Electrochemical production of chlorine is energy intensive. However, new technology promises to cut energy consumption by as much as 30% compared to conventional membrane technology, thus also reducing carbon dioxide emissions at power plants. A demonstration plant with a capacity of 20,000 metric tons of chlorine per year has just gone onstream in Uerdingen, Germany (Figure 1). Its lower electricity demand will decrease CO2 output from power plants by up to 10,000 metric tons/year.
The plant features oxygen depolarized cathodes (ODCs) developed by Bayer MaterialScience, Leverkusen, Germany, and new electrolysis technology from Uhdenora, Milan, Italy. It reflects eight years of work by Bayer to combine the two technologies, and marks the first teaming of ODCs with Uhdenora electrolysis cells on an industrial scale.
If the two-year trial proves successful, Bayer intends to gradually switch its chlorine production to the new combined approach. In addition, Bayer and Uhdenora will market the method.
ODC technology is based on the fuel cell principle and recycles chlorine from hydrochloric acid in a closed cycle. Bayer already uses the acid-based approach at production sites in Caojing, China, and Brunsbüttel, Germany.
"By feeding in gaseous oxygen, the new technology enables electrolysis to be performed at a lower voltage," explains Michael Schlösser, a Bayer spokesman. Energy savings may reach 30% versus membrane cells and 50% compared to mercury technology, he says.
"ODCs are highly complex structures. Different layers with different functions are needed inside the electrodes," he adds. (See Figure 2.)
The ODCs replace the standard cathodes used in the membrane process, which is the dominant route for making chlorine. They can be retrofitted in existing plants, even those using the newest generation membrane technology.
Currently, capital cost exceeds that of conventional cathodes but the energy savings over the life of the plant will more than make up for the higher investment, says Schlösser, who adds that Bayer is working to optimize the capital cost. In addition, government initiatives aimed at reducing CO2 emissions should favor the technology, he notes.