Coke Removal in Fuel-Cooled Thermal Management Systems

Ind. Eng. Chem. Res. 2005, 44, 267-278

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Coke Removal in Fuel-Cooled Thermal Management Systems He Huang* and Louis J. Spadaccini United Technologies Research Center, MS 129-29, 411 Silver Lane, East Hartford, Connecticut 06108

In hydrocarbon fuel cooling technology, the coke deposits, which may form in heat exchangers and reactors and on the inside surfaces of fuel system components, degrade heat-transfer, catalyst activity, and fuel-flow characteristics and can lead to system failure. Therefore, in situ regeneration of fouled surfaces was investigated as a practical approach for reducing the impact of coke formation on aircraft thermal management systems. Various surface regeneration techniques, such as carbon burnoff in air or oxygen and carbon gasification using CO2 or steam, were investigated. The most practical technique for in situ surface regeneration of the heat exchangers is the carbon burnoff method. Although the burnoff method is simple and cost-effective, care must be taken to control strong exothermic reactions. For this reason, a kinetic model has been developed and its successful application to regenerate a fouled multiple-channel heat-exchanger/reactor panel from a scramjet test engine is discussed in the paper. Introduction As aeropropulsion technologies advance into the 21st century, increasingly severe demands are being placed on engine power, fuel economy, and pollutant-emissions control. Furthermore, increasing the aircraft speed and engine thrust-to-weight ratio results in large simultaneous increases in the heat load and temperature of the air available for cooling, thereby shifting the burden for engine and vehicle cooling to the fuel. A promising approach to cooling advanced engines is the application of high heat sink fuel cooling technology. Utilization of this technology, however, requires heating of the fuel well above the maximum temperature (∼325 °F or 163 °C) allowable in conventional fuel systems and in some applications to supercritical temperatures, i.e., above 700 °F (371 °C). This heating is done in a heat exchanger or, for high heat sink applications, a catalytic heat exchanger/reactor (CHER).1,2 Degradation of hydrocarbon fuel at the elevated temperatures leads to coke deposition and fouling of the fuel passages. The rate of coke deposit buildup with increasing temperature for jet fuels is discussed extensively in the literature.3 In the temperature regime below approximately 800 °F (427 °C), oxygen dissolved in fuel is the major contributor to thermal decomposition and coke deposition. At temperatures above 900 °F (482 °C), the deposition mechanism is characterized by pyrolysis that is initiated by thermal (homogeneous) and/or catalytic (heterogeneous) cracking reactions and followed by polymerization. The coke deposits that form in the heat exchangers and reactors and on the fuel system component walls degrade heat-transfer and fuel-flow characteristics and can lead to system failure. Coke deposits also cause degradation of the catalyst on reactor surfaces. The extent to which the benefits of hydrocarbon fuel cooling technology can be realized depends on our ability to manage coke deposits. In gas-turbine applications, where duty cycles are thousands of hours, surface regeneration will be required despite the reduction in coking rates achievable through the other approaches.4 For regeneration to be practical, it must be done in situ (i.e., by a ground maintenance procedure without disassembling components from fuel systems). United

Technologies Research Center (UTRC) has performed catalyst regeneration by carbon burnoff with the reactor installed in the test apparatus (simulating in situ maintenance) and without affecting the surface structure or chemical composition. In previous work, UTRC has also shown that different elements (e.g., carbon and sulfur) are burned off at different temperatures and that further development of the regeneration process is required.5 It is important to note that there have been many books, papers, and patents on coke deposition research as well as coke removal and surface regeneration from catalyst surfaces.6-11 The objective of this study is to investigate and assess the practicality of various in situ surface regeneration techniques for realistic passage sizes and to develop a safe, simple, and cost-effective in situ surface regeneration technique for hydrocarbon fuel-cooled thermal management systems. Thermogravimetric analysis (TGA) was adopted in this study for quickly evaluating various coke-removal processes and compiling a database for kinetic analysis. A comprehensive kinetic model for the most practical technique identified in this study for onwing surface regeneration of heat exchangers and reactors was developed. A surface regeneration simulator was constructed and used to investigate the effectiveness of the in-situ regeneration techniques and to validate the kinetic model. A computer simulation was developed and successfully applied to select and/or optimize the key operating variables, and a real application, namely, regenerating a multichannel catalytic CHER panel, is discussed in this paper. Surface Regeneration Processes On the basis of consideration of the deposit thermochemistry and the thermal management system operation, three methods for on-wing surface regeneration of aircraft heat exchangers/catalytic reactors were identified and evaluated. Carbon Burnoff Method. The carbon burnoff method involves using heated air or oxygen to remove coke deposits from the aircraft heat exchangers/catalytic reactors by the following reactions:

