Heat Energy Recovery for Industrial Processes and Wastes 3031243730, 9783031243738

Table of contents :
Contents
Wastewater as a Source of Heat Energy
1 Introduction
2 Challenges in Waste Heat Recovery from Wastewater
3 Technologies for the Recovery of Heat from Residual Effluents
4 Methodology to Determine the Potential for Heat Recovery in an Effluent
5 Possible Options of Heat Recovery from Wastewater
6 Heat Recovery from Wastewater: Practical Applications
7 Conclusions
References
Heat Energy Recovery and Low CO2 Emission for Natural Gas Combined Cycle Power Plants Using Plasma Treatment
1 Introduction
2 The Total System
2.1 A Natural Gas Combined Cycle Power Plant
2.2 Energy Balance for a Gas Turbine Combined Cycle
2.3 Targeted Values for Zero CO2 Emission GTCC
2.4 Sub Research Topics for Zero CO2 Emission GTCC
3 Low-Calorie Gas-Fired Turbines
3.1 Fuel for Low-Calorie Gas-Fired Turbines
3.2 Gas Turbines that Use Low-Calorie Fuels
3.3 Gas Turbine Combined System with Low-Calorie Fuel and CO2 Capture
4 Fuel Conversion of CO2 Using Exhaust Gas Recirculation
4.1 Experimental Setup and Methods
4.2 Experimental Results and Discussions
5 Conclusions
References
Colloidal Technologies for Heat Energy Recovery
1 An Introduction: The Concept of Colloidal EneRgEtic System
2 Cinematic and Thermal Properties of Colloids
2.1 Complexity
2.2 Stability
2.3 Viscosity
2.4 Thermal Properties
3 Ferrohydrodynamics and Complex Phenomena in Ferrofluids
3.1 Conservation Equations of Hydrodynamics
3.2 Aspects of Non-isothermal Ferrohydrodynamics
3.3 Thermomagnetic Convection
3.4 Complex Phenomena in Ferrofluids
4 Triboelectric Colloids
4.1 Colloidal Triboelectric Nanogenerators (C-TENG)
5 Pyroelectric Colloids
5.1 Pyroelectric Coefficient
5.2 Pyroelectric Materials
6 Colloidal Materials for Phototermal and Thermoelectric Energy Conversion
6.1 Photothermal Conversion
6.2 Thermoelectric Nanofluids
7 Colloidal Devices for Energy Harvesting and Conversion
7.1 Ferrofluid Based Devices
7.2 Triboelectric Devices
7.3 Pyroelectric Devices
7.4 Photothermal Devices
7.5 Thermogalvanic and Thermo-Osmotic Devices
References
Techno-Economic Feasibility of Organic Rankine Cycles (ORC) for Waste Heat Recovery
1 Introduction
2 ORC Concept
3 Main ORC Applications
3.1 Renewable Energy Sources
3.2 Waste Heat Recovery
4 Architectures
5 Main Components
5.1 Expansion Device
5.2 Pump
5.3 Heat Exchangers
6 Working Fluids
7 Market Situation
8 Economic Feasibility
9 Case Studies
9.1 Geothermal Energy
9.2 Solar Energy
9.3 Biomass Energy
9.4 Waste Heat Recovery
9.5 Combined Heat and Power
10 Conclusions
References
New Techniques for Recovering Exhaust Heat from Gas Turbines Applied to Water-Energy Nexus
1 Introduction
1.1 Heat Recovery for Absorption Cycles
1.2 Absorption Chillers Powered by Low Temperature Hot Water Under 100 °C
1.3 Low Pressure (P  10 kPa) Steam Driven Absorption Chillers
1.4 Heat Recovery for Distillation
2 General Approach
2.1 Case Study
2.2 Energy Transition. H2 Economy
References
New Improvements in Existing Combined-Cycles: Exhaust Gases Treatment with Amines and Exhaust Gas Recirculation
1 Introduction
2 Theoretical Background
3 Materials and Methods
4 Discussion and Results
4.1 Existing CCGT Versus CCGT with EGR Close to 35–40%
4.2 Existing CCGT with PCC Versus CCGT with EGR 35%+ PCC
5 Conclusions
References
Energy Performance Assessment of a Polygeneration Plant in Different Weather Conditions Through Simulation Tools
1 Introduction
2 Simulation Methodology to Analyze a Polygeneration Plant
2.1 Meteorological Variables
2.2 Constructive Characterization
2.3 Building Energy Demands
2.4 Generation Systems and Operation Schedule
3 Thermal and Electrical Dynamic Simulation Models
3.1 Thermal Simulation Model
3.2 Electrical Simulation Model
4 Dynamic Evaluations of the Polygeneration Plant
4.1 Initial Optimizations
4.2 Global Simulation Results
5 Conclusions
References
Geothermal Heat Pumps for Slurry Cooling and Farm Heating: Impact and Carbon Footprint Reduction in Pig Farms
1 Introduction
1.1 Pig Production and Associated Impact
2 Theoretical Background
2.1 Environmental Impacts of Pig Farms
2.2 Slurry Environmental Impact
2.3 Slurry Technology
3 Proposal and Case Study
3.1 Heat Pumps for Slurry Management
3.2 Case Study
3.3 System Configuration
3.4 Slurry Management Proposal
3.5 Gaseous Emissions
4 Discussion
4.1 Reduction of Carbon and Hydric Footprint
4.2 Additional Advantages
5 Conclusions
References

Citation preview

Green Energy and Technology

David Borge-Diez Enrique Rosales-Asensio   Editors

Heat Energy Recovery for Industrial Processes and Wastes

Green Energy and Technology

Climate change, environmental impact and the limited natural resources urge scientific research and novel technical solutions. The monograph series Green Energy and Technology serves as a publishing platform for scientific and technological approaches to “green”—i.e. environmentally friendly and sustainable—technologies. While a focus lies on energy and power supply, it also covers “green” solutions in industrial engineering and engineering design. Green Energy and Technology addresses researchers, advanced students, technical consultants as well as decision makers in industries and politics. Hence, the level of presentation spans from instructional to highly technical. **Indexed in Scopus**. **Indexed in Ei Compendex**.

David Borge-Diez · Enrique Rosales-Asensio Editors

Heat Energy Recovery for Industrial Processes and Wastes

Editors David Borge-Diez Department of Electrical, Systems, and Automation Engineering University of León León, Spain

Enrique Rosales-Asensio Department of Electrical Engineering University of Las Palmas de Gran Canaria Las Palmas de Gran Canaria, Spain

ISSN 1865-3529 ISSN 1865-3537 (electronic) Green Energy and Technology ISBN 978-3-031-24373-8 ISBN 978-3-031-24374-5 (eBook) https://doi.org/10.1007/978-3-031-24374-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

Wastewater as a Source of Heat Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dolores Hidalgo, Jesús M. Martín-Marroquín, and Juan Castro Heat Energy Recovery and Low CO2 Emission for Natural Gas Combined Cycle Power Plants Using Plasma Treatment . . . . . . . . . . . . . . . Haruhiko Yamasaki, Hiroyuki Wakimoto, and Masaaki Okubo Colloidal Technologies for Heat Energy Recovery . . . . . . . . . . . . . . . . . . . . . M. Bevione, L. Cecchini, E. Garofalo, S. A. Suslov, and A. Chiolerio

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Techno-Economic Feasibility of Organic Rankine Cycles (ORC) for Waste Heat Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Adrián Mota-Babiloni, Marta Amat-Albuixech, Francisco Molés-Ribera, and Joaquín Navarro-Esbrí New Techniques for Recovering Exhaust Heat from Gas Turbines Applied to Water-Energy Nexus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Carlos Cuviella-Suárez, Antonio Colmenar-Santos, and David Borge-Diez New Improvements in Existing Combined-Cycles: Exhaust Gases Treatment with Amines and Exhaust Gas Recirculation . . . . . . . . . . . . . . . 181 David Borge-Diez, David Gómez-Camazón, and Enrique Rosales-Asensio Energy Performance Assessment of a Polygeneration Plant in Different Weather Conditions Through Simulation Tools . . . . . . . . . . . 203 S. Soutullo, L. A. Bujedo, J. Samaniego, D. Borge-Diez, J. A. Ferrer, R. Carazo, and M. R. Heras Geothermal Heat Pumps for Slurry Cooling and Farm Heating: Impact and Carbon Footprint Reduction in Pig Farms . . . . . . . . . . . . . . . . 221 Cristina Sáez Blázquez, David Borge-Diez, Ignacio Martín Nieto, Miguel Ángel Maté-González, Arturo Farfán Martín, and Diego González-Aguilera v

Wastewater as a Source of Heat Energy Dolores Hidalgo, Jesús M. Martín-Marroquín, and Juan Castro

Abstract To counteract climate change, the application of renewable energy sources and their efficient use are of crucial importance. In this context, wastewater has also gained increased attention in recent years. For decades, wastewater treatment plants have applied heat from digester gas combustion to meet internal demands. However, wastewater can be considered as a renewable heat source throughout its cycle, from production to disposal. Domestic, industrial, and commercial wastewaters retain considerable amounts of thermal energy after being discharged into the sewage system. It is possible to recover this heat through technologies such as heat pumps and exchangers and reuse it to meet heating demands, among others. This chapter provides an overview of existing opportunities for wastewater heat recovery and its potential at different scales within the sewerage system, including at the level of wastewater treatment plants. A systematic review of the benefits and challenges of wastewater heat recovery is provided, taking into account not only technical aspects, but also economic and environmental ones. This study analyzes important parameters, such as the temperature and flow dynamics of the sewage system, the impacts of heat recovery on the environment, and the legal regulations involved. The existing gaps in the field of harnessing the heat energy contained in residual effluents are also identified. The potential of wastewater to supply clean energy on a scale ranging from buildings to large communities and districts will be analyzed, assessing the role of administrations and other stakeholders in taking advantage of the full potential of this valuable renewable heat source.

