Regulating the combustion process (Basic principles of combustion)

For optimal combustion, it is necessary to use more air than the theoretical calculation suggests. chemical reaction(stoichiometric air).

This is caused by 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. Typically, excess air ranges from 5% to 50% depending on the type of fuel and burner.

Typically, the more difficult it is to oxidize the fuel, the more excess air is required.

The excess amount of air should not be excessive. Excessive combustion air supply reduces temperature flue gases and increases heat losses heat generator. In addition, at a certain limiting amount of excess air, the torch cools too much and CO and soot begin to form. Conversely, insufficient air causes incomplete combustion and the same problems noted above. Therefore, 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 verified by concentration measurements carbon monoxide CO in flue gases. If there is no carbon monoxide, then combustion has occurred completely.

Indirectly, the excess air level can be calculated by measuring the concentration of free oxygen O 2 and/or carbon dioxide CO 2 in the flue gases.

The amount of air will be approximately 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, but the percentage of volume will vary 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 a "combustion triangle" or Ostwald triangle, which is plotted for each fuel type.

Using 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. Figure 10 shows the combustion triangle for methane.

Figure 10. Combustion triangle for methane

The X-axis indicates the percentage of O2, and the Y-axis indicates the percentage of CO2. The hypotenuse goes from point A, corresponding to the maximum CO 2 content (depending on the fuel) at zero O 2 content, to point B, corresponding to zero CO 2 content and maximum O 2 content (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 represent different percentages of CO.

Let's assume that our system runs on methane and 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 maximum content CO 2 for different types fuel and the value that corresponds to optimal combustion. This value is recommended and based on experience. It should be noted that when the maximum value is taken from the central column, it is necessary to measure emissions according to the procedure described in Chapter 4.3.

Denis Ryndin,
chief engineer of "Water Technology"

Currently, the issues of increasing the efficiency of heating installations and reducing environmental pressure on the environment are particularly acute. The most promising, in this regard, is the use of condensation technology, which is capable of solving the outlined range of problems in the most complete manner. The Vodnaya Tekhnika company has always strived to introduce modern and efficient heating equipment. In light of this, her interest in condensation technology, as the most effective, high-tech and promising, is natural and justified. Therefore, in 2006, one of the priority areas for the company’s development was the promotion of condensing equipment on the Ukrainian market. To this end, a number of events are planned, one of which is a series of popularizing articles for those who are encountering such technology for the first time. In this article we will try to touch on the main issues of implementation and application of the principle of condensation of water vapor in heating technology:

• How is heat different from temperature?
• Can efficiency be more than 100%?

How is heat different from temperature?

Temperature is the degree of heating of a body (kinetic energy of body molecules). The value is very relative; this can be easily illustrated using the Celsius and Fahrenheit scales. In everyday life, the Celsius scale is used, in which 0 is the freezing point of water, and 100° is the boiling point of water at atmospheric pressure. Since the freezing and boiling points of water are not well defined, the Celsius scale is currently defined in terms of the Kelvin scale: a degree Celsius is equal to a degree Kelvin and absolute zero taken as −273.15 °C. The Celsius scale is practically very convenient because water is very common on our planet and our life is based on it. Zero Celsius is a special point for meteorology, since the freezing of atmospheric water changes everything significantly. In England and especially in the USA, the Fahrenheit scale is used. This scale divides the interval from the temperature of the coldest winter in the city where Fahrenheit lived to the temperature of the human body into 100 degrees. Zero Celsius is 32 Fahrenheit, and a degree Fahrenheit is equal to 5/9 degrees Celsius.

Table 1 Temperature units

In order to more clearly imagine the difference between the concepts of temperature and heat, consider the following example: Example with heating water: Let's say we have heated a certain amount of water (120 liters) to a temperature of 50°C, and how much water can we heat to a temperature of 40 °C, using the same amount of heat (burned fuel)? For simplicity, we will assume that in both cases the initial water temperature is 15 °C.

Figure 1 Example 1

As can be seen from clear example, temperature and amount of heat are different concepts. Those. bodies at different temperatures can have the same thermal energy, and vice versa: bodies with the same temperature can have different thermal energy. To simplify the definitions, a special value was invented - Enthalpy Enthalpy is the amount of heat contained in a unit mass of a substance [kJ/kg] Under natural conditions on Earth, there are three aggregate states of water: solid (ice), liquid (water itself), gaseous (water vapor) The transition of water from one state of aggregation to another is accompanied by a change in the thermal energy of the body at a constant temperature (the state changes, not the temperature, in other words, all the heat is spent on changing the state, and not on heating) Sensible heat is that heat at which a change in the amount of heat supplied to the body causes a change in its temperature. Latent heat - the heat of vaporization (condensation) is the heat that does not change the temperature of the body, but serves to change the state of aggregation of the body. Let us illustrate these concepts with a graph on which enthalpy (the amount of heat supplied) will be plotted along the ordinate axis, and temperature along the ordinate axis. This graph shows the process of heating a liquid (water).

