Francis Chinweuba Eboh EFFICIENCY IMPROVEMENTS IN WASTE-TO-ENERGY COMBUSTION PROCESSES

Thesis for the Degree of Doctor of Philosophy

EFFICIENCY IMPROVEMENTS

IN WASTE-TO-ENERGY COMBUSTION PROCESSES

Method Development and Evaluation

Francis Chinweuba Eboh

Efficiency improvements in waste-to-energy combustion processes: Method development and evaluation

Copyright 2019 © Francis Chinweuba Eboh Swedish Centre for Resource Recovery Faculty of Textiles, Engineering and Business University of Borås

SE-501 90 Borås, Sweden

Digital version: http://urn.kb.se/resolve?urn=urn:nbn:se:hb:diva-21801 ISBN 978-91-88838-47-6 (printed)

ISBN 978-91-88838-48-3 (pdf)

ISSN 0280--381X, Skrifter från Högskolan i Borås, nr. 100 Cover photo: Waste-to-energy plant (photo by David Castor) Printed in Sweden by Stema Specialtryck AB

Borås 2019

Abstract

The current increase being experienced in the generation of waste endangers human health and the environment. One possible way of addressing this issue is to minimise it by reusing or recycling large fractions of waste materials. A suitable approach for treating undesired end products remaining after recycling is the energy recovery method. The electrical efficiency of this technology, however, is generally low when compared with other solid fuel-fired combustion plants as a result of low steam properties.

Furthermore, there is lack of efficient methods to evaluate the performance of this system. The energy method, normally used, does not account for exergy destruction due to entropy generated within the system.

In this thesis, an exergy model for estimating the maximum available energy in a municipal solid waste and a modified exergy-based method for calculating the improvement potential in a waste-to-energy plant are developed. The exergy model was obtained from estimations of the higher heating value and standard entropy of municipal solid waste from the elemental compositions of the waste using statistical analysis.

The improvement potential was derived by comparing the exergy destruction of the real process with its corresponding theoretical process. It was applied in a solid-waste fired heat and power plant to investigate possible improvements in the system as well as the cost of the improvements. The different improvement modifications considered include the re-arrangement of air heaters, the introduction of a reheater, flue gas condensation and an integrated gasification-combustion process. Modelling, simulation and cost estimations were performed with the Aspen Plus software.

The results showed that the present proposed exergy model was more accurate than the previous models for estimating the maximum available energy in waste material, as the proposed model incorporates all the major elemental constituents as well as the physical composition of the solid waste. Moreover, the results obtained from the higher heating value model show a good correlation with the values measured, and are comparable with other recent and previous models. Furthermore, it was found that 64 % of the total exergy destruction in the process plant investigated can be reduced, while the boiler was identified as a component with the greatest potential for making improvements to the plant. Although the integrated gasification- combustion technology with flue gas condensation has the highest exergy efficiency, its higher capital cost exceeds all other alternatives. The improvement modifications with flue gas condensation not only provide the highest heat production but also the highest net present value. This indicates that flue gas condensation has a significant impact on the overall income generated by waste-to-energy combined heat and power industries.

Keywords: Solid waste, exergy, entropy, higher heating value, improvement potential, waste-to-energy plant, efficiency improvement, cost evaluation, simulation, modelling

SVANENMÄRKET

Trycksak 3041 0234

List of Publications

This thesis is based on the results presented in the following articles:

I. Eboh F.C., Ahlström P, Richards T. Exergy analysis of solid-fuel fired heat and power plants: A review. Energies 2017; 10(2).

II. Eboh F.C., Ahlström P., Richards T. Estimating the specific chemical exergy of municipal solid waste. Energy Science and Engineering 2016;4(3):217-231.

III. Eboh F.C., Ahlström P., Richards T. Evaluating improvements in a waste-to-energy combined heat and power plant. Case Studies in Thermal Engineering 2019;14.

IV Eboh F.C., Andersson B-Å., Richards T. Economic evaluation of improvements in a waste-to-energy combined heat and power plant. Waste Management 2019;100:75-83.

Statement of Contributions

The contributions made by Francis Chinweuba Eboh to the appended papers:

Paper I: Responsible for the idea, literature review, writing of the manuscript and its revision.

Paper II: Responsible for the idea, statistical analysis, model estimations, writing of the manuscript and its revision.

Paper III: Responsible for the idea, process modelling and simulation, efficiency evaluation, writing of the manuscript and its revision.

Paper IV: Responsible for the idea, process modelling and simulation, economic evaluation, writing of the manuscript and its revision.

Publication not included in this thesis

Eboh F.C., Ahlström P., Richards T. Method of Estimating Absolute Entropy of Municipal Solid Waste. International Journal of Power and Energy Engineering 2016;10.

Conference Contributions and Award

Eboh F.C., Ahlström P., Richards T. Method of Estimating Absolute Entropy of Municipal Solid Waste, 18th International Conference on Exergy, Exergy Systems Analysis and Optimization in Zurich (Switzerland) July 21-22, 2016.

Award: Certificate of Best Paper Award from World Academy of Science, Engineering and Technology (WASET).

Eboh F.C., Ahlström P., Richards T. Performance Evaluation of a Waste-to-Energy Power Plant: An Exergetic Approach, 25th European Biomass Conference and Exhibition in Stockholm (Sweden) June 12-15, 2017.

Eboh F.C., Ahlström P., Richards T. Development of an Exergy-Based Method to Evaluate the Improvement Potential of Fluidized Bed Waste-to-Energy Plant, 13th International Conference on Energy Sustainability: The American Society of Mechanical Engineers, (Washington) USA July 14-17, 2019.

Preface

This thesis is a partial requirement for a Ph.D. degree in Energy Technology at the Swedish Centre for Resource Recovery, Faculty of Textiles, Engineering and Business, University of Borås, Sweden. The dissertation was written under the supervision of Professor Tobias Richards, Associate Professor Peter Ahlström and Associate Professor Anita Pettersson.

The efficiency calculation of a waste combustion process can be evaluated by using not only the energy content in the waste but also using the maximum available energy as input to the system, as this will account for the irreversibility in the process due to the entropy generated in the waste material. Furthermore, the proper evaluation of efficiency improvement made to a process demands knowledge of possible improvements. This thesis presents methods for calculating the maximum energy available in municipal solid waste and for evaluating improvements made in a municipal waste-to-energy combined heat and power plant. Moreover, considering recent developments in this sector, these methods may also be used to evaluate the profitability of the energy recovered from waste technology.

