Oxy-fuel combustion technology, which is based on burning coal in a mixture of oxygen and recycled flue gas (RFG), is suggested as one of new promising technologies for capturing CO2 from power plants. In this thesis study, the pyrolysis and combustion behaviour of various fuels including imported coal, petroleum coke, two different types of indigenous lignites, olive residue and their blends with different proportions in air and oxy-fuel conditions were investigated by using non-isothermal thermogravimetric method (TGA) coupled with Fourier-transform infrared (FTIR) spectrometer. Pyrolysis tests were carried out in nitrogen and carbon dioxide environments, which are the main diluting gases of air and oxy-fuel environment, respectively. Pyrolysis results reveal that weight loss profiles are similar up to high temperature zone in both pyrolysis environments, indicating that CO2 behaves as an inert gas in this temperature range. However, further weight loss takes place in CO2 atmosphere v after 700oC due to CO2-char gasification reaction which is observed in pyrolysis of all fuel samples. Combustion experiments were carried out in four different atmospheres / air, oxygen-enriched air environment (30 % O2 &ndash / 70 % N2), oxy-fuel environment (21 % O2 &ndash / 79 % CO2) and oxygen-enriched oxy-fuel environment (30 % O2 &ndash / 70 % CO2). Combustion experiments show that replacing nitrogen in the gas mixture by the same concentration of CO2 does not affect the combustion process significantly but leads to slight delay (lower weight loss rate and higher burnout temperature) in combustion. Overall comparison of weight loss profiles shows that higher oxygen content in the combustion environment is the dominant factor affecting the combustion rather than the diluting gas. As O2 concentration increases profiles shift through lower temperature zone, peak and burnout temperatures decrease, weight loss rate increases and complete combustion is achieved at lower temperatures and shorter times. Pyrolysis and combustion behaviour of three different fuel blends were also investigated. Results reveal synergistic interactions in combustion tests of all blends in all combustion environments. During pyrolysis and combustion tests gaseous products CO2, CO, H2O, CH4, SO2 and COS were identified in flue gas and analyzed by using FTIR. Results indicate that higher CO and COS formation take place during pyrolysis tests due to gasification reaction in CO2 atmosphere at high temperature zone. Gaseous species evolution trends in combustion tests are found specific for each fuel. However, evolution trends slightly shift to lower temperatures in oxygen-enriched conditions.

Selective Exhaust Gas Recirculation (S-EGR) consists of selectively transferring CO2 from the exhaust gas stream of a gas-fired power plant into the air stream entering the gas turbine compressor. Unlike in “non-selective” Exhaust Gas Recirculation (EGR) technology, recirculation of, principally, nitrogen does not occur, and the gas turbine still operates with a large excess of air. Two configurations are proposed: one with the CO2 transfer system operating in parallel to the post-combustion carbon capture (PCC) unit; the other with the CO2 transfer system operating downstream of, and in series to, the PCC unit. S-EGR allows for higher CO2 concentrations in the flue gas of approximately 13-14 vol%, compared to 6.6 vol% with EGR at 35% recirculation ratio. The oxygen levels in the combustor are approximately 19 vol%, well above the minimum limit of 16 vol% with 35% EGR reported in literature. At these operating conditions, process model simulations show that the current class of gas turbine engines can operate without a significant deviation in the compressor and the turbine performance from the design conditions. Compressor inlet temperature and CO2 concentration in the working fluid are critical parameters in the assessment of the effect on the gas turbine net power output and efficiency. A higher turbine exhaust temperature allows the generation of additional steam which results in a marginal increase in the combined cycle net power output of 5% and 2% in the investigated configurations with S-EGR in parallel and S-EGR in series, respectively. With aqueous monoethanolamine scrubbing technology, S-EGR leads to operation and cost benefits. S-EGR in parallel operating at 70% recirculation, 97% selective CO2 transfer efficiency and 96% PCC efficiency results in a reduction of 46% in packing volume and 5% in specific reboiler duty, compared to air-based combustion CCGT with PCC, and of 10% in packing volume and 