Chemical Looping Hydrogen Generation Process Research Articles

Thermodynamic and kinetic analysis on the biomass syngas fueled chemical looping hydrogen generation process in fixed bed reactor

Because of its simple structure and easy operation, the fixed bed reactor is widely used in the chemical looping process to assess the performance of oxygen carriers (OC) or to evaluate new chemical looping technologies. However, the systematic investigation of the redox behavior and temperature evolution during the chemical looping hydrogen generation (CLHG) process in the fixed bed reactor under different conditions, which is crucial for process optimization and reactor design, has received less attention. In the present work, the redox characteristics of the CLHG process with biomass syngas as the fuel in a fixed bed reactor were investigated and analyzed from a perspective of thermodynamics and kinetics. Results showed that high redox temperature (>800 °C) favors the fuel conversion due to the improved reaction kinetics and favorable thermodynamics. A space velocity in the range of 0.12–0.5 s−1 and superficial velocity of 0.008–0.075 m/s can provide enough reaction time meanwhile enhance the external diffusion of fuel gas, achieving high reduction degree of OC and further high H2 yield (3.4 mmol/g Fe2O3) and high H2 energy efficiency (45 %). Moreover, a violent OC bed temperature rise (100 °C) was observed in the air oxidization stage, which can be mitigated by diluting the Fe2O3 content and decreasing the air flow rate. Finally, the reaction front and flow front moving behavior were discussed in detail for a better understanding of the CLHG process.

Particle-resolved simulation of Fe-based oxygen carrier in chemical looping hydrogen generation

Oxygen carrier capability plays an important role in the performance of chemical looping hydrogen generation (CLHG). In this work, particle-scale simulation of oxygen carriers during the CLHG process is carried out, where the coke deposition effect on pore structures is taken into consideration. Meanwhile, the impacts of porosity and active component distribution of oxygen carriers on the CLHG performance are also analyzed. The results demonstrate that the CLHG performance is sensitive to particle porosity and active component distribution, especially for fuel reactor (FR). The increase of particle porosity can promote the carbon formation rate and hydrogen production rate. The flow direction and the diffusion will lead to the discrepancy of temperature between Egg-shell and Egg-yolk particles.

New Process Combining Fe-Based Chemical Looping and Biomass Pyrolysis for Cogeneration of Hydrogen, Biochar, Bio-Oil and Electricity with In-Suit CO2 Separation

Fe-based chemical looping gasification is a clean biomass technology, which has the advantage of reducing CO2 emissions and the potential of self-sustaining operation without supplemental heating. A novel process combining Fe-based chemical looping and biomass pyrolysis was proposed and simulated using Aspen Plus. The biomass was first subjected to pyrolysis to coproduce biochar, bio-oil and pyrolysis gas; the pyrolysis gas was subjected to an Fe looping process to obtain high-purity hydrogen and carbon dioxide. The influences of the pyrolysis reactor operating temperature and fuel reactor operation temperature, and the steam reactor and air reactor on the process performance are researched. The results showed that, under the operating condition of the established process, 23.07 kg/h of bio-oil, 24.18 kg/h of biochar, 3.35 kg/h of hydrogen and a net electricity of 3 kW can be generated from 100 kg/h of rice straw, and the outlet CO2 concentration of the fuel reactor was as high as 80%. Moreover, the whole exergy efficiency and total exergy loss of the proposed process was 58.98% and 221 kW, respectively. Additionally, compared to biomass direct chemical looping hydrogen generation technology, the new process in this paper, using biomass pyrolysis gas as a reactant in the chemical looping hydrogen generation process, can enhance the efficiency of hydrogen generation.