Francis Chinweuba Eboh November 2019

Nomenclature

Ė energy rate (kW)

Ėx exergy rate (kW)

e specific exergy (kJ/kg)

ṁ mass flow rate (kg/s)

Q̇ heat transfer rate (kW)

R² coefficient of determination

S entropy (kJ/K)

s specific entropy (kJ/kg·K)

Ẇ work transfer rate (kW)

Subscripts

a available

c cold

D destruction

e energy

ex exergy

f flow

h hot

i input

k component of a process

L lost

max maximum

o output

p product

rp real process

tp theoretical process

0 reference temperature

Superscripts

AV available

ch chemical

UN unavailable

Abbreviations

AAE average absolute error ABE average bias error

BC boiler combustor

BCP base case plant

BFB bubbling fluidized bed

C carbon

CFB circulating fluidized bed

Cl chlorine

COND condenser

CP condensate pump

DH district heating

DRT deaerator

ECO economizer

EVA evaporator

FBB fluidized bed boiler

FG flue gas

FGC flue gas condenser

FWH feed-water heater

FWP feed-water pump

H hydrogen

HPT high-pressure turbine

IP improvement potential

IPT intermediate-pressure turbine IRR internal rate of return LPT low-pressure turbine

M modification

MIX mixer

MSW municipal solid waste

N nitrogen

NPV net present value

O oxygen

P pressure

PSAH primary steam air heater

S sulphur

SH superheater

SPLIT splitter

SSAH secondary steam air heater

STDRUM steam drum

SW steam and water

TP theoretical process

T temperature

W water

Greek letters

∆ change

𝜂𝜂 efficiency

List of Tables and Figures

Table 2.1. The physical classification of MSW………..………..5

Table 2.2. European Union average emission limit values for waste combustion …………...…….7

Table 4.1. Results from the analysis of a coal-fired thermal plant………..……..……...22

Table 4.2. Standard chemical exergy and standard entropies of various compounds..…………...24

Table 5.1. Design parameters of the plant……….………...………...30

Table 5.2. Evaluation of the improvement in efficiency in a combined heat and power plant…….33

Table 5.3. Evaluation of the improvement in efficiency in a power plant………...…....33

Table 6.1. The parameters of the base case plant and the different improvement modifications made………...………...…….35

Figure 2.1. Process diagram of waste combustion producing electricity and heat…....………8

Figure 2.2. Schematic diagram of a solid waste-fired grate boiler ………..………..…………9

Figure 2.3. Moving grate during combustion………...…...10

Figure 2.4. Waste reception (bunker) and the crane………10

Figure 2.5. Schematic diagram of a solid-waste fired fluidized bed boiler………..………....11

Figure 4.1. Comparison between the experimental and estimated HHV using the developed Correlation………...….………..26

Figure 4.2. Comparison between the experimental and estimated HHV using the Channiwala and Parikh correlation ………...………..26

Figure 4.3. Comparison between the experimental and estimated HHV using the Sheng and Azevedo correlation..………....………..26

Figure 4.4. Comparison between the experimental and estimated HHV using Dulong’s correlation………26

Figure 5.1. Schematic diagram of the municipal heat and power plant fired by solid waste………29

Figure 5.2. The base case plant, modelled in Aspen Plus………..…………..30

Figure 5.3. The exergy efficiency of the boiler and overall process plant………...………....31

Figure 6.1. The ratio of the increment in the capital cost to the increment in the exergy

efficiency and capital cost per total revenue………...…...……….….36

Figure 6.2. Net Present Value (NPV) and Internal Rate of Return (IRR) for the base case and the various modifications………..37

Figure 6.3. Net Present Value for plants producing electricity only………38

Table of Contents

List of Publications………..………..v

Statement of Contributions……….………….vi

Conference Contributions and Award………..………vii

Preface……….…………...………ix

Nomenclature………...………….…xi

List of Tables and Figures………...………..…………xiii

Chapter 1………..………1

Introduction………...………….1

1.1 Aim………...………2

1.2 Research Questions………...………3

1.3 Outline of the Thesis……….………3

Chapter 2………..5

Background………..………..5

2.1 Solid Waste as a Fuel………...………….5

2.2 Waste-to-energy Combustion Technology………...………6

2.3 Combustion Firing Systems………..8

2.3.1 Grate Boiler………...………..……9

2.3.2 Fluidized Bed Boiler……….………....11

Chapter 3………..…..13

Overview of Efficiency Evaluation Methods……….…13

3.1 Energy Efficiency Method………..………13

3.2 Exergy Efficiency Method………..………14

3.3 R1-formula Method………...……….16

3.4 Improvement Potential Method……….……….17

3.5 Chemical Exergy of Solid Fuels……….………18

3.6 Research Necessary………...………….18

Chapter 4………..………..21

Improving Efficiency Evaluation Methods………21

4.1 Using Exergy as a Method for Evaluating Efficiency…….…………...………21

4.2 Estimating the Chemical Exergy of Solid Waste………22

4.2.1 Estimating the Higher Heating Value and Standard Entropy of MSW……….24

4.3 Improvement Potential Methods……….……27

Chapter 5………..……..29

Evaluating Efficiency Improvement…….………...…..29

5.1 The Base Case Plant………29

5.2 Different Efficiency Improvement Methods………..……….32

Chapter 6………..…..35

Economic Evaluations of Efficiency Improvement……….………..…………35

6.1 Cost of Improving Efficiency………..…...…………36

6.2 Profitability Evaluation………...………37

Chapter 7………..………..39

Validation……….………….………..……….……39

Chapter 8………..………..41

Conclusions……….……….……….……41

Chapter 9..………...………...43

Future Work………...………43

Acknowledgements………....………..45

References……….…49

Chapter 1

Introduction

The generation of solid waste is an inevitable by-product of human activity and civilization in general, and increases in quantity as a result of growth in population, industrialization and urbanization [1, 2]. The traditional indiscriminate disposal of waste in landfill has become a major environmental problem that pollutes the air, land, ground water and endangers human health [3].

Landfilling contributes to about 5 % of total greenhouse gases (CH4, N2O, CO2) emissions that affect global warming and climate change [4]. The ability to provide solutions of how the large quantities of waste generated can be managed effectively is one of the greatest challenges facing both the present and future generations [5]. One possible approach is to minimize the amount of waste produced through reusing or recycling large fractions of waste materials [6]. A suitable method for treating undesired the end products remaining after recycling is the energy recovery approach [5]. Sweden, for instance, is an example of a country with efficient management of solid waste: landfilling has been significantly reduced to 0.5 % and about 50 % of the solid waste generated by households in 2017 was treated in waste-to-energy plants [7].

The energy recovery method of waste treatment does not only help in treating non-reusable and non-recyclable amounts of waste but also in the conversion of valuable energy resource into electricity and heat [8]. The technology used involves both thermochemical and biological processes [9], and is selected depending on the type, chemical composition and energy content of the waste and, finally, the overall efficiency. Thermochemical processes are more efficient than biological ones in terms of faster reaction rates and larger reductions in the mass and volume of the solid waste [10] and can be divided into three main groups: combustion, gasification and pyrolysis. Of these, combustion technology is the most widely used method for treating waste materials of different types and size [11,12]. It is a commercially viable option for the conversion of solid fuels into heat and power, and contributes to over 97 % of the world’s production of bio- energy [10,13].

