wet partial oxidation of jp8 in a well-insulated reactor

WET PARTIAL OXIDATION OF JP8 IN A WELL-INSULATED REACTOR

Richard Scenna

US Army CERDEC CPI Aberdeen Proving Ground, MD, United States

Ashwani K. Gupta Department of Mechanical Engineering

University of Maryland College Park, MD, United States

[email protected]

ABSTRACT This work investigates wet and dry non-catalytic partial

oxidation of JP8 under distributed reaction regime condition. Previous works have demonstrated the potential of the distributed reaction regime to increase hydrogen and carbon monoxide production over conventional non-catalytic reforming and suppress soot formation inside the reactor. Jet propellant 8 (JP8) has a high sulfur content (up to 3000ppm) and a tendency to form coke, making it an ideal candidate for this non-catalytic approach. Experimental results are reported with the reactor operated at fixed oxygen to carbon ratio of 1.08 and steam to carbon ratios varied from 0.0 to 0.23. Numerical simulations were used to determine flame regime and extent of distribution.

Steam provided favorable effects even with trace amounts (S/C=0.01), but more pronounced effects were observed at steam to carbon ratio of 0.17. Syngas composed of 22.5-24.6% hydrogen and 20.1-23.3% carbon monoxide was evolved. Of the hydrocarbons detected, only methane was seen in finite amounts (0.17-0.29%). The increase in performance in terms of reforming efficiency and conversion exceeded what can be ascribed to steam reforming reactions alone. Additional enhancement is attributed to distributed reaction in the reactor. Reforming efficiency of approximately 68-80% is comparable to that from catalytic reforming. Low steam to carbon ratio offers higher sustainability in mobile power systems at reduced costs from direct use of water recovered from fuel cells. INTRODUCTION

Reforming is chemical decomposition of hydrocarbon materials (fuels) into a hydrogen and carbon monoxide rich gas, known as syngas. Typically, catalytic reforming efficiency is in the range of 75-90%[1,2]. For stationary applications, natural gas is typically utilized due to its low cost and ease of reformation. Sulfur compounds present in natural gas are easily removed with an Amine/Claus process, which is suitable for stationary power applications[3]. However, in mobile power applications, natural gas is not as convenient as middle distillate fuels. Kerosene based fuels are more convenient in

commercial sectors, while JP8 fuel is more suitable in most defense applications due to convenience and logistics. JP8 has a higher hydrogen density as compared to high-pressure methane (at 3600 psi) or liquid hydrogen (at 20K, 1 atm), see Table 1.

Fuel Mole H2/L JP8 53.99 Methane (298K & 246 atm.) 23.43 Liquid hydrogen (20K & 1 atm.) 35.36

Table 1. Hydrogen content from conventional fuels on a volumetric basis

Certain users, such as the army are logistically restricted to

Jet Propellant 8 (JP8) middle distillate fuel. Reforming of middle distillate fuels such as JP8 has been a particular challenge for mobile power applications. High sulfur content in JP8 (up 3000ppm) provides some challenges as catalysts used in reforming (platinum, rhodium, and nickel) are quickly rendered inert when exposed to sulfur. Research has been focused on developing sulfur tolerant catalysts and sulfur removal, but these are still in their development stages[1,4]. Liquid phase desulfurization is particularly challenging since sulfur gets contained within the poly-aromatic compounds present in JP8[4]. The high carbon content of JP8 can also result in coke and soot formation, which blocks the active catalyst sites. Current catalytic efforts focus on developing catalysts that suppress the formation of coke or carbon formation[4,5]

Alternatively, operating conditions can be altered to reduce soot formation. This requires operating at higher oxygen to carbon ratio than ideal conditions (O/C=1.0), to subsequently reduce the reforming efficiency.

An alternative approach is to employ a non-catalytic reforming process, which negates issues arising from sulfur content and provides significant cost reduction. However, previous non-catalytic reactors have had issues with incomplete conversion and significant soot formation of up to 40%[6]. These reformers operate at lower reforming efficiency that ranges between 60-70%[6–8].

