Study On Kinetics Of Pyrolysis And Combustion And Ash Deposits Formation Mechanisms For The Refining And Chemicals Wastewater

Industries, such as petroleum refineries, petrochemical plants, as well as pulp and paper mills are a major anthropogenic source of hazardous industrial wastewater, which can pose serious and wide-ranging effects to the environment and human health. The incineration method is a well established technology which is often used to dispose industrial wastewater with highly toxicity, poor biodegradability, and complex composition. However, in actual applications, wastewater incineration treatment still faces many technical challenges, which include:(1) Owing to the complexity of chemical compositions and fluctuation of contents, wastewater incineration process is usually not stable, and then it can lead to incomplete thermal degradation of hazardous organic substances, even result in an undesirable increase in emissions of secondary pollutants.(2) Release and transformation of incombustible inorganic substances during wastewater incineration form a large number of ash-forming species which can cause some severe operation-related problems, such as fouling and slagging, which lead to low heat transfer and can, in serious cases, even result in unscheduled shutdown.In the present work, experimental analysis and numerical studies have been performed to investigate such technical difficulties during wastewater incineration process in order to obtain some useful mechanism conclusions, and they are:(1) Thermogravimetric analysis has been employed to investigate pyrolysis and combustion characteristics of the typical petrochemical industrial wastewater (PIW) and to establish the kinetic models of reactions. The results indicate that PIW is easy to achieve its ignition, and high heating rate enhances the stability of wastewater pyrolysis and combustion, furthermore improve synthetic combustion performance. The physical properties of wastewater have important role on reaction progress for both pyrolysis and combustion; thus, the kinetic parameters of pyrolysis and combustion are somehow similar. With increasing in heating rate, the activation energy of devolatilization stage decreases gradually, while the activation energy of the second devolatilization stage increases. Furthermore, the activation energy of fixed carbon combustion stage decreases with increasing heating rate.(2) Ash deposits characterization has been performed to investigate ash transformation and deposition behavior in a large-scale PIW incineration plant using XRF, SEM-EDS, and XRD for elemental composition, morphology, and mineralogy. The results show that this volatile element has a dominated contribution to form varied ash deposits in a wide temperature range. At high temperature section, ash deposits show a typical layer structure with major minerals of Na2SO4、NiO和Fe2O3. In the intermediate temperature range, eutectic mixtures such as Na3Fe(SO4)3and Na2Ni(SO4)2·4H2O which are temperature-dependent are formed in the molten phase and an "evolving" branched structure which indicates the formation of sintered deposits and high temperature corrosion is detected. Acidic salt sodium sesquisulfate is generated at low temperatures and should be responsible for low temperature corrosion. In addition, ash fusion tests (AFTs) show that ash fusion temperatures increase from sample1to sample2first, and then decrease gradually. Simultaneous thermal analysis (STA) indicates that typical melting process of ash samples collected at intermediate-and low-temperature sections is in a temperature range from650-710℃, which is about200℃lower than that of ash sample collected at high-temperature section. It can be explained by the fact that ash deposits at intermediate-and low-temperature sections have a high level of eutectic mixture whose melting temperature is much lower than each component.(3) The characteristics of the flow, combustion, temperature and NOx/SOx emissions in a large-scale PIW incineration plant have been numerically studied using FLUENT6.3.26. The results show that both air streamlines and combusting particle trajectories illustrate very complicated three-dimensional flow characteristics which promote the mixing of the air and combusting particles, and enhance thermal degradation of hazardous organic substances. On the basis of burn out characteristic as well as NOx and SOx emissions, the optimal operating condition is at ER=1.25. The average residence time of combusting particles and combustion air is5.80s and5.51s, respectively. Meanwhile, NOx emissions at the outlet of incineration plant are also relatively low and SOx emissions are at an acceptable level.(4) The melting curves of ash particles rich in alkali salts have been obtained by using FactSage thermochemical software and databases as well as XRD coupled with STA. And then numerical simulations of characteristics of ash particle deposition and distribution in an industrial-scale HRSG of the wastewater incinerator have been performed using the proposed numerical deposition model. Field measurements and ash samplings from an industrial-scale HRSG of the wastewater incinerator have been used to validate the model. The predicted results under different operating conditions are in good agreement with the measured data. The results show that ash deposition and distribution have significant particle size and temperature dependence, and high temperature will accelerate ash particle deposition in the furnace.