Reactivity of a spray-dried NiO/NiAl2O4 oxygen carrier for chemical-looping combustion

Kinetic data of a promising oxygen carrier of NiO/NiAl₂O₄ have been established from experiments in a small fluidized bed batch reactor using methane. The particles were prepared by spray-drying using commercially available raw material and selected as the best candidates from an earlier screening study. The particles clearly showed high reactivity, with a maximum gas yield between 86% and 93% in the temperature interval 750 °C to 950 °C when using a bed mass and a gas flow corresponding to only 6 kg/MW_fuel. A comparison of the reactivity with data from TGA experiments showed that the reactivity generally was faster in the batch fluidized bed in the investigated temperature interval. A simple reactor model using kinetic data from the batch fluidized bed reactor and the TGA predicted a minimum mass of 9–24 kg/MW_fuel of oxygen carrier particles for full gas yield of methane to carbon dioxide in the fuel reactor. Comparison with experiments performed in a 10 and 120 kW CLC reactor with the same type of oxygen carrier showed that even when employing 13 to 50 times the amount of oxygen carrier theoretically needed for complete gas conversion, full gas yield was not obtained in the circulating systems. Hence it is of great importance to consider the fluid dynamics and gas-solid contact when modeling the fuel reactor of a chemical-looping combustor.     

Carbon deposition characteristics and regenerative ability of oxygen carrier particles for chemical-looping combustion

For gaseous fuel combustion with inherent CO₂ capture and low NO_x emission, chemical-looping combustion (CLC) may yield great advantages for the savings of energy to CO₂ separation and suppressing the effect on the environment. In a chemical-looping combustor, fuel is oxidized by metal oxide medium (oxygen carrier particle) in a reduction reactor. Reduced particles are transported to the oxidation reactor and oxidized by air and recycled to the reduction reactor. The fuel and the air are never mixed, and the gases from the reduction reactor, CO₂ and H₂O, leave the system as separate streams. The H₂O can be easily separated by condensation and pure CO₂ is obtained without any loss of energy for separation. In this study, NiO based particles are examined from the viewpoints of reaction kinetics, carbon deposition, and cyclic use (regenerative ability). The purpose of this study is to find appropriate reaction conditions to avoid carbon deposition and achieve high reaction rate (e.g., temperature and maximum carbon deposition-free conversion) and to certify regenerative ability of NiO/bentonite particles. In this study, 5.04% methane was used as fuel and air was used as oxidation gas. The carbon deposition characteristics, reduction kinetics and regenerative ability of oxygen carrier particles were examined by TGA (Thermal Gravimetrical Analyzer).     

Syngas combustion characteristics of four oxygen carrier particles for chemical-looping combustion in a batch fluidized bed reactor

Syngas combustion characteristics of oxygen carrier particles have been investigated. Experiments were performed on four oxygen carrier particles in a fluidized bed reactor. All four oxygen carrier particles showed high gas conversion, high CO₂ selectivity, and low CO concentration in the reducer and very low NOx (NO, NO₂, N₂O) emissions in the oxidizer. Moreover, all particles showed good regeneration ability during successive reduction-oxidation cyclic tests up to the 10 ^th cycle. The results indicate that inherent CO₂ separation, NOx-free combustion, and long-term operation without reactivity decay of oxygen carrier particles are possible in a syngas fueled chemical-looping combustion system with NiO/bentonite, NiO/NiAl₂O₄, Co _x O _y /CoAl₂O₄, and OCN-650 particles. However, Co _x O _y /CoAl₂O₄ represented slight decay of oxidation reactivity with the number of cycles increased and the oxidation rate slower than other particles.     

Reactivity of a CaSO<Subscript>4</Subscript>-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. A reactivity study of CaSO₄ oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH₄ reacted with CaSO₄ to form CO₂ and H₂O. The oxidation was incomplete, possibly due to the CaSO₄ product layer. The reactivity of CaSO₄ oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO₂, CH₄, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H₂) in the product gas and slight CO₂ formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO₂ release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO₄ oxygen carrier. XRD analysis illustrated the phase change of CaSO₄ oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO₄ to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO₄ oxygen carrier may be an interesting candidate for oxygen carrier in CLC. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.     

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.     

