Chemical Looping Gasification Research Articles

External moisture enhanced synergistic conversion of biomass and iron sand for the green production of metallic iron

In this study, a novel approach combining chemical looping gasification (CLG) and direct reduced iron (DRI) was proposed for an efficient, sustainable, and green production of metallic iron by enhancing the synergistic conversion between biomass and iron sands via external moisture. In the process, the external moisture acts as a catalyst for pre-gasification of biomass at the early stage while iron sand, as an oxygen carrier (OC) and catalyst, contributes to the later biomass gasification, where biomass is used as a reducing agent for iron ore metallization. The effect of moisture on biomass gasification and roasting effectiveness was investigated, at a reduction temperature of 1080 °C, 10% H2O, and 80 min of roasting time. The total syngas yield was 84.13% and the reduction product with a metallization rate of 97.53% was obtained. The mechanism of synergistic conversion between biomass, external moisture, and iron sand was described. The iron ore and moisture in the initial roasting stage reduced the tar yield, significantly retarded the escape of hydrocarbons, and prolonged the high-pressure maintenance time. Thereafter, the biomass tar and biomass carbon were gasified under the action of oxygen elements from external moisture and iron ore, eventually generating CO and H2 with the final residual carbon being 9.73% comparing to the initial total C element. The reducing gas generated from biomass was used to reduce iron ore, ensuring a high metallization of iron ore and achieving efficient synergistic conversion of biomass and iron ore. Moisture and lattice oxygen of iron ore play significant roles in the gasification of biomass at the early stage. The formed metallic iron and moisture further promoted the gasification of biomass C in the subsequent catalysis. This mutually reinforcing and transformative approach shows great potential in promoting biomass gasification efficiency and reducing carbon emissions from DRI processes.

Dynamic optimal control of coal chemical looping gasification based on process modeling and complex risk computation

The safe and steady running of coal chemical looping gasification system (CCLGS) is directly related to the whole production benefit. CCLGS is composed of dryer, pyrolyzer, decomposer, fuel reactor (FR), and air reactor (AR), which reflects the complexity of the entire production process. In this paper, a novel dynamic safety control strategy based on process modeling and complex risk computation is put forward to ensure the stable operation of CCLGS. First, the process modeling of CCLGS is carried out by Aspen Plus, and the obtained simulation results are in agreement with experimental results. Then, the CCLGS complex network is formed by constructing the adjacency matrix and calculating comprehensive indicator (Ci) value to evaluate the importance of the variables and support risk computation. Subsequently, the risk values of five units are computed based on the unit maximum material coefficient (UMMC), unit general risk coefficient (UGRC), and unit special risk coefficient (USRC). The results prove that FR and AR have higher risk grades than other units. Finally, dynamic control study for FR and AR processes is conducted with the aim of evaluating pressure controllers by safety integrity level. The research will provide beneficial approaches for the safety control of actual plant.

Chemical looping gasification of benzene as a biomass tar model compound using hematite modified by Ni as an oxygen carrier

The effective removal of tar is a major challenge for biomass gasification. Schemes based on chemical looping gasification (CLG) offer a promising alternative for converting biomass into syngas with low tar content. The present work examined the influence of a Ni-modified oxygen carrier (OC) on the benzene conversion rate, hydrogen yield, carbon deposition rate, and yield of combustible gas products under various experimental conditions in the CLG of benzene as a model biomass tar compound. After modification, the NiFe2O4 OC exhibited good catalytic and oxidation performance. With the increase in reaction temperature and Ni loading, the hydrogen yield, benzene conversion rate, and carbon deposition rate all increased gradually. With the increase of weight hourly space velocity (WHSV), the benzene conversion rate decreased from 89.75% to 81.1%. which indicated that the oxidation capacity of the OC to benzene decreased. Meanwhile, the addition of steam significantly eliminated the carbon deposition, which decreased to 2.62% at an S/C ratio of 1.48. During the long-term experiment, the oxidation activity and catalytic activity of the OC each exhibit a slight decreased, which was possibly due to the sintering and agglomeration of the OC. Finally, the cracking mechanism of the tar model compound benzene was proposed.

