Tar and Condensable Hydrocarbons in Indirect Gasification Systems
Biomass gasification, which is a primary process step in the production of biofuels, yields a combustible gas mixture (raw gas). This raw gas consists of a wide range of species, from permanent gases to condensable hydrocarbons, which are collectively known as tar. Considered as the Achilles heel of biomass gasification, tar starts to condense at temperatures of around 350°C, causing blockage and fouling of downstream equipment. In addition to creating operational difficulties, tar is responsible for a loss of efficiency if it is not successfully converted into permanent gases. Consequently, there is a need to understand the concepts underlying tar formation and evolution, so as to guide efforts towards reducing its yield, as well as towards facilitating its removal once formed. This requires accurate quantification of all the components of the produced raw gas to evaluate the behavior of the tar. However, as the produced gas comprise such a wide range of species, several different measurement techniques are required. In this work, the effects of catalytic materials on tar were investigated in two different systems. The observed responses motivated the development of improved measurement systems, directed to fulfilling the mass balance of the gasifier. These systems were subsequently implemented to map the behaviors of the various species of the raw gas, for a range of process parameters, and to derive a reaction scheme for all the condensable species.
The concepts of primary and secondary measures were studied by introducing a catalytic material directly into the Chalmers dual fluidized bed (DFB) gasifier (primary measure) and by utilizing an additional reactor for catalytic reforming of the raw gas (secondary measure). Overall, both measures resulted in significantly decreased levels of tar. However, the composition of the remaining tar differed for the two measures, as did the added amounts of oxygen.
The SPA method for tar measurement was evaluated for reproducibility, which was found to be well within 10% for the majority of the measured species. In addition, the detection limits of the SPA method have been extended throughout this work and currently extend from benzene to coronene. A high-temperature reactor, for thermal cracking of all the gas species into CO, CO2, H2, and H2O, was constructed to measure the total elemental yields of C, H, O, and N in the raw gas. This measurement allowed a mass balance for the system to be constructed, which combined with the cold gas and tar measurements, was used to obtain information regarding the yields and composition of previously unmeasured condensable species. This group contained a level of carbon similar to that found in the SPA-measured tar, thereby underlining the need for quantification through standard measurements.
The developed measurement equipment was used to map the behavior of the gasifier under various temperatures, residence times, and steam-to-fuel ratios. The performed measurements showed that not only are the previously unmeasured species important for fulfilling the mass balance of the gasifier, but also for describing the formation of SPA-measureable tar. Subsequent modeling of the tar formation and evolution for the measured cases revealed that a substantial fraction of these species tends to form tertiary SPA tar directly, as these species are reformed. Furthermore, it was shown that additional factors, presumably related to the aging of the bed material, can significantly affect the reactivity of the gasifier and should be quantified to improve the functionality of the model.