Fig Steam reforming process [ Hydrogen Storage and Transportation

2.3.2.1 Compression and Liquefaction (Packaging)

Hydrogen still requires further preparation according to the demands of the intended end use. Firstly, cleaning of the hydrogen is required in order to ensure that the required quality is met. Following this it must be compressed, whereby the pressure level is dependent on either the end application or the intermediate storage method. Alternatively, liquefaction may be the reasonable option if transport over long distances is required or if the end users require a high energy density (small storage volume).

2.3.2.1.1 Compression of Hydrogen

Compression of hydrogen is carried out in the same way as for natural gas, though as hydrogen is less dense the compressors need better seals. It is sometimes even possible to use the same compressors, as long as the appropriate gaskets (e.g. Teflon) are used and provided the compressed gas can be guaranteed to be oil free. Energy is needed to compress gases. The compression work depends on the thermodynamic compression process. The ideal isothermal compression cannot be realized. The adiabatic compression equation [31, 190]:

where w

= specific compression work [J/kg] Po = initial pressure [Pa] pi = final pressure [Pa] Vo = initial specific volume [m3/kg] g = ratio of specific heats, adiabatic coefficient [-] The energy consumed by an adiabatic compression of Helium, hydrogen and methane from atmospheric conditions (1 bar = 105 Pa) to higher pressures is shown in Figure 2.7. Clearly, much more energy per kg is required to compress hydrogen than methane. Isothermal compression follows a simpler equation:

Adiabatic Compression Filling Tanks
Fig. 2.7 Adiabatic compression work for hydrogen, helium and methane

The compression work is the difference between the final and the initial energy state of the hydrogen gas.

Figure 2.8 illustrates the difference between adiabatic and isothermal ideal-gas compression of hydrogen. Multi-stage compressors with intercoolers operate between these two limiting curves. Also, hydrogen readily passes compression heat to cooler walls, thereby approaching isothermal conditions. Numbers provided by a leading manufacturer [31] of hydrogen compressors show that the energy invested in the compression of hydrogen is about 7.2% of its higher heating value (HHV). This number relates to a 5-stage compression of 1,000 kg of hydrogen per hour from 1 to 20 MPa. For a final pressure of 80 MPa the compression energy requirements would amount to about 13% of the energy content of hydrogen. This analysis does not include electrical losses in the power supply system.

Since hydrogen compression is carried out in the same way as the compression of natural gas, the procedure is well tested and readily available. New developments are mainly associated with the optimization of the individual units within the total concept, with the primary application here being the high pressure compression at service stations. Typical pressure levels are 3 - 4 MPa for pre-compression stages for filling of collecting tanks, and 25 - 30 MPa for storage tanks in fast fill applications. The fast fill process is achieved by an over pressure over the pressure level in the vehicle tank being filled (20 or even 25 MPa). The choice of the highest pressure level is primarily dependent on the maximum permitted pressure that the storage tank can withstand (modern tanks constructed from composite materials are rated for up to 700 bar). Because of the logarithmic relationship between pressure and work required for the isothermal compression, the increased energy required for a higher filling pressure is not that great. Thus the compression from 1 to 30 MPa needs only 10% more energy than the compression from 10 to 20 MPa [31].

Fi nal Pressure [bar]

Fig. 2.8 Energy required for adiabatic and isothermal ideal-gas compression of H2 [31].

Fi nal Pressure [bar]

Fig. 2.8 Energy required for adiabatic and isothermal ideal-gas compression of H2 [31].

2.3.2.1.2 Liquefaction of Hydrogen

In order to reduce the volume required to store a useful amount of hydrogen - particularly for vehicles - liquefaction may be employed. The advantage of liquid hydrogen is its high energy: mass ratio, three times that of gasoline. It is the most energy dense fuel in use (excluding nuclear reactions). That is why it is employed in all space programmes. Since hydrogen does not liquefy until it reaches -253°C (20 degrees above absolute zero), the process is both long and energy intensive. Up to 30% of the energy content in the hydrogen can be lost. Theoretically only about 14 MJ/kg (3.6 kWh/kg) have to be removed to cool hydrogen down to 20K (-253°C). The real energy needed to liquefy the hydrogen is about 40 MJ/kg (11 kWh/kg), compared to its energy content (high) of 142 MJ/kg.

