The dry adsorption process with sodium hydrogen carbonate as adsorbent has become a means of flue gas purification to remove various pollutants contained in flue gas. The purification effect can be compared with other known methods, such as the spray adsorption method using lime milk as adsorbent. Dry flue gas purification can be used not only in coal power plants, waste or alternative fuel incineration plants, but also in industrial furnaces in glass, cement, metallurgy and other industries. Using dry flue gas purification can economically meet the flue gas emission standard
Basic principle of sodium bicarbonate for flue gas desulfurization
Sodium bicarbonate (baking soda, NaHCO3) can be used as adsorbent for flue gas desulfurization. It can remove acidic pollutants in flue gas through chemical adsorption. At the same time, it can also remove some inorganic and trace substances through physical adsorption. In this process, the fine sodium bicarbonate powder is directly sprayed into the high-temperature flue gas. Sodium bicarbonate is divided into sodium carbonate Na2CO3, H2O and CO2 at high temperature.
The newly produced sodium carbonate Na2CO3 has a high degree of reaction * * at the moment of formation, and can spontaneously react with the acid pollutants in the flue gas
一般情况下，烟气温度在140和250 °C之间。由于碳酸氢钠吸附剂的高度**，通常略微过量的碳酸氢钠（化学计量因子在1.1和1.3之间）就足够 。
In general, the flue gas temperature is between 140 and 250 ° C. Due to the high * * of sodium bicarbonate adsorbent, a slight excess of sodium bicarbonate (stoichiometric factor between 1.1 and 1.3) is usually sufficient.
In principle, sodium carbonate (soda, Na2CO3) can also be used for flue gas desulfurization. The reaction * * of sodium carbonate is slightly lower, but the process flow is almost unchanged. The desulfurization of sodium carbonate is not further described in this paper.
由于运输和存储的原因，碳酸氢钠原料通常是粗颗粒（d50值约为200微米）。如要达到较高的反应性，吸附剂需要有较大的比表面积。因此在注入烟气管道前，碳酸氢钠研磨至一定细度。比如，要去除SO2碳酸氢钠细度须达到d90 < 20 µm。而去除HCl只要求d90 < 35 µm。如果系统操作正确，可以去除95%以上的SO2；HCl的去除率甚至可达 99%。
Due to transportation and storage, the raw material of sodium bicarbonate is usually coarse particles (D50 value is about 200 microns). In order to achieve high reactivity, the adsorbent needs to have a large specific surface area. Therefore, before injecting into the flue gas pipe, the sodium bicarbonate shall be ground to a certain fineness. For example, to remove SO2, the fineness of sodium bicarbonate must reach D90 < 20 & micro; m。 For HCl removal, only D90 < 35 & micro; m。 If the system is operated correctly, more than 95% SO2 can be removed; The removal rate of HCl can even reach 99%.
In order to maintain the required fineness of sodium bicarbonate in long-term operation, impact mill with classifier, also known as grading mill, such as zj-c grading impact mill, is usually used. After grinding, the adsorbent is transported by gas transportation and directly connected to the flue gas pipeline through multiple nozzles to ensure its uniform dispersion in the pipeline. The design of this equipment is simple and durable. Compared with other flue gas purification methods, its investment and operation cost are low.
This is a schematic diagram of the conventional negative pressure process in which sodium bicarbonate passes through a screw conveyor from the storage tank (a)
(1) The amount of adsorbent required by the downstream grading mill (2) can be calculated from the pollutant concentration trend curve before and after desulfurization, and the screw conveyor is controlled by the frequency converter. A rotary valve must be installed between the screw conveyor and the grading mill to isolate the air flow so as not to affect the quantitative feeding of the screw conveyor. After the sodium bicarbonate is ground in the graded mill, the fine powder is transported to the flue gas pipe (b) by a material conveying fan (3).
The crud problem in this operation mode is often the focus of discussion. The cleaning of equipment, especially the cleaning of material conveying fan, is too time-consuming. In addition, the scale on the fan impeller may peel off during startup and shutdown, resulting in the vibration of the fan and the oscillation of its operating state.
Therefore, it is considered to change from negative pressure process to positive pressure process. In the new working mode, the fan is installed before the grading mill, and the process air is pressed into the grinding system. The advantage of this working mode is that the ground sodium bicarbonate is not transported through the fan, which completely avoids the scaling problem of the fan. In addition, the power consumption of the fan is also reduced, making the operating cost lower
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