| Co-gasification of coal and biomass is a technical means to realize the coupling and efficient utilization of fossil and renewable energy,which is of great practical significance to achieve the goal of"double carbon".However,due to the high volatile composition and oxygen-rich characteristics of carbonaceous fuels such as biomass,the thermochemical conversion process inevitably involves volatile-char interaction and affects feedstock properties and equipment process parameters.Therefore,how to reduce the negative effects of interaction behavior to better match the coal and biomass reaction systems is a major technical challenge for co-gasification industrialization.This thesis focuses on the key issue of coal char deactivation induced by volatile-char interaction in the co-gasification process of coal and biomass.The interaction decoupling experiments were carried out based on the thermogravimetric analyzer and two-stage fixed bed reactor using Yangchangwan gasification bituminous coal and cow manure biomass as raw feedstock to investigate the interaction coal char reactivity variation in the co-gasification process.A variety of chemical structure characterization methods including atomic absorption spectroscopy,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,temperature program oxidation and laser Raman spectroscopy were used to investigate the effects of interaction on the migration behavior of alkali metals and alkaline earth metals(AAEM),chemical functional groups,carbon-oxygen compound,and carbon structure evolution of coal char,so as to relate the gasification deactivation behavior of interaction char and propose the reaction mechanism.On this basis,the oxygen-containing model compounds volatile-char interaction experiments were carried out,and chemical adsorption and bond-breaking reaction behaviors of different oxygen-containing structures during the interaction process were explored at the molecular level through reactive molecular dynamics(ReaxFF-MD)simulations and density functional theory(DFT)calculations,so as to reveal the mechanism of volatile-char interaction at the microscopic scale.The main contents are as follows:(1)The mechanism of intrinsic AAEM/organic components on coal and biomass co-gasification process was investigated by pyrolysis release of volatile and water elution AAEM pretreatment.The synergistic behavior of the coal and biomass co-gasification process complied with the competitive mechanism of AAEM and organic activity,and the biomass blending was beneficial to promote the synergistic effect of the whole gasification reaction process,which was more significant at a high blending ratio and high conversion rate.The specific reactivity index results showed that the increase in temperature was more conducive to promoting the gasification reaction of bituminous coal or low blending ratio biomass samples.AAEM components played the synergistic promoting role in coal and biomass co-gasification.The synergistic promoting effect was enhanced with the increase of carbon conversion rate X,and the greater the biomass blending proportion,the stronger the synergistic promoting effect was when X>70%in the late reaction stage.Organic components played the synergistic inhibitory role in coal and biomass co-gasification.Organic volatile did not participate in the CO2 gasification reaction at the gasification temperature less than 790℃ without synergistic effects.And the synergistic inhibitory behavior on the co-gasification process mainly occurred in the interval of 0<X<70%,which was attributed to the preferential consumption of polycyclic aromatic hydrocarbon(PAHs)completely as the gasification reaction proceeded.(2)The effects of volatile type and secondary pyrolysis on the structural evolution characteristics and gasification kinetics of coal char during volatile-char interaction was investigated by material multi-experimental group arrangement.The volatile-char interaction behavior induced coal char deactivation to inhibit the intrinsic reactivity of char particles,and the reactivity difference between interaction coal char decreased continuously with increasing gasification temperature.The inhibitory effect of biomass volatile on coal char gasification reactivity was more obvious.Compared to the secondary pyrolysis reaction,the secondary reaction behavior between volatiles was the main factor that determined the chemical structural properties of coal char.Pyrolysis volatiles was chemisorbed onto the coal char carbon skeleton and occupied active sites,resulting in increased aromatic carbon content and decreased aromatic C-O and C=O content,while inhibiting alkyl aromatic ether cleavage.Oxygen-containing functional groups can provide active sites for gasification reactions in interaction coal char,and the aromatic C-O content is particularly important.The activation energy Ea decreased first and then increased with the increase of carbon conversion in the gasification process.Due to the deposition effect of PAHs on the coal char surface,the inflection point of Ea of interaction coal char compared with blank char appears at the carbon conversion of 60%.The interaction between biomass volatiles and bituminous coal resulted in an increase of Ea by 22.12 kJ/mol,which seriously hindered the kinetic behavior of coal char CO2 gasification reaction.