The primary goal of a pilot scale gasification unit is to develop an in depth understanding of a proprietary gasification technology and demonstrate that the process works as designed. Pilot scale gasifiers must guide fine tuning of the reactor design for a specific feedstock. Alter NRG Corporation owns Westinghouse Plasma Corporation (WPC) which has operated a pilot gasifier for 20 years in Madison, Pennsylvania, USA. The WPC gasification technology can be categorized as wet bottom moving packed bed.
Some of the design parameters that are considered in the reactor design are: heat flux to the refractory inside surfaces, validation data for reaction rates, reaction bed height and voidage, density/PSD of the solid particulate cloud at the top of the fuel/charge bed and oxidant tuyere gas penetration depth. While some of the data can directly be measured, it is not possible or very difficult, given high temperature operating conditions and fast processes, to accurately measure bed inside temperatures, tuyere gas penetration depth, and particulate cloud density/PSD. The WPC pilot unit is well instrumented and allows the measurements of temperature at the reactor walls and along the gasification reaction column, pressure along the column, and product gas composition. However, this instrumentation alone provides an incomplete picture of the process. Computational fluid dynamics (CFD) models validated with measured pilot plant data can help to derive ‘difficult to measure' data.
A CFD model was developed by Hatch Ltd. to simulate the gasification column operation. Complete modeling of the gasification process is very challenging and therefore, a number of systematic simplifying assumptions were made. The packed solid fuel bed at the bottom of the reactor was treated as porous media where the bed porosity was defined as a function of flow momentum and distance from the reactor walls. The gasification process was defined using simplified global reactions. The temperature, pressure, and syngas composition measurement data were used to tune the reaction rate parameters, and bed properties. The tuned CFD model can be used to predict the heat flux at refractory inside surface, velocity and temperature distribution at the exit of the packed bed, and identify solids carry over characteristics. These results can then be used to model the commercial scale reactor and refine the design accordingly.