Heat Exchangers as Equipment and Integrated Items in Waste and Biomass Processing

Heat recovery systems play an important role in waste to energy and biomass processing. An efficient approach that follows a recommended hierarchy in design, process as a whole (e.g., incineration) → subsystem of the process (e.g., heat recovery system) → equipment (e.g., air pre-heater), is shown. Important factors have to be taken into consideration in processes for incineration (combustion of biomass), especially available energy, specific features of hot process fluid (flue gas), type of waste/biomass, fouling, and environmental impact. A combination of intuitive design, know-how, and a sophisticated approach based on up-to-date computational tools is shown. Some novel types of heat exchangers (e.g., air preheaters for high- and low-temperature applications, heat recovery steam generators and/or heaters, and those for specific applications) that can be substituted for conventional ones are presented. An improved or even optimum design of heat exchangers requires computational support in the following areas: a simulation based on energy and mass balance, the thermal and hydraulic calculation of heat exchangers, a CFD (computational fluid dynamics) approach, optimization, and heat integration. Some examples are presented. An approach that ranges from an idea to an industrial application is demonstrated on the novel design of integrated compact equipment (combustion chamber installed inside heat exchanger) for the thermal treatment of waste gases, including heat recovery. This approach involves simulation for obtaining basic process parameters, thermal and hydraulic calculations, design of experimental facility, the manufacture of the equipment and building of this facility, operation and functionality testing, data acquisition for validating and improving the CFD model, and the utilization of feedback from industrial applications.     

Different Strategies to Improve Industrial Heat Exchange

A variety of different strategies are available to process and equipment designers to improve industrial heat transfer. These range from the use of efficient forms of individual heat exchangers to the optimum utilization of the individual units in a network, generally referred to as "process integration." This article attempts to review these strategies with reference to the conventional and more recent forms of shell-and-tube heat exchangers. In the context of a heat exchanger network, process heat transfer intensification ( global intensification ), i.e., network design for maximum energy recovery, is a first important step. This needs to be then combined with heat transfer intensification in individual units/shells ( local intensification ). The benefits of global and local intensification are illustrated with examples of a helically baffled heat exchanger ( Helixchanger ), as a representative of a more recent form of shell-and-tube exchanger. Some aspects concerning the use of multistream heat exchangers are discussed, and finally, an example for optimization of a plate-type heat exchanger is presented.     

Process integration in bioprocess indystry: waste heat recovery in yeast and ethyl alcohol plant

The process integration of the bioprocess plant for production of yeast and alcohol was studied. Preliminary energy audit of the plant identified the huge amount of thermal losses, caused by waste heat in exhausted process streams, and reviled the great potential for energy efficiency improvement by heat recovery system. Research roadmap, based on process integration approach, is divided on six phases, and the primary tool used for the design of heat recovery network was Pinch Analysis. Performance of preliminary design are obtained by targeting procedure, for three process stream sets, and evaluated by the economic criteria. The results of process integration study are presented in the form of heat exchanger networks which fulfilled the utilization of waste heat and enable considerable savings of energy in short payback period.     

Energy recovery in petrochemical complexes through heat integration retrofit analysis

This paper proposes the principles of how to define a boundary for heat integration in petrochemical complexes which are composed of several interconnected processing units. In order to obtain retrofit schemes that offer significant energy saving potential and are easy to implement, heat integration strategies are also developed in this study. Two case studies based on an aniline plant and an aromatic hydrocarbon plant, each one comprising several processing units, are presented to illustrate the application of these principles and strategies. The boundary for heat integration in each plant can be the whole plant or its individual processing units, the choice of which is determined by their energy saving potentials. Based on energy saving potential, each processing unit in the aniline plant was selected as the boundary for heat integration. The boundary for heat integration in the aromatic hydrocarbon plant, by contrast, was the whole plant. Retrofit schemes for the heat exchanger networks of the two plants, developed using pinch analysis, revealed that significant heating utility savings could be realized with a small number of network structure modifications.     

