Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier

Chemical looping combustion is a novel technology that can be used to meet the demand on energy production without CO₂ emission. To improve CO₂ capture efficiency in the process of chemical looping combustion of coal, a prototype configuration for chemical looping combustion of coal is made in this study. It comprises a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam to prevent the contamination of the flue gas between the two reactors. The performance of chemical looping combustion of coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier in a 1 kW_th prototype. The experimental results show that the configuration can minimize the amount of residual char entering into the air reactor from the fuel reactor with the external circulation of oxygen carrier particles giving up to 95% of CO₂ capture efficiency at a fuel reactor temperature of 985 °C. The effect of the fuel reactor temperature on the release of gaseous products of sulfur species in the air and fuel reactors is carried out. The fraction of gaseous sulfur product released in the fuel reactor increases with the fuel reactor temperature, whereas the one in the air reactor decreases correspondingly. The high fuel reactor temperature results in more SO₂ formation, and H₂S abatement in the fuel reactor. The increase of SO₂ in the fuel reactor accelerates the reaction of SO₂ with CO to form COS, and COS concentration in the fuel reactor exit gas increases with the fuel reactor temperature. The SO₂ in the air reactor exit gas is composed of the product of sulfur in residual char burnt with air and that of nickel sulfide oxidization with air in the air reactor. Due to the evident decrease of residual char in the fuel reactor with increasing fuel reactor temperature, it results in the decrease of residual char entering the air reactor from the fuel reactor, and the decrease of SO₂ from sulfur in the residual char burnt with air in the air reactor.     

Reactivity deterioration of NiO/Al2O3 oxygen carrier for chemical looping combustion of coal in a 10 kWth reactor

A relatively long-term experiment for chemical looping combustion of coal with NiO/Al₂O₃ oxygen carrier was carried out in a 10 kW_th continuous reactor of interconnected fluidized beds, and 100 h of operation was reached with the same batch of the oxygen carrier. The reactivity deterioration of the oxygen carriers was present during the experimental period. The reactivity deterioration of reacted oxygen carriers at different experimental stages was evaluated using X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray fluorescence spectrometer. SEM analysis showed no significant change in the morphology of the nickel-based oxygen carrier at the fuel reactor temperature ?940 °C, but loss of surface area and porosity of reacted oxygen carriers was observed when the fuel reactor temperature exceeded 960 °C. The results show that the sintering effect have mainly contributed to the reactivity deterioration of reacted oxygen carriers in the CLC process for coal, while the effects of coal ash and sulfur can be ignored. The oxidization of reduced oxygen carrier with air was an intensive exothermic process, and the high temperature of oxygen carrier particles led to sintering on the surface of oxygen carrier particles in the air reactor. Attention must be paid to control the external circulation of oxygen carrier particles in the interconnected fluidized beds in order to efficiently transport heat from the air reactor to the fuel reactor, and reduce the temperature of oxygen carrier particles in the air reactor. Improvement of reactivity deterioration of reacted oxygen carriers was achieved by the supplement of steam into the fuel reactor. Nevertheless, NiO/Al₂O₃ is still one of the optimal oxygen carriers for chemical looping combustion of coal if the sintering of oxygen carrier is minimized at the suitable reactor temperature.     

Investigation of coal gasification hydrogen and electricity co-production plant with three-reactors chemical looping process

