Synergistic Effects on the Co-pyrolysis of Agricultural Wastes and Sewage Sludge at Various Ratios

This study investigated the co-pyrolysis of blends of sewage sludge (SS) with rice husk (RH) and with hemp straw (HS) at different ratios by using thermogravimetry (TG) and its rate (DTG, derivative TG) analysis at heating rates of 10, 20, and 30 K/min. The resulting kinetic parameters of activation energy (E_a) were calculated by both Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose models, followed by comparison of experimental values with calculated values to reveal the synergistic effects of SS/RH and SS/HS. With increasing additions of RH or HS to SS, a gradual decreasing trend in the experimental pyrolysis temperature range was evident, ranging from 144.5 to 95.2 °C for SS/RH and from 144.5 to 88.8 °C for SS/RH. Moreover, such temperature ranges were 6.7–20.4 °C less than the calculated values at the same blending ratio. The fitting results of the two kinetic models showed that with the same SS mass ratio, the experimental E_a^* (average activation energy) of both SS/RH and SS/HS were less than the calculated E_a^*. Especially, the experimental E_a^* of 7SS–3RH was lower around 43.8% than the calculated E_a^*, whereas the experimental E_a^* of 3SS–7HS was lower by about 39.4% than the calculated E_a^*. Synergistic analysis demonstrated that the co-pyrolysis of RH or HS with SS at various mass ratios presented obvious synergistic effects and then the decrease of E_a. The mechanism experiment showed that the co-pyrolysis of SS/HS may promote the decrease of E_a by changing the co-pyrolysis gas products or by increasing the overflow of volatile matter and then forming intermediate transition products, while SS/RH may accelerate the decrease of the E_a by using an appropriate K addition ratio from RH as a metal catalyst.