I suggest recycling activities for consideration flue gases. Flue gases are available in abundance in any town or city. The main part of smoke producers are steam and hot water boilers and engines internal combustion. I will not consider the flue gases of engines in this idea (although they are also suitable in composition), but I will dwell on the flue gases of boiler houses in more detail.

The easiest way is to use smoke from gas boiler houses (industrial or private houses), this is the most clean look flue gas, in which it is located minimal amount harmful impurities. You can also use smoke from boiler houses burning coal or liquid fuel, but in this case you will have to clean the flue gases from impurities (this is not so difficult, but still additional costs).

The main components of flue gas are nitrogen, carbon dioxide and water vapor. Water vapor is of no value and can be easily removed from the flue gas by contacting the gas with a cool surface. The remaining components already have a price.

Nitrogen gas is used in fire fighting, for the transportation and storage of flammable and explosive media, as a protective gas to protect easily oxidized substances and materials from oxidation, to prevent corrosion of tanks, for purging pipelines and containers, to create inert environments in grain silos. Nitrogen protection prevents the growth of bacteria and is used to clean environments from insects and microbes. In the food industry, a nitrogen atmosphere is often used as a means of increasing the shelf life of perishable products. Nitrogen gas is widely used to produce liquid nitrogen from it.

To obtain nitrogen, it is enough to separate water vapor and carbon dioxide from the flue gas. As for the next component of smoke - carbon dioxide(CO2, carbon dioxide, carbon dioxide), then the range of its applications is even greater and its price is much higher.

I suggest getting more complete information about him. Typically, carbon dioxide is stored in 40-liter cylinders painted black with the word “carbon dioxide” written in yellow. More correct name CO2, “carbon dioxide”, but everyone has already become accustomed to the name “carbon dioxide”, it is assigned to CO2 and therefore the inscription “carbon dioxide” on the cylinders is still preserved. Carbon dioxide is found in cylinders in liquid form. Carbon dioxide is odorless, non-toxic, non-flammable and non-explosive. It is a substance naturally formed in the human body. The air exhaled by a person usually contains 4.5%. Carbon dioxide is mainly used in the carbonation and sale of bottling drinks, and is used as shielding gas when carrying out welding work using semiautomatic welding machines, it is used to increase the yield (2 times) of agricultural crops in greenhouses by increasing the concentration of CO2 in the air and increasing (by 4-6 times when water is saturated with carbon dioxide) the production of microalgae when artificial cultivation, for preserving and improving the quality of feed and products, for the production of dry ice and its use in cryoblasting installations (cleaning surfaces of contamination) and for obtaining low temperatures during storage and transportation of food products, etc.

Carbon dioxide is a commodity in demand everywhere and the need for it is constantly increasing. In home and small businesses, carbon dioxide can be obtained by extracting it from flue gas in low-capacity carbon dioxide plants. It is easy for people involved in technology to make such an installation themselves. If the technological process standards are observed, the quality of the resulting carbon dioxide meets all the requirements of GOST 8050-85.
Carbon dioxide can be obtained both from the flue gases of boiler houses (or heating boilers of private households) and by special combustion of fuel in the installation itself.

Now the economic side of the matter. The installation can operate on any type of fuel. When burning fuel (especially to produce carbon dioxide), the following amount of CO2 is released:
natural gas (methane) – 1.9 kg CO2 from combustion of 1 cubic meter. m of gas;
hard coal, different deposits – 2.1-2.7 kg CO2 from burning 1 kg of fuel;
propane, butane, diesel fuel, fuel oil - 3.0 kg of CO2 from burning 1 kg of fuel.

It will not be possible to completely extract all the carbon dioxide released, but up to 90% (95% extraction can be achieved) is quite possible. The standard filling of a 40-liter cylinder is 24-25 kg, so you can independently calculate the specific fuel consumption to obtain one cylinder of carbon dioxide.

It is not that big, for example, in the case of obtaining carbon dioxide from combustion natural gas It is enough to burn 15 m3 of gas.

At the highest rate (Moscow) it is 60 rubles. for 40 liters. carbon dioxide cylinder. In the case of extracting CO2 from the flue gases of boiler houses, the cost of producing carbon dioxide is reduced, since fuel costs are reduced and the profit from the installation increases. The installation can operate around the clock, in automatic mode, with minimal human involvement in the process of producing carbon dioxide. The productivity of the installation depends on the amount of CO2 contained in the flue gas, the design of the installation and can reach 25 carbon dioxide cylinders per day or more.

