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author：yuumi 2019-12-27
In the transformation of coal-fired boilers into gas-fired boilers, the principle should be to keep the pressurized elements of the boiler body unchanged and reduce the modification of the original boilers.The transformation process should focus on the selection of gas burners, the determination of the number of burners, the arrangement of burners, the matching design of furnace layout, and the selection of explosion-proof measures.
Choice of gas burner
In the process of coal - fired boiler renovation, the first choice or design of the appropriate gas burner.The common gas burners can be divided into ejector burners, blast burners and spontaneous combustion.The air needed for ejector combustion is inhaled by the gas jet, the blast burner requires the blast equipment to send the air into the combustion system, and the natural draft burner relies on the negative pressure in the furnace to draw the air needed for combustion into the combustion system.
1. The selection of burners should be considered by comparing the characteristics of the three burners with the characteristics of the original furnace of the boiler.
1) the safety degree of the gas burner, that is, it is required to reduce the heat damage caused by incomplete combustion of gas.The completeness of combustion is mainly related to the mixture of gas and air and the adequacy of air.In general, it is not difficult to make the incomplete combustion loss of gas zero under the condition of sufficient air volume and good mixture.When burning high calorific value gas, the gas incomplete combustion damage should not exceed 0.5%;When burning low calorific value gas, incomplete combustion damage of gas should not exceed 1.5%.When using premix burner, it is easy to control the damage of incomplete combustion lower than that of diffusion combustion.
2) reducing the excess air coefficient in the smoke is an effective measure to reduce the smoke exhaust damage.The amount of excess air in the exhaust and the amount of air escaping through the flue gas passage.For the boiler running under micro-positive pressure, the air leakage in the flue gas channel is zero, at this point the main excess air quantity depends on the excess air quantity during combustion depends on the excess air quantity during combustion, in fact in any case, reducing the excess air quantity during combustion is beneficial to improve the thermal efficiency of the boiler.Whether the burner can run under the lowest excess air coefficient is one of the important indexes of burner combustion performance.
3) the flame characteristics of the burner are closely related to the heat transfer in the furnace and other characteristics of the boiler.For example, when the flame is diffused, the semi-luminous flame is more radiant than the flame without flame, which is beneficial to the propagation in the furnace.The burner nozzle airflow should be higher speed and greater range, so that the flame in the furnace full of good.In the use of refractory materials to enhance heat transfer in the furnace, the flame shape and flame velocity should be adapted to the radiation surface.
4) fully consider the combustion rate.Because high speed combustion is the development trend of modern small and medium-sized boilers, it can reduce the size of burner and furnace, is an important measure of boiler miniaturization, but also an important characteristic index of burner.However, in the actual production, because the small and medium-sized boilers are often used in the case of variable load, therefore, the burner is required to have a very wide range of load regulation.When the modified gas-fired boiler is in operation, it should make the furnace flame filling degree is better, do not form the gas dead Angle, avoid the flame interference of adjacent burners, at the same time, the mixture of unburned gas and air should not contact the heat exchange surface, so as to avoid the formation of gas incomplete combustion.However, high temperature flame should avoid high speed erosion of the heat exchange surface, so as to avoid excessive heat intensity of the surface so that the tube wall overheating.Therefore, when choosing burners, the length of combustion flame should be approximately estimated according to different gas and load.
2. Determination of the number of gas burners
The number of gas burners in the transformation of coal-fired boilers can be determined by the following formula:
N = Qgl/Qrq
Where, n is the number of gas burners;Qgl is the boiler heat load capacity (KW);Qrq is the heat load of a single burner, KW.In practice, the number of gas burners is generally not more than 4 in order to prevent the explosion or explosion caused by a single burner due to an accident when multiple burners are running at the same time.In order to solve the flameout and tempering problem caused by insufficient gas flow when the boiler is under low load, oil gun can be installed inside the burner to adapt to the change of boiler under low load, and flameout and explosion proof device should be installed in the furnace.
3. Burner arrangement
1) the flame should be located in the geometric center of the furnace, so that the flame can fill the furnace as much as possible and the combustion is stable.
2) the flame is located in the geometric center of the furnace, so that the heat in the furnace can be evenly distributed, and the stress will not increase due to local heating, so as to prevent uneven heating and local overheating of the boiler.
3) the horizontal boiler has a large depth of the furnace, and the burner is arranged on the front wall of the furnace to ensure that the front smoke box will not overheat.At present, the small and medium-sized coal-fired boilers in China can be divided into two combustion modes: layer combustion and chamber combustion.Most of the layer burners are chain burners, which are characterized by the combustion of fuel in a stationary or slowly moving furnace.When the reformed boiler is a chain furnace, the special shape of the front and rear walls and the arch of the furnace bring inconvenience to the installation of the burner, so the gas burner should be installed in the south side of the chain furnace.In order to make the heat exchange condition in the modified furnace similar to that before the modification and reduce the change of heat exchange equipment in the chain furnace, the gas burner should be installed below the center of the furnace wall.For pulverized coal furnace, a new burner can be installed at the nozzle location of the original pulverized coal burner in order to keep the aerodynamic field characteristics similar to that of the former furnace and reduce the amount of reconstruction work.
