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Determining the long-term performance of adsorbents is crucial for the design of air treatment systems. Heel buildup i.e., the accumulation of non-desorbed/ non-desorbable adsorbates and their reaction byproducts, on the surface/pores of the adsorbent is a primary cause of adsorption performance deterioration. However, due to the complexity of heel buildup mechanisms, theoretical models have yet to be developed to map the extent of heel buildup to the adsorption/desorption parameters. In this work, two machine learning (ML) algorithms (XGBoost and neural network (NN)) were applied to predict volatile organic compounds (VOCs) cyclic heel buildup on activated carbons (ACs) by considering the adsorbent characteristics, adsorbate properties and regeneration conditions. The NN algorithm showed better performance in prediction of cyclic heel buildup (R

A novel hydrothermal route was developed to synthesize crystalline and hierarchically porous TiO2 at mild conditions under acidic/basic reaction environments. A good control over the key features of catalysts could be achieved by varying the pH of the starting solution (pH = 2–12) and type of acid during preparation. Crystalline, textural, and optical properties of photocatalysts were characterized in order to explore the connections between catalysts’ features and photoactivity. The activities of catalysts were evaluated for decomposition of methyl ethyl ketone (MEK) in a continuous flow photoreactor. To gain insight into the impact of main operating conditions on photocatalytic oxidation (PCO) processes, experiments were conducted at wide ranges of inlet concentration, relative humidity, and residence time. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that yielded titania samples possess good crystallinity and mainly consist of anatase polymorph. Results of X-ray photoelectron spectroscopy (XPS) indicated that the population of hydroxyl groups on the surface of TiO2 declines as the acidity of hydrothermal solution increases. Hierarchical porous structure with ultra-long parallel macrochannels and large macropores (> 0.5 μm) could be directly observed in scanning electron microscopy (SEM). The multimodal porous structure offers higher light utilization, enhanced mass transport, and superior adsorption capacity for air pollutants. Photoluminescence spectroscopy revealed that the enhancement in crystallinity has an impact on the separation of photo-induced charge carriers. Moreover, a satisfactory linear correlation between the ability of photocatalysts to generate •OH and crystallinity could be found. Under the harshest operating condition (residence time = 0.012 s, relative humidity = 50%, concentration = 1000 ppb), the TiO2 synthesized in nitric acid (pH = 4) achieved the highest MEK removal efficiency, 36.9%, which substantially surpassed that of P25, 14.5%. Hierarchical porosity, optimum trade-off between surface area and crystallinity, existence of meso- and macropores, small crystal size, and high •OH generation ability could account for the superior performance. A tentative reaction pathway for MEK photodegradation and a health-risk index were put forward considering the by-products detected in the gas phase.

A series of porous TiO2 photocatalysts are prepared, by systematically varying the preparation conditions (time, temperature, or pressure (i.e. filling ratio)), characterized, and evaluated in photocatalytic oxidation of toluene and methyl ethyl ketone (MEK) to explore preparation-property-performance relationships. A detailed characterization has been conducted via X-ray diffraction (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), and UV–vis spectroscopy. Furthermore, hydroxyl radical ( OH) generation on the surface of TiO2 is measured by a photoluminescence (PL) method using terephthalic acid (TA) as probe molecule. All the hydrothermally-prepared samples possessed good crystallinity (79.5–89 %), large surface area (134.9–237.2 m2/g), small crystal size (5.9–10 nm), and mesoporous structure. SEM images revealed presence of macropores and marcochannels, and N2 adsorption-desorption and TEM analyses indicated a significant amount of mesopores. PL and XRD results demonstrated a good proportionality between surface area normalized OH generation and crystallinity. The complex interplay among various properties (especially crystallinity and surface area) led to appearance of activity optimums. Photocatalyst prepared at 12 h, 200 °C, and 80% filling ratio exhibited the best toluene and MEK removal efficiencies, which surpass those of P25 by factors of 2.08 and 1.85 times, respectively. The superior photocatalytic activity of developed TiO2 catalysts might be attributed to high surface area and existence of meso-/macropores that provide a large number of active sites, and facilitate light penetration and pollutants diffusion. The presented property-activity relationships can be utilized as potential design criteria for the development of new TiO2 photocatalysts for air purification.

