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As in SO2 mitigation, spontaneously remove CO2 from power plant flue gas using wet limestone scrubbing.



Carbonate mineral weathering is a major absorber of excess CO2 at planetary scales: CO2 + H2O + CaCO3 --> Ca(HCO3)2aq. However, relying on this very slow natural process to consume excess CO2 would in the interim commit us to many millennia of climate impacts and ocean acidity (1).  It is therefore relevant to find ways of cost-effectively accelerating this proven, natural (geo)chemistry in order to more quickly mitigate of our CO2 emissions, while also trying to rapidly transition to non-fossil energy sources.

Modeling and lab studies have shown that contacting CO2-enriched gas with water and limestone is an effective way of spontaneously capturing and storing CO2 as dissolved calcium bicarbonate (2-7). This is termed Accelerated Weathering of Limestone – AWL. In laboratory tests, up to 97% of the CO2 in a dilute gas stream was removed using this method (11). Seawater would appear the best option for such systems, although other non-potable water sources (wastewater, saline ground water) could also be relevant at inland sites.

An AWL total cost of <$30/tonne CO2 avoided has been estimated, with <$20/tonne being more likely at coastal power plants that already pump massive quantities of seawater for condenser cooling. The preceding mitigation cost ranges are a fraction of that reported for more conventional capture and underground storage of concentrated CO2 (CCS) when retrofitted to existing power plants (8).

CO2 mitigation is not the only potential benefit of AWL. As in natural carbonate weathering, the dissolved Ca(HCO3)2 added to the ocean by the process will help to chemically offset the effects of CO2-induced ocean acidification (9-11).

Despite its potential, AWL is lacking a demonstration at a scale that would prove its cost effectiveness, safety, and net environmental and societal benefit.  It is proposed that these issues be evaluated and tested at a relevant scale by a team of scientists, engineers, and environmental, economics, legal, and social experts.

Category of the action

Reducing emissions from electric power sector.

What actions do you propose?

Here we propose to assess the feasibility of AWL through the operation and monitoring of a demonstration facility.

Theoretical, modeling, and lab-scale studies (2-7) have all indicated that at coastal locations AWL can very significantly reduce CO2 emissions from power plants at a fraction of the cost of conventional CCS. Importantly, unlike CCS,  AWL avoids the very energy intensive capture and concentration of CO2, and avoids the risks inherent in concentrated CO2 transport and storage.  AWL does this by exploiting this spontaneous, exothermic chemical reaction: CO2 + H2O + CaCO3 ---> dissolved Ca(HCO3)2.  This is not to say that there is zero energy penalty. AWL requires that significant quantities of water, flue gas and limestone be moved and contacted in order for the reaction to occur and CO2 mitigation to proceed. Yet when the capital, operation and maintenance costs of the preceding are calculated for coastal power plants, total cost is typically <$30/tonne CO2 avoided (2-7), or less than 1/3 the cost of CCS retrofitted to existing power plants. This will especially pertain to coastal power plants where limestone is nearby, and where seawater, often massively pumped for condenser cooling, can be reused as a low cost water source.  Furthermore, the design and installation of AWL reactors can benefit from existing wet limestone or seawater scrubbing of flue gas, mature technologies already widely employed in the electric power industry to mitigate SO2 (12, 13).  Approximately 2.5 tonnes of crushed limestone are required for every tonne of CO2 avoided. The large limestone and water requirements of AWL suggest that a landfill sized reactor would be required. The minimum volume would be around 123,000 m3 to treat 1 million tonnes of CO2 per year, which is equivalent to a small (2 acre) landfill reactor.

After leaving the AWL reactor, the ‘hard’ seawater would mix with the existing bicarbonate ions in the ocean. When globally distributed, this is unlikely to have an appreciable impact. However, around the point of addition there will be elevated alkalinity, pCO2, and carbonate saturation. Such conditions are ideal for carbonate forming organisms and could be very beneficial in offsetting the effects of ongoing, CO2-induced ocean acidification (9-11).

