Pyrolysis of hydrothermal liquefaction algal biochar for hydrogen production in a membrane reactor

Algal biomass has recently emerged as a sustainable feedstock that can be converted to liquid fuels and other energy products. Hydrothermal liquefaction (HTL) is considered one of the most efficient thermochemical conversion techniques to produce high-quality biocrude oil that can be upgraded into a variety of liquid fuels. However, failure to identify practical uses for the HTL residual solids (biochar) could make the process economically less attractive especially with low lipid algae. This work investigates the conversion of HTL biochar of microalgae Galdieria sulphuraria to hydrogen under pyrolysis conditions in a membrane reactor capable of selectively separating hydrogen from the reaction zone. HTL biochar pyrolysis was first investigated using thermogravimetric analysis experiments and fixed bed reactor configuration. Batch membrane reactor pyrolysis experiments were then carried out using Pd₇₇Ag₂₃ hydrogen-selective membrane. The involvement of a Pd₇₇Ag₂₃ membrane in the reactor during the pyrolysis of biochar results in the recovery of hydrogen in the permeate stream (~2 times the hydrogen remaining in the retentate) and further facilitates the conversion of biochar to gaseous fuels. The retentate stream shows reduced CO and CO₂ as well as increased CH₄ content compared to pyrolysis conditions with no membrane.