This paper analyzes a novel process for producing hydrogen and electricity from coal, based on chemical looping combustion (CLC) and gas turbine combined cycle, allowing for intrinsic capture of carbon dioxide. The core of the process consists of a three-reactors CLC system, where iron oxide particles are circulated to: (i) oxidize syngas in the fuel reactor (FR) providing a CO₂ stream ready for sequestration after cooling and steam vapor condensation, (ii) reduce steam in the steam reactor (SR) to produce hydrogen, (iii) consume oxygen in the air reactor (AR) from air releasing heat to sustain the thermal balance of the CLC system and to generate electricity. A compacted fluidized bed, composed of two fuel reactors, is proposed here for full conversion of fuel gases in FR. The gasification CLC combined cycle plant for hydrogen and electricity cogeneration with Fe₂O₃/FeAl₂O₄ oxygen carriers was simulated using ASPEN^® PLUS software. The plant consists of a supplementary firing reactor operating up to 1350 °C and three-reactors SR at 815 °C, FR at 900 °C and AR at 1000 °C. The results show that the electricity and hydrogen efficiencies are 14.46% and 36.93%, respectively, including hydrogen compression to 60 bar, CO₂ compression to 121 bar, The CO₂ capture efficiency is 89.62% with a CO₂ emission of 238.9 g/kWh. The system has an electricity efficiency of 10.13% and a hydrogen efficiency of 41.51% without CO₂ emission when supplementary firing is not used. The plant performance is attractive because of high energy conversion efficiency and low CO₂ emission. Key parameters that affect the system performance are also discussed, including the conversion of steam to hydrogen in SR, supplementary firing temperature of the oxygen depleted air from AR, AR operation temperature, the flow of oxygen carriers, and the addition of inert support material to the oxygen carrier.     

Emissions from multiple-spouted and spout-fluid fluidized beds using rice husks as fuel

This paper presents the experimental results of the emissions of CO and CO₂ using rice husks as fuel on different configurations of spout-sluidized beds namely, multiple-spouted and spout-fluid fluidized bed. The emission of pollutants from the multiple-spouted bed and spout-fluid bed was investigated with rice husk fuel. The operating parameters considered were the different levels of excess air, different primary-to-secondary air ratios at each level of excess air and method of feeding. It was found that emission of CO from the multiple-spouted bed seemed to be lower with under-bed feeding of the rice husk fuel compared to over-bed feeding. However, the emission of CO₂ did not change significantly for both methods of feeding. Changes in excess air levels influenced the emissions of CO and CO₂ from the multiple-spouted bed within the excess air range investigated. It was found that emission of CO was less at 10% excess air with over-bed feeding; emission of CO in the case of under-bed feeding was lowest at 20% excess air level. It was found that the method of feeding had not significantly influenced the emission of CO and CO₂ in the spout-fluid bed. The combustion efficiency however, in general, was slightly higher in the case of under-bed feeding compared to over-bed feeding. Emission of CO was less in the spout-fluid bed compared with the emission of CO in the multiple-spouted bed. The result can be likely attributed to the higher combustion efficiency attained by the spout-fluid bed compared with that of multiple-spouted bed.     

煤基化学链燃烧串行流化床的冷态试验研究

采用“湍动床＋快速床”作为煤基化学链燃烧（CLC）系统的空气反应器（AR）,鼓泡床作为燃料反应器（FR）,设计了流动密封阀和旋风分离器,分别用于隔绝2个反应器之间的气氛和进行气固分离,在冷态试验装置上分析研究了CLC系统的压力分布、固体循环流量、气体泄漏率及煤灰与循环载体的分离效果．结果表明：该串行流化床反应器之间气氛隔绝性良好,气体泄漏率较低,固体循环流量达到甚至超过设计标准,FR二级旋风分离器的分离效率接近100％,FR中煤灰进入AR的质量分数小于1．55％,煤灰分离效果良好;装置可以长时间连续稳定运行,且操作气速范围较广,自行设计建造的循环流化床作为煤基化学链燃烧试验装置是可行的．     

