Feb-2022

Safe oxygen production for Giga-scale hydrogen generation

ASU design and operation relies on engineering expertise to ensure that the Giga-scale production of oxygen to make low carbon hydrogen is done safely.

Stephen B Harrison
sbh4

Article Summary

Oxygen is one of the main industrial gases and, at present, oil and gas processing and iron and steelmaking are the major consumers of oxygen worldwide. In the future, as there is a progressive decarbonisation of the energy sector, low carbon hydrogen production from natural gas and coal will be a major driver of growth in oxygen demand. Handling hydrogen comes with obvious hazards of flammability and explosion. The production of oxygen also requires the utmost respect for safety and best practices in operations and engineering.

Oxygen is required to make low carbon blue hydrogen
As hydrogen production ramps up, we are leaping orders of magnitude from Mega-scale to Giga-scale projects. The largest hydrogen electrolysers operating today are in the 20 Mega Watt (MW)range. Plans already exist for Giga Watt (GW)systems. Scale-up is also the order of the day for hydrogen production from natural gas and coal.

As an example, the H2H Saltend project that is planned to support decarbonisation of the Humber industrial cluster in East Yorkshire, UK. It proposes a 600 MW autothermal reformer (ATR) to make hydrogen-rich syngas. Conventional steam methane reformers (SMRs) do not require pure oxygen. On the other hand, ATRs do. To enable 600 MW of hydrogen production, the air separation unit (ASU) would need to produce about 1200 tonnes of oxygen per day. That would make it one of the largest in the UK.

Producing hydrogen from natural gas using reformers means that carbon dioxide (CO2) is generated from the process chemistry and the heat energy requirements. ‘Blue hydrogen’ is produced through the integration of carbon capture and storage (CCS) with the reformer. To qualify for the ‘blue’ hydrogen label, the captured CO2 will be sent to a CCS scheme under the North Sea, where it will be stored underground permanently in natural geological formations.

CCS in the North Sea is a proven technology. Equinor commenced capture and sequestration of CO2 on the Sleipner West field in the Norwegian sector more than 20 years ago. The components of a CCS scheme, from the absorption tower to the multi-stage CO2 compressor with integrated drying system, are all highly developed.

Beyond Norway, CCS, or carbon capture and usage (CCUS) combined with enhanced oil recovery (EOR), has also been used in Australia, Canada, and the United States for many years. Most major schemes have involved carbon capture retrofits onto existing carbon-intensive processes, such as decarbonisation of coal fired power plants and carbon capture from SMRs.

Since these have been retrofits projects, reforming technologies and CCS have been developed in parallel. They have never been optimised synergistically as an integrated process. Regarding blue hydrogen production, a paradigm shift is required: the system must be optimised in a holistic way.

Downstream of the reformer, some blue hydrogen schemes will also integrate a blue ammonia plant. Like hydrogen, ammonia is a carbon-free energy vector. Use of ammonia as a fuel will expand its range of applications and drive significant growth in ammonia demand. Ammonia production would require nitrogen as a feedstock to react with the hydrogen to make ammonia. The nitrogen could be produced on the ASU alongside the oxygen that feeds the ATR. The reforming, air separation and ammonia processes would be interdependent, and the integrated equipment can be viewed as a decarbonised ‘blue-energy island’.

Giga-scale oxygen for gas-to-liquids
The use of natural gas and ASUs to make syngas for fuels goes beyond their proposed application to make hydrogen. In 2006, the Oryx gas-to-liquids (GTL) project in Qatar was built to add value to natural gas and produce liquid fuels as energy-dense export products. Oryx has two large ATRs. Each is fed by a large ASU, rated at 3,500 tonnes per day of oxygen. The two ASU cold boxes for Oryx were built by Air Products at Acrefair in Wales. In a similar project, the Escravos GTL facility in Nigeria started up in 2014. It is of a similar configuration to Oryx, and it also uses two Air Products ASUs rated at 3,500 tonnes per day of oxygen.

Shell’s Pearl GTL facility was constructed at Ras Laffan in Qatar, close to the Oryx plant, and started up in 2011. It is fed by eight Linde ASUs, each one rated at around 3,500 tonnes per day of oxygen to produce almost 30,000 tonnes per day. In contrast to Oryx, Pearl uses partial oxidation (POX) to convert natural gas to syngas. The use of POX natural gas gasification technology for GTL production was pioneered by Shell in 1993 at Bintulu on the island of Sarawak. At Bintulu, oxygen for the POX gasification reactor is supplied by a 3,200 tonne per day ASU supplied by Air Liquide.

Partial oxidation is like autothermal reforming because the reaction takes place in one unit, to which oxygen and natural gas are supplied. But it differs from both the SMR and ATR processes because neither catalyst nor steam are used. When wood, coal, or petcoke are used as feedstock, this process is called ‘gasification’.
A subtle difference between the SMR, ATR, and POX processes is the pressure at which they operate. Whilst SMRs typically operate in the range of 15 to 40 bar, ATRs are more comfortable in the 30 to 50 bar range and POX reactors can operate up to 80 bar.

If hydrogen is intended for injection into gas transmission pipelines, producing it at high pressure is a tremendous benefit because a hydrogen compressor after the reformer can be avoided. This reduces both Capex and electrical power demand. This is one of the drivers for the selection of ATRs in proposed Giga-scale projects if an application of the hydrogen is to substitute natural gas to decarbonise domestic cooking and heating applications.

Hydrogen from coal gasification also pulls for Giga-scale oxygen supply
Beyond natural gas reforming and partial oxidation, coal and petcoke gasification is another Giga-scale pathway to make hydrogen-rich syngas. Gasification, like ATR and POX, requires oxygen. The use of pure oxygen, instead of air, is beneficial for precise control of the oxidation chemistry and avoids costly flue gas de-NOx systems. It also makes the integration of CCS more cost-effective because the system can be much smaller due to the avoidance of processing thousands of tonnes of nitrogen from the air.

One of the world’s largest gasification projects will come into operation at Saudi Aramco’s Jazan refinery where more than a dozen gasifiers built by Técnicas Reunidas will produce syngas from heavy refinery residues and petcoke. In total, the gasifiers at Jazan will be capable of producing 2 million normal cubic metres per hour of syngas.

At Jazan, the gasifiers will produce enough syngas to generate a total of 4 GW of power and steam. The syngas will be fired directly in gas turbines which produce 2.4 GW of electricity in an integrated gasification combined cycle, or IGCC power plant. The syngas-island will also export hydrogen and steam to the refinery. To feed the hungry gasifiers at Jazan, the process requires six Giga-scale ASUs supplied by Air Products, each one rated at 3,000 tonnes per day of oxygen.