Hydrogen for oil refining via biomass indirect steam gasification: energy and environmental targets

The energy and CO₂ consequences of substitution of a fossil-fuel-based hydrogen production unit with a biomass-based process in a large European refinery are studied in this study. In the base case, the biomass-based process consists in atmospheric, steam–blown indirect gasification of air-dried woody biomass followed by necessary upgrading steps. The effect of gradually substituting the current refinery hydrogen production unit with this process on global energy and CO₂ targets is estimated first. Few process concepts are studied in further detail by looking at different degrees of heat integration with the remaining refinery units and possible polygeneration opportunities. The proposed process concepts are compared in terms of energy and exergy performances and potential reduction in refinery CO₂ emission also taking into account the effect of marginal electricity. Compared to the base case, an increase by up to 8 % points in energy efficiency and 9 % points in exergy efficiency can be obtained by exploiting process integration opportunities. According to energy efficiency, steam production appears the best way to use excess heat available in the process while electricity generation through a heat recovery steam cycle appears the best option according to exergy efficiency results. All investigated cases yield to significant reduction in CO₂ emissions at the refinery. It appears in particular that maximal emission reduction is obtained by producing extra steam to cover the demand of other refinery units if high efficiency marginal electricity scenarios are considered.     

Exergy analysis of a thermal power plant using a modeling approach

The objective of this article is to perform exergy analysis for thermal power plants based on the simulation software Aspen Plus. In this study, the main sections of the system including the boiler and the steam turbine are modeled using the Aspen Plus software. Exergy analysis of the modeled system is studied to obtain the energy loss distribution of the system. The influences of various operating conditions on the exergy efficiency of the system are investigated including combustion temperature, excess air coefficient, steam temperature, and steam pressure. The results show that exergy loss mainly occurs in the boiler and the steam turbine. The major sources of irreversibility are combustion and the internal thermal energy exchange in the boiler. The high-pressure turbine has the lowest exergy efficiency in the steam turbine; however, it assumes the largest work output, accounting for 37.08%. Therefore, reducing the irreversible loss in the boiler and improving the performance of the high-pressure turbine are the requirements for improving the system.     

Exergy analysis of gas turbine trigeneration system for combined production of power heat and refrigeration

A conceptual trigeneration system is proposed based on the conventional gas turbine cycle for the high temperature heat addition while adopting the heat recovery steam generator for process heat and vapor absorption refrigeration for the cold production. Combined first and second law approach is applied and computational analysis is performed to investigate the effects of overall pressure ratio, turbine inlet temperature, pressure drop in combustor and heat recovery steam generator, and evaporator temperature on the exergy destruction in each component, first law efficiency, electrical to thermal energy ratio, and second law efficiency of the system. Thermodynamic analysis indicates that exergy destruction in combustion chamber and HRSG is significantly affected by the pressure ratio and turbine inlet temperature, and not at all affected by pressure drop and evaporator temperature. The process heat pressure and evaporator temperature causes significant exergy destruction in various components of vapor absorption refrigeration cycle and HRSG. It also indicates that maximum exergy is destroyed during the combustion and steam generation process; which represents over 80% of the total exergy destruction in the overall system. The first law efficiency, electrical to thermal energy ratio and second law efficiency of the trigeneration, cogeneration, and gas turbine cycle significantly varies with the change in overall pressure ratio and turbine inlet temperature, but the change in pressure drop, process heat pressure, and evaporator temperature shows small variations in these parameters. Decision makers should find the methodology contained in this paper useful in the comparison and selection of advanced heat recovery systems.     

Combined pinch and exergy analysis of an ethylene oxide production process to boost energy efficiency toward environmental sustainability

