Keywords: blast furnace gas; Coal-fired boiler; combined firing
Iron and steel enterprises consume a lot of coal, fuel oil and electric energy in the production process, and at the same time produce secondary energy such as blast furnace gas, coke oven gas and converter gas. The generated energy is used for other industries or civil use, except for the consumption of steel production itself.
Blast furnace gas is a by-product of ironmaking. It is a gas produced by partial combustion of coke and partial reduction of iron ore in blast furnace. It is colorless, tasteless and flammable. Its main combustible components are carbon monoxide and a small amount of H2. The nonflammable components are inert gas, carbon dioxide and N2. The volume fraction of CO is generally 2 1%-26%, and its calorific value is not high, and its low calorific value is generally 2760-3720 kJ/m3. The ignition temperature of blast furnace gas is about 600℃, and its theoretical combustion temperature is about 1 150℃, which is far lower than the theoretical combustion temperature of coal. The combustion temperature is low, so it is difficult to burn blast furnace gas completely, and the combustion stability is poor. Because blast furnace gas contains a lot of nitrogen and carbon dioxide, the combustion temperature is low, the speed is slow and the combustion is difficult, which makes the blast furnace gas emission rate of many iron and steel enterprises high. Using blast furnace gas to generate electricity, because of the low fuel cost and simple system, reduces the fuel transportation cost and infrastructure cost, can alleviate the power shortage of enterprises, reduce the pollution of CO to the environment, and achieve the dual effects of energy saving, power increase and environmental improvement, which can not only create considerable economic benefits for enterprises, but also create comprehensive social benefits.
According to the main utilization mode of blast furnace gas in iron and steel industry, the theoretical analysis and corresponding transformation measures are carried out to transform coal-fired boilers into all-fired blast furnace gas boilers.
1 Influence of mixing blast furnace gas on boiler performance
Effect of 1. 1 on combustion characteristics in furnace.
When the coal-fired boiler is mixed with blast furnace gas, the low calorific value of blast furnace gas is very low (2760
-3720 kj/m3), while the low calorific value of ordinary bituminous coal is about 18000 kj/kg. Therefore, the theoretical combustion temperature in the furnace will definitely drop, which will lead to the deterioration of pulverized coal combustion stability and the increase of incomplete combustion of pulverized coal particles, thus increasing the carbon content of fly ash, increasing the mechanical incomplete combustion loss and reducing the boiler efficiency. On the other hand, after adding blast furnace gas, the heat-absorbing medium sent into the furnace increases, the heat capacity of flue gas increases, the temperature level of the flame center decreases, and the position of the flame center moves up, resulting in shortening the residence time of pulverized coal in the furnace, incomplete combustion of pulverized coal and increasing the carbon content of fly ash. Thirdly, after adding blast furnace gas, the amount of flue gas in the furnace increases (table 1), and the velocity of flue gas in the furnace increases, thus shortening the residence time of pulverized coal particles in the furnace and causing incomplete combustion of pulverized coal. Fourthly, after mixing with blast furnace gas, a large number of inert gases such as nitrogen in blast furnace gas hinder the full mixing of combustible components with air, which reduces the probability of intermolecular collision in combustion reaction, leads to unstable and incomplete combustion of pulverized coal particles and increases the carbon content of fly ash. It can be seen that the carbon content of fly ash increases and the boiler efficiency decreases after mixing blast furnace gas. The test proves that [1], from the carbon content of fly ash, the optimum blending rate of blast furnace gas should be within 25% without raising the furnace temperature level.
Table 1 fuel production 1MJ flue gas combustion heat.
As we all know, the radiation ability of solid is much greater than that of gas, and the share of triatomic gas with radiation ability in flue gas produced by burning blast furnace gas is much lower than that of coal, while diatomic gas such as N2, which accounts for a large part of gas, has no radiation ability. Moreover, the triatomic gases in the flue gas produced by burning blast furnace gas are mainly CO2 and a small amount of H2O, and the radiation ability of CO2 is lower than that of H2O. Therefore, after adding blast furnace gas, the flame radiation ability in the furnace is weakened, and more heat flows to the post-superheater. After mixing with boiler gas, the heat transfer capacity in the furnace decreases. For boilers with furnace water wall as the main evaporation heating surface, if the boiler structure is not adjusted, the evaporation capacity of the boiler will decrease.
Effect of 1.2 on heat transfer characteristics of flue behind furnace
The absorption of flue gas heat by convection heat exchanger superheater system mainly depends on heat exchange temperature, pressure and heat exchange coefficient. For boilers burning coal mixed with blast furnace gas, there is little difference in flue gas temperature at the outlet of the furnace [2], so there is little difference in heat transfer temperature and pressure. However, the volume flow of flue gas in the boiler mixed with blast furnace gas is larger than that in the coal-fired boiler, and the velocity of flue gas on the convection heating surface increases, thus improving the heat transfer coefficient and increasing the heat absorption of the superheater, resulting in overheating of the superheater outlet temperature. Similarly, with the increase of flue gas volume, if the heating surface behind the furnace remains unchanged, the heat absorption of superheater, economizer and air preheater arranged in the flue behind the furnace will increase, but it is not enough to reduce the flue gas temperature to the previous temperature level, so the flue gas temperature will increase and the heat loss of flue gas will also increase.
