Online tar measurements as means to control the gasification process
Tar and tar related problems continue to remain the foremost obstacles in the development and implementation of gasification technologies into today‘s energy supply systems. Aromatic and polycyclic aromatic hydrocarbons (PAH) are by-products in most high temperature thermochemical conversion processes. They lower the efficiency of these processes and form tarry deposits when the gases are cooled and vapours begin to condense. It would be desirable to have a tool that is not only capable of monitoring the PAH composition and load of the product gas of a gasifier but is also sufficiently fast to be used for process control to minimize the formation of PAHs in the reactor. Up to now, no easy-to-use and reliable tool with a sufficiently low dead time has been available for tar monitoring.
LED technology for industrial applications
The new online tar measurement technique developed by Technische Universität Berlin is based on Light Emitting Diodes (LED) for Fluorescence Spectroscopy. The basic principle behind the measurement technique is simple. The light from the LED is used to excite different species in the hot product gas coming out of the gasifier. By detecting the emitted light from the analytes (tar forming aromatic species in the gas) with a spectrometer, information about the total amount and some information about the molecular size distribution can be obtained. Light emitting diodes have several advantages over lasers when it comes to industrial applications. Besides being more stable and reliable especially in a rough environment they are also much cheaper. It is possible to combine multiple diodes with different wavelengths in an array to take advantage of the diverse absorption-emission behaviour of different PAHs.
A technical breakthrough
The new and innovative online tar measurement technique was installed and tested in the 20 MWbioSNG GoBiGas plant in Gothenburg at the end of 2016. At this stage it’s possible to measure changes in the level of the detected tars and also if there is a shift towards heavier or lighter tars during operation. Ingemar Gunnarsson, Development engineer at Gothenburg Energy and no friend of big words stated that “It looks very promising” regarding the industrial use. In fact it’s a technical breakthrough that has potential to revolutionize monitoring and control of biomass gasification processes worldwide.
Photo taken inside the GoBiGas plant. A small flow of the hot and tar laden product gas is led by heated lines into the hot measurement cell where tar species are detected. The measurement cell is built into the shiny metal box on the floor in the center of the picture. Photo: Anton Larsson, Gothenburg Energy.