Evaluation of Multi-Utility Models with Municipal Solid Waste Combustion as the Primary Source under Specific Geographical and Operating Conditions

Abstract

Developments in waste incineration technology in terms of efficient fuel preparation, combustion, and emissions reduction, as well as the growing needs of the community in terms of electricity, water, and air conditioning loads, are the prime motive for this study. This study presents a novel approach, in which three models of the fluidized bed combustion of municipal waste for simultaneous power generation, freshwater production, and district cooling are analyzed for their energy and exergy performance. The three simultaneously evaluated utility models are different configurations of a fluidized bed combustion system with Rankine cycle power generation, cooling with a vapor absorption refrigeration system, and fresh water production using multiple effect desalination. The output from the turbine, cooling system, and desalination system is determined using the Engineering Equations Solver for different boiler operating pressures. Energy and exergy analysis data for different pressures are used to identify the best configuration. Two variants of the absorption cooling system, namely, single effect and double effect, are considered. The variants of the multiple-effect desalination are the three-stage and five-stage methods. Input parameters used in this study are municipal solid waste generation and composition data collected for an urban community in an arid climate zone with high demand for electric power, cooling, and fresh water. Model 2, which contains two turbines with the reheating and cooling systems connected to a high-pressure turbine and water desalination connected to a low-pressure turbine, gave the best overall performance. Significant savings in terms of the replacement of conventional energy were observed from these waste conversion plants with greater benefits in arid weather conditions. The results obtained by different models under different operating criteria constitute a guideline for municipal planners for the selection of appropriate waste utilization technology, as well as the appropriate operations.

1. Introduction

Escalating energy generation costs have resulted in planners considering the option of using municipal solid waste (MSW) for energy. The specific economic and geographical conditions of a community determine the distribution and utilization of this energy. A typical requirement in arid regions of the world is energy for cooling buildings and desalination for freshwater production. Power consumption for these utilities is seasonal in nature, with demand more than doubling in the summer compared to the winter. The incineration of waste to produce energy to meet these requirements and simultaneously eradicate waste is becoming acceptable due to recent developments in emissions reduction technology. Studies to quantify the energy requirement for cooling in Qatar were conducted after considering the impact of climate change and the emissions scenario in 2071, and a 13–53% increase in energy requirement was predicted [1]. About 20% of the world’s desalinated water is produced in Saudi Arabia and the highest total production is achieved by the Middle Eastern countries that are situated in regions with arid desert climates [2]. Studies on the utilization of MSW for combined cooling and freshwater production have been conducted in order to assess the potential of replacing fossil fuel energy consumption [3]. However, the waste-to-energy route constituted a gasification process and the cooling route was achieved by adsorption cooling technology. Studies on the assessment of using waste energy for cooling or waste energy for freshwater production have been conducted by researchers separately in several publications [4,5]. However, combined cooling and freshwater production using MSW combustion with a fluidized bed under different configurations has not yet been reported. The present study involves simultaneous cooling and desalination using the fluidized bed combustion of MSW, which has been achieved before. Additionally, the cooling mechanism adopted here is the vapor absorption (VAR) method.

Compared to GF, FB is more adaptable to variations in fuel qualities and also to co-firing with supplementary fuels [6]. This technique is considered to be more environmentally favorable, and emissions can be controlled better due to lower bed temperatures compared to GF systems. The bed material can be provided, along with certain chemical agents that can reduce undesirable emissions and agglomerate-forming materials [7]. Despite the above advantages, FB systems need pre-treatment in terms of size reduction and the removal of heavy particles [8]. Multi-stage flash (MSF) desalination, multiple-effect desalination (MED), and thermal vacuum compression (TVC) desalination are the most common kinds of thermally driven water desalination plants. The MED process consumes much less electrical power than the other methods and operates at much lower temperatures (<70 °C) and with lower corrosion and scaling problems [9]. Low-grade heat from power plants can act as the energy supply for the desalination process. Both GF and FB systems are currently used to drive power plants using the Rankine cycle [10]. The capacities of such units are determined based on the MSW generation rate and its quality in terms of the heating value and types of emissions. Centralized cooling for communities, also referred to as district cooling systems, are economically as well as environmentally feasible, since low-grade heat is used to operate these systems directly using vapor absorption refrigeration (VAR) machines. The application of lower-grade energy in the form of heat increases the exergy efficiency of the systems compared to conventional plants where electricity is used for cooling applications. The individual consumers extract the cooling from centralized systems, and the cooling load depends on the prevailing geographical conditions [5,11]. Two options are available to supply heat to the VAR: direct heating by the incinerator’s heat or using some of the heat from the steam generation unit [12].

