Sulfur crystal growth control in a biological gas desulfurization installation

In biological gas desulfurization, H 2 S is removed from gas and microbiologically converted to elemental sulfur. Particle size is important to harvest the sulfur particles. Here, we studied the crystallization of elemental sulfur in continuous experiments (14–31 days) with a reactor (20 L) consisting of a gas absorber, stirred bioreactor (anaerobic) and air-lift reactor (aerobic). Four experiments were performed in which the sulfur retention time (SRT) and hydraulic retention time (HRT) were varied by a high or low liquid inflow rate, and by operating with and without the stirred bioreactor. We found that sulfur particles produced at a HRT of 2 min and a SRT of 11 days had the poorest settleability (26 ± 5 % settled after 30 min). These particles were the smallest, had a uniform size (3.0 ± 2.0 µm) and were described as smooth, single bipyramidal crystals. Particle agglomerates, produced continuously at a HRT of 36 min and at both SRTs (6 and 11 days), and intermittently at a HRT of 2 min and SRT of 6 days, showed the best settleability (46–53 % settled after 30 min). HRT= 36 min led to more polysulfide formation, stimulating agglomeration. At HRT= 2 min, crystal growth proceeded via bipyramidal skeletons composed of nanoscale globules. We identified this growth mechanism as nearly oriented attachment. These results imply that scaling-up did not influence the crystallization phenomena we observed in smaller lab-scale set-ups. Thus, to reach good settleability for sulfur recovery in industrial scale desulfurization, we recommend increasing the HRT and decreasing the SRT. • Polysulfide formation stimulates stable sulfur particle agglomeration. • Biosulfur crystallization is not affected by upscaling. • Nearly oriented attachment is the main mechanism for biosulfur crystallization.