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 heat recovery of combustion products (CP) is ensured when they are cooled below the dew point temperature equal to the CP natural gas 50–55 0 C. 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 recycling for heating systems is carried out using condensation-type hot water boilers equipped with a condensation economizer.

Typically low temperature return water(30–40 0 C) with a typical temperature schedule, 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 condensate collection, removal and treatment unit (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 were received in the West wide application: 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. The main reason for Russia's lag in the development and implementation of deep recycling is low price 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 beyond 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, highlighted in 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 station’s automatic control system (see below), is divided into two streams: one supplied to the deep utilization unit 4, to the condensation heat exchanger 7, and the second to the heater low pressure(HDPE) 18, and then into the deaerator 15. The temperature of the steam condensate from the turbine condenser (about 20–35 0 C) makes it possible 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 by the drain pump 24 into the tank condensate reserve 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 - this is a generally accepted 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 this is not always the case) 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 technique 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.

Owners of patent RU 2606296:

The invention relates to thermal power engineering and can be used at any enterprise that operates boilers using hydrocarbon fuels.

Known commercially produced by the Kostroma Heating Plant are heaters of the KSk type (Kudinov A.A. Energy saving in heat-generating installations. - Ulyanovsk: UlSTU, 2000. - 139, p. 33), consisting of a gas-water surface heat exchanger, the heat exchange surface of which is made of finned bimetallic tubes, strainer, distribution valve, drip eliminator and hydropneumatic blower.

Heaters of the KSk type work as follows. Flue gases enter the distribution valve, which divides them into two streams, the main gas flow is directed through a mesh filter into the heat exchanger, the second through the bypass line of the flue. In the heat exchanger, water vapor contained in the flue gases condenses on finned tubes, heating the water flowing in them. The resulting condensate is collected in a pan and pumped into the heating network feed circuit. The water heated in the heat exchanger is supplied to the consumer. At the outlet of the heat recovery unit, the dried flue gases are mixed with the original flue gases from the flue bypass line and sent through a smoke exhauster into the chimney.

For the heat exchanger to operate in the condensation mode of its entire convective part, it is required that the heating temperature of the water in the convective package does not exceed 50°C. To use such water in heating systems, it must be additionally heated.

To prevent condensation of residual water vapor flue gases in the flues and chimney, part of the source gases through the bypass channel is mixed with the dried flue gases, increasing their temperature. With such an admixture, the content of water vapor in the exhaust flue gases also increases, reducing the efficiency of heat recovery.

A heat exchanger is known (RU 2323384 C1, IPC F22B 1/18 (2006.01), published on April 27, 2008), containing a contact heat exchanger, a droplet eliminator, a gas-gas heat exchanger connected according to a direct flow circuit, gas ducts, pipelines, a pump, temperature sensors, valves - regulators. Along the flow of circulating water of the contact heat exchanger, a water-to-water heat exchanger and a water-to-air heat exchanger with a bypass channel along the air flow are located in series.

A known method of operation of this heat exchanger. The exhaust gases through the gas duct enter the inlet of the gas-gas heat exchanger, sequentially passing through its three sections, then to the inlet of the contact heat exchanger, where, passing through a nozzle washed by circulating water, they are cooled below the dew point, giving off sensible and latent heat to the circulating water. Next, the cooled and moist gases are freed from most of the liquid water carried away in a droplet eliminator, heated and dried in at least one section of the gas-gas heat exchanger, sent into a chimney by a smoke exhauster and released into the atmosphere. At the same time, heated circulating water from the pan of the contact heat exchanger is pumped into the water-to-water heat exchanger, where it heats cold water from the pipeline. The water heated in the heat exchanger is supplied to the needs of process and domestic hot water supply or to a low-temperature heating circuit.

Next, the recycled water enters the water-air heat exchanger, heats at least part of the blown air coming from outside the room through the air duct, cooling to the minimum possible temperature, and enters the contact heat exchanger through the water distributor, where it takes heat from the gases, simultaneously washing them from suspended particles, and absorbs some of the oxides of nitrogen and sulfur. The heated air from the heat exchanger is supplied by a blower fan to a standard air heater or directly to the firebox. Recycled water is filtered if necessary and processed by known methods.

To implement this method, a control system is required due to the use of recovered heat for hot water supply purposes due to the variability of the daily schedule of hot water consumption.

Water heated in the heat exchanger, supplied for the needs of hot water supply or in a low-temperature heating circuit, requires it to be brought to the required temperature, since it cannot be heated in the heat exchanger above the temperature of the return circuit water, which is determined by the saturation temperature of water vapor in the flue gases. The low heating of the air in the water-to-air heat exchanger does not allow this air to be used for space heating.

The closest to the claimed invention are a device and method for utilizing heat from flue gases (RU 2436011 C1, IPC F22B 1/18 (2006.01), published 12/10/2011).

The flue gas heat recovery device contains a gas-gas surface plate heat exchanger made according to a counterflow circuit, a surface gas-air plate condenser, an inertial drop catcher, gas ducts, a smoke exhauster, air ducts, fans and a pipeline.

The feed flue gases are cooled in a gas-to-gas surface plate heat exchanger, heating the dried flue gases. The heating and heated medium move in countercurrent. In this case, the wet flue gases are deeply cooled to a temperature close to the dew point of water vapor. Next, the water vapor contained in the flue gases is condensed in a gas-air surface plate heat exchanger - a condenser, heating the air. The heated air is used to heat the premises and cover the needs of the combustion process. The condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle. To prevent condensation of residual water vapor carried away by the flow from the condenser, a portion of the heated, dried flue gases is mixed in front of the additional smoke exhauster. The dried flue gases are supplied by a smoke exhauster to the heater described above, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney.