Photocatalytic oxidation (PCO) is a well-known technology for air purification and has been extensively studied for removal of many air pollutants. Titanium dioxide (TiO2) is the most investigated photocatalyst in the field of environmental remediation owed to its chemical stability, non-toxicity, and suitable positions of valence and conduction bands. Various preparation techniques including sol-gel, flame hydrolysis, water-in-oil microemulsion, chemical vapour deposition, solvothermal, and hydrothermal have been employed to obtain TiO2 materials. Hydro-/Solvothermal (HST) synthesis, focus of the present work, can be defined as a preparation method in which crystal growth occurs in a solvent at relatively low temperature (

Ultraviolet photocatalytic oxidation (UV PCO) is regarded as one of the promising technologies for indoor air remediation. The present study examines the photocatalytic activity of four commercialized titanium dioxide photocatalysts (P25, PC500, UV100, and S5-300A) in a small-scale single-pass continuous flow reactor. Challenge compounds have been chosen from two prevalent VOC families in indoor environment: toluene (aromatics) and methyl ethyl ketone (ketones). The influence of key experimental conditions including concentration (100–1000 ppb), relative humidity (0–50% at 23 °C), light intensity (1.25–5 W m−2), and residence time (0.02–0.1 s) on removal efficiency are evaluated. Due to the fact that one of the main shortcomings of PCO air purifiers is by-products generation, a special emphasis is put on identification and quantification of gaseous by-products using HPLC and GC-MS methods. The obtained efficiencies on various photocatalysts are explained considering crystalinity, crystalline phase, crystal size, surface area, and population of surface hydroxyl groups. Despite possessing lower crystalinity, PC500, UV100 and S5-300A outperformed P25 in toluene and MEK removal efficiency, primarily owed to their larger surface area, smaller crystal size, and higher concentration of surface hydroxyl groups. Acetone, acetaldehyde, and formaldehyde were identified as the main gaseous by-products in PCO of toluene and MEK.

This article presents multi-objective optimization of an HT-PEM fuel cell based micro CHP system under steady-state operation by employing the mathematical model of the plant previously developed by our group. Different optimization procedures have been carried out to find the optimal points while considering two sets of objective functions: I) thermal power generation and net electrical output and II) net electrical efficiency and thermal efficiency. In the first part of the work, optimization has been performed at full load operation with electrical and thermal generation as objectives. The obtained Pareto frontier shows the capability of the system to cater a broad range of electrical demand (21.0 kW-29.4 kW) while offering the maximum achievable thermal generation. In the next step, in order to find the optimal operating conditions of the system while addressing specific thermal and electrical load profiles, a series of Pareto fronts have been acquired at different fuel partialization levels. Finally, using the primary energy saving (PES) index, the best operating points, in terms of electrical and thermal efficiency, have been determined. It was observed that the net electrical efficiency up to 32.3% and thermal efficiency as high as 61.1% can be reached through the optimization.

Adsorption of pollutants onto photocatalyst surface plays a critical role in the efficacy of photocatalytic oxidation (PCO) technology for air purification applications. In this article, gas phase adsorption of toluene and methyl ethyl ketone (MEK) was studied using a small-scale single-pass continuous flow system. Equilibrium adsorption isotherms for toluene and MEK on six commercially available TiO2 photocatalysts (P25, PC500, PC105, UV100, PC-S7, and S5-300A) coated on nickel foam substrates were determined. The selected photocatalysts cover a wide range of important photocatalytic properties including surface area, porosity, acidity, and population of surface hydroxyl groups. The TiO2 coated filters were characterized by SEM technique to evaluate the quality of coating and homogeneity of titania powder on the support material. Furthermore, a thorough Fourier transform infrared spectroscopy (FTIR) was performed to investigate the nature of surface hydroxyl (OH) groups on various photocatalysts. Using the adsorption isotherm data, Langmuir model adsorption coefficients for toluene and MEK on various photocatalysts and humidity levels have been determined. Additionally, the impacts of challenge compound polarity and air relative humidity (RH) on the adsorption efficiency and Langmuir constants were assessed. The adsorption capacity for MEK (a highly polar compound) was much higher than that of toluene (a non-polar compound) on all photocatalysts, regardless of the humidity content. The results vividly showed that the relative humidity negatively affects the adsorption capacity for both MEK and toluene, however to different extents due to the differences in water solubility and polarity. FTIR characterization of MEK/toluene saturated titania samples revealed that isolated OH groups (both terminal (Ti-OH) and bridged (Ti-OH-Ti)) serve as highly active adsorption sites for volatile organic compound (VOC) molecules.

