Towards higher energy efficiency in future waste-to-energy plants with novel latent heat storage-based thermal buffer system

Renewable and Sustainable Energy Reviews 112 (2019) 324–337

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Towards higher energy efficiency in future waste-to-energy plants with novel latent heat storage-based thermal buffer system

T

H. Xua, W.Y. Linb, F. Dal Magroc, T Lid, X. Pye, A. Romagnolia,b,∗ a

Department of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Energy Research Institute at NTU, 1 CleanTech Loop, #06-04, 637141, Singapore c Polytechnic Department of Engineering and Architecture, University of Udine, via delle Scienze 206, 33110, Italy d Singapore Institute of Manufacturing Technology (SIMTech), 73 Nanyang Drive, Singapore, 637662 e PROMES CNRS Laboratory, UPR 8521, Rambla de La Thermodynamique Tecnosud, 66100, Perpignan, France b

ARTICLE INFO

ABSTRACT

Keywords: Phase change materials High temperature thermal energy storage Waste-to-Energy plant Transient thermodynamic Efficiency upgrade Dynamic thermal network

Energy efficiency of current Waste-to-Energy plants is mainly limited by high temperature corrosion combined with temperature fluctuation of flue gas. This paper introduces a technology based on Phase Change Materials in the combustion chamber and its contribution to higher overall electrical efficiency. This technology encapsulates aluminium alloy-based Phase Change Materials in ceramic bricks similar to traditional refractory bricks in the combustion chamber. The proposed brick allows steam superheating on waterwall by absorbing temperature fluctuations and delivering a higher heat flux. Two studies are carried out to realize the technology development from refractory bricks to waterwall system. Study One adopts Dynamic Thermal Network method to model the heat transfer on waterwall with and without the novel brick. Real plant information is used as boundary condition to locate the design points of the novel bricks. Study Two conducts experiment to validate the numerical model, and performs a transient analysis of the waterwall to compare the thermal dampening and superheating effect of Phase Change Material-based waterwall. From the result, there is a 10% improvement in energy conversion efficiency on the waterwall by introducing the novel technology. Lastly, this paper introduces an integration scheme of three types of Phase Change Materials-based bricks in the waterwall to achieve continuous superheating of steam. A 34% electrical efficiency can be achieved by producing over 600 °C of superheated steam with this new plant configuration. The result shows that this new technology is highly applicable and promising to upgrade the overall efficiency of Waste-to-Energy plants.

1. Introduction 1.1. Waste-to-energy as a key step towards circular economy In 2015, the United Nations Member States adopted a shared blueprint for the future world, and published “The 2030 Agenda for Sustainable Development”, where 17 Sustainable Development Goals (SDGs) were specified [1]. These 17 SDG, including the 7th SDG (to ensure access to affordable, reliable, sustainable and modern energy to all), and 12th SDG (to ensure sustainable consumption and production patterns), are constantly challenged by the rapid global urbanization which has led to an excessive usage of energy and material resources and a growing concern over huge amount of waste generation. According to the World Bank [2], the Municipal Solid Waste (MSW) generation worldwide is expected to double from 1.3 billion tons in 2010 to approximately 2.2 billion tons in 2025. While presently the ∗

countries in Organisation for Economic Co-operation and Development (OECD) are responsible for producing nearly half of the world’s waste, the lower-middle income countries are projected to experience a more momentous growing of waste generation due to their accelerating economic development [2]. With respect to the Sustainable Development Goals and the rising waste generation issues, the European Commission has initiated a flagship program aiming at transforming Europe into a resource efficient economy [3]. A clear pathway to increase resource production efficiency and to decouple economic growth from resource consumption and environmental impact is established. The concept of a “Circular economy” is identified to be the driving strategy to close the loop of material lifecycle and to keep energy, material and economic value of resources within the economy for as long as it can be [4]. Waste management, comprises a hierarchy of implementation strategies from material recycle to landfill [5], plays a pivotal role in building towards

Corresponding author. Department of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore. E-mail address: [email protected] (A. Romagnoli).

https://doi.org/10.1016/j.rser.2019.05.009 Received 3 January 2019; Received in revised form 17 April 2019; Accepted 3 May 2019 Available online 04 June 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

Greek letters

Acronyms

ε λ ρ σ

CFD DTN MSW OECD PCMs PID TES TNM WtE

Computational Fluid Dynamic Dynamic Thermal Network Municipal Solid Waste Organisation for Economic Co-operation and Development Phase Change Materials Proportional–integral–derivative Thermal Energy Storage Thermal Network Method Waste to Energy

Subscripts b c g i liq n p pcm r s sol trad w wi wo

Latin letters A C″ c h H L Q R T t

Emissivity Thermal conductivity (W/m K) Density (kg/m3) Boltzmann constant (m2 kg∙s−2∙K−1)

Area Heat capacitance (J/K∙m2) Heat capacity (J/kg∙K) Heat transfer coefficient (W/m2∙K) Steam enthalpy (kJ/kg) Thickness (m) Heat flow (W) Thermal resistance (m2∙K/W) Temperature (°C) Time (s)

the circular economy. In this hierarchy, waste recycling is defined as the priority over other options such as Waste-to-Energy (WtE) or landfill. As compared with landfill which requires huge space to dispatch, waste incineration is capable of reducing the solid waste volume up to 90%, making the next step of landfilling much less space consuming. Additionally, being capable of recovering the embedded energy in the waste helps to keep the energy value within the economy as far as possible [6]. A good example of waste incineration is the WtE plants which recover the energy in the form of power and provide electricity supply to the grid as a primary energy carrier. WtE incineration plants have come on the stage since the middle of 20th century. The first batch was introduced primarily for the purpose of reducing the rising waste volume. It was later discovered the huge potential of energy recovery, which fostered the more advanced incinerator designs. It is estimated that 500 kWhe/ton of energy from waste can be harvested with an average calorific value of 10 MJ/kg from the MSW [7]. With MSW identified as a renewable energy source,

