Weighing up the options

While low-carbon blue hydrogen generation from fossil fuels currently offers lower costs, technology improvements in the coming decades will reduce production costs for zero-emission green hydrogen via electrolysis. In the near term, natural gas pipelines/storage already exist for blue hydrogen transport and storage. The nature of hydrogen storage will change going forward as the hydrogen sector moves towards producing more green hydrogen from excess renewable power. Methane pyrolysis, which produces grey hydrogen with a solid carbon byproduct (carbon black, carbon fibre, graphite, and coke), is a CO₂-free technology that can bridge the gap from fossil fuels to clean, renewable energy.

The appeal of decarbonised hydrogen energy comes with critical questions on how to adapt current compression technologies to fit the various means of production, transport, storage, and use.

Hydrogen production

The primary commercial methods for producing hydrogen are steam methane reforming (SMR), electrolysis, or methane pyrolysis. SMR is presently the most mature and least costly hydrogen production technology. It relies on a high amount of energy (heat generated via steam and supplied by boilers, nuclear, or turbines). SMR is also the current technology of choice for petrochemical hydrogen production.

SMR generally works through a catalytic conversion of methane and water to H₂ and CO. For higher efficiency, a second water-gas shift reaction uses additional steam to convert H2O and CO to more H₂ and CO₂. The CO₂ and H₂ are separated through pressure swing adsorption. H₂ produced by this method is referred to as grey hydrogen. When the CO₂ is captured from the SMR process, the hydrogen produced by this method is referred to as blue hydrogen.

In order to turn grey hydrogen to blue hydrogen, the CO₂ stream must be captured, compressed, and sequestered. Different processes, including partial oxidation and auto-thermal reforming, exist and/or are under development with widely varying CO₂ capture performance. Additional CO₂ capture from the exhaust steam generated from fossil fuels is another process that may be viable. Due to the growth of steam methane reformers for hydrogen, it is expected that carbon capture and sequestration needs will increase significantly until electrolysis costs can come down to compete.

To produce green hydrogen, electrolysis is powered by excess renewable power (typically at the site of a renewable energy farm, such as a solar field or wind turbine field). Excess power can also be transported via electric cables to power the electrolysis process closer to the point of hydrogen storage or usage.

Alkaline electrolysis produces hydrogen from water through electrochemistry. An electrolyser is comprised of an anode, cathode, electrolyte, and microporous membrane. In an alkaline cell, water is introduced to the cathode and decomposed into hydrogen and hydroxide (OH-). The OH- travels through the electrolyte and membrane to the anode, where O₂ is formed. The hydrogen is then left in an alkaline solution where it is separated from the water via gas to liquid separation. Commercial alkaline electrolysers have a range of reported efficiencies and working pressures, but nominally require 55.5 kWh/kg of hydrogen compared to 45.3 kWh/kg for SMR.

This article was originally published in the August 2024 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Marybeth McBain, Klaus Brun and Karl Wygant, Ebara Elliott Energy.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/07082024/weighing-up-the-options/