Inevitable presence of volatile organic compounds (VOCs) in indoor environment and their adverse impact on human health and productivity have encouraged the development of various technologies for air pollution remediation. Among these technologies, photocatalytic oxidation (PCO) is regarded as one of the most promising methods and has been the focus of many research works in the last two decades. Titanium dioxide (TiO 2 ) is by far the most investigated photocatalyst for photocatalytic degradation of gaseous VOCs. This review article is intended to provide a comprehensive overview of the application of commercial TiO 2 photocatalysts for removal of VOCs in air. First, the fundamentals of photocatalytic oxidation are briefly discussed and common TiO 2 -based photocatalysts are introduced. Then, the relations between the characteristics of photocatalysts (e.g. crystallinity, surface area and surface chemistry) and photocatalytic activity as well as the influence of key operating parameters on PCO processes are investigated. Afterwards, the reaction mechanisms and identified reaction intermediates/by-products for the most prevalent VOC families are reviewed. Finally, the paper discusses the deactivation of photocatalysts during PCO processes and some of the common regeneration techniques.

Electrospun Polyacrylonitrile/Cellulose nanocrystal (PAN-CNC) nanofibers with uniform diameter and smooth morphology were prepared. The nanofibers were stabilized, then activated to produce activated carbon nanofibers (ACnFs) and their physical and chemical properties were characterized. Last, the adsorption properties of ACnFs were tested for adsorption of methyl ethyl ketone (MEK) and cyclohexane. Scanning electron microscopy revealed that the addition of CNCs improves fiber uniformity and decreases fiber diameter due to enhanced electrical conductivity of electrospinning solutions. The addition of CNCs had minimal effect on fibers thermal stability; however, it brought large improvement in tensile strength. The pore size distribution indicated large volumes of micropores and mesopores in all samples. The maximum surface area and total pore volume were respectively 3497 m2/g and 2.62 cm3/g for 25%PAN-75%CNC nanofibers. The adsorption capacity for MEK and cyclohexane correlated well with the surface area of ACnF, reaching a maximum value of 1.7 and 1.8 gadsorbate/gadsorbent for MEK and cyclohexane, respectively. Compared to commercial activated carbons fiber and beads, the prepared ACnF exhibited higher capacities and faster adsorption kinetics due to larger surface area, higher mesoporosity, and better mass transfer. These results indicate that using CNC improves the mechanical and adsorption properties of electrospun PAN nanofibers. This article, for the first time, puts forward CNC reinforced activated carbon nanofibers as suitable adsorbents for environmental remediation applications.

Indoor air purification through photocatalytic oxidation (PCO) has been demonstrated to be an effective technique; nevertheless, further research is needed to address some of the existing shortcomings of PCO systems. In the present study, the influence of titanium dioxide (TiO2) features and the type of support material on the performance of PCO air purifiers are investigated. TiO2 photocatalysts are prepared via hydrothermal route followed by a calcination step (300–800 °C). Calcination temperature offers a good control over the characteristics of titania, in particular crystallinity and surface area, enabling fabrication of TiO2 with the most desirable set of properties. Two support materials with distinct adsorption properties, namely nickel foam filter (NFF) and activated carbon fiber (ACF) are examined for deposition of TiO2. The developed photocatalytic filters are evaluated by the removal efficiency of methyl ethyl ketone (MEK) and the amount of generated by-products. The photocatalyst calcined at 300 °C exhibited the highest activity, which was roughly 1.6 times higher than that of commercial P25. This can be attributed to the fact that at 300 °C the best trade-off between degree of crystallinity, crystal phase/size, surface area, and surface hydroxyl groups population was reached. The steady-state MEK removal efficiency and amount of by-products were 62.2% and 180 ppb for TiO2/ACF and 36.3% and 217.5 ppb for TiO2/NFF, respectively. This synergistic effect of TiO2/ACF and the concept of in-situ regeneration of ACF are also discussed.

