The goal of this study was to test the effect of metal-impregnated carbon-based catalysts on the conversion of methane to hydrogen gas and solid carbon using microwave reactor technology. Monometallic and bimetallic catalysts on activated carbon supports (Ni/AC, Fe/AC, NiFe/AC) are compared during methane pyrolysis testing. Catalytic methane pyrolysis was carried out in a microwave reactor at reaction temperatures of 600 degrees C and 800 degrees C. For comparison, one of the catalysts (NiFe/AC) was tested in a conventionally heated reactor at 800 degrees C. The prepared catalysts were characterized by X-ray diffraction (XRD), while post-reaction catalysts were characterized by XRD and SEM. During reaction testing, the monometallic Ni/AC catalyst exhibited the best catalytic activity (CH4 conversion: 46.0 and H2 yield: 46.9 %) when reacted in the microwave reactor, however, it suffered from rapid deactivation from carbon deposition (carbon yield: 0.39 g C/g catalyst). The bimetallic NiFe/AC catalyst was slightly less active (CH4 conversion: 36.9 and H2 yield: 40.5 %) but it was more resistant to carbon formation (carbon yield: 0.27 g C/g catalyst) suggesting it may have greater long-term stability. The NiFe/AC catalyst was also the most energy efficient as it required the least microwave power to maintain the 800 degrees C reaction temperature compared to the other catalysts tested. Methane conversion of the bimetallic NiFe/AC at 800 degrees C under microwave irradiation was three times the conversion under conventional heating at the same reaction temperature. This work demonstrates the use of microwave-specific catalysts for catalytic methane pyrolysis in a microwave reactor, and can be used as a foundation for further methane pyrolysis process and catalyst optimization for COx-free H2 production.

Co-gasification of waste plastic and corn stover into hydrogen is an effective way of reducing hydrogen cost and carbon footprint. However, current gasification technologies are energy intensive and cannot accommodate flexible feedstock. Microwave gasification offers several advantages including higher H2 yield and enhanced selectivity to syngas over tars. This work presents parametric optimization study of microwave co-gasification of mixed plastics and corn stover for hydrogen-rich syngas. High hydrogen yields of 30.5 mmolH2/gfeedat 700 degrees C were obtained using a microwave reactor, as opposed to 0.9 mmolH2/gfeedat 700-950 degrees C with conventional heating. Hydrogen yields are >98% of the theoretical extractable hydrogen from feedstock. Microwaves enhanced plastic and corn stover synergy and reduced tar yields. H-poor carbon formation resulting from the dehydrogenation of tar and coke, and light olefins decomposition, triggered by the external microwave electric fields, allowed further cracking of intermediate hydrocarbons to produce additional H2.

The structure and composition of residual coal char after pyrolysis dictates its reactivity towards gasification. The nature of the char is a result of the parent coal composition and structure as well as the reaction conditions during pyrolysis. Further, due to the unique characteristics of selective dielectric heating, char development during microwave CO2 gasification may differ from conventional thermal gasification, and little is known about the effect of microwave heating on the char structure development. In this work the heating method used is compared (microwave vs conventional thermal) to generate char samples from four different coal types. These coal chars generated by pyrolysis and gasification were characterized by ultimate analysis, surface area analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and dielectric characterization to investigate the chemical and structural differences between microwave- and conventionally-generated chars. The results reveal notable chemical and structural differences between the chars from microwave and conventional pyrolysis and gasification. In general, the conventional chars had greater pore structure development as seen by higher specific surface areas compared to microwave chars, possibly due to rapid thermal collapse of coal pore structure. Microwave pyrolysis chars had greater ordering of the carbon crystal structure for low-ash coals, while graphitization was inhibited for coals with high ash percentage yield. The observations from this study highlight the differences in char characteristics from microwave and conventional pyrolysis and gasification, which can aid microwave gasifier design, implementation, and integration for efficient syngas production and coal ash utilization.

