Continuous Carbon Capture using Stirling Coolers – a conceptual approach
Greenhouse gasses have been identified as the major culprits of climate change in recent years of which Carbon Dioxide constitutes as much as 70% of the greenhouse gasses emitted (U.S. Department of Energy, 2007). As a result, to try and curb the emission of carbon dioxide carbon taxes are increasingly instituted worldwide. Technologies able to capture carbon dioxide effectively and with a low parasitic load on the system will enable any operator emitting carbon dioxide to generate a financial saving, while contributing to the preservation of our environment.
Many processes exist on industrial scale which captures Carbon Dioxide, but all these processes are extremely energy intensive. The main reason for these processes being so energy intensive is due to the batch or cyclic nature of the processes themselves. The need to change the temperature or pressure in the carbon dioxide capture system requires large amounts of energy to be expended to drive these processes. A continuous Carbon Dioxide capture process will per definition be more energy efficient.
This article documents the current hypothesis for developing and testing a continuous Carbon Dioxide capture system using Stirling Coolers as the driving force.
A Stirling cycle cooler is a member of a family of closed-cycle regenerative thermal machines, including heat pumps and refrigerators, known collectively as Stirling cycle machines. In any refrigeration cycle, including the reversed Stirling cycle, net work input is necessary in-line with the second law of thermodynamics. This is achieved by shuttling the gas in the system backwards and forwards between the hot end and cold end spaces so that the temperature of the system during compression is, on average, higher than during expansion. As a result, the work done on the gas during compression is greater than the work done by the gas during expansion, as illustrated in Figure 1. Accordingly, the hot end and cold end gas spaces are also referred to as the compression space and the expansion space respectively. Furthermore, for operation as a refrigerator, heat must be rejected via a heat exchanger at the hot end, and heat must be absorbed from the space to be cooled via a heat exchanger at the cold end.
Figure 1. Piston and displacer movements during a Stirling refrigeration cycle.
Stirling engines have the advantage of being able to effectively convert heat energy as a driving force into mechanical work. Stirling coolers reverse the cycle, through supplying a mechanical driving force, heat energy is generated and removed by a heat exchanger, thus the cold end of the cylinder incrementally reduces its temperature to reach very low temperatures for relatively little work done. The below figure (source: Ray Radebaugh (NIST) 1999) illustrates Stirling cryocooler efficiency at 80K compared to traditional refrigerant driven cryogenic cooling systems.
Figure 2. Cryocooler performance comparison.
2. Conceptualisation of Continuous Carbon Capture Process
Cryogenic Carbon Capture processes have proven to work well regularly recovering in excess of 95% of the CO2 content in the gas stream, in basically a single step, due to the relatively high sublimation temperature of CO2. Typically, the condensation or sublimation temperatures for effluent or hydrocarbon gasses are much lower, enabling a very successful separation of the CO2.
As can be seen in the below phase diagram, for gas pressures below 5.1 1 atm, no liquid phase for CO2 exists and thus sublimates directly from gas to solid state. This enables cryogenic processes to directly freeze out the CO2 from a gas stream below 5.11 atm.
Figure 3. Carbon dioxide phase diagram.
Freezing out the CO2 directly from a gas phase requires a surface for the crystals to propagate on, which is the limiting factor in the existing cryogenic carbon capture processes, and causes them to have batch characteristics requiring energy state changes to function. To approximate a continuous cryogenic carbon capture process, to lower the energy requirements for the process and make it more feasible for industrial use, an infinite surface area for the propagation of the CO2(s) is required. Traditional heat exchanger design clearly doesn’t allow an infinite surface area which would in any case require an infinite amount of energy for driving the sublimation of the CO2.
Since a continuous process which doesn’t require cyclic changes in temperature or pressure to remove the CO2(s) from the gas stream is our aim, we clearly need to think out of the box when it comes to heat exchanger designs.
2.1 Intended Process
To ensure the heat exchanger conforms to the process requirements we will need to define or intended processes for which we will be removing the CO2. The current need within our organization is the enrichment of a bio-gas feed stream from an anaerobic digester (removal of CO2 and water vapor. The typical composition of the bio-gas contains in excess of 38% of CO2 and varying amounts of H2O vapor, with the main constituent being methane (CH4), less than 4% nitrogen (N2) and very low concentrations of hydrogen sulfide (H2S). Since the primary objective is the removal of CO2 and the removal of water vapor is well established we will only focus on the CO2 removal as process constraints. This process will typically be included after the H2O vapor has been expelled from the system.
2.2 Heat Exchanger Conceptualization
Since it has already been established that a conventional heat exchanger design will not meet our requirements, we will need to utilize a different approach for the required heat exchange. This author’s approach was to consider, instead of an energy (heat) transfer process, but rather to try and conceptualize a mass transfer process which exchanges energy as well and offers the opportunity of separating the different components, based on phase change properties.
