Gas burning. Conditions for ignition and combustion of gas fuel Combustion reaction of natural gas formula
The location of the automation device elements is shown in Fig. 7. It shows that the electromagnet is covered with a protective cap. The wires from the sensors are located inside thin-walled tubes. The tubes are attached to the electromagnet using union nuts. The body terminals of the sensors are connected to the electromagnet through the housing of the tubes themselves.
Now let’s look at the method for finding the above fault.
The check begins with the “weakest link” of the automation device - the traction sensor. The sensor is not protected by a casing, so after 6... 12 months of operation it becomes “overgrown” with a thick layer of dust. The bimetallic plate (see Fig. 6) quickly oxidizes, which leads to deterioration of contact.
The dust coat is removed with a soft brush. Then the plate is pulled away from the 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 “Contact” spray. It contains substances that actively destroy the oxide film. After cleaning, apply a thin layer of liquid lubricant to the plate and contact.
The next step is to check the serviceability of the thermocouple. She works hard thermal mode, since it is constantly in the pilot flame, naturally, its service life is significantly less than the other elements of the boiler.
The main defect of a thermocouple is burnout (destruction) of its body. In this case, the transition resistance at the welding site (junction) increases sharply. As a result, the current in the Thermocouple - Electromagnet circuit
- 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 rod (Fig. 5).
To check the thermocouple, unscrew the union nut (Fig. 7), located on the left 
sides of the electromagnet. Then turn on the igniter and use a voltmeter to measure the constant voltage (thermo-emf) at the thermocouple contacts (Fig. 8). A heated, serviceable thermocouple generates an EMF of about 25...30 mV. If this value is less, the thermocouple is faulty. To final check it, disconnect the tube from the electromagnet casing and measure the resistance of the thermocouple. 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 for the following reasons:
- clogging of the igniter nozzle (as a result, the heating temperature of the thermocouple may be lower than the nominal one). They “treat” such a defect by cleaning the igniter hole with any soft wire of a suitable diameter;
- shifting the position of the thermocouple (naturally, it may also not heat up enough). Eliminate the defect in the following way- loosen the screw securing the liner near the igniter and adjust the position of the thermocouple (Figure 10);
- low gas pressure at the boiler inlet.
If the EMF at the thermocouple terminals is normal (while the symptoms of malfunction indicated above remain), then check the following elements:
- integrity of contacts at the connection points of the thermocouple and draft sensor.
Oxidized contacts must be cleaned. The union nuts are tightened, as they say, “by hand.” In this case wrench It is not advisable to use it, since you can easily break the wires suitable for the contacts;
- integrity of the electromagnet winding and, if necessary, solder its terminals.
The functionality of the electromagnet can be checked as follows. Disconnect 
thermocouple connection. Press and hold the start button, then light the igniter. From a separate constant voltage source, a voltage of about 1 V is applied to the released electromagnet contact (from a thermocouple) relative to the housing (at a current of up to 2 A). For this, you can use a regular battery (1.5 V), the main thing is that it provides the necessary operating current. The button can now be released. If the igniter does not go out, the electromagnet and draft sensor are working;
- traction sensor. First, check the force of pressing the contact against the bimetallic plate (if indicated signs faults, it is often insufficient). To increase the clamping force, release 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 response temperature of the sensor. The sensor has a large margin of plate deflection angle, ensuring reliable breaking of the electrical circuit in the event of an accident.
Odorization
Combustible gases have no odor. To timely determine their presence in the air, quickly and accurately detect leakage points, the gas is odorized (gives a smell). For odorization, ethyl mercaptan (C 2 H 5 SH) is used. The odorization rate is 16 g of ethyl mercaptan per 1000 m3 of gas, 8 g of ethyl mercaptan sulfur per 1000 m³. Odorization is carried out at gas distribution stations (GDS). If present in the air 1% natural gas you should be able to smell it.
20% of the gas indoors causes suffocation
5-15% explosion
0,15 % carbon monoxide CO- poisoning; 0.5% CO = 30 min. breathing is fatal; 1% carbon monoxide is lethal.
Methane and other hydrocarbon gases are not poisonous, but inhaling them causes dizziness, and high levels in the air lead to suffocation due to lack of oxygen.
Complete and incomplete combustion of fuel:
To burn 1m³ of gas you need 10m³ of air.
The combustion of natural gas is a reaction that converts the chemical energy of the fuel into heat.
Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen.
When gas is completely burned, CO 2 is formed ( carbon dioxide), H 2 O
(water). When gas is incompletely burned, heat loss occurs. Lack of oxygen O 2 oxidizing agent.
The products of incomplete combustion of CO are carbon monoxide, poisonous, C carbon, soot.
Incomplete combustion is an unsatisfactory mixture of gas with air, excessive cooling of the flame before the combustion reaction is completed.
Combustion reaction of the main components of natural gas:
1:10 methane CH 4 + 20 2 = CO 2 + 2H 2 O = carbon dioxide + water
incomplete combustion of gas CH 4 + 1.5O 2 = 2H 2 O + CO - carbon monoxide
Advantages and disadvantages of natural gas over other types of fuel.
Advantages:
The cost of gas production is significantly lower than coal and oil;
High calorific value;
Complete combustion and easier conditions are ensured service personnel;
The absence of carbon monoxide and hydrogen sulfide in natural gases prevents poisoning from gas leaks;
When burning gas, a minimum air residue in the furnace is required and there are no costs due to mechanical afterburning;
When burning gas fuel provides more precise temperature control;
When burning gas, the burners can be placed in an accessible place in the furnace, which allows for better heat transfer and the need for temperature regime;
The ability to change the shape of the flame to heat in a specific place.
Flaws:
Explosion and fire hazard;
The gas combustion process is only possible when oxygen is displaced;
Explosion effect during spontaneous combustion;
Possibility of detonation of a mixture of gas and air.
The combustion products of natural gas are carbon dioxide, water vapor, some excess oxygen and nitrogen. Products are not complete combustion gases can be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, 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 occurs. On another scale, the gas analyzer needle should show the maximum CO 2max content in the combustion products. Complete combustion occurs at the maximum percentage of carbon dioxide, when the CO + H 2 scale pointer is at zero.
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 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 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 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 during their transfer 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 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 can 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 in women who cook on gas stoves, as well as 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 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 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.
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. That's why technical calculations lead by lower 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 is greater than the amount of air required for combustion calculated theoretically.