(3)The volatile-char interaction behavior of the coal-biomass co-gasification process was decoupled by the reactor internals setup,and the effects of different interaction modes and volatile-residence time on AAEM migration behavior,coal char chemical structure,and gasification reactivity were systematically investigated.The inhibition of coal char reactivity by volatile-char interaction during the actual coal and biomass co-pyrolysis process became more pronounced with increasing volatile-residence time,and significant gasification reaction rate fluctuations occurred at carbon conversions above 60%due to PAHs mass transfer effects.In contrast,the co-pyrolysis process showed the opposite reactivity inhibition pattern when only volatile-volatile interaction was present.The total Na+K content of alkali metal elements showed a linear correlation with coal char gasification reactivity for different interaction times.The prolonged volatile-residence time was not conducive to the decomposition of alkyl aryl ethers.The interaction reactions contributed to the increase in coal char aromatization and the number of macrocyclic aromatic hydrocarbons,which was detrimental to the graphitization process.The reduction of the oxygen-containing functional groups content led to the loss of active sites number,thus AAEM showed limited catalytic behavior.Oxygen-containing functional groups and crosslinking structures acted as active sites in the volatile-char interaction.(4)The effect of O-containing compound volatile species on the distribution of liquid-gas-solid three-phase products,coal char chemical structure,and gasification reactivity during the interaction reaction was systematically investigated through the selection of oxygen-containing model compounds.The volatile-char interaction was essentially a process of deoxygenation,dehydrogenation,and ring-opening repolymerization of oxygen-containing compounds.The interaction behavior inhibited the occurrence of deacidification and deketonization reactions,resulting in an increase of C=O content and a decrease of phenolic compounds in the liquid-phase component;and an increase of H2 proportion and a decrease of CO and CO2 proportion in the gas-phase component.The occupation of the active site by PAHs produced by the polymerization of oxygen-containing compounds during the interaction process was the main reason for coal char deactivation.PAHs were distributed as molecular clusters in the pore structure of the coal char particles,leading to the decrease in the total amount of H/C and oxygen-containing functional groups,and the C-O structure was the most intuitive expression of the char active site.The phenolic hydroxyl,carboxyl,aldehyde,and ether groups exhibited a gradually decreasing reaction inhibition behavior during the interaction.Phenol hydroxyl group was the main hydrogen source in the reaction process,which was more conducive to the dissociation of diaryl ether in coal char,reducing the C-O content,increasing the CO yield,and polymerizing to form the more complex π-conjugated aromatic ring system.The presence of carboxyl groups was more favorable to decarboxylation in the interaction process and the acceleration of the deoxidation process to increase the release of CO2.(5)ReaxFF-MD simulation and DFT calculation were used to analyze the activity characteristic behavior of each oxygen-containing group in the interaction reaction process under the electrostatic chemisorption effect from the microscopic perspective.The introduction of oxygen-containing compounds promoted the interaction behavior of coal reaction system,aggravated the generation of free radicals,increased the number of molecules and kinetic energy,and significantly reduced the reaction potential energy.Phenol hydroxyl group was more likely to interact with coal molecules.The structural properties of oxygen-containing functional groups determined the content and distribution of conventional gases.Volatile-char interaction promoted condensation reactions to increase H2 production,inhibited decarboxylation and deketone reaction to reduce CO2 and CO production,and intensified the formation of soot intermediates C2H2.Carboxyl and aldehyde groups mainly produced phenyl group byproducts,while ether and phenolic hydroxyl groups tended to produce cyclopentadienyl byproducts.The structural properties of the C-O active site greatly reduced the energy barriers required for the rearrangement of the carbocyclic structure and promoted the formation of cyclopentadienyl groups.Oxygen-containing compounds changed the frontier molecular orbital properties by chemisorption with coal molecules under electrostatic induced effects,and the electrons provided by the active sites of coal molecule fragments can effectively promote the activation of phenolic hydroxyl groups,thus reducing the orbital energy gap and accelerating the interaction process. |
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