Imperial College London

DrMarcosMillan-Agorio

Faculty of EngineeringDepartment of Chemical Engineering

Reader in Chemical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 1633marcos.millan

 
 
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Assistant

 

Mrs Sarah Payne +44 (0)20 7594 5567

 
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Location

 

501Roderic Hill BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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117 results found

Jin Z, Cui Z, Long X, Millan M, Yuan G, Dong Z, Cong Y, Zhang J, Li Y, Li Xet al., 2021, Understanding the correlation between microstructure and electrochemical performance of hybridized pitch cokes for lithium-ion battery through tailoring their evolutional structures from ordered soft carbon to disordered hard carbon, Journal of Alloys and Compounds, Vol: 887, ISSN: 0925-8388

Pitch-derived cokes (PCs) with different optical textures and microstructures were produced by thermo-polymerization and subsequent heat treatment of a mixture of graphitizable and non-graphitizable precursors (i.e., naphthalene pitch and C9 resin). The effects of weight fractions of C9 resin and heat-treating temperatures on the evolutional microstructure and electrochemical performance of different hybridized PCs used as an anode material for lithium-ion batteries were investigated. The results show that the macro-texture, microstructure and structural evolution of PCs could be controlled by facilely tailoring the synthetic precursors through pitch-resin co-polymerizing reaction. The versatile and tunable structure of PCs closely dominates the inserting and extracting capability of lithium ions in the resultant PCs. With the introduction of C9 resin in the pitches from 0 to 100 wt%, the microstructure of resulting PCs changes from a highly oriented lamellar texture to a fine-grained mosaic texture (i.e., from anisotropic soft carbon to isotropic hard carbon). In addition, the electrochemical performance (e.g., in the range of 200–370 mA h g−1 for the specific capacity) of the PCs varies according to the textural orientation, microcrystallite sizes and graphitization degrees. The relationship between preliminary microstructure and electrochemical performance of PCs with controllable microstructure and crystalline orientation has been studied to understand the importance of structure control. Furthermore, this work provides a new strategy to adjust the electrochemical performance of hybridized PCs through tailoring the liquid crystal development of texture-tunable pitch precursor synthesis.

Journal article

Spiegl N, Long X, Berrueco C, Paterson N, Millan Met al., 2021, Oxy-fuel co-gasification of coal and biomass for negative CO2 emissions, Fuel, Vol: 306, Pages: 1-8, ISSN: 0016-2361

A novel process has been investigated to enable CO2 to be isolated as a concentrated stream from the exhaust of a power station, without the use of separate, downstream capture technology. In this concept, a fluidised bed gasifier is operated with pure O2 and CO2, recycled from the combusted fuel gas stream, combining the advantages of gasification, fluidised bed operation and use of oxygen. With the combination of CO2 capture and biomass as the feedstock, net negative emissions can be achieved. This is the fifth in a series of papers which investigate the underlying science of the concept. It studies the influence of the use of different coals and mixtures containing coal/biomass on the process performance. A continuously fed, laboratory scale spouted bed reactor has been used for this study, and Daw Mill coal (DM), German lignite (GL), Polish coal (PC) and Olive bagasse (OB) were used as fuels. Carbon conversions for DM and PC were around 20–30% and dominated by release by pyrolysis. High conversions (60–80%) were observed for GL and the char showed an appreciable gasification reactivity. Co-processing of OB with GL proved to be a good way to further improve the overall process performance and complete conversion was achieved under some conditions. Processing biomass with lignite, with CO2 capture, is a technically viable way of producing energy from a waste material with a negative process carbon footprint.

Journal article

Bikane K, Yu J, Shankar R, Long X, Paterson N, Millan Met al., 2021, Early-stage kinetics and char structural evolution during CO2 gasification of Morupule coal in a wire-mesh reactor, CHEMICAL ENGINEERING JOURNAL, Vol: 421, ISSN: 1385-8947

Journal article

Long X, Spiegl N, Berrueco C, Paterson N, Millan Met al., 2021, Emission of species of environmental and process concern during simulated oxy-fuel gasification, Fuel, Vol: 299, ISSN: 0016-2361

The release of species of environmental concern during simulated oxy-fuel gasification in a laboratory scale fluidised bed have been investigated. Fluidising gases containing a high partial pressure of CO2 and in some tests, steam, were used. The species considered are tars, H2S and NH3 and the aim has been to gain an understanding of the impact of the gasifer operating conditions on their release. This is part of a programme of work on the potential of oxy-fuel gasification as a means of enabling the use of coal to continue as a source of energy, whilst capturing the CO2 for sequestration to minimise the impact on climate change. It has been shown that the tars released during pyrolysis are efficiently destroyed during their passage through the hot, char containing bed. The measured emission of tar was very low, particularly when compared to a conventional fixed bed gasifier. The extent of S release as H2S depends on the fuel-S and ash-Ca contents, and on the operating conditions. Increasing the partial pressure of CO2, steam and raising the temperature increased the proportion of S released. Adding dolomite, to retain S, decreased the emission level at atmospheric pressure, provided the temperature was above 950 °C. The use of this method to reduce S emissions at elevated pressure would require careful consideration, as the high partial pressure of CO2 in the fluidising gas can prevent the calcination of the dolomite and therefore reduce its effectiveness. Steam was found to increase the proportion of fuel-N converted to NH3 and the concentration also depended on the bed temperature. A peak in the concentration was noted at 850 °C, due to the opposing impacts on increased release as the temperature was raised and increasing equilibration of NH3 to N2 and H2.