In the present study, a detailed thermodynamic model for an internal-reforming solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system integrated with a Rankine (steam) cycle is developed, and exergetic, economic and environmental analyses have been carried out on the plant. Considering the exergetic efficiency and the total cost rate of the system as conflicting objectives, a multi-objective optimization of the system is conducted to determine the optimal design point of the plant. A set of optimal solutions (Pareto front) is achieved, each of which is a trade-off between the chosen objectives. Finally, TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution) decision-making method is used to choose the final optimal design parameters. The results demonstrate that the final optimal design of the proposed plant leads to an exergetic efficiency of 65.11% and total cost rate of 0.13745 €/s. Furthermore, the optimization results reveal that the integration of the Rankine cycle with the SOFC-GT system has led to an 8.84% improvement in the total exergetic efficiency of the plant, producing additional 8439.2 MW h of electricity and avoiding ∼5900 metric tons of carbon dioxide emissions annually.

Photocatalytic oxidation (PCO) is a well-known technology for air purification and has been extensively studied for removal of many air pollutants. Titanium dioxide (TiO

High-performance microporous titania–silica (TiO2–SiO2) nanocomposites with different TiO2 loadings of 0–100 wt% were synthesized using a sol–gel method and evaluated for ultra-deep oxidative desulfurization (ODS) of dibenzothiophene (DBT) using tert-butyl hydroperoxide (TBHP) as oxidant. The prepared catalysts were characterized by the N2 adsorption–desorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HR-TEM) and ammonia temperature-programmed desorption (NH3-TPD), and the ODS performances were evaluated in a batch reactor. The effects of titanium loading, calcination temperature, and reaction temperature on the catalyst performance were examined. The activity varied significantly with the amount of titanium in the TiO2–SiO2 nanocomposite with a nearly constant turnover frequency (TOF) of about 24.6 h−1. The TiO2–SiO2 nanocomposite containing 50 wt% titania loading (TS-50) with the highest total acidity was an excellent catalyst capable of removing more than 98% of DBT to less than 10 ppmw, after 20 min. DBT was oxidized to DBT-sulfone (DBTO2), a species with higher polarity that could be subsequently adsorbed on the TS-50 and therefore, the nanocomposite acts as both a catalyst and adsorbent simultaneously. The catalysts could be easily regenerated by calcination at 873 K. An empirical kinetic model was employed to interpret the reaction rate data; the apparent activation energy was 43.8 kJ/mol. Density functional theory (DFT) calculations revealed that DBT and TBHP reactants and DBTO2 product were more strongly adsorbed on (0 0 1) surface of β-cristobalite silica than on (1 0 1) surface of anatase titania. The adsorption energy of DBTO2 was larger than DBT on both surfaces.

The present study is dedicated to the investigation of applying different strategies on an HT-PEM fuel cell based micro cogeneration plant in order to evaluate the capability of this system to cope with intermittent electrical and thermal load profiles. The performance of the system under fuel partialization strategy is first studied and the thermal and electrical efficiencies of the plant at different partial loads are determined. It was found that due to partialization of the fuel down to 50% of the initial value, the electrical efficiency increases from 29.3% to 33.6% while the thermal efficiency decreases from 53.0% to 47.6%. Power to heat shifting strategy, as a faster approach, is then employed in which, by altering the anodic stoichiometric ratio, the electrical generation is decreased while higher thermal power is produced. The results showed that this partialization method leads to harsh drop, around 13%, in the electrical efficiency of the plant. In the last analysis, the previous strategies are combined and the power to heat shifting approach is implemented on the system operating at partial load. The main advantage of the last strategy is the flexibility of the system in covering a wide range of thermal demand.