A mechanistic investigation of a calcium-based oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) has been suggested as an energy-efficient method for the capture of carbon dioxide from combustion. It is indirect combustion by the use of an oxygen carrier, which can be used for CO₂ capture in power-generating processes. The possibility of CLC using a calcium-based oxygen carrier is investigated in this paper. In the air reactor air is supplied to oxidize CaS to CaSO₄, where oxygen is transferred from air to the oxygen carrier; the reduction of CaSO₄ to CaS takes place in the fuel reactor. The exit gas from the fuel reactor is CO₂ and H₂O. After condensation of water, almost pure CO₂ could be obtained. The thermodynamic and kinetic problem of the reduction reactions of CaSO₄ with CO and H₂ and the oxidization reactions of CaS with O₂ is discussed in the paper to investigate the technique possibility. To prevent SO₂ release from the process of chemical looping combustion using a calcium-based oxygen carrier, thermochemical CaSO₄ reduction and CaS oxidation are discussed. Thermal simulation experiments are carried out using a thermogravimetric analyzer (TGA). The properties of the products are characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffractometry (XRD), and the optimal reaction parameters are evaluated. The effects of reaction temperature, reductive gas mixture, and oxygen partial pressure on the composition of flue gas are discussed. The suitable temperature of the air reactor is between 1050 and 1150 °C and the optimal temperature of the fuel reactor between 900 and 950 °C.     

Parametric study of chemical looping combustion for tri‐generation of hydrogen,heat, and electrical power with CO2 capture

In this article, a novel cycle configuration has been studied, termed the extended chemical looping combustion integrated in a steam‐injected gas turbine cycle. The products of this system are hydrogen, heat, and electrical power. Furthermore, the system inherently separates the CO₂ and hydrogen that is produced during the combustion. The core process is an extended chemical looping combustion (exCLC) process which is based on classical chemical looping combustion (CLC). In classical CLC, a solid oxygen carrier circulates between two fluidized bed reactors and transports oxygen from the combustion air to the fuel; thus, the fuel is not mixed with air and an inherent CO₂ separation occurs. In exCLC the oxygen carrier circulates along with a carbon carrier between three fluidized bed reactors, one to oxidize the oxygen carrier, one to produces and separate the hydrogen, and one to regenerate the carbon carrier. The impacts of process parameters, such as flowrates and temperatures have been studied on the efficiencies of producing electrical power, hydrogen, and district heating and on the degree of capturing CO₂. The result shows that this process has the potential to achieve a thermal efficiency of 54% while 96% of the CO₂ is captured and compressed to 110 bar. Copyright © 2005 John Wiley & Sons, Ltd.     

Capturing and using CO2 as feedstock with chemical looping and hydrothermal technologies

Chemical looping technology for capturing and hydrothermal processes for conversion of carbon are discussed with focused and critical assessments. The fluidized and stationary reactor systems using solid, including biomass, and gaseous fuels are considered in chemical looping combustion, gasification, and reforming processes. Sustainability is emphasized generally in energy technology and in two chemical looping simulation case studies using coal and natural gas. Conversion of captured carbon to formic acid, methanol, and other chemicals is also discussed in circulating and stationary reactors in hydrothermal processes. This review provides analyses of the major chemical looping technologies for CO₂ capture and hydrothermal processes for carbon conversion so that the appropriate clean energy technology can be selected for a particular process. Combined chemical looping and hydrothermal processes may be feasible and sustainable in carbon capture and conversion and may lead to clean energy technologies using coal, natural gas, and biomass. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental Study on Coal Multi-generation in Dual Fluidized Beds

An atmospheric test system of dual fluidized beds for coal multi-generation was built.One bubbling fluidized bedis for gasification and a circulating fluidized bed for combustion.The two beds are combined with two valves:one valve to send high temperature ash from combustion bed to the gasification bed and another valve to sendchar and ash from gasification bed to combustion bed.Experiments on Shenhua coal multi-generation were madeat temperatures from 1112 K to 1191 K in the dual fluidized beds.The temperatures of the combustor are stableand the char combustion efficiency is about 98%.Increasing air/coal ratio to the fluidized bed leads to theincrease of temperature and gasification efficiency.The maximum gasification efficiency is 36.7% and thecalorific value of fuel gas is 10.7 MJ/Nm3.The tar yield in this work is 1.5%,much lower than that of pyrolysis.Carbon conversion efficiency to fuel gas and flue gas is about 90%.     

