Utilization of the cryogenic exergy of liquid natural gas (LNG) for the production of electricity

Liquid natural gas (LNG) delivered by means of sea-ships is compressed and then evaporated before its introduction to the system of pipelines. The possibilities of the utilization of cryogenic exergy of LNG for electricity production without any additional combustion of any its portion, have been analyzed. Three variants of the plant have been investigated. A cascade system with two working fluids has been analyzed in two first of them. The economic optimization proved that the optimum temperature difference in the LNG evaporation is higher than initially assumed. Therefore, a third variant of the plant has been analyzed, with ethane as a single working fluid. Only the third variant has been analyzed in detail.     

LNG冷yong动力系统的工质选择及系统分析方法

阐述了LNG冷yong动力系统的工质选择原则及系统分析的基本方法。     

Novel cogeneration power system with liquefied natural gas (LNG) cryogenic exergy utilization

We propose a new cogeneration power system with two energy sources of fuel chemical energy and liquefied natural gas (LNG) cryogenic energy, and two outputs of electrical power and cooling power. Due to the advanced integration of system and cascade utilization of LNG cryogenic energy, the system has excellent energy saving: chemical energy of fuel and LNG cryogenic energy are saved by 7.5–12.2% and 13.2–14.3%, respectively. As CO₂ is selected as working fluid and oxygen as fuel oxidizer, CO₂ is easily recovered as a liquid with LNG vaporization. In this paper, the typical recuperative Rankine cycle and the corresponding cogeneration system are described and a detailed thermodynamic analysis is carried out to reveal the principle of the cycle and system. Furthermore, the influence of key parameters on performance is discussed. Considering the engineering application, the technical advantages and concerns are pointed out.     

On the recovery of LNG physical exergy by means of a simple cycle or a complex system

The maximum and minimum temperatures available limit the usable fraction (or Carnot efficiency) of a power cycle. The construction of LNG terminals and the need to vaporize LNG offers a thermal sink at a very much lower temperature than seawater. By using this thermal sink in a combined plant, it is possible to recover power from the vaporization of LNG.To this purpose, in this paper combined systems using LNG vaporization as low-temperature thermal sink are considered and their pros and cons are presented. A system utilizing waste energy as heat source and with a single working fluid is analyzed in detail. However, the use of a single fluid is not the best solution from a thermodynamic point of view. Thus, a series of cascading cycles is also outlined. In these systems, both the thermal source and the thermal sink are exploited as exergy sources.     

Process Study and Exergy Analysis of a Novel Air Separation Process Cooled by LNG Cold Energy简

In order to resolve the problems of the current air separation process such as the complex process, cumbersome operation and high operating costs, a novel air separation process cooled by LNG cold energy is proposed in this paper, which is based on high-efficiency heat exchanger network and chemical packing separation technology. The operating temperature range of LNG cold energy is widened from 133K-203K to 113K-283K by highefficiency heat exchanger network and air separation pressure is declined from 0.5MPa to about 0.35MPa due to packing separation technology, thereby greatly improve the energy efficiency. Both the traditional and novel air separation processes are simulated with air handling capacity of 20t·h 1. Comparing with the traditional process, the LNG consumption is reduced by 44.2%, power consumption decrease is 211.5 kWh per hour, which means the annual benefit will be up to 1.218 million CNY. And the exergy efficiency is also improved by 42.5%.     

A real column design exergy optimization of a cryogenic air separation unit

Distillation columns are one of the main methods used for separating air components. Their inconvenient is their high energy consumption. The distillation process is simulated in three types of columns and the exergy losses in the different parts calculated. A sensitivity analysis is realized in order to optimize the geometric and the operational parameters of each type of column. A comparative exergy analysis between the distillation columns considered for cryogenic air separation shows that the exergy efficiency of a double diabatic column, with heat transfer all through the length of the column, is 23% higher than that of the conventional adiabatic double columns.     

Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes

A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency. A detailed model of a plate-fin heat exchanger is presented that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.     

An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes

Four processes for small-scale liquefied natural gas (LNG) production are evaluated. These include a single-stage mixed refrigerant (SMR), a two-stage expander nitrogen refrigerant and two open-loop expander processes. Steady-state simulations were undertaken to ensure that each process was compared on an identical basis, was fully optimised and was in agreement with published results. Composite curves for the feed and recycle streams and the refrigerant or cold recycle stream showed the degree of optimisation available within each process. The full exergy analysis showed the relative contributions to the total shaft work requirements, with the lowest being the SMR process. The lower efficiency of the expander-driven compressors is the main difference between processes. A more general comparison suggested that the nitrogen refrigerant process and the New LNG open-loop process are the leading candidates for offshore compact LNG production.     