Investigation of LD-slag as oxygen carrier for CLC in a 10 kW unit using high-volatile biomasses

A steel slag from the Linz-Donawitz process, called LD-slag, having significant calcium and iron-fractions, was investigated as an oxygen carrier in a recently developed 10 kWth chemical-looping combustor with three high-volatile biomass fuels. In order to improve operability, the LD-slag was found to require heat-treatment at high temperatures before being used in the unit. In total, operation with the biomasses was conducted for more than 26 h at temperatures of 870–980 °C. The fuel thermal power was in the range of 3.4–10 kWth. The operation involved chemical looping combustion (CLC), chemical looping gasification (CLG) and oxygen carrier aided combustion (OCAC). Around 12 h was in CLC operation, 13.3 h was conducted in CLG-conditions, while the remaining 0.7 h was OCAC. Here, the results obtained during the CLC part of the campaign is reported. Increased temperature in the fuel reactor and higher airflows to the air reactor both lead to better combustion performance. Steam concentration in the fuel reactor has little effect on the performance. The LD-slag showed higher oxygen demand (31.0%) than that with ilmenite (21.5%) and a manganese ore (19.5%) with the same fuel and normal solids circulation. However, with the LD-slag, there is possibility to achieve a lower oxygen demand (15.2%) with high solids circulation.

An innovative application of machine learning in prediction of the syngas properties of biomass chemical looping gasification based on extra trees regression algorithm

An innovative application of machine learning in prediction of the syngas properties of biomass chemical looping gasification based on extra trees regression algorithm

Chemical looping pyrolysis-gasification staged conversion of microalgae biomass with Ca-Fe composite oxygen carrier

Chemical looping pyrolysis-gasification staged conversion (CL-PGSC) of biomass was proposed in this work, which involves the separation of chemical looping gasification into two stages: biomass pyrolysis stage and bio-char gasification stage at different temperature. Pyrolysis and gasification behavior of Nannochloropsis sp., a microalga, with Ca-Fe composite oxygen carrier was investigated in a tubular reactor at varying temperatures and Ca-Fe molar ratios. The results indicate that all Ca-Fe composite oxygen carriers are capable of catalytically upgrading the quality of bio-oils in-situ during the pyrolysis stage. The complex metal oxides calcium ferrites facilitate the formation of aromatics and ketones, while both Fe2O3 and CaO components promote the production of alcohols. As for oxygen carrier with Ca-Fe molar ratio 1:1, the total peak area percentage of hydrocarbons and ketones reaches 66.5 area% within bio-oils, with 28.7 area% being toluene and 18.8 area% being 2-heptadecanone. Moreover, at the gasification stage, clean CO-rich or H2-rich syngas were generated from bio-chars produced at various pyrolysis temperatures with or without steam as a gasifying agent. In the proposed CL-PGSC process, Ca-Fe composite oxygen carrier serves as a catalysis for in-situ upgrading pyrolytic bio-oils, then as an oxygen donor for the gasification of pyrolytic char to achieve the simultaneous production of valuable pyrolytic bio-oils and clean syngas free from tar contamination. This provides a promising strategy for the full utilization of biomass.

Enhanced oxygen activity of LaFeO3 oxygen carriers in biomass chemical looping gasification coupled with CO2/H2O splitting by fragmented flaky structure

Considering that the conventional modification of metal doping may bring excessive cost escalation and more pollution, for the sol–gel method, a more cost-effective and eco-friendly modification method was proposed to enhance the oxygen activity of LaFeO3 and its doped oxygen carriers (OCs). In this work, LaFeO3 OCs with fragmented flaky structure was prepared by modification of adjusting the sol to weak alkalinity with ammonia and adding the complexing agent ethylene glycol. Then, it was applied in Biomass chemical looping gasification (BCLG) coupled with CO2/H2O splitting to produce high-quality syngas and achieve cascade conversion of carbon in biomass, and its performance and reaction mechanism were investigated by fixed bed test. The results showed that the fragmented flaky structure endowed the LaFeO3 OCs with more robust oxygen activity, thus realizing efficient lattice oxygen migration which increased the gas yield of BCLG stage from 695.6 mL/g to 851.2 mL/g at 900 °C, and the carbon conversion and gasification efficiency reached 65.6% and 64.4%, respectively. Despite the predominant role of residual char in the gas generation of splitting stage, the reduced OCs still possesses exceptional CO2 and H2O splitting ability. Among them, CO2 splitting was mainly carried out by the activation of Fe0 and alkaline active sites from the reduced OCs, while H2O splitting depended more on the decomposition of H2O molecules by oxygen vacancies. Moreover, this OCs exhibited outstanding stability in 10 cycles, and the total gas yield in two stages was consistently maintained around 1030.1 mL/g and 1069.6 mL/g, respectively. This work found that the LaFeO3 OCs with fragmented flaky structure is a prospective material for chemical looping technology.

