Use of flue gas heat in gas-fired industrial boiler houses

Use of flue gas heat in gas-fired industrial boiler houses

Candidate of Technical Sciences Sizov V.P., Doctor of Technical Sciences Yuzhakov A.A., Candidate of Technical Sciences Kapger I.V.,
Permavtomatika LLC, sizovperm@ mail .ru

Abstract: the price of natural gas varies significantly around the world. This depends on the country’s membership in the WTO, whether the country exports or imports its gas, gas production costs, the state of industry, political decisions, etc. The price of gas in the Russian Federation in connection with our country’s accession to the WTO will only increase and the government plans to equalize prices for natural gas both within the country and abroad. Let's roughly compare gas prices in Europe and Russia.

Russia – 3 rubles/m3.

Germany - 25 rubles/m3.

Denmark - 42 rubles/m3.

Ukraine, Belarus – 10 rubles/m3.

The prices are quite reasonable. In European countries, condensing-type boilers are widely used, their total share in the heat generation process reaches 90%. In Russia, these boilers are mainly not used due to the high cost of boilers, the low cost of gas and high-temperature centralized networks. And also by maintaining the system for limiting gas combustion in boiler houses.

Currently, the issue of more complete use of coolant energy is becoming increasingly relevant. The release of heat into the atmosphere not only creates additional pressure on environment, but also increases the costs of boiler house owners. At the same time, modern technologies make it possible to more fully utilize the heat of flue gases and increase the efficiency of the boiler, calculated based on the lower calorific value, up to a value of 111%. Heat loss with flue gases occupies the main place among the heat losses of the boiler and amounts to 5 ¸ 12% of generated heat. In addition, the heat of condensation of water vapor that is formed during fuel combustion can be used. The amount of heat released during condensation of water vapor depends on the type of fuel and ranges from 3.8% for liquid fuels and up to 11.2% for gaseous (for methane) and is defined as the difference between the higher and lower heat of combustion of the fuel (Table 1).

Table 1 - Values ​​of higher and lower calorific values ​​for various types fuel

Fuel type	PCS (Kcal)	PCI ( Kcal )	Difference (%)
Heating oil

It turns out that the exhaust gases contain both sensible and latent heat. Moreover, the latter can reach a value that in some cases exceeds sensible heat. Sensible heat is heat in which a change in the amount of heat supplied to a body causes a change in its temperature. Latent heat is the heat of vaporization (condensation), which does not change the temperature of the body, but serves to change the state of aggregation of the body. This statement is illustrated by a graph (Fig. 1, on which enthalpy (the amount of heat supplied) is plotted along the abscissa axis, and temperature is plotted along the ordinate axis).

Rice. 1 – Dependence of enthalpy change for water

In the section of graph A-B, water is heated from a temperature of 0 °C to a temperature of 100 °C. In this case, all the heat supplied to the water is used to increase its temperature. Then the change in enthalpy is determined by formula (1)

(1)

where c is the heat capacity of water, m is the mass of the heated water, Dt – temperature difference.

Section of the B-C graph demonstrates the process of water boiling. In this case, all the heat supplied to the water is spent on converting it into steam, while the temperature remains constant - 100 ° C. Plot graphics C-D shows that all the water has turned into steam (boiled away), after which the heat is spent to increase the temperature of the steam. Then the enthalpy change for section A-C characterized by formula (2)

Where r = 2500 kJ/kg – latent heat of vaporization of water at atmospheric pressure.

The biggest difference between the highest and lowest calorific values, as can be seen from table. 1, methane, so natural gas (up to 99% methane) gives the highest profitability. From here, all further calculations and conclusions will be given for methane-based gas. Consider the combustion reaction of methane (3)

From the equation of this reaction it follows that for the oxidation of one methane molecule, two oxygen molecules are needed, i.e. For complete combustion of 1 m 3 of methane, 2 m 3 of oxygen is required. It is used as an oxidizer when burning fuel in boiler units. atmospheric air, which represents a mixture of gases. For technical calculations, the conditional composition of air is usually taken as consisting of two components: oxygen (21 vol. %) and nitrogen (79 vol. %). Taking into account the composition of the air, to carry out the combustion reaction, complete combustion of the gas will require a volume of air 100/21 = 4.76 times more than oxygen. Thus, to burn 1 m 3 of methane it will take 2 ×4.76=9.52 air. As you can see from the oxidation reaction equation, the result is carbon dioxide, water vapor (flue gases) and heat. The heat that is released during fuel combustion according to (3) is called the net calorific value of the fuel (PCI).

If you cool water vapor, then under certain conditions they will begin to condense (transition from a gaseous state to a liquid) and at the same time an additional amount of heat will be released (latent heat of vaporization/condensation) Fig. 2.

Rice. 2 – Heat release during condensation of water vapor

It should be borne in mind that 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 °C. 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 and looks like Fig. 3.

Rice. 3 – Dew point for methane

Consequently, for flue gases, which are a mixture of gases and water vapor, the enthalpy changes according to a slightly different law (Fig. 4).

Figure 4 – Heat release from the steam-air mixture

From the graph in Fig. 4, two important conclusions can be drawn. First, the dew point temperature is equal to the temperature to which the flue gases were cooled. Secondly, it is not necessary to go through it as in Fig. 2, the entire condensation zone, which is not only practically impossible but also unnecessary. This, in turn, provides various implementation possibilities heat balance. In other words, almost any small volume of coolant can be used to cool flue gases.

From the above, we can conclude that when calculating the boiler efficiency based on the lower calorific value with subsequent utilization of the heat of flue gases and water vapor, the efficiency can be significantly increased (more than 100%). At first glance, this contradicts the laws of physics, but in fact there is no contradiction here. The efficiency of such systems must be calculated based on the higher calorific value, and determination of efficiency by lower calorific value it is necessary to carry out only if it is necessary to compare its efficiency with the efficiency of a conventional boiler. Only in this context does efficiency > 100% make sense. We believe that for such installations it is more correct to give two efficiencies. The problem statement can be formulated as follows. For more full use heat of combustion of flue gases, they must be cooled to a temperature below the dew point. In this case, the water vapor generated during gas combustion will condense and transfer the latent heat of vaporization to the coolant. In this case, cooling of flue gases should be carried out in heat exchangers special design, depending mainly on the temperature of the flue gases and the temperature of the cooling water. Use of water as intermediate coolant is the most attractive, because in this case it is possible to use water with the lowest possible temperature. As a result, it is possible to obtain a water temperature at the outlet of the heat exchanger, for example, 54°C, and then use it. If the return line is used as a coolant, its temperature should be as low as possible, and this is often only possible if there is low temperature systems heating as consumers.

Flue gases from high-power boiler units are usually discharged into a reinforced concrete or brick pipe. If special measures are not taken for the subsequent heating of partially dried flue gases, the pipe will turn into a condensation heat exchanger with all the ensuing consequences. There are two ways to solve this issue. The first way is to use a bypass, in which part of the gases, for example 80%, is passed through the heat exchanger, and the other part, in the amount of 20%, is passed through the bypass and then mixed with the partially dried gases. Thus, by heating the gases, we shift the dew point to the required temperature at which the pipe is guaranteed to operate in dry mode. The second method is to use a plate recuperator. In this case, the exhaust gases pass through the recuperator several times, thereby heating themselves.