District heating in case of power failure

Power failures in combination with harsh weather conditions during recent years have led to an increased focus on a safe energy supply to our society. Many vital functions are dependent on electricity; e.g., lighting, telephony, medical equipment, lifts, alarm systems, payment, pumps for town’s water and, perhaps the most critical of all, heating systems. In Sweden, district heating (DH) is the most common type of heating for buildings in town centres. Therefore, it is of great interest to investigate what happens in DH systems during a power failure. The present study shows that, by maintaining the DH production as well as the operation of the DH network, possibilities to supply connected buildings with space heat are surprisingly good. This is due to the fact that natural circulation will most often take place in radiator systems. In Sweden, and in many other countries, so-called indirect connection (heat supply across heat exchangers) of DH substations is applied. If a DH network operation can be maintained during a power failure, DH water will continue to pass the radiator system’s heat exchanger (HEX), provided that the control valve does not close. The radiator circulation pump will stop, causing the radiator water to attain a relatively high temperature in the HEX, which promotes a natural circulation in the hydronic heating system, due to an increased water density differential at different temperatures. Several field tests and computer simulations have been performed and have displayed that almost all buildings can achieve a space heat supply corresponding to 40–80% of the amount prior to the interruption. A sufficient heat load in the DH network can be vital in certain cases: e.g., for ‘island-operation’ of an electric power plant to be performed during a power failure. Furthermore, for many combined heat and power stations, a requirement involves that the DH network continues to provide a heat sink when no other cooling is available. Based on the findings presented herein, a set of recommendations have been set up to provide advice to, among others, DH utilities and owners of customer buildings.     

Heat transfer as an important subject in waste-to-energy systems

Characteristics of heat transfer equipment and/or heat exchangers used in waste to energy systems and their specific features are described and discussed in the paper. A combination of intuitive design, know how and sophisticated approach based on up-to-date computational tools is shown. Concrete examples involve e.g. heat exchangers of heat recovery systems (especially air pre-heaters and heat recovery steam generators) of units for the thermal processing of wastes, their design, arrangement, optimization etc. An application of CFD (computational fluid dynamics) both for improved design and troubleshooting (e.g. elimination of fouling) is demonstrated. There is also shown an approach ‘from idea to industrial applications’ on an example of a new unit for the thermal processing of gas waste. Design of this original compact equipment consists in a convenient integration of combustion chamber and heat exchanger. Experimental as well as CFD approaches largely contributed to an optimum design and operation.     

Two- and four-way coupling of cohesive poly-disperse particulate foulants on a metal foam fibre immersed in quiescent fluid

The ubiquity and complexity of the unsteadiness of fouling and multiphase flows in various engineering systems signify the need to develop advanced numerical methods to study the underlying phenomena of two-phase particle-laden fluid flows in heat exchanger systems such as, compact electronics cooling (i.e. heat sinks) and HVAC&R systems. Fouling is omnipresent in many industries such as power generation, chemical, petroleum, among others. The mechanisms governing fouling coupled with multiphase foulant-laden fluid flow in porous heat exchangers, such as metal foams, are very complex and poorly understood. This investigation forms the basis for addressing the implications of fouling for a myriad of industrial processes. This study will discuss the development of a coupled finite volume method and discrete element method (FVM-DEM) numerical framework to investigate the mechanisms governing particulate fouling in an idealized metal foam heat exchanger. This study resolves four-way and two-way coupled interactions based on poly-disperse cohesive foulants in fluid-saturated foam. The significance stems from the inclusion of cohesiveness between particle-particle and particle-wall contacts which play a decisive role in the foulant aggregation process prevalent in particles with a diameter smaller than 50 μm. The present results show that the cohesive foulants exhibit strong tendency to aggregate with time and form chain-like projections. A rigid aggregate stack is formed which alters the fluid velocity of the fluid-filled foam. Quantitative analysis of the foulant count and time-averaged aggregate count is discussed. The presented results and the numerical framework could potentially be used to optimize heat exchanger designs by considering operating conditions and foam morphology (i.e. pore diameter, ligament thickness, porosity) that is most susceptible to particulate fouling.     

Modelling and analyses of heat exchangers in a biomass boiler plant

A boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set‐values of the control system. Results of verification measurements and practical operation at a 40 kW_th pilot plant and a 500 kW_th demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved. Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed‐flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel‐flow type was found the most appropriate as the drying exhaust gas heat exchanger. Copyright © 2004 John Wiley & Sons, Ltd.     

Performances of a heat exchanger and pilot boiler for the development of a condensing gas boiler

In this research, design factors for a heat exchanger and boiler were investigated using a simplified model of a heat exchanger and pilot condensing boiler, respectively. Specifications of each heat exchanger component (e.g., upper heat exchanger (UHE) and lower heat exchanger (LHE); coil heat exchanger (CHE); baffles) were investigated using a model apparatus, and the comprehensive performance of the pilot gas boiler was examined experimentally. The heating efficiency of the boiler developed was about 90% when using the optimal designed heat exchangers. Compared to a conventional Bunsen-type boiler, the heating efficiency was improved about 10%. Additionally, NO_x and CO emissions were about 30 ppm and 160 ppm, respectively, based on a 0% O₂ basis at an equivalence ratio of 0.70, which is an appropriate operating condition. However, the pollutant emission of the boiler developed is satisfactory considering the emission performance of a condensing boiler, even though CO emission must be reduced.     

