# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 1 Introduction
Although the technology readiness levels (TRLs) of alkaline and PEM electrolyzers are 9 and 8, respectively [14], there is still room for improving materials' life-time and energy efficiency [16]. Existing challenges of hydrogen production by water electrolysis, such as high production costs, the cost of some rare materials, and corrosive environments, have led to other production methods being considered [17]. Hydrogen production using biomass has attracted much interest in recent years [18; 19; 20], as biomass resources can play a crucial role in reducing GHG emissions. Energy crops, agricultural residues, industrial and municipal waste, and forestry waste are examples of different types of biomass that can be used as feedstock to produce energy [21]. Based on the IRENA's renewable energy map, the predicted share of biomass among renewable resources in total final energy consumption in 2050 will be 30% [22]. Converting biomass feedstocks to heat or power is one of many possible routes to produce energy; other processes can convert biomass to secondary energy sources and energy carriers, such as hydrogen [23]. Therochemical and biological processes are the main routes for hydrogen production using biomass as feedstock. The thermochemical pathway is much faster than biological processes and the hydrogen yield is typically higher [24]. Conversely, biological conversion processes are typically considered more robust and technologically mature [25]. Thermochemical processes include gasification, pyrolysis, and hydrothermal liquefaction (HTL). The gasification process is designed to convert biomass to a combustible gas mixture in the controlled presence of oxidation agents, such as oxygen, air, or steam, and the high temperature of this process increases hydrogen yield [24; 26; 27]. The main products of this process include CO, H\({}_{2}\), H\({}_{2}\)O, CO\({}_{2}\), CH\({}_{4}\), and other hydrocarbons [23], and the temperature range for gasification is between 600 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C [28]. Many research studies at different scales have investigated H\({}_{2}\) production using the gasification method. The steam gasification of pine sawdust at the laboratory scale was performed by [PERSON] et al. to assess hydrogen production [29]. The temperature range was considered between 600 \({}^{\circ}\)C and 900 \({}^{\circ}\)C to assess its effects on hydrogen production. Results demonstrated that increasing temperature raised hydrogen production, and the maximum hydrogen yield was 7.3 wt% at 900 \({}^{\circ}\)C. [PERSON] et al. [30] investigated hydrogen production via the steam gasification of waste wood as a feedstock on a bench scale with a 2.5 kg/h capacity. The reaction temperature and steam-to-biomass ratio were set to 900 \({}^{\circ}\)C and 1, respectively. The obtained results showed a 5.9 wt% hydrogen yield. Also, the results of pine wood pellets' steam gasification at 800 \({}^{\circ}\)C at a pilot plant showed 1.8 wt% hydrogen production [31]. However, there are some disadvantages in this process which conversely affect hydrogen production. The need for additional catalytic processes to increase hydrogen production by gasification is another disadvantage of this route. Also, one of these drawbacks is catalyst deactivation by sintering at high temperatures [28]. Pyrolysis is a thermochemical process that converts biomass into various types of products in the absence of oxygen; process outputs include liquid oils (bio-oil), solids (biochar), and non-condensable gasses. While the condensable part is also referred to as bio-oil, and contains complex organic compounds, the non-condensable part includes gasses, such as carbon monoxide, carbon dioxide, hydrogen, and methane.