Oct-2014

Options for CO2 capture from SMR

As CO2 capture from steam methane reforming becomes increasingly important, the economics of retrofit for recovery from process gas streams need to be considered

GOUTAM SHAHANI, Linde Engineering North America
CHRISTINE KANDZIORA, Linde Clean Energy and Innovation Management

Article Summary

Crude oil is getting heavier and sourer on a global basis. In North America, this long term trend is being mitigated to some extent by the recent discovery and exploitation of light tight oil. In order to convert crude oil to transportation fuels, refiners need hydrogen (H₂). The dominant method of producing hydrogen is by steam methane reforming (SMR). Whenever a hydrocarbon is converted to hydrogen, there is a concurrent production of by-product carbon dioxide (CO₂). Historically, SMRs have been designed to optimise capital, operating costs and plant reliability. In the future, CO₂ emissions will also be an important consideration in the design of SMRs. This article will provide an overview of how CO₂ is produced in a SMR, describe where it can be captured, and provide a summary of different methods of capturing the CO₂ with the associated economics. Finally, some important considerations for process selection are presented along with recommendations for future work.

Process technology Hydrogen production
The predominant method of producing hydrogen on a large industrial scale is steam methane reforming. In this process, desulphurised hydrocarbon feed is mixed with steam and fed to tubes filled with nickel catalyst in a down-fired, down flow reformer. The following reactions occur:

CH₄ + H₂O → CO + 3H₂ Endothermic  (1)

CO + H₂O → CO₂ + H₂  Exothermic     (2)

Reaction (1) is reforming which is endothermic; reaction (2) is shift conversion which is exothermic. The shift reaction employs promoted iron oxide catalyst. Both reactions are equilibrium limited, based on the outlet temperature and pressure. The reaction products are a mixture of H₂, CO, CO₂ and H₂O. Hydrogen is recovered from this gas mixture by pressure swing adsorption (PSA). The PSA is a physical process that depends on the selective physical binding of gas molecules. Hydrogen, being non-polar and highly volatile is essentially not adsorbed by the adsorbent material. A simplified block flow diagram of the SMR process is shown in Figure 1.

It can be seen in Figure 1 that CO₂ is present in three streams which are depicted as syngas (1), PSA tail gas (2) and flue gas (3). Approximately 40% of the CO₂ is generated in the reformer furnace by combustion of fuel and tail gas and approximately 60% is present in the syngas, from which CO₂ is rejected by the PSA into the tail gas. Ultimately, all the carbon in the feed ends up in the flue gas. The characteristics of these three streams is shown in Table 1.

CO₂ capture
Several technologies are commercially available to capture CO₂ from the three streams described. These technologies include: pressure swing adsorption (PSA), absorption technologies, membranes, cryogenic processes and various combinations of these technologies (see Table 2). The choice of technology depends on how much CO₂ needs to be captured, the desired purity of the CO₂ and the cost of power and steam. This will determine which stream has to be processed and which technology is employed.

State of the art technologies for CO₂ capture in SMRs are amine absorption for the syngas and flue gas stream. Also, pressure swing adsorption for the syngas stream and the tail gas stream are technologies that are commercially available on an industrial scale.

Additionally, a range of technologies is currently being developed for CO₂ capture. These technologies include: new CO₂ membranes, new solvent systems, new adsorbents, or a combination of the above. In future, other technologies may be developed for post combustion CO₂ capture that could be applied to a SMR’s flue gas. These technologies could reduce the cost of CO₂ capture compared to today’s state of the art technologies. The best currently available choices are amine based absorption and PSA systems, based on recent studies.

Amine absorption
Amine absorption is a well- established technology whereby an acidic gas such as CO₂ is reacted with a basic liquid such as an alkyl amine. The choice of the amine depends mainly on the partial pressure of CO₂ and is proprietary to the solvent supplier. A typical process flow diagram is shown in Figure 2. The absorber is a gas-liquid contactor in which the amine solution absorbs CO₂ from the gas. The resulting ‘rich’ amine is then sent to a regenerator, which is a stripper with a reboiler to produce regenerated or ‘lean’ amine that is recycled for reuse in the absorber. The stripped overhead gas from the regenerator is concentrated CO₂. The CO₂ is dried and compressed.

In the context of a SMR, the amine absorption process can be applied to the syngas stream (1) and the flue gas (3). Since the partial pressure of the CO₂ is different in these two streams, different solvents are used in these two cases. The CO₂ is subsequently dried and compressed as required. This process produces very pure CO₂ with less than 20 ppm O2. Typically, steam consumption, which depends on CO₂ partial pressure is in the range 0.8-1.5 T/TCO₂.

Pressure swing adsorption
Pressure swing adsorption (PSA) technology is based on a physical binding of gas molecules to adsorbent material. The attractive forces between the gas molecules and the adsorbent material depends on the gas component, type of adsorbent material, partial pressure of the gas component and operating temperature. The PSA process works at basically constant temperature and uses the effect of alternating pressure and partial pressure to perform adsorption and desorption. Since heating or cooling is not required, short cycles within the range of minutes are utilised. Consequently, the PSA process allows the economical removal of large amounts of impurities.

In the context of a SMR, it should be noted that most modern SMRs require a H₂ PSA to produce pure H₂. If CO₂ is to be recovered by a PSA, an additional CO₂-PSA is needed. This PSA process can be applied to either the syngas stream (1) or the tail gas from the H₂ PSA (2). For high CO₂ purities, the CO₂-PSA is followed by a standard drying step and a cryogenic purification unit to reduce the level of the non-condensable gases in the CO₂. These two options are shown in Figures 3 and 4.
 
Impact on H₂ and steam production
The impact on SMR design and operation when CO₂ is captured from the three streams is described below.

Syngas
Fuel consumption decreases when CO2 is removed from the syngas. H2 production can be maintained. However, an assessment of the H2-PSA needs to be made. For retrofit applications, new burners may be needed since the tail gas composition has changed. For an absorption system, it is possible to integrate the regenerator steam system with the heat available from cooling the process gas. This would reduce the steam available for export.