Catalytic gasification of empty fruit bunch for tar-free hydrogen rich-gas production

Palm oil industry in Malaysia generates huge quantity of solid biomass every year including trunks, fronds, empty fruit bunches (EFB), shells and fibers as wastes from palm oil fruit harvest and oil extraction processing. These large volumes of wastes represent a big environmental threat for Malaysia. The great potential of oil palm biomass has motivated an increasing interest in the utilization of these wastes (i.e.EFB) as a source of clean energy. Fuel and chemical characteristics of the EFB undertaken in this study confirmed that it is a good candidate for gasification process as it is comparable to other lignocellulosic biomass. As a thermal process, gasification has been used to treat oil palm wastes due to its high conversion efficiency. This study evaluated the possibility to treat EFB via gasification process for hydrogen-rich gas production. In this study, the EFB obtained from a local palm oil mill was gasified in an atmospheric bench-scale fluidized-bed gasifier (FBG) using air as gasifying agent. The operating parameters,such as effects of gasifier temperature (700–1000 °C), equivalence ratio (0.15-0.35),feedstock particle size (<0.3, 0.3–0.5, 0.5–1.0 mm), and addition of catalysts (as a primary and secondary) were studied to evaluate the gasification yields and performance so as to reach maximum tar-free hydrogen-rich gas production. The main gas species generated, as identified by a gas chromatography (GC), were H2, CO, CO2 and CH4. With gasification temperature increases the total gas yield was enhanced greatly and reached the maximum value at 1000 °C with a large fraction of H2 (38.02 vol.%) and CO (36.36 vol.%). Equivalence ratio (ER) showed a significant influence on the upgrading of hydrogen production and product distribution. The optimum ER value of 0.25 was found to attain a higher H2 yield. Feedstock particle size showed an influence on the improvement of the gas yield. Smaller EFB particles size produced more H2, CO, CH4 and less CO2. Tar formation is a major drawback when EFB is converted via gasification to obtain fuel gas. Catalytic cracking is an efficient method to eliminate the tar content in the gas mixture. In this study, three types of Malaysian natural dolomites namely, P1, P2 and P3 in addition to spent mixed metal oxide (SMMO) were used as catalysts to reduce tar contents in the produced gas and for further improvement of hydrogen yield. Various types of analysis techniques such as X-ray fluorescence (XRF), thermogravimetry (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption-desorption isotherm have been used to characterize the catalyst morphology. The effect of the primary catalyst (dry-mixed with biomass) under different ratios of catalyst to biomass (C/B) varied from 0.05 to 0.3 was carried out in fluidized-bed gasifier. The performance of gasification process is improved by increasing C/B ratio. Malaysian dolomites as primary catalyst showed a better catalytic effect compared to spent mixed metal oxide. With 30% of P1 dolomite, the total gas yield increased by 8%, hydrogen content increased by 18%, and total tar content in flue gas decreased by 78% at gasification temperature of 850 °C. In the second part of the catalytic experiments, the calcined catalysts were placed in a fixed-bed catalytic cracking reactor located downstream from the fluidized-bed gasifier to investigate the effect of secondary catalyst at different cracking temperatures in the range of 700–900 °C. The results show that raising the temperature in the catalytic bed increases the cracking activity of the catalysts and then significantly improve the gasification yields and performance. As in primary position, Malaysian dolomites showed a stronger catalytic activity as a secondary catalyst compared to spent mixed metal oxide. As cracking temperature increasing to 900 °C, total gas yield increased by 20%, hydrogen increased by 66%, and almost 99% reduction in tar content were obtained with P1 dolomite. ASPEN PLUS simulation using thermodynamic equilibrium model based on minimization of Gibbs free energy used to predict the EFB gasification yields under selected experimental parameters and to compare the simulation results with experimental data. The analysis of data for product gases and carbon conversion efficiency obtained from the simulation agreed satisfactorily with the experimental data. As a conclusion, both EFB as untapped waste and Malaysian dolomite as a cheap catalyst that can significantly reduce tar content of the product gas, could greatly contribute to the Malaysian economy in terms of producing clean environmentally renewable energy.

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