In recent years, due to the improvement of the government's environmental protection requirements, in order to improve the quality of the atmospheric environment, many areas require the transformation of coal-fired boilers into gas-fired boilers. Coal-fired boilers are being gradually replaced by gas-fired boilers in urban heating due to their shortcomings such as low thermal efficiency, high energy consumption, high noise, and serious coal dust pollution. So how to transform a coal-fired boiler into a gas-fired boiler? The following editor will introduce the methods and precautions for transforming coal-fired boilers into gas-fired boilers.
1. The method of transforming coal-fired boilers into gas-fired boilers
the
1. Boiler body
the
Coal-fired boilers usually use layer combustion, while gas-fired boilers use chamber combustion. Due to different combustion methods, there are great differences in the structure of the furnace. Generally, the furnace volume of coal-fired boilers is relatively large, so the furnace size is not suitable for simple replacement of gas burners, and careful layout is required. Take the coal-fired DZL furnace type as an example:
① The size of the furnace structure is relatively large, but because of the front and rear arches, the length of the gas flame injection is limited. In order to reduce the transformation workload, the gas can be modified by arranging burners on both sides of the boiler or on the front wall.
②If the upper part of the furnace arch has a large space, two small burners can be arranged on the side walls of the boiler, so that there is no need to dismantle the grate, and only two layers of refractory bricks need to be laid on it, and then the brick joints should be compacted with refractory cement , forming a dry furnace bottom. If the upper space of the furnace arch is small, the chain grate and air chamber should be completely removed, and the dry furnace bottom should be constructed with refractory and heat-insulating materials between the base and the boiler support, and a combustion chamber should be formed at the lower part of the furnace arch.
③ Before upgrading the gas, the soot on all boiler heating surfaces and convection pipes should be cleaned, and the scale on the water side should be cleaned at the same time to ensure good heat transfer capacity of the heating surface. Since the flue gas volume of gas-fired boilers is much lower than that of coal-fired boilers, the flue gas flow rate in each heating surface will be relatively low after modification. If the boiler structure permits, the water cooling for double-sided exposure can be increased without increasing the size of the furnace body structure. Wall, convection tube, smoke tube spoiler, etc., to increase the heat exchange heating surface and heat transfer coefficient, so as to increase the full load output of the boiler. This is also the preferred option for increasing boiler output during transformation.
④ If the length of the boiler is longer than the length of the flame, a secondary reflection arch can be installed in the furnace to increase the radiation intensity.
⑤Remove the coal hopper, slag remover, grate reducer, air chamber damper on both sides, etc., keep 1 to 2 inspection doors, and build all other door openings with refractory bricks, and seal them tightly to form a closed furnace space. For gas explosion-proof requirements, the original water-sealed slag outlet can be used as a blasting outlet.
⑥ The furnace wall and flue should be overhauled to ensure tightness, and the furnace wall should be pushed and reinforced with a steel frame. When there are dead corners in the furnace and the flue, it is easy to cause the ventilation and purge to fail, and the remaining gas will cause an explosion. The dead corners can be blocked by masonry, and measures such as adding deflectors to the flue can be used.
2. Blower and induced draft fan
the
The excess air coefficient of the gas boiler is controlled at 1.1 to 1.15. Regardless of coal or natural gas, the ratio of the theoretical air volume required by the combustion chamber to the calorific value of the fuel is approximately equal, that is, the theoretical air volume corresponding to the low calorific value of 1MJ is about 0.27m3 under the standard state, and the heat of each MJ burned by various fuels Generate approximately the same amount of flue gas, about 0.30m3. Due to the large air leakage of coal-fired boilers, the excess air coefficient is much different from that of gas-fired boilers. The excess air coefficient at the furnace outlet of the layer-fired furnace is 1.4 to 1.6, so the actual flue gas volume and air volume are quite different. Generally, coal-fired boilers are 15% more than gas-fired boilers. ~60%. If the output remains unchanged after the gas is changed, the flue gas flow rate will be significantly reduced, the utilization rate of each heating surface will be greatly reduced, and the induced draft fan of the boiler will also have a margin of 15% to 60%. If the output is appropriately increased, the induced draft fan will Ability should be able to meet. Considering the reduction of renovation costs, the scope of increasing boiler output should be controlled within the output of the original drum and induced draft fan.