The disadvantages of this method are that predominantly the latent heat of condensation of water vapor contained in the flue gases is utilized. If the recuperative heat exchanger cools the source flue gases to a temperature close to the dew point of water vapor, then the heating of the exhaust dried flue gases will be excessive, which reduces the efficiency of recycling. The disadvantage is the use of only one medium for heating - air.

The objective of the invention is to increase the efficiency of heat recovery from flue gases by using the latent heat of condensation of water vapor and the increased temperature of the flue gases themselves.

In the proposed method of deep heat recovery from flue gases, as well as in the prototype, the flue gases are pre-cooled in a gas-gas surface plate heat exchanger, heating the dried flue gases, and the water vapor contained in the flue gases is condensed in the condenser, heating the air.

According to the invention, between the heat exchanger and the condenser, the flue gases are cooled to a temperature close to the dew point of water vapor, heating the water.

Gas boilers have a high temperature of flue gases (130°C for large energy boilers, 150°C-170°C for small boilers). To cool flue gases before condensation, two devices are used: a recuperative gas-to-gas heat exchanger and a recovery water heater.

The source flue gases are pre-cooled in a gas-gas surface plate heat exchanger, heating the dried flue gases 30-40°C higher than the saturation temperature of the water vapor contained in them, to create a temperature reserve for possible cooling of the flue gases in the pipe. This makes it possible to reduce the heat exchange area of ​​the recuperative heat exchanger compared to the prototype and usefully use the remaining heat of the flue gases.

A significant difference is the use of a contact gas-water water heater for the final cooling of wet flue gases to a temperature close to the dew point of water vapor. At the entrance to the water heater, the flue gases have a fairly high temperature (130°C-90°C), which allows water to be heated to 50°C-65°C with partial evaporation. At the exit from a contact gas-water water heater, the flue gases have a temperature close to the dew point of the water vapor they contain, which increases the efficiency of using the heat exchange surface in the condenser, eliminates the formation of dry zones of the condenser and increases the heat transfer coefficient.

The method of utilizing heat from flue gases is shown in Fig.1.

Table 1 shows the results of the verification calculation of the installation option for a natural gas boiler with a capacity of 11 MW.

The method of deep utilization of heat from flue gases is carried out as follows. The source flue gases 1 are pre-cooled in a gas-gas surface plate heat exchanger 2, heating the dried flue gases. Next, the flue gases 3 are finally cooled in a contact gas-water water heater 4 to a temperature close to the dew point of water vapor, spraying water, for which it is advisable to use the condensate obtained in the condenser. In this case, part of the water evaporates, increasing the moisture content of the flue gases, and the rest is heated to the same temperature. The water vapor contained in the flue gases 5 is condensed in a gas-air surface plate heat exchanger - a condenser 6 with a droplet eliminator 7, heating the air. Condensate 8 is supplied for heating to a contact gas-water water heater 4. The heat of condensation is used to heat cold air, which is supplied by fans 9 from environment through air duct 10. Heated air 11 is directed to the production room of the boiler shop for ventilation and heating. From this room, air is supplied to the boiler to ensure the combustion process. The dried flue gases 12 are supplied by a smoke exhauster 13 to a gas-gas surface plate heat exchanger 2 for heating and sent to the chimney 14.

To avoid condensation of residual water vapor carried away by the flow from the condenser, a portion of the heated, dried flue gases 15 (up to 10%) is mixed in front of the smoke exhauster 13 (up to 10%), the value of which is initially adjusted by the damper 16.

The temperature of the heated air 11 is regulated by changing the flow rate of the dried flue gases 1 or by changing the air flow rate by adjusting the speed of the smoke exhauster 13 or fans 9 depending on the outside air temperature.

Heat exchanger 2 and condenser 6 are surface plate heat exchangers made of unified modular packages, which are arranged in such a way that the coolant flows countercurrently. Depending on the volume of flue gases to be dried, the heater and condenser are formed from a calculated number of packages. Water heater 4 is a contact gas-water heat exchanger that provides additional cooling of flue gases and heating of water. Heated water 17 after additional processing is used to replenish losses in the heating network or steam turbine cycle. Block 9 is formed from several fans to change the flow of heated air.

Table 1 shows the results of the verification calculation of the installation option for a natural gas boiler with a capacity of 11 MW. Calculations were carried out for an outside air temperature of -20°C. The calculation shows that the use of a contact gas-water water heater 4 leads to the disappearance of the dry zone in the condenser 6, intensifies heat exchange and increases the power of the installation. The percentage of recovered heat increases from 14.52 to 15.4%, while the dew point temperature of water vapor in the dried flue gases decreases to 17°C. Approximately 2% of the thermal power is not utilized, but is used for recovery - heating the dried flue gases to a temperature of 70°C.

A method of deep utilization of heat from flue gases, according to which the flue gases are pre-cooled in a gas-gas surface plate heat exchanger, heating the dried flue gases, cooled in a water heater to a temperature close to the dew point of water vapor, heating the water, condensing the water vapor contained in the flue gases in the condenser, heating the air, characterized in that a surface tubular gas-water heater is installed between the heat exchanger and the condenser to cool wet flue gases and heat water, while the main heat recovery occurs in the condenser when heating the air, and additional heat recovery occurs in the water heater.

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The invention relates to thermal power engineering. The method of deep heat recovery from flue gases includes pre-cooling of flue gases in a gas-gas surface plate heat exchanger, heating dried flue gases with countercurrent to create a temperature reserve that prevents condensation of residual water vapor in the chimney. Further cooling of the flue gases to a temperature close to the dew point of water vapor is carried out in a contact gas-water water heater, which heats the water. Cooled wet flue gases are fed into a gas-air surface plate heat exchanger - a condenser, where the water vapor contained in the flue gases is condensed, heating the air. The dried flue gases are supplied by an additional smoke exhauster to a gas-gas surface plate heat exchanger, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney. Technical result: increasing the efficiency of flue gas heat recovery through the use of latent heat of condensation of water vapor and the increased temperature of the flue gases themselves. 1 ill., 1 tab.

