Natural gas releases when burned. Products of gas combustion and control of the combustion process. Gas combustion methods

CH 4 + 2 × O 2 +7.52 × N 2 \u003d CO 2 +2× H 2 O + 7.5× N 2 +8500 kcal

Air:

, hence the conclusion:

1 m 3 O 2 accounts for 3.76 m 3N 2

When burning 1 m 3 of gas, it is necessary to spend 9.52 m 3 of air (because 2 + 7.52). Complete combustion of gas releases:

· Carbon dioxide CO 2 ;

· Water vapor;

· Nitrogen (air ballast);

· Heat is released.

When burning 1 m 3 of gas, 2 m 3 of water is released. If the temperature of flue gases in the chimney is less than 120 ° C and the pipe is high and not insulated, then these water vapors condense along the walls of the chimney to its lower part, from where they enter the drainage tank or line through the hole.

To prevent the formation of condensate 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. it is dangerous to reduce the height of the chimney).

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 somewhat larger 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:

Excessively large excess of air reduces the efficiency. boiler.

In the city:

175 kg of reference fuel is spent on the production of 1 Gcal of heat.

By industry:

162 kg of standard fuel is spent on the production of 1 Gcal of heat.

Excess air is determined by flue gas analysis by the instrument.

Coefficientaalong the length of the furnace space is not the same. At the beginning of the furnace at the burner, and when the flue gases exit into the chimney, it is larger than the calculated one due to air leaks through the leaky lining (skinning) of the boiler.

This information applies to boilers operating under vacuum, when the pressure in the furnace is less than atmospheric pressure.

Boilers operating under excessive pressure of gases 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.

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 be concentrated 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, 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 impact from several compounds can 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 nature;
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.

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.

The main condition for gas combustion is the presence of oxygen (and therefore air). Without the presence of air, gas combustion is impossible. In the process of gas combustion, a chemical reaction of the combination of oxygen in the air with carbon and hydrogen in the fuel takes place. The reaction occurs with the release of heat, light, as well as carbon dioxide and water vapor.

Depending on the amount of air involved in the process of combustion of gas, its complete or incomplete combustion occurs.

With sufficient air supply, complete combustion of the gas occurs, as a result of which its combustion products contain non-combustible gases: carbon dioxide CO2, nitrogen N2, water vapor H20. Most of all (by volume) in the combustion products of nitrogen - 69.3-74%.

For complete combustion of gas, it is also necessary that it mixes with air in certain (for each gas) quantities. The higher the calorific value of the gas, the more air is required. So, for burning 1 m3 of natural gas, about 10 m3 of air is required, artificial - about 5 m3, mixed - about 8.5 m3.

In case of insufficient air supply, incomplete combustion of gas or chemical underburning of combustible components occurs; combustible gases appear in the combustion products - carbon monoxide CO, methane CH4 and hydrogen H2

With incomplete combustion of gas, a long, smoky, luminous, opaque, yellow torch is observed.

Thus, a lack of air leads to incomplete combustion of the gas, and an excess of air leads to excessive cooling of the flame temperature. The ignition temperature of natural gas is 530 °C, coke - 640 °C, mixed - 600 °C. In addition, with a significant excess of air, incomplete combustion of the gas also occurs. In this case, the end of the torch is yellowish, not completely transparent, with a blurry bluish-green core; the flame is unstable and breaks away from the burner.

Rice. 1. Gas flame i - without preliminary mixing of gas with air; b -with partial prev. fiduciary mixing of gas with air; c - with preliminary complete mixing of gas with air; 1 - inner dark zone; 2 - smoky luminous cone; 3 - burning layer; 4 - combustion products

In the first case (Fig. 1a), the torch is long and consists of three zones. Pure gas burns in atmospheric air. In the first inner dark zone, the gas does not burn: it is not mixed with atmospheric oxygen and is not heated to the ignition temperature. In the second zone, air enters in insufficient quantities: it is delayed by the burning layer, and therefore it cannot mix well with the gas. This is evidenced by the brightly luminous, light yellow smoky color of the flame. In the third zone, air enters in sufficient quantities, the oxygen of which mixes well with the gas, the gas burns in a bluish color.

With this method, gas and air are fed into the furnace separately. In the furnace, not only the combustion of the gas-air mixture takes place, but also the process of preparing the mixture. This method of gas combustion is widely used in industrial plants.

In the second case (Fig. 1.6), gas combustion is much better. As a result of partial preliminary mixing of gas with air, the prepared gas-air mixture enters the combustion zone. The flame becomes shorter, non-luminous, has two zones - internal and external.

The gas-air mixture in the inner zone does not burn, since it was not heated to the ignition temperature. In the outer zone, the gas-air mixture burns, while the temperature rises sharply in the upper part of the zone.

With partial mixing of gas with air, in this case, complete combustion of the gas occurs only with an additional supply of air to the torch. In the process of gas combustion, air is supplied twice: the first time - before entering the furnace (primary air), the second time - directly into the furnace (secondary air). This method of gas combustion is the basis for the construction of gas burners for household appliances and heating boilers.

In the third case, the torch is significantly shortened and the gas burns more completely, since the gas-air mixture was previously prepared. The completeness of gas combustion is evidenced by a short transparent blue torch (flameless combustion), which is used in infrared radiation devices for gas heating.

- Gas combustion process