A joint study by TMS Tankers Ltd and DNV explored the feasibility of retrofitting a liquid-absorption-based carbon capture and storage system on a s Suezmax tanker. The study revealed complex system interdependencies and yielded valuable insights.

Introduction Baseline vessel
The study explored how on-board carbon capture and storage (OCCS) technology could be deployed on an existing ship at the lowest possible investment cost (capex). It also examined its impact on other on-board machinery, the decarbonization potential and commercial viability of such an OCCS system on the given vessel. The baseline ship’s operational profile provided by TMS served as a referential data basis. This data was used to optimally fit the net emissions reduction to the trade. DNV applied its modelling suite COSSMOS [1] and the vessel's digital twin to simulate machinery integration scenarios and estimate the fuel consumption, demonstrating the way the OCCS system affects on-board equipment. [1] DNV COSSMOS stands for Complex Ship Systems Modelling and Simulation. COSSMOS has been extensively used for over 15 years for the assessment of new technologies in shipping.

Trade pattern characteristics
The baseline vessel in this case study most frequently operates at a speed range of 11 to 14 knots, but also spends time at port operations and anchorage, where the auxiliary engines and boilers are operating. The auxiliary boilers are dimensioned for the especially energy-intensive tank heating and offloading processes, providing a significant energy reserve for OCCS. The auxiliary engines are sized to cover electrical demands during sailing, including cold reserve supply.

System complexity and interaction of components
The OCCS system requires:
- Space for absorber and regeneration stacks, the liquefaction plant and CO2 tanks.
- Heat for solvent regeneration, to be provided by boiler(s).
- Electricity for pump and fan operation etc., to be provided by auxiliary engines (gensets). The performance limits of these systems - i.e. maximum steam production by the boiler, maximum genset loads, efficiency of auxiliary engine utilization - are constraints defining maximum and optimum performance of the OCCS.   To minimize fuel consumption and optimize maintainability, the gensets would run at optimum loads whenever possible, avoiding excessive or low loading. Furthermore, the increased load on the generators creates additional CO2 emissions which affect system performance.

Functional principles of a liquid-  absorption OCC system
A liquid-absorption OCCS system essentially comprises:
- The exhaust gas cleaning system, for SOx reduction and gas cooling.
- The CO2 capturing unit or absorber column that sprays an amine solution onto the engine exhaust gas.
- The amine regeneration column or stripper that separates the captured CO2 from the amine solution.
- The liquefaction plant that converts the CO2 gas into compressed liquid form.
- The storage tank for the liquefied CO2 for offloading at a shore-based receiving station.
- Plus associated equipment (heat exchanger, reboiler etc.). There are other types of OCCS systems on the market which were not the subject of this project.

Optional technical enhancements
Enhancement options to improve the performance of the carbon capture system include: (a) Optimized design of the composite boiler to cover the entire on-board heat demand at maximum capacity, including the carbon capture plant. (b) Optimized design of the auxiliary engines or use of power take-off to improve the efficiency of the electric power plant. (c) Implementation of auxiliary engine economizers to reduce energy consumption by boilers. (d) Reduction of the steam demand of the carbon capture plant; use of solvents that regenerate at less than 120 degrees Celsius. (e) Improvement of the liquefaction and CO2 treatment plants to reduce power requirement. Among the above solutions, the first two are relevant for newbuilds, the last two depend on the features of the capture technology, while only auxiliary engine economizers are relevant for a retrofit of a conventional vessel.

On-board integration scenarios
A variety of amine solvents can be used to separate the CO2 from the exhaust gas in the absorber column: a conventional solvent, a state-of-the-art solvent, or a solvent that regenerates at 40C in the presence of a catalyst. With the term state-of-the-art we describe recently developed solvents with improved thermodynamic and kinetic characteristics that improve their energy intensity footprint.    The study explored three scenarios:
- Scenario 1 Simple OCC retrofit: Baseline vessel plus OCC equipment; no modifications to the machinery. The carbon capture capacity is delimited by the vessel machinery.
- Scenario 2 Baseline vessel plus optimized OCC equipment plus auxiliary engine economizers for additional steam production. Sub-scenarios:  (a) Conventional versus state-of-the-art solvent. (b) Conventional versus state-of-the-art compression stage.
- Scenario 3 Baseline vessel plus optimized OCC equipment plus state-of-the-art CO2 compression train. There is no steam demand to regenerate the solvents used to scrub the exhaust from CO2.

