ON THE R E D U C T I O N OF TEMPERATURE DROP DURING D I S C H A R G E OF A D S O R B E D NATURAL GAS STORAGE SYSTEMS IN V E H I C U L A R

APPLICATIONS

JosO P. B. Mota Dept. Quimica, Centro de Quimica Fina e Biotecnologia, Faculdade de Ci6ncias e Tecnologia,

Universidade Nova de Lisboa, 2825-114 Caparica, Portugal

Introduction

Adsorption storage is the most promising low-pressure al- ternative for storing natural gas (NG), but some operational difficulties hinder its success. A factor determining the fea- sibility of this storage technology in mobile applications is the nonisothermal operation of the adsorption storage reser- voir [ 1,2].

The discharge of an on-board adsorption storage tank is dictated by the power requirements of the engine, and it is a slow process. Nevertheless, experimental and theoretical work has shown that under realistic discharge conditions the consumed heat of desorption is only partially compensated by the wall thermal capacity and by the heat transferred from the surrounding environment [2,3]. As a result, a radial tem- perature profile develops in the medium, the major temper- ature drop occurring at the center of the bed. This, in turn, leads to a net deliverable capacity that is lower than that for isothermal operation because more gas is retained in storage at depletion pressure.

This paper reports preliminary results of a feasibility study on the possibility of using the hot exhaust gases, downstream the combustion engine, to heat an adsorptive storage reservoir during normal vehicle operation. If the

Figure 1. Schematic drawing of a jacketed reservoir for nat- ural gas storage by adsorption.

reservoir is jacketed, so that the exhaust gas can flow along the annular space wrapping the vessel, then energy can be transferred to the carbon bed by forced convection. This ap- proach is illustrated in Figure 1.

Theoretical model

The model of the jacketed reservoir is an extension of pre- vious work, which has been validated experimentally, on modeling the dynamic behavior of adsorbed methane stor- age cylinders under discharge conditions [3,4].

In order to adapt the model to the new configuration, the boundary condition, which accounted for heat transfer by natural convection from the outside air, was changed. The new boundary condition for heat transfer at the external wall surface was interfaced with a Computational Fluid Dynam- ics code that predicts the velocity and temperature profiles of the exhaust gas in the annular space of the jacket. The heat transfer process to the carbon bed can be modeled accurately because, under normal discharge conditions, the flow in the jacket is laminar.

The heat capacity, viscosity, and flow rate of the exhaust gas can be related with the discharge flow rate of NG by a simple combustion model. Fuel (methane) and oxidant (air) are presumed to combine in a single step to form a product:

CH 4 +or (202 + 7.52N 2) = CO 2 + 2H20+ 2(a - 1 )02 + 7.52ctN 2 ,

where ot = 1.2 is the air-fuel ratio.

Results and discussion

Given that the jacket takes up space that would, otherwise, be filled with carbon, it seems reasonable to compare the performance of the proposed prototype with those of two non-jacketed reservoirs: one having the same volume as the prototype and the other having the same weight. These two cases are suitable benchmarks for mobile applications in which the critical parameter is, respectively, volume or weight of storage.

Figure 2 shows the dependence on discharge duration of the exhaust-gas inlet temperature that increases the net de- liverable capacity of the storage cylinder to isothermal per- formance. The results presented cover the two benchmark cases and refer to two different thicknesses of the annular space of the jacket.

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Figure 2. Exhaust-gas inlet temperature for isothermal per- formance as a function of discharge duration. 6 = thickness of annular space of jacket; R / L = 10/74; stainless-steel wall thickness = 5 mm; ~e = 2 x 10 -3 W. cm-1. K- l .

As expected, higher inlet temperatures are needed to at- tain isothermal performance when the comparison is made on a volume basis than on a weight basis. If the discharge duration is increased, which is equivalent to decreasing the discharge flow rate of NG, then isothermal performance can be attained with lower exhaust-gas inlet temperatures. De- creasing the annular space of the jacket improves the per- formance of the reservoir. This occurs because less space is occupied by the jacket and because heat transfer to the car- bon bed is enhanced.

The energy demand of a city vehicle, equipped with 3 cylinders like the one tested here and travelling at cruising speed, gives a discharge duration of about 3 hours. Figure 2 shows that, in this case, the required exhaust-gas inlet tem- peratures for isothermal performance are in a perfectly fea- sible range (80-100°C).

In an ordinary storage cylinder NG flows axially through the carbon bed, which means that radial heat transfer takes place mainly by conduction. This is not particularly effec- tive because the thermal conductivity of the carbon bed is low. Radially extruded internal fins have been investigated as a means of enhancing radial heat transfer in the carbon bed.

The fins effectively change the flow direction from axial to radial, which promotes some energy transfer by convec- tion from the hot wall to the central region. Furthermore, the high conductivity of the fins increases radial heat trans- fer by conduction. Obviously, the fins take up some space that could be filled with carbon; hence, the thickness, num- ber, and positioning of the fins must be optimized. This is a complex problem that is currently under study. Preliminary results suggest that net deliverable capacity can be increased

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by an extra 5% with fin insertion. Illustrative results of this approach are shown in Figure 3.

Conclusions

The results presented for a regular carbon bed (~-e = 2 x 10 - 3

W. cm-1. K- l ) show that the on-board adsorption storage reservoir can attain isothermal performance with perfectly feasible exhaust gas inlet temperatures. Dual-size particle packing or carbon monolith have larger thermal conductivi- ties, hence the gain in performance will be even higher. Ad- ditionally, laminar heat transfer in the annular space of the jacket can be increased further by a heat-transfer enhance- ment mechanism.

References 1. Parkins ND, Quinn, DE Natural gas adsorbed on carbon. In: Patrick JW, editor. Porosity in carbons, Arnold, London: Arnold, 1995;291. 2. Talu O. An overview of adsorptive storage of natural gas. Int. Conf. on Fundamentals o f Ad- sorption. Kyoto, Japan, 1992;655.3. Mota JPB, Rodrigues AE, Saatdjian E, Tondeur D. Dynamics of natural gas ad- sorption storage systems employing activated carbon. Car- bon 1997;35:1259-1270.4. Mota JPB. Impact of gas com- position on natural gas storage by adsorption. AIChE J. 1999 (in press).

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