Air gasification of digestate and its co-gasification with residual biomass in a pilot scale rotary kiln

In this study energy recovery of digestate from a biogas plant was investigated via air gasification. Gasification tests were executed in a pilot scale rotary kiln plant having a nominal biomass feeding rate of about 20kg/h. The equivalence ratio was varied from 0.22 to 0.39 with the goal to approach the autothermal condition. Tests were carried out for 5h in steady state condition. Syngas composition, char and gas yields were measured. To improve the cold gas efficiency of the process, a mixture of digestate and almond shells (60:40wt%) was gasified. Autothermal condition was reached with the mixture using equivalence ratio of 0.30 where the corresponding cold gas efficiency achieved the maximum value of 55%. The raw gas had a lower heating value of 4-5MJ/Nm(3). To evaluate possible improvements in the produced gas properties, in this work the effect of steam injection was also investigated.