But cryogenic refrigeration is a complex process involving Carnot-cycles and physical effects (e.g. Joule-Thomson) that do not obey the laws of heat engines. For the refrigeration between room temperature (Tr = 25°C) and liquid hydrogen temperature (Tl = -253°C) one obtains a Carnot efficiency of h =-—-= 0.072 (2-3)

or about 7%. The assumed single-step Carnot-type cooling process would consume at least 57 MJ/kg or 40% of the HHV energy content of hydrogen. This simple analysis does not include mechanical, thermal, flow-related or electrical losses in the multi-stage refrigeration process. But by intelligent process design the Carnot limitations may be partially removed. But the lower limit of energy consumption of a liquefaction plant does not drop much below 30% of the higher heating value of the liquefied hydrogen. As a theoretical analysis of the complicated, multi-stage liquefaction processes is difficult, we present the energy consumption of existing hydrogen liquefaction plants [31]. The liquefaction energy requirement depends on the process itself, the process optimization, the plant size, and on other parameters. Figure 2.9 shows typical energy requirements for the liquefaction of 1 kg hydrogen as a function of plant size and process optimization. The plants have a capacity between 1 to 10,000 kg of liquid hydrogen per hour.

Hydrogen Liquefaction Plant
Fig. 2.9 Typical energy requirements for the liquefaction [31, 212]
Hydrogen Liquefaction Linde
Fig. 2.10 Schematic of the Claude process for hydrogen liquefaction [96]

A commonly applied method in large-scale liquefaction plants is the Claude process (Fig.2.10). The necessary refrigeration is provided in four principal steps leading to the liquefaction of hydrogen [96]: (1) compression of hydrogen gas, removal of compression heat; (2) pre-cooling with liquid nitrogen (80 K); (3) expanding and thus cooling of a part of the hydrogen in an expander resulting in a further pre-cooling of the residual hydrogen (80-30K); and (4) expanding of residual hydrogen in a Joule-Thomson valve until liquefaction (30-20K). The method is now implemented in the Ingolstadt (Germany) liquefaction plant. The liquefaction plant supplied by Linde AG, has a capacity of 4.4 t/d. Today there are about 10 medium sized plants with production capacities of 10 - 60 t/d, in operation around the world. Liquefaction plants in USA, Japan and Europe with capacities in the range of 3 - 12 t/d are more recent [78].

2.3.2.2 Hydrogen Storage

As seen in Section 2.2, hydrogen has the lowest gas density and the second-lowest boiling point of all known substances, making it a challenge to store them as either a gas or a liquid. As a gas, it requires very large storage volumes and pressures. As a liquid, it requires a cryogenic storage system. Hydrogen's low density, both as a gas and a liquid, also results in very low energy density. Stated otherwise, a given volume of hydrogen contains less energy than the same volume of other fuels. This also increases the relative storage tank size, as more hydrogen is required to meet a, for example, given vehicle's range requirements. The amount of hydrogen needed for fuel cells is offset somewhat by the fact that it is used more efficiently than when burnt in an internal combustion engine, so less fuel is required to achieve the same result.

Despite its low volumetric energy density, hydrogen has the highest energy-to-weight ratio of any fuel. Unfortunately, this weight advantage is usually overshadowed by the high weight of the hydrogen storage tanks and associated equipment. Thus, most hydrogen storage systems are considerably bulkier and/or heavier than those used for gasoline or diesel fuels.

H2 gas at 350 bar

H2 gas at 250 bar

H2 Metal Hydrides

H2 gas at 350 bar

H2 gas at 250 bar

H2 Metal Hydrides

□ Fuel and total containment (kg)

J772

Fig. 2.11 Storage volume and weight of comparative fuels [31]

For all practical purposes, hydrogen can be stored as either a high-pressure gas, a liquid in cryogenic containers, or a gas chemically bound to certain metals (hydrides). The volume and weight of each of these systems is compared to gasoline, methanol and battery storage systems (each 1,044.5 MJ of stored energy; equivalent to 30 litres of gasoline) in Figure 2-11. Ironically, the best way to store hydrogen is in the form of hydrocarbon fuels although it requires additional systems to extract it.

