## 1 Introduction  
Nitrogen oxides (NOx), one of the major air pollutants, contribute to the formation of photochemical smog and the depletion of ozone [1; 2; 3]. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) by ammonia are the two typical methods for NOx removal [4; 5]. However, ammonia is also classified as a major air pollutant. Ammonia slip from the denitration process can cause secondary pollution [6; 7].  
In recent years, it is found that biomass char, a major renewable energy source [8; 9; 10], can be effective in removing NO without ammonia [11; 12]. The most likely mechanism of NO reduction using char can be described by the direct gasification reaction between NO and char [13; 14; 15; 16; 17] in:  
\[\mathrm{C+NO\to CO+1/2N_{2}} \tag{1}\]  
\[\mathrm{C+2 NO\to CO_{2}+N_{2}} \tag{2}\]  
and a catalytic reaction between NO and CO over the char surface [18; 19; 20]:  
\[\mathrm{CO+NO\to CO_{2}+1/2N_{2}} \tag{3}\]The heterogeneous NO reduction of char can be influenced by many factors, such as CO concentration, temperature, and initial NO concentration. These parameters have a strong effect on the carbon-consuming reduction and catalytic reduction mechanisms and on determining which mechanism dominates. [PERSON] et al. [21] thought that the presence of CO enhanced the NO conversion over char. However, [PERSON] et al. [22; 23] noted that the influence of CO was bidirectional, and excessive CO did not have any further appreciable effect on the NO reduction. [PERSON] et al. [24] investigated the coal char-NO interaction and found that, at low temperatures (\(<\)800 \({}^{\circ}\)C), the overall reaction scheme could be dominated by catalytic reaction (3). At higher temperatures, the direct gasification reactions (1) and (2) became obvious. [PERSON] et al. [25] reported that at high temperature, the catalytic reaction over coal char was the dominant mechanism when the concentration of NO was reasonably lower than that of CO (i.e., CO/NO ratio \(>>\) 1). There are still inconsistencies among these research works. Furthermore, fewer studies systematically report the influence of these reaction parameters on heterogeneous NO reduction using biomass char with and without CO, which can vary in a wide concentration range.  
Kinetics studies of the reaction between char and NO without CO present in the gas have been extensively carried out. The reaction of NO with char can be of first order [26; 27; 28] or fractional order [23; 29; 30] with respect to NO. The apparent activation energy of 23 kJ-mol\({}^{-1}\) was reported by [PERSON] et al. [31] and a higher value of 120 kJ-mol\({}^{-1}\) was reported by [PERSON] et al. [32], which mainly depends on the fuel type and the test conditions [33]. Compared with the sufficient kinetics studies of NO reduction by char, research on the kinetics of the char-NO-CO reaction is limited, especially for NO reduction with biomass char. Among the scarce kinetics studies, [PERSON] and [PERSON] [34] pointed out that when wood char was used for NO reduction, the activation energy for char-NO-CO reaction was in a slightly higher range compared with the char-NO reaction.  
In this work, rice husk biomass char--a byproduct in the pyrolysis of rice husk, which is the most common agricultural waste in China [35]--is used for investigation, considering that it is cost-effective and easily available [36]. Both experimental and kinetics studies of NO reduction with rice husk char were conducted, in the absence and presence of CO. The effects of CO concentration, reaction temperature, initial NO concentration, and char loading on the NO reduction using rice husk char were performed in a fixed-bed quartz reactor. In addition, the kinetic parameters, reaction order, and activation energy were also determined. Overall, through this paper, a reference that is expected to promote the industrial application of NO reduction using biomass char can be provided.