Enhanced coal gasification heated by unmixed combustion integrated with an hybrid system of SOFC/GT

For clean utilization of coal, enhanced gasification by in situ CO₂ capture has the advantage that hydrogen production efficiency is increased while no energy is required for CO₂ separation. The unmixed fuel process uses a sorbent material as CO₂ carrier and consists of three coupled reactors: a coal gasifier where CO₂ is captured generating a H₂-rich gas that can be utilized in fuel cells, a sorbent regenerator where CO₂ is released by sorbent calcination and it is ready for capture and a reactor to oxidize the oxygen transfer material which produces a high temperature/pressure vitiated air. This technology has the potential to eliminate the need for the air separation unit using an oxygen transfer material. Reactors' temperatures range from 750 °C to 1550 °C and the process operates at pressure around 7.0 bar. This paper presents a global thermodynamic model of the fuel processing concept for hydrogen production and CO₂ capture combined with fuel and residual heat usage. Hydrogen is directly fed to a solid oxide fuel cell and exhaust streams are used in a gas turbine expander and in a heat recovery steam generator. This paper analyzes the influence of steam to carbon ratio in gasifier and regeneration reactor, pressure of the system, temperature for oxygen transfer material oxidation, purge percentage in calciner, average sorbent activity and oxidant utilization in fuel cell. Electrical efficiency up to 73% is reached under optimal conditions and CO₂ capture efficiencies near 96% ensure a good performance for GHG's climate change mitigation targets.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

Nitrogen transfer of fuel-N in chemical looping combustion

Chemical-looping combustion (CLC) is a novel technique used for CO₂ separation that has been investigated for gaseous fuel and solid fuel. The nitrogen transfer of fuel-N in the coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier under a continuous operation in a 1 kW_th interconnected fluidized bed prototype. The effects of the fuel reactor temperature, coal type and operation conditions on the release of gaseous products of nitrogen species in the air reactor and the fuel reactor are carried out. Results show that the nitrogen transfer direction of fuel-N is toward N₂ formation in the fuel reactor independent of fuel type. In the fuel reactor N₂ is the sole product of nitrogen transfer of fuel-N. The concentration of N₂ in the fuel reactor exit gas increases with the fuel reactor temperature. The NO_x precursor of HCN can be oxidized by the oxygen carrier to form NO or N₂ in the fuel reactor. However, in the fuel reactor NO from coal devolatilization and HCN oxidization by oxygen carrier is completely reduced to N₂. The other NO_x precursor of NH₃ is completely converted to N₂ due to oxidization by NiO and the catalytic effect of Ni on the decomposition of NH₃. After coal devolatilization, char-N conversion in the fuel reactor is toward N₂ formation according to the investigation of solid–solid reaction between char and oxygen carrier. The amount of residual char has a potential to cause formation of nitrogen contaminants in the air reactor. In the air reactor, NO is the only nitrogen contaminant, and there is no NO₂ formation. The high fuel reactor temperature results in little residual char coming into the air reactor. The proportion of char-N converted to NO in the air reactor increases from 16.98% to 18.85% when the fuel reactor temperature changes from 850 to 950 °C. For the fuels containing more volatile matter, the possibility of NO formation in the air reactor is smaller than the fuels containing less volatile matter. For the fuels containing less volatile matter, char gasification rate is still a significant factor both for the carbon capture efficiency and NO formation.     

