B. V. Getman, N. V. Lezhneva

Key words: gas turbine plants, combined cycle plants

The paper considers various methods for utilizing the heat of exhaust gases from power plants in order to increase their efficiency, save organic fuel and increase energy capacities.

Keywords: gas-turbine installations, steam-gas installations

In work various methods of utilization of warmth of leaving gases from power installations for the purpose of increase of their efficiency, economy of organic fuel and accumulations of power capacities are considered.

With the beginning of economic and political reforms in Russia, first of all, it is necessary to make a number of fundamental changes in the country's electric power industry. The new energy policy should solve a number of tasks, including the development of modern highly efficient technologies for the production of electrical and thermal energy.

One of these tasks is to increase the efficiency of power plants in order to save fossil fuels and increase power capacities. Most

promising in this regard are gas turbine plants, with the exhaust gases of which up to 20% of heat is emitted.

There are several ways to increase the efficiency of gas turbine engines, including:

Increasing the gas temperature in front of the turbine for gas turbines with a simple thermodynamic cycle,

Heat recovery application,

The use of exhaust gas heat in binary cycles,

Creation of gas turbines according to a complex thermodynamic scheme, etc.

The most promising direction is the joint use of gas turbine and steam turbine units (GTP and STP) in order to improve their economic and environmental performance.

Gas turbines and combined plants created with their use, with parameters that are technically achievable at present, provide a significant increase in the efficiency of heat and electricity production.

The widespread use of binary CCGTs, as well as various combined schemes in the technical re-equipment of thermal power plants, will save up to 20% of fuel compared to traditional steam turbine units.

According to experts, the efficiency of the combined steam-gas cycle increases with an increase in the initial gas temperature in front of the gas turbine and an increase in the share of gas turbine power. Important

It also has the fact that, in addition to gaining efficiency, such systems require significantly lower capital costs, their specific cost is 1.5–2 times less than the cost of gas-oil steam turbine units and CCGTs with a minimum gas turbine power.

According to the data, three main directions for the use of gas turbines and combined cycle plants in the energy sector can be distinguished.

The first, widely used in industrialized countries, is the use of CCGT at large gas-fired condensing thermal power plants. In this case, it is most efficient to use a utilization-type CCGT with a large share of gas turbine power (Fig. 1).

The use of CCGT allows to increase the efficiency of fuel combustion at TPPs by ~ 11-15% (CCGT with gas discharge into the boiler), by ~ 25-30% (binary CCGT).

Until recently, extensive work on the introduction of CCGT in Russia has not been carried out. Nevertheless, single samples of such units have been successfully used for a long time, for example, a CCGT with a high-pressure steam generator (HPG) of the HSG-50 type at the head power unit CCGT-120 and 3 modernized power units with HSPG-120 at the CHPP-2 branch of JSC " TGC-1"; PGU-200 (150) with VPG-450 at the Nevinnomysskaya GRES branch. Three combined-cycle power units with a capacity of 450 MW each have been installed at Krasnodarskaya GRES. The power unit includes two gas turbines with a capacity of 150 MW each, two waste heat boilers and a steam turbine with a capacity of 170 MW, the efficiency of such an installation is 52.5%. Further

increasing the efficiency of a utilization-type CCGT is possible by improving

gas turbine plant and complicating the scheme of the steam process.

Rice. 1 - Scheme of CCGT with waste heat boiler

Combined-cycle plant with a boiler -

utilizer (Fig. 1) includes: 1-

compressor; 2 - combustion chamber; 3 - gas

turbine; 4 - electric generator; 5 - boiler-

utilizer; 6 - steam turbine; 7 - capacitor; eight

Pump and 9 - deaerator. In the waste heat boiler, the fuel is not reburned, and the generated superheated steam is used in the steam turbine plant.

