(399d) Thickening of Liquid Digestate: Integration of Vacuum Evaporator into a Biogas Plant

More than 17 thousand biogas plants (BGP), with a total installed capacity of more than 8,200 MWel, were established in Europe in 2015. The rise of number of BGP can be associated with increased amounts of available digestate. Digestate is a by-product of anaerobic fermentation and is predominantly used as a fertilizer in agricultural areas close to the BGP. Digestate in liquid form contains little nutrients, and its storage and application in the fields bring high costs to the operator. Promising method to improve nutrient concentrations in the liquid digestate is water separation via vacuum evaporation. Integration of vacuum evaporator (VE) into BGP should decrease the amount of fuels necessary for liquid digestate transport. The integration should further increase efficiency of waste heat utilization. Plausible payback period is a prerequisite of a successful integration of the technology on the large scale.

This paper evaluates the integration of VE in BGP. For this purpose a mathematical model was developed which provides details on mass and heat flows in the process and calculates the payback period too. The model reflects real operational and technological limitations. Limitations of liquid digestate thickening in VE include maximum admissible dry matter content which still allows for transport via common pumps.

It can be concluded that the integration of VE in BGP makes sense with the increase of transport distances, BGP capacity, thermal power consumption and dry matter content in the concentrate and with the decrease of feed-in tariffs and electrical power consumption. Integration of VE technology into BGP must be preceded by analysis of unit’s capacity in terms of produced digestate, and quality and amount of waste heat. Further, amounts of auxiliary chemical components and subsequent elimination of residual ammonia from the distillate must be also considered before the actual integration.