Open source codes for the development of multilevel modelling of entrained-flow coal gasifiers

This work illustrates how the open source OpenFOAM CFD library can be adopted to develop different numerical modelling approaches that can be jointly used to understand the fate of char particles in full-scale entrained-flow coal gasifiers operated in the slagging regime. Entrained-flow gasifiers are characterized by operating conditions that promote ash migration/deposition onto the reactor walls, whence the ash is drained as a molten phase. The optimal design of gasifiers, to control the flow pattern and therefore the residence times of fed coal particles, is essential to rise the performance and lower the environmental impact of such systems. Because of the wide range of spatial and temporal scales involved in these phenomena, numerical simulation of the fate of the flying fine char particles is a very difficult task. Numerical simulations of full scale gasifiers have to rely on the RANS approach. With this approach the particle-boundary interaction is modelled by means of empirical models that already postulate the leading interaction mechanism. To predict this mechanism more detailed models, coupling the Lagrangian particle tracking with a LES or a DNS approach, are required for the Eulerian gas phase. However, their application to full-scale configurations is computationally prohibitive but they allow to explore the physical behaviour of near-wall particle segregation and the role of relevant parameters if performed in simplified flow configurations. Adopting the OpenFOAM toolkit, a multi-level procedure is built to obtain a coupled RANSLES approach. RANS-based simulations of the full-scale geometry with coal particles injection and tracking aim to obtain the general behaviour of the flow field and particle trajectories. These simulations enabled to estimate the effect of swirl and tangential flow on the bulk-to-wall char particle deposition rate and to identify the input parameters for the detailed LES based models. The equations of particles motion were solved via a Lagrangian particle tracking algorithm. Simulations were performed involving a number of particles from 105 to 106, a level of detail that allowed to obtain a clear picture of the multiphase flow behaviour responsible for char deposition phenomena. This multi-level approach enabled the assessment of the char particle deposition rates and the nature of char/slag interaction (segregation/entrapment) that are likely to occur in full-scale slagging gasifiers, highlighting several features that have to be accounted in the design of green gasifiers.