Improvement of terrestrial storage-shelter facilities for natural gas storage as part of gas hydrates

Natural gas in the gas hydrates form is proposed to be stored in improved shell gas-resistant structures. The improvement of the construction and the operation method of these buildings consists in the use of liquid foam as a thermal insulation material. The design and operation technology of a terrestrial shelter facility for the accumulation and storage of natural gas as a gas hydrate is proposed, the main element of which is a frameless gas-resistant shelter in the form of at least two dome-shaped gas-tight soft shells on a thermally insulated base, the space between which is filled with liquid foam. The use of these hydrate shelters will significantly increase the efficiency and competitiveness of natural gas storage technology in the gas hydrates form.


Introduction
As is known, at relatively low temperatures and a certain pressure, water molecules form a three-dimensional structure that can be occupied by gas molecules (for example, methane, ethane, carbon dioxide, etc.) [1].This type of compounds are known as clathrates or inclusion compounds [2].Today, a number of technologies are known in which gas hydrates are an intermediate or target product.These are the so-called gas hydrate technologies (transportation and storage of natural gas in gas hydrate form [1], gas hydrate fractionation, concentration using gas hydrates of any aqueous solutions [3,4], etc.).
At the same time, the natural gas storage in gas hydrate form in many cases becomes a real alternative to traditional technologies.
Storage of gas hydrate requires maintenance of appropriate thermobaric conditions.However, gas hydrate can exist in a metastable state for some time as a result of the self-preservation effect or due to preservation by a ice layer of [4][5][6].
In addition, a compulsory condition for the reliable gas hydrate storage is the organization its high-quality thermal insulation and sealing.Such conditions should be ensured in specialized hydrate shelters.There are a number of projects, which involve the use of surface or partially buried capital structures built from traditional building materials.
However, warehouses made of traditional metal or reinforced concrete structures cannot provide effective sealing and thermal insulation.In fact, these structures can only perform the role of a skeleton for attaching insulating and sealing elements.At the same time, a relatively thin layer of porous material (foam, mineral wool, etc.) can provide effective thermal insulation, and a polymer film can provide high-quality sealing [7,8].Based on this, it is proposed to use gas-resistant pneumatic structures as hydrate storages [9].At the same time, modern coatings allow maintaining their operational characteristics for a long time (up to 15-20 years) [10].In addition, they can be easily dismantled, transported and quickly assembled, that is, they can be considered as mobile technological objects.
These buildings are structures supported by a gas cushion.However, the pressure in them is higher than atmospheric only to ensure the force for the formation of a dome-shaped form and to compensate for the mass of the shell itself (in the range of pressure -0.01-1.0MPa) [11,12].
However, the thermal resistance of such structures is insignificant [11].Therefore, they need their conditioning (cooling).Taking into account the costs of cold production, it is impractical to operate such buildings without the organization of additional thermal insulation.A variant of increasing the thermal resistance of these structures is the use of two-and three-layer coatings.However, the thermal resistance of the barrier gas layer increases noticeably only up to its thickness of 0.3 m [9].For example, the coefficient of thermal conductivity of the two-layer shelters considered in [11] was 2.8-3.4W/(m 2 K).Therefore, regardless of the external temperature, a significant heat flow will enter the storage even through a double-layer coating.