Waste-to-energy combustion technology has its own drawbacks. One of the major issues of this method is the low efficiency of utilizing the waste fuel as a result of the low heating values caused by the high contents of moisture and oxygen in the waste [13]. The average heating values of solid waste is about 10 MJ/kg [12] compared to 15-19 MJ/kg and 20-30 MJ/kg on dry basis for biomass materials and coals, respectively [10]. Furthermore, the electrical efficiency of waste combustion process plant is generally low when compared with other solid fuels due to the low steam properties that are employed in order to prevent fouling, slagging and corrosion of the heat exchanger tubes in the boiler [14, 15]. The typical electrical efficiency of a waste combustion plant is in the range of 18-26 % whereas average efficiencies of 35 %, 45 % and 38 % are reported for coal, natural gas and oil-fired power plants, respectively [12]. Moreover, the technology for recovering energy from waste is capital intensive when considering the high financial investments and high maintenance and operating costs involved [16]: the investment cost is about three times greater than for a woodchip CHP and four times greater than for a pulverized coal power plant [17]. This thesis addresses some of the challenges mentioned above by providing methods for evaluating efficiency and using them to evaluate various improvements, as well as their cost implications, to the state- of-the-art techniques employed in waste-to-energy combustion plants.

1.1 Aim

The aim of this thesis is to improve the methods used to evaluate the performance of a waste-to- energy combined heat and power plant that combusts both household and industrial waste with respect to enhancing efficiency and profitability. The specific objectives are:

1. To examine the existing efficiency evaluation methods used in a solid-fuel fired heat and power plant and develop an exergy model for calculating the maximum available energy in a municipal solid waste plant (Papers I and II).

2. To determine the potential for improvement in a municipal heat and power plant fired by solid waste (Paper III).

3. To evaluate the cost of making improvements in efficiency and profitability in a waste combustion plant (Paper IV).

1.2 Research Questions

This thesis is based on the following research questions:

 How can the maximum available energy and entropy generated during the solid waste conversion process be calculated?

 What is the best way to evaluate efficiency improvements made in a waste-to-energy combustion plant?

 How is the maximum theoretical efficiency of a waste combustion plant determined?

 What are the best options for improvement based on the state-of-the-art technology applied in municipal solid-waste fired heat and power plants with respect to the cost and economic viability of the different improvements?

1.3 Outline of the Thesis

This thesis is divided into the following chapters:

 Chapter 1 introduces the thesis and its aim.

 Chapter 2 presents the background and the research required.

 Chapter 3 is an overview of the various methods used for evaluating the efficiency of a process.

 Chapter 4 discusses the applications of exergy analysis in the evaluation of thermal conversion processes fired by solid fuels. It also presents the modified exergy improvements method introduced in this thesis.

 Chapter 5 describes the base case plant used in this thesis, along with the various efficiency improvement methods that are based on the state-of-the-art technology applicable to municipal waste-to-energy combined heat and power plants.

 Chapter 6 examines the costs involved with the different improvement modifications in order to ascertain the economic viability of process plants.

 Chapter 7 compares the key results obtained in this work with previous studies.

 Chapters 8 and 9 provide a summary of the key findings and suggestions for future research work, respectively.

Chapter 2

Background

2.1 Solid Waste as a Fuel

Municipal solid waste (MSW) is normally referred to as trash or garbage [18]. It is considered as unwanted material that is discarded as useless or worthless [17]. Its physical composition is heterogeneous and varies with time, life-style, economic status as well as geographical region [19, 20, 21]. The main constituents of municipal solid waste is shown in Table 2.1 [20].

Table 2.1[20]

The physical classification of MSW.

Physical classification Explanation Organics

Food residue Rice, cooked food remains, meat, vegetable, fruit waste

Wood waste Waste wood, disposable chopsticks, bamboo, flowers, grass, leaves, branches

Paper Tetrapak paperboard, office paper, toilet paper, newsprint, magazines Textiles Clothes, cloth shoes, cotton, chemical fibres

Plastics All kinds of plastic: film, bottles, tubes, bags, toys

Rubber Rubber shoes, worn out tyres

Inorganics

Metals Iron wire, cans, metal parts, pans Glass Glass: fragments, bottles, mirrors, balls Tiles Stones, tiles, cement, ceramic

Ash Slag, soil

Other Batteries, plaster

The energy recovered from solid waste depends on its physical composition [22], proximate and ultimate analyses and heating value. The proximate analysis determines the moisture content, along with the amounts of ash and volatile and fixed carbon in the waste, while the ultimate analysis provides the elemental constituents in the waste fuel, such as carbon, hydrogen, oxygen, nitrogen, sulphur and chlorine. The heating value is the energy content released during combustion of the organic components of solid waste [23]. Statistical modelling using correlation from the

applied to evaluate the heating value of municipal solid waste. According to Zhou et al. [20], the variation in heating value has a significant effect on the stable operation of a waste combustion plant and its monthly average lower heating value should therefore exceed 4.1 MJ/kg to ensure complete combustion.

2.2 Waste-to-energy Combustion Technology

The combustion of solid waste in industrial scale started around the end of the 19th century, with the first plants being constructed in England, the USA and Germany. The plants operated in an open environment, without energy recovery, with two main purposes: reducing the volume of the waste generated, so called incineration and promoting public health by controlling the spread of cholera that emanated from landfill [17, 26]. At that time, the technology used was very simple:

brick-lined cell ovens had a fixed metal grate over an ash pit, with one opening at the top or side of the oven for loading and another opening for removing the solid residues [27]. Since then, there has been significant advancement in the fields of energy efficiency and emission control of the system, one of which is a waste stream with increasing pollutants that now have the potential to be treated [26].

Waste-to-energy plants are well established in Europe, being the result of heavy taxes imposed on landfilling and bans on landfilling of unprocessed waste in some countries. The emission control from their operation has been driven by regulations passed by the European Union using the Directive for Waste Incineration in Europe, which means that all plants must be equipped with flue gas treatment technologies and operate within the average emission limit values as shown in Table 2.2 [28, 29]. This is to meet stringent regulatory standards to ensure that all emissions to air are well-controlled [30]. European countries had 429 waste-to-energy plants from which 96 million tonnes of waste were combusted in 2017 [31], with Sweden and Denmark having the highest incineration capacity of 591 and 587 kg/capita, respectively [32]. Sweden, for instance, has about 34 waste-to-energy combustion plants and recovers more energy from waste per capita than any other country in Europe [7]. The capacity of waste combustion plants in Sweden is, in fact, greater than the amount of combustible waste than the country produces: in 2017, a total of 6,150,150 tonnes of industrial and household waste were treated and converted into more than 18.3 TWh of energy, of which 2.2 TWh was for electricity and 16.1 TWh for heating [7]. An average

plant in Europe uses the grate-based combustion system that produces 546 kWh and 640 kWh of electricity per ton of waste with a gross energy efficiency of 18 % and 22 %, respectively [26]. In addition, these plants typically use steam parameters of 40 bar and 400 ^oC, respectively, for pressure and temperature, and use waste with the average net calorific value of 10. 44 MJ/kg [26].

