Study On The Composite Humidification Characteristics And Related Problems In Semi-Dry CFB-FGD System

Humidification is the precondition and key factor of achieving high desulfurization efficiency in semi-dry CFB-FGD technology. Humidification has a significant effect on the organization of desulfurization process, desulfurizer utilization efficiency and wall deposition in tower. The existing single humidification way can't achieve high efficient desulfurization process and high desulfurizer utilization efficiency as well as decreasing wall deposition effectively. A new composite humidification way is proposed in this paper. The distribution character of gas-liquid-solid phases was investigated under the condition of composite humidification way, the hydration activation of circulating desulfurization residues humidification and water droplets collision capture solid desulfurizer particles was also investigated in this paper. The concept of cooperativity was proposed and the cooperativity of gas-liquid-solid phases was also estimated under different humidification way.Firstly, the effect of composite humidification way on gas-liquid-solid phases distribution character was investigated under the conditions of straight flow fluidized way. The results show that, compared with in-tower humidification way, the distribution trend of gas-liquid-solid phases wasn't changed basically, but water droplet particle concentration and diameter were decreased in tower. The appropriate humidification water quality ratio of in-tower and out-tower was between 7/2 and 5.5/3.5 (the ratio of in-tower humidification water quality to total humidification water quality was between 61% and 78%). In the ratios mentioned previously, the average concentration of water droplets decrease less than 10% in the center area of tower and between 27% and 37% in the area near the wall respectively. Therefore it can decrease the possibility of wall deposition while ensure less change of water droplet concentration in main desulfurization reaction area in tower.In order to enhance desulfurizer utilization efficiency further, a kind of swirling-straight flow fluidization structure was adopted, and the effect of composite humidification way on gas-liquid-solid phases distribution character was investigated under swirling-straight flow fluidized way. Compared with straight flow fluidization way, the inner solid particle concentration was increased and tangential disturbance was enhanced in middle-lower part of tower under swirling-straight flow fluidization way, which is beneficial to accelerate heat and mass transfer and enhance desulfurizer utilization ratio and desulfurization efficiency. The impact of composite humidification way on water droplet particle concentration and diameter distribution rule was small. The distribution trend was still higher in center area of tower and lower in areas near the wall under the swirling-straight flow fluidization way. But compared with corresponding humidification condition of irrotational flow, the water droplet particle concentration was decreased and the water droplet particle diameter was increased in main desulfurization reaction area. The water droplet particle concentration was decreased further with the increase of out-tower humidification water quantity. Due to the cooperation of composite humidification way and swirling-straight flow fluidization way, the droplet concentration decreased very quickly in tower. Both the less possibility of wall deposition and higher water droplet concentration in tower were taken into consideration, an appropriate humidification water ratio of in-tower and out-tower as 7/2 (the ratio of in-tower humidification water to total humidification water is 78%) is proposed.The concept of gas-liquid-solid phases cooperativity was proposed. Good cooperativity was defined as having better tendency of gas-liquid-solid phases concentration distribution, better desulsurization reaction process and less possibility of wall deposition, among which the last two factors was more important. Compared with in-tower humidification way, the gas-liquid-solid phases concentration distribution cooperativity can be enhanced, the desulfurization reaction process can be rationally organized, high efficiency ion liquid phase desulfurization reaction and less possibility of wall deposition can be achieved simultaneously in appropriate humidification water quality ratio range of in-tower and out-tower; the better three phases cooperativity can be achieved under the condition of composite humidification way. Applying composite humidification way can make the best intensity disturbance, heat and mass transfer in middle-lower part of tower under swirling-straight flow fluidization way, which can make the desulfurizer of high concentration in the area of tower inlet and near the wall, and the ion reaction in liquid phase was still high efficiency because of less water droplet quality decrease in tower center area. All these were beneficial to high efficiency and complete desulfurization reaction process. In addition, the presence of swirling flow was beneficial to decrease wall deposition, and the water droplet evaporation can be enhanced further because of the combination with humidification way in the area of near tower wall, so the problem of wall deposition can be resolved obviously.Taking composite humidification way and the similarities and differences between circulation desulfurization residues composition and fly/Ca(OH)₂ hydration system as starting point, the effect of CaSO₃ on physical properties and desulfurization activity of hydration products in the fly ash/Ca(OH)₂ system is systematically investigated under different hydration conditions in this paper. The results indicates that the mineral species, BET SSA and middle-small pore volume of hydration products can be increased; the ability of SO₂ adsorption and Ca(OH)₂ utilization of hydration products can be enhanced if CaSO₃ is added to fly ash/Ca(OH)₂ system. According to these results, a new utilization way of using semi-dry FGD residues to prepare high activation desulfurizer directly was proposed and the feasibility was analyzed. The problems of this utilization way and the practicle humidification activation effect of composite humidification way on circulation FGD residues were also analyzed.The formation of alkaline slurry drops through water droplets collision to capture solid desulfurizer particles is the premise of high efficient ion desulfurization reaction in liquid phase. So the whole efficiency of water droplets capture solid particles must be enhanced in FGD tower. Based on water droplets collision and capture solid particles under straight flow fluidization way, the model of water droplets collided and capture solid particles under swirling-straight flow fluidization way was built. According to the related gas-liquid-solid phases parameters under swirling-straight flow fluidization way, the effect of different factors on capture efficiency was investigated. The results indicated that the capture efficiency was decreased at the beginning and then increased with the increase of water droplet diameter, solid particle diameter and water spray quantity. The capture efficiency was also increased with the increase of water droplet velocity, but the increase degree was not obvious when the water droplet velocity was faster than 10m/s. The general change trend of capture efficiency wasn't changed but capture efficiency was enhanced under swirling-straight flow fluidization way. The capture efficiency along the tower can be divided into three areas including high efficiency capture area, stable capture area and low efficiency capture area. The stable capture area was the most important area of water droplets capturing solid particles, and this area was also the main area for the defulfurization reaction in ion liquid phase. The effect of composite humidification way on water droplets collision and capture solid particles efficiency was small in the main defulfurization reaction area under the appropriate in-tower and out-tower humidification water quantity ratio. The whole desulfurization efficiency can be enhanced and the possibility of walldeposition can be decreased by means of rational organization of reaction process and water distribution. The effect of water spray on fly ash agglomeration was also investigated in this paper. By comparing solid particle diameter change of pre and post water spray, the diameter of ash particle at the bottom of the tower is the larger; the diameter of circulation ash particle is larger than that of ashes in dust collector, the distribution of ash particle diameter in dust collector was similar to that of the ashes in tower inlet. The solid particle diameter was significant enlarged and the phenomena of solid particle agglomeration was obvious after water spray humidification. The particle agglomeration types include superfine particle agglomerates, agglomerates carried by lager particle and coated body structure.Finally, a model to simulate the composite humidification way in semi-dry CFB-FGD system under straight flow fluidization way was built. Different conditions of temperature and humidity fields were simulated in this model. The results indicates that the simulated distribution trend of temperature, moisture concentration and water droplet concentration are very similar with the experimental results, and the average tower outlet temperature and average air humidity content also agree with the experimental measurement data. So this model can be used as a supplementary and a testifying method to the analysis of experimental data, which acts as a guide to the experiment and industrial application of composite humidification way.