Experimental study on micro modular combustor for micro-thermophotovoltaic system application

As one of the key components of micro modular thermophotovoltaic power generators, every micro combustor should be able to produce a high and uniform temperature distribution on the surface. In this work, three micro modular combustors with different fuel supply systems were designed and tested. The results indicated that the in-line design with only one fuel supply tube could not equally distribute H₂/air mixture to every combustor. The wall temperatures of the two central combustors were obviously higher than the two side combustors. However, both the in-line design with two fuel supply tubes and the parallel design could equally deliver the fuel/air mixture to every combustor, and an uniform temperature distribution could be obtained for every combustor. The total radiation energy and radiation efficiency of the micro modular combustors were also calculated for various flow speeds and H₂/air equivalence ratios. A radiation efficiency of 27.3% could be achieved when the H₂/air equivalence ratio was 0.8 and the flow speed was 6 m/s.     

Comparative investigation of combustion and thermal characteristics of a conventional micro combustor and micro combustor with internal straight/spiral fins for thermophotovoltaic system

The energy output and energy conversion efficiency of MTPV system are relatively low due to the energy loss. In order to improve the energy output of micro-thermophotovoltaic (MTPV) system, the internal straight and spiral fins are introduced into the micro combustor. The impact of hydrogen mass flow rate, equivalence ratio, and materials on the thermal performance are investigated. The increase of hydrogen mass flow rate brings higher average outer wall temperature, but the temperature difference also increases and the temperature uniformity becomes worse. The equivalence ratio of 1 is suggested to obtain higher average outer wall temperature and better temperature uniformity. The materials with higher thermal conductivity can obtain better thermal performance. Meanwhile, the higher thermal conductivity can also reduce the impact of introduction of internal fins.     

Exergy analysis and CO2 emission evaluation for steam methane reforming

This paper investigates the industrial production of hydrogen through steam methane reforming (SMR) from both exergy efficiency and CO₂ emission aspects. An SMR model is constructed based on a practical flow diagram including desulfurizer, furnace, separation unit and heat exchangers. The influence of reformer temperature (Tr) and steam to carbon (S/C) ratio is analyzed to optimize exergy efficiency and CO₂ emission. A clear correlation is obtained between exergy efficiency and CO₂ emission. Results also show optimal S/C ratio decreases with Tr. An exergy load distribution analysis which evaluates interactions between the system and its subsystems with parameter variations is employed to find promising directions for efficiency improvement. Results show that the greatest improvement lies in increasing efficiency of furnace without increasing its relative exergy load. Integration of oxygen-enriched combustion (OEC) with SMR is also evaluated. The integration of OEC can increase the system efficiency greatly when the reformer operates above critical point, while in other cases the system efficiency may decrease.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

Exergy analysis on the simulation of a small-scale hydrogen liquefaction test rig with a multi-component refrigerant refrigeration system

This study investigates the simulation of a proposed small-scale laboratory liquid hydrogen plant with a new, innovative multi-component refrigerant (MR) refrigeration system. The simulated test rig was capable of liquefying a feed of 2 kg/h of normal hydrogen gas at 21 bar and 25 °C to normal liquid hydrogen at 2 bar and −250 °C. The simulated power consumption for pre-cooling the hydrogen from 25 °C to −198 °C with this new MR cycle was 2.07 kWh/kg_GH2 from the ideal minimum of 0.7755 kWh per kilogram of feed hydrogen gas. This was the lowest power consumption available when compared to today’s conventional hydrogen liquefaction cycles, which are approximately 4.00 kWh/kg_GH2. Hence, the MR cycle’s exergy efficiency was 38.3%. Exergy analysis of the test rig’s cycle, which is required to find the losses and optimize the proposed MR system, was evaluated for each component using the simulation data. It was found that the majority of the losses were from the compressors, heat exchangers, and expansion valves. Suggestions are provided for how to reduce exergy in each component in order to reduce the exergy loss. Finally, further improvements for better efficiency of the test rig are explained to assist in the design of a future large-scale hydrogen liquefaction plant.     

