Todd H. Gardner

Dry Reforming of Hydrocarbon Feedstocks

Developments in catalyst technology for the dry reforming of hydrocarbon feedstocks are reviewed for methane, higher hydrocarbons and alcohols. Thermodynamics, mechanisms and the kinetics of dry reforming are also reviewed. The literature on Ni catalysts, bi-metallic Ni catalysts and the role of promoters on Ni catalysts is critically evaluated. The use of noble and transitional metal catalysts for dry reforming is discussed. The application of solid oxide and metal carbide catalysts to dry reforming is also evaluated. Finally, various mechanisms for catalyst deactivation are assessed. This review also examines the various process related issues associated with dry reforming such as its application and heat optimization. Novel approaches such as supercritical dry reforming and microwave assisted dry reforming are briefly expanded upon.

Investigation of a Novel Reciprocating Compression Reformer for Use in Solid Oxide Fuel Cell Systems

A novel reciprocating compression device has been investigated as a non-catalytic natural gas reformer for solid oxide fuel cell systems. The reciprocating compression reformer is a potential improvement over current reforming technology for select applications due to its high degree of heat integration, its homogenous gas phase reaction environment, and its ability to co-produce shaft work. Performance modeling of the system was conducted to understand component integration and operational characteristics. The reformer was modeled by utilizing GRI mech. in tandem with CHEMKIN. The fuel cell was modeled as an equilibrium reactor assuming constant fuel utilization. The effect on the reformer and the reformer – fuel cell system efficiencies and exit gas concentrations was examined over a range of relative air-to-fuel ratios, 0.2 to 1.0, and at compression ratios of 50 and 100. Results from this study indicate that the reformer – fuel cell system could approach 50% efficiency, if run at low relative air-to-fuel ratios (0.3 to 0.5). With higher air-to-fuel ratios, system efficiencies were shown to continuously decline due to a decrease in the quality of synthesis gas provided to the fuel cell (i.e. more power being produced by the reformer). Optimal operation of the system has been shown to occur at a relative air-to-fuel ratio of approximately 0.775 and to be nearly independent of the compression ratio in the reciprocating compression reformer. Higher efficiencies may be obtained at lower relative air-to-fuel ratios; however, operation below this point may lead to excessive carbon formation as determined from an equilibrium carbon formation analysis.Copyright

Systems Analysis of Diesel Based Fuel Cells for Auxiliary Power Units

The near-term commercial success for many fuel cell technologies will rely on their ability to utilize current infrastructure fuels. Several large ready-markets exist for fuel cell systems that utilize middle distillate petroleum fractions like diesel fuel. One particular application is diesel-based auxiliary power units (APU). Unfortunately, very little research and development has been devoted to this application. Ongoing research at the National Energy Technology Laboratory (NETL) and other organizations is trying to address this need. In order for a fuel cell to utilize diesel fuel, it must be reformed into a synthesis gas containing primarily hydrogen, carbon monoxide, carbon dioxide, steam and possibly methane. Because catalytic reforming of hydrocarbon fuels is conducted at the same relative operating temperatures of technologies like solid oxide fuel cells (800–1000°C) a high degree of thermal integration is possible. Unfortunately, carbon deposition and sulfur poisoning of catalysts in the reformer and fuel cell make system operation potentially complicated and costly. To help understand and quantify the impact of these issues on technology development and component, a number of systems analysis was conducted for a diesel-based fuel cell system. One particular system based on a hybrid combustor/reformer concept allowed for excellent utilization of available heat from the fuel cell and yielded an overall fuel to electric conversion efficiency of nearly 50%. This paper discusses its salient features and compares its characteristics to other possible system configurations.Copyright

