# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 1 Introduction
The composition of pyrogas in different studies are shown in Table 1. The main components of biomass include cellulose, hemicellulose, and lignin, which are decomposed at different temperatures; 200 \({}^{\circ}\)C to 327 \({}^{\circ}\)C, 327 \({}^{\circ}\)C to 450 \({}^{\circ}\)C, and 200 \({}^{\circ}\)C to 550 \({}^{\circ}\)C are the mass loss temperature ranges of hemicellulose, cellulose, and lignin, respectively [32]. Also, depolymerizing lignin components is more challenging due to its composition, which is mostly constituted by aromatic polymers [33]. The main products of decomposing lignin are hydrogen and methane [34]. It must be mentioned that the share among the products is significantly affected by the operating conditions for the pyrolysis process, such as temperature, residence time, and pressure. The heating rate (\({}^{\circ}\)C/s) is used to classify the pyrolysis process as slow, intermediate, and fast [23; 35]. Also, there are other conditions that are typically common in different types of pyrolysis process, such as residence time and hot vapor residence time [36]. In recent decades, pyrolysis has received considerable attention because of its advantages, such as energy efficiency and the possibility for direct biomass conversion into a wide range of products. Additionally, the lower operating temperature and pressure than those in gasification and HTL are other advantages often referred to in relation to pyrolysis [37]. A comparison of different hydrogen production processes is shown in Table 2.  
Different research studies related to hydrogen production via pyrolysis can be categorized based on the investigated production routes. While some of these studies have only one pyrolysis stage, other methods, including catalytic reforming, contain additional steps to increase hydrogen yield [28]. Reforming pyrolysis volatiles is one of the most interesting routes for hydrogen production which has attracted a lot of interest in recent years. Although there are many studies related to hydrogen production from gasification processes, the advantages of reforming pyrolysis volatiles compared to biomass gasification, such as the complete conversion of volatiles, avoiding catalyst deactivation by sintering, and more suitable conditions for optimizing processes in two separate reactors, highlight the importance of assessing this thermochemical method [28]. However, the existing body of knowledge lacks comprehensive and extensive reviews to assess its impacts on hydrogen yield compared to those of pyrolysis processes. Also, with a detailed observation of recent research studies, it can be inferred that there is a gap in the identification of effective factors and their impacts on hydrogen yield in the catalytic reforming of pyrolysis volatiles and its comparison with pyrolysis processes. The aim of this study is to fill existing gaps by covering all new studies about hydrogen production by the two-stage catalytic reforming of pyrolysis volatiles. This paper compares this technological option with hydrogen yield in standard pyrolysis processes to explore the possible advantages of this two-stage process. Also, important factors affecting hydrogen production are evaluated in this study to understand their role in this process and optimize it to maximize hydrogen yield. It is worth mentioning that the focus of this review is on the slow pyrolysis process for reforming pyrolysis volatiles, although a few studies relevant to other types of pyrolysis are also evaluated. The assessment of this process can play a key role in increasing hydrogen production and pave the way for large-scale economical hydrogen production from biomass feedstock.