1 Introduction Today the world is facing major challenges in terms of sustainable energy sources and depletion of energy resources. These, together with the worrying climate change, have been triggered by human action throughout the industrial development of recent D. Hidalgo (B) · J. M. Martín-Marroquín · J. Castro CARTIF Technology Centre, Area of Circular Economy, Valladolid, Spain e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 D. Borge-Diez and E. Rosales-Asensio (eds.), Heat Energy Recovery for Industrial Processes and Wastes, Green Energy and Technology, https://doi.org/10.1007/978-3-031-24374-5_1

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centuries, mainly sustained by conventional energy sources. There are, therefore, two concepts that motivate energy saving: global warming, the result of climate change, and sustainable development as a remedy for the depletion of resources. The phenomenon of global warming generates more and more impact on our environment and climate. This impact has already been studied by bodies such as the Intergovernmental Panel on Climate Change (IPCC), which in 2018 produced a report analyzing the consequences of a rise in the average global temperature of the planet of 1.5 °C compared to pre-industrial levels, and determined the trajectories that greenhouse gas emissions should follow to avoid these effects [1]. Faced with the danger of “turning the Earth into a greenhouse”, there are agreements and commitments, such as the Paris Agreement within the European framework, which promotes awareness and demands effective global action to reduce greenhouse gas emissions, with the goal of keeping global temperature rise well below 2 °C. Specifically, it is intended to reduce emissions by 2030 by at least 40% compared to 1990. Among these emissions, CO2 occupies a preferential place, due to their quantity and effect. Combustion processes are found in all types of industrial and human activity, and release large amounts of carbon dioxide into the atmosphere. The gases obtained in the combustion of fossil fuels with air contain between 4 and 16% CO2 [2]. Some of the most polluting activities in this sense, in addition to transport, are the processes of domestic heat generation; Boilers are, today, the most common heat generation system for heating and domestic hot water (DHW). In the case of the Netherlands, for example, the domestic demand for natural gas for heating DHW reaches 385 Nm3 /year on average in each household [3]. At this point, energy saving is key to being able to reduce the use of these systems, and with it CO2 emissions. Reusing the energy already produced is only the first step in decoupling energy consumption from CO2 emissions. On the other hand, energy saving is essential to guarantee sustainable development, in which the use and exploitation of energy resources do not condition the availability of energy for future generations. It is thus necessary to change an energy model that has historically been dependent on fossil fuels such as coal and oil, through the search for new resources and proper management of them. This management is where energy saving comes into play, a determining factor in improving the efficiency of processes and making the most of available resources. From this point of view, a recirculation of the energy contained in the wastewater would reduce the demand for new energy and avoid an unnecessary waste of resources. The word “residual heat” should be valued, since today waste must be considered as a source of renewable energy, which will never be lacking where there is vital activity.

2 Challenges in Waste Heat Recovery from Wastewater Thermal energy is a form of energy that is more degraded the lower its thermal level, and it is very expensive to convert it back into useful energy, capable of being reused

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in other forms of energy. However, in the case of study this concept changes, because what is sought is precisely to obtain the thermal energy lost in the processes to raise the thermal level of a fluid. It is therefore not a transformation of energy, but a recovery and adequate transport of it towards a heat transfer fluid. This transmission must be done with the lowest possible losses and with reasonable efficiency. Residual heat recovery is a promising method for saving energy and caring for the environment, as has been mentioned. However, this recovery can be difficult and costly, and in any case the extent to which it is worth implementing the recovery systems and methods in each specific case must be considered. The following aspects highlight the main problems when recovering residual heat from wastewater [4]: – The demand for recovered heat could not be continuous, but intermittent throughout the day. In general terms, it will be difficult to find a system that produces waste heat simultaneously with the demand for DHW, for example. This raises the search for residual heat storage alternatives for later use when hot water or any other energy demand is required. – Recovered heat is a basic resource that must meet a series of sanitary requirements, so it is necessary to investigate a clean way of providing heat that does not modify the characteristics of the receiving media. It will be necessary to take into account some aspects when recovering heat, especially if the thermal effluent comes from wastewater. The dirt that these bring with them negatively affects the exchange devices for multiple reasons: – Dirt tends to settle and collapse the ducts through which the residual water circulates, reducing the passage section of the hot flow through the recovery device. – The dirt not only obstructs the passage of the residual water flow, but also supposes an additional thermal resistance, causing the heat transfer coefficient of the exchange device to decrease. – If the device gets dirty, it is necessary to carry out regular and costly maintenance and cleaning work, since the stoppage of the system can entail significant additional costs. Once the main problems in the recovery of residual thermal effluents have been exposed, a review of the solutions adopted to solve them is carried out below. The main lines of work that will be focused on in this section are the recovery of heat from the gray water network and the recovery of heat from the refrigeration cycles used in air conditioning of buildings. In both cases there is a flow of heat to the outside of the building with the possibility of recovery. There are various devices capable of recirculating residual heat. The most mentioned stand out for their simplicity, their good behavior, or for being integrated into an existing installation. The essential element in any recovery process is the heat exchanger. Culha et al. [5] conducted an exhaustive review of wastewater heat exchangers. These authors presented a classification of the different configurations that it can take, as well as possibilities for its location and the design and construction process. The location of the exchanger with respect to the consumption point is a relevant aspect due to the

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Fig. 1 Possibilities of implementation of the wastewater heat exchanger. Adapted from Culha et al. [5]

existing heat losses during the fluid path. The device can be inside the building or in its vicinity, being restricted to a domestic level; outside the building, that is, in the sewage system, being able to continuously recover more heat from various buildings; and it can even be located after a wastewater treatment plant, taking advantage of the heat with which the treatment effluents come out, although very far from the DHW supply point (Fig. 1). As for the most common configurations, in the event that the exchanger is in the sewage system, these authors indicated three types: the external ones, which work through bypass systems; the integrated ones that adapt to the shape of the sewerage network; or the modular ones, which can be installed later in the waste disposal network. There are also studies about those exchangers designed to recover heat in situ in the drain. The research carried out with “Heat Pipe” technology in the recovery of heat from wastewater deserves a special mention. Heat pipes are devices in the form of a sealed and vacuum tube, which house a fluid inside whose function is to transport heat from one end to the other through successive phase changes (Fig. 2). The return of the fluid can be produced by capillarity, in the case of horizontal or inclined heat pipes; or by the effect of gravity, as in the case of thermosyphons. Thermosyphons have been studied as a powerful possibility for energy recovery from wastewater, due to their ability to transport heat between small temperature gradients [6]. The heat pipe is capable of transporting the heat from the wastewater to the DHW, through the evaporation and condensation of the internal refrigerant. Burlacu et al. [7] proposed a cylindrical battery of heat pipes with baffles, in which a maximum thermal jump of 49.6 °C was achieved in the DHW. The introduction of paraffin and phase change materials in the surroundings of the condensation zone made it possible to store the heat released in response to the non-simultaneous flow of wastewater and DHW demanded [8]. Vizitiu et al. [9] developed a prismatic battery performing an analysis using computational fluid dynamics (CFD), and achieving a thermal jump of 33.6 °C. Likewise, other simpler designs [10] put the wastewater and the heat pipes in direct contact, achieving a thermal jump of 28.4 °C in the DHW.

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Fig. 2 Heat pipe diagram showing heat flow

Finally, the use of heat exchangers in the field of heat recovery is not limited to the exchange between wastewater and DHW, but rather constitute an essential element in any system where energy inputs and outputs are produced, and key when it comes to recirculate residual thermal effluents. This is the case in the refrigeration cycles and heat pumps of air conditioning installations, an important source of residual thermal energy. In them, the exchangers can be direct, if they transfer heat directly from the residual water to the refrigerant (that is, they act as an evaporator), or indirect, if there is an intermediate water circuit between the residual water and the evaporator. A common classification of these exchangers can be seen in Fig. 3. On the other hand, among the different ways of extracting heat from a thermal effluent, heat pumps are a veteran technology, since they have been used to extract heat from wastewater since the 1980s, in centralized systems throughout Germany, Sweden and the Scandinavian countries. Among the different possibilities of recovery

Fig. 3 Classification of exchangers used in heat pumps with wastewater. Adapted from Iglesias [4]

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and applications that can be given by means of heat pumps, the main ones are the following: – Heat pump systems with source in wastewater. Hepbasli et al. [10] conducted an extensive review of these systems. The options are: (1) Extract heat from the wastewater to transfer it to the DHW; (2) Extract heat from wastewater to transfer it to the air conditioning system (heating) or other systems that require heat. – Heat pump systems with a source of another thermal energy carrier agent, such as the chillers of an air conditioning system, which extract heat from the glycol water (to cool it and send it to the cold demand points), and recirculate this heat in order to heat DHW. The standard configuration of heat pumps puts the wastewater in contact with the evaporator, and the DHW with the condenser [11]. Authors such as Li and Li [12], introduce a four-way valve that allows the refrigeration cycle to be inverted, and describe the plate and spray exchangers used. It is usual to introduce one more stage through an indirect exchanger, as Zhao et al. [13] to heat DHW, although sometimes the heat from wastewater is used for air conditioning purposes [14]. As mentioned in the previous classification of heat pumps, the application does not always have to be DHW heating. Farman and Gillich [15] applied the recovery to the district heating and cooling system of a city. Other authors apply the recovered heat to the biological treatment of sewage to give rise to purified water [16]. Finally, there are studies focused on the integration of systems, combining all the exposed alternatives. For example, systems have been proposed in which a single heat pump allows the extraction of thermal energy from different sources, as proposed by Ni et al. [17], who extract heat from residual water and from the condenser of air conditioners. There are other systems that take full advantage of the wastewater, and use it for a multifunctional purpose, such as heating water and air conditioning simultaneously [18]. Some even combine wastewater recovery heat with other systems for generating and capturing thermal energy, such as the electric resistance heater or the solar collector [19]. Research in this field has also focused on the possibilities to solve the problem of fouling in the exchanger ducts. Most of these problems are due to the sedimentation of suspended particles. In this line, Ni et al. [17] tried to characterize the critical size of these deposited particles, determining that all were below 4 mm. However, Song et al. [17] explain how the problem of dirt not only involves sedimentation, but also biofouling, as wastewater is an aqueous medium rich in nutrients and conducive to the proliferation of microorganisms, which accumulated form biofilms capable of reducing by 50% heat transfer. In the same article, a convective analysis is carried out that will be used later in this work to characterize the thermal resistance due to fouling. Some cleaning systems are included in the heat exchanger itself, such as the shell and tube evaporator presented by Shen et al. [20] and others are added to the installation in the step prior to heat exchange, as is the case with systems such as the automatic anti-clogging equipment presented by Liu et al. [21], or the reflux function sewage hydrocyclone developed by Ni et al. [22].