Effects of gasifier conditions (microwave or conventional) on the gasification characteristics of four different types of coal are evaluated by analysis of gaseous species generated during pyrolysis and gasification reactions. Four different coal samples are tested: a lignite (Mississippi), a low-ash subbituminous (Wyodak), a high-ash subbituminous (Usibelli), and a low volatile bituminous (Pocahontas #3). Gas composition, overall yields, carbon conversion efficiency, and cold gas efficiency are evaluated to compare gasification reactivity of the different coal types under microwave and conventional heating. During microwave pyrolysis (Ar atmosphere), greater selectivity of syngas species (H-2 + CO) and greater overall non-condensable gas yields are observed, compared to conventional pyrolysis for all coals tested. The high yield of syngas during microwave pyrolysis is attributed to primary pyrolysis gases subsequently gasifying the char of the same sample to produce greater amounts of H-2 and CO. During microwave gasification (CO2 atmosphere), selective heating and formation of hotspots within the coal enable the reverse Boudouard reaction to occur at a low bulk gasification temperature of 700 ?, which is less favorable under thermal gasification at this temperature. Yields of syngas are much higher under microwave gasification of all coals. Carbon conversion is found to have a linear correlation with volatile matter of the parent coal. Carbon conversion and cold gas efficiency are highest for coals gasified under microwave irradiation, and gasification reactivity of the four coals was determined to be dependent on the coal type.

Understanding microwave-material interactions will help facilitate the utilization of microwave technology in gasification and renewable energy production. In this study, cellulose was used as a model compound to simulate the organic matter in biomass, and its catalytic decomposition under a microwave (MW) field was studied to identify structural changes from the reaction. The study was conducted using a MW source coupled to a fixed-bed gas-flow reactor, mass spectroscopic, and Fourier transform infrared spectroscopy post-reaction analysis. Zeolite 13X was chosen as a microwave absorber to study the catalytic enhancement of the decomposition of cellulose. Density functional theory (DFT) was used to gain insights into the molecular transformations occurring in the presence of a static electric field, which was used to simulate the electric field component of the microwave electromagnetic radiation providing a theoretical basis for molecular sites to be selectively heated. Theoretical calculations demonstrated that both the positive and negative portion of the electric field interact with the permanent dipoles of the cellulose leading to Debye-type loss processes and localized heating indicated by the decomposition through the glycosidic bond breaking mechanism. The theoretical result was verified using infrared spectroscopic analysis of the pure cellulose during microwave heating. The theoretical calculations help to elucidate the dipoles and molecular bonds in the cellulose structure, which are more sensitive to selectively localized heating as compared to other bonds and conventional heating. Physically mixing Zeolite 13X with the cellulose led to a significant enhancement in the decomposition rate of the glycosidic bond. Zeolite 13X enhanced the glycosidic O-C decomposition at lower MW power (lower temperatures), whereas the O-H functional group required higher MW power (higher temperature) for its decomposition. The DFT study coupled with the reaction studies revealed that the electric field polarizability, and subsequent, localized heating, is dependent upon both the direction and the orientation of the cellulose. Gas products revealed that applying 250 W of MW power led to the production of CO, H-2, along with some CO2, CH4, and benzene at 305 degrees C. Reaction under 500, 750, and 1000 W of power at constant temperature (305 degrees C) revealed that higher power led to the complete decomposition of cellulose to mostly CO, H-2, and CO2.

Iron-based oxygen carriers with a CeO2 support have interesting redox applications in chemical-looping combustion (CLC). CeO2 behaves as an active support for many oxygen carrier applications because of the reversible release of lattice oxygen. High temperature processes may lead to improved process efficiency; however, the poor thermal stability of CeO2 gives rise to a need for oxygen carriers that can resist sintering and agglomeration and maintain reactivity after multiple reduction and oxidation (redox) cycles during in situ gasification chemical-looping combustion (iG-CLC) at 1100 degrees C. In the present study, Fe-based oxygen carriers on CeO2, ZrO2, and Ce0.75Zr0.25O2 supports were prepared by a coprecipitation method. The redox stability, reactivity, and sintering of the oxygen carriers were evaluated to investigate the effect of the CeO2-ZrO2 solid solution support. The oxygen transport capability was evaluated in a drop tube fixed-bed reactor under iG-CLC conditions with coal char at 1100 degrees C for 10 redox cycles. The CeO2-ZrO2 solid solution improved the oxygen mobility of the support from the creation of more oxygen defects. The Fe-Ce oxygen carrier had the highest oxygen transport capability because of the formation of cerium orthoferrite (CeFeO3) during high temperature reduction. The Fe-Ce-Zr oxygen carrier showed improved reactivity over the Fe-Ce oxygen carrier as the number of redox cycles increased. The oxygen carriers, before and after multiple redox cycles, were characterized by X-ray diffraction, scanning electron microscopy, and surface/pore analysis.

