Safety distances for the sour biogas in digestion plants

•Representation of a simplified process flow diagram of an anaerobic digestion plant.•Determination of unit operations involved in the accidental release of biogas.•Evaluation of the downwind safety distance of sour biogas using CFD and integral models.•Estimation of downwind safety distance for different wind velocity and acid gas content.•Quantification of the effects of the toxicity of H2S on the downwind safety distances. The presence of sulfur-based substances in the biogas (sour) produced from the digestion plant imposes several treatments to match the market and regulation requirements. Besides, it lays the production plants open to safety and environmental risks related to the accidental release of toxic species. This work is devoted to the investigation of the consequences of the accidental release of biogas containing hydrogen sulfide up to 10 %vol. To this aim, a schematic 3-D representation of a biogas productive plant was developed and implemented in a Computational Fluid Dynamics (CFD) model. The effects of the initial composition and the wind velocity on the cloud dispersion were evaluated. The calculated stand-off distances for lethality resulted from the numerical simulation were compared with the results of standard integral models commonly adopted in the process industry. Results indicated the dramatic effects of the toxicity of hydrogen sulfide on the downwind safety distance in the case of accidental release of sour biogas, and the negligible effects for the flammability concerns.