosion in a closed volume (room, furnace, tank, etc.), more destruction than with an explosion in the open air (rice. 5).
Maximum allowable concentrations ( MPC) harmful substances GGP in the air of the working area are established 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 the concentration equal to 20% lower flammable limit of 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 causes dizziness in a person, and their significant content in the inhaled air. When oxygen is reduced to 16% or less can lead to suffocation.
At burning gas with a lack of oxygen, i.e. with underburning, in the combustion products is formed carbon monoxide (CO), or carbon monoxide, which is a highly toxic gas.
Gas odorization - adding a strong-smelling substance (odorant) to the gas to give an odor GGP before delivery to consumers in city networks. At use for odorization of ethyl mercaptan (C 2 H 5 SH - according to 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 intensity of the smell of odorized HGP with a volume fraction of 1% in the air must be at least 3 points according to GOST 22387.5.
Non-odorized gas can be supplied to industrial enterprises, because The intensity of the smell of natural gas for industrial enterprises consuming gas from main gas pipelines is set in agreement with the consumer.
Burning gases. The furnace of a boiler (furnace) in which gaseous (liquid) fuel is burned in a flare corresponds to the concept of a “stationary boiler chamber furnace”.
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 \u003d CO 2 + 2H 2 O .
On complete combustion carbon is formed carbon dioxide (CO 2), but water kind - water vapor (H 2 O) .
In theory to burn 1 m 3 of methane, 2 m 3 of oxygen are needed, which are contained in 9.52 m 3 of air (Fig. 6). If a insufficient combustion air , then for a part of the molecules of combustible components there will not be enough oxygen molecules and in the combustion products, in addition to carbon dioxide (CO 2), nitrogen (N 2) and water vapor (H 2 O), products incomplete combustion of gas :
-carbon monoxide (CO), which, if released into the premises, can cause poisoning of the operating personnel;
- soot (C) , which, being deposited on the heating surfaces impairs heat transfer;
- unburned methane and hydrogen , which can accumulate in furnaces and flues (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. Burnout can also occur when poor mixing of gas with air and low temperature in the combustion zone.
For complete combustion of gas, it is necessary: the presence of air in the place of combustion in enough and good mixing of it with gas; high temperature in the combustion zone.
To ensure complete combustion of the gas, air is supplied in a larger quantity than theoretically required, i.e., in excess, while not all air will take part in 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 (should be a bluish-bluish flame with purple ends) or by analyzing the composition of flue gases.
Theoretical (stoichiometric) combustion air volume is the amount of air required for the complete combustion of a unit volume ( 1 m 3 of dry gas or mass of fuel, calculated from the chemical composition of the fuel ).
Valid (actual, required) Combustion air volume is the amount of air actually used to burn a unit volume or mass of fuel.
Combustion air ratio α is the ratio of the actual volume of air for combustion to the theoretical: α = V f / V t >1,
where: V f - actual volume of supplied air, m 3 ;
V t - theoretical volume of air, m 3.
Coefficient excess shows how many times the actual air consumption for gas combustion exceeds the theoretical depends on the design of the gas burner and furnace: the more perfect they are, the coefficient α less. When the excess air coefficient for boilers is less than 1, it leads to incomplete combustion of the gas. An increase in the excess air ratio reduces the efficiency. gas plant. For a number of furnaces where metal is melted, in order to avoid oxygen corrosion - α < 1 and after the furnace, an afterburning chamber for unburned combustible components is installed.
Guide vanes, gate valves, rotary dampers and electromechanical couplings are used to control the draft.
Advantages of gaseous fuels compared to solid and liquid– low cost, facilitating the work of personnel, low amount of harmful impurities in combustion products, improved environmental conditions, no need for road and rail transport, good mixing with air (less than α), full automation, high efficiency.
Gas combustion methods. Combustion air can be:
1) primary, is fed into the burner, where it is mixed with gas (a gas-air mixture is used for combustion).
2) secondary, enters directly into the combustion zone.
There are the following methods of gas combustion:
1. Diffusion method- gas and air for combustion are supplied separately and mixed in the combustion zone, i.e. all air is secondary. The flame is long, a large furnace space is required. (Fig. 7a).
