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 bimetallic 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 clamping force does not affect the sensor response temperature. 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.
Characteristics of methane
§ Colorless;
§ Non-toxic (not poisonous);
§ Odorless and tasteless.
§ The composition of methane includes 75% carbon, 25% hydrogen.
§ The specific gravity is 0.717 kg / m 3 (2 times lighter than air).
§ Flash point is the minimum initial temperature at which combustion begins. For methane, it is equal to 645 o.
§ combustion temperature- this is the maximum temperature that can be reached with complete combustion of the gas, if the amount of air required for combustion exactly corresponds to the chemical formulas of combustion. For methane, it is equal to 1100-1400 o and depends on the combustion conditions.
§ Heat of combustion- this is the amount of heat that is released during the complete combustion of 1 m 3 of gas and it is equal to 8500 kcal / m 3.
§ Flame spread rate equal to 0.67 m/s.
Gas-air mixture
In which the gas is located:
Up to 5% does not burn;
5 to 15% explodes;
Over 15% burns when additional air is supplied (all this depends on the ratio of the volume of gas in the air and is called explosive limits)
Combustible gases are odorless, for their timely detection in the air, quick and accurate detection of leaks, the gas is odorized, i.e. give off a scent. To do this, use ETHYLMERKOPTAN. The odorization rate is 16 g per 1000 m 3. If there is 1% natural gas in the air, its smell should be felt.
The gas used as fuel must comply with the requirements of GOST and contain harmful impurities per 100m 3 no more than:
Hydrogen sulfide 0.0 2 G /m.cube
Ammonia 2 gr.
Hydrocyanic acid 5 gr.
Resin and dust 0.001 g/m3
Naphthalene 10 gr.
Oxygen 1%.
The use of natural gas has several advantages:
absence of ash and dust and removal of solid particles into the atmosphere;
high calorific value;
· convenience of transportation and burning;
facilitating the work of maintenance personnel;
· Improvement of sanitary and hygienic conditions in boiler houses and adjacent areas;
Wide range of automatic control.
When using natural gas, special precautions are required, as possible leakage through leaks at the junction of the gas pipeline and fittings. The presence of more than 20% of gas in the room causes suffocation, its accumulation in a closed volume of more than 5% to 15% leads to an explosion of the gas-air mixture. Incomplete combustion produces carbon monoxide, which, even at low concentrations (0.15%), is poisonous.
Burning natural gas
burning is called the rapid chemical combination of combustible parts of the fuel with oxygen in the air, occurs at high temperature, is accompanied by the release of heat with the formation of a flame and combustion products. Burning happens complete and incomplete.
Full burning Occurs when there is sufficient oxygen. The lack of oxygen causes incomplete combustion, at which a smaller amount of heat is released than at full, carbon monoxide (poisonous effect on maintenance personnel), soot is formed on the surface of the boiler and heat losses increase, which leads to excessive fuel consumption, reduced boiler efficiency, atmospheric pollution.
The combustion products of natural gas are– carbon dioxide, water vapor, 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, because. is an integral part of air and does not take part in combustion.
The products of incomplete combustion of gas can be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, soot.
Methane reaction:
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
According to the formula 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. In practice, for burning 1 m 3 of methane, more air is needed, taking into account all kinds of losses, for this a coefficient is applied TO excess air, which = 1.05-1.1.
Theoretical air volume = 10 m 3
Practical air volume = 10*1.05=10.5 or 10*1.1=11
Completeness of combustion fuel can be determined visually by the color and nature of the flame, as well as using a gas analyzer.
Transparent blue flame - complete combustion of gas;
Red or yellow with smoky streaks - combustion is incomplete.
Combustion is controlled by increasing the air supply to the furnace or decreasing the gas supply. This process uses primary and secondary air.
secondary air– 40-50% (mixed with gas in the boiler furnace during combustion)
primary air– 50-60% (mixed with gas in the burner before combustion) gas-air mixture is used for combustion
Combustion characterizes flame spread rate is the speed at which the element of the flame front distributed by relatively fresh jet of air-gas mixture.
The rate of combustion and flame propagation depends on:
from the composition of the mixture;
on temperature;
from pressure;
on the ratio of gas and air.
