Experimental Study On Slag Deposition In An Entrained-Flow Membrane Wall Gasifier And Numerical Simulation Of The Membrane Wall

Entrained-flow gasification has become the preferred, efficient and clean technology for coal-based large-scale gas and syngas production due to its advantages, such as insensitivity to coal type, high operating pressure and temperature, high carbon conversion, high production intensity, large scale, etc. As one of the most important types of entrained-flow gasifier, membrane wall gasifier has the advantages of long service life and low maintenance cost, and hence, it has become the development tendency of entrained-flow gasification. Consequently, it is of great significance to study on the technology of membrane wall.(1) Gasification experiments were performed in a bench-scale membrane wall entrained-flow gasifier using Beisu coal and the slag deposition was studied. The surface morphology and composition of the slag layer were also investigated. Beisu coal ash has a low fusion temperature for its high alkaline oxide content. According to the XRD result, anorthite and maghemite generated during the gasification. The morphology of slag layer is strongly related to the temperature distribution in the gasifier. In general, the surface roughness of slag layer increases with the decrease of temperature.(2) Shenfu coal ash has a low fusion temperature due to its high content of CaO. The slagging in the gasifier is improved by using Shenfu coal ash as feed stock. During the gasification, a liquid slag layer was formed by molten slag over the solid slag layer. A large amount of pores were observed in the molten slag, and gas in the pores expanded with the increasing temperature, which resulted in the deformation of the molten slag layer. The bumped slag layer collapsed after the gas released from it. The slag sample mainly consists of anorthite, diopside and albite. The porosity of slag is 36.6%.(3) A heat transfer model of membrane wall was established and the temperature field was simulated using FEM. Under stable operation condition, the temperature distribution in the membrane wall can be accurately predicted with static method, while transient analysis gives more accurate results for variable conditions. The temperature distribution is closely related to the conductivities of materials. Great temperature gradient is observed in the material with a low conductivity. The maximum temperatures of stud, cooling tube and fin increase with the increase of slag surface temperature, while decrease with the increasing convection coefficient of working fluid. The effect of working fluid temperature on the maximum temperatures is insignificant. The static simulation result of an industrial membrane wall gasifier shows that the fin temperature in the region near the draw-out tube is higher for the poor cooling effect there. The maximum temperature obviously decreases after structure optimization by adding a cooling tube outside.(4) A thermal stress model of membrane wall was developed for the stress field study. The thermal stresses of the slag layer act as compressive stresses during temperature rising process, while tension during the cooling process. The maximum of thermal stresses decreases with the increase of conductivity and porosity, but increases with the increase of Young’s module, thermal expansive coefficient and slag thickness. The density and specific heat of slag have unconspicuous effects on the maximum thermal stresses. Both distribution and variation of thermal stresses are obviously different between the initial solid slag and the slag solidified, and the thermal stresses are discontinuous at the initial interface. For the slag layer with cracks, the maximum thermal stress is observed at the crack tip. There are great stress gradients in the region near the crack tip, while small gradients in others. The stress intensity factor of the crack tip has close relation with temperature distribution, feature size of crack and the angle between the crack face and the normal of slag surface. As the value of stress intensity factor exceeds fracture toughness of slag layer, the crack develops directionally.(5) A flow and heat transfer model of working fluid was built for the analysis of hydraulic characteristics of the membrane wall. The working fluid velocity gradually decreases along flow direction in the distribution header with the increase of static pressure. In the collection header, the velocity of working fluid gradually increases along flow direction, while the static pressure decreases in the same direction. Higher inlet-outlet pressure drop and flow rate are observed in the cooling tube which is farther from the header inlet. The maldistribution of working fluid can be reduced by diminishing the cooling tube diameter and enlarging the angle between the adjacent tubes. The maldistribution is enhanced by increasing the inlet velocity of working fluid. Changing the inlet temperature of working fluid has negligible effect on its maldistribution.