Pyrolysis of Macroalga Macrocystis Pyrifera for Production of Green Carbon-Negative Hydrogen

Abstract

Abstract Stringent emission norms worldwide have provided an impetus to explore alternative sustainable fuels that are carbon neutral. Hydrogen is touted as one of the potential fuels that aid decarbonization. Biomass, especially the ones that do not compete with the food needs are considered promising feedstock for hydrogen production by thermal conversion. In the current study, the performance of the macroalga Macrocystis pyrifera in the thermal conversion through pyrolysis as a potential biomass for hydrogen production was examined. The macroalga Macrocystis pyrifera is a giant brown seaweed commonly found in the Pacific Rim. It is characterized by its fast-growing ability and photosynthetic metabolism that generates carbon sources from atmospheric CO2. This alga is a potential biomass to be applied in bioenergy with carbon capture and storage (BECCS), which enables carbon-negative biofuels to avoid greenhouse emissions from biomass processing and use. Pyrolysis is a conventional method for the thermal conversion of biomass with low moisture into potential fuels. This process consists of decomposing the biomass into charcoal, light hydrocarbons, and non-condensable gases by the action of high temperatures (350-600°C) and the atmospheric pressure. The pyrolysis of the macroalga is applied to a process simulation in Aspen plus V12 with an optimization achieved by multiple sensitivity analyses. Additionally, to upgrade the hydrogen production from a carbon-neutral biofuel to BECCS, a carbon capture unit by physical absorption with dimethyl ethers of polyethylene glycol (DEPG) is included using a hierarchy user model of the software. The results showed a high sensitivity of the temperature. Additionally, a second reactor and a water gas shift unit were necessary to maximize the hydrogen production. The temperature profile showed a maximum production of hydrogen at 500°C with the following reduction of its yield at higher temperature values due to the enhanced carbon monoxide production. Additionally, a second reactor operating under the same conditions as a gasifier and a water gas shift unit based on the Le Chatelier principle successfully increased the hydrogen production by 50%. Finally, a hydrogen yield of 2.06% was reached. The study related to the thermal conversion of this alga is an opening to the study of the thermal conversion of biomass commonly found in desertic or semi-desertic climates such as halophytes or salicornia.