Ethylene oxide production process is one of the highest energy consumers in chemical industry, and therefore even a slight improvement in its overall efficiency can have a significant impact on the sustainability of the process. Efficiency improvement can be carried out using the exergy-aided pinch analysis outlined in this paper. The overall exergy loss distribution in different unit operations of an ethylene oxide process was first evaluated and mapped out in the form of “visualized exergetic process flowsheet”. An initial analysis of the four main functional blocks of the process showed that the exothermic reaction block contained the largest exergy loss (6043 and 428 kJ/kg of internal and external losses, respectively) which can be reduced by isothermal mixing, as well as increasing reaction temperature and reduction in pressure drop. The absorption block was also estimated to have the second highest contribution with total exergy losses of 3640 kJ/kg which were mainly due to the cooling column. These losses were then recommended to be reduced by improvements in the concentration and temperature gradients along the tower. Following the block-wise analysis, exergy analysis was then carried out for individual unit operations in each block to pinpoint the main sources of thermal exergetic inefficiency. Thermal solutions to reduce losses were also proposed in accordance with the identified sources of inefficiency, leading to a comprehensive list of cold and hot process streams that could be introduced to reduce losses. Finally, pinch analysis was brought into action to estimate the minimum energy requirements, to select utilities, and to design heat exchanger network. Thus, the methodology used in this work took advantage of both exergy and pinch analyses. The combined thermal-exergy-based pinch approach helped to set energy targets so that all the thermal possible solutions supported by exergy analysis were considered, preventing exclusion of any hot or cold process stream with high potential for heat integration during pinch analysis. Results indicated that the minimum cold utility requirement could be reduced from 601.64 MW (obtained via conventional pinch analysis) to 577.82 MW through screening of streams by the combined methodology.     

Critical analysis of a biogas powered absorption system for climate change mitigation

The importance of biogas as a renewable alternative is being studied because of an increase in the cost of conventional fuels. The present article suggests a numerical study of a biogas powered NH₃–H₂O absorption refrigeration system where biogas is used to heat the water which serves as an energy input to generator of an absorption system. A computational model has been developed for the analysis which involves the determination of effect of generator temperature on various performance parameters, i.e., exergy losses in the different components, COP_cooling, COP_heating and the exergy efficiency. The results indicate that COP_cooling and COP_heating lies in the range of 0.159–0.33 and 1.16–1.33, respectively, whereas exergetic efficiency lies in the range of 0.29–0.80 for the same variation in generator temperature ranging from 50 to 70 °C. The highest exergy loss is found in the generator while the lowest is found in the condenser and it is also found that with an increase in the evaporator as well as absorber and condenser temperature, the COP increases and decreases, respectively. The effect of ambient temperature on exergy loss in the different components is also studied. Exergy analysis is an excellent tool to pin point the losses in the system due to irreversibility which are the basis for the further improvement in the system components.     

Internal energy flow analysis within a single effect sorption heat pump

The knowledge of the characteristics of unused, excess and untapped exergy allows a thorough analysis of internal energy flows distribution within a sorption heat pump. It can be applied to any system based on gas–liquid absorption, adsorption or solid–gas reaction as well as to any process based on the internal recycling of the energy flux. It can also be applied for the case of a simple effect ideal machine, in particular in the definition of processes where the COP is larger than 2: the levels at which the initial exergy is downgraded on the one hand, as well as, the upgraded excess exergy produced on the other allows the designer to make a judicious choice of a system.     

Using biomass as an energy source with low CO2 emissions

This work deals with the carbon dioxide cycle and emissions from biomass incineration under a hydrogen production context. It is proposed to use the thermal energy obtained by biomass combustion to produce water steam, which afterwards would be converted into hydrogen by high temperature electrolysis (HTE). In France, the thermal energy potential from nonvalorised biomass reaches almost 6.5 Mtep. In this study, the potential avoided carbon emissions are quantified as well as the feasible hydrogen production capacity based on the steam supplied by the incineration units. Results show that carbon consumption in hydrogen production by steam methane reforming (SMR) or biomass incineration–HTE process is almost equivalent between both processes. However, the hydrogen produced by the biomass incineration–HTE process used to fuel vehicles, would lead to a decrease of 135 Mt of carbon from fossil origins yearly, in contrast to SMR.     

吸附式制冷循环的分析

以吸附式制冷循环的热力过程为依据，使用火用分析的方法对连续回热循环做了分析，对循环中各部分火用损进行了比较。指出了连续回热循环中火用损的主要部位，并探讨了回热率及吸附床的传热性能对循环火用效率的影响。     

Exergy analysis of a Gifford-McMahon cycle cryorefrigerator

Exergy analysis of a Gifford-McMahon cycle refrigerator is presented. Exergy losses occurring in various components are considered and the exergy balance is shown in tabular form. It is observed that the major losses occur in the compressor and at the cold end.     