船舶高低温冷库的(火用)分析应用与实验研究

船舶多温单机 (简称为高、低温冷库 )系统中 ,常用背压阀 (蒸发压力调节阀 )来调节各高温库的蒸发温度 ;但忽略了由此使系统的能量发生了无形的损耗 ,效率也有所下降。鉴于目前这种状况 ,提出了以喷射器为基础的船舶高、低温冷库制冷新循环 ,可使系统的火用效率提高 ,达到了节能的目的。     

Exergy analysis and optimization of a biomass gasification, solid oxide fuel cell and micro gas turbine hybrid system

A hybrid plant producing combined heat and power (CHP) from biomass by use of a two-stage gasification concept, solid oxide fuel cells (SOFC) and a micro gas turbine was considered for optimization. The hybrid plant represents a sustainable and efficient alternative to conventional decentralized CHP plants. A clean product gas was produced by the demonstrated two-stage gasifier, thus only simple gas conditioning was necessary prior to the SOFC stack. The plant was investigated by thermodynamic modeling combining zero-dimensional component models into complete system-level models. Energy and exergy analyses were applied. Focus in this optimization study was heat management, and the optimization efforts resulted in a substantial gain of approximately 6% in the electrical efficiency of the plant. The optimized hybrid plant produced approximately 290 kW_e at an electrical efficiency of 58.2% based on lower heating value (LHV).     

Exergy analysis of an integrated solid oxide fuel cell and organic Rankine cycle for cooling, heating and power production

The study examines a novel system that combined a solid oxide fuel cell (SOFC) and an organic Rankine cycle (ORC) for cooling, heating and power production (trigeneration) through exergy analysis. The system consists of an SOFC, an ORC, a heat exchanger and a single-effect absorption chiller. The system is modeled to produce a net electricity of around 500 kW. The study reveals that there is 3-25% gain on exergy efficiency when trigeneration is used compared with the power cycle only. Also, the study shows that as the current density of the SOFC increases, the exergy efficiencies of power cycle, cooling cogeneration, heating cogeneration and trigeneration decreases. In addition, it was shown that the effect of changing the turbine inlet pressure and ORC pump inlet temperature are insignificant on the exergy efficiencies of the power cycle, cooling cogeneration, heating cogeneration and trigeneration. Also, the study reveals that the significant sources of exergy destruction are the ORC evaporator, air heat exchanger at the SOFC inlet and heating process heat exchanger.     

Exergy analysis of Joule–Thomson cryogenic refrigeration cycle with an ejector

In this paper, exergy method is applied to analyze the ejector expansion Joule–Thomson (EJT) cryogenic refrigeration cycle. The exergy destruction rate in each component of the EJT cycle is evaluated in detail. The effect of some main parameters on the exergy destruction and exergetic efficiency of the cycle is also investigated. The most significant exergy destruction rates in the cycle are in the compressor and ejector. The ejector pressure ratio and compressor isothermal efficiency have a significant effect on the exergetic efficiency of the EJT cycle. The exergy analysis results show the EJT cycle has an obvious increase in the exergetic efficiency compared to the basic Joule–Thomson refrigeration cycle. A significant advantage from the use of the ejector is that the total exergy destruction of the EJT cycle can be reduced due to much more decreasing of the exergy destruction rates in the compressor and expansion valve. The exergy analysis also reconfirms that applying an ejector is a very important approach to improve the performance of the Joule–Thomson cryogenic refrigeration cycle.     

Exergy analysis on the irreversibility of rotary air preheater in thermal power plant

Energy recovery devices can have a substantial impact on process efficiency and their relevance to the problem of conservation of energy resources is generally recognized to be beyond dispute. One type of such a device, which is commonly used in thermal power plants and air conditioning systems, is the rotary air preheater. A major disadvantage of the rotary air preheater is that there is an unavoidable leakage due to carry over and pressure difference. There are gas streams involved in the heat transfer and mixing processes. There are also irreversibilities, or exergy destruction, due to mixing, pressure losses and temperature gradients. Therefore, the purpose of this research paper is based from the second law of thermodynamics, which is to build up the relationship between the efficiency of the thermal power plant and the total process of irreversibility in the rotary air preheater using exergy analysis. For this, the effects of the variation of the principal design parameters on the rotary air preheater efficiency, the exergy efficiency, and the efficiency of the thermal power plant are examined by changing a number of parameters of rotary air preheater. Furthermore, some conclusions are reached and recommendations are made so as to give insight on designing some optimal parameters.     

