Saurabh Bhavsar

Chemical looping beyond combustion: production of synthesis gas via chemical looping partial oxidation of methane

The recent surge in natural gas reserves has revived interest in the development of novel processes to convert natural gas into valuable chemical feedstocks. In the present work, we are applying “chemical looping”, a technology that has found much attention as a clean combustion technology, towards selective partial oxidation of methane to produce synthesis gas (CLPOM). By tailoring the composition of NixFe1−x–CeO2 oxygen carriers and carefully controlling the supply of oxygen, i.e., the extent of the carrier reduction and oxidation in redox cycles, the reactivity and selectivity of these carriers for partial oxidation was optimized. Addition of a small amount of Ni to iron oxides allowed the combination of the high reactivity of Ni for methane activation with the good syngas selectivity of iron oxides. An optimized carrier with the composition of Ni0.12Fe0.88–CeO2 demonstrated excellent stability in multi-cycle CLPOM operation and high syngas yields with a H2:CO ratio of ∼2 and minimal carbon formation. Finally, a simplified fixed-bed reactor model was used to assess the thermal aspects of operating the process in a periodically operated fixed-bed reactor. We found that the process is highly sensitive to the degree of carrier utilization, but that maximum temperatures can be easily controlled in CLPOM via control of the active metal content and oxygen utilization in the carriers. Overall, chemical looping partial oxidation of methane emerges as an attractive alternative to conventional catalytic partial oxidation, enabling the use of low-cost transition metal oxides and air as oxidant, and resulting in inherently safe reactor operation by avoiding mixed methane/air streams.

Accurate Amorphous Silica Surface Models from First-Principles Thermodynamics of Surface Dehydroxylation

Accurate atomically detailed models of amorphous materials have been elusive to-date due to limitations in both experimental data and computational methods. We present an approach for constructing atomistic models of amorphous silica surfaces encountered in many industrial applications (such as catalytic support materials). We have used a combination of classical molecular modeling and density functional theory calculations to develop models having predictive capabilities. Our approach provides accurate surface models for a range of temperatures as measured by the thermodynamics of surface dehydroxylation. We find that a surprisingly small model of an amorphous silica surface can accurately represent the physics and chemistry of real surfaces as demonstrated by direct experimental validation using macroscopic measurements of the silanol number and type as a function of temperature. Beyond accurately predicting the experimentally observed trends in silanol numbers and types, the model also allows new insights into the dehydroxylation of amorphous silica surfaces. Our formalism is transferrable and provides an approach to generating accurate models of other amorphous materials.

Chemical-looping processes for fuel-flexible combustion and fuel production

Abstract Chemical-Looping Combustion (CLC) is a rapidly emerging technology for clean combustion of fossil and renewable fuels. In CLC, the combustion of a fuel is broken down into two, spatially separated steps: The oxidation of fuel in contact with an ‘oxygen carrier’ (typically a metal oxide), and the subsequent reoxidation of the carrier with air. CLC thus produces sequestration-ready CO2 streams with only minor efficiency penalties for CO2 capture. While recent interest in chemical looping was almost exclusively focused on combustion, the underlying reaction engineering principle forms a flexible platform for fuel conversion with a long history in chemical engineering. This chapter gives a brief review of the status of chemical-looping processes for fuel conversion, focused predominantly on reforming and partial oxidation of fossil and renewable fuels and on the impact of fuel composition on combustion.