Coal-fired boilers generally use negative pressure combustion, which has poor sealing performance, while most gas-fired boilers use positive pressure combustion, because the air leakage coefficient is 0, which can improve the thermal efficiency of the boiler. If the sealing performance of the existing coal-fired boiler body is modified by forced positive pressure, the modification cost will be greatly increased. Therefore, after the transformation, a micro-negative pressure combustion method should be adopted, controlled at a negative pressure of 20Pa, and the amount of air leakage in the furnace can be reduced, so that a very good gas operation effect can also be obtained.
3. Selection and arrangement of burners
the
Select the burner according to the calorific value of the local gas supply and gas supply pressure, the performance of the boiler and the structure of the furnace. The output power of the burner should match the rated output of the boiler. Choose the shape of the flame, such as length and diameter, so that Adapt to the furnace structure, and have an adjustment range that can adapt to load changes. According to the rated output and boiler efficiency determined by the boiler transformation, the gas consumption is calculated, and then the number of burner configurations is determined according to the power of the selected burner. At present, most of the gas-fired boilers produced in China use imported burners with good quality and mature technology. Generally, there are: MM micro-proportional adjustment and EGA smoke detection, the two cooperate with each other to adjust the damper proportionally, and some parameters of the exhaust gas during the combustion process (such as exhaust gas temperature, oxygen, carbon dioxide, carbon monoxide) After continuous testing, the feedback is given to the microcomputer installed in the burner. After the analysis and calculation of the microcomputer, two servo motors are driven to automatically adjust the damper and fuel valve, which can accurately input and correct the ratio of air and gas to improve combustion efficiency.
The layout of the burner is closely related to the form of the furnace, and the principle should be that the flame in the furnace is full and the flame cannot wash out the water-cooled wall. The single burner has a simple system, convenient operation and management, and low investment. The two burners are often arranged on both sides of the furnace. The flames of the burners are sprayed obliquely upward, and the flames are in the symmetrical center of the furnace, which can ensure uniform heat exchange on the heating surfaces of each furnace. When the load decreases, one burner can be deactivated, the load adjustment range is wider, and the installation position is more convenient for operation, inspection and maintenance.
4. Utilization of latent heat of vaporization
the
The content of water vapor in flue gas after combustion of gas is relatively large, the proportion of water vapor in coal-fired flue gas is about 8%, and the proportion of water vapor in gas flue gas is nearly 18%. Therefore, the latent heat of vaporization of water vapor in the flue gas is very large, accounting for about 10% of the high calorific value of the gas. Making full use of the latent heat of vaporization is an effective measure to improve the utilization of gas. For gas-fired boilers, a tail flue gas-water heat exchanger can be added to increase the thermal efficiency of the boiler to 90% to 96%. When the sulfur content in the gas is high, it should be prevented that the temperature of the flue gas after heat exchange is lower than the acid dew point, so as to avoid the formation of sulfuric acid solution to corrode the heating surface.
5. Gas boiler furnace outlet temperature selection
the
The outlet of the gas boiler furnace will not be restricted by dust accumulation and high temperature corrosion. Generally, it is allowed to choose a higher range of about 1200 ° C. The smoke velocity of the convection heating surface of the gas boiler will not be limited by fly ash wear, and higher flue gas can be used. Flow rate, improve flue gas flow in the furnace, increase flue gas disturbance, make full use of the characteristics of the original coal-fired boiler with more convective heating surfaces, and increase boiler output without additional boiler heating surfaces.