Chemical looping combustion (CLC) allows combustion with less complex effluent than typical atmospheric combustion. CLC, which oxidizes the fuel with a regenerable solid oxygen carrier (OC), typically requires a step to separate unburned carbon and ash from the OC using a separation unit. This study investigated the use of high temperature combustion (1100°C) and low-cost OCs prepared using readily available natural and industrial waste materials to potentially reduce the need for the separation step. Two OC materials consisting of 1) iron-impregnated FCC, a spent fluid cracking catalyst and 2) red mud, the main waste product from the extraction of alumina from bauxite, were compared to a benchmark natural hematite. The high temperature conditions led to sintering and attrition of the hematite. Red mud was found to also suffer apparent sintering and attrition. Iron-impregnated FCC showed little morphology change or attrition when the fuel was limited to the amount that could be oxidized without reducing the iron beyond Fe3O4. The most promising candidate proved to be FCC loaded with a moderate amount of iron, and optimization may further improve the stability and performance.

Investigation of the redox reactivity of mixed-metal oxides in Chemical-Looping Combustion (CLC) can improve our understanding of the associated reaction mechanisms that are related to this technology. The Fe-Mn-based oxygen carrier supported on a spent fluid catalytic cracking catalyst (FCC) was characterized during CLC of CH4 by fixed bed reactor studies coupled with mass spectrometry, X-ray diffraction, and Raman spectroscopic analysis. Research was carried out to investigate Mn interaction with the FCC support during the high temperature (1100 degrees C) CLC and the potential impact on performance. The addition of Mn to Fe/FCC led to an increase in the oxygen transfer capacity at 900 degrees C and a decrease in oxygen transfer capacity at 1100 degrees C. Following 15 redox cycles, the average methane conversion was 85% at 900 degrees C and 68% at 1100 degrees C. The Fe-Mn/FCC carriers were selective for CO2 at 900 degrees C, and selective for CO at 1100 degrees C, which is consistent with the thermodynamic limitation of CO2 production at high-temperature. The Fe-Mn/FCC oxygen carrier's crystal structure remained stable at 900 degrees C, whereas at 1100 degrees C, XRD and Raman spectroscopic analysis revealed formation of MnAl2O4 and Al2Mn3Si3O12 phases due to reduced Mn metal alloying with the aluminosilicate. The decrease in reactivity and oxygen transfer capacity at 1100 degrees C was attributed to the formation of new Fe-Mn phases and alloying with the aluminosilicate, sintering, and agglomeration at high-temperature. These results demonstrate the need to stabilize the Mn-active phase of oxygen carriers on aluminosilicate supports being used for the high-temperature CLC application.

Addition of methane during microwave coal pyrolysis could greatly affect the product distribution to valuable products including the formation of char. In this work, low rank coal was exposed to microwave energy in the presence of different methane concentrations (0, 25, 50, and 90%) at 980 degrees C for 2 h. Increasing methane concentration was found to increase both the char and tar yields mainly due to carbon deposition during methane decomposition and hydrogenation of trapped carbon into tars. Analysis of the gas composition suggested that some of the methane was activated in the presence of the coal minerals, which could act as a catalyst, forming light hydrocarbons C-2-C-7 which account for up to 5% of the gaseous products at a methane concentration of 90%; they accounted for less than 0.5% in the absence of methane. Methyl groups may also have been substituted into aromatic compounds as observed in the tar analysis where the number of methyl substitutions in the detected parent phenol and naphthalene increased with methane concentration. Tar yield could also increase indirectly with methane addition through hydrogenation reactions with unsaturated coal compounds. The formation of char under different methane concentrations was examined by many characterization tools including Raman, dielectric properties, XRD, BET, SEM, and EDS. The results suggest that the addition of methane did not help in forming ordered carbon chars, not only due to amorphous carbon deposition on the char surface but also due to hydrogenation or alkylation reactions with char. The presence of methane enhanced the formation of C(2)s and benzene; a possible correlation between the formation rate of benzene and hydrogen during pyrolysis is proposed.

