연료전지 자동차용 가솔린 연료 개질기의 개발

: 상용 HT 촉매상에서 가솔린의 부분산화 개질반응

류종우, 문동주, K. Sreekumar, 우주영, 이상득, 안병성

CFC 대체연구센터, 한국과학기술연구원

Development of Gasoline Fuel Reformer for Fuel-Cell Powered Vehicles

: Partial Oxidation Reforming of Gasoline over a commercial HT catalyst

Jong Woo Ryu, Dong Ju Moon, K. Sreekumar, Joo Young Woo,

Sang Deuk Lee and Byoung Sung Ahn

CFC Alternatives Research Center, Korea Institute of Science & Technology

Introduction

Catalytic partial oxidation (POX) of hydrocarbons for generating hydrogen has gained much attraction in recent years owing to its direct application in proton exchanged membrane (PEM) fuel cells for generating clean electrical energy.  The successful development of fuel-cell powered vehicle is depending on the development of fuel processor.  Hydrogen is the ideal fuel for a PEM fuel cell because it simplifies the system integration [1]. However, a direct hydrogen system has many disadvantages such as the lack of widespread refueling infrastructure and high storage volume compared to the hydrocarbon based fuels [2-3].  A reformer based vehicle system has the advantage of using the existing hydrocarbon fuels that are more readily available.   There is growing interest in the development of gasoline reformers, as this would allow fuel-cell vehicles to fit into the existing fuel infrastructure instantly [4-5].  Considering the favorable factors required for the on-board generation of H2 in a fuel cell vehicle, POX reforming (or autothermal reforming, ATR) attracts much attention primarily because of a low energy requirement [6].

As a part of development of a gasoline fuel processor system for integration with PEM fuel cell, we have carried out the POX reforming of iso-octane over the commercial reforming catalyst obtained from Haldor Topsoe (HT). Various aspects of the POX reforming of gasoline for fuel-cell powered applications has already been reported by the authors [7-8]. iso-Octane is the principal component in gasoline and, therefore, the results obtained over the iso-octane feed can be extended to study a gasoline fuel processor system. Especially, the effects of reaction temperature, oxygen/carbon (O/C) molar ratio and water/carbon (H2O/C) molar ratio were investigated to minimize deactivation of the catalyst caused by coke deposition.  Since the key issue to be addressed in a commercial reforming system is the sulfur tolerance of the reforming catalysts, we have investigated the extent of sulfur tolerance of the HT catalyst using a standard iso-octane feed containing 100 ppm of S.

Experimental

1.  Catalyst

The commercial methane reforming catalyst (HP-R-67-7H, HT) (with surface area 24.9 m2/g) was supplied  from Haldor Topsoe INC.  The BET surface area and pore size distributions were measured by N2 physisorption using a Quantachrome I sorption analyzer (Autosorb-IC).

2.  Evaluation of catalytic properties

The POX reforming experiments were carried out using 1 g catalyst in the gasoline fuel processor system.  All the runs were conducted in the temperature range of 550∼750oC, space velocity of 4000∼43,000 h-1 and feed molar ratios of H2O/C = 0.5∼3 and O/C = 0.5∼2.0, at atmospheric pressure.  The gases were delivered by mass flow controllers, and H2O and iso-octane were fed by a liquid delivery pump (Young Lin Co., Model  M930).  Prior to the experiment, the catalyst was pretreated for 1 h at 700oC under hydrogen flow (flow rate, 60 cc/min.).  The gas effluent was analyzed by on-line gas chromatography (HP-5890 Series II) equipped with TCD and using a carbosphere 80/100, 10x1/8 SS column.

Results and discussion

The POX reforming reaction of iso-octane has been optimized with respect to the O/C molar ratio, H2O/C molar ratio, reaction temperature and space velocity.  Following the process optimization, a sulfur tolerance test has been carried out using a standard solution of iso-octane containing 100 ppm of sulfur.

Hydrogen concentration at the reaction condition of reaction temperature = 700oC, H2O/C molar ratio = 3.0 and space velocity = 8776 h-1 progressively decreased with increasing the O/C molar ratio with a concomitant rise in the CO2 concentration. A hydrogen concentration of 68 % was observed at an O/C molar ratio of 0.5 and it decreased up to 52.7 % at the molar ratio of 2.0. The CO2 concentration varied from 16.7 to 37.7 % along with the O/C molar ratio variation.  However, CH4 concentration remained almost unaffected, whereas CO concentration slightly decreased with increasing the O/C molar ratio.  Even though the H2 selectivity was a maximum at the O/C molar ratio of 0.5, there was a problem in the material balance of carbon for the production of H2 in the POX reforming reaction. Therefore, an O/C molar ratio of 1.0 is considered to be ideal for the reaction at the H2O/C molar ratio of 3.0.  The H2 concentration increased from 60.6 to 63.3 % as the H2O/C molar ratio increased from 0.5 to 3.0, at the reaction temperature of 700oC, space velocity of 8776 h-1 and O/C molar ratio of 1.0. The CO concentration decreased from 23.9 % to 13.8 % as the H2O/C molar ratio changes from 0.5 to 3.0. The CH4 concentration also decreased along with the increase in the H2O/C molar ratio.

