Kinetic analysis of solid fuel combustion in chemical looping for clean energy conversion

Chemical looping combustion (CLC) offers an advanced, eco-friendly method for converting solid fuels into energy with inherent CO2 capture, presenting a cost-efficient solution. The kinetics of solid fuel CLC, crucial for reactor design, are not well-understood, limiting its application and optimisation. Conducting a comprehensive kinetic analysis of solid fuel CLC is crucial, as it examines the combustion of both volatiles and fixed carbon, thereby addressing existing knowledge gaps and advancing clean conversion of solid fuels. In this study, a comprehensive kinetic analysis method for the first time has been applied to investigate the kinetics of CLC of low-volatile semi-anthracite coal with CuO at temperatures of 700–950 °C under varied oxygen excess ratios (0.5, 1.0, and 2.0). The results show that the combustion of coal in CLC can be divided into three distinct stages. The combustion of coal with CuO initiates with the combustion of volatiles interacting with solid CuO at 450–580 °C, where the combustion efficiency varies between 3–6 wt%. This is followed by a complex simultaneous mechanism involving gas-phase volatiles and solid-phase CuO, as well as gas-phase oxygen and solid-phase fixed carbon under non-isothermal conditions. The combustion efficiency is ranged 19–71 wt% at the temperature ranges from 580 °C to the isothermal temperatures of 750–950 °C. The activation energy for the combustion of volatiles was determined as Ea = 119 kJ/mol, whereas the initial combustion of fixed carbon in the non-isothermal stage ranged between Ea = 39–53 kJ/mol. The rate of combustion is initially limited by the oxygen diffusion rate from CuO, but with additional oxygen carriers and increased temperature, the reaction becomes constrained by first-order kinetics. Upon reaching the isothermal stage, the final combustion phase between fixed carbon and gas phase oxygen occurs, and the combustion efficiency increases to 77–96 wt% at 750–950 °C. The activation energy for fixed carbon combustion under the isotherm stage was approximately Ea = 234 kJ/mol, with a reaction rate constant (k0) of 7.84 × 109 min−1. This pioneering study not only clarifies the multi-stage kinetics of solid fuel CLC, bridging significant gaps in current knowledge but also sets the foundation for the enhanced design and efficiency of CLC systems for cleaner energy production

Demonstrating the applicability of chemical looping combustion for fluid catalytic cracking unit as a novel CO2 capture technology

