Tar Reforming over Low-cost Active Materials for Gasification Derived Syngas

Thesis from Yen-Hau Chen
University of Stuttgart, 2021

The present work investigated the potential of in-situ biochar mixture from gasification process (straw char containing fly ash), pyrolyzed biochars by self-production (wood char, straw char, and palm shell char), and metal impregnated biochars (potassium and iron-loaded palm shell char) to be reused as tar-reforming catalysts in gasification processes. The catalytic activity and reforming selectivity of different materials were evaluated with toluene and naphthalene as tar model compounds in the presence of steam and hydrogen (major composition in the syngas from sorption enhanced gasification and steam-oxygen gasification) in a lab-scale fixed bed reactor at high temperature up to 900 °C.

Straw char containing fly ash derived from the steam-oxygen gasification process was verified to be able to apply as a tar-reforming catalyst for enhancing the overall performance of the steam-oxygen gasification process. It was found that the significant effect of hydrogen on toluene reforming was demonstrated by the formation of benzene through hydrodealkylation reaction. The volumetric ratio of H2O to H2 was an essential parameter that decided the selectivity of toluene reforming. Although the coexist of toluene and hydrogen would inhibit the gasification, straw char containing fly ash was proved to be gasified during toluene reforming. The surface migration and agglomeration of inorganics due to gasification also resulted in the change of catalytic activity and reforming selectivity through time.

With regard to pyrolyzed biochars, the low-cost material, wood char, straw char, and palm shell char, can successfully be used as the tar-reforming catalyst after sorption enhanced gasification process. A theoretical space time to reach the complete naphthalene conversion was 0.07 kg h m-3 at 850 °C for wood char, demonstrating a promising catalytic activity. It was also found that potassium and iron-loaded palm shell chars exhibited much better catalytic activity than palm shell char, while the parallel reaction of gasification of K-loaded palm shell char influenced the conversion with its drastic mass loss.

Lastly, the spontaneous gasification and catalytic activity of wood char were thoroughly evaluated in simulated sorption enhanced gasification environments with toluene and naphthalene as tar model compounds. Besides, CaO was used as a reference catalyst for the comparison purpose since CaO is a commonly used sorbent in the sorption enhanced gasification process. A model of gasification reactivity during reforming of tar model compounds over wood char was developed in this work based on the reactivity at 20 % carbon conversion adopting the Extended Random Pore Model, where the pre-exponential factor, activation energy, and structural parameter were calculated to be 1.65·1010 min-1, 265.8 kJ mol-1, and 127, respectively. During the catalytic activity investigation, hydrogen was found to inhibit tar-reforming performance over CaO while the impact of hydrogen was insignificant over wood char. The results showed that spontaneous gasification during tar surrogates reforming led to mass loss, pore collapse, and inorganics agglomeration, which contributed to the catalytic deactivation of wood char. By considering gasification-caused deactivation, the carbon conversion was then used as a variable to modify the kinetic equations of the tar surrogates reforming. The activation energy and pre-exponential factor of naphthalene reforming (and toluene reforming) over wood char were calculated with the values of 422.5 kJ mol-1 and 2.92·1022 m3 kg-1 h-1 (284.8 kJ mol-1 and 1.90·1015 m3 kg-1 h-1), respectively, whereas the values for CaO were 126.9 kJ mol-1 and 6.79·104 m3 kg-1 h-1 (254.5 kJ mol-1 and 6.73·1011 m3 kg-1 h-1), respectively. The kinetic models developed in this study were later used for designing a tar reformer integrated with the sorption enhanced gasification process.

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