Brick Convection heat transfer Flue gas Sections i on the waterwall Liquidus Total number of sections on the waterwall Steel pipe PCM-based brick Radiation heat transfer Steam Solidus traditional brick Wall Inner wall (next to steel pipe) Outer wall (next to flue gas)

WtE can be used as a primary energy carrier to offset fossil fuels and achieve carbon mitigation goals [8]. Up till now, there are more than two thousands WtE plants installed in the global scale which are mostly situated in East Asia, Europe and the US according to previous reviews [9] and reports by Statisca [10] and the International Solid Waste Association [11]. Despite their least participation in WtE market, Latin America, Africa and other non-OECD countries have the potential to contribute to a five-folds increase of the WtE primary energy share worldwide by 2050, as projected by the World Bank [10]. Another forecast by Grandview Research Consultancy estimated a 34 million USD of global WtE incineration market revenue in 2024 compared with 20 million USD in 2014, with a prospective 70% increase [13]. 1.2. Challenges to increase waste-to-energy plant electrical efficiency The promising market outlook has driven WtE incineration industry to continuously investigate necessary measures to increase the plant

Fig. 1. (a) Corrosion diagram of steel surface [14] (Permitted for reproduction); (b) Fluctuation of temperature inside of combustion chamber at 2s after secondary air injection in a typical Waste-to-Energy plant in Southern Italy [15] (Permitted for reproduction).

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electrical efficiency, such as adopting advanced combustion control, increasing steam temperature and pressure, and reducing the condenser temperature [6]. Increasing the steam temperature and pressure allows more energy to be recovered in the turbine, thus resulting in a higher electrical efficiency. However, this approach is hindered by two major difficulties in the plant operation, the high temperature corrosion on the ferrous tubes, and the fluctuation of combustion gas. High temperature corrosion occurs when the heat exchanger metal surfaces are exposed to the high temperature flue gas with a significant concentration of acidic chlorides and sulphates. In MSW, the chlorides come from the gaseous products of the combustion of polyvinyl chloride plastic and food residues, and sulphates are derived from paper products and plastics. Instead of direct corrosion attack from the flue gas, the high temperature corrosion is mostly attributed to the aggressive species (chlorides of alkali metals and heavy metals) deposited on the ferrous surfaces, which triggers a series of chemical reactions and corrodes the surfaces. The degree of corrosion depends on the flue gas composition, the flue gas temperature and the steam temperature. In order to operate the heat exchangers in a safe condition and avoid the corrosion, the steam temperature and gas temperature are both regulated within certain levels. One common practice is to limit the gas temperature around 350–550 °C and the steam temperature at 250–400 °C in the superheaters (see Fig. 1(a)). Subsequently, the steam turbine works with limited steam parameters which will constrain the net power efficiency of whole plant. Another limitation faced by the WtE plants is the temperature fluctuation of the combustion gas. The waste feedstock is characterized with an extensive variation of proximate composition and the energy content. Even with most advanced mixing at the refuse storage bunker before combustion, there still exists the fluctuation in heating characteristics and uneven boiler firing in the combustion chamber [13]. Fig. 1(b) presents a typical temperature profile of the combustion chamber, extracted from operational data from a WtE plant in Vittore, Italy [15]. The temperature of combustion gas may vary over 70 °C in a span of 10 min. The challenge comes when the fluctuation from the gas side is transferred to the steam generation, which gives undesirable fluctuation in the steam temperature. This will induce complication to the energy recovery system and lead to inefficient operation of steam turbine. The temperature fluctuation of the flue gas is also associated with the corrosion rate of the metal surfaces, which may aggregate by several folds [16]. Since the corrosion attack directly impedes the electrical efficiency and safety operation of plants, more researches have been dedicated to the mitigation measures than those for fluctuation issue. One common approach is to overlay Inconel alloy 625 cladding on metal surfaces in the corrosion prone areas, such as part of the waterwall not covered by refractory lining and the superheater tubes. The Inconel alloy 625 is a corrosion resistant metal with 21Cr-9Mo-3.5Nb-Ni base. It not only offers corrosion protection at certain temperature range, but also provides higher thermal conductivity which allows better heat transfer than common refractory material [8]. The main disadvantage is the unpredictable lifetime associated with the working temperature. It is recommended by Lee et al. [17] not to be installed for superheater surfaces above 420 °C. Another issue is the higher cost of Inconel alloy 625, which might contribute to a 40% of increase in the maintenance cost of super-heater bundle unit compared with bare bundle unit. Another technology which is often applied with the corrosion resistant alloys is the thermal spray coatings, including flame, arc, plasma, detonation, high velocity air-fuel, and high velocity oxy-fuel and cold spray. These methods are used to produce thick and dense coatings on the metal surfaces to decrease the surface porosity and increase the corrosion resistancy to the acidic gas and its deposits [17]. Since thermal spray coatings can employ a wide range of coating and substrate materials, it is quite versatile to be applied for different purposes. The cost of this technology varies depending on the methods and materials. However, high initial cost may incur due to expensive

