Integrated combined cycle from natural gas with CO2 capture using a Ca–Cu chemical loop

The integration in a natural gas combined cycle (NGCC) of a novel process for H₂ production using a chemical Ca–Cu loop was proposed. This process is based on the sorption‐enhanced reforming process for H₂ production from natural gas with a CaO/CaCO₃ chemical loop, but including a second Cu/CuO loop to regenerate the Ca‐sorbent. An integration of this system into a NGCC was proposed and a full process simulation exercise of different cases was carried out. Optimizing the operating conditions in the Ca–Cu looping process, 8.1% points of efficiency penalty with respect to a state‐of‐the‐art NGCC are obtained with a CO₂ capture efficiency of 90%. It was demonstrated that the new process can yield power generation efficiencies as high as any other emerging and commercial concepts for power generation from NGCC with CO₂ capture, but maintaining competing advantages of process simplification and compact pressurized reactor design inherent to the Ca–Cu looping system. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2780–2794, 2013     

Hydrogen production via chemical looping dry reforming of methane: Process modeling and systems analysis

The present study presents and analyses a family of “chemical looping dry reforming” (CLDR) processes that produce inherently separated syngas (H₂ and CO) streams via a combination of methane cracking in a “cracker reactor” and the Boudouard reaction (i.e., conversion of the formed carbon with CO₂ to CO) in a “CO₂ reactor,” and then further maximize the H₂ yield via conversion of the produced CO via water-gas-shift. Remaining CO₂ emissions are minimized via CO₂ capture and sequestration. Four different configurations are evaluated which differ in how the heat required for the highly endothermic dry reforming reaction is supplied: (i) combustion of additional CH₄ feed (CLDR-CH₄); (ii) combustion of some of the CO produced in the CO₂ reactor (CLDR-CO); and combustion of some of the carbon produced in the cracker reactor with (iii) pure oxygen (CLDR-C-oxy); or (iv) with air (CLDR-C-air). Process models are developed to comparatively analyze the mass and energy balances of these configurations, and benchmark them against H₂-production via conventional dry reforming and steam reforming of methane. Our results show that CLDR-C-oxy is the most promising H₂-production pathway among the chemical looping and conventional technologies both in terms of chemical energy efficiency and in terms CO₂ emissions. Thus, the unique flexibility offered by the production of inherently separated syngas streams in CLDR enables overcoming the disadvantage of the strongly endothermic dry reforming reaction by combusting carbon internally in the reactor and thus achieving highly effective heat integration. Overall, the results support the technical viability and demonstrate the promise for strong process intensification of CLDR compared to conventional dry reforming and even steam reforming, the most widely used H₂-production pathway to-date.     

Carbon capture and utilization via chemical looping dry reforming

Chemical looping combustion (CLC) is a clean energy technology for CO₂ capture that uses periodic oxidation and reduction of an oxygen carrier with air and a fuel, respectively, to achieve flameless combustion and yield sequestration-ready CO₂ streams. While CLC allows for highly efficient CO₂ capture, it does not, however, provide a solution for CO₂ sequestration.Here, we propose chemical looping dry reforming (CLDR) as an alternative to CLC by replacing air with CO₂ as the oxidant. CLDR extends the chemical looping principle to achieve CO₂ reduction to CO, which opens a pathway to CO₂ utilization as an alternative to sequestration. The feasibility of CLDR is studied through thermodynamic screening calculations for oxygen carrier selection, synthesis and kinetic experiments of nanostructured carriers using cyclic thermogravimetric analysis (TGA) and fixed-bed reactor studies, and a brief model-based analysis of the thermal aspects of a fixed-bed CLDR process.Overall, our results indicate that it is indeed possible to reduce CO₂ to CO with high reaction rates through the use of appropriately designed nanostructured carriers, and to integrate this reaction into a cyclic redox (“looping”) process.     

