Behaviors Of Chlorine And Alkalis During Co- Hydrothermal Carbonization Of Polyvinyl Chloride And Fuels With High Alkaline Content

The extensive use of PVC?polyvinyl chloride?increases the chlorine content of solid waste,which makes the waste management become much more difficult,causing environmental pollution and threatening human health.Meanwhile,the fuels with high alkaline content such as biomass and Zhundong coal face the problems of slagging and corrosion of furnace in the thermal utilization.In order to solve the problem of chlorine pollution and alkaline corrosion in the thermal utilization of chlorine-containing plastic wastes and high-alkali fuels,this paper proposes a co-hydrothermal carbonization?co-HTC?process of PVC and fuels with high alkaline content to achieve the synergistic dechlorination and alkali removal,and improve the energy density of parent materials.The PVC,pine sawdust and high-alkali coal are selected as parent materials.The chlorine and alkalis behaviors during the co-HTC of PVC and fuels with high alkaline content were studied in virtue of potentiometric titration,inductively coupled plasma optical emission spectrometer?ICP-OES?,scanning electron microscope?SEM?,fourier transform infrared spectrometry?FTIR?and nitrogen adsorption isotherm.Meanwhile,the fuel properties of hydrochars were studied by means of thermogravimetry.Some conclusions are listed as follows:?1?The effects of hydrothermal temperature,residence time and particle size?PS?of pine sawdust on the dechlorination and removal of alkali and alkaline earth metallic species?AAEMs?in the co-HTC of PVC and pine sawdust were studied.It was shown that the increase of hydrothermal temperature or the prolongation of residence time can improve the dechlorination efficiency?DE?of PVC,while the hydrothermal temperature is the key factor affecting the DE.The importance of operating conditions for dechlorination is given as:hydrothermal temperature>residence time>PS;The removal of chlorine enhances the acidity of the hydrothermal reaction environment,significantly increasing the removal efficiency?RE?of the AAEMs.The increase of temperature is beneficial to the removal of potassium?K?and sodium?Na?.The RE of AAEMs for“HTC sawdust”follows the sequence:K>Fe?Al?Ca>Na>Mg,while the RE of AAEMs for co-HTC hydrochar follows the sequence:Al?Ca?Mg>Fe?K>Na.?2?At 225°C,the high-alkali coal will hinder the dechlorination process of PVC.The DE was decreased from 78.75%of the“PVC”blank test to 32.35%of“225-R0.04”,reduced by 58.92%.The DE was mainly affected by temperature,but is almost independent of the mass ratio of PVC to coal?Rp/c?.The RE of the“HCl”group containing equivalent chlorine is higher than that of the“Rp/c=0.20”group,indicating that the dechlorination pathway of PVC includes not only the substitution reaction,but also the elimination reaction.The change of"C=C"in the FTIR spectra shows that the elimination reaction is the dominating dechlorination path at 225°C;In the co-HTC process,the HTC temperature and Rp/c have significant positive effects on RE improvement and also the hydrochar morphology.Cracks on the surface of the hydrochar_coal become more and more obvious with the increase of Rp/c.The HCl derived from PVC hydrolysis produce parallel plate pores on the coal,and these pores have excellent openness and are favorable for the flow of sodium ions in steam.The ash content of hydrochar_coal is negatively correlated with the RE.When the RE increases,the ash content will decrease accordingly.?3?The pine sawdust and high-alkali coal could be upgraded by co-hydrothermal treated with PVC.The hydrochar_sawdust has low chlorine,low inorganics content and improved higher heating value?HHV?of 24-30 MJ/kg?similar to bituminous coal?with energy yield of 74-81%.The increase of Rp/c is beneficial to the upgrading of high-alkali coal.Due to the difference in the composition of high-alkali coal and pine sawdust,the volatile content is different.The volatile matter of pine sawdust is higher than that of high alkali coal.Therefore,the ignition temperature of hydrochar_sawdust is generally lower than the ignition temperature of hydrochar_coal,making the combustion characteristic index of hydrochar_sawdust is higher than that of hydrochar_coal.These results show that the co-HTC of PVC and fuels with high alkaline content was feasible for clean fuels production,since the chlorine and inorganics could be removed effectively by the positive synergistic effect.This work provides a new viewpoint for the development of Waste to Energy?WtE?and fuels upgrading technologies.