流化床中高水分煤的燃烧与排放试验研究

通过在一小型流化床中进行高水分煤的燃烧与排放的试验研究，表明水分含量和空气－燃料比对于高水分煤的燃烧与排放有较大影响。随着水分增加，流化床床温下降，ＮＯｘ、ＳＯｘ排放量也下降。空气－燃料比存在一最佳值，这时床温最高，而偏离此值，床温下降，随着空气量的增加，ＮＯｘ、ＳＯｘ排放量也增加。当空气－燃料比变化时，燃烧干煤与燃烧高水分煤有着类似的试验结果。     

Using steam reforming to produce hydrogen with carbon dioxide capture by chemical-looping combustion

In this paper, a novel process for hydrogen production by steam reforming of natural gas with inherent capture of carbon dioxide by chemical-looping combustion is proposed. The process resembles a conventional circulating fluidized bed combustor with reforming taking place in reactor tubes located inside a bubbling fluidized bed. Energy for the endothermic reforming reactions is provided by indirect combustion that takes place in two separate reactors: one for air and one for fuel. Oxygen is transferred between the reactors by a metal oxide. There is no mixing of fuel and air so carbon dioxide for sequestration is easily obtained. Process layout and expected performance are evaluated and a preliminary reactor design is proposed. It is found that the process should be feasible. It is also found that it has potential to achieve better selectivity towards hydrogen than conventional steam reforming plants due to low reactor temperatures and favorable heat-transfer conditions.     

Co-production system of hydrogen and electricity based on coal partial gasification with CO2 capture

To solve the problems of high cost and low efficiency of conventional co-production system of hydrogen and electricity with low hydrogen-to-electricity ratio, a novel co-production system based on coal partial gasification with CO₂ capture is proposed and thermodynamically analyzed. The new system integrates the conceptions of cascade conversion of coal and cascade utilization of syngas to realize the system with high efficiency, low cost, environmental friendliness and flexible hydrogen-to-electricity ratio. The performance of the new system is evaluated by an Aspen Plus model and effects of the operating conditions are also studied. It is found that the system with capturing CO₂ of 59.7% and hydrogen-to-electricity ratio of 4.76 holds a high exergy efficiency of 54.3% when the carbon conversion ratio of the pressurized fluidized bed (PFB) gasifier is equal to 0.7. The carbon conversion ratio of the PFB gasifier is a dominant factor to decide the performance of system. In comparison with the series-type co-production system, the parallel-type co-production system and separate production system, the new system proposed in this study has exergy-saving efficiency of 17.7%, 15.1% and 8.9%, respectively.     

Progress in Chemical-Looping Combustion and Reforming technologies

This work is a comprehensive review of the Chemical-Looping Combustion (CLC) and Chemical-Looping Reforming (CLR) processes reporting the main advances in these technologies up to 2010. These processes are based on the transfer of the oxygen from air to the fuel by means of a solid oxygen-carrier avoiding direct contact between fuel and air for different final purposes. CLC has arisen during last years as a very promising combustion technology for power plants and industrial applications with inherent CO₂ capture which avoids the energetic penalty present in other competing technologies. CLR uses the chemical looping cycles for H₂ production with additional advantages if CO₂ capture is also considered.     

Chemical‐looping combustion — an overview and application of the recirculating fluidized bed reactor for improvement

This paper propose recirculating fluidized bed (RCFB) reactor for chemical‐looping combustion (CLC) to overcome some of the issues associated with the existing interconnected reactors arrangements like low residence time of bed material in the air reactor, high attrition of bed material in the cyclone separator, cluster formation in the air reactor, complex operation involving loop seals and high heat losses. RCFB has high solid circulation rate, long residence time, efficient fuel–oxygen carrier contact, low heat losses and low gas leak in between the reactors, as compared to the existing reactor configurations. A cold model study was performed on a Perspex made, semicircular, transparent RCFB reactor. A single RCFB reactor was operated in the alternate oxidation and fuel burning cycles to simulate the interconnected reactors arrangement for CLC. The generated experimental data has been used to predict the optimal RCFB reactor configuration for a RCFB‐based CLC power plant. Copyright © 2014 John Wiley & Sons, Ltd.     

