With the growing interest in hydrogen fuel cells for energy generation, the need for a decentralized supply of hydrogen is also increasing. Partial oxidation is one of the processes currently in use for hydrogen production. The following article explains how this works and what types of partial oxidation are available.
Partial oxidation is one of the so-called reforming processes for hydrogen production. Even before electrolysis, reforming is one of the most important processes for producing hydrogen. More than 500 billion cubic meters of hydrogen are produced worldwide each year by reforming and around 15 billion cubic meters by electrolysis.
Reforming requires extremely high temperatures and a catalyst. An oxidizing agent is also required, for example steam, oxygen or a mixture of both starting materials. If oxygen is used, this is called partial oxidation.
Partial oxidation of methane or natural gas is also possible, producing the alcohol methanol, which is also used in direct methanol fuel cells..
The partial oxidation process for hydrogen is technically very mature. In this process, oxygen is added to the available raw material, i.e. natural gas or heavy hydrocarbons such as fuel oil or residual oils from petroleum processing. This process is exothermic, i.e. heat is released. The starting materials are the residues produced in the refinery process, i.e. liquid hydrocarbons. Although these are liquid, they have a high viscosity, i.e. they are very viscous or viscous.
In partial oxidation, the mixture of fuel and air is partially burned in a process furnace. 3 Via a dedicated burner, it enters the reactor, which is operated at 40 to 100 bar pressure. This process produces a synthesis gas rich in hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), making it suitable for further use in fuel cell technology.. The synthesis gas obtained is then cooled and the carbon dioxide is separated by amine scrubbing.
There are two types of partial oxidation: Thermal Partial Oxidation (TPOX) and Catalytic Partial Oxidation (CPOX). While TPOX requires temperatures of at least 1200 degrees Celsius for the reactions, CPOX uses a catalytic medium. Therefore, the temperature required for the latter is only 800 to 900 degrees.
Which technique is used in the reforming process depends on the amount of sulfur present in the fuel. If the sulfur content is less than 50 millionths (ppm), catalytic partial oxidation can be used. Since a higher sulfur content would lead to poisoning of the catalyst, Thermal Partial Oxidation is used for such fuels.
Partial oxidation with coal is also possible. The chemical process is similar to the reaction of the oil, but before that a coal treatment takes place. In this step, the coal is ground and mixed into a suspension with the addition of water.
The product of partial oxidation as a gasification reaction is a gas mixture that is used as a synthesis gas, as a heating gas or as a feedstock for hydrogen production. In addition to hydrogen, partial oxidation also produces carbon monoxide (CO), carbon dioxide (CO2) and water.
Partial oxidation is an efficient and recognized process for the recovery of synthesis gas. The advantage is that the ratio between hydrogen and carbon in the synthesis gas can be individually adjusted by various process steps – for example, the use of a membrane. The ratios can thus be adapted to the customer’s requirements without any problems.5 Another advantage of partial oxidation is that it allows the refinery residues used to be recycled particularly cleanly and economically.
A disadvantage of partial oxidation is its comparatively low efficiency due to the low hydrogen yield and the risk of coking, i.e. the conversion of coal into coke.