Modeling, simulation and optimization of a solar collector driven water heating and absorption cooling plant

A cogeneration system consisting of a solar collector, a gas burner, a thermal storage reservoir, a hot water heat exchanger, and an absorption refrigerator is devised to simultaneously produce heating (hot water heat exchanger) and cooling (absorption refrigerator system). A simplified mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer, is developed. The proposed model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. A system global optimization for maximum performance (or minimum exergy destruction) in the search for minimum pull-down and pull-up times, and maximum system second law efficiency is performed with low computational time. Appropriate dimensionless groups are identified and the results presented in normalized charts for general application. The numerical results show that the three way maximized system second law efficiency, η_{II,max,max,max}, occurs when three system characteristic mass flow rates are optimally selected in general terms as dimensionless heat capacity rates, i.e., (ψ_sp,s,ψ_wx,wx,ψ_H,s)_opt≅(1.43,0.23,0.14). The minimum pull-down and pull-up times, and maximum second law efficiencies found with respect to the optimized operating parameters are sharp and, therefore important to be considered in actual design. As a result, the model is expected to be a useful tool for simulation, design, and optimization of solar collector based energy systems.     

HYSYS流程模拟技术在丁辛醇装置用能优化方面的应用

分析了大庆石化公司丁辛醇装置工艺流程中用能优化的潜力,即可增没换热器,实现温度较低的精EPA与温度较高的粗EPA的换热,充分利用内部循环热能,同时可节约冷却水和加热蒸汽用餐。介绍了利用HYSYS流程模拟技术．通过采用定义虚拟组分来计算增设换热器换热面积的计算方法。根据模拟计算结果,采用换热面积为35m^2的新增换热器,每年节约1．3MPa蒸汽1×10^4t、冷却水28×10^4t。     

The effect of La oxide additive on the solubility of NiO in molten carbonates

A comprehensive analysis on a novel energy recovery system for reformate-based proton exchange membrane (PEM) fuel cell systems is presented. The energy recovery system includes a throttling valve, a heat exchanger, a compressor, and is coupled with a coolant loop for the fuel cell stack. The feed stock of the fuel reformer, which is primarily a mixture of water and fuel, is vaporized in the heat exchanger and is then compressed to a sufficiently high pressure before it is ducted into the fuel reformer. The analysis includes the throttling of two-phase fuel/water mixture and vaporization in the heat exchanger to obtain the temperature and pressure of the mixture at the inlet of the compressor. The results indicate that the power plant efficiency with the energy recovery system can be increased by more than 20% compared to that of a fuel cell power plant without the energy recovery system. Additionally, more than 25% of the waste heat generated by the fuel cell stack can be removed due to the energy recovery system, and the fuel burned for the fuel reforming purpose is reduced by more than 70%.     

Recent research development of process integration in analysis and optimisation of energy systems

The design of energy systems in a process plant requires a good understanding of each subsystem (e.g. processes, heat exchanger networks, utility systems) and their interactions in the context of an overall plant. An effective design method should be able to explore the synergy between the subsystems to the maximum extent and allow users to interact with the design process. To achieve this, the effective way is to combine physical insights with mathematical optimisation techniques. Physical insights are used as a wise man’s brain and eyes, while optimisation techniques are employed as a superman’s power in searching for optimal solutions. In the past, concepts and methods have been developed for handling grassroots design, operational management, retrofit and debottlenecking scenarios. This paper describes the recent research progress at UMIST in developing fundamental concepts and methodologies for analysis and optimisation of energy systems.     

Retrofitting Crude Oil Refinery Heat Exchanger Networks to Minimize Fouling While Maximizing Heat Recovery

Abstract

The use of fouling factors in heat exchanger design and the lack of appreciation of fouling in traditional pinch approaches have often resulted in crude preheat networks that are subject to extensive fouling. The development of thermal and pressure drop models for crude oil fouling has allowed its effects to be quantified so that techno-economic analyses can be performed and design options compared. The application of these fouling models is described here on two levels: the assessment of increasing heat recovery in stream matches (e.g., by adding extra area to exchangers) and the design of a complete network using the Modified Temperature Field Plot. Application to a refinery case study showed that, at both the exchanger and network levels, designing for maximum heat recovery (e.g., using traditional pinch approaches) results in a less efficient system over time due to fouling effects.     

