Synergistic Chemical Looping Process Coupling Natural Gas Conversion and NO<i>_x</i> Purification

We present a novel low-temperature chemical looping combustion scheme for simultaneous natural gas conversion into a sequestration-ready CO2 stream and NOx purification. The scheme employs nickel oxide (NiO) supported on ZrO2 as the oxygen carrier. In the process, CH4 reduces the oxidized carrier to Ni/ZrO2 in a co-current moving bed reactor, which is then oxidized back to NiO/ZrO2 by the NOx-laden flue gas in a fluidized bed reactor, completing the oxygen carrier loop. Thermodynamic studies demonstrate that the presence of CO2 does not significantly affect NOx purification performance at different flue gas flow rates. The operating temperatures of the reactors are selected based on NOx-temperature programmed oxidation (TPO) and CH4-temperature programmed reduction (TPR) experiments. Results show that the process can optimally operate at temperatures close to the combustion plants’ flue gas temperature of 400–500 °C, reducing the need for hot utilities. The study conducts comprehensive isothermal and autothermal analyses of the process to evaluate the effects of temperature and carrier flow rate on CH4 conversion, CO2 selectivity, carbon deposition, and NOx conversion. For the autothermal analysis, the CH4 reactor operates adiabatically, while the NOx reactor operates isothermally. Comparative studies with the conventional NOx selective catalytic reduction (SCR) process indicate an exergy efficiency and effective thermal efficiency (ETE) improvement of 9 and 18 percentage points, respectively. The findings suggest that this low-temperature chemical looping process is a promising solution for flue gas NOx treatment, utilizing cheaper natural gas as the reductant and eliminating environmental concerns, such as ammonia or urea slippage. Overall, this study contributes to the development of more efficient and sustainable methods for reducing NOx emissions

Thermodynamic Evaluation of the Cross-Current Moving-Bed Chemical Looping Configuration for Efficient Conversion of Biomass to Syngas

The rising chemical demand and its associated concern of climate change have put an impetus on converting diverse domestic sources to valuable products in a decarbonized manner. Lignocellulosic biomass, a viable feedstock, is garnering significant attention as a sustainable alternative to fossil fuels. However, challenges in handling biomass feed variability and effectively processing its char and tar contents have hampered its commercial deployment. However, the chemical looping-based biomass-to-syngas (BTS) technology being developed by The Ohio State University is among the most promising technologies for industrial biomass reforming. It utilizes proprietary iron oxide particles in a cocurrent moving-bed reactor, leveraging the flow dynamics to transform biomass to syngas, and has been proven to be more efficient than conventional processes. However, this cocurrent system suffers from a thermodynamic barrier, inhibiting the syngas yield. To overcome this barrier, a novel chemical looping cross-current system is developed and investigated through detailed thermodynamic ASPEN studies after accounting for practical constraints. The barrier in the cocurrent system can be attributed to the equilibrium between exiting syngas and solid streams, which limits the oxidation of oxygen carriers. The cross-current reactor system overcomes this issue by shifting the exit of the syngas stream to the middle of the reactor, thus not allowing the exiting syngas and solid streams to be in equilibrium and creating a cocurrent section above the syngas exit and a countercurrent section below it. Thermodynamic simulations conducted under autothermal conditions reveal that the cocurrent and cross-current systems perform similarly with steam and CO2 co-injection. However, under an isothermal condition, which is now feasible with cheaper and sustainable heating methods, the cross-current system achieves ∼34% higher syngas yield over the cocurrent system (∼0.074 in cross-current compared to ∼0.055 in cocurrent) for both steam and CO2 co-injection. The findings from this study justify the scale-up of the cross-current system and provide system-level insights into biomass valorization