Optimizing performance of iron-rich sludge ash as cost-effective oxygen carrier by calcium-based additive for syngas production from biomass chemical-looping gasification

Chemical-looping gasification tests were conducted on pine sawdust using thermogravimetric analyzer and horizontal sliding resistance furnace to investigate the regulation effects of calcium-based additive on iron-rich sludge ash oxygen carrier. The impacts of temperature, CaO/C in mole, multiple redox cycles, CaO addition modes on gasification performances were analyzed. The TGA results indicated that the CaO addition could effectively capture CO2 from syngas to from CaCO3, which subsequently decomposed at high temperatures. From in-situ CaO addition experiments, the temperature rise resulted in higher syngas yields, while a decrease in syngas LHV. With the CaO/C growing, the H2 yield grew from 0.103 to 0.256Nm3/kg at 800.0℃, and the CO yield boosted from 0.158 to 0.317Nm3/kg. Multiple redox manifested that the SA oxygen carrier and calcium-based additive kept higher reaction stability. The possible reaction mechanisms showed that the syngas variations from BCLG were influenced by the calcium roles and valence change of iron.

Design and control concept of a 1 MWth chemical looping gasifier allowing for efficient autothermal syngas production

Chemical looping gasification (CLG) is a novel gasification concept, allowing for the efficient production of a high calorific, N₂-free syngas with low tar content. Previous studies showed that the inherent process characteristics require a dedicated process control concept in order to allow for sufficient solid and thus heat transport between the two reactors (air and fuel reactor) of the gasification unit, while at the same time being able to accurately tailor the air-to-fuel equivalence ratio (λ), thus obtaining stable gasification conditions. To demonstrate its viability, a suitable control concept was implemented in the 1 MWth modular pilot plant located at the Technical University Darmstadt. In this paper, results obtained during the first ever autothermal CLG operation, achieved in this unit using biomass pellets as the feedstock, are presented, highlighting important process fundamentals. It is demonstrated that the novel process control concept allows for an accurate control of λ in semi-industrial scale, while at the same time guaranteeing stable hydrodynamics and thus solid and heat transport between the air and fuel reactor, making it a suitable control concept for large-scale implementation. Moreover, it is demonstrated that the underlying phenomena of the CLG process lead to substantial system inertia, as the solid bed inventory of the gasifier acts as an oxygen storage during transient periods, evoked by changes in the air-to-fuel equivalence ratio.

Oxygen uncoupling chemical looping gasification of biomass over heterogeneously doped La-Fe-O perovskite-type oxygen carriers

Biomass chemical looping gasification (BCLG) is a promising technology utilizes lattice oxygen for partial oxidation to produce high-value fuels. But in the field of BCLG, oxygen uncoupling chemical looping gasification (OU-CLG) is a novel concept that introduces molecular oxygen to optimize traditional gas-solid reactions. In this work, based on the most representative oxygen uncoupling elements successfully doped into La-Fe-O oxygen carriers, the results showed that the reaction performance not be positively correlated with the degree of oxygen uncoupling, only Co doping exhibited a better gasification performance than LaFeO3 did. Further, Co-LF differs from conventional LF in that it performs better in ex-situ gasification than in-situ gasification, with a total gas yield and carbon conversion efficiency of 1078.70 ml/g and 75.15% respectively. Further characterizations revealed that Co doping led to changes in the physical structure and reducibility. More importantly, enriched lattice oxygen and oxygen vacancies induced the formation of disordered reactive oxygen species in perovskite lattice, which promote the further oxygen uncoupling and the formation of oxygen uncoupling vacancies in ex-situ gasification. Moreover, doped but failure to promote the CLG might due to oxygen scarcity, impurity and elemental catalytic properties. Finally, a clear reaction mechanism regarding OU-CLG was proposed.

