Thermal energy storage by the chemical reaction augmentation of heat transfer and thermal decomposition in the CaOCa(OH)2 powder

The present paper is concerned with the utilization of a thermal decomposition reaction, Ca(OH)₂?CaO + H₂O, for energy storage. One of the important problems in this case is how to heat up and decompose the powder of Ca(OH)₂ effectively, where the thermal conduction is poor.In this study, the effect of copper plates, which are placed in the powder of Ca(OH)₂ as heat-transfer fins, is investigated experimentally and numerically. The results show that the Cu-plates are very effective for heat transfer and the thermal decomposition, and that the optimum configuration of the Cu-plate is 5–10 cm in height and 0.5–1 cm in interval for the condition of this study.     

The kinetics research of thermochemical energy storage system Ca(OH)2/CaO

As one of the most promising thermochemical energy storage medium, research on the Ca(OH)₂/CaO system provides an important way of understanding energy storage/release rates of the entire energy storage system. In this paper, a high‐precision thermogravimetric analysis is adopted to investigate thermal decomposition processes of the Ca(OH)₂ samples in pure N₂ atmosphere at different heating rates. The results demonstrate that during the thermal decomposition process, two weight loss processes respectively occur during 623.15 ~ 773.15 and 873.15 ~ 973.15 K, and the weight loss rates are close to 21% and 2% severally. Multi‐heating rate methods are applied to the study of thermal decomposition dynamics. Findings show that the obtained kinetic parameters are related to reaction conversion, heating rate, and the chosen model‐methods. To further understand the decomposition mechanism of Ca(OH)₂, differential method, integral method, and multiple scanning method are used to deal with the experimental data. Through the most probable mechanism function analysis, under certain experimental conditions, thermal decomposition kinetics model of Ca(OH)₂ accords well with the shrinking cylinder mechanism. These conclusions provide theoretical bases for applying the Ca(OH)₂/CaO system to the thermochemical energy storage field. Copyright © 2016 John Wiley & Sons, Ltd.     

Biogas quality upgrade by simultaneous removal of CO2 and H2S in a packed column reactor

Biogas from anaerobic digestion of biological wastes is a renewable energy resource. It has been used to provide heat, shaft power and electricity. Typical biogas contains 50–65% methane (CH₄), 30–45% carbon dioxide (CO₂), moisture and traces of hydrogen sulphide (H₂S). Presence of CO₂ and H₂S in biogas affects engine performance adversely. Reducing CO₂ and H₂S content will significantly improve quality of biogas. In this work, a method for biogas scrubbing and CH₄ enrichment is presented. Chemical absorption of CO₂ and H₂S by aqueous solutions in a packed column was experimentally investigated. The aqueous solutions employed were sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂) and mono-ethanolamine (MEA). Liquid solvents were circulated through the column, contacting the biogas in countercurrent flow. Absorption characteristics were examined. Test results revealed that the aqueous solutions used were effective in reacting with CO₂ in biogas (over 90% removal efficiency), creating CH₄ enriched fuel. H₂S was removed to below the detection limit. Absorption capability was transient in nature. Saturation was reached in about 50 min for Ca(OH)₂, and 100 min for NaOH and MEA, respectively. With regular replacement or regeneration of used solutions, upgraded biogas can be maintained. This technique proved to be promising in upgrading biogas quality.     

Solar energy storage by the reversible reaction: N2O42NO2. Theoretical and experimental results

The reversible reaction of N₂O₄2NO₂ has been experimentally studied at temperatures between 60 and 140°C in the gas phase, in a recirculating system including the decomposition reactor for N₂O₄, and the recombination apparatus for NO₂. Calculated thermal balances of heat exchanged in different experimental conditions agree well with experimental data. For the reaction to be carried out in the liquid phase, under pressure, some comparisons have been made among heat storage capacities (HSC) with respect to different processes. An hypotetical plant based upon the reversible reaction has HSC from 1.7 to 3 times greater than one employing direct heating of water; the latter being based upon a ΔT of 30–100°C. The investigated reaction has one of the lowest turning temperatures (about 60°C) among those useful for the storage of solar energy by means of flat collectors. These characteristics joined with a maximum HSC of 200 kcal −1⁻¹ (for the liquid-phase reaction) makes the above-mentioned reaction worthy of further studies.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