The increase in the capacity of these plants, the steam properties, integration processes (such as air heater, feed water heater and reheater) and co-generation (heat and power) have led to an enhancement in the overall efficiency of energy recovery from waste (Paper III).

Table 2.2[28, 29]

European Union average emission limit values for waste combustion plants.

Air emission Values

Particulate matter 10

Sulphur dioxide (SO2) 50

Nitrogen oxides (NOx) 200

Hydrochloric acid (HCl) 10

Fluorine acid (HF) 1

Carbon monoxide (CO) 50

Heavy metals 0.5

Cadmium (Cd) 0.05

Mercury (Hg) 0.05

Dioxin and furans 0.1

All values in mg/Nm³ (except for Dioxin and furans, ng/Nm³).

Advancement in the waste-to-energy sector has been observed in other countries as well. In the United States in 2014, 33 of 136 million tonnes of waste generated were treated in energy recovery plants with a total capacity of 2.5 GW that produced 14.3 TWh of electricity [33]. In the late 1980s, the waste combustion process was introduced in China; the technology has since made huge advancement, with the total capacity reaching 46 million tonnes a year between 2013 and 2014, and a power generation of 18.7 TWh [33]. In Japan, waste combustion is the most widely-used technology for waste treatment, due to the lack of land suitable for landfill sites [27]. Japan has the largest number of waste combustion plants in the world, according to Tan et al. [34]: over 80

% of their MSW is combusted in 1,900 plants.

Energy recovery has other advantages apart from the production of heat and power. The waste-to- energy combustion process not only reduces the volume and mass of solid waste by 90 % and 70

%, respectively [35] but is also an important additional source of renewable energy because half of the energy utilized in solid waste is biogenic [27]. Recovering energy from waste decreases the overall amount of CO2 emitted to the environment when compared to the CH4 and CO2 generated in landfill operations: the impact of CH4 as a greenhouse gas is 21 times higher than that of CO2

[36, 37]. Furthermore, waste combustion helps in the detoxification and the destruction of pathogenic organisms that are hazardous to public health [38].

2.3 Combustion Firing Systems

The combustion of a heterogeneous mixture of solid waste material involves several steps: drying and degassing (to remove moisture content and volatile organic compounds), followed by pyrolysis and gasification. The final step is oxidation, where CO2 and H2O are formed along with the release of energy as a result of the exothermic reactions [17]. The energy released is utilized in the Rankine steam cycle for the production of heat and electricity. The process diagram of a typical waste combustion for combined heat and power plant is shown in Figure 2.1.

There are two main types of combustion firing systems used in facilities where energy is recovered from waste: (i) stoker or grate-fired and (ii) fluidized bed boiler systems. The firing system selected depends on the extent of preparation of the solid waste to be used as fuel and the design of the combustion system [17]. Solid waste used as received, without size reduction and separation of metals, is more suited to a grate boiler than a fluidized-bed boiler (FBB) [39].

2.3.1 Grate Boiler

The grate-fired boiler is the waste combustion technology used most widely in the world [27]. It is normally designed to combust solid waste fuel without the need of major pre-treatment, which is placed on top of a grate with primary air passing up through it and secondary air passing over it. This technology is comprised of a stoker or fuel-feeding system, a moving grate assembly to support the burning mass of fuel, an underfire or primary air beneath the grate, a secondary air system to complete the combustion process and limit atmospheric emissions, and an ash or residual discharge system [39]. A schematic diagram of a grate boiler is shown in Figure 2.2, while the combustion process in a moving grate and the waste reception facility are presented in Figures 2.3 and 2.4, respectively.

Figure 2.3. Moving grate during combustion.

Figure 2.4. Waste reception (bunker) and the crane.

According to Kitto and Stultz [39], at normal operation, the primary air accounts for about 60 % of the total air flow whilst the rest comes from the secondary or overfire air, supplied through the air ports located in the front and rear walls of the furnace. Reduction of excess air in the boiler can be achieved by recirculating the flue gas, which combines with the secondary air and maintains the total flow volume of the combustion air while decreasing the amount of excess air: this helps to improve the boiler’s efficiency and reduce the formation of NOx [39]. A grate-fired boiler has

two essential advantages: the capacity to handle lump-sized waste and the ability to accommodate waste fuel with fluctuating properties [27].

2.3.2 Fluidized Bed Boiler

In a fluidized bed boiler (FBB), finely crushed and sorted waste fuel is required to ensure fluidization of the fuel within the bed. A schematic diagram of this technology is shown in Figure 2.5.

Figure 2.5. Schematic diagram of a solid-waste fired fluidized bed boiler: (1) fluidized bed, (2) fuel feed, (3) primary air, (4) secondary air, (5) empty gas pass, (6) superheaters, (7) cyclone and (8) economizer.

The operation of a fluidized bed boiler involves burning solid waste in an air-suspended bed of inert material such as sand (known as the bed material) and sometimes limestone (when it is added to control SO2) at the bottom of the combustion chamber. It consists of an air distributor with a large number of nozzles placed over the bottom of the furnace [17]. Fluidization occurs when the

air flow causes the bed particles to suspend in the upward stream and start behaving as a fluid [37].

Normally, the fluidized bed temperature is limited to the range of 800 to 900 ^oC in order to prevent the ash and bed particles from agglomerating [27].

There are two main types of fluidized beds in such boilers: bubbling fluidized bed (BFB) and circulating fluidized bed (CFB). In a BFB boiler, the velocity of the air does not cause the particles to move out of the bed, and is the preferred option for moderately-sized boilers [17]. In a CFB boiler, the velocity of the air distributor is sufficiently high to carry some bed material out of the bed container, i.e. it has higher fluidizing velocities, which increase erosion and the fan power required [27]. In this technology, the bed material is recirculated back to the boiler by means of a cyclone to ensure the continuous supply of particles [17].

Chapter 3

Overview of Efficiency Evaluation Methods

Evaluations and analysis of energy conversion processes are crucial for efficient utilization of energy resources in the system [40]. It enables adequate decisions to be made regarding efficiency improvement and the profitability of the process based on the inefficiencies and/or losses identified. Various different evaluation methods have been used in waste-to-energy conversions and thermal process plants and include, for example, the energy efficiency method, the exergy efficiency method, the R-formula and the improvement potential method.

