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 there is 1% natural gas in the air, you should 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 sufficient quantity oxygen.
With complete combustion of gas, CO 2 (carbon dioxide), H 2 O is formed
(water). When gas is incompletely burned, heat loss occurs. Lack of oxygen O 2 oxidizing agent.
Products of incomplete combustion CO - carbon monoxide, poisonous action, 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 for operating personnel are ensured;
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, more precise temperature control is provided;
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 possible only when oxygen is displaced;
Explosion effect during spontaneous combustion;
Possibility of detonation of a mixture of gas and air.
Characteristics of methane
§ Colorless;
§ Non-toxic (non-poisonous);
§ Odorless and tasteless.
§ Methane consists of 75% carbon, 25% hydrogen.
§ Specific gravity is 0.717 kg/m 3 (2 times lighter than air).
§ Flash point is the minimum initial temperature at which combustion begins. For methane it is 645 o.
§ Combustion temperature- This Maximum temperature, which can be achieved with complete combustion of the gas if the amount of air required for combustion exactly corresponds 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 than:
Hydrogen sulfide 0.0 2 G /m.cube
Ammonia 2 gr.
Hydrocyanic acid 5 g.
Resin and dust 0.001 g/m3
Naphthalene 10 gr.
Oxygen 1%.
Using natural gas has a number of advantages:
· absence of ash and dust and removal of solid particles into the atmosphere;
· high heat of combustion;
· ease of transportation and combustion;
· the work of service personnel is facilitated;
· sanitary and hygienic conditions in boiler houses and surrounding areas are improved;
· wide range of automatic control.
When using natural gas, special precautions are required because... leakage is possible through leaks at the junction of the gas pipeline and fittings. The presence of more than 20% of gas in a room causes suffocation; its accumulation in a closed volume of more than 5% to 15% leads to an explosion gas-air mixture. Incomplete combustion releases carbon monoxide, which is poisonous even at low concentrations (0.15%).
Natural gas combustion
Burning called the rapid chemical combination of combustible parts of the fuel with oxygen in the air, 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 (has a poisonous effect on service staff), 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 distributed by relatively fresh stream of gas-air mixture.
The rate of combustion and flame propagation depends on:
· on the composition of the mixture;
· on temperature;
· from pressure;
· on the ratio of gas and air.
The burning rate determines one of the main conditions for the reliable operation of the boiler room and characterizes it flame separation and breakthrough.
Flame break– occurs if the speed of the gas-air mixture at the burner outlet is greater than the combustion speed.
Reasons for separation: excessive increase in gas supply or excessive vacuum in the firebox (draft). Flame separation is observed during ignition and when the burners are turned on. 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. A slip 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 flue ducts for 10-15 minutes and re-ignite the fire.
The combustion process of gaseous fuel can be divided into 4 stages:
1. Gas leaking from the burner nozzle into the burner device under pressure at an increased speed.
2. Formation of a mixture of gas and air.
3. Ignition of the resulting combustible mixture.
4. Combustion of a flammable mixture.
Gas pipelines
Gas is supplied to the consumer through gas pipelines - external and internal– to gas distribution stations located outside the city, and from them via gas pipelines to gas regulatory points hydraulic fracturing or gas control device GRU industrial enterprises.
Gas pipelines are:
· high pressure first category over 0.6 MPa 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 yellow in boiler rooms, and medium-pressure gas pipelines are painted yellow with red rings.
Gas-burners
Gas-burners- a gas burner device designed to supply to the combustion site, depending on the technological requirements, a prepared gas-air mixture or separated gas and air, as well as to ensure stable combustion of gaseous fuel and control the combustion process.
The following requirements apply to burners:
· the main types of burners must be mass-produced in factories;
· burners must ensure the passage of a given amount of gas and the completeness of its combustion;
· ensure a minimum amount of harmful emissions into the atmosphere;
· must operate without noise, flame separation or breakthrough;
· must be easy to maintain, convenient for inspection and repair;
· if necessary, could be used for reserve fuel;
· samples of newly created and existing burners are subject to GOST testing;
The main characteristic 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.
Natural gas- This is the most common fuel today. Natural gas is called natural gas because it is extracted from the very depths of the Earth.
The process of gas combustion is a chemical reaction in which natural gas interacts with oxygen contained in the air.
In gaseous fuel there is a combustible part and a non-combustible part.
The main flammable component of natural gas is methane - CH4. Its content in natural gas reaches 98%. Methane is odorless, tasteless and non-toxic. Its flammability limit is from 5 to 15%. It is these qualities that have made it possible to use natural gas as one of the main types of fuel. A methane concentration of more than 10% is life-threatening; suffocation can occur due to lack of oxygen.
To detect gas leaks, the gas is odorized, in other words, a strong-smelling substance (ethyl mercaptan) is added. In this case, the gas can be detected already at a concentration of 1%.
In addition to methane, natural gas may contain flammable gases - propane, butane and ethane.
To ensure high-quality combustion of gas, it is necessary to supply sufficient air to the combustion zone and ensure good mixing of gas with air. The optimal ratio is 1: 10. That is, for one part of gas there are ten parts of air. In addition, it is necessary to create the desired temperature regime. In order for a gas to ignite, it must be heated to its ignition temperature and in the future the temperature should not fall below the ignition temperature.