10.1021/ie0401760 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004

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Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005

2C(solid) + O2 f 2CO and/or

C(solid) + O2 f CO2

Although the burnoff method is simple and cost-effective, several restrictions and additional procedures may be required for implementation because of the exothermic nature of the reactions. For example, any residual fuel in the heat exchangers/catalytic reactors must be purged out using an inert gas (e.g., nitrogen) before flowing hot air or oxygen over the surfaces. Carrying out the procedure also requires the ability to control the reaction rates to avoid runaway and to suppress sharp temperature increases. The burnoff method is probably not suitable for in-flight regeneration because of safety considerations. Carbon Gasification Method. The carbon gasification method uses either carbon dioxide or steam to remove coke deposits on the surfaces by the following reactions:

C(solid) + CO2 f 2CO or

C(solid) + H2O f CO + H2

These gasification methods are also simple and costeffective. In addition, they may be applicable for on-wing and in-flight regeneration because of the endothermic nature of the reactions. However, very low reaction rates may limit their implementation. Chemical Cleaning Method. Chemical cleaning methods have been developed for removing coke deposits from aircraft engines using potassium permanganate, a strong caustic oxidizer. The procedure consists of (1) degreasing, (2) caustic potassium permanganate cleaning, (3) acid cleaning, (4) checking for proper cleaning, and (5) drying. In operation 2, all cokedeposited surfaces must be exposed to a caustic potassium permanganate solution at 190 °F (88 °C) for up to 24 h. Operation 4 requires that the part be free of any residual coke deposit, caustic potassium permanganate solution, and acid. These operations are very difficult to implement for on-wing maintenance and might generate hazardous wastes. In addition, a caustic potassium permanganate solution and hydrochloric acid may contaminate and/or degrade catalysts coated on the CHER surfaces. Therefore, chemical cleaning was judged to be inappropriate for this application. Thermogravimetric Analysis TGA is an instrumentation technique that measures the change in mass of a sample as a function of temperature and/or time. It has been widely used as a convenient, sensitive, rapid, and accurate method for studying many physical and chemical processes that involve a mass change of a condensed phase (liquid or solid).12-14 The TGA technique was adopted in this study of surface regeneration to evaluate various coke-removal processes and compile a database for kinetic analysis. A schematic diagram of the TA Instruments (New Castle, DE) TGA 2950 thermogravimetric analyzer used in this study is shown in Figure 1. It features a vertical balance/horizontal purge design, high sensitivity (0.1 µg), and an ambient to 1832 °F (1000 °C) temperature range. During a TGA scan, the weight of a sample is continuously recorded as the temperature is raised at

Figure 1. Thermogravimetric analyzer.