Analysis of sulfur–iodine thermochemical cycle for solar hydrogen production. Part I: decomposition of sulfuric acid

The sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most studied cycles for hydrogen (H₂) production. S–I cycle consists of four sections: (I) acid production and separation and oxygen purification, (II) sulfuric acid concentration and decomposition, (III) hydroiodic acid (HI) concentration, and (IV) HI decomposition and H₂ purification. Section II of the cycle is an endothermic reaction driven by the heat input from a high temperature source. Analysis of the S–I cycle in the past thirty years have been focused mostly on the utilization of nuclear power as the high temperature heat source for the sulfuric acid decomposition step. Thermodynamic as well as kinetic considerations indicate that both the extent and rate of sulfuric acid decomposition can be improved at very high temperatures (in excess of 1000 °C) available only from solar concentrators. The beneficial effect of high temperature solar heat for decomposition of sulfuric acid in the S–I cycle is described in this paper. We used Aspen Technologies' HYSYS chemical process simulator (CPS) to develop flowsheets for sulfuric acid (H₂SO₄) decomposition that include all mass and heat balances. Based on the HYSYS analyses, two new process flowsheets were developed. These new sulfuric acid decomposition processes are simpler and more stable than previous processes and yield higher conversion efficiencies for the sulfuric acid decomposition and sulfur dioxide and oxygen formation.     

The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas

The synergistic effect of Ca(OH)₂ prepared by the wet-mixing method on lignite steam gasification process at different temperatures (700–900 °C) was analyzed in a spout-fluid bed reactor. Firstly, to avoid disturbance of volatile and tar, active carbon was used as a model compound. On the one hand, Ca(OH)₂ effectively catalyzed the water-gas shift (WGS) reaction to improve H₂ concentration, but the performance was weaker at higher temperature due to the enhancement of boudouard reaction and the weakening of WGS reaction. On the other hand, it was found that the (CO+2CO₂)/H₂ ratio of syngas produced at 700 °C in the presence of Ca(OH)₂ was 0.82, which was much lower than that of the other cases, owning to the absorption of CO₂. The synergistic effect was observed at this temperature, for the adsorption of CO₂ altered equilibrium of the WGS reaction and further improved H₂ concentration. Then two kinds of Chinese lignite (HLH and XM) were selected to further study the performance of Ca(OH)₂ on optimizing the lignite steam gasification process. In the presence of Ca(OH)₂, tar and char yields greatly reduced at the same reaction temperature, whereas the gas yields significantly increased. As a catalyst, Ca(OH)₂ can not only promote solid–gas reaction to decrease char yield, but also accelerate tar decomposition to reduce its yield in syngas. Based on GC–MS data, it can be deduced that Ca(OH)₂ has different catalytic activity on the steam reforming of tar with different molecular structures. Contrast to Class 4, tars of aliphatic hydrocarbons, Class 2 and Class 5 were clearly catalytic reformed. Hydrogen-rich gas can be produced at 800 °C and 900 °C owning to the catalytic effect of Ca(OH)₂, but the highest H₂ concentration was found at 700 °C due to the additional effect of CO₂ absorption, which was supported by the results of thermogravity experiments.     

Importance of pyrolysis and catalytic decomposition for the direct utilization of methanol in solid oxide fuel cells

There is interest in developing solid oxide fuel cells (SOFC) operated directly with liquid fuels such as methanol. This mode of operation increases the complexity of the anodic processes, since thermal and catalytic decomposition reactions are relevant. In this study, the pyrolysis and catalytic decomposition of methanol are investigated experimentally for conditions typical of SOFC. The results are compared to the thermodynamic equilibrium values and also to the predictions of a kinetics model. The main species of the thermal decomposition of methanol are H₂, CO, and HCHO; soot formation is relevant below 973 K. The presence of a catalyst allows the gas-phase composition to reach equilibrium. However, the catalysts tested – Ni/YSZ, Ni/CeO₂, Cu/CeO₂ and Cu–Co/CeO₂ – deactivate by coking so that the gas-phase composition reverts to that of pyrolysis alone. The results presented reveal part of the complex dynamics occurring within the anode compartment during the direct utilization of methanol.     

