Study On Syntheses, Capabilities And Mechanisms Of Dispersants Based On Humic Acid For Coal-water Slurry

In recent years, with the increasing shortage of petroleum resources andrising price of oil in the world, the comprehensive utilization of coal resources ismore and more interested. However, the direct combustion utilization of coal hasresulted in low efficiency, serious climate deterioration and environmentalpollution. The coal-water slurry (CWS), as a clean coal-based liquid fuel forenergy conservation and emission reduction, has emerged as the times require.The CWS is a suspension system of the coarse coal particles in water. Coalis hydrophobic substance, due to hydrophobic interaction, the coal particles inCWS are apt to agglomerate mutually and precipitate. The ideal CWS shouldhave good stability during its manufacture and storage, meantime have a lowviscosity in the process of its pipeline transportation and atomization. To achievethis, the selection of a dispersant plays a very important role.At present, the dispersants for CWS include naphthalene series, lignin series,humic acid series, polyolefin series, polycarboxylate series, sulfonatedacetone-formaldehyde and non-ionic dispersing agents etc.. However, most ofthe dispersants are synthesized using petroleum products as raw materials, andnot only have high prices, but also are very harmful to environment. The humicseries dispersants are obtained by modifying natural humic acid, and the rawmaterial for their syntheses are readily available, while they areenvironment-friendly, and have cheap prices and good dispersion properties.Whereas, the current research works on humic acid series dispersants are limitedto the primary modifications of humic acid, such as nitration, sulfonation,sulfomethylation etc.. Despite the modifications will help to improve thehydrophilicity and dispersion performance of humic acid, owing to relativelysmall molecular mass, the dispersants exhibit poor stability. The thesis adopts the graft copolymerization and condensation reactions tomodify humic acid for the syntheses of CWS dispersants by dispersant molecularstructure design. Firstly, using humic acid (HA), allylsulfonate (SAS), acrylicacid (AA), acrylamide (AM), allyl alcohol polyoxyethylene ether (APEG,relatively molecular mass is700,1000,1200and2400, respectively) andmethacryloyloxyethyl trimethyl ammonium chloride (DMC) as raw materials,persulfate as initiator, the seven new-style humic acid graft copolymerdispersants have been synthesized, namely, three binary graftcopolymers—HA-SAS (HAS), HA-AA (HAA) and HA-AM (HAM); twotribasic graft copolymers—HA-SAS-APEG1000(HSP1000) and HA-AA-APEG1200(HAP1200); two zwitterionic graft copolymers—HA-SAS-DMC (HSD)and HA-AA-DMC (HAD). Secondly, using humic acid, formaldehyde, urea andsodium sulfite as raw materials, two humic acid condensate dispersingagents—sulfonated humic acid-formaldehyde (SHF) and sulfonated humic acid-urea-formaldehyde (SHUF) have been synthesized by sulfomethylation andcondensation reactions, and the synthesis conditions are optimized. EmployingFT-IR, GPC, TG and DSC, the chemical structures of the nine dispersants arecharacterizated and their performances are also examined. Using Shenmu coal,Goukou coal, Binchang coal and Huolinhe coal as experimental coals, the effectsof the nine dispersants synthesized on the dispersion properties and stabilities ofthe CWSs are investigated, and the relationships between dispersant chemicalstructures and CWS capabilities are also discussed. The experimental resultsshow that the dispersion properties and stabilities of the CWSs prepared from thenine new-style dispersants synthesized are superior to those of the CWSs froman industrialized naphthalene sulfonate formaldehyde condensate (NSF)dispersant. As for a graft copolymer dispersant, the certain amount of sulfonicacid groups introduced in the dispersant molecule could improve the applicationproperties of the dispersant. While the moderate polyoxyethylene chains withspecific length and the quaternary amine cationic groups introduced in the abovemolecule could strengthen further the properties of the dispersant. In addition,the urea introduced in the condensate dispersant molecule may make themolecular chain become flexible, which is helpful to the application properties ofthe dispersant. The Shenmu CWSs (coal mass concentration=66wt%and dispersantdosage=0.5wt%) are prepared from HSP1000and HSD with relatively exellentdispersion properties and stabilities, and the rheological behaviors of the CWSshave been examined. The experimental results show that the apparent viscositiesof the CWSs prepared from HSP1000and HSD decrease significantly with anincrease of shear rate, and the CWSs exhibit pseudoplastic shear-thinning fluidcharacteristics. The relationships between shear stresses and shear rates of theCWSs are fitted by the Power-law model, Bingham model and Herschel-Bulkleymodel. The results show that the Herschel-Bulkley model has the highestmatching degree, and the correlation coefficient R2is0.9857and0.9988,respectively. With the model, the flow behavior index n of the CWS from HSDis smaller than that from HSP1000, which shows the CWS from HSD has moreevident pseudoplastic fluid characteristics.The adsorption amounts of the dispersants synthesized on Shenmu coalsurfaces, contact angles of the dispersant solutions on the coal surface and zetapotentials of the complex coal particle surface with adsorbed dispersantmolecules, are all investigated. The results show that the adsorption behaviors ofthe condensate dispersants, binary graft copolymer dispersants and tribasic graftcopolymer dispersants, are all according to the Langmuir isotherm equation, andthose of the zwitterionic graft copolymer dispersants are in line with theFreundlich model. The zwitterionic dispersants have the highest adsorptionamounts, which could be related not only to forceful electrostatic interactionbetween the cationic groups in the dispersant molecules and anionic groups ofcoal surface, but also to their multilayer adsorptions on the surface. Thedispersants synthesized could all diminish effectively the contact angles atcoal-water interface, which relies not only on the orientation tightness extent ofthe dispersant molecules adsorbed on the coal surface, but also on the numberand hydrophilic ability of the hydrophilic functional groups in dispersantmolecule. The nine new-style dispersants synthesized may all raise markedly thezeta potential absolute values of coal particle surfaces, which relates to the typeand number of the anionic groups in dispersant molecule, as well as theflexibility of the dispersant molecule. The surface morphology of the raw coalparticles and complex coal particles adsorbing HSD molecules is observed by SEM, while the specific surface and porosity of the complex coal particles is alsoinvestigated. Based on the above test results, the action mechanisms between thedispersants synthesized and coal have been described further.In summary, this thesis has studied the syntheses, characterizations,capabilities and action mechanisms of the humic acid-based dispersants for CWS,and a series of high-performance dispersants have been obtained. Therelationships between dispersant chemical structures and CWS capabilities havebeen explored, and the action mechanisms between the dispersants synthesizedand coal have also been disclosed. Therefore, the thesis is of theoretical andpractical significance in research on the molecular structure design and synthesisof a dispersant for CWS.