## 3 Results
### Determination of Reaction Order and Activation Energy  
The reaction rate for the heterogeneous NO reaction in the fixed bed can be expressed as follows [46; 47; 48; 49]:  
\[\gamma_{\text{NO}}=\ -\frac{\text{d}\text{C}_{\text{NO}}}{\text{dt}}=\ \text{k} \text{C}_{\text{NO}}^{\text{n}} \tag{8}\]  
where C\({}_{\text{NO}}\) is the NO concentration, t is the residence time of gas in the sample layer, k is the reaction rate constant, and n is the apparent reaction order. Figure 5: Effect of the initial NO concentration on NO reduction with rice husk char. Figure 6: Effect of char loading on NO reduction with rice husk char. Based on the analysis of the above experiments, the NO reduction reaction was the direct reaction between char and NO without additional CO. However, the overall reaction became the catalytic reaction over the char with a high concentration of CO (CO/NO = 10) present. According to this, \(\gamma\) in the char-NO system represented the reaction rate for NO reduction by char, whereas \(\gamma\) in the char-NO-CO system indicated the reaction rate for NO reduction with CO, catalyzed by char. During the NO-CO reduction experiment, an excess concentration of CO (60,000 ppm) was set to assume a constant CO concentration. As shown in Figure 7a,b, the NO reduction ratio decreased with increasing inlet NO concentration, whether in the char-NO system or in the char-NO-CO system. This indicates that the reaction order of NO should be fractional (\(\alpha<1\)) for NO reduction by char in the temperature range studied. Similarly, NO reduction by CO over char was a pseudo-fractional-order reaction with respect to NO. Thus, Equation (8) could be integrated as follows:  
\[\mathrm{C_{NO}}^{\mathrm{(1-n)}}-\mathrm{C_{NO,in}}^{\mathrm{(1-n)}}=\ -\ (1-n)\mathrm{kt}\ =\ -\ (1-n)\ \mathrm{kk}^{\star}/\mathrm{T} \tag{9}\]  
where T is the reaction temperature and k* can be estimated with Equation (10):  
\[\mathrm{k}^{\star}=\mathrm{HAT}_{0}/(\varepsilon\
u) \tag{10}\]  
where H is the bed height, A is the cross-sectional area of the reactor, \(\varepsilon\) is the bed porosity, \(\
u\) is the total gas flow rate, and T\({}_{0}\) is the ambient temperature. Taking the expression \(\mathrm{C_{NO}}=(1-\mathrm{X_{NO}})\mathrm{C_{NO,in}}\) into account allows Equation (9) to be further expressed as Equation (11):  
\[(1-\mathrm{X_{NO}})^{\mathrm{(1-n)}}=\ 1-\mathrm{C_{NO,in}^{\mathrm{(n-1)}}}(1-n) \mathrm{kk}^{\star}/\mathrm{T} \tag{11}\]  
A trial-and-error method [33] was used to determine the reaction order n. The calculated k is constant for each reaction temperature and variational inlet NO concentration. Accordingly, the reaction order n could be determined by varying the value of n (0-1). The reaction orders for all the test temperatures (750-900 \({}^{\circ}\)C) were determined, and an average order was allowed for the following calculation of apparent activation energies. Figure 7: NO reduction rate of rice husk char at different inlet NO concentrations, for the (**a**) char–NO system and (**b**) char–NO–CO system. In order to determine the activation energies, Equation (8) could be expressed as  
\[\gamma_{\text{NO}}=\ -\ \frac{\text{d}\text{C}_{\text{NO}}}{\text{dt}}=\ \text{k} \text{C}_{\text{NO}}^{\text{n}}=\ \text{k}_{0}\ \text{exp}(-\ \text{E}\alpha/\text{RT})\ \text{C}_{\text{NO}}^{\text{n}} \tag{12}\]  
where \(\text{k}_{0}\) is the pre-exponential factor, \(\text{E}_{\alpha}\) is the apparent activation energy, and R is the gas constant.