(125h) Mineral Matter Behavior during Coal Gasification: A New Approach to Evaluate the Slag Blockage Probability in Entrained Flow Coal Gasifiers

In commercial  IGCC  applications,  the predominant  type of coal gasifier is  the entrained-flow slagging gasifier  operating under  high  pressures and  at  high  temperatures.  Like other processes there are some issues during gasification process. Environmental issues due to production of various emissions such as GHG emission and fine aerosol particles, issues related to injection of feedstock during gasification process and issues related to mineral matters and slag formation. Slag flow during gasification process can cause various problems such as corrosion, erosion and tapping-hole blockage which may lead to emergency shout down.  All these problems affect strongly the reliability of gasification process. The phenomenon of micromechanical interactions between inorganic mineral matter and char with slag layer under various condition and at different locations on the wall of gasifier are pivotal keys in predicting and controlling slag flow. Surface tension, viscosity and mineral fusion temperature are essential parameters affecting this phenomenon. Despite the profusion of studies on viscosity and fusion temperature measurements as a function of temperature and mineral compositions, very less is known about the axial and radial variations of slag properties under realistic operating conditions. This research study tries to address these issues.

In this study, gasification experiments on two types of Canadian coal were performed under different operating conditions. The innovation of this study was using slag collector plates to take slag samples at different locations inside the gasifier at specific time intervals. At the first phase of experiments stationary non-continuous slag samples at different axial location were collected and the results showed that collected samples at the baffles of collection plate are different in color and morphology which is an indication of different composition and interaction at each elevation in the gasifier. The morphology and composition of slag at each location were determined by SEM/EDX analysis. Slag sample at higher elevation is smooth and bright but at lower elevation is granular and dark. Analysis of composition using EDX showed different mineral compositions at different axial length and also some un-reacted carbon in porous surface of some slag samples.

Proposing a method to investigate and characterize the blockage probability of slag at the bottom of gasifier is the main purpose of this study. So as the next step of experiments a combination of cylindrical and cone shape ceramic plates were used to collect flowing slag samples. This kind of configuration can be used for simulating the bottom of gasifier for modeling the blockage phenomenon. After installing the ceramic slag collector plates at specific location inside the gasifier gasification experiments were performed with feed stock with different size ranges from 28µm to 106 µm. One of the main advantages of this innovation is that the plate can be removed at desired time intervals (half an hour in these experiments), so the characteristics of minerals correspond to first slag layer are analyzed. The results showed that small iron and calcium particles are the first particles that stick to wall of gasifier. By putting the plates inside the gasifier for another time interval the interaction of new incoming particles with the previous layer were studied. SEM/EDX analysis showed that at vertical locations large alumina silicate particles can stick to particles in the first layer mostly if they have enough iron on their surface but in inclined locations the hydrodynamic of carrier gas, properties of first layer and conversion of char particle affect strongly the fate of incoming particles.

Utilizing the solid residue of each experiment as the feed stock of next gasification experiments showed that carbon conversion in coal particles plays a crucial role in interaction between char particles and slag layer. Char particles rarely stick to other particles on the wall and mostly stick to molten slag layer at high carbon conversion. The SEM/EDX analysis of char particles with different carbon conversion showed that in particles with high carbon conversion more sticky mineral particles are exposed to the surface of char particle making it very sticky which is likely to stick to wall at high temperatures.

In order to take into account the carrier gas hydrodynamic and slag layer properties two dimensionless number were used as the particle interaction indexes. Re number is used as index for gas hydrodynamic and We number is used as index for slag layer properties. The experiments were performed for different temperatures and different composition of gasifying agents and at the end of each experiment, Carbon conversion, Re and We number and slag layer thickness were calculated and all the data were gathered in a table. The results showed that when the temperature is lower than the T_cv (temperature of critical viscosity) only particle sintering is likely to occur and having more iron and calcium particles will increase the growth rate of sintered layer and carbon conversion, Re and We number don’t have an essential role. At temperatures higher than T_cv, size and composition of particles, carbon conversion, Re and We are important and there usually will be a critical number for each of these parameters beyond which particles behaviour will change. The results showed that for particles having high carbon conversion and at low Re, blockage is probable when We number is very low. But the role of Re number is different for vertical and inclined walls. Particles are not likely to stick to slag layer on vertical wall at high Re, but they are most likely to stick to slag layer at high Re on inclined slag surfaces. The results also showed that blockage is mostly probable working with feed stock with small size range at temperatures higher than T_cv , very high carbon conversion and very low We. (slag blockage occurred three times and slag  bridging occurred 2 times in our experiments at the bottom of slag collector plates).

Having a data bank for slag blockage probability as a function of operating condition parameters is essentially beneficial in controlling slag flow in gasifiers which is performed in this study. It’s also crucial to extend these data bank for various feed stock compositions which is the next aim of these study which is planned to be done by doing the gasification experiments with different coal blending.