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3 Technologies for the Recovery of Heat from Residual Effluents In most processes, both domestic and industrial, much of the energy that enters the process comes out in the form of effluents with high thermal energy. Being able to recover this type of energy is essential, as described in the previous section, to improve the total performance of the activity. For years, the recovery of surplus heat from processes, mainly industrial ones, but also from those that take place in residential buildings, for example, has been studied and analyzed to seek continuous improvement of the technologies involved [23]. These technologies can be categorized as passive technologies or active heat recovery technologies, according the classification showed in Fig. 4. In passive technologies, the equipment transfers thermal energy at the same or lower temperature. This type of technology includes the use of heat exchangers to provide heat to another current and the use of thermal storage equipment, which allows the heat generator source to be decoupled from the receiver. Active technologies for heat recovery transform thermal energy into another type of energy, or increase its temperature. In this section, the use of thermal energy to produce electricity will be analyzed as an active technology. A heat exchanger is an equipment designed to transfer energy, in the form of heat, between two or more fluids with different temperatures. These equipments are widely used for applications that involve increasing or decreasing the temperature of any fluid stream. In some of this equipment, the fluids involved in the process are

Fig. 4 Technologies for heat recovery. Adapted from Loma [24]

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in direct contact with each other. In other exchangers there is no contact between the fluids, so the heat transfer between them occurs through a surface called heat transfer surface, these teams are called indirect contact exchangers. There are two main types of indirect contact exchangers, those called direct transfer or recuperators and those called indirect transfer or regenerators. In the recuperators, the fluids go through the equipment with a continuous flow and the transfer is carried out through a fixed separation surface. In regenerators, heat transfer is carried out through a matrix through which the different currents circulate alternately, giving heat to the matrix in one case and absorbing heat from the matrix in another. Heat exchangers can be classified according to their heat transfer process, the number of fluids involved in the process, their function in the process, flow arrangement, heat transfer mechanism, or their construction structure [25]. Tubular heat exchangers are generally built using tubes with circular sections through which one of the fluids passes, although they can also be designed with cross sections that differ from circular, mostly elliptical and rectangular [26]. There is great design flexibility for this equipment, because the flow passage area, and therefore the heat transfer surface, can easily vary by changing the diameter of the tubes, the length and the arrangement of the set of tubes that make up the team. The design of the tubular exchangers can be adapted to any temperature and pressure of operation, the only existing limitation is the material of construction of the equipment and certain considerations of the system. Their high flexibility also makes them suitable for being designed for special operating conditions: corrosive fluids, with high viscosity, radioactive, etc. The main types of tubular exchangers are: shell-and-tube exchangers, double pipe exchangers, spiral exchangers and coils. Shell-and-tube exchangers are usually the most used, because it is possible to adapt them to a large number of processes thanks to their wide flexibility regarding pressure and temperature ranges, something that greatly determines the choice of the type of exchanger for each process. Shell-and-tube exchangers are mostly made up of a bank of tubes mounted inside a cylindrical shell. The use and design of this equipment for decades has led to the existence of well-established design criteria and a specific notation, developed by the Tubular Exchanger Manufacturers Association [27], for the denomination of the equipment according to its constructive parts. Using this notation each shell-and-tube exchanger can be defined using three letters that correspond to its front header type, shell type, and rear header type. On the other hand, double pipe exchangers are very basic equipment. Its main structure consists of two concentric tubes, mounted one inside the other, through which the two fluids between which heat is to be exchanged flow (Fig. 5). They are often used for applications where one or both fluids are at high pressure and for small capacities. These equipments have the advantage of having a pure countercurrent flow. The tubes in this type of equipment can be smooth or finned, in order to increase the transfer surface between the fluids. The choice between smooth or finned tubes is usually conditioned by cost. For small equipment where it is necessary to improve the transfer surface and the fluid in the inner tube has a relatively high transfer coefficient, the choice of finned tubes is usually more economical. As a general rule, these units are more economical if the external fluid transfer coefficient is less than 25% of the internal fluid coefficient.

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Fig. 5 Double pipe heat exchangers

Another group of exchangers are plate exchangers. This type of heat exchangers are usually made up of thin sheets, which make up the heat transfer surface. They are not usually used for high pressure or temperature differences between the fluids involved in the process. They can be classified into several groups: plate heat exchangers, spiral heat exchangers, coiled plate heat exchangers and lamella heat exchangers [28]. In plate heat exchangers, the fluids are separated by rectangular plates mounted on a frame. Generally, the plates have a corrugated design in order to increase the transfer surface between the fluids. The fluids enter through the ends of the plates, passing through them alternately, so that a section containing hot fluid is always in contact with one containing cold fluid. They are equipment that have a large contact surface in a small space. Within this group of exchangers, one can differentiate between those that use gaskets between the plates and those that use plates welded together. Plate heat exchangers that use gaskets have a very flexible design, which can be adapted from a standard design, so they can be produced in series. For common designs, the plates are made of stainless steel, but another commonly used material is titanium and they could be made of other materials if necessary. The gaskets between the plates, which are the main limitation in this equipment, are generally made of ethylene propylene rubber or nitrile elastomer, but there are a wide variety of materials that can be used for this purpose and they should be chosen considering the fluids in mind with which the equipment will work. One of the greatest advantages of this type of equipment is the possibility of increasing its size, simply by adding more plates to the equipment, which entails a small cost. The equipment in which the plates are welded together solve the problems that may arise from the use of gaskets. With welded plates, it is possible to work at a pressure of 60 bar inside the equipment, although there is a limitation when working with a pressure difference between the two fluids greater than 30 bar. The disadvantage with respect to the use of joints in

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this type of equipment is its higher price and the added difficulty of cleaning the equipment, requiring chemical cleaning. The basic structure of spiral exchangers consists of two metal sheets joined together, leaving an interior space for a fluid to circulate, and rolled around an axis, forming a series of spiral passages through which the fluids involved in the heat exchange will circulate, thus achieving a large transfer surface in a small space. In the countercurrent design of this type of equipment, the hot fluid enters through the central part of the equipment and exits through the outside, while the cold fluid travels in reverse, enters through the periphery of the equipment and flows towards the center, where he ends up leaving the team. There are alternative designs that can be used for the condensation or evaporation of fluids. The heat transfer coefficient in this equipment is lower than in a plate equipment if the plates that make it up are not corrugated, but this coefficient is greater than in a shell-tube equipment, so the required transfer area compared to a shell-with-tube unit is about 20% less. As main advantages, it can be highlighted that this equipment can work with fluids with high viscosity, with suspended particles, with sludge and with dirty fluids. The fouling of this equipment is less than in a shell-tube exchanger. They are compact units and do not present differential expansion problems. The main disadvantage is the need, generally, to use a chemical cleaning in the equipment and the difficulty when it comes to repairs due to its construction characteristics. Plate heat exchangers with coils or plate coils, are made up of a series of plates with a coil inside, through which one of the fluids flows, and these plates are submerged in a tank in which the other fluid is found. As main advantages we can highlight the high control that can be had on the transfer of heat and temperature within the equipment and the few maintenance problems, contamination between fluids and cleaning that they originate. Their use is limited and for the most part they are used in cryogenic processes, the food industry and the pharmaceutical industry. “Lamella” heat exchangers are hybrids between plate heat exchangers and shell-tube heat exchangers. In this type of design, the tubes are replaced by a series of plates and inserted into a casing similar to those of the casing-tube, fixing only one of the ends to allow expansion. They are very specific equipment and whose design is carried out by suppliers. Another group are the extended surface exchangers. In the tubular and plate type exchangers, already described, the heat transfer surface with respect to the volume occupied by the equipment is generally less than 700 m2 /m3 . On many occasions, a high effectiveness is needed in the equipment, having a limitation regarding the size of the equipment, which makes it necessary to increase its compactness (the transfer surface with respect to the occupied volume). A large transfer surface may also be necessary if one of the fluids involved in heat transfer has a low transfer coefficient. This increase in the transfer surface can be achieved by adding fins to the primary surfaces of the equipment, from which the two types of exchangers that will be detailed below arise: plate-fin exchangers and tube-fin exchangers. Plate-fin heat exchangers are made up of a series of parallel plates between which fins are placed, on whose edges side bars are installed to increase the transfer surface. The currents involved in the transfer process alternately pass between the plates. Aluminum is

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usually used as a construction material. These equipments are generally designed to work at moderate pressures ( 2.90/5.91 × 100 = 49% is the zero-emission condition, where a surplus power generation output of P = 5.91 − E v (eV/molecule) is obtained. The total efficiency of the GTCC, therefore, is as follows: ηgreen =

Fig. 3 Energy balance for a gas turbine combined cycle that illustrates the possibility of a zero CO2 emission power generation system

5.91 − E v × 100 (%) 9.23

(3)

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If ηgtcc = 60%, η > 52% is the required condition, and the total efficiency is as follows: ηgreen =

5.54 − E v × 100 (%) 9.23

(4)

A study by Spencer et al. [6] from the University of Michigan confirmed the conversion of nearly 90% of CO2 into CO using dilute argon plasma. Therefore, although the achievement of the goal of zero CO2 emissions (almost 100% reduction) in atmospheric plasma is challenging, it possible. The feasibility of this system was examined through elemental experiments in the laboratory [4, 5]. A target efficiency of 49% is determined by combining the utilization of the dielectric barrier discharge (DBD) and CO2 direct capture plasma concentration. Recent research with experimental results at the laboratory level, developed at Osaka Metropolitan University, is described in Sect. 4 of this chapter.