The second direction is the use of gas turbines to create a CCGT-CHP and GTU-CHP. In recent years, many options for technological schemes of CCGT-CHP have been proposed. At gas-fired CHPPs, it is advisable to use combined heat and power plants

recycling type. A characteristic example

a large CCGT-CHP of this type is Severo-Zapadnaya CHPP in St. Petersburg. One CCGT unit at this CHPP includes: two gas turbines with a capacity of 150 MW each, two waste heat boilers, a steam turbine. The main indicators of the unit are: electric power - 450 MW, thermal power - 407 MW, specific fuel consumption for electricity supply - 154.5 g of c.u. tons / (kWh), specific consumption of reference fuel for heat supply - 40.6 kg c.u. ton/GJ, efficiency of CHPP for the supply of electric energy - 79.6%, thermal energy - 84.1%.

The third direction is the use of gas turbines for the creation of CCGT-CHP and GTU-CHP of small and medium capacity on the basis of boiler houses. CCGT - CHP and GTU - CHP of the best options, created on the basis of boiler houses, provide efficiency for the release of electrical energy in the heating mode at the level of 76 - 79%.

A typical combined cycle plant consists of two gas turbines, each with its own waste heat boiler, which supplies the generated steam to one common steam turbine.

An installation of this type was developed for Shchekinskaya GRES. CCGT-490 was designed to generate electrical energy in the basic and partial modes of operation of the power plant with the release of heat to an external consumer up to 90 MW with a winter temperature schedule. The schematic diagram of the PGU-490 block was forced to focus on the lack of space when placing the waste heat boiler and

steam turbine plant in the power plant buildings, which created certain difficulties in achieving optimal modes of combined heat and power generation.

In the absence of restrictions on the location of the installation, as well as when using an improved gas turbine unit, it is possible to significantly increase the efficiency of the unit. A single-shaft CCGT-320 with a capacity of 300 MW is proposed as such an improved CCGT. The complete gas turbine unit for CCGT-320 is the single-shaft GTE-200, the creation of which is supposed to be carried out by switching to

double-support rotor, modernization of the cooling system and other units of the gas turbine in order to increase the initial gas temperature. In addition to the GTE-200, the CCGT-320 monoblock contains a K-120-13 STP with a three-cylinder turbine, a condensate pump, a seal steam condenser, a heater fed by heating steam supplied from the extraction before the last stage of the heat exchanger, and a two-pressure waste heat boiler containing eight heat exchange areas, including an intermediate steam superheater.

To assess the efficiency of the plant, a thermodynamic calculation was carried out, as a result of which it was concluded that when operating in the condensing mode of the CCGT-490 ShchGRES, its electrical efficiency can be increased by 2.5% and brought up to 50.1%.

Heating research

combined-cycle plants have shown that the economic indicators of CCGTs significantly depend on the structure of their thermal scheme, the choice of which is carried out in favor of the plant that provides the minimum temperature of the flue gases. This is explained by the fact that exhaust gases are the main source of energy losses, and in order to increase the efficiency of the circuit, their temperature must be reduced.

The model of a single-loop cogeneration CCGT, shown in fig. 2 includes a drum-type waste heat boiler with natural circulation of the medium in the evaporator circuit. In the course of gases in the boiler from bottom to top, heating surfaces are sequentially located:

superheater PP, evaporator I, economizer E and gas heating water heater GSP.

Rice. 2 - Thermal diagram of a single-circuit CCGT

The calculations of the system showed that when the live steam parameters change, the power generated by the CCGT is redistributed between the thermal and electrical loads. With the growth of steam parameters, the generation of electrical energy increases and the generation of thermal energy decreases. This is explained by the fact that with an increase in the parameters of live steam, its production decreases. At the same time, due to a decrease in steam consumption with a small change in its parameters in the extractions, the thermal load of the network water heater decreases.

A double-circuit CCGT, as well as a single-circuit one, consists of two gas turbines, two waste heat boilers and one steam turbine (Fig. 3). Network water is heated in two PGS heaters and (if necessary) in a peak network heater.

In the course of gases in the waste heat boiler

the following are in sequence

heating surfaces: high pressure steam superheater HDPE, HP high pressure evaporator, HDPE high pressure economizer, HDPE low pressure steam superheater,

low-pressure evaporator IND, low-pressure gas heater GPND, gas supply water heater GSP.

