Preliminary Study of a Gravimetric Approach to the 'Solid Phase Absorption' Methodology for Tars Sampling in Biomass Gasification Processes

Biomass gasification is one of the most promising and valid solutions to produce energy from renewable sources. However, today there are still problems that limit its diffusion at an industrial level. Among the main ones is the presence in the synthesis gas of condensable contaminants, the so-called tars, which accumulate in the parts of the system at low temperatures, and which can compromise normal operation, even causing the plant shutdown. Since it is not possible to eliminate tars formation, it is useful to know the quantity produced by each specific gasifier to plan maintenance and components replacement. The measurement of tars with traditional methods is complex, labor-expensive due to the long procedure and sample post-processing and provides non-real-time results. This work focuses on the “Solid Phase Adsorption” (SPA) sampling technique through adsorbent cartridges. Currently, the application of this method involves gas chromatographic analysis after dissolving the samplein a polar solvent. This study, instead, investigates the gravimetric approach and it is based on the weight of the tars captured by the cartridge. This approach maintains the advantages of SPA sampling (quick and easy sampling, compact system, low set-up time) and at the same time makes the analysis procedure more economical as no expensive equipment is used (e.g.: gas chromatographs), while ensuring accurate quantitative results.The tar content in the synthesis gas is highly variable depending on the operating conditions of the process and the used biomass. For this reason, synthesis gas was replaced by commercial cigarette smoke during this work. The aim was to use a standard fuel, containing a known and declared quantity of tars, to guarantee the total repeatability of the tests. An experimental apparatus that can hold five burning cigarettes at the same time has been created. Three repetitions of sampling were carried out with the SPA methodology and three with the standard “Tar Sampling Protocol” (TSP) methodology. The TSP is taken as the reference method as it is now the most reliable methodology. The results obtained from the "gravimetric SPA" method were compared with the values obtained from the TSP method. It can be concluded that the gravimetric SPA method represents a valid, probably even better, alternative to the current gravimetric cold trapping methods (TSP). The concentration of tars measured by SPA is on average higher than the concentration obtained by TSP: respectively 0.0019 g/l and 0.0016 g/l. This result could be motivated by the greater efficiency of the SPA method in retaining highly volatile tar compounds. In conclusion, the results obtained in this work lead to a positive evaluation of the gravimetric SPA method for the determination of tars in gasification systems and to proceed with further experiments on synthesis gas generated by real gasification processes to verify this new method.