The study reported in this paper involves using data obtained in an arid climate zone (BWh) as per the Koppen classification [13]. The MSW data collected are used to produce thermal energy using a fluidized bed incinerator-cum-boiler. Steam produced from this Rankine cycle is used to produce electricity from a turbo generator, cooling energy using a VAR cooling system and freshwater using a MED system. Three types of power generation scenarios using FB combustion boilers were considered in the study. Firstly, a single power turbine with intermediate steam extraction for the VAR system and exit steam utilization for the MED system is considered. Secondly, two turbines, high pressure (HP) and low pressure (LP), are used wherein the exit from the HP turbine is used for the VAR and the exit steam from the LP is used for the MED system. In the third case, a single turbine with the condensate supplied to a VAR followed by a MED system is considered. Two different configurations of VAR, namely, single effect and double effect, and three different configurations of MED, namely, double, triple and quadruple were analyzed in the study. The results of the performance parameters are obtained under different operating conditions. Results are compared with data available in other cities under different climate zones.

2. Component Specifications and Operation

2.1. Fluidized Bed Combustion (FBC) System

A fluidized bed combustion system of the bubbling type is considered as illustrated in Figure 1. Fuel is fed along with a jet of air at the bottom of the furnace which is normally square or rectangular in cross-sectional area. An array of nozzles supply the air that maintains the fluidization of the bed material and the fuel. The fuel constitutes only a part of the bed and silica sand is used as bed material, thereby maintaining a large thermal capacity in order to maintain a bed temperature between 800 and 900 °C [14]. The unburnt remains are normally reduced and some of the pollutants are reduced in the bed by adding certain additives. FB systems using MSW require pre-treatment in order to reduce their size and prevent large particles from entering the bed in order to prevent obstruction to the fluidization process [7]. The economizer heats up the boiler water and steam generation takes place in the generator bank. Saturated steam is separated in the steam drum followed by the super-heater which supplies super-heated steam to the turbines. In the case of reheating, outlet steam from the turbine re-enters for additional heating in the FB. Fuel preparation in FB systems involves shedding for size reduction and removal of heavy components using magnetic and eddy current separators [8].

2.2. VAR System

A VAR system operates on the principle of absorption and separation of the refrigerant by the absorbent fluid at different pressures as described in Figure 2. Absorption chillers use ammonia–water combination or lithium bromide combination normally. In the first plant, water acts as an absorbent and ammonia acts as refrigerant. In the second plant, lithium bromide is absorbent and water acts as refrigerant. The refrigerant and absorbent are separated inside the generator which uses heat from sources such as solar, waste heat from processes or combustion product gases from renewable or non-renewable fuels [15].

VAR systems can be single effect, double effect or triple effect, depending on the number of heating cycles performed in order to separate the absorber–refrigerant pair. In this study, single and double effect systems are considered for analysis and the energy from steam produced by the combustion of MSW is used as the generator heat source. Figure 2 gives the process description, in which strong solution from the absorber (1) is pumped (2) to the generator at state (3) where the refrigerant is separated from the absorbent (7) to form the weak solution (4). Weak solution then exchanges heat with strong solution and expands from generator pressure (5) to absorber pressure (6). The refrigerant is then condensed after which it is expanded in the expansion device (8) and then sent to the evaporator (9) to produce the cooling and leaving as saturated vapor to the absorber (10). The quantity of heat required depends on the generator’s operating temperature and the cooling capacity. Theoretical generator temperature depends on the coefficient of performance (COP) of the system as given in Equation (1).

$$\mathrm{COP}=\left(\frac{{\mathrm{T}}_{\mathrm{e}}}{{\mathrm{T}}_{\mathrm{o}}-{\mathrm{T}}_{\mathrm{e}}}\right)\times \left(\frac{{\mathrm{T}}_{\mathrm{g}}-{\mathrm{T}}_{\mathrm{o}}}{{\mathrm{T}}_{\mathrm{g}}}\right)$$

(1)

In the above equation, ${\mathrm{T}}_{\mathrm{e}}$,${\mathrm{T}}_{\mathrm{o}}$, ${\mathrm{T}}_{\mathrm{g}}$, represents the evaporator, condenser and generator temperatures, respectively. For example, the theoretical COP of a VAR with an evaporator temperature of 278 K, condenser temperature of 320 K and generator temperature of 363 K is found to be 0.78.