Currently, the temperature of the exhaust flue gases behind the boiler is taken to be no lower than 120-130 ° C for two reasons: to prevent condensation of water vapor on hogs, flues and chimneys and to increase natural draft, which reduces the pressure of the smoke exhauster. In this case, the heat of exhaust gases and the latent heat of vaporization of water vapor can be usefully used. The use of the heat of exhaust flue gases and the latent heat of vaporization of water vapor is called the method of deep utilization of the heat of flue gases. Currently there are various technologies implementations of this method, tested in Russian Federation and have found widespread use abroad. The method of deep utilization of heat from flue gases makes it possible to increase the efficiency of a fuel-consuming installation by 2-3%, which corresponds to a reduction in fuel consumption by 4-5 kg ​​of fuel equivalent. per 1 Gcal of generated heat. When implementing this method, there are technical difficulties and limitations associated mainly with the complexity of calculating the heat and mass transfer process during deep heat recovery of exhaust flue gases and the need to automate the process, however, these difficulties can be solved with the current level of technology.

For the widespread implementation of this method, it is necessary to develop guidelines for the calculation and installation of systems for deep heat recovery of flue gases and the adoption of legal acts prohibiting the commissioning of fuel-using installations on natural gas without the use of deep recovery of flue gas heat.

1. Formulation of the problem regarding the method (technology) under consideration for increasing energy efficiency; forecast of excessive consumption of energy resources, or description of other possible consequences on a national scale if the current situation is maintained

Currently, the temperature of the exhaust flue gases behind the boiler is taken to be no lower than 120-130 ° C for two reasons: to prevent condensation of water vapor on hogs, flues and chimneys and to increase natural draft, which reduces the pressure of the smoke exhauster. In this case, the temperature of the flue gases directly affects the value of q2 - heat loss with the flue gases, one of the main components of the boiler’s heat balance. For example, reducing the temperature of flue gases by 40°C when the boiler is operating on natural gas and an excess air ratio of 1.2 increases the gross efficiency of the boiler by 1.9%. This does not take into account the latent heat of vaporization of combustion products. Today, the vast majority of water heating and steam boiler units in our country that burn natural gas are not equipped with installations that use the latent heat of steam formation of water vapor. This heat is lost along with the exhaust gases.

2. Availability of methods, methods, technologies, etc. to solve the identified problem

Currently, methods of deep heat recovery from flue gases (WER) are used through the use of recuperative, mixing, and combined devices that operate using various methods of using the heat contained in the flue gases. At the same time, these technologies are used in the majority of boilers commissioned abroad that burn natural gas and biomass.

3. Short description the proposed method, its novelty and awareness of it, the availability of development programs; result with mass implementation nationwide

The most commonly used method of deep heat recovery from flue gases is that the combustion products of natural gas after a boiler (or after a water economizer) with a temperature of 130-150°C are divided into two streams. Approximately 70-80% of the gases are directed through the main gas duct and enter the surface-type condensing heat exchanger, the rest of the gases are sent to the bypass gas duct. In the heat exchanger, the combustion products are cooled to 40-50°C, and some of the water vapor condenses, which makes it possible to usefully use both the physical heat of the flue gases and the latent heat of condensation of some of the water vapor contained in them. The cooled combustion products after the droplet separator are mixed with the uncooled combustion products passing through the bypass flue and, at a temperature of 65-70°C, are discharged through the chimney into the atmosphere by a smoke exhauster. The heated medium in the heat exchanger can be source water for the needs of chemical water treatment or air, which is then supplied for combustion. To intensify heat exchange in the heat exchanger, it is possible to supply vapor from the atmospheric deaerator into the main gas duct. It is also necessary to note the possibility of using condensed desalted water vapor as source water. The result of the implementation of this method is an increase in the gross efficiency of the boiler by 2-3%, taking into account the use of the latent heat of steam formation of water vapor.

4. Forecast of the effectiveness of the method in the future, taking into account:
- rising energy prices;
- growth in the well-being of the population;
- introduction of new environmental requirements;
- other factors.

This method increases the efficiency of natural gas combustion and reduces emissions of nitrogen oxides into the atmosphere due to their dissolution in condensing water vapor.

5. List of groups of subscribers and objects where this technology can be used with maximum efficiency; the need for additional research to expand the list

This method can be used in steam and hot water boiler houses using natural and liquefied gas and biofuel as fuel. To expand the list of objects where this method can be used, it is necessary to conduct research into the processes of heat and mass transfer of combustion products of fuel oil, light diesel fuel and various grades of coal.

6. Identify the reasons why the proposed energy-efficient technologies are not applied on a mass scale; outline an action plan to remove existing barriers

Mass application of this method in the Russian Federation is not carried out, as a rule, for three reasons:

• Lack of awareness about the method;
• The presence of technical limitations and difficulties in implementing the method;
• Lack of funding.

7. The presence of technical and other restrictions on the use of the method at various sites; in the absence of information on possible limitations, they must be determined by testing

Technical limitations and difficulties in implementing the method include:

• The complexity of calculating the process of recycling wet gases, since the heat exchange process is accompanied by mass transfer processes;
• The need to maintain specified values ​​of temperature and humidity of exhaust flue gases, in order to avoid condensation of vapors in the flues and chimney;
• The need to avoid freezing of heat exchange surfaces when heating cold gases;
• In this case, it is necessary to test flues and chimneys treated with modern anti-corrosion coatings to determine the possibility of reducing restrictions on the temperature and humidity of the flue gases leaving the heat recovery unit.

