Study Examines Ability of Macroalga To Produce Green Hydrogen

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 210809, “Pyrolysis of Macroalga Macrocystis pyrifera for Production of Green Carbon-Negative Hydrogen,” by Carolina A. Gallego, Shashank S. Nagaraja, and Mani Sarathy, King Abdullah University of Science and Technology. The paper has not been peer reviewed. _ The macroalga Macrocystis pyrifera is a giant brown seaweed characterized by its fast growth rate 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. The study described in the complete paper is related to the thermal conversion of this alga and serves as an opening to the study of the thermal conversion of biomass commonly found in desertic or semidesertic climates. BECCS and M. pyrifera The relevance of BECCS and carbon-negative fuels is in the ability of plants and algae to capture atmospheric CO2 and simultaneously be a source of carbon. Second-generation biofuels—fuels derived from nonedible terrestrial crops or waste such as miscanthus, wheat straw, or paper waste—are a promising alternative. The disadvantages of their generation, however, include the high cost of related technologies and the risk of compromising food security in some areas because of the amount of land devoted to energy crops. Algae are classified as microalgae or macroalgae (also called seaweed). The harvesting methods of macroalgae are less energy-intensive and expensive than the methods used for microalgae. M. pyrifera, a brown macroalgae, presented several advantages because of its large size (12–22 m long), fast growth rate (7–30 cm/day), its high gas yield, and its potential sustainable cultivation. Its cultivation, however, currently takes place only in Pacific Rim countries. Thus, this alga must be imported in dry form to the Middle East. With thermal conversion methods a suboptimal choice for dry M. pyrifera, a conventional method for the thermal conversion of the algae and production of hydrogen as pyrolysis was applied. Pyrolysis is a thermochemical process for decomposition of biomass in absence of oxygen to produce oil, char, and gases; it is detailed in the complete paper. The carbon-capture process involves technologies that separate the CO2 from the products of reaction. In this way, greenhouse emissions can be reduced through the storage of captured CO2. When high quantities of CO2 compose the products of thermal conversion, the most-efficient applicable technology is physical absorption by Selexol or Rectisol.