2. Kinetic method - all air is mixed with gas inside the burner, i.e. all air is primary. Flame is short, small combustion space required (Fig. 7c).
3. mixed method - part of the air is supplied inside 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 products of combustion. The rarefaction in the furnace and the removal of combustion products are produced by the traction force that overcomes the resistance of the smoke path and arises due to the pressure difference between columns of cold outside air and lighter hot flue gas. In this case, flue gases move from the furnace into the pipe, and cold air enters the furnace in their place (Fig. 8).
The pull 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, rarefaction in the furnace .
Natural draft force - created by the height of the chimney, and artificial, which is a smoke exhauster with insufficient natural draft. The traction force is regulated by gates, guide vanes of smoke exhausters and other devices.
Excess air ratio (α ) depends on the design of the gas burner and 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.
Supercharging - removal of fuel combustion products due to the operation of blowers .When working “under supercharging”, a strong dense combustion chamber (furnace) is required that can withstand the excess pressure created by the fan.
Gas burners.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 declaration of conformity) or pass an industrial safety examination;
2) to 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 measuring the parameters of gas and air in front of the burner;
4) must have a simple design, be accessible for repair and revision;
5) work steadily within the working regulation, if necessary, have stabilizers to prevent separation and flashback of the flame;
Parameters of gas burners(Fig. 9). According to GOST 17356-89 (Burners gas, liquid fuel and combined. Terms and definitions. Rev. N 1) :Burner Stability Limit , at which not yet arise extinction, breakdown, detachment, burst of flame and unacceptable vibrations.
Note. Exist upper and lower limits of sustainability.
1) Heat output of the burner N g. - the amount of heat generated as a result of the combustion of fuel supplied to the burner per unit time, N g \u003d 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 not yet arise extinguishing, stalling, detachment, flashback and unacceptable vibrations . Note. Exist upper - N v.p . and lower -N n.p. limits of sustainability.
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. low limit power increased by 10%, N min. =1.1N n.p.
4) upper limit of stable operation of the burner N v.p. – the highest stable power, work without separation and flashover of the flame.
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 v.p.
6) rated power N nom - the highest thermal power of the burner, when the performance indicators comply with the established standards, i.e. the highest power with which the burner operates for a long time with high efficiency.
7) operating regulation range (burner heat output) – a regulated range in which the burner heat output can change during operation, i.e. power values from N min to N nom. .
8) coefficient of working regulation K rr. is the ratio of the rated heat output of the burner to its minimum operating heat output, i.e. shows how many times the rated power exceeds the minimum: K rr. = N rated / N min
Regime card.According to the "Rules for the use of gas ...", approved by the Government of the Russian Federation of May 17, 2002 No. 317(modified 06/19/2017) , upon completion of construction and installation works on the constructed, reconstructed or modernized gas-using equipment and equipment converted to gas from other types of fuel, commissioning and maintenance work is carried out. Launching gas to the constructed, reconstructed or modernized gas-using equipment and equipment converted to gas from other types of fuel for carrying out commissioning (integrated 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 object for connection (technological connection). The rules state that:
· gas-using equipment - boilers, production furnaces, process lines, waste heat recovery plants and other installations using gas as fuel in order to generate thermal energy for centralized heating, hot water supply, in technological processes of various industries, as well as other devices, apparatus, units, process equipment and installations using 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 efficiency optimization;
· regime and adjustment works- a set of works, including the adjustment of gas-using equipment in order to achieve the design (passport) efficiency in the range of operating loads, the adjustment of automatic control 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:Operating modes gas-using equipment at enterprises and in boiler houses must match the regime maps approved by the technical manager of the enterprise and P produced at least once every three years with adjustment (if necessary) of regime cards .
Unscheduled regime adjustment of gas-using equipment should be carried out in the following cases: after a major overhaul of gas-using equipment or making structural changes that affect the efficiency of gas use, as well as in case of systematic deviations of the controlled parameters of the gas-using equipment from regime maps.
