Regulation of the combustion process (Basic principles of combustion)
>> Back to contentFor optimal combustion, more air must be used than theoretically calculated. chemical reaction(stoichiometric air).
This is due to the need to oxidize all available fuel.
The difference between the actual amount of air and the stoichiometric amount of air is called excess air. As a rule, excess air is in the range from 5% to 50% depending on the type of fuel and burner.
Generally, the more difficult it is to oxidize the fuel, the more excess air is required.
Excess air should not be excessive. Excessive combustion air supply lowers the flue gas temperature and increases the heat loss of the heat source. In addition, at a certain limit of excess air, the flare cools too much and CO and soot begin to form. Conversely, too little air causes incomplete combustion and the same problems mentioned above. Therefore, in order to ensure complete combustion of the fuel and high combustion efficiency, the amount of excess air must be very precisely regulated.
The completeness and efficiency of combustion is checked by measuring the concentration of carbon monoxide CO in the flue gases. If there is no carbon monoxide, then combustion has occurred completely.
Indirectly, the level of excess air can be calculated by measuring the concentration of free oxygen O 2 and/or carbon dioxide CO 2 in flue gases.
The amount of air will be about 5 times greater than the measured amount of carbon in volume percent.
As for CO 2 , its amount in flue gases depends only on the amount of carbon in the fuel, and not on the amount of excess air. Its absolute amount will be constant, and the percentage of the volume will change depending on the amount of excess air in the flue gases. In the absence of excess air, the amount of CO 2 will be maximum, with an increase in the amount of excess air, the volume percentage of CO 2 in the flue gases decreases. Less excess air corresponds to more CO 2 and vice versa, so combustion is more efficient when the amount of CO 2 is close to its maximum value.
The composition of flue gases can be displayed on a simple graph using the "combustion triangle" or the Ostwald triangle, which is plotted for each type of fuel.
With this graph, knowing the percentage of CO 2 and O 2 , we can determine the CO content and the amount of excess air.
As an example, in fig. 10 shows the combustion triangle for methane.
Figure 10. Combustion triangle for methane
The X-axis indicates the percentage of O 2 , the Y-axis indicates the percentage of CO 2 . the hypotenuse goes from point A, corresponding to the maximum content of CO 2 (depending on the fuel) at zero content of O 2, to point B, corresponding to zero content of CO 2 and maximum content of O 2 (21%). Point A corresponds to the conditions of stoichiometric combustion, point B corresponds to the absence of combustion. The hypotenuse is the set of points corresponding to ideal combustion without CO.
Straight lines parallel to the hypotenuse correspond to different CO percentages.
Let's assume that our system is running on methane and the flue gas analysis shows that the CO 2 content is 10% and the O 2 content is 3%. From the triangle for methane gas, we find that the CO content is 0 and the excess air content is 15%.
Table 5 shows the maximum CO 2 content for different types fuel and the value that corresponds to optimal combustion. This value is recommended and calculated based on experience. It should be noted that when the central column is taken maximum value it is necessary to measure the emissions following the procedure described in chapter 4.3.
1. Description of the proposed technology (method) for improving energy efficiency, its novelty and awareness of it.
When fuel is burned in boilers, the percentage of "excess air" can be from 3 to 70% (excluding suction) of the volume of air, the oxygen of which is involved in the chemical reaction of oxidation (combustion) of the fuel.
The "excess air" involved in the combustion process is that part atmospheric air, whose oxygen is not involved in the chemical reaction of oxidation (combustion) of the fuel, but it is necessary to create the required speed mode for the outflow of the fuel-air mixture from the burner device of the boiler. “Excess air” is a variable value and for the same boiler it is inversely proportional to the amount of fuel burned, or the less fuel is burned, the less oxygen is required for its oxidation (burning), but more “excess air” is needed to create the required speed mode outflow of the fuel-air mixture from the burner device of the boiler. The percentage of "excess air" in the total air flow used for complete combustion of the fuel is determined by the percentage of oxygen in the flue gases.
If the percentage of “excess air” is reduced, then carbon monoxide “CO” will appear in the flue gases ( poisonous gas), which indicates underburning of the fuel, i.e. its loss, and the use of "excess air" leads to the loss of thermal energy for its heating, which increases the consumption of fuel burned and increases emissions greenhouse gases CO 2 into the atmosphere.
