Diseño de un sistema de reducción catalítica de óxidos de nitrógeno

Abstract

This thesis will focus on the removal of nitrogen oxides, usually referred as NOx, pursuing for their removal from a flue gas flow in a thermal power station. A viability study is carried out, in order to ensure the technical, environmental and economical feasibility of the study. This includes an analysis of the available technologies for the nitrogen oxides removal, choosing the Selective Catalytic Reduction, known as SCR. On this process, a monolithic catalyst reactor is used.for the chemical reduction of the NOx molecules, to harmless compounds as N2 and H2O. In addition, an assessment of Spanish electricity market is carried out. Viability study also includes project location, at the Aboño thermal power station, in Asturias, and reactor capacity, which will be set in nine hundred thousands kg per hour of flue gases. The design of the SCR reactor is carried out, plus the design of the auxiliary equipment needed for the conditioning of any flow of the system. Chemical analysis, kinetics, thermodynamics and operating conditions are studied in this part of the thesis. The heterogeneous catalytic reactor, where the reduction of nitrogen oxides is performed, is the main unit of the system. [46] It will carry the monolith inside, providing a large specific surface for the reaction. This catalyst will be divided in many layers, identical each other, with a one meter length, establishing the number of layers required for carrying out the reaction and reaching the NOx removal efficiency, settled as 81,5 %, by legal requirements. To modelling the reactor, mass and energy balance are applied to this heterogeneous system with axial diffusivity, whose resolution also requires the calculation of mass and heat transfer coefficients. Besides, physical and chemical characteristics of V2O5/TiO2, the vanadium monolithic used in the design, are established. The determination of the catalyst amount needed will allow to design the reactor chamber. Thus, technical report includes a P&ID (Piping and Instrumentation Diagram), in order to guarantee a safe and correct plant operation. To ensure the technical report developed, a security study is done. It includes a hazard and toxicity analysis of the chemical compounds involved in the SCR process, and a HAZOP study. This thesis also includes an environmental impact assessment, key of this study, since reducing the environmental impact of the power plant is the main reason to carry out the design. Initial conditions are analyzed, before and after the implementation of the SCR system. Identifying the impact to the environment, wildlife, flora, health and atmosphere will be crucial. By last, and economic analysis is developed. Profitability of the process will be given by environmental improvement, not just by economic factors.