Bio-derived Fuels Research Articles

Performance analysis and emission characteristics of VCR diesel engine using palm stearin oil biodiesel as fuel

Abstract The increasing depletion of fossil fuels has prompted international efforts to search for diesel substitutes. One way to considerably reduce fuel shortages would be to use bio-derived fuels as replacements. Transesterification of edible or nonedible vegetable oils yields biodiesel, which is one example of a bio-origin fuel and a workable solution. Biodiesel’s characteristics are often more advantageous than those of diesel. One well-known example of a nonedible vegetable oil is palm stearin. In this work, we investigate the potential use of transesterified palm stearin methyl esters (PSME) as a substitute fuel. A rated compression ratio (CR) of 17.5 and 210 bar of fuel injection pressure were employed in the experimental investigations, which were carried out at a constant speed of 1500 rpm. Our main goals were to reduce emissions and improve engine performance. The evaluation parameters encompassed brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and emission indices. It was observed that using PSME led to better performance and fewer emissions when compared to using conventional diesel, especially when the CR was 17.5. Increased BTE, decreased BSFC, lower air–fuel ratios, and decreased volumetric efficiencies were all correlated with higher CR. As a byproduct of palm oil, PSME shows promise as a seasonally and geographically specific feedstock for biodiesel production, providing benefits over conventional diesel fuels in terms of performance and the environment.

The Effects of the Use of Algae and Jatropha Biofuels on Aircraft Engine Exhaust Emissions in Cruise Phase

Due to environmental pressure and the prevailing political and economic situation in the world, alternatives to traditional fossil fuels are being sought. The use of bio-derived fuels may reduce the emission of pollutants present in jet engine exhausts. The presented research investigates the possibility of replacing the conventional fuel, which is kerosene, with plant-derived fuels from marine algae and jatropha. During the analysis, based on the available data, the emission indices of pollutants were computed, and then, for the adopted aircraft and route, the emissions for kerosene and alternative fuels were determined. A significant reduction in the emission of most analyzed compounds (even by 40% for CO) was achieved compared to the emission for kerosene. The obtained results are discussed in the conclusion section.

Symmetric Ethers as Bioderived Fuels: Reactivity with OH Radicals

Research reported in this publication was funded by the Office of Sponsored Research at King Abdullah University of Science and Technology (KAUST)

A Semi‐Batch Flow System for the Production of 5‐Chloromethylfurfural

AbstractThe bioplatform molecule 5‐chloromethylfurfural (CMF) has potential as an intermediate or precursor in the preparation of useful products, such as bioderived fuels, chemicals and polymers. The replacement of the halogenated solvents and the use of a semi‐batch system have been investigated in the synthesis to reduce the human and environmental safety concerns associated with current CMF production methods. The new method gives a high yield of CMF from fructose without chlorinated solvents, and can be extended to the synthesis of CMF from other feedstocks such as sugars, cellulose, seaweed and sawdust.

Comparison of methodologies used to determine aromatic lignin unit ratios in lignocellulosic biomass

BackgroundMultiple analytical methods have been developed to determine the ratios of aromatic lignin units, particularly the syringyl/guaiacyl (S/G) ratio, of lignin biopolymers in plant cell walls. Chemical degradation methods such as thioacidolysis produce aromatic lignin units that are released from certain linkages and may induce chemical changes rendering it difficult to distinguish and determine the source of specific aromatic lignin units released, as is the case with nitrobenzene oxidation methodology. NMR methods provide powerful tools used to analyze cell walls for lignin composition and linkage information. Pyrolysis-mass spectrometry methods are also widely used, particularly as high-throughput methodologies. However, the different techniques used to analyze aromatic lignin unit ratios frequently yield different results within and across particular studies, making it difficult to interpret and compare results. This also makes it difficult to obtain meaningful insights relating these measurements to other characteristics of plant cell walls that may impact biomass sustainability and conversion metrics for the production of bio-derived fuels and chemicals.ResultsThe authors compared the S/G lignin unit ratios obtained from thioacidolysis, pyrolysis-molecular beam mass spectrometry (py-MBMS), HSQC liquid-state NMR and solid-state (ss) NMR methodologies of pine, several genotypes of poplar, and corn stover biomass. An underutilized approach to deconvolute ssNMR spectra was implemented to derive S/G ratios. The S/G ratios obtained for the samples did not agree across the different methods, but trends were similar with the most agreement among the py-MBMS, HSQC NMR and deconvoluted ssNMR methods. The relationship between S/G, thioacidolysis yields, and linkage analysis determined by HSQC is also addressed.ConclusionsThis work demonstrates that different methods using chemical, thermal, and non-destructive NMR techniques to determine native lignin S/G ratios in plant cell walls may yield different results depending on species and linkage abundances. Spectral deconvolution can be applied to many hardwoods with lignin dominated by S and G units, but the results may not be reliable for some woody and grassy species of more diverse lignin composition. HSQC may be a better method for analyzing lignin in those species given the wealth of information provided on additional aromatic moieties and bond linkages. Additionally, trends or correlations in lignin characteristics such as S/G ratios and lignin linkages within the same species such as poplar may not necessarily exhibit the same trends or correlations made across different biomass types. Careful consideration is required when choosing a method to measure S/G ratios and the benefits and shortcomings of each method discussed here are summarized.