Research of boiler combustion regulation for reducing NOx emission and its effect on boiler efficiency

The effect of boiler combustion regulation on NO_x emission of two 1025t/h boilers has been studied.The re-searches show that NOx emission is influenced by coal species,operation conditions,etc,and can be reduced byregulating the combustion conditions.The effect of combustion regulation on boiler efficiency has also beenchecked.     

Exergy analysis of renewable light olefin production system via biomass gasification and methanol synthesis

The conceptual light olefin production system from biomass via gasification and methanol synthesis was simulated and its thermodynamic performance was evaluated through exergy analysis. The system was made up of gasification, gas composition adjustment, methanol synthesis, light olefin synthesis, steam & power generation and cooling water treatment. The in-depth exergy analysis was performed at the levels of system, subsystem and operation component respectively. The gasifier and the tail gas combustor were the main sources of irreversibility with exergy destruction ratios of 17.0% and 16.8% of the input exergy of biomass. The steam & power generation subsystem accounted for 43.4% of the overall exergy destruction, followed by 41.0% and 5.69% in the subsystems of gasification and gas composition adjustment respectively. The sensitivity evaluation of the operation parameters of gasifier indicates that the system efficiency could be improved by enhancing syngas yield and subsequent yield of light olefins. The overall exergetic efficiency of 30.5% is obtained at the mass ratios of steam to biomass and O₂-rich gas (95 vol%) to biomass (S/B and O/B) of 0.26 and 0.14 and gasification temperature at 725 °C.     

Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics

A mathematical model for the overall exergetic efficiency of two phase change materials named PCM1 and PCM2 storage system with a concentrating collector for solar thermal power based on finite-time thermodynamics is developed. The model takes into consideration the effects of melting temperatures and number of heat transfer unit of PCM1 and PCM2 on the overall exergetic efficiency. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the air which assumes that the temperature of the air varies in its flow path. The results show that the overall exergetic efficiency can be improved by 19.0-53.8% using two PCMs compared with a single PCM. It is found that melting temperatures of PCM1 and PCM2 have different influences on the overall exergetic efficiency, and the overall exergetic efficiency decreases with increasing the melting temperature of PCM1, increases with increasing the melting temperature of PCM2. It is also found that for PCM1, increasing its number of heat transfer unit can increase the overall exergetic efficiency, however, for PCM2, only when the melting temperature of PCM1 is less than 1150 K and the melting temperature of PCM2 is more than 750 K, increasing the number of heat transfer unit of PCM2 can increase the overall exergetic efficiency. Considering actual application of solar thermal power, we suggest that the optimum melting temperature range of PCM1 is 1000-1150 K and that of PCM2 is 750-900 K. The present analysis provides theoretical guidance for applications of two PCMs storage system for solar thermal power.     

Evaluation of a hybrid photovoltaic-wind system with hydrogen storage performance using exergy analysis

This article presents and discusses the results of an exergy analysis conducted during the operation of a test-bed hybrid wind/solar generator with hydrogen support, designed and constructed at the Industrial Engineering School of the University of Extremadura, Badajoz (Spain). An exergy analysis is made of the different components of the system, calculating their exergy efficiencies and exergy losses, and proposing future improvements to increase the efficiency of the use of the surplus energy produced by the wind/solar generator. The results show the electrolyzer to have an acceptable efficiency (η_ex = 68.75%), but the photovoltaic modules a low exergy efficiency (η_ex = 8.39%) as also is the case, though to a lesser extent, for the fuel cell (η_ex = 35.9%).     

Exergy analysis of the demonstration plant for co-production of hydrogen and benzene from biogas

The dehydro-aromatization reaction of methane is one of the methods to utilize biogas and co-produces hydrogen and benzene. To demonstrate the industrial co-production of hydrogen and benzene from biogas, we constructed a demonstration plant. The purpose of this study is to evaluate the feasibility of the demonstration plant, which can co-produce 134 N m³/day of hydrogen and 8.6 L/day of benzene from 200 N m³/day of biogas, by means of clearing energy requirement, exergy loss and CO₂ emission. Energy requirement was 3783 MJ/day. Only 21% of the input exergy was converted to hydrogen and benzene, and 79% was consumed. The sub-system of the dehydro-aromatization reformer showed the largest exergy loss among the eight sub-systems. Assuming electricity was supplied by thermal power generation, CO₂ emission was 634 kg/day. On the other hand, assuming electricity was supplied by biogas engine, fuel biogas requirement was 555 N m³/day. However, CO₂ emission was not counted in this case because of the carbon-neutral principle.     

