Effects Of Hydrothermal Upgrading On Low-rank Coal Properties And Catalytic Hydrothermal Gasification Of Wastewater

The resources of low-rank coals（LRCs）in China are abundant and low in price.However,the high moisture,low calorific value,high spontaneous combustion tendency and even high alkali metal content limit its large-scale utilization.Therefore,it is necessary for developing a safe,clean and efficient utilization technology of LRCs.In this paper,a novel process combining the hydrothermal（HT）upgrading of LRCs and the catalytic hydrothermal gasification（CHTG）of its wastewater（HT&CHTG-Process）was proposed to comprehensively improve the coal properties and remove the alkali metals duing the HT upgrading.Meanwhile,the wastewater will be continuously catalyzed to gasify into fuel gases rich in CH₄ and H₂.The main work and conclusions are as follows:Firstly,the effects of different HT conditions on the basic coal properties and the characteristics of solid,liquid and gas products of LRCs were investigated.Temperature is the main factor affecting the HT upgrading,while initial pressure and coal-water ratio have less influence.HT upgrading can effectively reduce the moisture,volatile matter,and oxygen contents of coal samples,increase calorific value and coal rank of LRCs.However,HT upgrading also resulted in a remarkable decline in solid recovery yield,and a remarkable increase in energy loss,especially for 300-350°C.At 250-300°C,the thermal energy loss could be controlled at about 7%.Comprehensively considering the effect of HT upgrading and energy loss,250-300°C was the optimal choice.With increasing HT temperature,the TOC concentration in wastewater increased significantly,and the gases generated increased gradually.Most of the carbon elements existed in the form of CO₂,and only a small amount of CO,CH₄ and H₂ were formed after the temperature exceeded 250°C.The carbon element had the largest proportion in the solid phase and gradually transfered to liquid and gas phases.More than 91%of the carbon remained in the solid phase at 300°C.Compared with Zhundong coal,lignite has a lower coal rank,and the effect of hydrothermal upgrading was more obvious.Secondly,the effect of HT upgrading on the physicochemical structure of LRCs was studied.With increasing HT temperature,the specific surface area and total pore volume increased first and then decreased,while the average pore size decreased first and then increased.The evolution of physical pore structure was the result of competition among many factors.HT upgrading could significantly modify the chemical structure of LRCs,and the changes of chemical structure in coal remained after pyrolysis coking.For the chemical structure,the aromatic carbon ratio increased,the condensation of aromatic rings intensified,and the contents of hydrophilic oxygen-containing functional groups such as hydroxyl and carboxyl groups decreased significantly.HT upgrading promoted the graphitization and aromatization of chars,and the chemical structure became dense,orderly and stable.Furthermore,the defects,disordered structures,amorphous carbon and internal reactive sites in coal chars decreased.Then,the effects of HT upgrading on the removal of AAEM and ash composition of Zhundong coal were studied,and the gasification characteristics of LRCs during HT upgrading was discussed.Na mainly existed in H₂O-soluble form,while K content was not much and mainly existed in insoluble form.The contents of Ca,Mg were also high,mainly existing in HCl-soluble form.HT upgrading can effectively remove AAEM in Zhundong coal.The highest removal rates of Na,K,Ca and Mg were 98.6%,82.7%,38.9%and 26.0%,respectively.HT upgrading can remove not only H₂O-soluble form of AAEM,but also the organic form of AAEM.HT upgrading can significantly reduce the contents of alkaline oxides,but increase the contents of acid oxides in coal ash.Na₂O content could been reduced to 0.32%,which met the requirements of power coal standards in China.With increasing HT temperature,the gasification reaction curve shifted to the high-temperature region,and the gasification reactivity exhibited a decreasing trend.For Zhuandong coal and lignite,98%and95%of the gasification reactivity of raw coal remained at 300°C,and the overall decline was not much.LRCs were suitable for clean utilization of gasification after HT upgrading.Kinetic analysis also confirmed this.The gasification reactivity of LRCs during the HT upgrading was comprehensively influenced by the coal maturity,alkali metal content,physical pore structure and chemical structure.A multiple linear regression model of gasification reactivity for LRCs was established and the influence order of each factor on gasification activity was obtained.For Zhundong coal,alkali metal content had the greatest impact on gasification activity,while coal maturity had the least impact.For lignite,coal maturity had the greatest impact,while physical pore structure had the least impact.Finally,a self-designed CHTG device and a large LHSV and liquid flow rate were employed under relatively mild conditions（≤350°C）.The effects of different catalysts,HT temperatures,CHTG temperatures,and LHSV values on CHTG process were evaluated.Ni/C and Ni/C/Al₂O₃ catalysts were prepared.Among them,Ni/C catalyst had a larger content of Ni element and a higher catalytic efficiency for wastewater.With increasing HT temperature,the TOC concentration of wastewater increased sharply.The CHTG efficiency also increased gradually,reaching 77.2%and 81.8%,respectively.And 300°C was the preferred choice.With increasing CHTG temperature,the CHTG efficiency and gas yields increased significantly.The proportion of CH₄ and H₂ in fuel gases was over 70%.With increasing LHSV,the CHTG efficiency decreased sharply,and a LHSV value of150?h^-1 corresponding liquid flow rate of 10?mL/min was the preferred choice.Ni was oxidized to NiO first and then reduced to Ni in the CHTG process,which acted as a catalyst.The synergistic effects of decomposition of organic matters,methanation reaction and water-gas shift reaction resulted in the gas products consisting mainly of CH₄,H₂ and CO₂ with little CO.