The price of 1 cylinder of carbon dioxide in most regions of Russia exceeds 500 rubles (December 2008). Monthly revenue from the sale of carbon dioxide in this case reaches: 500 rubles/ball. x 25 points/day. x 30 days. = 375,000 rub. The heat released during combustion can be used simultaneously for space heating, and in this case there will be no wasteful use of fuel. It should be borne in mind that the environmental situation at the site where carbon dioxide is extracted from flue gases is only improving, as CO2 emissions into the atmosphere are decreasing.

The method of extracting carbon dioxide from flue gases obtained from combustion also works well. wood waste(waste from logging and wood processing, carpentry shops, etc.). In this case, the same carbon dioxide installation is supplemented with a wood gas generator (factory or self-made) to produce wood generator gas. Wood waste (logs, wood chips, shavings, sawdust, etc.) is poured into the gas generator hopper 1-2 times a day; otherwise, the installation operates in the same mode as in the above.
The yield of carbon dioxide from 1 ton of wood waste is 66 cylinders. Revenue from one ton of waste is (at a carbon dioxide cylinder price of 500 rubles): 500 rubles/ball. x 66 points = 33,000 rub.

With the average amount of wood waste from one wood processing shop being 0.5 tons of waste per day, revenue from the sale of carbon dioxide can reach 500 thousand rubles. per month, and in the case of importing waste from other wood processing and carpentry shops, the revenue becomes even greater.

It is possible to obtain carbon dioxide from combustion car tires, which is also only beneficial for our environment.

In the case of producing carbon dioxide in quantities greater than the local market can consume, the produced carbon dioxide can be independently used for other activities, as well as processed into other chemicals and reagents (for example, simple technology into environmentally friendly carbon-containing fertilizers, dough leavening agents, etc.) up to the production of motor gasoline from carbon dioxide.

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 savings fuel.

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.

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. close to temperature dew point 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 a significantly higher temperature (up to 300-500°C and higher). 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 contribute significantly aerodynamic drag 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 system air air heating) 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 transfers the resulting heat through the wall of a surface built-in heat exchanger to clean water (for example, a heating system). 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, condensation of moisture 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 is achieved 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 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 represented

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 the air cools (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 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 flue gas purification 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.

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

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

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

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

Russia – 3 rubles/m3.

Germany - 25 rubles/m3.

Denmark - 42 rubles/m3.

Ukraine, Belarus – 10 rubles/m3.

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

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

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

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

Rice. 1 – Dependence of enthalpy change for water

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

(1)

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

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

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

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

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

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

Rice. 2 – Heat release during condensation of water vapor

It should be borne in mind that water vapor in flue gases has slightly different properties than pure water vapor. They are in a mixture with other gases and their parameters correspond to the parameters of the mixture. Therefore, the temperature at which condensation begins differs from 100 °C. The value of this temperature depends on the composition of the flue gases, which, in turn, is a consequence of the type and composition of the fuel, as well as the excess air coefficient.
The temperature of the flue gases at which condensation of water vapor in the products of fuel combustion begins is called the dew point and looks like Fig. 3.

Rice. 3 – Dew point for methane

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

Figure 4 – Heat release from the steam-air mixture

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

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

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

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

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

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

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

Rice. 8 – drawing of the heater tube.

coefficient

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

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

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

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

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

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

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

From Fig. 3 it is clear that with a very high-quality burner, it is possible to achieve a dew point temperature in the exhaust gases T dew = 60 0 C. In this case, the temperature of these gases is 130 0 C. The absolute moisture content in the gas (according to Table 2) at T dew = 60 0 C will be 129,70 g/m 3 . If water is sprayed into this gas, its temperature will drop sharply, its density will increase, and its enthalpy will rise sharply. It should be noted that spray water above relative humidity 100% does not make sense, because... When the relative humidity threshold exceeds 100%, the sprayed water will stop evaporating into gas. Let us carry out a small calculation of the required amount of sprayed water for the following conditions: Tg – initial gas temperature equal to 120 0 C, T rise - gas dew point 60 0 C (129.70 g/m 3), required IT: Tgk - the final temperature of the gas and Mv - the mass of water sprayed in the gas (kg.)