What is now needed is a demonstration of the AWL process at a scale that will realistically test it’s true cost effectiveness and competitiveness, safety, and environmental benefit in reducing power plant CO2 emissions. To this end an R&D team will be assembled to design and test AWL reactors at one or more relevant power plants. Natural gas fired power plants that already use seawater for once-through cooling are favored for such a demonstration, Dynegy’s 2.5 GW facility at Moss Landing , CA being a prime example. The test reactor will be designed to allow flexibility in operating procedures, e.g., gas and water flow rates, limestone particle size and mass, concurrent vs counter current flow, etc and to allow effective upstream, internal, and downstream monitoring of chemical and physical conditions of water and gas. The downstream impacts/benefits to marine biota would also be investigated by comparing biological outcomes between biota incubations in AWL effluent and ambient seawater. Another component of the study will be to identify all potential environmental impacts and permitting issues of the technology, such as that associated with increased limestone extraction, processing, and transport, and thermal, chemical, and biological impacts to seawater.  Finally, the results of the demonstration and testing will be used to calculate the full, life-cycle capital, operating, and maintenance costs and net environmnetal benefit of optimized AWL systems at full scale. This will then lead to an evaluation of  AWL’s potential global capacity and competiveness relative to other CO2 mitigation technologies.

Bottom line:  Despite billions of dollars invested, CCS is failing to quickly deliver critically needed, cost effective CO2 mitigation to the power industry.  This situation is unlikely to change as long as thermodynamically expensive and environmentally risky CO2 capture and underground storage (in molecular form) is viewed as the only valid approach. Given the urgency of the CO2 problem and our ongoing failure to adequately address it, it is now time to more broadly consider and evaluate alternative approaches to point source CO2 mitigation. AWL provides one such opportunity.  AWL will not singlehandedly solve the global, point-source CO2 problem, but neither will any other approach, short of a complete transition to non-fossil energy, which would appear to be many decades away. In the meantime we must quickly, broadly, and deeply consider and evaluate alternative approaches to point source CO2 emissions reduction. The proposed project therefore addresses the critical need to increase cost effectiveness and safety of such mitigation.

Who will take these actions?

To accomplish the above tasks, an R&D team will be formed comprising engineers, scientists, and environmental, economic, social, and legal experts with demonstrated experience in objectively evaluating new energy and environmental  technologies.  Expertise and partnering will be sought from companies that already have decades of experience developing and installing wet limestone and seawater scrubbing technologies in the electric power industry, e.g. Alstom, Babcock and Wilcox, Bechtel, Ducon, Mitsubishi, URS, etc. Regarding intellectual property, AWL methods and apparatus are covered by US patents 6890497 and 7655193.

Where will these actions be taken?

The R&D will be conducted in selected laboratories and at appropriate field sites. Live and virtual team meetings, conferences, public forums, and the Web will be used in this endeavor.

How much will emissions be reduced or sequestered vs. business as usual levels?

In the 1990’s, coastal power plants reportedly accounted for a total of about 174,000 MW  of power worldwide (14). Assuming: 1) this has now risen to 200,000 MW in 2013, 2) that these plants are on line 80% of the time, and 3) that an average of 0.75 tonnes of CO2 are emitted per MWhr, a total of about 1 GT of CO2/yr is then emitted globally by these plants. Assuming that an AWL reactor is installed at each of the preceding plants and that they can each mitigate 70% of a plant’s emissions, then the total potential mitigation is 0.7 GT CO2/yr. About 0.2 GT of carbon would then be added to the ocean in the form of Ca(HCO3)2aq, or an addition of about 0.0006% of the dissolved bicarbonate reservoir already in the ocean.  However, greater ocean chemistry impacts/benefits would occur near points of AWL discharge. Additional mitigation at inland power plants may be possible via use of wastewater or saline ground water with subsequent above- or below-ground Ca(HCO3)2aq storage. 

What are other key benefits?