8. The need for R&D and additional testing; topics and goals of work

The need for R&D and additional testing is given in paragraphs 5 and 7.

9. Existing measures of encouragement, coercion, incentives for the implementation of the proposed method and the need for their improvement

There are no existing measures to encourage and enforce the implementation of this method. Interest in reducing fuel consumption and emissions of nitrogen oxides into the atmosphere may stimulate the implementation of this method.

10. The need to develop new or amend existing laws and regulations

It is necessary to develop guidelines for the calculation and installation of systems for deep heat recovery of flue gases. It may be necessary to adopt legal acts prohibiting the commissioning of natural gas fuel-using installations without the use of deep recovery of flue gas heat.

11. Availability of regulations, rules, instructions, standards, requirements, prohibitive measures and other documents regulating the use of this 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

Questions regarding the application of this method in the existing regulatory framework are missing.

12. Availability of implemented pilot projects, analysis of their actual effectiveness, identified shortcomings and proposals for improving the technology, taking into account accumulated experience

There is no data on the large-scale implementation of this method in the Russian Federation; there is experience of implementation at the thermal power plants of RAO UES and, as mentioned above, accumulated great experience on deep utilization of flue gases abroad. The All-Russian Thermal Engineering Institute has completed design studies of installations for deep heat recovery of combustion products for PTVM (KVGM) hot water boilers. The disadvantages of this method and suggestions for improvement are given in paragraph 7.

13. 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 in economic indicators of energy production and transmission, etc.)

Mass implementation of this method will reduce fuel consumption by 4-5 kg ​​of fuel equivalent. per Gcal of generated heat and will affect the environmental situation by reducing emissions of nitrogen oxides.

14. Availability and sufficiency of production capacity in Russia and other countries for the mass introduction of the method

Profile production facilities in the Russian Federation are able to ensure the implementation of this method, but not in a monoblock design; when using foreign technologies, a monoblock design is possible.

15. The need for special training of qualified personnel to operate the technology being introduced and develop production

To implement this method, existing specialized training of specialists is required. It is possible to organize specialized seminars on the implementation of this method.

16. Proposed methods of implementation:
1) commercial financing (with cost recovery);
2) competition for the implementation of investment projects developed as a result of work on energy planning for the development of a region, city, settlement;
3) budget financing for effective energy-saving projects with long payback periods;
4) introduction of prohibitions and mandatory requirements for use, supervision of their compliance;
5) other offers.

Suggested implementation methods are:

• budget financing;
• attracting investments (payback period 5-7 years);
• introduction of requirements for the commissioning of new fuel-consuming installations.

In order to add a description of energy-saving technology to the Catalog, fill out the questionnaire and send it to marked “to Catalog”.

Heat recovery methods. Flue gases leaving working space ovens have a very high temperature and therefore carry away a significant amount of heat. In open-hearth furnaces, for example, about 80% of the total heat supplied to the working space is carried away from the working space with flue gases, in heating furnaces about 60%. From the working space of the furnaces, the flue gases carry away more heat with them, the higher their temperature and the lower the heat utilization coefficient in the furnace. In this regard, it is advisable to ensure the recovery of heat from exhaust flue gases, which can be done in principle by two methods: with the return of part of the heat taken from the flue gases back to the furnace and without returning this heat to the furnace. To implement the first method, it is necessary to transfer the heat taken from the smoke to gas and air (or only air) going into the furnace. To achieve this goal, heat exchangers of recuperative and regenerative types are widely used, the use of which makes it possible to increase the efficiency of the furnace unit, increase combustion temperature and save fuel. With the second method of utilization, the heat of exhaust flue gases is used in thermal power boiler houses and turbine plants, which achieves significant fuel savings.

In some cases, both described methods of heat recovery from flue gases are used simultaneously. This is done when the temperature of the flue gases after regenerative or recuperative heat exchangers remains sufficiently high and further heat recovery in thermal power plants is advisable. For example, in open-hearth furnaces, the temperature of the flue gases after the regenerators is 750-800 °C, so they are reused in waste heat boilers.

Let us consider in more detail the issue of recycling the heat of exhaust flue gases with the return of part of their heat to the furnace.

It should first of all be noted that a unit of heat taken from the smoke and introduced into the furnace by air or gas (a unit of physical heat) turns out to be much more valuable than units of heat obtained in the furnace as a result of combustion of fuel (a unit of chemical heat), since the heat of heated air ( gas) does not entail heat loss with flue gases. The value of a unit of sensible heat is greater, the lower the fuel utilization factor and the higher the temperature of the exhaust flue gases.

For normal operation of the furnace, it is necessary to supply the working space every hour. required amount heat. This amount of heat includes not only the heat of the fuel Q x, but also the heat of heated air or gas Q F, i.e. Q Σ = Q x + Q f

It is clear that for Q Σ = const an increase in Q f will allow you to decrease Q x. In other words, utilization of heat from flue gases makes it possible to achieve fuel savings, which depends on the degree of heat recovery from flue gases

R = N in / N d

where N in and N d are, respectively, the enthalpy of heated air and flue gases escaping from the working space, kW or

kJ/period.

The degree of heat recovery can also be called the heat recovery coefficient of the recuperator (regenerator), %

efficiency p = (N in / N d) 100%.

Knowing the degree of heat recovery, you can determine fuel economy using the following expression:

where N " d and N d are, respectively, the enthalpy of the flue gases at the combustion temperature and those leaving the furnace.

Reducing fuel consumption as a result of using the heat of exhaust flue gases usually provides a significant economic effect and is one of the ways to reduce the cost of heating metal in industrial furnaces.

In addition to saving fuel, the use of air (gas) heating is accompanied by an increase in the calorimetric combustion temperature T k, which may be the main purpose of recovery when heating furnaces with fuel with a low calorific value.