Direct conversion of a low-rank coal into valuable chemicals or improving its char's coking value became very demanding goals in coal utilization strategies. In this work, the co-pyrolysis of a low-rank lignite coal and pine wood sawdust biomass blended at a 3:1 coal-to-biomass ratio was investigated along with original coal and biomass samples by microwave assisted and conventional thermal methods at 550 degrees C under nitrogen and ambient pressure. The carbon structure and its reactivity in generated chars and the product distributions were greatly affected by the applied heating mechanism and the presence of biomass during coal pyrolysis. High gas and low tar yields were observed for all microwave chars in comparison to thermal chars, regardless of composition. The addition of biomass to coal increased the tar yield under both methods and to a higher extent under the microwave. This agrees with the high gas yield and high aromatic-to-aliphatic fraction observed under the microwave and the presence of biomass. The high O/C ratio and low fixed carbon content in a biomass structure relative to coal affect the product distribution during microwave pyrolysis. This could selectively heat the biomass in the sample, remove its polar groups, and convert it into an efficient microwave absorber biochar that can decompose coal efficiently during co-pyrolysis. The aromatic carbon stacking and its ordering in the generated chars were investigated by powder X-ray diffraction, Raman spectroscopy, dielectric property measurements, and electron spin resonance techniques. A synergistic effect was observed upon biomass addition during microwave coal pyrolysis. Electron spin resonance spectroscopy revealed that the microwave coal/biomass char is the most stable char with the lowest free radical concentration. This agrees with the highest I-G/band area ratio calculated from Raman analysis revealing a more graphitic nature for carbon in this char. Similarly, the dielectric properties confirmed that the addition of biomass to coal under the microwave has the highest loss tangent, indicating a high graphitic nature compared to pure biochar or coal char.

Chemical-looping combustion with oxygen uncoupling (CLOU) is a process using gaseous or solid hydrocarbon fuels and is a promising carbon capture and storage (CCS) technology. In CLOU, combustion of the fuel is achieved through the release of gaseous O-2 from an oxygen carrier material such as CuO and is favored at high temperatures and low O-2 partial pressures. The primary objectives of this study were to (1) compare values for the apparent rate constant k(ov)(T) for the overall rate of reduction of a CuO carrier, either alone or in the presence of coal char obtained from the pyrolysis of Powder River Basin (PRB) coal, and (2) develop and validate a computational fluid dynamics (CFD) model for the CuO/PRB coal char system based on known kinetics of the individual CuO and Cu2O reduction, combustion, and gasification reactions. Two oxygen carriers consisting of 20 wt % CuO/Al2O3 and 9 wt % CuO/Al2O3 were prepared by physical mixing and by incipient wetness impregnation (IWI), respectively. Kinetic analyses were conducted in the temperature range of 850-1100 degrees C. The CFD model typically reproduced experimental values of k(ov)(T) to within +/- 10%. The increase in k(ov)(T) caused by addition of only 0.075 g of PRB char per gram of CuO was modest at 1100 degrees C, but was more than 3-fold at 850 degrees C. Combustion and gasification of the coal char not only produced CO, opening another pathway for CuO reduction in addition to the CLOU reaction, but also resulted in the over-reduction of Cu2O to the undesired metallic Cu. Production of Cu metal increased with respect to char loading.

