A critical assessment of tar generated during biomass gasification - Formation, evaluation, issues and mitigation strategies

Biomass gasification receives attention as a promising method to utilize biomass, a fuel which is carbon neutral. The producer gas/syngas which is an energy carrier obtained through this method finds use in engines, fuel cells, Fischer-Tropsch reactors, methanol synthesis and as an input for chemical industries, after the required quality levels for the above applications are achieved. To use the producer gas/syngas for power generation on a commercial scale, the required gas quality has to be established. Producer gas obtained from biomass gasification has several contaminants like particulate matter, tar and gaseous species like H2S, NH3. The contaminants present in the producer gas, depending upon their nature and the amount, pose issues to power generation systems. Tar, which is a mixture of varying molecular weight hydrocarbon molecules, generated from the thermo-chemical conversion processes of organic materials, could condense at low temperatures, and lead to clogging or blockage in end-use application devices, filters, and fuel lines. So, it is essential to reduce or transform the tar present in the producer gas to utilize the biomass gasification systems for power generation. This paper attempts to provide a critical assessment of tar generated during biomass gasification, covering the sundry aspects of formation, evaluation, issues and mitigation strategies. The paper gives an introduction to biomass gasification systems, followed by a detailed description of tar, including the definition and the chemistry of formation and destruction. An explanation of the various aspects of tar sampling, characterization and analysis, is presented next. The suitability of different tar analysis approaches is compared from an end-use device perspective. Then the multifarious issues posed by the presence of tar in the syngas on the end-use devices is discussed. The last part of the paper describes several tar mitigation strategies used by researchers.