Fluid Catalytic Cracking (FCC) units are responsible for ~25% of CO2 emissions released from the oil refineries, which themselves account for 4-6% of total global CO2 emissions. Although post- and oxy-combustion technologies have been suggested to capture the CO2 released from the regenerator of FCC, Chemical Looping Combustion (CLC) may also be a potential approach to capture the CO2 released from the regenerators in FCC units with lower energy consumption. In this study, the applicability of CLC for the FCC unit was investigated as a novel approach for CO2 capture. In order to demonstrate the applicability of CLC, four main aspects were studied. A refinery FCC catalyst (Equilibrium Catalyst–ECat) was firstly modified with reduced oxygen carriers e.g. Copper (Cu), Copper (I) oxide (Cu2O), Cobalt (II) oxide (CoO), and Manganese (II, III) oxide (Mn3O4) and oxidised oxygen carriers e.g. Copper (II) oxide (CuO), Cobalt (II, III) oxide (Co3O4), and Manganese (II) oxide (Mn2O3) using mechanical mixing and wet-impregnation methods. Secondly, to identify any detrimental effects of the oxygen carriers on cracking, both reduced and oxidised oxygen carrier modified ECat formulations were tested for n-hexadecane cracking using the standard test method of FCC catalysts. Then, to investigate the CLC behaviour of coke with oxidised oxygen carriers (CuO, Co3O4 and Mn2O3), thermogravimetric analyses (TGA) were conducted on a low volatile semi-anthracite Welsh coal, which has a similar elemental composition to actual FCC coke. Finally, the CLC of coke deposited on the reduced oxygen carrier impregnated ECat was investigated with the stoichiometrically required amount of oxidised oxygen carrier impregnated ECat. The CLC tests were investigated in a fixed-bed and a fluidised-bed reactors equipped with an online mass spectrometer to monitor CO2 release. The results demonstrated that mechanical mixing of Cu with ECat was shown to have a negative impact on the cracking of n-hexadecane. However, the mixing of Cu2O, CoO, and Mn3O4 with ECat had no significant effect on gas, liquid and coke yields on product selectivity. Additionally, wet-impregnation of Cu, MnO, and Mn3O4 had a negligible impact on the cracking of n-hexadecane in terms of conversion, yields and product selectivity. In terms of the CLC tests of coke, complete combustion of the model coke was achieved with bulk CuO, Co3O4, and Mn2O3 when the stoichiometric ratio of oxygen carrier to coke was maintained higher than 1.0 and sufficient time was provided. Furthermore, although 90 vol.% combustion efficiency of the coke deposited on ECat was reached with bulk CuO and Mn2O3, the regeneration temperature required (800 °C) was not realistic in terms of commercial regenerator conditions. However, a relatively high combustion efficiency, (> 94 vol.%) of the coke deposited on reduced Cu and Mn3O4 impregnated ECat was achieved with the stoichiometrically required amount of CuO and Mn2O3 impregnated ECat at the conditions used in conventional FCC regenerators, 750 °C for 45 min. According to these results, CLC is a promising technology to incorporate into the next generation of FCC units to optimise CO2 capture. In addition to the application of CLC for FCC, the selective low temperature CLC of higher hydrocarbons was discovered during the cracking tests of n-hexadecane over oxidised oxygen carriers mixed ECat. Therefore, CLC of n-hexadecane and n-heptane with CuO and Mn2O3 was investigated in a fixed bed reactor to reveal the extent to which low temperature CLC can potentially apply to hydrocarbons. The effects of fuel to oxygen carrier ratio, fuel feed flow rate and fuel residence time on the extent of combustion were reported. Methane did not combust, while near complete conversion was achieved for both n-hexadecane and n-heptane with excess oxygen carrier for CuO. For Mn2O3, total reduction to Mn3O4 occurred, but the slower reduction step to MnO controlled the extent of combustion. Although the extent of cracking is negligible in the absence of cracking catalysts, for the mechanism to be selective for higher hydrocarbons suggests that the reaction with oxygen involves radicals or carbocations arising from bond scission. Sintering of bulk CuO occurred after repeated cycles, but this can easily be avoided using alumina support. The fact that higher hydrocarbons can be combusted selectively at 500 °C and below, offers the possibility of using CLC to remove these hydrocarbons and potentially other organics from hot gas streams

Progress in the CO2 Capture Technologies for Fluid Catalytic Cracking (FCC) Units—A Review

© Copyright © 2020 Güleç, Meredith and Snape. Heavy industries including cement, iron and steel, oil refining, and petrochemicals are collectively responsible for about 22% of global CO2 emissions. Among these industries, oil refineries account for 4–6%, of which typically 25–35% arise from the regenerators in Fluid Catalytic Cracking (FCC) units. This article reviews the progress in applying CO2 capture technologies to FCC units. Post combustion and oxyfuel combustion have been investigated to mitigate CO2 emissions in FCC and, more recently, Chemical Looping Combustion (CLC) has received attention. Post combustion capture can readily be deployed to the flue gas in FCC units and oxyfuel combustion, which requires air separation has been investigated in a pilot-scale unit by Petrobras (Brazil). However, in comparison, CLC offers considerably lower energy penalties. The applicability of CLC for FCC has also been experimentally investigated at a lab-scale. As a result, the studies demonstrated highly promising CO2 capture capacities for FCC with the application of post combustion (85–90%), oxyfuel combustion (90–100%) and CLC (90–96%). Therefore, the method having lowest energy penalty and CO2 avoided cost is highly important for the next generation of FCC units to optimize CO2 capture. The energy penalty was calculated as 3.1–4.2 GJ/t CO2 with an avoiding cost of 75–110 €/t CO2 for the application of post combustion capture to FCC. However, the application of oxyfuel combustion provided lower energy penalty of 1.8–2.5 GJ/t CO2, and lower CO2 avoided cost of 55–85 €/t CO2. More recently, lab-scale experiments demonstrated that the application of CLC to FCC demonstrate significant progress with an indicative much lower energy penalty of ca. 0.2 GJ/t CO2

Selective low temperature chemical looping combustion of higher alkanes with Cu- and Mn- oxides