两级双效溴化锂制冷-热泵复合循环

在热电冷联产系统中,溴化锂吸收式制冷机在制冷过程中排放了大量的废热,这些废热品味低,难以直接回收利用。在此提出了两级双效溴化锂制冷-热泵复合循环,该循环具有冷凝温度较高的特点,便于直接回收冷凝排放热。系统以背压汽轮机的背压蒸汽为热源,制冷的同时利用循环所排出的废热加热锅炉补充水至较高温度。以具有相同功效的双效溴冷机与单效溴化锂热泵联合运行作为对比循环,制冷-热泵复合循环系统省去了一台蒸发器与冷凝器,减少了两个换热温差,并且通过热力计算、能量分析和分析表明,该循环的能量利用率与效率均有很大的提高,效率比对比循环提高了45%。     

Performance comparison of single-stage mixed-refrigerant Joule–Thomson cycle and pure-gas reverse Brayton cycle at fixed-temperatures from 80 to 180 K

Performance comparison of single-stage mixed-refrigerant Joule–Thomson refrigeration cycle (MJTR) and pure-gas reverse Brayton cycle (RBC) at fixed-temperatures from 80 to 180 K was made in this paper. The simulation was mainly conducted under nonideal conditions with extrinsic irreversibilities. Exergy efficiency and volumetric cooling capacity are two main evaluation parameters. Exergy loss distributions along the cycles were analyzed. Under ideal conditions, RBC achieved the highest exergy efficiency at all temperatures, but lower volumetric cooling capacity than MJTR at middle-high temperatures. Under nonideal conditions, both the exergy efficiency and volumetric cooling capacity of MJTR were obviously superior to RBC from 100 to 180 K, but inferior to RBC at 80 K. Two reasons account for the sharp performance degradation of MJTR: The high fraction of neon resulted in large entropy generation and exergy loss in throttling process. Larger recuperator duty and WLMTD lead to larger losses in the recuperator.     

Economie d'énergie et récupération de chaleur notamment en transport maritime

In evaluating the energy performance of refrigerating systems, two aspects are considered: the efficiency with which cold is produced and the effectiveness of cold utilization. The first aspect is examined using exergy functions which allow the real losses of a plant to be quantified in the light of the first and second laws of thermodynamics. The exergetic efficiency is very low, particularly at partial operating loads. The second aspect is related to the difference between the total refrigerating effect available and the proportion usefully employed. Exergy losses in fans and pumps for circulating fluids, poor insulation of walls and pipes, bad defrosting methods and door opening, etc. are responsible for this difference, which constitutes the wasted refrigerating effect. The situation is critical, especially in small plants, such as domestic refrigerators, and in refrigerated transport. In the case of sea transport one must take into account that containerization is rapidly replacing conventional ships and that there is an increase in vessels which freeze and store fish at sea. These two aspects are discussed in the Paper from the viewpoint of energy economy. Heat recovery in refrigerated sea transport is also considered.     

Steel mill slags energy potential: the case of the steel factory of Arcelor-Mittal in Asturias (Spain)

Slag accounts for most of the residuals or by-products of the steel manufacturing process and represents a not inconsiderable amount of energy waste and CO₂ emissions. Energy recovery from steel mill slags is not actually performed because of the difficulty of the industrial implementation, but the actual demand and the incentives for new electricity generation plants based on renewable energies and on industrial waste heat recovery offer a new opportunity to evaluate the feasibility of this process. This article presents a review of the slag energy potential on a global scale, and a proposal for a recovery plant in the factories of Arcelor-Mittal in Asturias (Spain), based on a steam Rankine cycle for electricity production in a turbine. The plant production and viability have been analyzed using the typical technical and economic values for this kind of plant. Also, a parametric study has been performed on the heat recuperator efficiency and investment rate.     

First and second law investigation of waste heat based combined power and ejector-absorption refrigeration cycle

In the proposed cogeneration cycle, a LiBr-H₂O absorption refrigeration system is employed to the combined power and ejector refrigeration system which uses R141b as a working fluid. Estimates for irreversibilities of individual components of the cycle lead to possible measures for performance improvement. Results of exergy distribution of waste heat in the cycle show that around 53.6% of the total input exergy is destroyed due to irreversibilities in the components, 22.7% is available as a useful exergy output, and 23.7% is exhaust exergy lost to the environment, whereas energy distribution shows 44% is exhaust energy and 19.7% is useful energy output. Results also show that proposed cogeneration cycle yields much better thermal and exergy efficiencies than the previously investigated combined power and ejector cooling cycle. Current investigation clearly show that the second law analysis is quantitatively visualizes losses within a cycle and gives clear trends for optimization.     