apparatus. Similar to Inconel 625 alloy, the monolithic SiC concrete is currently employed for corrosion protection in some WtE plants which adopt high steam parameters to increase gross electric efficiency up to 30% (e.g. WtE plants in Stuttgart - Germany). By using monolithic SiC concrete, longer lifetime of superheaters can be achieved, given that regular inspections are ensured and direct repair of cracks are done before the tube itself shows corrosion attacks [18]. High maintainance cost is required with this technology installed. In addition to the strategy of employing protection layers on metal surfaces, some WtE researchers and engineers modified the plant configuration to address the issues. Two WtE plants in Schwanforf and Rosenheim in Germany installed radiant superheaters in the upper channel of the first boiler pass to make use of the high temperature of the flue gas [19]. In order to avoid corrosion, the super-heater tubes are protected with rear-ventilated tiles purged by sealing air. With this configuration, the Rosenheim WtE plant achieved steam parameter of 50 bars, 440-480 °C. Nonetheless, with higher steam operating parameters, it is increasingly difficult to regulate the super-heater output with the presence of fluctuations and to ensure the safety operation of the super-heater and the downstream equipment [20]. Another modification, featuring a prism-shaped body in the first boiler pass by Keppel Seghers, was designed to inject secondary combustion air from the nozzles embedded in the prism. The design allows an optimized mixing of the secondary combustion and was proven to produce a more uniform flue gas flow in velocity and temperature. A WtE plant in Netherlands was retrofitted with this prism design and is capable of generating the steam parameter of 100 bars and 400 °C. Follow-up research on this WtE plant indicated that this method was effective to suppress the corrosion phenomenon and to improve the quality of combustion, thus resulting in higher plant throughput and plant availability. Still, the installation and retrofitting always requires a long plant downtime, which may lead to lower economic return. Even though this prism design, to some extent, decreases the temperature fluctuation of flue gas, the fluctuation still exists due to the inhomogeneous nature of feedstock. Unlike the corrosion issue which has been addressed with efforts, there is much less improvement in the design to deal with the fluctuation issue through the years. It is commonly addressed by means of advanced mixing of feedstock, proportional–integral–derivative (PID) control or regulating the combustion process. Nonetheless, the fluctuation in steam production cannot be fully avoided even with the best control system installed [21]. In general, most of the technologies discussed above are primarily concerned of combating the corrosion issue. Prism-shaped body in the first boiler pass is quite promising as it re-distributes the combustion air, producing relatively more uniform flue gas temperature, but the fluctuation arising from the varying waste composition still exists. There still awaits the technology innovation which is able to address both the corrosion and the fluctuation issues simultaneously, and as well enhance the heat transfer of the boiler so as to push the steam parameters to even higher level. 1.3. Phase Change Materials as promising thermal buffer media In terms of absorbing thermal fluctuations, the Phase Change Materials (PCMs) undoubtedly offers a good solution, since they are able to absorb and release a large amount of latent heat during the phase change process while remaining at a nearly constant temperature [22]. This attractive characteristic has allowed PCMs to be applied in a wide range of temperature fluctuation scenarios, such as building thermal comfort management [23] and air conditioning [24], temperature regulating for solar photovoltaic [25] and solar thermal panels [26], and electronic thermal management [27]. In the high temperature PCMs range, molten salts in the form of packed-bed or shell-and-tube storage tanks are commercially available solutions to Concentrating Solar Power (CSP) plants [28]. Recently, metal alloys have been drawing researchers’ attention because of their competitive storage 326

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Fig. 2. Concept of Phase Change Materials-based refractory brick on the waterwall

capacity, high density, and appealing thermal conductivity [29]. Early stage researches have been carried out to conceptualize heat exchanger storage designs using aluminium-silicone series alloys as PCMs. Dal Magro et al. [30] has suggested a temperature smoothing devise based on Aluminium as PCMs to enhance the energy recovery in the Electric Arc Furnace in steelmaking process. Another study [31] uses Al-Si12% as PCMs to smoothen the fluctuation from a billet reheating furnace and allows the downstream Organic Rankine Cycle system to operate at a higher capacity factor. With all the expertise drawn from the past studies, in 2017, our research group proposed a novel refractory brick [32] based on latent heat storage (see Fig. 2) installed in the waterwall of the WtE plants to smoothen the temperature fluctuation, reduce the corrosion risk on the downstream metal surfaces and enhance the heat transfer. The proposed refractory brick is composed of high purity ceramic and Al-Si based PCMs. There are three major benefits associated with this design:

encapsulation materials at high temperature [33]. To demonstrate the compatibility of the proposed concept, another study [34] investigated the high temperature corrosion between aluminium based alloys and four different ceramic materials. The study confirmed the good compatibility between the Al-alloys and high purity alumina 99% at 1000 °C and recommended alumina 99% and blast furnace slag as potential encapsulation materials for molten Al-alloys. Even though the PCM-refractory brick is a very promising concept, the proposed geometry in [32] has several limitations which need to be further justified. The small holes in the encapsulating ceramic brick will present a challenge during manufacturing process, especially when the holes are to be filled up with molten aluminium alloys. Judging from the result of the thermo-fluid dynamic analysis, the first few rows of PCMs which are close to the gas side do not operate around melting temperature and face overheating problems, which means these PCMs were not working under the design conditions. Moreover, in the thermo-fluid dynamic analysis, only the temperature distribution within the brick itself was reported. It is not clear whether the PCMbrick will be able to fully buffer the fluctuation from the gas side and deliver a more constant and steadier output to the steam side. With the purpose of further justifying the feasibility and applicability of this technology, this paper adopts both numerical and experimental methods to realize a two-step approach from PCM-brick development to system level validation. The first step involves a parametric study by numerical methods to find the designed operational points of PCM-based refractory bricks, and to compare the transient response of it with traditional brick. Step two employs laboratory setup to validate the numerical model with results of the traditional waterwall performance, and then showcases the performance improvement by PCM-based waterwall in the system level in terms of fluctuation dampening and the steam superheating effects. In the end, this paper proposes a technology integration scheme for the future high steam parameter WtE plants, with three different PCM bricks installed in the waterwall of combustion chamber to realise the continuous superheating of steam at various temperature stages.

• Making use of the significant high latent heat and thermal con• •

ductivities of aluminium-alloys, this technology is expected to buffer heat fluctuation from the flue gas and produces a higher and steadier heat flux to the steam than traditional refractory. While enhancing the heat transfer, this design allows to superheat the steam flow directly on the secondary combustion chamber. Since the superheating will be achieved in the combustion chamber, it will reduce the risks of pushing high steam parameter in the traditional superheater unit, thus avoiding the typical corrosion issues.