a controlled rate (e.g., heating rate) and the primary data recorded by a computer are time, temperature, and weight. The secondary variable, such as the rate of weight change, can be numerically computed by a differential of the weight change against time. TGA may be carried out in a reactive environment such as air or under an inert atmosphere such as nitrogen. To simulate the carbon deposits on the CHER surfaces, coke samples were prepared from a well-characterized JP-8 fuel provided by Air Force Research Laboratory, Dayton, OH. Pyrolytic coking tests were carried out at approximately 1472 °F (800 °C) and 814 psia (55 atm) for 10 min in tubing bomb reactors. The coke samples were collected from the reactors, rinsed in hexane, dried in a vacuum oven at 176 °F (80 °C) for 24 h, and labeled as “jet fuel coke”. Coke deposits from hydrocarbon fuels normally have a complex structure containing carbon, oxygen, sulfur, nitrogen, and hydrogen as well as a trace amount of inorganic residue (i.e., ash). Although the coke deposit composition primarily depends on raw fuel composition/source and processing conditions, the pyrolytic coke sample prepared by the procedure used in this study is representative of the coke deposits that would be found in the fuel-cooled heat exchangers. There are three types of coke deposits: filamentous, amorphous, and graphitic. Their relative rates of formation strongly depend on the temperature.15 The coke sample prepared under this temperature (1472 °F or 800 °C) contains only filamentous and amorphous carbons (i.e., no graphitic carbon). The same carbon types (though in different proportions) are found in lower temperature aircraft engine heat exchangers.3 In each TGA scan, about 10 mg of the jet fuel coke sample was loaded in a quartz pan and then heated to a predetermined temperature (up to 1832 °F or 1000 °C) at a rate of either 3.6 or 18 °F/min (2 or 10 °C/min). For complete coke removal, some tests were maintained at the final temperature for up to 2 h. Coke-removal processes were studied in nitrogen, air, oxygen, and steam/nitrogen using TGA. Gas flow rates were 100 mL/ min at STP. The steam/nitrogen gas mixture was prepared by bubbling the nitrogen through a hot water reservoir. The water temperature controls the partial pressure of the steam. The transfer line between the water reservoir and the TGA sample chamber is heated to prevent steam condensation. All tests were duplicated, and the results were highly reproducible. Coke-Removal Process Evaluation The coke removal determined by TGA in different gaseous atmospheres (viz., nitrogen, air, oxygen, carbon dioxide, and steam/nitrogen) as a function of temperature is shown in Figure 2. (Note that the sample weight may actually increase prior to combustion in air or

Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 269

Figure 2. TGA of the jet fuel coke in different gases.

Figure 3. Comparison of the reactivity of coke and pyrolyzed coke in air.

oxygen as a result of oxygen adsorption.) The samples were heated at rates of either 3.6 or 18 °F/min (2 or 10 °C/min). The partial pressure of the steam in the steam/ nitrogen gas mixture was 4.1 psia (0.28 atm), determined by the partial pressure of water vapor at 154.4 °F (68.0 °C), the water reservoir temperature. As shown in the enlarged view of Figure 2, even in a nitrogen atmosphere, the weight of the jet fuel coke decreased as the temperature increased. (These results are consistent with those observed by Russian scientists.16) However, the data are not sufficient to ascertain whether the sample weight reduction in nitrogen resulted from carbon removal or the loss of other species (e.g., H2/CO/

CO2) in pyrolysis reactions that increase the C/H and/ or C/O ratios. Therefore, two additional TGA scans were performed at a lower heating rate (3.6 °F/min or 2 °C/ min): one on the original coke sample and the other on a sample of pyrolyzed coke (prepared by heating the jet fuel coke in a nitrogen atmosphere). As shown in Figure 3, the reactivity of the original jet fuel coke in air is significantly higher than that of the pyrolyzed coke. This result supports the premise that heating coke in a nitrogen atmosphere drives off volatile gases, creating heavier residues that are more difficult to remove. Consequently, it is recommended that the CHER not

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Figure 4. TGA of the jet fuel coke in CO2.

Figure 5. TGA of the jet fuel coke in steam.

be heated to high temperatures in a nitrogen (or other inert) atmosphere before regenerating. Coke gasification and removal profiles in heated carbon dioxide and steam are presented in Figures 4 and 5, respectively. With carbon dioxide, the jet fuel coke was heated to 1832 °F (1000 °C), the highest operating temperature allowed by the thermogravimetric analyzer, at a heating rate of 3.6 °F/min (2 °C). The test was then continued isothermally at 1832 °F (1000 °C) for more than 60 min for complete coke removal. With steam, the jet fuel coke was heated to 1112 °F (600 °C) at a heating rate of 36 °F/min (20 °C/min) and then to 1832 °F (1000 °C) at 18 °F/min (10 °C/min). (The faster heating rate was used in the low-temperature range to reduce the test time.) Finally, the test was continued isothermally at 1832 °F (1000 °C) for more than 100 min for complete coke removal. Compared to the oxygen and air burnoff processes, gasification of carbon using heated carbon dioxide or steam requires

much higher temperature and longer time for complete coke removal. The initial small reductions in the deposit weight shown in Figures 4 and 5 for carbon dioxide (4.0 0.5 0.2 3.2 0.3 0.1 3.6 0.4 0.1 1.4 0.2

