BG4278U1 - Installation for reducing co2 emissions during lignite combustion - Google Patents

Abstract

The utility model relates to an installation for CO2 emission reducing during lignite combustion, which will find application in the energy sector and in particular in thermal coal-fired power plants. The installation for CO2 emission reducing during lignite combustion comprises a boiler (1) to which a steam turbine (2) with an electric generator 3 is mounted in an energy mono-unit. A gas turbine (17) with an electric generator (19) is added to the energy mono-unit, as the flue gases leaving it are fed to the boiler (1). The gas turbine (17) is connected to the furnace chamber (4) of the boiler (1) via a flue gas outlet (18) of the gas turbine (17). The steam turbine (2) is connected via a steam extraction (15) to high pressure regenerative heaters (16), which have a connection to evaporation screens (5) in the furnace chamber (4) of the boiler (1). A shut-off valve (21) is fitted between the steam extraction (15) of the steam turbine (2) and the regenerative high pressure heaters (16).

Description

Field of technique

The utility model refers to an installation for reducing CO ₂ emissions during the combustion of lignite, which will find application in the energy sector and in particular in thermal electric coal-fired plants.

Prior art

In the current practice, conventional solid fuel thermal power plants use installations for obtaining electricity by burning lignite coal, which mostly include a coal-fired boiler, to which a steam turbine with an electric generator is installed in an energy monoblock. The boiler has a furnace chamber in which there are evaporation screens and a combustion system with powder burners. A horizontal gas duct is connected to the upper part of the furnace chamber, which has a connection to at least one convection shaft. The convective shaft is connected to an external air heater through a chimney for flue gases. One end of the vented air heater is connected to at least one air blower and the other end is connected to a duct for hot organized air. The air duct for hot organized air has a connection both to openings for organized air of the powder burners from the combustion system, and to air nozzles for overcombustion air. The air nozzles for super-combustion air are located in the upper part of the furnace chamber above the combustion device before the horizontal flue. The steam turbine is connected to the high-pressure regenerative heaters, which are connected to the evaporative screens in the furnace chamber, through a steam extractor.

Well-known thermal power plants for obtaining electricity must meet modern environmental requirements, which are becoming more and more strict. Along with these, there has also been an increase in the price of the carbon emission allowances that any conventional plant must purchase in order to operate. This increases the cost of the electricity they produce. One of the ways to reduce harmful substances released into the atmosphere, generated by coal-burning plants, is to implement a more efficient power generation cycle.

Technical nature of the utility model

The task of the present utility model is to improve the existing installations for obtaining electricity in conventional lignite-fired thermal power plants with minimal modifications, without disturbing the operability of the existing unit, while preserving its ability to operate independently.

The task of the present utility model has been solved by creating an installation for reducing CO ₂ emissions when burning lignite, including a boiler to which a steam turbine with an electric generator is installed in an energy monoblock. The boiler has a furnace chamber in which there are evaporation screens and a combustion system with powder burners. A horizontal gas duct is connected to the upper part of the furnace chamber, which has a connection to at least one convective shaft, which is connected to an external air heater through a flue for outgoing flue gases. One end of the vented air heater is connected to at least one air blower and the other end is connected to a duct for hot organized air. The air duct for hot organized air has a connection both to openings for organized air of the powder burners from the combustion system, and to air nozzles for overcombustion air. The air nozzles for super-combustion air are located in the upper private furnace chamber above the combustion system before the horizontal flue. The steam turbine is connected to high-pressure regenerative heaters through a steam manifold. Regenerative high-pressure preheaters are connected to the evaporative screens in the furnace chamber.

According to the present utility model, a gas turbine with an additional electric generator is connected to the furnace chamber of the boiler via a flue gas outlet from the gas turbine. A shut-off valve is installed between the steam intake of the steam turbine and the high-pressure regenerative heaters.

In preferred embodiments of the installation for reducing CO ₂ emissions during combustion of lignite, according to the utility model, the flue gas outlet of the gas turbine is connected to the furnace chamber of the boiler in one of the following ways or in combinations thereof, as follows : by means of the hot organized air duct to the organized air openings of the combustion system and/or by means of the hot organized air duct to the overhead air nozzles and/or directly to the nozzles installed under the combustion device in the furnace chamber.

The addition of a gas turbine to the power unit operating by burning lignite increases the efficiency of the unit, which is a consequence of the higher efficiency of the gas turbine module on the one hand, and on the other hand, the subsequent utilization of the flue gases leaving the high temperature gas turbine and their feed to the boiler.

