| Combustible solid waste (CSW) was a kind of organic carbon source, which could be developed to partially substitute the liquid fuel and chemicals from crude oil. Consequently, the problems on both energy shortage and waste disposal in our contury could be relieved. Among the various developing methods of CSW, producing liquid product via pyrolysis was a promising technology. In this paper, a comprehensive investigation was carried out from both fundamental and applicable perspectives. The pyrolytic kinetics, the pyrolytic mechanisms of products, the quality of pyrolytic products and their potential utilization methods, and the numerical simulation of pyrolysis process of CSW were studied.The weight loss and kinetics of two common CSW materials were studied using a thermo-gravimetric analyzer. The main weight loss of waste cardboard and waste tire each contained one peak which lay in the temperature range of 220 ?400? and 159?523?,respectively.These peaks were obviously influenced by the heating rate. The Gaussian peak fitting method was proposed to fit the differential thermo-gravimetric (DTG) curves. Results showed that the DTG curve of waste cardboard could be fitted by two peaks, while that of waste tire required three peaks. Each peak could represent a mechanism of weight loss. The kinetic parameters were calculated using Coats-Redfern method. Then the Flynn-Wall-Ozawa method and Malek method were adopted to verify the most reasonable mechanism function. Consequently, the kinetic models and parameters of the feedstocks at the main weight loss stages were obtained.The influence of catalysts and modified catalysts on the catalytic fast pyrolysis (CFP) of waste cardboard to produce aromatic hydrocarbons were analyzed in a pyrolyzer-gas chromatograph/ mass spectrum (Py-GC/MS) system. The non-catalytic pyrolysis products were oxygenates, including sugars, aldehydes and ketones, implying the poor quality of products.The microporous HY, HZSM-5 (HZ) and cheap mineral attapulgite (PA) catalysts increased the aromatic hydrocarbons yield and improved the product quality. In order to further improve the catalytic activities, the well-performed HZ was modified by alkaline treatment. Then the hierarchically porous HZ combining microporosity and mesoporosity were obtained, and the diffusion ability of the hierarchical HZ had been improved obviously. The mildly treated HZ(NaOH concentration<0.3 M) increased the yield of BTX (benzene, toluene and xylenes)dramatically. Meanwhile, the coking rate of HZ was also decreased. Consequently, the quality of pyrolytic products from waste cardboard was improved.The mechanisms on producing aromatic hydrocarbons via CFP of waste tire were explored using the Py-GC/MS system. As an important product from pyrolysis of waste tire, the degradation process of D-limonene was investigated. Results showed that aromatization of D-limonene became active at 600? and higher temperature. The main degrading pathways of D-limonene at different temperature were proposed, referring to the product distribution and literature reports. Then the pyrolysis characteristics of waste tire were explained using these pathways. Results from the catalytic pyrolysis of waste tire and its intermediates over HY, HZ and PA indicated that the catalytic mechanisms of each catalyst were not identical. In particular,HY promoted the aromatization of both D-limonene and the degradation products of polybutadiene rubber (BR), whereas HZ was only effective on the later one. PA was active in improving the yield of both aromatics and olefins. These results revealedpathways of transformation of CFP products of waste tire into aromatic hydrocarbons, which were beneficial for the valuable utilization of waste tire.Real CSW mixture was pyrolyzed in a fluidized bed reactor with the capacity of 10 kg/h.A multi-stage condensation system was adopted. Pyrolytic oils yielded 38.4 wt%? 56.5 wt%and separated into water soluble phases and organic phases. Water soluble phases were abundant in water, and the main organic compounds were acids, carbonyls, phenols and sugars,which needed further upgrading. Conversely, the organic phases were lack of water, principally contained aromatics and phenols, and had relatively low content of heavy metals (Cd, Pb and Zn). The fuel properties of organic phases were comparable to crude oil. The separation of water soluble phases and organic phases was beneficial for the utilization of pyrolytic oil. These oil phases could be used in different ways. Besides, pyrolytic chars and gases could be used by combustion to provide heat for pyrolysis process.Based on the experimental study, the modeling of CSW pyrolysis process in the fluidized bed was carried out. The multi-phase flowing was simulated by the Euler-Euler method. A multi-component and multi-step reaction method was adopted to describe the pyrolysis process of CSW. The user defined functions (UDF) of heterogeneous reactions were programmed and coupled with the computational fluid dynamics (CFD) software. Results indicated that the flow regime in the fluidized bed turned into stabilized fluidization gradually over pyrolysis time. The mass flow rate of pyrolytic products at the outlet fluctuated within the range of ± 10%. Pyrolysis temperature had the greatest effect on the product yield, while initial bed height had the most significant influence on product fluctuation. Compared to the experimental results, the simulated results of temperature distribution and that of of product yield changes with temperature were basically identical. The CFD modeling results of CSW pyrolysis process provided valuable reference for the process design and optimization of pyrolytic condition. |
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