The challenges to the design with using high temperature Al-Si alloys include the high thermal stress that may be induced by the huge difference between the thermal expansion rates of PCMs and encapsulation materials during the heating up process. It might directly result in cracks of this brittle ceramic encapsulation. To answer this critical question, one research [32] examined the thermal-mechanical stress of proposed brick with respect to different types of encapsulating ceramics. It was concluded that high purity alumina works well as the encapsulation material for aluminium-alloys, taking into consideration the mechanical strengths, energy density and their costs of those ceramics. Furthermore, the study fulfilled a thermo-fluid dynamic analysis, demonstrating that the PCM-based brick is able to provide a lower thermal gradient within the brick and higher thermal flux to the steam side, compared with the traditional brick in current WtE plants. Another challenge to the design is the compatibility between alloys and

2. Methods The methods and workflow of this paper is summarised in Fig. 3. Two main studies are carried out with respect to brick level development and subsequent system level development. In both studies, onedirectional heat transfer is adopted in the numerical simulation due to 327

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Fig. 3. Methods and workflow of developing Phase Change Materials-based superheater wall system

waterwall shall rectify the fluctuation of heat flux received from gas side in both frequency and magnitude, and produces smoothed heat flux to the steam. ⁃ The PCM brick-integrated waterwall shall provide higher average heat flux in order to superheat the steam. ⁃ The temperature gradient within the PCM brick shall be low to avoid high thermal stress.

the following considerations: (i) the heat transfer process from combustion chamber to the steam/water flow inside the waterwall is dominated by one directional heat transfer; (ii) at the early stage of brick development, considering the dominating heat transfer is onedirectional, it is important to adopt one geometry parameter of the PCMs along the heat transfer direction to quantify the PCMs needed to absorb the heat fluctuation; (iii) compared with three-dimentional analysis, more computational resources can be spared in one-directional analysis to construct a system model to simulate the heat transfer on waterwall. In Study One, a one-dimensional mathematical model is built to describe the thermodynamic process from the combustion chamber to the steam flow. Then a parametric study is conducted to explore how variables could affect the performance of PCM-bricks, and whether the PCM-brick is operating under the design point. The design point is defined as the operational condition when PCM inside the brick is undergoing phase change. This is a crucial step to ensure the PCM bricks are making use of the latent heat instead of only being good conductive materials. The third step is a comparative case study which aims to demonstrate the different performance of traditional brick and the PCMs-based brick. Study Two investigates the dynamic performance of waterwall heat exchange system as a whole, using second numerical model built on the basis of the brick heat transfer model to represent the waterwall heat exchange system. As a continuation from Study One, three different PCM-based refractory bricks to retrofit into different sections of the waterwall to superheat the steam at different temperature stages are proposed. Then, a waterwall heat exchange system is built in both numerical model and laboratory unit. The experimental result obtained is used to validate the numerical model of traditional waterwall. Then the steam superheating effect by PCM-based waterwall in comparison with that by traditional waterwall is presented. The last part of this paper consolidates three PCM-based refractory bricks and presents the retrofitting scheme of them into the existing WtE plant. This could also give insights into the future WtE plant configuration design. In the subsequent sections, detailed numerical model and laboratory setup are described.

2.1.1. One-dimensional heat transfer model of refractory bricks on waterwall Thermal Network Method (TNM) is a modelling technique of the heat transfer process analogous to the electrical networks. The physical objects in the thermal network are represented as thermal resistors and conductors. Driven by the “voltage”, the temperature difference, the “current” of the thermal network, the heat flow, travels through the resistances and capacitances [35]. TNM is widely used in the analysis of steady-state heat conduction process to calculate the heat flux and the temperature gradient. This study adopts the Dynamic Thermal Network (DTN) method to analyse the transient response of the waterwall heat transfer process. The DTN is an extension of TNM, which describes boundary temperature and heat flux by time-varying thermal resistances and capacitances. It is capable of dealing with dynamic boundary conditions, making it a good fit for the one-dimensional thermodynamic modelling. Fig. 4 presents the physical model (on the left) and the DTN model (on the right) of traditional and PCM-based waterwalls. Each of the resistances and capacitances are associated with a physical object, such as the gas, the brick and the pipe. The nodes represent the surface connections between two physical objects. For example, in the model of traditional waterwall, the node b,p represents the surface connection between brick and steel pipe. For each of the nodes, a thermal equilibrium equation can be established. Then with all the resistance and capacitances well-defined, the model is solved numerically with Modelica language in Dymola environment. For all the objects in these two waterwalls, the one-dimensional transient heat transfer process is governed by:

2.1. Study One - brick level development

C L

This paper proposes three objectives for the PCM-based waterwall to be examined for a satisfactory brick design in Study One:

T = t x

T x

(1)

C , L , and are respectively the thermal capacitance, thickness and thermal conductivity for the object modelled. Thermal resistance is introduced to describe the Fourier’s law:

⁃ When it is operating at designed points, the PCM brick integrated 328

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Fig. 4. Physical model (left) and resistance/capacitance model (right) of (a) traditional waterwall, and (b) Phase Change Materials-based waterwall

T x

A

=A x

T R

Rg, r =

(2)

where T is the temperature difference between the upstream and downstream nodes of one object. It is calculated continuously by the boundary condition described in section 2.1.2 and the values of previous time step. The capacitance and resistance model for each object are introduced below. Traditional waterwall - Heat travels from gas side to the steam side, passing through resistors (i.e. gas, brick, pipe and steam convection), which generate heat losses and temperature gradient. While passing by the brick and the pipe capacitors, the heat is absorbed or released depending on the local temperature level. The thermal equilibrium equations below are established consecutively for node (g,h), node (b,p) and node (p.s):

A

A

A

(Tg

Tg . b) Rg

(Tb . p

=A

Tp . s ) Rp

(Tp . s Rs

Ts )

(Tg . b

=A

=A

Tb . p) Rb

(Tb . p

Tp . s ) Rp

T t

ACb

ACp

T t

Tg

Tb

1

(7)

(8)

The total thermal resistance Rg can be expressed as:

1 1 1 = + Rg Rg. r Rg . c

(9)

The resistances for the brick, the pipe and the steam respectively are:

Rb =

Rp =

Lb

(10)

b

Lb

(11)

b

(12)

