Hydrogen and Hydrogen Storage

Hydrogen offers perhaps an obvious choice energy choice if the principal concern is the release of CO2. It would be the ideal commodity for energy, except that it is not widely available, it is difficult to store, and there is very little capacity to distribute hydrogen (unless it is chemically bound to another element, and usually that element is carbon). Most of the world's hydrogen is obtained from natural gas using thermal steam reforming of gas, and much of that is consumed not far from the point of production. About 90% of the hydrogen is generated and consumed within the refining sector to provide a product suite tailored to consumer and regulatory requirements. In a sense, refiners are producing a "hydrogen carrier" by adding H2 to the product stream. While it's the most abundant element in the universe, it is not abundantly available for such routine applications as vehicle refueling or power production.

Making hydrogen from inexpensive solid fuels would be ideal, but solid fuels are difficult to process, and conversion methods can be costly. Gasification of solid fuels usually requires large volumes of high purity oxygen, which in turn requires an expensive air separation technology. While this links hydrogen directly to the continued use of fossil fuels, there is an equally daunting challenge of the energy density of hydrogen—its volumetric energy density is so low as to limit its widespread commercial use. In a fuel such as methane, which is 25% hydrogen by weight, approximately xh of the total energy content of the fuel is obtained from oxidizing of the hydrogen (the balance from the carbon). For fuels such as gasoline,

Table 10.2 Energy storage capacity (weight and volumetric)

Energy density,

Energy density,

Storage mechanism

mass (kW h/kg)

volumetric (kW h/l)

5,000 psig tank

2.1

0.8

10,000 psig tank

1.9

1.3

Liquid hydrogen

2.0

1.6

Metal hydride

0.8

0.6

Chemical hydride

1.6

1.5

Ammonia-anhydrous

4.79

2.95

LNG(-259°F)

12.0

6.3

Kerosene/JP-8

12

10

kerosene, and methane, the carbon acts as the hydrogen carrier, providing a modestly dense pack of liquid energy. One way to carry a lot of hydrogen is to compress the gas to high pressure (Table 10.2). This fills in the voids between molecules, but it still does not yield the kind of energy density so readily found in liquid hydrocarbon fuels. Even at 5,000 psig, hydrogen only begins to approach 20% of the energy density found in kerosene (and with a very large penalty for compression and storage requirements).

The US Department of Energy has established targets for fuel systems that can store 6% by weight of hydrogen and 45 kg of hydrogen per cubic meter. Goals for 2015 are even higher: 9-wt % of hydrogen and 81 kg of hydrogen per cubic meter. Chemical hydrides, which store the hydrogen with less demanding material requirements, are expected to be one of the possible solutions to a method of storing hydrogen in a high-density form

Working with LiNH2, LiBH4, and MgH2, Pacific Northwest National Laboratories (PNNL) researchers have been able to form new compounds with desirable energy density features. Compressed hydrogen, peaks at a volumetric energy density of less than 60 kg of hydrogen per cubic meter, a limitation reached because of non-ideal gas effects that begin to dominate at the ultra-high pressures considered. Hydrogen stored chemically as a hydride (or a hydrocarbon) can achieve significantly higher densities than compressed gases without the requirement of a very bulky container to maintain the high-pressure conditions.

Recent breakthroughs came with boron compounds coupled with ammonia. By itself, the hydrogen content of ammonia (NH3) is 18 wt%. Ammonia borane can improve on this figure significantly, releasing between 12-wt% and 25-wt% equivalent, depending upon the reaction pathway. This compares quite favorably with conventional middle distillates such as kerosene, which are roughly 15-16% hydrogen by weight. The trick is to successfully cycle the hydrogen from gas phase where it can be used for combustion or in a fuel cell, to solid-state, and back again. One way of making the transition from solid-state hydrogen to gas phase is through thermal decomposition. Simply by heating the hydrogen is released. A research team including Ping Chen of the National University of Singapore; Thomas Autrey at Pacific Northwest National Laboratory in Richland, Wash.; William I. F. David at the Rutherford Appleton Laboratory in the U.K.; and their coworkers converted ammonia borane to lithium amidoborane (LiNH2BH3) finding that the amido compound releases nearly 11 wt % of hydrogen at just 90°C.[23] In addition, the amidoborane releases hydrogen that's free from borazine, an impurity and fuel-cell poison that evolves from ammonia borane. This process can be further enhanced using catalytic materials to accelerate the rate of release [24]. The prize here is to be able to successfully store, at high energy density, a fuel that is carbon free.

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