4 - Support for CFD and process simulation applications

Abstract

CFD has become the workhorse computational platform for engineering design work on coal utilization technology because CFD simulations can be the most economical route toward lower NOx emissions, and also diagnose important operational problems such as flame impingement, excessive LOI emissions, slagging in near-burner regions, and boiler-tube fouling in the convective passes. However, all CFD simulations must contend with severe computational restrictions that substantially erode their scientific fidelity. This chapter is devoted to only the chemistry submodels in CFD simulations of furnaces and gasifiers, which can only accommodate a handful of distinct chemical species. Nonetheless, CFD chemistry submodels do support global reaction rate expressions that are adequate for the essential aspects of solid fuel conversion, albeit only in pc furnaces, entrained flow gasifiers, and other systems that operate at the hottest temperatures. The strategy is to use comprehensive reaction mechanisms as a stand-alone “virtual fuels laboratory” that first simulates chemistry for the fuel properties and operating conditions of interest and then analyzes the simulation results to specify parameters in the global expressions that closely mimic the comprehensive reaction mechanisms. This chapter clearly demonstrates that all the important chemical stages of coal processing—drying, devolatilization, tar decomposition, and conversion of char and soot—can be accurately represented with global rate expressions, provided that the parameters in the global expressions are specified directly from the predictions from comprehensive mechanisms. The analysis to specify the kinetic parameters is no more complex than that used to specify them from laboratory test data, and is both more accurate and much more convenient to implement.