I’ve got a piece in Technology Review about a Quebec City-based company called CO2 Solution, which has developed a bioreactor for capturing CO2 from power plants and industrial facilities. At the heart of the reactor is an enzyme — extracted from genetically engineered E. coli but also present in all animals — that can absorb CO2 and convert it into bicarbonate. The bicarbonate-rich solution is then removed and the bicarbonate ions can be extracted for making everything from baking powder (sodium bicarbonate) to calcium carbonate (limestone). Alternatively, concentrated CO2 can be taken out of the solution and pumped into geological storage.
There are obviously plenty of alternatives for capturing CO2 from industrial and power-plant flue stacks, but separating the CO2 stream from other gases and pollutants makes most of these approaches energy-intensive and therefore quite costly. One of the oldest methods is to absorb the CO2 using a chemical solvent such as monoethanol amine, but it remains an expensive approach and requires significant pre-scrubbing of the gases.
Other options include cryogenic separation (complex and vulnerable to contamination), membrane separation (requires flue gas pressure and CO2 concentrations not characteristic of power plants), and absorption through porous solids such as alumina and activated-carbon (very energy-intensive to extract CO2 from solid afterwards).
Another, more recent method being tested in coal plants is to combust the fuel in a pure oxygen environment. The so-called oxyfuel approach achieves a high, almost pure concentration of CO2 with trace pollutants. But again, pre-separating pure oxygen from the air takes a lot of energy that drives up the cost. That said, power-plant equipment providers such as Babcock and Wilcox see potential in oxygen combustion, and at least two pilot plants — a Vattenfall lignite-fired coal plant in Germany and a Saskpower “clean coal” demonstration plant in Canada — hope to be the first to use the approach commercially.
Let us not forget CO2 capture for coal plants based on gasification, which turns coal into a synthetic gas made of carbon monoxide and hydrogen. Using steam and some catalysts that syngas can be made into hydrogen and CO2. The hydrogen is combusted in a gas turbine to generate electricity and the CO2 stream is easily separated. Unfortunately, the design of a gasification plant is generally considered 20 per cent more expensive than a conventional plant.
Of interest is that Babcock and Wilcox is working closely with CO2 Solution on its enzyme-based carbon-capture technology, which appears to be unique in the market. Babcock said it wants to broaden its porfolio of carbon-capture options and could see itself licensing the technology from CO2 Solution, assuming the company can drive down the price of its process. Development of the enzyme approach is still embryonic, but obviously has great potential given the interest from a major player such as Babock.