1

R s = hs

The capacitances for brick and the pipe are:

(4)

(5)

For the gas to wall heat transfer, both the radiation and convection are considered for better model accuracy. Thus the heat flow from gas to the wall is calculated as:

Qg = Qg , r + Q g, c

Tb4

R g . c = hg 1

(3)

(Tg . b

Tg4

And the convective resistance is:

Tb . p) Rb

g

Cb =

b

cb Lb

(13)

Cp =

p

c p Lp

(14)

PCM-based waterwall - The thermal equilibrium equations for nodes (g.wo), (wo, pcm), (pcm, wi), (wi,p), (p,s) are respectively:

A

(6)

where Qg , r is the radiation heat flow. Here the enclosure grey body radiation model is used. The corresponding thermal resistance is expressed as:

A

329

(Tg

Tg . wo) Rg

(Two . pcm

=A

(Tg . wo

Tpcm . wi )

Rpcm

Two . pcm ) (15)

Rwo =A

(Tg . wo

Two . pcm) Rwo

ACwo

T t

(16)

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Table 1 Material properties and design parameters of waterwall system in Study One

Universal Furnace gas

Traditional brick

PCM-based brick outer/inner wall

PCMs Steam pipe

Steam

Parameter

Value

Reference

Area depth of 1D study (m) temperature at 2s after second air injection (°C) Gas emissivity (−) Gas flow speed (m/s) Flue gas pressure (Bar) Traditional brick Refractory thickness(m) Refractory conductivity(W/m K) Refractory heat capacity(J/kg) Refractory density(kg/m3) Wall material Thickness (m) Thermal Conductivity (W/m•K) Specific Heat Capacity (J/kg•K) Density (kg/m3) Thickness (m) Pipe material Pipe dimension Pipe conductivity (W/m•K) Pipe heat capacity (J/kg) Pipe density (kg/m3) Superheated steam temperature (°C) Superheated steam pressure (Bar) Heat transfer coeff. (W/m2•K)

1 960(Fig. 5(a)) 0.3 (0.1-0.5) 1 1 high alumina 90% 0.06 16.7 920 3600 Alumina 99% 0.005 35 880 3890 0.005 Carbon steel 2 inch (schedule 40) 59 490 7690 400 42 250

[36] [15] [37] [38] [38] [38] [39] [39] [39] [39] [15] [40] [40] [40] [40] [36] [36] [8]

Table 2 Material properties of three Phase Change Materials considered in the novel bricks

A

A

A

Name

Standard Code ER

Melting point (°C)

Latent heat (kJ/kg)

Thermal conductivity (W/m•K)

Density (kg/m3)

Al99.5 AlSi5 AlSi12

1050 4043 4047

657 573–632 577–582

390 412 498

227 163 160

2750 2690 2657

(Tpcm . wi

Tb, p)

Rwi

(Tb . p

Tp . s ) Rp

(Tp . s

Ts )

Rs

=A

=A

=A

(Two . pcm

Tpcm . wi )

ACpcm

Rpcm

(Tpcm . wi

Tb, p)

(Tb . p

Tp . s ) Rp

ACp

wall, PCM layer and the inner wall are expressed as:

(17)

T t

ACwi

Rwi

T t

Cwo =

(18)

T t

Cpcm =

L wo

pcm

Lpcm

Rpcm = Lpcm

(

if Tpcm

sol

sol

+

(Tpcm

Tsol) ( liq Tsol

T liq

Lpcm liq

sol)

)

1

Tsol

if Tsol < Tpcm < Tliq if Tpcm

Tliq (21)

Rwi =

L wi wi

w

c w L wi

H Tsol

if Tpcm

Tsol

Lpcm if Tsol < Tpcm < Tliq

cliq Lpcm

if Tpcm

Tliq

(24) (25)

2.1.2. Materials and boundary conditions Table 1 shows the material properties and initial values of the design parameters used in Study One. Design parameters used in the modelling are collected from real cases or reasonably speculated based on literature results. A universal area depth of 1 m2 is adopted in this one-dimensional study. The average temperature of combustion gas is around 960 °C according to Costa et al. [15]. The gas emissivity is an unknown property, which is a variable depending on the fuel type and combustion process of the WtE plant. In general, it varies from 0.1 to 0.5. High alumina 90% is a common material for the refractory brick in incineration process [8], so it is used in the PCMs-based brick for PCMs encapsulation. Table 2 introduces the material properties of three aluminium based PCMs. Since AlSi12 has the lowest melting temperature range, it is used for the parametric study to find the design point of the first PCM-based brick to buffer the temperature fluctuation of steam flow superheated at temperature of 450 °C. The other two PCMs are studied to buffer temperature fluctuation of superheated steam at higher temperature. There are two types of boundary conditions applied in the study.

(20)

wo

csol Lpcm

pcm Tliq

(19)

Cwi =

(23)

c w L wo pcm

In the PCM-based waterwall, the resistances of the gas, pipe and steam are the same as in the traditional waterwall. Equations (5)–(7) and (9), (10) are used to represent the resistance models. Apart from those, the resistance models for the outer wall of the PCM-based brick, the PCM layer, and the inner wall of the PCM-based brick are:

Rwo =

w

(22)

Respectively, the capacitance of the pipe is the same in the traditional waterwall and the PCM-based waterwall, thus equation 12 is also used in the PCM-based waterwall model. The capacitances of outer

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Fig. 5. (a) First boundary condition which considers gas temperature fluctuation and (b) second boundary condition which considers both the start-up ramp and the fluctuation of gas temperature

Fig. 5(a) illustrates the fluctuating furnace gas temperature during the normal operation. These two hours of the fluctuating gas temperature profile is extracted from furnace monitoring data of a WtE plant in Southern Italy (see Fig. 5 (b)). This boundary condition is used in the parametric study to find the design point of PCM brick. Fig. 5(b) shows another boundary applied to the waterwall, including the starting up temperature ramp and the fluctuation of gas temperature. The second boundary condition is applied in the comparative study to examine the different performance of PCM brick and traditional brick.

