Study of temperature variation of walnut shell and solid heat carrier and their effect on primary pyrolysis and volatiles reaction

Abstract

Biomass pyrolysis by solid heat carrier is regarded to be efficient in heat transfer, high in tar yield and easier for reactor scale-up. However the transient behaviors in heat and mass transfer and complicate reactions, especially the volatiles reaction in the solid heat carrier phase, make it difficult to understand the relations between operating conditions and the conditions of actual reactions, and consequently the yield and composition of the products. This work studies these relations using a 4 mm layer of walnut shell as the biomass and some amounts of quartz sand as the solid heat carrier in a specially designed small reactor to simulate a finite element in a moving bed large reactor. The mean temperature of the walnut shell layer (T-WS) and the highest temperature of the quartz sand layer (T-QS-o) under various quartz sand preheating temperatures (PT, 600–900 °C) and various solid heat carrier-to-biomass mass ratios (Rs-b, 5:1–9:1) are determined and correlated with the walnut shell pyrolysis and volatiles reaction. The results show that T-QS-o and T-WS depend linearly on PT and Rs-b, but their difference varies from 39 to 108 °C. A reaction matrix is proposed and the oil yield is modeled by first order kinetics which decouples the roles of T-WS in WS pyrolysis and T-QS-o in oil reaction. It is found that the walnut shell pyrolysis is strongly influenced by heat transfer from quartz sand to walnut shell with activation energy of only 46.4 or 19.2 kJ/mol for varying PT or Rs-b, respectively. The oil reaction in the quartz sand layer reduces the oil yield and is dominated by the reaction of radical fragments that is weakly dependent on temperature, so the activation energy is also low, 4.8 or 2.9 kJ/mol for varying PT or Rs-b, respectively. The oil reaction in the quartz sand is extensive and more than 50% of it is converted to non-oil volatiles at T-WS of 350 °C.