1) Spontaneously removes CO2 from power plant flue gas

a) Captures point-source CO2 and converts it to Ca(HCO3)2aq

b) Avoids costly and risky capture, concentration, transport, and storage of molecular CO2

c) Accelerates natural, global scale, but slow CO2 mitigation by carbonate mineral weathering

d) Builds on mature SO2 removal technologies already widely deployed in the electric power industry  


2) Spontaneously produces environmentally beneficial calcium bicarbonate

a) Adds minimally to the massive, stable pool of dissolved Ca(HCO3)2aq already naturally present in the ocean

b) Locally/regionally increases carbonate saturation state in seawater, thus countering excess ocean CO2, acidity, and their negative effects on marine chemistry and biology

What are the proposal’s costs?

5 year R&D - $15,000,000.00 

Time line

Yr 1 

1) Assemble R&D team from academia, government, NGO's, and industry.

2) Host team and stakeholder workshops/webinars to further identify technical, environmental, legal, and social issues. 

3) Select power plant host for pilot scale AWL demonstration.

4) Model, design and construct experimental AWL reactor that will be used to test hypotheses about optimum reactor designs, performance, operating procedures, and costs.

5) Develop initial test protocols and operation procedures.

6) Initiate evaluation of potential economic, environmental, legal, social, political, and geopolitical constraints and issues of the technology.

Yr 2-4  

1) Extensively test AWL reactor under various configurations and conditions.

2) Refine reactor models, design, and operating procedures based on test results and re-test.

3) Refine global siting, economic, environmental, legal, social, political, and geopolitical analyses base on the preceding. 

Yr 5

1) Finalize data analysis.

2) Finalize recommendations for optimized reactor designs and operating procedures.

3) Finalize evaluation of global siting, capacity, economics, and environmental, legal, social, and political issues. 

4) Generate final public reports and presentations detailing and summarizing findings, making final recommendations for full-scale applications. Seek further R&D, funding, and partnering as needed. 

Related proposals



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2. Rau GH, Caldeira K. 1999. Enhanced carbonate dissolution: A means of sequestering waste CO2 as ocean bicarbonate. Energy Convers Manag 40:1803–1813.

3. Caldeira K, Rau  GH. 2000. Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications. Geophys Res Lett 27:225–228.

4. Sarv H, Downs W. 2002. CO2 Capture and Sequestration Using a Novel Limestone Lagoon Scrubber – A White Paper; McDermott Technology, Inc., Alliance, OH.

5. Rau GH, Knauss KG, Langer  WH, Caldeira K. 2007. Reducing energy-related CO2 emissions using accelerated weathering of limestone.  Energy 32:1471-1477.

6. Langer WH, San Juan CA, Rau GH, Caldeira K. 2009. Accelerated weathering of limestone for CO2 mitigation: Opportunities for the stone and cement industries. Mining Engineering 61:27-32.

7. Rau GH. 2011. CO2 mitigation via capture and chemical conversion in seawater. Environ Sci Technol 45:1088–1092.

8. Rubin ES, Zhai H. 2012. The cost of carbon capture and storage for natural gas combined cycle power plants. Environ Sci Technol 46:3076-3084.

9. Marubini F, Thake B. 1999. Bicarbonate addition promotes coral growth. Limnology and Oceanography 44:716 - 720.

10. Langdon C,  et al. 2000. Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef.  Glob Biogeochem Cycles 14:639-654.

11. Rau GH, Potts D. 2010. Enhanced conversion of point-source and atmospheric CO2 to ocean alkalinity: Concepts and experiments. EOS Trans AGU, 91(26), Ocean Sci. Meet. Suppl., Abstract IT35D-02.


13. Oikawa K, Yongsiri C, Kazuo K, Harimoto T. 2003. Seawater flue gas desulfurization: Its technical implications and performance results. Environmental Prog 22:67–73.

14. IEA. 2000. Capture of CO2 Using Water Scrubbing, Report PH3/26. International Energy Agency, Vienna.