Key performance indicators
Based on the premises specified above, the study focused on the following key performance indicators: (a)The CO2 reduction achievable by the vessel in per cent of its annual total CO2 emissions. (b)The "fuel penalty" resulting from additional steam and electricity consumption to operate the OCCS system. The extra fuel consumed to provide this energy increases fuel costs and CO2 emissions. (c)The efficiency of utilization (load) of the on-board systems determines the energy balance and commercial feasibility of an OCCS system. Furthermore, the model calculations were performed assuming three different nominal capacities of the OCC system: 1, 2 or 2.5 tonnes of CO2 per hour. The operation of the exhaust gas cleaning system is included in all scenarios assessed. The nominal capacity represents the maximum CO2 treatment potential of the OCC equipment.

Scenario 1 - On-board carbon capture using conventional OCC system
The first case examined is an OCC system retrofit without modifications to the existing machinery, using conventional amine solvents. The system with OCC uses the maximum power and heat resources available on board, including two gensets operating at 70% load and the auxiliary boiler for steam supply. Due to the resource limitations, the OCC system can treat up to 1 tonne of CO2 per hour. An OCC system design capturing up to 2.5 tonnes of CO2 per hour can reduce the emissions by 39% but would require three gensets to operate at 68% of their nominal capacity. Without additional heat resources, the additional energy consumption is significant, ranging from 9 to 24%.  The on-board storage capacity per round trip is less than 1,000 m3.

Scenario 1 - Findings
In annualized terms, the baseline vessel emits approximately 30,000 tonnes of CO2 per year, while the OCC system captures up to 6,000 tonnes per year. Including the fuel penalty, the net annual emission reduction achieved is 11%, based on the following specifications:
- A feasible OCC system nominal capture capacity of 1 tonne of CO2 treated per hour.
- The maximum available steam production from composite and auxiliary boilers.
- Utilization of two out of the three existing gensets to support operational and maintenance flexibility.

Scenario 2 - Impact of improving steam supply and OCC performance
In this scenario, a series of improvements are implemented:
- Advanced chemical solvent [2].
- Integration of auxiliary engine economizers (AEECO) for additional heat.
- Improvement of liquefaction plant (different technologies and liquefaction efficiencies). A conventional carbon capture system requires steam to regenerate the chemical solvent that captures the CO2 gas. Advanced solvents have a lower heat requirement, while some systems suggest regeneration at near-atmospheric temperatures. The heat requirement is defined in thermal energy (GJ per tonne of CO2). The model estimates that a reduction from 3 to 2 GJ of heat demand per tonne of CO2 leads to a significant reduction of the system fuel penalty of up to 17%. [2]  Damartzis et al. (2022): Solvents for Membrane-Based Post-Combustion CO2 Capture for Potential Application in the Marine Environment. Appl. Sci. 2022, 12(12), 6100; https://doi.org/10.3390/app12126100

Scenario 2 - Impact of improving steam and power supply
With improved solvents and integration of additional heat resources, the achieved emissions reduction is 15% with a 5% fuel penalty, respecting the operating limits of the engines. A set of two auxiliary engine economizers are used, optimized for operating conditions that match the requirements of the carbon capture system. Generator oversizing by approximately 18%, or a power take-out installation capable of providing the required electric load, could result in a reduction in the order of 25%. Finally, when an efficient liquefaction plant is chosen that consumes no more than 330 kWh per tonne of CO2, the system reduces emissions by up to 28%.

Scenario 3 - Further enhancement: System with zero steam demand
When a solvent is used that does not require a hot regeneration stage, the OCC case is delimited only by the capacity of the electric power generation plant. This scenario either utilizes three generator sets at a 58% load or two generator sets at an 86% load. Providing for bigger engines at the design stage or retrofitting/installing a shaft generator will increase the power reserve capacity. It should be noted, however, that a shaft generator retrofit is associated with major costs and therefore not assessed in this study.

