Introduction

Increasing energy efficiency and energy saving are the main indicators of the modernization and growth of the competitiveness of the economy of any state, its full-scale implementation in the modern world market of goods and services. It is necessary to save and save at all stages of energy conversion, from the extraction of energy carriers to the consumption of clean (adapted to energy consumers) energy by engineering equipment of buildings and structures.

One of the main characteristics of the energy efficiency of buildings is considered to be the specific energy consumption for heating and ventilation systems per year.

1. Problem Research

One of the company's energy-efficient measures is the use of heat recovery units in small and medium-sized boiler houses operating on flue gases.

At medium power boilers, the operating efficiency of the boiler unit can be increased to 7.5% and brought to a value of 97-98%. It must be assumed that an increase in the efficiency of any heat engine and a boiler unit in particular can be realized by reducing the heat of the exhaust gases (flue gases for the boiler). To achieve these goals, heat recovery units are used. There are two types of heat exchangers that can be used: these are contact type heat exchangers (direct contact of flue gases with the working fluid) and surface type.

There are several main aspects of using this system. This is, in particular, the environmental component. When using heat recovery, it is possible to reduce the temperature of flue gases from hot water boilers from 120-130 °C to 50-60 °C and lower and switch the heat exchanger to condensing mode.

The problem of increasing the efficiency of heat supply systems by reducing irrational heat losses with exhaust gases is relevant, since, at present, the temperature of exhaust gases in large power and industrial boiler units is 120 - 160 ° C. Accordingly, heat losses with exhaust gases when compiling the heat balance of these installations according to the lower calorific value of fuel fluctuate from 5-7% to 25-60%.

Gasified boiler houses have relatively high technical and economic indicators due to the absence of heat losses during the combustion of natural gas as a result of mechanical incompleteness of combustion, the proximity to zero chemical incompleteness of combustion and a very small loss of heat to the environment. Heat losses with exhaust gases are significant and in boilers without tail surfaces can reach 25%. At a nominal load of gas-oil steam boilers of the DE type, the temperature of the outgoing combustion products behind the economizer when operating on gas is 140 ... 160 ° C, and on fuel oil - 170 ... 190 ° C. For water-heating oil-fired boilers, this temperature is even higher: 140…190 and 180…230°C, respectively. Reducing flue gas temperatures is the main way to increase fuel utilization.

According to the technical literature, one of the components in the heat balance of a heat generating plant is losses with flue gases. Even after steel and cast iron economizers in steam boilers, the flue gas temperature can reach 150-200°C; in hot water boilers, additional cooling is not initially provided. Heat losses with exhaust gases in hot water boilers reach 10%, which is thousands of Gcal of thermal energy emitted into the atmosphere. Let's remember what the heat balance of the boiler consists of:

In widespread industrial, municipal and heating boilers that do not have tail heating surfaces, losses with the physical heat of exhaust gases are 15-25%. In addition, water vapor is emitted with the exhaust gases, the latent heat of vaporization of which is up to 10-15% of the lower calorific value of the fuel. The total heat loss with exhaust gases when compiling the heat balance according to the higher calorific value of the fuel, therefore, is 15-20% in the most advanced boiler units. Usually they are the largest of all heat losses that occur in thermal installations.

There are various methods of heat recovery, including deep heat recovery (using the heat of vaporization) of flue gases in gas boilers and the use of contact heat exchangers for operation in autonomous heating networks as a heat generating device. At the same time, fuel efficiency increases by 5-10%. In this regard, condensing heat exchangers of contact and surface types are becoming more widespread, allowing to cool the flue gases below the dew point and it is additionally useful to use the latent heat of condensation of the water vapor contained in the combustion products. The efficiency of using CT for utilizing the heat of combustion products of natural gas is explained by the increased content of water vapor in them and the high quality of condensate (demineralized water) released from combustion products.

Condensing heat exchangers of surface and contact types make it possible to cool combustion products below the dew point and additionally use the latent heat of condensation of water vapor contained in the combustion products.

2. WAYS AND METHODS FOR SOLVING THE PROBLEM

Reducing thermal energy losses is not only a matter of fuel economy, but also a reduction in harmful emissions into the atmosphere. Reduction of thermal energy losses is possible when using heat exchangers of various designs.

Most technological processes are accompanied by the formation of a large amount of outgoing heat. Examples are the exhaust pipes of boilers or furnaces; evaporation plants; dryers. Excess heat, in most cases, is released into the atmosphere and is not used for useful purposes. One of the ways to save energy is to use the thermal energy generated during various technological processes for the needs of the consumer. Currently, heat recovery is widely used, but the most commonly used methods do not fully use the thermal energy of flue gases.

The thermal energy of flue gases can be divided into two parts. The first is related to the high flue gas temperature. Utilization of this thermal energy occurs by lowering the temperature of the flue gases by heating another (water, air, etc.). In this case, the temperature of the flue gases can be reduced to the dew point temperature of the water vapor contained in the flue gases.

The second is the latent heat of water vapor condensation. Utilization of this heat in this case occurs only during the condensation of water vapor, and the heat released in this case heats the coolant. In conventional terminology, such recycling is called deep.

Deep heat recovery means lowering the temperature of flue gases below the dew point of water vapor with their subsequent condensation. At the same time, a significant part of the latent heat of condensation is utilized, and the condensate after additional processing can be used to replenish water losses in the energy cycle or heating network. Flue gas dehumidification reduces the dew point of residual water vapor and prevents moisture from escaping in the chimney, resulting in lower maintenance costs and longer life.

