## 1 Introduction  
Coal is still one of the most important energy resources in China; it accounts for \(\sim\)62% of the total energy demand nowadays and causes many environmental problems such as SOx, NOx, PM (particulate matter) and heavy metals [1; 2; 3; 4; 5; 6]. The newly discovered Zhundong coal field is the largest intact coalfield with a predicted reserve of \(\sim\)390 billion tons, located in Junggar Basin in the Xinjiang Autonomous Region of China. Zhundong coal was reported to show the typical combustion characteristics of a low-rank coal [7; 8], i.e., it consisted of sequential and overlapping stages of volatile release and char combustion [9; 10]. To better utilize it in a power boiler, Chinese researchers have paid attention to the obvious ash deposition problems, such as slagging, fouling and corrosion, which are mainly causedby the high sodium and calcium content of Zhundong low-rank coal [11; 12; 13; 14; 15; 16]. However, the emission of PM during Zhundong coal combustion, also as an important ash-related problem, has rarely been reported until now. The distinctive coal was reported to emit much more PM\({}_{10}\) (particulate matter with an aerodynamic diameter of \(\leq\)10 \(\upmu\)m) than other lignite and bituminous coals during combustion [12], which could cause both the operating problem of initial deposition and the environmental problem of haze [17]. Oxy-fuel (O\({}_{2}\)/CO\({}_{2}\)) combustion was proposed as a retrofit technology to allow for the capture and storage of CO\({}_{2}\) in new or existing air-fired coal boilers [18; 19; 20]. Recently, it reached a milestone 35 MW load [21]. In the near future, Zhundong low-rank coal is very likely to be utilized extensively in oxy-fired boilers, therefore the PM emission behaviors of Zhundong coal during oxy-fuel combustion need to be carefully studied before its large-scale use. As CO\({}_{2}\) replaces N\({}_{2}\) to dilute O\({}_{2}\) in oxy-fuel technology, the furnace temperature and heat transfer differ from those in air combustion because of the higher thermal capacity of CO\({}_{2}\) and the lower diffusivity of O\({}_{2}\) in CO\({}_{2}\)[22]. A higher initial O\({}_{2}\) fraction should be necessary to achieve a similar furnace temperature to air combustion. [PERSON] et al. reported that an O\({}_{2}\) fraction of 27% was calculated to match the radiant heat flux of the flue gas in air-fired combustion, and a fraction of 32% matched the adiabatic flame temperature [23; 24]. Therefore, the principal purpose of this study is to examine the possible difference in PM emission behaviors of Zhundong coal during oxy-fuel combustion in the three characterized O\({}_{2}\) input fractions comparable to air combustion, i.e., 21%, 27% and 32 vol.% of O\({}_{2}\) in a O\({}_{2}\)/CO\({}_{2}\) mixture. The effect of O\({}_{2}\) fractions on PM emissions during oxy-fuel coal combustion has been reported by several researchers, and some notable information can be found in the summary presented in Table 1. Firstly, studies on low-rank coals chose a furnace temperature of \(<\)1200 \({}^{\circ}\)C, which is much lower than that of the existing utility boiler (\(\sim\)1400-1600 \({}^{\circ}\)C). During coal combustion, a lower burning temperature would inhibit the vaporization of inorganic matters and the fragmentation of char particles [31], and even bring in the soot into ultrafine particles as a result of incomplete combustion, all of which would obviously affect the emission of inorganic PM. Moreover, most studies paid attention to O\({}_{2}\) fractions much higher than 30%; the above-mentioned three characterized O\({}_{2}\) fractions comparable