A new study published in Engineering has evaluated the staged, pressurized oxy-combustion (SPOC) process for boiler retrofits using woody biomass to generate carbon-negative power, with assessments covering both standalone power generation and cogeneration systems. First introduced in 2012, SPOC integrates fuel staging and pressurized oxy-combustion to deliver a more compact plant layout, improved energy efficiency, and lower costs for pollutant and greenhouse gas removal relative to conventional carbon capture setups. The modular boiler design supports flexible operation, which supports grid stability amid growing shares of intermittent renewable generation. While initially developed for coal, SPOC is well suited to biomass combustion because it can recover latent heat from the high moisture content in biomass flue gas, boosting performance compared with atmospheric-pressure biomass combustion systems.

Aspen Plus modeling confirms that SPOC outperforms baseline plants equipped with post-combustion capture (PCC) in both power and cogeneration configurations. The SPOC process relies on a series-parallel boiler arrangement that limits flue gas recirculation to less than 30 percent, far below rates used in first-generation atmospheric oxy-combustion, reducing auxiliary power demands. The integrated system includes an air separation unit, staged pressurized boilers, high-temperature heat recovery, a baghouse filter, a direct contact cooler (DCC), and a CO₂ processing unit, with heat recovery from flue gas moisture condensation and compression waste heat integrated into the steam cycle. The DCC simultaneously cools flue gas, condenses water vapor, and removes SOₓ, NOₓ, and HCl in a single pressurized column, cutting capital and operating expenses compared with conventional flue gas treatment systems. Carbon capture efficiency reaches 95 percent or higher, with the CO₂ stream compressed for sequestration at lower energy cost because the gas remains pressurized throughout the process.

Researchers at Washington University in St. Louis have successfully demonstrated 100 percent biomass firing in a 100 kWₜₕ pressurized oxy-combustor following minor adjustments to the dry feeding system. Tests showed stable startup, steady-state operation, and turndown performance with low excess oxygen, high particle burnout, and low NOₓ emissions, with no burner or combustor modifications needed between coal and biomass operation. The study indicates SPOC retrofits can convert existing coal-fired boilers to carbon-negative service using sustainable woody biomass, providing a practical pathway for industrial decarbonization and repurposing of thermal power assets.

The paper “The Staged, Pressurized Oxy-Combustion Technology: Status and Application to Boiler Retrofits to Yield Carbon-Negative Power via Biomass,” is authored by Duarte Magalhaes, Mao Cheng, Zachariah Wargel, Richard L. Axelbaum. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.11.016. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.