Online monitoring of industrial flue gases using tunable diode laser with a digital-control module

Increased demands on air quality have created incentives for new methods to monitor gas pollution. In this paper we will present an online gas analyzer for industrial flue pollution monitoring based on tunable diode laser absorption spectroscopy (TDLAS) technology. Signal measurements with a sensitive device inevitably suffer from the predictable or unpredictable sources such as intensity fluctuations and the laser output wavelength dithers. In order to eliminate or at least reduce the measurement uncertainty and gain high reliability, a close-circle digital-control module with functions of digital signal generator, digital lock-in-amplifier (D-LIA), data acquisition and data processing has been developed to substitute the previous independent signal generator board, analog lock-in-amplifier and data acquisition card in our TDLAS system. With the help of the digital-control module and the advanced digital signal processing techniques the performance of the TDLAS system has demonstrated great improvement in long term field tests.