Fuel characteristics and removal of AAEMs in hydrochars derived from sewage sludge and corn straw

Abstract Co-hydrothermal carbonization (Co-HTC) of sewage sludge (SS) and corn straw (CS) for fuel preparation is a waste treatment method that reduces the pre-treatment cost of solid waste and biomass fuel. Based on the response surface methodology (RSM), a test was designed to prepare SS and CS hydrochars using a hydrothermal high-pressure reactor. The test examined the higher heating value (HHV) and the concentrations of alkali metals and alkaline earth metals (AAEMs) and Cl. The HHV of SS-hydrochar decreased with an increase in reaction temperature, but that of CS-hydrochar increased. The yield of CS-hydrochar was substantially lower than that of SS-hydrochar. Co-hydrochar has the advantages of HHV and acceptable yield. In addition, the stabilities of AAEM and Cl in the co-hydrochar were Mg > Ca > K > Na > Cl. SS and CS had a synergistic effect on dechlorination efficiency (DE), which had a negative effect on the removal efficiency (RE) of Ca and Na. The optimal hydrocharization conditions were a temperature of approximately 246.14°C, a residence time of approximately 90 min, and a mixing ratio of SS–CS of approximately 57.18%. Considering the decreasing abundance of fossil fuels, this study offers a way to utilize SS and CS and convert them into fuel.