This article proposes a novel hybrid system, integrating high temperature MCFC-GT (molten carbonate fuel cell-gas turbine) and ORC (organic Rankine cycle), which provides the possibility to achieve high electrical and exergetic efficiencies owing to the subsequent electrical power output in the bottoming cycle. After developing a mathematical model, comprehensive energetic, exergetic, economic and environmental evaluations (4E analysis) are performed and a multi-objective optimization method is utilized to find optimal solutions while considering the exergetic and economic objectives simultaneously. Two conflicting objectives including total exergetic efficiency and total cost rate of the system in multi-objective optimization are taken into account to build a set of Pareto optimal solutions. This optimum solution results in the exergetic efficiencies of 35.6%, 44.3%, and 54.9% for the fuel cell system, ORC cycle and the whole hybrid system respectively, while the total cost of the plant is 0.294 M€ per year. The study reveals that introducing the ORC bottoming cycle leads to about 5% improvement in the exergetic efficiency of the proposed plant. Furthermore, a sensitivity analysis is conducted to investigate the effect of variation in economic parameters, the fuel unit cost and interest rate, on the Pareto optimal solutions.

The principal goal in the present work is to model a molten carbonate fuel cell-gas turbine (MCFC-GT) hybrid plant from energetic, exergetic, economic and environmental standpoints and to optimize the system through a multi-objective optimization scheme. Two conflicting objectives including exergetic efficiency and total cost rate of the system are introduced for multi-objective optimization. TOPSIS decision-making method is employed to determine the system final optimum design, leading to an overall exergetic efficiency of 51.7% and the total cost of 0.324 million USD per year. Moreover, a sensitivity analysis of the Pareto frontier to fuel unit cost and effective interest rate has been performed to investigate the variation of objective functions with economic parameters. Finally, a sensitivity analysis on the optimization results was performed for some of the key parameters, revealing the fact that operating pressure of the system has the highest impact on the exergetic efficiency compared to the other operating parameters.

In fuel cell based combined heat and power (CHP) plants, degradation within the fuel cell stack and the steam methane reformer significantly affects the generated electrical and thermal power. As a consequence, incorporating system’s degradation within the model of the plant could be of great importance in order to estimate the resulting variations in the electrical and thermal power generation and taking appropriate measures to mitigate such deviations. To this end, in the present article, a multi-objective optimization approach has been proposed and employed to find the optimal operating parameters of an HT-PEM fuel cell based micro-CHP system within the first 15,000 h of operation while considering the impact of degradation. Two different optimization procedures with the following objective functions have been applied: (I) net electrical efficiency and thermal generation; and (II) net electrical efficiency and electrical power generation. Steam to carbon ratio, auxiliary to process fuel ratio, fuel partialization level and anodic stoichiometric ratio are the design parameters. Based on the results of optimization procedure I, the highest achievable net electrical efficiency at the beginning of operation is 32.75% which, due to degradation, considerably declines to 29.51% in the last time interval. Moreover, in all time steps, optimal solutions cover a wide domain of thermal generation which assures the capability of the system to easily cope with the thermal demand of the user. On the other hand, optimization procedure II displays a steady decrease in both electrical efficiency and electrical generation through time which indicates the adverse effect of degradation on these two performance indices. Finally, it has been found that, using optimization procedure I, the cumulative average electrical efficiency of the plant improved from 26.03% at normal operation to 27.56% at optimized condition. Furthermore, it was determined that by employing the optimal points obtained in optimization procedure II, the average cumulative electrical power generation is increased from 25.4 kW (at normal operation) to 26.8 kW. It is noteworthy that, the study not only provides some insights into the long-term performance of such system, but can be more importantly perceived as a guideline to adaptively optimize the operating conditions of the system in order to alleviate the degradation’s effect and to guarantee optimal performance of the system throughout its lifetime.