The burning rate determines one of the main conditions for the reliable operation of the boiler house and characterizes it flame separation and breakthrough.
Flame break- occurs if the speed of the gas-air mixture at the outlet of the burner is greater than the combustion speed.
Reasons for separation: excessive increase in gas supply or excessive vacuum in the furnace (draught). Flame separation is observed during ignition and when the burners are turned on. The separation of the flame leads to the gas contamination of the furnace and gas ducts of the boiler and to an explosion.
Flashlight- occurs if the flame propagation speed (burning speed) is greater than the speed of the gas-air mixture outflow from the burner. The breakthrough is accompanied by the combustion of the gas-air mixture inside the burner, the burner heats up and fails. Sometimes the breakthrough is accompanied by a pop or explosion inside the burner. In this case, not only the burner, but also the front wall of the boiler can be destroyed. Overshoot occurs when the gas supply is sharply reduced.
When the flame breaks off and flashes, the maintenance personnel must stop the fuel supply, find out and eliminate the cause, ventilate the furnace and gas ducts for 10-15 minutes and rekindle the fire.
The combustion process of gaseous fuel can be divided into 4 stages:
1. Outflow of gas from the burner nozzle into the burner under pressure at an increased rate.
2. Formation of a mixture of gas with air.
3. Ignition of the resulting combustible mixture.
4. Combustion of a combustible mixture.
Gas pipelines
Gas is supplied to the consumer through gas pipelines - external and internal- to gas distribution stations located outside the city, and from them through gas pipelines to gas control points hydraulic fracturing or gas control devices GRU industrial enterprises.
Gas pipelines are:
· high pressure first category over 0.6 MPa up to 1.2 MPa inclusive;
· high pressure second category over 0.3 MPa to 0.6 MPa;
· medium pressure third category over 0.005 MPa to 0.3 MPa;
· low pressure category 4 up to 0.005 MPa inclusive.
MPa means Mega Pascal
Only medium and low pressure gas pipelines are laid in the boiler room. The section from the distribution gas pipeline of the network (city) to the premises, together with the disconnecting device, is called input.
The inlet gas pipeline is considered the section from the disconnecting device at the inlet, if it is installed outside the premises to the internal gas pipeline.
At the gas inlet to the boiler room in a lighted and convenient place for maintenance, there must be a valve. There must be an insulating flange in front of the valve to protect against stray currents. At each outlet from the gas distribution pipeline to the boiler, at least 2 disconnecting devices are provided, one of which is installed directly in front of the burner. In addition to fittings and instrumentation on the gas pipeline, in front of each boiler, an automatic device must be installed to ensure the safe operation of the boiler. In order to prevent the ingress of gases into the boiler furnace, if the shut-off devices are faulty, purge candles and safety gas pipelines with shut-off devices are required, which must be open when the boilers are inactive. Low-pressure gas pipelines are painted yellow in boiler houses, and medium-pressure gas pipelines are painted yellow with red rings.
Gas-burners
Gas-burners- a gas burner designed to supply to the place of combustion, depending on the technological requirements, a prepared gas-air mixture or separated gas and air, as well as to ensure stable combustion of gaseous fuel and control the combustion process.
Burners are subject to the following requirements:
· the main types of burners must be mass-produced at factories;
burners must ensure the passage of a given amount of gas and the completeness of its combustion;
ensure the minimum amount of harmful emissions into the atmosphere;
must work without noise, separation and flashover of the flame;
should be easy to maintain, convenient for revision and repair;
if necessary, could be used for reserve fuel;
· samples of newly created and operating burners are subject to GOST testing;
The main characteristic of the burners is its thermal power, which is understood as the amount of heat that can be released during the complete combustion of the fuel supplied through the burner. All these characteristics can be found in the burner data sheet.
General information. Another important source of internal pollution, a strong sensitizing factor for humans, is natural gas and its combustion products. Gas is a multicomponent system consisting of dozens of different compounds, including specially added ones (Table 1).
There is direct evidence that the use of appliances that burn natural gas (gas stoves and boilers) has an adverse effect on human health. In addition, individuals with increased sensitivity to environmental factors react inadequately to natural gas components and products of its combustion.