Chemical looping combustion (CLC) of n-hexadecane and n-heptane with copper and manganese oxides (CuO and Mn2O3) has been investigated in a fixed bed reactor to reveal the extent to which low temperature CLC can potentially be applicable to hydrocarbons. The effects of fuel to oxygen carrier ratio, fuel feed flow rate, and fuel residence time on the extent of combustion are reported. Methane did not combust, while near complete conversion was achieved for both n-hexadecane and n-heptane with excess oxygen carrier for CuO. For Mn2O3, complete reduction to Mn3O4 occurred, but the extent of combustion was controlled by the much slower reduction to MnO. Although the extent of cracking is relatively small in the absence of cracking catalysts, for the mechanism to be selective for higher hydrocarbons suggests that the reaction with oxygen involves radicals or carbocations arising from bond scission. Sintering of pure CuO occurred after repeated cycles, but this can easily be avoided using a support, such as alumina. The fact that higher hydrocarbons can be combusted selectively at 500 °C and below, offers the possibility of using CLC to remove these hydrocarbons and potentially other organics from hot gas streams

A novel approach to CO2 capture in Fluid Catalytic Cracking-Chemical Looping Combustion

Oil refineries collectively account for about 4–6% of global CO2 emissions and Fluid Catalytic Cracking (FCC) units are responsible for roughly 25% of these. Although post-combustion and oxy-combustion have been suggested to capture CO2 released from the regenerator of FCC units, Chemical Looping Combustion (CLC) is also a potential approach. In this study, the applicability of CLC for FCC units has been explored. A refinery FCC catalyst (equilibrium catalyst-ECat) was mixed mechanically with reduced oxygen carriers; Cu, Cu2O, CoO, and Mn3O4. To identify any detrimental effects of the reduced oxygen carriers on cracking, the catalyst formulations were tested for n-hexadecane cracking using ASTM D3907-13, the standard FCC microactivity test (MAT). To investigate the combustion reactivity of coke with physically mixed oxidised oxygen carriers, CuO, Co3O4 and Mn2O3, TGA tests were conducted on a low volatile semi-anthracite Welsh coal, which has a similar elemental composition to actual FCC coke, with various oxygen carrier to coke ratios over the temperature range 750–900 °C.The results demonstrated that, whereas Cu was detrimental for cracking n-hexadecane with the ECat, Cu2O, CoO, and Mn3O4 have no significant effects on gas, liquid and coke yields, and product selectivity. Complete combustion of the model coke was achieved with CuO, Co3O4 and Mn2O3, once the stoichiometric ratio of oxygen carrier/coke was higher than 1.0 and sufficient time had been provided. These results indicate that the proposed CLC-FCC concept has promise as a new approach to CO2 capture in FCC

Accelerated Corrosion Behaviors of Zn, Al AND Zn/15Al Coatings on a Steel Surface

Zn, Al and Zn/15Al coatings can be produced in optimum conditions by the twin wire arc (TWEA) spraying technique. The coatings are used for corrosion protection in a variety of industrial applications. In this study, the accelerated corrosion behavior of Zn, Al and Zn/15Al coatings on a steel surface during the salt-spray testing period was investigated. The surfaces of steel coupons were coated with Zn, Al and Zn/15Al using the TWEA spray-deposition system. The corrosion test was performed in a chloride atmosphere in a salt-spray test for over 2000 h. The corrosion of samples is assessed as the ratio of the corroded area of the specimens. The salt-spray test results showed that Al and Zn/15Al coatings have a better corrosion resistance than Zn coatings

Methylation of 2-methylnaphthalene over metal-impregnated mesoporous MCM-41 for the synthesis of 2,6-triad dimethylnaphthalene isomers

2,6-Dimethylnaphthalene (2,6-DMN) is one of the key intermediates for the production of polyethylene naphthalate (PEN), which demonstrates superior properties compared with the polyethylene terephthalate. However, the complex synthesis procedure of 2,6-DMN increases the production cost and decreases the commercialisation of PEN. In this study, selective synthesis of 2,6-triad DMN isomers (1,5-DMN, 1,6-DMN and 2,6-DMN) has been investigated by the methylation of 2-methylnaphthalene (2-MN) over mesoporous Cu/MCM-41 and Zr/MCM-41 zeolite catalysts. On the contrary of other DMN isomers, 2.6-triad isomers can effectively be converted to be profitable 2,6-DMN with an additional isomerisation reaction, which is a new approach to reach higher 2,6-DMN yield. The methylation reactions of 2-MN were investigated in a fixed-bed reactor at 400 °C and weight hourly space velocity of 1–3 h−1. The results showed that the activity of MCM-41 on the methylation of 2-MN has been enhanced with the impregnation of Cu. The conversion increased from about 17% to 35 wt% with the impregnation of Cu. Similarly, the 2,6-triad DMN selectivity and 2,6-/2,7-DMN ratio reached the maximum level (48 wt% and 1.95, respectively) over Cu-impregnated MCM-41 zeolite catalyst

