Chemical Looping Combustion Of Solid Fuels Research Articles

Thermodynamic analysis on the fate of ash elements in chemical looping combustion of solid fuels – Manganese-Based oxygen carriers

Chemical looping combustion (CLC) is an innovative technology suitable for converting waste-derived fuels into heat and power. The process inherently produces pure CO2, which is highly favorable for carbon capture and storage and could be instrumental for achieving negative emissions. CLC operates by utilizing solid oxygen carriers (OCs) to transfer heat and oxygen between two reactors. The OC play a crucial role in achieving an efficient combustion. Manganese-based OCs are particularly interesting, due to their ability to release gaseous oxygen. However, ash components from solid fuels could alter their oxygen transfer capacity, and cause problems related to corrosion and agglomeration. The objective of this work is to obtain in-depth insights about Mn-based OCs for CLC of waste-derived fuels. This is achieved by investigating phase transitions during CLC of solid fuels when utilizing two manganese-based OCs: manganese oxide and a representative manganese ore. For this purpose, thermodynamic modeling is employed, and a specific focus is given to K, Na, Cu, Zn, and Pb, due to their important role in corrosion and/or agglomeration. Thermodynamic databases are expanded by calculating properties from first-principles. It is shown that Mn-based OCs are suitable for effectively converting waste-derived fuels while limiting corrosion. Furthermore, the iron in manganese ores is found to have positive implications for oxygen-transfer reactions. In terms of alkali release to the gas phase, manganese ore seems to be a more promising material compared to manganese oxide. The pathways for the heavy metals Zn, Cu, and Pb were, meanwhile, independent of the OC type.

Enhanced cyclic redox reactivity of Fe2O3/Al2O3 by Sr doping for Chemical-Looping combustion of solid fuels

Aiming at the problems of low redox activity and easy sintering deactivation of iron-based oxygen carriers (OCs), Sr was used to modify Fe2O3/Al2O3 in chemical looping combustion (CLC) of solid fuels. The Sr doped Fe2O3/Al2O3 possessed high fuel combustion reactivity with different solid fuels (coal, coal char and biomass). During consecutive cycles, high cycling durability and morphological preservation were observed for Sr doped Fe2O3/Al2O3, signifying that the addition of strontium effectively enhanced the anti-sintering ability and inhibited the phase segregation of Fe2O3/Al2O3 during cyclic reactions. By comparing the coal char combustion reactivity of Fe2O3/Al2O3 samples doped with different SrO content (1 wt%, 5 wt% and 10 wt%), the optimum additive amount of SrO was 5 wt%. In addition, the characterization analysis proved that the Sr dopant suppressed the outward diffusion of Fe cations and maintained the surface micro-morphology of Fe2O3/Al2O3 in the process of CLC. This study presented important guiding principle and strategy for the synthesis of iron-based OCs with high cyclic redox reactivity in the CLC of solid fuels.

Chemical-Looping Combustion of Solid Fuels – Status and Recent Progress

Chemical-Looping Combustion of solid fuels has been studied for ten years and significant progress has been made. The paper discusses operational experiences and various aspects of up-scaling, including similarities to fluidized-bed combustion, key challenges, cost structure and strategies for reducing costs for demonstration. Based on more than 9000h of CLC operation in 34 pilots, of which >3000h with solid fuels, it is concluded that there are oxygen carrier materials suitable for solid fuels, and that the technology should be ready for scale-up.

CO2 Capture with Chemical Looping Combustion of Gaseous Fuels: An Overview

The world-wide consumption of natural gas (NG) and other fossil fuels (e.g., coal and crude oil) is ever increasing. However, most CO2 resulting from either NG combustion or other processes is released into the atmosphere without capture. Chemical looping combustion (CLC) is a two-step combustion technology for power and heat generation with inherent CO2 capture, using either gaseous fuels or solid and liquid fuels. A previous review focused on CLC of solid fuels or CLC of all types of fuels but did not give an in-depth and specific discussion of gaseous fuel CLC systems. China is one of the largest consumers of NG, coal, and crude oil in the world, and it is essential to develop an alternative technology to take the place of gaseous fuel (e.g., NG) combustion. This Review summarizes recent research and development work on CLC using gaseous fuels, including a technological and economic assessment, types of oxygen carriers (OCs), reactor types, coke formation and OCs poisoning, efficiency and exergy analys...

