Gas combustion products and combustion process control. Gas burning
Combustion is a reaction that converts the chemical energy of a fuel into heat.
Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen. Its lack causes incomplete combustion, which releases less heat than complete combustion, and carbon monoxide (CO), which has a poisonous effect on service staff, soot is formed, settling on the heating surface of the boiler and increasing heat loss, which leads to excessive fuel consumption and a decrease in the efficiency of the boiler, and atmospheric pollution.
To burn 1 m 3 of methane, you need 10 m 3 of air, which contains 2 m 3 of oxygen. For complete combustion natural gas air is supplied to the furnace with a slight excess. The ratio of the actually consumed air volume V d to the theoretically required V t is called the excess air coefficient = V d / V t. This indicator depends on the design of the gas burner and furnace: the more perfect they are, the smaller. It is necessary to ensure that the excess air coefficient is not less than 1, as this leads to incomplete combustion of the gas. An increase in the excess air ratio reduces the efficiency of the boiler unit.
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 - combustion is incomplete.
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), secondary air, under the influence of vacuum, enters the furnace through the purge doors.
In boilers equipped with injection burners: primary air enters the burner due to injection and is regulated by an adjusting washer, and secondary air enters through the purge doors.
In boilers with mixing burners, primary and secondary air is supplied to the burner by a fan and controlled by air valves.
Violation of the relationship between the speed of the gas-air mixture at the outlet of the burner and the speed of flame propagation leads to separation or jumping of the flame on the burners.
If the speed of the gas-air mixture at the burner outlet is greater than the speed of flame propagation, there is separation, and if it is less, there is breakthrough.
If the flame breaks out and breaks through, the maintenance personnel must extinguish the boiler, ventilate the firebox and flues, and re-ignite the boiler.
Gaseous fuels are found more and more every year wide application V various industries National economy. In agricultural production, gaseous fuel is widely used for technological (for heating greenhouses, greenhouses, dryers, livestock and poultry complexes) and domestic purposes. IN Lately it began to be increasingly used for internal combustion engines.
Compared to other types, gaseous fuels have the following advantages:
burns in a theoretical amount of air, which provides high thermal efficiency and combustion temperature;
upon combustion does not form undesirable products of dry distillation and sulfur compounds, soot and smoke;
it is relatively easily supplied through gas pipelines to remote consumption facilities and can be stored centrally;
ignites easily at any ambient temperature;
requires relatively low production costs, which means it is a cheaper type of fuel compared to other types;
can be used in compressed or liquefied form for internal combustion engines;
has high anti-knock properties;
does not form condensate during combustion, which ensures 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 in connections, etc. Therefore, when working with gaseous fuel, careful compliance with the relevant safety regulations is required.
The use of gaseous fuels is determined by their composition and properties of the hydrocarbon part. The most widely used natural or associated gas oil or gas fields, as well as factory gases from 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 little use of gaseous fuel for internal combustion engines. The continuously increasing fleet of vehicles requires more and more fuel. It is possible to solve the most important national economic problems of stable supply of automobile engines with efficient energy carriers and reducing 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-calorie gas, the engine does not develop the required power, and the vehicle’s range is also reduced, which is economically unprofitable. Pa). The following types of compressed gases are produced: natural, mechanized coke and enriched coke
The main flammable component of these gases is methane. As with 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 allows you to boost car engines by compression ratio (up to 10...12).
The presence of cyanogen CN in gas for cars is extremely undesirable. When combined with water, it forms hydrocyanic acid, under the influence of which tiny cracks are formed in the walls of the cylinders. The presence of resinous substances and mechanical impurities in gas leads to the formation of deposits and contaminants on gas equipment and engine parts.
Characteristics of methane
§ Colorless;
§ Non-toxic (non-poisonous);
§ Odorless and tasteless.
§ Methane consists of 75% carbon, 25% hydrogen.
§ 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 645 o.
§ Combustion temperature- this is the maximum temperature that can be achieved during complete combustion of gas if the amount of air required for combustion exactly corresponds to the chemical formulas of combustion. For methane it is 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 propagation speed equal to 0.67 m/sec.
