Behavior of ilmenite as oxygen carrier in chemical-looping combustion

For a future scenery where will exist limitation for CO₂ emissions, chemical-looping combustion (CLC) has been identified as a promising technology to reduce the cost related to CO₂ capture from power plants. In CLC a solid oxygen-carrier transfers oxygen from the air to the fuel in a cyclic manner, avoiding direct contact between them. CO₂ is inherently obtained in a separate stream. For this process the oxygen-carrier circulates between two interconnected fluidized-bed reactors. To adapt CLC for solid fuels the oxygen-carrier reacts with the gas proceeding from the solid fuel gasification, which is carried out right in the fuel-reactor. Ilmenite, a natural mineral composed of FeTiO₃, is a low cost and promising material for its use on a large scale in CLC.The aim of this study is to analyze the behavior of ilmenite as oxygen-carrier in CLC. Particular attention was put on the variation of chemical and physical characteristics of ilmenite particles during consecutive redox cycles in a batch fluidized-bed reactor using CH₄, H₂ and CO as reducing gases. Reaction with H₂ was faster than with CO, and near full H₂ conversion was obtained in the fluidized-bed. Lower reactivity was found for CH₄. Ilmenite increased its reactivity with the number of cycles, especially for CH₄. The structural changes of ilmenite, as well as the variations in its behavior with a high number of cycles were also evaluated with a 100 cycle test using a CO + H₂ syngas mixture. Tests with different H₂:CO ratios were also made in order to see the reciprocal influence of both reducing gases and it turned out that the reaction rate is the sum of the individual reaction rates of H₂ and CO. The oxidation reaction of ilmenite was also investigated. An activation process for the oxidation reaction was observed and two steps for the reaction development were differenced. The oxidation reaction was fast and complete oxidation could be reached after every cycle. Low attrition values were found and no defluidization was observed during fluidized-bed operation. During activation process, the porosity of particles increased from low porosity values up to values of 27.5%. The appearance of an external shell in the particle was observed, which is Fe enriched. The segregation of Fe from TiO₂ causes that the oxygen transport capacity, R_OC, decreases from the initial R_OC = 4.0% to 2.1% after 100 redox cycles.     

Design and operation of a 10 kWth chemical-looping combustor for solid fuels - Testing with South African coal

This paper presents the results obtained for the operation of a 10 kW_th chemical-looping combustor using a South African coal as the solid fuel and an oxygen carrier of ilmenite, a natural iron titanium oxide. A chemical-looping combustor for solid fuels was designed and built. It consists of two interconnected fluidized beds, an air reactor where the oxygen carrier is oxidized and a fuel reactor where the coal is gasified by steam and the syn-gases react with the oxygen carrier. A constant coal flow corresponding to a thermal power of 3.3 kW was introduced into the fuel reactor. The tests were conducted at temperatures above 850 °C and for a total test duration of 22 h. The particle integrity of ilmenite and the particle circulation between the two reactors were investigated and verified. The effects of particle circulation on coal conversion, gas conversion of the fuel reactor and carbon separation or CO₂ capture between the air and fuel reactors were investigated. The actual CO₂ capture ranged between 82.5% and 96% while the gas conversion from the fuel reactor was in the range 78-81%, based on measurements of unconverted CO and CH₄.     

Solid fuels in chemical-looping combustion using a NiO-based oxygen carrier

The feasibility of using three different solid fuels in chemical-looping combustion (CLC) has been investigated using NiO as oxygen carrier. A laboratory fluidized-bed reactor system for solid fuel was used, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions. In each reducing phase 0.2 g of fuel was added to the reactor containing 20 g oxygen carrier. The experiments were performed at 970 °C. Compared to previously published results with other oxygen carriers the reactivity of the used Ni-particles was considerably lower for the high-sulphur fuel and higher for the low-sulphur fuel. Much more unconverted CO was released and the fuel conversion was much slower for high-sulphur fuel such as petroleum coke, suggesting that the nickel-based oxygen carrier was deactivated by the presence of sulphur. The NiO particles also showed good reactivity with methane and a syngas mixture of 50% H₂ and 50% CO. For all experiments the oxygen carrier showed good fluidizing properties without any signs of agglomeration.     