In practice, heat exchangers cannot condense all water vapor. To estimate the depth of the process, it is convenient to use two coefficients. The flue gas drying coefficient is equal to the ratio of the condensate flow rate to its maximum recoverable value. So, with a coefficient equal to 1, the condensation of water vapor is theoretically the maximum possible, and with a coefficient equal to 0, no condensation occurs. Here, the determining factor is the end temperature of the flue gases, which is equal to the dew point temperature of the residual water vapor. It determines the final moisture content, the final condensate flow rate and the condenser capacity. Coefficient of heat recovery depth, defined as the ratio of the actual power of the heat exchanger to the theoretical (maximum possible). This coefficient is more preferable for comparing heat exchangers, as it takes into account the heat output, which includes the cooling of hot flue gases and heat losses with condensate and residual water vapor.

3. COOLING DEVICES FOR COMBUSTION PRODUCTS

Condensing heat exchangers of surface and contact types make it possible to cool combustion products below the dew point and additionally use the latent heat of condensation of water vapor contained in the combustion products.

The principle of operation of contact heat exchangers is to heat water with hot combustion products in direct contact. Heat and mass transfer between flue gases and water in their direct contact occurs due to the difference in temperature and partial pressures of water vapor. The heating surface in contact devices is the surface of the film, drops and streams of water, through which the heat exchange between gases and water takes place. At the same time, mass transfer between coolants is also carried out.

In contact heat exchangers, in contrast to surface heat exchangers, water heating is possible only up to a wet thermometer temperature t_m, approximately equal to the boiling point of water at a partial vapor pressure in flue gases. At wet bulb temperature, a dynamic equilibrium is reached between gases and water. In this case, all the heat from the combustion products is spent on the evaporation of water and returns to the flow of combustion products in the form of a vapor-gas mixture. Such a process is called adiabatic evaporation (without supply and removal of heat from the outside), and the temperature t_m is the water temperature during adiabatic evaporation. Thus, after the water has established and reached the temperature of the wet bulb, the flue gases are cooled only due to the evaporation of water at a constant value of this temperature.

Cooling in the contact heat exchanger of flue gases proceeds with a variable moisture content of the latter, since moisture exchange occurs between water and gases. It is known that the temperature at which saturation and precipitation (condensation) begins in the form of dew of water vapor contained in gases is called the dew point t_р.

A feature of the processes of deep cooling of combustion products is a change in their quantity due to the condensation of part of the water vapor.

Currently, deep flue gas heat recovery is implemented by using a heat exchanger in the flue section up to the chimney, which transfers the flue gas heat to the heated coolant. The medium to be heated is usually cold water used to feed the network circuit.

Despite the need to reduce the temperature of the exhaust gases, as well as the appropriate use of the extracted heat, deep cooling has its drawbacks:

1. The low flue gas temperature leads to intense corrosion. Relatively recently, technologies for the installation of gas ducts and chimneys made of stainless steel have appeared, and when the flue gases are cooled, condensate forms, which naturally causes corrosion of steel chimneys. In Udmurtia today, several companies offer heat-insulated prefabricated gas ducts made of corrosion-resistant materials.

2. Deep cooling produces sulfurous acid. With local cooling of flue gases to the dew point (60–80 °C, depending on the type of fuel [2]), condensate precipitates and, combining with combustion products of sulfur-containing fuels (coal, fuel oil), forms sulfurous acid. However, most of the boiler houses in the small-scale power industry of Udmurtia, for obvious reasons, burn exclusively natural gas, therefore, when the flue gases are cooled to the dew point, the condensate is free clean distilled water suitable for feeding the network or household needs. It's no secret that water costs are one of the cost items in the tariff structure.

3. With deep flue gas cooling, the temperature difference is too low to heat the water. In this regard, the question arises: "What to heat, cooling the smoke?" Indeed, even theoretically, cold water can only be heated up to 50 °C due to the cooling of flue gases, the residual temperature of the combustion products in this case will be about 70 °C. At first glance, in the absence of a centralized hot water supply and a significant consumption of source water for feeding networks, it is impossible to utilize the low-potential energy of flue gases. But this is only at first glance.

4. Cooler gases have poorer dispersion. As practice shows, the height of chimneys from the conditions of dispersion of emissions during the combustion of natural gas rarely exceeds 15-20 m even within the city, their draft is much less than the pressure of the burner fan or smoke exhauster.

Conclusions

When using heat recovery units, it was found that the flue gases are cooled to a temperature of 35-45°C. The drying coefficient does not exceed 0.7. The utilization depth coefficient also does not reach high values. This is due to the use of water as a heated coolant. In this case, the water temperature should not be higher than 40 °C.

Such a scheme is feasible in some foreign countries that use low-potential heating systems. In most cases, deep heat recovery is used to heat make-up water from 10 to 40°C.

It is possible to increase the disposal depth by using another heating medium that has negative operating temperatures, such as cold air. In winter, most territories experience negative temperatures. The use of air as a heated coolant makes it possible to reduce the final temperature of the flue gases to the limit values and bring the utilization depth coefficient closer to 1.

The air heated by flue gases can be used to supply it to the furnace of a boiler unit and provide a more efficient fuel combustion process or to heat rooms. Thus, the deep utilization of the thermal energy of exhaust gases solves the complex problem of saving natural resources and protecting the environment from pollution.

List of sources

1. Поздняков С. Р. Глубокая утилизация тепла топочных газов // В сборнике: Международная научнотехническая конференция молодых ученых.
2. Прохоров В. Б., Денищук Д. А. Влияние системы глубокой утилизации тепла дымовых газов с увлажнением первичного воздуха на работу мусоросжигательного котла // Новое в российской электроэнергетике. – 2020. – № 9.
3. Кудинов, А.А. Энергосбережение в теплоэнергетике и теплотехнологиях. – М.: Машиностроение, 2011. – 373 с.