Thermodynamic study of combining chemical looping combustion and combined reforming of propane

Existing energy generation technologies emit CO₂ gas and are posing a serious problem of global warming and climate change. The thermodynamic feasibility of a new process scheme combining chemical looping combustion (CLC) and combined reforming (CR) of propane (LPG) is studied in this paper. The study of CLC of propane with CaSO₄ as oxygen carrier shows thermodynamic feasibility in temperature range (400-782.95 °C) at 1 bar pressure. The CO₂ generated in the CLC can be used for combined reforming of propane in an autothermal way within the temperature range (400-1000 °C) at 1 bar pressure to generate syngas of ratio 3.0 (above 600 °C) which is extremely desirable for petrochemical manufacture. The process scheme generates (a) huge thermal energy in CLC that can be used for various processes, (b) pure N₂ and syngas rich streams can be used for petrochemical manufacture and (c) takes care of the expensive CO₂ separation from flue gas stream and CO₂ sequestration. The thermoneutral temperature (TNP) of 702.12 °C yielding maximum syngas of 5.98 mol per mole propane fed, of syngas ratio 1.73 with negligible methane and carbon formation was identified as the best condition for the CR reactor operation. The process can be used for different fuels and oxygen carriers.     

Polygeneration of hydrogen and power based on coal gasification integrated with a dual chemical looping process: Thermodynamic investigation

This paper assesses, from a thermodynamic perspective, the conversion of coal to power and hydrogen through gasification simultaneously with a dual chemical looping processes, namely chemical looping air separation (CLAS) and water–gas shift with calcium looping CO₂ absorption (WGS-CaL). CLAS offers an advantage over other mature technologies in that it can significantly reduce its capital cost. WGS-CaL is an efficient method for hydrogen production and CO₂ capture. The three major factors, oxygen to coal (O/C), steam to coal (S/C) and CaO to coal (Ca/C) were analyzed. Moreover, the comparisons of this suggested process and the traditional processes including integrated gasification combined cycle (IGCC), integrated gasification combined cycle with carbon capture and storage (IGCC-CCS) and integrated gasification combined cycle with calcium-based chemical looping (IGCC-CaL) were discussed. And, the exergy destruction analysis of this suggested process has also been calculated.     

Simulation and thermodynamic analysis of chemical looping reforming and CO2 enhanced chemical looping reforming

The production of hydrogen from methane via two chemical looping reforming (CLR) processes was simulated and thermodynamically analysed, one process being the conventional CLR process, the other being a CO₂ sorption enhanced process. The aim of the work was to identify suitable operating conditions for obtaining an optimum hydrogen gas purity and yield, whilst operating auto-thermally, at atmospheric pressure and with no carbon formation. In both simulations, the reactors were simulated using the Gibbs minimisation technique. NiO was used as the oxygen storing species, whilst CaO was used as the CO₂ adsorbent.     

Multiphase CFD-based models for chemical looping combustion process: Fuel reactor modeling

Chemical looping combustion (CLC) is a flameless two-step fuel combustion that produces a pure CO₂ stream, ready for compression and sequestration. The process is composed of two interconnected fluidized bed reactors. The air reactor which is a conventional circulating fluidized bed and the fuel reactor which is a bubbling fluidized bed. The basic principle is to avoid the direct contact of air and fuel during the combustion by introducing a highly-reactive metal particle, referred to as oxygen carrier, to transport oxygen from the air to the fuel. In the process, the products from combustion are kept separated from the rest of the flue gases namely nitrogen and excess oxygen. This process eliminates the energy intensive step to separate the CO₂ from nitrogen-rich flue gas that reduce the thermal efficiency.Fundamental knowledge of multiphase reactive fluid dynamic behavior of the gas-solid flow is essential for the optimization and operation of a chemical looping combustor.Our recent thorough literature review shows that multiphase CFD-based models have not been adapted to chemical looping combustion processes in the open literature. In this study, we have developed the reaction kinetics model of the fuel reactor and implemented the kinetic model into a multiphase hydrodynamic model, MFIX, developed earlier at the National Energy Technology Laboratory. Simulated fuel reactor flows revealed high weight fraction of unburned methane fuel in the flue gas along with CO₂ and H₂O. This behavior implies high fuel loss at the exit of the reactor and indicates the necessity to increase the residence time, say by decreasing the fuel flow rate, or to recirculate the unburned methane after condensing and removing CO₂.     

