May 2023

Carbon Capture/CO2 Mitigation

Carbon capture in syngas-based projects

Recent legislation, such as the Low Carbon Fuel Standard (LCFS) and the Inflation Reduction Act of 2022 (which was passed in the U.S.), provide financial incentives for projects generating power, transportation fuels or other forms of energy to reduce their overall carbon emissions.

Fair, D., KPE, A Shaw Group Company

Recent legislation, such as the Low Carbon Fuel Standard (LCFS) and the Inflation Reduction Act of 2022 (which was passed in the U.S.), provide financial incentives for projects generating power, transportation fuels or other forms of energy to reduce their overall carbon emissions. The carbon intensity (CI) score is the accepted measure of the effectiveness of carbon emissions reductions. In simple terms, the CI score of a project is the amount of carbon dioxide (CO2) equivalent emissions per unit of energy. Because of these financial incentives, many projects are under development in the U.S. and in other parts of the world to produce green energy. These include the production of renewable diesel and jet fuel from soybean oil, corn oil and animal tallow, and the generation of various energy products from non-fossil fuels such as woody biomass or municipal solid waste (MSW). This article will focus on the latter of these.

Many process technologies convert fossil fuels into various forms of energy. Gasification is a process that has been in practice for decades and generates a synthetic gas (syngas) from any hydrocarbon-based feed stream. Syngas is primarily comprised of a mixture of carbon monoxide (CO) and hydrogen in varying ratios, and it can be used to make hydrogen fuel, methanol, gasoline, diesel, aviation fuel and ammonia, among other energy products. This has typically been applied in parts of the world that are coal-rich and that do not have ready access to natural gas or liquid crude oil. While these projects have provided affordable energy products for decades, they are burdened with substantial amounts of CO2 emissions.

Clean for green

These same gasification-based technologies can be modified to accept renewable feeds in place of the more traditional fossil fuels. These renewable feeds include woody biomass, agricultural waste and MSW. If left to degrade naturally in forests or landfills, these feeds will generate a significant amount of methane. Like CO2, methane is a greenhouse gas (GHG). However, the GHG impacts of methane emissions are estimated to be approximately 80 times greater than CO2 over a 20-yr period and about 30 times that of CO2 over a 100-yr period (larger impacts are estimated in shorter time frames). Converting these feeds to fuels, instead of allowing them to degrade into methane, reduces their CI score. These fuels are referred to as renewable fuels, and they have a much lower CI score than their fossil fuel counterparts.

In the process of gasifying (i.e., creating syngas from) woody biomass or MSW, some amount of CO2 is produced. This CO2 can be removed from the syngas and vented to the atmosphere, or it can be captured. When the CO2 is captured, the CI score of the project can be significantly improved.

Carbon capture can also be used to improve the CI of the conversion of natural gas to H2 through steam methane reforming. Today, > 95% of the world’s H2 is produced using steam methane reforming without any carbon capture. In these projects, essentially 100% of the carbon in the natural gas is converted to CO2 and vented to the atmosphere. With the implementation of carbon capture on these technologies, approximately 60%–65% of the carbon in the natural gas can be captured.

The technologies used to capture carbon from syngas in the form of CO2 are very mature and have been used for more than 50 yr. Hundreds of gasifiers operating globally use these technologies to “clean up” the produced gas by removing CO2 [and hydrogen sulfide (H2S)] to generate syngas that meets the desired product requirements. In many cases prior to 2020, all CO2 captured in these projects was released into the atmosphere, as there was no financial incentive to sequester it. However, in today’s market, there are several economic incentives—along with environmental, social and governance (ESG) incentives—for sequestering the CO2. Decisions based on incentives can either result in additional income streams or the generation of carbon credits that can be sold or traded.

While these additional factors are also important considerations when the CO2 will be sequestered, the same basic technologies can be employed for the capture step. In addition, some newer technologies are being developed and/or optimized to improve the economics of the capture/sequestration combination.

Carbon capture technologies

Chemical solvent technologies have been used for decades to remove acid gases, including CO2 and H2S, in both syngas applications and refining applications. With a chemical solvent, the syngas is contacted with the solvent, and a chemical reaction occurs between the solvent and the acid gas components in the syngas. This traps these species in the solvent and results in clean syngas. The rich solvent containing the trapped acid gases is then regenerated by heating and decreasing the pressure to reverse the reaction and release the acid gases back into a gaseous stream. This process is very mature and well-understood and has many applications in technology used in projects involving syngas worldwide. Chemical solvents include various forms of nitrogen-based compounds known as amines. The most common amine applied for the removal of CO2 from syngas is methyl diethanolamine (MDEA). Producers of MDEA offer various versions of the solvent that can contain promoters or other additives to improve the performance of the MDEA for specific applications. MDEA can achieve the deep removal of CO2, but it is less effective at selective removal of CO2. This can be a very important consideration if sulfur species are present—such as H2S and carbonyl sulfide (COS), among others—in the syngas.

Is there an alternative?