Dynamic simulation of CaO/Ca(OH)2 chemical heat pump systems

Using energy and exergy analyses, a dynamic simulation is carried out with a CaO/Ca(OH)₂ chemical heat pump system for heating and cooling applications. The system consists of hydration/dehydration reactor connected to condenser/evaporator with a control valve in between. During the dehydration process, heat is supplied at 700 K for dehydration of Ca(OH)₂ and steam is condensed at 293 K. During evaporation/hydration process, heat is supplied at 290 K for evaporation of water at 273 K and heat of hydration is supplied to a load at 353 K. Duration of one cycle takes about 12 hours. Two subsystems are used to provide for heating/cooling demands in a continuous manner. Using synthetic demands of a residential dwelling, various performance parameters have been calculated for a 24 hour period. The results showed that CaO/Ca(OH)₂ chemical heat pump system could satisfy heating and cooling demands of a typical dwelling. Its energy and exergy efficiencies were 58.7% and 61.6% for heating and 12.7% and 4.5% for cooling respectively.     

Experimental results of a 10 kW high temperature thermochemical storage reactor based on calcium hydroxide

One promising possibility to store thermal energy is by means of reversible gas solid reactions. In this context, the endothermal dehydration of calcium hydroxide (Ca(OH)₂) to calcium oxide (CaO) is a well known, cycle stable reaction able to store heat at temperatures above 410 °C and pressures above 0.1 bar. Additionally, the storage material itself is a widely available low cost raw material which allows for low cost thermal energy storage capacities. Therefore, a multifunctional test bench for thermochemical storage reactors has been developed and set into operation. Simultaneously an indirect operated reactor for ∼20 kg Ca(OH)₂ was designed, manufactured and integrated into the test bench. Within this work the charge and discharge characteristics of the reactor concerning possible limitations due to heat and mass transfer were studied experimentally. Thereby, the possibility to store and release the heat of reaction at an adjustable temperature level was demonstrated in a technical relevant scale. The storage material remained stable and showed no degradation effects after ten cycles.     

Thermal stability of a new solid oxide fuel/electrolyzer cell seal glass

Long-term thermal stability of sealing glass is critical for hermetic seals of solid oxide fuel and electrolyzer cell stacks. In this work, a SrO–La₂O₃–Al₂O₃–SiO₂ glass (SABS-0 glass) has been evaluated as a high temperature sealant by thermal treatment. Powder and bulk SABS-0 glasses are studied in both air and H₂/H₂O atmospheres at 800 °C for up to 1000 h. Weight measurements show negligible SABS-0 glass vaporization during the thermal treatment. Both SABS-0 powder and bulk samples show some surface devitrification but the SABS-0 glass bulk remains amorphous at all the thermal treatment conditions. On the polished bulk SABS-0 surface, needle-shaped crystals are observed for both the air and the H₂/H₂O thermal treatment conditions. Polishing is believed to be the initiator for the SABS-0 glass surface devitrification. The crystalline phases, indentified as silicates and aluminates, increase with the thermal treatment time. However, the crystalline phases on the polished SABS-0 glass surface are very limited and only exist on the very surface of both the air and the H₂/H₂O atmosphere treated samples. The SABS-0 glass has excellent thermal stability in solid oxide fuel/electrolyzer operating environments and is a promising sealant material for such applications.     

Electrochromic characterization of Co(OH)2 thin film prepared by sol–gel process

Electrochemical, spectroscopic and structural measurements were used to characterize the electrochromic behavior and stability of sol–gel deposited Co(OH)₂ thin films. These films were prepared from polymeric solutions containing cobalt methoxyethoxide precursor by spin coating technique. The as-deposited films are amorphous and show crystalline structure after heat treatment at 450°C. Sol–gel-deposited cobalt hydroxide films show reversible electrochromic response in 1 M LiClO₄/ propylene carbonate solution beyond 500 cycles. The structural and chemical properties of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. Spectral transmittance change was T_p=29.9–60.2% for cobalt hydroxide films. It is argued that reversible lithium insertion capacity, good cyclic reversibility of Co(OH)₂ films make them suitable as counterelectrode layers in the electrochromic devices.     

