Investigation On Thermo-degradation And Catalytic Decomposition Characteristics Of Biomass

Topics on efficient conversion of biomass into available energy sources are currently aroused general interest due to the fact that the energy generated from renewables, especially biomass that is considered to be a highly promising alternative, is extremely limited in the global energy consumption portfolio, and at the meanwhile the conventional energy resources such as crude oil and coal tend to be depleted in the upcoming decades. This work provided an experimental study on the characteristics of the thermochemical conversion processes of biomass that derived from municipal solid waste（MSW） and its model compounds cellulose and sucrose, affording a preliminary insight on their mechanisms. It was initiated with the investigation on thermo-degradation of lignocellulosics by thermogravimetric analysis（TGA）, involved the processes of pinewood sawdust（WS） and paper which are typical biomass components widely scattering in MSW, with special concerns on the impacts of CO2 concentration in atmosphere on biomass and its chlorinated derivatives with all the particulars. Then a detailed analysis about the consequences of poly（vinyl） chloride（PVC） on WS pyrolysis was proceeded as a case study associated with the co-treatment circumstance of biomass with chlorinated contaminants that commonly occurred in practice, e.g. wallpaper with PVC coating. After that, simpler structure biopolymer（cellulose） were applied for obtaining detail information related to the thermo-degradation mechanism of biomass, and zeolites were introduced into, as catalysts, the thermo-degradation process of which the characteristics were investigated by coupled TG-FTIR（Fourier transform infrared） technique. At last, experiments of sucrose hydrolysis over several zeolite catalysts were performed as probe reactions for acquiring more mechanistic knowledge of the heterogeneous catalytic reaction within biomass conversion course. The results from the above mentioned aspects of current investigations can be concluded as follows:1) Experimental studies launched under atmospheres of different N₂/CO2 ratios for thermo-degradation of lignocellulosic biomass and its chlorinated derivative that typically contains 10 wt.% PVC over thermogravimetric analysis showed that: Two degradation stages were found where CO2 was inert in stage one but changed to be reactive in stage two. The values of apparent activation energy（Eapp） in stage one were 200²50kJ/mol with less variance but varied greatly in stage two for different scenarios concerning CO2 proportion and PVC presence. These values were used to determine the reaction mechanism of each stage by Master-plot method. Most processes were kinetically characterized by diffusion and reaction order models.2) With PVC presence, WS turned to be more reactive with less solid residue during thermally decomposing. Through the analysis based on superposition principle, interactions were confirmed to be existed when they decomposed together. Eapp calculated by Ozawa-Flynn-Wall or Vyazovkin method revealed that small amount PVC addition（<30wt.%） could decrease Eapp at low temperatures（<950K）, but at high temperature regime（>950K）, any PVC addition would increase Eapp, which conformed to the results from superposition analysis.3) Cellulose was mixed with different categories of zeolite and then thermally decomposed, and the decomposition process was monitored by TG-FTIR. It is showed that zeolite addition generally promote the dehydration of cellulose at low temperatures（<450K）. Smaller pore size facilitated the completion of the devolatilization of cellulose, prompted the release of carboxyl acids, CO2 and alcohols, but suppressed the generation of aliphatic alkyls and aldehydes. Moderate Si/Al would bring about a higher reactivity especially for the reactions relevant to deoxygenation. Also, the increase in blending ratio of zeolite could induce a better catalytic effects in terms of the weight loss characteristics of cellulose thermo-degradation.4) Mechanistic information related to a heterogeneous catalytic system included the effects of zeolite micropore topology, mesoporosity, and acidity on the hydrolysis of polysaccharides was probed in reactions of sucrose over a variety of zeolite catalysts. The reaction rate of sucrose hydrolysis increased with increasing zeolite micropore sizes and the presence of mesoporosity, and the Si/Al ratio of ⁷0- 150 provided a maximal rate constant per acid site in the catalysts. Eapp of the catalytic reactions in all zeolites were similar, the measured entropies, however, increased abruptly with increasing micropore sizes of zeolite and slightly with increasing mesoporosity in the zeolites. The results suggested that the hydrolysis of sucrose was driven primarily by the reaction entropies that were dominated consecutively by the micropore topology, acidity, and mesoporosity of the zeolite catalysts.