Physical solvents can also be used to remove CO2 from syngas. These solvents rely on the physical absorption of the CO2 into the solvent. These technologies are also widely used in the syngas and gasification industry. Because they rely on physical absorption, these solvents are more effective at elevated pressures. Like the chemical solvents, the syngas is contacted with physical solvent and the CO2 is absorbed into the solvent, resulting in a clean syngas stream. The solvent can also be regenerated for reuse. Much of the acid gas can be released simply by the flashing of the solvent, which reduces the pressure. This allows a portion of the CO2 to be released at elevated pressures, which can reduce the CO2 compression costs downstream. For full regeneration, the solvent pressure must be fully reduced, and some steam stripping applied. In general, the amount of steam/energy required for regeneration is less for physical solvents than it is for chemical solvents. Due to the differing absorption affinities for different components, physical solvent systems can be configured to remove CO2 and H2S selectively, resulting in a fundamentally H2S-free CO2 stream. Many well-known physical solvents are being used in industry today.

Cryogenic technologies are also mature and commercially proven technologies used to capture CO2. However, cryogenic technologies are capital and operating cost-intensive vs. solvent-based technologies. These cryogenic processes have been deployed on a commercial scale and are proven, mature and dependable. In general, starting with a higher fraction of CO2 in the gas mixture is a favorable condition for cryogenic separation. The gas mixture is dried, compressed, cooled to exceptionally low temperatures and fractionated to separate out CO2 by using the difference in boiling points of the components. With projects that intend to use onsite CO2 injection into a geologic formation, these technologies may be more economic, depending upon other project requirements and criteria.

The following innovative technologies are in various stages of development. While these technologies require further supporting data, along with pilot plant operating data for commercialization, they may be of interest in future projects as these technologies mature: 

  • Hydrophobic alkyl-ester solvent technology 
  • Amine-promoted buffer salts (APBS)a
  • Amino sulfonic acid technologies 
  • Solid sorbent technologies—metal organic framework, zeolite 13X activated carbon and alkali metal carbonate or hydrated solid sorbent processes.

What should be considered when selecting a technology?

Carbon capture technology selection criteria can include multiple gasification technologies, multiple feedstocks, multiple end uses for the syngas, and multiple sequestration or use options for the captured CO2. There is no simple answer as to which of the various carbon capture technologies is preferable for a given project. This important technology decision must be studied carefully in the early stages of project development.

Some key criteria that should be considered during this analysis include:

  • Feedstock/syngas sulfur content: The concentration of sulfur in the syngas (lb/lb), the total amount of sulfur in the syngas (lb/d) and the sulfur species that might be present in the syngas (H2S, COS, mercaptans, etc.) can impact the CO2 capture technology selection. For example, larger amounts of sulfur may drive an economic preference for physical solvents over chemical solvents.
  • Presence of heavy hydrocarbons (tars and oils) in the syngas: The gasification of biomass and MSW can result in some level of heavy hydrocarbons in the syngas. Each unique gasification technology will have its own tar reduction technology or strategy that may result in varying amounts of heavier hydrocarbons remaining in the syngas. Physical solvents can absorb heavy hydrocarbons (which can cause operational problems) from the syngas.
  • Final product requirements: Syngas can be converted into numerous end products, including H2, methanol, ammonia, gasoline, aviation fuel and natural gas. To achieve conversion to these final products, additional processing of the syngas is required to meet the key specifications for each downstream processing step. Some of these specifications include CO2 content, sulfur content, specific H2-to-CO ratios and heavy hydrocarbon limits. The various steps required to achieve the desired syngas concentration at the inlet of the downstream processing unit (often referred to as syngas conditioning) may drive the selection of the CO2 capture technology.
  • Operating pressure: Physical solvents are more effective as the pressure of the syngas increases. When the pressure drops below 400 psig–500 psig in syngas applications, chemical solvents may provide a more economical solution.
  • Abundance of low-level steam: Depending upon the other units in the facility, the plant may have either a shortage or an abundance of low-level steam. Chemical solvents require larger amounts of steam. If adequate steam is unavailable without additional consumption of fuel, moving to a physical solvent that uses less steam may be worthwhile.
  • Green power availability: Different technologies require various amounts of electricity. If green power is unavailable, then additional power consumption must be generated with fossil fuels, which can have a significant impact on the CI of a project.
  • CO2 disposition: Many current projects plan to sell CO2 into existing pipeline networks, while other projects are investigating onsite CO2 injection wells. The disposition of the CO2 will drive requirements (pressure and purity) that may influence the optimum CO2 capture technology.

Taking the first step

Carbon capture projects are complex, but they are achievable. Understanding the regulatory landscape for credits, determining what factors to consider when evaluating technology licensors, and finalizing the selection of the appropriate technology are all essential to a successful carbon capture project. An engineering, procurement and construction (EPC) partner with significant experience in various projects (e.g., renewable energy, syngas) with/without carbon capture can help navigate these project requirements. These EPC firms, such as the author’s company, help provide engineering services across the project lifecycle from early concept and investment studies to detailed engineering and final EPC phases. HP


a APBS is licensed by Carbon Clean

The Author

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