The use of NiO as an oxygen carrier in chemical-looping combustion

The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH₄ as fuel showed that the yield of CH₄ to CO₂ and H₂O was between 97.7 and 99.8% in the temperature range 700–1200 °C, with the yield decreasing as the temperature increases. Carbon deposition is not expected as long as sufficient metal oxide is supplied to the fuel reactor. Hydrogen sulfide, H₂S, in the fuel gas will be converted partially to SO₂ in the gas phase, with the degree of conversion increasing with temperature, but decreasing as a function of pressure. There is the possibility of sulfide formation as Ni₃S₂ at higher partial pressures of H₂S+SO₂ in the reactor. The reactivity of freeze granulated particles of NiO with NiAl₂O_4, MgAl₂O₄, TiO₂ and ZrO₂ sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH₄/50% H₂O and 5% O₂ at 950 °C. During the reducing period, the NiO initially reacted with the CH₄ to form CO₂ and H₂O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO₂, which was due to the thermodynamic limitations. Here, the ratio CO/(CO₂+CO+CH₄) was between 1.5 and 2.5% at 950 °C for the oxygen carriers with alumina based inert. A small amount of CH₄ was released from the reactor at high degrees of oxidation of the NiAl₂O₄ and MgAl₂O₄-based carriers. As the time under reducing conditions increased, steam reforming of CH₄ to CO and H₂ became considerable, with Ni catalyzing this reaction. Whereas the ZrO₂ particles showed similar behavior as the alumina-based carriers, the TiO₂-based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO₂.     

Effective methane reforming with CO2 and O2 under pressurized condition using NiO–MgO and fluidized bed reactor

Circulation of Ni_0.15Mg_0.85O catalyst particles in the fluidized bed reactor gave much higher CH₄ conversion in methane reforming with CO₂ and O₂ under pressurized condition than the case of the catalyst without moving in the fixed bed reactor. In addition, circulation of the catalyst particles in the fluidized bed reactor inhibited carbon deposition which is the serious problem in methane reforming.     

Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion

For combustion with CO₂ capture, chemical-looping combustion (CLC) with inherent separation of CO₂ is a promising technology. Two interconnected fluidized beds are used as reactors. In the fuel reactor, a gaseous fuel is oxidized by an oxygen carrier, e.g. metal oxide particles, producing carbon dioxide and water. The reduced oxygen carrier is then transported to the air reactor, where it is oxidized with air back to its original form before it is returned to the fuel reactor. The feasibility of using oxygen carrier based on oxides of iron, nickel, copper and manganese was investigated. Oxygen carrier particles were produced by freeze granulation. They were sintered at 1300 °C for 4 h and sieved to a size range of 125-180 μm. The reactivity of the oxygen carriers was evaluated in a laboratory fluidized bed reactor, simulating a CLC system by exposing the sample to alternating reducing and oxidizing conditions at 950 °C for all carriers except copper, which was tested at 850 °C. Oxygen carriers based on nickel, copper and iron showed high reactivity, enough to be feasible for a suggested CLC system. However, copper oxide particles agglomerated and may not be suitable as an oxygen carrier. Samples of the iron oxide with aluminium oxide showed signs of agglomeration. Nickel oxide showed the highest reduction rate, but displayed limited strength. The reactivity indicates a needed bed mass in the fuel reactor of about 80-330 kg/MW_th and a needed recirculation flow of oxygen carrier of 4-8 kg/s, MW_th.     

Experimental investigation of some metal oxides for chemical looping combustion in a fluidized bed reactor

Chemical looping combustion (CLC) is the process in which metal oxides, rather than air or pure oxygen, supply the oxygen required for combustion. In this process, different gaseous fuels can be burnt with the inherent separation of CO₂. The feasibility of the CLC system depends greatly on the selection of appropriate metal oxides as oxygen carriers (OC). In this study, NiO-NiAl₂O₄, Cu_0.95Fe_1.05AlO₄, and CuO-Cu_0.95Fe_1.05AlO₄ were tested experimentally in a fluidized bed reactor as a function of oxidation-reduction cycles, temperature, bed inventory and superficial gas velocity. The results showed that flue gases with a CO₂ concentration as high as 97% can be obtained. The flue gases should be suitable for transport and storage after clean-up and purification. With an increase in the bed inventory or a decrease in superficial gas velocity, the flue gas characteristics improved i.e. more CO₂ and fewer secondary components or less unreacted fuel were obtained. Carbon formation could occur during the reduction phase but it decreased with an increase in temperature and inventory and could be completely avoided by mixing steam with the fuel. The reactivity of NiO/NiAl₂O₄ was higher than the Cu- and Fe-based oxygen carriers. Increasing the CuO fraction in the oxygen carrier led to defluidization of the bed during the reduction and oxidation phases.     

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.     

