Study On Hydrothermal Treatment Of PVC Plastics:Dechlorination Mechanism And Combustion Characteristics Of The Hydrochars

High chlorine content is the difficulty to PVC（polyvinyl chloride）wastes treatment and disposal.Thus,exploring green and efficient dechlorination technology is of great significance to the downstream treatment and utilization of PVC wastes.As well known,hydrothermal treatment is an environmentally friendly thermochemical treatment technology which has certain development prospects in the field of degradation treatment of organic solid wastes.Based on the hydrothermal treatment method,this article firstly took PVC_P（PVC resin）as the object to explore the influence of Cu²⁺on the hydrothermal dechlorination of PVC_P;Then,the bagasse,HIPS（impact-resistant polystyrene plastic）and ABS（acrylonitrile-butadiene-styrene plastic）were used to co-hydrothermal treatment with PVC_R（PVC recycled plastics）to explore the effects of reaction temperature,residence time and mixing ratios on degradation and dechlorination.Furthermore,the combustion characteristics and behavior of chlorine release of hydrochars（solid products of PVC_R in the co-hydrothermal treatment）were studied.The results are as follows:（1）Cu²⁺obviously promoted the degradation and dechlorination of PVC_P.Under the condition of 220℃for 30 min,the dechlorination efficiency of hydrothermal treatment in 0.1 mol/L Cu²⁺solution was 58%,which was 9 times higher than that of pure water（6.24%）.When the Cu²⁺concentration increased from 0.01 mol/L to 0.2 mol/L,the dechlorination efficiency largely increased from 15.46%to 67.89%.According to the results of FTIR,SEM and XPS,it is inferred that the porosity and surface wettability of the hydrochars prepared by hydrothermal treatment in Cu²⁺solution were enhanced,which accelerated the elimination and the-OH substitution reaction of PVC_P,and enhanced the conversion of organic chlorine to inorganic chlorine then migrate to the liquid.As a result,the dechlorination of PVC_P was promoted.（2）For PVC_R co-hydrothermal treatment,reaction temperature was the main factor affecting the dechlorination.Under the condition of 220℃,60 min,the PVC_R dechlorination efficiency of PVC_R hydrothermal treatment in pure water,Cu²⁺solution,and the co-hydrothermal treatment of PVC_R-bagasse,PVC_R-bagasse-HIPS and PVC_R-bagasse-ABS were 4.88%,6.51%,34.09%,28.6%and 32.5%respectively.The bagasse significantly boosted the dechlorination of PVC_R.According to the results of FTIR,SEM and XPS,it can be inferred that bagasse provided abundant free-OH and boosted the elimination and substitution reaction.In addition,the bagasse formed a barrier between plastic particles which reduced the agglomeration then accelerated the migration of inorganic chlorine on the surface of hydrochars into the liquid,thereby facilitated the dechlorination efficiency.In addition,the debromination efficiency of HIPS and ABS has also been improved during the co-hydrothermal treatment.（3）The elemental analysis of PVC_R and hydrochars which prepared by the co-hydrothermal treatment of PVC_R-bagasse,PVC_R-bagasse-HIPS and PVC_R-bagasse-ABS（denoted as HC-PVC_R,HC-P-S,HC-P-SH and HC-P-SA respectively）found that the H/C and O/C molar ratios of HC-P-S,HC-P-SH and HC-P-SA were lower,consequently the degree of carbonization was higher.As well as the high calorific value and comprehensive combustion characteristic index of those hydrochars were obviously improved.The combustion and chlorine release characteristics of PVC_R and hydrochars were investigated by the thermogravimetric and infrared combined system.It is found that the combustion weight loss of PVC_R and hydrochars mainly occurred in two stages of 300℃and 500℃.The main combustion gas components of PVC_R and hydrochars were H₂O,CO₂,HCl,olefins and aromatics.A large amount of volatile gas released at about 300℃.The chlorine were released in two forms:HCl and C-Cl.The addition of bagasse,HIPS and ABS at hydrothermal treatment reduced the thermal stability of organic chlorine in hydrochars,and promoted the release of chlorine while combustion.The completely release of HCl and C-Cl could be achieved at a relatively lower temperature.In a word,PVC_R co-hydrothermal treatment with bagasse,HIPS and ABS improves the fuel properties and combustion characteries of hydrochars.In summary,hydrothermal treatment is a feasible method for preparing low-chlorine solid fuels from PVC.Both Cu²⁺and bagasse can significantly promote the dechlorination of PVC at a lower temperature of 220℃.PVC co-hydrothermal treatment with bagasse,HIPS and ABS has a synergistic effect on dechlorination and debromination.The results of this paper provide a data basis and scientific reference for the preparation of solid fuel from co-hydrothermal treatment of PVC,bagasse,HIPS and ABS solid wastes.