Gas-air mixture
Which gas contains:
Up to 5% does not burn;
From 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; in order to timely detect them in the air and quickly and accurately detect leaks, the gas is odorized, i.e. give off a smell. For this purpose, ETHYLMERCOPTAN is used. The odorization rate is 16 g per 1000 m 3. If there is 1% natural gas in the air, you should smell it.
Gas used as fuel must comply with GOST requirements and contain harmful impurities per 100m 3 no more than:
Hydrogen sulfide 0.0 2 G /m.cube
Ammonia 2 gr.
Hydrocyanic acid 5 g.
Resin and dust 0.001 g/m3
Naphthalene 10 gr.
Oxygen 1%.
Using natural gas has a number of advantages:
· absence of ash and dust and removal of solid particles into the atmosphere;
· high heat of combustion;
· ease of transportation and combustion;
· the work of service personnel is facilitated;
· sanitary and hygienic conditions in boiler houses and surrounding areas are improved;
· wide range of automatic control.
When using natural gas, special precautions are required because... leakage is possible through leaks at the junction of the gas pipeline and fittings. The presence of more than 20% of gas in a room causes suffocation; its accumulation in a closed volume of more than 5% to 15% leads to an explosion of the gas-air mixture. In case of incomplete combustion, it is released carbon monoxide, which even at low concentrations (0.15%) is poisonous.
Natural gas combustion
Burning is called the rapid chemical combination of flammable parts of the fuel with oxygen in the air, occurs at high temperatures, is accompanied by the release of heat with the formation of flame and combustion products. Combustion happens complete and incomplete.
Full combustion– Occurs when there is sufficient oxygen. Lack of oxygen causes incomplete combustion, in which less heat is released than with full carbon monoxide (has a poisonous effect on operating personnel), soot is formed on the surface of the boiler and heat loss increases, which leads to excessive fuel consumption, a decrease in boiler efficiency, and air pollution.
The products of natural gas combustion are– carbon dioxide, water vapor, some excess oxygen and nitrogen. Excess oxygen is contained in combustion products only in cases where combustion occurs with excess air, and nitrogen is always contained in combustion products, because is integral part air and does not take part in combustion.
Products incomplete combustion gas may be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, soot.
Methane reaction:
CH 4 + 2O 2 = CO 2 + 2H 2 O
According to the formula For the combustion of 1 m 3 of methane, 10 m 3 of air is required, which contains 2 m 3 of oxygen. In practice, to burn 1 m 3 of methane, more air is needed, taking into account all kinds of losses; for this, a coefficient is used TO excess air, which = 1.05-1.1.
Theoretical air volume = 10 m3
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 regulated by increasing the air supply to the firebox 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) a gas-air mixture is used for combustion
Combustion characterizes flame distribution speed is the speed at which the flame front element distributed by relatively fresh stream of gas-air mixture.
The rate of combustion and flame propagation depends on:
· on 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 room and characterizes it flame separation and breakthrough.
Flame break– occurs if the speed of the gas-air mixture at the burner outlet is greater than the combustion speed.
Reasons for separation: excessive increase in gas supply or excessive vacuum in the firebox (draft). Flame separation is observed during ignition and when the burners are turned on. Flame separation leads to gas contamination of the furnace and boiler flues and to an explosion.
Flame breakthrough– occurs if the speed of flame propagation (burning speed) is greater than the speed of outflow of the gas-air mixture from the burner. The breakthrough is accompanied by combustion of the gas-air mixture inside the burner, the burner becomes hot and fails. Sometimes a 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 there is a sharp decrease in gas supply.
If the flame comes off and breaks through, the maintenance personnel must stop the fuel supply, find out and eliminate the cause, ventilate the firebox and flue ducts for 10-15 minutes and re-ignite the fire.
The combustion process of gaseous fuel can be divided into 4 stages:
1. Gas leaking from the burner nozzle into the burner device under pressure at an increased speed.
2. Formation of a mixture of gas and air.
3. Ignition of the resulting combustible mixture.
4. Combustion of a flammable 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 via gas pipelines to gas regulatory points hydraulic fracturing or gas control device GRU industrial enterprises.
Gas pipelines are:
· high pressure first category over 0.6 MPa to 1.2 MPa inclusive;
· high pressure of the second category over 0.3 MPa to 0.6 MPa;
· average pressure of the third category over 0.005 MPa to 0.3 MPa;
· low pressure fourth category 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 network gas distribution pipeline (city) to the premises together with the disconnecting device is called input.