Rice. 3 - Thermal circuit diagram

double-circuit CCGT

Rice. 4 - Scheme of utilization of the heat of the gas turbine exhaust gases

In addition to the waste heat boiler, the thermal scheme includes a steam turbine with three cylinders, two heating water heaters PSG1 and PSG2, a deaerator D and PEN feed pumps. The exhaust steam from the turbine was sent to PSG1. The PSG2 heater is supplied with steam from the turbine extraction. All network water passes through PSG1, then part of the water is sent to PSG2, and the other part after the first stage of heating - to the GSP located at the end of the gas path of the waste heat boiler. The condensate of the heating steam of PSG2 is drained into PSG1, and then enters the HPPG and then to the deaerator. Feed water after the deaerator partly enters the economizer of the high pressure circuit, and partly - into drum B of the low pressure circuit. Steam from the superheater of the low pressure circuit is mixed with the main steam flow after the high pressure cylinder (HPC) of the turbine.

As a comparative analysis showed, when gas is used as the main fuel, the use of utilization schemes is advisable if the ratio of heat and electric energy is 0.5 - 1.0, with ratios of 1.5 or more, preference is given to CCGT according to the "discharge" scheme.

In addition to adjusting the steam turbine cycle to the gas turbine cycle, the utilization of the heat of exhaust gases

The gas turbine can be carried out by supplying steam generated by the waste heat boiler to the combustion chamber of the gas turbine, as well as by implementing a regenerative cycle.

The implementation of the regenerative cycle (Fig. 4) provides a significant increase in the efficiency of the installation, by a factor of 1.33, if the degree of pressure increase is chosen during the creation of the gas turbine in accordance with the planned degree of regeneration. Such a scheme includes a K-compressor; R - regenerator; KS - combustion chamber; TC - compressor turbine; ST - power turbine; CC - centrifugal compressor. If the gas turbine is made without regeneration, and the degree of pressure increase l is close to the optimal value, then equipping such a gas turbine with a regenerator does not lead to an increase in its efficiency.

The efficiency of the installation that supplies steam to the combustion chamber increases by a factor of 1.18 compared to the gas turbine, which makes it possible to reduce the consumption of fuel gas consumed by the gas turbine installation.

Comparative analysis showed that the greatest fuel savings are possible when implementing the GTU regenerative cycle with a high degree of regeneration, a relatively low value of the degree of pressure increase in the compressor l = 3, and with small losses of combustion products. However, in most domestic TKAs, aviation and marine gas turbine engines with a high degree of pressure increase are used as a drive, and in this case, exhaust gas heat recovery is more efficient in the steam turbine unit. Installation with steam supply to the combustion chamber is structurally the most simple, but less efficient.

One of the ways to achieve gas savings and solve environmental problems is the use of steam-gas plants at compressor stations. In research developments, two alternative options for using steam obtained by utilizing the heat of exhaust gases from gas turbines are considered: a combined-cycle plant driven by a steam turbine of a natural gas blower and from a steam turbine of an electric generator. The fundamental difference between these options lies in the fact that in the case of a CCGT with a supercharger, not only the heat of the exhaust gases of the GPU is utilized, but one GPU is replaced by a steam turbine pumping unit, while in a CCGT with an electric generator, the number of GPUs is preserved, and due to the utilized heat, electricity is generated by a special steam turbine. aggregate. The performed analysis showed that CCGT with a natural gas blower drive provided the best technical and economic indicators.

In the case of creating a steam-gas plant with a waste heat boiler on the basis of the CS, the GTU is used to drive the supercharger, and the steam power plant (SPU) is used to generate electricity, while the temperature of the exhaust gases behind the waste heat boiler is 1400C.

In order to increase the efficiency of the use of organic fuel in decentralized heat supply systems, it is possible to reconstruct heating boiler houses with the placement of gas turbine units (GTP) of small capacity in them and the utilization of combustion products in the furnaces of existing boilers. At the same time, the electrical power of the gas turbine depends on the operating modes according to the thermal or electrical load curves, as well as on economic factors.

The effectiveness of the reconstruction of the boiler house can be assessed by comparing two options: 1 - initial (existing boiler house), 2 - alternative, using a gas turbine. The greatest effect was obtained at the electrical power of the gas turbine equal to

maximum load of the consumption area.