Journal article

Ayala-Cortés A, Arcelus-Arrillaga P, Millan M, Arancibia-Bulnes CA, Valadés-Pelayo PJ, Villafán-Vidales HIet al., 2021, Solar integrated hydrothermal processes: A review, Renewable and Sustainable Energy Reviews, Vol: 139, ISSN: 1364-0321

Hydrothermal processes are attractive options for the transformation of mixtures of biomass with large amounts of water, i.e. above 20wt%. At hydrothermal conditions, the special properties of water makes it an attractive reaction medium to obtain several bio-based platform chemicals or fuel gases, such as hydroxymethilfurfural or fufurals, syngas, hydrogen, methane, etc. However, one of the main challenges is that a large amount of energy is required to heat reactants (mixture of water and biomass), which is usually achieved by combustion of a fraction of the bio-oil product. Therefore, to reduce this consumption, their integration with an external renewable energy source, such as concentrated solar radiation has been proposed. This approach has been recently analyzed by several research groups as an option to have sustainable and economically attractive processes. This work provides an overview of the different experimental and theoretical strategies to incorporate concentrated solar technologies into hydrothermal processing of biomass, including the main challenges of such integration for process technical feasibility.

Journal article

Daud ARM, Berrueco C, Hellgardt K, Millan M, Kandiyoti Ret al., 2021, Oxidative cracking of three to five-member ring polycyclic aromatic hydrocarbons in subcritical and supercritical water, The Journal of Supercritical Fluids, Vol: 167, Pages: 105050-105050, ISSN: 0896-8446

Polycyclic aromatic hydrocarbons (PAH) are refractory structures common in heavy hydrocarbons. Thermal cracking in supercritical water (SCW) is limited but PAH can be completely oxidised if an oxidant is added. By restricting oxidant supply to substoichiometric amounts, this study aims to achieve partial oxidation as a route to useful chemicals, such as mono- and bi-aromatics. Oxidative cracking reactions of anthracene, pyrene and benzo[a]pyrene in subcritical and SCW were studied. PAH conversions well above 90 % were achieved along a fast heating ramp in a batch reactor. This quick initial oxidation took place predominantly in inner rings, weakening the aromatic structure and increasing cracking reactivity. This oxidation-cracking pathway became dominant in the SCW region, producing mostly oxygenated compounds with fewer aromatic rings. On the other hand, competing reactions leading to polymerization were favoured in the subcritical water region. PAH reactivity was found to follow the order anthracene > benzo[a]pyrene > pyrene.

Journal article

Bikane K, Yu J, Long X, Paterson N, Millan Met al., 2020, Linking char reactivity to structural and morphological evolution during high pressure pyrolysis of Morupule coal, Chemical Engineering Science: X, Vol: 8

Understanding the influence of pressure on char physicochemical properties and reactivity during pyrolysis is principal for the design and optimisation of coal gasification technologies. This work investigated the pyrolysis behaviour of Morupule coal at various pressures and temperatures in a high-pressure wire-mesh reactor (HPWMR). The effect of pressure was pronounced at 600 and 800 °C, resulting in lower total volatile yields at higher pressures due to the repolymerisation of volatiles. Although graphitisation was limited during the heating period, an increase in the concentration of ordered structures was observed during holding at 1000 °C. Marked blowholes on the char surface suggested an explosive bubble transport phenomenon during pyrolysis. Chars produced during the ramping period under high pressure showed higher combustion reactivities than their low-pressure counterparts. However, prolonged holding at peak temperature produced chars of similar reactivity, with their CO2 gasification reactivity found to be independent of pyrolysis pressure.

Journal article

Long X, Spiegl N, Berrueco C, Paterson N, Millan Met al., 2020, Fluidised bed oxy-fuel gasification of coal: Interactions between volatiles and char at varying pressures and fuel feed rates, Chemical Engineering Science: X, Vol: 8

Fluidised bed gasification is a versatile technology in terms of load flexibility and ability to process various fuels. However, fluidised bed gasification processes are usually air-blown, making integration with CO2 capture difficult and expensive. Operation of a fluidised bed gasifier with O2/recycled CO2 mixtures has been proposed as a route to overcome this drawback. In this work, the effect of pressure and fuel feed rate on the extent of gasification of German lignite in a CO2 rich atmosphere has been examined in a fluidised bed reactor (FBR). A 19% decrease in carbon conversion with increasing pressure from 5 to 20 bara was observed. The higher fuel feeding rates needed to maintain the gasification agent to coal ratio in the FBR at high pressures produces a larger concentration of tars in the reactor, which seems to cause deposition, by intra and inter particle reaction, on the char and reduces its reactivity.