Let's consider an example of calculating a 150 m typical pipe (Fig. 5-7), which has a three-layer structure. Calculations were performed in the software package Ansys -CFX . It is clear from the figures that the movement of gas in the pipe has a pronounced turbulent character and, as a result, the minimum temperature on the lining may not be in the area of ​​the tip, as follows from the simplified empirical methodology.

Rice. 7 – temperature field on the surface of the lining

It should be noted that when installing a heat exchanger in a gas path, its aerodynamic resistance will increase, but the volume and temperature of the exhaust gases will decrease. This leads to a decrease in the current of the smoke exhauster. The formation of condensate imposes special requirements on the elements of the gas path in terms of the use of corrosion-resistant materials. The amount of condensate is approximately 1000-600 kg/hour per 1 Gcal of useful heat exchanger power. The pH value of the condensate of combustion products when burning natural gas is 4.5-4.7, which corresponds to acidic environment. In case of a small amount of condensate, it is possible to use replaceable blocks to neutralize the condensate. However, for large boiler houses it is necessary to use caustic soda dosing technology. As practice shows, small volumes of condensate can be used as make-up without any neutralization.

It should be emphasized that the main problem in the design of the systems noted above is the too large difference in enthalpy per unit volume of substances, and the resulting technical problem- development of the heat exchange surface on the gas side. The industry of the Russian Federation mass-produces similar heat exchangers such as KSK, VNV, etc. Let's consider how developed the heat exchange surface on the gas side is on the existing structure (Fig. 8). An ordinary tube in which water (liquid) flows inside, and air (exhaust gases) flows from the outside along the fins of the radiator. The calculated heater ratio will be expressed by a certain

Rice. 8 – drawing of the heater tube.

coefficient

K =S nar /S vn, (4),

Where S nar – outer area of ​​the heat exchanger mm 2, and S vn – internal area of ​​the tube.

In geometric calculations of the structure we obtain K =15. This means that the outer area of ​​the tube is 15 times larger than the inner area. This is explained by the fact that the enthalpy of air per unit volume is many times less than the enthalpy of water per unit volume. Let's calculate how many times the enthalpy of a liter of air is less than the enthalpy of a liter of water. From

enthalpy of water: E in = 4.183 KJ/l*K.

air enthalpy: E air = 0.7864 J/l*K. (at a temperature of 130 0 C).

Hence the enthalpy of water is 5319 times greater than the enthalpy of air, and therefore K =S nar /S vn . Ideally, in such a heat exchanger, the coefficient K should be 5319, but since the outer surface in relation to the inner surface is developed 15 times, the difference in enthalpy essentially between air and water is reduced to the value K = (5319/15) = 354. Technically develop the ratio of the areas of the internal and external surfaces to obtain the ratio K =5319 very difficult or almost impossible. To solve this problem, we will try to artificially increase the enthalpy of air (exhaust gases). To do this, spray water (condensate of the same gas) from the nozzle into the exhaust gas. Let's spray it in such an amount relative to the gas that all the sprayed water will completely evaporate in the gas and the relative humidity of the gas will become 100%. The relative humidity of the gas can be calculated based on Table 2.

Table 2. Values ​​of absolute gas humidity with a relative humidity of 100% for water at various temperatures and atmospheric pressure.

T,°C	A,g/m3	T,°C	A,g/m3	T,°C	A,g/m3
	86,74

From Fig. 3 it is clear that with a very high-quality burner, it is possible to achieve a dew point temperature in the exhaust gases T dew = 60 0 C. In this case, the temperature of these gases is 130 0 C. The absolute moisture content in the gas (according to Table 2) at T dew = 60 0 C will be 129,70 g/m 3 . If water is sprayed into this gas, its temperature will drop sharply, its density will increase, and its enthalpy will rise sharply. It should be noted that it makes no sense to spray water above 100% relative humidity, because... When the relative humidity threshold exceeds 100%, the sprayed water will stop evaporating into gas. Let us carry out a small calculation of the required amount of sprayed water for the following conditions: Tg – initial gas temperature equal to 120 0 C, T rise - gas dew point 60 0 C (129.70 g/m 3), required IT: Tgk - the final temperature of the gas and Mv - the mass of water sprayed in the gas (kg.)

Solution. All calculations are carried out relative to 1 m 3 of gas. The complexity of the calculations is determined by the fact that as a result of atomization, both the density of the gas and its heat capacity, volume, etc. change. In addition, it is assumed that evaporation occurs in an absolutely dry gas, and the energy for heating water is not taken into account.

Let's calculate the amount of energy given by the gas to water during the evaporation of water

where: c – heat capacity of gas (1 KJ/kg.K), m – gas mass (1 kg/m 3)

Let's calculate the amount of energy given up by water during evaporation into gas

Where: r – latent energy of vaporization (2500 KJ/kg), m – mass of evaporated water

As a result of substitution we get the function

(5)

It should be taken into account that it is impossible to spray more water than indicated in Table 2, and the gas already contains evaporated water. Through selection and calculations we obtained the value m = 22 g, Tgk = 65 0 C. Let's calculate the actual enthalpy of the resulting gas, taking into account that its relative humidity is 100% and when it is cooled, both latent and sensible energy will be released. Then according to we obtain the sum of two enthalpies. Enthalpy of gas and enthalpy of condensed water.

E voz = Eg + Evod

Eg we find from reference literature 1.1 (KJ/m 3 *K)

EvodWe calculate relative to the table. 2. Our gas, cooling from 65 0 C to 64 0 C, releases 6.58 grams of water. The enthalpy of condensation is Evod=2500 J/g or in our case Evod=16.45 KJ/m 3

Let's sum up the enthalpy of condensed water and the enthalpy of gas.

E voz =17.55 (J/l*K)

As we can see by spraying water, we were able to increase the enthalpy of the gas by 22.3 times. If before spraying water the gas enthalpy was E air = 0.7864 J/l*K. (at a temperature of 130 0 C). Then after sputtering the enthalpy is Evoz =17.55 (J/l*K). This means that to obtain the same thermal energy on the same standard heat exchanger type KSK, VNV, the heat exchanger area can be reduced by 22.3 times. The recalculated coefficient K (the value was equal to 5319) becomes equal to 16. And with this coefficient, the heat exchanger acquires quite feasible dimensions.

Another important issue when creating similar systems is the analysis of the spraying process, i.e. what diameter of a drop is needed when water evaporates in gas. If the droplet is small enough (for example, 5 μM), then the lifetime of this droplet in the gas before complete evaporation is quite short. And if the droplet has a size of, for example, 600 µM, then naturally it remains in the gas much longer before complete evaporation. The solution to this physical problem is quite complicated by the fact that the evaporation process occurs with constantly changing characteristics: temperature, humidity, droplet diameter, etc. For this process, the solution is presented in, and the formula for calculating the time of complete evaporation ( ) drops look like

(6)

Where: ρ and - liquid density (1 kg/dm 3), r – energy of vaporization (2500 kJ/kg), λ g – thermal conductivity of gas (0.026 J/m 2 K), d 2 – drop diameter (m), Δ t – average temperature difference between gas and water (K).

Then, according to (6), the lifetime of a droplet with a diameter of 100 μM. (1*10 -4 m) is τ = 2*10 -3 hours or 1.8 seconds, and the lifetime of a drop with a diameter of 50 µM. (5*10 -5 m) is equal to τ = 5*10 -4 hours or 0.072 seconds. Accordingly, knowing the lifetime of a drop, its flight speed in space, the speed of gas flow and the geometric dimensions of the gas duct, one can easily calculate the irrigation system for the gas duct.

