RU2411411C1 - Fuel combustion method - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: some portion of cooled flue gases is added to blast air, formed gas-air mixture is supplied by fan to gas-air mixture heater in which it is heated owing to heat of flue gases, and together with fuel it is supplied to combustion, and some part of cooled flue gases is supplied to be recirculated in order to add it to again coming cold air. At that, after gas-air mixture heater the condensate obtained in process heat exchanger is injected to flue gas flow; formed steam-gas mixture is condensed in process heat exchanger; control of temperature and saturation of steam-gas mixture supplied to process heat exchanger installed after gas-air mixture heater is performed by changing the quantity of injected condensate, and excess condensate formed due to condensation of water vapours of combustion products is extracted from closed condensate circuit and supplied to fuel heater to heat the natural gas.

EFFECT: effective heating of the main heat carrier by means of intermediate heat carrier represented by steam-gas mixture.

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Description

The method is intended for efficient heating of the main coolant using an intermediate coolant, which is a gas-vapor mixture. The invention relates to methods for the deep utilization of the heat of the exhaust products of combustion and can be used in stand-alone heat generators and in flue gas recovery systems.

There is a method of utilizing the heat of the exhaust gases by saturating the gas stream with water vapor, by passing it through a condensate film, and further condensing in a heat exchanger (RU 2069811 C1, 11.27.1996, F22B 1/18). Condensation of water vapor formed vapor-gas mixture can significantly reduce the temperature of the exhaust gases into the atmosphere.

The disadvantages of this method is the combustion process at a high temperature (1500-2000 ° C), which is accompanied by the formation of harmful impurities (MO _x , CO, etc.) that are released into the atmosphere along with the main part of the flue gases, as well as the lack of heating of the fuel ( gas) and blast air, which negatively affects the efficiency of the combustion process.

Closest to the invention is a method of burning fuel, according to which, after the passage of flue gases through a process heat exchanger and two radially spiral heat exchangers that heat the air-gas mixture and natural gas, part of these flue gases is fed to the formation of the specified gas-air mixture (RU 2347977 C1, 27.02 .2009, F23C 9/00). The specified method is adopted as a prototype.

The following disadvantages of this method were identified:

- the presence of restrictions on the minimum temperature of the heated medium (at least 180 ° C for liquid) at the entrance to the process heat exchanger;

- the need to clean the resulting condensate before its useful use;

- the need for a condensate consumer in the immediate vicinity;

- the need to use two expensive heat exchangers of radial-spiral type;

- the need to use a highly efficient or large-sized process heat exchanger (for a heated medium), which will be able to cool the flow of dry flue gases from 1100 ° C to 200 ° C.

The technical result of the invention is to increase the efficiency of heat transfer through the use of a gas-vapor mixture as an intermediate coolant, the beneficial use of the resulting condensate without purification, and also to reduce the total cost of implementing the proposed method.

The technical result is achieved by a method of burning fuel, namely, part of the cooled flue gas is mixed with the blast air, the resulting gas-air mixture is pumped with a fan into the gas-air mixture heater, in which it is heated by the heat of the flue gases and, together with the fuel, is supplied for combustion, and some of the chilled flue gas is sent to recirculation to mix with the newly incoming cold air.

In addition, after the heater of the gas-air mixture, the condensate obtained in the process heat exchanger is injected into the flue gas stream, and the resulting vapor-gas mixture is condensed in the process heat exchanger.

In addition, the temperature and saturation of the vapor-gas mixture supplied to the process heat exchanger installed after the gas-air mixture heater are controlled by changing the amount of injected condensate.

In addition, excess condensate resulting from the condensation of water vapor from the combustion products is taken from a closed condensate circuit and fed to a fuel heater for heating natural gas.

The injection of condensate from the process heat exchanger into the flue gas stream leads to the formation of a gas-vapor mixture, the saturation and temperature of which is controlled by a change in the amount of injected condensate. Evaporation of the injected condensate is accompanied by intensive absorption of the heat of the combustion products and a decrease in the temperature of the resulting mixture. Thus, the thickness of the insulating layer of the pipeline section with the gas-vapor mixture can be less than when using flue gases, without increasing heat loss to the environment.

The condensation of water vapor in a gas-vapor mixture in a process heat exchanger is accompanied by intense heat generation. Despite the lower temperature compared to flue gases, the heat content of the vapor-gas mixture flow is greater. In addition, to condense the vapor-gas mixture, a smaller heat transfer surface, i.e. a more compact heat exchanger, is required.

The use of a gas-vapor mixture as an intermediate heat carrier is especially effective for heating low-temperature heat-intensive media, for example, cold water. On the other hand, there is an upper limit to the range of permissible temperatures of the heated medium. So the proposed method is not applicable for heating the medium with an initial temperature of more than 100 ° C, since in this case the condensation of water vapor vapor-gas mixture will be difficult.