Hydrodynamic simulation of fuel-reactor in chemical looping combustion process

A multiphase CFD-based model with gas-solid hydrodynamics and chemical reactions is used to model flow behavior of gas and particles in the fuel reactor of chemical looping combustion process. The granular kinetic theory model is used to model the interaction of particle collisions. The friction stress of particles is considered to account for strain rate fluctuations and slow relaxation of the assembly to the yield surface. The reaction kinetics model of the fuel reactor is presented. The instantaneous mass fractions of both reactant and products are predicted, and the time averaged distributions are calculated in the fuel reactor. Simulated fuel reactor flows reveal a 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 and improve mixing in the fuel reactor using circulating fluidized bed technology.     

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     

Reactivity of a NiO/Al2O3 oxygen carrier prepared by impregnation for chemical-looping combustion

The reactivity of a Ni-based oxygen carrier prepared by hot incipient wetness impregnation (HIWI) on α-Al₂O₃ with a NiO content of 18 wt% was studied in this work. Pulse experiments with the reduction period divided into 4-s pulses were performed in a fluidized bed reactor at 1223 K using CH₄ as fuel. The number of pulses was between 2 and 12. Information about the gaseous product distribution and secondary reactions during the reduction was obtained. In addition to the direct reaction of the combustible gas with the oxygen carrier, CH₄ steam reforming also had a significant role in the process, forming H₂ and CO. This reaction was catalyzed by metallic Ni in the oxygen carrier and H₂ and CO acted as intermediate products of the combustion. No evidence of carbon deposition was found in any case. Redox cycles were also carried out in a thermogravimetric analyzer (TGA) with H₂ as fuel. Both tests showed that there was a relation between the solid conversion reached during the reduction and the relative amount of NiO and NiAl₂O₄ in the oxygen carrier. When solid conversion increased, the NiO content also increased, and consequently NiAl₂O₄ decreased. Approximately 20% of the reduced nickel was oxidized to NiAl₂O₄, regardless ΔX_s. NiAl₂O₄ was also an active compound for the combustion reaction, but with lower reactivity than NiO. Further, the consequences of these results with respect to the design of a CLC system were investigated. When formation of NiAl₂O₄ occurred, the average reactivity in the fuel reactor decreased. Therefore, the presence of both NiO and NiAl₂O₄ phases must be considered for the design of a CLC facility.     

Experimental Investigations of Catalytic Partial Oxidation of Methane to Synthesis Gas in Various Types of Fluidized‐Bed Reactors

The partial oxidation of methane to synthesis gas over Ni/α‐Al₂O₃ catalysts (1 and 5 wt.‐% Ni loading, 71–160 and 250–355 μm particle diameter) was investigated in different types of fluidized‐bed reactors, i.e., the bubbling fluidized bed (FlB), the spout fluid bed (SFB) and the internally circulating fluidized bed (ICFB). A methane‐to‐oxygen ratio of 2:1 was used in all experiments and the temperature was varied between 700 and 800 °C. Gas velocities and catalyst masses were adjusted to assure a stable and controllable reactor operation. A nearly isothermal operation was established in all reactors. The thermodynamic equilibrium values were achieved in the FlB and SFB reactor whereas in the ICFB reactor slightly lower conversions and selectivities were obtained. Taking the direct scale‐up concept of the ICFB reactor into account, significant higher space‐time yields were obtained in this reactor than in the industrial‐scale bubbling fluidized‐bed reactor. No increase of the space‐time yield in comparison to the FlB was obtained in the SFB reactor.     

Synthesis gas generation by chemical-looping reforming in a continuously operating laboratory reactor

Chemical-looping reforming is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. This paper describes continuous chemical-looping reforming of natural gas in a laboratory reactor consisting of two interconnected fluidized beds. Particles composed of 60 wt% NiO and 40 wt% MgAl₂O₄ are used as bed material, oxygen carrier and reformer catalyst. There is a continuous circulation of particles between the reactors. In the fuel reactor, the particles are reduced by the fuel, which in turn is partially oxidized to H₂, CO, CO₂ and H₂O. In the air reactor the reduced oxygen carrier is reoxidized with air. Complete conversion of natural gas was achieved and the selectivity towards H₂ and CO was high. In total, 41 h of reforming were recorded. Formation of solid carbon was noticed for some cases. Adding 25 vol% steam to the natural gas reduced or eliminated the carbon formation.     

Comparative study between fluidized bed and fixed bed reactors in methane reforming combined with methane combustion for the internal heat supply under pressurized condition

The effect of catalyst fluidization on the conversion of methane to syngas in methane reforming with CO₂ and H₂O in the presence of O₂ under pressurized conditions was investigated over Ni and Pt catalysts. Methane and CO₂ conversion in the fluidized bed reactor was higher than those in the fixed bed reactor over Ni_0.15Mg_0.85O catalyst under 1.0 MPa. This reactor effect was dependent on the catalyst properties. Conversion levels in the fluidized and fixed bed reactor were almost the same over MgO-supported Ni and Pt catalysts. It is suggested that this phenomenon is related to the catalyst reducibility. On a catalyst with suitable reducibility, the oxidized catalyst can be reduced with the produced syngas and the reforming activity regenerates in the fluidized bed reactor. Although serious carbon deposition was observed on Ni_0.15Mg_0.85O in the fixed bed reactor, it was inhibited in the fluidized bed reactor.     

