## 3 Results and Discussion  
### Effect of Aluminum Dross on Free Radical Generation during Catalytic Pyrolysis  
#### 3.1.1 Effect of Aluminum Dross on DTG Behavior  
The free radical reaction mechanism of biomass pyrolysis comprised radical generation by covalent bond cleavage and product formation by radical fragments coupling [18; 33]. The biomass, composed of lignin, cellulose, and hemicellulose, had a number of covalent bonds, including C\({}_{\text{al}}\)-C\({}_{\text{al}}\), C\({}_{\text{ar}}\)-C\({}_{\text{ar}}\), C\({}_{\text{ar}}\)-C\({}_{\text{al}}\), C\({}_{\text{ar}}\)-O, C\({}_{\text{al}}\)=C\({}_{\text{al}}\), C\({}_{\text{al}}\)-O, and C\({}_{\text{al}}\)=O bonds [34]. The cleavage of these bonds was correlated with the DTG behavior of the biomass. The TG and DTG curves of pine and the mixture are shown in Figure 2a,b.  
Figure 2 shows the cleavage of the covalent bond in the temperature range of 300-450 \({}^{\circ}\)C, which was mainly caused by the decomposition of carbonyl groups, the breakage of C-O-C, C\({}_{\text{al}}\)-C\({}_{\text{al}}\), and side chains of aromatics during primary pyrolysis. The C\({}_{\text{ar}}\)-C\({}_{\text{ar}}\), C\({}_{\text{al}}\)=C\({}_{\text{al}}\), and C\({}_{\text{al}}\)=O bonds could be not broken at a pyrolysis temperature between 300-450 \({}^{\circ}\)C. The DTG curves in Figure 2 show the overlapped signals for biomass pyrolysis. The signals were fitted with four sub-curves representing four major covalent bond cleavages [35]. Peak 1 is correlated to the decomposition of carbonyl groups, hydrogen bonds, weak C-O bonds, hemicellulose pyrolysis, and the disentanglement of cellulose, hemicellulose, and lignin in the biomass at about 320 \({}^{\circ}\)C [36]. The peak width is larger than the other peak. Peak 2 is dominated by cleavage of C-O bonds and weak C\({}_{\text{al}}\)-C\({}_{\text{al}}\) bonds with a bond energy range of 295-325 kJ/mol [34] at about 350 \({}^{\circ}\)C. This pyrolysis stage was the decomposition of glucosidic linkage and amorphous cellulose. Peak 3 is correlated to the breakage of C\({}_{\text{al}}\)-C\({}_{\text{al}}\) bonds at about 370 \({}^{\circ}\)C, associated with cellulose pyrolysis and weak aromatic side chain breakage. Peak 4 can be assigned to strong aromatic side chains and condensation of aromatic rings. The weight loss was the lowest at this terminal pyrolysis stage, as shown in Figure 2. The three parameters of the curve fitting operation, i.e., peak temperature (T\({}_{\text{p}}\)), peak width (W\({}_{\text{p}}\)) and peak area (A\({}_{\text{p}}\)), calculated from Figure 2a,b are listed in Table 2.  
The bond breakage behavior can be closely related to the peak temperature and peak area. The peak temperatures were similar with and without the aluminum dress, indicating little changes in the biomass pyrolysis pathway. Peak area was a quantitative description of the amount of bond cleaved at the respective temperatures. The peak area percentage of peak 1 decreased from 44.03% to 41.10% with AD addition, indicating that  
Figure 2: TG and DTG fitting curves between 100–500 \({}^{\circ}\)C of pine and mixture: (**a**) pine and (**b**) mixture with an aluminum dress to pine mass ratio of 1/4).  
the biomass pyrolysis rate was changed. The hemicellulose pyrolysis and macromolecular disentanglement were delayed. On the contrary, the peak area percentage of peak 2 increased from 23.69% to 25.95%, demonstrating that the cleavage of C-O bonds and weak C-C bonds as accelerated with AD addition. Pyrolysis of cellulose, especially amorphous cellulose, was activated by the catalytic effect of AD. Peak 3, associated with the breakage of C-C bonds, increased. Though the area percentages of peak 4 were similar, the residual content at the end of reaction was higher with AD addition, since the coking reaction during lignin pyrolysis was promoted.