Multi-objective optimization method using genetic algorithm is employed in order to optimize design and operating parameters of a high temperature proton exchange membrane (HT-PEM) fuel cell based combined heat and power system. Net electrical efficiency of the plant, indicating the system's performance (to be maximized) and the total capital cost (to be minimized) are considered as optimization objectives. Current density (indicating the stack size), steam to carbon ratio, burner outlet temperature and auxiliary to process fuel ratio have been chosen as design parameters. Two different multi-objective optimization approaches have been utilized: steady state (without degradation) and long-term optimization while considering the degradation in fuel cell stack and the fuel processor. The results of the optimization procedures are Pareto frontiers which are a set of optimal points each of which is a trade-off between the considered objective functions. The performance indexes and operating conditions of three points with the maximum cumulative net electrical efficiency, minimum capital cost, and the same fuel cell area as that of the initial design are compared. It can be observed that while attempting to maximize the electrical efficiency, the cumulative net electrical efficiency of 29.96% can be achieved although it results in a total capital cost of 115711 €. On the other hand, the capital cost can be reduced down to 39,929 € which significantly diminishes cumulative net electrical efficiency. Finally, by locating the point on the Pareto frontier in which the fuel cell area is the same as that of the initial design, a cumulative net electrical efficiency of 27.07% was achieved which is 1% higher than the value obtained using the operating conditions of the initial design.

Electrification to rural and remote areas with limited or no access to grid connection is one of the most challenging issues in developing countries like Colombia. Due to the recent concerns about the global climatic change and diminishing fuel prices, searching for reliable, environmental friendly and renewable energy sources to satisfy the rising electrical energy demand has become vital. This study aims at analyzing the application of photovoltaic (PV) panels, wind turbines and diesel generators in a stand-alone hybrid power generation system for rural electrification in three off-grid villages in Colombia with different climatic characteristics. The areas have been selected according to the “Colombia’s development plan 2011–2030 for non-conventional sources of energy”. First, different combinations of wind turbine, PV, and diesel generator are modeled and optimized to determine the most energy-efficient and cost-effective configuration for each location. HOMER software has been used to perform a techno-economic feasibility of the proposed hybrid systems, taking into account net present cost, initial capital cost, and cost of energy as economic indicators.

In the present study, long term performance of an HT-PEM fuel cell based micro CHP system, considering the degradation within the HT-PEM fuel cell stack and the steam methane reformer has been investigated. The variations in the generated electrical and thermal power and the corresponding efficiencies, in the first 15,000 h of operation of the plant, have been studied. Two strategies have been proposed and applied in order to remedy the excursion of thermal and electrical generation of the plant from the steady state production. In the partialization strategy, by means of reducing the fuel fed to the system, the thermal generation of the plant is kept in a specified range. On the other hand, in the recovery strategy, the supplied fuel is gradually increased to suppress the progressive reduction in the power production. The long term performance analysis of the system in normal condition reveals that, due to the degradation within the system, the power production diminishes from 28.2 kW to 23.4 kW while the thermal generation increases from 52.4 kW to 57.5 kW. The results of partialization strategy show that, in order to confine the thermal generation amplification, the partialization factor should be increased up to 7.2%. On the other hand, in the recovery strategy, the supplied fuel should be progressively increased up to 34.2% in order to preserve the electrical output at the initial level. Nevertheless, the recovery strategy has an adverse effect on the electrical efficiency as it diminishes the obtained efficiency to 21.6% compared to 24% obtained for the normal operation. In the last part of the study, the overall performance indexes of the plant, while operating in normal condition and under operational strategies, are compared. It is shown that operating under recovery strategy results in overall electrical efficiency of 24.7% which is notably lower than efficiencies of 26.1% and 26.4% obtained by operating in normal condition and under partialization strategy respectively. However, it was also demonstrated that applying this strategy results in generation of 422.6 MW h of electrical energy which is higher than the values obtained by normal operation (381.3 MW h) and partialization strategy (369.8 MW h).

A new configuration for an existing PEM fuel cell based residential micro-cogeneration system has been proposed in which the conventional low temperature PEM fuel cell is replaced with a high temperature one. Detailed mathematical models for the fuel processor, HT- PEM fuel cell stack and all other components of the plant have been developed. The electrical and thermal performance of the system have been determined and the corresponding results have been compared with the performance indices achieved for the previous plant. The electrical efficiency and the primary energy savings index obtained for the proposed system are 29.21% and 17.50% respectively which are considerably higher than the ones obtained for the existing LT-PEM based plant (21.18% and 6.07%). In order to enlighten our understanding of the behaviour of the system a parametric study on the key parameters of the system has been carried out.