2.4 Advances in the ANG Storage System
2.4.2 Experimental Investigations on the ANG Storage System
Enhancement on both the storage capacity and the thermal effects of the ANG storage system are experimentally investigated by several researchers. The potential advantages of this technology are motivating individuals for their indisputable efforts to overcome the limitations. At this point, we intend to review the efforts in experiments for the storage capacity improvement and the enhanced thermal management of the ANG storage system.
2.4.2.1 Experiments for storage capacity improvement
MacDonald and Quinn (1996) measured the adsorption capacity of methane onto powdered peach pit which was impregnated with various concentrations of phosphoric acid at high temperatures. The experimental results showed that the impregnated chars do not adsorb sufficient methane to be considered as suitable adsorbents for the ANG storage applications.
Activated carbons for ANG storage were developed from bituminous coals and scrap tires by both physical and chemical activation reported in the study of Sun et al.
(1997). The experimental results confirmed that pre-oxidation is a desirable step in the production of activated carbons for higher adsorption capacity. In addition, the optimum conditions for activation temperature, time, and agent usage for developing a better natural gas adsorbent are described by Chen et al (1999).
Pupier et al. (2005) investigated the impact of natural gas composition on storage capacity after successive cycles of the ANG system (filling and delivery). In the experiment, a 2 litre ANG storage vessel was cycled with natural gas composed of about 95 % of methane mixed with other heavier hydrocarbons. The gas composition at the outlet was determined as a function of the cycle numbers. The results depicted that there is a reduction in storage capacity by 50 % after 700 cycles corresponding approximately to 250,000 km for a vehicle with a fuel tank allowing 400 km distance run for each refill. To restore the initial performance, heating up of the storage vessel to a high temperature after every cycle was suggested for the residual species to be desorbed. Similar experimental studies were carried out by Amora et al. (2007) and Rios et al. (2011) showing the detrimental effect of the heavier hydrocarbons in long- term usage of the ANG vessel. Therefore, it is essential either to install a guard-bed in the charge/discharge steps as proposed by (Esteves et al., 2005) or to heat up the adsorbent bed to higher temperatures during the discharge process.
A comparative experimental study between the monolith and the powdered form of similar activated carbon (RP-20) was performed by Balathanigaimani et al. (2008).
The Monolith was prepared from RP-20 (90 % by weight) with the aid of polymeric binders (10 % by weight) to increase the packing density. The experimental results
confirmed higher adsorption capacity in case of the monolith compared to the powdered form. However, a higher temperature fluctuation was observed in the monolith compared to the powdered activated carbon bed because of its higher heat of adsorption and lower micropore size. These experimental results necessitate the existence of an effective thermal control in the monolith to avoid the temperature fluctuations and thus to obtain a higher storage capacity for the ANG system.
2.4.2.2 Experiments for effective thermal management
The detrimental effect of adsorption heat during the discharge process of the ANG storage system was investigated by Chang and Talu (1996). The temperature drop and the performance loss were measured under realistic conditions. The experimental data revealed that the temperature dropped by as much as 37 °C at practical discharge rates of vehicle applications with a performance loss of about 25 % of isothermal capacity. The performance loss was reduced to (15 to 20) % at moderate discharge rates. It was observed that the poor thermal conductivity of the packed activated carbon, thermal mass of the vessel wall, and external heat transfer conditions have a significant effect on the non-adiabatic and non-isothermal discharge process.
Methane Source
DischargeGas
ControllerFlow
RegulatorGas
Storage Cylinder
Perforated Tube Activated
Carbon
Figure 2.5 An ANG storage cylinder with a perforated tube inserted at the center of the adsorbent bed (Chang and Talu, 1996)
In this study, a perforated tube was inserted at the center of the storage cylinder as sketched in Figure 2.5 to change the flow direction during the discharge process from axial to radial and thus, the heat transfer was increased from the wall to the central region. Under identical conditions and at moderate discharge rates, the performance loss was reduced to 10 % due to the insertion of centred perforated tube.
Vasiliev et al. (2000) introduced another thermal control system with an internal source of energy input, based on heat pipe heat exchangers to enhance heat transfer between the adsorbent bed and the surroundings. Figure 2.6 shows the cross section of a multi-cell ANG vessel with internal heating elements (heat pipes). A performance improvement during the discharge process of the multi-cell ANG vessel was reported from the experimental results. However, a significant temperature drop of 25 °C was observed in the adsorbent bed during gas discharge from initial pressure of 3.5 MPa to depletion pressure of 0.3 MPa. The reason could be the inadequate supply of heat during the discharge process compared to the enthalpy of desorption.
1.Vessel envelope;2.Heating elements (Heat pipes);3.Adsorbent bed;
4.Gas channels;5.Metal ribs to heat/cool adsorbent bed
1 2 3 4 5
Figure 2.6 The cross-section of a multi-cell ANG vessel with internal heating elements (Vasiliev et al. 2000)
An adsorbent composite block, where the Maxsorb type activated carbon was consolidated with an inert graphite binder, was investigated by Biloé et al. (2001b) for
enhanced thermal behaviour of the ANG vessel during the discharge cycle. The thermal conductivity of the adsorbent composite block was increased by 30 times using highly conductive binder material compared to the Maxsorb only. The experimental results of the dynamic methane discharge revealed that both the heat exchange conditions and the delivered capacity of methane were significantly enhanced under various operating conditions. However, this technique lessens the amount of gas stored for same cylinder volume as the binder material occupies some volumes of the adsorbent bed.
Experimental studies on the dynamic performance of the ANG storage system were carried out by Yang et al. (2005). A one litre storage vessel with U-shaped heat exchanging pipe as shown in Figure 2.7 was packed with 320 gram of granular activated carbon to investigate the bed temperatures during the discharge cycle. The experimental results demonstrated that the central region of the adsorbent bed experiences severe temperature fluctuations in a short period of the initial discharge state and hot water flow through the heat exchanging pipe from engine radiator reduces the temperature fluctuations of the adsorbent bed.
G as C harg e/D isc ha rg e
H ot W at er In/ O ut
Activated Carbon bed U-shaped heat
exchanging pipe
Figure 2.7 An ANG storage cylinder with U-shaped heat exchanging pipe (Yang et al., 2005)
A similar experimental study was conducted by Sáez and Toledo (2009) showing severe temperature fluctuations at the central region of the activated carbon bed during both the charge and discharge processes. This study concluded that the temperature fluctuations can be mitigated with improved adsorbent properties and an efficient cooling/heating system is necessary to be installed in order to enhance the storage/delivery capacities.
At this instant, the following points can be highlighted from the above review of the experimental investigations reported in the literature.
1) The activated carbon must be developed from raw materials with high carbon content and synthesized with proper activation processes which are affected by activation agent, ratio, temperature, and time.
2) To avoid the capacity loss due to gas composition, another method of heating up of the adsorbent bed to a high temperature during the discharge process is suggested.
3) To increase the packing or bulk density of the activated carbon, monolith can be prepared using highly conductive binder material.
4) Since the adsorbent bed experiences severe temperature fluctuations due to the adsorption heat during both the charge and discharge processes, an effective and optimized cooling/heating arrangement based on internal thermal control has to be installed in the adsorbent bed of the ANG vessel.