Energy efficient simultaneous oxidative conversion and thermal cracking of ethane to ethylene using supported BaO/La2O3 catalyst in the presence of limited O2

An energy efficient conversion of ethane to ethylene involving simultaneous oxidative conversion (which is exothermic) and thermal cracking (which is endothermic) reactions of ethane in the presence of steam (steam/C₂H₆ mol RATIO=1.0) and limited O₂ (C₂H₆/O₂ mol ratio 4.0) over a BaO-promoted La₂O₃ supported on low surface area macroporous silica-alumina commercial catalyst carrier has been thoroughly investigated. Influence of various process parameters such as temperature (700–850°C), C₂H₆/O₂ feed ratio (4.0–8.0) and space velocity (50,000–200,000 cm³ g⁻¹ h⁻¹) on the conversion, product selectivity and net heat of reactions in the process has also been studied. At all the process conditions, there was no coke deposition on the catalyst. High selectivity ( 85%) for C₂₊ olefins (at 50–60% conversion) can be obtained in the process at a low contact time (<10 ms), particularly for the higher C₂H₆/O₂ ratios ( 6.0) and temperatures ( 800°C). The process exothermicity is decreased appreciably with increasing the temperature and/ or the C₂H₆/O₂ ratio. The net heat of reaction in the process can be controlled by manipulating the C₂H₆/O₂ ratio and reaction temperature. Also, because of simultaneously occurring endothermic and exothermic reactions, the process is highly energy efficient and non-hazardous.     

Thermal decomposition of doped calcium hydroxide for chemical energy storage

Thermal decomposition of Ca(OH)₂ with and without additives has been experimentally investigated for its application as a thermochemical energy storage system. The homogeneous reaction model gives a satisfactory fit for the kinetic data on pure and Ni(OH)₂---, Zn(OH)₂--- and Al(OH)₃---doped Ca(OH)₂ and the order of reaction is 0.76 in all cases except for the Al(OH)₃-doped sample for which the decomposition is zero order. These additives are shown not only to enhance the reaction rate but also to reduce the decomposition temperature significantly. Some models for solid decomposition reactions, and possible mechanisms in the decomposition of solids containing additives, are also discussed.     

Expanding performance limit of lithium-ion batteries simply by mixing Al(OH)3 powder with LiCoO2

Mixing a small amount of Al(OH)₃ powder with a LiCoO₂ cathode material is demonstrated to improve markedly the cycle performance and thermal stability of commercial grade LiCoO₂/graphite lithium-ion batteries. Al(OH)₃-mixed LiCoO₂/graphite prismatic cells exhibit excellent capacity retention as high as 95% after 400 cycles with negligible polarization build-up. Moreover, the thermal stability of the cells is greatly improved by Al(OH)₃ mixing, which is confirmed by higher residual and recovery capacity ratios after storage at 90 °C compared with a pristine cell. The beneficial effects of Al(OH)₃ are found to be related mainly to an improvement of the cathode side, which is ascribed to reduced unwanted side-reactions with the electrolyte.     

Heat recovery from a thermal energy storage based on the Ca(OH)2/CaO cycle

Thermal energy storage is very important in many applications related to the use of waste heat from industrial processes, renewable energies or from other sources. Thermochemical storage is very interesting for long-term storage as it can be carried out at room temperature with no energy losses.Dehydration/hydration cycle of Ca(OH)₂/CaO has been applied for thermal energy storage in two types of reactors. One of them was a prototype designed by the authors, and in the other type conventional laboratory glassware was used. Parameters such as specific heats, reaction rate and enthalpy, mass losses and heat release were monitored during cycles. Although in the hydration step water is normally added in vapour phase, liquid water, at 0 °C has been used in these experiences.Results indicated that the energy storage system performance showed no significant differences, when we compared several hydration/dehydration cycles. The selected chemical reaction did not exhibit a complete reversibility because complete Ca(OH)₂ dehydration, was not achieved. However the system could be used satisfactorily along 20 cycles at least. Heat recovery experiments showed general system behaviour during the hydration step in both types of reactors. The designed prototype was more efficient in this step.Main conclusions suggested carrying out one complete cycle at a higher dehydration temperature to recover total system reversibility. A modification of the prototype design trying to enhance heat transfer from the Ca(OH)₂ bed could also be proposed.     

Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

Sorption enhanced water gas shift reaction (SEWGS) based on calcium looping is an emerging technology for hydrogen production and CO₂ capture. SEWGS involves mainly two reactions, the catalytic WGS reaction and the bulk carbonation of CaO with CO₂, and the solid product is CaCO₃, and the Ca(OH)₂ may be formed from the reaction of CaO with H₂O with the presence of steam in gas phase. The effect of Ca(OH)₂ and CaCO₃ on the catalytic WGS reaction and carbonation reaction was studied in a fluidized bed reactor. It was found that the hydrated sorbent and CaCO₃ did not show any catalytic reactivity toward WGS reaction at 400 °C. When the temperature was increased to 500 °C and 600 °C, the catalytic reactivity of hydrated sorbent was recovered partially, but this will depend on the steam fraction in gas phase, the recovery of fresh CaO surface from dehydration of Ca(OH)₂ may be the reason of catalytic reactivity recovery. CaCO₃ can catalyze the WGS reaction at the high-temperature (>600 °C), this may due to the CaCO₃ decomposition and recarbonation processes in which the CaO is transiently formed. The possible mechanism was discussed.     

CaBr2 hydrolysis for HBr production using a direct sparging contactor

The calcium–bromine cycle being investigated is a novel continuous hybrid cycle for hydrogen production employing both heat and electricity. Calcium bromide (CaBr₂) hydrolysis generates hydrogen bromide (HBr) which is electrolyzed to produce hydrogen. The CaBr₂ hydrolysis at 1050 K (777 °C) is endothermic with the heat of reaction δG_T = 181.5 KJ/mol (43.38 kcal/mol) and the Gibbs free energy change is positive at 99.6 kJ/mol (23.81 kcal/mol). What makes this hydrolysis reaction attractive is both its rate and that well over half the thermodynamic requirements for water-splitting heat of reaction of δG_T = 285.8 KJ/mol (68.32 kcal/mol) are supplied at this stage using heat rather than electricity. Molten-phase calcium bromide reactors may overcome the technical barriers associated with earlier hydrolysis approaches using supported solid-phase calcium bromide studied in the Japanese UT-3 cycle. Before constructing the experiment two design concepts were evaluated using COMSOL™ multi-physics models; 1) the first involved sparging steam into a calcium-bromide melt, while 2) the second considered a “spray-dryer” contactor spraying molten calcium bromide counter-currently to upward-flowing steam. A recent paper describes this work [6]. These studies indicated that sparging steam into a calcium-bromide melt is more feasible than spraying molten calcium bromide droplets into steam. Hence, an experimental sparging hydrolysis reactor using a mullite tube (ID 70 mm) was constructed capable of holding 0.3–0.5 kg (1.5–2.5 × 10⁻³ kg mol) CaBr₂ forming a melt with a maximum 0.08 m (8 cm) depth. Sparging steam at a steam rate of 0.02 mol/mol of CaBr₂ per minute (1.2–2.3 × 10⁻⁵ kg/s), into this molten bath promptly yielded HBr in a stable operation that converted up to 25% of the calcium bromide. The kinetic constant derived from the experimental data was 2.17 × 10⁻¹² kmol s⁻¹ m⁻² MPa⁻¹ for the hydrolysis reaction. The conversion rate is highly dependent on melt depth and the design for steam sparging. This experimental data provides a basis for designing a larger-scale sparging hydrolysis reactor for the calcium bromide thermochemical cycle where the endothermic heat of reaction can be effectively supplied by heat transfer coils embedded in the melt.     

Spherical expanding flames in H2–N2O–Ar mixtures: flame speed measurements and kinetic modeling

Although ignition of hydrogen–nitrous oxide mixtures is a serious issue for nuclear waste storage and semi-conductor manufacturing, available flame speed data have not been recently updated and thermodiffusive stability is not known. In order to palliate this, the flame speed of a hydrogen–nitrous oxide mixture diluted in Ar (60% mol) was measured in a spherical bomb as a function of equivalence ratio. The initial pressure and temperature were held constant around ambient conditions. It is shown that the unstretched flame speed of the hydrogen–nitrous oxide mixture is relatively low for a hydrogen-based mixture, with a maximum of 56 cm/s for the stoichiometric condition. Further, hydrogen–nitrous oxide–argon flames appear unstable with respect to thermodiffusive effects at an equivalence ratio of 1. The downward flammability limit of hydrogen–nitrous oxide–argon was observed for hydrogen content of 8 mol%. The modeling of these experimental data has been performed with three recently developed models. All kinetic schemes give satisfactory predictions of the experimentally observed data. Sensitivity and reaction pathway analysis have demonstrated that the dynamic of the system is dominated by the reaction N₂O + H = N₂ + OH which governs the rate of energy release.