## 3 Results  
### Effect of CO Concentration  
Figure 2 presents the influence of CO on the evolution of outlet NO concentration and the amounts of CO consumed and CO\({}_{2}\) formed with time, for 400 ppm NO at 900 \({}^{\circ}\)C. As shown in Figure 2, a decrease in the outlet NO concentration and an increase in the amount of CO\({}_{2}\) formed and CO consumed could be found when increasing the CO concentration. The additional CO enhanced NO conversion, mainly due to the reaction between CO and the surface complexes C(NO), C(N-NO), and C(O) [37]:  
\[\text{C(NO)}+\text{CO}\rightarrow\text{C(N)}+\text{CO}_{2} \tag{5}\]  
\[\text{C(N-NO)}+\text{CO}\rightarrow\text{C(N-N)}+\text{CO}_{2} \tag{6}\]  
and the creation of more free carbon sites C* via reaction (7) [38,39].  
\[\text{C(O)}+\text{CO}\rightarrow\text{aC*}+\text{CO}_{2} \tag{7}\]  
It should be noted that no obvious variations were observed with a low concentration of CO (400 ppm), compared to the case without CO. This indicates that the catalytic reaction (3) would be ignored if CO was only present as the product of the gasification reaction (1) under the present  
Figure 2: Influence of CO on the evolution of (**a**) outlet NO concentration and the amounts of (**b**) CO\({}_{2}\) formed and (**c**) CO consumed with time.  
experimental condition. Still, the NO-CO reaction over char was insignificant when the concentration of additional CO was lower than that of NO. A consistent result was also found for other chars [24; 40].  
To examine the effect of CO concentration on carbon consumption, combustion tests of char residues after reduction reaction were performed by thermogravimetric (TG) analysis with a heating ramp of 10 \({}^{\circ}\)C min\({}^{-1}\). The equipment used was the STA Instruments model 490C. In this way, carbon mass (%) could be analyzed qualitatively against the effects of ash and quartz sand. The same amounts of residues after the experiments of NO reduction with different CO concentrations were weighed for TG analysis. A maximum temperature of 1200 \({}^{\circ}\)C was set to ensure the complete combustion of char. A blank test, using fresh mixtures of char and sand that were well-mixed, was carried out as well. Figure 3 shows the TG curves for the char residues, as well as the differential scanning calorimetry (DSC) curves that could be used for analyzing the exothermic heat of char samples. It could be seen that both the mass loss of char and exothermic heat were increased with the enhancement of the CO concentration, indicating the higher carbon content remaining in char residues. The reaction between CO and the oxygen in the C(NO), C(N-NO), and C(O) complexes was accelerated by increasing the CO concentration, leading to a diminished effect of gasification reactions (C(O) \(\rightarrow\) CO + bC*, C(O\({}_{2}\)) \(\rightarrow\) CO\({}_{2}\) + dC*). The gasification reactions were also carbon-consuming reactions considering that new active sites were produced by gasifying carbon atoms. Thus, the presence of CO could make the carbon consumption slower [41]. In addition, the detailed values of mass loss of chars are given in Table 3. It can be seen that the mass loss of char with the maximum CO concentration (4000 ppm) was about the same as that of fresh one. This reveals that the carbon consumption was nearly zero under the present condition (CO = 4000 ppm, NO = 400 ppm), and char just provided a catalytic surface for NO-CO reaction. This was consistent with the findings obtained by [PERSON] et al. [42]. These authors noted that the overall reaction became the catalytic reduction of NO by CO over a black liquor char at CO/NO = 6.1.