[Abstract] plate heat exchanger furnace gas exports recovered waste heat boiler for warm air into the fluidized bed boiler, steam production increased about 5%, and solved the sulfur trioxide condensation and dust clogging problems. The boiler for the center of the thermodynamic analysis and the use of low-temperature waste heat can be used in the process of losses.
Sulfuric acid production furnace waste heat boiler outlet temperature is generally 350 ~ 400 ℃, or the export of electric furnace dust temperature around 320 ℃. This part of the recycling furnace gas heat has been of concern to workers in sulfuric acid. But to recover this part of the heat has two problems to be solved: First, to prevent condensation of sulfur trioxide; the second is the impact of dust on heat transfer equipment. These two issues has been hampered to the field, I have been thinking about how to recycle this part of the heat. This part of the heat if we can use, not only to increase steam production and reducing the cleaning load. 2007 Ming (Zhanjiang) Co., Ltd. Chemical Machinery Engineering at 400 t / d sulfuric acid plant installed a boiler units exported plate heat exchanger used to preheat the air into the fluidized bed boiler, warm air from 40 ℃ to 130 ℃, recycling Steam heat can be prolific 1.1 ~ 1.20 t / h, that is, 9 000 t / a or so, to increase steam production by about 5%, while only 30 million investment (including pipes and insulation), to achieve better economic benefits, while a better solution to sulfur trioxide condensation and dust blockage.
1 with a plate heat exchanger, heat recovery boilers warm air furnace gas exports
1.1 should pay attention to problems
1) increase gas furnace wall temperature to prevent condensation of sulfur trioxide. Sulfur trioxide dew point, the nature of the sulfur trioxide into sulfuric acid vapor combined with water vapor and condensed into a certain concentration of sulfuric acid. Conditions in the furnace gas, condensed into a sulfuric acid if its w (H2SO4) is generally about 90%, the corrosion of ordinary steel is more serious, so it is economizer or heat exchangers are required to avoid condensation of sulfur trioxide gas furnace .
Temperature and dew point of sulfur trioxide gas in the furnace and water vapor concentration of sulfur trioxide content. The concentration of sulfur trioxide gas furnace and roasting conditions. For pyrite roasting, the oven gas sulfur trioxide concentration in the color of the main and dust, the dust color from black to brown-black room (SO3) 0.05% to 0.13%, the partial pressure of 50 ~ 130Pa.
The water vapor content of air and water on mine: If the air temperature at 30 ℃, relative humidity at 80% calculated; ore lump ore and water main difference between the tailings, water content by 1.0% to 6% . As a result, calculate the partial pressure of water vapor in the gas furnace is 4 000 ~ 9 000 Pa, the dew point on the map in sulfur trioxide sulfur trioxide Richard dew point temperature of 190 ~ 220 ℃. Of course, the dew point of sulfur trioxide on the map in Richard's dew point temperature for reference only. According to some empirical data using low-pressure boiler plant, pressure 0.8 MPa (gauge) steam temperature of 175 ℃, 185 ℃ wall temperature should be about, so long as reasonable structure, low-pressure boiler can also use 4 to 5 years. But in the actual design "board for" gas furnace should try to make the side of the wall temperature ≥ 200 ℃, more secure.
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How to enhance sulfur dioxide gas furnace side of the wall temperature it? This would be from the heat exchanger design solution. If, furnace gas into the plate heat exchanger temperature is 320 ℃, outlet temperature of 250 ℃, the air inlet temperature is 30 ℃. If the design of the normal gas furnace and air for the counter, then the low end of the average temperature of only a wall temperature (250 +30) / 2 = 140 ℃, so can not make the furnace gas side wall temperature ≥ 200 ℃. Therefore, the design used furnace gas and air and flow the way to improve air inlet furnace gas side wall temperature, so that the average wall temperature (320 +30) / 2 = 175 ℃, more than counter-current design improved 35 ℃, but still < 200 ℃, use is still unsafe. Therefore, in the design of "board for", the need to make the furnace gas side heat transfer coefficient is greater than the air side heat transfer coefficient, gas wall furnace temperature to increase.
