| Herb residues are the residual waste of herbs after decoction in Chinese medicine industry. As a typical industrial biomass, herb residues have the characteristics of large output and concentration, which make it suitable for industrial-scale and centralized utilization. The herb residues whose water content accounts for more than70%lead to serious environmental problems due to they are easy to decay. However, there is still no effective method for industrial-scale exploitation. The gasification technology is an effective method to convert herb residues to gas. This technology has significant environmental benefits for clean treatment and exploitation of herb residues, and the fuels converted from these residues release a small net emission of greenhouse gases. In this study, a pilot-scale experimental study was completed to examine the effects of operating parameters on gasification process, and the conservation of mass and energy was analyzed using herb residues as feed material. A new two-stage low-tar pyro lysis and gasification structure model was proposed coupling fluidized bed and tar oxidation reforming technologies, and the processes of fluidized gasification and tar cracking were simulated using CFD. All these studies provide practical and theoretical base for low-tar gasification of herb residues.Toward herb residues after decoction, the chemical composition was studied. Then pyro lysis experiments were done to study the generating rule of products and analyze the compositions of char and tar. The results indicated that higher temperature benefited more gas yield; tar yield reached a maximum at about500℃and decreased if temperature rose higher; char yield decreased with the increase of temperature. The particle size of char was mainly between1.5-3.5mm and became smaller at higher temperature. The compositions of char are volatile, fixed carbon and ash wich includes many metal oxides. The structure of char is porous, so the char has adsorption and catalytic role on tar. Tar is mainly composed by organic ring compound, the most abundant of which are naphthalene, benzene, and phenol. The content of phenols and benzene single ring derivatives decreased with increasing temperature for they are easily decomposed at high temperature.The gasification characteristics of herb residues in a fluidized were studied.The changing law of gas with the effect of equivalence ratio(ER), moisture content and steam feed(S/B) was investigated, and conservation of mass and energy was studied too. The results indicated ER heavily influenced the gasification process, which determined the temperature. When ER was lower than0.26, the gasification efficiency increased with the increase of ER; however, when ER was higher than0.26, the increasing of combustion reaction lowered the yield of H2and CO and more N2was feed into the gasifier, so the gasification efficiency decreased. When moisture content was greater than20%, the temperature reduced quickly for the water volatilization, which leaded to the decrease of gas quality and gasification efficiency. Extra steam supply promoted the reactions, which in particularly promoted the generation of H2and decreased the volume concentration of CO; however the carbon conversion efficiency was always at a higher level.An Euler-Lagrangian CFD model was proposed to simulate the gasification process in fluidized bed. The laws of the particle flow and gasification reactions were achieved, and the model was verified to the experimental results. It was found that CO and H2were mainly the product of volatile gasification, whose concentrations increased along the Y-axis direction. The concentration of CH4was effected obviously by temperature, and the mole fraction of CH4is higher at the bottom of the gasifier where the fast pyrolysis zone of feedstock was. Tar was produced fast at the gasifier entrance and occured a large number of secondary cracking at the higher zone which leaded to lower tar concentration. The results of simulation were similar to and experiment results when adjusted the equivalence ratio and steam feed, which confirmed the CFD model has good predictive role for H2, CO, CH4, CO2and tar generation. Based on the fast pyrolysis of fluidized bed and tar cracking, a tar oxidation reforming model was established, and an innovational agent feed system of reforming reactor was designed to realize tar efficiently cracking and get cleaner gas at a smaller agent feed. The study found when the upper and the lower oxygen feed ratio was at2:1, a large number of upper oxygen achieved efficient combustion of the tar combined with the lower oxygen feed, and efficient reforming reactions were achieved below the lower throat. Oxygen feed ratio was the decisive factor of the system, and all of the tar in crude gas was removed by cracking when ER at0.25. Steam has an important influence on the tar cracking and gas reforming too, which can enhance the reactions between tar and steam. When ER=0.15and S/B=0.2, the tar yield reached less than1.5%.Toward to the process of tar oxidation destruction and reforming process, benzene, naphthalene and phenol were selected as typical mold compounds, and a CFD mathematical model was proposed to intuitively reflect the mass and energy conversion. The results indicated the double throat structure combined with double oxygen supply was more conducive to rapid tar reactions. When the upper and the lower oxygen feed ratio was at2:1, and naphthalene and phenol were removed more fully by cracking. When ER was0.15, the export mass fraction of naphthalene is only about5%, and nearly no phenol left. When ER was0.2, the complete cracking of phenol and naphthalene could be achieved. The tar steam reforming reactions were strengthed when extra steam was feed, however the effect of steam was significantly lower than oxygen on tar cracking, and the mole fraction of CO decreased at steam atmosphere. |
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