Dechlorination and fuel gas generation in chemical looping conversion of waste PVC over inherently Na/Ca/K-containing bauxite residue-based oxygen carriers

The inert atmosphere in chemical looping (CL) technology can considerably inhibit the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans during the thermal treatment of polyvinyl chloride plastic (PVC) waste. In this study, PVC was innovatively converted to dechlorinated fuel gas via CL gasification under a high reaction temperature (RT) and the inert atmosphere by applying an unmodified bauxite residue (BR) as both a dechlorination agent and oxygen carrier. The dechlorination efficiency reached 49.98% at an oxygen ratio of only 0.1. Furthermore, a moderate RT (750 °C in this study) and an increased oxygen ratio enhanced the dechlorination effect. The highest dechlorination efficiency (92.12%) was achieved at an oxygen ratio of 0.6. Iron oxides in BR improved the generation of syngas from CL reactions. The yields of the effective gases (CH4, H2, and CO) increased by 57.13% to 0.121 Nm3/kg with an increase in oxygen ratio from 0 to 0.6. A high RT improved the production of the effective gases (an 809.39% increase to 0.344 Nm3/kg from 600 to 900 °C). Energy-dispersive spectroscopy and X-ray diffraction were used to study the mechanism, and formation of NaCl and Fe3O4 was observed on the reacted BR, indicating the successful adsorption of Cl and its capability as an oxygen carrier. Therefore, BR eliminated Cl in situ and enhanced the generation of value-added syngas, thereby achieving efficient PVC conversion.

Recent Progress on Hydrogen-Rich Syngas Production from Coal Gasification

Coal gasification is recognized as the core technology of clean coal utilization that exhibits significant advantages in hydrogen-rich syngas production and CO2 emission reduction. This review briefly discusses the recent research progress on various coal gasification techniques, including conventional coal gasification (fixed bed, fluidized bed, and entrained bed gasification) and relatively new coal gasification (supercritical water gasification, plasma gasification, chemical-looping gasification, and decoupling gasification) in terms of their gasifiers, process parameters (such as coal type, temperature, pressure, gasification agents, catalysts, etc.), advantages, and challenges. The capacity and potential of hydrogen production through different coal gasification technologies are also systematically analyzed. In this regard, the decoupling gasification technology based on pyrolysis, coal char–CO2 gasification, and CO shift reaction shows remarkable features in improving comprehensive utilization of coal, low-energy capture and conversion of CO2, as well as efficient hydrogen production. As the key unit of decoupling gasification, this work also reviews recent research advances (2019–2023) in coal char–CO2 gasification, the influence of different factors such as coal type, gasification agent composition, temperature, pressure, particle size, and catalyst on the char–CO2 gasification performance are studied, and its reaction kinetics are also outlined. This review serves as guidance for further excavating the potential of gasification technology in promoting clean fuel production and mitigating greenhouse gas emissions.

Evaluation of low-cost oxygen carriers for biomass chemical looping gasification

Biomass Chemical Looping Gasification (BCLG) is a cost-effective and efficient alternative to conventional gasification. The selection of appropriate oxygen carriers (OCs) is crucial for stable BCLG performance. These OCs need to possess high reactivity, selectivity, material strength, and resistance to sintering. The study investigated various OC materials, including industrial wastes (copper, nickel slag, desulphurization, LD, and ladle slags), residential waste (sewage sludge ash), and natural ore (manganese). The evaluation of OCs focused on reactivity, H2-selectivity, mechanical strength and sintering behaviour. Except for ladle slag, all OC samples exhibited favourable reactivity due to the presence of Fe- and Mn-oxides possessing high oxygen transport capacity (10–17.6%). Nickel slag, manganese ore, and desulphurisation slag displayed notable H2-selectivity (8.7 to 10.4). It can be attributed to the presence of less-active (lattice) oxygen, limiting strong oxygen agents such as Fe2O3, Fe3O4, and Mn2O3. Moreover, desulphurization slag contained highly selective Ca2Fe2O5, which falls within the partial oxidation zone of the Ellingham diagram. Furthermore, all OC samples exhibited desirable material strength (>20 MPa), suitable for fluidised bed reactors. However, nickel, LD, and ladle slags demonstrated limited sintering with sintering onset temperatures exceeding 963 °C. This limited sintering may be attributed to the absence of iron silicates, iron-bearing aluminium silicates, manganese silicates, and potassium that contributed to the low thermal stability observed in the remaining OCs. Altogether, nickel slag calcined at 1100 °C was identified as the most promising OC material with optimal reactivity, selectivity, material strength, and minimal sintering for BCLG. Overall, this study provides a detailed and scientific methodology for OC selection and can aid future OC development.