2.3.2.2.1 Compressed Gas (CGH2) Storage

The traditional way of storing hydrogen is in gaseous form in pressure vessels. Gaseous hydrogen can be stored either in above ground (in portable or stationary containers) or in underground (i.e. different kinds of earth caves) storages.

a. Above Ground Storage

Compressed gaseous hydrogen is stored above ground in a high pressure gas cylinder. It is classified based on material compositions, i.e. metal and composite (Table 2-3). In general, the less metal is used, the lower is the weight. For this reason, type 3 cylinders are usually used in hydrogen applications, and type 4 cylinders will likely gain prominence in the future. Specific weights depend on individual manufacturers, but as a point of reference, a 100 l type 1 (steel) cylinder weighs about 100kg, a type 3 (aluminum/composite) cylinder weighs about 65 kg, and a type 4 cylinder weighs about 30 kg. Type 3 cylinders derive most of their strength from the composite overwrap that is wound around the inner liner. This composite consists of high-strength fibers (usually carbon) that are wrapped around the cylinder in many layers and glued together by a resin such as epoxy.

In the industrial sector a standardization of type has already occurred. As a result, cylindrical tanks with a maximum operating pressure of 5 MPa and 2.8m diameter are now available in the following lengths (or heights): 7.3 m (max. capacity at 4.5 MPa: 1305 Nm3), 10.8 m (max. capacity 2250 Nm3) and 19m (max. capacity 4500 Nm3). Bottle type storage can also be used as stationary storage as long as the volume is sufficient. Such bottles are available in steel in sizes ranging from 2 to 50 l (corresponding to 0.35 - 8.9 Nm3 and weights of 5.3 - 68 kg) with operating pressures of 20 MPa. In these cases, calculations for energy density by weight (gravimetric) and volume (volumetric) including the storage device itself result in figures of 0.9 - 1.1 MJ/kg and 0.5 MJ/l, respectively. Gravimetric energy density of hydrogen is largely dependent on the material of the container since light materials usually do not tolerate pressure as well as heavier ones. The theoretical gravimetric energy density of hydrogen can be calculated with the molar mass of hydrogen molecule (2.016 g/mol) to be 141.8 MJ/kg [31, 212].

Table 2-3. High pressure gas cylinder classifications [190]

Design

Description

% Metal/ Composite

Weight (kg) for 100 l

Type 1 Type 2

A cylinder made wholely of steel or aluminum A cylinder with a metal line of steel or aluminum and a

100/0 55/45

100

Type 3

hoop-wrapped composite overwrap A cylinder with a thin metal liner of steel or aluminum

20/80

65

Type 4

and a fully wound composite overwrap A cylinder with a plastic liner and a fully wound composite overwrap

0/100

30

Note: % Load taken by metal vs. composite.

Note: % Load taken by metal vs. composite.

b. Underground Storage

Underground caves are an easy and relatively cheap method for large seasonal storage of hydrogen. This storage technique is already in use for natural gas. There are several kinds of caves that can be used, such as salt caverns, mined caverns, natural caves, and aquifer structures. For example, the city of Kiel in Germany has been storing town gas containing 60 - 65% of hydrogen in a gas cavern at a depth of 1330 m since 1971 [96; 207].

Salt is often found in the form of layers that can be hundreds of meters thick. These layers are practically impermeable to water and air. The cavity is made in the salt by dissolving and the surface is cemented before feeding the gas. Aquifers are located in porous geological formations. The gas is injected into the rock pores, initially filled with water, in which the gas is accumulated. The use of this technique requires special geological conditions and can thus be used only in certain regions. The pressure in the earth caves varies between 8 - 18 MPa and thus the volumetric energy density is about 900 - 1674 MJ/m3. In aquifer structures, the energy density is naturally significantly smaller. The losses caused by the leaks in the earth caves are about 1-3% of the total volume per year [31].

2.3.2.2.2 LH2 Storage

Liquid hydrogen (LH2) storage systems overcome many of the weight and size problems associated with high-pressure gas storage systems, albeit at cryogenic temperatures. Liquid hydrogen can be stored just below its normal boiling point of -253 °C (20 K) at or close to ambient pressure in a double-walled, super-insulating tank (Dewar). This insulation takes the form of a vacuum jacket, much like in a thermos bottle. Liquid hydrogen tanks do not need to be as strong as high-pressure gas cylinders although they do need to be adequately robust if used for automotive purpose.