Journal article

Chen Z, Kukushkin R, Yeletsky P, Saraev A, Bulavchenko O, Millan-Agorio Met al., 2020, Coupling hydrogenation of guaiacol with in-situ hydrogen production by glycerol aqueous reforming over Ni/Al2O3 and Ni-X/Al2O3 (X = Cu, Mo, P) catalysts, Nanomaterials, Vol: 10, ISSN: 2079-4991

Biomass-derived liquids, such as bio-oil obtained by fast pyrolysis, can be a valuable source of fuels and chemicals. However, these liquids have high oxygen and water content, needing further upgrading typically involving hydrotreating using H2 at high pressure and temperature. The harsh reaction conditions and use of expensive H2 have hindered the progress of this technology and led to the search for alternative processes. In this work, hydrogenation in aqueous phase is investigated using in-situ produced hydrogen from reforming of glycerol, a low-value by-product from biodiesel production, over Ni-based catalysts. Guaiacol was selected as a bio-oil model compound and high conversion (95%) to phenol and aromatic ring hydrogenation products was obtained over Ni/γ-Al2O3 at 250 °C and 2-h reaction time. Seventy percent selectivity to cyclohexanol and cyclohexanone was achieved at this condition. Hydrogenation capacity of P and Mo modified Ni/γ-Al2O3 was inhibited because more hydrogen undergoes methanation, while Cu showed a good performance in suppressing methane formation. These results demonstrate the feasibility of coupling aqueous phase reforming of glycerol with bio-oil hydrogenation, enabling the reaction to be carried out at lower temperatures and pressures and without the need for molecular H2.

Journal article

Puron H, Pinilla JL, Saraev AA, Kaichev VV, Millan Met al., 2020, Hydroprocessing of Maya vacuum residue using a NiMo catalyst supported on Cr-doped alumina, Fuel, Vol: 263, ISSN: 0016-2361

Mesoporous alumina was doped with Cr using a co-precipitation method to prepare a support for hydrocracking catalysts. Ni and Mo were then impregnated on Cr-doped mesoporous alumina (NiMo/Al2O3-Cr). Catalytic activity was tested in hydrodeasphaltenisation (HDA), hydrodesulfurisation (HDS) and hydrodemetallisation (HDM) of Maya vacuum residue in a batch reactor and compared with NiMo supported on mesoporous alumina not doped with Cr (NiMo/Al2O3). Following activation and initial carbon deposition, experiments involving contact of the catalysts with fresh feed were performed. Greater HDA extent and maltene to asphaltene ratios were obtained with NiMo/Al2O3-Cr than NiMo/Al2O3 despite a larger amount of coke deposits. Significant activity of both NiMo/Al2O3-Cr and NiMo/Al2O3 towards HDS and HDM were also observed. Alumina textural properties remained relatively unaffected by the co-precipitation of Cr. X-ray photoelectron spectroscopy (XPS) showed that the catalysts contained Cr3+ and after reduction all Ni was present in metallic state at least in the near-surface region, while Mo6+ was reduced to Mo4+ and Moδ+ (0 ≤ δ ≤ 3) to a larger extent in NiMo/Al2O3-Cr. Lower reduction temperatures in the presence of Cr were determined, suggesting a larger number of metal sites available in reduced form at operating conditions. It was found that Cr aided metal dispersion in catalyst synthesis and coke dispersion during reaction. Spent catalysts showed reductions in surface area and pore volume. However, while spent NiMo/Al2O3 catalysts had a decrease in average pore diameter (APD), NiMo/Al2O3-Cr maintained the fresh material APD, which may explain the sustained catalytic activity.

Journal article

Spiegl N, Berrueco C, Long X, Paterson N, Millan Met al., 2020, Production of a fuel gas by fluidised bed coal gasification compatible with CO<inf>2</inf> capture, Fuel, Vol: 259, ISSN: 0016-2361