Below we will consider the implementation of the system design taking into account the relations obtained above. It is believed that the flue gas heat exchanger must operate depending on the outside temperature, otherwise the house pipe will be destroyed when condensation forms in it. However, it is possible to manufacture a heat exchanger that operates regardless of the street temperature and has a better heat removal from exhaust gases, even to subzero temperatures, despite the fact that the temperature of the exhaust gases will be, for example, +10 0 C (the dew point of these gases will be 0 0 C). This is ensured by the fact that during heat exchange the controller calculates the dew point, heat exchange energy and other parameters. Let's consider the technological diagram of the proposed system (Fig. 9).

According to the technological diagram, the following are installed in the heat exchanger: adjustable dampers a-b-c-d; heat exchangers d-e-zh; temperature sensors 1-2-3-4-5-6; o Sprinkler (pump H, and a group of nozzles); control controller.

Let us consider the functioning of the proposed system. Let the exhaust gases escape from the boiler. for example, a temperature of 120 0 C and a dew point of 60 0 C (indicated in the diagram as 120/60). The temperature sensor (1) measures the temperature of the boiler exhaust gases. The dew point is calculated by the controller relative to the stoichiometry of gas combustion. A gate (a) appears in the path of the gas. This is an emergency shutter. which closes in the event of equipment repair, malfunction, overhaul, maintenance, etc. Thus, the damper (a) is fully open and directly passes the boiler exhaust gases into the smoke exhauster. With this scheme, heat recovery is zero; in fact, the flue gas removal scheme is restored as it was before the installation of the heat exchanger. In operating condition, the gate (a) is completely closed and 100% of the gases enter the heat exchanger.

In the heat exchanger, the gases enter the recuperator (d) where they are cooled, but in any case not below the dew point (60 0 C). For example, they cooled down to 90 0 C. No moisture was released in them. The gas temperature is measured by temperature sensor 2. The temperature of the gases after the recuperator can be adjusted with a gate (b). Regulation is necessary to increase the efficiency of the heat exchanger. Since during condensation of moisture, the mass of it in gases decreases depending on how much the gases have been cooled, it is possible to remove from them up to 2/11 of total mass gases in the form of water. Where did this figure come from? Let's consider chemical formula methane oxidation reactions (3).

To oxidize 1m 3 of methane, 2m 3 of oxygen is needed. But since the air contains only 20% oxygen, 10 m 3 of air will be required to oxidize 1 m 3 of methane. After burning this mixture, we get: 1 m 3 of carbon dioxide, 2 m 3 of water vapor and 8 m 3 of nitrogen and other gases. We can remove from the exhaust gases by condensation just under 2/11 of all exhaust gases in the form of water. To do this, the exhaust gas must be cooled to outside temperature. With the release of the appropriate proportion of water. The air taken from the street for combustion also contains minor moisture.

The released water is removed at the bottom of the heat exchanger. Accordingly, if the entire composition of gases (11/11 parts) passes along the path of the boiler-recuperator (e)-heat recovery unit (e), then only 9/11 parts of the exhaust gas can pass along the other side of the recuperator (e). The rest - up to 2/11 parts of the gas in the form of moisture - can fall out in the heat exchanger. And to minimize the aerodynamic resistance of the heat exchanger, the gate (b) can be opened slightly. In this case, the exhaust gases will be separated. Part will pass through the recuperator (e), and part through the gate (b). When the gate (b) is fully opened, the gases will pass through without cooling and the readings of temperature sensors 1 and 2 will coincide.

An irrigation system with a pump H and a group of nozzles is installed along the path of the gases. Gases are irrigated with water released during condensation. Injectors that spray moisture into the gas sharply increase its dew point, cool it and compress it adiabatically. In the example under consideration, the gas temperature drops sharply to 62/62, and since the water sprayed in the gas completely evaporates in the gas, the dew point and the gas temperature coincide. Having reached the heat exchanger (e), latent thermal energy is released on it. In addition, the density of the gas flow increases abruptly and its speed decreases abruptly. All these changes significantly change the heat transfer efficiency for the better. The amount of water sprayed is determined by the controller and is related to the temperature and gas flow. The gas temperature in front of the heat exchanger is monitored by temperature sensor 6.

Next, the gases enter the heat exchanger (e). In the heat exchanger, the gases cool down, for example, to a temperature of 35 0 C. Accordingly, the dew point for these gases will also be 35 0 C. The next heat exchanger on the path of the exhaust gases is the heat exchanger (g). It serves to heat combustion air. The air supply temperature to such a heat exchanger can reach -35 0 C. This temperature depends on the minimum outside temperature air in this region. Since some of the water vapor is removed from the exhaust gas, the mass flow of exhaust gases almost coincides with the mass flow of combustion air. Let the heat exchanger, for example, be filled with antifreeze. A gate (c) is installed between the heat exchangers. This gate also operates in discrete mode. When it warms up outside, there is no point in extracting heat from the heat exchanger (g). It stops its operation and the gate (c) opens completely, allowing exhaust gases to pass through, bypassing the heat exchanger (g).

The temperature of the cooled gases is determined by the temperature sensor (3). These gases are then sent to the recuperator (d). Having passed through it, they are heated to a certain temperature proportional to the cooling of the gases on the other side of the recuperator. The gate (g) is needed to regulate the heat exchange in the recuperator, and the degree of its opening depends on the outside temperature (from sensor 5). Accordingly, if it is very cold outside, then the gate (d) is completely closed and the gases are heated in the recuperator to avoid the dew point in the pipe. If it is hot outside, then gate (d) is open, as is gate (b).

CONCLUSIONS:

An increase in heat exchange in a liquid/gas heat exchanger occurs due to a sharp jump in gas enthalpy. But the proposed water spraying should occur in strictly measured doses. In addition, dosing of water into the exhaust gases takes into account the outside temperature.

The resulting calculation method allows one to avoid moisture condensation in the chimney and significantly increase the efficiency of the boiler unit. A similar technique can be applied to gas turbines and other condenser devices.

With the proposed method, the design of the boiler does not change, but is only modified. The cost of modification is about 10% of the cost of the boiler. The payback period at current gas prices is about 4 months.

This approach can significantly reduce the metal consumption of the structure and, accordingly, its cost. In addition, the aerodynamic resistance of the heat exchanger drops significantly, and the load on the smoke exhauster is reduced.

LITERATURE:

1.Aronov I.Z. Use of heat from flue gases of gasified boiler houses. – M.: “Energy”, 1967. – 192 p.

2.Thaddeus Hobler. Heat transfer and heat exchangers. – Leningrad: State scientific publication of chemical literature, 1961. – 626 p.

Proceedings of Instorf 11 (64)

UDC 622.73.002.5

Gorfin O.S. Gorfin O.S.

Gorfin Oleg Semenovich, Ph.D., prof. Department of Peat Machines and Equipment of Tver State Technical University (TvSTU). Tver, Academicheskaya, 12. [email protected] Gorfin Oleg S., PhD, Professor of the Chair of Peat Machinery and Equipment of the Tver State Technical University. Tver, Academicheskaya, 12

Zyuzin B.F. Zyuzin B.F.

