6.8.1 Electrolysis
The most attractive method of producing hydrogen is by hydroelectricity at off-peak load. The minimum voltage required for water splitting at 25C and 1 atm is 1.23 V (reversible voltage).
Under these conditions, hydrogen would be produced only if heat is added. If no heat exchange with the surroundings takes place, the cell cools down. The thermoneutral potential at 25C is 1.48 V.
Table 6.4 Autoignition temperatures and explosive limits of some gases and vapors in air as vol. %
Autoignition Autoignition
Substance Temp.C LEL/UEL Substance Temp.C LEL/UEL
Acetone 465 2.6–12.8 Hexane 225 1.1–7.5
Acetylene 305 2.5–8.0 Hydrogen 400 4.1–74.0
Ammonia 651 16.0–26.0 Isopropanol 456 2.0–12.0
Benzene 562 1.3–7.1 Jet fuel 1.3–80.0
Butane 405 1.8–8.4 Kerosene 210 1.3–80.0
Carbon disulfide 125 1.3–50.0 Methane 650 5.3–14.0
Carbon monoxide 609 12.5–74.0 Methyl bromide 537 13.5–14.5
Cyclo propane 500 2.4–10.4 Naphthalene 277 0.9–6.0
1,2-Dichloroethylene 460 9.7–12.8 Naphthalene 568 0.9–5.9
Dimethyl ether 350 3.4–27.0 n-Octane 220 0.8–3.2
Ethane 515 3.2–12.5 Propane 450 2.4–9.5
Ethanol 423 3.3–19.0 Toluene 480 1.2–7.1
Ethylene 490 3.0–36.0 Water gas 6.0–9.0, 55.0–70.0
Fuel oil 0.7–5.0 Xylene 530 1.0–6.0
Gasoline 280–456 1.4–7.6 Dust ~50 mg/L
94 6 Gaseous Fuels
The electrolysis cells draw thousands of amperes (current densities of 1–4 kA/m2) (100–400 A/ft2) at high temperatures (up to 300C) and pressures (above 200 atm). The electrolyte is usually potassium hydroxide (KOH), but acidic solutions (H2SO4) are also used occasionally.
Table 6.5 Selected physical properties of hydrogen (H2)
Molecular weight (g/mol) 2.01594
Natural isotopic abundance*
1H 1.00794 g/mol 99.985 %
2H (deuterium) 2.01355 g/mol 0.015 %
3H (tritium)a 3.01550 g/mol 0
Melting point (K) 14.2
Boiling point (K) para-H2 20.27
Boiling point (normal-H2; 25 % p-H2) (K) 20.39
Triple point (K, n-H2) 13.96
Triple point liquid density (kg/m3) 77.20 Triple point solid density (kg/m3) 86.71 Triple point vapor density (kg/m3) 0.131 Boiling point liquid density (kg/m3) 71.0 Boiling point vapor density (kg/m3) 1.33
Critical temperature (K, n-H2) 33.19
Critical pressure (atm, n-H2) 12.98
Critical volume (kg/m3, n-H2) 30.12
Latent heat of fusion at T.P.; p-H2(J/mol) 117.6 Latent heat vaporization at B.P. (J/mol, n-H2) 897.3 Heat of combustion (gross) liquid H2O (kJ/mol) 285.8 Heat of combustion (net) gaseous H2O (kJ/mol) 241.8 Limits of flammability in air (vol.%) 4.0–75.0 Limits of detonability in air (vol.%) 18.0–59.0
Burning velocity in air (m/s) Up to 2.6
Burning velocity in oxygen Up to 8.9
Limits of flammability in oxygen (vol.%) 4.0–95.0 Limits of detonability on oxygen (vol.%) 15.0–90.0 Detonation velocity
15 % H2in O2(m/s) 1,400
90 % H2in O2(m/s) 3,600
Nonflammable limits, air-H2 <8 % H2
Nonflammable limits, O2-H2 <5 % H2
Maximum flame temperature at 31 % H2in air (K) 2,400
Autoignition temperature in air (K) 847
Autoignition temperature in oxygen (K) 833
*The radioactive decay constantl1/2¼12.26 year; n-H2refers to normal H2(25% p-H2 and 75% o-H2)
Table 6.6 Preparation of H2
1. Electrolysis of water H2O!H2+ ½O2
2. Thermal methods 2H1!H2+ I2
3. Natural gas CH4!C + 2H2; CH4+ H2O!CO + 3H2 CH4+ ½O2!CO + 2H2
4. Thermal, nuclear, electrical CH4+ H2O!CO + 3H2
5. Photoelectrolysis of water H2O +hv +catalyst!H2+ ½O2
2
At 25C,DG0¼ 237.13 kJ for water, but at 900C,DG0¼ 182.88 kJ for steam. Hence, the minimum voltageEth(900C)¼0.906 V at the higher temperature and the electrolysis of steam at high temperatures and pressures are also being studied as a means of producing hydrogen efficiently.
At 200C, the voltage of 1.3 V is the minimum voltage at which electrolysis would occur. Above this voltage, heat is produced due to the IR drop in the cell and due to the overvoltage on the electrodes.
