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 deep recycling heat of exhaust 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.

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% at the lowest level calorific value gas.

In addition, during deep utilization in the presence of water vapor in the PS, the emission 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 the advanced Western countries deep utilization for heating systems is carried out using condensation-type hot water boilers equipped with a condensation economizer.

The generally low return water temperature (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 ( and then using it 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 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 only possible 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 recycling - 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 Oh.

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 recycling 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 tail surfaces boiler, economizer) on 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 technological scheme(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 is used by 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 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 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 stream of pure condensate is supplied by a feed pump 16 to the high-pressure heater 17, and from it to the boiler 1.

Thus, the heat of combustion products utilized in the condensation heat exchanger saves fuel spent 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.

Various types of heat exchangers are applicable: shell-and-tube, straight-tube, knurled fins, plate or efficient design With new form 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 within a wide range thermal power system, maintaining its profitability: the degree of bypass, the temperature of the combustion products behind the condensation heat exchanger, etc. The thermal load of the condensation 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 the 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. Optimal mode– 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 carryover of splashes 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.

V.S.Galustov, Doctor of Technical Sciences, Professor, CEO SE NPO "Polytechnika"
L.A. Rosenberg, engineer, director of the Yumiran Unitary Enterprise.

Introduction.

With flue gases of various origins, thousands and thousands of Gcal of heat are released into the atmosphere, as well as thousands of tons of gaseous and solid pollutants and water vapor. In this article we will focus on the problem of heat recovery (about cleaning gas emissions We'll talk about it in the next post). The deepest use of fuel combustion heat is carried out in thermal power boilers, for which in most cases economizers are provided in their tail section. The temperature of the flue gases after them is about 130-190°C, i.e. is close to the dew point temperature of acid vapors, which, in the presence of sulfur compounds in the fuel, is lower limit. When burning natural gas, this limitation is less significant.

Flue gases from various types of furnaces can have significantly higher high temperature(up to 300-500°C and above). In this case, heat recovery (and gas cooling) is simply mandatory, if only to limit thermal pollution of the environment.

Heat recovery units.

Even in the first message, we limited the range of our interests to processes and devices with direct phase contact, but to complete the picture, let us recall and evaluate other options. All known heat exchangers can be divided into contact, surface, and devices with an intermediate coolant. We will dwell on the first in more detail below. Surface heat exchangers are traditional air heaters that are placed directly in the flue after the furnace (boiler) and have serious disadvantages that limit their use. Firstly, they introduce significant aerodynamic resistance into the gas path and worsen the operation of furnaces (the vacuum decreases) with a designed smoke exhauster, and replacing it with a more powerful one may not compensate for the accompanying costs by saving heat. Secondly, low heat transfer coefficients from gas to the surface of the tubes determine large values required surface contact.

Devices with an intermediate coolant are of two types: periodic with solid coolant and continuous with liquid. The first are at least two columns filled, for example, with crushed granite (packing). Flue gases pass through one of the columns, giving off heat to the nozzle, heating it to a temperature slightly lower than the temperature of the gases. Then the flue gases are switched to the second column, and the first is supplied with a heated medium (usually air supplied to the same furnace, or air from an air heating system), etc. The disadvantages of such a scheme are obvious (high resistance, bulkiness, temperature instability, etc.), and its application is very limited.

Devices with a liquid intermediate coolant (usually water) were called contact heat exchangers with an active nozzle (CTAN), and after minor improvements the authors called them heat exchangers with a saturated coolant and condensation (TANTEC). In both cases, the water heated by the flue gases then releases the resulting heat through the wall of the surface built-in heat exchanger clean water(eg heating systems). Compared to air heaters, the resistance of such heat exchangers is much lower, and in terms of heat exchange in the flue gas - water system, they are completely similar to the direct-flow atomization devices that interest us. However, there are significant differences, which we will discuss below.

The developers of KTAN and TANTEC devices do not consider in their publications the features of heat transfer during direct contact of flue gases and water, so we will dwell on them in a little more detail.

The main processes in the flue gas - water system.

