Carbon Capture and Storage for Marine Vessels

Hannu Jääskeläinen

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• Introduction
• Amine-based CCS Process
• Performance and Fuel Penalty
• Economic Analysis
• Conclusions

Introduction

Carbon capture and storage (CCS) and carbon capture utilization and storage (CCUS) technologies involve the capture of CO₂, primarily from large point sources like power generation. The captured CO₂ could be utilized in a range of applications or else injected into deep geological formations such as depleted oil and gas reservoirs or saline aquifers. Most climate policy centers consider a wide-scale deployment of CCS/CCUS technologies to be necessary to achieve ‘net zero’ GHG emission goals and to decarbonize sectors with hard-to-abate emissions [5968].

Carbon capture technologies still require much work to be deployed on a wider scale. As CCS processes require high energy input, they tend to have a significant negative impact on the efficiency and the economics of the associated industrial process. The existing experience shows that CCS technologies also present a range of technical and scalability challenges [5753]. In the United States, CCS demonstration projects resulted in varying levels of success, in spite of sizable government subsidies [5969].

The idea of using CCS to reduce CO₂ emissions from marine vessels, Onboard Carbon Capture and Storage (OCCS), is commonly discussed as an option to meet the IMO greenhouse gas reduction objectives such as the 2030 goal of a 40% reduction in carbon intensity. Other options for these objectives include low carbon fuels, vessel efficiency improvements and operational efficiency improvements.

There are several approaches for CO₂ capture and storage from the combustion of carbon-based fuels.

• Pre-combustion capture involves first converting the fuel into a synthesis gas (CO + H₂) and then using a shift reactor to yield a mixture of CO₂ and H₂ from which the CO₂ is separated.
• Oxy-fuel combustion capture uses oxygen instead of air for the combustion of the primary fuel to produce a flue gas that is mainly water vapor and CO₂.
• Post-combustion capture separates CO₂ from the flue gases produced by the combustion of fossil fuels in air. Separation can be achieved with chemical absorption, membrane separation, cryogenic capture or adsorption technologies. The chemical absorption process using amine is considered mature for shore-based applications and is being developed for use on ships [5957][5958][5959][5960].

Amine-based technologies have a high technology readiness level but suffer from some significant disadvantages. For land-based power generation, these amine-based absorption technologies result in a nearly 30% drop in net power production, an 11% drop in efficiency and, due to the low concentration of CO₂ in the flue gas, increase the cost of power production by approximately 60–70% for new infrastructure and by 220–250% for retrofitting [5966].

While application of these amine-based technologies to marine vessels is technically feasible, the outlook from a cost perspective varies from optimistic [5959] to pessimistic [5957][5958]. The more optimistic outlooks either neglect to fully account for the total system costs or use simplified costing assumptions. Further development to limit energy consumption and costs is required before carbon capture can be considered a viable option to reduce greenhouse gas emissions from marine vessels.

Amine-based CCS Process

Marine vessels typically employ a low- or medium-speed engine for propulsion and several generator sets for auxiliary power. Some may also employ a boiler that utilizes waste exhaust heat or that burns additional fuel. All this combustion equipment contributes to carbon dioxide emissions from the ship and a carbon capture system should be designed to handle their collective exhaust gas. Chemical absorption using an amine solvent requires additional energy input in the form of heat and electricity that imposes a fuel consumption penalty and additional CO₂ emissions that must be captured.

Figure 1 shows an amine-based system that could be used to capture carbon in marine vessels [5957]. With heavy fuel oils, exhaust cleaning to remove sulfur is also required.

First, water quenching is used to lower the temperature of the exhaust gas to approximately 40°C, a temperature at which carbon dioxide is readily absorbed in the next stage by monoethanolamine (MEA), a first-generation amine solution widely used in carbon capture applications. In this system, a blower compensates for back-pressure induced by the overall system to avoid negative performance impacts on the two-stroke propulsion engine. The cooled exhaust gas then enters the absorber column where it is exposed to the amine sorbent and carbon dioxide is absorbed into the solution. Most of the volatile amine carried out of the absorber is removed from the exhaust gas by the water wash and returned to the column.

The carbon dioxide-enriched amine is then pumped from the bottom of the absorber and sent through a heat exchanger to scavenge energy from the carbon dioxide-lean amine returning from the stripper. At the bottom of the stripper the temperature of the amine solution is increased to about 120°C at 2 bar. A reboiler raises part of the amine solution to the boiling point to introduce sufficient vapor to strip the carbon dioxide from the solvent. Concentrated carbon dioxide and water vapor exit the top of the stripper and are then cooled and flashed to remove residual water and amine, which is returned to the main loop. The almost pure gaseous carbon dioxide is then sent to a final quench station where remaining impurities are removed and finally to a liquefaction system, Figure 2, where it is compressed, liquefied and pumped into holding tanks at a pressure of 16 to 20 bar [5958][5957].

Exhaust is supplied to the capture system from the SOx scrubber. This is where SOx concentration is lowest. SOx is preferentially absorbed by the amine and forms heat stable salts which deactivate the amine group and prevent it from absorbing CO₂. As a result, periodic replacement of the solvent is necessary. Figure 3 shows the theoretical effect of different SOx concentrations, as SO₂, on solvent deactivation assuming that all SOx species are absorbed into the liquid phase [6313].

The major energy consumers in the process are the heat required in the reboiler and the electricity required for the liquefaction plant’s compressor and chiller. Depending on the vessel’s needs, some of the heat required for the reboiler could be supplied using waste heat from the engine. Any additional heat would be supplied by a fuel-fired boiler. The electricity consumption, however, will be an additional load on the auxiliary engines and will result in additional fuel consumption. Engines burning LNG can save on liquefaction costs by using CO₂ from the stripper to vaporize LNG supplied to the engine [5961][5965].

The energy penalty of the process is dependent on the solvent used in the absorption process. Monoethanolamine (MEA) requires around 3.5 GJ/t-CO₂ captured. Solvents requiring less energy have been identified such as Siemen’s amino acid salt which requires 2.7 GJ/t-CO₂ and an amine mixture by Mitsubishi that requires less than 3 GJ/t-CO₂ [5965]. Other solvents are also under development [5962].

In addition to the increased fuel consumption, loss of cargo capacity and the structural design of the vessel need to be considered. The need for onshore infrastructure to offload and dispose of CO₂ and suitable market mechanisms would be required for carbon capture to be a viable option.

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