Development of a First-Principles Hybrid Boiler Model for Oxy-Combustion Power Generation System

The design of an oxy-combustion system for CO2 capture involves the integration of multiple devices including air separation unit, coal-fired boiler, steam turbine, flue gas cleanup, recycle, and compression units. Thousands of design parameters for the entire system need to be optimized to achieve the lowest cost per kilowatt-hour of electricity generated. An appropriate first-principles based boiler model with short computer execution time but yet reasonable accuracy in both air-fired and oxy-fired configurations is highly desired. To this end, a hybrid boiler model with 1-D resolution for main flow and reaction related calculations and 3-D resolution for radiative heat transfer was developed as a part of the oxy-combustion subtask of the Carbon Capture and Simulation Initiative (CCSI) sponsored by U.S. Department of Energy. The developed model is able to automatically generate a 3-D mesh based on user-specified furnace shape for the calculation of radiative heat transfer using discrete ordinates method. The 3-D cells are assigned to individual regions along the furnace height, forming 1-D zones in which conservations of mass and energy in both gas and particle phases are enforced. The kinetics of heterogeneous reactions between char particles and gas reactants are modeled zone by zone in Lagrangian framework. Gas phase chemistry in each zone is simplified based on chemical equilibrium. The submodels for calculating the radiative properties of the gas and particle phases and those for calculating the heterogeneous char reactions are suitable for both air-fired and oxy-fired conditions. A typical boiler model can be converged in approximately one minute on a personal computer and, therefore, is suitable for the generation of reduced order models in algebraic form which could be used for the large-scale multivariable optimization of the oxy-combustion systems. By modeling a utility boiler in both air-fired and oxy-fired configurations and comparing the results to those from computational fluid dynamics simulations, the hybrid boiler model was found to give very comparable predictions in major boiler performance parameters such as furnace exit gas temperature, heat losses to walls, and unburned carbon in fly ash. By modeling a small test furnace, it was found that with the adjustments on a “mixedness” related effectiveness factor for the char combustion and gasification reactions, the boiler model is able to correctly predict the trends of incident radiation heat flux profile along the furnace axial length as well as the trend of the unburned carbon in both air-fired and oxy-fired configurations.