ChemInform Abstract: Chemical‐Looping Combustion and Gasification of Coals and Oxygen Carrier Development: A Brief Review

AbstractReview: 95 refs.

Chemical-Looping Combustion of Solid Fuels Using Manganese Ores as Oxygen Carriers

In chemical looping combustion (CLC), the choice of the oxygen carrier material is crucial with respect to overall system performance and cost. Materials based on manganese ores are promising candidates due to their favorable thermodynamic properties, high availability, and low price. As these ores tend to be comparably soft and prone to attrition, the challenge is to find materials which combine the above-mentioned advantages with sufficient mechanical durability. In this study, three manganese materials were screened for their suitability as oxygen carriers in the chemical looping process. The materials were subjected to continuous operation with fuel in a 10 kW chemical looping unit and evaluated in terms of gas conversion, carbon capture efficiency, and particle lifetime. All oxygen carriers showed good performance and reached more than 90% gas conversion at relevant conditions. Particle lifetime based on fines production was in the range of 99–284 h, which is a considerable improvement compared to a ...

Chemical-looping combustion of solid fuels in a 100 kW dual circulating fluidized bed system using iron ore as oxygen carrier

Chemical-looping combustion (CLC) is an innovative carbon-capture technology with potential to drastically reduce the cost of capture. By using a circulating bed material to transfer oxygen from the combustion air to the fuel, air and fuel are never mixed and the CO2 can be obtained as a separate flue gas stream, undiluted by N2. In other words, carbon capture is inherent to the CLC process. Here, we present findings from a 100kW chemical-looping combustor for solid fuels. The oxygen carrier used in the present tests was an iron ore from Tierga, Spain. In total, 26h of operation using bituminous coal and wood char as fuel was achieved. The highest gas conversion with bituminous coal was 87%, and the highest gas conversion using wood char as fuel was 93%. The carbon capture efficiency with bituminous coal was 94–98%, which is lower than what has been observed with ilmenite. For the wood char, the carbon capture was even lower. The fate of nitrogen and sulfur was investigated. It was found that 84wt% of the S-containing gas was SO2, and only 16wt% was H2S. The nitrogen analysis indicates that the fuel-N converted to gas was distributed as 10wt% NO, 37wt% NH3, and 53wt% N2. It was found that the oxygen-carrier particles retained their high reactivity throughout the operational period. Furthermore, the expected lifetime of the iron ore was found to be approximately 300h.

Chemical-Looping Combustion and Gasification of Coals and Oxygen Carrier Development: A Brief Review

Chemical-looping technology is one of the promising CO2 capture technologies. It generates a CO2 enriched flue gas, which will greatly benefit CO2 capture, utilization or sequestration. Both chemical-looping combustion (CLC) and chemical-looping gasification (CLG) have the potential to be used to generate power, chemicals, and liquid fuels. Chemical-looping is an oxygen transporting process using oxygen carriers. Recently, attention has focused on solid fuels such as coal. Coal chemical-looping reactions are more complicated than gaseous fuels due to coal properties (like mineral matter) and the complex reaction pathways involving solid fuels. The mineral matter/ash and sulfur in coal may affect the activity of oxygen carriers. Oxygen carriers are the key issue in chemical-looping processes. Thermogravimetric analysis (TGA) has been widely used for the development of oxygen carriers (e.g., oxide reactivity). Two proposed processes for the CLC of solid fuels are in-situ Gasification Chemical-Looping Combustion (iG-CLC) and Chemical-Looping with Oxygen Uncoupling (CLOU). The objectives of this review are to discuss various chemical-looping processes with coal, summarize TGA applications in oxygen carrier development, and outline the major challenges associated with coal chemical-looping in iG-CLC and CLOU.

