Enhancing the quality of syngas from biomass gasification via methane conversion

Abstract

One promising way of utilizing renewable energy is to gasify biomass into syngas and convert it to methanol, an essential and versatile chemical and fuel. High-quality syngas required for methanol synthesis can be produced by sorption-enhanced gasification (SEG) of biomass. In recent years, the SEG process based on a dual fluidized bed system has received much attention due to its potential for carbon capture, utilization, and storage (CCUS). Using a CO2-active sorbent as bed material enables in-situ CO2 capture during gasification, resulting in hydrogen-rich and tailored syngas suitable for chemical and fuel production. An attractive option is the SEG process with oxy-fuel combustion (Oxy-SEG), which produces a flue gas with a high CO2 concentration. A challenging area in the field of methanol from biomass is the remaining hydrocarbons in syngas, including gaseous hydrocarbons, which are unusable in methanol synthesis, and condensable hydrocarbons (tar), which foul downstream equipment and deactivate reforming or methanol synthesis catalysts. This research thus examined how to improve the syngas quality to make it more suitable for methanol synthesis by converting hydrocarbons into usable gases (H2 and CO) if possible, focusing on methane conversion, the most abundant hydrocarbon in syngas. Experiments were conducted on the non-catalytic partial oxidation method (POX) in a non-premixed burner system and the catalytic steam reforming method (CSR) in a fixed bed reactor at atmospheric pressure under various conditions, including SEG-derived syngas and purge gas exiting the methanol synthesis. Based on experimental data, process simulation models were developed and simulated in Aspen Plus® to evaluate the impact of methane conversion concepts on the overall process. Biochar and CaO, which have promising catalytic activity for tar reforming, were found to be ineffective as methane reforming catalysts in experiments. As a result of its high activity, a commercial Ni-based catalyst was investigated further. The findings of this research demonstrated the significance of syngas composition (mainly H2 and H2O) on reforming processes. Increasing hydrogen and decreasing steam concentrations were suggested for a high CO yield. In POX, highly reactive hydrogen allowed syngas with high steam concentrations of up to 0.60 m3 m−3 to ignite at a preheating temperature of approximately 670 °C. This temperature is within the range of gasification temperatures. Unfortunately, high steam concentration had almost no effect on CH4 conversion, but it was useful for adjusting product distribution (H2/CO ratio and CO/CO2 ratio). In CSR, hydrogen inhibited methane reforming, whereas steam promoted it. The high steam content of SEG-derived syngas enabled reaching the optimum CH4 conversion and slowing catalyst deactivation. In addition to hydrogen and steam, this research found that the oxygen-to-fuel ratio 𝑛O2 is critical for POX. However, a trade-off existed between high CH4 conversion at high 𝑛O2 and high H2/CO ratio at low 𝑛O2. Although the preheating temperature had a negligible effect on methane, it increased the conversion of more reactive hydrocarbons such as C2H4 and CO yield. Unfortunately, the conversion of the desired hydrogen could not be avoided under methane oxidation in excess hydrogen. Nevertheless, desirable CO could be produced at high steam concentrations (0.50 m3 m−3 to 0.60 m3 m−3) and low 𝑛O2 (up to 0.43). As a result of this study, it is suggested to use POX to provide the heat required for CSR (known as autothermal reforming or ATR). Furthermore, this research highlighted the importance of simultaneously investigating gaseous and condensable hydrocarbons in syngas reforming. When tar model compounds were introduced into the feed at 600 °C, CH4 conversion dropped by about 30%, and catalyst deactivation due to carbon formation was observed. The deactivation reduced tar conversion over time, while CH4 conversion stayed nearly constant up to 240 min. The presence of naphthalene, a heavier tar model compound, inhibited toluene and methane reforming. A high temperature above 800 °C is recommended to maximize CH4 conversion and CO yield while minimizing catalyst deactivation due to carbon formation. The simulation findings also revealed that the reformer benefits the gasification and methanol synthesis process chains. The utilization of purge gas exiting the methanol synthesis is interesting but inefficient. Although methanol production per biomass input could be slightly improved, this route requires a larger reactor and two recycle loops, complicating the process in practice. Direct conversion of hydrocarbons in the syngas is more appealing and was investigated using CSR and ATR in optimized cases and compared with the reference case using a rapeseed methyl ester (RME) scrubber. Regarding carbon conversion, the optimized cases clearly outperformed the reference case, which simply discharged hydrocarbons from the process. The optimized case had nearly twice the biomass-to-methanol conversion efficiency as the reference case (0.53 and 0.31, respectively). At the same outlet temperature, CSR appeared to be a better option for producing usable gases, whereas ATR was more efficient in terms of energy. Although both methods had a trade-off, the results are auspicious.