Reactor Technology Research Group University of KwaZulu-Natal Highly dispersed zinc based sorbents for hot gas desulphurization: synthesis and application David Lokhat Reactor Technology Research Group University of KwaZulu-Natal

Introduction High-temperature processing of most fossil fuels containing sulphur results in the formation of hydrogen sulphide (H2S) and sulphur dioxide (SO2) impurities in the flue gas.

Introduction Conventional methods of H2S desulphurization involve absorption of the acid component using regenerative solvents, e.g. amine treatment, or more recently biological desulphurization. † Shell Global Solutions. Sour gas processing. Hydrocarbon Processing special supplement

Introduction Desulphurization may also be accomplished using solid sorbents such as metal oxides. Advantages: Use of less toxic materials, suitable for high-temperature and high-throughput processes. Disadvantages: Low sulphur sorption capacity, requires very large processing units. † Bakker, W.J.W. et al. Chem Eng J (2003) 96: 223-235

Rationale and Motivation The removal of sulphur compounds from coal gas is very important for the correct operation of Integrated Gasification Combined Cycle (IGCC) processes for power generation.

Rationale and Motivation Removal of H2S by metal oxide sorbents is a potential technique for hot coal gas cleaning. Zinc oxide (ZnO) based sorbents are regarded as amongst the best materials. High affinity for H2S absorption and reaction. Thermodynamically more favourable than other metal oxides (lower temperatures employable). ZnO (s) + H2S (g)  ZnS (s) + H2O (g)

Rationale and Motivation The H2S capacity of metal oxide sorbents can be improved by dispersing the active metal components over various porous inert supports, and using suitable metal dopants that enhance performance. † Yang, H.Y. and Tatarchuk, B. AIChE J (2010) 56: 2898-2904 Montes, D. et al. Micro Meso Mat (2013) 168: 111-120 ZnO (s) + H2S (g)  ZnS (s) + H2O (g)

Rationale and Motivation It is believed that during wet impregnation of the support, ultrasound can assist with the insertion and hence dispersion of metal particles into support pores due to microjet and shockwave formation accelerating metal ions into the support material. Inert Support Sonication M+ Dissolved metal ions Acoustic bubble Bubble implodes Bubble forms Bubble grows Bubble reaches unstable size † Bianchi, CL. et al. Chem Lett (1993) 22: 319-325. ZnO (s) + H2S (g)  ZnS (s) + H2O (g)

Research Questions This research project aims to answer the following research questions: 1. Can the metal dispersion of ZnO/SiO2 sorbents prepared by wet impregnation be improved by employing ultrasonic irradiation during preparation? 2. Does the improved dispersion of ZnO/SiO2 sorbents result in enhanced sulphur removing capacity for hot gas desulphurization when compared to conventional ZnO/SiO2 sorbents?

Zinc chloride + deionized water Ultrasonic irradiation 30 W Experimental Sorbent preparation Zinc chloride + deionized water Drying 80 °C under vacuum Wet impregnation 4 hours Drying 105 °C overnight Silica gel (300 m2/g) 250-500 µm Ultrasonic irradiation 30 W Calcination 550 °C 4 hours

Experimental Desulphurization equipment and experiments Gas analysis Shimadzu GC 2014 Inertcap 5MS/NP capillary column (30m x 0.25 mm x0.25 µm) Oven temp: 40 °C Temp: 350, 450 and 550 °C Pressure: 1 bar Reactor ID: 27 mm Reactor length: 400 mm Feed gas: 1% H2S in N2

Experimental Desulphurization equipment and experiments

Results and Discussion 30 wt% un-sonicated (A) and sonicated (B) sorbents Sorbent characterizations TEM (JEOL 1010) A A B B 10 wt% un-sonicated (A) and sonicated (B) sorbents

Results and Discussion H2S breakthrough tests Breakthrough curves for H2S absorption were constructed from temporal measurements of the H2S concentration in the exit gas stream. When the sorbent becomes saturated and can no longer capture and convert the H2S, the exit concentration of H2S rises rapidly. This is referred to as breakthrough.

Results and Discussion H2S breakthrough tests 10 wt% ZnO/SiO2 20 wt% ZnO/SiO2 30 wt% ZnO/SiO2 Un-sonicated Sonicated

Results and Discussion H2S breakthrough tests 30 wt% ZnO/SiO2 Average deviation = 2.8 min Initial Repeat

Conclusions Ultrasound assists with the controlled insertion of metal particles into support pores due to microjet and shockwave formation accelerating metal ions into the support material The metal dispersion (spread of metal particles over the solid support) and loading capability (total amount of metal taken up by the support) of ZnO/SiO2 sorbents prepared by wet impregnation was improved by employing ultrasonic irradiation during preparation. The use of ultrasonic irradiation also resulted in less agglomeration of metal particles on the support structure.

Conclusions The improved dispersion and reduced agglomeration of active material on sonicated ZnO/SiO2 sorbents resulted in enhanced sulphur removing capacity for hot gas desulphurization when compared to conventional ZnO/SiO2 sorbents. The effects were more pronounced at higher metal loadings, with an average relative increase in breakthrough time of approximately 74%. Comparison of the results for repeated experiments using the 30 wt% ZnO/SiO2 showed that the improvement in breakthrough times for sorbents prepared using ultrasonic irradiation were statistically significant.

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