A 1000 MWth boiler for chemical-looping combustion of solid fuels – Discussion of design and costs

More than 2000h of solid-fuel CLC operation in a number of smaller pilot units clearly indicate that the concept works. A scale-up of the technology to 1000MWth is investigated in terms of mass and heat balances, flows, solids inventories, boiler dimensions and the major differences between a full-scale Circulating Fluidized-Bed (CFB) boiler and a Chemical-Looping Combustion CFB (CLC–CFB). Furthermore, the additional cost of CLC–CFB relative to CFB technology is analysed and found to be 20€/tonne CO2. The largest cost is made up of compression of CO2, which is common to all capture technologies. Although the need for oxygen to manage incomplete conversion is estimated to be only a tenth of that of oxy-fuel combustion, oxygen production is nonetheless the second largest cost. Other significant costs include oxygen-carrier material, increased boiler cost and steam for fluidization of the fuel reactor.

ChemInform Abstract: Chemical‐Looping Combustion of Solid Fuels ‐ Status of Development

AbstractReview: 68 refs.

Synergistic effects of mixtures of iron ores and copper ores as oxygen carriers in chemical-looping combustion

Iron ore is a cheap and nontoxic oxygen carrier in chemical-looping combustion (CLC). However, it performs good recyclability while low reactivity in the CLC of solid fuels. Copper ore exhibits very high reactivity and oxygen uncoupling behavior, while suffers from tendency towards sintering and agglomeration during consecutive redox cycles at a higher temperature (e.g., 900–1000°C). In this work, mixtures of hematite and copper ore were used as oxygen carriers for CLC of syngas and coal. Through the isothermal redox experiments at 950°C in a thermogravimetric analyzer, it is found that there are synergistic effects between iron ore and copper ore, and copper ore could be more efficiently utilized when the mixing ratio of copper ore is maintained 10–20wt%. As the mixing ratio of copper ore is 20wt%, the reduction reaction of the mixer OC is no longer endothermic, which is beneficial to the controllability of the fuel-reactor temperature. The fluidized-bed experiments were carried out to verify the reactivity of the mixing ore OCs at 950°C. It is observed that the mixing OC with 20wt% copper ore has a better reactivity with the gasification products (especially H2) of low-volatile anthracite than the pure hematite, and leads to a higher fuel conversion rate and CO2 yield. The mixtures of iron ore and copper ore are expected to address simultaneously reactivity, recyclability, cost and environmental concerns of oxygen carriers.

Chemical-looping Combustion of Solid Fuels – Technology Overview and Recent Operational Results in 100 kW Unit

Chemical-looping combustion is a new combustion principle that uses metal oxides for oxygen transfer from air to fuel. Fuel is never mixed with combustion air and the combustion products, CO2 and steam, are recovered in a separate flow without gas separation. Thus, CO2 capture is inherent in the process and costs and energy penalties for gas separation are avoided. The paper includes:•an overview of results from a 100-kW chemical-looping combustor for solid fuel.•a discussion of technology scale-up and costs based on comparison to conventional circulating fluidized-bed boilers.The added cost for a CLC power plant, compared with a conventional CFB plant, should be in the range 12-27 €/tonne of CO2 avoided, and likely around 19-20 €/tonne.

Chemical-looping combustion of solid fuels – Status of development

Chemical-looping combustion (CLC) of solid fuels is a technology with the potential of reducing the costs and energy penalty dramatically for CO2 capture. The potential for low costs is based on the similarity to coal combustion in fluidized beds. However, this assumes reaching high performance with respect to fuel and gas conversion, or that inadequate performance can be readily mitigated by downstream options. There are uncertainties with respect to the performance that can be reached in large-scale units, as well as with the extra costs needed to compensate for inadequate performance. Performance will be dependent on both reactor design and oxygen carrier properties. The status of chemical-looping combustion of solid fuels is discussed with respect to performance and experiences from pilot operation.

Chemical-looping combustion of solid fuels in a 10 kW reactor system using natural minerals as oxygen carrier

Chemical-looping combustion (CLC) is an unmixed combustion concept where fuel and combustion air are kept separate by means of an oxygen carrier, and the CO2 capture is inherently achieved. This work presents findings from a continuously operated 10kW pilot for solid fuels. Using petcoke as fuel, the following oxygen carriers are compared: (a) ilmenite, (b) ilmenite + lime, (c) manganese ore, and (d) manganese ore + lime. Compared to ilmenite, the use of manganese ore as oxygen carrier greatly enhanced the rate of gasification. By adding lime particles to the Mn ore, performance improved further. The addition of lime to ilmenite had a small beneficial effect on gas conversion and char gasification rate.

