Combustion of natural gas is a complex physical and chemical process of interaction of its combustible components with an oxidizing agent, while the chemical energy of the fuel is converted into heat. Burning can be complete or incomplete. When gas is mixed with air, the temperature in the furnace is high enough for combustion, the fuel and air are continuously supplied, complete combustion of the fuel is carried out. Incomplete combustion of fuel occurs when these rules are not observed, which leads to less heat release, (CO), hydrogen (H2), methane (CH4), and as a result, to soot deposition on heating surfaces, worsening heat transfer and increasing heat loss, which in turn, leads to excessive fuel consumption and a decrease in the efficiency of the boiler and, accordingly, to air pollution.
The excess air ratio depends on the design of the gas burner and furnace. The excess air coefficient must be at least 1, otherwise it may lead to incomplete combustion of the gas. And also an increase in the excess air coefficient reduces the efficiency of the heat-using installation due to large heat losses with the exhaust gases.
The completeness of combustion is determined using a gas analyzer and by color and smell.
Complete combustion of gas. methane + oxygen \u003d carbon dioxide + water CH4 + 2O2 \u003d CO2 + 2H2O In addition to these gases, nitrogen and the remaining oxygen enter the atmosphere with combustible gases. N2 + O2 If the combustion of gas is incomplete, then combustible substances are emitted into the atmosphere - carbon monoxide, hydrogen, soot.CO + H + C
Incomplete combustion of gas occurs due to insufficient air. At the same time, soot tongues appear visually in the flame. The danger of incomplete combustion of gas is that carbon monoxide can cause poisoning of boiler room personnel. The content of CO in the air 0.01-0.02% can cause mild poisoning. A higher concentration can lead to severe poisoning and death. The resulting soot settles on the walls of the boilers, thereby impairing the transfer of heat to the coolant and reducing the efficiency of the boiler room. Soot conducts heat 200 times worse than methane. Theoretically, 9 m3 of air is needed to burn 1 m3 of gas. In real conditions, more air is needed. That is, an excess amount of air is needed. This value, denoted alpha, shows how many times more air is consumed than theoretically necessary. The alpha coefficient depends on the type of a particular burner and is usually prescribed in the burner's passport or in accordance with the recommendations of the commissioning organization. With an increase in the amount of excess air above the recommended one, heat losses increase. With a significant increase in the amount of air, flame separation can occur, creating an emergency. If the amount of air is less than recommended, then combustion will be incomplete, thereby creating a risk of poisoning the boiler room personnel. Incomplete combustion is determined by:
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 to 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 reference 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 greater than the calculated one due to air leaks through the leaky lining (skin) 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.
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
Gas combustion is a combination of the following processes:
Mixing combustible gas with air
heating the mixture
thermal decomposition of combustible components,
Ignition and chemical combination of combustible components with atmospheric oxygen, accompanied by the formation of a torch and intense heat release.
The combustion of methane occurs according to the reaction:
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
Conditions required for gas combustion:
Ensuring the required ratio of combustible gas and air,
heating up to ignition temperature.
If the gas-air mixture of gas is less than the lower flammable limit, then it will not burn.
If there is more gas in the gas-air mixture than the upper flammable limit, then it will not burn completely.
The composition of the 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.
Combustion of 1 m 3 of natural gas requires 9.5 m 3 of air. 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 expense;
L t - theoretically required flow.
The excess air coefficient is always greater than one. For natural gas, it is 1.05 - 1.2.
2. Purpose, device and main characteristics of instantaneous water heaters.
Flowing gas water heaters. Designed for heating water to a certain temperature during drawdown. Flowing water heaters are divided according to the load of thermal power: 33600, 75600, 105000 kJ, according to the degree of automation - into the highest and first classes. efficiency water heaters 80%, oxide content is not more than 0.05%, the temperature of the combustion products behind the draft interrupter is not less than 180 0 C. The principle is based on heating water during the drawdown period.
The main units of instantaneous water heaters are: a gas burner, a heat exchanger, an automation system and a gas outlet. The low pressure gas is fed into the injection burner. The combustion products pass through the 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 breakers, which, in the event of a short-term violation of draft, prevent the flame of the gas burner from extinguishing. There is a flue pipe for connection to the chimney.
Gas instantaneous water heater - VPG. On the front wall of the casing there are: a gas cock control knob, a button for turning on the solenoid valve and a viewing window for observing the flame of the pilot and main burners. At the top of the device there is a smoke exhaust device, at the bottom there are branch pipes for connecting the device to the gas and water systems. The gas enters the solenoid valve, the gas shut-off valve of the water and gas burner block sequentially turns on the pilot burner and supplies gas to the main burner.
Blocking the flow of gas to the main burner, with the obligatory operation of the igniter, is carried out by an electromagnetic valve operating from a thermocouple. Blocking the gas supply to the main burner, depending on the presence of water intake, is carried out by a valve driven through the stem from the membrane of the water block valve.
Combustion is a chemical reaction that proceeds rapidly in time, combining combustible fuel components with atmospheric oxygen, accompanied by an intense release of heat, light and combustion products.
For methane, the combustion reaction with air is:
CH4 + 2O2 = CO2 + 2H2O + Qn
C3H8 + 5O2 = 3CO2 + 3H2O + Qn
For LPG:
C4 H10 + 6.5O2 = 4CO2 + 5H2O + Qn
The products of complete combustion of gases are water vapor (H2 O), carbon dioxide (CO2 ) or carbon dioxide.
With complete combustion of gases, the color of the flame, as a rule, is bluish-violet.
The volumetric composition of dry air is taken:O2 ≈ 21%, N2 ≈ 79%, from this it follows that
1m3 of oxygen is contained in 4.76m3 (≈ 5 m3) of 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,
- 1 m3 of butane - 6.5 m3 of oxygen or about 32.5 m3 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 lower calorific value Qn.
Conditions required for combustion:
1. availability of fuel (gas);
2. the presence of an oxidizing agent (air oxygen);
3. the presence of a source of ignition temperature.
Incomplete combustion of gases.
The cause of incomplete combustion of gas is insufficient air.
The products of incomplete combustion of gases are carbon monoxide or carbon monoxide (CO), unburned combustible 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 for human health (with a content of 1% CO in the air, 2-3 breaths for a person is enough to poison with a fatal outcome).
Incomplete combustion is uneconomical (soot interferes with the process of heat transfer; with incomplete combustion of gas, we receive less heat for which we burn gas).
To control the completeness of combustion, pay attention to the color of the flame, which should be blue during complete combustion, and yellowish-straw if incomplete combustion. The most perfect way to control the completeness of combustion is the analysis of 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 enters the combustion zone from the burner. The air required for combustion is mixed with the gas in the combustion zone. This air is called secondary.
The flame is elongated, yellow.
a= 1.3÷1.5t≈ (900÷1000) о С
Kinetic method - a method with complete pre-mixing of gas with air.
Gas is supplied to the burner and air is supplied by a blower device. The air necessary for combustion and which is supplied to the burner for pre-mixing with gas is called primary.
The flame is short, greenish-bluish in color.
a= 1.01÷1.05t≈ 1400о С
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≈1200о С
Excess air ratioa= Letc./L theor. is the ratio of the amount of air required for combustion in practice to the amount of air required for combustion and theoretically calculated.
Should always bea>1, otherwise there will be underburning.
Lex.=a∙ L theor., 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 and calculated theoretically.