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

The feasibility of using ilmenite as oxygen carrier in chemical-looping combustion has been investigated. It was found that ilmenite is an attractive and inexpensive oxygen carrier for chemical-looping combustion. A laboratory fluidized-bed reactor system, simulating chemical-looping combustion by exposing the sample to alternating reducing and oxidizing conditions, was used to investigate the reactivity. During the reducing phase, 15 g of ilmenite with a particle size of 125–180 μm was exposed to a flow of 450 mL_n/min of either methane or syngas (50% CO, 50% H₂) and during the oxidizing phase to a flow of 1000 mL_n/min of 5% O₂ in nitrogen. The ilmenite particles showed no decrease in reactivity in the laboratory experiments after 37 cycles of oxidation and reduction. Equilibrium calculations indicate that the reduced ilmenite is in the form FeTiO₃ and the oxidized carrier is in the form Fe₂TiO₅ + TiO₂. The theoretical oxygen transfer capacity between these oxidation states is 5%. The same oxygen transfer capacity was obtained in the laboratory experiments with syngas. Equilibrium calculations indicate that ilmenite should be able to give high conversion of the gases with the equilibrium ratios CO/(CO₂ + CO) and H₂/(H₂O + H₂) of 0.0006 and 0.0004, respectively. Laboratory experiments suggest a similar ratio for CO. The equilibrium calculations give a reaction enthalpy of the overall oxidation that is 11% higher than for the oxidation of methane per kmol of oxygen. Thus, the reduction from Fe₂TiO₅ + TiO₂ to FeTiO₃ with methane is endothermic, but less endothermic compared to NiO/Ni and Fe₂O₃/Fe₃O₄, and almost similar to Mn₃O₄/MnO.     

Long-term integrity testing of spray-dried particles in a 10-kW chemical-looping combustor using natural gas as fuel

Chemical-looping combustion, CLC, is a combustion concept with inherent separation of CO₂. The fuel and combustion air are kept apart by using an oxygen carrier consisting of metal oxide. The oxygen carriers used in this study were prepared from commercially available raw materials by spray-drying. The aim of the study was to subject the particles to long-term operation (>1000 h) with fuel and study changes in particles, with respect to reactivity and physical characteristics. The experiments were carried out in a 10-kW chemical-looping combustor operating with natural gas as fuel. 1016 h of fuel operation were achieved. The first 405 h were accomplished using a single batch of NiO/NiAl₂O₄-particles. The last 611 h were achieved using a 50/50_mass-mixture of (i) particles used for 405 h, and (ii) a second batch of particles similar in composition to the first batch, but with an MgO additive. Thus, at the conclusion of the test series, approximately half of the particles in the reactor system had been subjected to >1000 h of chemical-looping combustion. The reason for mixing the two batches was to improve the fuel conversion. Fuel conversion was better with the mixture of the two oxygen carriers than it was using only the batch of NiO/NiAl₂O₄-particles. The CO fraction was slightly above the equilibrium fraction at all temperatures. Using the oxygen carrier mixture, the methane fraction was typically 0.4-1% and the combustion efficiency was around 98%. The loss of fines decreased slowly throughout the test period, although the largest decrease was seen during the first 100 h. An estimated particle lifetime of 33 000 h was calculated from the loss of fines. No decrease in reactivity was seen during the test period.     

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.     

Interaction of mineral matter of coal with oxygen carriers in chemical-looping combustion (CLC)

The chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) processes are novel solutions for efficient combustion with direct separation of carbon dioxide. These processes use a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor, where the fuel reacts with the solid oxygen carrier. When utilizing coal in CLC, the oxygen carrier particles could be affected through interaction with the ash-forming mineral matter found in coal, causing deactivation and/or agglomeration. In this work, possible interactions between minerals commonly encountered in coal and several promising oxygen carriers that are currently under investigation for their use in CLC are studied by both experiment and thermodynamic equilibrium calculations. Possible interaction was studied for both highly reducing and oxidizing conditions at 900 °C. Under highly reducing conditions pyrite was found to have by far the most deteriorating effect on the oxygen carrier particles, as the sulfur in the pyrite reacted with the oxygen carrier to form sulfides. Quartz and clay minerals were found to have a rather low influence on the oxygen carriers. Out of the oxygen carriers investigated, CuO/MgAl₂O₄ and the Mn₃O₄/ZrO₂ oxygen carriers tended to be quite reactive towards mineral matter whereas ilmenite has been shown to be the most robust oxygen carrier. Although sulfur can clearly deactivate Ni, Cu and Mn based oxygen carriers under sub-stoichiometric conditions, when the fuel is converted fully to CO₂ and H₂O, sulfides are only expected for Ni-based oxygen carriers.     

