| Experimental research and numerical simulation are conducted to investigate the flow field characteristic in the cold-state experimental scale combined gasifier, and the gasification reactivity and heat recovery effect are also studied in a hot-state experimental scale device. Furthermore, the gasification efficiency of coal water slurry (CWS) injecting from the second stage burner in the opposed multi-burner (OMB) gasifier is assessed and compared with original OMB gasifier. Also the influence of various operating conditions on the gasification indicators in a novel updraft two-stage gasifier modified from the OMB (TS-OMB) gasifier is analyzed.1. The pressure drop of second stage fixed-bed shows an increased trend with the increase of Reynolds number and bed height. A smaller porosity for the bed layer turns to be a larger pressure drop. Compared with the classical Ergun equation, the Montiller empirical formula can predict the pressure drop in the combined gasifier more accurately. With the form of top-burner, an impacting zone is formed above the second stage fixed bed, and a faster central velocity decay in the impacting zone for a higher bed height. Among the axial velocity of radial distribution, the central velocity turns to be the largest and decays fastest. In the position of r=0.5R, the axial velocity increases firstly and then decreases due the presence of recirculation flow in the furnace. The velocity has been the form of plug flow in the distance of2.9D below the burner plane. With the form of four-burner, impinging-jet flows are formed above and below the burner plane. The velocity of impinging-jet flow increases from nil to the maximum velocity, and then decays with the axial position. The distance for the downward impinging-jet flow to be plug flow is1.1D. In the condition of four-burner, the average residence time shows a decreasing tendency with the increase of superficial gas velocity and bed layer residence. The mathematical model of Gamma distribution shows good agreement with the experimental data for fitting the residence time density function. By analyzing the model parameters, almost no short-circuit is existed in the furnace. With the increase of superficial gas velocity and decrease of bed layer resistance, the flow pattern in the furnace is close to complete mixing flow. The influence of plug flow becomes more significant with the increase of superficial gas velocity and bed layer resistance.2. A3-dimensional model is established for the cold-stage two-stage gasifier, in which the Realizable k-ε model is adopted for the turbulent flowing of first stage, and a porous media model for the second stage fixed-bed. With the form of top-burner, the central velocity decays gradually with the axial position, and the plug flow is formed below the burner plane of2.7D. The recirculation flow with the opposite direction from the central is formed surrounding the jet flow, and the reflux ratio turns to be an increasing firstly and then decreasing trend. With the form of four-burner, the central velocity above and below the burner plane shows decreasing trend after first increasing. The position of maximum impingping-jet velocity is located at about0.21L. The recirculation flows are also formed surround the impinging-jet flows, in which the reflux ratio above the burner plane is much larger than that below the burner plane. The recirculation formed by four-burner is much more remarkable than that by top-burner. In the condition of four-burner, with the increase of bed layer height the velocity decays faster in the impacting zone, and the reflux ratio and recirculation zone becomes larger. With the increase of gas velocity at burner outlet, the maximum velocity of impinging-jet flow increases and then shows a faster decaying rate, while its position has almost no change. The reflux ratio and recirculation flow zone increases for a higher bed layer height and smaller burner diameter, while unchanged with the gas flow rate.3. As for the hot-state two-stage gasifier, the metallurgical coke of15-20mm shows a better gasification reactivity than10-15mm and20-25mm. Compared with the initial syngas from first stage, the average effective gas concentration (EGC) of second stage increased by2.52%,2.27%for average low heat value (LHV) and8.9%for CO2conversion. The final carbon conversion is32.4%after a three-hour reaction time. When the oxygen/diesel ratio reaches the highest (1.92m3-kg-1), the average EGC and LHV increased by7.43%and5.26%, respectively, and18.1%for the CO2conversion. The final carbon conversion of coke is41.72%. The gasification reaction rate can be improved effectively after adding5%potassium nitrate or calcium nitrate in Inner Mongolia lignite, and the catalytic effect of5%potassium nitrate is better than calcium nitrate. When adding5%potassium, the average EGC increases by3.43%, LHV by4.72%, and CO2conversion by9.3%. The final carbon conversion of Inner Mongolia lignite is85.5%after a three-hour reaction time. With the increase of calcium loading in the lump coal, the gasification reactivity becomes better. When the calcium loading is larger than5%, catalytic effect for the gasification reaction is much more significant. When the calcium loading of lump coal is8%, the average EGC is increased by3.74%, LHV by4.95%, and CO2conversion by9.4%. The final carbon conversion of8%calcium loading is90.8%.4. Based on the former hot-state experimental scale two-stage gasifier, a fixed-bed gasification reaction model is established. The simulated result shows a good agreement with the experimental data. When the high temperature syngas flows through the bed layer, the gas velocity increases and the gas temperature reduces significantly. The content of CO2+H2O in the syngas is reduced, and the effective content of CO+H2is increased due to the gasification reaction. With the proceeding of gasification reaction, the reaction zone shows a moving down trend from the upper to the bottom in the coal layer, and the varying of gas composition and temperature becomes smaller. With the decrease of coal amount in the fixed-bed, it turns to be a faster heating rate in the initial stage, which will take the lead of devolatilization and gasification reaction. The final carbon conversion decreases with the increase of coal amount. With the decrease of coal particle size, the EGC, reaction rate and carbon conversion are enhanced, as well as a leading gasification reaction, which is propitious to the gasification reaction. The gasification reaction rates and carbon conversion are also enhanced with the increase of CO2+H2O content of syngas. Furthermore, the heating rate and reaction temperature of fixed-bed coal layer are enhanced with the increase of syngas temperature, which results in a faster devolatilization rate and gasification rate, and is propitious to the second stage gasification reaction.5. A3-dimensional numerical model for an OMB gasifier is established, and the simulated results show good agreement with practical values. The gasification reaction in the impinging and impinging-jet flow zones turns to be significant in OMB gasifier. As for OMB-staged gasifier, the injecting of additional CWS in the second stage reduces the temperature in the whole furnace, resulting in a relative low reaction rate and carbon conversion. As for TS-OMB gasifier, the productivity of effective gas increases by31.1Nm3·h-1, and a slight higher cold gas efficiency (CGE) and carbon conversion compared with OMB gasifier, which shows that a two-stage modified for the gasifier is a potential way to improve the gasification efficiency. In the typical condition of updraft TS-OMB gasifier, the cold gas efficiency reaches77.47%with the outlet gas temperature of1353.5K, and final carbon conversion of96.25%. With the increase of coal distribution in the second stage, the average temperature and carbon conversion are reduced, and the EGC and CGE turn to be peak values. With the increase of oxygen/coal ratio in the first stage, the average temperature and carbon conversion are enhanced, while the EGC and CGE show decreasing trends after the increasing. With the increase of CWS concentration, the average temperature, EGC, CGE and carbon conversion are all increased, which is propitious to the gasification reaction. |
PDF Full Text Request