6. Efficiency
the
After the general layer coal-fired boiler is changed to a gas-fired boiler, the heat transfer conditions are improved due to the obvious reduction of pollution and ash accumulation on the heating surface; the excess air coefficient and exhaust temperature during the smoke exhaust process are reduced, and the incomplete loss can also be controlled, so Boiler efficiency can be increased by more than 10%. The gas boiler exhaust gas temperature determines the thermal efficiency of the boiler, the lower the exhaust gas temperature, the higher the boiler efficiency. Therefore, it is generally necessary to increase the low-temperature heat exchanger at the tail of the boiler to transform the coal-fired boiler, and use the heat exchanger with a larger heating area to reduce the exhaust gas temperature as low as possible. After the transformation, the output of the boiler has been greatly improved. Attention should also be paid to the capacity of the steam-water separation device in the boiler, the temperature change of the superheater, the change of the water circulation resistance, and the change of the safe flow rate of the water circulation. Therefore, various factors for the safe operation of the boiler should be considered more comprehensively.
2. Precautions for transforming coal-fired boilers into gas-fired boilers
the
Since natural gas is flammable and explosive, when designing the automatic control system of a gas boiler, the reliability of the system must be given priority, and the interlock protection system and the detection control system should be set separately. When selecting a microcomputer system, measures such as backup manual operation and dual indication of important parameters can be installed to improve the reliability of the control and protection system. Interlock protection measures such as automatic ignition, flameout protection, gas leakage protection, and automatic combustion adjustment should be installed on the modified gas boiler.
1. Automatic ignition
the
Generally, the electric spark automatic ignition method is adopted, and the low voltage is raised to high voltage by the ignition transformer, and a spark discharge is formed at the head of the igniter, and the natural gas is ignited by the electric spark energy. The ignition control program is realized by PLC. The program is to start the induced draft fan first, and then start the blower. The indicator light of the fan is on and the furnace is purged. Open the solenoid valve of the ignition gun, light a small fire, and after confirming that the small fire is established, open the high fire solenoid valve to establish the main torch. The induced draft fan is increased at the same time, the main gas regulating valve is opened, and the ignition gun is closed. The boiler is burning normally and the flame is monitored by the flameout protection device.
2. Flame protection
the
Flame monitoring adopts plasma, infrared or ultraviolet photosensitive detector monitoring and relay control, with sound and light alarm system. The main part of the monitor is composed of a power transformer, an ultraviolet light probe, and a relay. The photosensitive probe responds to the flame signal and turns it into an electrical signal. The amplifier amplifies the input signal and controls the switch of the solenoid valve through the relay. Once the boiler is turned off, the gas solenoid valve is automatically cut off to cut off the gas supply.
3. Chain protection
the
Due to the adoption of the original balanced ventilation method of the coal furnace, the frequency conversion control method is generally used for the induced draft fan in the control system, and the frequency conversion of the induced draft fan is automatically adjusted according to the set furnace negative pressure and boiler load changes to maintain a stable negative pressure in the furnace. .
Corresponding chain protection control: when the induced draft fan stops suddenly, the blower and gas supply are automatically cut off, the gas supply is automatically cut off when the gas pressure is lower than the specified value, and the gas supply is cut off when the steam boiler is overpressured and the boiler water level is too high or too low.
How to transform a coal-fired boiler into a gas-fired boiler? In fact, it is not difficult, as long as you refer to each of the above steps. In fact, the transformation needs to formulate a reasonable transformation plan through comprehensive consideration of the coal-fired boiler furnace structure and local gas conditions, and control the selection and configuration of the burner. It can achieve the effects of improving efficiency, saving operating costs and improving the environment. At the same time, the automatic control, chain protection and safety protection measures of gas-fired boilers are much higher than those of coal-fired boilers. The operation is safe and stable, which greatly improves the working environment, reduces labor intensity, and helps to enhance the corporate image. With the support of policies, the price of gas is moderate and the gas source is stable. The conversion of coal-fired boilers to gas has a wide range of application markets, which is conducive to achieving a win-win situation of economic benefits and environmental protection standards.