Complete coke removal.

tube, shown in Figure 13, illustrates the relative mass of coke deposited along the reactor tube. These deposits primarily represent pyrolytic coke formation because the test was conducted under high temperatures. The coke-removal experiments were then conducted using oxygen and air at elevated pressures. The results are summarized in Table 3. The most important finding is that the coke-removal rates are controlled by the partial pressure of oxygen, irrespective of whether air or oxygen is used. Reaction rates increase with increasing oxygen partial pressure, consistent with the previous observations in the TGA tests (see Figures 7 and 8). Besides, the surface regeneration undergoes the same reaction mechanism (i.e., CO formation mechanism) whether carried out in air or oxygen. As discussed in the previous section, coke deposits can be removed by gasification reactions with carbon dioxide and steam. The reactions are strongly endothermic (about 5000 Btu/lb‚m or 140 kJ/mol for steam gasification and 6000 Btu/lb‚m or 170 kJ/mol for carbon dioxide gasification) and thermodynamically feasible above 1000 °F (538 °C). Thus, they may provide an in situ method


Figure 15. Multichannel hydrocarbon fuel-cooled heat-exchanger panel.

Figure 14. Map of regeneration conditions in the air burnoff method: 99% coke-removal efficiency as a function of the temperature, pressure, and time.

for regenerating a CHER using the engine exhaust gas. For this reason, the feasibility of using high-temperature, high-pressure steam and carbon dioxide to remove carbon deposits was investigated. A test was first conducted at a steam pressure of 615 psia (41.8 atm) and a temperature range of 1000-1400 °F (538-760 °C) for 4 h. The results indicated that less than 20% of the coke deposit was removed in the whole operation, and the coke that remained seemed to become much harder to remove. Then, a test using 585 psia (39.8 atm) carbon dioxide was carried out in the temperature range of 800-1400 °F (427-760 °C) for more than 1 h. The results indicate that, based on the reactivity, there are at least two types of coke deposits, distinguished by the peaks in the CO formation. About 10% of the coke deposit was removed from the reactor at a wall temperature of 900 °F (482 °C) in less than 20 min. Removal of the remainder of the coke deposit required much higher temperature (∼1400 °F or 760

Figure 16. Liquid-crystal paint tests.

°C). Even at this high temperature, a long time was required for complete coke removal. In conclusion, because of the higher temperature and longer time requirements, neither steam nor carbon dioxide appears to be practical for surface regeneration. However, there

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and time) are correlated using the kinetic model developed in this study, and the results are mapped in Figure 14a. For these simulations, the coke-removal efficiency was set at 99% and it was assumed that air could be flowed freely through the passages. The correlation for a narrower range of practical temperatures and pressures is shown in Figure 14b. To strike a balance between the allowable surface temperature rise and the burnoff rate in a regeneration process, it is recommended that the procedure be carried out under high air pressure to permit lower temperature operation. CHER Panel Regeneration

Figure 17. Gas flow/pressure measurements.

may be catalysts that can be applied to the surface to enhance the carbon gasification rates. Investigation of these catalysts is desirable but beyond the scope of the current program. A rigorous numerical validation of the kinetic model is very difficult because of nonuniform coke deposit and temperature distributions along the reactor tube. However, when the regeneration times for a limited number of nearly isothermal tests are compared with model predictions, using the average carbon deposition, the results agree to within 15%. Surface Regeneration Simulation The most practical method for on-wing surface regeneration of heat exchangers and reactors is to use air to burn off the coke deposits. For a specific case, such as a CHER with a specified maximum allowable temperature, computer simulations may be required to guide the selection of regeneration conditions. The primary operating variables (i.e., temperature, pressure,

Figure 18. CHER panel regeneration.