2.3 Targeted Values for Zero CO2 Emission GTCC The three goals of research and development of a zero CO2 emission system are as follows: The first goal is to directly collect CO2 at a concentration of 400 ppm from the atmosphere and increase its concentration by 20% (=20,000 ppm) with 10% thermal energy at a rate 10 times faster than that of conventional techniques, such as heating. The technology is currently under development. The second goal is to achieve an energy conversion efficiency of 49% using the combined adsorbent/plasma technologies into CO using an environmental plasma system. This is the minimum efficiency at which a zero CO2 emission GTCC can be established. It has been experimentally demonstrated globally that the ultimate goal of achieving a zero CO2 emission GTCC thermal power plant with an energy efficiency of 49% or higher is quite reasonable. At present, an energy efficiency of 34% has been achieved in a laboratory at the Osaka Metropolitan University. Therefore, by increasing the efficiency of plasma conversion to electrical energy, understanding the mechanism of the high-concentration process reduction, and using a scaled-up plasma reactor, we achieved an efficiency ≥49%. The third goal is to establish a pollution-free zero CO2 emission thermal power plant by creating and operating a conceptual design and demonstrating a prototype of an actual GTCC system that includes environmental plasma equipment, small gas turbines, heat exchangers, and exhaust gas treatment systems.

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2.4 Sub Research Topics for Zero CO2 Emission GTCC Sub-themes for zero CO2 emission thermal power generation are explained below. First, a CO2 direct air capture (DAC) [8] plasma concentration system is explained as sub-theme I. Figure 4 shows a DAC system that captures and absorbs CO2 directly from the atmosphere with an adsorbent at a high flow rate. After capture, air is blown at a low flow rate toward the adsorbent while applying nonthermal plasma and waste heat. CO2 is then desorbed from the gas fuel. Figure 4 shows a conceptual diagram of the fuel conversion. The process consists of two steps. In operation (1), CO2 in the atmosphere (approximately 400 ppm) flows through an adsorbent at a high flow rate Q to concentrate CO2 and release gas without no CO2 . In operation (2), atmospheric air with a low flow rate q (99.95% [15]. However, the

Fig. 8 Design concepts of the 1773 K-class low-calorie fuel combustor [15]

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conversion rate is influenced by the CH4 concentration of the fuel. When CH4 is present in the fuel, the CHi (i = 1, 2, 3) produced by the decomposition of CH4 is involved in the decomposition of NH3 , rapidly producing HCN and NHi (i = 0, 1, 2, 3) in the near-flame zone. When the primary equivalence ratio is less than one, the conversion rate to NOx also decreases as the CH4 concentration decreases. When the primary equivalent ratio is >1, there is an optimal primary equivalent ratio that minimizes the conversion ratio. In other words, low-calorie fuels contain CO and H2 as major combustion components. However, they only contain small amounts of CH4 ; therefore, the conversion rate of NOx in the fuel N component, which determines fuel NOx production, can be greatly reduced by setting the primary equivalent ratio to a fuel enrichment condition of one or higher. Consequently, gas turbines (Fig. 8) can be used to generate electric power using low-calorie fuels (Fig. 7a).

3.3 Gas Turbine Combined System with Low-Calorie Fuel and CO2 Capture A conceptual diagram of the proposed combined system with the CO2 plasma reduction technology, IGCC, and CO2 capture is shown in Fig. 9. Here, IGCC is considered an example of a gas turbine combined system that uses low-calorie fuels. Low-calorie fuels, such as those shown in Fig. 7a flow into the combustor of a gas turbine after gasification, heat exchange, desulfurization, and other gas purification processes. In a typical IGCC system, the exhaust gas from the gas turbine is discharged through the stack after the heat exchange in the boiler. A typical IGCC emits a high amount of CO2 per unit of electric power generated; therefore, there is a need to increase the efficiency of power generation technologies and introduce CO2 capture and storage (CCS) technologies, from a global environmental conservation perspective. There are two types of CO2 recovery methods for coal-fired power plants: one is to recover CO2 from the coal combustion exhaust gas and the other is to recover CO2 from the fuel gas before combustion. In coal gasification systems, CO2 is separated from the fuel gas, allowing the recovery of high concentrations of CO2 under pressurized conditions. Therefore, the reduction in plant efficiency is expected to be lower than that of CO2 separation from the combustion exhaust gas. The CO in the gas fuel produced by the gasifier is converted to CO2 and H2 by water in the CO shift reactor, and CO2 is separated and recovered from the gas fuel using an acid gas removal facility [17, 18]. In recent years, demonstration tests for oxygen-blown IGCC with CO2 separation and recovery have been conducted in Japan [19]. Here, a portion of the fuel gas containing CO and H2 is converted to CO2 and H2 by a shift reactor. CO2 is separated by physical adsorption, and the H2 -rich gas is mixed with the fuel gas and fed to the fuel tank of a gas turbine (Fig. 9). The technology is associated with increased power generation efficiency and increased CO2 emission concentrations at low flow rates. The concept of the system (Fig. 9) shows that a portion of the electric power

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Fig. 9 Conceptual diagram of the IGCC and CO2 capture system with the plasma CO2 reduction reactor

generated by the gas turbine is fed into a plasma reactor, where the CO2 separated by the adsorption column is reduced to CO, which is then mixed with H2 -rich gas and reused as fuel for the gas turbine. In the next section, CO2 plasma reduction is performed in a simulated exhaust gas with a CO2 concentration of 10%, which is assumed to be a combustion gas, with the aim of decomposing CO2 in the exhaust from thermal power generation into CO fuel. The energy efficiency of plasma CO2 reduction is closely related to the concentration of the treated CO2 . The latest gasification-fueled power generation systems are expected to improve the energy efficiency of the plasma reduction technology owing to their high energy efficiency and high CO2 emission concentrations. The CO2 discharged from the CO2 adsorption tower or directly from the IGCC gas turbine is reduced to CO by the plasma reduction system. H2 is produced by a water–gas shift reaction between a portion of the steam produced by the boiler and the reduced CO. A mixture of CO and H2 is reintroduced into the gas turbine combustor. The mixture of CO and H2 can be reused as a fuel in gas turbine combustors to save coal and other fuels.

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4 Fuel Conversion of CO2 Using Exhaust Gas Recirculation 4.1 Experimental Setup and Methods This section explains the technology for converting CO2 into fuel by recirculating the exhaust gas with inert gas/catalytic plasma treatment. To reduce CO2 emissions, the method of converting CO2 , using reduction technologies, into CO by NTP is explained. Through an experiment, the CO2 reduction performance is evaluated for the two processes using different gas mixtures. First, CO2 is adsorbed from a gas flow mixture comprising Ar He, N2 , and CO2 (approximately 10% concentration) onto an adsorbent, after which CO2 is desorbed and reduced by an Ar or He NTP flow at a higher concentration of 10–22% under the same experimental conditions. An energy efficiency of 20% is achieved in the laboratory, indicating that a CO2 reduction of 41% is possible for GTCC plants by scaling up the NTP system to the pilot scale. In this experiment, NTP is generated using a mixture of Ar, He, and N2 and the effects of changing the concentration ratio of NTP gas [20] and introducing various catalysts downstream of the plasma reactor are evaluated to determine the effect of NTP in reducing the CO2 in exhaust gas [21]. A schematic of the experimental setup is shown in Fig. 10. The experiment consists of two steps: the first involves the adsorption of CO2 by the adsorbent and the other involves the desorption and reduction of CO2 by NTP flow. In the adsorption process, carbon dioxide (CO2 ) gas with a concentration of 10% is flowed through the equipment at a total flow rate of 10 L/min, considering that the combustion exhaust gas from boilers and internal combustion engines contains a concentration of almost 10% CO2 . The flow rate and concentration are regulated using a mass flow controller (SFC281E; Hitachi Metals Ltd.). The mixed gas is passed through the adsorbent packed in the adsorption chamber, with a volume of 1.8 L, which is located downstream of the plasma reactor. The CO2 concentration upstream of the chamber is measured every 10 min using gas detector tubes with glass syringes (2H and 2HH; Gastec Co., Ltd.) at a measurement point (MP1). Similarly, the downstream CO2 concentration is measured every 10 min using a CO2 analyzer (COZY-1; non-dispersive infrared absorption method, accuracy ± 0.25%, Ichinen Jiko Co., Ltd.) on the pumped gas using a gas detector tube with a glass syringe (2H, Gastec Co., Ltd.) at a measurement point (MP2). Molecular sieve zeolite 13X (APG-III, Union Syowa K. K.), which is spherical with a diameter of 2 mm, is used as the adsorbent. The main components of the adsorbent are Na+ , Al2 O3 , and SiO2 . The volume that can be adsorbed on the adsorbent decreases with an increase in temperature. To evaluate the adsorption and desorption performances of the adsorbent, the gas temperature is measured every second at three points in the adsorption chamber (upstream, midstream, and downstream) using a thermocouple connected to a data logger (Model 8430, Hioki Electric Co.). During the desorption and reduction processes, NTP flow is generated using a plasma reactor and blower (roots-type blower; BSS25Gz; maximum flow rate

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Fig. 10 Schematic of the experimental setup

540 L/min at gauge pressure 10 kPa; Anrhet Corporation) to create a circulation flow path with a total channel capacity of 86.7 L. The direction of the circulating flow is illustrated in Fig. 10. The power applied to the plasma reactor is measured using a wattmeter (POWER HiTESTER 3333, accuracy ± 0.1%, Hioki Electric Co.). Under NTP flow, the CO2 adsorbed on the adsorbent is desorbed by heat and reduced mainly to CO by the following plasma chemical reaction (5): CO2 → CO +

1 O2 2

(5)

