Chemical Looping Combustion of Hematite Ore with Methane and Steam in a Fluidized Bed Reactor

Samuel Bayham,Justin Weber,Ronald Breault

doi:10.3390/en10081179

Abstract

Chemical looping combustion is considered an indirect method of oxidizing a carbonaceous fuel, utilizing a metal oxide oxygen carrier to provide oxygen to the fuel. The advantage is the significantly reduced energy penalty for separating out the CO2 for reuse or sequestration in a carbon-constrained world. One of the major issues with chemical looping combustion is the cost of the oxygen carrier. Hematite ore is a proposed oxygen carrier due to its high strength and resistance to mechanical attrition, but its reactivity is rather poor compared to tailored oxygen carriers. This problem is further exacerbated by methane cracking, the subsequent deposition of carbon and the inability to transfer oxygen at a sufficient rate from the core of the particle to the surface for fuel conversion to CO2. Oxygen needs to be readily available at the surface to prevent methane cracking. The purpose of this work was to demonstrate the use of steam to overcome this issue and improve the conversion of the natural gas to CO2, as well as to provide data for computational fluid dynamics (CFD) validation. The steam will gasify the deposited carbon to promote the methane conversion. This work studies the performance of hematite ore with methane and steam mixtures in a 5 cm fluidized bed up to approximately 140 kPa. Results show an increased conversion of methane in the presence of steam (from 20–45% without steam to 60–95%) up to a certain point, where performance decreases. Adding steam allows the methane conversion to carbon dioxide to be similar to the overall methane conversion; it also helped to prevent carbon accumulation from occurring on the particle. In general, the addition of steam to the feed gas increased the methane conversion. Furthermore, the addition of steam caused the steam methane reforming reaction to form more hydrogen and carbon monoxide at higher steam and methane concentrations, which was not completely converted at higher concentrations and at these residence times.

Highlights

According to the Intergovernmental Panel on Climate Change, the unprecedented rate of global warming in the past few centuries is due to anthropogenic greenhouse gas emissions, and these emissions need to be curtailed to prevent a further increase in temperature [1]
The reactor consists of three sections: the plenum, where the gases enter through the bubblecaps, the fluidized bed, which is the raw hematite material, and the freeboard region, which expands at a region higher up to minimize the loss of oxygen carrier
For the overall methane conversion, the values for different methane and steam feeds as a function of temperature mostly overlap due to the conversion of methane to different products

Summary

Introduction

According to the Intergovernmental Panel on Climate Change, the unprecedented rate of global warming in the past few centuries is due to anthropogenic greenhouse gas emissions, and these emissions need to be curtailed to prevent a further increase in temperature [1]. While developments in solar and wind power generation are occurring, there is still a need for baseload power on the electric grid, which cannot be attained using only renewable technologies. There will be a need for baseload plants, which in the United States are mainly based on fossil fuel technology. A proposed solution for utilizing baseload fossil fuel plants under a carbon-constrained scenario is carbon capture and sequestration. Examples of technologies are categorized into post-combustion, pre-combustion, and oxy-combustion capture [2,3]. Chemical looping combustion splits the conventional combustion reaction into two Energies 2017, 10, 1179; doi:10.3390/en10081179 www.mdpi.com/journal/energies two thereby steps, thereby resulting in a separate stream ofdioxide carbon flue dioxide flue is gas that free of nitrogen

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