Zyuzin Boris Fedorovich, Doctor of Technical Sciences, Prof., Head. Department of Peat Machines and Equipment TvSTU [email protected] Zyuzin Boris F., Dr. Sc., Professor, Head of the Chair of Peat Machinery and Equipment of the Tver State Technical University

Mikhailov A.V. Mikhailov A.V.

Mikhailov Alexander Viktorovich, Doctor of Technical Sciences, Professor of the Department of Mechanical Engineering, National Mineral Resources University "Mining", St. Petersburg, Leninsky Prospect, 55, bldg. 1, apt. 635. [email protected] Mikhailov Alexander V., Dr. Sc., Professor of the Chair of Machine Building of the National Mining University, St. Petersburg, Leninsky pr., 55, building 1, Apt. 635

THE DEVICE FOR DEEP

FOR DEEP UTILIZATION OF HEAT

HEAT RECYCLING OF COMBUSTION GASES

FLUE GASES OF SUPERFICIAL TYPE

Annotation. The article discusses the design of a heat exchanger, in which the method of transferring recovered thermal energy from the coolant to a heat-receiving environment has been changed, making it possible to utilize the heat of vaporization of fuel moisture during deep cooling of flue gases and completely use it to heat cooling water, directed without additional processing to the needs of the steam turbine cycle. The design allows, in the process of heat recovery, to purify flue gases from sulfuric and sulfurous acids, and use the purified condensate as hot water. Abstract. The article describes the design of heat exchanger, in which new method is used for transmitting of recycled heat from the heat carrier to the heat receiver. The construction allows to utilize the heat of the vaporization of fuel moisture while the deep cooling of flue gases and to fully use it for heating the cooling water allocated without further processing to the needs of steam turbine cycle. The design allows purifying of waste flue gases from sulfur and sulphurous acid and using the purified condensate as hot water.

Key words: CHP; boiler installations; surface heat exchanger; deep cooling of flue gases; recovery of the heat of vaporization of fuel moisture. Key words: Combined heat and power plant; boiler installations; heater of superficial type; deep cooling of combustion gases; utilization of warmth of steam formation of fuel moisture.

Proceedings of Instorf 11 (64)

In boiler houses of thermal power plants, the energy of vaporization of moisture and fuel along with flue gases is released into the atmosphere.

In gasified boiler houses, heat losses from exhaust flue gases can reach 25%. In boiler houses operating on solid fuel, heat loss is even higher.

For the technological needs of the TBZ, milled peat with a moisture content of up to 50% is burned in boiler rooms. This means that half the mass of the fuel is water, which during combustion turns into steam and energy losses due to vaporization of fuel moisture reach 50%.

Reducing thermal energy losses is not only a matter of saving fuel, but also reducing harmful emissions into the atmosphere.

Reducing thermal energy losses is possible by using heat exchangers of various designs.

Condensation heat exchangers, in which the flue gases are cooled below the dew point, make it possible to utilize the latent heat of condensation of water vapor and fuel moisture.

The most widespread are contact and surface heat exchangers. Contact heat exchangers are widely used in industry and energy due to their simplicity of design, low metal consumption and high heat exchange intensity (scrubbers, cooling towers). But they have a significant drawback: the cooling water becomes contaminated due to its contact with combustion products - flue gases.

In this regard, more attractive are surface heat exchangers that do not have direct contact of combustion products and coolant, the disadvantage of which is the relatively low temperature its heating, equal to the temperature of the wet thermometer (50...60 °C).

The advantages and disadvantages of existing heat exchangers are widely covered in specialized literature.

The efficiency of surface heat exchangers can be significantly increased by changing the method of heat exchange between the medium that gives off heat and receives it, as is done in the proposed heat exchanger design.

The diagram of a heat exchanger for deep utilization of heat from flue gases is shown

on the image. The body 1 of the heat exchanger rests on the base 2. In the middle part of the body there is an insulated tank 3 in the form of a prism, filled with pre-cleaned running water. Water enters from above through pipe 4 and is removed at the bottom of housing 1 by pump 5 through gate 6.

On the two end sides of the tank 3 there are jackets 7 and 8, isolated from the middle part, the cavities of which through the volume of the tank 3 are connected to each other by rows of horizontal parallel pipes forming bundles of pipes 9 in which gases move in one direction. Shirt 7 is divided into sections: lower and upper single 10 (height h) and the remaining 11 - double (height 2h); shirt 8 has only double sections 11. The lower single section 10 of shirt 7 is connected by a bundle of pipes 9 to the bottom of the double section 11 of shirt 8. Next top part this double section 11 of the jacket 8 is connected by a bundle of pipes 9 to the bottom of the next double section 11 of the jacket 7 and so on. Consistently, the upper part of the section of one jacket is connected to the lower part of the section of the second jacket, and the upper part of this section is connected by a bundle of pipes 9 to the bottom of the next section of the first jacket, thus forming a coil of variable cross-section: the bundles of pipes 9 periodically alternate with the volumes of the sections of the jackets. In the lower part of the coil there is a pipe 12 for supplying flue gases, in the upper part there is a pipe 13 for the exit of gases. Connections 12 and 13 are connected to each other by a bypass flue 4, in which a gate 15 is installed, designed to redistribute part of the hot flue gases bypassing the heat exchanger into chimney(not shown in the figure).

The flue gases enter the heat exchanger and are divided into two streams: the main part (about 80%) of the combustion products enters the lower single section 10 (height h) of the jacket 7 and is sent through the pipes of the bundle 9 to the heat exchanger coil. The rest (about 20%) enters bypass flue 14. Redistribution of gases is carried out to increase the temperature of the cooled flue gases behind the heat exchanger to 60-70 ° C in order to prevent possible condensation of residual fuel moisture vapor in the tail sections of the system.

Flue gases are supplied to the heat exchanger from below through pipe 12, and removed to

Proceedings of Instorf 11 (64)

Drawing. Diagram of the heat exchanger (type A - connection of pipes with jackets) Figure. The scheme of the heatutilizer (a look A - connection of pipes with shirts)

upper part of the installation - pipe 13. Pre-prepared cold water fills the tank from above through pipe 4, and is removed by pump 5 and gate 6, located in the lower part of housing 1. The counterflow of water and flue gases increases the efficiency of heat exchange.

The movement of flue gases through the heat exchanger is carried out by a technological smoke exhauster of the boiler room. To overcome the additional resistance created by the heat exchanger, it is possible to install a more powerful smoke exhauster. It should be borne in mind that the additional hydraulic resistance is partially overcome by reducing the volume of combustion products due to the condensation of water vapor in the flue gases.

The design of the heat exchanger ensures not only effective utilization of the heat of vaporization of fuel moisture, but also removal of the resulting condensate from the flue gas flow.

The volume of sections of jackets 7 and 8 is greater than the volume of the pipes connecting them, so the speed of gases in them is reduced.

The flue gases entering the heat exchanger have a temperature of 150-160 °C. Sulfuric and sulfurous acids condense at a temperature of 130-140 °C, so the condensation of acids occurs in the initial part of the coil. When the speed of the gas flow in the expanding parts of the coil - sections of the jacket decreases and the density of the condensate of sulfuric and sulfurous acids in the liquid state increases compared to the density in the gaseous state, and the direction of movement of the flue gas flow changes multiple times (inertial separation), the acid condensate precipitates and is washed out of gases, part of the condensate of water vapor, into the acid condensate collector 16, from where, when the shutter is activated, 17 is removed into the industrial sewer.

