Thermodynamic and Process Analyses of Syngas Production Using Chemical Looping Reforming Assisted by Flexible Dicalcium Ferrite-Based Oxygen Carrier Regeneration

Syngas production is highly critical to the manufacturing of many value-added products, and its economic prospects can be increased through the enhancement of fuel conversion and the syngas yield. This study explores the thermodynamic characteristics of syngas production through chemical looping reforming (CLR) of natural gas using dicalcium ferrite (Ca2Fe2O5) as the oxygen carrier in a cocurrent moving-bed reactor. The effects of temperature, pressure, and steam addition are studied for both isothermal and adiabatic conditions. A natural gas conversion of 99.78% and a yield of 2.86 mol of syngas/mol of natural gas are obtained for CLR as compared to 95.97% and 2.70, respectively, for autothermal reforming (ATR). A fluidized bed and a countercurrent moving bed are employed for the regeneration of reduced solids using air and a steam/CO2 mixture, respectively, thereby achieving operational flexibility. The syngas yield increases by ∼41% using the steam/CO2 mixture, whereas a high-purity H2 is obtained from the oxidation of reduced solids in pure steam. The process analyses indicate an increase in the effective thermal efficiency from 86.4% to 92.2% and the exergy efficiency from 79.5% to 85.3% on using the Ca2Fe2O5-based CLR over ATR, rendering the syngas production using CLR economically attractive

Autothermal Operation Strategies of Chemical Looping Processes for Hydrogen Generation: Process Simulation, Parametric Studies, and Exergy Analysis

Chemical looping is an advanced material and energy conversion technology that can achieve both high-level process intensification and efficiency. To analyze chemical looping processes, it is essential to include process conditions that are realistic and comparable to those that are expected in industrial systems. Relevant variations in these conditions as occurred in bench versus industrial-scale systems include isothermal versus adiabatic operation of the reactors and local versus global process heat integration. Naturally, the types of reactors employed dictate how the reactor operation is to be conducted from the heat integration viewpoint in the overall process arrangement. As an example, in industrial applications, a fluidized bed reactor is operated near uniform temperature conditions. A fixed bed or a moving bed reactor, on the other hand, is typically operated adiabatically, and thus under the autothermal operation, the nonisothermal condition prevails, leading to different strategies for process simulations and heat integration requirements. This study presents the chemical looping process simulation based on a moving bed reactor used as a reducer for two H2 generation process configurations under autothermal operating conditions. The two process configurations are represented by the two-reactor (reducer–combustor followed by the water–gas shift reaction) and the three-reactor (reducer–oxidizer–combustor with water splitting for H2 generation in the oxidizer) chemical looping systems with each configuration producing H2 in a different operating scheme. The simulation results are compared with the conventional steam methane reforming (SMR) system as a baseline case to underscore the attractiveness of the chemical looping configurations. Specifically, for each configuration, the parametric study under the adiabatic conditions is used to optimize the operating conditions that can satisfy the heat balance requirements and can achieve a maximum H2 yield. The exergy analysis indicates that the two-reactor chemical looping and three-reactor chemical looping systems can achieve, respectively, a 4.0 and 11.4% increase in relative percentage in the overall process exergy efficiency over the conventional steam methane reforming system

A Novel 2‑Reactor Chemical Looping System for Hydrogen Production with Biogas as the Feedstock: Process Simulation and Comparison with Conventional Reforming Processes

With increasing environmental concerns because of greenhouse gas (GHG) emissions from conventional sources of energy, a tremendous shift in momentum is observed toward clean combustion sources and carbon-neutral feedstocks. Hydrogen (H2) as a fuel negates carbon emissions during energy generation and hence is gaining attention as a valuable source of energy. The existing processes for producing H2 use non-renewable fossil fuels as feedstock. Biogas derived from the decomposition of waste organic matter is a renewable and carbon-neutral feedstock that can be utilized for fossil-free H2 generation. In this study, a novel 2-reactor chemical looping water-splitting process (CLWS-2R) is introduced, simulated, and analyzed for the production of H2 from biogas. Process simulations are carried out for the CLWS-2R system to achieve optimized process parameters for the desired system operating conditions. The process evaluation parameters of this scheme are compared with three established processes for H2 production from biogas, including the 3-reactor chemical looping water-splitting (CLWS-3R), the steam reforming (SR), and the mixed reforming (MR) process. The simulation results from this study indicate that for a biogas feedstock composition of 25% CO2 by volume, the CLWS-2R system can achieve the highest cold gas efficiency (CGE: 75%) and the highest effective thermal efficiency (ETE: 71%). The sensitivity analysis on biogas composition indicates that CLWS-2R achieves its highest ETE for biogas with a low CO2 content (25–30 vol %), which is almost equivalent to the ETE obtained using MR. For all the remaining biogas compositions with high CO2 content, MR achieves the highest ETE