Reaction (5) requires energy (ΔH) > 2.9 eV/molecule, which is the negative value of the standard enthalpy of the reaction. The CO concentration in the pumped gas is measured using a gas detector tube (1H, GASTEC) with a glass syringe, and the concentration of O2 is measured using an oxygen analyzer (OXY-1S; galvanic cell method; accuracy = 0.1%; Ichinen Jiko Co., Ltd.) every 10 min. The height, width, and length of the plasma reactor are 100, 90, and 200 mm, respectively. It consists of 12 surface discharge elements (Fig. 11), with six on each of the left and right sides. The surface discharge elements consist of a tungsten discharge electrode on the surface of an alumina ceramic tube, and a counter electrode embedded inside the tube wall. NTP is generated by the surface discharge from the surface discharge elements. The plasma reactor is energized using a high AC voltage power supply with an input power of 300 W and frequency of 10 kHz. The discharge power in the plasma reactor is measured with an oscilloscope (DLM2054, Yokogawa

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Fig. 11 Detailed diagram of the discharge element

Electric Corporation) using a current probe (P6021, Tektronix, conversion factor = 10 A/V) and high-voltage probe (P6015A, Tektronix, conversion factor = 1 kV/V). The experiment is conducted over four cycles with 140 min for adsorption and 140 min for desorption. The conversion efficiency α of the reaction (5) indicates the ratio of CO2 that is reduced to CO and is defined by Eq. (6): α=

CO × 100 CO2 + CO

(6)

The energy efficiency η indicates the ratio of the input power used for CO2 reduction, and is defined by Eq. (1). In this system, E v is the specific input energy of the plasma (eV/molecule) [2], which can be calculated using Eq. (7): Ev =

P (W) 1.602×10−19 (J/eV) Q (L/min) 60 (sec/min)

×

1 22.4 (L/mol)

× 6.02 × 1023 (molecule/mol) ×

273.15 (K) T (K)

(7)

where P (W) is the input energy, Q (L/min) is the CO2 flow rate during adsorption, and T (K) is the ambient temperature. In the present study, the input energy is P = 300 W, the CO2 flow rate during the adsorption process is Q = 1.0 L/min, and the environmental temperature is T = 20 °C = 293.15 K, which yields a specific input energy of E v = 4.49 eV/molecule. Notably, the experiment is conducted so that the CO concentration does not exceed 12.5% of the lower explosive limit.

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4.2 Experimental Results and Discussions 4.2.1

Experimental Evaluation of Gas Types and Concentration Ratios

Discharged forms vary when different gases are used [22–25]. Several studies have experimented with CO2 and Ar/He mixtures to achieve high efficiency [26–28]. Ramakers et al. [20] reported that the CO2 conversion in DBD is higher for a mixed gas of CO2 and Ar than that for a mixed gas of CO2 and He, when the CO2 concentration is below 30%. Xu et al. [21] also achieved high energy efficiency by diluting CO2 with Ar and N2 , and reducing it under atmospheric pressure in a barium titanatepacked-bed NTP flow reactor. Therefore, assuming reduction is performed by mixing Ar or He with exhaust gas (mainly N2 and CO2 ), after moisture removal, experiments are conducted using Ar/He, N2 , and CO2 mixed gases to simulate exhaust gas to evaluate the effects of gas type and concentration ratios on CO2 reduction performance. During the adsorption process, (a) N2 = 9.0 L/min, (b) Ar = 1.0 L/min and N2 = 8.0 L/min, (c) Ar = 4.5 L/min and N2 = 4.5 L/min, (d) Ar = 8.0 L/min and N2 = 1.0 L/min, (e) He = 4.5 L/min and N2 = 4.5 L/min, and (f) He = 8.0 L/min and N2 = 1.0 L/min, are mixed with CO2 = 1.0 L/min, and used as the simulated exhaust gas for each condition. The simulated exhaust gas flows into an adsorption chamber with a volume of 1.8 L, CO2 is adsorbed using 1.2 kg of zeolite as the adsorbent, and the remainder is exhausted. Notably, condition (a) is not performed in this experiment, but the results of a previous study using the same experimental setup [5] are used. The typical time-dependent CO2 concentrations during the adsorption process (under condition (a)) are shown in Fig. 12. The upstream CO2 concentration is approximately 10% when the total flow rate is 10 L/min, while the CO2 flow rate is 1.0 L/min. In the first cycle as shown in the figure, the downstream CO2 concentration increases gradually with the elapsed time and approaches 10% because the adsorbent is fresh with a sufficient CO2 adsorption capacity. However, after the second cycle, the downstream CO2 concentration increases rapidly and reaches 10% after approximately 30 min, indicating a plateau. Similar results are obtained under other experimental conditions. During the desorption process, the plasma reactor generates NTP at an input power of 300 W, and a circulating flow of 540 L/min is generated by the blower. The time-dependent CO2 , CO, and O2 concentrations during the desorption process under conditions (a)–(f) are shown in Fig. 13a–f. In the first cycle, the CO2 concentration increases with the elapsed time and then plateaus. After the second cycle, the CO2 concentration shows a trend similar to that during the first cycle, but plateaued at a value higher than that in the first cycle. The CO2 concentrations during the desorption process are 16, 16, 17, 17, 18, and 18% at 140 min under conditions (a)–(f). The CO concentration increases with the elapsed time, averaging 4.1, 2.6, 2.5, 3.0, 3.7, and 2.3% at 140 min under conditions (a)–(f), respectively. The O2 concentration monotonically increases, averaging 3.4, 4.2, 5.3, 3.1, 3.3, and 5.0% at 140 min under conditions (a)–(f). Notably, the first and second cycles of experimental condition

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Fig. 12 Time-dependent CO2 concentrations during the adsorption process under condition a N2 flow rate at 9.0 L/min

(c) are excluded from this discussion because the O2 concentration increases significantly owing to outside air contamination caused by a defect in the experimental setup. During CO2 reduction (reaction (1)), CO2 is the reactant and CO and O2 are the products. The rate of the chemical reaction is more rapid under higher concentrations of reactants and slower under higher concentrations of the products. Therefore, a higher concentration of O2 suppresses the forward reaction, resulting in lower CO concentrations. The adsorbent temperature is increased until 10 min into the desorption process and then plateaus. In the first cycle, the adsorbent temperatures at 140 min of the desorption process are 54.7, 54.0, 52.6, 49.7, and 55.2 °C under conditions (b)–(f), respectively. Notably, under condition (a), the temperature is not measured. Similar results are obtained for the second cycle. The relationship between the ratio of gases Ar/(Ar + N2 ), He/(He + N2 ), and the conversion and energy efficiencies are shown in Fig. 14. The plot shows the average value of four conversion cycles, with the energy efficiency at 140 min. The error bars in the figure correspond to the standard deviations of the four cycles. The maximum conversion efficiencies are 20, 14, 13, 18, 14, and 11% for conditions (a)– (f), respectively. The energy efficiency is proportional to the conversion efficiency for a constant applied NTP energy and CO2 flow rate, and 13, 9, 8, 12, 9, and 7% for conditions (a)–(f), respectively. When using the Ar NTP flow, the efficiency increases when the value of Ar/(Ar + N2 ) is 0.50. When using the He NTP flow, the efficiency increases as the flow of He/(He + N2 ) decreases. When using the N2 NTP flow, filamentary discharges occur in the plasma reactor [22, 23]. As filamentary discharges are non-uniform, the number of CO2 molecules irradiated by the NTP flow decreases, but the electrons are concentrated and the CO2 molecules are strongly reduced, as follows: e− + CO2 → e− + CO∗2 → CO + O

(8)

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Fig. 13 Time-dependent CO2 , CO, and O2 concentrations during the desorption process (flow rate = 540 L/min) under conditions (a–f), respectively

where CO2 * represents CO2 radicals and e− represents electrons. When using the Ar NTP flow, in addition to reaction (8), charge transfer from Ar to CO2 occurs, as follows: + − Ar+ + CO2 → Ar + CO+ 2 , CO2 + e → CO + O

(9)

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Fig. 14 Relationship between the ratio Ar/(Ar + N2 ) and He/(He + N2 ) ratios, conversion, and energy efficiencies

resulting in glow discharge in the plasma reactor [24]. Because glow discharge occurs uniformly, the number of CO2 molecules affected by the NTP flow is high, although there is no region with an extremely high electron density. When Ar/(Ar + N2 ) reaches 0.50, some electrons are dispersed and the number of streamers decreases; thus, the advantages of filament discharge cannot be realized. In addition, the abundance of electrons is not sufficient to reduce CO2 molecules; therefore, the advantages of glow discharge cannot be realized. When using the He NTP flow, in addition to reaction (8), the charge transfer from He to CO2 occurs as follows: + − He+ + CO2 → He + CO+ 2 , CO2 + e → CO + O

(10)

resulting in glow discharge in the plasma reactor [24, 25]. He has a higher metastable excitation energy and is more likely to generate glow discharge than Ar. Therefore, the efficiency when the He/(He + N2 ) ratio is 0.50 is higher than that when the Ar/(Ar + N2 ) ratio is 0.50. However, He has high ionization energy and produces fewer electrons than Ar or N2 . Therefore, when He/(He + N2 ) is increased to more than 0.50, the glow discharge becomes weaker, thus decreasing the efficiency.

