Liquid Hydrogen (LH2), as a liquid form of hydrogen gas, is widely used in aerospace, energy storage, clean fuels, and various other fields. Due to hydrogen’s high energy density and the fact that its combustion only generates water, it is considered a key element for future sustainable energy. However, the process of converting hydrogen gas into liquid hydrogen is highly technical and requires strict control. This process involves multiple steps, including hydrogen production, purification, and liquefaction. Below, we will detail the production process of liquid hydrogen and provide additional information on onsite hydrogen production and related precautions.

Hydrogen Production

The production of liquid hydrogen depends on the preparation of hydrogen gas. The main methods of hydrogen production are as follows:

Steam Methane Reforming (SMR)

Steam methane reforming is the most commonly used industrial method for hydrogen production. This process uses methane (CH₄) from natural gas to react with high-temperature steam to generate hydrogen (H₂) and carbon monoxide (CO). The chemical equation for this reaction is:

CH₄ + H₂O → CO + 3H₂ (ΔH = +206 kJ/mol)

In this reaction, methane reacts with steam under the influence of a catalyst (such as nickel catalyst) at high temperatures (around 800°C to 900°C), generating hydrogen and carbon monoxide. The carbon monoxide is then further converted into carbon dioxide and hydrogen through a water-gas shift reaction:

CO + H₂O → CO₂ + H₂

This reaction also requires a catalyst (such as platinum or iron-based catalysts) and high temperatures (around 400°C to 500°C). Finally, gas separation methods are used to remove impurities like carbon dioxide, yielding purified hydrogen.

Although this method is cost-effective, it involves carbon dioxide emissions, which poses environmental challenges.

Electrolysis of Water

Electrolysis of water is a process in which water (H₂O) is decomposed into hydrogen and oxygen by passing an electric current through it. The chemical equation for this reaction is as follows:

 2H₂O→2H₂ + O₂

The process occurs in an electrolyzer, where energy from an external power source is used to overcome the molecular bonding energy of water. During electrolysis, hydrogen is produced at the cathode, and oxygen is produced at the anode. By adjusting the current and the concentration of the electrolyte (usually sodium hydroxide or potassium hydroxide solution), hydrogen of varying purity can be obtained. The advantage of electrolysis is that it produces hydrogen of very high purity (close to 99.999%) and does not emit greenhouse gases, making it a clean method of hydrogen production, especially when driven by renewable energy sources.

Coal Gasification

Coal gasification is a process in which coal reacts with steam under high temperature and pressure to produce synthesis gas (syngas), which contains hydrogen, carbon dioxide, carbon monoxide, methane, and other components. The syngas is then further processed to separate hydrogen. The simplified chemical equation for coal gasification is:

C+H₂O→CO+H₂C+H₂O→CO+H₂

This method is suitable for areas rich in coal resources but faces the environmental challenge of high carbon dioxide emissions. To reduce the environmental impact, carbon capture and storage (CCS) technology is often integrated.

Biomass Gasification

Biomass gasification is a process that converts organic materials (such as wood, agricultural waste, etc.) into synthesis gas under high-temperature, oxygen-limited conditions. Similar to coal gasification, the synthesis gas produced contains hydrogen, carbon monoxide, methane, and other components. This method is more environmentally friendly, but its economic viability and large-scale application still face challenges.

Hydrogen Purification

The purity of hydrogen is crucial for its subsequent applications, especially in the liquefaction process. Common methods for hydrogen purification include:

Pressure Swing Adsorption (PSA)

PSA is a method that separates gases based on their different adsorption abilities on the surface of adsorbents. The process involves changing the system’s pressure to separate hydrogen from other impurities (such as carbon monoxide, methane, carbon dioxide, etc.). Hydrogen is adsorbed by the adsorbent and released when the pressure is reduced, achieving hydrogen purification.