Chemical looping reforming of toluene as bio-oil model compound via NiFe2O4@SBA-15 for hydrogen-rich syngas production

Hydrogen-rich syngas was a clean energy and an important industrial material. Based on the decoupling strategy of biomass chemical looping gasification process, this paper proposed a strategy of metal oxides embedded into molecular sieves to prepare highly dispersed and nanosized oxygen carriers for producing hydrogen-rich syngas. NiO@SBA-15, Fe2O3@SBA-15, and NiFe2O4@SBA-15 were prepared by the impregnation method, and the reaction conditions on the chemical looping reforming of toluene were investigated. The results showed that NiFe2O4@SBA-15 had the highest toluene conversion rate of 93.4% and a relatively high CO selectivity rate of 80.7%. It was confirmed that the embedding strategy can effectively enhance the nanocrystallization and dispersion of metal oxides in oxygen carriers, which could effectively reduce sintering. The inverse spinel structure of NiFe2O4 made the oxygen carrier have more metal adsorption sites and a closer reaction distance, which were beneficial to the adsorption and reaction of the fuel. After testing, the optimum reaction temperature was 750 °C, and the optimum weight hourly space velocity was 1.168 h−1. In the 10 cycles of testing of 20 NiFe2O4@SBA-15, the average conversion rate of toluene was 95.34%, the moderate selectivity of CO in the gaseous product was 94.83%, the average H/C ratio was 1.97, which indicated that the cycle stability is good. It provided a reference for developing and designing future oxygen carriers of biomass chemical looping reforming.

Process modelling integrated with interpretable machine learning for predicting hydrogen and char yield during chemical looping gasification

Chemical looping gasification (CLG) is a promising thermochemical process for the production of H2. CLG process is mainly based on oxygen transfer from an air reactor to a gasification reactor using solid metal oxides (also called oxygen carriers, (OC)) as oxidants. The unique oxygen separation system of CLG makes it an advanced process with a smaller carbon footprint compared to the conventional gasification process. The other advantages of CLG includes increased efficiency, reduced greenhouse gas emissions, and improved process stability compared to conventional biomass gasification. Although CLG is a promising technology, it still faces several challenges such as high capital cost, OC durability, complex reaction mechanism and scalability issues. Some of these challenges can be addressed by understanding the impact of various process conditions on H2 yield and char formation during CLG. The present study proposes a novel integrated process simulation and experimental studies to generate large dataset used for interpretable machine learning (ML) analysis. Three different ML models including support vector machine (SVM), random forest (RF), and gradient boost regression (GBR) were used to develop models for predicting the H2 and char yield during CLG. The GBR outperformed other models for the prediction of H2 and char yield during CLG with R2 value > 0.9. Among the experimental conditions, the temperature (T) and steam to biomass ratio (SBR) were the most relevant parameters affecting H2 and char production. Biomass ash, C, volatile matter (VM) and H content also influenced H2 and char formation. Overall, a combination of SHAP and partial dependence plot helped address the black box challenges of ML models.

Generalized analysis of optimal performance of chemical looping gasification

Thermodynamic modeling of chemical looping gasification (CLG) is carried out using Gibbs free energy minimization approach to, determine the optimal operating condition, analyze the optimal performance and generalize to any feedstock. Optimal operating condition is identified by analyzing the effect of mole Fe2O3/mole C (OC/C) on carbon conversion and Cold Gas Efficiency (CGE). Simulations using four different oxygen carriers (OC) reveal that, CGE shows a maximum with increase in OC/C. The OC/C at which CGE attains a maximum is proposed to be called as the Maximum CGE Point (MCP). It is shown that, unlike conventional gasification, the carbon conversion in CLG is less than 100 % at the point of maximum CGE. MCP is shown to be the optimal operation point since CGE is higher at MCP than that at carbon boundary point. Finally, it has been demonstrated that the optimal performance analysis at MCP can be generalized for feedstock of any H/C and O/C in the form of contour plots.