Poisoning of SCR Catalysts by Alkali and Alkaline Earth Metals

SCR still represents the most widely applied technique to remove nitrogen oxides from flue gas from both stationary and mobile sources. The catalyst lifetime is greatly affected by the presence of poisoning compounds in the exhaust gas that deactivate the catalysts over time on stream. The progressive and widespread transition towards bio-derived fuels is pushing research efforts to deeply understand and contrast the deactivating effects of some specific poisons among those commonly found in the emissions from combustion processes. In particular, exhaust gases from the combustion of bio-fuels, as well as from municipal waste incineration plants and marine engines, contain large amounts of alkali and alkaline earth metals that can severely affect the acid, redox, and physical properties of the SCR catalysts. This review analyzes recent studies on the effects of alkali and alkaline earth metals on different types of SCR catalysts divided into three main categories (conventional V2O5-WO3/TiO2, supported non-vanadium catalysts and zeolite-based catalysts) specifically focusing on the impact of poisons on the reaction mechanism while highlighting the different type of deactivation affecting each group of catalysts. An overview of the different regeneration techniques aimed at recovering as much as possible the original performance of the catalysts, highlighting the pros and cons, is given. Finally, current research trends aiming to improve the tolerance towards alkali-poisoning of SCR catalysts are reported.

A detailed chemical kinetic modeling and experimental investigation of the low‐ and high‐temperature chemistry of n‐butylcyclohexane

AbstractChemical kinetic models of gasoline, jet, and diesel fuels and their mixtures with bioderived fuels are needed to assess fuel property effects on efficiency, emissions, and other performance metrics in internal combustion and gas turbine engines. As these real fuels have too many fuel components to be included in a chemical kinetic model, surrogate fuels containing fewer components are used to represent them. These surrogate fuels mimic the chemical classes or molecular structures contained in the real fuel. One of the important chemical classes in gasoline, jet, and diesel fuels comprises cyclohexanes. Cyclohexanes comprise about 30% or more by weight in diesel fuel. Also, Mueller et al (Energy Fuels. 2012;26(6):3284‐3303) proposed n‐butylcyclohexane (nBCH) as a component in a nine‐component surrogate palette to represent the ignition properties, distillation curve, density, and molecular structures of a diesel certification fuel. In this work, experimental measurements of the ignition delay times (IDTs) of nBCH in a shock tube and in a rapid compression machine are reported over a wide range of temperature, pressure, and equivalence ratio important for enabling the validation of a chemical kinetic model for nBCH for combustion in diesel engines. The range of conditions are temperatures of 630–1420 K, pressures of 10, 30, and 50 bar, and equivalence ratios of 0.3, 0.5, 1.0, and 2.0 in ‘air’. A detailed chemical model is developed for nBCH to simulate its ignition at both low and high‐temperature conditions and at relevant elevated pressures. The experimentally measured IDTs are used to improve and validate the chemical kinetic model.

