Sulphur based thermochemical cycles: Development and assessment of key components of the process

HycycleS was a cooperation of nine European partners and further non-European partners and aimed at the qualification and enhancement of materials and components for key steps of solar and nuclear powered thermochemical cycles for hydrogen generation from water. The focus of HycycleS was the decomposition of sulphuric acid (H₂SO₄) which is the central step of the sulphur-based family of those processes. Emphasis was put on materials and components for H₂SO₄ evaporation, decomposition, and sulphur dioxide separation. The suitability of materials and components was demonstrated by decomposing H₂SO₄ and separating its decomposition products in scalable prototypes.     

Status report on thermochemical iron/chlorine cycles: a chemical engineering analysis of one process

An iron/chlorine process with detailed flowsheeting is discussed. Mass and energy balances had been done to gain high efficiency. Results of 28–35% are achieved by an optimization of the HCl boiler system and by the high temperature heat balance requiring two paths for FeCl₂ hydrolysis. The dechlorination operates in two steps by means of a subcycle with carbon compounds. 35% efficiency is too low to guarantee economic feasibility but the development steps learned in this cycle should be valuable to other cycle studies.     

Solar heating and cooling by a thermochemical process. First experiments of a prototype storing 60 kW h by a solid/gas reaction

The chemical heat pumps using monovariant solid/gas reactions and thermal solar energy are potentially interesting for the air-conditioning of building (heating in winter or mid-season and refreshing in summer). They provide a function of storage without loss and potentially at high energy density. The selected reaction involves SrBr₂ as reactant and H₂O as refrigerant fluid. It is adapted to the thermodynamic constraints in temperature (heat provided by plane solar collector, heating and cooling on the level of the floor) and uses reagents having a weak impact for the environment and health. The reactive salt SrBr₂ is implemented with an expanded natural graphite in the form of a consolidated material which has acceptable thermal conductivity and permeability adapted to low pressure. The prototype reactor has a total volume of 1 m³. It is able to store, with a complete reaction, 60 kW h or 40 kW h for the heating or cooling function respectively. This prototype was tested under conditions representative of summer or mid-season; the mean heating or cooling powers, typically about 2.5–4 kW, are still insufficient because of a low heat transfer at the interface between the reactive layer and the exchanger wall. However this limitation can be clearly attenuated; that is the subject of current work in following these first experiments.     

Economic comparison of solar hydrogen generation by means of thermochemical cycles and electrolysis

Hydrogen is acclaimed to be an energy carrier of the future. Currently, it is mainly produced by fossil fuels, which release climate-changing emissions. Thermochemical cycles, represented here by the hybrid-sulfur cycle and a metal oxide based cycle, along with electrolysis of water are the most promising processes for ‘clean’ hydrogen mass production for the future. For this comparison study, both thermochemical cycles are operated by concentrated solar thermal power for multistage water splitting. The electricity required for the electrolysis is produced by a parabolic trough power plant. For each process investment, operating and hydrogen production costs were calculated on a 50 MW_th scale. The goal is to point out the potential of sustainable hydrogen production using solar energy and thermochemical cycles compared to commercial electrolysis. A sensitivity analysis was carried out for three different cost scenarios. As a result, hydrogen production costs ranging from 3.9–5.6 €/kg for the hybrid-sulfur cycle, 3.5–12.8 €/kg for the metal oxide based cycle and 2.1–6.8 €/kg for electrolysis were obtained.     

Quantitative evaluation and factors of gas-wetness in gas/liquid/solid systems

After altering the wetness of porous media from preferentially liquid-wet to preferentially gas-wet, gas well production will increase greatly in gas condensate reservoirs. Since the gas-wet degree seriously affects the production, it has both theoretical and practical value to study quantitative measurement and effect factors for gas wettability. In the study, using the sessile drop and the captive bubble, the gas-wet quantitative evaluation methods and criteria have been put forward. Glass slides, treated by the fluorocarbon polymer Zonyl8740, were used as gas-wetness measurement substrates in order to eliminate the roughness effect. The calculation of slide surface free energy and slide/water interface free energy was determined by two models, respectively, Owens-Wendt and the captive bubble two-probe method. The results show that, if the solid surface free energy is less than solid/liquid interface free energy, the surface or the interface will be preferential gas-wetting, otherwise, non gas-wetting; with the decrease of surface free energy and interface free energy, the gas-wetting gets strong. Thus, it is effective to realize gas-wetness by lowering the value of surface free energy or surface tension.     