In recent years, due to the improvement of the government's environmental protection requirements, in order to improve the quality of the atmospheric environment, many areas require the transformation of coal-fired boilers into gas-fired boilers. Coal-fired boilers are being gradually replaced by gas-fired boilers in urban heating due to their shortcomings such as low thermal efficiency, high energy consumption, high noise, and serious coal dust pollution. So how to transform a coal-fired boiler into a gas-fired boiler? The following editor will introduce the methods and precautions for transforming coal-fired boilers into gas-fired boilers.
1. The method of transforming coal-fired boilers into gas-fired boilers
the
1. Boiler body
the
Coal-fired boilers usually use layer combustion, while gas-fired boilers use chamber combustion. Due to different combustion methods, there are great differences in the structure of the furnace. Generally, the furnace volume of coal-fired boilers is relatively large, so the furnace size is not suitable for simple replacement of gas burners, and careful layout is required. Take the coal-fired DZL furnace type as an example:
① The size of the furnace structure is relatively large, but because of the front and rear arches, the length of the gas flame injection is limited. In order to reduce the transformation workload, the gas can be modified by arranging burners on both sides of the boiler or on the front wall.
②If the upper part of the furnace arch has a large space, two small burners can be arranged on the side walls of the boiler, so that there is no need to dismantle the grate, and only two layers of refractory bricks need to be laid on it, and then the brick joints should be compacted with refractory cement , forming a dry furnace bottom. If the upper space of the furnace arch is small, the chain grate and air chamber should be completely removed, and the dry furnace bottom should be constructed with refractory and heat-insulating materials between the base and the boiler support, and a combustion chamber should be formed at the lower part of the furnace arch.
③ Before upgrading the gas, the soot on all boiler heating surfaces and convection pipes should be cleaned, and the scale on the water side should be cleaned at the same time to ensure good heat transfer capacity of the heating surface. Since the flue gas volume of gas-fired boilers is much lower than that of coal-fired boilers, the flue gas flow rate in each heating surface will be relatively low after modification. If the boiler structure permits, the water cooling for double-sided exposure can be increased without increasing the size of the furnace body structure. Wall, convection tube, smoke tube spoiler, etc., to increase the heat exchange heating surface and heat transfer coefficient, so as to increase the full load output of the boiler. This is also the preferred option for increasing boiler output during transformation.
④ If the length of the boiler is longer than the length of the flame, a secondary reflection arch can be installed in the furnace to increase the radiation intensity.
⑤Remove the coal hopper, slag remover, grate reducer, air chamber damper on both sides, etc., keep 1 to 2 inspection doors, and build all other door openings with refractory bricks, and seal them tightly to form a closed furnace space. For gas explosion-proof requirements, the original water-sealed slag outlet can be used as a blasting outlet.
⑥ The furnace wall and flue should be overhauled to ensure tightness, and the furnace wall should be pushed and reinforced with a steel frame. When there are dead corners in the furnace and the flue, it is easy to cause the ventilation and purge to fail, and the remaining gas will cause an explosion. The dead corners can be blocked by masonry, and measures such as adding deflectors to the flue can be used.
2. Blower and induced draft fan
the
The excess air coefficient of the gas boiler is controlled at 1.1 to 1.15. Regardless of coal or natural gas, the ratio of the theoretical air volume required by the combustion chamber to the calorific value of the fuel is approximately equal, that is, the theoretical air volume corresponding to the low calorific value of 1MJ is about 0.27m3 under the standard state, and the heat of each MJ burned by various fuels Generate approximately the same amount of flue gas, about 0.30m3. Due to the large air leakage of coal-fired boilers, the excess air coefficient is much different from that of gas-fired boilers. The excess air coefficient at the furnace outlet of the layer-fired furnace is 1.4 to 1.6, so the actual flue gas volume and air volume are quite different. Generally, coal-fired boilers are 15% more than gas-fired boilers. ~60%. If the output remains unchanged after the gas is changed, the flue gas flow rate will be significantly reduced, the utilization rate of each heating surface will be greatly reduced, and the induced draft fan of the boiler will also have a margin of 15% to 60%. If the output is appropriately increased, the induced draft fan will Ability should be able to meet. Considering the reduction of renovation costs, the scope of increasing boiler output should be controlled within the output of the original drum and induced draft fan.
