Incomplete understanding of biogas chemical hazards — Serious gas poisoning accident while unloading food waste at biogas plant

Confined space hazards: Plain seawater, an insidious source of hydrogen sulfide

In 2022, a confined space entry accident occurred on a Danish product tanker in which two technicians died from hydrogen sulfide poisoning during the inspection of an empty cargo tank that had contained vegetable cooking oil. The source of the hydrogen sulfide was enigmatic. About three weeks before the accident, the cargo tank was prewashed with seawater. The wash water did not seem likely to present a toxic hazard and was left in the tank. However, the seawater's natural content of dissolved sulfate was converted to sulfide by sulfate-reducing bacteria, and the low-sulfur vegetable oil residue provided the nutrients necessary for bacterial growth. Calculations confirm that sulfate in just 10 m3 of plain seawater is sufficient to create an immediately fatal concentration of H2S gas in the 4,500 m3 cargo tank of the product tanker. Accident statistics show that fatal accidents within enclosed spaces are a serious and stubborn problem. Strict adherence to routine forced ventilation and extensive gas testing of cargo tanks before entry would offer simple and effective preventive measures.

Heat-transfer performance of twisted tubes for highly viscous food waste slurry from biogas plants

Background

The use of food waste as feedstock shows high production of biogas via anaerobic digestion, but requires efficient heat transfer in food waste slurry at heating and cooling processes. The lack of rheological properties hampered the research on the heat-transfer process for food waste slurry. Referentially, the twisted hexagonal and elliptical rubes have been proved as the optimal enhanced geometry for heat transfer of medium viscous slurries with non-Newtonian behavior and Newtonian fluids, respectively. It remains unknown whether improvements can be achieved by using twisted geometries in combination with food waste slurry in processes including heating and cooling.

Results

Food waste slurry was observed to exhibit highly viscous, significant temperature-dependence, and strongly shear-thinning rheological characteristics. Experiments confirmed the heat-transfer enhancement of twisted hexagonal tubes for food waste slurry and validated the computational fluid dynamics-based simulations with an average deviation of 14.2%. Twisted hexagonal tubes were observed to be more effective at low-temperature differences and possess an enhancement factor of up to 2.75; while twisted elliptical tubes only exhibited limited heat-transfer enhancement at high Reynolds numbers. The heat-transfer enhancement achieved by twisted hexagonal tubes was attributed to the low dynamic viscosity in the boundary layer induced by the strong and continuous shear effect near the walls of the tube.

Conclusions

This study determined the rheological properties of food waste slurry, confirmed the heat-transfer enhancement of the twisted hexagonal tubes experimentally and numerically, and revealed the mechanism of heat-transfer enhancement based on shear rate distributions.