Numerical simulations of condensing heat exchangers with oxyfuel flue gas

Previous work had been done at the Energy Research Center to develop and test numerical software for modeling condensing heat exchangers. This software was used to numerically simulate the results from an experimental condensing heat exchanger which used bare tube heat exchangers, with coal fired flue gas on the outside of the tubes, and cooling water in the interior. This program initially was designed to simulate flue gas from a laboratory scale coal or oil fired boiler, and was eventually scaled up to model commercial size heat exchangers. The limitations of this program were based in inherent simplifications limiting the ability to simulate diverse flue gas compositions accurately.;The heat exchanger code failed to be able to provide simulations consistently or accurately for varied compositions of flue gas relating to several cases, such as in an oxyfuel fired power plant, or a CO2 and water vapor flue gas as produced by a post combustion carbon capture unit. The changes of composition in the dry fuel gas caused intrinsic errors in the simulations due to the method of calculation of transport properties and the high water content sometimes experienced in post combustion carbon capture systems caused numerical instabilities.;Modifications were made to improve accuracy and convergence over a wider range of possible flue gas compositions. Each modification was verified by checking congruency with the original code, as well as experimental data. Due to a lack of experimental data for heat exchangers with a flue gas composition of an oxyfuel fired power plant, or a post combustion capture system, additional verification occurred only from simulated changes matching theoretically expected results.;Changes made and verified on the lab scale simulation code were transferred to a full scale version of the simulation code. This code was used to simulate the potential changes and efficiency losses involved with running oxyfuel flue gas through a heat exchanger that was optimized for regular flue gas (and regular power plant flue gas flow rates). The code was also used to provide a performance benchmark to estimate heat duty changes resulting from deviations in composition from regular flue gas and alternative flue gas compositions.