CO2 capture from fluid catalytic crackers via chemical looping combustion: Regeneration of coked catalysts with oxygen carriers

Oil refineries are responsible for ∼5% of total global CO2 emissions and approximately 25–35% of these emissions are released from a single unit called Fluid Catalytic Cracking (FCC). Chemical Looping Combustion (CLC) has been recently proposed as a novel CO2 capture method from the regenerator of FCC units as an integrated process of CLC-FCC. In this study, for the first time, the combustion behaviour of three types of cokes, a model FCC coke (which is a low volatile semi-anthracite coal), and cokes deposited on commercial FCC catalysts by n-hexadecane cracking and Vacuum Gas Oil, were comprehensively investigated with oxygen carriers (Co3O4, CuO, and Mn2O3) in a fixed-bed reactor at 700–850 °C. The results demonstrate that a high coke combustion efficiency was achieved with CuO (98 vol. %), Co3O4 (91 vol. %), and Mn2O3 (91 vol. %) at 800 °C for 30 min. CuO was the most effective oxygen carrier, at temperatures greater than 750 °C for 45 min of residence time. These are the regeneration conditions used in the conventional FCC regenerators

Predictability of higher heating value of biomass feedstocks via proximate and ultimate analyses – A comprehensive study of artificial neural network applications

Higher heating value (HHV) is a key characteristic for the assessment and selection of biomass feedstocks as a fuel source. The HHV is usually measured using an adiabatic oxygen bomb calorimeter; however, this method can be time consuming and expensive. In response, researchers have attempted to use artificial neural network (ANN) systems to predict HHV using proximate and ultimate analysis data, but these efforts were hampered by varying case specific approaches and methodologies. Based on the complex ANN structures, a clear state of the art ANN understanding must be required for the prediction of biomass HHV. This study provides a comprehensive ANN application for HHV prediction in terms of how the activation functions, algorithms, hidden layers, dataset, and randomisation of the dataset affects the prediction of HHV of biomass feedstocks. In this paper we present a comparative qualitative and quantitative analysis of thirteen different algorithms, four different activation functions (logsig, tansig, poslin, purelin) with a wide range of hidden layer (3–15) for ANN models, used to predict the HHV of the biomass feedstocks. ANN models trained by the combination of ultimate-proximate analyses (UAPA) datasets provided more accurate predictions than the models trained by ultimate analysis or proximate analysis datasets. Regardless of the used datasets, sigmoidal activation functions (tansig and logsig) provide better prediction results than linear activation function (poslin and purelin). Furthermore, as training activation functions, “Levenberg-Marquardt (lm)” and “Bayesian Regularization (br)” algorithms provide the best HHV prediction. The best average correlation coefficients of 30 randomised run were observed with tansig as 0.962 and 0.876 for the ANN model developed by the UAPA dataset with a relatively high confidence levels of ∼96% for training and ∼92% for testing

Prediction of Biomass Pyrolysis Mechanisms and Kinetics: Application of the Kalman Filter

In order to predict the pyrolysis mechanisms of four different biomasses (Asbos (Psilocaulon utile), Kraalbos (Galenia africane), Scholtzbos (Pteronia pallens), and palm shell), a novel method called Kalman filter was investigated and the results were compared by regression analysis. Both analyses were applied to five different generalized biomass pyrolysis models consisting of parallel and serial irreversible-reversible reaction steps. The models consisting of reversible reactions in addition to parallel pyrolysis steps demonstrated a better fit with the experimental results. The pyrolysis step from biomass to bio-oil has the highest reaction rates compared with the other pyrolysis steps defined in the models. The Kalman filter is thus defined as a promising filtering and prediction method for the estimation of detailed pyrolysis mechanisms and model parameters, using minimum experimental data