Ni, Co and Cu-promoted iron-based oxygen carriers in methane-fueled chemical looping hydrogen generation process

Oxygen carrier (OC) is essential in methane based chemical looping hydrogen generation (CLHG) process. Fe-based oxygen carriers have attracted lots of attention and attempts have been made to improve the CH4 conversion and hydrogen productivity. However, the influence of different promoters has yet to be compared. In this work, the interaction of promoters (Ni, Co and Cu) with Fe and the feasibility of tiny doping were investigated. Results showed that the oxygen activity and CH4 conversion increased after the addition of Ni, Co and Cu. Ni-promoted oxygen carriers presented the highest CH4 conversion and H2 yield. Co dopant benefitted the oxygen capacity, but Cu dopant restricted the deep reduction of Fe-based oxygen carriers. H2-TPR results revealed the oxygen activity was improved significantly at the intermediate temperature. A low concentration of Ni addition worked on hydrogen productivity as Mg-Fe-Al-O spinel with 1.7 wt% of Ni produced 1.3 mmol/gOC comparing with 1 mmol/gOC for Mg-Fe-Al-O. The promoters also facilitated the Fe segregation from the spinel phase. And the surface sintering of the promoted OCs was more serious, possibly resulting from the impregnation method. These findings could provide guidance for selection of promoters in chemical looping hydrogen generation process fueled by CH4.

Hydrogen element flow and economic analyses of a coal direct chemical looping hydrogen generation process

Abstract Coal direct chemical looping hydrogen generation (CDCLHG) is known as an attractive technique that converts coal into high purity H2 with inherent CO2 separation. In this paper, a CDCLHG system is developed and modeled. According to the prediction results, hydrogen element flow and economic performance of the CDCLHG system are researched. The study demonstrate that utilization efficiency of hydrogen element is 51.89%. By economic analysis, the results indicate that the calculated H2 producing cost of the CDCLHG system is 11.55 CNY/kg with coal price of 797 CNY/t and Fe based oxygen carrier price of 1986 CNY/t. Sensitivities to variables such as coal price, oxygen carrier price, water price and operating labor are researched. Coal price represents the most important sensitivity, while the sensitivity to water price and operating labor is relatively small. Additionally, H2 production from the CDCLHG process is only 3.0% more expensive than H2 production from natural gas chemical looping reforming with Fe based oxygen carrier. The CDCLHG process therefore appears to be an ideal option to obtain competitive hydrogen production.

Cu–Fe–Al–O mixed spinel oxides as oxygen carrier for chemical looping hydrogen generation

The reduction level for a metal oxides carrier determines the final hydrogen yield for a chemical looping hydrogen generation process. Nevertheless, when the oxygen carrier is reduced at a high level, the sintering of materials would be accelerated. In this paper, we prepare a Cu0·2Fe0·8(FeAl)Ox spinel material and investigate its hydrogen production performance. The results indicate that it exhibits a good redox stability even at reduction level of 0.75 during 20 cycles. In contrast, the deactivation of Fe2O3 oxygen carrier can be obviously observed in the first few cycles. This enable the material with stable hydrogen production at a high yield about 7 mmol/g, which is 3.5 times higher than that of Fe2O3. Upon SEM and XRD characterization techniques, we find that the reason of the both good stability and high hydrogen yield is that the ability of spinel support to inhibit sintering of Cu and Fe active compositions.

Using High/Low WHSV Value to Uncover the Reaction Behavior between Methane and Iron Oxide in Packed Bed for Chemical Looping Hydrogen Generation Process

In the chemical looping hydrogen generation (CLHG) system, methane acts as an important reducing gas, whereas iron oxide has poor reactivity with methane, resulting in the inefficiency of the syste...

