New algorithm for calculating CF4 emissions from high voltage anode effects

Abstract

Current Tier 2 algorithms calculate CF4 emission factors based on average anode effect minutes or average millivolts Overvoltage per cell day. Both the “slope” and “overvoltage” algorithms are based on the assumption that CF4 emission rates remain constant for anode effect durations ranging from less than one second to more than 1000 s. Detailed analysis of PFC measurements made over the past decade has shown conclusively that CF4 emission rates decrease rapidly with increasing reported anode effect duration. Average duration or average overvoltage can be skewed by changes in the mix of anode effect durations, particularly the mix of anode effects mitigated by automatic process control and those much longer anode effects requiring manual intervention. This study analyzes measurement data from individual anode effects from more than 2000 individual anode effects from more than 30 different measurements to develop a new exponential algorithm that better reflects changes in emission rate with anode effect duration. The exponential calculation should prove to be more robust resulting in more accurate calculation of PFC emissions than previous models.