Decarbonisation Technology - August 2023 Issue

This example shows both the magnitude of the challenge and the potential benefit of reducing CO₂ emissions from hydrogen production via decarbonising existing steam methane reforming (SMR) facilities. Hydrogen production unit under consideration Given its significant near-term positive potential, this article focuses on four options that Wood has developed for decarbonising an existing hydrogen production unit based on steam reforming. The unit on which this analysis has been performed was designed by Wood in 2017 and has been operational since 2020. It is designed to produce 80,000 Nm³/h (approximately 173 TPD) of hydrogen at 99.9% purity by reforming natural gas and does not have a pre-reforming section or any provision for carbon capture installation. It works on high-temperature (HT) shift and is equipped with an HT combustion air pre-heating system. In each of the four cases, reduction in CO₂ emissions/carbon impact, capital expenditure, operational expenditure, and project implementation time are compared with a reference case. The reference case is the revamp of the existing unit by adding a post-combustion carbon capture system. To better understand the solutions in this article, it is important to properly define some terms that will be used:  Post-combustion carbon capture : this refers to a solvent-based carbon absorption system added to the flue gas stream leaving the hydrogen production unit by means of the stack installed on top of the furnace.  Pre-combustion carbon capture : this refers to a carbon removal system installed on the

syngas stream leaving the reaction section of the hydrogen unit (meaning, it removes the CO 2 before the syngas enters the final purification section).  Gas heated reformer : a convection-type steam reforming reactor in which the process heat required is provided by cooling down the syngas leaving the steam reforming furnace.  Carbon impact : is the parameter used by UK regulations to measure the CO₂ footprint of the facility under assessment. For the purpose of this article, Wood has considered both the carbon capture options to be a solvent-based absorption system. The results described are not fully exhaustive of the options available on the market, and each unit will have its own unique characteristics. Reference case The reference case is based on the installation of a post-combustion carbon capture on the reference hydrogen unit (see Figure 1 ). The main advantage of this system is the option to add carbon capture without making major modifications to the existing hydrogen unit. A post-combustion carbon capture system is, in fact, a completely independent unit for which the sole major modification is to create a flue gas connection point to the existing hydrogen unit (typically on the stack of the steam reforming furnace). Flue gas is then sent to the carbon capture system by means of a blower, treated to separate the CO₂ from the other components, and finally delivered to battery limits in two streams: i) a pure CO₂ stream that goes to storage or to reuse, ii) a stream of decarbonised flue gases to be sent back to the stack of the hydrogen unit – or to a new one – which is discharged to the atmosphere. While the application of post-carbon capture is the simplest conceptually, it is a high Capex solution, which includes washing columns, exchangers, and pumps. The high cost is related to the fact that the system works at very low pressure (just a little more than atmospheric pressure). Therefore the CO₂ concentration (its partial pressure) is low, making the specific volumes of flue gas to be processed very high. This also requires a high electric power consumption, which comes with additional

CO for use or sequestration

Clean ue gas to atmosphere

Post-capture

Flue gas

Feed

Steam reforming

Hydrogen product

Shift

PSA

PSA tail gas

Fuel

Figure 1 Reference case: post-combustion carbon capture