Explanation of the attached figures

The installation for reducing CO ₂ emissions during the combustion of lignite, according to the present utility model, is disclosed in the attached figures, where:

Figure 1 presents a general view of a combustion plant with a solid fuel boiler;

Figure 2 presents a general view of the thermal scheme of the combined heat and power plant for obtaining electricity with a gas turbine;

Figure 3 graphically presents part of the results obtained from the simulation studies, where the change in the flow rate of organized air, supplied for combustion to the boiler's combustion systems after the air heater removed, is visualized as a function of the electrical energy produced by the energy unit (steam turbine + gas turbine) and at different power of the gas turbine connected to it (20 MW; 37.5 MW; 75 MW);

Fig. 4 shows the change in the temperature of the outgoing flue gases after the removed air heater, as a function of the electrical energy produced by the unit;

Figure 5 shows the variation of the specific consumption of carbon dioxide (CO ₂ ) as a function of the electrical energy produced;

Figure 6 presents the variation of the specific consumption of sulfur dioxide as a function of the electrical energy produced.

Example implementation of the utility model

The attached figure 1 shows an exemplary embodiment, according to the present utility model, in which the installation for reducing CO ₂ emissions during the combustion of lignite has a boiler 1 with a furnace chamber 4. The walls of the furnace chamber 4 are shielded with heating surfaces in the form of evaporation screens 5 located along its height. The furnace chamber also houses a combustion device 6 with powder burners 7. The furnace chamber 4 has the shape of a four-sided prism, the lower part of which ends with a narrowing in the form of a quadrangular truncated pyramid, called a cool cone 5.1 . The upper part of the furnace chamber 4 is connected to a horizontal gas duct 8. The horizontal gas duct 8 has a connection to at least one convective shaft 9, which is connected to an external air heater 10 through a chimney for outgoing flue gases. One end of the external air heater 10 has a connection to at least one air fan 11, and the other end is connected to an air duct for hot organized air 12. Air required for the combustion process is sucked in by the air fans 11 and directed through the external air heater 10, where it is heated by the flue gases leaving the convection shaft 9 , to the air duct for hot organized air 12. Then the air through the air duct for hot organized air 12 is supplied to openings for organized air 13 of the powder burners 7. Part of the organized air is taken from the powder burners 7 and supplied to air nozzles for overburning air 14, located in the upper part of the furnace chamber 4 above the combustion device 6 before ho the horizontal gas pipe 8. In this way, a combustion process is organized with a lack of air in the area of the powder burners 7, which leads to the incomplete combustion of the carbon in the fuel and to the production of large amounts of carbon monoxide. This, in turn, leads to a decrease in the temperatures in the upper part of the furnace chamber 4. In the section between the combustion device 6 and the air nozzles for overburning air 14, a zone is obtained that is poor in oxygen and in which the reduction of nitrogen oxide takes place , formed in the area of the powder burners 7. Through the air supplied through the air nozzles for overburning air 14, the combustion of carbon monoxide to carbon dioxide takes place.

A steam turbine 2 with an electric generator 3 is installed to the boiler 1 in an energy monoblock. The steam turbine 2 is connected through a steam manifold 15 to regenerative high-pressure heaters 16, which are connected to the evaporation screens 5 in the furnace chamber 4. In order to increase the efficiency of the installations for obtaining electricity in conventional lignite-fired thermal power plants, a gas turbine 17 with an electric generator 19 is added to the existing power unit. The flue gases leaving the gas turbine 17 are fed to the boiler 1 (Fig. 2). The gas turbine 17 is connected to the furnace chamber 4 of the boiler 1 by means of a flue gas outlet 18. In this case, a shut-off valve 21 is installed between the steam outlet 15 of the steam turbine 2 and the high-pressure regenerative heaters 16. The gas turbine 17 is designed to burn natural gas , as this fuel emits the least carbon emissions of all fossil fuels.

