Skeptical of Calera
Document Actions
- Share this:
- Like
- Tweet
- Print this
- Comments (5)
I have read several positive reports (including the one in HCN on March 15) about Calera Corporation's presumed process that uses seawater or brine to sequester carbon dioxide, particularly from coal-fired power plants. Calera claims to produce a mixture of calcium and magnesium carbonates (limestone, dolomite, etc.) that can be used as a substitute for Portland cement.
I have a Ph.D. in chemistry (Oregon State University, 1970), worked in chemical research and development for 30 years, and am currently on a technical advisory group for Colorado Springs Utilities.
I have looked extensively on Calera's Web site and there is relatively little quantitative information about seawater usage, energy use in the Mineralization via Aqueous Precipitation (MAP) process, energy pumping costs and what is done with the final wastewater. I am concerned that the performance of the carbon dioxide capture part of the process has been exaggerated.
A coal-fired power plant consumes approximately 10,000 Btu of energy per kilowatt-hour of electricity produced and emits approximately 2.2 lb of carbon dioxide/kwh. Only about one-third of this energy becomes electricity; the rest (6,600 Btu) is waste heat. The waste heat is only enough to vaporize less than a gallon of water or raise the temperature of 100 gallons of water about 10 degrees Fahrenheit (possibly not accomplishing much if it's used to heat seawater or brine in the Calera process).
Also, if 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 one kwh; a 500 MW plant would require about 415 billion gallons of seawater/year under this scenario.
Space allotted for this letter limits discussion of other issues such as pumping energy, fate of wastewater, etc. Given the importance of energy and greenhouse gas problems, I suggest that Calera 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.
Jerry D. Unruh, Ph.D.
Chemical Industry Senior Research Associate (retired)
Manitou Springs, Colorado
Calera CEO responds:
The Calera Mineralization via Aqueous Precipitation (MAP) technique is a robust technology that can be applied to a wide range of applications, including power plants near the coast as well as inland. Coastal plants are often seawater-cooled, with existing infrastructure for seawater pumping that exceeds the Calera process needs. In this case, the pumping requirements are already part of the power plant process and efficiencies. For inland plants, high-salinity brines and wastewaters can be used, which have much higher concentrations (100 to 1,000 times higher calcium than in seawater) with therefore reduced pumping requirements.
Extensive studies have shown that the energy demand of the Calera process is in the range of 15-30 percent of the power plant output for capture of the carbon dioxide and conversion to building material.
The more important consideration is the total carbon life cycle of the Calera process ... including the energy and carbon offset by the avoidance of the building material produced by the conventional cement kilns. Studies by researchers at the University of California, Santa Barbara investigated the total lifecycle greenhouse gas of implementation of the Calera process at plants in Australia ... including the energy demand of the Calera MAP processes and the transportation of the Calera product to suitable markets as far away as China. The avoided production of cement by the traditional methods and the net capture by the MAP process results in a huge reduction in greenhouse gas emissions when a total lifecycle analysis including all energy demands and transportation impacts is considered.
Also, fresh-water production is a critical need in many locations around the world. Traditional desalination production of fresh water from saline water involves the use of technologies to remove hardness combined with technologies to remove salinity. These traditional technologies are very energy-intensive. The Calera process involves hardness removal directly by precipitation of the calcium and magnesium from the material via the MAP process. Therefore, the MAP process also reduces the overall energy requirements for desalination of brackish and saline water.
Brett Constantz
Consulting Professor
Department of Geological and Environmental Sciences
Stanford, California
High Country News in your inbox!
The chemistry is well known and indisputable. Carbon dioxide added to seawater reacts with carbonate ions to form bicarbonate ions which are soluble. Stated simply
Ca++CO3-- plus CO2 plus H2O goes to Ca++(HCO3-)2
The process thus removes carbonate ions allowing more solid calcium carbonate to dissolve to reach the saturation solubility product (calcium ion concentration x carbonate ion concentration).
Of course when you heat water containing dissolved calcium bicarbonate, as in your kettle, carbon dioxide gas is driven off and the reverse process happens. Calcium carbonate precipitates as the scale that we are all familiar with.
For Calera’s process to work as described the liquor used to absorb carbon dioxide must be much more alkaline than seawater, just like more conventional carbon capture processes. I am not an expert but I suppose it is conceivable that there may be a few locations where such liquids or solid equivalents can be found underground (a recent article mentions magnesium hydroxide) but Calera also propose to increase the alkalinity of the water used by an electrochemical process.
The electrolysis process Calera describe in their patent produces two moles of hydrochloric acid for every mole of carbon dioxide captured. Quoting from their website
“Q. What products does Calera produce?
A. The outputs from the Calera process are clean air, building materials (sand, aggregates, and/or supplementary cementitious products), fresh water, and excess carbonated water for re-injection. If the proprietary low voltage electrochemical process is required for additional alkalinity, hydrochloric acid will also be produced.
Q. What will happen to the hydrochloric acid?
A. The hydrochloric acid can be sold on the open market for industrial uses, used for oil well or brine production or be injected in deep geologic reservoirs”
As the lowest pH mentioned in table 1 of their patent
http://v3.espacenet.com/pub[…];DB=EPODOC&locale=en_gb
is 3.115 this hydrochloric acid is likely to be very dilute, 0.000767 moles per litre based on 3.115. The volume of acidic liquid to be disposed of is therefore a massive 59,235 cubic metres per tonne of captured carbon dioxide.
Calera do suggest other ways of increasing alkalinity. Their patent DESALINATION METHODS AND SYSTEMS THAT INCLUDE CARBONATE COMPOUND PRECIPITATION
http://v3.espacenet.com/pub[…]&NR=2155350A2&KC=A2
exemplifies addition of dolime (calcined dolomite) or sodium hydroxide. How clever is that?
I have also checked out the energy cost of Calera's low voltage electrochemical process again based on table 1 on page 14 of their patent
http://v3.espacenet.com/pub[…];DB=EPODOC&locale=en_gb
The measured 1 volt across the electrodes corresponding to the massive 59235 cubic metres of waste acid per tonne of captured carbon dioxide gives an electrical power requirement of 1218 KWH per tonne. Other examples in table 1 use less power, 487 KWH/tonne is the lowest, but they produce an even bigger volume of waste acid. At say 5 cents per KWH the 1 volt example would cost $61/tonne of carbon dioxide captured just for the electricity needed for electrolysis.