Study On Relationship Between Flow Patterns And Pipeline Pressure Signals In Dense-Phase Pneumatic Conveying Of Pulverized Coal

Dense-phase pneumatic conveying of pulverized coal has been applied widely in dry coal feeding entrained-flow pressured gasification. However, lower gas velocity easily causes unsteady flow and even leads to blockage of pipe, which influences the efficiency and operation safety of gasifier. The dense-phase conveying is a complicated nonlinear system which is hard to be predicted and optimized without an accurate theoretical model. Pressure signals contain abundant information on flow, reflecting flow patterns and flow stability of pulverized coal. The pressure signals were analyzed using various signal processing methods to objectively identify flow patterns and obtain conveying mechanism for basic research of the conveying and industrial application. The main research contents are as followings:1. The different modes of aeration had an impact on flow stability which was represented through fluctuation amplitude of pressure signals. The results indicated that supplemental gas and fluidizing gas could make the flow steady, while pressurizing gas may cause instability of the flow. The flow patterns were closely related with flow stability. Gas grap flow, dune flow and slug flow were unsteady, but plug flow, annular flow and stratified flow were steady. The experiments indicated that the main influence on flow instability originated from pressure fluctuations of the feeding vessel, bad fluidization condition of pulverized coal and lower gas velocity in the pipeline. Under the condition of the steady pressure and good aeration, the criterion of flow stability was established through non-dimensional parameter Fr, which reflected the relationship between flow stability and gas velocity. The three types of blockage were found in the experiments and a simple model was proposed to predict the critical gas velocity.2. Experiments were carried out to research the conveying characteristics (phase diagrams, pressure drop models, pressure fluctuations and flow patterns) of pulverized coal using CO2and N2as carrier gas. The experimental results indicated that the conveying characteristics were basically the same for CO2and N2at larger pressure of the feeding vessel, while there were significant differences in the conveying characteristics between CO2and N2at lower pressure of the feeding vessel. This is because the permeability of CO2and N2in pulverized coal is related to the pressure of the feeding vessel. The permeability coefficient was introduced to establish the unified formulas for CO2and N2, predicting economic gas velocity and pressure drop. The pressure fluctuations indicated that the flow stability for CO2was lower than that for N2, but their difference was not obvious. The ECT results also revealed that the flow patterns were similar for CO2and N2.3. Electric Capacity Tomography (ECT) was used to help us to observe flow patterns in horizontal pipes of20mm and50mm diameter and a vertical pipe of20mm diameter. There are gas grap flow, plug flow, slug flow, dune flow and stratified flow in the horizontal pipe, while flow patterns in the vertical pipe are gas grap flow, plug flow, slug flow and annular flow. The characteristics of stratified flow are more obvious at larger pipe. The empirical formulas were established to predict flow patterns and their transitions through the relationship between the Ar and the Re.4. Characteristic values of pressure signals were extracted using different signal processing methods (Standard Deviation, Averaged Cycle Frequency, Power Spectrum Density, Wavelet and Chaos Analysis) for correlating the fluctuations to the flow patterns. The result indicated that Standard Deviation (SD) and Power Spectrum Density (PSD) as simple methods presented fluctuation characteristics of the flow pattens. Wavelet and Chaos Analysis were complicated, but they respectively revealed flow characteristics in different scales and chaotic characteristics of pulverized coal flow.