Concept design and techno-economic performance of hydrogen and ammonia co-generation by coke-oven gas-pressure swing adsorption integrated with chemical looping hydrogen process

The coke-oven gas direct chemical looping hydrogen generation is a promising process for chemical industry; however, it still suffers from high energy consumption and low hydrogen production. The chemical looping-derived H2 and N2 can be used to produce ammonia. Therefore, a new process for coke-oven gas chemical looping hydrogen and ammonia co-generation integrated with pressure swing adsorption technology, which can flexibly adapt to market demand, is proposed in this paper. The new process has two extreme configurations to produce hydrogen or ammonia only. A thorough analysis of key operational parameters of the proposed systems has been conducted to optimize coke-oven gas utilization, and maximize hydrogen and subsequent ammonia production. The maximal hydrogen and ammonia productions are 7126 and 4784 kmol/h of 5532 kmol/h coke-oven gas consumption for the two extreme configurations, respectively. Moreover, an exergy efficiency of 68.5–73.6% and about 100% direct CO2 capture efficiency are obtained when switching between hydrogen and ammonia production in the novel process. These values are compared to those of the coke-oven gas chemical looping hydrogen generation process (hydrogen production of 6613 kmol/h per 6276 kmol/h coke-oven gas consumption, 66.0% exergy efficiency, and 73.3% CO2 capture efficiency). Finally, the economic and sensitive analyses of the novel process are also conducted in this paper.

Ca2Fe2O5: A promising oxygen carrier for CO/CH4 conversion and almost-pure H2 production with inherent CO2 capture over a two-step chemical looping hydrogen generation process

Chemical looping hydrogen generation (CLHG) is a promising technology for high-purity hydrogen production with inherent CO2 separation. The selection of a high-performance oxygen carrier capable of being reduced and oxidized over multiple redox cycles against deactivation is a key issue for CLHG technology. In this work, a two-step chemical looping hydrogen generation (TCLHG) process is proposed by using a novel calcium ferrite, Ca2Fe2O5, as an oxygen carrier which is synthesized with applied a citric acid assisted sol–gel method. The experimental results indicate that the reduced oxygen carrier achieves one-step oxidation from Fe0 to Fe3+ by using steam as an oxidizing agent. Thus, higher yields of hydrogen could be generated compared with Fe2O3. The fresh and reacted Ca-Fe based oxygen carriers were characterized using different methods such as XRD, SEM/EDS, TEM, N2 adsorption, H2-TPR, XPS, and Mossbauer spectroscopy test etc. The oxygen release and storage capacity, cyclic stability, and carbon deposition characteristics of the Ca-Fe based oxygen carriers were investigated using TGA and a fixed bed reactor with multicycles of CO/CH4 reduction and H2O/O2 oxidation. Ca2Fe2O5 is proved to be a more stable formation of the calcium ferrite compounds and a promising oxygen carrier for TCLHG process which shows perfect reducibility, oxidation activity, and cyclic stability. The existence of Ca appears to perform a significant effect on the Fe3+ reduction and Fe0 oxidation and the reduction from Fe3+ to Fe0 was concluded to be a simple one-step reaction.

Carbon formation on iron-based oxygen carriers during CH4 reduction period in Chemical Looping Hydrogen Generation process

Chemical Looping Hydrogen Generation (CLHG) is promising hydrogen production technology with inherent capture CO2 and high-purity hydrogen production simultaneously. Although the reactivity and durability of oxygen carriers can be improved by the combination of inert materials. Carbon deposition over oxygen carrier in the reduction period is still a serious problem in the process of CLHG with CH4 as fuel, and it affects the purity of hydrogen in steam oxidation period and the carbon capture efficiency. In this study, oxygen carriers, Fe2O3/Al2O3 and Fe2O3/MgAl2O4, composed of 60wt% Fe2O3, were prepared by mechanical mixing method. Moreover, different contents of K2CO3 were incorporated into the Fe2O3/Al2O3 oxygen carriers by using impregnation method. Experiments were performed in a fluidized bed reactor by exposing the oxygen carriers to alternating methane reducing, steam oxidizing and air oxidizing conditions. The effects of inert supports and alkali additive on the carbon formation performance of iron oxides with different methane concentration were investigated. Results indicate that carbon was formed on the oxygen carriers Fe2O3/Al2O3 during the reduction, the cracking reaction of CH4 on the surface of iron-based oxygen carrier starts as active phase Fe or Fe3C formed. However, the methane decomposition promoted the reduction rate of oxygen carriers and it increased with the temperature and CH4 concentration. Lower carbon deposition rate was found on Fe2O3/MgAl2O4 compared with Fe2O3/Al2O3. Furthermore, the addition of K2CO3 exhibited a beneficial effect on suppressing carbon formation but decreased the reduction activity of oxygen carrier.