3.1 Energy Efficiency Method

The energy efficiency method is the one used most often to evaluate the performance of a thermal system. Also known as the first law efficiency method, it is based on the first law of thermodynamics or the conservation of energy [41, 42]. It is defined as the ratio of the useful energy output rate to the energy input rate in the system [43], and is calculated using Equation (3.1):

𝜂𝜂e=^𝐸𝐸̇_𝐸𝐸̇^o

i = 1 −^𝐸𝐸̇_𝐸𝐸̇^L

i (3.1)

where 𝐸𝐸̇i and 𝐸𝐸̇o are the energy input and useful energy output rates, respectively, and 𝐸𝐸̇L is the rate of energy loss.

The energy method has been employed to evaluate improvements made to waste-to-energy processes. Main and Maghon [44] used energy analysis to access different improvement options for enhancing the efficiency of modern energy from waste (EFW) facilities located in Hameln/Germany, Arhus/Denmark, Heringen/Germany, Naples/Italy and Ruedersdorf (Berlin)/Germany. In the Afval Energie Bedrijf waste-to-energy plant in Amsterdam, a net electrical efficiency exceeding 30 % was achieved when improvement modifications were applied [45]. Evaluation has also been carried out for a typical EFW plant in Germany operating with

mbar and a net plant efficiency of 20.6 % [46]. Increases in net efficiency of 21.3 %, 23.4 %, 24.0

% and 28.1 % were calculated for an excess air ratio reduced to 1.4, condensate pressure reduced to 30 mbar, steam parameters increased to 74 bar/ 480 °C and steam parameters increased to 130 bar/ 480 °C, respectively, using intermediate reheat.

Even though the energy method is mostly used for system evaluation in a waste combustion plant, the use of this analysis alone for performance criteria in a thermal process is deemed inadequate:

it is bound to lead to misconception, misevaluation and poor decision-making [47]. It does not account for irreversibilities within the system, providing only information of inputs and outputs of energy in the process [48]. Hence, the concept of second law analysis, based on exergy efficiency, was introduced.

3.2 Exergy Efficiency Method

Exergy analysis has been shown to be an effective tool in furthering the goal of attaining a more efficient use of energy resources [49]. Its aim is to identify the locations and magnitudes of thermodynamic irreversibilities in a process. Exergy analysis explicitly takes the effects of the surroundings into account: it provides a more realistic picture of improvement potentials compared to a pure energy analysis. On the other hand, a definition of the state of the surroundings is not always unambiguous, leaving some uncertainties in the analysis. Exergy is the maximum amount of work that can be obtained from a stream of matter as it comes to equilibrium with a reference environment [50]. It combines the utilization of the first and second laws of thermodynamics, using the concepts of irreversibilities identified as a result of entropy generated within the system [51].

Furthermore, exergy is not subject to conservation law, hence it accounts for losses due to irreversibility during any process [50].

For the steady-state process, assuming that the changes in kinetic and potential energy in this particular system can be neglected, the exergy destruction rate for the overall system can be determined from the exergy rate balance, and is given in Equation (3.2):

𝐸𝐸̇𝑥𝑥_D= ∑ 𝐸𝐸̇𝑥𝑥_{i i}− ∑ 𝐸𝐸̇𝑥𝑥_{o o}+ ∑ [1 −^𝑇𝑇_𝑇𝑇⁰

j]

j 𝑄𝑄̇_j− Ẇ (3.2)

where 𝐸𝐸̇xi and 𝐸𝐸̇xo are the input and output, respectively, of the system’s exergy rate, 𝑄𝑄̇j is the heat transfer rate in at position j through the boundary at temperature 𝑇𝑇j and Ẇ is the net work transfer rate out across the boundary of the system.

In the case of two separate fluid streams interacting (such as in boiler heat exchangers, condensers, air preheaters and feedwater heaters), the exergy destruction rates are obtained from Equation (3.3), assuming no heat loss to the surroundings, as follows:

𝐸𝐸̇𝑥𝑥D= ∑ (𝑚𝑚̇i h,i𝑒𝑒𝑥𝑥h,i+ 𝑚𝑚̇c,i𝑒𝑒𝑥𝑥c,i)− ∑ (𝑚𝑚̇o h,o𝑒𝑒𝑥𝑥h,o+ 𝑚𝑚̇c,o𝑒𝑒𝑥𝑥c,o) (3.3)

where 𝑚𝑚̇h and 𝑚𝑚̇c are the mass flow rate of the hot and cold stream, respectively.

The process exergy efficiency for a heat and power plant, 𝜂𝜂ex, is expressed as Equation (3.4):

𝜂𝜂ex=^{𝐸𝐸̇𝑥𝑥Qh}_{∑ 𝐸𝐸̇𝑥𝑥}^{+𝑊𝑊̇net}

i

i =∑ (Exergy available)a

∑ (Exergy input)_i (3.4)

where 𝐸𝐸̇𝑥𝑥Q_h is the exergy flow rate associated with the production of heat and Ėxi is the exergy input rate given in Equation (3.5).

𝐸𝐸̇𝑥𝑥i= 𝑚𝑚̇f· 𝑒𝑒𝑥𝑥^ch (3.5)

where 𝑒𝑒𝑥𝑥^ch is the specific exergy of the fuel. For a solid-waste fuel, such as municipal solid waste, the specific chemical exergy can be calculated by Equation (3.6): this is taken from the model developed in Paper II and is based on the elemental composition of waste fuel.

𝑒𝑒𝑥𝑥^ch= 376.461C + 791.018H − 57.819O + 45.473N − 1536.242S + 100.981Cl (kJ/kg) (3.6)

where C, H, O, N, S and Cl are the content of carbon, hydrogen, oxygen, nitrogen, sulphur and chlorine, respectively, in solid waste in wt %.

Exergy analysis has been used widely in the evaluation of thermal processes. Bejan et al. [52]

investigated the application of the exergy method in the thermal process of a co-generation system that included a gas turbine and heat-recovery steam generator. This method has been applied in the coal combustion process [53-70], the biomass conversion process [71-76] and the coal-biomass co-combustion process [77-79]. In the case of waste-to-energy combustion, Solheimslid et al. [8]

used different methods to calculate the chemical exergy of solid waste by employing correlations, the chemical exergy obtained from the combustion equation and the absolute entropy to determine the exergy efficiency of a combined heat and power plant fired by municipal solid waste in Bergen, Norway. They found the results of the different methods to be in good agreement. Grosso et al.