A continuously fed, laboratory scale spouted bed gasifier has been used to study oxy-fuel gasification of German lignite. In this paper, the influence of different gasification agents and bed temperature on the process performance, during tests at atmospheric and elevated pressure are studied. Two gasification agents have been used, CO2 (with different CO2/C ratios) and mixtures of CO2/steam. The results show that despite the relatively slow CO2-char reaction, good gasification performance could be achieved with German lignite by adjusting the operating conditions at atmospheric pressure: complete carbon conversion, high energy conversion and a medium heating value fuel gas (8–10 MJ m−3). The CO2/C ratio was found to have a large effect on the gasification performance. Increasing the ratio increased the carbon conversion, but the CO2 conversion decreased. At 950 °C, maximum carbon conversion was already achieved with pure CO2, therefore using steam at this temperature could not increase the conversion, but did increase the H2/CO ratio in the fuel gas. At 850 °C, replacing 25% of CO2 with steam increased the carbon conversion to the level achieved at 950 °C without steam. Replacing more than 25% of CO2 with steam increased the H2/CO ratio further. Therefore, with the addition of steam, the operating temperature could be reduced from 950 °C to 850 °C while maintaining the gasification performance. The changes of gasification performance with steam addition at pressures up to 10 bara followed the same trends achieved at atmospheric pressure.

Journal article

Zhu HL, Pastor-Pérez L, Millan M, 2020, Catalytic steam reforming of toluene: Understanding the influence of the main reaction parameters over a reference catalyst, Energies, Vol: 13

Identifying the suitable reaction conditions is key to achieve high performance and economic efficiency in any catalytic process. In this study, the catalytic performance of a Ni/Al2O3 catalyst, a benchmark system-was investigated in steam reforming of toluene as a biomass gasification tar model compound to explore the effect of reforming temperature, steam to carbon (S/C) ratio and residence time on toluene conversion and gas products. An S/C molar ratio range from one to three and temperature range from 700 to 900 °C was selected according to thermodynamic equilibrium calculations, and gas hourly space velocity (GHSV) was varied from 30,600 to 122,400 h-1 based on previous work. The results suggest that 800 °C, GHSV 61,200 h-1 and S/C ratio 3 provide favourable operating conditions for steam reforming of toluene in order to get high toluene conversion and hydrogen productivity, achieving a toluene to gas conversion of 94% and H2 production of 13 mol/mol toluene.

Journal article

Arcelus-Arrillaga P, Pinilla JL, Millan M, 2020, CHAPTER 3: Catalytic Conversion of Fossil and Renewable Fuel Resources: Approaches Using Sub and Supercritical Water as a Reaction Medium, RSC Energy and Environment Series, Pages: 46-79

Access to energy is one of the main challenges society will face in the decades to come. Liquid fuels are expected to remain one of the main sources of energy, despite the depletion of conventional fossil fuel reserves. The development of cleaner technologies to transform unconventional energy resources such as heavy oil, biomass and organic residues into fuels is crucial to meeting the world's future energy demand. Water in hydrothermal conditions near or above the critical point can provide an efficient route to obtain fuels from unconventional sources in a clean and efficient way. For instance, due to its particular physicochemical properties, near-critical water and supercritical water are considered excellent solvents for catalytic organic chemical reactions. In hydrothermal processes, the addition of a catalyst has the purpose of increasing rates of reaction and product yields, suppressing the formation of solids, reducing re-polymerization of intermediate species and promoting cracking reactions. Successful implementation of catalytic processes in hydrothermal conditions requires the development of highly active catalysts that are stable under these conditions without deactivation. In this work, a thorough review of the literature on the application of catalytic hydrothermal processes for the conversion and upgrading of fossil and renewable unconventional fuel resources is presented.

Book chapter

Durkin A, Millan-Agorio M, Guo M, 2020, Process Systems Design Framework for Resource Recovery from Wastewater, Editors: Pierucci, Manenti, Bozzano, Manca, Publisher: ELSEVIER SCIENCE BV, Pages: 1039-1044

Book chapter

Volpe R, Menendez JMB, Reina TR, Volpe M, Messineo A, Millan M, Titirici M-Met al., 2019, Free radicals formation on thermally decomposed biomass, FUEL, Vol: 255, ISSN: 0016-2361

Journal article

Yu J, Paterson N, Millan M, 2019, The primary products of cellulose pyrolysis in the absence of extraparticle reactions, FUEL, Vol: 237, Pages: 911-915, ISSN: 0016-2361

Journal article

Almeida RSR, Taccini MM, 2019, Effect of Storage Time on the Chemical Characterization of Pyroligneous Liquor From Eucalyptus Wood, Waste and Biomass Valorization, Pages: 1-7, ISSN: 1877-2641

© 2017 Springer Science+Business Media B.V. Abstract: This research focused on the effect of storage time on the chemical composition of the pyroligneous liquor (PL) produced during the pyrolysis of eucalyptus wood in a laboratory furnace at a relatively low heating rate (1 °C min−1) and a maximum temperature of 400 °C. The chemical compounds present in PL were identified through gas chromatography-mass spectrometry. This analysis was repeated over 19 months to observe changes in PL composition. Compounds such as 1,2-butanediol, 2-methoxytetrahydrofuran, 1,2-cyclopentanedione were only detected in the fresh PL and not after 11 or 19 months of storage. On the other hand, in PL stored for 11 and 19 months, new compounds were found, such as propanoic acid, butanediol, 5,9-dodecadien-2-one,6-10-dimethyl cyclopentanone, which were not observed in fresh PL. This suggests that PL from eucalyptus wood pyrolysis contains reactive compounds, mainly oxygenated, that interact during storage. Regarding the moment of stabilization of the qualitative composition of the PL, this research suggests that it is only reached after 11 months of storage. Phenol and furan derivatives were found to be stable, only reacting in their side chains without affecting the central ring. On the other hand, derivatives of tetrahydrofuran showed significant reactivity and tended to disappear with storage time. Graphical Abstract: [Figure not available: see fulltext.]

