energies Article Numerical Simulation of Combustion of Natural Gas Mixed with Hydrogen in Gas Boilers Yue Xin 1 , Ke Wang 1 , Yindi Zhang 1,2, *, Fanjin Zeng 1 , Xiang He 1,3, *, Shadrack Adjei Takyi 1 and Paitoon Tontiwachwuthikul 4   Citation: Xin, Y.; Wang, K.; Zhang, Y.; Zeng, F.; He, X.; Takyi, S.A.; Tontiwachwuthikul, P. Numerical Simulation of Combustion of Natural Gas Mixed with Hydrogen in Gas Boilers. Energies 2021, 14, 6883. https://doi.org/10.3390/ en14216883 Academic Editor: Vladislav A. Sadykov Received: 21 September 2021 Accepted: 15 October 2021 Published: 20 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Petroleum Engineering, Yangtze University, Wuhan 430100, China; Xinyue@yangtzeu.edu.cn (Y.X.); Wangke@yangtzeu.edu.cn(K.W.); Zengfanjin@yangtzeu.edu.cn (F.Z.); Takyi@yangtzeu.edu.cn (S.A.T.) 2 Measurement Science and Standards, National Research Council Canada, Building M-9, 1200 Montreal Road, Ottawa, ON K1A OR6, Canada 3 Tianyuan Holding Group Co., Ltd., Wuhan 430062, China 4 Faculty of Engineering and Applied Science, Clean Energy Technologies Research Institute (CETRI), University of Regina, Regina, SK S4S 0A2, Canada; paitoon@uregina.ca * Correspondence: zhangyindi@yangtzeu.edu.cn (Y.Z.); Hexiang@yangtzeu.edu.cn (X.H.) Abstract: Hydrogen mixed natural gas for combustion can improve combustion characteristics and reduce carbon emission, which has important engineering application value. A casing swirl burner model is adopted to numerically simulate and research the natural gas hydrogen mixing technology for combustion in gas boilers in this paper. Under the condition of conventional air atmosphere and constant air excess coefficient, the six working conditions for hydrogen mixing proportion into natural gas are designed to explore the combustion characteristics and the laws of pollution emissions. The temperature distributions, composition, and emission of combustion flue gas under various working conditions are analyzed and compared. Further investigation is also conducted for the variation laws of NOx and soot generation. The results show that when the boiler heating power is constant, hydrogen mixing will increase the combustion temperature, accelerate the combustion rate, reduce flue gas and CO 2 emission, increase the generation of water vapor, and inhibit the generation of NOx and soot. Under the premise of meeting the fuel interchangeability, it is concluded that the optimal hydrogen mixing volume fraction of gas boilers is 24.7%. Keywords: hydrogen-blended combustion; numerical simulation; combustion characteristics; pollu- tant generation; flue gas analysis 1. Introduction In recent years, with the rapid development in economy and society, the world’s energy demand is increasing, and the energy structure is changing. According to the BP 2020 [1], the overall growth of primary energy consumption has slowed down, but renewable energy consumption has increased significantly. In addition to that, the develop- ment of the natural gas industry has also increased steadily. Natural gas will account for 18% of China’s primary energy consumption by 2050 [2]. As important renewable energy, hydrogen energy is being jointly promoted by the global trend of clean and low-carbon energy and the progress of the hydrogen energy technology. Due to the large-scale use of hydrogen energy, global CO 2 emission will be reduced by 20% by 2050 [3]. At present, the development and construction of the natural gas pipeline network have experienced a maturity, and it is feasible to mix hydrogen in it for utilization, and the energy storage is huge. Melaina [4] indicated that risk increases slightly when 50% hydrogen is mixed in the gas transmission pipeline but increases significantly when the gas is mixed with more than 50% hydrogen. Polman [5] et al. analyzed the risk of hydrogen mixing in pipelines and found that under the premise of effective supervision, there is no risk of explosion when the volume fraction of hydrogen mixed in gas pipelines is less than 25%. Melaina [4] et al. Energies 2021, 14, 6883. https://doi.org/10.3390/en14216883 https://www.mdpi.com/journal/energies