Atmospheric air consists of 79% nitrogen (N 2 is an inert gas without color, taste and odor), which performs the main function of creating the required speed mode for the outflow of the fuel-air mixture from the burner of the power plant for complete and sustainable combustion of fuel and 21% oxygen (O 2), which is a fuel oxidizer. Outgoing flue gases at the nominal mode of natural gas combustion in boiler units consist of 71% nitrogen (N 2), 18% water (H 2 O), 9% carbon dioxide(CO 2) and 2% oxygen (O 2). The percentage of oxygen in the flue gases equal to 2% (at the outlet of the furnace) indicates a 10% content of excess atmospheric air in the total air flow involved in creating the required speed mode for the outflow of the fuel-air mixture from the burner device of the boiler unit for complete oxidation (combustion) fuel.
In the process of complete combustion of fuel in boilers, it is necessary to utilize flue gases, replacing them with “excess air”, which will prevent the formation of NOx (up to 90.0%) and reduce emissions of “greenhouse gases” (СО 2), as well as the consumption of combusted fuel (up to 1.5%).
The invention relates to power engineering, in particular to power plants for combustion various kinds fuel and methods of utilization of flue gases for fuel combustion in power plants.
The power plant for fuel combustion contains a furnace (1) with burners (2) and a convective gas duct (3) connected through a smoke exhauster (4) and a chimney (5) to a chimney (6); an outside air duct (9) connected to the chimney (5) through a flue gas bypass pipe (11) and an outside air/flue gas mixture duct (14) connected to a draft fan (13); a throttle (10) mounted on the air duct (9) and a damper (12) mounted on the flue gas bypass pipeline (11), the throttle (10) and the damper (12) being equipped with actuators; air heater (8) located in the convective flue (3), connected to the draft fan (13) and connected to the burners (2) through the air duct (15) of the heated mixture of outside air and flue gases; a flue gas sampling sensor (16) installed at the inlet to the convective flue (3) and connected to a gas analyzer (17) for determining the content of oxygen and carbon monoxide in flue gases; electronic control unit (18), which is connected to the gas analyzer (17) and to the actuators of the throttle (10) and valve (12). The method of utilization of flue gases for fuel combustion in power plant includes taking part of the flue gases with a static pressure greater than atmospheric from the chimney (5) and supplying it through the bypass pipe (11) of the flue gases into the air duct (9) of the outside air with a static pressure of the outside air less than atmospheric; regulation of the supply of outdoor air and flue gases by the actuators of the throttle (10) and damper (12), controlled electronic unit control (18), so that the percentage of oxygen in the outside air is reduced to a level at which, at the inlet to the convective flue (3), the oxygen content in the flue gases is less than 1% in the absence of carbon monoxide; subsequent mixing of flue gases with outside air in the air duct (14) and draft fan (13) to obtain a homogeneous mixture of outside air and flue gases; heating the resulting mixture in the air heater (8) by utilizing the heat of flue gases; supply of the heated mixture to the burners (2) through the air duct (15).
2. The result of increasing energy efficiency during mass implementation.
Up to 1.5% savings in fuel burned in boiler houses, CHPPs or SDPPs
3. Is there a need for additional research to expand the list of objects for the introduction of this technology?
Exists, because the proposed technology can also be applied to engines internal combustion and for gas turbine plants.
4. Reasons why the proposed energy efficient technology is not applied on a mass scale.
The main reason is the novelty of the proposed technology and the psychological inertia of specialists in the field of thermal power engineering. It is necessary to mediatize the proposed technology in the Ministries of Energy and Ecology, energy companies generating electricity and heat.
5. Existing incentives, coercion, incentives for the introduction of the proposed technology (method) and the need to improve them.
Introduction of new more stringent environmental requirements for NOx emissions from boiler units
6. Availability of technical and other restrictions on the use of technology (method) at various facilities.
Expand the scope of clause 4.3.25 of the "RULES FOR THE TECHNICAL OPERATION OF ELECTRIC STATIONS AND NETWORKS OF THE RUSSIAN FEDERATION ORDER OF THE MINISTRY OF ENERGY OF THE RUSSIAN FEDERATION OF JUNE 19, 2003 No. 229" for boilers burning any type of fuel. In the following edition: “...On steam boilers, burning any fuel, in the control range of loads, its combustion should be carried out, as a rule, with excess air coefficients at the outlet of the furnace less than 1.03 ... ".
7. The need for R&D and additional testing; themes and goals of the work.
The need for R&D is to obtain visual information ( educational film) to familiarize employees of thermal power companies with the proposed technology.
8. Availability of decrees, rules, instructions, standards, requirements, prohibitive measures and other documents regulating the use of this technology (method) and mandatory for execution; the need to make changes to them or the need to change the very principles of the formation of these documents; presence of pre-existing normative documents, regulations and the need for their restoration.