[80] found that exergy analysis was a more reliable measure of performance criteria in waste incineration plants in Europe than the energy recovery efficiency analysis proposed in the Waste Frame Directive [81].

The exergy method has been used as an effective method of evaluating the performance of a system because it enables the main sources of inefficiency to be identified and provides directions for improvement enhancement within the system This method alone, however, does not provide information of the improvements that are possible (improvement potential) in a real process.

3.3 R1-formula Method

The R1-formula method is proposed by the European Union, under the new Waste Frame Work Directive (WFD), used as a guideline for improving the energy performance of waste-to-energy combustion plants. The directive allows waste combustion plants to be classified as recovery operations rather than disposals, provided than the energy recovery efficiency is higher than a designated threshold of equal to or above 0.60 for installations in operation and sanctioned before 1^st January 2009, and 0.65 for installations sanctioned after 31^st December 2008 [81]. The R1- formula is given in Equation (3.7) thus:

Energy efficiency =_{0.97∗(𝐸𝐸}^{𝐸𝐸p−(𝐸𝐸f+𝐸𝐸i)}

w+𝐸𝐸_f) (3.7)

(GJ/year). 𝐸𝐸f is the annual energy input to the system from fuels contributing to the production of steam (GJ/year). 𝐸𝐸w is the annual energy contained in the treated waste calculated using the net calorific value of the waste (GJ/year). 𝐸𝐸i is the annual energy imported, excluding 𝐸𝐸w and 𝐸𝐸f

(GJ/year). 0.97 is a factor accounting for energy losses due to bottom ash and radiation.

The use of the R1-formula for the evaluation of the energy performance of waste combustion may be inadequate because it does not consider the size of the plant and its operation based on climate regions. In addition, the energy efficiency method used does not account for the quality of the energy in the heat produced during district heating.

3.4 Improvement Potential Method

The need to improve the exergy-based method of evaluation led to the introduction of the exergy improvement potential, which was proposed by van Gool [82] for industrial processes, given in Equation (3.8) as:

IPk= (1 − 𝜂𝜂𝑒𝑒𝑒𝑒,𝑟𝑟𝑟𝑟)ĖxD_k,rp (3.8)

This method, which has been applied to the evaluation of energy systems in the UK [83] in a coal thermal plant [84] and for solar energy [85], relates the improvement potential to the inefficiency experienced in a system with its exergy efficiency. Nevertheless, the improvement is limited to the current performance of the specific real process system without taking any future development in the system into consideration. The method does not compare a specified process with its theoretical process for potential advancement and relative progression in the system.

Improvement that has been made to exergy analysis has led to the development of advanced exergy analysis [86], which involves dividing the destruction of exergy into two parts: avoidable and unavoidable. Avoidable exergy destruction is defined as the irreversibility that can be prevented through performance enhancement, whilst unavoidable exergy destruction occurs as a result of physical, technological and economical constraints. Here, the improvement potential lies in the former, i.e. avoidable, part of exergy destruction. The avoidable exergy destruction rate for a

Eẋ𝐷𝐷,𝑘𝑘AV = Eẋ𝐷𝐷,𝑘𝑘− Eẋ𝐷𝐷,𝑘𝑘UN (3.9)

This method has been applied to a gas turbine co-generating system [86], a combined cycle power plant [87], a fluidized bed boiler [88] and a geothermal power plant [89]. The unavoidable exergy destruction rate is determined by selecting the most important thermodynamic parameters of the component being studied to give its maximum achievable efficiency [86]. Although this method compares the real process with an advanced process, the efficiency improvement is limited to current technological constraints. Moreover, efficiency limited by technology is not predictable: it may change over time for a given process [90] as a result of subjective decisions [86].

3.5 Chemical Exergy of Solid Fuels

The chemical exergy of the fuel in question is a basic property to be considered in the system analysis of energy conversion processes: it is used to estimate the maximum available energy entering the system for performance evaluation and optimization of the process. However, the exergy values of some solid fuels with unknown structure and chemical compositions cannot be determined directly due to the lack of standard absolute entropy values [91]. Hence, many models have been proposed based on the characteristics of known homogeneous organic substances in the fuel. Rant [92] was the first person to model the chemical exergy of a structurally complicated material by using organic substances of known absolute entropies. Future improvement of Rant’s model was carried out by Szargut and Styrylska [93], who took the elemental composition of the fuels into consideration. Some elements, such as sulphur and chlorine, however, were not considered. The use of standard entropy and chemical constituents of solid fuels to model the chemical exergy have also been investigated specifically for coal conversion [94-97] and biomass conversion processes [98].

3.6 Research Necessary

The use of an efficient evaluation method to assess the performance of a thermal process, specifically a waste-to-energy combustion process, may ensure that adequate decisions are taken towards making improvements in the efficiency of the system. Although some improvements have been made in this area, previous efficiency evaluation methods have at least one of the following

limitations: inability to consider the entropy generated within a system; inability to consider what improvements are possible in a process; limiting improvements based on the current performance and technological constraints of the system. In addition, as mentioned previously, the chemical exergy of solid waste is vital for the performance analysis and optimization of a combustion process. Here, not only the energy content of the solid waste is considered but also the entropy generated in the waste conversion process. Previous models have specifically considered the chemical exergy of solid fuels such as coal and biomass. These models cannot be used for estimating the chemical exergy of substances containing elements other than C, H, O, N, and S or for combustible materials, such as certain categories of leather, plastic and rubber, that all form a part of municipal solid waste.

Two exergy-based methods, applicable to the waste combustion process, are therefore proposed in this thesis: an exergy model to calculate the maximum available energy in a solid waste fuel and a modified exergy method to determine the improvement that is possible in a process. Both methods were applied to a solid-waste fired heat and power plant to evaluate improvements, taking into consideration the state-of-art-technologies used in the system. Furthermore, the cost and profitability of different improvement measures were evaluated to ascertain the economic viability of the process modifications.

Chapter 4

Improving Efficiency Evaluation Methods

Improving the efficiency of a process is important for the advancement of energy recovery and productivity; it can only be accurate when a suitable method is used. In this chapter, an overview is given of exergy analyses used for the efficiency evaluation of solid fuel conversion process.

This is followed by the introduction of methods for estimating the maximum available energy in a solid waste and calculating improvements possible in waste-to-energy combustion plant.

4.1 Using Exergy as a Method for Evaluating Efficiency

A comprehensive study was carried out on the use of exergy analysis with respect to evaluating, comparing performance and suggesting improvements for solid fuel-fired heat and power plants that use coal, biomass and a combination of these feedstocks as fuels (Paper I). The exergy and energy efficiency, the exergy destruction in each component and the overall process based on the mass, energy, entropy and exergy balance were compared. This was done by reviewing research work from the literature, which includes both existing plants and simulated processes.