Proceedings of the ASME 2016 Power Conference POWER2016

June 26-30, 2016, Charlotte, North Carolina

POWER2016-59515

1 Copyright © 2016 by ASME

This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government’s contributions.

mailto:[email protected]

The distributed reaction regime has been shown to enhance reformate quality in non-catalytic reformers[9–11]. The distributed reaction regime suppresses soot formation, enhances reaction rates, and develops a uniform thermal field[9–11]. The distributed reaction regime is believed to promote both steam and dry reforming reactions to improve reformate quality though entraining heat, carbon dioxide, and steam into the premixed fuel air charge.

Steam is a known additive that can be used to enhance the reforming process[12]. Steam acts as an additional oxidizer to enhance the conversion of hydrocarbons, while simultaneously providing additional hydrogen content. In the context of reforming, the distributed reaction regime steam offers additional benefits. Steam will delay ignition, which allows for additional time for mixing and a more distributed reaction. In addition, steam enhances turbulent velocity and reduces laminar flame speed to promote distributed reaction conditions. Previous work on the impact of air preheats on partial oxidation in a well-insulated indicated higher reformate quality under enhanced distributed condition[11]. Steam also aids in heat transfer, which enhances uniformity of the thermal field. Steam has also been shown to further reduce soot formation in non-catalytic reactors [12].

However, steam reforming reactions are endothermic in nature, which limits the amounts of steam that can be added before degradation occurs. Steam acts as thermal diluent, reducing reactor temperature, which in turn hinders kinetics. Excess steam reduces reactor residence times, increasing hydrocarbon formation. These effects hinder the benefits of steam. This paper investigates the thermochemical effects of steam addition to distributed reaction regime. Emphasis is placed on understanding how steam can be used to further increase reformate quality and reformer efficiency. Numerical simulations were also conducted to determine flame regime.

EXPERIMENTAL DESIGN

The experimental design is of the same construction and design as previous works[11]. The reactor consisted of two major components, an outer steel pressure vessel and internal alumina insulation. The outer pressure vessel retained system pressure and provided a controlled environment. The internal insulation, in addition to retaining heat, protected the external pressure vessel. A high purity alumina insulation was employed to avoid reduction by reformate gases. The insulation was divided into four segments to relieve thermal stresses and extend reactor life. The reactor was encased in a tube furnace that replicates thermal conditions found in the hot box in a fuel cell system (T of about 800˚C). This results in near adiabatic heat transfer.

Reactor was configured to operate in reverse flow configuration. Arghode [13] showed that this configuration provides longer residence time of gases in the reactor. A premixed fuel air charge was injected through a single central injection point (ID=0.152”) at the base of the reactor. Injecting from the center of the cylindrical shaped reactor allowed equal

entrainment from all sides of the reactor. Reformate was exhausted through two outlets from the same plane as the injector. A spark igniter located opposite to the central injector location in the reactor was used to cause ignition in the reactor.

Reactants (fuel, air, and steam) were independently heated and mixed in a three bladed static mixer prior to injection into the reactor. Per manufactures, its mixing efficiency is greater than 99.99% under experimental conditions. Reactants were injected at 375˚C. Airflow rates (30.0±0.25slpm) were regulated by a mass flow controller and preheated using a potted air heater. JP8 (6.81g/min) and water (0-2.0 g/min) flow rates were regulated by prismatic pumps. Fuel and water streams were independently vaporized in a coiled tube vaporizers. Fuel and air were injected at fixed oxygen to carbon ratio of 1.08. Steam to carbon ratio was varied from 0.0 to 0.23. Heat tape and insulation were wrapped around the fuel/steam lines and mixer to prevent condensation of fuel and steam. Residence time of the mixer was estimated to be between 3.19 to 3.46 ms. Prior to injection the reactant temperature was recorded. Air preheats were used to control and maintain constant injection temperature.

After reformate was exhausted from the reactor, it was immediately cooled in a coiled loop heat exchanger, which prevented flaring of the exhaust. A liquid/gas separator was used to remove the condensed liquid and water vapors. The sample was passed through an additional condenser and 0.3-micron filter to protect the gas chromatograph from water vapor and particle contamination. Prior to exhaust, the reformate composition was analyzed for gas composition using a gas chromatograph. The remaining reformate was exhausted into a hood.

