## 1 Introduction  
Coal combustion in utility power plants is one of the largest contributors to the anthropogenic emission of carbon dioxide (CO\({}_{2}\)), which also brings about many environmental pollution problems, such as nitrogen oxides (NO\({}_{x}\)), sulfur oxides (SO\({}_{x}\)), particulate matters (PMs), mercury (Hg), and so on [1; 2; 3; 4; 5; 6; 7]. Several approaches can be utilized to reduce CO\({}_{2}\) emissions originating from coal-fired power plants, including the improvement of thermal efficiency, the replacement of coal with low-carbon fuels, the introduction of combined cycles, and the capture and storage of CO\({}_{2}\) (CCS) [8; 9; 10; 11; 12]. Oxy-fuel combustion can not only capture CO\({}_{2}\) in the large-scale but also obviously lower NO\({}_{x}\) emission in coal-fired power plants [13; 14; 15; 16; 17; 18; 19; 20], which is a potential clean coal technology in the near future and is drawing an ever-increasing attention nowadays. Numerous studies have been carried out on NO\({}_{x}\) emission in oxy-fuel combustion using various coals from different regions [21; 22; 23; 24; 25; 26]. [PERSON] et al. [22] indicated that compared to the air condition, nitric oxide (NO) emission was reduced obviously during oxy-fuel combustion, while the presence of steam could further lower the formation of NO from coal nitrogen and diminish the recycled NO. [PERSON] et al. [27] believed that the NO\({}_{x}\) emission per unit of energy in oxy-fuel combustion was nearly 70% lower than that in the air case. Notwithstanding, [PERSON] et al. [28] reported close emissions of NO\({}_{x}\) per energy unit between the air and oxy-fuel conditions using a bubbling fluidized bed combustor, while [PERSON] et al. [29] found out that the NO content in oxy-fuel combustion was less than one-third in air using a pressurized fluidized bed combustor. Under the oxy-fuel condition with gas recirculation, the emission of NO per energy unit could be reduced by 70-75% compared to air combustion from experiments performed by [PERSON] et al. [30]. However, [PERSON] et al. [31] pointed out that the conversion rates of NH\({}_{3}\)-to-NO\({}_{x}\) in the air and a 20.9% O\({}_{2}\)/79.1% CO\({}_{2}\) atmosphere showed similarity, but the increase of O\({}_{2}\) concentration had an obvious promotion on NO formation. The NO\({}_{x}\) emission in O\({}_{2}\)/CO\({}_{2}\) atmosphere was roughly 20% lower than that in the air, while the NO\({}_{x}\) emission under the condition with recycled flue gas was considerably reduced by approximately 50% [27]. Conclusions drawn by different investigators were still inconsistent possibly due to the complexity and differences of coal samples. The previous research showed that the influences of combustion temperature on NO formation from volatile-N were contrary between reducing and oxidizing atmospheres [32]. The increase in pressure did not significantly affect the NO creation but generated preferential conditions for NO reduction [29]. [PERSON] et al. [21] believed that the flue gas recirculation could result in a reduction of NO\({}_{x}\) emission by 40-50%, and that wet recycling was preferable in term of NO\({}_{x}\) reduction due to the inhibition effect of H\({}_{2}\)O on the oxidation of intermediates to form NO. [PERSON] and [PERSON] [33] concluded that NO reburn fraction was between 20% and 40% in oxy-fuel combustion. The stream staging was more efficient for the reduction of NO\({}_{x}\) in oxy-CFB combustion than that in air staging from experiments conducted by [PERSON] et al. [34] The little formation of thermal-NO\({}_{x}\), the destruction of recycled NO\({}_{x}\), and the reduction of NO\({}_{x}\) on char surfaces by the high concentration of local carbon monoxide (CO) all have contributions to the reduction of NO\({}_{x}\) during oxy-fuel combustion. Sometimes, experimental results from various researchers could differ significantly from one another due to the inherent complexity of coal compositions.