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

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

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

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

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

As a result of substitution we get the function

(5)

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

E voz = Eg + Evod

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

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

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

E voz =17.55 (J/l*K)

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

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

(6)

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

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

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

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

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

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

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

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

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

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

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

CONCLUSIONS:

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

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

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

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

LITERATURE:

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

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

Description:

Bryansk heating network together with the design institute LLC VKTIstroydormash-Proekt, we developed, manufactured and implemented in two boiler houses in Bryansk installations for heat recovery of flue gases (UUTG) coming from hot water boilers

Flue gas heat recovery plant

N. F. Sviridov, R. N. Sviridov, Bryansk thermal networks,

I. N. Ivukov, B. L. Turk, LLC "VKTIstroydormash-Project"

Bryansk Heat Networks, together with the design institute VKTIstroydormash-Proekt LLC, developed, manufactured and implemented installations for flue gas heat recovery (UHTG) coming from hot water boilers in two boiler houses in Bryansk.

As a result of this implementation, the following was obtained:

Additional capital investments per 1 Gcal/h of heat received is more than 2 times lower compared to if a new boiler house was being built, and pays off in approximately 0.6 years;

Due to the fact that the equipment used is extremely easy to maintain and uses a free coolant, i.e. flue gas (FG) previously emitted into the atmosphere, the cost of 1 Gcal of heat is 8–10 times lower heat cost, produced by boiler houses;

Coefficient useful action boilers increased by 10%.

Thus, all costs in March 2002 prices for the implementation of the first UTG with a capacity of 1 Gcal of heat per hour amounted to 830 thousand rubles, and the expected savings per year will be 1.5 million rubles.

Such high technical and economic indicators are understandable.

There is an opinion that the efficiency of the best domestic boilers with a thermal power of 0.5 MW and above reaches 93%. In reality, it does not exceed 83% and here's why.

There are lower and higher heating values ​​of fuel combustion. The lowest calorific value is less than the highest by the amount of heat that is expended on evaporation of water formed during the combustion of fuel, as well as the moisture contained in it. An example for the cheapest fuel - natural gas: DGs formed during its combustion contain water vapor, occupying up to 19% of their volume; the higher heat of combustion exceeds the lower heat by approximately 10%.

To improve the performance of chimneys through which diesel generators are emitted into the atmosphere, it is necessary that the water vapor present in the diesel generator does not begin to condense in the chimneys at the most low temperatures environment.

UUTG projects have revived and improved long-forgotten technical solutions aimed at recycling heat from diesel generators.

UUTG contains contact and plate heat exchanger and with two independent circuits of circulating and waste water.

The design and operation of the UTG are clear from the diagram shown in the figure and the description of its positions.

In a contact heat exchanger, DG and sprayed circulating water move in a vertical countercurrent, i.e. DG and water are in direct contact with each other. To maintain uniform spraying of circulating water, nozzles and a special ceramic nozzle are used.

The heated circulating water, pumped in its water circuit by an independent pump, transfers the heat acquired in the contact heat exchanger to the supply water in the plate heat exchanger.

For the required cooling of circulating water, only cold water should be used. tap water, which, after heating in the UTG, is brought to the required temperature in the boilers of existing boiler houses and is then used for hot water supply to housing.

In the contact heat exchanger, the cooled diesel generators additionally pass through a droplet eliminator and, as a result, lose more than 70% of the moisture in the form of condensate water vapor, are connected to a part of hot diesel generators (10–20% of the volume of diesel generators leaving the boiler), directed directly from the boiler into the chimney, forming a mixture of diesel generators with low moisture content and with a temperature sufficient for the passage of the chimney without condensation of the remaining water vapor .

The volume of circulating water continuously increases due to the condensate of water vapor present in the diesel generator. The resulting excess is automatically drained through a valve with an electromechanical drive and can be used as preparation. extra water in the boiler room heating system. The specific consumption of drained water per 1 Gcal of recovered heat is about 1.2 tons. The drainage of condensate is controlled by level meters B and H.

The described method and equipment for heat recovery of diesel generators are capable of working with dust-free fuel combustion products that have an unlimited maximum temperature. Moreover, the higher the temperature of the flue gas, the more high temperature The supply water will heat up. Moreover, in this case it is possible to partially use recycled water for heating heating water. Considering that the contact heat exchanger simultaneously works as a wet dust catcher, it is possible to practically utilize the heat of dusty diesel generators by purifying the circulating water by known methods from dust before feeding it into the plate heat exchanger. It is possible to neutralize circulating water contaminated with chemical compounds. Therefore, the described UTG can be used to work with DGs that participated in technological processes during melting (for example, open-hearth, glass furnaces), during calcination (for example, bricks, ceramics), during heating (ingots before rolling), etc.

Unfortunately, in Russia there are no incentives to encourage energy conservation.