Investigation on gradient thermal cycle for power and refrigeration cogeneration

In order to improve energy utilization efficiency of low grade heat, a novel gradient thermal cycle for power and refrigeration cogeneration is proposed. The cycle is cascaded with two stages based on different thermal driven temperature. The first stage is pumpless Organic Rankine Cycle (PRC) while the second stage is two-stage sorption refrigerator. R245fa is selected as the working fluid of PRC, whereas CaCl₂-BaCl₂-NH₃ working pair is chosen for two-stage sorption refrigerator. Different heat source temperatures from 80°C to 95°C are adopted for analysis and comparison. Results indicate that the highest average power output and cooling effect are able to reach 204 W and 0.91 kW under the condition of 95°C heat source temperature and 10°C refrigeration temperature. For different heat source temperatures, total energy and exergy efficiency of the gradient thermal cycle for power and refrigeration cogeneration range from 9.49% to 9.9% and 10.9% to 11.8%, respectively. For gradient thermal cycle exergy efficiency of heat utilization ranges from 24% to 18.8% which is 126.5% and 70.9% higher than the PRC and two-stage sorption refrigerator, respectively, when the heat source temperature is 80°C.     

回收PAFC废热与LNG冷能的动力循环分析

在液化天然气(LNG)作为磷酸型燃料电池(PAFC)燃料的电站中,大量的LNG冷能和PAFC废热被释放.提出一种利用LNG和环境作为冷源的双冷源动力循环和LNG直接膨胀做功相结合的回收PAFC废热和LNG冷能的能量系统.对循环的分析表明,当工质为水时,该循环对PAFC废热和LNG冷能的可用能的回收效率最大可达47%左右.     

能源总线系统多热源混水和热回收过程变分析

在能源互联网时代,区域供冷供热系统将由原本单一形式的热源向多种形式热源并存转变,尤其是可再生能源和未利用能源。不同形式不同品位的热源集成必将引起系统能量变化。能源总线系统是集成化规模化应用区域内可再生能源及未利用能源的多源多用户能源系统。本文针对能源总线系统相对常规分散系统而言特有的多源多用户特征进行系统混水和热回收过程的变分析,将能源总线系统抽象为一系列工作在高温热源和低温热源之间的劳伦兹循环的集成,通过建立能源总线系统与常规分散系统的理想热力学模型,找到能源总线系统混水和热回收过程变的规律及影响因素。结果表明:系统的变化与各子系统低温热源进出口温度、高温热源进口温度以及高低温热源质量流量比相关,不同的设计参数会导致混水过程能量发生增加或者减小,亦或不变。通过分析得到热回收过程影响源侧总线热量变化的相关参数并找到热量变化规律,并得到最佳总线供水温度TEBS1的确定方法。     

Exergy analysis of R1234yf and R1234ze as R134a replacements in a two evaporator vapour compression refrigeration system

Exergy analysis is a useful way for determining the real thermodynamic losses and optimising environmental and economic performance in the systems such as vapour compression refrigeration systems. The present study deals with the exergy analysis on a two evaporator vapour compression refrigeration system using R1234yf, R1234ze and R134a as refrigerants. In the calculation of losses occurring in different system components, besides the exergy efficiency of the refrigeration cycle, a computer code was developed by using Engineering Equation Solver (EES-V9.172-3D) software package program. The effects of the evaporator and condenser temperatures on the exergy destruction and exergy efficiency of the system were investigated. R1234yf and R1234ze, which are good alternatives to R134a concerning their environmentally friendly properties and this is the most significant finding emerging from this study.     

Thermodynamic performance assessment of a novel air cooling cycle: Maisotsenko cycle

This study presents energy and exergy analyses and sustainability assessment of the novel evaporative air cooling system based on Maisotsenko cycle which allows the product fluid to be cooled in to a dew point temperature of the incoming air. In the energy analysis, Maisotsenko cycle’s wet-bulb and dew point effectiveness, COP and primary energy ratio rates are calculated. Exergy analysis of the system is then carried out for six reference temperatures ranging from 0 °C to 23.88 °C as the incoming air (surrounding) temperature. The specific flow exergy, exergy input, exergy output, exergy destruction, exergy loss, exergy efficiency, exergetic COP, primary exergy ratio and entropy generation rates are determined for various cases. Furthermore, sustainability assessment is obtained using sustainability index method. As a result, maximum exergy efficiency is found to be 19.14% for a reference temperature of 23.88 °C where the optimum operation takes place.     

多联机空调系统的分析

应用？分析方法,从能量的量和质两个方面对多联机空调系统进行了节能分析。首先建立了多联机空调系统的？分析模型,进而利用该模型模拟出了各部分的？损失,最后,提出了系统的节能方法和应采取的措施,以期实现整个系统的？损失最小,？效率最高,节能效果最佳,这为多联机空调系统的设计提供了有益的参考。