The place for the supply of the flue gases from the gas turbine 17 depends on the design features of the boiler 1. According to the present utility model, the following options are possible for the supply of the flue gases from the gas turbine 17 to the boiler 1:

• Variant 1 - by means of the duct for hot organized air 12 to the holes for organized air 13 of the combustion system 6. In this variant, the flue gases from the gas turbine 17 are mixed with the heated air after the removed air heater 10 of the boiler 1. The mixture of flue gases and preheated air is supplied to the furnace chamber 4 through the holes for supply of organized air 13;

• Variant 2 - the flue gases from the gas turbine 17 by means of the duct for hot organized air 12 are supplied to the air nozzles for super-combustion air 14. This variant is suitable in the presence of realized low nitrogen combustion of the boiler 1;

• Variant 3 - the flue gases from the gas turbine 17 are fed to the furnace chamber 4 of the boiler 1 directly to nozzles 20 installed under the combustion device 6 in the furnace chamber 4. In some of the existing lignite coal boilers, such nozzles 20 are already provided, separated in the cool cone 5.1. In cases where none are provided, the nozzles 20 are specially designed and re-installed in the cool cone 5.1 of the furnace chamber 4;

• Option 4 - the flue gases from the gas turbine 17 are fed into the furnace chamber 4 in a combination of two or three of the previous options.

After the gas turbine 17, the flue gases have a temperature V = 550 ^ 600°C and an oxygen concentration O2 = 15 ^ 16 vol%. These gases can no longer be used in the gas turbine 17, but they contain a sufficient amount of heat and oxygen. For this reason, the flue gases from the gas turbine 17 can be fed to the boiler 1, which achieves two effects:

1. The oxygen in the flue gases from the gas turbine 17 participates in the combustion process taking place in the furnace chamber 4 of the boiler 1. This, in turn, reduces the amount of organized combustion air supplied to the combustion devices 6 of the boiler 1. As a result, a reduction in the own needs of the energy unit, since the air fan (s) 11 consumes less electrical energy.

2. The heat contained in the flue gases after the gas turbine 17 is utilized by the evaporation screens 5 in the furnace chamber 4 of the boiler 1. In this way, the amount of burned organic fuel is reduced. This results in a reduction in the amount of carbon dioxide CO2 generated per unit of energy generated [tco2/kWh(MWh)], as well as a number of other benefits, such as:

• Fewer fuel systems in operation;

• Smaller own needs of the energy unit;

• Smaller emissions of harmful substances released into the atmosphere, such as:

o Sulfur dioxide SO ₂ ;

o Nitrogen oxides ΝΟχ;

o Dust, etc.

Although the present utility model has been described in connection with the preferred exemplary embodiment, it should be noted that other alternative embodiments are possible which will be apparent to a person of ordinary skill and will achieve the desired effects through the features disclosed in the present description. and which do not go beyond the scope of the present utility model defined by the claims.

Application of the utility model

In a specific application of the utility model, the technical-economic and environmental indicators of a power unit burning Bulgarian lignite coal were evaluated. The circuit thus described in Figure 2 was modeled in the GateCycle software environment. The energy unit simulated in the GateCycle software environment is based on the most widely used energy boiler 1, namely P-62. It is installed in a monoblock with a steam turbine 2. To increase its efficiency, a gas turbine 17 is added to the existing power unit, to which there is a generator 19, and the flue gases leaving it from the flue gas outlet 18 are fed to the now existing energy boiler 1 (fig. 1). Gas Turbine 17 burns natural gas, the fuel that produces the least carbon emissions of all fossil fuels. The choice of unit power of the gas turbine 17 is consistent with the following restrictions: the existing energy unit can work independently, i.e. its operation does not depend on the operation of the gas turbine 17; the operating range of the lignite-fired power unit to remain in its current form (from 145 to 225 MW); not to make significant changes to the existing facilities in the energy block.

In the proposed scheme of connecting the gas turbine 17 to an existing power unit, the flue gases after the gas turbine 17, which are at a high temperature (above 585°C), are fed to the furnace chamber 4 of the energy boiler 1. It performs the role of a recovery boiler, which absorbs the heat carried by the flue gases after the gas turbine 17. This leads to several positive effects, namely: the consumption of fuel from lignite coal is reduced (while preserving the electrical power); the concentration of sulfur dioxide in the flue gases is reduced; the concentration of carbon dioxide in flue gases is reduced; the concentration of other pollutants in flue gases, such as dust, heavy metals, mercury and others, is also reduced; reduction of the amount of organized air supplied for combustion to the fuel systems of the energy boiler; reduction of the energy unit's own needs and others.