Interaction between Fe-based oxygen carriers and n-heptane during chemical looping combustion

Chemical looping combustion (CLC) uses a metal oxide (the oxygen carrier) to provide oxygen for the combustion of a fuel and gives an inherent separation of pure CO2 with minimal energy penalty. In solid-fuel CLC, volatile matter will interact with oxygen carriers. Here, the interaction between iron-based oxygen carriers and a volatile hydrocarbon (n-heptane) was investigated in both a laboratory-scale fluidised bed and a thermogravimetric analyser (TGA). Experiments were undertaken to characterise the thermal decomposition of the n-heptane occurring in the presence and in the absence of the oxygen carrier. In a bed of inert particles, carbon deposition increased with temperature and acetylene appeared as a possible precursor. For a bed of carrier consisting of pure Fe2O3, carbon deposition occurred once the Fe2O3 was fully reduced to Fe. When the Fe2O3 was doped with 10mol% Al2O3 (Fe90Al), deposition started when the carrier was reduced to a mixture of Fe and FeAl2O4, the latter being very unreactive. Furthermore, when pure Fe2O3 was fully reduced to Fe, agglomeration of the fluidised bed occurred. However, Fe90Al did not give agglomeration even after extended reduction. The results suggest that Fe90Al is promising for the CLC of solid fuels.

Rate Analysis of Chemical-Looping with Oxygen Uncoupling (CLOU) for Solid Fuels

Chemical-looping with oxygen uncoupling (CLOU) offers a promise to reduce energy penalty by facilitating the capture of CO2 emitted from power plants. It has a potential to lower the oxygen carrier inventory of the fuel reactor in contrast to chemical-looping combustion (CLC). The primary mechanism in CLOU for the combustion of solid fuels is their reaction with gaseous oxygen released by the decomposition of a metal oxide, which differs from CLC of solid fuels where the solid fuel has to be gasified first. The slower gasification reaction in CLC is subsequently followed by combustion of the fuel with a circulating oxygen carrier. The present study is concerned with the rate analysis from reported batch fluidized bed CLOU experimental data of Mexican petcoke particles by a CuO/ZrO2 oxygen carrier. The methodology to determine the kinetic parameters for CuO decomposition and solid fuel oxidation during the fuel reactor stage and for Cu2O oxidation in the air reactor stage have been discussed. The results of the study are expected to help in the development of a process model for CLOU, furthering the development of a pilot scale process.

Chemical-looping combustion of solid fuels – Operation in a 10 kW unit with two fuels, above-bed and in-bed fuel feed and two oxygen carriers, manganese ore and ilmenite

Chemical-looping combustion (CLC) is a combustion concept with inherent separation of CO2. The process uses a solid oxygen carrier, which consists of metal oxide, to transfer the oxygen from air to fuel. The chemical-looping combustor used in the present experiments features two interconnected fluidized beds; a fuel reactor (FR) and an air reactor (AR). In the FR, fuel is gasified with steam whereupon gasification products react with the oxygen carrier to form, ideally, CO2 and H2O. This study concerns CLC of solid fuels in a continuously operating 10kW unit using two natural ores as oxygen carrier: (a) ilmenite, an iron–titanium mineral and (b) a manganese ore containing smaller amounts of Fe, Al and Si. The fuel feed was re-designed in order to increase contact between oxygen carrier and fuel. The new in-bed fuel feed was found to significantly improve gas conversion, mainly caused by increased contact between the oxygen carrier and volatile gases released in the fuel chute. Two fuels were used to evaluate the effect of fuel feed; a bituminous coal and a pet coke. The in-bed fuel feed was used when ilmenite and manganese ore were compared. The use of a manganese ore as oxygen carrier was shown to significantly enhance the rate of char gasification and also improve gas conversion. A concern with the manganese ore is the large production of fines.