Expand the scope of the "RULES FOR THE TECHNICAL OPERATION OF ELECTRIC STATIONS AND NETWORKS OF THE RUSSIAN FEDERATION ORDER OF THE MINISTRY OF ENERGY OF THE RUSSIAN FEDERATION OF JUNE 19, 2003 No. 229"
clause 4.3.25 for boilers burning any type of fuel. In the next edition: "… On steam boilers that burn fuel, in the control range of loads, its combustion should be carried out, as a rule, with excess air coefficients at the outlet of the furnace less than 1.03 ...».
clause 4.3.28. "... Fire-up of the boiler on sulphurous fuel oil must be carried out with the air heating system (heaters, hot air recirculation system) switched on beforehand. The air temperature in front of the air heater during the initial period of kindling on an oil-fired boiler should, as a rule, not be lower than 90°C. The kindling of the boiler on any other type of fuel must be carried out with the air recirculation system turned on beforehand»
9. The need to develop new or change existing laws and regulations.
Not required
10. Availability of implemented pilot projects, analysis of their real effectiveness, identified shortcomings and proposals for improving the technology, taking into account the accumulated experience.
The test of the proposed technology was carried out on a wall-mounted gas boiler with forced draft and the output of flue gases (natural gas combustion products) to the facade of the building with a rated power of 24.0 kW, but under a load of 8.0 kW. Flue gases were supplied to the boiler through a duct installed at a distance of 0.5 m from the flare emission of the coaxial chimney of the boiler. The box delayed the outgoing smoke, which in turn replaced the "excess air" necessary for the complete combustion of natural gas, and the gas analyzer installed in the outlet of the boiler flue (regular place) controlled emissions. As a result of the experiment, it was possible to reduce NOx emissions by 86.0% and reduce emissions of "greenhouse gases" CO2 by 1.3%.
11. The possibility of influencing other processes during the mass introduction of this technology (change environmental situation, possible impact on people's health, increasing the reliability of energy supply, changing daily or seasonal load schedules power equipment, change economic indicators generation and transmission of energy, etc.).
Improving the environmental situation that affects people's health and reducing fuel costs in the production of thermal energy.
12. The need for special training of qualified personnel for the operation of the introduced technology and the development of production.
It will be sufficient to train the existing service personnel of boiler units with the proposed technology.
13. Suggested methods of implementation:
commercial financing (at cost recovery), as the proposed technology pays off within a maximum of two years.
Information provided by: Y. Panfil, PO Box 2150, Chisinau, Moldova, MD 2051, e-mail: [email protected]
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The composition of the flue gases is calculated based on the combustion reactions constituent parts fuel.
The composition of flue gases is determined using special devices called gas analyzers. These are the main instruments that determine the degree of perfection and efficiency of the combustion process, depending on the content of carbon dioxide in the flue gases, the optimal value of which depends on the type of fuel, type and quality of the combustion device.
The composition of flue gases in the steady state changes as follows: the content of H2S and S02 is steadily decreasing, 32, CO2 and CO - changes slightly / In layer-by-layer combustion of oxa, the upper layers of the catalyst are regenerated earlier than the lower ones. A gradual decrease in temperature in the reaction zone is observed, and oxygen appears in the flue gases at the outlet of the reactor.
The composition of flue gases is controlled by samples.
The composition of the flue gas is determined not only by the content of water vapor, but also by the content of other components.
The composition of flue gases varies along the length of the flame. It is not possible to take this change into account when calculating the radiative heat transfer. Therefore, practical calculations of radiative heat transfer are based on the composition of flue gases at the end of the chamber. This simplification is to a certain extent justified by the consideration that the combustion process usually proceeds intensively in the initial, not very large part of the chamber, and therefore most of chamber is occupied by gases whose composition is close to its composition at the end of the chamber. At the end of it almost always contains very few products of incomplete combustion.
The composition of the flue gases is calculated based on the combustion reactions of the constituent parts of the fuel.
The composition of flue gases during the complete combustion of gas from different fields differs slightly.
Flue gases include: 2 61 kg CO2; 0 45 kg H2O; 7 34 kg of N2 and 3 81 kg of air per 1 kg of coal. At 870 C, the volume of flue gases per 1 kg of coal is 45 m3, and at 16 C it is 113 m3; the density of the flue gas mixture is 0 318 kg/l3, which is 103 times greater than the density of air at the same temperature.