Multiphase CFD Modeling for a Chemical Looping Combustion Process (Fuel Reactor)

There are growing concerns about increasing emissions of greenhouse gases and a looming global warming crisis. CO₂ is a greenhouse gas that affects the climate of the earth. Fossil fuel consumption is the major source of anthropogenic CO₂ emissions. Chemical looping combustion (CLC) has been suggested as an energy‐efficient method for the capture of carbon dioxide from combustion. A chemical‐looping combustion system consists of a fuel reactor and an air reactor. The air reactor consists of a conventional circulating fluidized bed and the fuel reactor is a bubbling fluidized bed. The basic principle involves avoiding direct contact of air and fuel during the combustion. The oxygen is transferred by the oxygen carrier from the air to the fuel. The water in combustion products can be easily removed by condensation and pure carbon dioxide is obtained without any loss of energy for separation. With the improvement of numerical methods and more advanced hardware technology, the time required to run CFD (computational fluid dynamic) codes is decreasing. Hence, multiphase CFD‐based models for dealing with complex gas‐solid hydrodynamics and chemical reactions are becoming more accessible. To date, there are no reports in the literature concerning mathematical modeling of chemical‐looping combustion using FLUENT. In this work, the reaction kinetics models of the (CaSO₄ + H₂) fuel reactor is developed by means of the commercial code FLUENT. The effects of particle diameter, gas flow rate and bed temperature on chemical looping combustion performance are also studied. The results show that the high bed temperature, low gas flow rate and small particle size could enhance the CLC performance.     

Carbon-negative syngas production: A comprehensive assessment of biomass pyrolysis coupling chemical looping reforming

This article introduces a pyrolysis chemical looping reforming (PCLR) process that produces carbon-negative syngas in autothermal operation. To enhance the system's carbon negativity, a process configuration with oxygen carrier two-stage regeneration is adopted, enabling internal CO₂ utilization. The PCLR process is systematically evaluated and compared to chemical looping gasification and steam gasification processes in the process performance, energy efficiency, and environmental impact using a process model. Results reveal significant improvements over chemical looping gasification, including 69%, 45%, and 4% improvement in syngas yield, energy efficiency, and environmental benefit. Process analysis demonstrates that decoupling volatile reforming from pyrolysis and combustion enhances syngas quality, energy efficiency, and process flexibility. While the two-stage regeneration sacrifices syngas production, it contributes to a 4% increase in carbon negativity and a 15% reduction in carbon emissions. Thus, the PCLR process effectively overcomes the limitations of chemical looping gasification systems and exhibits excellent process intensification performance.     

Effect of Gasifying Medium on the Coal Chemical Looping Gasification with CaSO4 as Oxygen Carrier

The chemical looping gasification uses an oxygen carrier for solid fuel gasification by supplying insufficient lattice oxygen. The effect of gasifying medium on the coal chemical looping gasification with CaSO₄ as oxygen carrier is investigated in this paper. The thermodynamical analysis indicates that the addition of steam and CO₂ into the system can reduce the reaction temperature, at which the concentration of syngas reaches its maximum value. Experimental result in thermogravimetric analyzer and a fixed-bed reactor shows that the mixture sample goes through three stages, drying stage, pyrolysis stage and chemical looping gasification stage, with the temperature for three different gaseous media. The peak fitting and isoconversional methods are used to determine the reaction mechanism of the complex reactions in the chemical looping gasification process. It demonstrates that the gasifying medium (steam or CO₂) boosts the chemical looping process by reducing the activation energy in the overall reaction and gasification reactions of coal char. However, the mechanism using steam as the gasifying medium differs from that using CO₂. With steam as the gasifying medium, parallel reactions occur in the beginning stage, followed by a limiting stage shifting from a kinetic to a diffusion regime. It is opposite to the reaction mechanism with CO₂ as the gasifying medium.     