Natural gas in the home is a source of many different pollutants. These include compounds that are directly present in the gas (odorants, gaseous hydrocarbons, toxic organometallic complexes and radioactive gas radon), products of incomplete combustion (carbon monoxide, nitrogen dioxide, aerosol organic particles, polycyclic aromatic hydrocarbons and small amounts of volatile organic compounds). All of these components can affect the human body both by themselves and in combination with each other (synergistic effect).
Table 12.3
Composition of gaseous fuel
Odorants. Odorants are sulfur-containing organic aromatic compounds (mercaptans, thioethers and thio-aromatic compounds). They are added to natural gas in order to detect it in case of leaks. Although these compounds are present in very low, sub-threshold concentrations that are not considered toxic to most individuals, their odor can cause nausea and headaches in otherwise healthy individuals.
Clinical experience and epidemiological data indicate that chemically sensitive individuals react inappropriately to chemicals present even at subthreshold concentrations. Individuals with asthma often identify odor as a promoter (trigger) of asthmatic attacks.
Odorants include, for example, methanethiol. Methanethiol, also known as methylmercaptan (mercaptomethane, thiomethylalcohol), is a gaseous compound commonly used as an aromatic additive to natural gas. The malodor is experienced by most people at a concentration of 1 part per 140 million, but this compound can be detected at much lower concentrations by highly sensitive individuals.
Toxicological studies in animals have shown that 0.16% methanethiol, 3.3% ethanethiol, or 9.6% dimethyl sulfide can induce comatose states in 50% of rats exposed to these compounds for 15 minutes.
Another mercaptan, also used as an aromatic additive to natural gas, is mercaptoethanol (C2H6OS) also known as 2-thioethanol, ethyl mercaptan. Severe irritant to eyes and skin, capable of exerting a toxic effect through the skin. It is flammable and decomposes when heated to form highly toxic SOx fumes.
Mercaptans, being indoor air pollutants, contain sulfur and can capture elemental mercury. In high concentrations, mercaptans can cause impaired peripheral circulation and increased heart rate, can stimulate loss of consciousness, the development of cyanosis, or even death.
Aerosols. Combustion of natural gas results in the formation of fine organic particles (aerosols), including carcinogenic aromatic hydrocarbons, as well as some volatile organic compounds. DOS are suspected sensitizing agents that are able to induce, together with other components, the "sick building" syndrome, as well as multiple chemical sensitivity (MCS).
DOS also includes formaldehyde, which is formed in small quantities during the combustion of gas. The use of gas appliances in a home where sensitive individuals live increases exposure to these irritants, subsequently exacerbating the signs of illness and also promoting further sensitization.
Aerosols formed during the combustion of natural gas can become adsorption centers for a variety of chemical compounds present in the air. Thus, air pollutants can concentrate in microvolumes, react with each other, especially when metals act as catalysts for reactions. The smaller the particle, the higher the concentration activity of such a process.
Moreover, the water vapor generated during the combustion of natural gas is a transport link for aerosol particles and pollutants when they are transferred to the pulmonary alveoli.
During the combustion of natural gas, aerosols containing polycyclic aromatic hydrocarbons are also formed. They have adverse effects on the respiratory system and are known carcinogens. In addition, hydrocarbons can lead to chronic intoxication in susceptible people.
The formation of benzene, toluene, ethylbenzene and xylene when burning natural gas is also unfavorable to human health. Benzene is known to be carcinogenic at doses well below the threshold. Exposure to benzene has been correlated with an increased risk of cancer, especially leukemia. The sensitizing effects of benzene are not known.
organometallic compounds. Some natural gas components may contain high concentrations of toxic heavy metals, including lead, copper, mercury, silver, and arsenic. In all likelihood, these metals are present in natural gas in the form of organometallic complexes of the trimethylarsenite (CH3)3As type. The association with the organic matrix of these toxic metals makes them lipid soluble. This leads to a high level of absorption and a tendency to bioaccumulate in human adipose tissue. The high toxicity of tetramethylplumbite (CH3)4Pb and dimethylmercury (CH3)2Hg suggests an impact on human health, as the methylated compounds of these metals are more toxic than the metals themselves. Of particular danger are these compounds during lactation in women, since in this case there is a migration of lipids from the fat depots of the body.
