With the need for decarbonization and emission reduction, hydrogen energy is being used more and more widely. As an energy source, hydrogen is gradually entering our daily lives, so the demand for hydrogen is growing. Due to the unique properties of hydrogen, it rarely exists in nature as elemental hydrogen (i.e., H₂ gas). Therefore, we need to produce hydrogen from hydrogen compounds. Methanol-based hydrogen production is one such method.
There are two common methods for producing hydrogen from methanol: methanol reforming for hydrogen production and methanol cracking for hydrogen production. In addition, there are some niche and novel methods, such as partial oxidation of methanol, electrolysis of methanol for hydrogen production, and ultrasonic decomposition of methanol‑water solutions for hydrogen production. Since the latter three methods are not mainstream due to various drawbacks or immaturity, we will not discuss them here.
At present, in China's hydrogen energy field, where methanol-based hydrogen production is needed, methanol reforming is the dominant approach. So why is methanol reforming used instead of methanol cracking? Let me explain.
Methanol Reforming for Hydrogen Production
Methanol reforming for hydrogen production is also known as the methanol steam reforming method. The chemical reaction is as follows:
In the reaction formula, CH₃OH represents methanol. As can be seen from the reaction equation, water needs to be added during the reaction. In practice, deionized water (pure water) is first mixed with methanol at a ratio of approximately 1:1 to obtain a methanol solution. The methanol solution is then vaporized at a temperature of 250–300°C and fed into the reactor. Inside the reactor, water molecules react with methanol molecules to produce hydrogen and carbon dioxide, along with a small amount of carbon monoxide. The gas composition is as follows:
- H₂: approximately 75%
- CO₂: approximately 25%
- CO:<1%
The process flow is shown in the figure below.
Process flow diagram of methanol steam reforming for hydrogen production
Methanol steam reforming hydrogen production technology has the advantages of convenient operation, mild reaction conditions, and few by-products that are easy to separate. Therefore, it is the most mature and widely used method for hydrogen production from methanol.
Methanol Cracking for Hydrogen Production
Methanol cracking for hydrogen production uses the direct decomposition of methanol to produce hydrogen. The chemical reaction is as follows:
As can be seen from the reaction equation, this method does not require the addition of water. The products of the reaction are hydrogen and carbon monoxide, with the following gas composition:
- H₂: approximately 66%
- CO: approximately 33%
- CO₂ and other gases: ≤1%
The process is as follows (see figure below): The raw material methanol enters a buffer tank, then passes through a heat exchanger for temperature increase, and then enters a vaporization superheater where it is vaporized using high-temperature thermal oil. Afterwards, it enters a shell-and-tube reactor, where cracking and shift reactions take place in the presence of a catalyst, producing carbon monoxide and hydrogen.
Process flow diagram of methanol cracking for hydrogen production
From the product gas composition obtained by the two methods above, it can be seen that the reforming method yields 75% hydrogen, with by-product gases consisting of 25% carbon dioxide and less than 1% carbon monoxide. In contrast, the cracking method yields only 66% hydrogen — less than that of the reforming method. More critically, the by-product gas contains as much as 33% carbon monoxide. Carbon monoxide is a highly undesirable gas in hydrogen energy applications, because it poisons the catalyst in fuel cells that generate electricity from hydrogen. This can render expensive catalysts ineffective, reduce the lifespan of the fuel cell, or even cause it to be scrapped. Removing carbon dioxide from the gas mixture is relatively easy, whereas removing carbon monoxide is difficult and costly.
Therefore, from both a technical and economic perspective, producing hydrogen by the methanol reforming method is more economical and more widely adopted than by the methanol cracking method.