Chemical‐looping combustion process: Kinetics and mathematical modeling

Chemical Looping Combustion technology involves circulating a metal oxide between a fuel zone where methane reacts under anaerobic conditions to produce a concentrated stream of CO₂ and water and an oxygen rich environment where the metal is reoxidized. Although the needs for electrical power generation drive the process to high temperatures, lower temperatures (600–800°C) are sufficient for industrial processes such as refineries. In this paper, we investigate the transient kinetics of NiO carriers in the temperature range of 600 to 900°C in both a fixed bed microreactor (WHSV = 2‐4 g CH₄/h/g oxygen carrier) and a fluid bed reactor (WHSV = 0.014‐0.14 g CH₄/h per g oxygen carrier). Complete methane conversion is achieved in the fluid bed for several minutes. In the microreactor, the methane conversion reaches a maximum after an initial induction period of less than 10 s. Both CO₂ and H₂O yields are highest during this induction period. As the oxygen is consumed, methane conversion drops and both CO and H₂ yields increase, whereas the CO₂ and H₂O concentrations decrease. The kinetics parameter of the gas–solids reactions (reduction of NiO with CH₄, H₂, and CO) together with catalytic reactions (methane reforming, methanation, shift, and gasification) were estimated using experimental data obtained on the fixed bed microreactor. Then, the kinetic expressions were combined with a detailed hydrodynamic model to successfully simulate the comportment of the fluidized bed reactor. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Innovative Internally Circulating Reactor Concept for Chemical Looping‐Based CO2 Capture Processes: Hydrodynamic Investigation

An experimental hydrodynamic investigation has been carried out for a novel internally circulating chemical looping (ICCL) reactor concept proposed to reduce the technical complexities encountered in conventional chemical looping combustion (CLC) and reforming (CLR) technologies. The concept consists of a single reactor with internal physical separations dividing it into two sections, i.e., the fuel and air sections. The trade‐off for this reduction in process complexity is increased gas leakage between the two reactor sections, so a pseudo‐2D cold‐flow experimental unit was designed. The ICCL concept remains highly efficient in terms of CO₂ separation while ensuring significant process simplifications. The solids circulation rate also proved easy to control by adjusting the fluidization velocity ratio and the bed loading. In the light of the excellent hydrodynamic performance, the ICCL concept appears to be well‐suited for further development as a CLC/CLR reactor model.     

Physical mixtures as simple and efficient alternative to alloy carriers in chemical looping processes

Chemical looping combustion is a clean combustion technology for fossil or renewable fuels. In a previous demonstration, chemical looping was applied to CO₂ activation via reduction to CO with concurrent production of synthesis gas (CO + H₂) from CH₄ via rationally designed Fe‐Ni alloys. Here, it is demonstrated that that a simple physical mixture can even outperform the equivalent alloy based on an intricate gas phase mediated coupling between the two metals: Ni cracks methane to carbon and H₂. The latter then reduces iron oxide carrier, forming steam, which gasifies the carbon deposits on Ni to produce a mixture of CO + H₂, thus regenerating the active Ni surface. It was suggested that the principle demonstrated here—the gas phase‐mediated coupling of two solid reactants with distinct functionalities—should be applicable broadly toward oxidation reactions and hence opens a new avenue for rational design of chemical looping processes. © 2016 American Institute of Chemical Engineers AIChE J, 63: 51–59, 2017     

Effect of solid residence time on CO<Subscript>2</Subscript> selectivity in a semi-continuous chemical looping combustor