Kinetics of redox reactions of ilmenite for chemical-looping combustion

The objective of this study was to establish the kinetic of both reduction and oxidation reactions taking place in the chemical-looping combustion (CLC) process using ilmenite as an oxygen carrier. Because of the benefits of using of pre-oxidized ilmenite and the activation of the ilmenite during the redox cycles, the reactivity of both the pre-oxidized and activated ilmenite was analyzed. The experimental tests were carried out in a thermogravimetric analyzer (TGA), using H₂, CO or CH₄ as reducing gases, and O₂ for the oxidation step. Thus, the reactivity with the main reacting gases was analyzed when natural gas, syngas or coal are used as fuels in a CLC system. The changing grain size model (CGSM) was used to predict the evolution with time of the solid conversion and to determine the kinetic parameters. In most cases, the reaction was controlled by chemical reaction in the grain boundary. In addition, to predict the behaviour of the oxidation during the first redox cycle of pre-oxidized ilmenite, a mixed resistance between chemical reaction and diffusion in the solid product was needed. The kinetic parameters of both reduction and oxidation reactions of the pre-oxidized and activated ilmenite were established. The reaction order for the main part of the reduction reactions of pre-oxidized and activated ilmenite with H₂, CO, CH₄ and O₂ was n=1, being different (n=0.8) for the reaction of activated ilmenite with CO. Activation energies from 109 to 165 kJ mol⁻¹ for pre-oxidized ilmenite and from 65 to 135 kJ mol⁻¹ for activated ilmenite were found for the different reactions with H₂, CO and CH₄. For the oxidation reaction activation energies found were lower, 11 kJ mol⁻¹ for pre-oxidized and 25 kJ mol⁻¹ for activated ilmenite.Finally, simplified models of the fuel and air reactors were used to do an assessment of the use of ilmenite as an oxygen carrier in a CLC system. The reactor models use the reaction model in the particle and the kinetic parameters obtained in this work. Taking into account for its oxygen transport capacity, the moderated solids inventory and the low cost of the material, ilmenite presents a competitive performance against synthetic oxygen carriers when coal or syngas are used as fuel.     

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).     

Chemical-looping with oxygen uncoupling using CuO/ZrO2 with petroleum coke

Oxygen carrier particles of CuO/ZrO₂ were reacted with petroleum coke using chemical-looping with oxygen uncoupling (CLOU). The fuel was burnt in gas-phase oxygen released from the oxygen carrier particles during the fuel oxidation. The particles were then regenerated in 5-21% oxygen. In this process, the carbon dioxide from the combustion is inherently separated from the rest of the flue gases without the need for an energy intensive air separation unit. Copper oxide has thermodynamic characteristics that make it suitable as an oxygen carrier in CLOU. Particles were prepared by freeze granulation and were exposed cyclically with petroleum coke and oxygen in a laboratory fluidized bed reactor of quartz. The reaction temperature and oxygen concentration during the oxidation were varied. The average conversion rate of petroleum coke was a function of temperature and varied between 0.5%/s and 5%/s in the set-point temperature interval 885-985 °C. The conversion rate is considerably higher than rates obtained with the same fuel using iron-based oxygen-carrier in chemical-looping combustion. As for the regeneration with oxygen, the reduced particles reacted at low oxygen concentrations, with a considerable part of the reaction occurring near the thermodynamic equilibrium.     

Batch testing of solid fuels with ilmenite in a 10 kWth chemical-looping combustor

