Solid phase adsorption method for tar sampling – How post sampling treatment affects tar yields and volatile tar compounds?

The Solid Phase Adsorption (SPA) methodology is the method of choice for sampling tar from biomass gasifiers and pyrolysers. Reliable and robust tar measurement techniques are essential for having an efficient implementation of these thermochemical processes. This paper critically evaluates the SPA tar sampling method from the following perspectives: (i) the efficiency of tar extraction from the aminopropyl silica sorbent; (ii) the effect of transport and storage period on tar adsorbed on the SPA sorbents; (iii) off-line SPA vs. the on-line µGC method for quantitative determination of benzene and toluene. The present study showed that the tar extraction efficiency for aminopropyl silica sorbent SPA cartridges was typically 95% or higher. The 30% of tar was lost from these sorbent materials during the airfreight and 21.9% during 5 days of ambient storage, respectively. When two sorbents were employed into the SPA sampling device, the loss was reduced to 15%. The loss of SPA detectable tar was 16.4% when aminopropyl silica sorbents were stored in a freezer for 5 days prior to analysis. A significant portion of light tar compounds (benzene and toluene) was lost from the aminopropyl silica sorbent during the airfreight, while, under either ambient or frozen storage, these compounds were preserved relatively well. The best performance using the SPA method was achieved when tar was extracted from the sorbent immediately on-site. The study also demonstrated the superiority of on-line µGC measurement over the off-line SPA method for quantitative determination of light tar compounds. Light tar sampling efficiency, such as for benzene and toluene, was improved by introducing the second activated charcoal sorbent. Benzene yields measured by on-line µGC were up to 10.4 times higher than those extracted from the SPA aminopropyl silica sorbent solely, and, even when two sorbents were employed, they were up to 3.4 times higher. Regarding toluene yields, these values were 2.6 and 1.7 times higher.