Increase in Q F at leads to an increase in combustion temperature. If it is necessary to provide a certain amount T k, then an increase in the temperature of heating the air (gas) leads to a decrease in the value , i.e., to reduce the share of gas with a high calorific value in the fuel mixture.

Since heat recovery allows for significant fuel savings, it is advisable to strive for the highest possible, economically justified degree of recovery. However, it must immediately be noted that recycling cannot be complete, i.e. always R< 1. Это объясняется тем, что увеличение поверхности нагрева рационально только до определенных пределов, после которых оно уже приводит кочень незначительному выигрышу в экономии тепла.

Characteristics of heat exchange devices. As already indicated, the recovery of heat from exhaust flue gases and their return to the furnace can be carried out in heat exchange devices of regenerative and recuperative types. Regenerative heat exchangers operate in a non-stationary thermal state, while recuperative heat exchangers operate in a stationary thermal state.

Regenerative type heat exchangers have the following main disadvantages:

1) cannot provide a constant temperature for heating air or gas, which drops as the bricks of the nozzle cool, which limits the possibility of using automatic control of the furnace;

2) cessation of heat supply to the furnace when the valves are switched;

3) when heating the fuel, gas is carried out through the chimney, the value of which reaches 5-6 % full flow rate;

4) very large volume and mass of regenerators;

5) inconveniently located - ceramic regenerators are always located under the furnaces. The only exceptions are cowpers placed near blast furnaces.

However, despite very serious disadvantages, regenerative heat exchangers are sometimes still used in high-temperature furnaces (open hearth and blast furnaces, in heating wells). This is explained by the fact that regenerators can operate at very high temperature flue gases (1500-1600 °C). At this temperature, recuperators cannot yet operate stably.

The recuperative principle of heat recovery from exhaust flue gases is more progressive and perfect. Recuperators provide a constant temperature for heating air or gas and do not require any changeover devices - this ensures smoother operation of the furnace and greater opportunity for automation and control of its thermal operation. Recuperators do not carry gas into the chimney; they are smaller in volume and weight. However, recuperators also have some disadvantages, the main ones being low fire resistance (metal recuperators) and low gas density (ceramic recuperators).

General characteristics of heat exchange in recuperators. Let's consider general characteristics heat exchange in the recuperator. The recuperator is a heat exchanger operating under stationary conditions thermal state, when heat is constantly transferred from cooling flue gases to heating air (gas) through the dividing wall.

The total amount of heat transferred in the recuperator is determined by the equation

Q = KΔ t av F ,

Where TO- total heat transfer coefficient from smoke to air (gas), characterizing the overall level of heat transfer in the recuperator, W/(m 2 -K);

Δ t avg- average (over the entire heating surface) temperature difference between flue gases and air (gas), K;

F- heating surface through which heat is transferred from flue gases to air (gas), m2.

Heat transfer in recuperators includes three main stages of heat transfer: a) from flue gases to the walls of recuperative elements; b) through the dividing wall; c) from the wall to the heated air or gas.

On the smoke side of the recuperator, heat from the flue gases to the wall is transferred not only by convection, but also by radiation. Therefore, the local heat transfer coefficient on the smoke side is equal to

where is the heat transfer coefficient from the flue gases to the wall

convection, W/(m 2 °C);

Heat transfer coefficient from flue gases to the wall

by radiation, W/(m 2 °C).

Heat transfer through the dividing wall depends on the thermal resistance of the wall and the condition of its surface.

On the air side of the recuperator, when heating the air, heat is transferred from the wall to the air only by convection, and when heating the gas - by convection and radiation. Thus, when air is heated, heat transfer is determined by the local convection heat transfer coefficient; if the gas is heated, then the heat transfer coefficient

All noted local heat transfer coefficients are combined into the total heat transfer coefficient

, W/(m 2 °C).

In tubular recuperators, the total heat transfer coefficient should be determined for a cylindrical wall (linear heat transfer coefficient)

, W/(m °C)

Coefficient TO called the heat transfer coefficient of the pipe. If it is necessary to attribute the amount of heat to the area of ​​the internal or external surface of the pipe, then the total heat transfer coefficients can be determined as follows:

,

Where a 1 - heat transfer coefficient per inside

pipes, W/(m 2 °C);

a 2 - the same, on the outside of the pipe, W/(m 2 °C);

r 1 and r 2 - respectively, the radii of the inner and outer

pipe surfaces, m. In metal recuperators, the value of the thermal resistance of the wall can be neglected , and then the total heat transfer coefficient can be written in the following form:

W/(m 2 °C)

All local heat transfer coefficients necessary to determine the value TO, can be obtained based on the laws of heat transfer by convection and radiation.

Since there is always a pressure difference between the air and smoke sides of the recuperator, the presence of leaks in the recuperative nozzle leads to air leakage, sometimes reaching 40-50%. Leaks sharply reduce the efficiency of recuperative installations; the more air sucked in, the lower the proportion of heat usefully used in the ceramic recuperator (see below):

Leakage, % 0 25 60

Final flue gas temperature,

°C 660 615 570

Air heating temperature, °C 895 820 770

Recuperator efficiency (without taking into account

losses), % 100 84 73.5

Air leakage affects the value of local heat transfer coefficients, and air trapped in the flue gases not only

Rice. 4. Schemes of movement of gaseous media in recuperative heat exchangers

reduces their temperature, but also reduces the percentage of CO 2 and H 2 0, as a result of which the emissivity of gases deteriorates.

Both with an absolutely gas-tight recuperator and with a leak, the local heat transfer coefficients change along the heating surface, therefore, when calculating recuperators, the values ​​of the local heat transfer coefficients for the top and bottom are determined separately and then the total heat transfer coefficient is found using the average value.

