Liquid chemical looping gasification

Abstract

Combustion of fossil fuel for energy production is not a sustainable method since it releases CO2, particulate materials and greenhouse gases such as NOx into the atmosphere, which causes environmental pollution and global warming. Additionally, fossil fuel resources are limited, thus reliance on fossil fuels is not sustainable. To address this, special attention has recently been paid to renewable energy resources as alternatives for fossil fuels. However, it requires the development of new processes, or to integrate systems to produce energy through clean technologies aimed at the reduction of carbon dioxide emissions. One promising method is to convert fossil fuels, or biomass, to synthetic fuel referred to as “syngas”. Gasification is an established method for producing syngas from a carbonaceous fuel. The conventional gasification pathways employ air to supply the required oxygen for the reactions, however, due to the presence of the nitrogen in the gaseous products, the quality of the syngas (molar ratio of H2: CO) is relatively low. Thus, a new process for the production of syngas has been developed, referred to as a “chemical looping gasification” process, which uses solid metal oxide as the oxygen carrier. This process prevents direct contact between the feedstock and the air, addressing the challenge of the presence of nitrogen in the product. However, there are some disadvantages associated with the use of solid metal oxides, such as sintering, breakage of the particle, agglomeration and the deposition of the carbon on the oxygen carrier particles. Therefore, one potential solution to address the aforementioned challenges is to use a liquid metal oxide as an oxygen carrier instead of solid particles in a new process referred to as Liquid Chemical Looping Gasification (LCLG). To assess the LCLG system, a thermodynamic model was developed to simulate the reactions occurring in a chemical looping gasification system with a liquid metal, such as copper oxide, as the oxygen carrier. To identify other suitable oxygen carriers, a thermodynamic model and a selection procedure were also developed to assess the chemical performance of the system with various metal oxides. Copper, lead, antimony and bismuth oxides were potential options. Amongst them, lead oxide was assessed for integration of the system with a supercritical steam turbine cycle for the co-production of work and syngas. Bismuth oxide was thermodynamically and experimentally assessed for the gasification of biomass, coal and natural gas. To validate the developed models and to demonstrate the liquid chemical looping process, a series of experiments were conducted using a thermo-gravimetric analyser. Experiments were performed to assess the reduction and oxidation reactions of bismuth oxide with a graphitic carbon and air by measuring the mass change of the samples in the nitrogen and the air environments. The activation energy and reaction constant for the reduction and oxidation reactions were measured experimentally. The results obtained with the thermodynamic models for the bismuth oxide were in good agreement with those obtained with the experiments.