2. Influence of burning blast furnace gas on boiler performance
2. Influence of1on combustion characteristics in furnace
A large number of inert gases such as N2 and CO2 in blast furnace gas do not participate in the combustion reaction during combustion, on the contrary, they also absorb the heat released by a large number of combustible gases during combustion, which makes the combustion temperature of blast furnace gas lower. Although blast furnace gas is a gas fuel, the theoretical combustion temperature (-1 150℃) is much lower than that of pulverized coal particles (1800℃-2000℃), but a large number of inert gases contained in blast furnace gas will hinder the flame propagation and slow it down (for example, the laminar flame propagation speed is only 0.3-/kloc-0) There is almost no ash in blast furnace gas. When burning, the flame basically does not produce radiant energy. Only triatomic gas in the flue gas produced by combustion has radiation ability, and a large amount of nitrogen in blast furnace gas has no radiation ability. Therefore, the radiation heat transfer capacity of flue gas in boiler burning blast furnace gas is lower than that of coal-fired boiler. Therefore, the heat absorption of the water wall in the furnace is reduced, which leads to the decrease of boiler evaporation.
2.2 Influence on heat transfer characteristics of flue behind furnace
Because there is almost no dust in blast furnace gas, the fly ash in the flue gas produced by burning blast furnace gas can be ignored, so the pollution coefficient ξ of convection heating surface is very low, only 0.0043, while for coal-fired boilers, when the flue gas velocity is 10m/s, the pollution coefficient ξ is 0.0 19[3], which shows that burning blast furnace gas.
Blast furnace gas contains a lot of inert gases, and the amount of flue gas produced by blast furnace gas with the same combustion energy is larger than that produced by pure coal combustion. Therefore, the amount of flue gas flowing through the convection heating surface increases, and the velocity of flue gas increases, resulting in an increase in the heat transfer coefficient of convection heat and convection heat absorption. Therefore, the temperature of superheated steam absorbing heat from the convection heating surface increases. Similarly, with the increase of flue gas volume, if the heating surface behind the furnace remains unchanged, the heat absorption of superheater, economizer and air preheater arranged in the flue behind the furnace will increase, but it is not enough to reduce the flue gas temperature to the previous temperature level, increase the flue gas temperature and increase the heat loss of flue gas.
3 transformation measures after mixing blast furnace gas
From the above analysis, in order to solve a series of problems after mixing blast furnace gas: the furnace temperature drops; The temperature of superheated steam rises; The carbon content of fly ash increases; The exhaust temperature becomes larger, etc. , and put forward the following solutions.
3. 1 Rebuild burner
Blast furnace gas burner is generally arranged in the lower part of pulverized coal burner. When the blast furnace gas burner has the function of starting the boiler, this arrangement can obtain the benefits of burning and adjusting the air temperature. If the combustion of coal is used to stabilize the combustion of blast furnace gas, it will only be beneficial to the adjustment of temperature. Because the flame center position in the furnace moves up after the mixed combustion of blast furnace gas, resulting in incomplete combustion of pulverized coal and rising exhaust temperature, measures such as moving down the burner position as far as possible and inclining the burner nozzle downward can be taken to reduce the flame center position and increase the fuel residence time in the furnace. Choose a burner that can strengthen pulverized coal combustion, such as pulverized coal burner with stable combustion chamber [4], to strengthen the combustion of pulverized coal particles, reduce the carbon content of fly ash and improve boiler efficiency.
3.2 Superheater transformation
After mixing with blast furnace gas, the radiation heat absorption in the furnace decreases and the convection heat absorption increases. Therefore, if it is feasible, adding more platen superheaters will reduce the heating surface of convection superheaters accordingly. In this way, the temperature adjustment performance of superheater can be taken into account under the working conditions of completely burning coal and mixing blast furnace gas, and excessive increase of desuperheating water can be avoided.
3.3 Transformation of Economizer
After mixing with blast furnace gas, the radiation and heat absorption in the furnace are reduced, which directly affects the boiler evaporation and boiler output. In addition, after mixing with blast furnace gas, the amount of flue gas increases and the exhaust temperature increases. Therefore, the heat exchange area of economizer is increased in the flue behind the furnace, and the boiling economizer is adopted to ensure that its boiling degree does not exceed 20%. Otherwise, due to the increase of working fluid volume and flow rate in the economizer, the flow resistance of the economizer will increase greatly, thus affecting the economy of the boiler. Increase the heat exchange area of economizer, improve the heat absorption of economizer, reduce the excessive exhaust gas temperature, reduce the exhaust gas loss and improve the boiler efficiency.