Figure 2 Graph of Enthalpy – Temperature, for water

A-B water is heated from a temperature of 0 ºС to a temperature of 100 ºС (in this case, all the heat supplied to the water goes to increase its temperature)
B-C water boils (in this case, all the heat supplied to the water is used to convert it into steam, the temperature remains constant at 100 ºС)
C-D all the water turned into steam (boiled away) and now the heat is coming to increase the steam temperature.

Composition of flue gases when burning gaseous fuels

The combustion process is the process of oxidation of combustible components of fuel with the help of atmospheric oxygen, which releases heat. Let's look at this process:

Figure 3 Composition of Natural Gas and Air

Let's see how the combustion reaction of gaseous fuel develops:

Figure 4 Combustion reaction of gaseous fuel

As can be seen from the oxidation reaction equation, the result is carbon dioxide, water vapor (flue gases) and heat. The heat that is released during the combustion of fuel is called the lower calorific value of the fuel (PCI). If we cool the flue gases, then under certain conditions water vapor will begin to condense (transition from a gaseous state to a liquid).

Figure 5 Latent heat release during condensation of water vapor

At the same time it will stand out additional quantity heat (latent heat of vaporization/condensation). The sum of the lower heating value of a fuel and the latent heat of vaporization/condensation is called the higher heating value of the fuel (PCS).

Naturally, the more water vapor is in the combustion products, the greater the difference between the Higher and Lower heat of combustion of the fuel. In turn, the amount of water vapor depends on the composition of the fuel:

Table 2 Values ​​of higher and lower calorific values ​​for various types fuel

As can be seen from the table above, we can obtain the greatest additional heat by burning methane. The composition of natural gas is not constant and depends on the field. Average composition natural gas is shown in Figure 6.

Figure 6 Composition of natural gas

Interim conclusions:

1. Using the latent heat of vaporization/condensation, you can obtain more heat than is released when burning fuel

2. The most promising fuel, in this regard, is natural gas (the difference between the higher and lower calorific values ​​is more than 10%)

What conditions must be created for condensation to begin? Dew point.

Water vapor in flue gases has slightly different properties than pure water vapor. They are in a mixture with other gases and their parameters correspond to the parameters of the mixture. Therefore, the temperature at which condensation begins differs from 100 ºС. The value of this temperature depends on the composition of the flue gases, which in turn is a consequence of the type and composition of the fuel, as well as the excess air coefficient. The temperature of the flue gases at which condensation of water vapor in the products of fuel combustion begins is called the Dew Point.

Figure 7 Dew point

Interim conclusions:

1. The task of condensation technology is to cool combustion products below the dew point and remove the heat of condensation, using it for useful purposes.

Can the efficiency of a gas boiler be more than 100%?

Let's take technical characteristics some arbitrary mounted boiler:

Total boiler power = 23,000 Kcal/h (26.7 KW);

Net boiler power = 21,000 Kcal/h (24.03 KW);

In other words, the maximum thermal power of the burner is 23,000 Kcal/h (the amount of heat released during fuel combustion), and the maximum amount of heat received by the coolant is 21,000 Kcal/h.

Where does the difference between them go? Some amount of generated heat (6-8%) is lost with the exhaust flue gases, and another (1.5-2%) is dissipated into the surrounding space through the walls of the boiler.

If we add these values, we can write the following equation:

If we divide the useful power of the boiler by the total and multiply the result by 100%, we get the coefficient useful action boiler (efficiency) in%.

If we carefully read the text of the definition, we will see that full power boiler is equal to the amount of heat that is released during the combustion of fuel per unit time.

Thus, this value directly depends on the lower heating value of the fuel, and does not take into account the heat that can be released during the condensation of water vapor from the combustion products.

In other words, this is the efficiency of the boiler, relative to the lower heating value of the fuel.

If we take into account the value of the heat of condensation of water vapor (see Table 1), then we can present the following picture of the distribution of heat flows in a non-condensing boiler.