Recirculation of part of the cooled combustion products into the stream of blast air allows to reduce the adiabatic temperature of fuel combustion, which is inversely proportional to the fraction of flue gases supplied to the combustion of the air-gas mixture. A decrease in the adiabatic combustion temperature leads to a reduction in the amount of harmful substances formed (NO _x , CO) and a decrease in the aggressiveness of flue gases and gas-vapor mixture with respect to the contact surfaces of pipelines and heat exchangers.

Catalytic or flameless burners should be used to burn fuel in highly diluted flue gas air, since flare burners cannot provide a stable combustion process.

The gas-air mixture can be heated in the gas-air mixture heater, through which flue gases are passed to recover heat. On the one hand, the component of the fuel mixture is heated, which contributes to the efficient combustion of fuel, and on the other hand, the combustion products are pre-cooled before condensate injection.

Condensate is injected after the gas-air mixture heater, since the gas-vapor mixture is somewhat more aggressive to contact surfaces than dry combustion products, and the heat exchange rates with flue gases and gas-vapor mixture without condensation are comparable.

Deep cooling of the combustion products leads to an increase in the amount of condensate introduced into the closed loop after each evaporation / condensation cycle per mass of water vapor generated during combustion. Thus, the resulting excess condensate in the circuit can be used to heat the fuel.

Figure 1 presents a schematic flow chart of fuel combustion using a gas-vapor mixture as an intermediate coolant and deep heat recovery of the exhaust combustion products.

Figure 2 shows the temperature dependence of the resulting vapor-gas mixture on the amount of condensate injected into the flue gas stream.

Figure 3 shows the dependence of the ratio of the required heat exchange surface areas with condensate injection and without injection from the amount of condensate injected into the flue gas stream.

The proposed method of using a gas-vapor mixture as an intermediate coolant is as follows.

The basic technological scheme of the installation with recirculation of part of the exhaust flue gases, heating of the fuel and air-gas mixture and deep heat recovery through the use of gas-vapor mixture obtained by injection of condensate into the flue gas stream (Fig. 1) is presented.

Natural gas and air-gas mixture (air with a part of the exhaust flue gases added to it) by the fan 5 are pumped into the burner 1, where the fuel is burned. The resulting combustion products pass through the gas-air mixture heater 2. Then, condensate is injected into the gas-air duct from the process heat exchanger 3. The pressure necessary for efficient atomization of the condensate is created by the pump 6. Condensate injection leads to active vaporization and the formation of a gas-vapor mixture (ASG). The temperature of the ASG is controlled by the amount of condensate injected. The resulting vapor-gas mixture enters the process heat exchanger 3, where it completely condenses. Condensation of the ASG results in the formation of condensate, which is pumped to the injection pump 6, and dry flue gases with a temperature of less than 100 ° C, which are vented to the atmosphere. Excess condensate is taken from the condensate circuit and sent to the fuel heater 4.

The dependence of the temperature of the obtained vapor-gas mixture on the amount of condensate injected into the flue gas stream at a ratio of "flue gases - natural gas" equal to 11: 1 is presented. As follows from the diagram, the larger the amount of injected condensate, the lower the temperature of the resulting vapor-gas mixture.

The dependence of the ratio of the required heat exchange surface areas with and without injection of condensate on the amount of condensate injected into the flue gas stream with a ratio of "flue gases - natural gas" equal to 11: 1 is presented. As follows from the diagram, under the indicated conditions (adiabatic combustion temperature 1200 ° С, injected condensate temperature 90 ° С, heated water is heated from 10 ° to 80 ° С), the maximum increase in heat transfer efficiency occurs when 5 kg of condensate is injected per 1 nm ^{3 of} natural gas . Injection of condensate in an amount of more than 17 kg per 1 nm ^{3 of} natural gas leads to a decrease in heat transfer efficiency.

The calculation results illustrated in the diagrams (figure 2 and 3) show the following:

a) by condensate injection, it is possible to achieve a significant decrease in the temperature of the heat carrier (gas-vapor mixture) without significant losses in the thermal efficiency of the process;

b) with a ratio of "flue gases - natural gas" equal to 11: 1 (adiabatic combustion temperature of 1200 ° C) and injection of 5 kg of condensate per 1 nm ^{3 of} natural gas (temperature of a gas-vapor mixture of 667.666 ° C), a reduction in the required surface area can be achieved technological heat exchanger, compared with the mode without injection, by 11.4%.

Claims (1)

The method of burning fuel, which consists in the fact that part of the cooled flue gas is mixed with the blast air, the resulting air-gas mixture is pumped by a fan into the gas-air mixture heater, in which it is heated by the heat of the flue gas, and, together with the fuel, it is supplied for combustion, and part of the cooled flue gas gases are sent to recirculation to mix with the newly incoming cold air, characterized in that after the gas-air mixture heater, condensate is injected into the flue gas stream obtained in the process heat exchanger, the resulting gas-vapor mixture is condensed in the process heat exchanger, the temperature and saturation of the gas-vapor mixture supplied to the process heat exchanger installed after the gas-air mixture heater are controlled by changing the amount of injected condensate, and the excess condensate generated due to condensation of the water vapor of the combustion products , selected from a closed loop of condensate and served in a fuel heater for heating natural gas.

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