Rh assisted catalytic oxidation of jet fuel surrogates in a meso-scale combustor

Highlights

• Observed global combustion behavior for dodecane and dodecane/m-xylene mixtures.

• Analyzed fuel/oxygen conversion, product selectivities, and reaction kinetics.

• Stable hybrid flame seen for all fuels over a range of operating conditions.

• Addition of aromatic increased conversion, CO₂ selectivity, and activation energy.

• Rh shown to promote total oxidation over partial oxidation for all fuels.

Abstract

Oxidation behavior of dodecane and two mixtures of dodecane and m-xylene (90/10 wt.% and 80/20 wt.%) over an Rh catalyst in a meso-scale heat recirculating combustor was examined to isolate the effect of aromatic content on performance. The fuel conversion, product selectivities, and reaction kinetics were calculated, and the global combustion behavior observed. The results showed that increasing the amount of m-xylene in the fuel increased the fuel conversion from 85% (pure dodecane) to 92% (90/10) and further to 98% (80/20). The presence of xylene also significantly increased CO₂/H₂O selectivity and decreased CO/H₂ selectivity. Global activation energy increased linearly with increase in xylene content, supporting that addition of aromatic species to fuel lowers the overall reactivity. The non-catalytic reaction was also simulated using Chemkin software to determine the effect of the Rh catalyst on the combustor performance and to analyze the difference in chemical mechanisms. The results revealed that the catalyst promotes total oxidation over partial oxidation, and lowers the global activation energy by up to 70%.

Introduction

Meso-scale combustion systems can be used for a plethora of applications, including micro thrust and propulsion generation and local power/heat production, resulting in much attention form the technical community. Hydrocarbon fuels are more energy dense than batteries, suggesting that even devices with poor thermal-to-electric conversion efficiencies can provide superior electrical outputs [1], and in turn allow for other performance improvements in portable electrical devices, such as reduced weight and a longer lifetime [2]. Several research groups have already demonstrated successful integration of meso-scale combustion systems and thermoelectric/thermophotovoltaic devices to generate electric power [3], [4], [5], [6], [7]. However, an intrinsic limitation to meso-scale systems exists in that, as the system is scaled down, the heat loss to heat generation (or surface area to volume) of the system increases, which may result in flame quenching. To diminish this effect, a prevalent technique is to utilize a heat recirculating swiss roll combustor design, which facilitates heat transfer from the products to the reactants. This enables stable combustion in channels with a characteristic dimension smaller than the flame quenching distance, as well as expands the range of stable operating conditions by lowering the lean flammability limit [8]. Many studies have examined the effects of geometry, scaling, combustor material, and operating conditions on the performance of micro- and meso-scale swiss roll combustors [9], [10], [11], [12], [13], [14], but Shirsat and Gupta [15] found that the optimal combustor geometry to maximize exhaust enthalpy while minimizing heat loss is a single-turn design.

Liquid hydrocarbon fuels are ideal for use in meso-scale systems, as they have very high specific energies and are easily stored. However, they present several challenges when used at this scale. Liquid fuels need to be vaporized prior to mixing with the oxidizer, leading to higher energy input required at start up. Also, flow through the channels of meso-scale combustors is typically laminar, which, when coupled with shorter residence times, causes insufficient mixing and thus incomplete and unstable combustion [11], [15], [16]. Blended fuels with components that are vaporized/oxidized at different rates, particularly those containing both alkanes and aromatic species, are especially challenging, as the slower kinetics governing aromatic oxidation result in lower overall reactivity, causing increased soot formation and flame instabilities [17], [18], [19], [20]. Won et al. [20] examined the kinetic effects of toluene addition on the extinction limit of n-decane flames. It was found that the extinction strain rate of the toluene/n-decane flames depended linearly on the maximum concentration of OH radicals, which was directly affected by the concentration of toluene; as toluene content increased, the maximum OH concentration decreased (via H abstraction and chain termination reactions involving toluene), thus decreasing the extinction strain rate. Seshadri et al. [21] studied the reaction kinetics of JP-8, a military jet fuel, and two surrogates in detail. The researchers found that the lower reactivity of the aromatic components was attributable to the formation of resonantly stabilized radicals, which have a scavenging effect—the aromatic radicals participate in recombination reactions with H radicals, which act as chain termination steps and thus the global reactivity is reduced.

The utilization of a catalyst can help to improve performance in a meso-scale system. Since catalytic combustion relies on surface reactions, the high surface area of a meso-scale combustor provides a larger number of active sites upon which the reactants can adsorb. Additionally, chemical reactions will theoretically only occur on catalyst surfaces, so the location of the heat source is fixed, unlike in homogenous combustion, where the flame may anchor at different locations depending on the operating conditions [9]. This thermal stability makes a catalytic system much more attractive for integration with a thermoelectric/thermophotovoltaic device than a comparable non-catalytic system [22]. Several studies have shown that catalysts can help stabilize liquid fuel combustion at very short residence times, even for petroleum-based jet fuels [23], [24], [25], [26]. Most of the existing literature investigates the role of the catalyst in fuel reformation (i.e. partial oxidation), particularly for fuel cell applications [25], [26], [27], [28], [29], [30], [31], [32]. Very few studies have been conducted on catalytic combustion (i.e. complete oxidation) of liquid fuels at the meso scale.

The current study examines the catalytic combustion behavior of dodecane and two jet fuel surrogates: 90 wt.% dodecane/10 wt.% m-xylene and 80 wt.% dodecane/20 wt.% m-xylene. The effect of increasing the amount of xylene in the fuel will be analyzed in terms of global combustion behavior, fuel conversion, product selectivity, and reaction kinetics.