Batch experiments were conducted in a 10 kW_th chemical-looping combustor for solid fuels using ilmenite, an iron titanium oxide, as the oxygen carrier with two solid fuels: a petroleum coke from Mexico and a bituminous coal from South Africa. The purpose of these batch tests was to attain detailed information on fuel conversion, complementary to previous continuous operation of the unit. At steady-state, a fuel batch of typically 25 g was introduced in the fuel reactor and gas concentrations were measured at the outlet of both air and fuel reactors. The fuel reactor was fluidized with steam and the amount of bed material was typically 5 kg. The fuel introduced devolatilizes rapidly while the remaining char is gasified and the resulting syngases H₂ and CO react with the oxygen carrier. Operation involved testing at different fuel reactor temperatures from 950 to 1030 °C, and investigation of the influence of particle circulation between air and fuel reactors.The fuel conversion rate was increased at higher temperature: at 950 °C the instantaneous rate of conversion for petroleum coke averaged at 17.4%/min while at 1030 °C, the value was 40%/min. For the much more reactive South African coal, the averaged rate at 970 °C was 47%/min and increased to 101%/min at 1000 °C. For petroleum coke testing with particle circulation, the oxygen demand - defined as oxygen lacking to fully convert the gases leaving the fuel reactor - was typically 12-14% for the gasified char including H₂S, in line with previous experiments with the same unit and fuel. If only syngases are considered, the oxygen demand for char conversion was 8.4-11%. Similar or even lower values were seen for the char of South African coal. This is in line with expectations, i.e. that it is possible to reach fairly high conversion, although difficult to reach complete gas conversion with solid fuel. It was also seen that the volatiles pass through the system essentially unconverted, an effect of feeding the fuel from above. Moreover, the oxygen demand for char conversion decreased with increasing temperature. Finally, the CO₂ capture - defined as the proportion of gaseous carbon leaving the fuel reactor to total gaseous carbon leaving the system - decreased at higher particle circulation and a correlation between capture and circulation index was obtained.     

The use of petroleum coke as fuel in chemical-looping combustion

Chemical-looping combustion is a novel technique used for CO₂ separation that previously has been demonstrated for gaseous fuel. This work demonstrates the feasibility of using solid fuel (petroleum coke) in chemical-looping combustion (CLC). Here, the reaction between the oxygen carrier and solid fuel occurs via the gasification intermediates, primarily CO and H₂. A laboratory fluidized-bed reactor system for solid fuel, simulating a CLC-system by exposing oxygen-carrying particles to alternating reducing and oxidizing conditions, has been developed. In each reducing period, 0.2 g of petroleum coke was added to 20 g of oxygen carrier composed of 60% active material of Fe₂O₃ and 40% inert MgAl₂O₄. The effect of steam and SO₂ concentration in the fluidizing gas was investigated as well as effect of temperature. The rate of reaction was found to be highly dependent on the steam and SO₂ concentration as well as the temperature. Also shown was that the presence of a metal oxide enhances the gasification of petroleum coke. A preliminary estimation of the oxygen carrier inventory needed in a real CLC system showed that it would be below 2000 kg/MW_th.     

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.     

A 300 W laboratory reactor system for chemical-looping combustion with particle circulation

Chemical-looping combustion (CLC) is a method to burn gaseous fuels with inherent separation of carbon dioxide. A continuously operated laboratory reactor system for chemical-looping combustion with two interconnected fluidized beds was designed and built. This chemical-looping combustor was designed to operate with a fuel flow corresponding to 100–300 W. The CLC system was operated successfully using a highly reactive nickel-based oxygen-carrier. Furthermore, tests were carried out to determine the degree of gas leakage between the reactors. Although there was some leakage between the fuel and air reactors, it is low enough to enable evaluation of the combustion results. The combustion tests showed a high conversion of the natural gas to carbon dioxide, indicating that the particles are suitable for chemical-looping combustion. No methane was detected in the gas from the fuel reactor, and the fraction of carbon monoxide was in the range 0.5–3%.     

Fe2O3 on Ce‐, Ca‐, or Mg‐stabilized ZrO2 as oxygen carrier for chemical‐looping combustion using NiO as additive

Oxygen‐carrier particles for chemical‐looping combustion have been manufactured by freeze granulation. The particles consisted of 60 wt % Fe₂O₃ as active phase and 40 wt % stabilized ZrO₂ as support material. Ce, Ca, or Mg was used to stabilize the ZrO₂. The hardness and porosity of the particles were altered by varying the sintering temperature. The oxygen carriers were examined by redox experiments in a batch fluidized‐bed reactor at 800–950°C, using CH₄ as fuel. The experiments showed good reactivity between the particles and CH₄. NiO was used as an additive and was found to reduce the fraction of unconverted CH₄ with up to 80%. The combustion efficiency was 95.9% at best and was achieved using 57 kg oxygen carrier per MW fuel. Most produced oxygen carriers appear to have been decently stable, but using Ca as stabilizer resulting in uneven results. Further, particles sintered at high temperatures had a tendency to defluidize. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

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.     