Air-side heat transfer coefficient α, the furnace gas side heat transfer coefficient 1.5α, and set the wall temperature is t. If you do not take into account the thermal resistance of steel and dirt effects, the import side, there are: (t-30) α = (320-t) 1.5α, was t = 204 ℃; in the export side, according to the heat balance to calculate the air temperature is about 130 ℃, there are: (t-130) α = (250-t) 1.5α, was t = 202 ℃. If the actual operation, the air temperature <130 ℃, can be installed in the air side of the Vice-line, the plate heat exchanger air outlet temperature ≥ 130 ℃.
The above data shows that the use of gas and the flow of air and furnace operation, and design "board for" furnace gas side heat transfer coefficient is greater than the air side heat transfer coefficient, can make the furnace wall temperature is higher than sulfur trioxide gas dew point temperature .
In exceptional circumstances, such as the northern winter Blower outlet air temperature may be only 0 ℃, even lower than 0 ℃ how to do? Can design a low-pressure steam heated with air preheater, the first air heated to 30 ~ 40 ℃, and then into the plate heat exchanger, plate heat exchanger can be sure that the furnace gas temperature higher than the wall of sulfur trioxide dew point temperature.
2) to prevent adhesion of dust in the furnace gas wall caused by blockage in the board.
Export or use of the boiler furnace gas heat ESP exit, we must consider the impact of dust on the heat exchanger. Plate heat exchanger can solve this problem, because the top-down flow of gas with dust, but also with the air and dust movement. There is also a gravity and dust, so long as the acid does not produce condensation, dust is not easy to adhere to the plate wall. In the design of the gas flow channel is also equipped with a device to enhance gas turbulence, the dust easily attached to the wall.
1.2 plate heat exchanger gas heat recovery boiler plate heat exchanger effect parameters:
Sulfuric acid production 17.0 t / h
Furnace gas dust 30 g/m3 (standard state)
Furnace gas temperature of 317 ~ 250 ℃ out
Air temperature of 40 ~ 130 ℃ out
Furnace gas side pressure drop 450Pa
Acid production calculated according to the amount of air 26 000 ~ 27 000 m3 / h heat recovery 3.0 × 106 ~ 3.15 × 106 kJ / h this plate heat exchanger using more than six months out of the gas temperature does not change, indicating that heat transfer factor has not declined, there is no dust adheres to the basic wall plate. Can also explain the wall of the plate heat exchanger without condensation acid, otherwise the dust will adhere to the above. From this analysis, we design the plate heat exchanger should be long-term use, of course, but also by the longer test.
2 low level heat energy using the thermodynamic analysis
Sulfuric acid production process a large number of low bit of waste heat, waste heat utilized this part or part utilized for improving the sulfuric acid production process steam production, thereby enhancing the power output has a very good value for money. Mentioned in this article: the use of boiler exit gas or electric furnace dust export into the boiling furnace heat to preheat the air; with dry heat to preheat turbine suction out of soft water, to warm up to about 100 ℃; use into the boiler economizer to pump out water heated to 104 ℃ 170 ~ 180 ℃. This number can be very good if you do use heat, steam production can be increased about 20%, and the saving of steam deaerator, the actual increase in volume and value-added generating more than 20%.
2.1 Thermodynamic Analysis of preheated air
Let the air into the fluidized bed boiler is 40 ℃, the roasting process can be made (1-T0 / T)-Q (see Figure 1).
Furnace gas can be used for: Ex = ∫ (1-T0 / T) dQ where T0 --- ambient temperature, 298K;
T --- medium temperature, K;
Q --- medium heat, kJ / h.
Points can be illustrated (see Figure 1), the figure: AB for the furnace gas heat, can be used for ABB3A this area; AD for the boiler exit flue gas heat away, it can be used for ADD1A this area; CB section layer evaporation tube removed for the boiling heat, can be used for CBB3C1C this area; to double-dotted curve above this area of the roasting process exergy loss. Now to preheat the air, the heat of AA2, the boiling layer of furnace gas to increase the heat to BB1, apparently AA2 = BB1. But this can be the difference between the two large air can be used after the warm-up area of AA1A2A, but due to the increase in air preheat furnace gas available for BB1B2B3B, obviously a lot of the latter than the former. 2.2 boiler steam drum heat transfer process and the thermodynamic analysis of the above mixed gas furnace is not available we can get to the actual steam that can be used for energy, there's the drum in the boiler heat transfer and mixing soft drinks The exergy loss, shown in Figure 2.