Journal article

Ji G, George A, Skoulou V, Reed G, Millan M, Hooman K, Bhatia SK, Diniz da Costa JCet al., 2018, Investigation and simulation of the transport of gas containing mercury in microporous silica membranes, Chemical Engineering Science, Vol: 190, Pages: 286-296, ISSN: 0009-2509

© 2018 Elsevier Ltd This work investigates the effect of condensable Hg vapour on the transport of N2gas across cobalt oxide silica (CoOxSi) membranes. Experimental results suggest that Hg significantly affects N2permeation at 100 and 200 °C, though this effect is negligible at 300 °C. This effect was found to have a correlation with Hg adsorption on CoOxSi xerogels. In order to understand the Hg effect in the transport phenomena of N2permeation, the oscillator model was used to model gas transport through pores with different sizes. By including effective medium theory (EMT), the oscillator model fitted well the experimental results and gave good prediction of mass transfer in ultra-microporous materials with a tri-modal pore size distribution, such as silica membranes. It is postulated that Hg seeks lower level potentials in micro-pores, and therefore Hg molecules tend to block small pores (2.5–4 Å from 2.9 Å), or reduce the average pore size of larger pores (6.7–7.8 Å and 12–14 Å). Although N2permeation decreased with the presence of Hg, it did not decrease when the Hg load was increased by a factor of ten; this strongly suggests the adsorption of Hg molecules in the smaller pores (2.5–4.0 Å), or along the pore wall for the larger pore ranges (6.7–7.8 Å and 12–14 Å).

Journal article

Yeletsky PM, Reina TR, Bulavchenko OA, Saraev AA, Gerasimov EY, Zaikina OO, Bermúdez JM, Arcelus-Arrillaga P, Yakovlev VA, Millan Met al., 2018, Phenanthrene catalytic cracking in supercritical water: effect of the reaction medium on NiMo/SiO<inf>2</inf> catalysts, Catalysis Today, ISSN: 0920-5861

© 2018 A series of NiMo/SiO2 catalysts was synthesized by sol-gel method for heavy oil upgrading in supercritical water (SCW). Phenanthrene was used as substrate as it represents polyaromatic structures present in asphaltenes. No phenanthrene conversion was observed in a blank (non‐catalytic) experiment. However, phenanthrene conversions up to 24% after 1 h of reaction in SCW at 425 °C and 230 bar were observed in the presence of NiMo/SiO2, underlining the role of the catalysts in the process. Conversion was found to be dependent mainly on Ni content and the Ni/Mo ratio in the catalysts. The liquid products obtained are thought to be the result of both oxidation and hydrogenation processes. Characterization of the fresh and spent catalysts using X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) was performed. It was revealed that catalysts are not completely stable in SCW, showing that NiMo intermetallic compounds formed the initial catalysts were decomposed, Mo0 and Ni0 were oxidised and the latter formed Ni2SiO4. In addition, MoO2 phase domain size in the catalysts increased and the surface of the spent catalysts appeared to be enriched with Ni and depleted with Mo.

Journal article

Cardoso A, Ramirez Reina T, Suelves I, Pinilla JL, Millan M, Hellgardt Ket al., 2018, Effect of carbon-based materials and CeO<inf>2</inf>on Ni catalysts for Kraft lignin liquefaction in supercritical water, Green Chemistry, Vol: 20, Pages: 4308-4318, ISSN: 1463-9262

© The Royal Society of Chemistry 2018. Kraft lignin (KL) is a by-product from cellulose production typically treated as a waste or used as a low-value fuel in heat and power generation in the pulp and paper industry. This study explores KL upgrading to monoaromatic compounds using supercritical water (SCW) as reaction medium. The effect of Ni-CeO2catalysts supported on carbon nanofibers (CNF) and activated carbon (AC) on the product distribution was investigated. These catalysts were prepared by a wet-impregnation method with acetone, and reduced Ni was observed without the use of H2. CNF presented a high degree of stability in SCW. Ni in its reduced state was still present in all spent catalysts, mainly when CNF were the support. While catalysts supported in AC led to high yields of char and gas, a 56 wt% yield of a light liquid fraction, recovered as dichloromethane (DCM)-soluble product and consisting mainly of (methoxy)phenols (>80 mol%), was obtained in a batch reactor at 400 °C, 230 bar, with Ni-CeO2/CNF as a catalyst. A short reaction time was key to avoid the formation of gas and char. This study demonstrates that high yields of DCM-soluble products from KL and low char formation can be obtained by using only SCW and catalysts, an alternative to widely reported approaches like the addition of organic co-solvents (e.g., phenol) and/or H2.