LITERATURE

1. B.A.Arutyunov, V.I. Mitkalinny, S.B. Stark. Metallurgical heat engineering, vol. 1, M, Metallurgy, 1974, p. 672
2. V.A. Krivandin and others. Metallurgical heat engineering, M, Metallurgy, 1986, p. 591
3. V.A.Krivandin, B.L. Markov. Metallurgical furnaces, M, Metallurgy, 1977, p.463
4. V.A. Krivandin, A.V. Egorov. Thermal work and designs of ferrous metallurgy furnaces, M, Metallurgy, 1989, p.463

Owners of patent RU 2436011:

The invention relates to thermal power engineering and can be used at any enterprise that operates boilers using hydrocarbon fuels. The objective of the invention is to increase the efficiency of using low-grade heat of condensation of water vapor contained in flue gases. The flue gas heat recovery device contains a gas-gas surface plate heat exchanger, in which the original flue gases are cooled, heating the dried flue gases in a countercurrent manner. Cooled wet flue gases are fed into a gas-air surface plate heat exchanger-condenser, where the water vapor contained in the flue gases is condensed, heating the air. The heated air is used to heat the premises and cover the needs of the gas combustion process in the boiler. The condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle. The dried flue gases are supplied by an additional smoke exhauster to the heater described above, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney. 2 n.p. f-ly, 1 ill.

The invention relates to thermal power engineering and can be used at any enterprise that operates boilers using hydrocarbon fuels.

A known boiler installation contains a contact water heater connected at the inlet to the exhaust gas duct of the boiler, and at the outlet through a gas outlet duct equipped with a smoke exhauster to the chimney, and an air heater with heating and air paths (USSR Copyright Certificate No. 1086296, F22B 1/18 dated 15.04. 1984).

The installation works as follows. The main part of the gases from the boiler enters the exhaust flue, and the rest of the gases enter the heating duct. From the exhaust flue, the gases are directed to a contact water heater, where condensation of water vapor contained in the flue gases occurs. The gases then pass through the droplet eliminator and enter the gas outlet channel. Outside air enters the air heater, where it is heated by gases passing through the heating path, and is sent to the gas outlet channel, where it mixes with cooled gases and reduces the moisture content of the latter.

Flaws. Unacceptable quality of heated water for use in the heating system. The use of heated air only to supply it to the chimney in order to prevent condensation of water vapor. Low degree of heat recovery from flue gases, since the main task was to dry the flue gases and reduce the dew point temperature.

Known commercially produced by the Kostroma Heating Plant are heaters of the KSk type (Kudinov A.A. Energy saving in heat-generating installations. - Ulyanovsk: UlSTU, 2000. - 139, p. 33), consisting of a gas-water surface heat exchanger, the heat exchange surface of which is made of finned bimetallic tubes, strainer, distribution valve, drip eliminator and hydropneumatic blower.

Heaters of the KSk type work as follows. Flue gases enter the distribution valve, which divides them into two streams, the main gas flow is directed through a mesh filter into the heat exchanger, the second through the bypass line of the flue. In the heat exchanger, water vapor contained in the flue gases condenses on finned tubes, heating the water flowing in them. The resulting condensate is collected in a pan and pumped into the heating network feed circuit. The water heated in the heat exchanger is supplied to the consumer. At the outlet of the heat recovery unit, the dried flue gases are mixed with the original flue gases from the flue bypass line and sent through a smoke exhauster into the chimney.

Flaws. For the heat exchanger to operate in the condensation mode of its entire convective part, it is required that the heating temperature of the water in the convective package does not exceed 50°C. To use such water in heating systems, it must be additionally heated.

To prevent condensation of residual water vapor of flue gases in the flues and chimney, part of the source gases is mixed through a bypass channel with the dried flue gases, increasing their temperature. With such an admixture, the content of water vapor in the exhaust flue gases also increases, reducing the efficiency of heat recovery.

A known installation for recycling heat from flue gases (RF patent No. 2193727, F22B 1/18, F24H 1/10 dated April 20, 2001), containing a sprinkler installed in the gas duct with distribution nozzles, a recovery heat exchanger and an intermediate coolant heat exchanger, the heated path of which is connected at the inlet to the moisture collector. The sprinkler is located in front of the specified heat exchangers, installed one opposite the other at the same distance from the sprinkler, the nozzles of which are directed in the direction opposite to the heat exchangers. The installation is additionally equipped with a heat exchanger for reheating the irrigation water installed in the gas duct and located above the sprinkler, the heated path of which is connected at the inlet to the heat exchanger of the intermediate coolant, and at the outlet to the sprinkler. All heat exchangers are surface, tubular. The tubes can be finned to increase the heating surface.

There is a known method of operation of this installation (RF patent No. 2193728, F22B 1/18, F24H 1/10 dated April 20, 2001), according to which the flue gases passing through the flue duct are cooled below the dew point and removed from the installation. In the installation, water is heated in a recovery heat exchanger and distributed to the consumer. The outer surface of the recovery heat exchanger is irrigated with an intermediate coolant - water from a sprinkler with distributing nozzles directed towards the flow of gases. In this case, the intermediate coolant is preheated in a heat exchanger installed in the gas duct opposite the recovery heat exchanger and at the same distance from the sprinkler as the recovery heat exchanger. Then the intermediate coolant is supplied to a heat exchanger for reheating the irrigating water installed in the gas duct and located above the sprinkler, heated to the required temperature and sent to the sprinkler.