2.2. Study Two – system level development In Study Two, a laboratory scale test unit is built to reproduce the traditional waterwall performance in which the steam is re-directed to the secondary combustion chamber after being superheated in the traditional superheater. The result is used to validate the simulation of traditional waterwall. Based on the validated model, the PCM-based waterwall performance is presented by the simulation.

Fig. 6. Schematic drawing of the lab setup of the high temperature steam loop for brick testing, located in the Thermal Energy System Lab of Nanyang Technological University (www.thermalenergysystemslab.com). The thermal network model of waterwall heat exchange system 331

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2.2.1. The laboratory setup of waterwall heat exchange system Fig. 6 illustrates the schematic diagram of the lab setup. The main components include the steam boiler, superheater, the electric furnace which further superheats the steam, and the condenser. The feed water gets heated to saturated steam by the steam boiler at a mass flow rate of 40 to 50 kg/h. Then the superheater heats up the saturated steam further to 400 °C which is directed into the simulated waterwall inside the furnace. There, the refractory bricks are installed on the surface of the steam pipes, facing the electric heater which provides the radiating heating power. As seen in Fig. 6, there are three steam pipes inside of the furnace, and each pipe has 13 bricks installed, which contributes to a small-scale waterwall system comprising a total number of 39 bricks. The furnace can be programed to produce different temperature ramping profiles. The pressure is set to 3 bar due to the pressure limit by the laboratory regulation, and the tradeoff between maintaining steam mass flow rate and high pressure. Investigation on the pressure and mass flow influence on the heat transfer indicates that the pressure has much less influence than the mass flow rate. So, it is decided to decrease the pressure to 3 bars to keep the mass flow rate at a reasonable high level. The other detailed material and boundary conditions used in this lab test is described in section 2.2.3.

corresponding part of the steam pipe. Here the BRICK, represented by the dashed box, can be either traditional brick or the PCM-based brick. The heat transfer starts from the radiation between gas and brick, then heat conduction from brick to pipe, and lastly the convection to steam. At the same time, the steam passes the entire n sections of bricks/pipe, while getting heated up continuously. The physical model is translated into the thermal network model shown on the right side in Fig. 7. The 1D heat transfer model described in Section 2.1 is used for each of the n sections. All sections share the same gas temperature. The heat transfer driven by the difference between Tg and Ts, i is different for each of the sections. On the right side, the incoming steam flows through the pipe and passes by nodes Ts,1, Ts,2 ,…and Ts, n , and finally exits at temperature Ts, out . The resistance and capacitance models for bricks inside the dashed boxes originate from the dashed boxes shown in Fig. 4, so as their mathematical models. Each section i shares the same mathematical governing equations from equation 1 -25. The sections are connected at nodes Ts, i with

ms Hs, i = ms Hs, i

1

(26)

+ Q s, i

where ms denotes the steam flow rate. Whereas Hs, in and Hs, i 1 are the steam enthalpy after and before the node Ts, i . Q s, i represents the heat flow from brick number i to the steam at node Ts, i .The total enthalpy gain for steam passing through waterwall can be summarized as

2.2.2. The thermal network model of waterwall heat exchange system In order to represent the heat exchange process of the waterwall system, the 1D heat transfer model described in section 2.1.1 is expanded into a thermal network system, allowing the study of the continuous effect of PCM-based brick superheating the steam in a simulated waterwall system. As illustrated in Fig. 7, the left side presents the physical model of the waterwall system. The waterwall is divided into n sections, where each section represents one brick and one

ms Hs, out = ms Hs, in +

n i=1

Q s, i

(27)

where Hs, out and Hs, in are the steam enthalpy at outlet and inlet of the waterwall heat exchange system. The steam temperature at the exit can be obtained with the known enthalpy value.

Fig. 7. Dynamic thermal network model of the waterwall. 332

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Table 3 Type of Phase Change Materials-based bricks used in the numerical study of the waterwall system PCM Brick

PCMs applied

Thickness of PCM layer (mm)

Thickness of wall layers (mm)

Number of bricks

1 2 3

AlSi12 AlSi5 Al99.5

3 5 10

3 5 10

13 13 13

3.1. Design point of Phase Change Materials-based refractory brick

Table 4 Different wall thickness considered in the parametric study Case

In the parametric study, the design points of PCM-brick are located by varying three parameters, including the thicknesses of PCM and wall layer, the steam temperature, and the emissivity of the combustion gas. Table 4 presents five different cases of PCM and encapsulating wall thicknesses. Case A, B and C have the same PCM thickness, while the wall thickness increases. Case C, D and E have the same wall thickness, while the PCM thickness increases. With the first boundary condition in Fig. 5 (a) applied at the gas side, the results are obtained and presented in Fig. 8. Fig. 8(a) presents the average temperature of PCM bricks for different cases, while Fig. 8 (b) illustrates the gas-to-wall heat flow and the steam-convection heat flow in each case. Since different wall thickness will result in different thermal resistance of PCM bricks, it influences the PCM working temperature and the heat delivered to the steam. Ideally the PCM should work within the melting temperature range (shaded in yellow colour in Fig. 8 (a)). Case A and E fulfil the requirement. Consequently, the steam-convection heat flow is smoothed. In case B, C and D, the temperature of PCM exceeds the melting range and fluctuates. This leads to fluctuation in heat flow to steam. Comparing case A, B and C, it is evident that the thicker the encapsulation wall is, the more likely the PCM will not be at the melting temperature range, since the thicker walls “push” the temperature of PCMs off the melting range. For case C, D and E, when increasing the PCM thickness, the PCM temperature goes back to operate in the melting range and produces smoothed heat flow to steam. In the result for case C, there is the highest and longest fluctuation from 4000s to 8000s, while case D has only a small jump-up around 8000s. Though PCMs in case A and E both operate at the design point, case E delivers much lower heat to the steam side. This is due to higher thermal resistance with thicker PCM bricks. Based on the principle of reducing the material cost as much as possible, this study adopts case A as one standard design condition for the following studies. Besides the thickness of PCM brick materials, other parameters that could lead to the change of operating temperature of PCM include the emissivity of gas and the steam temperature. A parametric study is performed to correlate the PCM operating temperature with these two