Journal article

Bermudez JM, Garcia-Fayos J, Reina TR, Reed G, Persoon ES, Görtz D, Schroeder M, Millan M, Serra JMet al., 2018, Thermochemical stability of La<inf>x</inf>Sr<inf>1-x</inf>Co<inf>y</inf>Fe<inf>1-y</inf>O<inf>3-Δ</inf>and NiFe<inf>2</inf>O<inf>4</inf>-Ce<inf>0.8</inf>Tb<inf>0.2</inf>O<inf>2-Δ</inf>under real conditions for its application in oxygen transport membranes for oxyfuel combustion, Journal of Membrane Science, Vol: 562, Pages: 26-37, ISSN: 0376-7388

© 2018 This work addresses the thermochemical stability of ceramic materials –typically used in oxygen transport membranes– under the harsh gas environments found in oxyfuel combustion processes. Specifically, a dual-phase NiFe2O4-Ce0.8Tb0.2O2-δ(NFO-CTO) composite and a single-phase La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) were studied. The effect of the main contaminants present in this kind of processes (CO2, SO2and H2O) has been tested. NFO-CTO composite remains stable under all the conditions studied whereas LSCF presents a poor stability in the presence of CO2and SO2. Regardless of the treatment, NFO-CTO conserves its crystalline structure, without giving rise to new species due to segregation or incorporation of sulphur and/or carbon. On the contrary, LSCF is prone to degradation in contact with CO2and SO2, segregating Sr in the form of SrCO3and SrSO4and Co and Fe in the form of CoO and Fe3O4. It is also shown that SO2interaction with LSCF is stronger than in the case of CO2. A concentration of just 2000 ppm of SO2in CO2is enough to subdue the formation of SrCO3, promoting the segregation of Sr only in the form of SrSO4. With the results presented in this work, it is possible to conclude that the NFO-CTO is a suitable candidate from the thermochemical viewpoint to be used as membrane material in 4-end modules for oxygen generation integrated into oxyfuel combustion processes whereas the use of LSCF should be dismissed.

Journal article

Bizkarra K, Bermudez JM, Arcelus-Arrillaga P, Barrio VL, Cambra JF, Millan Met al., 2018, Nickel based monometallic and bimetallic catalysts for synthetic and real bio-oil steam reforming, International Journal of Hydrogen Energy, Vol: 43, Pages: 11706-11718, ISSN: 0360-3199

© 2018 Hydrogen Energy Publications LLC Catalysts based on Ni supported on alumina were studied for steam reforming (SR) of a synthetic bio-oil/bio-glycerol mixture and a real bio-oil. Catalyst tests were carried out in a continuous fixed bed reactor at atmospheric pressure and steam to carbon (S/C) ratio of 5.0. In the case of experiments with the bio-oil/bio-glycerol mixture the initial temperature was 1073 K, then it was successively changed to 973 K and 1073 K again to assess catalyst deactivation. Experiments with the bio-oil sample were run at 1073 K. First, the effect of modifications to the alumina support with CeO2 and La2O3 was studied in monometallic catalysts. Ni/CeO2–Al2O3 was identified as the catalyst more resistant to deactivation, likely due to its higher oxygen mobility, and selected for further tests. Then, bimetallic catalysts were produced by impregnation of noble metals (Pd, Pt or Rh) on the Ni catalyst supported on CeO2–Al2O3. Co-impregnation of Rh and Ni on the CeO2–Al2O3 support represented a further improvement in the catalytic activity and stability respect to the monometallic catalyst, leading to stable gas compositions close to thermodynamic equilibrium due to the favourable Rh–Ni interactions. Rh–Ni/CeO2–Al2O3 is therefore a promising catalyst to produce a hydrogen-rich gas from bio-oil SR.

Journal article

Na BT, Ovtar S, Yu JH, Hendriksen PV, Kaiser A, Reina TR, Millan M, Kiebach Ret al., 2018, Performance and stability of (ZrO<inf>2</inf>)<inf>0.89</inf>(Y<inf>2</inf>O<inf>3</inf>)<inf>0.01</inf>(Sc<inf>2</inf>O<inf>3</inf>)<inf>0.10</inf>-LaCr<inf>0.85</inf>Cu<inf>0.10</inf>Ni<inf>0.05</inf>O<inf>3-Δ</inf>oxygen transport membranes under conditions relevant for oxy-fuel combustion, Journal of Membrane Science, Vol: 552, Pages: 115-123, ISSN: 0376-7388