Conversion of poplar biomass into high-energy density tricyclic sesquiterpene jet fuel blendstocks

BackgroundIn an effort to ensure future energy security, reduce greenhouse gas emissions and create domestic jobs, the US has invested in technologies to develop sustainable biofuels and bioproducts from renewable carbon sources such as lignocellulosic biomass. Bio-derived jet fuel is of particular interest as aviation is less amenable to electrification compared to other modes of transportation and synthetic biology provides the ability to tailor fuel properties to enhance performance. Specific energy and energy density are important properties in determining the attractiveness of potential bio-derived jet fuels. For example, increased energy content can give the industry options such as longer range, higher load or reduced takeoff weight. Energy-dense sesquiterpenes have been identified as potential next-generation jet fuels that can be renewably produced from lignocellulosic biomass.ResultsWe developed a biomass deconstruction and conversion process that enabled the production of two tricyclic sesquiterpenes, epi-isozizaene and prespatane, from the woody biomass poplar using the versatile basidiomycete Rhodosporidium toruloides. We demonstrated terpene production at both bench and bioreactor scales, with prespatane titers reaching 1173.6 mg/L when grown in poplar hydrolysate in a 2 L bioreactor. Additionally, we examined the theoretical fuel properties of prespatane and epi-isozizaene in their hydrogenated states as blending options for jet fuel, and compared them to aviation fuel, Jet A.ConclusionOur findings indicate that prespatane and epi-isozizaene in their hydrogenated states would be attractive blending options in Jet A or other lower density renewable jet fuels as they would improve viscosity and increase their energy density. Saturated epi-isozizaene and saturated prespatane have energy densities that are 16.6 and 18.8% higher than Jet A, respectively. These results highlight the potential of R. toruloides as a production host for the sustainable and scalable production of bio-derived jet fuel blends, and this is the first report of prespatane as an alternative jet fuel.

Oxidation of 2,6-dimethylheptane at low temperature: Kinetic modeling and experimental study

Branched alkanes represent an important class of compounds in conventional fuels and some bio-derived fuels. This study is dedicated to the investigation of the low-temperature oxidation chemistry of 2,6-dimethylheptane using a combination of experimental and computational methods. All the reactants, transition states, and products in the first oxidation stage, which are crucial to the initiation reactions in the low-temperature reaction chain, were optimized through the B3LYP/CBSB7 level of theory and a kinetic mechanism that included the new reaction pathways was assembled. Ignition delay time measurements were carried out in a rapid compression machine and the results were compared with modeling predictions. The kinetic mechanism is able to capture both the first and total ignition delay times with a root-mean-square deviation of 39.6%. In addition, sensitivity analysis is performed to quantify the impact of newly developed chemistry of 2,6-dimethylheptane on ignition delay time. Rate parameters found in this study may be applicable to other branched alkanes with similar molecular structure.

Spray Flame Characteristics of Bio-Derived Fuels in a Simulated Gas Turbine Burner

Abstract Fuel injection plays an important role in liquid-fueled gas turbine combustion. The strong interdependence of liquid breakup and atomization, turbulent dispersion of these droplets, droplet evaporation, and fuel–air mixing make the spray modeling an extremely challenging task. The physical processes are even more difficult to predict for alternative fuels with different thermophysical properties. In this study, spray flames of unheated and preheated vegetable oil (VO) produced by an air-blast (AB) atomizer in a swirl stabilized combustor are investigated experimentally. Phase Doppler particle analyzer (PDPA) is used to measure the instantaneous diameter and axial velocity of droplets at different axial and radial locations in both flames. Experiments are conducted at an equivalence ratio of 0.79 and atomizing air to liquid ratio by a mass of 2.5 to obtain stable VO flames. Radial profiles of mean axial velocity and Sauter mean diameter (SMD) are presented to show the effect of fuel preheating. Joint probability density functions (joint PDF) are presented to show the correlation between droplet diameter and axial velocity. Results are analyzed to show that both sprays exhibit self-similar droplet diameter distributions at different axial and radial locations when normalized properly. Thus, the vast amount of PDPA data in the spray can be reduced to simple distribution functions. A method to reconstruct the joint PDF from experimentally determined distribution functions is presented. We envision that the joint PDF approach outlined in this study could be implemented in high-fidelity computational fluid dynamic models to improve spray predictions in future studies.