Comparative analysis of a gas turbine unit with a CHP generating steam in the amount of 0.144 kg/kg s. , condensing specifications and gas turbines without CHP and with dry heat exchange specifications showed the following: useful

electric power - 1.29, natural gas consumption - 1.27, heat supply - 1.29 (respectively 12650 and 9780 kJ/m3 of natural gas). Thus, the relative increase in GTU power when steam was introduced from the CHP was 29%, and the consumption of additional natural gas was 27%.

According to the data of operational tests, the temperature of the flue gases in hot water boilers is 180 - 2300C, which creates favorable conditions for the utilization of the heat of gases using condensing heat exchangers (TU) . In TU, which

are used for preliminary heating of network water in front of hot water boilers, heat exchange is carried out with the condensation of water vapor contained in the exhaust gases, and the water is heated in the boiler itself already in the “dry” heat exchange mode.

According to the data, along with fuel economy, the use of technical specifications also provides energy savings. This is explained by the fact that when an additional flow of circulating water is introduced into the boiler, in order to maintain the calculated flow through the boiler, it is necessary to transfer part of the return water from the heating network in an amount equal to the recirculation flow from the return pipe to the supply pipe.

When completing power plants from separate power units with a gas turbine drive

generators, there are several options for utilizing the heat of exhaust gases, for example, using a utilizing

heat exchanger (UTO) for heating water, or using a waste heat boiler and

steam turbine generator to increase power generation. An analysis of the plant operation, taking into account heat recovery with the help of UTO, showed a significant increase in the heat utilization factor, in some cases by 2 times or more, and experimental studies of the EM-25/11 power unit with an NK-37 engine made it possible to draw the following conclusion. Depending on the specific conditions, the annual supply of utilized heat can vary from 210 to 480 thousand GJ, and the real savings in gas amounted to 7 to 17 thousand m3.

Literature

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© V.V. Getman - Cand. tech. Sciences, Assoc. cafe automation of technological processes and production FGBOU VPO "KNRTU", 1ega [email protected] yaMech; N. V. Lezhneva - Ph.D. tech. Sciences, Assoc. cafe automation of technological processes and production FGBOU VPO "KNRTU", [email protected]

The flue gas condensing system makes it possible to recover and recover the large amount of thermal energy contained in the moist flue gas from the boiler, which is usually emitted through a chimney to the atmosphere.

The system of heat recovery/flue gas condensation allows to increase by 6 - 35% (depending on the type of fuel burned and plant parameters) heat supply to consumers or reduce natural gas consumption by 6-35%

Main advantages:

• Fuel economy (natural gas) - the same or increased boiler heat load with less fuel combustion
• Emission reduction - CO2, NOx and SOx (when burning coal or liquid fuels)
• Receiving condensate for the boiler feeding system

Principle of operation:

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

In the condenser, the flue gases are cooled with the return water from the heating network. When the flue gas temperature drops, a large amount of water vapor contained in the flue gas condenses. The thermal energy of vapor condensation is used to heat the heating system return.

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

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

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

In the flue gas condensing plant, condensate is also produced, which, depending on the composition of the flue gas, will be further purified before being fed into the boiler system.

Economic effect.

Comparison of thermal power under the conditions:

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

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

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

Savings depending on the temperature of the coolant in the return pipeline:

Waste flue gas heat recovery

The flue gases leaving the working space of the furnaces have a very high temperature and therefore carry away with them a significant amount of heat. In open-hearth furnaces, for example, about 80% of all 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 with them the more heat, the higher their temperature and the lower the heat utilization factor in the furnace. In this regard, it is advisable to ensure the recovery of the heat of flue gases, which can be carried out 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 the 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 method of utilization, the heat of flue gases is used in thermal power boilers and turbine plants, 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 the heat exchangers of the regenerative or recuperative type remains high enough and it is advisable to further utilize the heat in thermal power plants. So, for example, in open-hearth furnaces, the flue gas temperature after regenerators is 750-800 °C, so they are reused in waste heat boilers.

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

First of all, it should 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 a unit of heat obtained in the furnace as a result of fuel combustion (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 physical heat is the greater, the lower the fuel utilization factor and the higher the temperature of the flue gases.