© 2018 Elsevier B.V. Self-standing, planar dual-phase oxygen transport membranes consisting of 70 vol% (ZrO2).89(Y2O3).01(Sc2O3).10(10Sc1YSZ) and 30 vol% LaCr.85Cu.10Ni.05O3-δ(LCCN) were successfully developed and tested. The stability of the composite membrane was studied in simulated oxy-fuel power plant flue-gas conditions (CO2, SO2, H2O). The analyses of the exposed composites by X-ray diffraction (XRD), X-ray fluorescence (XRF), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy revealed an excellent stability. Oxygen permeation fluxes were measured across 1000 µm thick and 110 µm thick self-supported 10Sc1YSZ-LCCN (70–30 vol%) membranes from 700 °C to 950 °C using air as the feed gas and N2or CO2as the sweep gas. The 110 µm thick membrane, prepared by tape-casting and lamination processes, showed oxygen fluxes up to 1.02 mLNcm−2min−1(950 °C, air/N2). Both membranes demonstrated stable performances over long-term stability tests (250–300 h) performed at 850 °C using pure CO2as the sweep gas.

Journal article

Yu J, Sun L, Berrueco C, Fidalgo B, Paterson N, Millan Met al., 2018, Influence of temperature and particle size on structural characteristics of chars from Beechwood pyrolysis, Journal of Analytical and Applied Pyrolysis, Vol: 130, Pages: 127-134, ISSN: 0165-2370

This work investigates the effect of temperature and particle size on the product yields and structure of chars obtained from the pyrolysis of Beechwood Chips (BWC), a lignocellulosic biomass. BWC of three different size fractions (0.21-0.50 mm, 0.85-1.70 mm and 2.06-3.15 mm) were pyrolyzed at atmospheric pressure and temperatures ranging from 300 to 900 °C in a fixed bed reactor. Tar and gas yields increased with increasing temperature, while char yield decreased, particularly between 300 and 450 °C. The effect of particle size was mostly observed at temperatures lower than 400 °C as a larger char yield for larger particles due to intraparticle reactions. At higher temperatures the larger surface area in the char fixed bed favoured reactions increasing char and gas yields from the smaller particles. Pyrolysis chars were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. Loss in oxygenated functional groups and aliphatic side chains with increasing temperature was revealed, along with an increase in the concentration of large aromatic systems, leading to a more ordered char structure but no significant graphitization. The changes in char nature at high temperature led to a loss in their combustion reactivity. Raman spectra indicated that the temperature needed to completely decompose the cellulose structure increased with biomass particle size and the enhanced intraparticle reactions in pyrolysis of large particles was likely to give rise to amorphous carbon structures with small fused ring systems.

Journal article

Torres D, Arcelus-Arrillaga P, Millan M, Pinilla JL, Suelves Iet al., 2017, Enhanced reduction of few-layer graphene oxide via supercriticalwater gasification of glycerol, Nanomaterials, Vol: 7

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. A sustainable and effective method for de-oxygenation of few-layer graphene oxide (FLGO) by glycerol gasification in supercritical water (SCW) is described. In this manner, reduction of FLGO and valorization of glycerol, in turn catalyzed by FLGO, are achieved simultaneously. The addition of glycerol enhanced FLGO oxygen removal by up to 59% due to the in situ hydrogen generation as compared to the use of SCW only. Physicochemical characterization of the reduced FLGO (rFLGO) showed a high restoration of the sp2-conjugated carbon network. FLGO sheets with a starting C/O ratio of 2.5 are reduced by SCW gasification of glycerol to rFLGO with a C/O ratio of 28.2, above those reported for hydrazine-based methods. Additionally, simultaneous glycerol gasification resulted in the concurrent production of H2, CO, CH4and valuable hydrocarbons such as alkylated and non-alkylated long chain hydrocarbon (C12–C31), polycyclic aromatic hydrocarbons (PAH), and phthalate, phenol, cresol and furan based compounds.

Journal article

Arcelus-Arrillaga P, Hellgardt K, Millan M, 2017, Effect of process conditions on the hydrothermal partial oxidation of phenanthrene as a heavy oil model structure, FUEL, Vol: 209, Pages: 434-441, ISSN: 0016-2361

Journal article

Pirou S, Bermudez JM, Hendriksen PV, Kaiser A, Reina TR, Millan M, Kiebach Ret al., 2017, Stability and performance of robust dual-phase (ZrO<inf>2</inf>)<inf>0.89</inf>(Y<inf>2</inf>O<inf>3</inf>)<inf>0.01</inf>(Sc<inf>2</inf>O<inf>3</inf>)<inf>0.10</inf>-Al<inf>0.02</inf>Zn<inf>0.98</inf>O<inf>1.01</inf>oxygen transport membranes, Journal of Membrane Science, Vol: 543, Pages: 18-27, ISSN: 0376-7388