Most of the condensate - condensate of water vapor - is released with a further decrease in the temperature of the gases to 60-70 ° C in the upper part of the coil and enters the moisture condensate collector 18, from where it can be used as hot water without additional treatment.

Proceedings of Instorf 11 (64)

Coil pipes must be made of anti-corrosion material or with an internal anti-corrosion coating. To prevent corrosion, all surfaces of the heat exchanger and connecting pipelines should be gummed.

In this heat exchanger design, flue gases containing fuel moisture vapor move through the coil pipes. The heat transfer coefficient in this case is no more than 10,000 W/(m2 °C), due to which the efficiency of heat transfer sharply increases. The coil pipes are located directly in the coolant volume, so heat exchange occurs constantly by contact. This allows for deep cooling of flue gases to a temperature of 40-45 ° C, and all the recovered heat of vaporization of fuel moisture is transferred to cooling water. Cooling water does not come into contact with flue gases, therefore it can be used without additional treatment in the steam turbine cycle and by hot water consumers (in the hot water supply system, heating of return network water, technological needs of enterprises, in greenhouses and greenhouse farms, etc.). This is the main advantage of the proposed heat exchanger design.

The advantage of the proposed device is also that in the heat exchanger the time of heat transfer from the environment of hot flue gases to the coolant, and therefore its temperature, is regulated by changing the liquid flow rate using a gate.

To check the results of using a heat exchanger, thermal and technical calculations were carried out for a boiler installation with a boiler steam output of 30 tons of steam/h (temperature 425 °C, pressure 3.8 MPa). 17.2 t/h of milled peat with a moisture content of 50% is burned in the firebox.

Peat with a moisture content of 50% contains 8.6 t/h of moisture, which, when peat is burned, turns into flue gases.

Dry air (flue gas) consumption

Gfl. g. = a x L x G,^^ = 1.365 x 3.25 x 17,200 = 76,300 kg d.g./h,

where L = 3.25 kg dry. g/kg peat - theoretically required amount combustion air; a =1.365 - average air leakage coefficient.

1. Heat of flue gas recovery Flue gas enthalpy

J = cm x t + 2.5 d, ^zh/kgG. dry gas,

where ccm is the heat capacity of the flue gases (heat capacity of the mixture), ^l/kg °K, t is the temperature of the gases, °K, d is the moisture content of the flue gases, G. moisture/kg. d.g.

Heat capacity of the mixture

ссМ = сг + 0.001dcn,

where sg, cn are the heat capacity of dry gas (flue gases) and steam, respectively.

1.1. Flue gases at the inlet to the heat exchanger are at a temperature of 150 - 160 °C, we take C. g. = 150 °C; cn = 1.93 - heat capacity of steam; сг = 1.017 - heat capacity of dry flue gases at a temperature of 150 °C; d150, G/kg. dry d - moisture content at 150 °C.

d150 = GM./Gfl. g. = 8600 /76 300 x 103 =

112.7 G/kg. dry G,

where Gvl. = 8600 kg/h - mass of moisture in the fuel. scm = 1.017 + 0.001 x 112.7 x 1.93 = 1.2345 ^f/kg.

Flue gas enthalpy J150 = 1.2345 x 150 + 2.5 x 112.7 = 466.9 ^l/kg.

1.2. Flue gases at the outlet of the heat exchanger at a temperature of 40 °C

scm = 1.017 + 0.001 x 50 x 1.93 = 1.103 ^f/kg °C.

d40 =50 G/kg dry g.

J40 = 1.103 x 40 + 2.5 x 50 = 167.6 ^f/kg.

1.3. In the heat exchanger, 20% of the gases pass through the bypass flue, and 80% through the coil.

The mass of gases passing through the coil and participating in heat exchange

GzM = 0.8Gfl. g = 0.8 x 76,300 = 61,040 kg/h.

1.4. Heat recovery

exc = (J150 - J40) x ^m = (466.9 - 167.68) x

61,040 = 18.26 x 106, ^f/h.

This heat is spent on heating the cooling water

Qx™= W x w x (t2 - t4),

where W is water consumption, kg/h; sv = 4.19 ^l/kg °C - heat capacity of water; t 2, t4 - water temperature

Proceedings of Instorf 11 (64)

respectively at the outlet and inlet of the heat exchanger; we take tx = 8 °C.

2. Cooling water flow, kg/s

W=Qyra /(st x (t2 - 8) = (18.26 / 4.19) x 106 / (t2 - 8)/3600 = 4.36 x 106/ (t2 -8) x 3600.

Using the obtained dependence, you can determine the flow rate of cooling water at the required temperature, for example:

^, °С 25 50 75

W, kg/s 71.1 28.8 18.0

3. Condensate flow rate G^^ is:

^ond = GBM(d150 - d40) = 61.0 x (112.7 - 50) =

4. Checking the possibility of condensation of residual moisture from fuel vaporization in the tail elements of the system.

Average moisture content of flue gases at the outlet of the heat exchanger

^р = (d150 x 0.2 Gd.g. + d40 x 0.8 Gd.g.) / GA g1 =

112.7 x 0.2 + 50 x 0.8 = 62.5 G/kg dry. G.

According to the J-d diagram, this moisture content corresponds to a dew point temperature equal to tp. R. = 56 °C.

The actual temperature of the flue gases at the outlet of the heat exchanger is

tcjmKT = ti50 x 0.2 + t40 x 0.8 = 150 x 0.2 + 40 x 0.8 = 64 °C.

Since the actual temperature of the flue gases behind the heat exchanger is above the dew point, condensation of fuel moisture vapor in the tail elements of the system will not occur.

5. Efficiency

5.1. Efficiency of utilization of the heat of vaporization of fuel moisture.

The amount of heat supplied to the heat exchanger

Q^h = J150 x Gft g = 466.9 x 76 300 =

35.6 x 106, M Dj/h.

Efficiency Q = (18.26 / 35.6) x 100 = 51.3%,

where 18.26 x 106, МJ/h is the heat of utilization of vaporization of fuel moisture.

5.2. Efficiency of fuel moisture utilization

Efficiency W = ^cond / W) x 100 = (3825 / 8600) x 100 = 44.5%.

Thus, the proposed heat exchanger and its method of operation provide deep cooling of flue gases. Due to the condensation of fuel moisture vapor, the efficiency of heat exchange between flue gases and coolant increases dramatically. In this case, all the recovered latent heat of vaporization is transferred to heat the coolant, which can be used in the steam turbine cycle without additional processing.

During the operation of the heat exchanger, the flue gases are purified from sulfuric and sulfurous acids, and therefore the vapor condensate can be used for hot heat supply.

Calculations show that the efficiency is:

When utilizing the heat of vaporization

fuel moisture - 51.3%

Fuel moisture - 44.5%.

Bibliography

1. Aronov, I.Z. Contact heating of water by natural gas combustion products. - L.: Nedra, 1990. - 280 p.

2. Kudinov, A.A. Energy saving in heat power engineering and heat technologies. - M.: Mechanical Engineering, 2011. - 373 p.

3. Pat. 2555919 (RU).(51) IPC F22B 1|18 (20006.01). Heat exchanger for deep heat recovery of surface-type flue gases and its method of operation /

O.S. Gorfin, B.F. Zyuzin // Discoveries. Inventions. - 2015. - No. 19.