The result of the interaction of heated flue gases (in composition and properties this is actually moist air) and water (in the form of droplets of one size or another), which we will call a heat-accumulating medium (it can be used as the main or intermediate coolant), is determined by a whole complex of processes.

Simultaneously with heating, moisture condensation on the surface of the droplets or evaporation may occur. In fact, there are three possible options for the mutual direction of heat and moisture flows (heat transfer and mass transfer), which depend on the ratio of phase temperatures and the ratio of partial vapor pressures in the boundary layer (near the drop) and in the core of the gas flow (Fig. 1a).

In this case, the first (upper) case, when the flows of heat and moisture are directed from the droplets to the gas, corresponds to evaporative cooling water; the second (middle) - heating the droplets while simultaneously evaporating moisture from their surface; the third (lower) option, in which heat and moisture are directed from the gas to the droplets, reflects the heating of water with condensation of vapor. (It would seem that there should be a fourth option, when cooling of droplets and heating of gas are accompanied by moisture condensation, but in practice this does not occur.)

All the described processes can be clearly represented on the Ramzin diagram of the state of humid air (H - x diagram, Fig. 1b).

Already from what has been said, we can conclude that the third option is the most desirable, but in order to understand how to ensure it, it is necessary, in addition to what has been stated above, to recall:

- the amount of water vapor contained in 1 m3 of humid air is called absolute air humidity. Water vapor occupies the entire volume of the mixture, therefore the absolute humidity of the air is equal to the density of water vapor (under given conditions) pp

— when the air is saturated with steam, the moment comes when condensation begins, i.e. the maximum possible vapor content in the air at a given temperature is achieved, which corresponds to the density of saturated water vapor pH;

— the ratio of absolute humidity to the maximum possible amount of steam in 1 m3 of air at a given pressure and temperature is called relative air humidity f;

- the amount of water vapor in kg per 1 kg of absolutely dry air is called air moisture content x;

— moist air as a coolant is characterized by enthalpy / (heat content), which is a function of the temperature and moisture content of the air and is equal to the sum of the enthalpies of dry air and water vapor. In the most convenient form for practical use, the formula for calculating enthalpy can be presented

I= (1000 + 1.97 . 103x) t+ 2493 . . 103x J/kg dry air, where 1000 - specific heat dry air, J/kg*deg); 1.97*103 - specific heat capacity of steam, J/(kg*deg); 2493*103 - constant coefficient, approximately equal to the enthalpy of steam at 0°C; t—air temperature, °C;

I = 0.24t + (595 + 0.47t) Xkcal/kg dry air; where 595 is a constant coefficient approximately equal to the enthalpy of steam at 0°C; 0.24—specific heat capacity of dry air, kcal/(kgtrad); 0.47 — heat capacity of steam, kcal/(kgtrad);

— when cooling the air (in conditions of constant moisture content) relative humidity will increase until it reaches 100%. The corresponding temperature is called the dew point temperature. Its value is determined solely by the moisture content of the air. On the Ramzin diagram, this is the point of intersection of the vertical line x = const with the line φ = 1.

Air cooling below the dew point is accompanied by moisture condensation, i.e. air drying.

Some confusion is caused by publications that give dew point values ​​for various solid and liquid fuels about 130-150°C. It must be borne in mind that this concerns the beginning of condensation of vapors of sulfuric and sulfurous acids (denoted by eetpK), and not water vapor (tp), which we discussed above. For the latter, the dew point temperature is much lower (40-50°C).

So, three quantities - flow rate, temperature and moisture content (or wet-bulb temperature) - fully characterize flue gases as a source of secondary energy resources.

