Microwave co-pyrolysis of sewage sludge and rice straw

•Sewage sludge and rice straw can be co-pyrolyzed by using microwave heating.•Microwave power level of 250 W can be satisfactory for the co-pyrolysis.•Maximum temperature was up to 500 °C at the rice straw addition ratio of 20 wt.%.•Fixed carbon content was as high as 33 wt.% at the rice straw ratio of 30–40 wt.%.•Atomic H/C and O/C ratios can be very close to those of anthracite.

This study focused on the co-pyrolysis of sewage sludge and rice straw using microwave heating. The input microwave power level was a critical parameter. Sewage sludge was pyrolyzed without the addition of rice straw at the microwave power levels of 200–300 W, while lower or higher than this range led to only drying or over-heating. The addition of rice straw increased the performance of microwave heating, which allowed a higher maximum temperature. The calorific value of the pyrolyzed biomass increased with the addition of 30–40 wt.% rice straw. A maximum temperature of up to 500 °C was measured for blends containing 20 wt.% rice straw, which could be attributed to the synergetic effect of the addition of rice straw and microwave heating. This high temperature may provide another way of thinking for the thermal treatment of waste sewage sludge. A fixed carbon content of up to 33 wt.% was obtained for blends containing 30–40 wt.% rice straw. The atomic H/C and O/C ratios were very close to those of anthracite coal. Therefore, the pyrolyzed blends should have a high potential to be co-fired with coal or even to replace it.