CH 4+ 2 × O 2 +7.52 × N 2 = CO 2 +2× H 2 O + 7.5× N 2 +8500 Kcal
Air:
, hence the conclusion:
per 1 m 3 O 2 there are 3.76 m 3N 2
When burning 1 m 3 of gas, 9.52 m 3 of air must be consumed (since 2 + 7.52). At complete combustion gas is released:
· Carbon dioxide CO 2;
· Water vapor;
· Nitrogen (air ballast);
· Heat is released.
When 1 m 3 of gas is burned, 2 m 3 of water is released. If the temperature of the exhaust flue gases in the chimney is less than 120 °C and the pipe is high and uninsulated, then these water vapors condense along the walls of the chimney into its lower part, from where they enter a drainage tank or line through a hole.
To prevent the formation of condensation in the chimney, it is necessary to insulate the chimney or reduce the height of the chimney, having previously calculated the draft in the chimney (i.e., reducing the height of the chimney is dangerous).
Products of complete combustion of gas.
· Carbon dioxide;
· Water vapor.
Products of incomplete combustion of gas.
· Carbon monoxide CO;
· Hydrogen H 2;
· Carbon C.
In real conditions, for gas combustion, the air supply is slightly greater than calculated by the formula. The ratio of the actual volume of air supplied for combustion to the theoretically calculated volume is called the excess air coefficient (a). It should not be more than 1.05...1.2:
Excessive excess air reduces efficiency. boiler
By city:
175 kg of standard fuel is spent to generate 1 Gcal of heat.
By trade:
162 kg of standard fuel is spent to generate 1 Gcal of heat.
Excess air is determined by flue gas analysis with a device.
Coefficientathe length of the combustion space is not the same. At the beginning of the firebox at the burner, and when the flue gases exit into chimney it is greater than calculated due to air leaks through the leaky lining (casing) of the boiler.
This information refers to boilers operating under vacuum, when the pressure in the firebox is less than atmospheric.
Boilers operating under excess gas pressure in the boiler furnace are called pressurized boilers. In such boilers, the lining must be very tight to prevent flue gases from entering the boiler room and poisoning people.
Combustion products natural gas are carbon dioxide, water vapor, some excess oxygen and nitrogen. Products of incomplete combustion of gas can be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, and soot.
The more carbon dioxide CO 2 in the combustion products, the less carbon monoxide CO will be in them and the more complete the combustion will be. The concept of “maximum CO 2 content in combustion products” was introduced into practice. The amount of carbon dioxide in the combustion products of some gases is shown in the table below.
The amount of carbon dioxide in gas combustion products
Using the table data and knowing the percentage of CO 2 in the combustion products, you can easily determine the quality of gas combustion and the excess air coefficient a. To do this, using a gas analyzer, you should determine the amount of CO 2 in the gas combustion products and divide the CO 2max value taken from the table by the resulting value. So, for example, if when burning gas the products of its combustion contain 10.2% carbon dioxide, then the coefficient of excess air in the furnace
α = CO 2max / CO 2 analysis = 11.8/10.2 = 1.15.
The most advanced way to control the flow of air into the furnace and the completeness of its combustion is to analyze combustion products using automatic gas analyzers. Gas analyzers periodically take a sample of exhaust gases and determine the content of carbon dioxide in them, as well as the amount of carbon monoxide and unburned hydrogen (CO + H 2) in volume percent.
If the gas analyzer needle reading on the scale (CO 2 + H 2) is zero, this means that combustion is complete and there is no carbon monoxide or unburned hydrogen in the combustion products. If the arrow deviates from zero to the right, then the combustion products contain carbon monoxide and unburned hydrogen, that is, incomplete combustion. On another scale, the gas analyzer needle should show the maximum CO 2max content in the combustion products. Complete combustion occurs when maximum percentage carbon dioxide when the CO + H 2 scale pointer is at zero.
Combustion is a chemical reaction that occurs quickly over time, combining combustible fuel components with oxygen in the air, accompanied by an intense release of heat, light and combustion products.
For methane, combustion reaction with air:
CH4 + 2O2 = CO2 + 2H2 O + Qn
C3 H8 + 5O2 = 3CO2 + 3H2 O + Qn
For LPG:
C4 H10 + 6.5O2 = 4CO2 + 5H2 O + Qn
The products of complete combustion of gases are water vapor (H2 O), carbon dioxide (CO2 ) or carbon dioxide.
When gases are completely burned, the color of the flame is usually bluish-violet.
The volumetric composition of dry air is assumed to be:O2 ≈ 21%, N2 ≈ 79%, from this it follows that
1m3 of oxygen is contained in 4.76m3 (≈ 5 m3) air.
