Investigation of the NOx and SOx Emissions from Large-Scale Coal-Based Power Plants

Abstract

Coal-fired power plants are a pivotal source of energy in Bangladesh, a country with abundant coal reserves. Yet, the environmental and health risks linked to the greenhouse gas emissions from these plants are significant contributors to climate change. The international framework, including agreements like the Kyoto Protocol, enforces strict limits on emissions, focusing the need for improved environmental compliance. Specific attention is given to the temporal analysis of COx emissions, revealing trends in global CO2 emissions from fuel sources between 2001 and 2022, alongside a detailed examination of NOx, SOx, and other volatile organic compound emissions in relation to socio-economic factors such as GDP, population, and per capita metrics. This thesis focuses on the Barapukuria Thermal Power Plant (BTPP), Bangladesh's first coal-based power station, which operates with sub-critical steam technology. The plant utilizes bituminous coal and is equipped with boilers that handle a flow rate of 40 tonnes per hour at 80% load conditions, featuring a detailed configuration of burners and air nozzles. The primary objective of this thesis is to analyze the current GHGs emission characteristics of BTPP. Using the commercial CFD software ANSYS 2020R2 Fluent, the thesis investigates on various oxidizing cases including standard air-firing (AF) and multiple oxy-firing (OF) conditions, across three loading levels: 50%, 80%, and 100%. The emission pattern which are investigated in this study can be implemented in modern coal-fired power stations to reduce emissions of recalcitrant pollutants like COx (Carbon oxides), NOx (Nitrogen Oxides), and SOx (Sulfur oxides). This comprehensive study utilizes advanced numerical methodology to model the chemical kinetics of combustion, detailing the dynamic interactions between turbulence and reaction mechanisms in various oxidizing environments. The model’s accuracy is validated by comparing the predicted flue gas temperatures with actual plant data, demonstrating a satisfactory alignment. The results reveal distinct variations in flame temperature across different oxy-fired (OF) scenarios, with the lowest temperatures observed in the OF23 case and the highest in OF31. Notably, the maximum flame temperatures increase with higher coal and airflow at 100% fuel loading, correlating directly with the loading levels. Velocity patterns of the flue gases also indicate that higher loadings accelerate the flow, contributing to increased velocity, particularly in the OF31 scenario. The study examines the highest CO2, NO, NO2, N2O and SO2 concentrations in the AF (Air Fire) scenario in all loads conditions, e. g. CO2, NOx and SOx emission exceeds 400 ppm (AQI standard, DOE, Bangladesh) which is attributed to enhanced pollutants capture techniques which reduce emissions patterns in OF23 to OF31 compared to AF23. This thesis explores the heat reaction rates, labeled as Heat Reaction Rate 1(devolatilization rate) and Heat Reaction Rate 2 (Char burnout rate), crucial for understanding the kinetics of coal combustion under different firing conditions. These rates are instrumental in optimizing combustion efficiency and reducing emissions. The analysis of NOx formation includes fuel, prompt, thermal, and intermediate N2O, NOx rates, comparing scenarios from AF to OF31. The outcomes are graphically represented, providing a clear comparison of NOx emissions in parts per million (PPM), enhancing the understanding of emission patterns under varying operational conditions. Hence, the sustainable scrubbing approach is a progressive new direction with exciting potential in the fields of technology and the economy. Adapting to the climate concern and clean energy is the focus of Sustainable Development Goal (SDG) 7 (Affordable and Clean Energy) and 13 (Climate Action). It is of the utmost importance to consider the long-term goal of reducing emissions by the year 2050 or it will get worse. This would also pave the way for the retrofitting of existing power stations to open the gateway of sustainable generation of green electricity.