Study On Preparation Of Coal Water Slurry Using Chlorine-containing Wastewater With High-salts And Chlorine Migration And Transformation Mechanisms During Its Gasification

Wastewater from coal chemical industry contains a certain amount of chloride,carbonate,sulfate,organic matter,etc.,causing difficulties in selecting and designing the process for wastewater treatment.Clean,efficient,and economical utilization is a potential approach to treat the wastewater in future.With the implementation of more severe environmental protection policies,close attention has been paying to coal water slurry(CWS)preparation using industrial wastewater.However,few studies on the recycling of wastewater in coal chemical industry have been conducted.Particularly,the influence of such wastewater on the performance of CWS and chlorine release behavior during gasification is still unclear.Therefore,it is vital to investigate the CWS preparation using industrial wastewater,slurrying performance,characteristics of combustion and gasification,mode of occurrence of chlorine,and migration and transformation of chlorine during gasification.The findings can be employed to evaluate(1)the feasibility of CWS preparation using the wastewater;and(2)the effects of chlorine migration on the gasification system.In this paper,two kinds of chlorine-containing wastewater were collected from a coal chemical enterprise;their effect on the slurrying performance and rheological characteristics of the prepared CWS were firstly investigated.Slurrying mechanisms of the CWS were deduced by using simulated CaCl2 or NaCl-containing wastewater.Secondly,the combustion and gasification characteristics and reaction kinetics were deeply investigated.Finally,the release and transformation behavior of chlorine during the CWS gasification was revealed through the determination of chlorine-bearing species in the CWS and gasification residuals.The main conclusions are drawn as follows:(1)The calorific value,ash melting point,and particle size distribution of the coal samples collected from 16 mines in Dongsheng area of Inner Mongolia are analyzed.According to the basic properties of these coal samples,Saimengteer coal is selected as a coal sample for CWS preparation using wastewater in this study.By comparing the slurrying performance with the addition of ?-naphthalene sulfonate-based additive,sodium lignosulfonate,and polycarboxylic acid-based additive,it is found that the ?-naphthalene sulfonate-based additive is the best one for the CWS preparation.Relative to the CWS prepared using deionized water,the viscosity of CWS prepared using industrial and simulated wastewater is slightly higher.Additionally,the concentration of CWS decreases by 1%,and the pseudoplastic of the CWS prepared using chlorine-containing wastewater is better than that of the CWS prepared using deionized water.The mechanisms of the preparation of the CWS using wastewater are explored by investigating the effect of industrial and simulated wastewater on the slurrying performance.The cation content in wastewater is the main factor affecting the slurry concentration and stability.(2)Use of the industrial and simulated wastewater to prepare the CWS results in a decrease in apparent activation energy based on the analysis of TG-DTG and DSC.The combustion characteristics of the prepared CWS are improved.During the combustion stage,an increase in the content of NaCl or CaCl2 in the simulated wastewater leads to a narrow combustion temperature range,which is reflected by higher temperature of ignition point and lower burn out temperature.A higher combustion rate of the prepared CWS is achieved.In addition,the combustion characteristic of the CWS becomes better with the increase of wastewater based on the combustion characteristic parameters.Kinetic calculation by the Coats-Redfern model indicates that the reaction order of the combustion mechanism function is 1(n=1).According to the methods of Kissinger,Starink,and Flynn-Wall-Ozawa,a kinetic equation of the combustion of CWS prepared using industrial wastewater is d?/dT=2.20×1011 exp(-24099/T)?-1.CaCl2 or NaCl exhibits a catalytic effect on fixed carbon conversion during the CWS gasification.The catalytic effect of CaCl2 is better than that of NaCl.The shrinking core model(SCM)can be employed to evaluate the kinetic during the CWS gasification.The calculated kinetic parameters show the active energy E is decreased,and the pre-exponential factor is increased with the addition of CaCl2 or NaCl-containing wastewater.(3)The calculation results of chlorine balance in two kinds of gasification process show that about 96%of chlorine in feed coal transfers into slag water in the pulverized coal gasification process,and about 90%of chlorine transfers into slag water in the CWS gasification process.The chlorine in coarse slag and fine slag mainly comes from slag water.Approximately 8.42%of chlorine in raw coal is organic chlorine,and the remaining chlorine is inorganic.The chlorine in coarse slag,fine slag,and fly ash exists in the form of inorganic chlorides.Both organic and inorganic chlorine are detectable in raw coal,coarse slag,and fine slag collected from the CWS gasification process.The content of organic chlorine follows a descending order of raw coal>coarse slag>fine slag.The decomposition ratio of chlorine in the pulverized coal gasification process is higher than in the CWS gasification process.(4)The vaporization rate of chlorine in the CWS prepared using chlorine-containing wastewater increases with the increase of reaction temperature.The chlorine almost releases at 800?.The free chlorine in the CWS or coal micropores and internal fissures readily releases at<400?.The chlorine released at 400?600? is attributed to the breakage of organically-bond chlorine in coal maceral.The inorganic chlorine vaporizes at a temperature above 600 ?.Such observations are in line with that from the SEM-EDX analysis.