4. Gorfin, O.S., Mikhailov, A.V. Machines and equipment for peat processing. Part 1. Production of peat briquettes. - Tver: TvSTU 2013. - 250 p.

Evaluation of Efficiency of Deep recuperation of Power Plant Boilers’ Combustion Productions

E.G. Shadek, Candidate of Engineering, independent expert

Keywords: combustion products, heat recuperation, boiler plant equipment, energy efficiency

One of the methods to solve the problem of fuel economy and improvement of energy efficiency of boiler plants is development of technologies for deep heat recuperation of boiler exhaust gases. We offer a process scheme of a power plant with steam-turbine units (STU) that allows for deep recuperation of heat from boiler combustion products from STU condenser using cooler-condensate with minimum costs without the use of heat pump units.

Description:

One of the ways to solve the problem of saving fuel and increasing the energy efficiency of boiler plants is to develop technologies for deep utilization of the heat of exhaust gases from boilers. We propose a technological scheme of a power plant with steam turbine units (STU), allowing minimal costs, without the use of heat pump units, to carry out deep utilization of the heat of combustion products leaving the boiler due to the presence of a cooler - condensate from the PTU condenser.

E. G. Shadek, Ph.D. tech. sciences, independent expert

One of the ways to solve the problem of saving fuel and increasing the energy efficiency of boiler plants is to develop technologies for deep utilization of heat from flue gases from boilers. We offer a technological scheme of a power plant with steam turbine units (STU), which allows, at minimal cost, without the use of heat pump units, to carry out deep utilization of the heat of combustion products leaving the boiler due to the presence of a cooler - condensate from the STU condenser.

Deep utilization of heat from combustion products (CP) is ensured when they are cooled below the dew point temperature, equal to 50–55 0 C for CP of natural gas. In this case, the following phenomena occur:

• condensation of water vapor (up to 19–20% of the volume or 12–13% of the weight of combustion products),
• utilization of physical heat from PS (40–45% of total heat content),
• utilization of latent heat of vaporization (60–55%, respectively).

It was previously established that fuel savings during deep utilization in comparison with a boiler with a passport (maximum) efficiency of 92% is 10–13%. The ratio of the amount of recovered heat to the thermal power of the boiler is about 0.10–0.12, and the efficiency of the boiler in condensing mode is 105% based on the lower calorific value of the gas.

In addition, during deep recycling in the presence of water vapor in the PS, the emission of harmful emissions is reduced by 20–40% or more, which makes the process environmentally friendly.

Another effect of deep recycling is the improvement of the conditions and service life of the gas path, since condensation is localized in the chamber where the recovery heat exchanger is installed, regardless of the outside air temperature.

Deep recycling for heating systems

In advanced Western countries, deep utilization for heating systems is carried out using condensation-type hot water boilers equipped with a condensation economizer.

Low, as a rule, return water temperature (30–40 0 C) at typical temperature chart, for example 70/40 0 C, in the heating systems of these countries allows for deep heat recovery in a condensation economizer equipped with a unit for collecting, discharging and processing condensate (with its subsequent use to feed the boiler). This scheme ensures the condensation mode of operation of the boiler without artificial coolant, i.e., without the use of a heat pump unit.

The effectiveness and profitability of deep recycling for heating boilers does not need proof. Condensing boilers are widely used in the West: up to 90% of all manufactured boilers are condensing. Such boilers are also used in our country, although we do not produce them.

In Russia, unlike countries with warm climates, the temperature in the return line of heating networks is usually higher than the dew point, and deep utilization is possible only in four-pipe systems (which are extremely rare) or when using heat pumps. main reason Russia's lag in the development and implementation of deep utilization - low price of natural gas, high capital costs due to the inclusion of heat pumps in the scheme and long payback periods.

Deep recycling for power plant boilers

The efficiency of deep utilization for power plant boilers (Fig. 1) is significantly higher than for heating boilers, due to the stable load (KIM = 0.8–0.9) and large unit capacities (tens of megawatts).

Let us estimate the heat resource of combustion products of station boilers, taking into account their high efficiency (90–94%). This resource is determined by the amount of waste heat (Gcal/h or kW), which is uniquely dependent on the thermal power of the boiler Q K, and temperature behind gas boilers T 1УХ, which in Russia is accepted at no lower than 110–130 0 C for two reasons:

• to increase natural draft and reduce pressure (energy consumption) of the smoke exhauster;
• to prevent condensation of water vapor in hogs, flues and chimneys.

Extended analysis of a large array 1 of experimental data from balance and commissioning tests carried out by specialized organizations, performance maps, reporting statistics of stations, etc. and the results of calculations of heat loss values ​​​​with exhaust combustion products q 2, the amount of reclaimed heat 2 Q UT and their derivative indicators in a wide range of station boiler loads are given in Table. 13 . The goal is to determine q 2 and ratios of quantities Q K, q 2 and Q UT under typical boiler operating conditions (Table 2). In our case, it does not matter which boiler: steam or hot water, industrial or heating.

Indicators table. 1, dedicated blue, were calculated using the algorithm (see help). Calculation of the deep recycling process (definition Q UT, etc.) were carried out according to the engineering methodology given in and described in. The heat transfer coefficient “combustion products – condensate” in the condensation heat exchanger was determined according to the empirical methodology of the heat exchanger manufacturer (OJSC Heating Plant, Kostroma).

The results indicate the high economic efficiency of deep utilization technology for station boilers and the profitability of the proposed project. The payback period of the systems ranges from 2 years for a minimum power boiler (Table 2, boiler No. 1) to 3–4 months. The resulting ratios β, φ, σ, as well as savings items (Table 1, lines 8–10, 13–18) allow you to immediately assess the capabilities and specific indicators of a given process, boiler.

Heat recovery in a gas heater

The usual technological scheme of a power plant involves heating the condensate in a gas heater (part of the tail surfaces of the boiler, economizer) using the flue gases leaving the boiler.

After the condenser, the condensate is sent by pumps (sometimes through a block desalting unit - hereinafter referred to as BOU) to a gas heater, after which it enters the deaerator. When the quality of the condensate is normal, the water treatment unit is bypassed. To prevent condensation of water vapor from the flue gases on the last pipes of the gas heater, the temperature of the condensate in front of it is maintained at least 60 0 C by recirculating heated condensate to the inlet.

To further reduce the temperature of the flue gases, a water-to-water heat exchanger cooled by make-up water from the heating network is often included in the condensate recirculation line. Heating of network water is carried out by condensate from a gas heater. With additional cooling of the gases by 10 0 C, about 3.5 Gcal/h of heating load can be obtained in each boiler.

To prevent condensate from boiling in the gas heater, control feed valves are installed behind it. Their main purpose is to distribute condensate flow between boilers in accordance with the thermal load of the steam turbine unit.

Deep recovery system with condensing heat exchanger

As can be seen from the flow diagram (Fig. 1), steam condensate from the condensate collector is supplied by pump 14 to the collection tank 21, and from there to the distribution manifold 22. Here, the condensate using the system automatic regulation station (see below) is divided into two streams: one is supplied to the deep utilization unit 4, to the condensation heat exchanger 7, and the second to the low pressure heater (LPH) 18, and then to the deaerator 15. The temperature of the steam condensate from the turbine condenser (approx. 20–35 0 C) allows you to cool the combustion products in the condensation heat exchanger 7 to the required 40 0 ​​C, i.e., ensure deep utilization.