Thickness (mm)

A B C D E

PCM

inner/outer wall

5 5 5 10 15

5 10 15 15 15

2.2.3. Materials and boundary conditions In Study Two, the materias and boundary conditions are modified based on the ones in Study One to adapt to laboratory environment. Firstly, as introduced by section 2.2.1, the operational pressure in steam supply is downsized to 3 bars. Secondly, the steam pipe in the laboratory setup is built with Stainless Steel 316, different from Study One. Thirdly, a 17mm alumina 99% brick with a 5mm of air gap in the middle of the brick is used to represent the traditional brick in the lab test, instead of using the high alumina 90% as in Study One. The numerical model is subsequently adapted to include this air gap effect. Furthermore, in the lab environment, the furnace is manually programed to create significant temperature fluctuation (maximum 100 °C/10 min as reported by Costa et al. [15]) to test the waterwall and steam response. In the numerical study, three PCM-based bricks are used on each of the three sections of the steam pipe (see Table 3). The thickness of PCMs and encapsulation wall layers are determined by the method of finding brick design point introduced in Study One. 3. Results and discussions This section presents the results obtained from Study One and Two, discusses how the PCMs-based refractory can be conceptualized from the brick level to the system level, then introduces the technology integration scheme to realize the plant level implementation.

Fig. 8. (a) Average temperature of PCM bricks and (b) gas-to-wall heat flow and steam convection heat flow for different cases

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240 min), the difference in performance between traditional brick and PCM-based brick is very small. After 300 min, during the fluctuation phase, the PCM-based brick delivers rectified heat flow to the steam. The average heat flow to the steam by the PCM-based brick is around 30.7 kW, while the average heat flow by the traditional brick is 27.8 kW. The PCM case allows a 10% upgrade of energy harvesting on the waterwall. 3.3. The continuous steam superheating effect of waterwall heat exchange system In the previous studies, the PCM-based brick is capable of smoothing the fluctuation of heat flow, providing higher energy output to the steam while maintaining a low temperature gradient within the brick itself. To provide superheated steam with higher parameters to the inlet of steam turbine, it is necessary to properly integrate the PCM-based bricks into the waterwall to achieve continuous superheating of steam from 400 °C to higher parameters. Fig. 11 presents the temperature profiles of furnace and steam during the numerical and laboratory test. Three cases are investigated here, which are traditional waterwall in laboratory test, traditional waterwall in simulation, and PCM-based waterwall in simulation. All three cases share the same steam inlet temperature profile. Case traditional (both laboratory and simulation) share the same furnace temperature input. For case PCM, since the system with PCM-based waterwall has a much higher system thermal inertia which requires a much longer charging time, the first 70 min of the furnace temperature input profile for PCM case is changed to steady temperature instead of following the laboratory furnace input as the traditional case (see the grey solid line in Fig. 11). However, from 70 min to 160 min, the important fluctuation profile remains the same as that for the traditional cases. As we can see in the traditional cases, the steam outlet temperature from the lab (orange dashed line) and from the simulation (yellow solid line) almost overlap with each other, especially in the fluctuation period (from 109 min to 160 min). This indicates the numerical model has a rather accurate prediction of the laboratory result. With the confidence in the numerical model, we could see that the steam outlet temperature from the PCM case (red solid line) has a much higher and steadier temperature profile than the traditional cases. From 110 min to 160 min, the steam outlet temperature remains at a nearly constant temperature of 657658 °C, which proves: (i) first, PCM-based waterwall provides a good buffering effect by the PCM-based waterwall as compared with the steam outlet temperature fluctuation of more than 12 °C in the traditional cases; (ii) second, PCM-based waterwall delivers much better superheating effect than the traditional waterwall to the steam and is able to produce steady superheated steam at 658 °C to the turbine.

Fig. 9. Different steam temperature and corresponding PCM temperature at different conditions of gas emissivity

parameters. As presented in Fig. 9, the result shows that the higher the gas emissivity is, the higher the PCM temperature is; for the same gas emissivity, the higher the steam temperature is, the higher the PCM temperature is. The yellow shaded area highlights the design points which allow PCMs to operate at melting temperature range. The points are (Ts =410 °C, g =0.4), (Ts =450 °C, g =0.3), (Ts =490 °C, g =0.2), (Ts =530 °C, g =0.1). Emissivity of 0.5 is not included in the figure since the results does not coincide with the yellow shaded zone. 3.2. Comparison of dynamic performance of traditional and Phase Change Materials refractory bricks In the comparative study, second boundary condition (Fig. 5(b)) is applied to compare the transient thermal performances of the traditional and PCM-based brick. Fig. 10(a) illustrates the temperature profiles on the outer and inner walls of traditional brick and PCM-based brick along with time. It is obvious from the result that after 250 min at fluctuating phase, there is a huge temperature difference between the outer and inner wall of the traditional brick, which is about 70 °C. Contrary to the traditional case, the PCM-based brick has much lower temperature difference between the outer and inner walls, which signifies a much smaller thermal resistance compared with traditional brick. Also the temperature profiles of PCM brick are much steadier than those of the traditional brick. Fig. 10(b) illustrates the corresponding gas-to-wall and steam-convection heat flow. As can be seen, during the start-up phase (0-

Fig. 10. (a) Outer wall and inner wall temperature at the traditional and PCM bricks; (b) gas to wall heat flow and steam convection heat flow for traditional and PCM-based cases