Combustion characteristics of butanol, butyl butyrate, and Jet A-1 in a swirl-stabilized combustor

Significant growth in the aviation industry has also raised concern on the impact of this sector in terms of environmental pollution. Bio-derived fuels are going to play an important role as potential alternative aviation fuels in the near future to mitigate the carbon footprint. Ethanol is a candidate biofuel which is widely used in many countries in the petrol-ethanol blend for S.I. engines. However, the interest in utilizing biofuels in aviation gas turbine engines is relatively recent. In comparison to ethanol, n-butanol possesses some better fuel properties such as higher energy density, less hygroscopic, lower vapor pressure, higher flash point. Butyl butyrate is another potential candidate which can fulfill the chemical property requirements of alternative aviation fuels. However, compatible substitutes for currently used aviation fuel (such as Jet A-1) are still at the early stage of development. The present study deals with experimental investigations of combustion characteristics of aviation kerosene (Jet A-1), butanol, butyl butyrate, and blends of the last two fuels with Jet A-1 in a swirl-stabilized gas turbine type combustor. Visualization of flame, measurement of temperature inside the flame, and exhaust emissions at the combustor exit are carried out to compare the performance of various fuels relative to Jet A-1. Spray penetration length, ligament length, droplet diameter, and spray cone angle of various fuels obtained using high-speed imaging technique are analyzed to understand the combustion characteristics. Fuel properties of individual fuels and blends are also measured and compared to probe the variation in spray and combustion performances.

Sooting tendencies of 20 bio-derived fuels for advanced spark-ignition engines

The sooting tendencies of 20 bio-derived fuels suggested as potential blendstocks for spark-ignition engines by the Co-Optimization of Fuels & Engines (Co-Optima) Initiative are studied. The Yield Sooting Index (YSI) is used to quantify the sooting tendencies. The Co-Optima Chemical Model is used to predict the numerical YSIs for all test fuels, and these results are compared with measurements. Experimental YSIs are newly measured for 2 furans and are taken from our previous work for the other compounds. Overall, the predicted YSIs agree well with measurements, within the experimental uncertainties, except for 2,5-dimethylfuran (2,5-DMF). It is found that Polycyclic Aromatic Hydrocarbons (PAH) growth reactions have little influence on the relative soot production from each fuel and therefore on YSI predictions. A modified sensitivity coefficient formulation is proposed to evaluate reaction sensitivities specifically for YSI. This formulation is applied to the 2,5-DMF-doped flame, and six 2,5-DMF initial decomposition reactions, which lead to different major soot precursors, are identified as the most sensitive reactions. The impact of the chemical kinetic uncertainties embedded in these reactions is quantified by randomly perturbing their reaction rates within a factor of two. The resulting prediction uncertainty in the 2,5-DMF-doped flame is found to be ±8 YSI units (±11%), which demonstrates that the test-fuel-specific decomposition reactions indeed have considerable influence on its YSI prediction. This work suggests that more accurate kinetic parameters for fuel initial decomposition reactions can potentially improve YSI predictions effectively, without altering the YSI predictions of other fuels with significantly different molecular structures.

Bifunctional rice husk-derived SiO2-Cu-Al-Mg nanohybrid catalyst for one-pot conversion of biomass-derived furfural to furfuryl acetate

Developing one-pot reaction methodologies, which typically require multifunctional catalyst systems, is crucial for sustainable production of bio-derived fuels and chemicals. This work reports one-pot hydrogenation-esterification of furfural to furfuryl acetate using a bifunctional metal-based nanohybrid catalyst, composed of rice husk (RH) derived SiO2, Cu, Al, and Mg species (RHSiO2-Cu-Al-Mg). For comparison, the catalytic efficiency of RHSiO2-Cu and RHSiO2-Cu-Al were tested under similar reaction conditions. Various analytical techniques were used to elucidate the physicochemical, textural, and acid-redox properties of the catalysts. It was found that the RHSiO2-Cu-Al-Mg catalyst contains an optimum amount of acid and redox sites, as illustrated by NH3-TPD and H2-TPR studies, respectively. Especially, Mg addition played a vital role in tailoring acidity of the RHSiO2-Cu-Al catalyst to promote in-situ esterification of furfuryl alcohol with acetic acid to yield furfuryl acetate. As a result, the RHSiO2-Cu-Al-Mg catalyst exhibited the best performance in one-pot conversion of furfural to furfuryl acetate, outperforming various noble metal/silica based catalysts. This study offers potential opportunities for the rational design of novel, bifunctional heterogeneous catalysts for efficient production of bio-derived fuels and value added chemicals.