Post combustion CO2 capture by carbon fibre monolithic adsorbents

As generation of carbon dioxide (CO₂) greenhouse gas is inherent in the combustion of fossil fuels, effective capture of CO₂ from industrial and commercial operations is viewed as an important strategy which has the potential to achieve a significant reduction in atmospheric CO₂ levels. At present, there are three basic capture methods, i.e. post combustion capture, pre-combustion capture and oxy-fuel combustion. In pre-combustion, the fossil fuel is reacted with air or oxygen and is partially oxidized to form CO and H₂. Then it is reacted with steam to produce a mixture of CO₂ and more H₂. The H₂ can be used as fuel and the carbon dioxide is removed before combustion takes place. Oxy-combustion is when oxygen is used for combustion instead of air, which results in a flue gas that consists mainly of pure CO₂ and is potentially suitable for storage. In post combustion capture, CO₂ is captured from the flue gas obtained after the combustion of fossil fuel. The post combustion capture (PCC) method eliminates the need for substantial modifications to existing combustion processes and facilities; hence, it provides a means for near-term CO₂ capture for new and existing stationary fossil fuel-fired power plants.This paper briefly reviews CO₂ capture methods, classifies existing and emerging post combustion CO₂ capture technologies and compares their features. The paper goes on to investigate relevant studies on carbon fibre composite adsorbents for CO₂ capture, and discusses fabrication parameters of the adsorbents and their CO₂ adsorption performance in detail. The paper then addresses possible future system configurations of this process for commercial applications.Finally, while there are many inherent attractive features of flow-through channelled carbon fibre monolithic adsorbents with very high CO₂ adsorption capabilities, further work is required for them to be fully evaluated for their potential for large scale CO₂ capture from fossil fuel-fired power stations.     

200‐MW chemical looping combustion based thermal power plant for clean power generation

The present study demonstrates a possible configuration of a 200 MW chemical looping combustion (CLC) system with methane (CH₄) as fuel. Iron oxide‐based oxygen carriers were used because of its non‐toxic nature, low‐cost, and wide availability. We analyzed the effects of different variables on the design of the system. For the air reactor (oxidizer), bed mass is independent, and for the fuel reactor (reducer), it decreases with increase in the conversion difference between the air and fuel reactors. On the other hand, the pressure drop in the air reactor is unchanged, whereas for the fuel reactor, it decreases with the same increase of conversion difference between air and fuel reactors. Also, entrained solid mass flow rate from the air to fuel reactor shows a decreasing trend. Bed mass, bed height, pressure drop, and residence time of the bed materials decrease with increase in the conversion rates in the air and fuel reactors. Residence time of bed material in the air and fuel reactor reduces with increase in the temperature of the air reactor. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrogen production with integrated CO2 capture in a novel gas switching reforming reactor: Proof-of-concept

This paper reports an experimental investigation on a novel reactor concept for steam-methane reforming with integrated CO₂ capture: the gas switching reforming (GSR). This concept uses a cluster of fluidized bed reactors which are dynamically operated between an oxidation stage (feeding air) and a reduction/reforming stage (feeding a fuel). Both oxygen carrier reduction and methane reforming take place during the reduction stage. This novel reactor configuration offers a simpler design compared with interconnected reactors and facilitates operation under pressurized conditions for improved process efficiency.The performance of the bubbling fluidized bed reforming reactor (GSR) is evaluated and compared with thermodynamic equilibrium. Results showed that thermodynamic equilibrium is achieved under steam-methane reforming conditions. First, a two-stage GSR configuration was tested, where CH₄ and steam were fed during the entire reduction stage after the oxygen carrier was fully oxidized during the oxidation stage. In this configuration a large amount of CH₄ slippage was observed during the reduction stage. Therefore, a three-stage GSR configuration was proposed to maximize fuel conversion, where the reduction stage is completed with another fuel gas with better reactivity with the oxygen carrier, e.g. PSA-off gases, after a separate reforming stage with CH₄ and steam feeds. A high GSR performance was achieved when H₂ was used in the reduction stage. A sensitivity analysis of the GSR process performance on the oxygen carrier utilization and target working temperature was carried out and discussed.