Figure 9 Distribution of heat flows in a non-condensing boiler

Then, as in a condensing boiler, the distribution of heat flows will look like this:

Figure 10 Distribution of heat flows in a condensing boiler

Interim conclusions:
1. Efficiency of 100% or more is possible if the Lower, and not the Higher, calorific value of combustion is taken as the reference point.
2. We cannot fully use all the heat (sensible and latent) for technical reasons, therefore the boiler efficiency cannot be equal to or greater than 111% (relative to the lower heating value of the fuel).

Operating modes of condensing boilers

Gas condensing boilers can be installed in any heating system. The amount of condensation heat used and the efficiency factor, depending on the operating mode, depend on the correct calculation heating system.

To make effective use of the heat of condensation of water vapor contained in flue gases, it is necessary to cool the flue gases to a temperature below the dew point. The degree of use of condensation heat depends on the calculated temperatures of the coolant in the heating system and on the number of hours worked in condensation mode. This is shown in graphs 11 and 13, where the dew point temperature is 55 °C.

Heating system 40/30 °C

Figure 11 Low-temperature system operating schedule

The productive capacity of the condensing boilers of such a heating system throughout the entire heating period is of great importance. Low return temperatures are always below the dew point temperature, so condensation occurs constantly. This occurs in low temperature panel heating systems or under floor heating. A condensing boiler is ideal for such systems.

Figure 12 Temperature conditions of the room when using floor and convector heating

There are many advantages of water underfloor heating systems over traditional ones:

• Increased comfort. The floor becomes warm and pleasant to walk on, since heat transfer occurs from a large surface with a relatively low temperature.
• Uniform heating of the entire area of ​​the room, and therefore uniform heating. A person feels equally comfortable near a window and in the middle of a room.
• Optimal temperature distribution along the height of the room. Figure 12 illustrates the approximate distribution of temperatures along the height of the room when using traditional heating and floor heating. The temperature distribution with underfloor heating is perceived by a person as the most favorable. It is also necessary to note a reduction in heat loss through the ceiling, since the temperature difference between internal air and external air is significantly reduced, and we receive comfortable heat only where needed, rather than heating the environment through the roof. This allows the underfloor heating system to be used effectively for buildings with high ceilings - churches, exhibition halls, gyms, etc.

• Hygiene. There is no air circulation, drafts are reduced, which means there is no dust circulation, which is a big plus for people’s well-being, especially if they suffer from respiratory diseases.
• A significant portion of the heat from the floor is transferred in the form of radiant heat transfer. Radiation, unlike convection, immediately spreads heat to surrounding surfaces.
• There is no artificial dehumidification of air near heating devices.
• Aesthetics. There are no heating devices, there is no need for them design or selection of optimal sizes.

Heating system 75/60 ​​°C

Figure 13 High-temperature system operating schedule

Effective use of condensation heat is also possible at design temperatures of 75/60 ​​°C for a time of 97% of the duration of the heating period. This applies to outside temperatures between – 11 °C and + 20 °C. Old heating systems, which were designed for temperatures of 90/70 °C, today operate at temperatures of almost 75/60 ​​°C. Even in systems with 90/70 °C heating water and with an operating mode in which the boiler water temperature is controlled depending on outside temperature, the time of use of condensation heat is 80% of the duration of the annual heating period.

High standardized efficiency

In the examples in Figures 11 and 13 it is clearly visible that the different but at the same time high percentage of condensation heat used for these two options has a direct impact on the energy consumption of a gas condensing boiler. To indicate fuel efficiency heating boilers The concept of standardized efficiency was introduced. Figure 14 shows the dependence of energy consumption on various design temperatures of the heating system.

Figure 14 Dependence of efficiency on return temperature

The high standardized efficiencies of gas condensing boilers are explained by the following factors:

– Implementation high value CO2. The higher the CO 2 content, the higher the dew point temperature of the heating gases.

– Maintenance low temperatures return line. The lower the return temperature, the more active the condensation and the lower the flue gas temperature.

Interim conclusions:

The efficiency of a condensing boiler depends very much on temperature regime operation of the heating system.
In new installations, all possibilities for optimal operation of the gas condensing boiler must be taken advantage of. A high efficiency is achieved when the following criteria are met:
1. Limit the return temperature to a maximum of 50 °C
2. Strive to maintain a temperature difference between flow and return of at least 20 K
3. Do not take measures to increase the temperature of the return line (these include, for example, installing a four-way mixer, by-pass lines, hydraulic switches).