The hydrocarbon reformed here was jet propellant 8 (JP8). The one fuel forward mandates the Army to utilize JP8 as the primary fuel source. It is chemically similar to Jet A1 except for the following additives: antifungal, dielectric constant, and anti-corrosion[14]. JP8 has allowed sulfur content of up to 3000ppm. However characteristic fuel found in the continental United States tends to be much lower in sulfur content (below 200ppm). JP8 used in this experiment had a sulfur concentration of 30ppm. The fuel had a hydrogen composition of 14.4% by mass and a heating value of 43.6MJ/kg.

INSTRUMENTATION Gas chromatography was used to determine reformate’s

chemical composition generated in the experiment. A four-channel micro GC was employed, which could detect fixed gases and hydrocarbon up to C6. The GC was calibrated against three primary standards, each containing 10-14 hydrocarbons at different concentrations. Reformate was sampled every 3.2 minutes. Each data point presented herein represents the average of four or more samples. Reactor stabilized before results were recorded, as judged from reformate concentration and temperature profiles. Under wet partial oxidation conditions, the carbon balance was between 95.5-98.5%.


During dry partial oxidation, the carbon balance was only 86.44%.

REACTOR OPERATING CONDITION The reactor was operated at a thermal load of 5.1kWth. The

premixed mixture was injected into the reactor at 375˚C. Fuel and air were injected at a fixed O/C ratio of 1.05. The steam to carbon (S/C) ratio was increased until noticeable degradation of reformate quality occurred, which was determined from gas chromatography. The reactor was operated under dry partial oxidation (at S/C=0.0) and under wet partial oxidation (at S/C=0.01-0.23). For the reactor to operate under auto-thermal conditions, a higher steam to carbon (S/C) ratio of 1.0 to 2.0 would be required. Operating under these conditions, while common in catalytic reforming, could quench a non-catalytic reactor.

PREMIXED TURBULENT FLAME REGIME

The reaction regime was determined through numerical simulations, which were conducted in the same manner as Scenna and Gupta[10]. Here, numerical simulations were used to predict the development of the distributed flame regime in a quartz reactor. A brief summary of the approach is provided below, see Eq. 1-5. A Borghi plot presented in Fig. 1, presents the relevant flame regime for the various experimental conditions.

Flame regimes were determined from the Damkohler number (𝐷𝐷𝐷𝐷) and the turbulent Reynolds number (𝑅𝑅𝑒𝑒𝑜𝑜). The Damkohler number (𝐷𝐷𝐷𝐷) represents the ratio of the characteristic mixing time scale to characteristic chemical time scales. The turbulent Reynolds number, given by Glassman[15] and Law[16], is based on integral length scale(𝑙𝑙𝑜𝑜). Transport is primarily governed by turbulent mixing rather than diffusion in distributed reactor designs[17]. The Damkohler and turbulent Reynolds numbers are functions of integral length scale (𝑙𝑙𝑜𝑜), turbulent velocity (𝑢𝑢′), laminar flame speed (𝑆𝑆𝑙𝑙), and laminar flame thickness (𝛿𝛿). The distributed reaction regime occurs when the integral length scale is sufficiently shorter than the laminar flame thickness, which only occurs at Damkohler number significantly less than unity. This indicates that the transport is sufficiently faster than the chemistry, and the turbulent eddies are sufficiently small enough to reside within the flamelet itself.

Integral length scale and turbulent velocity are functions of volume averaged turbulent kinetic energy (𝑘𝑘) and turbulent energy dissipation (𝜀𝜀). These properties were calculated using a commercial CFD code, Fluent. Results were allowed to converge to 10-5. The mesh was composed of 511k elements. Laminar flame thickness (𝛿𝛿) and flame speed (𝑆𝑆𝑙𝑙) were determined through chemical analysis package Chemkin[18]. A JP8 surrogate was used which has previously been verified to predict flame speed[19]. A reduced mechanism kinetic mechanism was employed that consisted of 121 species and 2,673 reactions [20].