CFD simulation of a chemical-looping fuel reactor utilizing solid fuel

A computational fluid dynamic (CFD) study has been carried out for the fuel reactor for a new type of combustion technology called chemical-looping combustion (CLC). CLC involves combustion of fuels by heterogeneous chemical reactions with an oxygen carrier, usually a granular metal oxide, exchanged between two reactors. There have been extensive experimental studies on CLC, however CFD simulations of this concept are quite limited. In the present paper we have developed a CFD model for the fuel reactor of a chemical-looping combustor described in the literature, which utilized a Fe-based carrier (ilmenite) and coal. An Eulerian multiphase continuum model was used to describe both the gas and solid phases, with detailed sub-models to account for fluid–particle and particle–particle interaction forces. Global reaction models of fuel and carrier chemistry were utilized. The transient results obtained from the simulations were compared with detailed experimental time-varying outlet species concentrations (Leion et al., 2008) and provided a reasonable match with the reported experimental data.     

Comparison of natural ilmenites as oxygen carriers in chemical-looping combustion and influence of water gas shift reaction on gas composition

Chemical-looping combustion (CLC) is a promising technology for CO₂-capture for storage or reuse as a method to mitigate CO₂ emissions from the use of fossil fuels. In a CLC system the oxygen carrier is of great importance. Environmentally sound and low cost materials seem to be preferable especially for CLC of solid fuels. The natural occurring ore ilmenite has already been the target of different studies in order to work out its feasibility as oxygen carrier for different fuels. The initial part of this work is a screening of five commercial available ilmenite minerals as oxygen carrier, crushed and sieved to 125–180 μm. The screening includes an examination of the sulfur released during the first heat up and the activation of the oxygen carrier, indicated by the fuel conversion using alternating reduction (syngas 50 vol.% CO in H₂) and oxidation conditions (10 vol.% O₂ in N₂). The five first cycles were carried out at 850 °C to avoid initial agglomeration whereas the main activation cycles have been performed at 950 °C in a tubular quartz reactor under fluidized bed conditions. From these experiments it is concluded that rock ilmenites are preferable as oxygen carriers since they revealed an improved fuel conversion, although offering a higher sulfur content, which is released during the initial heat up.     

Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers

Chemical-looping combustion (CLC) is a novel technology that can be used to meet demands on energy production without CO₂ emissions. The CLC-process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N₂ from the air with CO₂ from the combustion. The combustion gases consist almost entirely of CO₂ and H₂O. Therefore, the technique reduces the energy penalty that normally arises from the separation of CO₂ from other flue gases, hence, CLC may make capture of CO₂ cheaper.Iron ore and oxide scale from steel production were tested as oxygen carriers in CLC batch experiments with solid fuels. Petroleum coke, charcoal, lignite and two bituminous coals were used as fuels.The experiments were carried out in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction phases. The exhaust gases were led to an analyzer where the contents of CO₂, CO, CH₄ and O₂ were measured. Gas samples collected in bags were used to analyze the content of hydrogen in a gas chromatograph.The results showed that both the iron ore and the oxide scale worked well as oxygen carrier and both oxygen carriers increased their reactivity with time.     

Co-firing of sugar cane bagasse with rice husk in a conical fluidized-bed combustor

This paper presents experimental results on co-firing of ‘as-received’ sugar cane bagasse and rice husk in a conical fluidized-bed combustor (FBC) using silica sand as the bed material. Axial temperature, O₂, CO₂, CO and NO concentration profiles in the conical FBC operated at 82.5-82.8 kg/h fuel feed rate and various values of excess air (of about 40, 60, 80 and 100%) for different rice husk energy fractions (of 0.60, 0.85 and 1.0) are discussed. The bed temperature, CO and NO emissions from the combustor, as well as the heat losses and combustion efficiency, are also provided for the above operating conditions. The axial temperature profiles in the conical FBC were almost independent of excess air but noticeably affected by the rice husk energy fraction. The CO emissions were found to reduce for higher values of excess air and rice husk energy fractions. Meanwhile, the NO concentrations at all the points over the combustor volume and, accordingly, NO emissions from the reactor increased with higher excess air and energy contributions by rice husk. The co-firing of these fuels in the conical FBC at the rice husk energy fractions greater than 0.6 resulted in the sustainable combustion, with 95-96% combustion efficiency, and lower NO emissions compared with those for firing pure rice husk. Through co-firing with rice husk, an effective use of ‘as-received’ sugar cane bagasse becomes feasible for energy conversion in the fluidized-bed combustion systems.