Transformation measures after complete combustion of blast furnace gas
4. 1 furnace transformation
The radiant heat transfer energy in the coal-fired boiler is very large, and a large number of water walls are arranged in the boiler to absorb the radiant heat. After the combustion of blast furnace gas, the radiant energy in the furnace decreases. Too much water wall absorbs a lot of radiant heat energy, which will further reduce the temperature in the furnace and aggravate the instability of blast furnace gas combustion. Therefore, laying fireproof belt will reduce the heat absorption of the lower furnace in the combustion zone, further increase the temperature of the combustion zone and improve the combustion stability of blast furnace gas. After increasing the flame protection zone, the water wall area decreases and the boiler evaporation decreases. In order to ensure the boiler evaporation, it is necessary to increase the blast furnace gas quantity and increase the furnace heat load. However, the high heat load of the furnace also increases the flue gas quantity and the outlet temperature of the furnace, which leads to the overheating of superheated steam, the increase of flue gas temperature and the decrease of boiler efficiency. Therefore, it is impossible to increase boiler evaporation by infinitely increasing furnace heat load. After the boiler is changed to burn blast furnace gas, the heat transfer capacity in the boiler is obviously reduced. For the boiler with the furnace water wall as the total evaporation heating surface, if the boiler structure is not allowed to be greatly changed, the evaporation will definitely decrease.
4.2 burner modification
For blast furnace gas, power combustion, that is, flameless combustion, is a suitable combustion mode with short flame length, fast combustion speed, high intensity and high temperature. However, it is rarely used in practice because of its large volume, high tempering degree, high noise, inflexible load adjustment, complicated flow passage and high cost. Diffusion combustion is not only too long flame, but also poor mixing and incomplete combustion, which is not suitable for blast furnace gas. In fact, most burners are premixed with some air. This burner is simple in structure, not easy to backfire, sensitive in load adjustment, stable in the case of fluctuation of gas calorific value and air preheating temperature, wide in adjustment range, and stable in the case of minimum load to maximum load of boiler.
The arrangement of burner mainly considers the following points: the flame should be located in the geometric center of the furnace, so that the flame can fill the furnace as much as possible, so that the heat distribution in the furnace is uniform and the load on the heating surface is uniform, and the internal stress will not be increased due to local heating, so as to prevent uneven heating. As for the arrangement height, under the condition of not affecting the flame diffusion angle, the burner is arranged at a low position, which is beneficial to increase the gas combustion time and keep the furnace temperature uniform.
4.3 Modification of Superheater
After burning blast furnace gas, the increase of flue gas quantity will lead to overheating of superheated steam, and the temperature of superheated steam can be controlled within the specified range by appropriately reducing the area of superheater. The temperature of superheated steam can also be controlled by increasing the temperature adjustment ability of desuperheater.
4.4 Increase the gas preheating device
On the one hand, installing a gas preheater can further reduce the exhaust gas temperature and improve the boiler efficiency; On the other hand, it can increase the energy into the furnace, raise the combustion temperature, enhance the radiation ability of the flame, and improve the ignition and burnout conditions of blast furnace gas. It is proved that the theoretical combustion temperature can be increased by 4℃ every time the blast furnace gas temperature is increased by 10℃. However, due to the flammability and toxicity of blast furnace gas, the heat exchange process between blast furnace gas and flue gas is required to be strict and there can be no leakage, so in theory, only separate heat pipe heat exchangers can be used.
4.5 Transformation of Economizer
After burning blast furnace gas, the flue gas temperature rises and the boiler evaporation decreases. Therefore, increasing the area of economizer and adopting boiling economizer can improve the heat absorption capacity of economizer, reduce excessive flue gas temperature, reduce flue gas loss and improve boiler efficiency. On the other hand, the flame blackness and temperature in the blast furnace gas boiler are low, so it is not appropriate to increase the boiler evaporation simply by increasing the heating surface area, but to use boiling economizer to make up for the reduction of boiler evaporation, which is an effective measure to improve the boiler output.
4.6 Tail flue reconstruction
Due to the low calorific value and high inert gas content of blast furnace gas, the flue gas volume and resistance of boiler will increase when burning blast furnace gas. Therefore, it is generally necessary to consider expanding the circulation area of the tail flue to reduce the flow resistance and increase the induced air volume of the induced draft fan.
4.7 Gas safety and explosion-proof measures
Considering the safety, it is necessary to establish the combustion system of gas boiler, including automatic control of automatic ignition, flameout protection, automatic combustion adjustment and necessary chain protection. At the same time, in order to reduce the damage to the furnace and flue during explosion, explosion-proof devices should be installed on the furnace and flue of gas boiler. In addition, the gas system should be equipped with radiation pipes, and the radiation points should be arranged at the entrance side of the main cut-off valve for the boiler room, the end of the main pipeline, the highest point of pipelines and equipment, and in front of the burner. After taking the above safety measures, the safe operation of the boiler can be ensured.
References:
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