Using continuous and pulse experiments to compare two promising nickel-based oxygen carriers for use in chemical-looping technologies

Chemical-looping technologies have obtained widespread recognition as power or hydrogen production units with inherent carbon capture in a future scenario where CO₂ capture and storage (CCS) is reality. In this paper three different techniques are described; chemical-looping combustion and two categories of chemical-looping reforming. The three techniques are all based on oxygen carriers that are circulating between an air- and a fuel reactor, providing the fuel with undiluted oxygen. Two different oxygen carriers; NiO/NiAl₂O₄ (40/60 wt/wt) and NiO/MgAl₂O₄ (60/40 wt/wt) are compared. Both continuous and pulse experiments were performed in a batch laboratory fluidized bed working at 950 °C using methane as fuel. It was found that pulse experiments offer advantages in comparison to continuous experiments, particularly when evaluating suitable particles for autothermal chemical-looping reforming. Firstly, smaller conversion ranges can be investigated in more detail, and secondly, the onset and extent of carbon formation can be determined more accurately. Of the two oxygen carriers, NiO/MgAl₂O₄ offers several advantages at elevated temperatures, i.e. higher methane conversion, higher selectivity to reforming and lesser tendency for carbon formation.     

Sol-gel synthesis of Co-W promoted NiAl2O4 spinel nanocatalyst used in combined reforming of methane: Influence of tungsten content on catalytic activity and stability

A stable process for green fuel production by means of a combined CO₂ reforming of methane and partial oxidation was evaluated during this research work. The goal was achieved by using Co-W promoters with NiO spinel on Al₂O₃ support. Sol-gel technique based on the privileged characterization such as homogeneous distribution was used for the synthesis of enhanced Co-W catalyst on NiAl₂O₄ spinel. Structural and morphological attributes of fabricated samples were investigated through XRD, FESEM, EDX, BET and FTIR techniques. Eventually, homogeneous distribution of particles, less than 50?nm, high specific surface area and homogeneous structure were observed. Catalysts stability and resistance to carbonaceous deposits in operated conditions are evaluated by TG-DTG technique. In this study, reduction of deposited coke after the reaction was achieved by raising the tungsten amount in catalyst. According to obtained results, CoNiAl₂O₄ promoter with 1?wt%?W (CoW1/NiAl₂O₄) nanocatalyst has obtained higher conversion along with high H₂ and CO yields. Also, CoW1/NiAl₂O₄ was considered as an optimum nanocatalyst for methane combined reforming by illustrating the tremendous stability at 750?°C and during the 48?h time on stream performance.     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion

In chemical-looping combustion (CLC) a gaseous fuel is burnt with inherent separation of the greenhouse gas carbon dioxide. The oxygen is transported from the combustion air to the fuel by means of metal oxide particles acting as oxygen carriers. A CLC system can be designed similar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return side. Thus, the system consists of a riser (fast fluidized bed) acting as the air reactor. This is connected to a cyclone, where the particles and the gas from the air reactor are separated. The particles fall down into a second fluidized bed, the fuel reactor, and are via a fluidized pot-seal transported back into the riser. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the reaction products, carbon dioxide and water, come out from the fuel reactor. The water can easily be condensed and removed, and the remaining carbon dioxide can be liquefied for subsequent sequestration.The gas leakage between the reactors must be minimized to prevent the carbon dioxide from being diluted with nitrogen, or to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the fuel reactor to the cyclone and to the pot-seal, (ii) from the cyclone down to the fuel reactor, (iii) from the pot-seal to the fuel reactor. These gas leakages were investigated in a scaled cold model. A typical leakage from the fuel reactor was 2%, i.e. a CO₂ capture efficiency of 98%. No leakage was detected from the cyclone to the fuel reactor. Thus, all product gas from the air reactor leaves the system from the cyclone. A typical leakage from the pot-seal into the fuel reactor was 6%, which corresponds to 0.3% of the total air added to the system, and would give a dilution of the CO₂ produced by approximately 6% air. However, this gas leakage can be avoided by using steam, instead of air, to fluidize the whole, or part of, the pot-seal. The disadvantages of diluting the CO₂ are likely to motivate the use of steam.