Reducing global carbon emissions requires a a diverse portfolio of low-emissions technologies, including renewable energy and carbon capture and storage (i.e., CCS and CCUS).1,2 Without using the full portfolio of low-emission options, the costs for reducing global emissions will be higher and the probability of successful climate change mitigation decreases. Each technology, however, faces its own set of challenges. For example, although the deployment of renewables has accelerated in recent years, the issue of intermittency remains a major challenge. Similarly, CCS is lagging behind the projected amount of demonstration projects needed. Sustainable Energy Solutions (SES) has developed a low-cost, integrated energy storage and CO2 capture technology, called Cryogenic Carbon CaptureTM (CCC), that can help address the major challenges faced by renewables and CCS.
While the CCC process relies on refrigeration process principles, the products are at nominally the same temperature as the incoming flue gas, and thus the energy efficiency is much higher than for typical refrigeration processes. For comparison, the energy efficiency of an air conditioner could be similarly high if it delivered air at the same temperature as the outdoor air, which, of course, defeats the purpose for that application. However, since the purpose of the CCC process is to separate CO2 from the other constituents in flue gas, with cooling as only an intermediate step, recuperative heat exchange drives most of the temperature change.
THE CCC TECHNOLOGY
The foundation of the CCC process relies on refrigeration to cryogenic temperatures, rather than a chemical reaction, to separate CO2 from flue gas from a power plant or industrial source. Typically, refrigeration cycles consume large amounts of energy, but this is only true if the final products are at lower temperature than the incoming streams, e.g., air conditioning.
While the CCC process relies on refrigeration process principles, the products are at nominally the same temperature as the incoming flue gas, and thus the energy efficiency is much higher than for typical refrigeration processes. For comparison, the energy efficiency of an air conditioner could be similarly high if it delivered air at the same temperature as the outdoor air, which, of course, defeats the purpose for that application. However, since the purpose of the CCC process is to separate CO2 from the other constituents in flue gas, with cooling as only an intermediate step, recuperative heat exchange drives most of the temperature change.
“Without using the full portfolio of low-emission technologies, the costs for reducing global emissions will be higher and the probability of successful climate change mitigation decreases.”
There are two possible implementations of the CCC process. Figure 1 illustrates the major process steps of the external cooling loop (CCCECLTM) version, which is the implementation that enables large-scale energy storage. Alternatively, the compressed flue gas (CCC-CFGTM) version of the process differs from the ECL version in that it does not include an external refrigeration loop but rather uses the flue gas as its own refrigerant. This article focuses on the ECL process to highlight the opportunity to meet the dual challenge of CCS deployment and energy storage; more information on the CFG process is provided elsewhere.
FIGURE 1. Simplified flow diagram of the CCC-ECL™ process
CO2 Capture
The flue gas enters the capture system and cools in a series of heat exchangers until it reaches a temperature at which the CO2 freezes to form a nearly pure solid that separates easily from the remaining gases. The process pressurizes the solid CO2 to force out all the gases from between the solid particles. Two separate streams exist at this point in the process: the pressurized solid CO2 stream and the CO2-lean flue gas stream at ambient pressure. Both streams warm to ambient temperature by cooling the incoming gases in recuperative heat exchangers. These recuperative heat exchangers are important because they accomplish most of the sensible cooling in the process. As the solid CO2 warms, it melts to form a liquid. The process delivers a liquid stream of nearly pure CO2 at 150 bar and a gas stream at atmospheric pressure, with both streams near ambient temperature. This process can capture more than 99% of CO2 from a large-point source emitter. One substantial advantage of this approach is the ease with which emission sources can be retrofit. Although the process uses electricity, it does not require the extraction of steam or any upstream modifications.
Simultaneous Emissions Control
As the flue gas cools in a series of heat exchangers (for simplicity, only one is shown in Figure 1), most gases other than N2 and O2 condense at component-specific temperatures. Thus, as part of the CO2 capture process, the CCC process also captures SOx, NOx, Hg, HCl, particulate, VOCs, etc. In fact, the CCC process removes all gas constituents less volatile than carbon monoxide (CO), which includes nearly all other currently and foreseeably regulated emissions.
Energy Storage
The CCC-ECLTM process stores energy in the form of cold, condensed refrigerant. If there is excess power from renewables on the grid, the extra electricity generates and stores excess refrigerant. The CO2 capture process recovers this energy in periods of high power demand by increasing the net power plant input, using the stored refrigerant, rather than compressor power, to drive the carbon capture and reduce parasitic losses. Refrigerant generation represents over 80% of the energy required in the CCC-ECLTM process (see Table 1). The same approach allows dispatchable power plants to follow dynamic load without changing steam generation rates or temperatures.
TABLE 1. Summary of energy demands in the CCC-ECLTM process
SES has completed detailed transient analyses of the energy storage and recovery processes.7 For example, an 800-MWe power plant can stabilize up to a ±400-MWe swing in power demand on a typical U.S. grid with intermittent wind and dispatchable gas and coal power. The estimated economic benefit of the energy storage exceeds $20/MWh, because the system can utilize energy which would otherwise be curtailed or is generated using low-cost baseload resources during off- peak times.1 The process also largely decreases the need for spinning reserve and other high-cost backup systems. The value of the energy storage nearly equals the carbon capture cost in many markets.