Dimethylmercury (CH3)2Hg is a particularly dangerous organometallic compound due to its high lipophilicity. Methylmercury can be incorporated into the body through inhalation as well as through the skin. The absorption of this compound in the gastrointestinal tract is almost 100%. Mercury has a pronounced neurotoxic effect and the ability to influence the human reproductive function. Toxicology does not have data on safe levels of mercury for living organisms.
Organic arsenic compounds are also very toxic, especially when they are metabolically destroyed (metabolic activation), resulting in the formation of highly toxic inorganic forms.
Combustion products of natural gas. Nitrogen dioxide is able to act on the pulmonary system, which facilitates the development of allergic reactions to other substances, reduces lung function, susceptibility to infectious diseases of the lungs, potentiates bronchial asthma and other respiratory diseases. This is especially pronounced in children.
There is evidence that N02 produced by burning natural gas can induce:
- inflammation of the pulmonary system and a decrease in the vital function of the lungs;
- increased risk of asthma-like symptoms, including wheezing, shortness of breath and asthma attacks. This is especially common in women cooking on gas stoves, as well as in children;
- a decrease in resistance to bacterial lung diseases due to a decrease in the immunological mechanisms of lung protection;
- providing adverse effects in general on the immune system of humans and animals;
- impact as an adjuvant on the development of allergic reactions to other components;
- increased sensitivity and increased allergic response to side allergens.
The combustion products of natural gas contain a rather high concentration of hydrogen sulfide (H2S), which pollutes the environment. It is poisonous at concentrations lower than 50.ppm, and at concentrations of 0.1-0.2% it is fatal even with short exposure. Since the body has a mechanism to detoxify this compound, the toxicity of hydrogen sulfide is related more to the exposure concentration than to the duration of exposure.
Although hydrogen sulfide has a strong odor, continuous exposure to low concentrations leads to a loss of the sense of smell. This makes a toxic effect possible for people who may unknowingly be exposed to dangerous levels of this gas. Insignificant concentrations of it in the air of residential premises lead to irritation of the eyes, nasopharynx. Moderate levels cause headache, dizziness, as well as coughing and difficulty breathing. High levels lead to shock, convulsions, coma, which ends in death. Survivors of acute toxic exposure to hydrogen sulfide experience neurological dysfunctions such as amnesia, tremors, imbalance, and sometimes more severe brain damage.
The acute toxicity at relatively high concentrations of hydrogen sulfide is well known, however, unfortunately, little information is available on the chronic low-dose effects of this component.
Radon. Radon (222Rn) is also present in natural gas and can be transported through pipelines to gas stoves, which become sources of pollution. Since radon decays to lead (210Pb has a half-life of 3.8 days), this results in a thin layer of radioactive lead (on average 0.01 cm thick) that coats the interior surfaces of pipes and equipment. The formation of a layer of radioactive lead increases the background value of radioactivity by several thousand disintegrations per minute (over an area of 100 cm2). Removing it is very difficult and requires the replacement of pipes.
It should be borne in mind that simply turning off the gas equipment is not enough to remove the toxic effects and bring relief to chemically sensitive patients. Gas equipment must be completely removed from the premises, as even a non-working gas stove continues to release aromatic compounds that it has absorbed over the years of use.
The cumulative effects of natural gas, aromatic compounds, and combustion products on human health are not exactly known. It is assumed that the effects from several compounds may be multiplied, while the response from exposure to several pollutants may be greater than the sum of the individual effects.
Thus, the characteristics of natural gas that are of concern to human and animal health are:
- flammability and explosive character;
- asphyxic properties;
- pollution by products of combustion of the indoor air;
- the presence of radioactive elements (radon);
- the content of highly toxic compounds in the combustion products;
- the presence of trace amounts of toxic metals;
- the content of toxic aromatic compounds added to natural gas (especially for people with multiple chemical sensitivities);
- the ability of gas components to sensitize.
Combustion of a gas is a reaction of the combination of combustible gas components with oxygen in the air, accompanied by the release of heat. The combustion process depends on the chemical composition of the fuel. The main component of natural gas is methane, but ethane, propane and butane are also combustible, which are contained in small quantities.
