Flame ionization detection as a simple real-time tar monitoring device for biomass downdraft gasification

A new, simple online tar- and performance measurement method for biomass gasifiers, based on flame ionization detection (FID), is presented. An empirical correlation between the total hydrocarbon content (THC) and the gravimetric tars in the product gas is established. The evaluation is done on a commercially available 85 kW throated downdraft gasifier fed with wood pellets. The product gas composition is determined with a gas analyzer for CO, CO2, CH4 and H2, as well as the tar protocol. This data is compared to the total hydrocarbon content measured with a flame ionization detector. To investigate different tar levels, different operating points of the gasifier are analyzed. The gathered data of THC versus gravimetric tars are best fitted by an exponential function. The repeatability of the method is proven with data from previous measurements with wood pellets and wood chips, although the employed correlation has to be adapted for different fuels and gasifiers. The presented FID based method has three main advantages compared to other state-of-the-art tar detection techniques: 1) in contrast to the widely used tar protocol the method delivers real-time results, hence the investigation of fast process changes is possible; 2) while other online detection devices are expensive, highly complex and often maintenance intensive, the presented method is simple, cheap and robust; and 3) the FID is flexible and convenient not only for tar detection, but for numerous other applications, which increases its attractiveness.