Carbon Emissions | January 27, 2009 |
Capturing Carbon Emissions…in Cement?
By Ben BlockStepped on for more than 2,300 years, cement rarely gets the respect it deserves.
The versatile construction ingredient has been commonplace since the Roman Empire, but production has boomed in recent decades, rising nearly 400 percent since 1970. Global cement output may reach 5 billion metric tons by 2030, driven mainly by growth in the top two producing countries, China and India, according to a December WWF report.
In much of the world, replacing traditional wooden or earthen structures with concrete - the hardened mix of the cement powder, aggregate, and water - is a sign of wealth and prosperity. But cement manufacture is among the most energy-consuming and polluting industrial processes.
WWF estimates that unless cement plants become more efficient or transition to renewable energy sources, the industry - now responsible for some 6 percent of human-caused greenhouse gas emissions - is likely to become a larger contributor to climate change than the European Union by 2030.
Alternatives have emerged this past year, however, that may redefine the future of cement. One company, California-based Calera Corp., claims to have found a way for cement production to absorb more carbon dioxide than it emits, acting as a vehicle for carbon capture and sequestration, or CCS.
Similar to how certain corals produce reefs by excreting dissolved calcium carbonate, or limestone, Calera sends carbon dioxide emissions through seawater to create a chalky carbonate byproduct. This cement substance is then mixed with aggregate and water to create concrete. Its production avoids the need to heat the cement materials in coal-fired kilns, while sequestering carbon throughout the process, the company says.
For every ton of cement produced, two-fifths of a ton of carbon dioxide is stored, according to founder Brent Constantz.
"Calera appears to be the only viable, economically feasible, no CO2-footprint way to capture, separate, and sequester billions of tons of carbon dioxide," said Constantz, a Stanford University biomechanical engineer who developed bone-fracture cement before turning his attention to climate change solutions. "We're converting carbon dioxide into carbonate, and we're putting it into the built environment."
Constantz opened his company's first demonstration site in August 2008. Located about 90 meters away from a natural gas-fired power plant, the Calera facility aims to filter more than 90 percent of the plant's carbon dioxide emissions through seven 11-million liter seawater tanks that sit adjacent to the Pacific Ocean.
"Gas-powered plants are not our principal aim," Constantz said. "Our principal aim is coal plants around the world. We're spending lots of time interviewing different coal plants in the U.S. and other parts of the world as well.... The challenge is to decide how many plants to develop in parallel."
Although the test facility so far produces at most five tons of cement per day, the technique is quickly gaining attention around the world. Aided by the connections of its main investor, venture capitalist Vinod Khosla, Calera is discussing projects in Australia, the Netherlands, Saudi Arabia, and the United Arab Emirates (UAE), which is considering using the material in its Masdar City project, Constantz said.
Calera is not alone, however. California-based Carbon Sciences plans to mix cement with power-plant emissions and waste material from coal and steel production. Carbon Sense Solutions in Nova Scotia, Canada, redirects emissions from pre-cast concrete plants into new cement. And London-based Novacem says its magnesium silicate cement will absorb enough emissions to make it carbon negative.
But Calera appears to be gaining the most attention. The company received interest from the U.S. Department of Energy last year - Constantz met with former agency Secretary Sam Bodman. Constantz is also speaking at next month's World of Concrete trade fair in Las Vegas, the commercial concrete and masonry construction industry's only annual international event.
In addition, Khosla promoted his investment at the World Future Energy Summit in Abu Dhabi, UAE, on Thursday as a better CCS strategy than geological storage. The world's eight largest industrialized nations, the G8, agreed last year to establish 20 CCS demonstration projects by 2010 [PDF], most of which would inject filtered carbon dioxide emissions into underground reservoirs.
Calera and other emerging cement companies still need to prove that the carbon-sequestering cements can be cost efficient and durable compared to traditional cement.
Meanwhile, the cement industry's interest in reducing energy costs has led many countries to replace small-scale cement plants with larger, more efficient models. Cement kilns are also turning to greater use of renewable energy sources, such as biomass, which typically provides only 5 percent or less of kiln fuel in developing countries.
But green building designers are searching for more dramatic changes in cement options. "Let's ...begin to lessen the CO2 consequences of our current dependence on cement," said Gail Vittori, co-director of the Austin, Texas-based Center for Maximum Potential Building Systems, whose headquarters is built with concrete made from recycled sources. "Part of that is how we identify opportunities for some more-benign cement materials such as naturally occurring elements or recycled materials."
Alternative blends have included ash from coal-fired power plants, known as fly ash, and pozzolana, a volcanic ash. Although limited in their supply, these natural materials replace some of the fused minerals used in traditional cement, and therefore avoid the need to heat limestone to 1,450 degrees Celsius. The substitutes are becoming more common, mostly in continental Europe, according to the International Energy Agency.