A comparative CFD assessment study of cryogenic hydrogen and LNG dispersion

The introduction of hydrogen to the commercial market as alternative fuel brings up safety concerns. Its storage in liquid or cryo-compressed state to achieve volumetric efficiency involves additional risks and their study is crucial. This work aims to investigate the behavior of cryogenic hydrogen release and to study factors that affect the vapor dispersion. We focus on the effect of ambient humidity and air's components (nitrogen and oxygen) freezing, in order to identify the conditions under which these factors have considerable influence. The study reveals that the level of influence depends highly on the release conditions and that humidity can reduce conspicuously the longitudinal distance of the Lower Flammability Limit (LFL). Low Froude (Fr) number (<1000) at the release allows the generated by the humidity phase change buoyancy to affect the dispersion, while for higher Fr number - that usually are met in cryo-compressed releases - the momentum forces are the dominant forces and the buoyancy effect is trivial. Simulations with liquid methane release have been also performed and compared to the liquid hydrogen simulations, in order to detect the differences in the behavior of the two fuels as far as the humidity effect is concerned. It is shown that in methane spills the buoyancy effect in presence of humidity is smaller than in hydrogen spills and it can be considered almost negligible.     

Design and Optimization of a Full-Generation System for Marine LNG Cold Energy Cascade Utilization

This paper took a 100,000 DWT LNG fuel powered ship as the research object. Based on the idea of "temperature matching, cascade utilization" and combined with the application conditions of the ship, a horizontal three-level nested Rankine cycle full-generation system which combined the high-temperature waste heat of the main engine flue gas with the low-temperature cold energy of LNG was proposed in this paper. Furthermore, based on the analysis and selection of the parameters which had high sen...     

Analysis of a combined power and refrigeration cycle by the exergy method

The exergy analysis method was applied in order to evaluate the new combined cycle proposed by Goswami [Solar thermal technology: present status and ideas for the future. Energy Sources 1998;20:137–45], using Hasan–Goswami–Vijayaraghavan parameters. This new combined cycle was proposed to produce both power and cooling simultaneously with only one heat source and using ammonia–water mixture as the working fluid. The simulation of the cycle was carried out in the process simulator ASPEN Plus. The Redlich–Kwong–Soave equation of state was used to calculate the thermodynamic properties. The cycle was simulated as a reversible as well as an irreversible process to clearly show the effect of the irreversibilities in each component of the cycle. At the irreversible process two cases were considered, changing the environmental temperature. However, in order to know the performance of the new cycle at different conditions of operation, the second irreversible case was analyzed varying the rectification temperatures, the isentropic efficiency of the turbine and the return temperature of the chilled water. Exergy effectiveness values of 53% and 51% were obtained for the irreversible cycles; with heat input requirements at temperatures of 125 and 150 °C. Solar collectors or waste heat are suggested as heat sources to operate the cycle.     

Evaluation of a chemical-looping-combustion power-generation system by graphic exergy analysis

A loop of chemical reactions is introduced to reduce the exergy loss caused by the conversion of fuel energy to thermal energy in conventional LNG powerplants. By applying this chemical loop and the graphic simulator which generates energy-utilization diagrams, a new gasturbine power-generation system called a chemical-looping-combustion system (CLCS) is composed. Exergy analysis of a model system shows an expected thermal powerplant efficiency as high as 50.2%(LHV).     

Classification of geothermal resources by exergy

Geothermal resources have been classified as low, medium and high enthalpy resources according to their reservoir fluid temperatures. There is no general agreement on the arbitrary temperature ranges used. Classification of a geothermal resource by its reservoir fluid temperature can be ambiguous because two independent properties are required to define the thermodynamic state of a fluid. Geothermal resources should be classified to reflect their ability to do thermodynamic work. In this paper, it is proposed that geothermal resources be classified as low, medium and high quality resources with reference to their specific exergy indices (SExI), SExI<0.05, 0.05SExI<0.5 and SExI0.5, respectively. The demarcation limits for these indices are exergies of saturated water and dry saturated steam at 1 bar absolute. These demarcation lines can be plotted on a Mollier diagram to form a classification map of geothermal resources.     