Application of ozone treatment and pinch technology in cooling water systems design for water and energy conservation

Re‐circulating cooling water systems offer the means to remove heat from a wide variety of industrial processes that generate excess heat. Such systems consist of a cooling tower and a heat‐exchanger network that conventionally has a parallel configuration. However, reuse of water between different cooling duties allows cooling water networks to be designed in a series arrangement. This results in performance improvement and increased cooling tower capacity. In addition, by the integration of ozone treatment into the cooling tower, the cycle of concentration can be increased. The ozone treatment also dramatically reduces the blow‐down that, in turn, is environmentally constructive. In this study, a new environmental‐friendly and cost‐effective design methodology for cooling water systems was introduced. Using this design methodology, Integrated Ozone Treatment Cooling System (IOTCS), achievement of minimum environmental impacts and total cost were afforded through a simultaneous integration of the cooling system components using an ozone treatment cooling tower and optimum heat‐exchanger network configuration. Moreover, in the proposed method, the cooling tower optimum design was achieved through a mathematical model. The IOTCS design method is based upon a complex design approach using a combined pinch analysis and mathematical programming that provides an optimum heat‐exchanger configuration while maximizing water and energy conservation and minimizing total cost. Related coding in MATLAB version 7.3 was used for the illustrative example to obtain optimal values in the IOTCS design method computations. The results of the recently introduced design methodology were compared with the conventional method. Copyright © 2009 John Wiley & Sons, Ltd.     

Premixed hydrogen-air combustion system for fuel cell systems

The advance of efficient hydrogen-air combustion systems has increasingly become of interest in the framework of the development of fuel cell systems, especially for the automotive sector. Therefore, compact modulating systems are required, with the additional demand of low emissions, to be integrated in a fuel cell system. A modulating combustion system based on combustion within inert porous media and an integrated heat exchanger has been developed and investigated. The system is able to handle premixed combustion of lean H₂/air mixtures at a surface load range of 1075 kW/m²-2150 kW/m², and a global equivalence ratio of ?=0.5. The special hydrogen-air mixing concept eliminates the risk of flame flashback and enables operation with very low NO_x emissions.     

Optimization of heat exchanger designs in metal hydride based hydrogen storage systems

Design of the heat exchanger in a metal hydride based hydrogen storage system influences the storage capacity, gravimetric hydrogen storage density, and refueling time for automotive on-board hydrogen storage systems. The choice of a storage bed design incorporating the heat exchanger and the corresponding geometrical design parameters is not obvious. A systematic study is presented to optimize the heat exchanger design using computational fluid dynamics (CFD) modeling. Three different shell and tube heat exchanger designs are chosen. In the first design, metal hydride is present in the shell and heat transfer fluid flows through straight parallel cooling tubes placed inside the bed. The cooling tubes are interconnected by conducting fins. In the second design, heat transfer fluid flows through helical tubes in the bed. The helical tube design permits use of a specific maximum distance between the metal hydride and the coolant for removing heat during refueling. In the third design, the metal hydride is present in the tubes and the fluid flows through the shell. An automated tool is generated using COMSOL-MATLAB integration to arrive at the optimal geometric parameters for each design type. Using sodium alanate as the reference storage material, the relative merits of each design are analyzed and a comparison of the gravimetric and volumetric hydrogen storage densities for the three designs is presented.     

Improving heat recovery using retrofitted heat pump in air handling unit with energy wheel

The world is facing a challenge to reduce energy use to meet the environmental goals set for the future. One factor that has a great impact on the energy performance of buildings is the ventilation losses. To handle these losses, heat recovery systems with rotating heat exchanger are often implemented. These systems have been shown to recover about 60–70% of the energy in the exhaust air on an annual basis.After a heat recovery system is installed it is hard to improve the efficiency of the installed recovery system with an acceptable economic payback period. In the present paper one way to improve the energy performance of a building with this type of heat recovery system by the use of a heat pump is investigated by simulations in TrnSys.The heat pump system is arranged so that the evaporator is connected to a heat exchanger mounted in the exhaust airstream after the energy wheel, and the condenser of the heat pump is mounted so that the temperature of return water from the heating coil is increased.The simulations show that there is a possibility to increase the heat recovery rate of the air handling unit in a significant way by retrofitting a heat pump to the system.     

Design of biomass district heating systems

The biomass exploitation takes advantage of the agricultural, forest, and manure residues and in extent, urban and industrial wastes, which under controlled burning conditions, can generate heat and electricity, with limited environmental impacts.Biomass can – significantly – contribute in the energy supplying system, if the engineers will adopt the necessary design changes to the traditional systems and become more familiar with the design details of the biomass heating systems.The aim of this paper is to present a methodology of the design of biomass district heating systems taking into consideration the optimum design of building structure and urban settlement around the plant. The essential energy parameters are presented for the size calculations of a biomass burning-district heating system, as well as for the environmental (i.e. Greenhouse Gas Emissions) and economic evaluation (i.e. selectivity and viability of the relevant investment). Emphasis has been placed upon the technical parameters of the biomass system, the economic details of the boiler, the heating distribution network, the heat exchanger and the Greenhouse Gas Emissions.