Figure 2 can be used in the steam area DD2E1G1H2H1B4B1D, which H1B4B1HH1 boiling furnace to generate steam used to; G1H2H1G1 for the superheated steam and saturated steam available to the poor. This process can be used to heat loss D2D1B2B4B5H1H2G1D2 this area; and drum mixed in soft drinks does not change the heat, but can be used to produce the loss, the area of DD2E1D, because here the water into the steam drum water at room temperature, saturated steam temperature is below temperature, mixing entropy generated, causing the loss of available energy.
If the drum into the water to warm up, because this part of the heat from the boiler to get outside, so the heat can be left to extend the line, assuming the water into the drum of the steam in the boiler to preheat outside the saturation temperature, then the time there is no drum in the exergy loss, and increased FD this heat can be used to increase the FD2DF this area. Usually in boiler feed water deaerator preheated to 104 ℃, and now will use the waste heat into warm water to 170 ~ 180 ℃, only increasing the heat, while reducing the drum can be used in soft drinks mixed losses. Boiler feed water temperature to improve the above analysis is to reduce the amount of condensing the steam drum, thereby increasing the steam production, thus reducing the time of the drum in the soft drink mixed entropy, can be used to reduce the loss of boiler feed water temperature increase, thereby increasing the steam production process, and did not increase the boiler's evaporation. Evaporation from the evaporation of the boiler tube area, the saturation temperature of water (or saturated steam temperature) and the oven temperature decision. Therefore, the water temperature in the steam drum level only with the mixing process, whereas the process of evaporation and evaporation tube has nothing to do. Therefore, the conversion system is installed economizer, both old and new boilers, the boiler's evaporation area were unchanged. But because the water temperature has increased, the actual steam production increases. Must be noted that the boiler water temperature increased, the warm water of the condenser used to produce reduced size would need to be adjusted. Because the original cut is 104 ℃ warm water and saturated steam around 250 ℃ heat exchange, so that saturated steam condensing into water, now with 170 ~ 180 ℃ of water and saturated steam for heat exchange, Δt is reduced, and therefore produce less warm water of the condenser area to be increased.
2.3 steam heat into mechanical energy available to analyze
Used to preheat furnace gas into the boiling furnace air, the conversion of waste heat to preheat boiler feed water, steam pressure increase in the production of about 20%. Because the steam production increases, deaerator steam consumption also increased, if the dry heat absorption to replace oxygen consumed by steam, then increase the generating capacity of more than 20% of the overall economic efficiency is more evident. Of course, not all the heat of steam as mechanical energy, steam can be effective area of FD2G1H2H1B4B1, minus the exhaust steam is available to area FMNL, the size of the difference between the steam heat that can be converted to mechanical energy acting outside the area (see Figure 3).
3 Conclusion
The use of low-temperature waste heat boiler for the center is, and its utilization by the boiler parameters restrictions. Preheat oven air temperature by the boiler exit temperature and dew point temperature limit sulfur trioxide; and boiler feed water temperature of saturated steam by the boiler temperature limit, of course, in the pyrite acid system into waste heat to preheat boiler feed water is unlikely to to the steam saturation temperature. Boiler feed water preheating air and preheat the two parts can directly increase the heat output of medium pressure steam. And although a large number of low-temperature waste heat drying and absorption are available, but the amount of steam produced by the boiler comprehensive restrictions, air preheat and preheat boiler feed water improves boiler steam production, which also increased the low-temperature waste heat drying and absorption of utilization. Of course, the use of dry heat absorption by the acid temperature limitations and some technical issues to be addressed. In short, the use of sulfuric acid waste heat should be considered in order to improve their economic efficiency.
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