Energetic and Cost Parameters

In Table 5.2 we compare ammonia as hydrogen source with other fuels (gasoline, compressed natural gas [CNG]) and pure hydrogen based on the following energetic and economic indicators: specific energy per mass and per volume, respectively, and gravimetric, volumetric and, respectively energetic costs. Gasoline, stored in non-pressurized tank in liquid phase with a density of 736 kg/m3, is listed in Table 5.2 as a reference fuel.

Compressed natural gas (CNG) also represents an interesting path toward hydrogen economy. Usually, CNG, which is formed mainly from CH4, is combusted directly in internal combustion engines. The combustion products are cleaner than the gasoline products: lower CO2 emissions and no SOx emissions. We take here the opportunity to comment on a recently promoted solution by Wesley (2008) that is suitable for future CH4 zero-emission transportation. That is to decompose methane into carbon and hydrogen according to the following reaction:

Table 5.2 Comparison of ammonia as hydrogen source with other options.

Fuel/storage

(bar)

(kg/m3)

(GJ/m3)

(CN$/m3)

c/HHV (CN$/GJ)

Gasoline/liquid

1

736

46.7

34.4

1.36

1,000

29.1

Hydrogen/CH4 pressurized tank

250

188

35.5

6.6

1.20

226

33.8

Hydrogen/metal hydrides

14

25

142

3.6

4.00

100

28.2

Hydrogen/NH3 pressurized tank

10

603

25.0

15.1

0.30

181

12.0

In such supposed layout, the carbon is recycled in the form of nanotubes or carbon fibers. Carbon can be collected on vehicles after decomposition and delivered to fueling stations where it is combined with hydrogen from water to form synthetic CH4 according to the following reactions

Here, the ideal (neglecting the reaction irreversibilities) energetic cost to produce synthetic methane is thus 211 MJ for 1 kmol of H2 equivalent. Based on stoi-chiometry and the energy of formation, it results that, on per mass basis, the cost of methane is 0.185 which differs from the cost of hydrogen for equivalent energy content. On the other hand, since 16 kg of CH4 contains 4 kg of H2, it results that in order to be competitive the methane costs must be 0.25 which differ from hydrogen cost on a per mass basis. Since 0.185<0.25 it results that, on an ideal basis, using CH4 as hydrogen source for zero-emission engines is feasible. Compared to ammonia as a hydrogen source, one must observe that CH4 decomposition needs about 22% from its HHV for decomposition vs. 12% in the case of NH3. Further comparison of CH4 with NH3 can be made regarding the onboard storage.

Fig. 5.5 Comparison of specific energy densities (a) and volumetric energy densities (b) of various fuels and NH3.

Typically, CNG is stored under 250-350 bar pressure on special "integrated storage systems" on vehicles. This system consists of a number of tubular tanks interconnected to each other and embedded in safety foam to minimize the damage of a possible tank fracture during a crash that can produce serious damage because CH4 is toxic, flammable, has explosion danger, and greenhouse gas effect.

The current cost of natural gas in Ontario is ~CN $0.3 for a standard cubic meter, i.e., ~CN $0.45/kg. However, the compression work is significant and this raises the CNG price about three times. Current price for CNG in Ontario is around CN $1.2/kg. The HHV of hydrogen-stored CH4 can be estimated taking in to account that 4 kg of methane contains 1 kg of H2. Because of its gaseous phase the energy density in the CNG tank is low (i.e., 6.6 GJ/m3) and this fact leads to an expensive specific energy (i.e., $33.8/GJ). Since this estimation is based on CNG price, more cost is expected if synthetic CH4 is used as a hydrogen source.

Ammonia, if used as a hydrogen source appears more attractive than CH4 in several aspects (see Table 5.2): it stores more hydrogen energy per tank volume, energy cost is about three times less, despite of its toxicity it presents less danger because ammonia is not flammable and does not present explosion risk.

In Fig. 5.5 we compare the gravimetric and volumetric energy content of various fuels and ammonia, where ammonia is viewed as a hydrogen source. As it is made clear by the figure, ammonia is the best hydrogen source among those considered for analysis. Note that in Fig. 5.5 the gasoline and LPG (liquefied petroleum gas - propane) are indicated for comparison purpose only - they are treated as fuels and not H2 sources.

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