For normal operation of the furnace, the required amount of heat should be supplied to the working space every hour. This amount of heat includes not only the heat of the fuel, but also the heat of the heated air or gas, i.e.

It is clear that with = const the increase will allow to decrease . In other words, waste heat recovery from flue gases allows to achieve fuel savings, which depends on the degree of heat recovery from flue gases.

where - 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, it is possible to determine the fuel economy by the following expression:

where I "d, Id - respectively, the enthalpy of flue gases at the combustion temperature and leaving the furnace.

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

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

An increase in at leads to an increase in the 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 proportion of gas with a high calorific value in the fuel mixture.

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

V.S. Galustov, Doctor of Technical Sciences, Professor, General Director of SE NPO "Polytechnic"
L.A. Rozenberg, engineer, director of UE Yumiran.

Introduction.

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

Flue gases after various types of furnaces can have a significantly higher 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, however, to complete the picture, we will also recall and evaluate other options. All known heat exchangers can be divided into contact, surface, and devices with an intermediate coolant. The first one will be discussed in more detail below. Surface heat exchangers are traditional heaters that are placed directly in the flue after the furnace (boiler) and have serious drawbacks 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 design smoke exhauster, and replacing it with a more powerful one may not compensate for the accompanying costs by saving heat. Secondly, the low coefficients of heat transfer from the gas to the surface of the tubes determine the large values ​​of the required contact surface.

Apparatuses with an intermediate heat carrier are of two types: intermittent operation with a solid heat carrier and continuous operation with a liquid one. The first ones 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 heated medium is supplied to the first (usually air supplied to the same furnace, or air from the air heating system), etc. The disadvantages of such a scheme are obvious (high resistance, bulkiness, temperature instability, etc.), and its application is very limited.

Apparatuses with a liquid intermediate heat carrier (usually water) were called contact heat exchangers with active packing (KTAN), and the authors, after a slight improvement, called them heat exchangers with saturated coolant and condensation (TANTEK). In both cases, the water heated by flue gases then gives off the received heat through the wall of the surface built-in heat exchanger to clean water (for example, heating systems). Compared to heaters, the resistance of such heat exchangers is much lower, and in terms of heat exchange in the flue gases - water system, they are completely similar to the direct-flow spray apparatus of interest to us. However, there are significant differences, which we will discuss below.

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

The main processes in the system flue gases - water.

The result of the interaction of heated flue gases (by composition and properties, this is actually humid 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 heat carrier), is determined by a whole range of processes.

Simultaneously with heating, condensation of moisture on the surface of the droplets or evaporation can occur. In fact, there are three 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 droplet) and in the core of the gas flow (Fig. 1a).

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

All the described processes can be visually represented on Ramzin's 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 most desirable, but in order to understand how to ensure it, it is necessary to recall in addition to what was stated in:

- the amount of water vapor contained in 1 m3 of moist air is called the absolute humidity of the air. Water vapor occupies the entire volume of the mixture, so 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, there comes a moment when condensation begins, i.e. the maximum possible vapor content in the air is reached at a given temperature, 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 humidity f;

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

- moist air as a heat carrier is characterized by enthalpy / (heat content), which is a function of 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 application, the formula for calculating enthalpy can be represented

I \u003d (1000 + 1.97. 103x) t + 2493. . 103x J / kg of dry air, where 1000 is the specific heat capacity of dry air, J / kg * deg); 1.97 * 103 - specific heat capacity of steam, J / (kg * deg); 2493*103 is a constant coefficient approximately equal to the enthalpy of steam at 0°C; t is the air temperature, °С;

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 is the specific heat capacity of dry air, kcal/(kgtrad); 0.47 is the heat capacity of steam, kcal/(kgtrad);

- when the air is cooled (under conditions of constant moisture content), the 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 of the order of 130-150 ° C. It must be borne in mind that this refers to the beginning of the condensation of sulfuric and sulfurous acid vapors (we denote 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 vapors condense on the surface of cold drops (corresponds to the 3rd option in Fig. 1a) until the temperature corresponding to the dew point for the gas is reached, i.e. the boundary of the transition to the second regime (variant 3 in Fig. 1a). Further, as the water is heated and the partial pressure of vapor 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 droplets to 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 the heat of evaporation of the liquid. After that, further heating of the liquid is impossible, and it evaporates at a constant temperature. The temperature reached in this case is called the wet bulb temperature tM (in practice, it is defined as the temperature indicated by a thermometer, the bulb of which is covered with a damp cloth, from which moisture evaporates).