PROJECTED PERFORMANCE AND ECONOMIC COMPARISONS
Economic analysis completed by SES, based on application of the technology in the U.S., indicates that, even without considering the economic advantages of energy storage, the CCC process is more efficient and cost effective than leading alternative approaches to CO2 capture.
SES has completed quantitative estimates for the energy consumed by its CCC processes and compared them to that of a post-combustion liquid amine CO2 capture system. The results based on the CFG and ECL systems appear in two forms: a bolt-on version and implementation with some integration. The bolt-on versions consume about 0.71 GJe/tonne of CO2 captured. An integrated system (1) uses a portion of the heat collected in the first condensing heat exchanger to preheat boiler feedwater and (2) reduces the energy demand associated with the control of other emissions (e.g., SOx, NOx, etc.) by capturing them as part of the CCC process. These integration steps reduce the effective energy demand to a little less than 0.6 GJe/ tonne of CO2. In both the bolt-on and integrated configurations, CCC is predicted to consume significantly less energy than post- combustion liquid amine-based CO2 capture (see Figure 2).
The primary sources of energy savings compared to liquid amine systems come from two factors: (1) the CCC process does not require large thermal swings or recycling materials (e.g., water and amine in the liquid amine CO2 capture process, distillation reflux in oxyfuel systems, etc.) and (2) the CCC process pressurizes the CO2 in a condensed phase, rather than as a gas. Condensed-phase compression requires far less expensive equipment and far less energy than gas compression.
FIGURE 2. Estimated parasitic load for amine6 and CCC capture processes
While the parasitic energy is a major component of costs, the economics of all CO2 capture processes also depend strongly on financing and capital costs. To provide some means of comparison with other technology options, SES obtained vendor quotes for major equipment and otherwise made stride-for- stride identical assumptions and used the same software (to the greatest extent possible) as used in detailed cost estimates provided by the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) (see Figure 3).
In all configurations, the CCC CO2 capture cost estimates per unit of electricity fall well below those of leading alternatives. The CCC processes are predicted to increase electricity costs by about 2.5 ¢/kWh, possibly much less if the processes are fully integrated and/or the energy storage option is used.4 The energy storage, as previously discussed, might provide up to 2 ¢/kWh of additional savings, which is close to the total CO2 capture cost for the fully integrated systems.8 For context, the average U.S. residential retail electricity price is about 13 ¢/kWh.
DEVELOPMENT STATUS AND CHALLENGES
SES has built and successfully tested the CCC-CFGTM and CCC- ECLTM versions of the process at lab, bench, and skid scales up to 7–8 tonnes of flue gas/day (1 tonne of CO2 per day). The largest of these test systems occupies two shipping containers and is mobile. Field tests have included flue gas slipstreams from subbituminous coal, bituminous coal, biomass, natural gas, municipal waste, tires, and blends of these fuels. These field tests occurred at utility-scale power plants, industrial heat plants, cement kilns, and pilot-scale reactors. SES is actively seeking technology partners capable of constructing the equipment for the next two phases of the project: a 5-MWe equivalent (100 tonnes/day of CO2) pilot plant and ultimately a 150–200-MWe demonstration plant.
FIGURE 3. Incremental increases in the cost of electricity relative to a non-capture supercritical (SC) plant for an amine system6 and for CCC with varying degrees of integration. The bars represent estimated cost of electricity for a new SC coal plant with no carbon capture, a new SC plant with 90% capture via aqueous amines, a new SC plant with 90% capture by CCC, the cost of power for an existing plant with paid-off capital (i.e., most existing plants in the U.S.), and cost of power from an existing SC plant retrofitted with CCC. The first two of these bars are based on results published by NETL6 and the others are SES results using the same assumptions.
CCC-ECLTM process test skid
Several of the essential components of the CCC processes are in commercial use in the power and other industries. Examples include the condensing heat exchanger, many of the intermediate heat exchangers, slurry and cryogenic liquid pumps, dryers, and water treatment facilities. The primary equipment that is not currently available commercially, and thus the focus of current and future technology development efforts, includes cryogenic solid-fluid separations equipment and desublimating heat exchangers that continuously process solids-forming streams without fouling or plugging.
The remaining challenges in the scale-up of the CCC technology include assessing potential long-term issues with construction materials and engineering details related to solids handling at large scale. Water purification, multi-pollutant handling, and other process steps also still require demonstration, but should be manageable using currently available commercial technologies.
CONCLUSIONS
The CCC-ECL™ process affordably reduces emissions from fossil-fueled power plants while enabling more and better use of renewables on the grid. The CCC process offers major advantages over alternative capture technologies, including lower energy consumption, lower costs, optional energy storage, easier retrofit, lower water use, and optional criteria emission control. Based on its multiple advantages, the CCC process could become one of the most strategically important components of a low-carbon power industry.
Source: Larry Baxter - Cofounder, Sustainable Energy Solutions (SES)
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