Chemical looping combustion with oxygen uncoupling (CLOU) is a promising carbon capture and storage (CCS) technology for conversion of gaseous and solid hydrocarbon fuels where the release of gaseous O-2 from an oxygen carrier is favored at high temperature and low O-2 partial pressure. One promising CLOU material is the copper oxide redox system (CuO-Cu2O). The primary objective of this study was to examine the use of a drop tube fluidized bed reactor (DT-FBR) for evaluating the kinetics of oxygen uncoupling with Cu-based oxygen carriers. Appropriate rate expressions from redox experiments are needed to model and scale up CLOU systems. Additionally, the determined oxygen uncoupling kinetic parameters were validated using a computational fluid dynamic model. Two oxygen carriers consisting of 20 wt % CuO/Al2O3 and 9 wt % CuO/Al2O3 were prepared by physical mixing. A third oxygen carrier sample was prepared at 9 wt % CuO/Al2O3 by incipient wetness impregnation. The reaction rates of the copper oxide redox system are strongly dependent on thermodynamic effects of the oxygen partial pressure relative to the equilibrium oxygen partial pressure. The results presented in this paper offer an alternative and simplified kinetic analysis compared to that traditionally presented in the literature for the thermal reduction of copper oxide or other CLOU oxygen carriers. The reactor system used in this study allows for operating parameters to be adjusted, minimizing thermodynamic and mass transfer limitations, which eliminates the need for more complex kinetic/thermodynamic reaction models. The reaction kinetics measured in this study is compared based on the preparation method and the CuO weight percent loading. These results may aid in the development of CLOU technologies during reactor design and process modeling.

Chemical looping combustion is based on the transfer of oxygen from air to fuel by means of an oxygen carrier using a metal oxide. This study investigated the development of an oxygen carrier by adding iron to a spent commercial fluid catalytic cracking catalyst (Fe/FCC) and comparing to Fe/Al2O3, and Fe/SiO2 oxygen carriers that may be suitable for the high-temperature (1100 degrees C) process. The FCC material was chosen as a low-cost oxygen carrier. The FCC support strongly modified the reduction behavior of the Fe-based carrier as compared to the Fe/Al2O3 and Fe/SiO2 carriers. Multicycle studies with CH4 revealed the FCC support impacted the CO2 selectivity (13.5%) at 1100 degrees C to produce more gaseous CO as compared to the 900 degrees C (89.7%) due to the formation of FeAl2O3 and Fe2SiO4 phases when the Fe2O3 was reduced at 1100 degrees C. The FeAl2O3 and Fe2SiO4 phases also considerably slowed the CO2 formation rate on the Fe/FCC oxygen carrier at 1100 degrees C as compared to the 900 degrees C reaction tests. The higher-temperature (1100 degrees C) decreased the oxygen transfer capacity of the Fe/FCC carrier, did not significantly increase the capacity of either the Fe/SiO2 and Fe/Al2O3 carriers. These results suggest that high-temperatures (1100 degrees C) may lead to carrier transformations through sintering and agglomeration that significantly reduce the kinetics and performance of the oxygen carrier. This research suggests at higher temperatures, shorter reduction times may be necessary to prevent the formation of these phases and the subsequent deactivation of the carrier for total oxidation.

Pyrolysis conditions greatly affect the structure-reactivity relationship of char during coal gasification. This work investigated the effect of temperature and microwave heating on the structural properties of the chars generated during pyrolysis, as well as gaseous and tar products. Results showed that microwave pyrolysis of Mississippi coal produced more gaseous products and less tars compared to conventional pyrolysis. Higher CO/CO2 ratio (> 1) was observed under microwave pyrolysis compared to conventional pyrolysis (CO/CO2 < 1), which may be explained by a greater extent of gasification between solid carbon and the CO2 formed during microwave pyrolysis. Additionally, in microwave pyrolysis, the oil tars generated exhibited lower concentrations of polar oxygenates, while the wax tars showed higher concentrations of non-polar alkanes, as observed from the intensity of C-H vibrations in FTIR. The product compositions and FTIR analysis of the tars (oils and waxes) suggest that the microwave interacted preferentially with these polar species, which have relatively higher dielectric properties compared to alkanes. The structure-reactivity relationship of the chars produced was also investigated using a variety of characterization tools such as XRD, BET, SEM, EDS, and FTIR. Finally, the char reactivity towards combustion suggested that microwave-produced chars have a higher thermal stability, likely due to lower O/C ratios, and could be utilized in the metallurgical industry.