Traditionally, the road map for the exhaust gas-air mixture (GAM) purification plant at a rotary lime kiln is as follows:
GAM with a temperature of 1000 ÷ 1100ºС and dust content of up to 40 g / m3 comes from a rotary kiln into a large dust collector, then it is fed to a waste-heat boiler, then to cyclones, installed after the waste-heat boiler and after that – to electrostatic precipitators. In this cleaning scheme the dust in the gas-air mixture mainly consists of the burnt lime dust.
At the present time, manufacturers of gas-cleaning equipment, including ESP, have improved the quality of manufactured equipment that provides a residual dust content of 70 mg / nm3, which is practically impossible to achieve in previous-generation electrostatic precipitators.The analysis of the technical data of the electrostatic precipitator shows that the reasons for non-obtaining the expected technical result are as follows:
Due to the specific properties of the dust appearing during the limestone burning, the use of electrostatic precipitators does not guarantee the high degree of process gas purification that meets the requirements of modern environmental standards.
The use of electrostatic precipitators requires special measures to intensify the deposition process in an electric field, and the growth of non-shaking layers of lime particles on the cathode and anode electrodes leads to such negative processes as “back corona” and a decrease (cut-off) of the corona current having the consequence that some sections of the electrostatic precipitator electrodes do not take part in the process of electrostatic dust sedimentation, and that decreases the cleaning efficiency. Intensification of the process of the limestone dust particles sedimentation in the electric field by conditioning of the process gas flow requires additional costs for selection of the appropriate reagent, a method for its metered supply, as well as additional costs for evaluating the interaction factor of the reagent with the duct material and electrostatic precipitator itself.
In addition, the degree of suspended particles trap in an electrostatic precipitator depends on their size which, due to the fine dispersion and high resistance of dust while limestone burning, also does not allow us to reduce the final dust content in the process gases. The use of an ESP for purification of process gases from a limestone burning kiln is characterized by high energy consumption, high capital and operating costs, as well as by difficult maintenance and insufficient reliability.
At the same time, the use of bag filters can reduce residual dust content to less than 20 mg / nm3.
The heater, installed in the complex of the rotary lime kiln, makes it possible not only to use the heat of the flue gases (the temperature of the GAM decreases from 1100 ° C to 350 ° C), but also to reduce the GAM dust content from 25 g / nm3 to 12.5 g / nm3. The shaft limestone heater can be regarded as a granular-bed filter, filled with classified limestone with a fraction of 20 ÷ 40 mm and a layer height of 8.5 m. It should be noted, that burnt lime dust, having high reactivity, is captured in the shaft heater, and limestone dust is evacuated. The use of a shaft limestone heater enables us to stabilize the gas-dust flow temperature and the dust load parameters.
Thus, the shaft limestone heater can be regarded both as the first cleaning stage and a mixing type recuperator, after which a bag filter can be installed in the rotary kiln complex.
As a rule, the dust and gas emissions characteristics show that the process gas temperature after a cooler is 350 ° C, dust content – 12.5% and humidity – 12.8%.
Experts of Aurum Engineering GmbH offer to use a bag filter in the process gas purification system of the limestone burning kiln with an integrated limestone heater as a primary purification facility. The filter is used for fine filtering and its operation temperature range exceeds the dew point value, the mesh of the filter material corresponds to the size of the particles in the cooled gas stream after primary purification. Filtration area provides filtration capacity of 1,5 -2,0m3/m2 min.
Despite a high dust load of process gases and presence of relatively large particles (the content of particles with the size over 16 microns is 60%), Aurum Engineering GmbH specialists have experience in designing single-stage gas treatment plants, installed after rotary limestone burning kilns, which operate in the conditions of a high dust load and presence of relatively large particles in the stream (the content of particles with the size over 16 microns is 60%).
Implementation of such solutions can reduce capital costs for construction of the first stage of the gas purification plant and save the areas for its installation. The absence of the first stage — inertial dust catchers — also reduces acquisition and operation costs of the draught equipment by excluding the first stage hydraulic resistance from the scheme.