When water comes into contact with hot gases, the liquid is initially heated and vapor condenses on the surface of cold drops (corresponds to option 3 in Fig. 1a) until the temperature corresponding to the dew point for the gas is reached, i.e. boundary of transition to the second regime (3rd option in Fig. 1a). Further, as the water heats up and the partial vapor pressure at the surface of the droplets increases, the amount of heat transferred to them due to heat transfer Q1 will decrease, and the amount of heat transferred from the droplets to the flue gases due to evaporation Q2 will increase. This will continue until equilibrium is reached (Q1 = Q2), when all the heat received by water from the flue gas will be returned to the gas in the form of heat of evaporation of the liquid. After this, further heating of the liquid is impossible, and it evaporates at a constant temperature. The temperature achieved in this case is called the wet bulb temperature tM (in practice, it is defined as the temperature indicated by a thermometer whose ball is covered with a damp cloth, from which moisture evaporates).

Thus, if water is supplied to the heat exchanger with a temperature equal to (or greater than) tM, then adiabatic (at constant heat content) cooling of the gases will be observed and there will be no heat recovery (not counting negative consequences- loss of water and humidification of gases).

The process becomes more complex if we consider that the composition of the droplets is polydisperse (due to the mechanisms of liquid disintegration during spraying). Small drops instantly reach tM and begin to evaporate, changing the gas parameters towards increasing moisture content, medium drops can be between tp and tM, and large drops can be below tp, i.e.

heat up and condense moisture. All this occurs simultaneously in the absence of clear boundaries.

It is possible to comprehensively analyze the results of direct contact between droplets of a heat-accumulating medium and hot flue gases only on the basis of a mathematical model that takes into account the entire complex of phenomena (simultaneous heat and mass transfer, changes in environmental parameters, aerodynamic conditions, polydisperse composition of the droplet flow, etc.).

A description of the model and the results of analysis based on it is given in the monograph, which we recommend that the interested reader refer to. Here we note only the main thing.

For most flue gases, the wet bulb temperature is in the range of 45-55°C, i.e. water in the zone of direct contact with flue gases, as noted above, can only be heated to the specified temperature, although with fairly deep heat recovery. Preliminary humidification of gases, as provided for by the TANTEC design, not only does not lead to an increase in the amount of utilized heat, but even to its decrease.

And finally, it should be taken into account that when recovering heat even from gases that do not contain sulfur compounds, they should not be cooled below 80°C (it makes it difficult to evacuate them to environment through the flue and chimney).

Let us explain what was said on specific example. Let the flue gases after the boiler in an amount of 5000 kg/h, having a temperature of 130°C and a moisture content of 0.05 kg/kg, contact the heat recovery medium (water, tH = 15°C). From the H-x diagram we find: tM= 49.5°C; tp= 40°С; I = 64 kcal/kg. Calculations using the model showed that when gases are cooled to 80°C by a polydisperse flow of droplets with an average diameter of 480 μm, the moisture content actually remains unchanged (evaporation of small droplets is compensated by condensation on large ones), tM becomes equal to 45°C, and heat content I = 50 kcal/kg . Thus, 0.07 Gcal/h of heat is utilized, and the heat-accumulating medium in the amount of 2.5 m3/h is heated from 15 to 45°C.

If you use TANTEC and preliminarily humidify - adiabatic cooling of gases to t-100°C, and then cool to 80°C at X = const, then the final gas parameters will be: tM = 48°C; I = 61.5°C. And although the water will heat up slightly higher (up to 48°C), the amount of heat utilized is reduced by 4 times and will be 0.0175 Gcal/h.

Options for organizing heat recovery.

Solution specific task utilization of flue gas heat depends on a number of factors, including the presence of pollutants (determined by the type of fuel burned and the object heated by flue gases), the presence of a heat consumer or directly hot water etc.

At the first stage, it is necessary to determine the amount of heat that, in principle, can be extracted from the existing flue gases, and evaluate the economic feasibility of heat recovery, since the capital costs for it are not proportional to the amount of heat recovered.

If the answer to the first question is positive, then you should evaluate the possibility of using moderately heated water (for example, when burning natural gas, use it to prepare make-up water for boilers or heating systems, and if the target product is contaminated with dust particles, use it to prepare raw materials, for example, in the production of ceramic products and so on.). If the water is too polluted, you can provide a double-circuit system or combine heat recovery with flue gas purification (get higher (above 45-5СРС) temperatures or a surface stage).

