Absorption analysis of CO2, SO2 and NOx from flue gases and performance evaluation of IGCC with and without CO2 capture

Abstract

Fossil fuels are the most widely used source of electricity generation due to their low price, abundant availability, and wide distribution around the globe. However, combustion of fossil fuels produces environment pollutants like carbon dioxide, sulfur dioxide, nitrogen oxides, particulates, and mercury. The direct emission of these components into atmosphere is creating economic losses from weather and climate related disasters. The main focus of research related to CO2 capture, SO2 removal and NOx abatement is to increase the absorption efficiency of flue gases and to reduce the overall cost of absorption system. The conventional techniques for removing CO2, SO2, and NOx are not cost-effective, the additional energy requirements, operational costs and capital costs of these technologies can reduce power plant efficiency. For example, the conventional use of representative solvent monoethanolamine (MEA) requires a large heat input for its regeneration in the stripper, has fairly poor CO2 absorption capacity, tends to corrode equipment, and can be degraded by oxidants in the flue gases, chiefly SO2, NO2 and O2. Worldwide, 87% of the installed SO2 control technologies use a wet process with 97% using a calcium-based sorbent. These processes have a typical SO2 removal efficiency of 90% and reaction products are gypsum which is relatively inexpensive product. This means that the conventional SO2 removal process may allow fraction of the SO2 to move into CO2 absorber. The commercially available amine-based CO2 capture process such as MEA requires that SO2 concentration must be maintained below 10 ppm to prevent solvent degradation. MEA solvent is degraded with SO2 and oxygen to produce insoluble salts that cannot be regenerated by thermal decomposition. Likewise, for NOx removal, Selective Catalytic Reduction (SCR) has been most highly researched and is considered as an effective technology to control NOx emissions into the atmosphere. However, it has certain disadvantages, such as catalyst poisoning due to the presence of SO2 in flue gases, the risk of ammonia slip, the requirement of additional heat exchangers to recover energy due to the high operating temperature (430-470°C) and the limited catalyst lifetime due to intensive fly ash erosion. To overcome these shortcomings, alternative solvents such as aqueous ammonia have been proposed in the literature. The aqueous ammonia has many advantages over the conventional MEA solvent such as the ability to capture multiple pollutants, the low cost of absorbent, high CO2 loading capacity, no degradation with SO2 or oxygen, and can be regenerated in stripper at relatively low energy penalty. The capture of SO2 and NOx with aqueous ammonia results in ammonium sulphate and ammonium nitrate which are useful fertilizers. In this dissertation, an aqueous ammonia-based CO2 absorption-desorption process integrated with a washing column is modelled and simulated in Aspen Plus®. The predicted results agreed with published experimental results. The effect of performance parameters such as the ammonia concentration in lean solution, CO2 fraction in the flue gas, inlet feed gas temperature, lean solution temperature and flow rates of the flue gas and lean solution were investigated to predict the optimized operating conditions. The capability of ammonia to capture multi-pollutant was analysed by combined removal of SO2 and CO2 in an aqueous ammonia-based system. A closed-loop CO2 absorption-desorption process integrated with flue gas desulfurization system is modelled and simulated. To optimize the operating conditions, the impacts are investigated of performance parameters including flue gas temperature, concentration of CO2 and SO2 in flue gases, and the temperatures of the lean solution and ammoniated water. Furthermore, the performance efficiency of the stripper column is analysed in terms of reboiler heat duty and CO2 regeneration rate. For NOx removal, due to aforementioned drawbacks of SCR, the alternate method such as ozone injection integrated with wet absorption is presented in this thesis. In this thesis, the ozone injection process integrated with an absorber column is numerically modelled and simulated at various operating conditions. The predicted results of NOx oxidation with ozone injection and absorption in water agreed with the published experimental results. The influence of performance parameters (such as feed gas flow rate, inlet gas temperature, reactor configurations, ozone concentration, and NO/NO2 molar ratio) on the oxidation efficiency of NOx in the reactor and absorber column is investigated to predict the optimal operating conditions. Although, aqueous ammonia based CO2 capture offers several advantages over the conventional MEA solvent, however aqueous ammonia is highly volatile in nature. In conventional aqueous ammonia-based CO2 capture, the process either needs to operate at very low temperatures or must include wash-water columns to mitigate ammonia slip, which increase the capital and operational costs of the system. The sterically hinder amine such as 2-amino-2-methyl-1-propanol (AMP) is thermally stable, has low heat capacity and high CO2 loading, but it has slow reaction rate compared to ammonia. On the other hand, ammonia has a very fast reaction rate with CO2 but relatively low CO2 loading capacity than AMP. By mixing two solvents, the deficiency of two solvents can be overcome. In this thesis, a blended solution of AMP and ammonia was used to analyze the CO2 capture efficiency, ammonia slip, and stripper heat duty. Finally, this dissertation presents performance evaluation of IGCC integrated with blended solution of ammonia and AMP. The performance evaluation of IGCC is carried out in terms of power produced and efficiency penalty due to CO2 capture. Furthermore, exergy analysis of IGCC with and without CO2 capture is carried out to predict the loss of useful work in major sections of the IGCC.