# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 3 Hydrogen Production by Catalytic Reforming of Pyrolysis Volatiles  
Using two-stage pyrolysis and catalytic reforming to produce hydrogen has been gaining momentum because of its advantages compared to other biomass-to-hydrogen routes. Under this approach, the pyrolysis of biomass takes place in the first step, where the feedstock is converted into pyrolysis volatiles, including condensable and non-condensable fractions, and biochar. In the following step, pyrolysis volatiles may be reformed in the catalytic reformer. Figure 1 shows the concept of a two-stage process to reform pyrolysis volatiles. This two-stage configuration has the advantage of a considerable increase in gas production and a decrease in bio-oil production. The advantage of the two-stage approach is related to the opportunity to convert pyrolysis volatiles in the second reactor, including the non-condensed bio-oil fraction. For instance, in comparing gas product distribution between pyrolysis and the reforming of pyrolysis volatiles for palm kernel shell, performed by [PERSON] et al. [33], a 66.1% increase in gas yield could be observed. The results of another research study showed that gas distribution increased by 7.64% without any catalyst. Also, Ni-based and Co-based catalysts with 10% loading enhanced gas production (wt%) by 25% and 43.76% compared to the pyrolysis process, respectively [55]. Additionally, based on the literature, an increase in hydrogen yield is another important result of reforming pyrolysis volatiles. In the case of palm kernel shell, hydrogen yield was enhanced from 6.62 mmol/g to 25.35 mmol/g by reforming pyrolysis volatiles [33]. Figures 2 and 3 show the effects of reforming pyrolysis volatiles on hydrogen yield and gas product distribution compared to the single pyrolysis process. Figure 1: Catalytic reforming of biomass pyrolysis volatiles. Figure 2: Comparing gas product yield between processes of pyrolysis and reforming of pyrolysis volatiles [33]. The separation of pyrolysis and reforming steps provides an opportunity to control and optimize these processes more efficiently [56]. This configuration has the additional advantage of avoiding direct contact between the feedstock impurities and the reforming catalyst [57]. Additionally, preventing catalyst sintering and achieving the full conversion of pyrolysis volatiles using highly active reforming catalysts, such as the commercial Ni catalyst, are other significant advantages of this configuration [28; 58; 59]. Moreover, considering the process description and the possibility of using a highly active reforming catalyst to achieve complete conversion, there is no need to condense pyrolysis products and design complicated steps to cope with them. Additionally, the simplicity of scaling up this two-stage process with continuous feed is its most significant advantage [28]. Producing syngas by consuming greenhouse gasses (CO\({}_{2}\) and methane) in the dry reforming process is another significant advantage of it [60]. A two-stage fixed-bed reactor was used for experimental studies of this process on a laboratory scale [33; 61; 62] and the schematic of the pyrolysis and catalytic reforming process is shown in Figure 4.  
Figure 4: Commonly used configuration of reactor to reform pyrolysis volatiles [33]. Figure 3: Comparing hydrogen yield between processes of pyrolysis and reforming of pyrolysis volatiles [33]. Different reactions take place in this process, such as pyrolysis, the catalytic reforming of hydrocarbons and tars, the catalytic cracking of tar, char gasification, etc. Table 3 shows the most significant reactions occurring in the two-stage process of reforming pyrolysis volatiles. Considering the existing literature, different factors affect this two-stage process, such as the heating rate, the pyrolysis temperature, and catalysts. Identifying and analyzing these factors and their effects play a vital role in better and more accurately understanding the process.