Chemical Looping Hydrogen Generation Using Potassium-Modified Iron Ore as an Oxygen Carrier

Chemical looping hydrogen generation (CLHG) consists of an oxidation process, a reduction process, and a hydrogen generation process. Achieving deep reduction of the oxygen carrier is the challenge for the CLHG process. In this paper, experiments on CLHG using K-modified iron ore as an oxygen carrier and CO as a fuel were carried out in a laboratory-scale fluidized bed reactor. A high temperature improved the reduction reactivity. However, at the same reduction condition, a higher temperature did not improve the hydrogen generation process, which means that a higher temperature mainly benefited the reduction process and then elevated hydrogen generation in a CLHG process. Adding KNO3 improved the rate of reduction and hydrogen generation. With the KNO3 loading in iron ore increasing from 0 to 10%, not only the carbon conversion but also the hydrogen production was accelerated. A high KNO3 loading in iron ore can also maintain longer reaction time. The 10% K-modified iron ore could decrease carbon depositi...

Use of Spinel Nickel Aluminium Ferrite as Self-Supported Oxygen Carrier for Chemical Looping Hydrogen Generation Process

Chemical looping hydrogen generation (CLHG), as a new technology involved in chemical looping combustion (CLC) process and steam-iron process, has attracted attention for clean energy generation and efficient energy conversion. A systematic investigation of spinel NiFeAlO4 has led to characterization of a well-defined self-supported oxygen carrier which was prepared by solid state reaction. The redox behavior of NiFeAlO4 shows the enhancement on the resistance to agglomeration exceeding previously reported CLC process using similar compositions including Fe2O3, NiO and NiFe2O4 without any inert support. Moreover, the high CO2 conversion and H2 generation by NiFeAlO4 oxygen carrier under a fixed-bed reactor (FxBR) were obtained. The reaction mechanism of NiFeAlO4 subjected to the isothermal stepwise reduction in a sequence of varying durations by XRD was also demonstrated, which provided with a novel function of self-supported and agglomeration-resistant characteristic in the oxygen carrier system.

Thermodynamic analysis of a combined chemical looping-based trigeneration system

Energy and exergy analyses of a newly developed three-reactor chemical looping hydrogen generation process are performed for trigeneration of power, hydrogen, and heating. The present integrated system consists of an (a) air separation unit (ASU), (b) gasification sub-system, (c) chemical looping hydrogen generation unit in connection with SOFC assisted gas turbine (CLHG-SOFC/GT), (d) an extended heat recovery steam generation unit (HRSG) to supply heat for Steam cycle, organic Rankine cycle and space heating, (e) a two stage steam Rankine cycle (SRC) for power generation with reheat and regeneration, and (f) an organic Rankine cycle (ORC) to produce power. The gasified coal is separated and purified in quench chamber and syngas cleaner; CO2 and H2 are generated from fuel and steam reactors of chemical looping unit, and both are then compressed after separated from water and ready for transportation. A specified amount of H2 produced from steam reactor is also used to produce electricity with SOFC/GT. A comprehensive parametric study is performed, and the effects of multi-generation and system integration, environmental conditions, and system parameter variations on overall efficiencies are investigated. Overall electrical, hydrogen, energy and exergy efficiencies are comparatively determined for different cases. Overall energy and exergy efficiencies of proposed system are found to be 56.9% and 45.05%, respectively, with a total exergy destruction rate of 15,421kW. The highest exergy destruction occurs in the gasifier and CLHG due to high temperature chemical processes.

