2.4 Advances in the ANG Storage System
2.4.1 Theoretical Studies on the ANG Storage System
A large number of theoretical works are reported in the literature for enhanced storage capacity and thermal management of the ANG storage system. Some of these studies were validated with experimental investigations while others were evaluated only by numerical simulations. Here, the previous theoretical efforts are reviewed for the storage capacity improvement and the enhanced thermal management of the ANG storage technique.
2.4.1.1 Theoretical efforts on storage capacity improvement
In 1992, Matranga et al. conducted the first molecular simulations using the Monte Carlo method to predict adsorption capacity of pure methane on simplified slit- model carbon. The simulation results predicted that the theoretical maximum storage capacity of methane at 3.4 MPa is 209 V/V and the theoretical maximum delivery is 195 V/V for monolithic carbon with the assumption of isothermal charge and discharge processes.
Talu (1992) provided an overview of the ANG technology mentioning the theoretical limits and operational problems. Based on the ideas, it was suggested to develop better carbon surfaces optimized for high storage capacity, to find out better packing technology and to reduce residual amount of gas left at depletion pressure.
Mota (1999) developed a detailed mathematical model in order to study the impact of natural gas composition on cycling efficiency of the ANG storage systems.
The simulation results demonstrated substantial reduction in net deliverable capacity on extended cyclic operation due to the heavier hydrocarbons which are present in small amounts in natural gas. In order to recover the capacity loss, specially tailored carbon-based filter or guard-bed was proposed by the same research group (Esteves et al., 2005) to use in the charge and discharge steps as shown in Figure 2.3.
(α) ExchangerHeat
StorageMain Guard bed Tank
(1–α) StorageMain
Guard bed Tank
ExchangerHeat
(a) Charge Step (b) Discharge Step
Figure 2.3 Schematic diagram of two-step operation of an ANG storage system: α is the bypass flow fraction during discharge and (1–α) fraction is heated before passing to guard bed (Esteves et al., 2005)
The effect of residual moisture content in microporous activated carbon for methane adsorption was studied by Zhou et al. (2001). The theoretical study confirmed the negative effect of moisture on the adsorption capacity for a wide range of moisture content at temperatures from (273 to 298) K and therefore, a minimum drying period of 6 hours was suggested for the adsorbent sample before packing in the storage vessel.
Another study (Zhou et al., 2010) of the same research group presented a review on the works of natural gas storage with wet adsorbents where the methane molecules were stored by hydrate formation instead of adsorption. Although they reported higher storage capacity in wet carbons than the adsorptive storage, it is not a preferred technique for storing natural gas due to high operating pressure that is almost twice that of the ANG method and also because of wet discharged gas that is not favourable for engine combustion.
An optimal design for carbon adsorbent was theoretically developed by Biloé et al. (2002) from the dynamic performance criterion of the ANG storage system. This study suggested that the activated carbon must be highly conductive (with an average micropore diameter of 15 Å) for the charge cycle, permeable and sufficiently conductive for the discharge process (with an average micropore diameter of 25 Å).
The microporous characteristics of the activated carbon type Maxsorb was found to be suitable for the ANG application where the thermal conductivity was increased by introducing highly conductive expanded natural graphite.
2.4.1.2 Theoretical efforts on effective thermal management
Barrett and Jon (1995) demonstrated multi-cell ANG storage system filled with microporous carbon matrix for potential use in vehicles, river boats, forklift trucks, and portable generator systems while Cardenas et al. (1996) emphasized the development
of heat management and gas mixture conditioning in order to increase the mileage range. Wegrzyn and Gurevich (1996) compared the ANG storage with the other conventional technologies, providing estimates of costs associated with each technology.
A simple one-dimensional, non-adiabatic model was established for the ANG system by Fu and Zhou (2003) to study the detrimental effect of adsorption heat on storage capacity during the charge cycle. The simulation results revealed that the adsorbent bed reaches only up to 76 % of maximum adsorption capacity in 10 minutes of charge period with no cooling arrangement and furthermore, cooling of inlet gas was found not to be a feasible way in enhancing the charge performance. Hirata et al.
(2009) also employed a transient one-dimensional nonlinear formulation to study the adverse effect of adsorption heat on delivery capacity during the discharge cycle. A slow discharge process was suggested to avoid adsorbent bed temperature fluctuations when there is no arrangement for enhanced heat exchange to the surroundings.
Santos et al. (2009) presented a numerical study of an ANG vessel made up of several tubes to minimize the adsorption heat effects. A schematic of the gas charging system for the multi-tube adsorbent tank is sketched in Figure 2.4. The gas was circulated through the multi-tube tank and the non-adsorbed gas was passed through an external heat exchanger installed close to the gas station. Hence, the adsorbent bed heat transfer rate was increased during the charge process through forced advection. The simulation results claimed rapid temperature drop of the adsorbent bed and thus, the storage capacity was maximized. However, there was no analysis reported regarding the capacity loss due to the tubes inserted in the adsorbent bed and also the discharge process was not analyzed.
Compressor
Heated Gas ColdGas
Heat Exchanger
Multi-tube Tank
Activated Carbon
Cooling Fluid
Reservoir Gas
Gas Station
Figure 2.4 An ANG storage system with multi-tube tank where gas circulates through the tank and the heat exchanger (Santos et al., 2009)
Basumatary et al. (2005) presented two-dimensional thermal modeling of the ANG storage system where the heat and fluid flow inside the porous adsorbent bed were modeled using a volume averaging technique and Darcy–Brinkman formulation.
The effective thermal conductivity of the methane/activated carbon system was estimated as a function of uptake according to the Luikov model. The simulation results evaluated that both the gas charging rate and pre-cooling of gas have no positive impact on bed temperature rise. Another detailed parametric study was performed by Loh et al. (2010b) to analyze the mass flow rate, heat transfer process, and cylinder cooling and heating requirements during both the charge and discharge processes of the ANG storage system. Subsequently, Sacsa Diaz and Sphaier (2011) proposed a methodology that comprises a set of physically meaningful dimensionless groups for analyzing heat and mass transfer processes of the ANG storage system. The simulation results using this dimensionless methodology demonstrated that higher heat of adsorption value causes reduction in discharge capacity, whereas larger Biot and Fourier numbers can increase the amount of gas recovered.
Now, the following points can be summarized from the above reviews of the theoretical studies on the ANG storage system.
1) The activated carbons with larger surface area and an average micropore diameter of (10 to 20) Å are required for high storage capacity.
2) To avoid the negative effect of gas composition, a guard-bed can be installed in the charge/discharge line of the ANG storage system.
3) Pre-cooling of gas or circulation of cold gas through the adsorbent bed is not an effective way for proper thermal management during charge cycle.
4) An effective cooling/heating arrangement based on internal thermal control is necessary in order to minimize the adverse effect of adsorption heat during both the charge and discharge processes.