There are many options for organizing the heat recovery process. From choice optimal solution depends economic efficiency Events.

Literature:

1. Galustov B.S. Heat and mass transfer processes and devices with direct phase contact in heat and power engineering // Energy and management. - 2003. - No. 4.

2. Galustov B.S. Direct-flow spraying devices in thermal power engineering. - M.: Energoatomizdat, 1989.

3. Sukhanov V.I. and others. Installations for heat recovery and purification of flue gases of steam and hot water boilers. - M.: AQUA-TERM, July 2001.

4. Planovsky A.N., Ramm V.M., Kagan S.Z. Processes and apparatus chemical technology.— M.: Goskhimizdat, 1962.—P.736-738.

Heat recovery from flue gases

The flue gases leaving the working space of the furnaces 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 furnaces, 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 two ways: 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 the combustion temperature and save fuel. With the second recovery method, the heat of exhaust flue gases is used in thermal power boiler houses and turbine units, which achieves significant fuel savings.

In some cases, both described methods of waste heat recovery 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 be noted, first of all, 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 a unit of heat obtained in the furnace as a result of combustion of fuel (a unit of chemical heat), since the heat of the 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 the oven should be fed into the workspace every hour required amount heat. This amount of heat includes not only the heat of the fuel, but also the heat of heated air or gas, i.e.

It is clear that with = const an increase will reduce . In other words, utilization of heat from flue gases makes it possible to achieve fuel savings, which depends on the degree of heat utilization from flue gases

where is, respectively, the enthalpy of heated air and flue gases leaving the working space, kW, or kJ/period.

The degree of heat recovery can also be called efficiency. recuperator (regenerator), %

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

where I"d, Id 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 calorimetric combustion temperature, which may be the main purpose of recovery when heating furnaces with fuel with a low calorific value.

An increase in at leads to an increase in combustion temperature. If it is necessary to provide a certain value, then an increase in the air (gas) heating temperature leads to a decrease in the value, i.e., to a decrease in the share in fuel mixture gas with a high calorific value.

Since heat recovery allows for significant fuel savings, it is advisable to strive for the highest possible, economically justified degree of utilization. However, it must immediately be noted that recycling cannot be complete, i.e. always. This is explained by the fact that increasing the heating surface is rational only up to certain limits, after which it already leads to a very insignificant gain in heat savings.

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 implementation 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 utilization of heat from flue gases and the need to automate the process, however, these difficulties can be solved with the current level of technology.

For widespread implementation of this method, it is necessary to develop methodological instructions on the calculation and installation of systems for deep recovery of flue gas heat 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 exhausted 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 implementing this method is increased efficiency boiler gross by 2-3%, taking into account the use of latent heat of vaporization 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 c 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, biofuel. To expand the list of objects where this method can be used, it is necessary to conduct research on the processes of heat and mass transfer of combustion products of fuel oil, light diesel fuel and various brands coals.

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 gas ducts and chimneys treated with modern anti-corrosion coatings regarding the possibility of reducing restrictions on the temperature and humidity of 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. The introduction of this method may be stimulated by interest in reducing fuel consumption and emissions of nitrogen oxides into the atmosphere.

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 fuel-using installations using natural gas 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

There are no questions regarding the application of this method in the existing regulatory framework.

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, extensive experience has been accumulated in 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 load schedules energy equipment, change economic indicators generation and transmission of energy, 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 implementation 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 bans and mandatory requirements on application, 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.

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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 the working space of the furnace in the form of heat from heated combustion components (gaseous fuel and air). In fact, increased heat recovery leads to a reduction in fuel consumption and to intensification and improvement of the 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 smelting furnaces, in copper smelting reverberatory furnaces, in rotary kilns for burning cement clinker, in the dry method of 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. 6 shows a diagram of the placement of coil heating surfaces in vertical chimneys. The movement of the steam-water mixture is carried out by a 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 usually 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 (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.

Heat transfer 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 case of equipment high temperature furnaces It is advisable to install waste heat boilers with metal recuperators 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.