Use of an Fe-Based Residue from Alumina Production as an Oxygen Carrier in Chemical-Looping Combustion

Chemical-looping combustion (CLC) has been emerging in recent years as a promising CO2 capture technology. In CLC, the oxygen needed in the combustion is supplied by an oxygen carrier (OC), normally a metal oxide, which circulates between fuel and air reactors. In the fuel reactor, the fuel is oxidized to CO2 and H2O, which facilitates CO2 capture once the water has been condensed. The reduced OC is then transported to the air reactor, where it is reoxidized before the start of a new cycle. Gaseous fuel combustion has been extensively studied to date, and now attention has been directed to solid fuels, such as coal. In this case, the OC should be inexpensive, as some losses are expected with the ashes. A bauxite waste from alumina production, containing Fe2O3, could fulfill the requirements for CLC of solid fuels. In this work, the reactivity of this waste material to different gases present in coal combustion was studied. Thermogravimetric analysis (TGA) together with experiments in a batch fluidized-bed reactor (FB) were performed in the 1100–1223 K temperature range using H2, CO, H2/CO mixtures, and CH4 as reducing gases. Similar to other natural minerals or waste materials, a gain in the bauxite residue reactivity with the number of cycles in TGA was observed. TGA experiments at 1173 K showed that the bauxite waste samples reacted more rapidly with H2 than with CO and CH4. The reactivity of this material was found to be comparable to those of other Fe-based synthetic materials and even higher than those of some minerals, such as ilmenite. In batch FB experiments, H2 and CO were completely converted. CH4 was partially converted at all temperatures and reducing times tested. With regard to the fluidization behavior of this waste, the material presented moderate attrition rates and no defluidization problems were observed during operation in the fluidized bed after >40 h. Therefore, this material has suitable properties to be considered as an OC for CLC with solid fuels.

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 125–180μm. 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 50vol.% CO in H2) and oxidation conditions (10vol.% 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.One ilmenite mineral was used to investigate the influence of the catalyzed and non-catalyzed water gas shift reaction (WGS) on the fuel gas conversion at 950°C. It was shown, that the WGS has an influence on the final gas composition. Experiments with only H2 or CO separately as fuel diluted with N2 have been performed to eliminate the effect of the WGS. Experiments with syngas and therefore activated WGS allow determining its influence on gas composition. To quantify the reaction progress, the law of mass action for the gas consumption is compared to the equilibrium constant at the corresponding temperature. The results from these experiments agree with the results from catalyzed WGS experiments. Therefore, CaO in the size fraction 180–250μm was added to the oxygen carrier since it is a well-known WGS catalyst. As a result the overall conversion of hydrogen and carbon monoxide were improved.

Chemical-looping combustion of solid fuels in a 10 kWth pilot–batch tests with five fuels

Chemical-looping combustion (CLC) is a combustion concept with inherent separation of CO2. The process uses a solid oxygen carrier, which consists of metal oxide, to transfer the oxygen from air to fuel. This paper presents findings from batch tests performed in a 10 kWth CLC pilot for solid fuels. The pilot, which is the world’s first chemical-looping combustor for solid fuels, is based on interconnected fluidized-bed technology and features a fuel reactor (FR) and an air reactor (AR) as the principal reaction chambers. In the FR, fuel is gasified with steam whereupon gasification products react with the oxygen carrier to form, ideally, CO2, H2O and SO2. Oxygen-carrier particles exit the FR through a weir and are led to the AR, where they are regenerated to their oxidized state. The pilot has been operated using a natural iron-titanium ore called ilmenite as oxygen carrier. Previous continuous tests have demonstrated a need for batch tests in order to obtain complementary information on system performance. In this study, five fuels were fed to the fuel reactor in batches of 20-25 g at four temperatures; 940°C, 970°C, 1000°C and 1030°C. By using devolatilized fuel, it was possible to determine (a) oxygen demand associated with syngas from char gasification as well as (b) kinetics of char conversion to gas. Rates of char conversion were found to be temperature dependent, as expected, whereas no temperature dependence was found for the oxygen demand. Activation energies for conversion of char to gas were calculated using Arrhenius plots. The minimum oxygen demand for char was found to be around 5%.