4. Matching of furnace layout
When the type and position of burner are selected, the matching calculation of furnace should be carried out, which is mainly composed of four parts.
1) the exhaust smoke volume should match with the induced fan.The amount of smoke discharged by the induced draft fan of a coal-fired boiler is arranged in accordance with the smoke produced by burning coal.After the change to burning gas, the density and flow of the flue gas change, it is necessary to re-check whether the air intake matches, otherwise it will be difficult to ignite, exhaust gas temperature height isotherm.
2) the furnace air leakage system shall match the air volume of the burner.In the process of replacing coal with gas, the air leakage coefficient should be below 0.1.Air leakage in coal-fired boilers is mainly concentrated in front of the coal scuttle, exhaust, blower inlet.After the use of gas fuel, it shall be closed to reduce the heat loss and power consumption of smoke exhaust, otherwise it is easy to cause air leakage.
3) the size of the furnace should match the layout of the burner.Furnace layout should consider a single burner.According to the flame length and diameter of a single burner, determine the distance between burners to ensure that the flames do not wash the furnace wall, do not interfere with each other, and is conducive to heating surface matching.
5. Selection of explosion protection measures for gas-fired boilers
After the coal - fired boiler is transformed into gas - fired boiler, the following aspects should be taken to prevent the occurrence of furnace explosion accident.
1) must be equipped with reliable safety protection control measures, such as automatic ignition device, quick cut-off valve, flame monitoring system (FSSS) chain protection.
2) for water tube boilers, explosion-proof doors shall be set at the smoke outlet location of the furnace chamber (or directly opposite the center of the furnace chamber) and on the flue.The action of explosion-proof door is that it can automatically open when the mixture gas in the furnace or flue explodes and release certain pressure in the furnace to protect the furnace wall from serious damage.
3) the safety operation rules must be strictly formulated and implemented, especially the purge work must be done at each ignition start and the purge time must be well controlled.It must be ensured that after the damper is opened, the volume of purge air calculated based on the flow of the ventilator shall be greater than or equal to 3 times the volume of the furnace and flue, and the required time shall be delayed for over 30S.In boiler operation, pay attention to the proportion of air regulation, prevent the phenomenon of deignition, tempering, ensure the complete combustion of gas.
4) the combustion system of the gas-fired boiler shall be automated, including ignition and flameout protection, automatic combustion regulation, necessary interlock protection and automatic start-up by program.
5) when several boilers share a flue, each boiler shall have a flue door, and each flue door shall have a limit switch.At the same time, it is also necessary to ensure that the boiler can only be put into use after the flue door is opened before the boiler is started, in case the flue door is not opened and ventilation is not free due to wrong operation.
Differences between coal - fired boilers and gas - fired boilers
1. Combustion mode
Circulating fluidized bed coal-fired boiler is the coal through the crushing equipment, through the belt into the bunker, through the feeder using the coal seeding air, into the furnace;The combustion process is fluidized by circulating ash heating.After coal is changed into gas, coke oven gas and chi vent gas are mixed and injected into direct combustion by four burners.
2. Combustion products
Coal is mainly composed of C, H, O, N, S and other elements, ash and water. After the coal burns out heat, SO2, SO3, NOx ash and water are generated, while SO2, SO3, NOx are discharged into the atmosphere by smoke, polluting the environment and causing harm to human beings.The main components of coke oven gas and exhaust gas are CO and H2. After combustion, CO2 and H2O are mainly generated, and the emission of SO2, SO3 and NOx is much less than that of coal-fired boiler gas-fired boiler.
3. Ventilation mode
Coal-fired boilers generally adopt negative pressure combustion, and the basic combustion process is completed by the cooperation of the blower and induced fan. In the combustion process of coal, a large amount of air is needed, and due to the air leakage in the flue of the furnace wall, the excess air coefficient can reach 2.1-2.5.The gas-fired boiler USES the micro positive pressure or micro negative pressure combustion, needs the air volume is small, in the burner air can be better premixed with the natural gas.Micro-positive pressure combustion has no air leakage factors of furnace wall and flue, and the air coefficient of flue outlet in operation can be 1.05-1.2.
4. Explosive of fuel
Coal - fired boiler combustion safety, furnace explosion risk is not easy.But the gas boiler is in the explosion concentration limit, encounters the open fire to be able to produce the explosion.Greater risk.
5. Boiler automatic control
It is difficult for coal-fired boilers to automatically adjust the operating parameters of boilers according to the load due to the variety of coal, the thickness of material layer, the volume of blast air, the volume of draft air and the ratio of wind-coal.While the gas boiler is less affected by the factors, the size of the burner valve can be adjusted according to the load, easy to achieve automatic control.
conclusion
After the actual transformation of coal to gas boiler, and operation analysis.The technical index and combustion characteristic index of the modified boiler meet the design requirements, and can realize the integration of use and monitoring, which greatly reduces the potential danger of the boiler.At the same time, on the basis of reducing the cost of buying new furnaces, the emission of gas pollutants was reduced.Achieved economic benefits and environmental protection of the win-win.