Classification of gas burners According to GOST gas burners are classified according to: method of supplying the component; the degree of preparation of the combustible mixture; the speed of the expiration of combustion products; the nature of the flow of the mixture; nominal gas pressure; degree of automation; the ability to control the coefficient of excess air and the characteristics of the torch; localization of the combustion zone; possibility of using the heat of combustion products.
AT chamber furnace of a gas-using plant gaseous fuel is burned in a flare.
According to 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 to diffusion and gas jet energy (rice. 10 ), all air is secondary .
Advantages of the burner : simplicity of design, reliability of work ( no flashover possible ), silent operation, good regulation.
Flaws: low power, uneconomical, high (long) flame, flame retardants are needed to prevent burner flame from going out at separation .
b. injection - air is injected, i.e. sucked into the inside of the burner due to the energy of the gas jet coming out of 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, there are injection burners: with incomplete and complete pre-mixing of gas and air.
The burner medium and high pressure gas all the necessary air is sucked in, i.e. all air is primary, there is a complete pre-mixing of gas with air. A fully prepared gas-air mixture enters the combustion zone and there is no need for secondary air.
The burner low pressure part of the air necessary 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 ratio "gas - air" in these burners is controlled by the position of the air washer 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 ).
Advantages of burners: simplicity of design and power regulation.
Disadvantages of burners: high noise level, possibility of flame flashback, small range of operating regulation.
2) Forced air burners - These are burners in which the combustion air is supplied from a fan. The gas from the gas pipeline enters the inner chamber of the burner (Fig. 14 ).
The air forced 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 gas outlets 3 .Combustion takes place in a ceramic tunnel 7 .
 Rice. 14. Burner with forced air supply: 1 - gas channel; 2 - air channel; 3 - gas outlets; 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 holes; 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. |
Advantages of burners: high thermal power, wide range of operating regulation, possibility of regulation of excess air ratio, possibility of gas and air preheating.
Disadvantages of burners: sufficient design complexity; 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 supply to the burner.
3) block burner – it is an automatic burner with forced air supply (rice. 16 ), arranged with a fan in a single unit. The burner is equipped with an automatic control system.
The fuel combustion process in block burners is controlled by an electronic device called a combustion manager.
For oil burners, this unit includes the fuel pump or the fuel pump and the fuel preheater.
The control unit (combustion manager) controls and controls the operation of the burner, receiving commands from the thermostat (temperature controller), the flame control electrode and the gas and air pressure sensors.
The gas flow is regulated by a butterfly valve located outside the burner body.
The retaining washer is responsible for mixing the gas with the air in the conical part of the flame tube and is used to control the inlet air (adjustment on the pressure side). Another possibility for changing the amount of air supplied is to change the position of the air butterfly valve in the air regulator housing (adjustment on the suction side).
Regulation of gas-air ratios (control of gas and air butterfly valves) can be:
connected, from one actuator:
· frequency regulation of air flow, by changing the frequency of rotation of the fan motor using an inverter, which consists of a frequency converter and a pulse sensor.
The ignition of the burner is carried out automatically by the ignition device using the ignition electrode. The presence of a flame is monitored by a flame control electrode.
The operating sequence for turning on the burner:
request for heat production (from the thermostat);
· inclusion of the electric motor of the fan and preliminary ventilation of a fire chamber;
Enabling electronic ignition
opening of the solenoid valve, gas supply and ignition of the burner;
signal from the flame control sensor about the presence of a flame.
Accidents (incidents) on burners. Flame break - moving the root zone of the torch from the burner outlets in the direction of fuel or combustible mixture flow. Occurs when the speed of the gas-air mixture or gas becomes greater than the speed of flame propagation. 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 can form in the furnace.
Separation occurs when: increasing the gas pressure above the allowable, a sharp increase in the supply of primary air, increasing the rarefaction in the furnace. For tear protection apply combustion stabilizers (rice. 17): brick slides and posts; ceramic tunnels of various types and brick slots; poorly streamlined bodies that heat up during burner operation (when the flame goes out, a fresh jet will ignite from the stabilizer), as well as special pilot burners.