Cryogenic Distillation

Cryogenic distillation relies on the differences in boiling points of different gases. By cooling the hydrogen mixture to extremely low temperatures, hydrogen can be separated from other gases. Hydrogen has a boiling point of -252.87°C, while other gases (such as oxygen, nitrogen, methane) have higher boiling points. By precisely controlling the temperature, hydrogen can be separated from the gas mixture.

Membrane Separation

Membrane separation utilizes the differences in the ability of hydrogen and other gas molecules to pass through a membrane. Hydrogen molecules, due to their smaller molecular weight, are more likely to pass through specific types of membranes, achieving efficient separation.

The Liquid Hydrogen Process

The process of producing liquid hydrogen mainly involves the cooling, compression, and expansion of gas. The key to liquid hydrogen production is to lower the temperature of hydrogen to below its boiling point (-252.87°C) and transform it into a liquid. The liquid hydrogen process typically uses the following techniques:

Gas Compression and Expansion

The first step in liquefying hydrogen is to compress the hydrogen gas to high pressure (typically 100 to 200 bar). The compressed hydrogen gas is then rapidly expanded, which causes the gas to cool down. According to thermodynamic principles, when a gas expands, it loses internal energy, causing the temperature to drop.

Multistage Refrigeration Cycle

Hydrogen liquefaction usually involves a multistage refrigeration cycle, using different refrigerants and multistage coolers to progressively lower the temperature of hydrogen until it becomes liquid. Common refrigerants include helium, nitrogen, and ammonia. In each stage, hydrogen and refrigerant exchange heat in heat exchangers, gradually reducing the temperature of hydrogen.

Carnot Cycle

The Carnot cycle is an ideal thermodynamic cycle that uses a series of compression, expansion, and heat exchange steps to achieve low-temperature liquefaction. In the hydrogen liquefaction process, the Carnot cycle is often used to maximize efficiency, especially under high-pressure and low-temperature conditions.

Storage and Transport

Storage of liquid hydrogen typically requires high-strength insulated containers. These containers use vacuum insulation technology to reduce heat conduction and prevent the hydrogen from vaporizing. The storage system design must also consider hydrogen’s evaporation loss and temperature control, ensuring that liquid hydrogen remains at low temperatures for extended periods.

Onsite Hydrogen Production Process and Precautions

In industrial applications, hydrogen needs to be produced onsite based on demand. Common methods for onsite hydrogen production include steam methane reforming and electrolysis. Below are detailed processes and precautions:

Steam Methane Reforming Onsite

When using steam methane reforming onsite, natural gas (mainly methane) first reacts with steam at high temperatures to produce carbon monoxide and hydrogen. This process requires the use of a high-temperature reactor, with the reaction temperature controlled between 700°C to 900°C. The selection of catalysts is crucial, with nickel-based catalysts commonly used. The resulting hydrogen must undergo water-gas shift reactions and gas separation to achieve high-purity hydrogen.

Electrolysis Onsite

The principle of onsite hydrogen production via electrolysis is to generate hydrogen and oxygen by electrolyzing water. This process requires sufficient electrical energy, and the electrolyzer should be regularly maintained to prevent electrode damage. During electrolysis, the concentration of the electrolyte, temperature, and current intensity need to be strictly controlled to ensure the production and purity of hydrogen.

Precautions

Equipment Safety: Hydrogen is an extremely flammable gas, so hydrogen production equipment must adhere to high safety standards. All equipment should have leak prevention, explosion protection, and pressure control functions.

Purity Monitoring: The purity of hydrogen must be monitored in real-time during onsite production to ensure it meets usage standards, especially for high-precision applications such as fuel cells.

Waste Gas Treatment: Some hydrogen production methods may generate waste gases like carbon dioxide. Effective waste gas treatment systems must be in place to avoid environmental pollution.

The production of liquid hydrogen involves multiple complex steps, from hydrogen preparation and purification to liquefaction, each requiring strict technical and safety management. As hydrogen energy technology advances, the application prospects of liquid hydrogen in energy storage, transportation, and other fields will become increasingly promising. The maturity of onsite hydrogen production technology and safety guarantees will be key factors in driving the development of the hydrogen economy.