4.2.2

Experiments with the Introduction of Catalysts

In the next stage, a CO2 plasma-catalyzed fuel processing system is researched and developed. We have conducted exploratory research on zero CO2 emissions from GTCC natural gas power generation by applying plasma reduction technology. It is necessary to determine how the efficiency of CO2 nonthermal plasma catalysis reduction technology can be improved at atmospheric pressure and room temperature. Most mainstream CO2 reduction methods use metal catalysts, which are substances that promote specific chemical reactions and are widely used in a variety of applications because they do not degrade during the reaction. Most CO2 reduction catalysts contain precious metals such as ruthenium, silver, platinum, and rhenium

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[29–32], and their high economic costs poses a challenge for their widespread application. Some CO2 reduction catalysts without precious metals, and with copper as the base metal, can provide high selectivity and reduction performance [33–36]. However, the use of catalysts generally requires CO2 to be heated to high temperatures [37]. In addition, catalyst-only reduction methods are currently in their infancy because of the high activation energy of the catalyst and its low conversion efficiency at atmospheric pressure and room temperature. However, there have been reports of the plasma catalytic effect, wherein the catalyst can be activated by the action of NTPs. In particular, plasma application to copper-doped alumina has been shown to improve the CO2 treatment performance [38]. Applying NTP on CO2 directly, as well as on the copper catalyst, allows reduction to be performed by both NTP and the catalyst, enabling high CO2 reduction performance at room temperature and atmospheric pressure. In the adsorption process, 1.2 L of the downstream side of the 1.8 L adsorption chamber is filled with zeolite as the adsorbent, and 0.6 L of the upstream side is filled with a catalyst: under conditions with (I) zeolite, (II) γ alumina, (III) copper, and (IV) copper-doped alumina, adsorption is conducted with a simulated exhaust gas mixture, N2 = 9.0 L/min and CO2 = 1.0 L/min. Notably, condition (I) is not performed in this experiment; rather, the results of a previous study using the same experimental setup [39] are adopted. The time-dependent CO2 concentrations during the adsorption process exhibit almost the same trend under all the conditions (Fig. 12). However, the volume of adsorbed CO2 is comparatively lower under conditions (II), (III), and (IV) (Fig. 12). This is because in conditions (II), (III), and (IV), the amount of adsorbent in the adsorption chamber is low, two-thirds of the amounts under other conditions. During the desorption process, the plasma reactor generates NTP at an input power of 300 W and a circulating flow rate of 90 L/min is generated by the blower. The circulation flow rate is decreased to 90 L/min to increase the temperature and activate the catalyst by adding simulated exhaust gas to the plasma reactor at a lower flow rate. The time-dependent CO2 , CO, and O2 concentrations under conditions (I)–(IV) are shown in Fig. 15a–d. The CO2 concentration increases rapidly with time from an initial value of 10% and eventually plateaus. The CO2 concentration during the desorption process average 25, 20, 19, and 19% at 140 min under conditions (I)–(IV), respectively. Under condition (I), most of the CO2 is desorbed because the entire adsorption chamber is filled with zeolite. In condition (II), not only is CO2 desorbed from the zeolite, but a small amount of CO2 adsorbed on the γ-alumina is desorbed. Under conditions (III) and (IV), the volume of desorbed CO2 is less than that under condition (II) because copper and copper-doped alumina have no adsorption effect. The CO concentration increases with the elapsed time, averaging 4.7, 3.8, 3.0, and 4.0% at 140 min under conditions (I)–(IV), respectively. The O2 concentrations increase monotonously, averaging 3.7, 3.1, 5.6, and 5.4% at 140 min under conditions (I)–(IV), respectively. Condition (I) had the highest CO2 concentration under all conditions, thus promoting the reduction reaction and achieving the highest CO concentration. Under condition (II), the CO2 concentration is lower than that under condition (II); therefore, the CO concentration is also lower than that

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under condition (I). Under condition (III), the CO2 concentration is lower than that in condition (II), and the O2 concentration is higher, so that the reduction reaction is suppressed and the CO concentration is lower than that in condition (II). In other words, under conditions (I), (II), and (III), the CO concentration can be explained based on the relationship between the CO2 and O2 concentrations. However, condition (IV) results in a higher CO concentration than condition (II), despite the low CO2 and high O2 concentrations. This result cannot be explained based only on the relationship between CO2 and O2 concentration, indicating that the copper-doped alumina acts as a catalyst to promote the reduction reaction. The adsorbent and catalyst temperatures increase for 10 min in the desorption process and then plateau. In the first cycle, the adsorbent temperatures at 140 min in the desorption process are 79.5, 75.9, 78.6, and 82.5 °C under conditions (I)–(IV), respectively. The catalyst temperatures at 140 min of the desorption process are 84.9, 76.5, 62.9, and 83.5 °C under conditions (I)–(IV), respectively. Similar results are obtained for the second cycle. Temperature measurements (Fig. 15) indicate that zeolite, γ-alumina, and copper does not function as catalysts at temperatures as low

Fig. 15 Time-dependent CO2 , CO, and O2 concentrations during the desorption process (flow rate = 90 L/min) under conditions (I)–(IV)

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Fig. 16 Conversion and energy efficiencies for each type of catalyst (zeolite, γ alumina, copper, and copper-doped alumina)

as 80 °C. Conversely, Cu-doped alumina still acts as a catalyst at 80 °C, suggesting that it is activated by NTP flow. The conversion and energy efficiencies for each type of catalyst (zeolite, γ-alumina, copper, and copper-doped alumina) are shown in Fig. 16. The average conversion and energy efficiencies of the four cycles at 140 min are shown along the top of the bars. The error bars in the figure correspond to the standard deviations of the four cycles. The average conversion efficiencies of zeolite, γ-alumina, copper, and copper-doped alumina are 16, 16, 13, and 17% at 140 min, respectively. The energy efficiencies are proportional to the conversion efficiencies in the present study, where the input NTP energy and CO2 flow rate during vthe adsorption process are constant, with averages of 10, 10, 9, and 11%, respectively, at 140 min. Condition (I) has the highest CO concentration; however, this is because of the high CO2 concentration. The CO concentration is not high relative to the CO2 concentration, resulting in almost the same efficiency as that in condition (II). Condition (III) has the lowest efficiency owing to the high O2 concentration, which suppresses the reduction reaction, and a low CO concentration relative to the CO2 concentration. Condition (IV) has the highest efficiency because the reduction reaction is promoted by the catalytic action of Cu-doped alumina, and the relatively high CO2 concentration.

4.2.3

Analysis of Adsorbent Deterioration and Atomic Carbon Generation

Some of the CO produced by CO2 reduction is reduced further into atomic C via reaction (11). CO → C +

1 O2 2

(11)

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Previous studies have only considered the reduction of CO2 into CO, which may underestimate the conversion energy efficiency. Therefore, we measured the mass of atomic C and calculated its conversion and energy efficiencies by considering the reduction of CO into atomic C. The analyses were performed by an analytics company (Union Showa Co.). The resulting measurements of the mass of atomic C in the experiments that evaluated the gas types and concentration ratios are listed in Table 1. Notably, under condition (a), the atomic C is not measured. Under conditions (b)–(f), 0.5, 0.6, 0.8, 0.7, and 0.6 g of atomic C is existed in 100 g of adsorbent after the four cycles. Therefore, the possible CO concentrations considering the reduction of CO into atomic C are 5.9, 6.3, 9.0, 7.8, and 6.2% for conditions (b)–(f), respectively. The possible conversion efficiencies are 26, 26, 34, 30, and 25%, and the energy efficiencies are 17, 17, 22, 19, and 16%, respectively. The reason for the greatest abundance of atomic C under condition (d) is that a uniform glow discharge occurs in the plasma reactor, with a high probability for the CO molecules obtained by CO2 reduction to be acted upon again by the NTP flow before the gas exits the plasma reactor. Although glow discharge also occurs when the He NTP flow is used, the probability for the CO molecules to be reworked by the NTP flow is low because He has a high ionization energy and the number of free electrons generated by discharge is low. As the detected atomic C increases with an increase in Ar/(Ar + N2 ) ratio, it is likely that the N2 = 9.0 L/min condition results in less atomic C being detected than those under other Ar concentrations. This is due to filamentary discharges being non-uniform, which reduces the probability for the reworking of CO molecules by the NTP flow, thus decreasing the efficiency of reducing low concentrations of CO into atomic C. Table 1(ii) shows the mass of atomic C in the experiments with the introduction of catalysts. Notably, atomic C is not measured under condition (I). Under conditions (II)–(IV), 0.66, 0.81, and 0.71 g of atomic C is existed in 100 g of adsorbent, while 0.21, 0.00, and 0.12 g of atomic C exists in 100 g of the catalyst after four cycles. Thus, the possible CO concentrations, considering the reduction of CO into atomic C, are 7.2, 6.7, and 7.5% under conditions (II)–(IV), respectively. The possible conversion efficiencies are 25, 24, and 27%, and the energy efficiencies are 16, 16, and 17%, under conditions (II)–(IV), respectively. The relationships between conversion and energy efficiency obtained in previous studies [5, 40] and in the present study are depicted in Fig. 17. In experiments evaluating gas types and concentration ratios, the best results, with maximum conversion and energy efficiencies, are 19 and 12%, respectively, which are obtained under the following flow conditions: Ar = 8.0 L/min, N2 = 1.0 L/min, and CO2 = 1.0 L/min. The efficiencies are similar to the results of previous studies on DBD, thermal plasma, and gliding arcs. When reduction to both CO and atomic C is considered, the maximum conversion and energy efficiencies improve to 34 and 22%, respectively, which is better than the results obtained with other methods. Conversely, in the experiments with the introduction of catalysts, the optimal conditions lead to maximum conversion and energy efficiencies of 17 and 11%, respectively, which are obtained with the use of copper-doped alumina. When reduction into both CO and atomic C is considered, the maximum conversion and energy efficiencies improve to

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Table 1 Measurement results for atomic C mass (i) The mass of atomic C in experiments evaluating gas types and concentration ratios Flow condition

Detected atomic C per adsorbent of 100 g (g)

(b) Ar = 1.0 L/min, N2 = 8.0 L/min

0.5

(c) Ar = 4.5 L/min, N2 = 4.5 L/min

0.6

(d) Ar = 8.0 L/min, N2 = 1.0 L/min

0.8

(e) He = 4.5 L/min, N2 = 4.5 L/min

0.7

(f) He = 8.0 L/min, N2 = 1.0 L/min

0.6

(ii) The mass of atomic C in experiments with the introduction of catalysts Catalyst condition

