# Advances in Bio-Hydrogen Production: A Critical Review of Pyrolysis Gas Reforming
## 2 Hydrogen Production by Using Pyrolysis Process  
Different types of pyrolysis processes, including slow, intermediate, fast, and flash ones, have been used to produce hydrogen. Many studies in the literature report the composition and yield of pyrolysis gas and hydrogen content, considering different conditions in the pyrolysis process, such as the temperature, residence time, and heating rate. Experimental results of cotton stalk slow pyrolysis at 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C showed the product compositions and distribution. The heating rate and residence time of this process were set to 10 \({}^{\circ}\)C/min and 1 h. While the maximum char yield was 58.54 wt% at 300 \({}^{\circ}\)C, maximum gas and bio-oil production was achieved at 500 \({}^{\circ}\)C, which yielded 36.6 wt% and 25.25 wt%, respectively. Analysis of the pyrolysis gas showed that it mainly contained carbon dioxide and hydrogen, in the range of 0.26, 0.35, and 0.54 wt% at 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C, respectively [46]. A slow pyrolysis experiment of waste wood with a 30 \({}^{\circ}\)C/min heating rate was performed at different temperatures, and the main components of the pyrolysis gas composition were measured. Hydrogen production results showed that it slightly increased between 500 \({}^{\circ}\)C and 700 \({}^{\circ}\)C, but it was enhanced considerably in the 700-800 \({}^{\circ}\)C temperature range. The hydrogen concentration at 800 \({}^{\circ}\)C increased to 42.7 vol% [47]. Rice straw slow pyrolysis was explored to evaluate the effect of temperature on product yield distribution and product compositions. The results of pyrolysis gas composition showed a low increasing trend in hydrogen from 300 \({}^{\circ}\)C to 500 \({}^{\circ}\)C, which was enhanced from 1.9% to 9.08%, and after that, it increased up to 17.72% by increasing the pyrolysis temperature from 500 \({}^{\circ}\)C to 600 \({}^{\circ}\)C [48]. The pyrolysis of wood sawdust at 500 \({}^{\circ}\)C was performed to explore the product's distribution and composition, and the results of the pyrolysis gas composition (mol) showed 6.7% hydrogen [49]. Hydrogen production was investigated using the pyrolysis process with rice straw and sawdust at 500 \({}^{\circ}\)C, 750 \({}^{\circ}\)C, and 850 \({}^{\circ}\)C. The obtained results demonstrated that increasing the pyrolysis temperature enhanced gas product distribution by up to 40.6%. Also, the results showed that the hydrogen yield by pyrolyzing the rice straw and sawdust at 500 \({}^{\circ}\)C was 41.2 wt% and 40.6 wt% of pyrolysis gas, respectively. Additionally, the highest hydrogen yield for the pyrolysis of sawdust and rice straw was 47 and 48.2 mmol/g, respectively [50]. Rice husk slow pyrolysis results considering the temperature range between 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C demonstrated that hydrogen yield increased up to 15 vol% at 600 \({}^{\circ}\)C, and the minimum hydrogen yield was less than 5 vol% at 400 \({}^{\circ}\)C. Also, these results show the beneficial effect of increasing temperature on hydrogen yield in the slow pyrolysis process [51]. Sugar can bagasse slow pyrolysis showed that hydrogen volume in produced volatiles was increased considerably from 9.6 vol% to 28.8 vol% by increasing temperature from 480 \({}^{\circ}\)C to 680 \({}^{\circ}\)C. Also, pyrolysis gas yield was enhanced from 25.1 wt% to 35.67 wt% in this temperature range. Additionally, fast pyrolysis results for sugar can bagasse were reported in this study. While these obtained results showed that hydrogen volume in the produced syngas increased dramatically from 8.7 vol% to 45.3 vol% in the mentioned temperature range, a slight increase in pyrolysis gas yield was observed [52]. An experiment was performed by [PERSON] et al. to investigate the effects of woody biomass fast pyrolysis on product distribution and mass balance.