Inhibition effect of H2 on char gasification during chemical looping gasification of biomass

Chemical looping gasification (CLG) of biomass produces high contents of syngas, which would have inhibition effect on the gasification of its biomass char. Experiments using a rice husk char as fuel and a low-cost red mud as oxygen carrier for CLG investigation were performed, and effects of temperature, concentrations of steam and H2 on gasification rate were evaluated. Meanwhile, the mathematical models coupling with reaction and diffusion were established focusing on the H2 inhibition on syngas distributions inside and surrounding a single char particle. The results indicated that H2 in the reaction atmosphere has an inhibition effect on its char conversion, and at a high temperature the inhibition effect tends to be stronger. The shrinking core model (spherical symmetry) was found to be suitable to describe the char conversion under the present conditions with the reaction kinetic parameters of E = 128.8 kJ mol−1 and A = 451.2 s−1. In the internal diffusion of a single char particle, the concentrations of CO and H2 both decrease with the increase of dimensionless radius due to the consumption of carbon. In the external diffusion of the char particle, the concentrations of CO and H2 decrease with the increase of the dimensionless radius. The accumulation of H2 inside the char particle prevents CO production, thus inhibiting char gasification.

Effect of water/acetic acid washing pretreatment on biomass chemical looping gasification (BCLG) using cost-effective oxygen carrier from iron-rich sludge ash

Effect of water/acetic acid washing pretreatment on biomass chemical looping gasification (BCLG) using cost-effective oxygen carrier from iron-rich sludge ash

Biomass pellet fueled chemical looping gasification: Reaction kinetics study with a fluidized bed - thermogravimetric analysis

Biomass pellets show the advantages of higher energy density, lower transportation and storage costs compared to powder biomass, making them attractive for future industrial applications. In this work, a fluidized bed-thermogravimetric analysis (FB-TGA) is developed. The gas-solid process for gram-scale biomass pellets under fluidization state at high temperature is characterized. The effects of gasification agents, bed material components and operating conditions on devolatilization and char gasification are investigated. The devolatilization is revealed to be a rapid phase transition process of volatiles, of which the rate is mainly affected by the intense heat and mass transfer with increasing temperature. While the char gasification is significantly slower, taking more than 600 s for all conditions, therefore be considered as the rate-control step. The hematite oxygen carrier can effectively promote the char gasification by breaking the concentration balance of gas components near the char surface. The char reaction kinetics is fitted based on the isothermal differential method. Results show that the cylindrical geometrical contraction model exhibits an excellent linear correlation. With a mass ratio of oxygen carrier/quartz sand of 1:9, the average activation energies of 130.58 kJ/mol and 104.17 kJ/mol are obtained when CO2 and steam as gasifying agents.

Improving bio aviation fuel yield from biogenic carbon sources through electrolysis assisted chemical looping gasification

The second-generation bio aviation fuel production via Chemical Looping Gasification (CLG) of biomass combined with downstream Fischer-Tropsch synthesis is a possible way to decarbonize the aviation sector. Although CLG has a higher syngas yield and conversion efficiency compared to the conventional gasification processes, the fraction of biogenic carbon which is converted to biofuel is still low (around 28%). To increase carbon utilization and biofuel yield, incorporation of two types of electrolyzers, Polymer Electrolyte Membrane (PEM) and Molten Carbonate Electrolysis Cell (MCEC), for syngas conditioning has been investigated. Full chain process models have been developed using an experimentally validated CLG model in Aspen Plus for Iron sand as an oxygen carrier. Techno-economic parameters were calculated and compared for different cases. The results show that syngas conditioning with sustainable hydrogen from PEM and MCEC electrolyzers results in up to 11.5% higher conversion efficiency and up to 8.1 % higher biogenic carbon efficiencies in comparison to the syngas conditioning with water gas shift reactor. The study shows that the lowest carbon capture rates are found in the configurations with the highest biogenic carbon efficiency which means up to 14% more carbon ends up in FT crude compared to the case with conventional WGS conditioning. Techno-economic analysis indicates that syngas conditioning using PEM and MCEC electrolyzers would result in an increase of the annual profit by a factor of 1.4 and 1.7, respectively, when compared to using only WGS reactors.