As compared with pressurized gas storage, this method is more expensive, because of the high cost of insulation. Despite the high price, however, in the case of large tanks the increased storage density of liquid hydrogen outweighs the benefit of reduced material costs associated with compressed gas storage. The containers usually combine different kinds of insulating methods. These include vacuum insulation, vapour-cooled radiation shields (VCS), and multi-layer insulation (MLI). A schematic of an insulated vessel is given in Figure 2.12. Larger containers are to some extent produced with perlite vacuum insulation.

T2=300 K Outer vessel

T2=300 K Outer vessel

Fig. 2.12 Combined insulation of vacuum, MLI, and VCS techniques [170]

Hydrogen cannot be stored in liquid form indefinitely. All tanks, no matter how good the insulation, allow some heat to be transferred from the ambient surroundings. The heat leakage rate depends on the design and size of the tank — in this case, bigger is better. This heat causes some of the hydrogen to vaporize and the tank pressure to increase. Stationary liquid hydrogen storage tanks are often spherical since this shape offers the smallest surface area for a given volume, and therefore presents the smallest heat transfer area. For example, the largest LH2 tank belongs to NASA located at Cape Canaveral is a spherical tank with a storage volume of 3800 m3 (approx. 270 t LH2) and the outer spherical diameter of 20 m. The evaporation rate is under 0.03% per day, allowing for a storage period of several years.

Although liquid hydrogen storage systems eliminate the danger associated with high pressures, they introduce dangers associated with low temperatures. A severe frostbite hazard exists in association with the liquid hydrogen, its vapour and contact surfaces. Carbon steel exposed to temperatures below -30°C, either directly or indirectly becomes brittle and is susceptible to fracture. Air may liquefy on the outside of exposed liquid hydrogen lines or under insulation resulting in an oxygen concentration that poses a fire or explosion hazard if it drips onto combustible materials.

The gravimetric energy density of liquid hydrogen including the storage container is about 25.9 wt% (5 MJ/kg), and the volumetric energy density about 9936 MJ/m3. Improvements in insulation techniques and the pressurization of the vessel will have some effect on these figures [31].

2.3.2.2.3 Metal Hydride

Metal hydride storage systems are based on the principle that some metals readily absorb gaseous hydrogen under conditions of high pressure and moderate temperature to form metal hydrides. These metal hydrides release the hydrogen gas when heated at low pressure and relatively high temperature. In essence, the metals soak up and release hydrogen like a sponge. The advantages of metal hydride storage systems revolve around the fact that the hydrogen becomes part of the chemical structure of the metal itself and therefore does not require high pressures or cryogenic temperatures for operation. The high weight of a metal hydride storage device is its disadvantage. Since hydrogen is released from the hydride for use at low pressure (0.25 to 1 MPa depending on material choice), hydrides are the most intrinsically safe of all methods of storing hydrogen. At the same time it gives a high volumetric storage density of approx. 0.8 - 1.4 MJ/kg and 3.6 - 5.4 MJ/l.

There are many types of specific metal hydrides, but they are primarily based on metal alloys of magnesium, nickel, iron and titanium. In general, metal hydrides can be divided into those with a low or high hydrogen release temperature [213]. The high temperature hydrides may be less expensive and hold more hydrogen than the low temperature hydrides, but require significant amounts of heat in order to release the hydrogen. Low temperature hydrides can get sufficient heat. In Germany, metal hydride storage is being further developed and supplied by GfE (Gesellschaft für Elektrometallurgie) [207].

2.3.2.3 Transport and Distribution

Hydrogen transportation issues are directly related to hydrogen storage issues. In general, compact forms of hydrogen storage are more economical to transport and diffuse forms are more costly. The technologies for routine handling and delivery of large quantities of hydrogen have been developed in the chemical industry. Liquid hydrogen is delivered by truck or rail over distances of up to several hundred miles. Compressed gas hydrogen pipelines (up to several hundred kilometers in length) are used commercially today to bring hydrogen to large industrial users like refineries. For a large-scale hydrogen energy system, it would probably be less expensive to transport a primary energy source (like natural gas or coal) to a hydrogen plant located at the "city gate," rather than making hydrogen at the gas field or coal mine and piping it to the city. In the long term, transcontinental hydrogen pipelines seem unlikely, unless there were a compelling reason to make hydrogen in a particular location far from demand.