Synthesis and Regeneration of Sustainable CaO Sorbents from Chicken Eggshells for Enhanced Carbon Dioxide Capture

Eggshell waste, which contains 95% calcium carbonate (CaCO₃), presents itself as an inexpensive calcium-based sorbent for removal of carbon dioxide (CO₂) in combustion streams used to generate electricity. The utilization of eggshell waste in CO₂ capture via cyclic carbonation-calcination reactions (CCR) was investigated in this work. Using thermogravimetric analysis, the CO₂ capture capacity for multiple acetic acid pretreated eggshell samples was studied. This pretreatement generates a mesoporous structure, allowing the eggshell-derived sorbent to reach higher conversions over more CCR cycles while also removing the eggshell’s protein-rich membrane. Six acetic acid treatments were also explored for regeneration of spent sorbents after multiple cycles. The regeneration of spent sorbents with acetic acid provided a 38% improvement in CaO conversion over untreated shells after ten cycles. The eggshell membrane contained highly valuable Type X collagen, which can be recovered through the course of shell pretreatment to increase process feasibility. This scheme allows for sustainable generation of CaO sorbents while also transforming a current waste material into a value-added product

Mo-Doped FeS Mediated H₂ Production from H₂S via an In Situ Cyclic Sulfur Looping Scheme

Decomposition of H2S into sulfur and clean fuel H2 is an attractive process and requires a design concept with a maximum H2S conversion and a minimal energy consumption. Herein, we demonstrate a sulfur looping scheme in a one-reactor system using a low-cost and environmentally safe iron-based sulfur carrier. H2S decomposition is split into cyclic sulfidation and regeneration of sulfur carriers, which overcomes the inherent thermodynamic constraint, allowing in situ H2 generation. We experimentally obtained 24% higher sulfur uptake in 2% Mo-doped iron-based sulfur carriers compared with undoped sulfur carriers. The reaction mechanisms unveiled by the density functional theory indicate that surface hydrogen diffusion is the rate-determining step for sulfidation of the sulfur carrier. Compared with the undoped sulfur carrier, Mo dopant facilitates the surface hydrogen diffusion, thus promoting the overall H2S conversion. This work demonstrates a novel strategy for high-yielding H2S removal through low-percentage dopant modification sulfur carrier and provides new insights for an effective dopant screening strategy aiding the future carrier design

Life Cycle Comparison of Coal Gasification by Conventional versus Calcium Looping Processes

This work evaluates the environmental impact of conventional and calcium looping processes implemented with CO₂ separation to gain broad insight and identify opportunities for future improvement. These systems are assessed at multiple scales: equipment, value chain and economy, and emissions of CO₂, water use, land use, and energy return on investment are estimated. The difference in the energy quality of hydrogen and electricity products is considered in developing aggregate metrics. Calcium looping is found to be superior due to its smaller life cycle impacts. However, this process has a smaller energy return on investment due to the higher energy and resource requirements in the calcination and air separation steps. Future efforts for reducing the energy intensity of these steps by developing new technologies and optimizing existing methods are recommended

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

Operating Strategy of Chemical Looping Systems with Varied Reducer and Combustor Pressures

In chemical looping technology when applied to gasification, reforming, and chemical syntheses, the operating pressure is an important factor that dictates the reactant conversion and product formation economics of the technology for the processes. Common consideration of the operating pressure for the chemical looping system that includes reducer/fuel reactor and combustor/air reactor operation is based on the condition in which the reducer and the combustor are operated under similar pressures. This study considers another operating condition, characterizing the Fan chemical looping system, where the reducer and the combustor are operated under different pressures while solids are continuously or discontinuously circulating through the loop system. As an example, the chemical looping natural gas conversion to syngas with the eventual production of liquid fuels is given in this study. This example illustrates the thermodynamic limits and their associated compression duty with chemical looping syngas production at pressures between 1 and 30 atm, with a goal of obtaining syngas suitable for cobalt based Fischer–Tropsch synthesis at 30 atm. The adaptation of thermodynamic operating conditions to maximize syngas yield and balance between the syngas compression and the air compression are discussed in this study over a range of operating pressures and temperatures. Further, a novel operating strategy characterized by differential operating pressures between the fuel reactor and the air reactor in a continuous solid flow system is presented. Such a strategy allows the fuel reactor to operate at elevated pressures, closer to the syngas requirements of downstream units, while the air reactor operates near the ambient pressure conditions. This strategy has broad implications for other process system applications such as reaction–regeneration and adsorption–desorption processes where pressure variations are a key part of the optimum operation considerations. Considering the cost of the key components of the system including the reactors, valves, and compressors, such a strategy is shown to reduce the capital cost for the chemical looping system by 29% compared to equal pressure chemical looping reforming

Enhanced Light Absorption and Radiative Forcing by Black Carbon Agglomerates

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh–Debye–Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3–5 W/m2 in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75–1.25 °C, solely due to BC emissions