Coal-fired boilers generally use negative pressure combustion, which has poor sealing performance, while most gas-fired boilers use positive pressure combustion, because the air leakage coefficient is 0, which can improve the thermal efficiency of the boiler. If the sealing performance of the existing coal-fired boiler body is modified by forced positive pressure, the modification cost will be greatly increased. Therefore, after the transformation, a micro-negative pressure combustion method should be adopted, controlled at a negative pressure of 20Pa, and the amount of air leakage in the furnace can be reduced, so that a very good gas operation effect can also be obtained.
3. Selection and arrangement of burners
the
Select the burner according to the calorific value of the local gas supply and gas supply pressure, the performance of the boiler and the structure of the furnace. The output power of the burner should match the rated output of the boiler. Choose the shape of the flame, such as length and diameter, so that Adapt to the furnace structure, and have an adjustment range that can adapt to load changes. According to the rated output and boiler efficiency determined by the boiler transformation, the gas consumption is calculated, and then the number of burner configurations is determined according to the power of the selected burner. At present, most of the gas-fired boilers produced in China use imported burners with good quality and mature technology. Generally, there are: MM micro-proportional adjustment and EGA smoke detection, the two cooperate with each other to adjust the damper proportionally, and some parameters of the exhaust gas during the combustion process (such as exhaust gas temperature, oxygen, carbon dioxide, carbon monoxide) After continuous testing, the feedback is given to the microcomputer installed in the burner. After the analysis and calculation of the microcomputer, two servo motors are driven to automatically adjust the damper and fuel valve, which can accurately input and correct the ratio of air and gas to improve combustion efficiency.
The layout of the burner is closely related to the form of the furnace, and the principle should be that the flame in the furnace is full and the flame cannot wash out the water-cooled wall. The single burner has a simple system, convenient operation and management, and low investment. The two burners are often arranged on both sides of the furnace. The flames of the burners are sprayed obliquely upward, and the flames are in the symmetrical center of the furnace, which can ensure uniform heat exchange on the heating surfaces of each furnace. When the load decreases, one burner can be deactivated, the load adjustment range is wider, and the installation position is more convenient for operation, inspection and maintenance.
4. Utilization of latent heat of vaporization
the
The content of water vapor in flue gas after combustion of gas is relatively large, the proportion of water vapor in coal-fired flue gas is about 8%, and the proportion of water vapor in gas flue gas is nearly 18%. Therefore, the latent heat of vaporization of water vapor in the flue gas is very large, accounting for about 10% of the high calorific value of the gas. Making full use of the latent heat of vaporization is an effective measure to improve the utilization of gas. For gas-fired boilers, a tail flue gas-water heat exchanger can be added to increase the thermal efficiency of the boiler to 90% to 96%. When the sulfur content in the gas is high, it should be prevented that the temperature of the flue gas after heat exchange is lower than the acid dew point, so as to avoid the formation of sulfuric acid solution to corrode the heating surface.
5. Gas boiler furnace outlet temperature selection
the
The outlet of the gas boiler furnace will not be restricted by dust accumulation and high temperature corrosion. Generally, it is allowed to choose a higher range of about 1200 ° C. The smoke velocity of the convection heating surface of the gas boiler will not be limited by fly ash wear, and higher flue gas can be used. Flow rate, improve flue gas flow in the furnace, increase flue gas disturbance, make full use of the characteristics of the original coal-fired boiler with more convective heating surfaces, and increase boiler output without additional boiler heating surfaces.
