Study On Synergistic Synthesis Hierarchical Porous Materials From Residual Carbon/Minerals In Coal Gasification Fine Slag And CO₂ Capture

Coal gasification is a key technology for efficient and clean utilization of coal.Coal gasification fine slag is a type of industrial waste produced from coal gasification.Now,almost all of them are unused and deposited in landfills,which causes an environmental problem and is unfavorable from the point of view of economics.Fine slag consists of residual carbon with porous structure and mineral particles.Therefore,fine slag can be used to prepare porous materials.However,a thorough separation between minerals and residual carbon can hardly be achieved because the two components are mixed and even embedded with each other.It aims to propose a novel approach of residual carbon/minerals synergistic pore building to achieve utilization of full-component fine slag.The porous materials synthesized from fine slag can be used as adsorbent to capture CO₂,which shows the method of waste control by waste.The hierarchical pore structure is adjusted by chemical activation-hydrothermal methods using residual carbon/minerals in fine slag as raw material to realize fast CO₂ adsorption.The properties and occurrence characteristics of residual carbon and minerals in fine slag were studied.The properties of porous carbon derived from residual carbon using chemical activation method and CO₂ adsorption performance were researched.The pore building mechanism of minerals during chemical activation for fine slag was clarified.The porous carbon/minerals synergistic synthesis porous materials from fine slag treated by hydrothermal method was studied.The properties of porous materials and its CO₂ adsorption performance were researched.The mechanism of porous carbon/mineral synergistic building pores was clarified.1.The properties and occurrence of residual carbon and minerals in fine slag were studied.Fine slag consisted of residual carbon（29.79%）and minerals particles（70.21%）.Most of mineral particles were spherical particles and solid with few pores.The elements in most of spherical mineral particles were uniformly distributed.Most of minerals were composed of aluminosilicate which is beneficial to the dissolution during hydrothermal process.There were three occurrence characteristics for mineral particles:（1）discrete mineral particles;（2）agglomerated slag particles;（3）mineral particles embedded in the inner cavities（pore structure）of the residual carbon.Residual carbon was comprised of floccules and irregularly porous particles with many submicron and micron pore structure which assists to formation of hierarchical pore structure.Residual carbon in fine slag may be broadly divided into three categories:（1）discrete residual carbon particles;（2）residual carbon particles embedded in the mineral particle matrix;（3）residual carbon associated in some way with the inorganic matter within the mineral particle matrix.There are two association methods:a:physical association with the inorganics:carbon adsorbed to the inner surfaces or pores within the mineral particle;b:chemical association with inorganics via covalent or ionic bond in mineral matrix.Based on the inter-embedded occurrence characteristics of the two components,the strategy of residual carbon/minerals collaborative built pores was proposed2.The properties of porous carbon derived from residual carbon using chemical activation method and CO₂ adsorption performance were studied.The optimum activation condition for preparation of porous carbon was:activation temperature:800^oC,the ratio of raw materials and activated agent:1:2,activation time:90 min.The surface area and pore volume of RC2-800-90 under this condition was 1596 m²/g and1.297 cm³/g,respectively.The porous carbon was also proved to be attractive for CO₂capture.The highest CO₂ adsorption capacity of 4.9,2.75and 0.86 mmol/g at 0,25,and 50 ^oC was achieved with RC2-800-90 sample.The isosteric heat of adsorption values was 21.2-28.7 k J/mol for RC2-800-90,indicated that the adsorption is physical adsorption.Furthermore,the sample had not only fast adsorption kinetics that it reached saturation levels in about 1.5 min,but also excellent regeneration efficiency after ten cycles.3.The pore building mechanism of minerals during chemical activation for fine slag and CO₂adsorption performance was clarified.Firstly,porous materials were prepared using chemical activation method from fine slag as raw material.The optimized fine slag-based porous material had a specific surface area of 1187 m²/g and a pore volume of 0.89 cm³/g.The ash content was about 59.4%in this sample.During chemical activation for fine slag,the minerals could played synergistic role to improve pore structure and the mechanism was:（1）The metallic oxides in minerals could participate the activation reaction between activated agent and carbon improving the activation reaction.In this process,the eutectic mixture was produced.（2）The eutectic mixture could improve the dispersibility of activated agent,which accelerated the activation reaction.（3）The eutectic mixture had catalytic effect,which promote the pyrolysis of residual carbon and the activation reaction.The activated materials showed well CO2 adsorption performance.FS2-800-60 exhibited a high CO₂ capture of 2.53mmol/g at 25 ^oC.Additionally,the sample showed a high adsorption rate that CO₂adsorption uptake almost reached constant values within two minutes.It was not any loss in the efficiency after 5 adsorption-desorption cycles.Secondly,the effect of mineral particles on pore structure of activated materials was studied.The changes of impregnation conditions could be effective for developing pore structure of activated materials and the surface area increased by 200 m²/g.The increase contents of leached metallic elements in the process of KOH impregnation could decrease the mesopore.Additionally,the existing position and content of mineral particles could affect the pore structure of activated materials.The acid washing process could change the position and content of mineral particles.The change of surface area of activated materials treated by different acid washing conditions could reach to 407 m²/g.The pore size of activated materials distributed in the range of 3.796-29.57 nm under different acid washing conditions,which is mainly due to the embedded position of mineral particles into porous carbon.There may be two occurrence characteristics for mineral particles:（1）The mineral matters embedded into those pores and occupied the spaces;（2）The mineral matters only blocked at the entrance of those pores.Finally,the mesopore formation for mineral particles was studied during chemical activation.The pore structure can be developed in the process of impregnation and acid washing.The surface area and pore volume of mineral particles increased to 137 m²/g and 0.138cm³/g,respectively.The sample had high mesoporosity up to 75%of total pore volume.The reason for forming mesopores is as follows.In the process of KOH impregnation and acid washing,the elements were dissolved starting from the surface of mineral particles and then extending into the interior.The elements uniformly distributed in the mineral particles.The dissolution of metallic elements developed longitudinally to the interior to create mesopores.The mesoporous material with silica-aluminum network structure derived from mineral particles was generated.4.The synergistic building pore mechanism of porous carbon/minerals during hydrothermal treatment using activated materials as raw material was clarified.The CO₂ adsorption performance of porous materials was also studied.The porous material derived from activated material had the surface area of 1713 m²/g.The yield of this sample is about 61%and it contains about 17.2%ash.The mechanisms for the enhancement of pore structure by hydrothermal treatment are:（1）Dissolution.The mineral particles are dissolved into alkali solution;（2）The dissolved species inter-reacted to generate aluminosilicate.（3）Attach to the nucleation sites provided by carbon surface.（4）Growth.While nuclei are generated on the carbon surface,the silicate/aluminosilicate continued to polymerization and aggregation centered on them and formed intensely polymerized aluminosilicate structure.Due to covering the silicate/aluminosilicate film on porous carbon surface,the meso-macropore transformed into micro-/mesopores.The porous materials showed remarkable CO₂adsorption performance.The sample exhibited the highest CO₂ uptake of 2.97 mol/kg at 25 ^oC.The increase of CO2 adsorption capacity for porous mateiral is 1.03 mol/kg after hydrothermal treatment.It also exhibited good selectivity of CO₂ over N₂ of 8.1-15.3.4-FSHPC-NM sample had the best CO₂ adsorption rate,whose CO₂ adsorption equilibrium is achieved within 4.4 min.CO₂ uptake of the 4-FSHPC-NM sample was well maintained above 98%after 10 cycles.All these results suggest that hierarchical porous materials have an enormous potential to be reused for practical and industrial CO₂ adsorption applications.