It is necessary to organize the removal of combustion products into the atmosphere.
Complete combustion is achieved if there are no flammable substances in the combustion products released into the atmosphere. In this case, carbon and hydrogen combine together and form carbon dioxide and water vapor.
Visually, with complete combustion, the flame is light blue or bluish-violet.
Complete combustion of gas.
methane + oxygen = carbon dioxide + water
CH 4 + 2O 2 = CO 2 + 2H 2 O
In addition to these gases, nitrogen and remaining oxygen are released into the atmosphere with flammable gases. N2+O2
If gas combustion does not occur completely, then flammable substances are released into the atmosphere - carbon monoxide, hydrogen, soot.
Incomplete combustion of gas occurs due to insufficient air. At the same time, tongues of soot visually appear in the flame.
The danger of incomplete combustion of gas is that carbon monoxide can cause poisoning of boiler room personnel. A CO content in the air of 0.01-0.02% can cause mild poisoning. Higher concentrations can cause severe poisoning and death.
The resulting soot settles on the walls of the boiler, 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, 9m3 of air is needed to burn 1m3 of gas. In real conditions, more air is required.
That is, an excess amount of air is needed. This value, designated alpha, shows how many times more air is consumed than is theoretically necessary.
The alpha coefficient depends on the type of specific burner and is usually specified in the burner passport or in accordance with the recommendations for organizing the commissioning work being carried out.
As the amount of excess air increases above the recommended level, heat loss increases. With a significant increase in the amount of air, flame rupture may occur, creating emergency situation. If the amount of air is less than recommended, combustion will be incomplete, thereby creating a risk of poisoning for boiler room personnel.
For more accurate control of the quality of fuel combustion, there are devices - gas analyzers, which measure the content of certain substances in the composition of exhaust gases.
Gas analyzers can be supplied complete with boilers. If they are not available, the corresponding measurements are carried out commissioning organization using portable gas analyzers. A regime map is drawn up in which the necessary control parameters are prescribed. By adhering to them, you can ensure normal complete combustion of the fuel.
The main parameters for regulating fuel combustion are:
- the ratio of gas and air supplied to the burners.
- excess air coefficient.
- vacuum in the furnace.
- Boiler efficiency factor.
In this case, the efficiency of the boiler means the ratio of useful heat to the amount of total heat expended.
Air composition
| Gas name | Chemical element | Contents in the air |
| Nitrogen | N2 | 78 % |
| Oxygen | O2 | 21 % |
| Argon | Ar | 1 % |
| Carbon dioxide | CO2 | 0.03 % |
| Helium | He | less than 0.001% |
| Hydrogen | H2 | less than 0.001% |
| Neon | Ne | less than 0.001% |
| Methane | CH4 | less than 0.001% |
| Krypton | Kr | less than 0.001% |
| Xenon | Xe | less than 0.001% |
The main condition for gas combustion is the presence of oxygen (and therefore air). Without the presence of air, gas combustion is impossible. During gas combustion, chemical reaction compounds of oxygen in the air with carbon and hydrogen in the fuel. 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 gas combustion process, 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 C02, nitrogen N2, water vapor H20. Most of all (by volume) in the combustion products of nitrogen is 69.3-74%.
For complete combustion of gas it is also necessary that it be mixed with air in certain (for each gas) quantities. The higher the calorific value of the gas, the greater the amount of air required. Thus, to burn 1 m3 of natural gas, about 10 m3 of air is required, artificial - about 5 m3, mixed - about 8.5 m3.
If there is insufficient air supply, incomplete combustion of gas or chemical underburning of combustible materials occurs. components; Combustible gases appear in combustion products: carbon monoxide CO, methane CH4 and hydrogen H2
With incomplete combustion of gas, a long, smoky, luminous, opaque, yellow color torch.
Thus, lack of air leads to incomplete combustion gas, and excess - to excessive cooling of the flame temperature. The ignition temperature of natural gas is 530 °C, coke gas - 640 °C, mixed gas - 600 °C. In addition, with a significant excess of air, incomplete combustion of gas also occurs. In this case, the end of the torch is yellowish in color, not completely transparent, with a vague bluish-green core; the flame is unstable and comes off the burner.
Rice. 1. Gas flame - without preliminary mixing of gas with air; b -c partial prev. verifiable 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 longer and consists of three zones. IN atmospheric air pure gas burns. In the first inner dark zone, the gas does not burn: it is not mixed with oxygen in the air and is not heated to the ignition temperature. Air enters the second zone in insufficient quantities: it is retained by the burning layer, and therefore it cannot mix well with the gas. This is evidenced by the brightly glowing, light yellow, smoky color of the flame. Air enters the third zone in sufficient quantities, the oxygen of which mixes well with the gas, the gas burns bluish.
With this method, gas and air are supplied to the furnace separately. In the firebox, not only the combustion of the gas-air mixture occurs, but also the process of preparing the mixture. This method of gas combustion is widely used in industrial installations.