2.3.2.3.1 Road Transport

A hydrogen economy also involves hydrogen transport by road. There are other options for hydrogen distribution (such as rail, barge, etc.), but road transport will always play a role, be it to serve remote locations or to provide back-up fuel to filling stations at times of peak demand.

a. Liquid Transport

Hydrogen can be transported on the road by truck as a cryogenic liquid in double-walled, super-insulated vacuum-lined tanks. Transporting liquid hydrogen is far more efficient than transporting a high-pressure gas, particularly where larger quantities are needed. On the downside, maintenance costs are much higher for liquid transportation. Today, LH2 is transported in cryo-containers or trailers of typically up to 41 m3 or 53 m3 at cryogenic temperatures of - 253°C. Larger quantities of LH2 have been transported in NASA's space program in barges over distances of about 100 km. LH2 road transport in large cylindrical containers of 270 m3 and 600 m3 has been performed in the framework of ESA's Ariane space program. In most cases the transport is weight-limited, it is limited by volume for liquid hydrogen. For example, the useful volume of a LH2 tanker truck with dimension of 2.4 m wide, 2.5 m high and 10 m long, is 60 m3. Only 4.2 tons of liquid hydrogen can be filled into this box, because the density of the cold liquid is only 70 kg/m3 or slightly more than that of heavy duty Styrofoam. The rest of the space is needed for container, thermal insulation, equipment etc. In fact, there is room for only about 2.1 tons of liquid hydrogen on a large-size truck.

b. Gas Transport

Hydrogen as a high-pressure gas can be transported in cylinders at pressures ranging from 15 to 40 MPa. For trucks, specially designed tube trailers carry a number of large, high-strength steel tubes linked together through a common manifold. This design works well in providing small quantities of hydrogen, but is very inefficient in terms of transport energy. The weight of the cylinders required is such that the gas is only 2 to 4% of the cargo weight. A hydrogen pressure tank can be emptied only from 20 MPa to about 4.2 MPa to accommodate for the 4 MPa pressure systems of the receiver. As a consequence, it delivers only 80% of its freight, while 20% of the load remains in the tanks and is returned to the gas plant. Today, at 20 MPa pressure only 320 kg of hydrogen can be carried and only 288 kg are delivered by a 40-ton truck. This is a direct consequence of the low density of hydrogen, as well as the weight of the pressure vessels and safety armatures.

2.3.2.3.2 Pipeline Delivery

Gaseous hydrogen can be transported by pipeline in a similar fashion as natural gas. Hydrogen, being less dense than natural gas, results in less mass transport for a given pipeline size and operating pressure. In addition, the energy density of hydrogen is only one third that of natural gas on a volumetric basis; hence, three times the amount of hydrogen gas must be pumped through a pipeline to transmit an equivalent amount of energy. To compensate for both of these properties, hydrogen pipelines need to be designed to operate at higher pressure in order to be practical. All pumps and other equipment must be hydrogen compatible. Furthermore, hydrogen pipelines must be resistant to hydrogen embrittlement in order to prevent cracking.

Existing hydrogen gas pipelines operate in some parts of the world. In the US there are 725 km of pipelines, including those in Texas, Indiana, New Jersey and Louisiana. In Europe, pipelines are operated in Germany (210 km) and between Belgium and France (400 km), among several others. Compared to pipelines for others gases, these lengths are very short. However, they indicate that the high cost of transporting hydrogen by gas pipeline is already worth while in some areas [207].

The theoretical pumping power requirement N [W] is presented by [31]:

N = VaDp = Ap v Dp = p/4 d2 v Dp = p /4d2v >2 p v2e = 1 p ■ d2 • v3 • p ■ e (2-4a)

with e = 0.31164/^e"; and Re = p ■ v ■ d / h (2-4b)

where,

Vo

= volumetric flow rate [m3/s]

P

= density of

Ap

= cross section of pipe [m2]

the

gas [kg/m3]

v

= flow velocity of the gas [m/s]

Re

= Reynolds number

Dp

= pressure drop [Pa]

n

= 0.25, for turbulent pipe flow

d

= pipeline diameter [m]

(Blasius equation)

e

= resistance coefficient

h

= dynamic viscosity [kg/(m s)]

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