Chemical looping combustion (CLC) is a promising technology for fossil fuel combustion with inherent CO₂ capture and sequestration, which is able to mitigate greenhouse gases (GHGs) emission. In this study, to design a 0.5MW_th pressurized chemical looping combustor for natural gas and syngas the effects of solid residences time on CO₂ selectivity were investigated in a novel semi-continuous CLC reactor using Ni-based oxygen carrier particle. The semi-continuous chemical looping combustor was designed to simulate the fuel reactor of the continuous chemical looping combustor. It consists of an upper hopper, a screw conveyor, a fluidized bed reactor, and a lower hopper. Solid circulation rate (G _s ) was controlled by adjusting the rotational speed of the screw conveyor. The measured solid circulation rate increased linearly as the rotational speed of the screw increased and showed almost the same values regardless of temperature and fluidization velocity up to 800°C and 4 U _mf , respectively. The solid circulation rate required to achieve 100% CH₄ conversion was varied to change G _s -fuel ratio (oxygen carrier feeding rate/fuel feeding rate, kg/Nm³). The measured CO₂ selectivity was greater than 98% when the Gs-fuel ratio was higher than 78 kg/Nm³.     

Integrating desulfurization with CO2-capture in chemical-looping combustion

Chemical looping combustion (CLC) is an emerging technology for clean combustion. We have previously demonstrated that the embedding of metal nanoparticles into a nanostructured ceramic matrix can result in unusually active and sinter-resistant nanocomposite oxygen carrier materials for CLC which maintain high reactivity and high-temperature stability even when sulfur contaminated fuels are used in CLC. Here, we propose a novel process scheme for in situ desulfurization of syngas with simultaneous CO₂-capture in chemical looping combustion by using these robust nanocomposite oxygen carriers simultaneously as sulfur-capture materials. We found that a nanocomposite Cu-BHA carrier can indeed strongly reduce the H₂S concentration in the fuel reactor effluent. However, during the process the support matrix is also sulfidized and takes part in the redox process of CLC. This results in SO₂ production during the reduction of the oxygen carrier and thus limits the degree of desulfurization attainable with this kind of carrier. Nevertheless, the results suggest that simultaneous desulfurization and CO₂ capture in CLC is feasible with Cu as oxygen carrier as long as appropriate carrier support materials are chosen, and could result in a novel, strongly intensified process for low-emission, high efficiency combustion of sulfur contaminated fuel streams.     

基于赤铁矿载氧体的煤化学链燃烧试验

化学链燃烧是一种具有CO₂内分离特性的燃烧方式。以赤铁矿为载氧体,在1 kW_th级串行流化床上进行了煤化学链燃烧试验。讨论了燃料反应器温度对气体产物组分的影响;比较了各反应参数对煤气化效率、煤气化产物的转化效率及碳捕集效率的影响情况,分析了煤中硫的排放问题。试验结果表明：温度由900℃升高到985℃,燃料反应器中CO体积份额逐渐增加,CO₂体积份额逐渐减小,空气反应器中CO₂浓度呈线性下降。燃料反应器温度的升高促进煤气化效率及碳捕集效率大大提高。载氧体量和系统负荷是煤气化产物转化效率的主要影响因素,载氧体量的增加和负荷的增加分别会使煤气化产物转化效率提高和下降。燃料反应器中的硫主要以SO₂形式存在于燃料反应器,随温度的升高,SO₂浓度由515×10^-6逐渐增加到562×10^-6相似文献   