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

The process becomes more complicated if we take into account that the composition of the droplets is polydisperse (due to the mechanisms of liquid decomposition during spraying). Small droplets instantly reach tM and begin to evaporate, changing the gas parameters towards an increase in moisture content;

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 drops 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 (simultaneously occurring heat and mass transfer, changes in the parameters of the media, aerodynamic conditions, polydisperse composition of the droplet flow, etc.).

The description of the model and the results of analysis based on it is given in the monograph, which we recommend to the interested reader. 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 a sufficiently deep heat recovery. Preliminary humidification of gases, as provided for by the TANTEK 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 borne in mind that when utilizing heat, even from gases that do not contain sulfur compounds, they should not be cooled below 80 ° C (their evacuation to the environment through the flue and chimney is difficult).

Let us explain what has been said with a specific example. Let the flue gases after the boiler in the amount of 5000 kg/h, having a temperature of 130°C and a moisture content of 0.05 kg/kg, contact with a heat recovery medium (water, tH= 15°C). From the H-x diagram we find: tM= 49.5°C; tp= 40°C; I \u003d 64 kcal / kg. Model calculations 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 storage medium in the amount of 2.5 m3/h is heated from 15 to 45°C.

If we use TANTEK and preliminarily carry out humidification — adiabatic cooling of gases to t-100°C, and then cool to 80°C at X = const, then the final parameters of the gas will be: tM = 48°C; I = 61.5°C. And although the water will heat up a little higher (up to 48 ° C), the amount of heat utilized decreases by 4 times and will be 0.0175 Gcal/h.

Options for organizing heat recovery.

The solution of a specific problem of flue gas heat utilization depends on a number of factors, including the presence of pollutants (determined by the type of fuel burned and the object of flue gas heating), 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 available 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 yes, then the possibility of using moderately heated water should be assessed (for example, when burning natural gas, send 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 the raw mass, for example, in the production of ceramic products etc.). If the water is too polluted, it is possible to provide a two-circuit system or combine heat recovery with flue gas cleaning (to obtain higher (above 45-5 CPC) temperatures or a surface stage).

There are many options for organizing the process of heat recovery. The economic efficiency of the event depends on the choice of the optimal solution.

Literature:

1. Galustov B.C. Heat and Mass Transfer Processes and Apparatuses with Direct Phase Contact in Thermal Power Engineering // Energy and Management.— 2003.— No. 4.

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

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

4. Planovsky A.N., Ramm V.M., Kagan S.Z. Processes and apparatuses of chemical technology.— M.: Goshimizdat, 1962.—S.736-738.

Usage: energy, waste heat recovery. Essence of the invention: the gas flow is moistened by passing it through a condensate film formed on a dihedral perforated sheet 4, where the gases are saturated with water vapor. In chamber 2 above sheet 4, volumetric condensation of water vapor occurs on dust particles and tiny droplets of the vapor-gas flow. The prepared gas-vapor mixture is cooled to the dew point temperature by transferring heat from the flow of the heated medium through the wall of the heat exchange elements 8. The condensate from the flow falls onto inclined partitions 5 with chutes 10 and then enters the sheet 4 through the drain pipe 9. 1 sludge.