The inlet gas pipeline is considered to be the section from the disconnecting device at the inlet if it is installed outside the room to the internal gas pipeline.
There should be a valve at the gas inlet into the boiler room in a lighted and convenient place for maintenance. There must be an insulating flange in front of the valve to protect against stray currents. At each branch from the gas distribution pipeline to the boiler, at least 2 shut-off 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 safe work boiler To prevent gases from entering the boiler furnace in the event of faulty shut-off devices, purge candles and safety gas pipelines with shut-off devices are required, which must be open when the boilers are idle. Low pressure gas pipelines are painted in boiler rooms in yellow, and medium pressure in yellow with red rings.
Gas-burners- a gas burner device designed to supply to the combustion site, 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.
The following requirements apply to burners:
· the main types of burners must be mass-produced in factories;
· burners must ensure the passage of a given amount of gas and the completeness of its combustion;
· ensure a minimum amount of harmful emissions into the atmosphere;
· must operate without noise, flame separation or breakthrough;
· must be easy to maintain, convenient for inspection and repair;
· if necessary, could be used for reserve fuel;
· samples of newly created and existing burners are subject to GOST testing;
The main characteristic of burners is its thermal power, which is understood as the amount of heat that can be released during 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 those specially added (Table
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 the components of natural gas and its combustion products.
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, aerosolized organic particles, polycyclic aromatic hydrocarbons and small amounts of volatile organic compounds). All of these components can affect the human body either on their own or in combination with each other (synergy effect).
Table 12.3
Composition of gaseous fuel
Odorants. Odorants are sulfur-containing organic aromatic compounds (mercaptans, thioethers and thio-aromatic compounds). Added to natural gas to detect leaks. Although these compounds are present in very small, subthreshold concentrations that are not considered toxic to most individuals, their odor can cause nausea and headaches in healthy individuals.
Clinical experience and epidemiological data indicate that chemically sensitive people react inappropriately to chemical compounds 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 methyl mercaptan (mercaptomethane, thiomethyl alcohol), is a gaseous compound that is commonly used as an aromatic additive to natural gas. Unpleasant smell is experienced by most people at a concentration of 1 part in 140 ppm, however this compound can be detected at significantly lower concentrations by highly sensitive individuals.
Toxicological studies in animals have shown that 0.16% methanethiol, 3.3% ethanethiol, or 9.6% dimethyl sulfide are capable of inducing coma 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. Strong irritant to eyes and skin, capable of causing toxic effects through the skin. It is flammable and decomposes when heated to form highly toxic SOx vapors.
Mercaptans, being indoor air pollutants, contain sulfur and are capable of capturing elemental mercury. In high concentrations, mercaptans can cause impaired peripheral circulation and increased heart rate, and can stimulate loss of consciousness, the development of cyanosis, or even death.
Aerosols. The combustion of natural gas produces small organic particles (aerosols), including carcinogenic aromatic hydrocarbons, as well as some volatile organic compounds. DOS are suspected sensitizing agents that, together with other components, can induce the “sick building” syndrome, as well as multiple chemical sensitivity (MCS).
DOS also includes formaldehyde, which is formed in small quantities during gas combustion. Usage gas appliances in a home where sensitive individuals live increases exposure to these irritants, subsequently increasing symptoms of illness and also promoting further sensitization.
Aerosols generated during the combustion of natural gas can become adsorption sites for a variety of chemical compounds present in the air. Thus, air pollutants can concentrate in microvolumes and react with each other, especially when metals act as reaction catalysts. The smaller the particle, the higher the concentration activity of this process.
Moreover, water vapor generated during the combustion of natural gas is a transport link for aerosol particles and pollutants as they are transferred to the pulmonary alveoli.
The combustion of natural gas also produces aerosols containing polycyclic aromatic hydrocarbons. 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 during the combustion of natural gas is also unfavorable for human health. Benzene is known to be carcinogenic at doses well below threshold levels. Exposure to benzene is correlated with an increased risk of cancer, especially leukemia. The sensitizing effects of benzene are not known.