𝐷𝐷𝐷𝐷 = 𝜏𝜏𝑚𝑚𝑚𝑚𝑚𝑚𝜏𝜏𝑐𝑐ℎ𝑒𝑒𝑚𝑚

= �𝑙𝑙𝑜𝑜𝛿𝛿� �𝑆𝑆𝑙𝑙

𝑢𝑢′� (1)

𝑢𝑢′ = �23𝑘𝑘�

1/2 (2)

𝑙𝑙𝑜𝑜 =�23𝑘𝑘�

1.5

𝜀𝜀 (3)

𝑅𝑅𝑒𝑒𝑜𝑜 = 𝑢𝑢′𝑙𝑙𝑜𝑜𝑆𝑆𝑙𝑙𝛿𝛿

(4)

𝛿𝛿 = 2𝛼𝛼𝑆𝑆𝑙𝑙

(5)

The addition of steam reduced the laminar flame speed by 14%, but it increased volumetric flow rate, which in turn elevated turbulent velocity by approximately 9%. This resulted in a decrease in Damkohler number, but an increase in turbulent Reynolds number. Steam also tends to delay ignition, which resulted in an enhanced time for mixing.

Figure 1. Numerical simulations of flame regime under dry and wet partial oxidation conditions EXHAUST TEMPERATURE

With the addition of steam, there was a rapid decrease in exhaust temperature as compared to dry reforming condition, see Fig. 2. The decrease in exhaust temperatures indicates endothermic reactions to occur in the reactor.

At steam to carbon ratios exceeding 0.1, the exhaust temperatures began to move upwards with increase in steam content. The slight increase in temperature is attributed to the role of steam in assisting development of uniform thermal field in the reactor. It is thought that peak reactor temperature decreases with increase in steam content.

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er N

umbe

r(Da

)

Turbulent Reynolds Number (Reo)

Flamelets in Eddies Regime

Distributed Reaction Regime

Lo/δ=1

Wrinkled Flame Regime

Increasing S/C

Ratio

Lk/δ=1


Figure 2. Average exhaust temperature under dry and wet partial oxidation condition REFORMATE COMPOSITION

The reactor demonstrated high quality reformate over all the conditions examined here. The addition of steam showed noticeable improvement over the dry partial oxidation condition. Syngas composition consisted of 22.5-24.6% hydrogen and 20.1-23.3% carbon monoxide. These concentrations exceed the characteristic syngas composition generated from non-catalytic reforming (only about 13% hydrogen and 19% carbon monoxide) or catalytic reforming (24% hydrogen and 24% carbon monoxide). The high concentrations of carbon monoxide and low carbon dioxide indicated complete mixing.

Steam was found to have a strong positive effect on reformate quality. Even with trace amounts of steam (at S/C=0.01), the reformer demonstrated a drastic increase in both hydrogen and carbon monoxide concentrations, see Fig 3. Hydrogen concentrations reached a maximum at steam to carbon ratio of 0.11, after which it degraded with increase in steam content. This degradation was expected due to the combined effects of quenching and reduced residence times. Higher steam to carbon ratios increased carbon monoxide concentrations but decreased carbon dioxide concentrations. Operating at a low steam to carbon ratio increases sustainability in a mobile power system (e.g., from water recovery from fuel cell) and reduced costs. Lower water usage reduces both storage and water recovery, which subsequently reduces system weight and cost.

Steam was also found to impede the formation of lower molecular weight hydrocarbons. Of the hydrocarbons detected, only methane was seen in finite amounts (only 0.17-0.29%). Acetylene and ethylene were only detected in trace amounts (0.01-0.04%), see Fig. 4. Hydrocarbon formation occurred at conditions involving lower steam content (S/C=0.0-0.01). Trace amounts of steam(S/C=0.01) resulted in a small initial increase

in lower molecular weight hydrocarbons over dry partial oxidation conditions. It is conjectured to be from pulsation in the pump that results in fluctuation in steam content. At higher steam to carbon ratios (S/C>0.01), steam was found to impede hydrocarbon formation. Hydrocarbons greater than ethylene were not detected.

Figure 3. H2, CO, and CO2 concentration under dry and wet partial oxidation conditions

Figure 4. Lower molecular weight hydrocarbons concentrations under dry and wet partial oxidation conditions REFORMING EFFICENCY AND FUEL CONVERSION

Reforming efficiency and fuel conversion represents a direct measure of reformate quality. Fuel conversion is defined as carbon conversion to carbon monoxide and carbon dioxide over the initial carbon content of the fuel, as defined by Eq. 6.