Methods for implementing the principle of condensation in mounted boilers

IN this moment There are two main ways to implement the principle of condensation of water vapor in flue gases: a remote economizer and a stainless steel heat exchanger with a built-in economizer

In the first case, the main heat of combustion products is utilized in a conventional convection heat exchanger, and the condensation process itself takes place in a separate unit - a remote economizer. This design allows the use of components and assemblies used in conventional, not condensing boilers, however, does not make it possible to fully realize the potential of condensation technology

Figure 17 Condensing boiler with remote economizer

A heat exchanger with a built-in economizer consists of 4-7 heat exchange elements (coils). Each heat exchange element, in turn, consists of 4 turns of a smooth rectangular pipe, made of of stainless steel with wall thickness approx. 0.8mm (See Figure 18).

Figure 18 Diagram of the movement of flue gases between the turns of the heat exchanger

In front of the insulating plate there are several heat exchange elements. They play the role of the “first stage”, since only minor condensation occurs here. The fourth and, accordingly, the fifth heat exchange element is located behind the insulating plate. In this “condensation stage” the main condensation process takes place.

The advantages of this principle are a very efficient heat transfer and, on the other hand, the elimination of boiling noises caused by high flow rates in smooth pipes.
The next advantage of this heat exchanger is its low tendency to liming, since due to the small cross-sections of the pipes it is created high level swirls.
The smooth surface of the stainless steel pipes and the vertical flow direction ensure a self-cleaning effect.
The heat exchanger return connection is located at the rear, the flow connection is at the front. A condensate drain is installed on the heat exchanger.
The exhaust gas collector before connecting the “air supply / exhaust gas removal” pipeline is made of plastic.

Figure 19 Hydraulic diagram of a condensing boiler with built-in economizer

Figure 20 Sectional view of the heat exchanger of a condensing boiler with a built-in economizer

Conventional gas combustion and full premix combustion

Most boilers with an open combustion chamber have the same principle of gas combustion. Due to the kinetic energy of the gas jet, air is sucked into it.

Figure 19 Principle of gas combustion in atmospheric burners (Venturi nozzle)

Combustible gas is supplied under pressure to the nozzle. Here, due to the narrowing of the passage, the potential pressure energy is converted into the kinetic energy of the jet. Due to the special geometric section of the Venturi nozzle, primary air is mixed. Directly in the nozzle, gas and air are mixed (a gas-air mixture is formed). At the exit from the nozzle, secondary air is mixed. The burner power changes due to changes in gas pressure; the speed of the gas jet and the amount of sucked air change accordingly.
The advantages of this design are its simplicity and noiselessness.
Limitations and disadvantages: large excess air, limited modulation depth, abundance of harmful emissions.

In boilers with a closed combustion chamber, the principle of gas combustion is similar to that described above. The difference lies only in the forced emission of combustion products and the supply of air for combustion. All the advantages and disadvantages of atmospheric burners are valid for boilers with a closed combustion chamber.

Condensing boilers use the principle of “Complete pre-mixing of gas and air”. The essence of this method is the admixture of gas to the air stream, due to the vacuum created by the latter in the Venturi nozzle.

Gas fittings and blower
After the electronic unit recognizes the starting speed of the blower, the gas valves located in series open.
On the suction side of the blower there is a double-walled air supply/exhaust gas outlet fitting (Venturi system). Due to the annular slot, in accordance with the Venturi principle, a suction phenomenon occurs in the chamber above the control membrane main gas in gas fittings.

Figure 20 Burner mixing unit with full premix

Ignition process
The gas passes through channel 1 under the control membranes. The main gas control valve opens due to the resulting pressure difference. The gas then flows through the Venturi system into the blower and mixes with the intake air. The gas-air mixture enters the burner and is ignited.
Modulation mode
The stroke of the main gas control valve depends on the position of the control valve. By increasing the blower speed, the pressure behind the main gas control valve is reduced. Channel 2 continues to change the pressure until the pressure is below the control valve diaphragm. The outlet flow hole continues to close, due to which the intensity of the decrease in gas pressure through channel 2 decreases. Thus, through channel 1, the pressure under the membrane of the main gas control valve increases. The main gas control valve continues to open, thus allowing more gas to flow to the blower and therefore more gas to the burner.
The burner is thus modulated continuously by changing the air flow of the blower. The amount of gas tracks the amount of air in a pre-specified ratio. Thus, it is possible to maintain the excess air ratio at an almost constant level throughout the entire modulation range.