Vittori said she is also keeping an eye on companies such as Calera. Even if they cannot replace cement entirely, their impact could be meaningful, she said. "Adding 1 percent here or there could make a big difference in CO2 emissions," she said. "It's certainly a worthy pursuit."
Ben Block is a staff writer with the Worldwatch Institute.
Photo courtesy howzey/Flickr
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Comments By Readers
The Calera story sounds good - but upon close examination, and a reading of their patent, it is clear that it is a complete Greenwash.
Their patent application US 20090020044 (search under patent number at http://appft1.uspto.gov/netahtml/PTO/search-a... ). The patent describes taking dolomite/dolomitic limestone and calcining it (releasing CO2) and then reacting this with seawater and CO2 to regenerate calcium/magnesium carbonate!!!
There is very little if any capture of CO2 by their process since the calcining step generates so much CO2, and there is CO2 generated from the processing operations - it is highly probable that the technology has a net positive carbon footprint.
Where is the biological process for capturing CO2? Calera is using decades old and very dirty and polluting chemistry for extracting magnesium/calcium from seawater - a very similar process was used by the magnesium factory which formerly occupied the Moss Landing site.
There is nothing Green about Calera's process - basically, they are burning/calcining limestone, and using it to make artificial limestone from seawater. All smoke and mirrors.
And, Calera's product is no cement - at best it is a poor mineral admixture. The patent shows that at a 20% replacement of Portland cement, the strength is only 50%(3000 psi) of that of 100% Portland cement - to get close to the strength development of Portland cement, the replacement level has to be reduced to 5%. Also, the drying shrinkage is doubled at 20% replacement of Portland cement.
Not only is Calera's product not a cement, but its addition to Portland cement is very deleterious - it greatly reduces strength and increases shrinkage - and almost certainly decreases long-term durability, corrosion resistance, freeze-thaw resistance, etc..
This is very disappointing - first Calera said that it had a 100% replacement for Portland cement that would capture 1 ton of CO2 per ton of cement (impossible, unless the cement is pure CO2!)- they then amended this to a 50% replacement for Portland cement and a carbon neutral cement. Now what - 5% replacement for Portland cement?
Total Greenwash!
And note for comparison that (natural) limestone is commonly used as an aggregate for concrete, and is used as an admixture for cement in Europe at up to 45%.
A quick analysis of Calera’s process:
Assuming Calera captures calcium and magnesium from seawater as carbonates, one ton of carbonate cement would require at least 500 tons of seawater (> 80% capture efficiency) - or about 300 tons of desalination brine. So, to supply just US cement demand (over 120 million Mt per year), you would need to process 50 billion cubic meters of seawater! The most economic method would be to piggyback the process onto desalination plants, but even with desalination capacity increases, desalination brines could supply at most 6% of US cement demand. And, processing seawater for cement production alone is neither economic (Portland cement sells at $100-120 per Mt in the US) nor environmentally friendly.
Also, such a process will generate a calcium/magnesium-stripped brine rich in sodium/potassium. Many, many studies have shown that such brines have severe environmental impacts when discharged into the ocean - the high salinity kills flora and fauna in the brine plume - so much so that regulations now dictate that such brines have to be diluted with seawater prior to discharge, or have to be landfilled. Also, processing seawater produces large amounts of a toxic sludge containing copper, nickel and chromium (leached from metal piping and processing tanks) as well as cleaning agents and disinfectants (used in daily cleaning operations) – this sludge is highly hazardous and has to be landfilled.
It is puzzling that Calera does not appear to be talking about a biological process, and in fact their patent is a pure chemical process that essentially reprocesses natural limestone to make artificial limestone. Originally, Calera was talking about making calcium/magnesium carbonates via a biological path - ie. use carbonate-forming marine organisms to form magnesian calcite, and collect/process the resulting biomass/skeletons (the precursor to limestone). Of course, the chemistry is such that this product could never be a cement (it does not undergo a hydraulic reacting with water and does not set) – this would explain why Calera has now given up on making a cement (as they had initially claimed). Similarly, this product would be a poor mineral admixture for cement due to its biomass content and the crystalline form of the magnesian calcite – this would explain the poor results reported in the Calera patent application.
Now, presumably as a last-ditch effort to salvage something, Calera is talking about making an aggregate. Well, if you want a strong, resilient and time-tested carbonate aggregate, you simply use limestone. You do not set up a limestone-to-calcined limestone-to artificial limestone process that is dirty and polluting and carries huge environmental consequences, makes a much inferior product to the natural material, and then claim it is Green!
More Greenwash!
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