Conclusion: for burning
- 1m3 of methane requires 2m3 of oxygen or about 10m3 of air,
- 1m3 of propane - 5m3 of oxygen or about 25m3 of air,
- 1m3 of butane - 6.5m3 of oxygen or about 32.5m3 of air,
- 1m3 LPG ~ 6m3 oxygen or about 30m3 air.
In practice, when gas is burned, water vapor, as a rule, does not condense, but is removed along with other combustion products. Therefore, technical calculations are based on the lowest calorific value Qn.
Conditions required for combustion:
1. availability of fuel (gas);
2. presence of an oxidizing agent (air oxygen);
3. presence of a source of ignition temperature.
Incomplete combustion of gases.
The reason for incomplete combustion of gas is insufficient air.
The products of incomplete combustion of gases are carbon monoxide or carbon monoxide (CO), unburned flammable hydrocarbons (Cn Hm) and atomic carbon or soot.
For natural gasCH4 + O2 → CO2 + H2 O + CO+ CH4 + C
For LPGCn Hm + O2 → CO2 + H2 O + CO + Cn Hm + C
The most dangerous is the appearance of carbon monoxide, which has a toxic effect on the human body. The formation of soot gives the flame a yellow color.
Incomplete combustion of gas is dangerous to human health (with 1% CO in the air, 2-3 breaths for a person are enough to cause fatal poisoning).
Incomplete combustion is uneconomical (soot interferes with the heat transfer process; with incomplete combustion of gas, we do not receive the heat for which we burn the gas).
To control the completeness of combustion, pay attention to the color of the flame, which with complete combustion should be blue, and with incomplete combustion - yellowish-straw. The most advanced way to control the completeness of combustion is to analyze combustion products using gas analyzers.
Gas combustion methods.
The concept of primary and secondary air.
There are 3 ways to burn gas:
1) diffusion,
2) kinetic,
3) mixed.
Diffusion method or method without preliminary mixing of gas with air.
Only gas flows from the burner into the combustion zone. The air required for combustion is mixed with gas in the combustion zone. This air is called secondary.
The flame is elongated and yellow.
a= 1.3÷1.5t≈ (900÷1000) o C
Kinetic method - a method with complete preliminary mixing of gas with air.
Gas is supplied to the burner and air is supplied by a blowing device. The air required for combustion and which is supplied to the burner for pre-mixing with gas is called primary air.
The flame is short, greenish-bluish in color.
a= 1.01÷1.05t≈ 1400o C
Mixed method - a method with partial preliminary mixing of gas with air.
The gas injects primary air into the burner. A gas-air mixture with an insufficient amount of air for complete combustion enters the combustion zone from the burner. The rest of the air is secondary.
The flame is medium in size, greenish-blue in color.
a=1,1 ¸ 1,2 t≈1200o C
Excess air ratioa= Letc./L theory is the ratio of the amount of air required for combustion in practice to the amount of air required for combustion theoretically calculated.
Should always bea>1, otherwise there will be underburning.
Lex.=a∙ L theoretical, i.e. the excess air coefficient shows how many times the amount of air required for combustion in practice more quantity air required for combustion and calculated theoretically.
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 various connections, 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 hypersensitivity to environmental factors they 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 radon gas), 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 people.
Clinical experience and epidemiological data indicate that chemically sensitive people react inadequately to chemical compounds present even in 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 are capable of inducing, 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 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 an adverse effect on 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 with the organic matrix of these toxic metals 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 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 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 in women who cook on gas stoves, as well as in children;
- decrease in resistance to bacterial diseases lungs due to a decrease in the immunological mechanisms of lung defense;
- causing adverse effects in general on the immune system of 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 strong smell, its 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, coma, which ends 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 taken into account 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, since even non-working gas stove continues to release aromatic compounds it has absorbed over 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 by combustion products air environment premises;
- 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.
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 is the maximum temperature that can be achieved by complete combustion of gas if the amount of air required for combustion is exactly the same chemical formulas 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:
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, it is required special measures caution, 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 gas-air mixture. Incomplete combustion releases carbon monoxide, which is poisonous even at low concentrations (0.15%).
Natural gas combustion
Burning called fast chemical compound combustible parts of fuel with air oxygen, occurs when high temperature, 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 at full carbon monoxide (poisonous effect on service 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 of incomplete combustion of gas can 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 spreads 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. The separation of the flame leads to gas contamination of the furnace and gas ducts of the boiler 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 supplying fuel, find out and eliminate the cause, ventilate the firebox and flues 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 through 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 up 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, it is necessary to install automatic device, providing 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 - gas burner device, intended for supply to the combustion site, depending on technological requirements, a prepared gas-air mixture or separated gas and air, as well as to ensure stable combustion of gaseous fuel and regulate 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;
· provide a minimum amount 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 burner is hers 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.