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AcetyleneEtheyleneMethane


The formation of lower molecular weight hydrocarbons and soot will result in less than 100% conversion. 𝜂𝜂𝑐𝑐𝑜𝑜𝑐𝑐𝑐𝑐 = 𝐶𝐶𝐶𝐶+𝐶𝐶𝐶𝐶2

𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑜𝑜𝑐𝑐 𝑐𝑐𝑜𝑜𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑓𝑓𝑢𝑢𝑐𝑐𝑙𝑙 (6)

Reforming efficiency is a measure of the energy retained in

reformate after the reforming process, see Eq. 7. At oxygen to carbon ratio of one and assuming only carbon is oxidized, the maximum reforming efficiency was found to be 85.6%. 𝜂𝜂𝐶𝐶𝑐𝑐𝑓𝑓𝑓𝑓 = 𝐿𝐿𝐿𝐿𝐿𝐿(𝐿𝐿2+𝐶𝐶𝐶𝐶)

𝐿𝐿𝐿𝐿𝐿𝐿(𝐹𝐹𝑢𝑢𝑐𝑐𝑙𝑙) (7)

Reforming efficiency is a function of the hydrogen and

carbon monoxide yields. Even with trace amounts of steam, noticeable impacts on the reforming reformate quality was observed, see Fig. 5. When the reactor operated under dry partial oxidation, it demonstrated a reforming efficiency of 67.74% and fuel conversion of 85.18%. This indicates that soot deposition occurred in the exhaust line under dry partial oxidation conditions. While the distributed reaction regime suppresses soot in the reactor, reformate exhausting through exhaust line may not be under the distributed reaction regime resulting in the formation of soot.

Figure 5. Reforming efficiency under dry and wet partial oxidation conditions

Under wet partial oxidation conditions, reformer performance was drastically improved. Figures 5 and 6 show rapid increases in both reforming efficiency and fuel conversion even with trace amounts of steam (S/C=0.01). An increase in steam to carbon ratio increased fuel conversion, which reached a maximum at steam to carbon ratio of 0.23. At a steam to carbon ratio of 0.17, the reactor reached peak efficiency (𝜂𝜂𝐶𝐶𝑐𝑐𝑓𝑓𝑓𝑓 = 80.35%), with negligible change at higher steam to

carbon ratios (𝜂𝜂𝐶𝐶𝑐𝑐𝑓𝑓𝑓𝑓 =80.15%). Peak reactor efficiency occurred in between peak hydrogen amounts (at S/C=0.11) and carbon peak monoxide amounts (at S/C=0.23). This is expected, as reforming efficiency is the product of hydrogen and carbon monoxide yields. The addition of steam can reduce deposition in the exhaust line.

Figure 6. Conversion under dry and wet partial oxidation conditions

EFFECTS OF STEAM

It was originally conjectured that steam would enhance both the steam reforming (R1) and water gas shift (R2) reactions. The corresponding increase in both carbon monoxide and hydrogen concentrations, along with an overall reduction in hydrocarbons and carbon dioxide concentrations, indicate enhanced steam reforming reactions (R1). With the steam addition, exhaust temperatures were lower than in the dry partial oxidation case. This indicates the presence of endothermic steam reforming reactions.

The addition of steam did not enhance the activity of the water gas shift reaction (R2). An active water gas shift reaction would have increased carbon dioxide emission with increase in steam content. Instead, carbon dioxide concentrations decreased, which indicates that additional steam content did not promote water gas shift reaction.

𝐶𝐶𝑥𝑥𝐻𝐻𝑦𝑦 + 𝑥𝑥𝐻𝐻2𝑂𝑂 → 𝑥𝑥𝐶𝐶𝑂𝑂 + �𝑦𝑦

2+ 𝑥𝑥� 𝐻𝐻2 (R1)

𝐶𝐶𝑂𝑂 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶𝑂𝑂2 + 𝐻𝐻2 (R2)

The increased reforming efficiency, fuel conversion, and hydrogen concentrations cannot be accounted for by only steam reforming reactions. At steam to carbon ratio of 0.01, fuel conversion increased by ~9%, but oxidizer content increased by ~1%. This indicates there must be an additional method of enhancement beyond that from steam reforming reactions.