Detected atomic C per adsorbent or catalyst of 100 g (g) Adsorbent

Catalyst

(II) γ-alumina

0.66

0.21

(III) Copper

0.81

0.00

(IV) Copper dopped alumina

0.71

0.12

27 and 17%, respectively. The results are inferior to those obtained under the aforementioned conditions of Ar = 8.0 L/min, N2 = 1.0 L/min, and CO2 = 1.0 L/min; however, the performance is superior to that observed in previous studies. In the experiments evaluating gas types and concentration ratios, the maximum energy yields are 10.6, 6.4, 6.0, 9.4, 8.0, and 5.7 g(CO2 )/kWh under conditions (a)–(f), respectively. In the present study, the CO2 concentration during desorption is approximately 20%, and if the concentration is 100%, the energy yield Fig. 17 Relationships between conversion and energy efficiency obtained in various studies

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increases to 53, 32, 30, 47, 40, and 28 g(CO2 )/kWh, respectively. Similarly, in experiments with catalysts introduced, the maximum energy yields are 11.6, 9.4, 7.8, and 9.7 g(CO2 )/kWh under conditions (I)–(IV), respectively; if the CO2 concentration during the desorption process is 100%, the energy yields will increase to 58, 47, 39, and 48 g(CO2 )/kWh, respectively. The latest LNG GTCC has an overall efficiency of 64%. To achieve zero CO2 emissions in the process, an energy efficiency of 49% must be achieved for reduction [5]. When the CO2 concentration is 100%, an energy yield of 18 g(CO2 )/kWh is required to achieve an energy efficiency of 49%. This indicates that a CO2 zero-emission system for power plants can be achieved by further increasing the adsorbed and desorbed CO2 concentrations. In the present study, experiments are conducted using dry gas, assuming that the moisture in the exhaust gas emitted from the GTCC should be first removed. In industrial applications, it is desirable to remove moisture from the exhaust gas using an exhaust gas dehumidifier or a similar device. In addition, because the technology circulates CO2 , it is practical to reuse noble gases, such as Ar and He, to reduce CO2 emissions.

5 Conclusions Part of this project’s research and development comprises a grand plan that aims to address and prevent future global warming by facilitating the achievement of the zero CO2 emissions targets. In such a context, research on gas turbine combined cycle (GTCC) natural gas power generation by adoption of CO2 plasma-catalyzed fuel processing technologies are highly relevant. Compared with thermal processes, nonthermal plasma leads to greater CO2 desorption and its conversion into fuel. Thus, research and development, in parallel with basic experiments, analyses, actual performance tests, and improvements of plasma reactors, adsorption systems, and CO2 concentration systems, are necessary to achieve the zero CO2 emissions targets. The technologies mentioned herein have so far been tested at small-scale in laboratories. Future tests will be conducted on slightly scaled bench-scale machines to enable further process development and examination of the novel technologies. We plan to introduce the technology extensively into industries in Japan and promote its expansion into evidential research facilities. These companies are world leaders in the development of high-temperature gas turbines, and adopt CO2 capture technologies based on physical adsorbents. According to the results of the present study, it may be possible to achieve zero CO2 emissions in GTCC thermal power plants using plasma reduction technologies. Although such research encounters substantial technical difficulties, the development of novel techniques could be a game changer for the realization of a zero-carbon society in the current context of global warming.

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References 1. Komori T, Shiozaki S, Yamagami N, Kitauchi Y, Akizuki W (2007) CO2 emission reduction method through various gas turbine fuel applications. Mitsubishi Heavy Ind Ltd, Tech Rev 44(1):1–5 2. Okubo M (2021) Recent development of technology in scale-up of plasma reactors for environmental and energy applications. Plasma Chem Plasma Process 42:3–31. https://doi.org/10. 1007/s11090-021-10201-7 3. Okubo M, Kuwahara T (2019) New technologies for emission control in marine diesel engines. ISBN: 9780128123072, Elsevier, p 296 4. Yamasaki H, Kuroki T, Okubo M (2020) Another author, adsorbed CO2 dissociation using argon and helium nonthermal plasma flows. IEEE Trans Ind Appl 56:6983–6989 5. Okubo M, Takahashi K, Kamiya S, Kuroki T (2018) High-efficiency carbon dioxide reduction using nonthermal plasma desorption. IEEE Trans Ind Appl 54:6422–6429 6. Spencer LF, Gallimore AD (2011) Efficiency of CO2 dissociation in a radio-frequency discharge. Plasma Chem Plasma Process 31:79–89 7. Morimoto K, Matsumura Y, Iijima T, Wakazono S, Kataoka M, Yuri M (2021) Validation results of 1650 ºC class JAC gas turbine at T-point 2 demonstration plant. Mitsubishi Heavy Ind Tech Rev 58(1):1–12 8. Hanna R, Abdulla A, Xu Y, Victor DG (2021) Emergency deployment of direct air capture as a response to the climate crisis. Nat Commun 12:368. https://doi.org/10.1038/s41467-020-204 37-0 9. Okubo M, Kuroki T, Yamada H, Yoshida K, Kuwahara T (2017) CO2 Concentration using adsorption and nonthermal plasma desorption. IEEE Trans Ind Appl 53:2432–2439 10. Oki Y, Hamada H, Kobayashi M, Nakao Y, Hara S (2016) Development of high-efficiency oxy-fuel IGCC system. Mech Eng J 3(5):16-00351 (total 6 pages) 11. Mitsubishi Power Ltd. (2022) Press Release. https://power.mhi.com/news/20210419.html. Accessed 29 May 2022 12. Hashimoto T, Sakamoto K, Ishii H, Fuji T, Koyama Y (2010) Commercialization of clean technology with CO2 recovery. Mitsubishi Heavy Ind Tech Rev 47(1):9–14 13. Giuffrida A, Romano MC, Lozza F (2011) Thermodynamic analysis of air-blown gasification for IGCC applications. Appl Energy 88:3949–3958 14. Sato M, Hasegawa T (2001) Denchuken Rev 44:74–81, Chapter 5 (in Japanese) 15. Hasegawa T (2006) Development of gas turbine combustor for utilizing various gasified fuels with high-efficiency and minimal pollutant emissions. J Combust Soc Jpn 48(146):46–61 (in Japanese) 16. Walsh PM (1979) A review of ammonia and hydrogen cyanide concentrations in low and medium-btu coal gases, Contract No. EF-77-S-01-2762. Princeton University, Princeton, NJ, USA 17. Amamoto M, Takahara M, Fujii T, Kumagai T (2019) Current status of integrated coal gasification combined cycle projects. Mitsubishi Heavy Ind Tech Rev 56(3):1–6 18. Moioli S, Giuffrida A, Romano MC, Pellegrini LA, Lozza G (2016) Assessment of MDEA absorption process for sequential H2 S removal and CO2 capture in air-blown IGCC plants. Appl Energy 183:1452–1470 19. Shidao (2015) Outline of the OSAKI Coolgen Project. In: Proceedings of international conference on power engineering 15 (ICOPE-15), Yokohama, ID: ICOPE-15–1034 (total 7 pages) 20. Wakimoto H, Yamasaki H, Kuroki T, Okubo M (2022) Effect of argon and helium concentrations on adsorbed CO2 dissociation using nonthermal plasma flow. Int J Plasma Environ Sci Technol 16(1):e01006 21. Wakimoto H, Yamasaki H, Kuroki T, Okubo M High-efficiency carbon dioxide reduction using catalytic nonthermal plasma desorption. Mech Eng J (submitted) 22. Takaki K, Fujiwara T, Tochikubo F (2003) Production of atmospheric-pressure glow discharge. J Plasma Fusion Res 79(10):1002–1008 (in Japanese)

48

H. Yamasaki et al.

23. Gherardi N, Gouda G, Gat E, Ricard A, Massines F (2000) Transition from glow silent discharge to micro-discharges in nitrogen. Plasma Sour Sci Technol 9(3):340–346 24. Kogoma M (2003) Generation of atmospheric-pressure glow discharge and its applications. J Plasma Fusion Res 79(10):1000–1001 (in Japanese) 25. Tochikubo F, Chiba T, Watanabe T (1999) Structure of low-frequency helium glow discharge at atmospheric pressure between parallel plate dielectric electrodes. Jpn J Appl Phys 38(9):5244– 5250 26. Ramakers M, Michielsen I, Aerts R, Meynen V, Bogaerts A (2015) Effect of argon or helium on the CO2 conversion in a dielectric barrier discharge. Plasma Process Polym 12(8):755–763 27. Xu S, Chansai S, Shao Y, Xu S, Wang Y, Haigh S, Mu Y, Jiao Y, Stere CE, Chem H, Fan X, Hardacre C (2020) Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide. Appl Catal B: Environ 268(118752):1–12 28. Willems G, Hecimovic A, Sgonina K, Carbone E, Benedikt J (2020) Mass spectrometry of neutrals and positive ions in He/CO2 non-equilibrium atmospheric plasma jet. Plasma Phys Control Fusion 62(034005):1–12 29. Ge H, Kuwahara Y, Kusu K, Yamashita H (2021) Plasmon-induced catalytic CO2 hydrogenation by a nano-sheet Pt/Hx MoO3-y hybrid with abundant surface oxygen vacancies. J Mater Chem A 9:13898–13907 30. Kumagai H, Nishikawa T, Koizumi H, Yatsu T, Sahara G, Yamazaki Y, Tamaki Y, Ishitani O (2019) Electrocatalytic reduction of low concentration CO2 . Chem Sci 10:1597–1616 31. Pang R, Teramura K, Morishita M, Asakura H, Hosokawa S, Tanaka T (2020) Enhanced CO evolution for photocatalytic conversion of CO2 by H2 O over Ca modified Ga2 O3 . Commun Chem 3(137):1–8 32. Yamada K, Ogo S, Yamano R, Higo T, Sekine Y (2020) Low-temperature conversion of carbon dioxide to methane in electric field. Chem Lett 49(3):303–306 33. Makimura JI, Higo T, Kurosawa Y, Murakami K, Ogo S, Tsuneki H, Hashimoto Y, Sato Y, Ishitani O (2021) Fast oxygen ion migration in Cu-ln-oxide bulk and its utilization for effective CO2 conversion at lower temperature. Chem Sci 12:2108–2113 34. Song Y, Hensley DK, Bonnesen PV, Liang L, Wu Z, Meyer HM, Chi M, Sumpter BG, Rondinone AJ (2016) High-Selectivity electrochemical conversion of CO2 to ethanol using a copper nanoparticle/N-doped graphene electrode. Chem Select 1:6055–6061 35. Watari R (2014) The trend of catalyst technologies related to carbon dioxide transformation. Environ Sci Res Lab Rep 13006:1–19 (in Japanese) 36. Wu Y, Iwase K, Harada T, Nakanishi S, Kamiya K (2021) Sn atoms on Cu nanoparticles for suppressing competitive H2 evolution in CO2 electrolysis. ACS Appl Nano Mater 4:4994–5003 37. Jwa E, Lee SB, Lee HW, Mok YS (2013) Plasma-assisted catalytic methanation of CO and CO2 over Ni-zeolite catalysts. Fuel Process Technol 108:89–93 38. Brune L, Ozkan A, Genty E, Visart de Bocarme T, Reniers F (2018) Dry reforming of methane via plasma-catalysis: influence of the catalyst nature supported on alumina in a packed-bed DBD configuration. J Phys D Appl Phys 51(23):234002 39. Kamiya S (2018) Reduction of carbon dioxide by argon and nitrogen nonthermal plasmas. Master’s thesis in fiscal year 2017, Department of Mechanical Engineering, Osaka Prefecture University, pp 1–49 (in Japanese) 40. Li J, Zhang X, Shen J, Ran T, Chen P, Yin Y (2017) Dissociation of CO2 by thermal plasma with contracting nozzle Quenching. J CO2 Util 21:72–76