Thermo-economic modeling of a solid oxide fuel cell/gas turbine power plant with semi-direct coupling and anode recycling

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency in order to improve system efficiencies and economics. The SOFC system is semi-directly coupled to the gas turbine power plant, with careful attention paid to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 21.6 MW at 49.2% efficiency. The model also predicts a breakeven per-unit energy cost of USD 4.70 ¢/kWh for the hybrid system based on futuristic mass generation SOFC costs. Results show that SOFCs can be semi-directly integrated into existing GT power systems to improve their thermodynamic and economic performance.     

Role of the contact layer between liquid and solid on a solidification process

The object of this paper is the presentation of a theoretical solidification model of heat-conducting liquid metal flowing near of a cold plate. The problem of the unsteady behaviour both of the frozen metal layer and of the cold plate is studied analytically. Influence of the contact layer between the frozen layer and the cold plate on the solidification process is also studied. The results in the form of the analytical formula and the graphs are presented and compared with the other results.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

A numerical modeling of an absorption process on a liquid falling film

A simple and promising numerical model is developed for simultaneous heat and mass transfer in smooth falling-film absorption. The systems of LiCl---H₂O and LiBr---H₂O are solved because they are the most popularly used materials in absorption heat pumps and chillers. The results are in good agreement with those of complicated formulations in the literature and the experimental data.     

Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents

A systematic investigation is made of the two-stage vapour absorption refrigeration system employing the refrigerant absorbent combinations of NH₃---H₂O and NH₃---LiNO₃. The system consists of coupling two conventional absorption cycles so that the first-stage evaporator produces cooling water to circulate in the absorber of the second stage. The effect of operating variables such as generator temperature, evaporator temperature, absorber temperature and condenser temperature on the coefficient of performance (COP), heat transfer rates and relative circulation have been studied for both single-stage and two-stage absorption refrigeration systems. It is found that the COP is higher for NH₃---LiNO₃ than for NH₃---H₂O, in both single-stage and two-stage absorption systems, especially at higher generator temperatures. Furthermore, the minimum evaporator temperature achieved is lower for NH₃---LiNO₃, and the system can be operated at lower generator temperatures.     

Available power generation cycles to be coupled with the liquid natural gas (LNG) vaporization process in a Spanish LNG terminal

The boil off gas in Spanish LNG terminals is managed using recondensers. The electricity consumed by these terminals is bought in the Spanish wholesale market. Several power generating options using current available equipment and assuring the availability of the current terminal process have been analyzed thermoeconomically. A new combined cycle using a gas turbine and a pure NH₃ Rankine cycle coupled with the natural gas vaporization process has been chosen as the most advisable one to be installed, due to the lower thermoeconomic cost obtained as shown in a new graphical representation similar to the existing exergetic cost diagrams.     

A study on the advanced performance of an absorption heater/chiller with a solution preheater using waste gas

A large amount of consumed energy is released to the environment as waste heat, which may be used directly in some applications for useful purposes. Thus, from the standpoint of energy conservation, it will be meaningful to investigate systems with waste heat utilization. It has long been indicated that absorption cycles may have good potential applications for enhancing energy conservation via waste heat recovery. As such they have often been identified as an appropriate subject for research and development. Along this line, this paper also examines the development of an absorption cycle with waste heat utilization. More specifically, this study investigates a double-effect LiBr–water absorption cycle which uses exhaust gases from the burner of the high-temperature generator to preheat the weak absorbent solution on its way from the absorber to the low-temperature generator. The overall performance of the absorption heater/chiller system is analyzed and discussed on the basis of experimental results.     

Comparison of a Reaction Front Model and a Finite Difference Model for the Simulation of Solid Absorption Process

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Theoretical comparison of single stage and advanced absorption heat transformers operating with water/lithium bromide and water/Carrol mixtures

A thermodynamic analysis was carried out to compare the theoretical performance of single stage, two stage and double-absorption heat transformers operating with the water/lithium bromide and the water/Carrol mixtures, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH₂OH)₂] in the ratio 1:4·5 by weight. A mathematical model to predict the theoretical performance of single stage and the advanced heat transformers is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system for both mixtures. The water/Carrol mixture showed in general to have a better performance than the water/lithium bromide mixture. © 1998 John Wiley & Sons, Ltd.     