Figure 21 Burner thermal module with full premix

Content of harmful substances in flue gases and ways to reduce their concentration

Currently, pollution environment is acquiring alarming proportions. The amount of emissions from the heat and power sector ranks second, after road transport place.

Figure 22 Percentage emissions

Therefore, the issue of reducing harmful substances in combustion products is especially acute.

Main pollutants:

1. • Carbon monoxide CO
   • Nitrogen oxides NOx
   • Acid fumes

It is advisable to combat the first two factors by improving the combustion process (exact gas-air ratio) and reducing the temperature in the boiler furnace.

When burning gaseous fuel, the following acids may form:

Acid vapors are perfectly removed along with condensate. They are quite easy to dispose of in liquid form. Typically, this is done by neutralizing an acid with an alkali.

Disposal of acid condensate

As can be seen from the methane combustion reaction:

When 1 m3 of gas is burned, 2 m3 of water vapor is formed. Under normal operating conditions of a condensing boiler, about 15-20 liters are generated per day. condensate This condensate has low acidity (about Ph = 3.5-4.5), which does not exceed the permissible level of household waste.

Figure 23 Acidity level of gas boiler condensate

Table 3 Contents heavy metals in condensate

Therefore, it is allowed to discharge condensate into the sewer, where it will be neutralized using alkaline household waste.
Please note that house drainage systems are made of materials that are resistant to acidic condensate.
According to worksheet ATV A 251, these are the following materials:
_ Ceramic pipes
_ Rigid PVC pipes
_ PVC pipes
_ High density polyethylene pipes
_ Polypropylene pipes
_ Pipes made of a copolymer of acrylonitrile, butadiene and styrene or a copolymer of acrylonitrile, styrene and acrylic esters (ABS/ASA)
_ Stainless steel pipes
_ Borosilicate pipes

Figure 24 Condensate disposal

According to Italian standards, the above condensate discharge scheme can be used for boiler systems with a total power of no more than 116 kW (according to the German standard ATV A 251, no more than 200 kW). If this value is exceeded, it is necessary to install special granulator condensate neutralizers.

Figure 25 Neutralization of condensate using a condensate pump

1. Boiler condensate drain outlet
2. Neutralizer inlet pipe
3. Condensate neutralizer
4. Neutralizer outlet pipe
5. Condensate supply hose to the condensate collector
6. Condensate collector
7. Condensate outlet fitting
8. Condensate drain hose
9. Adapter
10. Sewerage
11. Mounting clamps

Figure 25 shows an example of a neutralization installation. The condensate entering the neutralizer is first filtered through a layer activated carbon, and then undergoes neutralization in the main volume. A condensate pump is installed when it is necessary to remove condensate above the level of the condensate siphon in the boiler. This design used for neutralizing condensate from boilers with a total power of 35 to 300 kW (depending on the power of the installation, the length of the neutralizer varies). If the installation power exceeds 300 kW, then several neutralizers are installed in parallel.
The neutralizer is extremely easy to maintain and requires inspection and replenishment of granulate no more than once a year. As a rule, the acidity of the condensate is also assessed using litmus paper.

The argument for condensation technology

1. Description of the proposed technology (method) for increasing energy efficiency, its novelty and awareness of it.

When burning fuel in boilers, the percentage of “excess air” can range from 3 to 70% (excluding suction cups) of the volume of air, the oxygen of which participates in the chemical reaction of oxidation (combustion) of the fuel.

The "excess air" involved in the combustion process is the part atmospheric air, the oxygen of which does not participate in the chemical reaction of fuel oxidation (combustion), but it is necessary to create the required speed regime for the outflow of the fuel-air mixture from the boiler burner device. “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 (combustion), but more “excess air” is needed to create the required speed regime leakage of the fuel-air mixture from the boiler burner device. 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 exhaust flue gases.

If you reduce the percentage of “excess air”, then carbon monoxide “CO” (a poisonous gas) will appear in the exhaust flue gases, which indicates underburning of the fuel, i.e. its loss, and the use of “excess air” leads to the loss of thermal energy for heating it, which increases the consumption of burned fuel and increases emissions of greenhouse gases “CO 2 ” into the atmosphere.

Atmospheric air consists of 79% nitrogen (N 2 - an inert gas without color, taste and odor), which performs the main function of creating the required speed regime for the flow of the fuel-air mixture from the burner device of the power plant for complete and stable combustion of fuel and 21% oxygen (O2), which is a fuel oxidizer. Exhaust flue gases at nominal combustion of natural gas 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 exit from the furnace) indicates a 10% content of excess atmospheric air in the total air flow involved in creating the required speed regime for the flow 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 “excess air” with them, which will prevent the formation of NOx (up to 90.0%) and reduce emissions of “greenhouse gases” (CO 2), as well as the consumption of burned fuel (up to 1.5%).