Model studies were carried out with 3 different gas turbines: 20 MW; 37.5 MW; 75 MW. The flue gases after the gas turbine 17 supplied to the furnace chamber 4 of the boiler 1 contain about 16% oxygen. This leads to a reduction in the amount of organized air supplied to the boiler 1. For this reason, the external air heater 10 is in danger of being left without a sufficient flow of air to cool it, which may lead to its destruction. Therefore, when choosing the power of the gas turbine 17, one of the main conditions is to have a sufficient volume of organized air to cool the flue gases in the exhaust air heater 10. If this condition is not met and a larger power is set, the gas turbine 17 , this will lead to the need to carry out a reconstruction in the tail part of the boiler 1 connected to the removed air heater 10. From the results presented in figures 3 and 4 it is clear that the choice of power of 37.5 MW of the gas turbine unit is the most suitable for the purposes thus set because: the operating range of the power unit is from 180 to 262 MW. If we consider electric power obtained only from steam turbine 2, it is from 143 to 225 MW; there is a sufficient flow rate of organized air to pass through the exhaust air heater 10; the temperature of the flue gases after the removed air heater 10 changes in the range from 197.96°C at minimum load to 189.21°C at maximum load.

The technical-economic evaluation of the operation of the thermal power plant for obtaining electricity with a combined cycle of a lignite power unit and a gas turbine 17 was carried out in the GateCycle software environment. Three thermal schemes were simulated with minimal changes to them: Scheme 1 - operation of a power unit only on lignite coal; Scheme 2 - combined operation of a power unit (lignite power unit with a gas turbine 17 with a power of 37.5 MW added to it); Scheme 3 - combined power unit operation - lignite power unit with 37.5 MW gas turbine 17 added to it and high pressure heaters stopped. The obtained results are summarized in tabular form in Table 1.

It shows data for minimum, average and maximum block load for each of the schemes.

From the results thus presented, we can draw the following conclusions:

• The more we reduce the supply of steam to the high-pressure regenerative heaters 16, the more the temperature of the outgoing flue gases after the removed air heater 10 decreases. In the case where we shut off the high-pressure regenerative heaters 16 through the shut-off valve 21, 1dg takes values of 181, 4°C at minimum load to 174.3°C at maximum. These temperatures come closest to the values we have when the power unit burns only lignite;

• From the change in the temperature of the feed water, it can be seen that the lower the value it enters the boiler 1, the more the temperature of the flue gases after the removed air heater 10 decreases, which leads to a reduction in the losses with exhaust gases generated by the energy boiler 1 .

TG hell

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From the power unit efficiency values, it can be seen that when we add a gas turbine 17 to the lignite power unit, the efficiency of the unit increases. This is explained by the higher efficiency factor of the gas turbine module on the one hand, and on the other hand the subsequent utilization of the flue gases leaving the gas turbine 17 with a high temperature and their supply to the energy boiler. The average increase in the efficiency of the energy unit is about 3.5%.

When implementing a scheme in which a gas turbine 17 with _Мел = 37.5 MW is added to an existing lignite power unit, along with an increase in the installed power and an increase in the efficiency of the unit, a reduction in pollutant emissions is also observed and greenhouse gases. The greatest reduction is observed in the emission of the following gases: carbon dioxide COg - it is a greenhouse gas and the reduction in its emissions is due to the following reasons: a reduction in the amount of lignite coal burned, on average by about 36 t/h (for one and same amount of electrical energy produced); generating lower emissions when burning natural gas; increasing the efficiency of the thermal scheme thus proposed; sulfur dioxide SO2 - the reduction in emissions is due to: a reduction in the consumption of burnt lignite coal (for the same amount of electricity produced).

With the help of the data presented in table 1, what will be the mass flows of carbon dioxide COg and sulfur dioxide SO2 generated by the energy unit in the considered 4 cases were calculated. The results are presented in Table 2.

Size Vp₽ Gi Mini Smoke is silent MW t/h t/h t/h t/h t/MW t/MW Combustion of lignite only 145.74 216.72 0.00 163.27 a,56 1.1203 0.0587 168.53 252.72 0.00 188.81 9.89 190.79 288.72 0.00 213.74 11,20 212.53 324.78 0.00 238.10 12.48 225.28 346.32 0.00 252.38 13,23 coal 4 Gas turbine = 37.S MW (without high pressure subheaters) 180.70 180.72 7.20 151.51 6.90 0.8385 0.0382 204.38 216.72 7.20 177.75 3.28 0.8697 0.0405 227.41 252.72 7.20 203.99 9.65 0.8970 0.0424 249.85 288.72 7.20 230.23' 11.03 0.9215 0.0441 262.76 310.32 7.20 151.51 6.90 0.9361 0.0451

Table 2 - Determination of the mass and specific consumption of carbon dioxide (CO2) and sulfur dioxide (SO2), in different modes of operation of an energy unit.

