# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 3 Hydrogen Production by Catalytic Reforming of Pyrolysis Volatiles
### Slow Pyrolysis
#### 3.1.5 Catalyst Type
The pyrolysis and reforming temperatures were set to 950 \({}^{\circ}\)C, and three different feedstocks, including rice husk, sugar cane, and wheat straw were selected for this experimental study. Hydrogen production, by reforming pyrolysis volatiles and using the Ni-dolomite catalyst, was raised up to 23.71, 21.18, and 21.59 mmol/g for the rice husk, sugar cane bagasse, and wheat straw, respectively [63]. Ni-exchange resin char was another catalyst used to investigate the two-stage pyrolysis and reforming process of corncob, considering 900 \({}^{\circ}\)C and 650 \({}^{\circ}\)C for the pyrolysis and reforming temperatures, respectively. Gas composition analysis showed a significant gas yield increase due to using the Ni-exchange resin char of 3.32 times compared to that of the non-catalytic reforming. The obtained results showed that the hydrogen yield was increased up to 25 mmol/g using Ni-exchange resin char [77]. A research study performed by [PERSON] et al. [68] evaluated the effects of nanosized NiZnAlO\({}_{\rm x}\) catalysts on hydrogen production in two-stage pyrolysis and steam reforming. While the ratio of gas yield to wood sawdust when using a sand bed was 33 wt%, adding the NiZnAlO\({}_{\rm x}\) catalyst with a 5% Ni molar ratio increased the gas yield to 49.3%. Also, the hydrogen yield, by using this catalyst, was increased from 2.4 mmol/g to 8.2 mmol/g. It is worth mentioning that hydrogen production, due to using NiZnAlO\({}_{\rm x}\) with a 35% Ni molar ratio, increased to 20.1 mmol/g, which was about 737.5% more than that from the process without using a catalyst [68]. Figure 7 shows the effects of various catalysts on hydrogen yield. The impacts of using other types of materials as catalysts have been investigated by research studies. Tire char, which can be produced by the pyrolysis of waste tires, is one of the materials used as a catalyst for the reforming pyrolysis of wood pellet volatiles. While the pyrolysis temperature was set to 500 \({}^{\circ}\)C, a reforming temperature range between 700 \({}^{\circ}\)C and 900 \({}^{\circ}\)C was considered. Also, the acid treatment of tire char by HCl was performed to examine the effects of ash on this two-stage process. Results for the hydrogen yield showed that it could be enhanced by up to 39.2 mmol/g by increasing the reforming temperature to 900 \({}^{\circ}\)C in the presence of the tire char catalyst. Although the hydrogen yield increased considerably due to using the acid-treated tire char, the results showed the negative effect of tire char treatment on the hydrogen yield. For instance, the hydrogen yield, due to using acid-treated tire char, was reduced by 5.7 mmol/g, 3.9 mmol/g, and 8.7 mmol/g at reforming temperatures of 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C, respectively [70]. Hydrogen yield results with various types of catalysts are shown in Figure 8.  
An experimental research study performed by [PERSON] et al. [66] assessed the feasibility of using biochar as a catalyst and poplar wood, catalpa wood, elm wood, and pine wood as feedstocks, and the pyrolysis and reforming temperatures were set to 700 \({}^{\circ}\)C and 650 \({}^{\circ}\)C, respectively. Product yield results showed that hydrogen production, due to using catalpa char, poplar char, pine char, and elm char, increased to 17.1 mmol/g, 14.4 mmol/g, 14.4 mmol/g, and 15.7 mmol/g, respectively. Another section of this ex  
Figure 8: Effect of different catalysts on hydrogen yield [66; 70; 71; 72; 73; 76; 77]. Figure 7: Effect of using different catalysts on hydrogen yield [64; 68]. perimental study examined the effects of using Ni/biochar as a catalyst with different nickel loadings. The results of using 5% Ni/poplar char, 10% Ni/poplar char, and 15% Ni/poplar char as catalysts for the two-stage pyrolysis and reforming process showed 21.4 mmol/g, 27.2 mmol/g, and 18.9 mmol/g hydrogen yields, respectively [66]. Natural limonite ore is another material used to catalyze the reforming process of corncob pyrolysis volatiles.