(329d) Chemical Looping Combustion (CLC)-Aided Biomass Gasification for Co-Production of Hydrogen and Electricity

This paper presents a comprehensive thermodynamic evaluation of multiple carbon capture pathways of utilizing chemical looping combustion (CLC) to aid biomass gasification for co-production of electricity and fuels.

Gasification of carbonaceous fuels produces synthesis gas (syngas), consisting predominantly of H₂ and CO, which could be used as a feedstock for a wide range of chemical processes, including production of fuels such as hydrogen or Fischer-Tropsch (FT) liquids. Biomass gasification for production of syngas, in combination with carbon capture and storage (CCS), potentially leads to net negative CO₂ emissions. Biomass gasification technology, using oxygen, steam or CO₂ has developed over the years with a few large-scale demonstration plants being operated. The composition of syngas depends on the gasifying agent and operating conditions of the gasifier. Gasification using high-purity O₂, instead of air, produces a high heating value syngas. However, the cryogenic air separation unit (ASU) required to produce high-purity O₂ is an energy-intensive and a costly process. Gasification using steam also increases the syngas heating values by generating more hydrogen. However, the overall thermal conversion efficiency of steam gasification of biomass was reported to be in the range of 50 – 70% owing to the high demand of energy for producing steam and maintaining high gasification temperatures. On the other hand, CO₂ gasification has been reported to attain higher carbon and energy efficiency. Thermodynamic analysis has shown higher conversion of biomass gasification through recycling of CO₂.

Chemical looping combustion (CLC) is an indirect combustion process in which fuel is combusted without direct contact with air. Transfer of oxygen between air and fuel takes place with the aid of an oxygen-carrier (OC), usually oxides of transient metals. The reduced form of metal oxide extracts O₂ from air in one reactor and then transfers it to fuel in a subsequent reactor. Since the fuel does not come in direct contact with air, the products of combustion contain only carbon dioxide (CO₂) and water (H₂O). CO₂ stream of very high purity can be obtained by condensing the water vapor. Gaseous fuels such as natural gas or solid fuels have been demonstrated as fuels in CLC, for inherent CO₂ capture.

In this study, a novel approach in integrating biomass gasification, aided by CLC is investigated, with a view of attaining lower carbon emissions. The hot CO₂ and H₂O products from the fuel reactor of CLC are used as gasifying agents for biomass, while enabling auto-thermal operation and eliminating the need for expensive processes for O₂ or steam production. This will also greatly reduce the demand for process water required to generate steam. The hot depleted air stream from the air reactor is used to further generate steam.

Separate performance models are developed to perform a detailed thermodynamic analysis of:

(1) CLC reactor system using natural gas or syngas as fuel and Ni-/Cu-/Fe-based materials as oxygen carriers;

(2) biomass gasification using CO₂ and steam as gasifying agents; and

(3) combined CLC and biomass gasifier system.

The effect of varying operating conditions (temperature, steam-carbon and CO₂-carbon ratio, etc.) on performance metrics such as the syngas composition and heating value, overall thermal efficiency, and overall CO₂ emissions is studied. A comparison will be made with a conventional biomass gasification system in order to understand the relative feasibility of CLC-aided biomass gasification. The performance models are developed using Aspen Plus and MATLAB.

Using the results of the combined models, we will explore the technical feasibility of different utilization pathways of the syngas generated in the above process, specifically for co-production of H₂ and electricity in an integrated gasification combined cycle power plant (IGCC), with pre-combustion CO₂ capture and storage.

This work will help identify potential pathways to utilize CLC and biomass to achieve net-negative CO₂ emissions.