The invention relates to thermal power engineering, in particular to power plants for burning various types of fuel and methods for recycling flue gases for burning fuel in power plants.

A power plant for burning fuel contains a furnace (1) with burners (2) and a convective flue (3) connected through a smoke exhauster (4) and a chimney (5) to a chimney (6); an air duct (9) of outside air connected to the chimney (5) through a bypass pipe (11) of flue gases and an air duct (14) of a mixture of outside air and flue gases, which is connected to a blower fan (13); a throttle (10) installed on the air duct (9) and a valve (12) mounted on the flue gas bypass pipeline (11), wherein the throttle (10) and valve (12) are equipped with actuators; air heater (8), located in the convective flue (3), connected to the blower fan (13) and connected to the burners (2) through the air duct (15) of the heated mixture of outside air and flue gases; sensor (16) for sampling flue gases, installed at the entrance to the convective flue (3) and connected to a gas analyzer (17) for determining the content of oxygen and carbon monoxide in the 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). A method for utilizing flue gases for burning fuel in a power plant includes selecting part of the flue gases with a static pressure greater than atmospheric from a chimney (5) and feeding it through a flue gas bypass pipeline (11) into an outdoor air duct (9) with a static pressure of external air less than atmospheric ; regulation of the supply of outside air and flue gases by the throttle (10) and valve (12) actuators, controlled by an electronic control unit (18), so that the percentage of oxygen in the outside air is reduced to a level at which at the entrance to the convective flue (3 ) the oxygen content in the flue gases was less than 1% in the absence of carbon monoxide; subsequent mixing of flue gases with outside air in the air duct (14) and the blower fan (13) to obtain a homogeneous mixture of outside air and flue gases; heating the resulting mixture in the air heater (8) by recycling the heat of the flue gases; supplying the heated mixture to the burners (2) through the air duct (15).

2. The result of increasing energy efficiency with mass implementation.
Saving of burned fuel in boiler houses, thermal power plants or state district power plants up to 1.5%

3. Is there a need for additional research to expand the list of objects for the implementation 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 heat and power engineering. It is necessary to mediatize the proposed technology in the Ministries of Energy and Ecology, energy companies generating electric and thermal energy.

5. Existing measures encouragement, coercion, incentives for the implementation of the proposed technology (method) and the need for their improvement.
Introduction of new, more stringent environmental requirements for NOx emissions from boiler units

6. The presence of technical and other restrictions on the use of technology (method) at various sites.
Expand the validity of clause 4.3.25 of the “RULES FOR TECHNICAL OPERATION OF POWER PLANTS AND NETWORKS OF THE RUSSIAN FEDERATION ORDER OF THE MINISTRY OF ENERGY OF THE RF OF JUNE 19, 2003 No. 229” for boilers burning any types of fuel. In the following edition: “...On steam boilers burning any fuel, within the control load range, 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; topics and goals of the work.
The need for R&D is to obtain visual information (educational film) to familiarize employees of heat and power companies with the proposed technology.

8. Availability of regulations, 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 regulatory documents, regulations and the need for their restoration.
Expand the scope of the “RULES FOR TECHNICAL OPERATION OF POWER PLANTS AND NETWORKS OF THE RUSSIAN FEDERATION ORDER OF THE MINISTRY OF ENERGY OF THE RF 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, within the control load range, its combustion should be carried out, as a rule, with excess air coefficients at the furnace outlet of less than 1.03...».

clause 4.3.28. "... The sulfur fuel oil boiler should be fired with the air heating system (air heaters, hot air recirculation system) pre-switched on. The air temperature in front of the air heater during the initial period of firing in an oil-fired boiler should, as a rule, not be lower than 90°C. Ignition of a boiler using any other type of fuel must be done with the air recirculation system previously turned on»

9. The need to develop new or amend 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 proposed technology was tested on a wall-mounted gas boiler with forced draft and discharge of exhaust flue gases (products of natural gas combustion) to the facade of the building with a nominal power of 24.0 kW, but under a load of 8.0 kW. The supply of flue gases to the boiler was carried out through a box installed at a distance of 0.5 m from the flare emission of the coaxial chimney of the boiler. The box retained the escaping smoke, which in turn replaced the “excess air” necessary for complete combustion of natural gas, and a gas analyzer installed in the boiler flue outlet (standard location) monitored emissions. As a result of the experiment, it was possible to reduce NOx emissions by 86.0% and reduce greenhouse gas emissions CO2 by 1.3%.

