Wet desulfurization flue gas reheating system in cement plant
Date：2019-08-23 15:40 Source：cementepcViews:
With the restrictions on mining and the reduction of high-quality limestone stock, many cement plants have to use low-grade limestone, resulting in excessive SO2 in the flue gas of kiln tail, and some cement plants with substandard discharge began to adopt the wet method (kiln ash-gypsum method) for desulfurization.
The net after desulfurization flue gas exhaust smoke temperature is low (45 ° ~ 60 ° C) C, with saturated vapor absorption tower export net part of the condensation in the process of flue gas in the discharge of droplet, smoke from the chimney mouth cannot effectively uplift after discharge, spread to the atmosphere, especially when the area of temperature, pressure is low or in cloudy weather, time, carry the dust in flue gas and droplet gathered near the chimney forming acid rain or rain gypsum, cause pollution in the power plant and the surrounding environment, even corrosion of equipment.Therefore, it is very important to improve the smoke temperature and reduce the "gypsum rain".
1. Ideas for reducing "gypsum rain
From the genesis of "gypsum rain", it can be seen that its formation mechanism is complex, which has a great relationship with the amount of smoke and the velocity of smoke in the tower, the selection of mist eliminator, the liquid-gas ratio of reaction tower (sulfur content), the form and velocity of chimney barrel.
Generally speaking, the higher the velocity in the absorption tower and the chimney, the more likely to produce "gypsum rain";The higher the concentration of SO2 in flue gas, the more likely it is to produce gypsum rain.Flat plate type mist eliminator is easy to produce "gypsum rain", roof ridge type cement plant wet desulfurization flue gas reheat system discussion effect is relatively good;The chimney walls are smoother than the rough ones that produce "gypsum rain", and adding a tubular mist eliminator is better than not adding one.But at present, due to the difference of environmental conditions, quality of limestone and the difference in the choice of design parameters, "gypsum rain" occurs from time to time, one of the solutions is to heat the net smoke.
2. Comparison of heating net flue gas schemes
The schemes of wet desulfurization flue gas reheating in cement plant mainly include gas-gas heater, gas-gas heater, bypass flue gas heating, net flue gas heating of kiln head, etc.
The comparison of smoke raising and exhaust temperature of wet flue gas is shown in the table below.It can be seen from the following table that GGH or SGH flue gas heating scheme in the cement plant application is very small, may use the bypass flue gas method or kiln head net flue gas heating method.
3. Bypass flue gas heating
When the raw material mill is in operation, the SO2 at the outlet of most kiln tail precipitators is less than 600mg/Nm3. When the SO2 limit is 200mg/Nm3 or the SO2 limit is 100mg/Nm3 for enterprises in key areas, the bypass heating scheme is feasible.When the emission requirement is 30mg/Nm3 or lower, it is easy to exceed the emission standard by adopting bypass heating scheme.
Assuming that completely mixed flue gas, flue gas in about 1% of the total bypass can improve the hot flue gas absorption tower outlet flue gas temperature of 0.9 ° C, flue gas reheat degree will be affected by the net amount of liquid drop in flue gas, net gas exists in the water, the more mixed flue gas temperature is lower, most of the heat is used for the droplet evaporation, so high performance mist system is also the important factors influencing the bypass heating scheme is successful or not.
4. Net flue gas heating of kiln head
The newly added pipeline extracts part of the hot flue gas (120°C ~ 160°C) from the kiln head exhaust fan.In order to monitor and control the parameters of the flue gas heating system, adjustable baffle doors and corresponding temperature and pressure measuring devices should be set up on the flue gas heating system.
According to experience, the air volume of kiln tail of the 5000t/d clinker production line is about 500,000 Nm3/h. After desulfurization, the smoke volume increases to 550,000 Nm3/h, and the flue gas temperature is 45°C ~ 55°C.The furnace smoke volume is about 300,000 Nm3/h and the temperature is about 150°C. Considering the pipe heat dissipation and other factors, the temperature entering the mixer is about 140°C.Only by accounting that 45% ~ 55% of the net flue gas of the kiln head should be extracted into the flue gas mixer can the net flue gas after desulfurization be heated to 70°C to reduce the "gypsum rain" phenomenon.
The flue gas mixer is set at the outlet of the reactor tower. Considering the harsh conditions such as moisture, corrosion, alternating hot and cold, erosion and so on, the metal materials of the mixer are selected to be resistant to sulfuric acid dew point corrosion steel, such as s-ten or domestic 09CrCuSb (ND) steel.
The temperature of net flue gas after wet desulfurization is low, which is easy to produce gypsum rain.Based on the process characteristics of cement plant, two heating schemes are put forward to improve the flue gas exhaust temperature after desulfurization, without additional energy consumption.
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