As part of the Hypersonic Scramjet Engine Technology (HySET) program (AF Contract F33615-96-C-2694), a 6 in. × 15 in. fuel-cooled multichannel CHER panel (see Figure 15) was constructed and tested in a scramjet combustor. Although this panel was designed for missile applications where the engine is not exposed to multiple cycles, the panel was exposed to multiple hot combustion runs in the test facility. After several high-temperature tests (Tfuel exit > 1250 °F or 677 °C) with jet fuel, the panel was examined for coke deposition by two methods, i.e., liquid-crystal paint tests and gas flow/pressure measurements. In the liquid-crystal paint test, the CHER panel surface was first painted with a thermotropic liquid-crystal film that changes colors with changing temperatures. The hot fluid was then flowed through the multiple channels. The color distribution on the CHER panel surface was used to determine if there was any blockage and/or flow restriction in the multiple channels. Both test results are shown in Figures 16 and 17, indicating that coke deposits formed in the fuel passages and blocked one or two channels. It is believed that these coke deposits were generated during the posttest purging and cooldown of the panel. Following the successful completion of the test program, it was decided to evaluate our ability to remove coke deposits from the panel. The motivation for this action was to assess the coke-removal process for future potential reusable applications of this technology. For


this purpose, an air burnoff regeneration procedure was designed based on the computer simulation described in the previous section. To minimize a sharp temperature increase and avoid uneven thermal stress on the panel at high concentrations of coke deposit, a 350-psig air pressure and a stepwise temperature-increase technique were utilized. Four thermocouples were mounted on the surfaces of the panel to monitor the temperatures from the inlet to outlet during the regeneration. The panel was first heated to about 140 °F (60 °C) using strip heaters attached to the surface and purged with nitrogen for about 1 h to remove any residue fuel that remained in the panel. Then 350 psig of air at a flow rate of 4.4 lb/h (2 kg/h) was preheated and introduced to start the surface regeneration. The temperature profiles during the regeneration are shown in Figure 18a. The temporal profiles of the reaction product gas concentration (CO + CO2) and the cumulative carbon removal are presented in Figure 18b. To confirm that the coke deposits were completely removed, both checking methods (e.g. liquid-crystal paint tests and gas flow/pressure measurements) were conducted again after the regeneration. The liquidcrystal paint tests after the regeneration positively showed that all blocked channels had been cleared (see Figure 16). The gas flow/pressure measurements of the panel before and after regeneration also indicated that complete removal of the coke deposits was achieved (see Figure 17). Concluding Remarks The study conducted in this program addresses a practical approach for reducing the impact of coke formation on aircraft thermal management systems, viz., in situ regeneration of fouled surfaces. Key results and conclusions regarding the effectiveness of this approach and procedures required for implementation are summarized below. (i) Three surface regeneration processes, viz., carbon burnoff, carbon gasification, and chemical cleaning, have been evaluated. The most practical technique for onwing surface regeneration of heat exchangers and reactors is the carbon burnoff method. Although the carbon gasification method is attractive because of its endothermic nature, very low reaction rates limit its application without addition of a gasification catalyst. Chemical cleaning is complicated, costly, and timeconsuming. (ii) TGA was shown to be a valuable tool for evaluating various coke-removal processes and acquiring a database for kinetic analysis. Two characteristic parameters, viz., the initial coke-removal temperature and the peak-removal temperature, determined from the TGA test provide quantitative descriptions of the cokeremoval processes. (iii) An Arrhenius form kinetic correlating expression has been validated experimentally for the coke-removal processes in different gas atmospheres (viz., air, oxygen, carbon dioxide, and steam). The apparent activation energies for burnoff (air or oxygen), gasification in carbon dioxide, and gasification in steam are 185, 220, and 128 kJ/mol, respectively. The coke-removal rates in air or oxygen are controlled by the partial pressure of oxygen, irrespective of whether air or oxygen is used. (iv) Regeneration conditions (i.e., temperature, pressure, and time) can be determined using a computer simulation developed herein based on the kinetic model