Organometallic compounds. Some components of natural gas 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 such as trimethylarsenite (CH3)3As. The association of these toxic metals with the organic matrix makes them lipid soluble. This leads to high levels 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, since the methylated compounds of these metals are more toxic than the metals themselves. These compounds pose a particular danger during lactation in women, since in this case lipids migrate from the body’s fat depots.
Dimethylmercury (CH3)2Hg is a particularly dangerous organometallic compound due to its high lipophilicity. Methylmercury can be incorporated into the body through inhalation and also 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 affect 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 destroyed metabolically (metabolic activation), resulting in the formation of highly toxic inorganic forms.
Natural gas combustion products. 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 lungs, potentiates bronchial asthma and other respiratory diseases. This is especially pronounced in children.
There is evidence that NO2 produced by burning natural gas can induce:
- inflammation of the pulmonary system and decreased vital function of the lungs;
- increased risk of asthma-like symptoms, including wheezing, shortness of breath and attacks. This is especially common among women who prepare food for gas stoves ah, and also in children;
- decreased resistance to bacterial lung diseases due to a decrease in the immunological mechanisms of lung defense;
- causing adverse effects in general on immune system humans and animals;
- influence as an adjuvant on the development of allergic reactions to other components;
- increased sensitivity and increased allergic response to adverse allergens.
Natural gas combustion products contain a fairly high concentration of hydrogen sulfide (H2S), which pollutes environment. It is poisonous in concentrations lower than 50.ppm, and in concentrations of 0.1-0.2% 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 its exposure concentration than to the duration of exposure.
Although hydrogen sulfide has a strong odor, continuous low concentration exposure leads to loss of the sense of smell. This makes it possible for toxic effects to occur in people who may be unknowingly exposed to dangerous levels of this gas. Minor concentrations of it in the air of residential premises lead to irritation of the eyes and nasopharynx. Moderate levels cause headache, dizziness, as well as coughing and difficulty breathing. High levels lead to shock, convulsions, comatose state that end in death. Survivors of acute hydrogen sulfide toxicity experience neurological dysfunction such as amnesia, tremors, imbalance, and sometimes more severe brain damage.
The acute toxicity of relatively high concentrations of hydrogen sulfide is well known, but unfortunately little information is available on chronic LOW-DOSE exposure to this component.
Radon. Radon (222Rn) is also present in natural gas and can be carried through pipelines to gas stoves, which become sources of pollution. As radon decays to lead (210Pb has a half-life of 3.8 days), it creates a thin layer of radioactive lead (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 decays per minute (over an area of 100 cm2). Removing it is very difficult and requires replacing the 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 room, since even a gas stove that is not working continues to release aromatic compounds that it has absorbed over the years of use.
The cumulative effects of natural gas, the influence of aromatic compounds, and combustion products on human health are not precisely known. It is hypothesized that effects from multiple compounds may be multiplying, and the response from exposure to multiple pollutants may be greater than the sum of the individual effects.
In summary, the characteristics of natural gas that cause concern for human and animal health are:
- flammable and explosive nature;
- asphyxial properties;
- pollution of indoor air by combustion products;
- presence of radioactive elements (radon);
- content of highly toxic compounds in combustion products;
- the presence of trace amounts of toxic metals;
- toxic aromatic compounds added to natural gas (especially for people with multiple chemical sensitivities);
- the ability of gas components to sensitize.
Gas combustion is a combination of the following processes:
mixing of flammable gas with air,
· heating the mixture,
thermal decomposition of flammable components,
· ignition and chemical combination of flammable components with atmospheric oxygen, accompanied by the formation of a torch and intense heat release.
Methane combustion occurs according to the reaction:
CH 4 + 2O 2 = CO 2 + 2H 2 O
Conditions necessary for gas combustion:
· ensuring the required ratio of combustible gas and air,
· heating to ignition temperature.
If the gas-air mixture contains less than the lower flammable limit, it will not burn.
If there is more gas in the gas-air mixture than the upper flammability limit, then it will not burn completely.
Composition of products of complete combustion of gas:
· CO 2 – carbon dioxide
· H 2 O – water vapor
* N 2 – nitrogen (it does not react with oxygen during combustion)
Composition of products of incomplete combustion of gas:
· CO – carbon monoxide
· C – soot.
To burn 1 m 3 of natural gas, 9.5 m 3 of air is required. In practice, air consumption is always higher.