Emergy as a function of exergy

This paper aims to clarify some aspects of the discussion between “emergists” and “exergists”. First, we address the problem of the differences between energy-based emergy and exergy-based emergy: we show that the two are proportional, having the exergetic equivalent of solar energy as scale factor. In the second part, we show that emergy and transformity can be written as a function of exergy alone, in particular of “partial” efficiencies of the processes involved in a production system, from solar energy to the final product.     

Solution of the inverse heat conduction problem described by the Poisson equation for a cooled gas-turbine blade

The presented paper describes a method of solving the inverse problems of heat conduction, consisting in solving the Poisson equation for a simply connected region instead of the Laplace equation for a multiply connected one, like a gas-turbine blade provided with cooling channels. The considered method consists in determining unknown values of the source (heat sink) power in the cooling channels for a given external heat transfer situation to achieve as close as possible an isothermal outer surface. Afterwards the temperature and heat flux distributions at the cooling channel walls are determined. Since the unknown source power is sought, the problem is an inverse one. Taking into account the sought values the method is reckoned among the class of the fictitious source methods and presents an optimization scheme. Using an exemplary gas turbine blade cooling configuration, the results of the calculation obtained with this method have been compared to the results achieved with an inverse method using the boundary element method for a multiple connected region.The results obtained with both methods within the optimization scheme approximated each other. Nevertheless, the results for the inverse method shown in the present paper gave nearly no oscillations, which is important in case of the blades with other geometric features of the cooling channels.     

Clustering of chaotic dynamics of a lean gas-turbine combustor

This work deals with the dynamic behaviour of a lean premixed gas turbine combustor. The study aims to achieve a classification of experimental burner dynamic behaviour and is based on the geometrical properties of the attractors of the system variables. Several experiments were performed varying the flame stoichiometric ratio λ and the pilot fuel percentage PFP. The dynamics of the experimental time series of the flame front heat release were described by using vectors collecting information on the topological distribution of the attractors. Therefore, unsupervised Kohonen associative memories were trained to create clusters of operating conditions characterised by similar dynamical behaviours. Kohonen associative memories were able to divide the experimental operating conditions into different clusters according to the different values of the flame stoichiometric ratio. The results of the clustering underline the possibility of being able to define an algorithm for combustion-instability pattern recognition that takes into account the highly non-linear effects which govern combustion processes.     

On the exergy of radiation

In the domain of heat radiation, and particularly of solar energy use, the notion of exergy (or alternatively entropy) of radiation has given rise to a fairly abundant literature; unfortunately, incoherencies and discrepancies between the various authors could lead to a complete disuse of the notion in this context. The aim of this paper is to contribute to a clarification of this issue.We propose here a derivation of the exergy of radiation, based solely on classical thermodynamics notions, making thus possible a ready check of the validity of the results. Results are given first for the blackbody case, then extended to a radiation with arbitrary spectrum. Finally, application of the notion of radiation entropy to a few simple examples shows the consistency of this notion with some well-known physical laws, and can even give some insight into the real signification of these laws.     

Performance and parametric investigation of a binary geothermal power plant by exergy

Exergy analysis of a binary geothermal power plant is performed using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy diagram, and compared to the energy diagram. The sites with greater exergy destructions include brine reinjection, heat exchanger and condenser losses. Exergetic efficiencies of major plant components are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the plant are 4.5% and 21.7%, respectively, based on the energy and exergy of geothermal water at the heat exchanger inlet. The energy and exergy efficiencies are 10.2% and 33.5%, respectively, based on the heat input and exergy input to the binary Rankine cycle. The effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained.     

Application of the meta-heuristics for optimizing exergy of a HT-PEMFC

The aim of this study is to analyze the exergy efficiency of a high-temperature proton exchange membrane fuel cell. To do this purpose, meta-heuristic technique has been used. First, the model of a membrane fuel cell is simulated and the polarization diagram shows a potent agreement with empirical data. Then, a new improved version of collective animal behavior algorithm is utilized for evaluating and optimizing the thermodynamic irreversibility, exergy efficiency, and work of the fuel cell. The algorithm uses opposition-based learning and Lévy flight for improving the algorithm's premature convergence shortcoming. The result of this study shows that by comparison with standard collective animal behavior algorithm, genetic algorithm, and empirical results, the proposed algorithm has better achievements for both terms of optimal value finding and convergence strength.