Study on the co-combustion process and mercury emission characteristics of sewage sludge-coal slime coupled fuel

To realize the clean and efficient resource utilization of sewage sludge (SS) and coal slime (CS), single-component and coupled fuels were analyzed. The micro-characteristics of combustion products were analyzed using multiple characterization means. The structure–activity relationship between the physicochemical properties and the mercury emission characteristics of SS and CS during the complex and changeable co-combustion process was established. The influence of coupling multiparameter conditions on the mercury emission process was revealed along with the combustion characteristics and evolution process using a distributed activation energy model. The results showed that α-HgS, HgSO₄ and HgO with α-type hexagonal structures are the main forms of inorganic mercury compounds contained in SS and CS. The pyrolysis and combustion process of the sample can be divided into three stages. In the SS combustion process, the activation energy increases first and then decreases with increasing conversion rate α, and the activation energy of CS shows a U shaped trend distribution. For the influence of O₂ on the combustion and mercury emission process, a critical concentration exists between O₂ concentrations of 5% and 7%, which dominates the pyrolysis and heterogeneous combustion of carbonaceous substances and volatiles. When the temperature is above 750 °C, the Boudouard gasification reaction occurs directly between CO₂ and the sample in the semicoke state, promoting combustion and mercury emission processes. Due to the strong interaction between the SS and CS, the mercury emission during the co-combustion process of the coupled fuel is inhibited and promoted at 400–500 °C and 500–600 °C, respectively. The iron mineral components and fixed carbon in the sample play a catalytic role during the decomposition process of mercury-containing compounds that are difficult to decompose.