2% in specific reboiler duty, compared to 35% EGR. S-EGR in series operating at 95% selective CO2 transfer efficiency and 32% PCC efficiency results in a reduction of 64% in packing volume and 7% in specific reboiler duty, compared to air-based, and of 40% in packing volume and 4% in specific reboiler duty, compared to 35% EGR. An analysis of key performance indicators for selective CO2 transfer proposes physical adsorption in rotary wheel systems as an alternative to selective CO2 membrane systems. A conceptual design assessment with two commercially available adsorbent materials, activated carbon and Zeolite X13, shows that it is possible to regenerate the adsorbent with air at near ambient temperature and pressure. Yet, a significant step change in adsorbent materials is necessary to design rotary adsorption systems with dimensions comparable to the largest rotary gas/gas heat exchanger used in coal-fired power plants, i.e. approximately 24 m diameter and 2 m height. An optimisation study provides guidelines on the equilibrium parameters for the development of materials. Finally, a technical feasibility study of configuration options with rotary gas/gas heat exchangers shows that cooling water demand around the post-combustion CO2 capture system can be drastically reduced using dry cooling systems where gas/gas heat exchangers use ambient air as the cooling fluid. Hybrid cooling configurations reduce cooling and process water demand in the direct contact cooler of a wet cooling system by 67% and 35% respectively, and dry cooling configurations eliminate the use of process and cooling water and achieve adequate gas temperature entering the absorber.

Le captage du CO2 en postcombustion par absorption dans des solutions aqueuses d'amines est la technologie la plus mature pour réduire les émissions de gaz à effets de serre. Cependant, les amines utilisées sont susceptibles de réagir avec l'oxygène présent dans les fumées pour former de nouveaux composés qui peuvent être émis à l'atmosphère et avoir des conséquences sur l'environnement et la santé humaine.. L'objectif de cette thèse était donc d'identifier le maximum de produits de dégradation des amines grâce au développement de différentes techniques analytiques et d'échantillonnage, notamment pour l'analyse de la phase gaz. Ainsi plus de soixante produits issus de la dégradation de la monoéthanolamine (MEA) en pilote de captage du CO2 ont été identifiés. Une trentaine de ces produits sont nouveaux, ils sont souvent issus d'une même famille comme les pyrazines ou les oxazolines ou ils peuvent être caractérisés par l'allongement de la chaine carbonée (C2 entre deux hétéroatomes à C5).Des mécanismes basés sur des réactions d'alkylation/de désalkylation, la formation d'aldéhydes ou de cétones, l'amidification, l'aldolisation, la réaction d'Eschweiler Clarke, la formation de pyridines ont été proposés pour expliquer la formation de tous les nouveaux produits de dégradation et validés, dans la plupart des cas, en mélangeant les réactifs proposés dans le mécanisme. Finalement, il a été montré que la transposition de ces schémas réactionnels à trois autres amines (N-méthylaminoéthanolamine, 1-aminopropan-2-ol, 3-aminopropan-1-ol) a permis de prédire leurs produits de dégradation. / The CO2 post-combustion capture with aqueous solutions of amines is the most mature technology to reduce greenhouse gases emissions. However chemical absorption is suffering from the degradation of amines mainly due to the presence of O2 in flue gases. Formed products, which could be rejected to atmosphere, may be detrimental to environment and human health. The aim of this thesis was to identify as many degradation products as possible thanks to the development of different sampling and analytical methods especially for gas phase analysis. Thus more than sixty products issued from monoethanolamaine (MEA) degradation were observed in pilot plant samples. Thirty of them are novel, they often belong to the same family as pyrazines or oxazolines, or they could be characterized by the increase of carbon chain lengths (C2 between two heteroatoms to C5).Mechanisms such as alkylation/dealkylation, aldehydes/ketones formation, amidification, aldolisation, Eschweiler Clarke, pyridines formation were proposed to explain the formation of novel products and were, most of the time, validated by mixing the reactants proposed in the mechanism. Finally, it has been shown that the transposition of these reactions to three other amines (N-methylaminoethanolamine, 1-aminopropan-2-ol, 3-aminopropan-1-ol) enabled us to predict their degradation products.