It is known (T. Totev Ignatov, B Technical-Economic and Environmental Assessment of the Operation of a Lignite Power Unit, Energy Forum 2019) that during the operation of a lignite power unit, its specific consumption of carbon dioxide shCOg is 1 ,1203 tco2/MWh. When modeling the operation of gas turbine 17 with a power of 37.5 MW, we used pure CH4 methane as fuel. The specific carbon dioxide consumption of gas turbine 17 is 0.528 tco2/MWh. Therefore, when we replace part of the lignite burning with pure methane burning, the total amount of carbon dioxide emissions generated by the combined power unit decreases. This can be seen both from the results presented in table 2 and from figure 5. It also shows that with a decrease in the produced 9

BG 4278 UI electrical energy from the energy unit, the specific consumption of CO ₂ decreases. This is because the block load change occurs only with a reduction in lignite consumption, while the gas turbine 17 operates at a constant power. The reduction in carbon dioxide emissions is from 16.44% at maximum load to 25.17% at minimum.

As mentioned above, the reduction of sulfur dioxide SO ₂ emissions is mainly due to the lack of sulfur content in the gas fuel burned by the gas turbine 17. Figure 6 shows how the specific consumption of sulfur dioxide changes, as a function of the electrical energy produced by the unit. At maximum load, this reduction is 23.19% and increases to 34.90% at minimum load. This is again due to the fact that the reduction of the load of the block is carried out at the expense of the reduction of the combustion of lignite coal, namely, they also contain sulfur, during the combustion of which sulfur dioxide is generated in the flue gases.

Combining one power unit with a steam turbine 2 and a boiler 1 burning lignite with a gas turbine 17 with a capacity of 37.5 MW has the following advantages:

1. The proposed scheme, according to which the gas turbine 17 and the lignite energy unit work in combination, has minimal changes. The presence of gas turbine 17 does not impair the operability of the existing unit and it retains its ability to work independently;

2. It increases the power of the power unit by 37.5 MW and reaches 262.7 MW, while preserving the operating range of the conventional unit (from 145 to 225 MW);

3. The efficiency of the combined cycle of the power unit of lignite and gas turbine 17 (from 37.5 MW) increases by an average of 3.5%;

4. The consumption of lignite coal is reduced by about 36.0 t/h at maximum load, which will also lead to a reduction of the unit's own needs for fuel preparation;

5. The generation of carbon dioxide CO2 emissions from the energy unit is reduced from 16.44% at maximum load to 24.17% at minimum load. This will reduce the bloc's costs of purchasing carbon allowances;

6. The generation of sulfur dioxide SO ₂ emissions is reduced from 23.19% at maximum load to 34.90% at minimum load. This will result in additional savings from: limestone supply; reducing the unit's own needs for the preparation of limestone and its subsequent use in a desulfurization plant;

7. The reduction in the consumption of lignite coal will also lead to a reduction in emissions of other pollutants, such as: dust, mercury, fluorides, chlorides, etc.;

8. Reduction of own needs of the energy block as a whole.

Claims (1)

An installation for reducing CO2 emissions during the combustion of lignite, including a boiler to which a steam turbine with an electric generator is mounted in an energy monoblock, where the boiler has a furnace chamber in which evaporation screens and a combustion device with powder burners are located, as in the above a portion of the furnace chamber is connected to a horizontal flue that has a connection to at least one convection shaft that is connected to an exhaust air heater through a flue, wherein one end of the exhaust air heater is connected to at least one air fan and the other end is connected to a hot organized air duct, the hot organized air duct having a connection both to the organized air openings of the powder burners from the combustion system and to the supercombustion air nozzles located in the upper part of the furnace chamber above the combustion system before the horizontal gas pipeline, in which the steam turbine is connected to the regenerator through a steam outlet tive high-pressure preheaters which have a connection to the evaporation screens in the furnace chamber, characterized in that a gas turbine (17) with an additional electric generator (19) is connected to the furnace chamber (4) of the boiler (1) by means of a flue gas outlet (18) from the gas turbine (17), and a shut-off valve (21) is installed between the steam outlet (15) of the steam turbine (2) and the high-pressure regenerative heaters (16)