The first process which came to mind is a traditional scrubber column which contacts liquids and gas streams, to separate components from one of the streams with those components being absorbed in the other. This process also exchanges energy between the two streams. Should the liquid stream selected for a scrubbing column not dissolve any of the components of the gas stream, nor be dissolved into the gas stream only energy transfer will occur, thus serving as a “Direct Contact Heat Exchanger”.
The liquids stream, cooled down sufficiently low, and if sufficiently dispersed (while ensuring sufficient mass to ensure heat exchange from the liquid to the gas can still occur) will increase the surface volume of the “heat exchanger” substantially, supplying a very large area for the propagation of the CO2 crystals. Thus forming the CO2 crystals on the surface of the heat exchanger liquid droplets. Should the liquids stream and the solid CO2 be separated, with the liquid stream cooled down again and recycled to the scrubber column, an infinite surface area can be supplied for the propagation of the CO2 crystals and thus enable true continuous Cryogenic Carbon Capture.
2.3 Heat Exchanger Component Conceptualization
Since the preliminary idea for a Direct Contact Heat Exchanger (DCHE) with a theoretical infinite surface are has been established we now need to conceptualize the supporting equipment to enable the successful operation of the DCHE.
As discussed in the above section the preliminary equipment will be based around an absorption or scrubber column. Typically, these columns come in various arrangements depending on the intended separation of components from the different feedstocks. For the DCHE arrangement a counter current scrubbing column is recommended to optimize the temperature difference (DT) between the gas stream and the heat exchange liquid (similar to the concentration difference in a mass transfer process).
Since the propagation of the CO2 crystals occurs on any possible surface and can consequently create system blockages, the internal design of the scrubbing column should not include any trays and only compose of a single spray head (possibly multiple nozzles to maximize the liquid dispersion).
Figure 4. Proposed scrubber column (single head layout).
Next step would be to conceptualize the heat exchange liquid. This approach is very straight forward. As mentioned in the preliminary constraints the heat exchange fluid should not allow any mass transfer from the gas stream, including the CO2 to it, or allow any of its mass to be transferred to the gas stream or the CO2 stream. Further since we will be operating the column below the sublimation temperature of the CO2 the heat exchange liquid should have a lower freezing point, while remaining sufficiently viscous at the operating temperature, to allow for easy pumping and dispersion.
A detailed investigation needs to be conducted to identify a suitable heat exchange liquid, but Hydrocarbon rings bindings, which include a fluorine (Polytetrafluoroethylene) or chlorine atom in the molecule, immediately spring to mind.
It is safe to anticipate that the CO2 crystals which will form on the surface of the cold heat exchange liquid will form a slurry with the heat exchange liquid when collected. The next step would be to conceptualize the liquids and solids separation process.
Yet again it is easier to turn to existing technology and possibly change it slightly to fit our process’ requirement. A 2-phase decanter centrifuge is a type of centrifuge, which employs a rotational moment to separate components of different densities, this makes them ideal for use in slurry separation.
In typical horizontal decanter centrifuges, the rotating assembly is mounted horizontally with bearings on each end of a rigid frame, which provides a good sealing surface. For the purpose of separating the CO2 crystals from the heat exchange liquid the typical arrangement will need to be adapted. Instead of the typical horizontal arrangement with the feed being fed in the middle of the centrifuge, the proposal is to orientate the decanter centrifuge at 45°, with the scroll and housing submerged under the liquid level, while employing a preliminary porous scroll housing to allow the heat exchange liquid to drain. The scroll discharge screw then forces the solid CO2 crystals to one end of the bowl to be discharged as solid CO2 ice. This arrangement will allow the centrifuge to function like a scroll feeder with a higher than normal rotational force and possibly compress all the liquid out of the solid CO2 by changing the scroll vane geometry.
The cooling source for the heat exchanger liquid will be a stirling engine as discussed in Section 2 above.
3. Conceptual proposal for further development
The conceptual idea for the “direct contact heat exchanger” and its supporting equipment has been established and needs to be defined for a feasibility development. This design process set point is required for the evaluation of the preliminary concept and will assist in developing it into a feasible proposal. From this feasible proposal a working pilot facility can be constructed to ultimately prove the concept and develop it into an acceptable process for industrial applications, where the need exists.
The process layout to be tested for feasibility is illustrated below:
Figure 5: Conceptual layout of envisioned continuous carbon capture process.
The above layout, together with the descriptions in the above sections, will serve as as the conceptualization set point from which the further feasibility studies will proceed. All components will be theoretically assessed for their ability to deliver on their intended requirements individually and then the process will be assessed as a unit.
4. Next Steps
EPCM Consultants have proposed the above concept to the University of Pretoria, South Arica engineering department to assist in the feasibility development of the concept. Further development of the concept will be completed by a team selected by the university to deliver a feasible proposal, and ultimately a working prototype. All progress will be included in future revisions of this article.