At 1 ¢/kWh, the cost of 1 GJ of H2is 3.8 ¢ at 1.3 V and 4.9 ¢ at 1.8 V. Hence, great care is required in designing cells, electrodes, and electrode separators in order to reduce costs.
The production of oxygen which accompanies the hydrogen is a surplus product since oxygen is readily separated from nitrogen in liquid air or by the use of molecular sieves or membranes. Hence, by using a carbon anode, the following reactions would occur:
H2Oð1ÞþC!H2þCO DG0¼100:0 kJ=mol (6.6) H2OðaÞþC!H2þCO2 DG0¼40 kJ=mol (6.7) This can be compared with DGj0(H2O)1¼ 237.2 kJ/mol and represents a substantial energy saving. However, when hydrogen is prepared by this method, it no longer is an environmentally friendly fuel since CO and CO2will be formed.
The main problem involved in hydrogen production by electrolysis is the materials of construction since reliability is essential if explosions are to be avoided.
6.8.2 Thermal Methods
Several closed-cycle thermal processes have been developed whereby hydrogen can be produced from water using, e.g., coal as the source of heat. Three common schemes are presented in Table6.7.
Table 6.7 Three schemes for the thermal generation of hydrogen by closed-cycle processes with thermal efficiency (TE) and Carnot efficiency (CE)
Agnes
TE¼41–58% 3FeCl2+ 4H2O¼Fe3O44-6HCl + H2 450–750C
CE¼58% Fe3O4+ 8HCl¼FeCl2+ 2FeCl3+ 4H2O 100–110C
2FeCl3¼2FeCl2+ Cl2 300C
Cl2+ Mg(OH)2¼MgCl2+ ½O2+ H2O 50–90C
MgCl2+ 2H2O¼Mg(OH)2+ 2HCl 350C
Behiah
TE¼53–63% 2Cu + 2HCl¼2CuCl + H2 100C
CE¼63% 4CuCl¼2CuCl2+ 2Cu 30–100C
2CuCl2¼2CuCl + Cl2 500–600C
Cl2+ Mg(OH)2¼MgCl2+ H2O½O2 80C
MgCl2+ 2H2O¼Mg(OH)2+ 2HCl 350C
Catherine
TE¼64–83% 3I2+ 6LiOH¼5Lil + LiIO3+ 3H2O 100–190C
CE¼83% LiIO3+ Kl¼KIO3+ Lil 0C
KlO3¼Kl + 1½O2 650C
6LiI + 6H2O¼6HI + 6LiOH 450–600C
6HI + 3Ni¼3NiI2+ 3H2 150C
3NiI2¼3Ni + 3I2 700C
96 6 Gaseous Fuels
The corrosive nature of some of the products implies rather elaborate precautions in construction and design. The total energy required for the reaction
H2O!H2OðgÞ!H2þ1=2O2 DH0¼285:8 kJ (6.8) is 285.8 kJ if one starts with liquid H2O and 241.8 kJ if steam is available. In either case, this energy can be supplied in a series of steps as shown in Table6.7. The efficiencies given represent the thermal efficiencies of the sum of the steps as well as the Carnot (reversible) efficiency.
6.8.3 Natural Gas
The major use of hydrogen today is in the production of ammonia via the Haber reaction:
1 2N2þ3
2H2!Catalyst
½500C;300atmNH3 DH0¼ 46:2 kJ (6.9) The nitrogen is prepared by the fractional distillation of liquid air (b.p. N2¼ 196C, b.p.
O2¼ 183C), whereas the hydrogen is usually prepared by the thermal cracking of natural gas:
CH4!Cþ2H2 DH0¼74:8 kJ (6.10)
Hydrogen, as a component of synthesis gas, is also made from carbon and methane by the following reactions:
CþH2O!COþH2 DH0¼131:3 kJ (6.11)
CH4þH2O!COþ3H2 DH0¼206 kJ (6.12)
CH4þ1=2O2!COþ2H2 DH0¼ 36 kJ (6.13)
CH4þ2H2O!CO2þ4H2 DH0¼165 kJ (6.14)
The CO2can be separated from H2by its solubility in water at 25 atm.
6.8.4 Thermal-Nuclear-Electrical
Water is readily dissociated into its elements H2and O2by an electrical discharge or by thermal energy. The separation of H2from O2is then required. An interesting proposal presented several years ago was to detonate a nuclear explosive device in a landfill (garbage dump). A calculated yield of the products H2, CO, CO2, and CH4showed that as the organic content of the garbage reaches 80%, the H2and CO each approach 50%. The obvious problem in such a scheme is that the gases cannot be used until the radioactivity of the gases has decayed to acceptable levels.
2
6.8.5 Photoelectrolysis
About 25 years ago, it was first shown that ultraviolet light can dissociate water on a suitable semiconductor catalyst. Since then, it has been possible to photocatalytically dissociate water into H2 and O2at separate electrodes (hence, not requiring a separation of the gases) or to generate electrical energy for direct use. This rapidly expanding field will eventually achieve what nature does in plants, i.e., using visible light in multiple steps to convert CO2and water into cellulose. Efficiencies are usually low, and long-term stability has not as yet been achieved, but with continued efforts, it will be possible to produce hydrogen from sunlight and water.