High temperature flue gas desulfurization: experiments and modelling.
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As stringent environmental regulations regarding SO2 emissions have been enacted in many countries, the removal of SO2 from flue gas has become a necessity. Conventional methods for flue gas desulfurization (SO2 removal) involve cooling of the flue gas followed by scrubbing and subsequent reheating of the treated gas. To minimize cooling/heating requirements, thus optimizing energy consumption and related expenses, high temperature flue gas desulfurization technologies utilizing dry sorbent injection have recently been emphasized. Experimental studies reported in literature dictate that possible formation of sulfur trioxide (SO3) in situ has been largely ignored for the high temperature desulfurization process. Since the performance of desulfurization systems is based on the amount of SO2 removed, any conversion to SO3 would result in an overestimated efficiency with respect to SO2 removal. This study focused on determining if significant amounts of SO3 are formed at the excessive temperatures of high temperature flue gas desulfurization and what effect this has on the actual amount of desulfurization achieved. The experiment was designed with one independent variable (desulfurization temperature) and three levels for that variable. The aim of the experimental design was to evaluate the effect of desulfurization temperature (700oC, 800oC and 900oC) on the amount of SO2 removed using limestone as the solid sorbent – through determination of breakthrough time and sorbent sorption capacity (based on saturation time) – and the amount of SO3 formed – estimated via the Isopropanol Absorption Bottle Method. Reaction of SO2 within a packed bed of limestone sorbent particles was modelled by considering two descriptions with regards to the morphological structure of the sorbent surface: a non-porous surface and a porous surface. Solution of the developed model was undertaken utilizing the Method of Lines (MOL). Experiments were carried out using a bed height of 3 cm and fixing the inlet flowrate of flue gas at 6Lmin, resulting in a residence time within the sorbent bed of 0.3 seconds. A gas chromatograph (GC) was utilized to generate the transient SO2 concentration profile at the reactor inlet and outlet. This ultimately determines breakthrough time, saturation time and sorbent sorption capacity. The GC was thus required to be calibrated. A third-degree polynomial was found to best fit the calibration curve data. A dead time of three seconds was estimated for flue gas to propagate through the system to the sample extraction point. Sieve tray analysis of the limestone sorbent particles revealed an average particle size of 362.76μm. Experimental results for SO2 removal indicate that breakthrough time, saturation time and SO2 sorption capacity of limestone sorbent all increase with an increase in operating temperature. This trend is attributed v to formation of larger volume product CaSO4 layers on active sorbent surfaces effecting a more gradual decline in overall diffusivity (Dz) with increasing operating temperature, as expressed by results of the model analysis. Breakthrough times of 100s, 210s and 240s were achieved for operating temperatures of 700 ℃, 800 ℃ and 900℃ respectively; saturation times of 890s, 1060s & 1200s were achieved for operating temperatures of 700 ℃, 800 ℃ and 900℃ respectively; SO2 sorption capacities of 13.50mg ofSO2gram of Sorbent, 16.20mg ofSO2gram of Sorbent and 18.73mg ofSO2gram of Sorbent were achieved for operating temperatures of 700 ℃, 800 ℃ and 900℃ respectively. The model analysis proves further that the limestone sorbent particles are porous in nature due to a conservative fit between experimental data and the predicted model solution. Results for SO3 formation indicate that the generation of SO3 is predominantly due to the heterogeneous catalytic reaction of SO2 with the stainless steel walls of the reactor at such elevated temperatures utilized during high temperature flue gas desulfurization. In the absence of stainless steel the mass of sulfur entering the system which is converted to SO3 was calculated to be 0.251%, 0.249% and 0.247% for the operating temperatures of 700 ℃, 800 ℃ and 900℃ respectively. In the presence of stainless steel the mass of sulfur entering the system which is converted to SO3 was calculated to be 3.24%, 5.60% and 9.30% for operating temperatures of 700 ℃, 800 ℃ and 900℃ respectively. It can thus be concluded that as operating temperature shifts away from 700oC towards 900oC the formation of SO3 becomes much more significant in the presence of stainless steel. The amount of desulfurization achieved however is similar for the respective operating temperatures. Since stainless steel is a requirement it is recommended to operate at a temperature of 700oC since similar performance is achieved by the limestone sorbent in terms of SO2 removal, relative to 800oC and 900oC, while SO3 production is relatively minimal.