Étude expérimentale et numérique des processus de dévolatilisation et d'oxydation du charbon pulvérisé à fort gradient de chauffe sous air et sous atmosphères enrichies en oxygène

Abstract

Coal is expected to remain largely used to meet the global primary energy needs, and particularly in the electricity generation field. A current political and economic challenge is to supply the growing energy needs without exceeding the carbon dioxide emission limitations. To do so, pulverized coal can be burnt under unconventional atmospheres to ease the capture of carbon dioxide. In this context, this thesis aims at better understanding the impact of switching from conventional air to oxy-fuel combustion conditions on the physical-chemical processes involved in pulverized coal combustion. This thesis particularly focuses on the modeling of pulverized coal devolatilization and volatile oxidation under air, oxygen-enriched air and oxy-fuel combustion conditions by means of a Chemical Reactor Network (CRN) modeling approach. The first part of this thesis focuses on devolatilization kinetics. Empirical and network models have been analyzed. This study underlines that the atmosphere influences devolatilization kinetics mainly indirectly, by modifying heat transfer to particles, under the moderate temperature conditions investigated herein. Two kinetic models have then been selected to be used together in the CRN, namely the two-competing rate and the Chemical Percolation Devolatilization (CPD) models. The second part of this thesis has been dedicated to the fluid dynamics modeling of the experimental combustion chamber. Computational Fluid Dynamics (CFD) simulations have been conducted using the ANSYS FLUENT software. Obtained results have then been used into the CFD to CRN approach presented in the third part of this thesis. CFD results led to estimate the reactors’ properties (such as their volume) together with the heat and mass transfers between each reactor. The use of a CRN approach allowed implanting a detailed reaction mechanism including 190 species involved in 869 reactions so as to study the oxidation of devolatilized matters including nitrogen- and sulfur-containing species likewise tar. The CRN predictions allowed explaining the changes observed in terms of CO, NO, SO2 and soot precursor concentrations when changing the combustion atmosphere. The analysis of the reaction pathways especially showed that the increased CO concentrations measured under oxy-fuel combustion conditions was due to both a heightened HCO production and an increased tar oxidation. Under such a type of atmosphere, an increased NO production has also been noted and related to alternative NO formation pathways such as the HCN → NCO → HNCO → HNO → NO route.

Date	23 Jan 2019
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Patrice Seers (Supervisor) & Romain Lemaire (Co-supervisor)