Comparison of various desulfurization processes
1 Comparison of desulfurization processes
Currently, the flue gas desulfurization process can be divided into dry process, semi-dry process and There are three categories of wet methods.
1.1 Dry method
Dry method commonly used include calcium injection (lime/limestone) in the furnace, metal absorption, etc. Dry desulfurization is a traditional process, and the desulfurization rate is generally not high (< 50), with few industrial applications.
1.2 Semi-dry method
The most commonly used semi-dry method is the spraying method in the tower, which is to make lime into lime slurry and absorb SO2 in the tower. However, due to the lime The speed of dissolving SO2 is slow, the calcium spray reaction efficiency is low, and the Ca/S ratio is large, generally above 1.5 (general warm-method desulfurization Ca/S ratio is 0.9 to 1.2). There are not many applications.
1.3 Wet method
Wet desulfurization is currently the most widely used desulfurization method, accounting for 80% of the total desulfurization. Diffuse desulfurization can be divided into limestone/lime method, ammonia method, sodium-alkali method, soda-calcium double alkali method, metal oxide method, alkaline aluminum sulfate method, etc. according to the different desulfurization raw materials. Among them, limestone/lime method, ammonia method , sodium-alkali method, sodium-calcium double-alkali method and magnesium oxide method in metal oxides are more commonly used.
1.3.1 Limestone/lime method
The limestone method uses limestone crushed into lime powder of 200 to 300 mesh size, which is made into lime slurry, and is sprayed in the absorption tower. Atomization makes it come into contact with the flue gas to achieve the purpose of desulfurization. This process requires a limestone crushing system and a limestone pulverizing slurry system. Since the limestone activity is low, it is necessary to increase the spray volume of the absorbing liquid and increase the liquid-gas ratio to ensure sufficient desulfurization efficiency, so the operating cost is high. The lime method uses lime powder instead of limestone. The activity of lime is much higher than that of limestone, which can improve the desulfurization efficiency. The main problem of the lime method is that it is easy to scale in the tower, causing the gas-liquid contactor (nozzle or tray) to be blocked.
1.3.2 Sodium-alkali method
The sodium-alkali method uses alkaline substances such as sodium carbonate or sodium hydroxide to absorb SO2 in the flue gas, and can by-produce high-concentration SO2 gas or Na2SO3 has the advantages of non-volatile absorbent, high solubility, high activity, and no clogging of the absorption system. It is suitable for waste gas SO2 absorption treatment where the flue gas SO2 concentration is relatively high. However, there are also shortcomings such as difficulty in recovering by-products, high investment, and high operating costs.
1.3.3 Ammonia method
The ammonia method uses ammonia water as the absorbent of SO2. The reaction between SO2 and NH3 can produce ammonium sulfite, ammonia bisulfite and part of the ammonia produced by oxidation. Ammonia sulfate. According to the different absorption liquid regeneration methods, the ammonia method can be divided into ammonia-acid method, ammonia-ammonium sulfite method and ammonia-ammonia sulfate method.
The main advantage of the ammonia method is high desulfurization efficiency (same as the sodium-alkali method), and the by-products can be used as agricultural fertilizers.
Because ammonia is easy to volatilize, the consumption of absorbent increases and the utilization rate of desulfurizer is not high; desulfurization has certain requirements on the concentration of ammonia water. If the concentration of ammonia water is too low, it will not only affect the desulfurization efficiency, but also affect the water circulation system. It is huge, which increases operating costs; an increase in concentration will inevitably lead to an increase in evaporation, which will have an impact on the working environment. Moreover, ammonia easily reacts with SO2 in the purified flue gas to form aerosol, making the flue gas unable to meet the emission standards.
The recovery process of the ammonia method is also more difficult and the investment cost is high. It needs to be equipped with an acid making system or a crystallization recovery device (needs to be equipped with a neutralizer, crystallizer, dehydrator, dryer, etc.). The system Complex, with numerous facilities and high management and maintenance requirements.
1.3.4 Metal oxide method
The commonly used metal oxide method is the magnesium oxide method. Magnesium oxide reacts with SO2 to obtain magnesium sulfite and magnesium sulfate, which can be obtained by calcining Re-decompose magnesium oxide and recover purer SO2 gas at the same time, and the desulfurizer can be recycled. Since the magnesium oxide activity is higher than that of lime water, the desulfurization efficiency is also higher than that of the lime method. Its disadvantage is that the magnesium oxide recovery process requires calcination and the process is complicated. However, if the discarding method is directly used, the magnesium salt will cause secondary pollution and the overall operating cost will be higher.
1.3.5 Sodium-calcium double-alkali method
Sodium-calcium double-alkali method (Na2CO3/Ca(OH)2) is based on the lime method combined with the sodium-alkali method, using sodium salt Easily soluble in water, sodium alkali is used inside the absorption tower to absorb SO2. The absorbed desulfurization liquid is regenerated with cheaper lime in the regeneration tank, thereby recycling sodium ions for absorption and utilization.
This process combines the characteristics of the lime method and the sodium-alkali method, solves the problem of easy scaling in the tower of the lime method, and has the advantage of high absorption efficiency of the sodium-alkali method.
The by-product of desulfurization is calcium sulfite or calcium sulfate (after oxidation). Calcium sulfite is combined with synthetic resin to produce a new type of composite material called calcium plastic; it can be oxidized to make gypsum; or it can be directly mixed with fly ash to increase the plasticity of fly ash and increase fly ash. The strength of ash as a base material for paving. Compared with the magnesium oxide method, calcium salt is not polluting, so it does not produce secondary pollution of waste residue.