When it comes to generating hydrogen, there is a great saying from the EWEA (European Wind Energy Association - http://www.ewea.org) - H2 is only as clean as the energy source used to obtain it. If a coal burning arrangement was used to make the H2 (directly or indirectly), and the CO2 pollution is not properly disposed on, it's filthy H2. Same goes for nuke derived H2, and for natural gas derived H2.
However, lets say you have a renewable energy source - whether biomass based or wind and/or hydroelectric made electricity. You can take that electricity as pass it though an ionic water solution (water with a salt of some sort dissolved in it) - and water splits, with H2 going to the cathode. If ordinary salt is used, H2 comes off at the cathode and chlorine gas (Cl2) comes off at the anode, and sodium hydroxide is made in the electrolysis cell. Millions of tons of NaOH and Cl2 are made each year in the U.S, along with that H2, which always finds a use, even if it is just as a boiler fuel.
If potassium hydroxide is used as the salt, H2 comes off at the cathode and O2 comes off at the anode. H2 has been made this way commercially for over 100 years - usually KOH is the preferred electrolyte. For efficient systems, it takes about 21.4 to 25 kw-hr of electricity to make 1 lb of H2 and 8 lbs of oxygen (O2). The efficiency of the best systems is usually between 85 % (21.4 kw-hr/lb H2) to 80%. And if the electricity was green, the H2 is clean.....
To do this on a large scale, special electrolysis cells are needed, and these operate at atmospheric pressure and about 80 to 85 C (176 to 185 F). Smaller units can make H2 under pressure, but the entire apparatus must be enclosed in a stainless steel pressure vessel, which gets expensive when the vessel has to be 10 ft in diameter and about 25 ft long. The water fed to the unit must be extremely pure (18 megaohm), and this is done via reverse osmosis. Even the CO2 dissolved in the water has to be removed, and any impurity (calcium salts, for example) would gunk up the electrodes. But, the H2 and O2 made are extremely pure, aside from the water content, which is easily removed via molecular sieves. Since 10 to 20% of the electricity going into the cell does not make product but instead heat, that heat has to be removed (basically this is electrical resistance of the 25 wt% KOH solution).
Anyway, that's the summary of how to make really pure H2 from clean electricity. The trick is getting the clean electricity...but this can be done. And its an important way to make "green" ammonia,
( Note - not hydrolysis)
When it comes to generating hydrogen, there is a great saying from the EWEA (European Wind Energy Association - http://www.ewea.org) - H2 is only as clean as the energy source used to obtain it. If a coal burning arrangement was used to make the H2 (directly or indirectly), and the CO2 pollution is not properly disposed on, it's filthy H2. Same goes for nuke derived H2, and for natural gas derived H2.
However, lets say you have a renewable energy source - whether biomass based or wind and/or hydroelectric made electricity. You can take that electricity as pass it though an ionic water solution (water with a salt of some sort dissolved in it) - and water splits, with H2 going to the cathode. If ordinary salt is used, H2 comes off at the cathode and chlorine gas (Cl2) comes off at the anode, and sodium hydroxide is made in the electrolysis cell. Millions of tons of NaOH and Cl2 are made each year in the U.S, along with that H2, which always finds a use, even if it is just as a boiler fuel.
See http://en.wikipedia.org/wiki/Water_electrolysis for a primer on this.
If potassium hydroxide is used as the salt, H2 comes off at the cathode and O2 comes off at the anode. H2 has been made this way commercially for over 100 years - usually KOH is the preferred electrolyte. For efficient systems, it takes about 21.4 to 25 kw-hr of electricity to make 1 lb of H2 and 8 lbs of oxygen (O2). The efficiency of the best systems is usually between 85 % (21.4 kw-hr/lb H2) to 80%. And if the electricity was green, the H2 is clean.....
To do this on a large scale, special electrolysis cells are needed, and these operate at atmospheric pressure and about 80 to 85 C (176 to 185 F). Smaller units can make H2 under pressure, but the entire apparatus must be enclosed in a stainless steel pressure vessel, which gets expensive when the vessel has to be 10 ft in diameter and about 25 ft long. The water fed to the unit must be extremely pure (18 megaohm), and this is done via reverse osmosis. Even the CO2 dissolved in the water has to be removed, and any impurity (calcium salts, for example) would gunk up the electrodes. But, the H2 and O2 made are extremely pure, aside from the water content, which is easily removed via molecular sieves. Since 10 to 20% of the electricity going into the cell does not make product but instead heat, that heat has to be removed (basically this is electrical resistance of the 25 wt% KOH solution).
Anyway, that's the summary of how to make really pure H2 from clean electricity. The trick is getting the clean electricity...but this can be done. And its an important way to make "green" ammonia,
Nb41