Optimal analysis of the exothermic process of a Mg/MgH2 thermochemical heat storage system

基于镁/氢化镁热化学储热系统,建立了二维非稳态数学模型.对吸氢放热过程中的传热传质现象进行了数值模拟,主要研究了壁面温度和反应床当量导热系数对系统反应速率的影响.结果表明,放热过程中存在最佳的壁面温度使反应速率达到最快,过高或者过低的壁面温度都将使反应床的温度偏离理论上的最佳值,从而降低反应速率.针对不同当量导热系数的反应床,最佳壁面温度也不相同;反应床的当量导热系数并非越大越好,应该根据具体的边界温度以及氢气压力情况进行合理的选择以获得最佳的反应速率.     

Operational strategy of a two-step thermochemical process for solar hydrogen production

A two-step thermochemical cycle process for solar hydrogen production from water has been developed using ferrite-based redox systems at moderate temperatures. The cycle offers promising properties concerning thermodynamics and efficiency and produces pure hydrogen without need for product separation.     

Performance analysis and optimization of a trigeneration process consisting of a proton-conducting solid oxide fuel cell and a LiBr absorption chiller

In this work, the trigeneration system, consisting of a proton-conducting solid oxide fuel cell (SOFC–H⁺) and a single-stage LiBr absorption chiller, was proposed. The SOFC–H⁺ and single-stage LiBr absorption chiller models were developed through Aspen Plus V10. From the sensitivity analysis, the results show that increases in temperature and fuel utilization can improve the performance of the SOFC–H⁺. Conversely, the air to fuel (A/F) molar ratio and pressure negatively affect the electrical efficiency and overall system efficiency. In the case of the absorption chiller, the coefficient of performance was increased and made stable according to a constant value when the generator temperature was increased from 90 to 100 °C. When the optimization was performed, it was found that the SOFC–H⁺ should be operated at 700 °C and 10 bar with fuel utilization of 0.8 and A/F molar ratio of 2 to achieve a maximum overall efficiency of 93.34%. For the energy and exergy analysis, a combined heat and power SOFC–H⁺ was found to have the highest energy and exergy efficiencies, followed by the trigeneration process. This indicates that the integration of the SOFC–H⁺ and LiBr absorption chiller is possible to efficiently produce electricity, heating and cooling.     

Development of a numerical model for natural gas steam reforming and coupling with a furnace model

This paper presents a numerical model (SRP) that was developed to describe the steam reforming process within tubes or channels. This model was implemented in C language and is used as a User-Defined Function (UDF) in the commercial program Fluent. The SRP model is one-dimensional representing mass and energy balances along the tubes/channels assuming uniform conditions in the cross section, except for temperature within porous regions. The model calculates the gas species concentrations and temperature profiles along the tubes/channels and since it is coupled with the Fluent furnace calculation, the boundary temperature is continuously updated and is a model result.     

Catalytic thermochemical reactor/receiver for solar reforming of natural gas: Design and performance

The advantages of thermochemical conversion of concentrated solar energy using catalytic processes are discussed. The design of a solar volumetric thermochemical reactor/receiver (TCRR) with catalytic absorber, method for synthesis of catalytically activated ceramics, and preparation of catalytic absorber have been described. The prototype TCRR was tested in the high flux solar furnace at the DAC, Cologne by using the dioxide reforming of methane. The tests were performed to check the main concept of the TCRR design and catalytic absorber, to study the influence of solar flux distribution, the reagent flows and their ratio on the productivity or conversion, determine the reagent's conversion depending on the focal point disposition with respect to the absorber, and to study the efficiency of the thermochemical conversion. The chemical and total efficiencies of the CO₂–methane conversion were calculated using the experimentally measured concentrations of the reaction products. The highest overall efficiency achieved in these experiments was 30% with the Ni–Cr catalytic absorber.     

Testing of gas and liquid fuel burners for power and process industries

The object of the present paper is a review of issues related to the testing of gas and liquid fuel burners which are amongst the most important items of equipment in materials processing and energy producing industries. The text gives basic information about fuels, types of burners and their testing and also about modelling of combustion and formation of pollutants, mainly nitrogen oxides. The first two sections of the text provide an overview of fuels and burner types. The most part of the paper deals with an assessment of conditions and equipment required for testing of gas and liquid fuel burners. Conditions that must be satisfied in burner tests in order to preserve comparable operating conditions as in the real application are stressed. Last part provides an outline of the utilisation of statistical analysis methods and modelling by computational fluid dynamics (CFDs) including the formation of pollutants.