In the second case (Fig. 1.6), gas combustion occurs 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, and 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 in the upper part of the zone the temperature rises sharply.
With partial mixing of gas with air, in this case, complete combustion of the gas occurs only with additional air supply to the torch. During 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 design 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 has been previously prepared. A short transparent flame indicates the completeness of gas combustion blue color(flameless combustion), which is used in infrared radiation devices for gas heating.
- Gas combustion process
Combustion is a reaction that converts the chemical energy of a fuel into heat.
Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen. Its lack causes incomplete combustion, during which less heat is released than during complete combustion, and carbon monoxide (CO), which has a poisonous effect on operating personnel, soot is formed, settling on the heating surface of the boiler and increasing heat loss, which leads to excessive fuel consumption and a decrease in boiler efficiency, air pollution.
To burn 1 m 3 of methane, you need 10 m 3 of air, which contains 2 m 3 of oxygen. To ensure complete combustion of natural gas, air is supplied to the furnace with a slight excess. The ratio of the actually consumed air volume V d to the theoretically required V t is called the excess air coefficient = V d / V t. This indicator depends on the design gas burner and fireboxes: the more perfect they are, the smaller they are. It is necessary to ensure that the excess air coefficient is not less than 1, as this leads to incomplete combustion of the gas. An increase in the excess air ratio reduces the efficiency of the boiler unit.
The completeness of fuel combustion can be determined using a gas analyzer and visually - by the color and nature of the flame:
transparent bluish - complete combustion;
red or yellow - combustion is incomplete.
Combustion is regulated by increasing the air supply to the boiler furnace or decreasing the gas supply. This process uses primary (mixed with gas in the burner - before combustion) and secondary (combined with gas or gas-air mixture in the boiler furnace during combustion) air.
In boilers equipped with diffusion burners (without forced submission air), secondary air under the influence of vacuum enters the furnace through the blow-off doors.
In boilers equipped with injection burners: primary air enters the burner due to injection and is regulated by an adjusting washer, and secondary air enters through the purge doors.
In boilers with mixing burners, primary and secondary air is supplied to the burner by a fan and controlled by air valves.
Violation of the relationship between the speed of the gas-air mixture at the outlet of the burner and the speed of flame propagation leads to separation or jumping of the flame on the burners.
If the speed of the gas-air mixture at the burner exit is greater than the speed of flame propagation, there is separation, and if it is less, there is breakthrough.
If the flame breaks out and breaks through, the maintenance personnel must extinguish the boiler, ventilate the firebox and flues and re-ignite the boiler.
Gaseous fuels are found more and more every year wide application in various sectors of the national economy. In agricultural production, gaseous fuel is widely used for technological (for heating greenhouses, greenhouses, dryers, livestock and poultry complexes) and domestic purposes. IN Lately it is increasingly being used for engines internal combustion.
Compared to other types, gaseous fuels have the following advantages:
burns in a theoretical amount of air, which provides high thermal efficiency and combustion temperature;
upon combustion does not form undesirable products of dry distillation and sulfur compounds, soot and smoke;
it is relatively easily supplied through gas pipelines to remote consumption facilities and can be stored centrally;
ignites easily at any ambient temperature;
requires relatively low production costs, which means it is a cheaper type of fuel compared to other types;
can be used in compressed or liquefied form for internal combustion engines;
has high anti-knock properties;
does not form condensate during combustion, which ensures a significant reduction in wear of engine parts, etc.
However, gaseous fuel also has certain negative properties, which include: poisonous effect, formation of explosive mixtures when mixed with air, easy flow through leaks in connections, etc. Therefore, when working with gaseous fuel, careful compliance with the relevant safety regulations is required.
The use of gaseous fuels is determined by their composition and properties of the hydrocarbon part. The most widely used are natural or associated gas from oil or gas fields, as well as industrial gases from oil refineries and other plants. The main components of these gases are hydrocarbons with the number of carbon atoms in a molecule from one to four (methane, ethane, propane, butane and their derivatives).
Natural gases from gas fields almost entirely consist of methane (82...98%), with little use of gaseous fuel for internal combustion engines. The continuously increasing fleet of vehicles requires more and more fuel. Solve the most important national economic problems of stable supply of automobile engines with efficient energy resources and reduction of consumption liquid fuel petroleum origin is possible through the use of gaseous fuel - liquefied petroleum and natural gases.
For cars, only high-calorie or medium-calorie gases are used. When running on low-calorie gas, the engine does not develop the required power, and the vehicle’s range is also reduced, which is economically unprofitable. Pa). The following types of compressed gases are produced: natural, mechanized coke and enriched coke
The main flammable component of these gases is methane. As with liquid fuel, the presence of hydrogen sulfide in gaseous fuel is undesirable due to its corrosive effect on gas equipment and engine parts. The octane number of gases allows you to boost car engines in terms of compression ratio (up to 10...12).
The presence of cyanogen CN in gas for cars is extremely undesirable. When combined with water, it forms hydrocyanic acid, under the influence of which tiny cracks are formed in the walls of the cylinders. The presence of resinous substances and mechanical impurities in gas leads to the formation of deposits and contaminants on gas equipment and engine parts.
