Capturing CO2 from a conventional coal-fired power plant is a very expensive approach for reducing CO2 emissions because of the low CO2 partial pressure in combustion products. One option to reduce the capture cost is to burn coal with O2 and recirculated exhaust gas. In this way the main products of combustion are H2O and CO2, which can be easily separated. Furthermore, NOx emissions are smaller owing to the lower N2 fraction in the gas and the lower flame temperature, while producing pure O2 is also highly expensive. In this paper the performances of pulverized coal combustion with exhaust gas recirculation are evaluated by means of the CFD commercial code Fluent, using advanced mathematical models for coal devolatilization and for turbulence-chemistry interaction in the gas phase. Advanced combustion models, accounting for finite reaction rates, are required to simulate pulverized coal oxycombustion since the high fraction of CO2 reduces the flame temperature due to the higher specific heat, thus determining combustion instabilities related to a low reaction rate, such as extinction and ignition. The pulverized coal combustion with recirculated exhaust gas is evaluated by considering the same burner geometry and operating condition used for the conventional air combustion test-case in IFRF no.1 furnace. The secondary air stream is replaced by a O2/CO2 mixture with the same O2 volume fraction and the same molar flow rate, and keeping the same velocity profile. The performances of oxy-coal combustion with exhaust gas recirculation are evaluated and compared with conventional air combustion in terms of burner efficiency, flame characteristics, emissions and other combustion parameters. Two oxycombustion cases are considered, characterized by the same O2-to-coal ratio: in the first case the secondary molar flow rate is kept constant as in the air combustion case (21%O2-79%CO2 by volume), in the second one the secondary mass flow rate is kept constant (30%O2-70%CO2 by volume). The first case is characterized by similar fluid dynamic behaviour of the air combustion, but by a lower flame temperature related to the lower CO2 specific heat. The second case has similar flame temperature, but fluid dynamic behaviour is different because of the different inlet velocity of secondary stream. Oxycombustion flame are deeply different respect to conventional flame because of the different physical properties of the O2-CO2 mixture. Therefore they require an optimization of the burner geometry and of the O2-CO2 mixture composition in order to improve flame characteristics and reduce pollutants emissions.

Numerical simulation of pulverized coal oxy-combustion with exhaust gas recirculation

CAU, GIORGIO
2009

Abstract

Capturing CO2 from a conventional coal-fired power plant is a very expensive approach for reducing CO2 emissions because of the low CO2 partial pressure in combustion products. One option to reduce the capture cost is to burn coal with O2 and recirculated exhaust gas. In this way the main products of combustion are H2O and CO2, which can be easily separated. Furthermore, NOx emissions are smaller owing to the lower N2 fraction in the gas and the lower flame temperature, while producing pure O2 is also highly expensive. In this paper the performances of pulverized coal combustion with exhaust gas recirculation are evaluated by means of the CFD commercial code Fluent, using advanced mathematical models for coal devolatilization and for turbulence-chemistry interaction in the gas phase. Advanced combustion models, accounting for finite reaction rates, are required to simulate pulverized coal oxycombustion since the high fraction of CO2 reduces the flame temperature due to the higher specific heat, thus determining combustion instabilities related to a low reaction rate, such as extinction and ignition. The pulverized coal combustion with recirculated exhaust gas is evaluated by considering the same burner geometry and operating condition used for the conventional air combustion test-case in IFRF no.1 furnace. The secondary air stream is replaced by a O2/CO2 mixture with the same O2 volume fraction and the same molar flow rate, and keeping the same velocity profile. The performances of oxy-coal combustion with exhaust gas recirculation are evaluated and compared with conventional air combustion in terms of burner efficiency, flame characteristics, emissions and other combustion parameters. Two oxycombustion cases are considered, characterized by the same O2-to-coal ratio: in the first case the secondary molar flow rate is kept constant as in the air combustion case (21%O2-79%CO2 by volume), in the second one the secondary mass flow rate is kept constant (30%O2-70%CO2 by volume). The first case is characterized by similar fluid dynamic behaviour of the air combustion, but by a lower flame temperature related to the lower CO2 specific heat. The second case has similar flame temperature, but fluid dynamic behaviour is different because of the different inlet velocity of secondary stream. Oxycombustion flame are deeply different respect to conventional flame because of the different physical properties of the O2-CO2 mixture. Therefore they require an optimization of the burner geometry and of the O2-CO2 mixture composition in order to improve flame characteristics and reduce pollutants emissions.
978-92-9029-467-2
Coal; CFD; Oxycombustion; CO2 removal
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11584/25284
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