contain known components (H 2 O, CH 4 , CO, CO 2 , and N 2 ), so impuri- ties such as oxygen and sulphur are not present and will not contribute to the degradation of the amine solution. Pressure control exists at the PSA inlet, meaning this stream will be available at high, defined pressure. These factors simplify the design and reduce the size of the removal system. In addition, the operat- ing cost of the liquid amine based contact system will be lower for pre-combustion carbon capture. While these systems benefit from smaller plot space (see Figure 2 ) and solvent stability, pre-com- bustion capture means only the process-side CO 2 is removed. The total CO 2 generated in a hydrogen plant can typically represent up to 60% of the total carbon emissions, but will vary depending on plant conditions. CO 2 capture efficiency 2 Against this backdrop of conditions and challenges, there is a proven way to generate syngas at scale, in which all the CO 2 comes out in a single stream at high pressure within the process, making it easy and economical to capture it with very high efficiency (i.e., 95% and above). Advanced Reforming technology, utilising heat exchange such as gas heated reforming (GHR) and other non-fossil fuel fired technologies, has been used at an industrial scale for decades and removes the need for a separate stream of methane fuel to be used to generate the tem- perature to drive the reactions. This in turn eliminates the dilute, low pressure outlet stream contain- ing CO 2 (post-combustion CO 2 ) that results from the fossil fuel based firing. Some technologies generate heat with process-side combustion using oxygen or enriched air to drive the reforming reactions that generate the byproduct CO 2 . This requires a source of oxygen, which is typically produced through air separation. A GHR can be combined with an SMR or other Advanced Reforming technologies for much greater heat integration, lower byproduct CO 2 ,
processed in a WGS reactor to con- vert the bulk of the CO process stream (containing greater than 70% H 2 ) into a PSA process to reach high level (99+%) purity H 2 for use in hydroprocessing and isomerisa- tion units to produce cleaner fuels. The CO 2 generated is at high pressure, and the process stream composition is simpler with min- imal impurities, making it easier to utilise. Consequently, capturing this process-side byproduct CO 2 is less complex and costly, and estab- lished solvent and absorbent based technologies can provide cost-effec - tive solutions. Capturing post-combustion CO 2 In a post-combustion scheme, CO 2 is removed from the flue gas stream. Amine based post-com- bustion technology has previously been deployed at a commercial scale, but uptake has been low due to high capture costs. 4,5 New technologies, including amine based as well as cryogenic and other novel forms of post-com- bustion, focus on minimising cost and improving reliability. Using carbon capture at this location can achieve CO 2 reductions of greater than 90%. However, since the SMR furnace operates at a negative pres- sure, the flue gas pressure is quite low and will complicate the design of the solvent based system. Lower pressure requires larger equipment, needing more space.
Plot space is likely at a premium at existing facilities. Fired heaters operate with excess O 2 , which will pass into the flue gas stream, while the make-up fuel will likely contain sulphur and other impurities. The amine solutions used for carbon capture are prone to oxidative and sulphur degradation. Fresh amine will have to be added more often on a post-com- bustion system, increasing operat- ing costs. There is also a significant amount of wastewater created by removing some impurities and reducing flue gas temperature. Capturing pre-combustion CO 2 CO 2 in the pre-combustion scheme is removed from the process stream after the WGS reactor and upstream of the PSA. The process stream will Figure 2 Relative plot space differential between different carbon capture systems compared to a hydrogen plant
Figure 3 Minimising post-combustion CO 2 : comparison of conventional to Advanced Reforming technologies
24 PTQQ 2 2022
Powered by FlippingBook