Bio-fuels generation and the heat conversion mechanisms in different microwave pyrolysis modes of sludge

Abstract

In order to explore the different characteristics of bio-fuels generated by microwave pyrolysis of sludge in the temperature and power control modes, the differences in the distribution, composition, lower heating value and energy utilization efficiency have been analyzed. The heat conversion mechanism was preliminarily explored via changes in the dielectric constants of the sludge. The results showed that: First, there were differences between the two modes during the pyrolysis process. This was the main reason that the lower heating value of the products in the temperature control mode was higher than it in the power control mode. For instance, the lower heating values of bio-gas and bio-oil generated at 800 °C were 16.04 MJ/kg and 35.96 MJ/kg, respectively. Second, the energy utilization efficiency of the temperature control mode was higher than that of the power control mode. This was because the required microwave energy input in the temperature control mode (40.32–65.52 MJ) was lower than that in the power control mode (72.80–88.00 MJ). This can be proven by the integral calculation results of output power and reaction time. Third, during the process of microwave pyrolysis, the ability of sludge to absorb microwave energy and convert it into heat (tanθ) first decreased and then increased due to the water evaporation and subsequent formation of pyrolysis residues. Therefore, choosing a suitable pyrolysis mode (temperature control mode) and condition (800 °C) is crucial to improving the efficiencies of resource recovery and energy utilization.