Improvement of the storage-shelter thermal insulation system
It is known that the thermal insulation properties of materials are determined by their porosity.The pores are filled with gas of low thermal conductivity.As thermal insulation, among others, materials are used, which are polymers, foamed before the start of hardening.Hardening, in this case, is required for the production of heat-insulating panels of a certain shape and size for the convenience of their use and transportation.
However, the phase state of material bubbles does not affect its thermal insulation properties.Therefore, the use of solid porous thermal insulation materials is due to the convenience of their use.In the case of gas-resistant shell structures, the use of solid heat-insulating materials will be unacceptable, as this will significantly complicate the construction and duration of the shelter installation.
Therefore, it is suggested to use liquid polymer foams as a material for thermal insulation of gas-resistant shell structures -shelters for gas hydrates.At the same time, in the case of using a transparent shell-shelter, the foam will allow part of the scattered sunlight into the storage-shelter.This will make it possible to regulate the supply of energy to the storage to some extent.The schematic diagram of such a storage-shelter, supplemented by a complex of appropriate equipment for the implementation of the technological process of gas hydrate storage and regasification, is presented in figure 1.
Its main elements are shelter, base and auxiliary equipment.The shelter of the hydrate storage-shelter consists of a liquid foam layer between several impermeable to gas and water canvases 2 and 4 with a sun-reflective layer on top.The shelter is fixed by a net made of ropes 5.The storage-shelter is equipped with the following systems: foam generation and selection of products its destruction; storage conditioning (cooling and heating); gas and water selection.The foam generation system involves supplying the produced foam to the upper part of the space between the inner and outer shells.At the same time, a system for selecting of foam destruction products is placed between the shells at the base level.
For the maximum energy efficiency of the technology, the level of gas hydrate cooling in the production process is determined, taking into account the duration and parameters of transportation and storage.
The flow of thermal energy from the ground through the base of the storage at its average value of 17 W/m 2 [13] will be 0.03 MW.Thermal insulation of the base will allow to reduce it to 9 kW.After the preparation of the site, the base of the storage-shelter is arranged by sequentially laying a layer of thermal insulation 9 (figure 1), a coating of water-and gas-tight material 8, a heat exchanger in the form of a system of pipes 16, a system of gutters and perforated pipes 6 Figure 1.Schematic diagram of hydrate storage-shelter at the gas hydrate storage stage: 1gas hydrate; 2 -the lower fabric for the shelter; 3 -a liquid foam layer; 4 -the upper fabric for the shelter; 5 -a system of external reinforcement of the shelter in the form of a net made of ropes; 6 -perforated pipes of the gas and water extraction system from under the gas hydrate stack; 7 -a hermetic connection system of the cover sheets and the base; 8 -base covering made of material impermeable to gas and water; 9 -heat-insulating coating of the base; 10 -water collection tank; 11 -system of heating the coolant based on the solar collector; 12 -coolant heating block; 13 -unit of the refrigerating unit; 14 -air cooling system; 15 -circulation pump; 16 -heat exchanger pipe system; 17 -gas selection system; 18 -gas compression unit; 19 -gas consumer [9] for the removal of gas and water.
For reliable sealing of the hydrate storage-shelter, system 7 seals the connection between the shelter sheets and the base.The isolated space formed is connected to the compressor 18 by the gas discharge line 17.The temperature regime of the hydrate storage is ensured by the accumulated cold in the gas hydrate and the additional air conditioning system.Hydrate dissociation is proposed to be carried out directly in the storage at the expense of solar energy.The temperature of the gas hydrate (cooling during storage and heating in the process of gas extraction) is maintained by pumping the cooled or heated coolant through the heat exchanger 16 at the base of the hydrate storage-shelter.
The coolant is cooled by a refrigerating unit 13 or an air cooling system 14.Cooling or heating of the coolant is carried out in the solar collector 11, heater 12, refrigerating unit 13, air cooling system 14.Gas selection is carried out as a result of controlled dissociation of gas hydrate when pumping the coolant through the heat exchanger 16.Water is discharged from the storage-shelter to the tank 10 by the collector 6.(In addition, it is possible to organize the inflow of solar energy through the transparent areas of the shelter).Gas through the selection line 17 enters the compression unit 18.Then it is consumed by the gas distribution network.The pressure in the line is limited by the mechanical strength of the shelter and lies within 0.2-0.4MPa.The mobility of the storage-shelter will allow them to be placed directly near the objects of consumption.This will allow the gas during pressure dissociation to be sent to the gas distribution network without additional compression.

