Study On Preparation And Mercury Removal Characteristics Of Formed Petroleum Coke-based Sulfur Loaded Activated Carbon

The energy consumption and demand of natural gas in China and the world are in a rising period,whereas the naturally contained and highly toxic mercury in raw natural gas poses a great threat to the safe production of natural gas,and thus has gained widespread concern.At present,fixed-bed adsorption is the most widely used and mature process for treating mercury from natural gas.However,mercury removal adsorbents are generally expensive,the key technology and patents are controlled by foreign companies,and rare research of mercury removal adsorbents used during natural gas production process are reported in China.Based on this,this paper used the cheap high-sulfur petroleum coke as precursor to prepare a formed petroleum coke-based sulfur loaded activated carbon with excellent mercury removal performance and regenerative capacity,and deep research was conducted to investigate the preparation process,mercury removal characteristics and mercury removal mechanism etc.,in order to provide experimental and theoretical support for industrial application of natural gas fixed bed adsorption used for mercury removal.Firstly,the preparation process and sulfur loading methods of formed petroleum coke-based sulfur loaded activated carbon were screened and studied,mainly including three preparation steps:activation to expand pores,granulation to form adsorbent,modification to load sulfur.Two sulfur loading methods were selected including SO₂ activation modification and elemental sulfur impregnation modification.Among them,SO₂ activation modification had dual functions of pore expansion and sulfur loading,while elemental sulfur impregnation was combined with KOH to complete pore expansion and sulfur loading.The preparation and mercury removal evaluation were carried out in a horizontal tube furnace and fixed-bed mercury adsorption experimental device.At the same time,the physicochemical properties of the adsorbent were analyzed with characterization methods such as BET,elemental and XPS analyses.The results showed that,compared with the raw petroleum coke,the mercury removal efficiency of formed petroleum coke-based sulfur loaded activated carbon obtained from KOH activation and sulfur impregnation was 67%,SO₂ activation modification improves the mercury removal performance by 25%.Sulfur content,non-oxidized sulfur,C=O and COOH/C（O）-O-C,and rich pore structure were the key factors to promote mercury removal.Among those factors,developed pore structure became a necessary condition,whereas chemical properties,such as sulfur content and forms,exhibited as dominant factors for mercury removal.Eventually,preparation process was optimized,i.e.,raw high-sulfur petroleum coke was sequentially subjected to the three steps of KOH activation and expansion,granulation and forming,and sulfur-loaded modification.Sulfur loading method was selected meanwhile,i.e.,the combination of using KOH activation for pore structure expansion and elemental sulfur impregnation for sulfur loading.Then,the influence of preparation parameters（KOH/coke ration,modification temperature,sulfur/coke ratio）and operating parameters（empty bed contact time,adsorption temperature,inlet mercury concentration）on the performance of mercury removal was investigated.Combined with elemental and BET analyses,the change of bulk sulfur content and specific surface area were explored.The adsorption kinetics was used to analyze the mercury adsorption rate and adsorption mechanism,and temperature programmed desorption（TPD）experiment was used to study the mercury species.The mechanism of mercury removal by adsorbent was analyzed in depth.The results indicated that the increase of KOH/coke ratio,modification temperature and sulfur/coke ratio all promoted the adsorption of mercury,but the promotion effect was different.The increase of KOH/coke ratio would greatly enhance the physisorption of mercury molecules through the increase of specific surface area of the adsorbent.Increasing the modification temperature would cause cyclic sulfur at room temperature broken into more reactive short-chain sulfur,specific surface area increased as well,and the adsorption effect of mercury was more obvious.Sulfur/coke ratio enhanced chemical adsorption by increasing the sulfur loading of the adsorbent.The increase of KOH/coke ratio and modification temperature would both reduce sulfur loading,while the sulfur loading was positively correlated with sulfur/coke ratio.Due to the blockage of sulfur in pores,the bulk content of sulfur was always negatively correlated with the BET specific surface area.According to the results of adsorption kinetics,external mass transfer and chemical adsorption were the control steps of the adsorption rate.The results of TPD analysis showed that mercury species was mainly composed of Hg S and Hg-OM combining with oxygen containing functional groups,which again proves that the sulfur forms and oxygen-containing functional groups on the surface of the adsorbent were the main reasons for the mercury adsorption promotion.Finally,the study on the regeneration characteristics of the modified adsorbent was carried out.The effects of three thermal regeneration methods and the number of regeneration cycles on the mercury removal performance were investigated,the physicochemical properties of the regenerated adsorbent were analyzed in depth through BET,elemental,SEM and XPS analyses.The results indicated that the order of the mercury removal performance between the original modified adsorbent and the three thermal regeneration adsorbents is:sulfur thermal regeneration≈SO₂ thermal regeneration>direct thermal regeneration>original modified adsorbent.Although compared with the original modified adsorbent,the direct thermal regeneration performance was improved,the mercury removal performance decreased as the number of regeneration cycles increased,indicating that the active substance must be loaded again.Direct thermal regeneration and sulfur thermal regeneration both improved the pore structure of the adsorbent,but the high temperature during regeneration would reduce the active sulfur substance.With regard of adsorbent regenerated by sulfur thermal regeneration,the sulfur loading increased with the increase of regeneration cycles,thus offsetting the negative effects caused by the escape of sulfur at high temperature.The sulfur content started to become uniform within the adsorbent body meanwhile.The surface of the adsorbent contained abundant sulfur forms and oxygen-containing functional groups,and as the number of regeneration cycle increased,the absolute content of non-oxidized sulfur,carbonyl and carboxyl/ester groups increased,which greatly promoted the removal of mercury.