Hydrogen production from plastic waste pyrolysis syngas: A review on progresses and challenges

Globally, 380 million tons of plastic are produced annually, and more than 75 % of these plastics are rejected as waste in the environment. The increasing accumulation of plastic waste poses significant environmental, and health risks and urges the need for sustainable waste management strategies. One promising solution is the conversion of plastic waste into valuable energy products (oil, char, and syngas) through thermochemical processes, such as pyrolysis/gasification. This review focuses on hydrogen production from syngas derived from the pyrolysis of plastic waste. PET and HDPE emerge as highly effective feedstocks, producing gaseous yields of up to 77 % and 66 %, respectively, during pyrolysis. Researchers favour pyrolysis because of its ability to yield syngas from 35 % to 40 % from plastic waste, depending on the type of pyrolysis process used. This review evaluates the impact of key pyrolysis process parameters (temperature, pressure, residence time, heating rate, catalyst, and feedstock composition) on hydrogen yield and its quality. Pyrolysis, particularly using different types of catalysts, increases hydrogen output to as high as 219 mmol/g to 730.6 mmol/g. Advances on palladium-based membranes have been discussed for their role in separating hydrogen from syngas efficiently, achieving 99.99 % purity. Among palladium-based membranes, performance of PdAg is reviewed in detail as it shows good effectiveness against impurities, carbon dioxides (C $O_2)$, carbon monoxide (CO), methane (C $H_4)$, etc. and efficiency against poisoning (sulphur and Chlorine) that reduce the membrane thickness, durability, quality, stability and selectivity. The highest permeability of palladium silver (PdAg) membrane was found 3.2 × ${10}^{-8}$ mol. $m^{-1}.s^{-1}.{\text{pa}}^{-0.5}$ at 450 °C and the lowest thickness was found 0.8 μm. Plastic waste pyrolysis produces significantly lower C $O_2$ (1–3 kg C$O_2$/kg $H_2$) emissions compared to traditional methods such as landfilling, incineration and recycling. The review concludes with a roadmap for future research directions aimed at optimising hydrogen separation from plastic waste syngas to develop cost-effective, sustainable solutions for plastic waste management and clean energy production, thereby contributing to a cleaner environment and sustainable energy future.