Section content
When burning organic fuels in boiler furnaces, various combustion products are formed, such as carbon oxides CO x \u003d CO + CO 2, water vapor H 2 O, sulfur oxides SO x \u003d SO 2 + SO 3, nitrogen oxides NO x \u003d NO + NO 2 , polycyclic aromatic hydrocarbons (PAHs), fluorides, vanadium compounds V 2 O 5 , particulate matter, etc. (see Table 7.1.1). With incomplete combustion of fuel in furnaces, exhaust gases may also contain hydrocarbons CH 4, C 2 H 4, etc. All products of incomplete combustion are harmful, however, when modern technology fuel combustion, their formation can be minimized [1].
Table 7.1.1. Specific emissions from flaring of organic fuels in power boilers [3]
Symbols: A p, S p – respectively, the content of ash and sulfur per working mass of fuel, %.
The criterion for the sanitary assessment of the environment is the maximum permissible concentration (MPC) of a harmful substance in the atmospheric air at ground level. MPC should be understood as such a concentration various substances And chemical compounds, which, when exposed daily for a long time to the human body, does not cause any pathological changes or diseases.
Maximum allowable concentrations (MPC) of harmful substances in the atmospheric air of populated areas are given in Table. 7.1.2 [4]. The maximum one-time concentration of harmful substances is determined by samples taken within 20 minutes, the average daily - per day.
Table 7.1.2. Maximum permissible concentrations of harmful substances in the atmospheric air of populated areas
| Pollutant | Maximum allowable concentration, mg / m 3 | |
| Maximum one-time | Average daily | |
| Dust non-toxic | 0,5 | 0,15 |
| sulphur dioxide | 0,5 | 0,05 |
| carbon monoxide | 3,0 | 1,0 |
| carbon monoxide | 3,0 | 1,0 |
| nitrogen dioxide | 0,085 | 0,04 |
| Nitric oxide | 0,6 | 0,06 |
| Soot (soot) | 0,15 | 0,05 |
| hydrogen sulfide | 0,008 | 0,008 |
| Benz(a)pyrene | - | 0.1 μg / 100 m 3 |
| Vanadium pentoxide | - | 0,002 |
| Fluorine compounds (for fluorine) | 0,02 | 0,005 |
| Chlorine | 0,1 | 0,03 |
Calculations are carried out for each harmful substance separately, so that the concentration of each of them does not exceed the values given in Table. 7.1.2. For boiler houses, these conditions are tightened by the introduction additional requirements on the need to sum up the impact of sulfur and nitrogen oxides, which is determined by the expression
At the same time, due to local air deficiencies or unfavorable thermal and aerodynamic conditions, incomplete combustion products are formed in the furnaces and combustion chambers, consisting mainly of carbon monoxide CO (carbon monoxide), hydrogen H 2 and various hydrocarbons, which characterize heat losses in boiler unit from chemical incompleteness of combustion (chemical underburning).
In addition, during the combustion process, a number of chemical compounds are obtained, which are formed as a result of the oxidation of various components of the fuel and nitrogen in the air N 2. The most significant part of them is nitrogen oxides NO x and sulfur SO x .
Nitrogen oxides are formed by oxidation as molecular nitrogen air and nitrogen contained in the fuel. Experimental studies have shown that the main share of NO x formed in the furnaces of boilers, namely 96÷100%, falls on nitrogen monoxide (oxide) NO. Nitrogen dioxide NO 2 and hemioxide N 2 O are formed in much smaller quantities, and their share is approximately: for NO 2 - up to 4%, and for N 2 O - hundredths of a percent of the total NO x emission. Under typical conditions of fuel flaring in boilers, nitrogen dioxide concentrations NO 2 are, as a rule, negligible compared to the NO content and usually range from 0÷7 ppm up to 20÷30 ppm. At the same time, the rapid mixing of hot and cold regions in a turbulent flame can lead to relatively large concentrations of nitrogen dioxide in the cold zones of the flow. In addition, partial emission of NO 2 occurs in the upper part of the furnace and in the horizontal flue (at T> 900÷1000 K) and at certain conditions can also reach noticeable sizes.
Nitrogen hemoxide N 2 O, formed during the combustion of fuels, is, apparently, a short-lived intermediate. N 2 O is practically absent in the combustion products behind the boilers.
The sulfur contained in the fuel is a source of formation of sulfur oxides SO x: sulfurous SO 2 (sulfur dioxide) and sulfuric SO 3 (sulfur trioxide) anhydrides. The total mass emission of SO x depends only on the sulfur content in the fuel S p , and their concentration in flue gases also depends on the air flow coefficient α. As a rule, the share of SO 2 is 97÷99%, and the share of SO 3 is 1÷3% of the total output of SO x . The actual content of SO 2 in the gases leaving the boilers ranges from 0.08 to 0.6%, and the concentration of SO 3 - from 0.0001 to 0.008%.
