Study On Slurry Ability Of Low-quality Coal With Particle Size Control And Interface Modification

Coal water slurry（CWS）has the advantages of convenient transportation,high combustion efficiency and low pollutant emission,which is generally regarded as pseudo-liquid fuel for coal clean utilization with high efficiency.According to coal resources in China,it is attractive to prepare CWS by using low-quality coals,which are abundant and cheap.It can not only reduce the production cost,but also improve the utilization efficiency of low-quality coals.Nevertheless,the high mineral content and complex components of low-quality coals make them difficult to prepare a qualified CWS directly.In this thesis,two low-quality coals,i.e.,Jinda and Jisuo coal in southwest Shandong,were chosen as experimental samples.First,the low-quality coals were upgraded by mineral separation equipment,and the influence of components and surface properties on the slurry ability of coal particles were studied after the low-quality coals were ungraded.Subsequently,the particle size control technology was utilized to improve CWS performance,and the effect of micro-particles on slurry ability for gradation CWS preparation was investigated.Finally,the interface modification of the dispersant was further used to improve the slurry concentration,and the strengthening dispersion mechanism and adsorption mechanism on refined coals were established.A mineral separation equipment was employed to upgrade low-quality coals.The composition,surface properties and particle morphology of low-quality coals and their refined coals were analyzed;subsequently,the difference in the slurry ability of different coal particles was studied.With mineral separation treatment,most kaolinite and quartz in low-quality coals were removed,and the removal rates of minerals in JD and JS coal were 84.04 and 77.77%,respectively.The surface of refined coals particles became smoother,while the specific surface area and porosit y in refined coals were greatly reduced;in addition,the refined coals had showed stronger hydrophobicity.The maximum slurry concentration of two refined coals was6%higher than that of responding low-quality coals.Meanwhile,the calorific value on CWS prepared with each refined coal was 2.99 and 1.58 times of the value on CWS prepared by the corresponding low-quality coal.To further improve the slurry ability of coal particles,the particle size control was conducted by the addition of micro particles.The effect of different paritcle gradation on the performance of CWS was investigated.The results demonstrated that the stability and flowability of CWS were enhanced with the increment of micro particles in the slurry.In contrast,the slurry concentration increased first and then decreased with the addition of the micro particles.The maximum concentration of CWS with triple-peak could reach 70%when the proportion of micro particles was16%.The micro particles had a double-sided effect,i.e.,packing efficiency improvement and free water decrease.On one hand,the appropriate introduction of micro particles could fill particle gap to improve packing efficiency,which was beneficial to prepare CWS with high solid content.On the other hand,the micro particles had a strong influence on binding water,and excessive addition of micro particles would decrease free water content,which was unfavorable for CWS preparation.The slurry concentration of coal particles could be further increased by dispersant modification.The performance and dispersing mechanism of anionic dispersants,i.e.,sodium lignosulfonate（SLF）,naphthalene sulfonate formaldehyde（NSF）and sodium polystyrene sulfonate（PSS）,were investigated systematically.In addition,the adsorption mechanism of dispersant on each component of coal particles was revealed.The results demonstrated that the refined coal modified by NSF showed the highest concentration.The improvement in the slurry ability of refined coals was mainly attributed to the enhancement of particles’negative charge after dispersant adsorption.The calculation on colloid stability theory exhibited that the electrostatic repulsion energy between particles modified by dispersant was increased 1-3 times than the initial value,which could promote the stable dispersion state of particles.The refined coal particles mainly consisted of combustible organic matter,kaolinite,quartz and pyrite.The modification of NSF on coal-water interface was related to the adsorption on organic matter,kaolinite and pyrite instead of quartz.XPS and contact angle results indicated that the adsorption of dispersant on organic matter mainly depended on the interaction between hydrophobic groups of the dispersant and hydrophobic surface of organic matter;besides,the adsorption of dispersant on kaolinite and pyrite was attributed to the combination of hydrophilic sulfonic groups in NSF molecular with the metal elements Al and Fe.To analyze the economic benefit of increasing slurry concentration,the effects of CWS concentration,CO₂ capture and storage（CCS）equipment on the production cost of the coal-to-methanol process were systematically studied.The results demonstrated that when the sold content of CWS increased from 63%to 68%,the production cost could be saved by 186 million CNY/a,and the CO₂ emission could be decreased by 0.65 Mt/a for a methanol factory with a 1.8 million t/a scale.The production cost would increased by 7-8%after adding CCS equipment.However,CCS could effectively alleviate the increasing methanol cost caused by the carbon tax.The economic equilibrium point of CCS equipment and carbon tax on methanol production cost was 60 CNY/t CO₂.