Ash and slag properties for co-gasification of sewage sludge and coal: An experimentally validated modeling approach

Regulatory changes in the EU and Germany, and environmental aspects direct the attention of the agricultural and energy sector towards sewage sludge as a new resource. The present study evaluates the co-gasification of sewage sludge with brown coal in a slagging gasifier. Currently, a lack of experimental data on phosphorous containing ashes and slags under reducing atmospheres limits our understanding of gasification of this fuel. To address this gap, in this study the ash and slag properties of one brown coal and its mixtures with sewage sludge were experimentally investigated under a reducing atmosphere and predicted with FactSage. An ASPENplur' simulation of the co-gasification is conducted to evaluate the impact of the changing feedstock characteristics on major process parameters when different mixtures of sewage sludge and brown coal are fed into a Siemens gasifier. The flow temperature of the coal ash can noticeably be reduced if the sewage sludge ash content is increased up to 50 wt%. Thermochemical calculations using FactSage are in good agreement with the experimental ash fusion behavior. This paper describes a method to improve the calculation to better fit the experimental values. The elemental liberation of the main ash components are modeled using FactSageTM and experimentally validated. Except for potassium, no significant release into the gas phase was detected in the experiment. This is in contrast to the predicted evaporation of phosphorous. The addition of sewage sludge results in a lower viscosity of the slags of the mixtures in this study. As a result, the studied entrained-flow gasifier can be operated at lower temperatures while staying at an optimal viscosity for slag tapping. On the other hand, it decreases the temperature working range inside a slagging gasifier, where the slag viscosity lies between 2 and 25 Pa s. The resulting implications on the process parameters of entrained flow gasification have been modeled with ASPENplus (TM). Even low sewage sludge additions to the brown coal allow for lower gasifier temperatures, decreasing the specific oxygen consumption. Moreover, the cold gas efficiency and the H-2/CO ratio are increased.