developed in this program. In general, to strike a balance between the allowable surface temperature rise and the burnoff rate in a regeneration process, the procedure should be carried out under high air pressure to permit lower temperature operation. A real application of regenerating a multichannel CHER panel was successfully accomplished using this approach. (v) Heating the coke deposit in an inert atmosphere (e.g., nitrogen) drives off volatile gases, creating heavier residues that are more difficult to remove. Thus, purging with very hot (e.g., >400 °F or >204 °C) nitrogen should be avoided. Acknowledgment This paper is based on work performed by the Air Force Research Laboratory, Aero Propulsion and Power Directorate, under Contract F33615-97-D-2784, administered by Dr. Tim Edwards. The authors gratefully acknowledge the technical contributions of the following individuals: J. C. Hawkins Jr. for reactor testing and Dr. D. R. Sobel for discussions regarding coke deposits and surface regeneration. The support of the CHER regeneration by the Hypersonic Scramjet Engine Technology (HySET) program under Contract F33615-96-C2694 is also acknowledged. Literature Cited (1) Huang, H.; Spadaccini, L. J.; Sobel, D. R. Fuel-Cooled Thermal Management for Advanced Aero Engines. J. Eng. Gas Turbines Power 2004, 126, 284. (2) Huang, H.; Sobel, D. R.; Spadaccini, L. J. Endothermic HeatSink of Hydrocarbon Fuels for Scramjet Cooling. 38th AIAA/ ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Indianapolis, IN, July 2002; AIAA Paper 2002-3871. (3) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Publication Code Number 21-001092-12; ASTM: West Conshohocken, PA, 1991. (4) Spadaccini, L. J.; Sobel, D. R.; Huang, H. Coke Formation and Mitigation in Aircraft Fuels. J. Eng. Gas Turbines Power 2001, 123, 741. (5) Spadaccini, L. J.; Szetela, E. J. Approaches to the Prevaporized-Premixed Combustor Concept for Gas Turbines. 20th International Gas Turbine Institute Conference, Houston, TX, Mar 1975; ASME Paper 75-GT-85. (6) Sacco, A., Jr.; Caulmare, J. C. Growth and initiation mechanism of filamentous coke. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202;American Chemical Society: Washington, DC, 1982; p 177. (7) Ngomo, H. M.; Susu, A. A. Investigation of Prolonged Deactivation-Regeneration Regimes on the Dehydrogenation Activity of Platinum/Alumina Reforming Catalyst. Pet. Sci. Technol. 2001, 19, 283. (8) Camacho, L. A.; Park, C.; Rodriguez, N. M. Novel Regeneration Method for Deactivated Noble Metal Catalysts. Stud. Surf. Sci. Catal. 1997, 111, 665. (9) Figueiredo, J. L. Carbon Deposition Leading to Filament Growth on Metals. Mater. Corros. 1998, 49, 373. (10) Bibby, D. M.; McLellan, G. D.; Howe, R. F. Effects of Coke Formation and Removal on the Acidity of ZSM-5. Stud. Surf. Sci. Catal. 1987, 34, 651. (11) Lomas, D. A.; Thompson, G. J. Fluid catalyst regeneration. U.S. Patent 4,353,812, 1982. (12) Huang, H.; Calkins, W. H.; Klein, M. T. Use of a Novel Short Contact Time Batch Reactor and Thermogravimetric Analysis to Follow the Conversion of Coal-Derived Resids during Hydroprocessing. Ind. Eng. Chem. Res. 1994, 33, 2272. (13) Huang, H.; Wang, K.; Wang, S.; Klein, M. T.; Calkins, W. H. A Novel Method for the Determination of the Boiling Range of Liquid Fuels by Thermogravimetric Analysis. Am. Chem. Soc., Div. Fuel Chem. 1995, 40, 485.

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(14) Huang, H.; Wang, K.; Wang, S.; Klein, M. T.; Calkins, W. H. Applications of the Thermogravimetric Analysis in the Study of Fossil Fuels. Prepr. Pap., Am. Chem. Soc., Div. Fuel Chem. 1996, 41, 1. (15) Baker, R. T. K.; Yates, D. J. C.; Dumesic, J. A. Filamentous carbon formation over iron surfaces. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; p 1.

(16) Yanovski, L. S. Central Institute of Aviation Motors, Moscow, Russia, personal communication, 1998.

Received for review June 14, 2004 Revised manuscript received September 27, 2004 Accepted October 29, 2004 IE0401760

Coke Removal in Fuel-Cooled Thermal Management Systems

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