Scenario 3 - Findings: Impact of optimized OCC equipment and no demands for solvent regeneration
When using a solvent that can be regenerated at ambient temperature, a best-case result is reached at a nominal capture capacity of 2.5 tonnes of CO2 per hour, which can reduce the CO2 emissions of the reference vessel by as much as 38%.

Impact of enhanced performance on CO2 reduction
The illustration shows the net CO2 emission reduction rates that can be achieved by the different OCC system scenarios. This is shown for various OCC system nominal capacities (1, 2 or 2.5 tonnes per hour) and power demands for liquefaction.  It was observed that:
- The on-board heat integration and various technology innovations can increase the CO2 reduction potential of OCC systems.
- Improving heat and power resources on board, e.g. with the addition of auxiliary engine economizers, bears the biggest improvement benefit to emissions reduction for all OCC system sizes.  • The technology innovations are more effective with regards to emission reduction with bigger OCC capacities.
- The maximum possible electric power supply on board delimits the emissions reduction potential in all cases.

Impact of enhanced performance on the fuel penalty
The figure shows how the fuel penalty can be reduced incrementally by optimizing the steam and power demand. Similar to the previous slide:
- The fuel penalty is impacted by both technology innovations and on-board integration.
- State-of-the-art OCC systems with on-board heat and power integration can result in half the fuel penalty impact, compared to conventional ones.
- Technology innovations, in combination with on-board integration, can reduce the fuel penalty by up to 63%.

Financial performance of OCC scenarios vs biofuel
In the study, the financial performance of the investment in OCC (blue colour) was compared against the case of biofuel use (green colour) without any OCC technology. To have a fair comparison between the cases, we assumed that the amount of biofuel used was as needed to achieve the same CO2 emissions reduction as for the OCC scenarios. For all these cases, the net present value (NPV) of the decarbonization costs was estimated, assuming that:
- All solutions are applied for the remaining vessel lifetime, from 2030 to 2035.
- OCC capital cost is included within a range of USD 150 to 250 per tonne captured annually.
- The additional fuel operating cost (FOPEX) and CO2 disposal costs (approx. USD 70 per tonne) are accounted for. Biofuels are assumed to have a price of EUR 1,157 per tonne.
- The fuel penalty caused by the OCC operation is included. No CO2 taxation benefits were considered, nor was compliance with a CII trajectory imposed. The results revealed good performance of the OCC scenarios as an alternative to using biofuel.

Estimated decarbonization cost per tonne of CO2 - biofuel vs OCC
The estimated cost to reduce the vessel emissions by 1 tonne of CO2 when using biofuel was estimated at USD 180 per tonne.  The costs for the basic OCC scenario without additional heat generation is close to this value. For the scenario that uses additional heat generation and innovative technology to make the system more efficient, the net life emissions reduction potential reaches up to almost 20% and the cost of decarbonizing 1 tonne of CO2 drops to around USD 80 per tonne.
- The more CO2 is abated, the lower the cost per tonne.
- The more innovative technology is used, the higher the cost.

Conclusions
The study examined the decarbonization capabilities of OCCS technology from an engineering viewpoint, identifying optimal conditions for its use in various scenarios. It applied a simulation model to compare the baseline vessel with and without carbon capture and storage, assessing rising fuel consumption and CO2 reduction potential. It has been shown that:
- The emissions reduction capacity of OCC technologies of different levels of on-board integration and innovations is delimited by the capacity of the existing on-board machinery, due to the energy penalty and the available electricity supply for CO2 liquefaction.
- In retrofit cases, the above limitations need to be considered in the feasibility assessment of OCC on board ships.
- For newbuilds with OCC-ready features, feasibility analyses should consider the expected electric supply to the OCC and liquefaction systems.
- Despite limitations, all OCC scenarios are competitive against the biofuel cases, when considering the same emissions reduction effect. The cost to reduce emissions by 1 tonne of CO2 is lower with OCC than using biofuels.
- The currently lowest capital investment in OCC enables a CO2 reduction of 11 per cent.
- Optimizing the integration of an innovative OCC system on board can significantly reduce carbon emissions by as much as 38 per cent. Notably, the feasibility of OCC also depends on the availability of onshore offloading infrastructure, which is expected to begin being available around 2030.

Source: DNV

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	Issue 92 of Robban Assafina (July/Aug. 2024)