Coal-fired power plants have been around for a long time to meet global electricity generation needs. Needless to say, there are environmental and human health concerns that need to be addressed on this front. Although efforts to switch to renewable energy resources continue, coal-fired power plants may not be obsolete just yet. Against this background, it is relevant to investigate how the efficiency of these coal-fired boilers can be improved while reducing their harmful environmental impacts, namely greenhouse gas emissions, acid rain and photochemical smog formation, and human health .
For this purpose, various combustion methods such as air and stepped and swirling flow have been proposed. However, the efficacy of these technologies to mitigate pollutant emissions while maximizing combustion efficiency remains unclear. Now, in a recent study published online on December 31, 2022 and to be published in Volume 268, Issue 1 of the journal Energy on April 01, 2023, an international team of researchers led by Prof. Gyungmin Choi of the National University of Busan, Korea, analyzed the effectiveness of combining eddy flow and air stage to improve combustion efficiency and reduce pollution. “The exhaust pipe vortex (ETV) structure accompanying the swirling flow improves flame stability and combustion efficiency, but has the disadvantage of generating a large amount of NOx emissions. In contrast, air stage technology creates a fuel-rich environment in the primary combustion zone, which has a positive effect on NOx reduction, but has a negative impact on combustion efficiency,” explains Prof. Choi. “Therefore, if these two technologies are appropriately combined and applied in real life, a synergistic effect can be expected that reduces air pollutant emissions , as well as improves combustion efficiency.”
Accordingly, the team used both simulations and experiments to investigate the combined effects of different swirl configurations and air distribution within the 16 kWth converted down-burning pulverized coal boiler. A coal-fired boiler consists of three sections: a vortex burner, a boiler and a discharge pipe. For stage combustion, stage air is split into two sides and injected tangentially into the boiler. Liquefied Petroleum Gas (LPG) is used to preheat and stabilize the flame. The flow rate of staged air and LPG is adjusted and for each setting the temperature is measured using thermocouples. Additionally, the amount of gas-phase species was measured using a multigas analyzer.
Air stages with two swirl configurations, namely co-swirl and flame counterswirl, were evaluated to find out which one is more beneficial in reducing pollutant emissions. In the case of the co-vortex burner, where air and fuel circulate in the same way, the coal particles are evenly distributed thanks to the formation of an internal circulation zone and ETV – two vital characteristics for optimizing the design of coal-fired boilers.
Additionally, the team observed a uniform burn zone for the co-swirl configuration, which ensures complete combustion of the fuel, reducing emissions of gaseous species. It also facilitated increased conversion of chemical energy into heat energy, increasing combustion efficiency. In contrast, reverse swirl burners show uneven distribution of coal particles, uneven combustion and increased NOx emissions, suggesting that the co-vortex configuration is the better option. Additionally, the team showed that the air handling technology reduced environmental costs from $0.003 to $0.015 per day.
Overall, the insights from this study could prove extremely valuable in addressing the environmental issues and health hazards associated with coal-fired power plants. “We have identified and studied the structure and flame of ETV for the first time and will continue to explore and seek to use it in combustion-based industry.” concluded the optimistic Prof. Choi.