Hydrogen Purification and Production

 

Hydrogen gas is generated from multiple different ways and is typically produced mixed with extraneous matter. Two of the most typical ways to create hydrogen are chemical synthesis by steam reforming of natural gas, partial oxidation of methane, and gasification processing where hydrogen gas is produced as a by-product, and petrochemical applications such as hydrocracking or desulphurization; while the other method of hydrogen generation is made by putting a lot of electricity into water to separate hydrogen from water (water electrolysis). However, steam reforming of natural gas has only 70-85% efficiency for hydrogen production.

To generate purified hydrogen in the 99.99% purity levels, a purification process is necessary to create purified hydrogen gas. The purification process involved Pressure Swing Adsorption (PSA). PSA is a process that involves the production of contaminants or side products that need to be removed. It includes compounds such as carbon monoxide, carbon dioxide, nitrogen, water and unreacted hydrocarbons. PSA eliminates other compounds from the hydrogen stream to yield purified hydrogen.

The most used method that is capable of producing high purity level of 99.99%, namely Pressure Swing Adsorption, concentrates on Palladium Membrane Diffusion comes into consideration.

Hydrogen purification by palladium membrane diffusion is the proven technology for hydrogen purity. The palladium membrane comprises a palladium and silver alloy material possessing the unique property of only allowing hydrogen gas molecule coming into contact with the palladium membrane surface dissociates into atoms to pass through its crystal lattice when it is heated approximately 400ºC. The atoms are small enough to diffuse through the palladium membrane, driven by differential hydrogen pressure across the membrane. The hydrogen atoms recombine into molecules after passing through the membrane. Only hydrogen atoms, no other materials, can diffuse through the palladium diffuser and so the permeation is only hydrogen.

Advantages and Disadvantages of Hydrogen Production

Hydrogen generated from electrolysis
Advantages	Disadvantages
If the electricity for electrolysis is sourced from renewables with low CO2, then the net energy cycle is very low carbon. No negative emissions at end point of use.	Electrolysis is about 70% efficient, meaning about 30% of the energy in the electricity is wasted. If the electricity for electrolysis is sourced from fossil fuels, then the net energy cycle is higher carbon. Fuel cells are only 40% to 60% efficient and waste heat is generated. If the waste heat is used as well, overall efficiency at point of generation can be greater, but the theoretical maximum is 85%. At minimum, 15% of stored electricity is thrown away. In reality, no automotive fuel cell captures the waste heat, so 40% to 60% of the stored electricity is thrown away. If the hydrogen is burned in a Carnot or steam cycle, then efficiency is even lower than via a fuel cell, closer to gasoline where efficiency is in the range of 20%.
Hydrogen generated from steam reforming of Methane
Advantages	Disadvantages
Net positive source of energy. No negative emissions at end point of use. Less expensive than hydrogen from electrolysis.	Processing emits just as much CO2 as burning the methane directly, so it contributes to global warming. It’s energy inefficient compared to burning the methane in a combined cycle gas generator to get much more of the energy. Nitrous and sulfur oxides are emitted by processing and create air pollution. Extracting methane is shown to have significant leakage of methane to the atmosphere, and methane is a much more potent greenhouse gas than CO2. There are local impacts of fracking such as minor earthquakes and in some cases ground water pollution to consider.