Calculation of storage-shelter operational parameters
The thermal resistance of the coating, which characterizes the heat-shielding characteristics of the gas-resistant storage-shelter, is determined by the formula: where R stor -reduced thermal resistance of the storage-shelter shell, m 2 K/W; α 1 , α 2coefficients of heat transfer of external and internal air, respectively,W/K m 2 ;δ shel -thickness of the gas-tight shell fabric, m; R barr -thermal resistance of the barrier layer, m 2 K/W; λ shelcoefficient of thermal conductivity of the shell fabric, W/m K.The average temperature for the forest-steppe zone of Ukraine for January is 265 K, July -293 K [14], [15].The thermal resistance of the storage-shelter cover R stor in summer will be 0.27 m 2 K/W, in winter -0.35 m 2 K/W.Then, during the winter period, energy will flow into the storage-shelter: where t tout ,t instor -air temperature, respectively, outside and in the storage-shelter, K.
The heat flow in the hydrate storage in the summer (q s ) will be [16]: where A d -daily fluctuation of heat energy flow, W/m 2 ; q av.d -average daily heat energy input to the storage, W/m 2 ; k -daily coefficient of change of heat flow A d ; t s , t instor -the temperature of the outside air in July and inside the storage, respectively, K; p rad -coefficient of absorption of heat of solar radiation by the external surface of the storage; I rad -average daily amount of solar radiation reaching the storage surface in the warmest month, MJ/m 2 ; α surf -coefficient of heat absorption of the external surface of the storage for the warm period of the year; K mthe maximum amplitude of daily air temperature fluctuations in the warmest month of the year, K; I m , I av.d -maximum and average daily value of the amount of solar radiation, respectively, MJ/m 2 .The results of calculations of thermodynamic parameters for gas hydrate storage in a storageshelter with a two-layer coating and an air barrier layer are given in table 1.Therefore, such a coating is inertialess.Let's consider the option of reducing the heat flow through a two-layer coating when filling the space between them with stable foam (foam density 4.0 kg/m 3 , layer thickness δ coatl -1.5 m, coefficient of thermal conductivity λ coatl -0.041 W/(m•K)) using an example storage with a capacity of 3,000 tons of gas hydrate (5.4 million cubic meters of natural gas).
The coefficient of thermal conductivity λ coatl of a two-layer reinforced coating with a thickness of 2 mm is 0.16 W/m K [16].Then its thermal resistance R coat1 will be at the level of 0.0125 m 2 K/W.The thermal balance of the storage-shelter is described by the equation: where Q 1 -heat flow into the storage through the shell, J; Q 2 -heat flow from the base of the storage, J; Q 3 -cold accumulated by gas hydrate, J, Q add -additional heat removal (cooling), J.
The reduced thermal resistance of the coating is determined by the formula: where R stor.coat -reduced thermal resistance of the storage-shelter shell, m 2 K/W; α outcoefficients of heat transfer of outside air, W/K m 2 ; δ coat1 -coating layer thickness, m; λ coat1thermal conductivity coefficient coating, W/m K; δ coat2 -thickness of the foam layer, m; λ coat2coefficient of thermal conductivity of the foam layer, W/m K. Therefore, the reduced thermal resistance of the storage-shelter cover (R stor.coat1 and R stor.coat2 ) for winter and summer will be 37.78 and 37.62 m 2 K/W, respectively.Then, in winter, the heat flow into the storage from the outside will be: where t tout ,t instor -air temperature, respectively, outside and in the storage, K.
The total heat flow into the storage-shelter through the coating (Q w1 ) in the winter period will be 1.76 kW.The heat flow to the storage in the summer (q s1 ) was determined by the formula: The total heat flow into the storage through the coating (Q s1 ) in the summer period will be 5.23 kW.
The heat flow that enters the gas hydrate from the base (Q 2 ), with its average annual value for mid-latitudes of 0.17 W/m 2 [15], will be 0.3 kW.Its insulation will reduce heat input to 0.25 kW.
In the table 2 shows a comparison of the calculated parameters of gas hydrate storage in ground-based gas-resistant storages in variants of the air layer between shells and foam.
Table 2. Comparison of gas hydrate storage parameters in ground gas-resistant storage-shelters depending on the level of thermal insulation.

Thermodynamic parameters Winter Summer
Thermal resistances, R stor.coat , m 2 K/W: -air between the fabrics of the shell 0.27 0.35 -a layer of foam between the fabrics of the shell 37.78 37.62 Heat flow to storage-shelter, q, W/m 2 : -air between the fabrics of the shell 48.6 48.6 -a layer of foam between the fabrics of the shell 0.45 1.33 Heat flow to storage-shelter, Q 1 , kW: -air between the fabrics of the shell 189 408.0 -a layer of foam between the fabrics of the shell 1.76 5.23 Energy costs for cooling, Q 3 , kW: -air between the fabrics of the shell 190.2 409.2 -a layer of foam between the fabrics of the shell 2.1 5.5 Therefore, the required capacity of the additional cooling system of the storage-shelter for storage of gas hydrate without its dissociation (at a temperature of 258 K) is 0.9 kW in the winter period, and 4.44 kW in the summer period.

