## 1 Introduction  
The emission of NOx (including NO, N\({}_{2}\)O, NO\({}_{2}\), etc.) harms seriously humans health and the environment by causing acid rain and photochemical reactions. With the expansion of construction in China, the NOx emissions of the cement industry have also increased year by year. In 2017, the amount of NOx emissions by the cement industry was 2.4 million tons [1,2]. The denitration technologies currently used in the cement industry are Selective Non-catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). The SNCR technology does not require a catalyst and does not induce the conversion of SO\({}_{2}\) to SO\({}_{3}\), which is suitable for the temperature reaction window of the cement kiln (800-1100 \({}^{\circ}\)C). However, the SNCR denitration efficiency can only reach about 50%, and SNCR causes the release of ammonia, thus promoting pollution [3], in contrast with the current emission regulations. The SCR technology requires precious metals or heavy metals as catalysts, such as titanium, vanadium, manganese, and other metals oxides, and uses ammonia or urea as a reducing agent in a low temperature range (350-450 \({}^{\circ}\)C). The SCR technology reduces nitrogen oxides to N\({}_{2}\) and H\({}_{2}\)O under the action of a catalyst [4]. The shortcomings of SCR are that the catalyst is extremely susceptible to poisoning, secondary pollution is caused by ammonia escape, and it requires high investments and operating costs [4]. Therefore, in order to meet the requirements of national environmental protection in the future, it is necessary to develop new technologies for nitrogen oxide reduction. In recent years, biochar, a largely available and easily accessible renewable resource, has been used as a new energy source in various fields such as agriculture and industry [5]. Biochar-based materials are porous solid particulate materials produced by pyrolysis of a carbon-rich biomass material under anaerobic or anoxic conditions [6]. The main reactions leading to the reduction of nitrogen oxides by biochar are [7]  
\[2\text{C}+2\text{NO}\to 2\text{CO}+\text{N}_{2} \tag{1}\]  
\[\text{C}+2\text{NO}\to\text{CO}_{2}+\text{N}_{2} \tag{2}\]  
\[2\text{CO}+2\text{NO}\to 2\text{CO}_{2}+\text{N}_{2} \tag{3}\]  
Recently, many scholars have studied the denitration process occurring when using biochar as a reducing agent to reduce nitrogen oxides. [PERSON] et al. [8] found that eucalyptus charcoal had an obvious reduction effect on NO. Due to its larger specific surface area and oxygen-containing functional groups on its surface, it is more active than charcoal and presents more active sites at high temperatures. [PERSON] and [PERSON] [9] studied the C-NO reduction reaction with straw charcoal and found that carbon prepared at 1073 K exhibited the best pore structure, the largest specific surface area, and the best oxidation activity. The NO removal efficiency was more than 80%. Some researchers [10; 11; 12; 13] found that the surface of biochar has highly catalytic and reactive sites and is rich in oxygen-containing functional groups such as carboxyl groups, hydroxyl groups, phenolic hydroxyl groups, which probably promote denitration. A number of studies on the surface functional groups of carbonaceous materials have been published. It has been reported that the nitrogen oxide (NOx) adsorption capability of carbon materials is influenced greatly by the content of nitrogen and carbon-containing functional groups (CFGs) [10; 11; 14]. [PERSON] et al. [15], [PERSON] et al. [16], and [PERSON] et al. [17] proved that the concentration of such oxygen-containing functional groups (OFGs) corresponded to the presence of acidic oxygen-containing groups, and the nitrogen-containing functional groups (NFGs) increased significantly after high-temperature treatment. [PERSON] et al. [18] also analyzed the surface chemical composition of heat-treated bamboo slivers, showing that the O/C ratio decreased as temperature increased, while the amount of both C-C and C-O bonds increased.