In order to produce carbon neutral or carbon negative hydrogen from gasifier operations to support our transition to a net-zero greenhouse gas emissions economy requires an ability to manage complex feedstock properties (such as coal waste, biomass, plastics and municipal waste). The operating temperatures in these gasifiers can range from 600 C − 1000 C and need to be carefully monitored and controlled. Hydrogen production, CO and CH₄ concentrations, ash fusion, tar formation all vary quite sharply within this temperature range. High-fidelity CFD simulations that rigorously incorporates all of the physio-chemical processes within these gasifiers can greatly assist towards the design, scale-up and fuel-blending operations to optimize their performance. While simulations of gasifiers have predominantly been carried out using equilibrium chemistry approaches in process models, there have only been a handful of studies that have carried out and successfully validated multiphase CFD simulations of these gasifiers where the fluidization characteristics and finite rate chemistry effects have been resolved accurately. In addition, the effects of radiative transfer in these gasifier simulations have either been traditionally ignored or approximated crudely using simple fourth power of temperature (optically thin) relationships. This has been primarily due to the assumption that radiative effects are likely not that significant at these lower temperature range (600 C − 1000 C) and small optical path lengths. However, radiative transfer effects can significantly impact the product syngas composition as it is cooled and can also impact the location of tar/ash condensation along the sampling train.
These effects are demonstrated in this study, where the impact of radiative transfer associated with the cooling of syngas arising from bubbling bed gasification of a highly reactive lignite coal is examined through numerical simulations. First, CFD modeling fidelity was established by the ability to identify the optimum conditions for hydrogen production as a function of the oxygen (oxidizer) to carbon (fuel) ratios at the gasifier inlet as previously identified by experiments. The syngas composition predictions compared favorably with measurements obtained from this gasifier and a maximum hydrogen concentration of 50% (dry-basis) was predicted at low oxygen to carbon ratios. However, there were significant variations in the H₂O/CO₂ ratios within the gasifier and the CO mole fractions were as high as 0.5 in some regions. This points to a need for developing and validating customized models for the gas radiative properties that are able to account for CO contributions as well as handle the wide variations in H₂O/CO₂ ratios within the gasifier. The water-gas-shift reaction persisted throughout the gasifier and along the syngas sampling train as reflected by its compositional variations during the cooling process. The radiative properties of the CO-CO₂-H₂O gas mixture was estimated employing a Planck mean absorption coefficient and used in conjunction with the P1 radiation model to estimate the radiative source terms and couple it with the energy/temperature equation. Accounting for radiative losses accelerated the syngas cooling process and the attainment of equilibrium concentrations along the sampling train. The study therefore highlights the need to account for radiative transfer phenomena during the syngas cooling process. This not only has implications on its compositions but also on the locations along the sampling line where ash and tar condensation processes occur thereby potentially presenting operational challenges.