Module8:Engine Fuels and Their Effects on Emissions
  Lecture 40:Alternative Fuels (contd.)
 

Hydrogen

Interest in hydrogen as a potential alternative automotive fuel has grown due to need of reducing   greenhouse gas, CO2 emissions and to minimize dependence on fossil fuels. Hydrogen can be produced from a variety of fossil and non-fossil sources.
Hydrogen is a colourless, odourless and non-toxic gas. It burns with an invisible and smokeless flame. The combustion products of hydrogen consist of water and some nitrogen oxides. The major hurdles in the use of hydrogen as a fuel are lack of production, distribution and storage infrastructure. On board storage of hydrogen is another major challenge.  Hydrogen has very low boiling point (–253º C) and very low volumetric energy density.
Volumetric energy density of compressed hydrogen is just one-third of energy density of natural gas. Liquid hydrogen also has a very low volumetric energy density, which is about one-fourth of gasoline.  Hydrogen can be stored as compressed gas, as iron, magnesium, titanium or nickel hydride, or in liquefied form. The liquid, hydride and compressed hydrogen storage methods are compared in Table 8.20 for storing 19 litres of gasoline equivalent in energy. Hydrogen storage space required is at least 10 to 12 times higher than for gasoline.  Storage and fuel weight for hydrides is 27 times and for compressed H2 is 4 to 5 times of gasoline.

                            Table 8.20
Comparison of Hydrogen Storage Methods

 

Gasoline

Liquid H2

Hydride Fe-Ti (1.2%)

Compressed H2 (70MPa)

Energy (LHV) stored, MJ
Fuel mass, kg
Tank mass, kg
Total Fuel System mass, kg
Volume, l

600
14
6.5
20.5
19

600
5
19
24
178

600
5
550
555
190

600
5
85
90
227

Combustion characteristics of hydrogen and its impact on emissions are given below;

  • Hydrogen octane rating is 106 RON making it more suitable for spark-ignited engines.
  • The laminar flame speed of hydrogen is 3 m/s, about 10 times that of gasoline and methane.
  • Hydrogen has very wide flammability limits ranging from 5 to 75% by volume (f = 0.07 to 9), which may lead to pre-ignition and backfiring problems.
  • Its adiabatic flame temperature is higher by about 110º C compared to gasoline.
  •  If inducted along with intake air,   the volume of hydrogen is nearly 30% of the stoichiometric mixture decreasing maximum engine power.
  • Hydrogen on combustion produces water and there are no emissions of carbon containing pollutants such as HC, CO and CO2 and air toxics.
  • Trace amounts of HC, CO and CO2 however, may be  emitted as a result of combustion of lubricating oil leaking into engine cylinder.
  • NOx is the only pollutant of concern from hydrogen engines. Very low NOx emissions can be obtained with extremely lean engine operation (f < 0.05) and/or injection of water into intake manifold or exhaust gas recirculation which in this case consists primarily of water vapours. NOx emissions of 0.013 g/km have been obtained which are about 1/10th of the US Tier 2 regulations.
  • Hydrogen fuelled engines produces almost no CO2 and its global warming potential is insignificant.

Hydrogen fuelled IC engines however are not considered a long term option when compared to fuel cell. Hydrogen fuel-cell vehicles are expected to have more commercial potential in the long run. Though it is believed that significant production volumes for customers will not be available until the 2017-2020 time frame.