© 2017 Elsevier B.V. Dual-phase composite oxygen transport membranes consisting of 50 vol% Al0.02Zn0.98O1.01and 50 vol% (ZrO2)0.89(Y2O3)0.01(Sc2O3)0.10were successfully developed and tested. The applicability of the membrane in oxy-fuel power plants schemes involving direct exposure to flue gas was evaluated by exposing the membrane to gas streams containing CO2, SO2, H2O and investigating possible reactions between the membrane material and these gases. The analyses of the exposed composites by x-ray diffraction (XRD), x-ray fluorescence (XRF), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy revealed excellent stability. Additionally, an electrical conductivity measurement over 900 h confirmed that the composite is stable under prolonged exposure to CO2. However, an instability of the dual-phase membrane under oxygen partial pressures below PO2~10−4atm. was found. Oxygen permeation tests on a 1 mm thick self-standing membrane resulted in an oxygen flux of 0.33 mLNmin−1cm−2at 925 °C in air/N2. Stability tests in CO2with 3 vol% O2demonstrated the potential for the use of 10Sc1YSZ-AZO dual-phase membranes in oxy-combustion processes involving direct exposure to flue gas.

Journal article

Arcelus-Arrillaga P, Pinilla JL, Hellgardt K, Millan Met al., 2017, Application of Water in Hydrothermal Conditions for Upgrading Heavy Oils: A Review, ENERGY & FUELS, Vol: 31, Pages: 4571-4587, ISSN: 0887-0624

Journal article

Millán M, 2017, Effect of Metal Loading in NiMo/Al<inf>2</inf>O<inf>3</inf> Catalysts on Maya Vacuum Residue Hydrocracking, Energy and Fuels, Vol: 31, Pages: 4843-4850, ISSN: 0887-0624

© 2017 American Chemical Society. Hydrocracking catalysts with large porosity need to be developed to treat heavy oil feedstocks rich in large molecular weight components such as asphaltenes and withstand deactivation due to coke formation. In this work, catalysts were prepared by impregnation of varying NiMo loadings on a mesoporous Al2O3 support. The effect of metal loading on the hydrocracking of a vacuum residue at three temperatures (400, 425, and 450 °C) was studied in a batch microbomb reactor. Catalysts were reutilized in a second reaction with fresh feed to assess their activity following the initial period where results are dominated by a large carbon deposition. The textural properties and the coke content on the spent catalysts were evaluated after both reactions. It was found that the reaction temperature had an important effect on the conversion of the fraction with a boiling point above 450 °C, whereas metal loading had minimal impact. On the other hand, metal loading had a significant effect on hydrodeasphaltenization (HDA); higher asphaltene conversions were obtained with higher metal loading. Reaction temperature had an influence on HDA, particularly for lower metal loadings, as catalysts with higher loadings showed significant activity at the lower temperatures studied. It was observed that coke deposits were mainly formed during the initial hour of reaction with little additional coke being formed in the reutilization of the catalysts. More deposits were obtained at lower reaction temperature, as coke precursors, polyaromatic hydrocarbons, polymerize into coke. No evidence of pore mouth plugging was observed, indicating that catalysts could accommodate coke while retaining most of their textural properties. Catalysts with higher metal loadings took longer to reach a stable amount of deposits, but they stabilized at an overall smaller coke deposition and retained significant HDA activity.

Journal article

Yu J, Paterson N, Blamey J, Millan Met al., 2017, Cellulose, xylan and lignin interactions during pyrolysis of lignocellulosic biomass, Fuel, Vol: 191, Pages: 140-149, ISSN: 0016-2361

The three primary lignocellulosic biomass components (cellulose, xylan and lignin), synthetic biomass samples (prepared by mixing the three primary components) and lignocellulosic biomass (oak, spruce and pine) were pyrolysed in a thermogravimetric analyser and a wire mesh reactor. Different reactivities were observed between the three biomass components. Cellulose mainly produced condensables and was less dependent on heating rate, while xylan and lignin contributed most char yields and were significantly affected by heating rate. While xylan and lignin pyrolysed over a large temperature range and showed the behaviour characteristic of solid fuels, cellulose decomposition is sharp in a narrow temperature range, a behaviour typical of linear polymers. Comparison of the pyrolysis behaviour of individual components with that of their synthetic mixtures showed that interactions between cellulose and the other two components take place, but no interaction was found between xylan and lignin. No obvious interaction occurred for synthetic mixtures and lignocellulosic biomass at 325 °C, before the beginning of cellulose pyrolysis, in slow and high heating rate. At higher pyrolysis temperatures, more char was obtained for synthetic mixtures containing cellulose compared to the estimated value based on the individual components and their proportions in the mixture. For lignocellulosic biomass, less char and more tar were obtained than predicted from the components, which may be associated with the morphology of samples. The porous structure of lignocellulosic biomass provided a release route for pyrolysis vapours.

Journal article

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