## 3 Results  
### Effect of Operation Conditions on HT Dechlorination Behavior  
#### 3.1.1 Effect of Temperature on HT Dechlorination of PVC  
Comparatively, the color of PVC changed from white to light brown for all these hydrochars and got darker and darker with the HT temperature, residence time and also the NaOH dosage. Significant degradation of PVC took place because of the hydrolysis and difunctionalization such as dehydration, recondensation and aromatization [13]. Figure 2 indicates that the HT temperature played a dominant role in the HT dechlorination among all these experimental conditions investigated. Whether adding NaOH or not, the DE kept increasing with the HT temperature. After the HT temperature was higher than 200 \({}^{\circ}\)C, the DE was dramatically increased. The highest DE of 94.3% was obtained at a HT temperature of 240 \({}^{\circ}\)C in the presence of 1% NaOH. This result indicates that the PVC dechlorination rate was controlled by the HT temperature, which was in accordance with that reported by [PERSON] et al. [21]. It is reasonable because the chemical reaction rate was usually determined by the temperature because of the activation energy.  
Figure 2: Effect of temperature on HT dechlorination of PVC.  
Although a growth trend with the HT temperature was presented in the DE for all conditions, a small difference could be still observed. At a lower NaOH dosage of 0% and 1%, the DE was nearly the same when the HT temperature was below 220 \({}^{\circ}\)C. It was slightly increased from 1.6% to around 10%, and then dramatically improved to 55% as the temperature was increased from 180 \({}^{\circ}\)C to 220 \({}^{\circ}\)C. After the HT temperature was increased to 240 \({}^{\circ}\)C, the DE for 1% NaOH was sharply increased from 14% to 94.3%, higher than that without NaOH (the DE was about 76.6%). The DE increase rate kept increasing with the HT temperature. At a dosage of 3% and 5%, the trend of the DE variation with the temperature was almost the same. After the temperature was higher than 220 \({}^{\circ}\)C, the DE increase rate reduced a little, which was in contrast to those of 0% and 1%. This is because the chlorine transformation behavior was also affected by some other parameters, such as the alkalis dosage, rotating speed, holding time, etc.  
The slope of the curve representing the DE increase rate could be expressed as the dechlorination rate constant, k, as a function of the HT temperature. In the HT dechlorination of PVC at 3% and 5% NaOH dosages, the DE could be described as an S-type curve. The dechlorination rate was increased with the HT temperature when the HT temperature was lower than 220 \({}^{\circ}\)C. However, it decreased a little after the HT temperature was increased from 220 \({}^{\circ}\)C to 240 \({}^{\circ}\)C. For the sample hydrothermally treated at 0% and 1% NaOH dosages, it kept increasing with the HT temperature. This difference might be ascribed to the variation in the NaOH dosage, which changed the viscosity of the reaction medium. The reaction between the PVC particle and dissolved NaOH happened in a multi-phase hybrid system. After the surface reaction between the PVC particle and NaOH, the OH- then penetrates into the PVC particles [21]. A high NaOH concentration may change the surface morphology of the PVC particle, preventing the penetration of OH- and slowing down the dechlorination rate.