11. The possibility of influencing other processes with the mass introduction of this technology (changes in the environmental situation, possible impact on human health, increased reliability of energy supply, changes in daily or seasonal loading schedules of energy equipment, changes economic indicators generation and transmission of energy, etc.).
Improving the environmental situation, which affects people’s health, and reducing fuel costs when generating thermal energy.

12. The need for special training of qualified personnel to operate the technology being introduced and develop production.
Training of existing service personnel boiler units with the proposed technology.

13. Estimated methods of implementation:
commercial financing (with cost recovery), since 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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composition of complete combustion products

The products of complete combustion also include ballast components - nitrogen (N2) and oxygen (O2).

Nitrogen always enters the furnace with air, and oxygen remains from air flows not used during the combustion process. Thus, flue gases formed during complete combustion of gaseous fuel consist of four components: CO2, H2O, O2 and N2

When gaseous fuel burns incompletely, combustible components, carbon monoxide, hydrogen, and sometimes methane appear in the flue gases. With a large chemical underburning, carbon particles appear in the combustion products, from which soot is formed. Incomplete combustion of gas can occur when there is a lack of air in the combustion zone (cst>1), unsatisfactory mixing of air with gas, or contact of the torch with cold walls, which leads to the termination of the combustion reaction.

Example. Let us assume that the combustion of 1 m3 of Dashavsky gas produces dry combustion products Kci-35 m3/m3, while the combustion products contain flammable components in the amount of: CO = 0.2%; H2=0.10/o; CH4= = 0.05%.

Determine heat loss from chemical incomplete combustion. This loss is equal to Q3 = VC, g ("26, 3SO + Yu8N3 + 358CH4) = 35 (126.3-0.2 + 108-0.1 + 358-0.05) =

1890 kJ/m3.

The dew point of combustion products is determined as follows. First, find the total volume of combustion products

and, knowing the amount of water vapor Vhn that they contain, determine the partial pressure of water vapor Pngo (the pressure of saturated water vapor at a certain temperature) using the formula

P»to=vmlVr, bar.

Each value of the partial pressure of water vapor corresponds to a certain dew point.

Example. Combustion of 1 m3 of Dashavi natural gas at at = 2.5 produces combustion products Vr = 25 m3/m3, including water vapor Vsn = 2.4 m3/m3. It is required to determine the dew point temperature.

The partial pressure of water vapor in combustion products is equal to

^0=^/^ = 2.4/25 = 0.096 bar.

The found partial pressure corresponds to a temperature of 46 °C. This is the dew point. If the flue gases of this composition have a temperature below 46 "C, then the process of condensation of water vapor will begin.

The efficiency of operation of household stoves converted to gas fuel, is characterized by a coefficient of performance (efficiency), the efficiency of any thermal apparatus is determined from the thermal balance, i.e., the equality between the heat generated by burning fuel and the consumption of this heat for useful heating.

When operating gas household stoves, they have place cases when the flue gases in the chimneys are cooled to the dew point. The dew point is the temperature to which air or other gas must be cooled before the water vapor it contains reaches saturation.

Toxic (harmful) are chemical compounds that negatively affect human and animal health.

The type of fuel affects the composition of harmful substances formed during its combustion. Power plants use solid, liquid and gaseous fuels. The main harmful substances contained in boiler flue gases are: sulfur oxides (SO 2 and SO 3), nitrogen oxides (NO and NO 2), carbon monoxide (CO), vanadium compounds (mainly vanadium pentoxide V 2 O 5). TO harmful substances also includes ash.

Solid fuel. In thermal power engineering, coal (brown, stone, anthracite coal), oil shale and peat are used. The composition of solid fuel is schematically represented.

As you can see, the organic part of the fuel consists of carbon C, hydrogen H, oxygen O, organic sulfur Sopr. The combustible part of the fuel from a number of deposits also includes inorganic pyrite sulfur FeS 2.

The non-combustible (mineral) part of the fuel consists of moisture W and ash A. The main part of the mineral component of the fuel turns into fly ash during combustion, carried away by flue gases. Another part depending on the design of the firebox and physical features mineral component of the fuel can turn into slag.

The ash content of domestic coals varies widely (10-55%). The dust content of flue gases changes accordingly, reaching 60-70 g/m 3 for high-ash coals.