Two independent flows of water flow in the installation: clean, heated through a heat transfer surface, and irrigating, heated as a result of direct contact with exhaust gases. A clean stream of water flows inside the tubes and is separated by walls from the contaminated stream of irrigation water. A bundle of tubes performs the function of a nozzle designed to create a developed contact surface between irrigation water and exhaust gases. The outer surface of the nozzle is washed by gases and irrigating water, which intensifies heat exchange in the apparatus. The heat of the exhaust gases is transferred to the water flowing inside the tubes of the active nozzle in two ways: 1) due to the direct transfer of heat from the gases and irrigating water; 2) due to condensation on the surface of the nozzle of part of the water vapor contained in the gases.

Flaws. The final temperature of the heated water at the outlet of the nozzle is limited by the wet thermometer temperature of the gases. When burning natural gas with an excess air ratio of 1.0-1.5, the wet bulb temperature of the flue gases is 55-65°C. This temperature is not sufficient to use this water in the heating system.

Flue gases leave the apparatus with a relative humidity of 95-100%, which does not exclude the possibility of condensation of water vapor from the gases in the exhaust tract after it.

The closest to the claimed invention in terms of use, technical essence and achieved technical result is a heat exchanger (RF patent No. 2323384, F22B 1/18 dated 08/30/2006), containing a contact heat exchanger, a droplet eliminator, a gas-gas heat exchanger connected according to a direct flow circuit, gas ducts, pipelines, pump, temperature sensors, control valves. Along the flow of circulating water of the contact heat exchanger, a water-to-water heat exchanger and a water-to-air heat exchanger with a bypass channel along the air flow are located in series.

Method of operation of the heat exchanger. The exhaust gases through the gas duct enter the inlet of the gas-gas heat exchanger, sequentially passing through its three sections, then to the inlet of the contact heat exchanger, where, passing through a nozzle washed by circulating water, they are cooled below the dew point, giving off sensible and latent heat to the circulating water. Next, the cooled and moist gases are freed from most of the liquid water carried away in a droplet eliminator, heated and dried in at least one section of the gas-gas heat exchanger, sent into a chimney by a smoke exhauster and released into the atmosphere. At the same time, heated circulating water from the pan of the contact heat exchanger is pumped into the water-to-water heat exchanger, where it heats cold water from the pipeline. The water heated in the heat exchanger is supplied to the needs of process and domestic hot water supply or to a low-temperature heating circuit.

Next, the recycled water enters the water-air heat exchanger, heats at least part of the blown air coming from outside the room through the air duct, cooling to the minimum possible temperature, and enters the contact heat exchanger through the water distributor, where it takes heat from the gases, simultaneously washing them from suspended particles, and absorbs some of the oxides of nitrogen and sulfur. The heated air from the heat exchanger is supplied by a blower fan to a standard air heater or directly to the firebox. Recycled water is filtered if necessary and processed by known methods.

The disadvantages of this prototype are:

The need for a control system due to the use of recovered heat for hot water supply purposes due to the variability of the daily schedule of hot water consumption.

Water heated in the heat exchanger, supplied for the needs of hot water supply or in a low-temperature heating circuit, requires it to be brought to the required temperature, since it cannot be heated in the heat exchanger above the temperature of the return circuit water, which is determined by the saturation temperature of water vapor in the flue gases. The low heating of the air in the water-to-air heat exchanger does not allow this air to be used for space heating.

The task set is to simplify the heat recovery technology and increase the efficiency of using low-grade heat of condensation of water vapor contained in flue gases.

This problem is solved in the following way.

A device for heat recovery from flue gases is proposed, containing a gas-gas heat exchanger, a condenser, an inertial drip eliminator, gas ducts, air ducts, fans and a pipeline, characterized in that the gas-gas surface plate heat exchanger is made according to a counterflow circuit, a surface gas-air plate heat exchanger is installed as a condenser, in An additional smoke exhauster is installed in the gas duct of cold dried flue gases; a gas duct for mixing part of the heated dried flue gases is installed in front of the additional smoke exhauster.

A method of operation of a flue gas heat recovery device is also proposed, according to which the flue gases are cooled in a gas-gas heat exchanger, heating the dried flue gases, water vapor contained in the flue gases is condensed in the condenser, a part of the blast air is heated, characterized in that in the gas-gas in a heat exchanger, the dried flue gases are heated by cooling the original flue gases using a counterflow scheme without regulating the gas flow rate, water vapor is condensed in a surface gas-air plate heat exchanger-condenser, heating the air and the heated air is used for heating and covering the needs of the combustion process, and the condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle, in the gas duct of cold dried flue gases, the aerodynamic resistance of the gas path is compensated by an additional smoke exhauster, in front of which part of the heated dried flue gases is mixed, excluding the condensation of residual water vapor carried away by the flow from the condenser, the temperature of the heated air is regulated using changes in the speed of the smoke exhauster depending on the outside air temperature.

The source flue gases are cooled in a gas-gas surface plate heat exchanger, heating the dried flue gases.

The difference is the use of a surface plate heat exchanger without any gas flow control devices, where the heating medium (the entire volume of wet flue gases) and the heated medium (the entire volume of dried flue gases) move in countercurrent. In this case, a deeper cooling of the moist flue gases occurs to a temperature close to the dew point of water vapor.

Next, the water vapor contained in the flue gases is condensed in a gas-air surface plate heat exchanger-condenser, heating the air. The heated air is used to heat rooms and cover the needs of the combustion process. The condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle.

The difference between the proposed method is that the heated medium is cold air supplied by fans from the environment. The air is heated by 30-50°C, for example from -15 to 33°C. The use of air with a negative temperature as a cooling medium allows you to significantly increase the temperature pressure in the condenser when using counterflow. Air heated to 28-33°C is suitable for space heating and supply to the boiler to ensure the combustion of natural gas. Thermal calculation of the circuit shows that the flow rate of heated air is 6-7 times higher than the flow rate of the original flue gases, which makes it possible to completely cover the needs of the boiler, heat the workshop and other premises of the enterprise, and also supply part of the air to the chimney to reduce the dew point temperature or to a third-party consumer .