(Invited) Recent Developments in Intermediate-Temperature Reversible Fuel Cells

Intermediate-temperature fuel cells have potential to be the cleanest and most efficient option for direct conversion to electricity of a wide variety of fuels, from hydrogen to hydrocarbons, coal gas, and bio-derived fuels. When operated in the reverse mode, on the other hand, they are very efficient for low-cost production of hydrogen from splitting water. Thus, a reversible fuel cell is ideally suited for large-scale energy storage and conversion, which is vital to the deployment of renewable energies. However, the commercialization of these systems hinges on rational design of novel materials of exceptional functionalities at lower temperatures to dramatically reduce the cost while enhancing performance and durability. A key challenge is the development of bifunctional catalysts, especially those for the oxygen electrode, for efficient dual-mode operation. Further, the reversible fuel cells must have high durability when cycled between the fuel cell and the electrolysis modes while maintaining high Faraday efficiency and roundtrip energy efficiency. The electrolyte materials must have high ionic transference number and excellent stability under the operating conditions. To accomplish these goals, it is imperative to gain a fundamental understanding of the mechanisms of charge and mass transport along surfaces, across interfaces, and through porous electrodes. Further, new protocols must be developed to control materials structure, composition, and morphology over multiple length scales. This presentation will highlight the critical scientific challenges facing the development of a new generation of reversible fuel cells based on oxygen ion- or proton- conducting electrolyte, the latest development of bifunctional electrodes and electrolyte materials, the strategies for enhancing electrode activity and durability of hydrogen evolution/oxidation, and oxygen reduction/evolution reactions in reversible fuel cells, the latest developments in modeling, simulation, and in situ characterization techniques for unraveling charge and mass transport mechanisms, and the outlook for future-generation energy storage systems that exploit nano-scale materials of significantly improved performance.

Cyclic Ketones as Future Fuels: Reactivity with OH Radicals.

For a sustainable energy future, research directions should orient toward exploring new fuels suitable for future advanced combustion engines to achieve better engine efficiency and significantly less harmful emissions. Cyclic ketones, among bio-derived fuels, are of significant interest to the combustion community for several reasons. As they possess high resistance to autoignition characteristics, they can potentially be attractive for fuel blending applications to increase engine efficiency and also to mitigate harmful emissions. Despite their importance, very few studies are rendered in understanding of the chemical kinetic behavior of cyclic ketones under engine-relevant conditions. In this work, we have conducted an experimental investigation for the reaction kinetics of OH radicals with cyclopentanone and cyclohexanone for the first time over a wide range of experimental conditions ( T = 900-1330 K and p ≈ 1.2 bar) in a shock tube. Reaction kinetics was followed by monitoring UV laser absorption of OH radicals near 306.7 nm. Our measured rate coefficients, with an overall uncertainty (2σ) of ±20%, can be expressed in Arrhenius form as (in units of cm3 molecule-1 s-1): (902-1297 K); (935-1331 K). Combining our measured data with the single low-temperature literature data, the following three-parameter Arrhenius expressions (in units of cm3 molecule-1 s-1) are obtained over a wider temperature range: k1(CPO + OH) = (298-1297 K); (298-1331 K). Discrepancies between the theoretical and current experimental results are observed. Earlier theoretical works are found to overpredict our measured rate coefficients. Interestingly, these cyclic ketones exhibit similar reactivity behavior to that of their linear ketone counterparts over the experimental conditions of this work.

Automated Transformation of Lignin Topologies into Atomic Structures with LigninBuilder

Lignin is an abundant aromatic heteropolymer found in secondary plant cell walls and is a potential feedstock for conversion into bioderived fuels and chemicals. Lignin chemical diversity complicat...

Storage and Oxidation of Oxygen-Free and Oxygenated Hydrocarbons on a Pt–Pd Series Production Oxidation Catalyst

Within the Research Cluster of Excellence “Tailor-Made Fuels from Biomass” at RWTH Aachen University, novel fuels from biomass for internal combustion engines are investigated. The new fuels tend to soot less, which allows to increase exhaust gas recirculation for reduction of nitrogen oxides (NOx) emissions. This increases the emissions of unburnt hydrocarbons (HC), while the composition of the HC emissions changes because of the changed fuel composition. The impact of 16 different oxygen-free and oxygenated hydrocarbons (1-octanol, di-n-butyl ether (DNBE), 2-butanone and ethanol as examples for bio-derived fuels; toluene and n-heptane as examples for conventional fuels’ components; propane, propene, ethane, ethene, ethyne, methane, n-butane, isobutane, n-pentane and 2-propanol as HC known to be in exhaust gases) on a series production Pt–Pd/Al2O3 oxidation catalyst on a cordierite substrate has been investigated regarding adsorption and temperature programmed oxidation (TPO). In general, due to higher polarity oxygenated HC are stronger adsorbed than oxygen-free ones. For 1-octanol, an extraordinary high adsorption could be observed associated with vigorous exothermal oxidation reaction during TPO. Additionally, the temperature programmed desorption of 1-octanol and ethanol has been investigated by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (TP-DRIFTS). Adsorbed alcohol species as well as oxidized products were demonstrated to be present.

