The following is a guest post by Dr. Jerry Unruh. Jerry is a Ph.D. chemist that I had the pleasure of working with for several years. (More on Jerry in A Conversation on Energy Issues). Below Jerry presents his concerns that Calera’s claimed process for carbon sequestration may be grossly exaggerated. Calera responded to Jerry’s initial letter to the editor of High Country News, but unfortunately they provided no calculations to refute what Jerry has presented. Jerry’s analysis was quantitative, but Calera’s response was qualitative.
Calera Corporation’s presumed Carbon Capture and Sequestration (CCS) Process
by Dr. Jerry Unruh, May 31, 2010
Calera Corporation has made several high profile statements recently about their process to capture carbon dioxide (CO2), particularly from coal-fired power plants using seawater. These reports (e.g., in the NYT and High Country News) have been highly positive. Calera claims to produce a mixture of calcium and magnesium carbonates (limestone, dolomite, aragonite, etc) from the calcium and magnesium in seawater. Presumably the product can be used as a substitute for Portland cement. I am concerned that the performance of the CO2 capture part of the process has been exaggerated. Specifically, I think much more energy may be required than has been indicated, which could negate the value of the process.
The Calera website is more nuanced than the news articles, but the energy balance problems remain. This concerns me for two basic reasons. First, if my concerns are correct, it leaves the public with a false sense of security that the greenhouse gas problem is solved. Second, public money, if sought, takes funds that could be used for other energy/climate change solutions that have the potential for solving both our energy and climate issues. For these reasons I urge Calera Corporation to do a “black box” energy and material balance around the carbon capture part of their process so it can be fairly judged. A potentially simple way to do that balance is around production of 1 kWh of electricity. My concerns using this method are listed below.
A coal-fired power plant consumes approximately 10,000 Btu of energy per kWh of electricity produced and produces approximately 1 kg (2.2 lb) of carbon dioxide/kWh. Only about 1/3 of this energy is converted to electricity; the rest (6,600 Btu) is waste heat. The waste heat is enough to vaporize less than a gallon of water (about 8,000 Btu are required to evaporate a gallon of water) or raise the temperature of 100 gallons of water about 8 degrees F.
The concentrations of calcium and magnesium in seawater are fairly constant and are 0.411 g and 1.29 g respectively (Handbook of Physics and Chemistry, 64th Ed and/or http://www.seafriends.org.nz/oceano/seawater.htm#salinity). If absolutely all the calcium and magnesium in seawater reacted with carbon dioxide to produce the respective carbonates, it would take approximately 95 gallons of seawater to remove the carbon dioxide for just 1 kWh. A 500 MW plant would require about 415 billion gallons of seawater/year under this scenario. If only CaCO3 is precipitated, about 600 gallons of seawater would be required to remove byproduct CO2 from 1 kWh of electricity production!
A coal-fired power plant with cooling water towers consumes about 1 gallon of water for cooling with about two thirds evaporated and one third blown down (see for example “WaterReport_IGCC_Final_August2005.pdf”, pp.62 ff). Once through cooling using ocean water is used in a number of power plants along the California coast. These appear to require about 20-45 gallons/kWh and may be phased out for environmental reasons, both from federal and state government mandates. In any event the cooling water use in such plants is one fifth to one half of what would be required for capture of CO2 from a coal-fired power plant (assuming precipitation of all the Ca and Mg). At least one plant, El Segundo, has already converted to air-cooling (see below).
I think it important to make a short digression here. The April 26, 2010 issue of High Country News contains my letter to the editor and Brett Constanz’s (Calera corp) response. In the Calera response Dr. Constanz indicates that for power plants along the coast, there is already seawater cooling that exceeds their process requirements. Therefore I tried to find once-through cooling requirements for coastal plants in California. Unfortunately, I could not find direct data so I used the information in: http://www.cpuc.ca.gov/Environment/info/esa/divest-edison/tables/tab4_4_1.pdf and calculated the flow rates/kWh based on published information for the plants shown in the table. My calculated rates varied from 18 to 48 gallons, but none came close to the 95 gallons above. In addition, once through seawater cooling may well be phased out in California (http://www.sanluisobispo.com/2010/03/23/1077678/state-plan-would-end-seawater.html). The plants listed are natural gas fired, but the efficiencies should be close to the same for coal-fired plants. I cannot reconcile these numbers with Dr. Constanz’s comments.
The calcium, magnesium carbonates can be precipitated in several ways, one of which is evaporation of the water. Given the chemical equilibria involved, significant water would probably have to be evaporated, but there is only enough waste heat to vaporize less than one gallon of water. Further evaporation would require more heat input, and if this comes from fossil fuels, the energy balance could rapidly become negative. For example, seawater is evaporated to produce sea salt with calcium carbonate precipitating as the first half of the water is evaporated (http://www.seafriends.org.nz/oceano/seawater.htm#salinity). Simply heating the seawater could precipitate some calcium carbonate, but more than a 8-9 degree F heat rise would be required (the amount the waste heat could raise the temperature of 95 gallons of seawater).
One can also add base, e.g. sodium hydroxide (NaOH), to raise the pH high enough to precipitate calcium, magnesium carbonates (> ~ 9). Calera Corp presumably has an electrochemical process that can produce sodium hydroxide at ½ to ¼ the electrical energy of the current chlor-alkali process, but even at ¼ the energy usage it would still require about 0.025 kWh/mole of NaOH = ~0.6 kWh/kg. In addition equimolar quantities of HCl are produced – more about this later.
In world patent WO2009006295A2 assigned to Calera Corporation, the patent examples indicate that about 5 moles of NaOH are required to precipitate 1 mole of CaCO3 from seawater treated with CO2. This is outrageous since approximately 1 kg of CO2 is produced/kWh from a coal-fired power plant. Therefore, approximately 2.7 kWh of electricity would have to be consumed to produce enough NaOH to trap 100% of the CO2 byproduct from 1 kWh of electricity! There must be better examples than those presented in the above patent.
The Calera website also indicates that inland power plants could use calcium-containing brines for capture of CO2 from power plants. In general, calcium concentrations in groundwater are relatively low. However, there are brines in the U.S. that contain more than 4% Ca as CaCl2 or about 1 mole/liter (Kirk-Othmer, Concise Encyclopedia of Chemical Technology, Fourth Ed, John Wiley 7 Sons, NY, 1999, p. 392). However, it is hard to imagine that CaCl2 could trap CO2; reaction of the CaCl2 with 2 moles of NaOH would convert it to Ca(OH)2, which could than capture the CO2. Under this scenario, the full 1kWh would have to be consumed to produce the NaOH required to capture the byproduct CO2. Of course, pumping costs would have to be considered here as well.
There is also the problem of HCl byproduct from the Calera process to produce NaOH. The annual world production of HCl is about 20 million tons, most of which is captive (about 5 million tons on the merchant market). The new 750 MW Comanche 3 addition to the Xcel’s Comanche coal-fired power plant complex in Pueblo, CO will generate almost 6 million metric tons of CO2/yr (assuming 90% capacity factor). Assuming stoichiometric consumption of NaOH, this one plant would produce half the world’s entire annual demand for HCl – clearly a problematic issue.
In summary, my analysis may be way off base, but it is not obvious to me how. Given the importance of energy and greenhouse gas problems, I again suggest that Calera Corp. treat the carbon capture part of the process as a “black box”, but show all energy and material flows in and out so the technology can be fairly judged. I submit that this can be done without providing proprietary information. My concerns are due to Calera’s high profile promotion of its process that can lull the public into a false sense of security.
Jerry D. Unruh, Ph.D.