Flashlight - moving the torch zone towards the combustible mixture, in which the flame penetrates into the burner . This phenomenon occurs only in burners with a preliminary mixture of gas and air and occurs when the speed of the gas-air mixture becomes less than the speed of flame propagation. The flame jumps into the inside of the burner, where it continues to burn, causing the burner to deform from overheating.
The 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. During the slip, a small pop may occur, as a result of which the flame will go out, while gas may continue to flow through the idle burner and an explosive mixture may form in the furnace and gas ducts of the gas-using installation. To protect against slippage, plate or mesh stabilizers are used., since through narrow slots and small holes there is no breakthrough of the flame.
Actions of personnel in case of an accident at the burners
In the event of an accident on the burner (separation, slippage or extinction 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 problem; report to the responsible person; after eliminating the causes of the malfunctions and checking the tightness of the shut-off valve in front of the burner, at the direction of the responsible person, according to the instructions, re-ignite.
Burner load change.
There are burners with various ways to change the heat output:
Burner with multi-stage heat output control- this is a burner, during which the fuel flow regulator can be installed in several positions between the maximum and minimum operating positions.
Burner with three-stage heat output regulation- this is a burner, during the operation of which the fuel flow regulator can be set in the positions "maximum flow" - "minimum flow" - "closed".
Burner with two-stage heat output control- a burner operating in the "open - closed" positions.
Modulating burner- this is a burner, during which the fuel flow regulator can be installed in any position between the maximum and minimum operating positions.
It is possible to regulate the thermal power of the installation by the number of operating burners, if provided by the manufacturer and regime card.
Changing the heat output manually, in order to avoid flame separation, it is carried out:
When increasing: first increase the gas supply, and then the air.
When decreasing: first reduce the air supply, and then the gas;
To prevent accidents on burners, changing their power must be done smoothly (in several stages) according to the regime map.
Ld. - the actual amount of air supplied to the furnace, it is usually supplied in excess. The relationship between theoretical and actual flow is expressed by the equation:where α is the excess air coefficient (usually greater than 1).
Incomplete combustion of gas leads to excessive fuel consumption and increases the risk of poisoning by products of incomplete combustion of gas, which also include carbon monoxide (CO).
Products of gas combustion and control over the combustion process.
The combustion products of natural gas are carbon dioxide (carbon dioxide), water vapour, some excess oxygen and nitrogen. Excess oxygen is contained in combustion products only in those cases when combustion occurs with excess air, and nitrogen is always contained in combustion products, since it is an integral part of air and does not take part in combustion.
The products of incomplete combustion of gas can be carbon monoxide (carbon monoxide), unburned hydrogen and methane, heavy hydrocarbons, soot.
The combustion process can be most correctly judged by flue gas analysis devices that show the content of carbon dioxide and oxygen in it. If the flame in the boiler furnace is elongated and has a dark yellow color, this indicates a lack of air, and if the flame becomes short and has a dazzling white color, then its excess.
There are two ways to regulate the operation of the boiler unit by changing the thermal power of all burners installed in the boiler, or by turning off part of them. The method of regulation depends on local conditions and must be specified in the production instructions. A change in the thermal power of the burners is permissible if it does not go beyond the limits of stable operation. Deviation of thermal power beyond the limits of stable operation can lead to separation or flashback of the flame.
Adjust the operation of individual burners in two steps, slowly and gradually changing the flow of air and gas.
When reducing the heat output, first reduce the air supply, and then gas; with an increase in thermal power, first increase the gas supply, and then air.
In this case, the vacuum in the furnace should be regulated by changing the position of the gate valve with the boiler or the blades of the guide vane in front of the smoke exhauster.
If it is necessary to increase the heat output of the burners, increase vacuum in the furnace; with a decrease in thermal power, the operation of the burners is first regulated, and then the vacuum in the furnace is reduced.
Gas combustion methods.
Depending on the method of education DHW combustion methods can be divided into diffusion, mixed and kinetic.
At diffusion In this method, gas enters the combustion front under pressure, and air from the surrounding space due to molecular or turbulent diffusion, mixture formation proceeds simultaneously with the combustion process, therefore the rate of the combustion process is determined by the rate of mixture formation.