6. Efficiency
the
After the general layer coal-fired boiler is changed to a gas-fired boiler, the heat transfer conditions are improved due to the obvious reduction of pollution and ash accumulation on the heating surface; the excess air coefficient and exhaust temperature during the smoke exhaust process are reduced, and the incomplete loss can also be controlled, so Boiler efficiency can be increased by more than 10%. The gas boiler exhaust gas temperature determines the thermal efficiency of the boiler, the lower the exhaust gas temperature, the higher the boiler efficiency. Therefore, it is generally necessary to increase the low-temperature heat exchanger at the tail of the boiler to transform the coal-fired boiler, and use the heat exchanger with a larger heating area to reduce the exhaust gas temperature as low as possible. After the transformation, the output of the boiler has been greatly improved. Attention should also be paid to the capacity of the steam-water separation device in the boiler, the temperature change of the superheater, the change of the water circulation resistance, and the change of the safe flow rate of the water circulation. Therefore, various factors for the safe operation of the boiler should be considered more comprehensively.
2. Precautions for transforming coal-fired boilers into gas-fired boilers
the
Since natural gas is flammable and explosive, when designing the automatic control system of a gas boiler, the reliability of the system must be given priority, and the interlock protection system and the detection control system should be set separately. When selecting a microcomputer system, measures such as backup manual operation and dual indication of important parameters can be installed to improve the reliability of the control and protection system. Interlock protection measures such as automatic ignition, flameout protection, gas leakage protection, and automatic combustion adjustment should be installed on the modified gas boiler.
1. Automatic ignition
the
Generally, the electric spark automatic ignition method is adopted, and the low voltage is raised to high voltage by the ignition transformer, and a spark discharge is formed at the head of the igniter, and the natural gas is ignited by the electric spark energy. The ignition control program is realized by PLC. The program is to start the induced draft fan first, and then start the blower. The indicator light of the fan is on and the furnace is purged. Open the solenoid valve of the ignition gun, light a small fire, and after confirming that the small fire is established, open the high fire solenoid valve to establish the main torch. The induced draft fan is increased at the same time, the main gas regulating valve is opened, and the ignition gun is closed. The boiler is burning normally and the flame is monitored by the flameout protection device.
2. Flame protection
the
Flame monitoring adopts plasma, infrared or ultraviolet photosensitive detector monitoring and relay control, with sound and light alarm system. The main part of the monitor is composed of a power transformer, an ultraviolet light probe, and a relay. The photosensitive probe responds to the flame signal and turns it into an electrical signal. The amplifier amplifies the input signal and controls the switch of the solenoid valve through the relay. Once the boiler is turned off, the gas solenoid valve is automatically cut off to cut off the gas supply.
3. Chain protection
the
Due to the adoption of the original balanced ventilation method of the coal furnace, the frequency conversion control method is generally used for the induced draft fan in the control system, and the frequency conversion of the induced draft fan is automatically adjusted according to the set furnace negative pressure and boiler load changes to maintain a stable negative pressure in the furnace. .
Corresponding chain protection control: when the induced draft fan stops suddenly, the blower and gas supply are automatically cut off, the gas supply is automatically cut off when the gas pressure is lower than the specified value, and the gas supply is cut off when the steam boiler is overpressured and the boiler water level is too high or too low.
How to transform a coal-fired boiler into a gas-fired boiler? In fact, it is not difficult, as long as you refer to each of the above steps. In fact, the transformation needs to formulate a reasonable transformation plan through comprehensive consideration of the coal-fired boiler furnace structure and local gas conditions, and control the selection and configuration of the burner. It can achieve the effects of improving efficiency, saving operating costs and improving the environment. At the same time, the automatic control, chain protection and safety protection measures of gas-fired boilers are much higher than those of coal-fired boilers. The operation is safe and stable, which greatly improves the working environment, reduces labor intensity, and helps to enhance the corporate image. With the support of policies, the price of gas is moderate and the gas source is stable. The conversion of coal-fired boilers to gas has a wide range of application markets, which is conducive to achieving a win-win situation of economic benefits and environmental protection standards.