The heated steam condensate from the condensation heat exchanger 7 is fed through the HDPE 18 (or bypassing 18) into the deaerator 15. The combustion product condensate obtained in the condensation heat exchanger 7 is drained into the pan and tank 10. From there it is fed into the contaminated condensate tank 23 and pumped drain pump 24 into the condensate reserve tank 25, from which the condensate pump 26 through the flow regulator is supplied to the combustion products condensate purification section (not shown in Fig. 1), where it is processed using known technology. The purified condensate of combustion products is supplied to HDPE 18 and then to deaerator 15 (or directly to 15). From the deaerator 15, a flow of clean condensate is supplied by a feed pump 16 to the heater high pressure 17, and from it to boiler 1.

Thus, the heat of combustion products utilized in the condensation heat exchanger saves fuel consumed in the power plant process flow diagram for heating the station condensate in front of the deaerator and in the deaerator itself.

The condensation heat exchanger is installed in chamber 35 at the junction of boiler 27 with the gas duct (Fig. 2c). The thermal load of the condensation heat exchanger is regulated by bypassing, i.e., by removing part of the hot gases in addition to the condensation heat exchanger through the bypass channel 37 with a throttle valve (gate) 36.

The simplest would be the traditional scheme: a condensing economizer, more precisely the tail sections of the boiler economizer, such as a gas heater, but operating in condensation mode, i.e., cooling the combustion products below the dew point temperature. But at the same time, structural and operational difficulties arise (maintenance, etc.), requiring special solutions.

Applicable Various types heat exchangers: shell-and-tube, straight-tube, with knurled fins, plate or efficient design with a new shape of the heat exchange surface with a small bending radius (regenerator RG-10, NPC "Anod"). In this scheme, heat exchange block sections based on a bimetallic heater of the VNV123-412-50ATZ brand (OJSC Heating Plant, Kostroma) are used as a condensation heat exchanger.

The choice of section layout and water and gas connections allows you to vary and ensure the speed of water and gases within the recommended limits (1–4 m/s). The flue, chamber, gas path are made of corrosion-resistant materials, coatings, in particular stainless steels, plastics is a common practice.

* There are no heat losses due to chemical incomplete combustion.

Features of deep recycling with a condensing heat exchanger

The high efficiency of the technology makes it possible to regulate the thermal power of the system within a wide range, maintaining its profitability: the degree of bypass, the temperature of the combustion products behind the condensation heat exchanger, etc. The thermal load of the condensing heat exchanger QUT and, accordingly, the amount of condensate supplied to it from the collector 22 (Fig. 1 ), is determined as optimal (and not necessarily maximum) according to technical and economic calculations and design considerations, taking into account operating parameters, capabilities and conditions of the technological scheme of the boiler and the station as a whole.

After contact with natural gas combustion products, the condensate retains high quality and requires simple and inexpensive cleaning - decarbonization (and not always) and degassing. After treatment at the chemical water treatment site (not shown), the condensate is pumped through a flow regulator into the station’s condensate line - to the deaerator, and then into the boiler. If the condensate is not used, it is drained into the sewer.

In the condensate collection and processing unit (Fig. 1, pos. 8, 10, Fig. 2, pos. 23–26), well-known standard equipment of deep recycling systems is used (see, for example,).

The installation produces a large amount of excess water (condensate of water vapor from the combustion of hydrocarbons and blown air), so the system does not need to be recharged.

Temperature of combustion products at the outlet of the condensing heat exchanger T 2УХ is determined by the condition of condensation of water vapor in the exhaust combustion products (in the range of 40–45 0 C).

In order to prevent the formation of condensate in the gas path and especially in the chimney, bypassing is provided, i.e. bypassing part of the combustion products through a bypass channel in addition to the deep utilization unit so that the temperature of the gas mixture behind it is in the range of 70–90 0 C. Bypassing worsens all process indicators. The optimal mode is to work with bypass in the cold season, and in the summer, when there is no danger of condensation and icing, without it.

The temperature of the boiler flue gases (usually 110–130 0 C) allows the condensate to be heated in the condensation heat exchanger in front of the deaerator to the required 90–100 0 C. Thus, the temperature requirements of the technology are satisfied: both heating the condensate (about 90 0 C) and cooling the products combustion (up to 40 0 ​​C) until condensation.

Comparison of combustion product heat recovery technologies

When making a decision on the utilization of heat from boiler combustion products, one should compare the effectiveness of the proposed deep utilization system and the traditional scheme with a gas heater as the closest analogue and competitor.

For our example (see reference 1), we obtained the amount of heat recovered during deep utilization Q UT equal to 976 kW.

We assume the temperature of the condensate at the inlet to the gas condensate heater is 60 0 C (see above), while the temperature of the combustion products at the exit from it is at least 80 0 C. Then the heat of the combustion products utilized in the gas heater, i.e., heat savings, will be equal to 289 kW, which is 3.4 times less than in the deep recycling system. Thus, the “issue price” in our example is 687 kW, or, on an annual basis, 594,490 m 3 of gas (with KIM = 0.85) costing about 3 million rubles. The gain will increase with the boiler power.

Advantages of deep recycling technology

In conclusion, we can conclude that, in addition to energy saving, with deep utilization of combustion products from a power plant boiler, the following results are achieved:

• reducing the emission of toxic oxides CO and NOx, ensuring the environmental cleanliness of the process;
• obtaining additional, excess water and thereby eliminating the need for boiler make-up water;
• condensation of water vapor from combustion products is localized in one place - in the condensation heat exchanger. Apart from the slight splash carryover after the droplet eliminator, condensation in the subsequent gas path and the associated destruction of gas ducts from the corrosive effects of moisture, the formation of ice in the path and especially in the chimney are eliminated;
• in some cases, the use of a water-to-water heat exchanger becomes optional; there is no need for recirculation: mixing part of the hot gases with cooled ones (or heated condensate with cold ones) in order to increase the temperature of the exhaust combustion products to prevent condensation in the gas path and chimney (saving energy and money).

Literature

1. Shadek E., Marshak B., Anokhin A., Gorshkov V. Deep recovery of heat from waste gases of heat generators // Industrial and heating boilers and mini-CHPs. 2014. No. 2 (23).
2. Shadek E. Trigeneration as a technology for saving energy resources // Energy saving. 2015. No. 2.
3. Shadek E., Marshak B., Krykin I., Gorshkov V. Condensation heat exchanger-recovery – modernization of boiler plants // Industrial and heating boilers and mini-CHP. 2014. No. 3 (24).
4. Kudinov A. Energy saving in heat generating installations. M.: Mechanical Engineering, 2012.
5. Ravich M. Simplified method of thermotechnical calculations. M.: Publishing House of the USSR Academy of Sciences, 1958.
6. Berezinets P., Olkhovsky G. Advanced technologies and power plants for the production of thermal and electrical energy. Section six. 6.2 gas turbine and combined cycle gas plants. 6.2.2. Combined-cycle plants. JSC "VTI". “Modern environmental technologies in the energy sector.” Information collection ed. V. Ya. Putilova. M.: MPEI Publishing House, 2007.

1 Primary source of data: inspection of hot water boilers (11 units in three boiler houses of heating networks), collection and processing of materials.

