Autothermal reforming of simulated gasoline and diesel fuels

https://doi.org/10.1016/j.ijhydene.2006.04.004Get rights and content

Abstract

Autothermal reforming (ATR) of simulated fuels was studied with n-heptane, n-dodecane, toluene, and methylcyclohexane as model compounds for different fractions of gasoline and diesel on zirconia-supported Rh and Pt. A performance comparison was made on commercial nickel catalyst. No inhibiting effect was observed between the hydrocarbons on any catalyst, even though aliphatic hydrocarbons are more reactive than aromatics. The product selectivities obtained in ATR of simulated fuels could be calculated from the selectivities obtained in ATR of single hydrocarbons.

ATR of fuels has to be operated at high temperatures, which promotes thermal cracking. To prevent these side reactions active and selective catalysts are needed. The Rh/ZrO2 catalyst showed high ATR activity and selectivity. Moreover, less coke was accumulated on the zirconia-supported noble metal catalysts than on the commercial nickel catalyst. Some deactivation of the rhodium and nickel catalysts was, however, observed.

Introduction

Demand for hydrogen will increase dramatically in the future when new applications for fuel cells are commercialized. Hydrogen and synthesis gas (H2 + CO) are already widely produced by steam reforming (STR) of methane in stationary systems. Although, methane can easily be utilized in the vicinity of natural gas pipelines, it must be compressed when it is transported and stored in vehicles. An easily delivered and safely storable hydrogen source, such as gasoline or diesel, would thus be preferable in mobile applications. The infrastructure for these fuels already exists, moreover. Hydrogen generation could be carried out anywhere using an on-board reformer, and internal combustion engines could be replaced with more efficient fuel cell engines [1], [2].

STR of higher hydrocarbons has been practiced for the last 40 years in locations where natural gas is not available [3]. Unfortunately, coke is formed in large amount on the conventional nickel catalysts [4]. In addition, nickel catalysts do not tolerate sulfur compounds [5], [6]. Thus, more stable catalysts are required for commercial fuels to be used as a hydrogen source. Stable reforming catalysts would also find application in autothermal reforming (ATR), where the highly endothermic STR is combined with the exothermic partial oxidation (POX) reaction [7], [8]. ATR is the preferred choice for vehicle applications of higher hydrocarbons owing to its short start-up time [6], [8], [9]. Moreover, the ATR product gas, containing mainly H2, CO, CO2, CH4, and H2O, can be utilized, for example, in solid oxide fuel cells (SOFC), albeit the selectivity to hydrogen is lower in ATR than in STR [10]. In addition, operation of ATR of n-heptane, at least, is thermally more stable than STR [11]. Still, ATR of hydrocarbon mixtures and liquid fuels has not been studied sufficiently.

In this work, ATR of higher hydrocarbons and their mixtures was studied in order to gain more insight into the suitability of liquid fuels as feed for ATR. Moreover, the interaction of hydrocarbons in simulated fuels was evaluated. n-Heptane and n-dodecane were used as model compounds for the n-alkane fractions of gasoline and diesel, and toluene and methylcyclohexane (MCH) as model compounds for the aromatic and cycloalkane fractions, respectively.

Zirconia-supported rhodium and platinum catalysts, and for comparison a commercial NiO/Al2O3 catalyst, were studied. Zirconia is noted for its stability [12]. In addition, as an acid–base bifunctional oxide it is less acidic than alumina [13]. Despite of these good features, zirconia has enjoyed only limited use as a support due to its low surface area. In recent years, especially mixed zirconia oxides have attracted much attention in various reactions [14], [15]. The noble metals were chosen for their tolerance against coke formation and sulfur compounds [4], [16], [17], [18].

Section snippets

Thermodynamics

Thermodynamics of the ATR reactions of n-heptane, n-dodecane, toluene, and MCH were calculated with HSC Chemistry version 5.11 [19] to determine the reaction conditions under which a thermoneutral total reaction in ATR could be achieved.

Catalysts

Two noble metal catalysts, 0.5 wt% Rh/ZrO2 and 0.5 wt% Pt/ZrO2, were prepared by dry impregnation using a 10 wt% Rh(NO3)3 solution in diluted (5 wt%) nitric acid (Sigma-Aldrich) and a Pt(NH4)4(NO3)2 solution (99%, Strem Chemicals). The ZrO2 support (MEL Chemicals

Thermodynamics of thermoneutral ATR

The ATR feed consisted of hydrocarbons, water, and air. Consequently, in addition to the endothermic STR (Eq. (2)), exothermic POX (Eq. (3)) takes place in the reformerCxHy+xH2Ox+12yH2+xCO,ΔH2980>0kJ/mol,CxHy+12xO212yH2+xCO,ΔH2980<0kJ/mol.

The absolute value of STR and POX reaction enthalpies increases noticeably with the chain length of the hydrocarbon (Table 1). Since STR is highly endothermic, the reactor has to be heated substantially, especially when water is fed in excess (H2O/C>1mol/mol

Conclusions

ATR of higher hydrocarbons can be operated thermoneutral, which is an important issue when large-scale applications are designed. Moreover, coke accumulation on the catalyst is decreased compared to STR.

Since aromatics are more stable hydrocarbons than aliphatics, the ATR reaction temperature has to be raised when simulated fuels are used as feedstock. However, thermal cracking accelerates with temperature and takes place especially when the conversion of aliphatic hydrocarbons is incomplete.

Acknowledgments

Financial support was received from VTT (Technical Research Centre of Finland) and Tekes (Finnish Funding Agency for Technology and Innovation).

References (21)

There are more references available in the full text version of this article.

Cited by (0)

View full text