| The increasing consumption of fossil energy worldwide has caused severe environmental pollution,energy shortage,and climate warming,which have attracted intense attention globally.The development and optimization of resource recovery techniques for renewable energies,especially waste biomass,has become research hotspots.In decades,pyrolysis has been developed to be a promising,simple,flexible,green,and economical technique that can convert waste biomass into biofuels,chemical products,and multifunctional carbon materials.However,the chemical behavior during waste biomass pyrolysis is very complex,there is still a lack of investigation on multiple subtle chemical reactions,reaction pathways within different components,and intercomponent interactions,thus limiting its scale-up.Hence,it is crucial to perform an indepth study of biomass pyrolysis behavior and mechanism,which is significant for pyrolysis pollution control,preparation of targeted products,and further industrial applications.In this thesis,typical waste biomass(peanut shell,rice husk,walnut shell,wheat straw,and municipal sewage sludge)was used as the raw materials to produce biochars under different pyrolysis conditions.By combining the theories of environmental chemistry,mineralogy,thermochemistry,thermodynamics,spectroscopy,statistics,carbon emission management,and techno-economic analysis,as well as applying a variety of techniques:Fourier Transform Infrared Spectroscopy,Thermogravimetric-IrMass Spectroscopy,In-situ Pyrolysis Infrared Spectroscopy,X-Ray Diffraction,X-Ray Fluorescence Spectroscopy,and pore structure analysis,the systematic investigation on the chemical behavior and mechanisms during the pyrolysis of typical waste biomass from the macroscopic pyrolysis conditions to the microscopic molecular level were conducted.Furthermore,the environmental impacts and economic profits of the waste biomass pyrolysis system were analyzed using life cycle assessment and cost-benefit analysis models.The main results obtained are as follows:(1)Through multivariate pyrolysis of peanut shells with different particle sizes,biochar samples at different pyrolysis temperatures,retention times,heating rates,and gas flow rates were produced.The distributions of pyrolysis products and physicochemical properties of biochars were investigated.The results showed that the pyrolysis temperature was the main influencing factor for biomass pyrolysis with the proper pyrolysis temperature>400℃.Meanwhile,the variations in feedstock particle size and pyrolysis heating rate were proved to have significant influences on the product yield and pore structure of biochar.Consequently,the response mechanism of biomass pyrolysis process to pyrolysis conditions,especially extremely low heating rate and small particle size,was elucidated.(2)Based on the in-situ FTIR and TG-FTIR-MS results of peanut shell,the evolutions of biomass solid matrix and volatiles during the pyrolysis process were studied at the molecular level.The real-time concentrations of pyrolytic volatiles were in the sequence of C=O>CO2>aliphatic C-O-(H)>C-O-(C)in esters>aromatics>H2O>phenolic hydroxyl>aliphatic hydrocarbons>CO,reaching a maximum at 410-433℃.Moreover,the pyrolysis kinetics of the subtle functional groups in the biomass solid matrix were quantitatively resolved,revealing that the pyrolysis reaction mechanisms of total OH at 20-380℃,aliphatic C-Hn groups at 20-500℃,C=O groups at 260-500℃,and C-O groups at 300-500℃ were dominant by twodimension/three-dimension diffusion and second/third order-based chemical reactions.The pyrolysis mechanism of the functional groups in the biomass was further clarified.And a novel methodology was established for in situ analysis of biomass pyrolysis mechanisms at the molecular level.(3)Four typical waste biomass(wheat straw,rice husk,walnut shell,and sewage sludge),with significant differences in mineral fractions,were treated with acid washing for demineralization.The obtained demineralized samples were comparatively studied for the variations in their physicochemical properties,pyrolysis behaviors,and pyrolytic volatile compositions.It was found that the C contents,H contents,and the strength of cellulose were increased after acid demineralization,while no changes in the surface chemical functional groups.Meanwhile,the effects of biomass inherent minerals on the pyrolysis behaviors,thermodynamic mechanisms,and pyrolytic characteristic parameters were investigated.The results indicated that the activation energy of biomass pyrolysis could be reduced by the existence of inherent minerals,which exhibited catalytic effects on biomass pyrolysis.In addition,the inherent minerals could significantly affect the volatile compositions and yields,particularly the formation of high value-added short-chain hydrocarbons and nitrogen-/sulfurcontaining substances.The influencing mechanisms of inherent minerals on the pyrolysis process of various biomass.(4)By applying the update model parameters,the life cycle carbon footprint,carbon emission reduction potential,and techno-economic assessment for the above biomass pyrolysis to biochar system under a typical pyrolysis condition were comparatively performed with two scenarios of biochar returned to the soil and combusted for power.The results suggested that the average annual carbon emission reduction potential per unit biomass for the biochar-soil scenario was 3.73 times higher than the biochar-power scenario,while the average annual net benefit and 25-year net present value(NPV)were negative.In contrast,biochar combusted for power was economically beneficial,which was industrially advantageous under the current energy and carbon trading patterns in China.In addition,the sensitive factors of the carbon sequestration potential and economic benefits for the biomass pyrolysis system were explored via the multi-factor sensitivity analysis,with the system optimization proposed accordingly. |
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