(303n) Advancements in the OSU Moving Bed Chemical Looping Biomass Gasification: >700hrs Sub-Pilot (15kWth) Operation, Modularization, and Cross Current Reactor Design

Abatement of the effects of anthropogenic climate change remains one of the most imposing problems confronting the scientific community. Commercializing biomass-based technologies as an alternative to fossil fuels for a sustainable future faces several challenges, including high pretreatment cost, tar formation, high moisture content, process complexity, ash separation, etc.

This current study presents advancements in OSU's biomass-to-syngas (BTS) technology, a robust chemical looping gasification process for producing syngas from biomass. The technology utilizes iron-based oxygen carriers, which donate their lattice oxygen for lignocellulosic biomass gasification. The BTS technology consists of a moving bed reducer, where the biomass is gasified into syngas, and a fluidized bed combustor for regeneration in the air of the oxygen carriers.

Xu et al., in 2018, established the BTS process with a moving bed bench scale reactor which showed high-purity syngas formation with a variable H₂:CO ratio. The current work presents the scale-up results that prove the BTS technology's commercial viability on a sub-pilot scale (~15 kWth capacity) with continuous operation for over 700 hours. The demonstration successfully tested three types of untreated biomass, including pelletized corn cob, woody pellets, and unpelletized loose corn cob, for a prolonged duration with a variable syngas H2/CO ratio quality ~2 and a syngas purity greater than 70%. Process simulations show that integrating the BTS system with the Fischer Tropsch process for liquid fuel production can achieve a ~14% reduction in biomass requirement compared to conventional biomass gasifiers. Heat integration can lead to an overall autothermal process using combustor exhaust heat for biomass drying.

Further bench-scale studies are done to determine the minimum required reactor volume and the minimum enhancer gas flow rate for efficient gasification. A volume reduction of nearly 67% was achieved, with a minimum steam flow rate as low as 5% of the inlet carbon flow rate for woody biomass pellets. An increase in enhancer gas injection leads to higher tar concentrations due to decreased gas residence time in the reactor.

Furthermore, the study introduces a novel cross-current moving bed reducer design, which involves outletting the syngas from the middle of the reactor and inletting the enhancer gas from the bottom. This design significantly improves the char gasification rate and syngas yield, as confirmed by experimental and process simulation results.

The study also proposes a modular system for generating high-purity syngas from biomass and capturing CO₂ by operating two moving bed reducers in parallel with a common fluidized bed combustor. This system eliminates the need for a separate CO₂ capture unit and recovers additional hydrogen/heat, boosting the process's effective thermal efficiency (ETE) by 17%. The modular system can be integrated with other syngas-requiring processes and can recover energy from tail gas streams, producing additional hydrogen or heat output while capturing CO₂.

Finally, the study highlights future research directions, including co-gasification with carbon-rich sources such as post-consumer plastic waste, to enhance syngas quality via experimental results and process simulations.