| Energy is the basis for development of modern society, exploit and use of clean renewable energy relate to the sustainable development of the national economy, while the pyrolysis conversion and utilization of biomass can effectively alleviate the worsening of energy and environmental issues. In this paper, a series of basic problems in the process of producing bio-fuel by using straw biomass as the object were studied. Firstly, the prediction model of biomass pyrolysis kinetics parameters was established. Then, the biomass pyrolysis liquefaction experiments were carried out in a vacuum pyrolysis reactor. The process was analyzed and optimized, the physicochemical properties and composition of bio-oil crude were analyzed, and the mechanism of bio-oil instability was revealed. On this basis, in-situ catalytic cracking of pyrolysis vapors using HZSM-5 was conducted. The process was also analyzed and optimized, the physicochemical properties and composition of the bio-oil before and after upgrading were compared, and the reaction paths via HZSM-5 catalysis were discussed. Combined with modern analysis technology to explore the coking characteristics and deactivation mechanism of HZSM-5. In further, HZSM-5 modified by phosphorus and metals and catalytic selectivity and anti-coking performance of the modified HZSM-5 were investigated. Finally, the test bench of regeneration of deactivated HZSM-5 via non-thermal plasma injection was built. The effects of active substances on the removal of coke at different temperatures were studied, and the mechanism of chemical reaction in the process of regeneration was analyzed. The specific research works are as follows:(1) Using the model compounds of cellulose, hemicellulose and lignin as raw materials, and the simple-lattice mixture experiments were carried out in a thermos-gravimetric analyzer. The pyrolysis characteristics of the mixtures of three components were investigated. The mathematical models for predicting the kinetics parameters by the three component ratio were established and verified. The results showed that there are mutual influences between the three components, the reaction activation energy and pre-exponential factor were mainly influenced by the cellulose pyrolysis, and the reaction order was mainly influenced by the pyrolysis of hemicellulose and lignin. The accuracy of the model was high, which can predict kinetics parameters of biomass pyrolysis effectively.(2) The experimental study on the pyrolysis liquefaction of rape straw in the vacuum pyrolysis reactor was conducted. The effects of pyrolysis temperature, reactor pressure, heating rate and holding time on the yield of bio-oil were investigated. Response surface methodology was employed to optimize the liquefaction process, the physicochemical properties, molecular composition and volatile performance of bio-oil crude were analyzed, and the mechanism of the bio-oil instability was revealed. The results showed that the influence of pyrolysis temperature, system pressure and heating rate on the yield of bio-oil crude was great. 43.5% of bio-oil crude yield was obtained at pyrolysis temperature of 494.0℃, reactor pressure of 5.0k Pa, heating rate of 18.4℃/min and holding time of 60 min. Bio-oil crude had the characteristics of high moisture content(33.85%), strong acid(p H value 2.32) and low calorific value(18.65MJ/kg), which composed of phenols, alcohols, ketones, aldehydes, acids, esters and aromatic compounds. The volatile performance of bio-oil crude was good, but the high oxygen content made the difference of weight loss curves in inert and oxidizing atmosphere small. The violent movement of electron on different functional groups intensified with the change of temperature and the extension of storage time, which led to the reaction of esterification, condensation, polymerization, etc.(3) The refined bio-oil was prepared via in-situ catalytic cracking of biomass pyrolysis vapors using HZSM-5 zeolite. The influence of catalytic temperature, height of catalytic layer, Si/Al molar ratio of HZSM-5 on the upgrading process was investigated and the oil phase yield was optimized by response surface methodology. The physicochemical properties and molecular composition of bio-oil before and after upgrading were compared and the reaction paths of organics on HZSM-5 were discussed. The results showed that the catalytic temperature, the height of catalytic layer and the Si/Al molar ratio had great influences on the yield of the products. 9.80% of the oil phase yield was obtained at catalytic temperature of 491.0℃, catalytic layer height of 27 mm, Si/Al molar ratio of 50. The oil phase had a high fuel added value, acid(p H=5.15) was significantly lowered, calorific value(33.80MJ/kg) was significantly increased and dynamic viscosity(5.12mm2/s) was in the range of diesel viscosity. The aromatic hydrocarbon content in the oil phase was significantly increased and its volatile performance was part better than that of diesel. The calorific value of the aqueous phase was 29.69MJ/kg, but it still contained a certain amount of ketones, aldehydes, acids, etc. The removal of oxygen in organic compounds was in terms of COx and H2 O, and transformation reactions on HZSM-5 to form aromatic hydrocarbons followed carbonium ions reaction mechanism.(4) The coking characteristics of HZSM-5 were studied. The coking rate, crystal structure, surface physical properties and acidity changes with usage time were analyzed, and the HZSM-5 particle status change and coke shape were also characterized. In further, the soluble organic composition inside and outside the channels were analyzed to explore the coking deactivation mechanism. The results showed that the service life of HZSM-5 was short, the coking rate first increased and small change was observed after deactivation, the catalyst skeleton was well, but the surface physical properties and acidity were obviously deteriorated, and the catalyst particle size was increased due to the cover of coke. The deposited amorphous coke I can be removed at a lower temperature, but the graphite-like coke II had a higher decomposition temperature. The soluble organic compounds in the channels were mainly aromatics while those on the surface were more. The deactivation started from the inner of the catalyst, and pores were blocked by high molecular compounds, which led to deactivation.(5) The HZSM-5 was modified by P, Zn, Ti and the upgrading experiments were conducted to study the effects on product yields, physicochemical properties and composition of the organic phase, and the catalytic selectivity and anti-coking performance were analyzed. The results showed that the yield of organic phase was decreased while the physicochemical properties were further promoted. P, Zn modification contributed to the hydrogen atom transfer reactions which strengthened their aromatization performance and obtain more polycyclic aromatic hydrocarbons. Otherwise, the cracking performance of Ti/HZSM-5 was enhanced so that obtained relatively high contents of monocyclic aromatic hydrocarbons and aliphatic hydrocarbons. The anti-coking performance of the modified HZSM-5 had been enhanced vary degrees, anti-coking performance of P, Zn/HZSM-5 for graphite coke was poor, while that of Ti/HZSM-5 for amorphous and graphite coke was both well.(6) The experimental study on regeneration of the deactivated HZSM-5 via non-thermal plasma injection was carried out. The effect of temperature on the regeneration process was investigated and the chemical reaction mechanism was analyzed. The results showed that lower temperature led to the coke not being completely activated or being activated slowly, while higher temperature resulted in the active substances generated by non-thermal plasma generator easily getting annihilated. The optimal regeneration temperature was around 250℃ and the regeneration process can be finished in a shorter time. The crystal structure, surface physical properties, acidity and particle size of the regenerated HZSM-5 were restored to the fresh state. |
PDF Full Text Request