The various studies of solid fuel-fired plants identify the boiler as the component with the highest exergy destruction, which is a result of irreversible combustion reactions, and the large temperature difference between the combustion gas and the feedwater. The results also showed that the overall exergy efficiency and exergy destruction are lower than the overall energy efficiency and heat loss, respectively. Table 4.1 shows the exergy destruction, heat loss and entropy generation of different components in a coal-fired thermal plant [53]. It can be seen that the higher the entropy generated, the greater the exergy destruction. According to energy analysis, the major energy losses in the plant were due to heat rejection in the condenser as a result of the large enthalpy difference between the turbine and the condenser, whereas exergy analysis showed that less than 20 % of the total exergy destruction is to be found in this component. It substantiates the fact that using the energy method for efficiency evaluation is bound to lead to misconception, misevaluation and poor decision-making [47].

Table 4.1[53]

Results from the analysis of a coal-fired thermal plant.

Component Exergy destruction

(kW) Heat Loss

(kW) Entropy Generation (kW/K)

Boiler 73046 12663 3312.0

Turbine 6403 3242 17.2

Air-cooled condenser 1622 33283 3.3

De-aerator 886 71 1.4

LP heater 552 336 2.4

HP heater 759 65 3.7

Boiler feed pump 375 140 0.0

Generator 550 656 0.9

Total 84193 50456 3339.9

An decrease in the amount of excess air, with a reduction in flue gas losses, and an increase in the temperature of the combustion air have shown to improve both the boiler performance and the overall exergy efficiency [56, 99]. The boiler’s efficiency can also be enhanced by using a feedwater heater to reduce the temperature difference between the flue gas inside the boiler and the working fluid [58].

4.2 Estimating the Chemical Exergy of Solid Waste

Solid waste is a heterogeneous substance in nature, with a complex structure and lacking an exact value of its chemical exergy. The direct value of the chemical exergy of a material with a complicated structure is difficult to determine [100] when the standard chemical exergy of a substance that is in the environment is unavailable. Therefore, the standard chemical exergy of a substance that is not present in the environment can be evaluated by considering a reaction of the substance with other substances for which the chemical exergies are known [52,101]. In this section, a model for calculating the maximum available energy in solid waste is proposed (Paper II). It is based on the elemental composition of the waste that involves C, H, O, N, S and Cl on a dry ash-free (daf) basis. The model was obtained from the standard exergy values of waste combustion products as well as from estimations of the higher heating value and standard entropy of MSW using statistical analysis. Here, 1 kg of MSW (daf), expressed as 𝐶𝐶m𝐻𝐻n𝑁𝑁p𝑂𝑂q𝐶𝐶𝐶𝐶r𝑆𝑆t, is considered to undergo complete combustion at standard state under steady conditions to produce carbon dioxide, water, nitrogen, hydrogen chloride and sulphur dioxide, the standard chemical exergies of which are known. The standard chemical exergy and standard entropies of various

compounds are presented in Table 4.2, while the combustion reaction of the solid waste is given in Equation (4.1) as follows:

𝐶𝐶m𝐻𝐻n𝑁𝑁p𝑂𝑂q𝐶𝐶𝐶𝐶r𝑆𝑆t+ (m + t −^q₂+^n−r₄ ) O2→ m𝐶𝐶𝑂𝑂2+ (^n−r₂ ) 𝐻𝐻2O +^P₂𝑁𝑁2+ rHCl + t𝑆𝑆𝑂𝑂2 (4.1)

where m, n, p, q, r and t are the numbers of atoms of C, H, N, O, Cl, and S, respectively, in kmol/kg MSW.

The maximum work that occurs when there is no irreversibility in a system is obtained from the energy and entropy balances in Equation (4.1) for the steady state under standard conditions, and is expressed in Equation (4.2) thus:

𝑊𝑊max= 𝐻𝐻𝐻𝐻𝐻𝐻msw− To[𝑠𝑠mswo + (m + t −^q₂+ⁿ⁻¹₄ ) 𝑠𝑠_O^o₂− m𝑠𝑠_CO^o ₂− (ⁿ⁻¹₂ ) 𝑠𝑠_H^o_2O−^p₂𝑠𝑠_N^o₂− r𝑠𝑠_Hcl^o − t𝑠𝑠_SO^o₂] (4.2)

where 𝐻𝐻𝐻𝐻𝐻𝐻msw is the higher heating value obtained from the heat of reaction of the combustion process, ∆𝐻𝐻𝑟𝑟𝑜𝑜, presented in Equation (4.3) [96]:

∆𝐻𝐻𝑟𝑟𝑜𝑜= −𝐻𝐻𝐻𝐻𝐻𝐻 (4.3)

Considering the waste combustion reaction in Equation 4.1 at adiabatic process with no irreversibility, the exergy balance equation is given in Equation (4.4) as:

0 = 𝑊𝑊max+ 𝑒𝑒MSW+ (m + t −^q₂+ⁿ⁻¹₄ ) 𝑒𝑒_O^o₂− m𝑒𝑒_CO^o ₂− (ⁿ⁻¹₂ ) 𝑒𝑒_H^o_2O−^p₂𝑒𝑒_N^o₂− r𝑒𝑒_Hcl^o − t𝑒𝑒_SO^o ₂ (4.4)

where 𝑒𝑒 is the specific chemical exergy. By substituting Equation (4.2) into Equation (4.4), the specific chemical exergy of MSW (daf), 𝑒𝑒MSW, can be presented as Equation (4.5):

𝑒𝑒MSW= 𝐻𝐻𝐻𝐻𝐻𝐻MSW− To[𝑠𝑠mswo + (m + t −^q₂+ⁿ⁻¹₄ ) 𝑠𝑠Oo2− m𝑠𝑠COo 2− (ⁿ⁻¹₂ ) 𝑠𝑠Ho2O−^p₂𝑠𝑠No2− r𝑠𝑠Hclo − t𝑠𝑠SOo2] + m𝑒𝑒COo 2+ (ⁿ⁻¹₂ ) 𝑒𝑒_H^o_2O+^p₂𝑒𝑒_N^o₂+ r𝑒𝑒_Hcl^o + t𝑒𝑒_SO^o₂− (m + t −^q₂+ⁿ⁻¹₄ ) 𝑒𝑒_O^o₂ (4.5)

where e^o and s^o are the standard exergy in kJ/mol and the standard entropy in kJ/mol·K, respectively, and given in Table 4.2.

Table 4.2

Standard chemical exergy and standard entropies of various compounds.

