ABSTRACT
Hydrogen is an odorless, colorless gas. With a molecular weight of 2.016, hydrogen is the lightest element. Its density is about 14 times less than air (0.08376 kg/m3 at standard temperature and pressure). Hydrogen is liquid at temperatures below 20.3 K(at atmospheric pressure). Hydrogen has the highest energy content per unit mass of all fuels-higher heating value is 141.9 MJ/kg, almost three times higher than gasoline. However, because of its low density, its heating value on volumetric basis is almost one-third of natural gas. Some important properties of hydrogen are compiled in Table 52.1. It exists in three isotopes:protium, deuterium, and tritium. Astandard hydrogen atom (protium) is the simplestof all the elements and consists of one proton and one electron. Molecular hydrogen (H2) exists in two forms: ortho-and para-hydrogen. Both forms have identical chemical
properties, but due to different spin orientation have somewhat different physical properties. At room temperature, hydrogen consists of approximately 75% ortho-and 25% parahydrogen.Since para-hydrogen is more stable at lower temperatures, its concentration increases at lower temperatures, reaching virtually 100% at liquid hydrogen temperatures.
While hydrogen is the most plentiful element in the universe, making up about three quarters of all matter, free hydrogen is scarce. The atmosphere contains trace amounts of it (0.07%), and it is usually found in small amounts mixed with natural gas in crustal reservoirs. A few wells,however,havebeenfoundtocontainlargeamountsofhydrogen,suchassomewells in Kansas that contain 40% hydrogen, 60% nitrogen, and trace amounts of hydrocarbons [1]. The Earth’s surface contains about 0.14% hydrogen (10th most abundant element), most of which resides in chemical combination with oxygen as water. Hydrogen, therefore, must be produced. Logical sources of hydrogen are hydrocarbon (fossil) fuels (CXHY) and water (H2O). Presently, hydrogen is mostly being produced from fossil fuels (natural gas, oil, and coal). However, except for the space program, hydrogen is not being directly used as a fuel or an energy carrier. It is being used in reŽneries to upgrade crude oil (hydrotreating and hydrocracking), in the chemical industry to synthesize various chemical compounds (such as ammonia and methanol), and in metallurgical processes (as a reduction or protection gas). The total annual hydrogen production worldwide in 1996 was about 40 million tons (5.6 EJ) [2]. Less than 10% of that amount was supplied by industrial gas companies; the rest is being produced at consumer-owned and operated plants (so-called captive production), such as reŽneries, and ammonia and methanol producers. Production of hydrogen as an energy carrier would require an increase in production rates by several orders of magnitude.