Exergy analysis of the woody biomass Stirling engine and PEM-FC combined system with exhaust heat reforming

The woody biomass Stirling engine (WB-SEG) is an external combustion engine that outputs high-temperature exhaust gases. It is necessary to improve the exergy efficiency of WB-SEG from the viewpoint of energy cascade utilization. So, a combined system that uses the exhaust heat of WB-SEG for the steam reforming of city gas and that supplies the produced reformed gas to a proton exchange membrane fuel cell (PEM-FC) is proposed. The energy flow and the exergy flow were analyzed for each WB-SEG, PEM-FC, and WB-SEG/PEM-FC combined system. Exhaust heat recovery to preheat fuel and combustion air was investigated in each system. As a result, (a) improvement of the heat exchange performance of the woody biomass combustion gas and engine is observed, (b) reduction in difference in the reaction temperature of each unit, and (c) removal of rapid temperature change of reformed gas are required in order to reduce exergy loss of the system. The exergy efficiency of the WB-SEG/PEM-FC combined system is superior to EM-FC.     

Exergy cost analysis of a micro-trigeneration system based on the structural theory of thermoeconomics

In this paper, an exergy cost analysis method based on the structural theory of thermoeconomics is applied to a gas-fired micro-trigeneration system, which uses a small-scale generator set driven by a gas engine and a new small-scale adsorption chiller (ADC). The thermoeconomic model for the system based on the fuel–product concept is defined to quantify the productive interaction between various components. The distribution of the resources and costs of all flows in the productive structure are calculated by solving a set of equations according to the experimental data. By adopting the exergy cost analysis method, the production performance of components at design and variable conditions of combined cooling and power are evaluated in detail. Moreover, a comparison between the method of conventional exergy analysis and exergy cost analysis is presented. The results not only reflect that the structural theory is a powerful and effective tool for performance evaluation of complex system, but also prove that the micro-trigeneration system is efficient in utilizing the low-grade waste heat.     

Exergy, economic & environmental (3E) analysis of inlet fogging for gas turbine power plant

In the present paper, the effects of inlet fogging system on the first and second law efficiencies are investigated for a typical power plant (Shahid Rajaee) which is located near Ghazvin in Iran. Also a new function is proposed for system optimization that includes the social cost of air pollution for power generating systems. The new function is based on the first law efficiency, energy cost and the external social cost of air pollution for an operational system. Social cost of air pollution is based on the negative effects of air pollution on the health of society and environment. The economic aspect of these effects is called external social cost of air pollution. Other pollution sources such as water, soil, etc. produced by an operational power generation system are ignored. The theoretical results obtained from the model are validated by registered practical performance results from Shahid Rajaee power plant. It is concluded that using of inlet fogging system, increases the average output power production, the first and the second law efficiencies through three months of year (June, July and August) by 7%, 5.5% and 6% respectively and reduces the objective function value by about 4%.     

Performance analysis of a novel heat recovery system with hydrogen production designed for the improvement of boiler effectiveness

In this study, a multi-generation system is designed for the waste recovery of a 150 MW coal-fired power plant. The waste heat from the boiler system of the power plant is recycled in the power block with supercritical organic Rankine Cycle to obtain the required energy for the proton exchange membrane electrolyzer block. Then, two different cases are handled to utilize the products of the proton exchange membrane electrolyzer block. In the first case, H₂ is stored as an energy carrier to be used for external operations where O₂ was used for the enrichment of the combustion air. In the second case, H₂ is used for the enrichment of the fuel where O₂ is used for the enrichment of the combustion air as in the first case. It is determined that it is available to produce H₂ in an amount of 0.0417–0.0433 kmol/s. The energy efficiency of the overall system is determined as 25.37% and 24.05% where the exergy efficiency of the overall system is determined as 31.56% and 29.80% for the first and second cases, respectively.