The aerodynamic resistance of the gas path in the flue of cold, dried flue gases is compensated by an additional smoke exhauster. To prevent condensation of residual water vapor carried away by the flow from the condenser, a portion of the heated, dried flue gases (up to 10%) is mixed in front of the additional smoke exhauster. The temperature of the heated air is regulated by changing the flow rate of the dried flue gases, by adjusting the speed of the smoke exhauster depending on the outside air temperature.

The dried flue gases are supplied by a smoke exhauster to the heater described above, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney.

The flue gas heat recovery device shown in the drawing contains a gas duct 1 connected to a heat exchanger 2, which is connected to a condenser 4 through a gas duct 3. The condenser 4 has an inertial drop catcher 5 and is connected to the condensate discharge pipeline 6. The fan 7 is connected to a cold air duct 8 with condenser 4. Condenser 4 is connected by air duct 9 to the heat consumer. The dry flue gas duct 10 is connected to the heat exchanger 2 through a smoke exhauster 11. The dry heated flue gas duct 12 is connected to the heat exchanger 2 and directed into the chimney. The gas duct 12 is connected to the gas duct 10 by an additional gas duct 13, which contains a damper 14.

Heat exchanger 2 and condenser 4 are surface plate heat exchangers made of unified modular packages, which are arranged in such a way that the coolant flows countercurrently. Depending on the volume of flue gases to be dried, the heater and condenser are formed from a calculated number of packages. Block 7 is formed from several fans to change the flow of heated air. Condenser 4 at the outlet of the dried flue gases has an inertial drop catcher 5, made in the form of vertical louvers, behind which a gas duct 10 is embedded. A damper 14 is installed on the gas duct 13 for the initial adjustment of the temperature reserve, which prevents condensation of residual water vapor in the smoke exhauster 11.

Method of operation of a flue gas heat recovery device.

Wet flue gases enter heat exchanger 2 through flue 1, where their temperature is reduced to a temperature close to the dew point. The cooled flue gases through the flue 3 enter the condenser 4, where the water vapor contained in them is condensed. The condensate is discharged through pipeline 6 and, after additional processing, is used to replenish losses in the heating network or steam turbine cycle. The heat of condensation is used to heat cold air, which is supplied by fans 7 from the environment. Heated air 9 is directed to the production room of the boiler room for its ventilation and heating. From this room, air is supplied to the boiler to ensure the combustion process. Dried flue gases 10 pass through an inertial drip eliminator 5, are supplied by a smoke exhauster 11 to a heat exchanger 2, where they are heated and directed into the chimney 12. Heating of the dried flue gases is necessary to prevent condensation of residual water vapor in the flues and chimney. To prevent drops of moisture from falling out in the smoke exhauster 11, carried away by the dried flue gas flow from the condenser, part of the heated dry flue gases (up to one tenth) from the flue 12 through the flue 13 is supplied to the flue 10, where the entrained moisture evaporates.

The temperature of the heated air is regulated by changing the flow rate of the dried flue gases by changing the speed of the smoke exhauster 11 depending on the outside air temperature. With a decrease in the flow rate of wet flue gases, the aerodynamic resistance of the gas path of the device decreases, which is compensated by a decrease in the speed of the smoke exhauster 11. The smoke exhauster 11 provides a difference in the pressure of the flue gases and air in the condenser in order to prevent flue gases from entering the heated air.

A verification calculation shows that for a natural gas boiler with a power of 6 MW, with a flow rate of wet flue gases of 1 m 3 / s with a temperature of 130 ° C, the air is heated from -15 to 30 ° C, with a flow rate of 7 m 3 / s. The condensate flow rate is 0.13 kg/s, the temperature of the dried flue gases at the outlet of the heater is 86°C. The thermal power of such a device is 400 kW. The total heat exchange surface area is 310 m2. The dew point temperature of water vapor in flue gases decreases from 55 to 10°C. The boiler efficiency increases by 1% only due to the heating of cold air in the amount of 0.9 m 3 /s required for the combustion of natural gas. At the same time, heating this air accounts for 51 kW of the device’s power, and the rest of the heat is used for air heating of the premises. The results of calculations of the operation of such a device at various outdoor temperatures are given in Table 1.

Table 2 shows the results of calculating variants of the device for other flow rates of dried flue gases, at an outside air temperature of -15°C.

1. A device for recovering heat from flue gases, containing a gas-gas heat exchanger, a condenser, an inertial drop eliminator, gas ducts, air ducts, fans and a pipeline, characterized in that the gas-gas surface plate heat exchanger is made according to a counterflow circuit, and a surface gas-air heat exchanger is installed as a condenser plate heat exchanger, an additional smoke exhauster is installed in the gas duct of cold dried flue gases, a gas duct for mixing part of the heated dried flue gases is installed in front of the additional smoke exhauster.

2. The method of operation of the flue gas heat recovery device, according to which the flue gases are cooled in a gas-gas heat exchanger, heating the dried flue gases, condensing water vapor contained in the flue gases in the condenser, heating part of the blast air, characterized in that in the gas-gas in a heat exchanger, the dried flue gases are heated by cooling the original flue gases using a counterflow scheme without regulating the gas flow rate, water vapor is condensed in a surface gas-air plate heat exchanger-condenser, heating the air and the heated air is used for heating and covering the needs of the combustion process, and the condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle, in the gas duct of cold dried flue gases, the aerodynamic resistance of the gas path is compensated by an additional smoke exhauster, in front of which part of the heated dried flue gases is mixed, excluding the condensation of residual water vapor carried away by the flow from the condenser, the temperature of the heated air is regulated using changes in the speed of the smoke exhauster depending on the outside air temperature.

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