Attitude actual consumption air to theoretically required flow is called the excess air coefficient: α = L/L t.,
Where: L - actual consumption;
L t is the theoretically required flow rate.
The excess air coefficient is always greater than one. For natural gas it is 1.05 – 1.2.
2. Purpose, design and main characteristics of instantaneous water heaters.
Instantaneous gas water heaters. Designed to heat water to a certain temperature when drawing water. Instantaneous water heaters are divided according to the thermal power load: 33600, 75600, 105000 kJ, according to the degree of automation - into the highest and first classes. Efficiency water heaters 80%, oxide content no more than 0.05%, temperature of combustion products behind the draft breaker no less than 180 0 C. The principle is based on heating water during water withdrawal.
The main components of instantaneous water heaters are: gas burner device, heat exchanger, automation system and gas outlet. Low pressure gas is supplied to injection burner. Combustion products pass through a heat exchanger and are discharged into the chimney. The heat of combustion is transferred to the water flowing through the heat exchanger. To cool the fire chamber, a coil is used, through which water circulates, passing through the heater. Gas instantaneous water heaters are equipped with gas exhaust devices and draft interrupters, which, in the event of a short-term loss of draft, prevent the flame of the gas burner from going out. There is a smoke outlet pipe for connection to the chimney.
Gas instantaneous water heater–HSV. On the front wall of the casing there are: control handle gas tap, a button for turning on the solenoid valve and an observation window for observing the flame of the pilot and main burner. At the top of the device there is a smoke exhaust device, at the bottom there are pipes for connecting the device to the gas and water systems. Gas enters solenoid valve, the gas block valve of the water-gas burner unit sequentially turns on the pilot burner and supplies gas to the main burner.
Blocking the flow of gas to the main burner, when the igniter is required to operate, is carried out by an electromagnetic valve powered by a thermocouple. Blocking the gas supply to the main burner, depending on the presence of water supply, is carried out by a valve driven through a rod from the membrane of the water block valve.
Gas combustion is a reaction between the flammable components of a gas and oxygen in the air, accompanied by the release of heat. The combustion process depends on chemical composition fuel. The main component of natural gas is methane; ethane, propane and butane, which are contained in small quantities, are also flammable.
Natural gas produced from Western Siberian fields almost entirely (up to 99%) consists of CH4 methane. Air consists of oxygen (21%) and nitrogen and a small amount of other non-flammable gases (79%). Simplified, the reaction of complete combustion of methane looks like this:
CH4 + 2O2 + 7.52 N2 = CO2 + 2H20 + 7.52 N2
As a result of the combustion reaction, complete combustion produces carbon dioxide CO2 and water vapor H2O, substances that do 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 believed that for complete combustion of 1 m³ of natural gas, at least 10 m³ of air is required. However, if you supply only the theoretically required amount of air, then it is impossible to achieve complete combustion of the fuel: it is difficult to mix the gas with air so that the required number of oxygen molecules is supplied to each of its molecules. In practice, more air is supplied to combustion than is theoretically necessary. The amount of excess air is determined by the excess air coefficient a, which shows the ratio of the amount of air actually consumed for combustion to the theoretically required amount:
α = V actual/V theoretical
where V is the amount of air actually consumed 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 gas-using equipment. But burning 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 and air, the excess air coefficient lies in the range of 1.05 - 1.1”, that is, air consumed for combustion is 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 indoor air (when using equipment without exhausting combustion products into the atmosphere - gas stoves, low-heat water heaters) and has a poisonous 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, the dishes become contaminated with soot, which requires considerable effort to remove. In water heaters, soot contaminates the heat exchanger, in “neglected” cases, until the transfer of heat from combustion products almost completely stops: the column burns, and the water heats up by several degrees.
Incomplete combustion occurs:
- when there is insufficient air supply for combustion;
- with poor mixing of gas and air;
- when the flame cools excessively before the combustion reaction is completed.
The quality of gas combustion can be controlled by the color of the flame. Poor gas combustion is characterized by a yellow, smoky flame. At complete combustion gas flame is a short torch of bluish-violet color with high temperature. Used to control the operation of industrial burners special devices, analyzing the composition flue gases and temperature of combustion products. Currently, when setting up certain types of household gas-using equipment, it is also possible to regulate the combustion process by temperature and analysis of flue gases.