The combustion process begins after the formation of contact between gas and air and the formation of hot water of the required composition. In this case, air diffuses to the gas jet, and gas diffuses from the gas jet into air. Thus, a hot water supply is created near the gas jet, as a result of the combustion of which a zone of primary gas combustion is formed (2) . The combustion of the main part of the gas occurs in the zone (Z), in the zone (4) moving combustion products.
This combustion method is mainly used in everyday life (ovens, gas stoves, etc.)
With the mixed method of gas combustion, the burner ensures that the gas is premixed with only a part of the air necessary for complete combustion of the gas. The rest of the air comes from the environment directly to the torch.
In this case, only part of the gas mixed with primary air (50%-60%), and the rest of the gas, diluted with combustion products, burns out after the addition of oxygen from the secondary air.
The air surrounding the flame is called secondary .
With the kinetic method of gas combustion, DHW is supplied to the combustion site fully prepared inside the burner.
Classification of gas burners .
A gas burner is a device that provides stable combustion of gaseous fuel and regulation of the combustion process.
The main functions of gas burners:
Supply of gas and air to the combustion front;
mixture formation;
Stabilization of the ignition front;
Ensuring the required intensity of the gas combustion process.
According to the gas combustion method, all burners can be divided into three groups:
Diffusion - without preliminary mixing of gas with air;
Diffusion-kinetic - with incomplete preliminary mixing of gas with air;
Kinetic - with complete pre-mixing of gas with air.
According to the method of air supply, the burners are divided into:
Blow-free - in which air enters the furnace due to the discharge in it.
Injection - in which air is sucked in due to the energy of the gas jet.
Blast - in which air is supplied to the burner or furnace using a fan.
According to the pressure of the gas on which the burners operate:
- low pressure up to 0.05 kgf/cm 2 ;
- medium pressure over 0.05 to 3 kgf/cm 2 ;
- high pressure over 3 kgf/cm 2 .
General requirements for all burners:
Ensuring the completeness of gas combustion;
Stability when changing thermal power;
Reliability during operation;
Compactness;
Serviceability.
A similar defect is associated with a malfunction of the boiler automation system. Note that it is strictly forbidden to operate the boiler with the automation turned off (for example, if the start button is forcibly jammed in the pressed state). This can lead to tragic consequences, since if the gas supply is interrupted for a short time or if the flame is extinguished by a strong air flow, the gas will begin to flow into the room. To understand the causes of such a defect, let us consider in more detail the operation of the automation system. On fig. 5 shows a simplified diagram of this system. The circuit consists of an electromagnet, a valve, a draft sensor and a thermocouple. To turn on the igniter, press the start button. The rod connected to the button presses on the valve membrane, and the gas begins to flow to the igniter. After that, the igniter is lit. The igniter flame touches the body of the temperature sensor (thermocouple). After some time (30 ... 40 s), the thermocouple heats up and an EMF appears on its terminals, which is enough to trigger the electromagnet. The latter, in turn, fixes the rod in the lower (as in Fig. 5) position. Now the start button can be released. The draft sensor consists of a bimetallic plate and a contact (Fig. 6). The sensor is located in the upper part of the boiler, near the pipe for the removal of combustion products into the atmosphere. In the event of a clogged pipe, its temperature rises sharply. The bimetallic plate heats up and breaks the voltage supply circuit to the electromagnet - the rod is no longer held by the electromagnet, the valve closes, and the gas supply stops.
The location of the elements of the automation device is shown in fig. 7. It shows that the electromagnet is closed with a protective cap. The wires from the sensors are located inside thin-walled tubes. The tubes are attached to the electromagnet using cap nuts. The body leads of the sensors are connected to the electromagnet through the body of the tubes themselves.
And now consider the method of finding the above fault.
The check begins with the “weakest link” of the automation device - the thrust sensor. The sensor is not protected by a casing, therefore, after 6 ... 12 months of operation, it “overgrows” with a thick layer of dust. The bimetallic plate (see Fig. 6) quickly oxidizes, which leads to poor contact.