2 Calculation methodology, in particular Q UT, given in.

Flue gas condensation system for the company's boilers “ AprotechEngineeringAB” (Sweden)

The flue gas condensation system allows the capture and recovery of large amounts of thermal energy contained in the wet boiler flue gas, which is usually discharged through the chimney into the atmosphere.

The heat recovery/flue gas condensation system makes it possible to increase heat supply to consumers by 6–35% (depending on the type of fuel burned and installation parameters) or reduce natural gas consumption by 6–35%

Main advantages:

• Fuel economy (natural gas) - the same or increased heat load of the boiler with less fuel combustion
• Reduction of emissions - CO2, NOx and SOx (when burning coal or liquid fuels)
• Obtaining condensate for the boiler make-up system

Principle of operation:

The heat recovery/flue gas condensation system can operate in two stages: with or without the use of an air humidification system supplied to the boiler burners. If necessary, a scrubber is installed before the condensation system.

In the condenser, the exhaust flue gases are cooled using return water from the heating network. When the temperature of the flue gases decreases, a large amount of water vapor contained in the flue gas condenses. Thermal energy Vapor condensation is used to heat the return heating network.

Further cooling of the gas and condensation of water vapor occurs in the humidifier. The cooling medium in the humidifier is the blast air supplied to the boiler burners. Since the blast air is heated in the humidifier, and warm condensate is injected into the air flow in front of the burners, an additional evaporation process occurs in the exhaust flue gas of the boiler.

The blown air supplied to the boiler burners contains an increased amount of thermal energy due to increased temperature and humidity.

This leads to an increase in the amount of energy in the exhaust flue gas entering the condenser, which in turn leads to more efficient use of heat by the district heating system.

The flue gas condensation unit also produces condensate, which, depending on the composition of the flue gases, will be further purified before being fed into the boiler system.

Economic effect.

Comparison of thermal power under the following conditions:

1. No condensation
2. Flue gas condensation
3. Condensation together with humidification of the air supplied for combustion

The flue gas condensation system allows the existing boiler house to:

• Increase heat production by 6.8% or
• Reduce gas consumption by 6.8%, as well as increase revenue from the sale of CO,NO quotas
• Investment size is about 1 million euros (for a boiler house with a capacity of 20 MW)
• Payback period is 1-2 years.

Savings depending on the coolant temperature in the return pipe:

The heat of flue gases leaving furnaces, in addition to heating air and gaseous fuel, can be used in waste heat boilers to generate water steam. While the heated gas and air are used in the furnace unit itself, the steam is sent to external consumers (for production and energy needs).

In all cases, one should strive for the greatest heat recovery, i.e., to return it to working space furnaces in the form of heat from heated combustion components (gas fuel and air). In fact, increased heat recovery leads to a reduction in fuel consumption and to intensification and improvement technological process. However, the presence of recuperators or regenerators does not always exclude the possibility of installing waste heat boilers. First of all, waste heat boilers have found application in large furnaces with a relatively high temperature of exhaust flue gases: in open-hearth steel furnaces, in copper smelting reverberatory furnaces, in rotary kilns for burning cement clinker, in dry cement production, etc.

Rice. 5.

1 - steam superheater; 2 - pipe surface; 3 - smoke exhauster.

The heat of flue gases leaving the regenerators of open-hearth furnaces with a temperature of 500 - 650 ° C is used in gas-tube waste heat boilers with natural circulation working fluid. The heating surface of gas-tube boilers consists of smoke tubes, inside which flue gases pass at a speed of approximately 20 m/sec. Heat from gases to the heating surface is transferred by convection, and therefore increasing the speed increases heat transfer. Gas-tube boilers are easy to operate, do not require lining or frames during installation, and have high gas density.

In Fig. Figure 5 shows a gas-tube boiler of the Taganrog plant with an average productivity D av = 5.2 t/h with the expectation of passing flue gases up to 40,000 m 3 / h. The steam pressure produced by the boiler is 0.8 Mn/m2; temperature 250 °C. The gas temperature before the boiler is 600 °C, behind the boiler 200 - 250 °C.

In boilers with forced circulation, the heating surface is made up of coils, the location of which is not limited by the conditions of natural circulation, and therefore such boilers are compact. The coil surfaces are made from small diameter pipes, for example d = 32×3 mm, which lightens the weight of the boiler. With multiple circulation, when the circulation ratio is 5 - 18, the water speed in the tubes is significant, at least 1 m/sec, as a result of which the precipitation of dissolved salts from the water in the coils is reduced, and crystalline scale is washed off. Nevertheless, boilers must be fed with water that is chemically purified using cation exchange filters and other water treatment methods that meet the feed water standards for conventional steam boilers.

Rice. 6.

1 - economizer surface; 2 - evaporation surface; 3 - steam superheater; 4 - drum-collector; 5 - circulation pump; 6 - sludge trap; 7 - smoke exhauster.

In Fig. Figure 6 shows a diagram of the placement of coil heating surfaces in vertical chimneys. The movement of the steam-water mixture is carried out circulation pump. Boiler designs of this type were developed by Tsentroenergochermet and Gipromez and are manufactured for flue gas flow rates of up to 50 - 125 thousand m 3 / h with an average steam output of 5 to 18 t / h.

The cost of steam is 0.4 - 0.5 rub/t instead of 1.2 - 2 rub/t for steam selected from steam turbines CHP and 2 - 3 rubles/t for steam from industrial boiler houses. The cost of steam is made up of energy costs for driving smoke exhausters, costs for preparing water, depreciation, repairs and maintenance. The gas speed in the boiler ranges from 5 to 10 m/sec, which ensures good heat transfer. Aerodynamic drag gas path is 0.5 - 1.5 kN/m2, so the unit must have artificial draft from a smoke exhauster. The increased draft that accompanies the installation of waste heat boilers, as a rule, improves the operation of open-hearth furnaces. Such boilers are widespread in factories, but for their good operation it is necessary to protect the heating surfaces from being carried over by dust and slag particles and to systematically clean the heating surfaces from entrainment by blowing with superheated steam, washing with water (when the boiler is stopped), by vibration, etc.

Rice. 7.

To use the heat of flue gases coming from copper smelting reverberatory furnaces, water-tube boilers with natural circulation are installed (Fig. 7). The flue gases in this case have a very high temperature (1100 - 1250 °C) and are contaminated with dust in amounts up to 100 - 200 g/m3, some of the dust has high abrasive (abrasion) properties, the other part is in a softened state and can slag boiler heating surface. It is the high dust content of the gases that is forcing us to abandon heat recovery in these furnaces for the time being and limit ourselves to the use of flue gases in waste heat boilers.

The transfer of heat from gases to the screen evaporation surfaces proceeds very intensively, due to which intensive vaporization of slag particles is ensured, when cooled, they granulate and fall into the slag funnel, which prevents slagging of the convective heating surface of the boiler. Installation of such boilers for the use of gases with a relatively low temperature (500 - 700 ° C) is impractical due to weak heat transfer by radiation.

In the case of equipping high-temperature furnaces with metal recuperators, it is advisable to install waste heat boilers directly behind the working chambers of the furnaces. In this case, the temperature of the flue gases in the boiler drops to 1000 - 1100 °C. At this temperature, they can already be sent to the heat-resistant section of the recuperator. If the gases carry a lot of dust, then the recovery boiler is arranged in the form of a screen boiler-slag granulator, which ensures separation of entrainment from gases and facilitates the operation of the recuperator.