Colloidal Technologies for Heat Energy Recovery M. Bevione, L. Cecchini, E. Garofalo, S. A. Suslov, and A. Chiolerio

Abstract This chapter provides an overview of the physics, materials and devices conceived so far for the conversion of waste heat into power, particularly based on the Colloidal Energetic Systems concept. Colloids can be used exploiting their huge variety of phenomena embedded at the nanoscale, to convert low grade energy into electrical power, by exploiting thermomagnetic advection, triboelectric, pyroelectric, photothermal, thermoelectric and thermogalvanic effects.

1 An Introduction: The Concept of Colloidal EneRgEtic System The huge availability of low-grade, low-enthalpy sources, i.e. those below 200 ◦ C that typically are discarded in industrial processes dealing with transformation of raw materials, energy production and even computation, poses a big question on if and how they could be harvested [1]. This question, based on solid opportunities, still opens a big debate about the economic feasibility, where thermodynamic cycles currently represent the most explored solution though their high maintenance costs are reflected in a very low degree of adoption of the technology and less important maturity level. Therefore research on alternative approaches has led to the development of M. Bevione Laboratory for High Performance Ceramics—Empa. Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland Laboratory of Nanoscience for Energy Technologies (LNET), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland L. Cecchini · E. Garofalo · A. Chiolerio (B) Center for Convergent Technologies—Bioinspired Soft Robotics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy e-mail: [email protected] S. A. Suslov Department of Mathematics, Swinburne University of Technology, H38, PO Box 218, Hawthorn, VIC 3122, Australia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 D. Borge-Diez and E. Rosales-Asensio (eds.), Heat Energy Recovery for Industrial Processes and Wastes, Green Energy and Technology, https://doi.org/10.1007/978-3-031-24374-5_3

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new concepts, including what was baptised Colloidal EneRgEtic System (CERES), a system entirely based on a liquid state active core (see for example Fig. 1), that exchanges only thermal or electromagnetic energy with the environment, and which is contained inside a skin transparent to the form of energy that is being exchanged, and protected against the environment and against diffusion of its constitutive components toward the outside [2]. Why liquid? Because a liquid is reconfigurable and intrinsically fault-tolerant, suitable for creating the next generation of autonomous devices [3], and complex phenomena like those described in the following sections, can be enabled simply working on the composition of the liquid suspension/solution. Relevant to the CERES paradigm is that owing to the reconfigurability of internal structures within a liquid body, the system is also intrinsically adaptable to fluctuations of the external source of power, making them particularly interesting for practical applications where the low enthalpy waste is not continuous in flux/heat density. In the following text of this chapter, we will base our discussion on some assumptions/concepts: • a Liquid features no shape retention and an almost perfect volume retention, when undergoes any pressure variation; it still belongs to the condensed matter studies; • a Fluid is a state of matter such that applied shear forces result in a flow without compromising the continuity of matter, and includes liquids and gases; • a Colloid is between completely homogeneous systems (solutions) and completely heterogeneous systems (suspensions), being composed by a fine dispersion of solids in a fluid solvent; we will focus on solid dispersion inside a liquid solvent; • a Smart fluid is a liquid featuring smart functionalities such as memory and information processing capabilities [4], sensing capabilities, movement, and energy capabilities, including conversion and storage; • a Ferrofluid is a colloid featuring magnetic properties, particularly superparamagnetism and/or ferromagnetism. In order to describe the behaviour of a CERES in the most general form possible, we might suggest that a complete and closed set of equations includes ferrohydrodynamics (kinematics and kinetics) of a magnetically (or electrically) charged liquid, plus thermal diffusion equations [5]. We know that nanoscale matter follows the fluctuation-dissipation theorem, where local entropic reductions are possible following a well known probability distribution [6]. Moreover, quantum mechanics plays also a relevant role at the nanoscale, where confinement effects are experienced: quantum thermodynamics considers Markovian processes, where no dependence upon previous system history can be found [7]. We are able to extract work by “destroying" a microstate (direct conversion between information and energy) and also to increase the amount of work extracted, thanks to the non-Markovianity of a process [8]. In the following parts, we will attempt to create a comprehensive summary of the most important physical properties of colloids, and describe devices for waste heat to power applications, following this order:

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Fig. 1 Visualisation of mixing and diffusion of liquids during perfusion in a two-phase flow experiments on a CERES featuring toroidal volume coupled to a hot tubing. The heat flow is oriented across the torus walls (see the cooling fins on the outer surface). Experimental setup by A. Chiolerio and E. Garofalo, CSFT-IIT, 2019

• basic physical properties of colloids, particularly the cinematic and thermal ones, paragraph no 2; • advanced physical properties of magnetic colloids, including ferrohydrodynamics and other complex phenomena, paragraph no 3; • materials for triboelectric generation, paragraph no 4; • materials for pyroelectric generation, paragraph no 5; • materials for photothermal and thermoelectric generation, paragraph no 6; • devices for the waste heat to power application based on the previous materials and effects, and additionally thermogalvanic and thermo-osmotic effects, paragraph no 7.

2 Cinematic and Thermal Properties of Colloids 2.1 Complexity Even if colloidal suspensions represent the simplest complex fluids, important questions in confining geometries and external fields are still open since soft matter reacts sensitively to external perturbations and stable colloids are rare in real case scenarios. The sensitivity of colloidal soft matter to external fields has been exploited empirically in many different applications but the systematic scientific understanding is still immature. In contrast to molecular condensed matter systems, working with colloids provides the fascinating possibility to control and tailor the external perturbation and

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Fig. 2 Schematic diagram of complexity: the x-axis shows the complexity of the system, the y-axis the complexity of the problem. The problems associated with different kinds of external field are indicated [9]

study their influence on scales of length and time, which are typically associated with the particle structure [9]. Distinguishing between the complexities of the colloid itself (complexity of the system) and that of the circumstances under which the colloid is investigated (complexity of the problem), it is possible to schematically visualize (Fig. 2) possible research directions (arrows in Fig. 2 parallel to x and y-axis). The simplest situation is a system composed of equally sized spherical particles in equilibrium corresponding to a one-component classical system with a radially symmetric pair potential in the absence of any external field. Problem complexity is enhanced analysing the same system in an external potential (confining geometries, magnetic and laser optical fields). Then, by turning from equilibrium to steadystate non-equilibrium problems and finally, to fully non-equilibrium situations with explicit time-dependent processes (shear flow, electric fields). On the other hand, system complexity is increased in a binary or multicomponent (polydisperse) mixture of spherical particles. Even more complexity arises from cylindrically symmetric (rod-like or plate-like) particle shapes featuring orientational degrees of freedom, mixtures of spheres/rods/plates, flexible particle shapes [9].

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2.2 Stability Colloidal phase systems are mainly characterized by their ability to suspend particles evenly in a carrier fluid, or rather to maintain stable homogeneity at thermal equilibrium. Considering a generic colloidal system there are two main effects that can guarantee the stability of the system: the Brownian motion, i.e. the random movement of fine particles in a carrier liquid and the steric or the ionic repulsion. Nevertheless, external perturbations tends to destabilize the suspension by agglomeration and sedimentation by internal or external forces. Therefore, a colloidal system is in a stable state if there is a balance of the energies associated to the stabilizing and destabilizing mechanisms.

2.2.1

How to Avoid Sedimentation

The thermal energy associated to colloidal system is modeled considering the vibrational energy of colloidal harmonic oscillators, where the suspended particles are assumed spherical and identical: ET = k B T ,

(1)

where k B is the Boltzmann constant (1.38 × 10−23 m2 kg s−2 K−1 ) and T is the temperature. The thermal energy is the expression of the Brownian motion that permits to the particles to move freely. At thermal equilibrium, sedimentation is present due to gravitational effects, where the effective potential energy ΔE p associated with the reduced weight (sum of the Archimedes forces and gravity) of a particles is: ) ( ΔE p = ρ p − ρb f V gh,

(2)

where ρ p is the density of nanoparticles (NPs), ρb f is the density of the base fluid (solvent), V is the volume of the spherical particle, g is the gravity constant and h is the relative height. To provide sufficient mixing is it simply necessary that the gravitational energy or the magnetic energy is lower than the thermal energy. It is then possible to define the critical diameter, i.e. the maximum diameter that the particle can have in order to be in a stable state due to gravitational effects at defined temperature T. [ ET > E p ,

D
ΔE H ,

2.2.2

D


min |E dd |,

D