Shell biorefinery: A comprehensive introduction

Biomass refinery is considered to be a key technology in the 21st century due to the importance of the sustainable production of various bio-derived fuels and fine chemicals. Besides the synthesis of oxygen-containing chemicals mainly from lignocellulosic biomass, nitrogen-containing chemicals belong to some of the most important commodity and fine chemicals. In this introductory short review, the main similarities and difficulties between petroleum oil- and biorefinery will be discussed and future challenges will be highlighted. As a particular example, recent developments in the shell biorefinery – the utilization of shell waste – will be reviewed. Particular emphasis will be placed on the structure of shell biomass, the current and emerging fractionation methods and the conversion of chitin and chitosan to various heteroatom-containing chemicals. This review is meant to provide an introduction to beginners in the field of biorefinery as well as a comprehensive discussion of recent proceedings in the field of shell biorefinery. An outlook on the future potential and challenges will be given.

Exploring the negative temperature coefficient behavior of acetaldehyde based on detailed intermediate measurements in a jet-stirred reactor

Acetaldehyde is an observed emission species and a key intermediate produced during the combustion and low-temperature oxidation of fossil and bio-derived fuels. Investigations into the low-temperature oxidation chemistry of acetaldehyde are essential to develop a better core mechanism and to better understand auto-ignition and cool flame phenomena. Here, the oxidation of acetaldehyde was studied at low-temperatures (528–946 K) in a jet-stirred reactor (JSR) with the corrected residence time of 2.7 s at 700 Torr. This work describes a detailed set of experimental results that capture the negative temperature coefficient (NTC) behavior in the low-temperature oxidation of acetaldehyde. The mole fractions of 28 species were measured as functions of the temperature by employing a vacuum ultra-violet photoionization molecular-beam mass spectrometer. To explain the observed NTC behavior, an updated mechanism was proposed, which well reproduces the concentration profiles of many observed peroxide intermediates. The kinetic analysis based on the updated mechanism reveals that the NTC behavior of acetaldehyde oxidation is caused by the competition between the O2-addition to and the decomposition of the CH3CO radical.

Pressurised chemical-looping combustion of an iron-based oxygen carrier: Reduction kinetic measurements and modelling

Chemical-looping combustion (CLC) is a novel combustion techology offering the potential to provide uninterrupted and reliable heat and power production from fossil or bio-derived fuels with integrated, intrinsic CO2 capture and minimal energy penalty. Operation of CLC at elevated pressures provides the potential for integration with a combined cycle, which makes the use of solid fuels significantly more feasible. To date, only a few experimental studies investigating CLC processes and oxygen carrier performance under pressurised conditions have been reported in the open literature. This article reports findings from investigations into the effect of pressure, temperature and CO concentration on the intrinsic reaction kinetics of an Al2O3-supported Fe-based oxygen carrier. Our study employed an innovative pressurised fluidised-bed reactor, designed for operation at temperatures up to 1273K and pressures up to 20bara, to simulate ex-situ gasification of solid fuels at elevated pressures. An intrinsic reaction model was developed and pseudo-intrinsic rate constants were derived. Differences in the activation energies and pre-exponential factors of the Al2O3-supported Fe2O3 and a pure Fe2O3 oxygen carriers were observed, indicating a change in reaction mechanism when Al2O3 was present. Subsequently, an adapted random pore model was developed to describe the variation of reaction rate with solid conversion. The good agreement between the adapted random pore model and empirical measurements indicated that the change in mechanism was due to a significantly higher product layer diffusivity for the Al2O3-supported Fe2O3 oxygen carrier compared with the pure Fe2O3 material. When pressurised, the observed reaction order with respect to CO was slightly lower than 1. The model developed using atmospheric pressure measurements was successfully applied to predict reaction kinetics at elevated pressures up to 5bara providing further validation of the model.