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This discrepancy is attributed to steam causing a more distributed reaction, to promote enhanced conversion and reformate quality. Previous work on the impact of air preheats on partial oxidation in a well-insulated distributed reactor showed higher reformate quality under more distributed conditions[11]. Numerical simulations shown in the Borghi plot in Fig. 1 demonstrate that steam caused reactions to occur under more distributed conditions. The addition of steam to a premixed fuel/air mixture caused an increase in ignition delay[21], which allowed greater mixing of premixed charge with the reactive exhaust species present inside the reactor. This allows for a more distributed reaction than would be possible without steam.

CONCLUSIONS This paper investigates the thermochemical behavior of

steam addition to a distributed reaction regime under reforming conditions. The reactor was operated under both dry and wet partial oxidation. Numerical simulations were used to determine flame regime. Steam improved reformate quality even with trace amounts of steam addition (S/C=0.1). Reformate quality achieved was comparable to catalytic reformers.

Reformate quality improvement with steam addition is attributed to two primary effects. Steam acts as an additional oxidizer to enhance the conversion of hydrocarbons, through steam reforming reactions. This is supported by increased conversion, reforming efficiency, hydrogen concentrations, and decreased lower molecular weight hydrocarbons. This is further supported by decreased reactor temperatures indicative of endothermic reactions. However, the increased performance in terms of reforming efficiency, conversion and hydrogen concentration exceeded what can be ascribed to steam reforming reactions alone.

The addition of steam promoted a more distributed conditions within the reactor. Previous work on air preheats has shown that reformate quality improves with enhanced distribution[11]. Literature indicates that steam will delay ignition [21], which promotes a more distributed reaction regime. This is supported by numerical simulations, which also show the reactor shifting to a more distributed reaction regime.

Steam enhanced the activity of steam reforming reactions, however the water gas shift reaction was unaffected. This was inferred from the decrease in carbon dioxide concentrations, since active water gas shift must enhance carbon dioxide concentration.

The amount of steam added to the reactor must be limited, since excess steam can degrade reformate quality. Reaction kinetics are hindered by lower reactor temperatures, which resulted from steam promoting endothermic reactions (steam reforming) and acting as thermal diluent. Steam amount can also reduce residence time to limit the reactor capacity.

Syngas obtained had 22.5-24.6% hydrogen and 20.1-23.3% carbon monoxide. These concentrations are more representative of reformate produced in a catalytic reforming than a non-catalytic reformer. Methane was the only hydrocarbon detected

in finite amounts (0.17-0.29%). Ethylene and acetylene detected were only in trace amounts (0.01-0.04%). The reformate composition was analyzed for fixed gases and hydrocarbons up to C6.

Peak efficiency was achieved at low steam to carbon ratios of S/C=0.17. The reactor demonstrated reforming efficiency of ~80%, which is superior to previously reported results with non-catalytic reforming. The reformate quality obtained was more comparable to catalytic reforming. Operating at a low steam to carbon ratio offers increased sustainability in mobile power systems, where water recovery from fuel cells can be utilized to reduce costs.

NOMENCLATURE 𝐷𝐷𝐷𝐷 Damkohler number Lk Kolmogorov length scale 𝑙𝑙𝑜𝑜 Integral length scale O/C Oxygen to carbon ratio 𝑅𝑅𝑒𝑒𝑜𝑜 Turbulent Reynold number 𝑆𝑆𝑙𝑙 Laminar flame speed S/C Steam to carbon ratio u' Turbulent velocity α Thermal diffusivity δ Laminar flame thickness 𝑘𝑘 Turbulent kinetic energy 𝜀𝜀 Turbulent energy dissipation 𝜂𝜂𝑐𝑐𝑜𝑜𝑐𝑐𝑐𝑐 Fuel conversion 𝜂𝜂𝐶𝐶𝑐𝑐𝑓𝑓𝑓𝑓 Reforming efficiency

ACKNOWLEDGMENTS The authors acknowledge the support of United States

Army’s Research Development Engineering Command Independent Laboratory Innovative Research program. The authors also gratefully acknowledge the help and support provided by Reaction Design for the Chemkin code used here.

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wet partial oxidation of jp8 in a well-insulated reactor

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