Modeling of heat exchange processes
Let's set the temperature on the surface of the gas hydrate as a result of the arrival of heat flow, as a process of heat transfer through a multilayer coating.On one side of the coating is the external environment with a temperature of T p , and on the other -cooled to a temperature of T g gas hydrate (figure 2).
Formulation of the problem.Let's assume: the initial temperature of the gas hydrate (T 0 ) is 248 K; the gas hydrate stack has the shape of a hemisphere with a base radius (R g ) of 23.5 m; the surface of the gas hydrate is covered with a 1.5 m layer of foam (R s -R g = 1.5 m); the initial temperature of the foam is 248 K; there is thermal contact between the foam and the hydrate; the temperature of the outer surface of the storage-shelter (T out ) is constant and is 293 and 265 K for summer and winter, respectively.It is necessary to find the temperature distribution over time and establish the moment when the temperature of the surface of the gas hydrate reaches 258 K (equilibrium temperature).
This process is described by a system of differential equations.The change in gas hydrate temperature is described by the equation: (τ > 0, 0 < r < R g ) Temperature change in the foam layer as a result of heat exchange with the gas hydrate and the external environment: Initial conditions of the process: Boundary conditions of the process: IOP Publishing doi:10.1088/1755-1315/1254/1/012012 where r -variable (current) value of the radius of the storage base, m; R g -radius of the base of the storage-shelter located under the gas hydrate embankment, m; R s -the radius of the base of the storage-shelter, which is located under the gas hydrate mound and the foam layer, m; T o -storage temperature of gas hydrate, initial temperature of gas hydrate and foam layer, (T o = 248), K; T out -ambient temperature (293 K in summer, 255 K in winter); T g -variable over time (current) gas hydrate temperature, K; T s -variable over time (current) temperature of the foam layer, K; τ -the time of the heating (cooling) process of gas hydrate and foam; λ g -coefficient of thermal conductivity of gas hydrate, W/(m K);λ s -coefficient of thermal conductivity of foam, W/(m K); a g -coefficient of thermal conductivity of gas hydrate, m 2 /s; α -heat transfer coefficient, W/K m 2 ; a s -coefficient of gas hydrate thermal conductivity, m 2 /s.In order to evaluate the thermal insulation characteristics of the foam and the dynamics of the temperature change of the gas hydrate in the gas-resistant storage-shelter, its computer simulation was carried out (figures 3-6).The above storage calculation parameters were taken as the starting point.Therefore, with an initial temperature of gas hydrates of 248 K, its additional cooling when stored in a storage-shelter facility insulated with foam, even in the summer months, will be necessary after 30 days of storage (to maintain the temperature of the gas hydrate at a level not higher than 258 K).

Conclusions
Thus, gas storage in gas hydrate form is proposed to be implemented in improved shell gasbearing structures.This improvement consists in the use of liquid stable foams as a thermal   insulation material.The main design elements of this storage-shelter facility are a frameless gas-resistant shelter in the form of at least two dome-shaped gas-tight soft shells on a thermally insulated base, the space between which is filled with liquid foam.The use of these hydrate storages will significantly increase the efficiency and competitiveness of natural gas storage technology in the form of gas hydrates.

Figure 2 .
Figure 2. Scheme of the gas hydrate heating process in the storage-shelter.

Figure 3 .
Figure 3. Simulation of heat exchange in summer (external temperature 293 K) of gas-resistant hydrate storage-shelter thermally insulated with liquid foam without additional cooling after 30 days of storage.

Figure 4 .
Figure 4. Dynamics of changes in the temperature of the gas hydrate surface and the foam layer in the hydrate storage-shelter in the summer (T = 293 K) without additional cooling.

Figure 5 .
Figure 5. Dynamics of changes in the temperature of the gas hydrate (with an initial temperature of 248 K) from the surface to the center of the sole of the stack in the summer period of storage (T = 293 K) without additional cooling, provided that it is thermally insulated with a layer of foam 1.5 m thick: a) at the end of the first days of storage; b) at the end of the thirtieth day of storage.

Figure 6 .
Figure 6.Dynamics of changes in the temperature of the gas hydrate surface (with an initial temperature of 248 K) during thirty days in the summer storage period (T = 293 K) without additional cooling, provided that it is thermally insulated with a layer of foam 1.5 m thick.

Table 1 .
Gas hydrate storage parameters in a double-layered storage with an air barrier layer.Taking into account the thermal resistance, the thermal inertia of the two-layer cover of the hydrate reservoir will be[11]:D ther.in=0.27R stor λρc,where D ther.in -thermal inertia; R stor -thermal resistance, m 2 K/W.