Among harmful components flue gases occupies a special place large group polycyclic aromatic hydrocarbons (PAH). Many PAHs have high carcinogenic and (or) mutagenic activity, activate photochemical smog in cities, which requires strict control and limitation of their emissions. At the same time, some PAHs, such as phenanthrene, fluoranthene, pyrene, and a number of others, are almost physiologically inert and are not carcinogenic.
PAHs are formed as a result of incomplete combustion of any hydrocarbon fuels. The latter occurs due to the inhibition of the reactions of oxidation of fuel hydrocarbons by the cold walls of the combustion devices, and can also be caused by an unsatisfactory mixture of fuel and air. This leads to the formation in the furnaces (combustion chambers) of local oxidizing zones with low temperature or areas with excess fuel.
Due to a large number different PAHs in flue gases and the difficulty of measuring their concentrations, it is customary to estimate the level of carcinogenic contamination of combustion products and atmospheric air by the concentration of the most powerful and stable carcinogen, benzo(a)pyrene (B(a)P) C 20 H 12 .
Due to the high toxicity, special mention should be made of such fuel oil combustion products as vanadium oxides. Vanadium is contained in the mineral part of fuel oil and, when burned, forms vanadium oxides VO, VO 2 . However, during the formation of deposits on convective surfaces, vanadium oxides are present mainly in the form of V 2 O 5 . Vanadium pentoxide V 2 O 5 is the most toxic form of vanadium oxides, therefore their emissions are accounted for in terms of V 2 O 5 .
Table 7.1.3. Approximate concentration of harmful substances in combustion products during flaring of organic fuels in power boilers
| Emissions = | Concentration, mg / m 3 | ||
| Natural gas | fuel oil | Coal | |
| Nitrogen oxides NO x (in terms of NO 2) | 200÷ 1200 | 300÷ 1000 | 350 ÷1500 |
| Sulfur dioxide SO 2 | - | 2000÷6000 | 1000÷5000 |
| Sulfuric anhydride SO 3 | - | 4÷250 | 2 ÷100 |
| Carbon monoxide SO | 10÷125 | 10÷150 | 15÷150 |
| Benz (a) pyrene C 20 H 12 | (0.1÷1, 0) 10 -3 | (0.2÷4.0) 10 -3 | (0.3÷14) 10 -3 |
| Solid particles | - | <100 | 150÷300 |
During the combustion of fuel oil and solid fuels, emissions also contain particulate matter, consisting of fly ash, soot particles, PAHs and unburned fuel as a result of mechanical underburning.
The ranges of concentrations of harmful substances in flue gases during the combustion of various types of fuels are given in Table. 7.1.3.
Natural gas is the most widely used fuel today. Natural gas is called natural gas because it is extracted from the very bowels 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 combustible 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 made it possible to use natural gas as one of the main types of fuel. The concentration of methane is more than 10% dangerous for life, so suffocation can occur due to lack of oxygen.
To detect a gas leak, the gas is subjected to odorization, 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, combustible gases such as propane, butane and ethane may be present in natural gas.
To ensure high-quality combustion of gas, it is necessary to bring air into the combustion zone in sufficient quantities and achieve good mixing of gas with air. The ratio of 1: 10 is considered optimal. That is, ten parts of air fall on one part of the gas. In addition, it is necessary to create the desired temperature regime. In order for the 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 combustible 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 \u003d CO 2 + 2H 2 O
In addition to these gases, nitrogen and the remaining oxygen enter the atmosphere with combustible gases. N 2 + O 2
If the combustion of gas is not complete, then combustible substances are emitted into the atmosphere - carbon monoxide, hydrogen, soot.
Incomplete combustion of gas occurs due to insufficient air. At the same time, tongues of soot 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. Higher concentrations can lead to severe poisoning and death.
The resulting soot settles on the walls of the boilers, thereby worsening the transfer of heat to the coolant, which reduces the efficiency of the boiler house. 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 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 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.
To more accurately control the quality of fuel combustion, there are devices - gas analyzers that measure the content of certain substances in the composition of exhaust gases.
Gas analyzers can be supplied with boilers. If they are not available, the relevant measurements are carried out by the commissioning organization using portable gas analyzers. A regime map is compiled in which the necessary control parameters are prescribed. By adhering to them, you can ensure the normal complete combustion of the fuel.
The main parameters for fuel combustion control are:
- the ratio of gas and air supplied to the burners.
- excess air ratio.
- crack in the furnace.
- Boiler efficiency factor.
At the same time, the efficiency of the boiler means the ratio of useful heat to the value of the total heat expended.
Composition of air
| Gas name | Chemical element | Content 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% |