One of the most important features ash is that its particles have different sizes, which range from 1-2 to 60 microns or more. This feature as a parameter characterizing ash is called dispersion.

Chemical composition Solid fuel ash is quite diverse. Typically, ash consists of oxides of silicon, aluminum, titanium, potassium, sodium, iron, calcium, and magnesium. Calcium in ash can be present in the form of free oxide, as well as in the composition of silicates, sulfates and other compounds.

More detailed analyzes mineral part solid fuels show that the ash may contain other elements in small quantities, for example, germanium, boron, arsenic, vanadium, manganese, zinc, uranium, silver, mercury, fluorine, chlorine. Microimpurities of the listed elements are distributed unevenly in fly ash fractions of different particle sizes, and usually their content increases with decreasing particle size.

Solid fuel may contain sulfur in the following forms: pyrite Fe 2 S and pyrite FeS 2 in the molecules of the organic part of the fuel and in the form of sulfates in the mineral part. As a result of combustion, sulfur compounds are converted into sulfur oxides, with about 99% being sulfur dioxide SO 2 .

The sulfur content of coals, depending on the deposit, is 0.3-6%. The sulfur content of oil shale reaches 1.4-1.7%, peat -0.1%.

Compounds of mercury, fluorine and chlorine are present behind the boiler in a gaseous state.

The composition of solid fuel ash may contain radioactive isotopes of potassium, uranium and barium. These emissions have virtually no effect on the radiation situation in the area of ​​the thermal power plant, although their total amount may exceed the emissions of radioactive aerosols at nuclear power plants of the same power.

Liquid fuel. IN Thermal power engineering uses fuel oil, shale oil, diesel and boiler and furnace fuel.

There is no pyrite sulfur in liquid fuel. The composition of fuel oil ash includes vanadium pentoxide (V 2 O 5), as well as Ni 2 O 3, A1 2 O 3, Fe 2 O 3, SiO 2, MgO and other oxides. The ash content of fuel oil does not exceed 0.3%. When it is completely burned, the content of solid particles in the flue gases is about 0.1 g/m3, but this value increases sharply during the period of cleaning the heating surfaces of boilers from external deposits.

Sulfur in fuel oil is found primarily in the form of organic compounds, elemental sulfur and hydrogen sulfide. Its content depends on the sulfur content of the oil from which it is obtained.

Depending on the sulfur content in them, heating oils are divided into: low-sulfur S p<0,5%, сернистые S p = 0.5+ 2.0% and high sulfur S p >2.0%.

Diesel fuel is divided into two groups based on sulfur content: the first - up to 0.2% and the second - up to 0.5%. Low-sulfur boiler and furnace fuel contains no more than 0.5 sulfur, sulfur fuel contains up to 1.1, shale oil contains no more than 1%.

Gaseous fuel represents the “cleanest” organic fuel, since when it is completely burned from toxic substances only nitrogen oxides are formed.

Ash. When calculating the emission of solid particles into the atmosphere, it is necessary to take into account that unburned fuel (underburning) enters the atmosphere along with ash.

Mechanical underburning q1 for chamber furnaces, if we assume the same combustible content in the slag and entrainment.

Due to the fact that all types of fuel have different calorific values, the given ash content Apr and sulfur content Spr are often used in calculations.

The characteristics of some types of fuel are given in table. 1.1.

The proportion of solid particles carried away from the firebox depends on the type of firebox and can be taken according to the following data:

Chambers with solid slag removal., 0.95

Open with liquid slag removal 0.7-0.85

Semi-open with liquid slag removal 0.6-0.8

Two-chamber fireboxes................... 0.5-0.6

Fireboxes with vertical pre-furnaces 0.2-0.4

Horizontal cyclone furnaces 0.1-0.15

From the table 1.1 shows that oil shale and brown coal, as well as Ekibastuz coal, have the highest ash content.

Sulfur oxides. The emission of sulfur oxides is determined by sulfur dioxide.

As studies have shown, the binding of sulfur dioxide by fly ash in the flues of power boilers depends mainly on the content of calcium oxide in the working mass of the fuel.

In dry ash collectors, sulfur oxides are practically not captured.

The proportion of oxides captured in wet ash collectors, which depends on the sulfur content of the fuel and the alkalinity of the irrigation water, can be determined from the graphs presented in the manual.

Nitrogen oxides. The amount of nitrogen oxides in terms of NO 2 (t/year, g/s) emitted into the atmosphere with the flue gases of a boiler (casing) with a productivity of up to 30 t/h can be calculated using the empirical formula in the manual.