The present invention relates to the field of boiler technology, and more specifically to the field of exhaust gas heat recovery. A known method for utilizing the heat of exhaust gases (USSR ed.St. N 1359556, MKI F 22 B 33/18, 1986), which is the closest analogue, in which the combustion products are sequentially forcibly moistened, compressed in a compressor, cooled to a temperature below the dew point temperature together with condensation of water vapor at a pressure above atmospheric, are separated in the separator, expanded with a simultaneous decrease in temperature in the turbo expander and removed to the atmosphere. A known method of waste gas heat utilization (GDR, Pat. N 156197, MKI F 28 D 3/00, 1982) is achieved by countercurrent movement in the waste gas heat exchanger and an intermediate liquid medium heated to a temperature higher than the dew point temperature of the exhaust gases, which are cooled to a temperature below the dew point. A known method of low-temperature heating using the gross calorific value of the fuel (Germany, application N OS 3151418, MKI F 23 J 11/00, 1983), which consists in the fact that fuel is burned in the heating device with the formation of hot gases that enter the heating device forward and to the side. On part of the flow path, the fuel gases are directed downward with the formation of condensate. Fuel gases at the outlet have a temperature of 40 45 o C. The known method allows the cooling of exhaust gases below the dew point temperature, which slightly increases the thermal efficiency of the installation. However, in this case, condensate is sprayed through the nozzles, which leads to additional power consumption for own needs and increases the content of water vapor in the combustion products. The inclusion of a compressor and a turbo expander in the circuit, which compress and expand the combustion products, respectively, does not improve efficiency, and, in addition, leads to additional power consumption associated with losses in the compressor and turbo expander. The objective of the invention is to intensify heat transfer with deep utilization of the heat of exhaust gases. The problem is solved due to the fact that the humidification of the gas flow is carried out by passing it through the condensate film with saturation of the flow with water vapor, followed by condensation of the latter, as well as condensate falling onto the said film and draining the unevaporated part. The proposed method can be implemented in the device shown in the drawing, where: 1 condensate collector, 2 chamber, 3 housing, 4 dihedral unequal inclined perforated sheet, 5 inclined partitions, 6 - tapering two-dimensional diffuser, 7 expanding diffuser, 8 heat exchange surface, 9 drain pipe, 10 gutter, 11 mating surface, 12 - separator, 13 overheating heat exchanger, 14 smoke exhauster, 15 chimney, 16 water seal, 17 horizontal axis. The operation of the device according to the proposed method for utilizing the heat of combustion products is similar to an atmospheric type heat pipe. Its evaporative part is located in the lower part of the chamber 2, from which the prepared vapor-gas mixture rises, and the condensing part is on the heat exchange surfaces 3, from which the condensate flows down the inclined partitions 5 with gutters 10 through the drain pipes 9 onto the dihedral uneven-sided perforated sheet 4, and the excess - into condensate collector 1. The combustion products coming from the overheating heat exchanger 13 bubble a condensate film on a dihedral unequal inclined perforated sheet 4. The condensate is sprayed, heated and evaporated, and its excess flows into the condensate collector 1. The flue gases are saturated with water vapor at a pressure approximately equal to atmospheric. It depends on the mode of joint operation of the fan and smoke exhauster 14. In chamber 2, water vapor is in a supersaturated state, since the vapor pressure in the gas mixture is greater than the saturated vapor pressure. The smallest droplets, dust-like particles of combustion products become condensation centers, on which the process of volumetric condensation of water vapor takes place in chamber 2 without heat exchange with the environment. The prepared gas-vapor mixture condenses on the heat exchange surfaces 8. When the surface temperature of these heat exchange elements 8 is significantly lower than the dew point temperature, the moisture content of the combustion products after the heat exchanger is lower than the initial one. The final phase of this continuous process is the loss of condensate on the inclined partitions 5 with complaints 10 and its entry onto the perforated sheet 4 through the drain pipe 9. The achievement of the task is confirmed by the following: 1. The value of the heat transfer coefficient increased to 180 250 W / m 2 o C, which sharply reduces the area of ​​the heat exchange surface and, accordingly, reduces the weight and size indicators. 2. A 2.5-3 times decrease in the initial moisture content of water vapor in the exhaust gases reduces the intensity of corrosion processes in the gas path and chimney. 3. Load fluctuation of the steam generator does not affect the efficiency of the boiler plant.

Claim

A method for utilizing the heat of exhaust gases, which consists in the fact that the gas flow is humidified and cooled to the dew point temperature by transferring the heat of the flow to the heated medium through the wall, characterized in that the gas flow is humidified by passing it through a condensate film with saturation of the flow with water vapor, followed by condensation of the latter, as well as the precipitation of condensate on the mentioned film and the runoff of its unevaporated part.