Mar-2023

Mineralisation to capture and use CO2 from steam methane reforming

Where CCS with underground CO2 storage is not possible, mineralisation of the CO2 to chemicals such as soda ash or sodium bicarbonate is an emerging solution.

Stephen Harrison
sbh4

Article Summary

Carbon capture is an essential tool in the global decarbonisation tool kit
Much has been said about CCS – carbon capture and storage. The need to decarbonise is clear. Renewable power generation and green hydrogen may do much of the heavy lifting as they scale up in coming decades, but there are legacy assets that must also be decarbonised, and there are several processes that release CO2 from within the process chemistry.

Neither renewable power nor green hydrogen will be able to avoid CO2 release from calcium carbonate rock during the calcination of limestone to make lime or cement. Also, steam methane reformers (SMR) release CO2 from the process chemistry. So, even if the required heat for the endothermic reaction is provided by renewable power, CO2 emissions from the conversion of the methane to hydrogen-rich syngas would still occur (see Figure 1).

While the benefits of capturing CO2 emissions before they reach the atmosphere are widely accepted, the possibility of storing CO2 underground in depleted gas fields or saline aquifers relies heavily on having the right sub-surface geological conditions. In addition to geological constraints, public opinion and political will must also be aligned before underground CO2 storage can be considered a sequestration method.

Carbon capture and mineralisation (CCM) is an alternative to CCS. The starting point is, in principle, the same: CO2 emissions are captured before they are released into the air. However, instead of storing the CO2 underground, it is reacted with chemicals to form inert mineral salts. CCM is, in essence, an example of carbon capture and utilisation.

The chemicals that react with the CO2 to form the minerals can also be used to capture the CO2 from the flue gas, thereby differentiating from established carbon capture technologies that consume either high amounts of steam or electricity at the CO2 capture location.

Airovation Technologies, based in Israel, has developed an innovative CCM process that reacts commonly available chemicals, such as sodium hydroxide, with CO2 from flue gases to produce mineral salts, such as sodium carbonate and sodium bicarbonate, which have a wide range of commercial applications.

CO2 generation during steam methane reforming
Renewable power supplied to an electrolyser can produce green hydrogen, and many new-build hydrogen plants will operate this way. However, the most common hydrogen production process, used to generate about 80% of the world’s hydrogen, is steam methane reforming. There is a legacy of more than 1,000 operational SMRs around the world. Retrofitting CCM, or another carbon capture technology, can decarbonise these assets and extend their life.

Steam methane reforming uses natural gas, refinery gas, or naphtha as feedstocks. When these fossil feedstocks are used to generate hydrogen without capturing the CO2 emissions, it is called ‘grey’ hydrogen. If most of the CO2 from the SMR is captured, the hydrogen is referred to as ‘blue’.

CO2 is released from the SMR in two locations: firstly, as the feedstock is transformed to hydrogen, CO2 is produced as an unavoidable by-product. Secondly,

CO2 emissions from the combustion of fossil fuels (generally a portion of the natural gas feedstock) create the heat required to drive the reforming chemical reactions that convert the feedstock to hydrogen (see Figure 2).

CO2 is unavoidably released from the process during reforming, cement, glass and steel production. The flue gas streams from these processes are rich in CO2 at about 15%. This can double to 30% if oxy-fuel combustion has been used. This compares with flue gas from a normal gas- or coal-fired combustion process, which typically contains 2-6% CO2 (see Figure 3).

Whilst these processes are responsible for high CO2 emissions globally, they are also some of the best processes to target for CO2 capture because removing CO2 from these flue gases is more cost effective per tonne of CO2 sequestered than capturing CO2 from a very dilute flue gas stream, such as the emissions from a gas-fired turbine power plant.

The Airovation Technologies CCM process is ideal for treating steam methane reforming flue gases and other flue gases with similarly elevated CO2 concentrations. It benefits from the process intensity, and the reactions respond well to the elevated CO2 concentrations.

Conventional SMR carbon capture technologies can stress utilities infrastructure
The most widespread technology for carbon capture uses a twin tower process, where CO2 from the flue gas is absorbed into an amine-based solvent in the first tower. The CO2-lean flue gas flows to the atmosphere. The CO2-rich amine is pumped to a second stripper tower, where steam is used in vast quantities to boil the CO2 away from the amine solvent. The regenerated, CO2-lean amine solvent is pumped back to the absorber tower to collect more CO2, and the process operates continuously, with the amine being recirculated from the absorber to the stripper.

Variations of the amine-based carbon capture process use chilled ammonia, methanol or potassium bicarbonate as the solvent to absorb the CO2. The process configuration is similar. All these processes require a huge heat energy input to boil the CO2 out of the solvent. Unfortunately, this additional energy is not always available at the site where the CO2 must be captured. The ramp-up in natural gas or fuel oil supplies may stress the local infrastructure beyond its capability (see Figure 4).

Vacuum swing adsorption (VSA) and temperature swing adsorption have also been used for refinery SMR CO2 capture. VSA locks the CO2 into a solid molecular sieve adsorbent. In the VSA system, a rapid pressure reduction releases the CO2 from the adsorbent. Electrical power is required in large quantities, and the local power supply infrastructure may not be sized for the additional demand.