Natural gas produced from West Siberian deposits almost completely (up to 99%) consists of CH4 methane. Air consists of oxygen (21%) and nitrogen and a small amount of other non-combustible gases (79%). Simplified, the reaction of complete combustion of methane is as follows:
CH4 + 2O2 + 7.52 N2 = CO2 + 2H20 + 7.52 N2
As a result of the combustion reaction during complete combustion, carbon dioxide CO2 is formed, and water vapor H2O is a substance that does not have a harmful effect on the environment and humans. Nitrogen N does not participate in the reaction. For complete combustion of 1 m³ of methane, 9.52 m³ of air is theoretically required. For practical purposes, it is considered that for the complete combustion of 1 m³ of natural gas, at least 10 m³ of air is needed. However, if only the theoretically necessary amount of air is supplied, then it is impossible to achieve complete combustion of the fuel: it is difficult to mix the gas with air in such a way that the required number of oxygen molecules is supplied to each of its molecules. In practice, more air is supplied to combustion than theoretically necessary. The amount of excess air is determined by the coefficient of excess air a, which shows the ratio of the amount of air actually consumed for combustion to the theoretically required amount:
α = V fact./V theor.
where V is the amount of air actually used for combustion, m³;
V is the theoretically required amount of air, m³.
The excess air coefficient is the most important indicator characterizing the quality of gas combustion by the burner. The smaller a, the less heat will be carried away by the exhaust gases, the higher the efficiency of the gas-using equipment. But burning the gas with insufficient excess air results in a lack of air, which can cause incomplete combustion. For modern burners with complete pre-mixing of gas with air, the excess air coefficient lies in the range of 1.05 - 1.1 ", that is, air is consumed for combustion by 5 - 10% more than theoretically required.
With incomplete combustion, the combustion products contain a significant amount of carbon monoxide CO, as well as unburned carbon in the form of soot. If the burner works very poorly, then the combustion products may contain hydrogen and unburned methane. Carbon monoxide CO (carbon monoxide) pollutes the air in the room (when using equipment without exhausting combustion products into the atmosphere - gas stoves, columns of low thermal power) and has a toxic effect. Soot contaminates heat exchange surfaces, sharply reduces heat transfer and reduces the efficiency of household gas-using equipment. In addition, when using gas stoves, dishes are contaminated with soot, which requires considerable effort to remove. In water heaters, soot pollutes the heat exchanger, in “neglected” cases, almost to the complete cessation of heat transfer from combustion products: the column burns, and the water heats up by several degrees.
Incomplete combustion occurs:
- with insufficient air supply for combustion;
- with poor mixing of gas and air;
- with excessive cooling of the flame before the completion of the combustion reaction.
The quality of gas combustion can be controlled by the color of the flame. Poor-quality gas combustion is characterized by a yellow smoky flame. When the gas is completely burned, the flame is a short torch of a bluish-violet color with a high temperature. To control the operation of industrial burners, special devices are used that analyze the composition of flue gases and the temperature of the combustion products. At present, when adjusting certain types of household gas-using equipment, it is also possible to regulate the combustion process by temperature and analysis of flue gases.
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The combustion of gaseous fuel is a combination of the following physical and chemical processes: mixing combustible gas with air, heating the mixture, thermal decomposition of combustible components, ignition and chemical combination of combustible elements with atmospheric oxygen.
Stable combustion of a gas-air mixture is possible with a continuous supply of the necessary amounts of combustible gas and air to the combustion front, their thorough mixing and heating to the ignition or self-ignition temperature (Table 5).
The ignition of the gas-air mixture can be carried out:
- heating the entire volume of the gas-air mixture to the auto-ignition temperature. This method is used in internal combustion engines, where the gas-air mixture is heated by rapid compression to a certain pressure;
- the use of foreign sources of ignition (igniters, etc.). In this case, not the entire gas-air mixture is heated to the ignition temperature, but part of it. This method 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
|
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 |
|||||
|
oxygen |
dioxide carbon |
||||||
|
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 increases above certain limits, the gas-air mixture does not ignite at any ratio of gas and air (table below).
The number of volumes of inert gas per 1 volume of combustible gas at which the gas-air mixture ceases to be explosive
The smallest amount of air required for complete combustion of gas is called the theoretical air flow and is denoted by Lt, that is, if the net calorific value of gas fuel is 33520 kJ / m 3 , then 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 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.