Effect of Oxygen on Methane Steam Reforming in a Sliding Discharge Reactor

Hydrogen‐rich gas can be efficiently produced in compact plasma reformers by the conversion of a variety of hydrocarbon fuels, including natural gas and gasoline. This article describes experimental and modeling progress in plasma reforming of methane using a sliding discharge reactor (SDR). Experiments have been carried out in a compact device operating at low consumed power (1–2 kW). Previous studies of methane steam reforming using a SDR at atmospheric pressure show promising results (H₂ concentration higher than 55 %). In order to study the effect of oxygen on the methane conversion and thus hydrogen production, a small amount of oxygen in the range of 7–20 % was added to the CH₄‐H₂O mixture. An unexpected result was that under our experimental conditions in the SDR oxygen did not have any influence on the methane conversion. Almost the totality of added oxygen is recovered intact. Moreover, part of the H₂ produced was transformed into water by reaction with O₂. A model describing the chemical processes based on classical thermodynamics is also proposed. The results indicate that the reactor design has to be improved in order to increase conversion and hydrogen production.     

Biomass Gasification Integrated with Chemical Looping System for Hydrogen and Power. Coproduction Process – Thermodynamic and Techno‐Economic Assessment

Three biomass gasification‐based hydrogen and power coproduction processes are modeled with Aspen Plus. Case 1 is the conventional biomass gasification coupled with a shift reactor, cases 2 and 3 involve integration of biomass gasification with iron‐based and calcium‐based chemical looping systems. The effects of important process parameters on the performance indicators such as hydrogen yield and efficiencies are evaluated by sensitivity analyses. These parameters include gasification temperature, molar ratios of steam to biomass in the gasifier, Fe₂O₃ to syngas in the fuel reactor, Fe/FeO to steam in the steam reactor, CaO to CO, and steam to CO in the carbonator. The energy and exergy balance distributions for the above three cases are comprehensively discussed and compared. Furthermore, techno‐economic assessments are performed to evaluate the three cases in terms of capital cost, operating cost, and leveled cost of energy.     

10 kWth级串行流化床中木屑化学链燃烧试验

设计并建立了10 kW_th级串行流化床化学链燃烧反应器系统,以NiO/Al₂O₃为载氧体,在该系统上进行生物质（松木木屑）化学链燃烧分离CO₂的试验研究,探讨了燃料反应器温度T、水蒸气/生物质比率S/B对两个反应器（空气反应器和燃料反应器）气体产物组成以及燃烧效率的影响。试验结果表明,燃料反应器温度是影响生物质化学链燃烧过程的重要因素,随着温度的升高,燃料反应器气体产物中CO₂浓度不断上升,CH4浓度显著降低,CO浓度先升高而后迅速下降;较高的反应器温度有助于燃烧效率的提高。随着S/B的增加,燃料反应器气体产物中CO和CH₄浓度均会增大,CO₂浓度以及燃烧效率有所降低。在100 h的连续试验过程中,采用共沉淀法制备的NiO/Al₂O₃载氧体展现出良好的氧化-还原性能和较强的持续循环能力,是生物质化学链燃烧理想的载氧体。     

Hydrogen production through chemical looping using NiO/NiAl2O4 as oxygen carrier

A new autothermal route to produce hydrogen from natural gas via chemical looping technology was investigated. Tests were conducted in a micro-fixed bed reactor loaded with 200 mg of NiO/NiAl₂O₄ as oxygen carrier. Methane reacts with a nickel oxide in the absence of molecular oxygen at 800 °C for a period of time as high as 10 min. The NiO is subsequently contacted with a synthetic air stream (21% O₂ in argon) to reconstitute the surface and combust carbon deposited on the surface. Methane conversion nears completion but to minimize combustion of the hydrogen produced, the oxidation state of the carrier was maintained below 30% (where 100% represents a fully oxidized surface). Co-feeding water together with methane resulted in stable hydrogen production. Although the carbon deposition increased with time during the reduction cycle, the production rate of hydrogen remained virtually constant. A new concept is also presented where hydrogen is obtained from methane with inherent CO₂ capture in an energy neutral 3-reactors CFB process. This process combines a methane combustion step where oxygen is provided via an oxygen carrier, a steam methane reforming step catalyzed by the reduced oxygen carrier and an oxidizing step where the O-carrier is reconstituted to its original state.