Figure 1 shows the product distribution as a function of the reaction temperature over the catalysts.  The reaction is carried out in the temperature range of  550∼750oC at the fixed reaction conditions as space velocity = 8776 h-1, O/C molar ratio = 1.0 and H2O/C = 3.0.  The H2 concentration increased up to 700oC and remained almost unaffected with further rise in temperature.  A rapid increase in the CO concentration was observed up to 650oC, and, thereafter, only a slight increase was noticed up to 750oC. The CO2 and CH4 concentrations decreased with increasing the temperature.  Thus, for the maximum H2 selectivity, it is always desirable to operate the POX reforming system at high temperature; however, there is an adverse affect due to rapid increase in the CO concentration. A reaction temperature of 700oC is found to be ideal for operating the system.  The effect of space velocity variations at the reaction conditions of reaction temperature of 700oC, H2O/C molar ratio of 3.0 and O/C molar ratio of 1.0 is shown in Fig. 2.  The H2 and CO2 concentrations decreased while that of both CO and CH4 concentrations increased with increasing the space velocity.

In sulfur free iso-octane feed the catalyst displayed prolonged stability.  This result indicates that the catalyst is coke resistant under the autothermal conditions.  The sulfur resistance of the HT catalyst has been tested using a standard iso-octane solution containing 100 ppm of sulfur.  The catalytic run is performed at 700oC with a space velocity of 8776 h-1 and by using the feed molar ratios as H2O/C = 3.0 and O/C = 1.0.  The result is presented in Fig. 3.  An increase in reaction pressure with time-on-stream was observed during the experiment.  The sulfur present in the iso-octane feed deposits on the catalyst surface.  Such sulfur deposits on the catalyst surface increase carbon deposition and, thereby, causing an increase in the reaction pressure.  Even though the catalyst surface is coked with sulfur and carbon deposits, the product distribution remained more or less unaffected during the period of investigation.  The observed trend can be interpreted by considering the progressive increase in the contact time along with the reactor pressure and thereby compensating the effect of catalyst deactivation during the reaction.  The pressure gradually increased up to 40 psig at the time-on-stream of 13.5 h.  Hydrogen is purged to remove the sulfur deposits and the run was restarted when the pressure reached nearly to 0 psig.  It was observed that the reactor pressure was developing quickly with increasing the reaction time over the regenerated catalyst.  This result indicates that, in the case of the HT catalyst, the accumulated carbon on the catalyst surface facilitates sulfur deposition.

Figure 4 represents the X-ray diffraction (XRD) patterns of the fresh and used catalysts. The catalysts recovered after 24 hours of run are compared with the fresh catalyst.  We could not observe any major changes in the phase pattern of the catalyst after the POX reforming reaction using both sulfur-free and sulfur containing iso-octane feeds. This is evident from the identical nature of the XRD patterns of the used and fresh systems.

The discussion so far made has revealed that even though the HT catalyst is performing well as a POX reforming catalyst in sulfur-free iso-octane feed, they are not promising systems in operations using feeds containing sulfur. This drawback makes this system unfavorable for applying in the gasoline fuel processor system for commercial applications.  This is because gasoline itself is a mixture of many hydrocarbons and usually contains sulfur impurities with their amount may vary depending on the gasoline source.  The fuel needed to be desulfurized before passing through this catalyst. However, such ancillary subsystems might increase the size of the fuel processor system and reduce the overall efficiency. Thus, development of high performance POX reforming catalysts with sulfur resistance would be the main challenge in the coming years.

References

[1]	P.J. de Wild, M.J.F.M. Verhaak, Catalysis Today, 60 (2000) 3.
[2]	J.M. Ogden, M.M. Steinbugler, T.G. Kreutz, J. Power Sources, 79 (2000) 143-168.
[3]	V. Raman, Hydrogen Fuel Option for Fuel Cell Vehicle Fleets in Fuel Cell Power for Transportation, SP-1425, Published by: Society of Automotive Engineers Inc., PA, USA, Page 1, March 1999.
[4]	A.D. Little, in Multifuel Reformers for Fuel Cells Used in Transportation Assessment of Hydrogen Storage Technologies, U.S DOE Report, DOE/CE/50343-1, March 1994.
[5]	Y. Jamal, M.L. Wyszynski, Int. J. Hydrogen Energy, 19(7) (1994) 557-572.
[6]	A. Docter, A. Lamm, J. Power Sources, 84 (1999) 194.
[7]	D.J. Moon, K. Sreekumar, S.D. Lee, B.G. Lee and H.S. Kim, Studies on Gasoline Fuel Processor System for Fuel-Cell Powered Vehicles Applications, Appl. Catal. A, General (in press) (2001)
[8]	D.J. Moon, S.D. Lee, B.G. Lee, Partial Oxidation (POX) Reforming of Gasoline for Fuel-Cell Powered Vehicles Applications, submitted to Appl. Catal. A, General (2001).

Fig. 1. Product distribution as a function of reaction temperature over ICI catalyst in the POX reforming of iso-octane. Space velocity = 8776 h-1; feed molar ratios H2O/C = 3 and O/C = 1.		Fig. 2. The effect of space velocity variations for the POX reforming of iso-octane over HT catalyst at reaction temperature of 700oC and molar ratios of H2O/C = 3 and O/C = 1

Fig. 3. Sulfur tolerance test of the HT catalyst catalyst for the POX reforming of iso-octane with 100 ppm of sulfur. Reaction temperature = 700oC ; O/C molar ratio = 1.0 ; H2O/C molar ratio = 3.0, space velocity = 8776 h-1.		Fig. 4. The XRD patterns of the fresh and used catalyst. The catalysts recovered after 24 hours of the run are compared with the fresh catalyst.