An integrated system combining chemical looping hydrogen generation process and solid oxide fuel cell/gas turbine cycle for power production with CO2 capture

In this paper, the solid oxide fuel cell/gas turbine (SOFC/GT) cycle is integrated with coal gasification and chemical looping hydrogen generation (CLHG) for electric power production with CO2 capture. The CLHG-SOFC/GT plant is configurated and the schematic process is modeled using Aspen Plus® software. Syngas, produced by coal gasification, is converted to hydrogen with CO2 separation through a three-reactors CLHG process. Hydrogen is then fueled to SOFC for power generation. The unreacted hydrogen from SOFC burns in a combustor and drives gas turbine. The heat of the gas turbine exhaust stream is recovered in HRSG for steam bottoming cycle. At a system pressure of 20 bar and a cell temperature of 900 °C, the CLHG-SOFC/GT plant has a net power efficiency of 43.53% with no CO2 emissions. The hybrid power plant performance is attractive because of high energy conversion efficiency and zero-CO2-emission. Key parameters that influence the system performance are also discussed, including system operating pressure, cell temperature, fuel utilization factor, steam reactor temperature, CO2 expander exhaust pressure and inlet gas preheating.

Design and Fluid Dynamic Analysis of a Three-Fluidized-Bed Reactor System for Chemical-Looping Hydrogen Generation

Chemical-looping hydrogen generation (CLHG) can produce hydrogen from fossils fuels with inherent separation of CO2. Iron oxide is a suitable oxygen carrier for this process. The CLHG process basically involves three reactors, a fuel reactor (FR), a steam reactor (SR), and an air reactor (AR). In the FR, the carbon-containing fuel gases react with hematite (Fe2O3). The product solids are wustite (FeO), and the product stream is a mixture of carbon dioxide and water vapor. After water condensation, pure carbon dioxide can be obtained. FeO then enters the SR and react with steam, giving the gas product hydrogen and the solid product magnetite (Fe3O4). In the AR, Fe3O4 is reoxidized to Fe2O3. Through this cycle, hydrogen is generated with inherent separation of CO2. In this article, a novel compact fluidized-bed fuel reactor is proposed. It integrates a bubbling fluidized bed and a riser to obtain full conversion of unreacted fuel gases through the thermodynamic equilibrium limit. Based on this fuel reactor,...

Experimental investigation of chemical-looping hydrogen generation using Al2O3 or TiO2-supported iron oxides in a batch fluidized bed

Chemical-looping hydrogen generation (CLHG) is a novel technology for hydrogen production with inherent separation of CO2. Three oxygen carriers Fe2O3 using inert materials Al2O3 or TiO2 as support were prepared by mechanical-mixing method, i.e., Fe90Al10 (90%Fe2O3 + 10%Al2O3), Fe60Al40 (60%Fe2O3 + 40%Al2O3) and Fe60Ti40 (60%Fe2O3 + 40%Al2O3). Reactivity of the three oxygen carriers was first determined under CO reduction, steam oxidation and air oxidation atmospheres at 900 °C in a thermogravimetric analyzer. Then experiments to simulate the CLHG process were carried out in a batch fluidized bed. In the fluidized bed, all of the three oxygen carriers showed good reactivity over the multi-cycle experiments at 900 °C, and Fe60Al40 had the highest hydrogen yield. The reactivity of the oxygen carrier supported on Al2O3 was higher than that on TiO2, which interacted with iron oxide forming FeTiO3. The reactivity of Fe60Al40 was better than that of Fe90Al10. No deterioration of the oxygen carrier occurred after the multiple cycles, but for Fe90Al10 some agglomeration was detected. At 600–900 °C, higher temperature favored deeper reduction of iron oxide and increased the hydrogen production, while carbon deposition in the reduction period was suppressed with the rise of temperature. In the reduction, the conversion of fuel gas was constrained by thermodynamics in a single-stage reactor, and a compact fuel reactor was proposed for a full conversion of gaseous fuels.