# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 1 Introduction  
Energy demand has increased considerably all over the world during recent decades and it originates from different reasons such as the increasing population, changing lifestyles due to technological advancement, economic development, and rising life standards [1]. Hence, providing required energy while securing supply is currently an important challenge, for many regions in the world. Fossil-based fuels are still the primary energy resource; recently published data show that the share of fossil fuels in primary energy consumption worldwide was about 81.5% in 2023 [2]. At the same time, environmental concerns related to the use of fossil-based resources for energy production are growing. The use of fossil-based fuels is associated with a considerable amount of greenhouse gas (GHG) emissions [3]. Considering the current use of fossil fuels globally, the importance of pushing for sustainable alternatives has been stressed over recent decades. As a result of policies aiming at curbing the environmental impact of energy production, the share of renewable and sustainable fuels increased consistently during these years: the share of renewable energy in primary energy consumption from 2012 to 2023 increased from 9.42% to 14.56%, worldwide [4]. Among the potential substitutes for fossil fuels, hydrogen has been identified for its potential to be a carbon-free energy carrier. As an innovative energy carrier, hydrogen can perform a vital role in environmental decarbonization, when sustainably produced [5]. A key function of the hydrogen economy is the possibility to support the implementation of the growing contribution of renewable resources such as wind and solar energy, by addressing the critical aspect of their intermittent nature [6]. Furthermore, hydrogen can play a key role in the decarbonization of hard-to-abate industrial sectors, like steel and cement making [7]. These convincing advantages stimulate experts to research the various aspects of the hydrogen value chain, including production, transport, storage, etc. Hydrogen can be produced by using different kinds of resources. It must be mentioned that hydrogen production from fossil fuels accounted for 96% of total production in 2021, and renewable sources had only a 4% share of global production [8]. Reforming oil and gas is the main technology for hydrogen production, covering 78% of the total [9]. However, large-scale clean hydrogen production methods have gained interest during these past years. The increasing trend of renewable energy production, such as wind, solar, and biofuel energy, provides a great opportunity to develop clean and environmentally friendly routes to produce hydrogen [10]. The water electrolysis process can be implemented to produce hydrogen by decomposing water, and the electricity produced by renewable energies such as wind and solar can be used for this purpose. A polymer electrolyte membrane (PEM) electrolyzer is one of the technologies that can be used for water electrolysis. Producing high-purity hydrogen (99.99%), a compact design, and enabling fast responses during load variation are the advantages of this technology [11; 12]. Alkaline electrolyzers are another technology for water electrolysis and they can also reach high purity levels. This technology has the capability to produce hydrogen in a few hundred MW, although gas permeation caused by using a diaphragm is one of the significant drawbacks of it. It must be noted that the operation temperature of PEM electrolyzers and alkaline electrolyzers is relatively low [12; 13]. A solid oxide electrolyzer (SOE) is another electrolysis technology with an operating temperature higher than 500 \({}^{\circ}\)C, which results in an efficiency increase. Also, an oxygen ion can be used for steam decomposition, which operates up to 1000 \({}^{\circ}\)C. However, this temperature leads to some problems, such as degradation and challenges in selecting materials for electrodes and catalysts [14; 15].