Elsevier

Journal of Catalysis

Volume 215, Issue 2, 25 April 2003, Pages 332-343
Journal of Catalysis

Catalytic partial oxidation of higher hydrocarbons at millisecond contact times: decane, hexadecane, and diesel fuel

https://doi.org/10.1016/S0021-9517(03)00011-3Get rights and content

Abstract

The catalytic partial oxidation of n-decane and n-hexadecane with air over a Rh-coated monolith produces synthesis gas (H2 and CO) in selectivities exceeding 80%, with > 99% conversion of fuels and 100% conversion of oxygen at catalyst contact times from 5 to 25 ms. The high boiling hydrocarbons were delivered as liquids using an automotive fuel injector into a heated chamber where they vaporized in the presence of air. This system creates temperature and concentration gradients that allow safe mixing of fuel with air at temperatures above the autoignition of the fuel. Fuel-rich feeds beyond the syngas ratio produced olefins with selectivities as high as 80% in the case of n-hexadecane. The distribution of these olefins goes from primarily ethylene to large α-olefins as oxygen feed is decreased. Partial oxidation of low sulfur diesel fuel was also carried out successfully, producing synthesis gas at > 98% fuel conversion with several hours of stable operation.

Introduction

Reforming hydrocarbons is important in many applications to produce fuels, such as H2, and chemical intermediates, such as synthesis gas, and olefins [1], [2], [3], [4], [5], [6]. It is accomplished either by steam reforming or steam cracking which involve reaction with H2O in endothermic processes or by partial oxidation which involves reaction with O2 in exothermic processes. Conversion of methane to syngas [1], [2], [3] and ethane to olefins [4], [5] by both processes is well established, and reactions of alkanes up to isooctane have been demonstrated [6], [7].

While steam reforming and steam cracking of higher alkanes, such as diesel fuel, can be accomplished under suitable conditions, the partial oxidation of higher alkanes presents several problems, such as flames during vaporization and mixing, soot formation associated with combustion of fuel-rich gases, and coke formation on reactor walls and on catalysts.

Currently there is considerable interest in reforming logistic fuels such as diesel and JP-8 (similar to kerosene and used as a military fuel) into light alkanes and especially H2 for devices such as fuel cells, which function either exclusively on H2 (the proton-exchange membrane fuel cell) or which function best with H2 in the fuel (the solid oxide fuel cell). Since a major interest is in fuel cells for transportation vehicles, gasoline and diesel are essential fuels in the next generation of fuel cell vehicles.

There is also considerable interest in fuel reforming for pollution abatement in automotive applications with internal combustion engines. Reforming of gasoline or diesel into H2 and other small molecules creates a fuel that burns very efficiently, thus reducing or eliminating exhaust emissions of hydrocarbons, CO, and particulate matter [8], [9], [10]. The abatement of NOx in diesel engines is especially difficult because, unlike a spark ignited engine, in a lean burn environment there is insufficient H2, CO, and small hydrocarbons to react with NOx in the catalytic converter. Therefore, reforming part of the fuel and using it to react with NOx could be important in diesel emissions control.

We describe here the reforming by partial oxidation of two of the major components of diesel fuel: n-decane and n-hexadecane. These fuels can be quantified by detailed mass balances to determine conversions and selectivities to various products. We also demonstrate the partial oxidation of a low sulfur grade of diesel fuel. Since this is a mixture, quantitative analysis of reactants and products is more complicated, as is specification of the C/O ratio.

Our primary objective in this paper is to demonstrate the feasibility of this reaction in short contact time reactors using a fuel injector for fuel vaporization and mixing with air. We offer qualitative arguments regarding the mechanisms that lead to successful operation, but further experiments and modeling will be required to characterize these partial oxidation processes quantitatively, especially with diesel fuel mixtures.

Section snippets

Fuel injector for fuel vaporization and mixing

For methane and other light alkanes the fuel is a gas at room temperature or it can be vaporized before mixing by heating to < 100 °C [1], [11]. However, the autoignition temperature of normal alkanes decreases as the chain length increases and is as low as ∼ 200 °C for alkanes above n-decane. Since the boiling point increases with the hydrocarbon chain length, the boiling point exceeds the autoignition temperature for alkanes higher than n-decane. During vaporization and mixing of fuel with

n-Decane

n-Decane and air were fed to the Rh-monolith reactor at several flow rates. The carbon/oxygen feed ratio was varied from 0.7 to 2.5 at total flow rates of 2, 4, 6, and 8 SLPM. These flow rates correspond to catalyst contact times of 24, 12, 8, and 6 ms, respectively, estimated at an average catalyst temperature of 800 °C. The reactor pressure was kept at 1 atm and the feed temperature was 250 °C which is above the boiling point of n-decane, 174 °C. The combustion ratio for n-decane is C/O=0.323

Discussion

This is a complicated process in terms of reactions, reactor configuration, range of operation, and products. More detailed experiments are required to decide definitively how each of these variables affects the process, because we have only considered the variation of composition and total flow rate with fixed geometry and sufficient preheat to vaporize the fuels. In this discussion we will consider some of these subjects, but we can only speculate on detailed mechanisms that may be operative.

Summary

These experiments show that it is possible to oxidize higher alkanes to syngas and to olefins by partial oxidation in air at contact times of 5 to 25 ms. The process is fairly robust in that the catalyst can be operated successfully over a wide range of C/O as long as the fuel is vaporized and mixed with air and the catalyst is kept sufficiently hot.

The process appears to be fairly insensitive to catalyst form and loading, although it can certainly be “tuned” beyond results shown here by

Acknowledgements

This research was sponsored by the ARL Collaborative Technology Alliance in Power and Energy, Cooperative Agreement No. DAAD19-01-2-0010 and by Caterpillar Inc.

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