| The main energy consumption in China is coal. Coal fired plants are the main source of SO2 pollution because of their dominant role in coal consumption. With the environmental regulations increasingly strict, the wet flue gas desulfurization (WFGD) which main sorbent was calicium based-limestone, became the preferred desulfurization way. As one kind of natural source, the great amount of limestone consumption will influence the earth’s ecological system inevitably.While a great quantity of alkaline industrial wastes(AIW) such as carbide slag(CS), rud mud(RM), paper white mud(PWM), alkali slag(AS) and salt slag(SS) were discharged by numerous chemical companies and non-ferrous metals industries in China, which resulted in the pollution of local environment and became the constraints of the above companies in sustainable development. Previous studies proved that AIW had the ability of desulfurization, and can be used as WFGD sorbent. However, the mechanism for AIW dissolution which is one of the rate control step in WFGD has not been well acknowledged.Desulfurization behaviors of collected AIW are screened through chemical analysis, XRD analysis and comparative study. Results show that there are three kinds of sorbents which have better desulphurization properties:1) CaCO3 based sorbent such as RM, PWM, AS and SS, et al.,2) Ca(OH)2 based sorbent for example CS and 3) calcium-aluminum oxide and calcium magnesium silicate based sorbent like magnesium slag(MS). While some sorbents such as fly ash, furnace slag, blast furnace slag and water treatment waste can not be used as desulfurization sorbent alone. The pH value analysis of AIW slurries show that the higher the pH value of AIW initial slurry, the more obvious of "three stages" in desulfurization process. The concentration of free OH- is increased with the increasing of the pH value of AIW initial slurry, which makes the first stage gently. The peak value of the changing rate of pH value has direct contact with the dissolution of sorbent particles, the lower the pH value of AIW initial slurry, the faster dissolution of the desulfurization sorbent. The AIW which have better desulfurization properties are mainly mentioned in this study and their dissolution characteristics are investigated through pH static method. The dissolution mechanisms of AIW are analyzed by modified unreacted shrinking core model(MUSCM) in order to obtain the data base for designing the desulfurization systems which fit to the AIW. The key parameters which influence AIW dissolution such as pH value, reaction temperature and particle size are discussed by conversion rate and conversion rate differential. pH values have the important effect on sorbents dissolution, the lower the pH value is, the easier the AIW sorbents dissolute. According to dissolution conversion rate differential, the higher pH value the CaCO3 based AIW initial slurry has, the higher the initial conversion rate, while its decay rate is faster also. The magnesium slag need more time to obtain the same desulfurization effect compared to other AIW because the separating of Ca2+ is a slower process. Conversion rate increases with the increasing of reaction temperature although the final curves of conversion rate closed to each other. This proves that reaction temperature affect the reaction velocity only and has slightly influence on final conversion rate. Particle sizes contribute to the AIW conversion rate. The smaller the particle size is, the higher the conversion rate in the same reaction time. According to the decending order, the contributions to AIW dissolution because of the decreasing of particle size are as follows:magnesium slag, Ca(OH)2 based AIW and CaCO3 based AIW.The dissolution mechanisms of CS, RM, PWM, AS, SS and MS are analyzed by MUSCM and their kinetic parameters are calculated. Results show that the dissolution reactions of PWM, AS and CS are under control of film diffusion and particle surface chemical reaction. The activation energies of PWM, AS and CS in film diffusion control step are 11.94±0.7,4.51±0.8 and 7.64±1.8kJmol-1, respectively. While in surface chemical reaction control step the activation energies are 13.69±1.3,36.34±3.1 and 14.41±1.0 kJ mol-1, respectively. The dissolution reactions of SS and MS are controlled by surface chemical reaction and their activation energies are 9.69±1.9and 14.42±1.5 kJ mol-1, respectively. The optimal operation and configuration parameters of impinge stream scrubber experimental bench are gained through CFD simulation analysis. The inlet angle has important effect to the distribution of gas flow in scrubber, when the inlet angle is 13°, the optimal flow and gas velocity distribution can be got. Perforated baffle has the obvious impact on gas flow straightening as the porosity is 0.5. Droplets have significant influence on gas flow straightening. To a given spray flow, droplets distribution have direct contact with gas velocity in scrubber, the higher the velocity is, the lower number of droplets under the baffle, however, the droplets distribution above the baffle are influenced slightly by gas velocity when it is above 3m/s. the optimal gas velocity 3m/s is got by taken the amount of droplets above and under the baffle, demister load and induced draft fan output into account.The impinge stream scrubber experimental bench is established, based on the above simulation results. The five main parameters such as pH value, gas velocity, spray density, liquid gas ratio and inlet gas temperature which influence the AIW sorbents wet desulfurization performances are investigated. The desulfurization behaviors of compound AIW sorbents are discussed also. Desulfurization efficiency is very sensitive to the change of gas velocity when the slurry has a high pH value. Under high spray density conditions, desulfurization efficiency is dependent on droplets distribution, while the flow around action of gas has slightly influence. To the low pH value sorbents, the changing of gas velocity has no significant effect on desulfurization efficiency because of the certain driving force of chemical sorption to SO2. It is recommended that to CS slurry tank the optimal pH value is 6-7, while the circulating slurry pH value is 8-9. To PWM, AS and RM slurry tank, the slurry tank pH vaule is 5-6, while the circulating slurry is 6.5-7.5,6.5-8 and 6.5-8, respectively.Desulfurization efficiency increases with the increasing of lquid gas ratio when gas velocity is stable, while the decreasing of the lquid gas ratio has different effects on Ca(OH)2 based and CaCO3 based AIW sorbents when increasing gas velocity. To CS sorbent, the lower the lquid gas ratio is, the lower the desulfruzation efficiency. While to PWM, RM, AS and limestone sorbents, the desulfurization efficiency rises with the decreasing of lquid gas ratio when gas velocity is 2.1-2.7m/s. The critical desulfurization properties and optimal gas liquid flow can be obtained to CaCO3 based AIW sorbents when gas velocity is 3m/s. When liquid gas ratio is 16L/m3 and gas velocity is 3m/s, the desulfurization efficiencies of CS, PWM, RM, AS and limestone are 97%,87%,84%,91% and 95%, respectively.The desulfurization efficiency is slightly increased with the increasing of inlet temperature under the experimental conditions.The properties of three kinds of compound desulfurization AIW sorbents are conducted in impinge stream scrubber such as limestone primary type, CS primary type and CaCO3 based AIW type.The mixing of CS in limestone results in CaSO3·1/2H2O adhering to the surface of limestone particles because of its low solubility, which will decrease the dissolution of CaCO3. When the compound ratio of limestone and AS is 8:2, the improvement of magnesium and the lowest negative effect of Cl- to desulfurization process can be got. Desulfurization efficiency of limestone-RM type sorbents decreases with the increasing compound ratio of RM. The desulfruzation efficiency of limestone-PWM type changes slightly regardless of the variation of PWM compound ratio.To CS primary type compound desulfurization sorbents, it is the quantity of high solubility material after desulfurization and Al3+, Fe3+concentration in AIW that are the two key factors when choosing CaCO3 based AIW as supplementary material in sorbents.Results on CaCO3 based AIW type show that, the best compound ratio 7:3 of RM-AS type compound sorbent is got because of the improvement of Mg2+ and the inhibition of Cl- to desulfurization process. |
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