Substance e^𝑜𝑜(kJ/mol) s^𝑜𝑜(kJ/mol·K)

CO2 19.87 0.214

H2Ol 0.95 0.070

O2 3.97 0.205

N2 0.72 0.192

SO2 310.93 0.248

SiO2 1.636 0.041

HCl 85.5 0.187

CaO 129.881 0.038

K2O 412.544 0.102

P2O5 377.155 0.117

Al2O5 4.479 0.051

MgO 62.417 0.027

Fe2O3 17.656 0.087

SO3 242.003 0.257

Na2O 296.32 0.075

MnO 122.390 0.060

ZnO 37.080 0.042

Cr 538.610 0.024

Pb 226.940 0.065

As 477.040 0.035

Cd 290.920 0.052

Cl 163.940 0.166

l, liquid phase.

Source: [49, 98]

4.2.1 Estimating the Higher Heating Value and Standard Entropy of MSW

The higher heating value of the waste was derived statistically using a regression model based on the elemental composition of six categories of combustible waste fractions, namely food, wood, paper, textiles, plastics and rubber. Here, 56 and 30 data points of MSW were used for derivation and validation of the correlation, respectively. For further validation, the HHV of waste derived was compared with the experimental values and the previous models.

The standard entropy of MSW was derived from organic compounds of known standard entropies located in the molecular structure of the various waste fractions mentioned earlier. The molecular structure was obtained from the organic polymers in each of the waste fractions. For instance, cellulose, hemicellulose and lignin are the three major polymers found in wood waste, in which organic compounds such as glucose, lactose, galactose, sorbose, sucrose, xylose and hydroxybenzoic acid are present. Polymers such as proteins and lipids are seen in food waste, in which urea, lactic acid, malic acid, maleic acid, citric acid and stearic acid, all with known standard entropies, are found. Based on the standard entropies and the elemental compositions of the selected organic substances, a correlation was statistically obtained for the standard entropy of the waste fractions and the mixture.

The models derived for the higher heating value, the standard entropy and the exergy of solid waste are presented in Equations (4.6), (4.7) and (4.8), respectively.

HHV = 0.364C + 0.863H − 0.075O + 0.028N − 1.633S + 0.062Cl (MJ/kg) (4.6)

𝑠𝑠mswo = 0.0101C + 0.0630H + 0.0106O + 0.0108N + 0.0155S + 0.0084Cl (kJ/K^·kg) (4.7)

𝑒𝑒msw= 376.461C + 791.018H − 57.819O + 45.473N − 1536.242S + 100.981Cl (kJ/kg) (4.8)

The results of the validation of the derived model of HHV and comparisons with published correlations using the experimental values of 30 samples of MSW in the different categories of food, wood, plastic, textile, rubber and paper waste are depicted in Figures 4.1–4.4. R², AAE and ABE denote the coefficient of determination, the average absolute error and average bias error of a correlation, respectively. These three parameters are the important statistical criteria employed to assess correlations [102]. A higher R² value with smaller values of AAE and ABE mean a higher accuracy of the estimation.

Figure 4.1. Comparison between the experimental and Figure 4.3. Comparison between the experimental and estimated HHV using the developed correlation. estimated HHV using the Sheng and Azevedo [104] correlation.

Figure 4.2. Comparison between the experimental and estimated Figure 4.4. Comparison between the experimental and HHV using the Channiwala and Parikh [103] correlation. estimated HHV using Dulong’s correlation [103].

The developed correlation for HHV shows a better estimation than the previous models where the average absolute error, AAE, and average bias error, ABE are concerned. The ratio of exergies to heating values of 1.036 of the developed model is similar to the 1.047 obtained from the Szargut and Styrylska comparison [93], which is the model commonly used for evaluating the chemical exergy of solid fuels. It shows that the present model is reliable and accurate, while the slight variation in the ratio is due to the different types of fuel used.

0 10 20 30 40 50

0 10 20 30 40 50

HHVest (MJ/kg)

HHVexp (MJ/kg) R²= 0.95

AAE = 5.7%

ABE = 0.03%

0 10 20 30 40 50

0 10 20 30 40 50

HHVest (MJ/kg)

HHVexp (MJ/kg)

0 10 20 30 40 50

0 20 40 60

HHVest (MJ/kg)

HHVexp (MJ/kg)

0 10 20 30 40 50

0 10 20 30 40 50

HHVest (MJ/kg)

HHVexp (MJ/kg) R² = 0.92

AAE= 9.7%

ABE = -3.7%

R² = 0.95 AAE= 11.8%

ABE = -4.8%

R²= 0.95 AAE= 6.5%

ABE = 2.3%

4.3 Improvement Potential Method

The improvement potential provides information of the improvement that is possible to attain in both the components and the overall process. It gives a more realistic description of the changes that are possible with respect to the constraints of the conversion pathway selected. In Paper III, a modified exergy-based method is introduced that links the exergy efficiency to the total exergy destruction and compares the exergy destruction of the real process with its theoretical process. It is presented in Equation (4.9) thus:

𝐼𝐼𝐼𝐼_k= 𝐸𝐸̇𝑥𝑥_Dk,rp−^{(1−𝜂𝜂}_{𝜂𝜂𝑒𝑒𝑒𝑒,𝑡𝑡𝑡𝑡}^{𝑒𝑒𝑒𝑒,𝑡𝑡𝑡𝑡)}𝐸𝐸̇𝑥𝑥_Pk,rp (4.9)

where 𝐸𝐸̇𝑥𝑥D_k,rp and 𝐸𝐸̇𝑥𝑥P_k,rp are the exergy destruction and product exergy, respectively, of a particular component in the real process and ^{(1−𝜂𝜂}_𝜂𝜂 ^ex,tp)

ex,tp is the exergy destruction per unit of product exergy under theoretical conditions.

The theoretical process can be determined when the optimal values of the real process have been reached. It is achieved by using the parameters of each component of the process plant that give its maximum efficiency without being limited technologically by the cost and properties of the materials used in the system. These parameters are subject to constraints: the combustion process in the boiler that converts the chemical energy in the fuel into thermal energy in its heat exchanger, and a Rankine cycle that converts the energy in the steam into electricity and district heat. In the boiler combustion process, the adiabatic flame temperature (the maximum temperature that can be achieved for the given fuel) is chosen and used to optimize the overall system based on variations in steam pressure and extraction pressures. Although it is not anticipated that technological enhancements will reach their theoretical limits the latter do, however, provide information of the progress that is possible and the improvements that are necessary in the former.

Chapter 5

Evaluating Efficiency Improvement

The modified exergy-based method, Equation 4.9, was used to evaluate the improvement potential of both the individual components and the overall base case plant. The results were then used to decide upon which technical modifications should be introduced.

5.1 The Base Case Plant

The base case plant is a typical grate boiler of waste-to-energy combined heat and power plant. A schematic diagram of the plant is shown in Figure 5.1, while Figure 5.2 shows the Aspen Plus model of the process plant.

Figure 5.1 Schematic diagram of the municipal heat and power plant fired by solid waste.