(17a) Process Analyses of Selective NOx Decomposition Employing Chemical Looping Scheme

Rapid technological advances have led to precipitous rise in energy consumption which have contributed to environmental degradation. The pollutant NO_x generated because of exorbitant use of fossil fuels are primary concern for photochemical smog, acid rain, tropospheric ozone, and ozone layer depletion. This pollutant needs to be removed from the stack gases prior to releasing it into the atmosphere. Additionally, the removal or purification process needs to be sustainable and economical to reduce the plant’s operating cost. Hence, NO purification especially at lower temperatures is widely investigated across the globe. In detail, NO direct decomposition is favorable below 1000^oC; however, the reaction has a high activation energy barrier of 335 kJ mol^−1. Although the current state-of-the-art process, selective catalytic reduction (SCR) exhibits good selectivity and conversion towards NOx purification, it faces several drawbacks such as poor thermal stability, catalyst poisoning due to the presence of other acidic gases in the stack gas like CO₂ and suffers from limited operational flexibility. Additionally, the industrial SCR catalysts are made of noble metals like Pt, Pd, and Rh, which have recently become very expensive thus resulting in an overall increase in the process economics. Furthermore, the operation of SCR requires use ammonia which is expensive when compared to other abundant and cheaper reducing gases like natural gas.

Methane contained in natural gas is a well-known greenhouse gas and is about 25 times more potent than CO₂ at trapping heat in the atmosphere. Due to these reasons, economic and sustainable pathways for natural gas utilization alongside addressing the environmental concerns are recently being explored. This work presents a unique scheme for NOx purification alongside natural gas utilization.

The proposed process is based on a chemical looping scheme that involves carrying out a redox reaction in two different reactors assisted by a metal oxide carrier cyclically looping between two reactors. The process scheme for CLND involves the operation of the NOx purification process operated autothermally. In the first reactor i.e., reducer, the carrier containing an oxidized metal oxide carrier is reduced using natural gas primarily containing methane. It is ensured that the natural gas is completely oxidized to CO₂, with 100% purity. After carrier is reduced, the flue-gas stream containing NO is sent to the reactor, where it selectively reacts with NO to decompose into N2. In addition, the process is studied over a varied temperature range of 300-700 ^oC and is not affected by components like CO₂ and H₂O in the flue-gas stream. Moreover, the methane utilization for regeneration produces a sequestration- ready pure CO₂ stream. The reaction scheme for CLND is as follows:

Reactor 1(Reducer): 4NiO + CH4 → 4Ni + CO₂ + 2H₂O (1)

Reactor 2(Oxidizer): 4Ni + 4NO → 4NiO + 2N₂ (2)

Overall: 4NO + CH₄ → 2N₂ + CO₂ + 2H₂O

Process simulations involving sensitivity studies varying oxygen carrier flow rates, operation temperature and inert support composition are conducted. Operating zone of the reducer is chosen based on near 100% methane conversion and CO₂ selectivity alongside no carbon output. Moreover, the reducer and oxidizer are operated in adiabatic and isothermal mode respectively showing the autothermal operating condition. Since reducer is adiabatic and reduction reaction is endothermic, a drop in temperature across the reducer’s length is observed. Therefore, it is ensured the reducer outlet temperature is beyond the activation temperature of the carrier to ensure maximum reactivity. To improve heat transfer from the oxidizer to the reducer, addition of inert with higher heat capacity is investigated. Based on exergy analysis, the Effective thermal efficiency (ETE) of the CLND process is calculated ~18 percentage points higher than SCR process.

Besides, the experiments were conducted to determine the activation of CH₄. NO_x reactivity was assessed at different temperatures to determine the best operating temperature for simulation studies. The carrier was tested in a TGA across 20 cycles to assess its redox performance. X-ray diffraction was used to identify the phase changes during the redox process. SEM combined with EDS was performed on fresh and post cycles samples to observe changes in surface morphology. BET analysis showed no significant change in surface area for fresh and post 20 redox cycles. This unique scheme comprising of purifying a pollutant and utilizing natural gas while generating heat for electricity is highly promising.