CFD modelling of slagging fixed-bed gasification of Turkish lignite and biomass blends for hydrogen-enriched syngas

Abstract

This study presents a validated computational fluid dynamics (CFD) model of the Slagging Fixed-Bedgasifier, developed in OpenFOAM to simulate pure lignite, pure biomass, and coal-biomass co-gasification under pressurised oxygen-steam conditions. The main problem addressed in this study is the lack of a validated CFD-based engineering framework for assessing the fuel-flexible operation of slagging fixed-bed gasifiers using high-ash Turkish lignites and biomass blends under pressurised oxygen-steam conditions. The study therefore aims not only to predict syngas composition but also to identify operating conditions that improve hydrogen yield, reduce CO2 emissions, and maintain cold gas efficiency. Two Turkish lignites (Tun & ccedil;bilek and Soma) and selected biomass feedstocks (spruce wood, corn cob, wheat straw) were characterised by proximate and ultimate analyses. The model employs detailed devolatilisation, char oxidation, and gas-phase reaction submodels coupled with the Partially Stirred Reactor (PaSR) turbulence-chemistry interaction framework. Numerical predictions for syngas composition, hydrogen-to-carbon monoxide ratio (H-2/CO), and cold gas efficiency (CGE) were compared with experimental reference bands from peer-reviewed Slagging Fixed-Bed gasification studies. Across all cases, CFD predictions fell within experimental ranges, with relative deviations of +/- 3-4% for H-2, +/- 2-3% for CO, +/- 5% for CO2, and +/- 2% for CGE; CH4 showed slightly higher deviations (up to +/- 10%) due to its low concentration. Pure spruce wood achieved the highest hydrogen content (45.1 vol%), while Tun & ccedil;bilek lignite produced 34.2 vol% H-2 with CGE of 78%. Biomass co-feeding (20-40 wt%) increased hydrogen yield by 15-25% and reduced CO2 emissions by up to 21.8% without compromising CGE (>80%). The validated model demonstrates robust predictive capability for both single-feed and co-gasification scenarios, providing a reliable basis for optimisation studies. Parametric analysis confirmed that optimal O-2/C ratios were similar to 0.8 for coal and similar to 0.6 for biomass, with S/C above 1.0 increasing H-2 but slightly reducing CGE. These findings support the use of fuel-flexible Slagging Fixed-Bed units for hydrogen-oriented syngas production and partial decarbonisation in coal-based energy infrastructure.