Modelling coal gasification with CFD and discrete phase method S.-P. Shi* 1 , S. E. Zitney 2 , M. Shahnam 1 , M. Syamlal 2 and W. A. Rogers 2 In the present paper the authors describe a computational fluid dynamics model of a two-stage, oxygen blown, entrained flow, coal slurry gasifier for use in advanced power plant simulations. The discrete phase method is used to simulate the coal slurry flow. The physical and chemical processing of coal slurry gasification is implemented by calculating the discrete phase trajectory using a Lagrangian formulation. The particle tracking is coupled with specific physical processes, in which the coal particles sequentially undergo moisture release, vaporisation, devolatilisation, char oxidation and char gasification. Using specified plant boundary conditions, the gasification model predicts a synthesis gas composition that is very close to the values calculated by a restricted equilibrium reactor model tuned to represent typical experimental data. The char conversions are 100 and 86% for the first stage and second stage respectively. Keywords: CFD, DPM, Coal gasification, Chemical reaction, Kinetics Introduction Because of deregulation, rapidly changing market demands, fluctuations in natural gas prices and increased environmental concerns, gasification will become the centrepiece of tomorrow’s advanced power plants. Large improvements in the efficiency, reliability and feedstock flexibility of gasification systems are necessary for the success of gasification based power plants. To address these challenges, the US Department of Energy (DOE) is sponsoring a broad spectrum of gasification research and demonstration projects. For example, the DOE’s $1 billion, 10-year, FutureGen project is aimed at creating the world’s first coal fired, gasification based, near zero emissions electricity and hydrogen production power plant. 1 Gasifiers involve complex physical and chemical phenomena including fluid flow, heat and mass transfer and chemical reactions. Combined with data from existing pilot and commercial scale gasifiers, computa- tional fluid dynamics (CFD) models offer a powerful method for understanding and improving gasification systems. Over the past decade, CFD modelling has played an important role in optimising the performance of the current fleet of pulverised coal fired electric utility boilers; e.g. Ref. 2. Likewise, CFD modelling can provide in- sights into the flow field within the gasifier, which can be used to enhance its design, analysis and operation. Coal gasification takes place when coal reacts with an oxidising agent such as air, oxygen, steam, or carbon dioxide (CO 2 ) to form a carbon monoxide (CO)/hydrogen (H 2 ) rich synthesis gas that is sent to downstream plant sections such as gas cleaning and CO 2 separation before entering gas turbines, or fuel cells for power production. Developments in coal gasification have been reviewed by Vamvuka 3 and recently by Niksa et al. 4,5 Owing to the simplicity of the geometry, lower pollutant generation and wide fuel compatibility, the entrained flow gasification technique is very attractive and will be the focus of the present paper. The entrained flow gasifier considered here is modelled using the commercial finite volume CFD software, Fluent. 6 Coal gasification modelling The coal gasification model used in the present study evolved from earlier models developed for fixed bed gasifiers 7 and dilute 8 and dense 9–11 transport gasifiers. Coal contains four pseudocomponents: ash, moisture, volatile matter and fixed carbon. Ash does not take part in any reaction. Moisture is released in the initial stage reaction of drying. Volatile matter in the coal produces several gas phase species through devolatilisation. Fixed carbon takes part in combustion and gasification reactions. In entrained flow gasification, the coal particles mainly follows the gas flow and the gasifier is typically in a dilute flow regime, where the volume occupied by the particles and the particle–particle interactions are negligible. A criterion often used in CFD for dilute flow is that the particle volume fraction is ,10%. In this case, a discrete phase method (DPM) can be applied to model the particle flow. Also the ratio of the mass flow rate of solids to that of the gas is less than or equal to one, which is required for ensuring the stability of DPM calculations. 8 Using DPM, the particle trajectories, along with mass and energy transfer to/from the particles, are computed 1 Fluent, Inc., 3647 Collins Ferry Road, Suite A, Morgantown, WV 26505, USA 2 US Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV 26507, USA *Corresponding author, email ssp@fluent.com ß 2006 Energy Institute Published by Maney on behalf of the Institute Received 27 October 2005; accepted 26 June 2006 DOI 10.1179/174602206X148865 Journal of the Energy Institute 2006 VOL 79 NO 4 217