Carbon capture with least opex and capex

Recommendations on how to optimise the mass and heat balance of the process and equipment designs of a typical post-combustion CO2 capture unit.


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Article Summary

Carbon or, more correctly, carbon dioxide (CO2) capture is considered a key enabling technology option for industrial decarbonisation in order to meet the required CO2 emission reductions for a 1.5°C development according to the Paris Agreement. The most commonly used process to remove CO2 from industry flue gases and process streams is still a solvent based absorption/stripping system, as per the simplified process scheme shown in Figure 1. Various kinds of chemical solvents (basic amines, proprietary solvents, amine solvent blends) can be used for post-combustion CO2 capture.

Even if solvent based absorption/stripping CO2 capture processes have existed for more than 30 years, the capex and opex, such as specifically the steam consumption in the solvent stripper reboiler, have always limited the implementation of these processes in the market. With increased focus and interest to integrate new CO2 capture units in existing industrial plants, additional optimisation of the process and equipment designs is required to make such investments more economically attractive and feasible.

In this study, several optimisations are presented for a typical post-combustion CO2 capture plant, using an open art activated MDEA solvent (42 wt% MDEA + 8 wt% PZ), recovering 376000 t/y CO2 from combined refinery flue gases from crude distillation and diesel hydrotreatment gas fired furnaces, with a 90% CO2 capture efficiency. For flue gas composition, see Table 1.

In general, the CO2 capture efficiencies of solvent based absorption/stripping systems vary between 85% and 95%, but steam consumption at the stripper reboiler increases with higher CO2 capture efficiency. For recoveries above 85%, there is a significant (exponential) increase in reboiler steam consumption. This means that there exists an optimum in selecting the CO2 capture efficiency to minimise opex which typically depends on the financial targets of the organisation and the regulatory framework in the country of implementation. This optimisation has been excluded from this study.

The study has been carried out by developing and comparing two process designs, a base and an optimised design, for a new grassroots CO2 capture plant, requiring investment in a new cooling water system and an on-purpose steam boiler. The optimisations are mainly based on utilising available waste heat and the full capacity of the heat transfer equipment, thereby improving the performance of the process in terms of energy efficiency, water management, and investment cost. For each optimisation, a cost-benefit analysis (CBA) is presented, covering costs with more than €5000 difference. On-stream availability of 365 days/year has been used in the calculations, and the results of the most interesting optimisations are presented in the following sections.

Optimisation 1: Waste heat recovery from flue gas
In this optimisation, energy from the furnace flue gas is recovered upstream of the direct contact cooler (DCC) by implementing a waste heat recovery (WHR) system. This system generates a maximum of low pressure steam to be used as energy source in the stripper reboiler (E303), thereby reducing investment cost and fuel consumption in a new steam boiler.

This also reduces the investment cost in the DCC equipment since the flue gas will enter the DCC at a much lower temperature and thus lower volumetric flow rate.

The lower the flue gas inlet temperature to the DCC, the more steam is generated in the WHR system and the lower the boiler and the DCC investment cost.    

A simplified process scheme is shown in Figure 2.

The limit to how much flue gas cooling can be achieved in the WHR system is set by the steam saturation pressure and temperature required in the stripper reboiler. It means, firstly, that the optimal stripper pressure must be set.

In a chemical solvent system, it is normally not optimal to reduce the stripper pressure as this means that more water is evaporated and a higher reboiler duty is required. In addition, this also means that increased capacity of the CO2 compressor system downstream of the stripper is required. Instead, a higher stripper pressure is beneficial to both reduce the reboiler duty and the CO2 compressor capacity.

On the other hand, higher stripper pressure means that the solvent will boil at a higher temperature and, since most CO2 capture solvents are temperature sensitive, the optimal stripper pressure is therefore set by the maximum temperature allowed to avoid severe degradation of the solvent.

In this study, this means that a stripper pressure of 1.9 bara is selected, and the solvent boils at around 120°C.

The next step is to select a reboiler type that allows for minimal temperature difference between the boiling temperature of the solvent and the steam. For this, a welded plate heat exchanger called Compabloc is chosen (see Figure 3). It is able to boil the solvent using only 3 bara steam, with a saturation temperature of around 133.5°C. In addition to maximising the amount of steam that can be generated in the WHR system, the low steam temperature also reduces the wall temperature in the reboiler. This, in combination with minimised hold-up time of the solvent and no dead zones in a Compabloc reboiler, reduces the risk of solvent degradation even further. Furthermore, the minimal hold-up volume allows for a quick response time to changes in operating parameters, such as at start-up and shutdown of the plant, and the corrugated plates provide efficient wetting of the heat transfer surface, thereby minimising the reboiler fouling tendency.

With only 3 bara steam required in the stripper reboiler, maximum energy is recovered from the flue gas, thereby generating around 40% of the steam required by the process. This reduces the size and fuel consumption of the on-purpose steam boiler while the investment cost in DCC equipment is minimised.

Another important benefit is reduced CO2 emissions from the steam boiler, which works in favour of the investments in a CO2 capture plant to reduce emissions to the atmosphere.

Generating 3 bara steam in the WHR system also means that the acid dew point can be avoided as the flue gas leaves the system at 140°C before entering the DCC. As such, the cost of the WHR system can also be reduced as no high-grade materials are required.

The WHR system selected must still provide maximal reliability to cool sufficiently the flue gases upstream of the DCC, and as such, a tailor engineered and optimised Aalborg solution with two smaller heat recovery boilers in parallel is selected. This both maximises the performance of the system and provides improved reliability in terms of boiler capacity redundancy. Such a WHR system can also be supplied as a modular solution, thereby minimising both cost and time for integration in the plant.

In addition to the investment cost of the WHR system, the stripper reboiler cost increases with the lower than normal temperature difference between the solvent and the steam. However, as per the CBA carried out (see Table 2), the increased capex of the WHR system and the larger reboiler size is by far offset by the annual savings in steam boiler fuel consumption and investment cost and reduced DCC equipment cost.

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