## 1 Introduction  
With the rapid development of modern society, the energy crisis and environmental issues are becoming increasingly serious. It is therefore urgent to reduce the use of fossil fuels as well as to develop renewable energy and clean energy. China has put forward the 'carbon peaking and carbon neutrality' goals to limit CO\({}_{2}\) emissions. The disadvantages of renewable energy such as photovoltaic and wind power include difficulties with storage and low efficiency [1, 2]. Hence, some scholars have used unstable photoelectric and wind power to produce hydrogen by water electrolysis for storage [3, 4]. However, hydrogen is explosive, and also expensive to store and transport [5, 6]. Ammonia is an ideal renewable carbon-free fuel and hydrogen carrier, whose combustion with oxygen produces only nitrogen and water, without emitting SO\({}_{x}\) or particulate matter [7, 8]. If those green hydrogens and the nitrogen separated from air are used to synthesize ammonia, complete carbon-free emissions can be achieved [9, 10, 11]. In comparison to hydrogen, ammonia is easy to liquefy, and the cost of storage and transportation is low [12]. Ammonia has a pungent smell, so it is easy to detect leaks and avoid danger [13]. Liquid ammonia has high volume energy density [14], and ammonia is safe because it is not easy to ignite at ambient temperature [13]. However, compared with conventional hydrocarbon fuels, pure ammonia has a slower laminar combustion rate and is harder to ignite in air [15, 16]. In practice, it is challengingto achieve complete combustion of ammonia, and the combustion process has a risk of exceeding NO\({}_{x}\) emissions [17]. Blending known fuels with other fuels for combustion is a common practice [18; 19]. Co-firing ammonia with another combustible gas is a potential approach to improving the combustion rate and performance of ammonia, which had been studied by some scholars. [PERSON] et al. [20] found that the mixed combustion of ammonia and hydrogen produced by the decomposition of ammonia by catalysts can improve the combustion performance of ammonia engines and reduce exhaust emissions. [PERSON] et al. [21] studied the laminar flame propagation characteristics of an ammonia-hydrogen mixture and found that when the volume fraction of H\({}_{2}\) mixing reached 40%, the laminar flame velocity of the mixture reached 35 cm/s, which was roughly equivalent to the laminar flame velocity of CH\({}_{4}\) when it was burned in air. [PERSON] et al. [6] also found a similar variation trend in laminar flame velocity. [PERSON] et al. [22] also found that the laminar combustion velocity increased with the increase in hydrogen fraction. In order to reduce costs, ammonia can be mixed with some by-product gases produced in agricultural or industry, such as landfill gas, steel gas, or biogas. The main flammable component of these gases is CH\({}_{4}\) or CO. One study found that when ammonia was mixed with conventional hydrocarbon fuels, the flame speed, heat release rate, and radiation intensity of the fuel were higher than that of pure ammonia [3]. [PERSON] et al. [23] conducted experimental and numerical studies on methane-NH\({}_{3}\)-air premixed flames, and found that the unstretched laminar combustion velocity of the mixture decreased with the increase in ammonia concentration. [PERSON] et al. [24] conducted premix combustion experiments and modeling studies on NH\({}_{3}\)/CH\({}_{4}\)/O\({}_{2}\)/Ar under 11 kinds of excess air coefficients, and then proposed a comprehensive chemical mechanism of combustion of fuel mixtures with different ratios of ammonia/methane. [PERSON] et al. [25] studied the ignition process of 60% NH\({}_{3}\)/40% CH\({}_{4}\) fuel mixture by using a shock tube, and found that the ignition performance of NH\({}_{3}\)/CH\({}_{4}\) was significantly improved with the increase in temperature, pressure, and methane mixing ratio, but the influence of the excess air coefficient on ignition performance was not obvious. [PERSON] et al.