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provide much smoother heat flow to the steam side in comparison with the fluctuated heat flow received from gas side. With the proposed three PCM-based refractory bricks for different superheating temperature, Fig. 13 presents the schematic design of the future WtE plant with high steam parameters facilitated by the PCMbased waterwalls. As shown, the waterwall of combustion chamber is divided into two parts. Upper part features lower chamber temperature so that it is used to evaporate water into saturated steam. After steam being superheated in the convectional superheater, it reaches 400 °C. Then it goes through the PCM-based superheater on the lower part of the waterwall to be superheated from 400 °C to higher parameters. Three types of PCM-based bricks are installed consecutively from top to the bottom to superheat the steam at 400 °C, 500 °C and 545 °C. An overall steam output of 600 °C is expected to be achieved for the turbine inlet. As such, the WtE plant is upgraded to deliver an electrical efficiency of 34% under the current turbine technologies. This waterwall with new PCM-based refractory lining can also be retrofitted into existing WtE plants without too much alternation of their original configurations to achieve high steam parameters. Fig. 11. Temperature profiles of furnace and steam during the laboratory and simulation test

3.5. Future research trend of Phase Change Materials buffering

Table 5 Type of PCM-based bricks to be applied in the WtE plant and the corresponding designed steam superheating temperature PCM Brick

PCMs applied

Thickness of PCM layer (mm)

Thickness of wall layers (mm)

Temperature of steam received for further superheating (°C)

1 2 3

AlSi12 AlSi5 Al99.5

5 15 10

5 15 10

450 500 545

In order to completely realize the future WtE plant concept as proposed in Figure 13, further research shall be done in the design, operational and economical aspects. Firstly, in the design aspect, PCMsbased evaporator shall be designed for the upper part of the first boiler pass to realize the smoothening effect during the steam evaporation. Phase change temperature around 200-300 °C could be used as a criterion to select the suitable PCMs. PCMs-based refractory lining on evaporating waterwall further ensures temperature-dampened steam generation for more efficient and safer operation of downstream equipment in WtE plants. Secondly, in the operational aspect, it is necessary to study the material durability and possible service duration of PCMs-based bricks under long term cyclic effect in harsh plant conditions. Also, with the PCMs-based waterwall installed, the plant control strategy shall be updated. Lastly, in the economical aspect, a technoeconomical study with a life cycle perspective shall be conducted to evaluate the investment and savings to understand the market

3.4. The concept of future waste-to-energy plants with new waterwall configuration Previous results have shown us the potential of using PCM-based refractory lining to replace the traditional waterwall to achieve continuous superheating of steam. In this section, we take a typical WtE plant configuration described in [15] as an example to showcase how the future WtE plant could evolve with the installation of PCM-based waterwall. Firstly, three different PCM-based bricks are proposed (as shown in Table 5) with respect to the boundary condition of this typical plant to superheat the steam at three temperature levels (450 °C, 500 °C and 545 °C). Each brick contains one type of aluminium alloy which melts at different temperature range. Parametric study has been performed for the three bricks to find their design points. Fig. 12 shows the temperature of outer and inner walls of the three PCM-based bricks, and their heat buffering performance. As shown, all of them are able to

Figure 12. (a) Outer wall and inner wall temperature at different bricks; (b) Gas-to-wall heat flow and steam convection heat flow for three different PCM-bricks

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Fig. 13. Technology integration of PCM-based superheater wall in the furnace of WtE plant: (left) the three types of PCM-based bricks superheating steam at different temperature range; (right) the proposed plant configuration to use PCM-based waterwall to superheat steam for higher temperature output

investigate the response of the PCMs-based waterwall system. With three types of PCMs-based bricks superheating the steam at various temperature stages, the steam achieves a smoothened temperature around 657 °C–658 °C at the waterwall exit, which proves an extraordinary dampening effect of less than 1 °C as compared with fluctuation of more than 12 °C by traditional waterwall, and an enhanced superheating effect of producing steam 90 °C higher than that produced by traditional waterwall. Lastly, the paper presents a novel WtE plant scheme with three types of PCMs-based bricks installed into the waterwall to achieve continuous superheating of the steam from 400 °C to 600 °C. This contributes to a high steam efficiency of 34% in the WtE plant. This PCMs-based waterwall can also be retrofitted into existing plants without changing the majority of the plant configuration. In conclusion, the PCMs-based refractory brick demonstrates a highly applicable technology to buffer thermal fluctuation and upgrade the energy conversion efficiency of waterwall. The proposed WtE configuration is estimated to achieve high steam parameters and contribute to a more sustainable future in the long run.

penetration potential of this new WtE configuration. Sensitivity analysis shall be performed to investigate how different types of bricks will affect the total cost of power plants. Besides application in the WtE plants, this PCM buffering concept may provide an alternative solution to temperature fluctuation scenarios in many thermal processes. Such processes include combined heat and power plants which burn inhomogeneous fuel, or concentrating thermal power tower which face the issue of fluctuating solar irradiance causing uneven absorption in the solar receiver. Future research can explore the technology innovation to fully unleash the potential of PCM-buffering concept for higher energy efficiencies in industrial processes. 4. Conclusion This study presents a novel concept of combustion chamber waterwall featuring Phase Change Materials-based refractory brick to upgrade the thermodynamic efficiency of the energy conversion on the waterwall. Three aluminium-based alloys are suggested as the Phase Change Materials, encapsulated by high purity alumina. Then, two studies are carried out to realize the complete technology development from brick to system. Firstly, Study One employs Dynamic Thermal Network method to construct a one-dimensional transient model with all material compositions represented as thermal resistances and capacitances. Real plant data are extracted from a Waste-to-Energy plant in Southern Italy for boundary conditions. Then, a parametric study is performed accordingly to locate the design points where the PCM-based bricks are able to operate around their melting temperature. Then the thermal performance of traditional and PCMs-based bricks is compared when subjected to fluctuation boundary conditions. The results from Study One suggests that the PCMs-based waterwall delivers a 10% higher and smoother heat flux to the steam side, while maintaining a low temperature gradient across the brick itself. In order to understand the collective performance of PCMs-based waterwall in the system level, in Study Two, a laboratory setup is employed to replicate a small scale waterwall system to validate the numerical model. Then the validated numerical model is used to

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