The dust coat is removed with a soft brush. Then the plate is pulled away from contact and cleaned with fine sandpaper. We should not forget that it is necessary to clean the contact itself. Good results are obtained by cleaning these elements with a special spray "Contact". It contains substances that actively destroy the oxide film. After cleaning, a thin layer of liquid lubricant is applied to the plate and contact.
The next step is to check the health of the thermocouple. It works in heavy thermal conditions, as it is constantly in the igniter flame, naturally, its service life is much less than the rest of the boiler elements.
The main defect of the thermocouple is burnout (destruction) of its body. In this case, the transition resistance at the welding site (junction) sharply increases. As a result, the current in the circuit Thermocouple - Electromagnet
- The bimetal plate will be lower than the nominal value, which leads to the fact that the electromagnet will no longer be able to fix the stem (Fig. 5).
To check the thermocouple, unscrew the union nut (Fig. 7), located on the left 
side of the electromagnet. Then the igniter is turned on and the constant voltage (thermo-EMF) at the thermocouple contacts is measured with a voltmeter (Fig. 8). A heated serviceable thermocouple generates an EMF of about 25 ... 30 mV. If this value is less, the thermocouple is faulty. For its final check, the tube is undocked from the casing of the electromagnet and the resistance of the thermocouple is measured. The resistance of the heated thermocouple is less than 1 ohm. If the resistance of the thermocouple is hundreds of ohms or more, it must be replaced. The low value of thermo-EMF generated by a thermocouple can be caused by the following reasons: - clogging of the igniter nozzle (as a result, the heating temperature of the thermocouple may be lower than the nominal one). A similar defect is “treated” by cleaning the igniter hole with any soft wire of a suitable diameter;
- by shifting the position of the thermocouple (naturally, it can also not heat up enough). Eliminate the defect in the following way - loosen the screw fastening the eyeliner near the igniter and adjust the position of the thermocouple (Fig. 10);
- low gas pressure at the boiler inlet.
If the EMF at the thermocouple leads is normal (while maintaining the symptoms of the malfunction indicated above), then the following elements are checked:
- the integrity of the contacts at the connection points of the thermocouple and the draft sensor.
Oxidized contacts must be cleaned. Union nuts are tightened, as they say, "by hand". In this case, it is undesirable to use a wrench, since it is easy to break the wires suitable for the contacts;
- the integrity of the electromagnet winding and, if necessary, solder its conclusions.
The performance of the electromagnet can be checked as follows. Disconnect 
thermocouple lead. Press and hold the start button, then ignite the igniter. From a separate source of constant voltage to the released contact of the electromagnet (from the thermocouple), a voltage of about 1 V is applied relative to the housing (at a current of up to 2 A). To do this, you can use a regular battery (1.5 V), as long as it provides the necessary operating current. Now the button can be released. If the igniter does not go out, the electromagnet and draft sensor are working;
- thrust sensor. First, the force of pressing the contact to the bimetallic plate is checked (with the indicated signs of a malfunction, it is often insufficient). To increase the clamping force, loosen the lock nut and move the contact closer to the plate, then tighten the nut. In this case, no additional adjustments are required - the pressure force does not affect the temperature of the sensor response. The sensor has a large margin for the angle of deflection of the plate, ensuring reliable breaking of the electrical circuit in the event of an accident.
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 self-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 is used when burning gases in the 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 energy necessary to break molecular bonds and create new ones.
The chemical formula for the combustion of gas fuel, indicating the entire reaction mechanism associated with the emergence and disappearance of a large number of 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 for the chemical reaction of 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, 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
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To burn 1 m 3 of gas is required, m 3 |
When burning 1 m 3 gas is released, m 3 |
Heat of combustion He, kJ / m 3 |
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oxygen |
dioxide carbon |
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carbon monoxide |
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If the gas-air mixture contains gas less than the lower flammable limit, then 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 rises 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 the theoretically required amount of air for burning 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 combustion plant operates 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 a minimum value of a, since with a decrease in excess air, heat losses with exhaust gases decrease. 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.
