Torrefaction pretreatment has recently gained attention for the potential improvement in biomass properties. Otherwise, visible image-processing technology for analyzing properties of torrefied biomass was evaluated for possible use in the future online process control. In this study, torrefied rice husk from different torrefaction temperatures (180–330 °C) was obtained. After torrefaction, the biochar was characterized to determine the effects of torrefaction temperature on the properties, including the proximate analysis, solid yield (SY), and higher heating values. In addition, the color values, including red-green-blue (RGB) values, and grayscale (GS) of torrefied rice husk, were measured. The results show that the fixed carbon and ash increased from 17.39 to 35.13 and 7.06 to 38.41%, respectively, while volatile matters decreased from 71.47% to a minimum of 22.89% with the increase of torrefaction temperature from 105 to 330 °C. The SY remained higher than 46% even at the most severe torrefaction condition because of the high ash content and high remaining lignin. Moreover, the higher heating values of torrefied rice husk were increased from 14.80 to 17.82 MJ/kg when increased the pretreatment temperature. RGB values were decreased with the increase of torrefaction temperature. The GS analysis results show that the color of torrefied rice husk changed from yellow to brown at light torrefaction and black at severe torrefaction. GS of torrefied rice husk shows a good correlation (R = 0.9998) with torrefaction temperature. Prediction equations with higher fitting degree between GS and proximate analysis (R2 > 0.9900), high heat values (R2 = 0.9999), and SY (R2 = 0.9979), which are developed to reflect the changing characteristics of torrefied rice husk. The results show that the prediction method based on GS is a promising technology to measure the properties of torrefied rice husk.

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.

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