One of the challenges in attempting to reduce emissions of greenhouse gases (such as carbon dioxide and methane) is the considerable variety of emission sources. Some, like the burning of fossil fuels for power generation and transportation, are reasonably obvious, but there are others that aren’t. For example, it has been estimated that, in some California counties, cattle flatulence produces more greenhouse gas than motor vehicles.
Another source that might not spring immediately to mind is the production of cement, which is estimated to account for 5-6% of man-made greenhouse gas emissions. Conventionally, cement is made by heating crushed limestone (which is basically calcium carbonate, CaCO3) to a temperature of about 1500°C, where it breaks down to give calcium oxide (CaO, also called quicklime), the key ingredient of cement, and carbon dioxide.
CaCO3 -> CaO + CO2
About 60% of the carbon dioxide produced in the production of cement comes from this reaction; the balance comes from the fossil fuel burned to heat the limestone.
According to an article at Technology Review, a team at George Washington University has developed a new process that produces calcium oxide from limestone without the emission of carbon dioxide.
The new process changes the chemistry. Rather than emitting carbon dioxide, it converts the gas, using a combination of heat and electrolysis to produce oxygen and either carbon or carbon monoxide, depending on the temperatures employed.
In this process, the crushed limestone is mixed with lithium carbonate and heated to about 900°C. The application of a relatively small electric current causes calcium oxide to form as a precipitate.
The research team believes that the heating required for the process could be provided by solar energy, and has constructed a proof-of-concept apparatus to demonstrate this. The device uses two large Fresnel lenses to concentrate sunlight on the mixture of limestone and lithium carbonate, and a third to focus sunlight on a solar cell, which provides the electricity required. The device makes use of about 50% of the available solar energy, which compares favorably with the ~15% efficiency of solar cells.
The technique is still at a very early stage of development, and considerable work remains to check that the process can be scaled up to industrial size. Nonetheless, it is an intriguing idea, and another example of the value of encouraging research into alternative technologies. Cement is not very exciting or sexy, after all; but I suspect that progress in reducing greenhouse gases will, in the end, involve a number of changes like this.