Modeling of Reformers for Fuel Cell Applications

The lack of an existing hydrogen infrastructure makes fuel processing of hydrocarbon fuels a critical component of fuel cell systems. The reformer is the principle reactor in the fuel processing subsystem and converts various hydrocarbon fuels into hydrogen, carbon monoxide, and other products that can then be utilized in the fuel cell. To help understand and quantify diesel fuel reforming by partial oxidation, a computational fluid dynamic model was developed that included the following sub-processes: liquid fuel atomization, fuel drop evaporation, fuel drop boiling and vaporization, and gas-phase chemical reaction. This work focused on n-heptane as a representative diesel fuel and reduced reaction mechanisms from the literature for fuel rich oxidation of n-heptane were used to analyze reactions in the partial oxidation reformer. Turbulent chemistry was modeled using the eddy dissipation concept. Reactor performance and parametric analysis based on model simulations are presented and discussed.Copyright

Numerical Simulation of Partial Oxidization Processing of Diesel for Fuel Cells

The application of a reduced, fuel-rich chemistry mechanism for n-Heptane partial oxidation was valid over the temperature range from 900 K to 1200 K and at the O/C ratio of 1.57. In this work, n-Heptane was utilized as a single fuel representative for diesel fuel to quantify CFD – Chemistry interation. The turbulence-reaction model is one of the standard combustion models developed in FLUENT, a commercial CFD (Computational Fluid Dynamic) software package. In this model, n-Heptane reduced chemistry mechanisms were coupled with the multi-dimensional CFD solver. The interaction between the chemical reactions and turbulence has been considered. In this paper, some preliminary results are presented and comparisons between the experiments and the simulations are made.Copyright

Support Effects for Pt and Rh-Based Catalysts for Partial Oxidation of n-Tetradecane

Catalytic partial oxidation (CPOX) of liquid fuels is an attractive option for producing a hydrogen-rich gas stream for fuel cell applications. However, the high sulfur content along with aromatic compounds present in liquid fuels may deactivate reforming catalysts. Deactivation of these catalysts by carbon deposition and sulfur poisoning is a key technical challenge. The relationship between catalyst supports and deactivation have been studied here for three catalysts (Rh/Ce0.5 Zr0.5 O2 , Pt/Ce0.5 Zr0.5 O2 , and Pt/Al2 O3 ) in a fixed bed catalytic reactor using a mixture of n-tetradecane, 1-methylnaphthalene, and dibenzothiophene to simulate logistic fuels. Carbon production during CPOX reforming was directly related to olefin formation. Olefins, which are known coke precursors, were observed on the Pt catalysts during CPOX of n-tetradecane with no sulfur (particularly from Pt/Al2 O3 ), but not on Rh/Ce0.5 Zr0.5 O2 . For the Rh/Ce0.5 Zr0.5 O2 , yields of H2 and CO dropped to a stationary level after the introduction of sulfur-containing feed (1000 ppm sulfur) or aromatic-containing feed (5 wt%), however, the catalyst activity was restored after removing the sulfur or aromatics from the feed. For the Pt catalysts, H2 and CO yields dropped continuously over time in the presence of sulfur or aromatics in feed. The superior performance of Rh/Ce0.5 Zr0.5 O2 can be attributed to the higher oxygen-ion conductivity of the Ce0.5 Zr0.5 O2 support as well as the activity of the Rh sites.Copyright

Synthesis Gas by Partial Oxidation and the Role of Oxygen-Conducting Supports: A Review

The objective of this review is to bring together results from experimental studies to better understand issues and the role of catalyst types and supports in the conversion of hydrocarbon fuels to synthesis gas. Fuel conversion, selectivity and carbon formation as function of type of support, metal, dispersion and temperature are presented. Reaction mechanisms for syn-gas (a mixture of CO and H2 ) production and formation of inactive carbon are reviewed. This review resulted in two primary conclusions: 1) Catalyst supports based on oxygen-ion conductors such as doped ceria may be used to mitigate carbon formation due to their high oxygen mobility; and 2) A more detailed and comprehensive study of these systems leading to mechanistic understanding may be of significant benefit in developing low-cost, effective, and long-duration reforming catalysts for POM and possibly higher hydrocarbon fuels.Copyright