DEM simulations of coal particles in entrained-flow slagging gasifiers: particle-particle interaction at different burning levels

To allow the future use of coal for power generation a very low level of emissions is required, as expected in well-designed IGCC gasification power plants. This study focuses on an entrained-flow gasifier unit operated in slagging mode, where the conversion of coal into syn-gas occurs together with the separation of mineral ashes that experience a molten state into the gasifier due to the high operating temperatures. These two processes need to be both carefully controlled to achieve high gasification efficiencies: flow configurations promoting the centrifugal motion of particles enhance the separation of ashes, but at the same time unburnt char particles can in this way be transferred to the gasifier's walls covered by a flowing molten slag layer possibly layered by previously impacted char particles. As highlighted in previous studies of this research group, different interaction mechanisms may establish upon impact of char/ash particles with the slag layer. Entrapment of char particles into the slag leads to a decrease in the particle burn-off degree, while segregation of char particles into a "dense-dispersed phase" upon interaction with other particles layered onto the slag phase can enhance the overall particle residence time and, thus, the conversion efficiency. Adopting numerical simulations, it has been shown that the occurrence of desired segregation phenomena is possible for both unconverted (non-sticky char) and converted (sticky ash) particles. Following this path, in this paper numerical DEM simulations of a prototype entrained-flow slagging gasifier have been developed to establish the role of the level of particle burning, and therefore of the properties relevant to particle-particle interaction (e.g. particle stickiness degree and deriving characteristics), on the distribution of char particles in the whole reactor. Even in presence of simplifying assumptions, the results reveal the importance to take into account the particle-particle interaction for a correct prediction of the effective residence time distribution of particles in the reactor. The effective flux of particles towards the wall slag layer appears indeed strongly dependent on the initial particle-particle contact characteristics in the region of the gasifier where the dense phase develops. In the proposed configuration of a downward gasifier, this is the result not only of the interaction among particles in the region of injection but also of the distribution pattern that establishes after the first impact of particles with the walls.