2.3. MED System

The MED process involves evaporation of brackish water using more than one stage by utilizing heat at very low temperatures, which makes them suitable for utilizing low grade heat. The process is illustrated in Figure 3.

The thermal energy required for desalination in the MED process depends on the quantity of fresh water to be generated. The energy for the operation of a 1 m³ fresh water production unit is 1.5–2.5 kWh of electrical energy and 5–8.5 kWh equivalent of thermal energy [16]. The energy produced from waste combustion can be utilized for water production. The heat from the steam produced by MSW is used only for the initial stage. Subsequent stages make use of heat from the preceding stage.

3. Methods and Criteria

Three different models for the utilization of MSW are considered in this study. MSW is received from the collection system and its preparation for combustion starts with separation of non-combustible and hazardous materials. This also ensures trouble free operation in the size reduction machinery. MSW which is stored after preparation is supplied to the FBC by pneumatically assisted feeding mechanism. The arrangement of the power generation turbine-generator, the VAR cooling machine and the MED desalination process for models 1, 2 and 3 are shown in Figure 4, Figure 5 and Figure 6, respectively. In the case of Model 1, a single turbine expands the steam (superheated) and condenses it at the exit of turbine. In between the inlet and exit, 50% of the steam is utilized in a superheated state for heating in the generator of the VAR system. Condensed steam from the turbine exit is used to operate the MED system. The steam from the MED is then pumped using pump 1 to the VAR exit pressure and then further pumped to the boiler pressure using pump 2. The open feed water heater (OFWH) is used to raise the temperature before feeding into the boiler.

Figure 5 gives the schematic of Model 2 wherein two turbines are used. The high-pressure turbine supplies 50% of the steam to the VAR and the remaining steam is reheated and supplied to the MED system. The steam from the outlet of the VAR and MED are mixed in the OFWH and further pumped to the boiler using pump 2. The heat produced by the combustion of the fuel is supplied to the water entering at state point 10 and leaves the boiler at 1 as superheat steam and at 4 as reheated superheat steam. The HP turbine expands the steam from state 1 to 2 and the LP turbine expands steam from 4 to 5.

Model 3, as shown in Figure 6, gives the arrangement of the VAR and MED arranged in series at the exit of the turbine. Steam leaves the turbine in a saturated state and a part of the heat is used for VAR and the remaining heat is used up in the MED. The water re-enters the boiler at a much lower temperature compared to the previous two models in this case.

3.1. Energy and Exergy Analysis

Energy output from the turbine is the electricity generated, a process which depends on turbine efficiency and generator efficiency. All the models in this study use multistage condensing turbines which produce maximum power and electrical generation efficiency. In Model 1, steam is extracted at an intermediate pressure to supply the VAR and then condensed to supply the MED. The pressure of the condensed steam is below atmospheric pressure and the corresponding saturation temperature is sufficient to operate the MED system. The efficiency of steam turbines (η_T) used for high capacities of the order of 1000 kW and more is 0.9 [17]. Different energy losses, such as heat dissipation due to friction, prevent the steam turbine from having 100% efficiency. The electricity generator efficiency is assumed as 1 in all cases. Energy balance equations are based on the first law of thermodynamics and all the mass balances are given for steady state conditions. Equation (2) gives the continuity equation and Equation (3) gives the energy equation.

$${{\displaystyle \sum }}_{\mathrm{inlet}} \dot{\mathrm{m}} ={{\displaystyle \sum }}_{\mathrm{outlet}} \dot{\mathrm{m}}$$

(2)

$${\dot{\mathrm{Q}}}_{\mathrm{in}}+{{\displaystyle \sum }}_{\mathrm{inlet}} \dot{\mathrm{m}} \mathrm{h}= {\dot{\mathrm{w}}}_{ \mathrm{out}}+{{\displaystyle \sum }}_{\mathrm{outlet}} \dot{\mathrm{m}} \mathrm{h}$$

(3)

The heat input ${\dot{\mathrm{Q}}}_{\mathrm{in}}$ depends on the mass flow rate,$\dot{\mathrm{m}}$, the lower heating value (LHV) of MSW and the combustion efficiency of the FBC system as given in Equation (4). ${\dot{\mathrm{w}}}_{\mathrm{out}}$, represents the work output, ‘h’ represents specific enthalpy and ${\mathrm{\eta }}_{\mathrm{C}}$ is the combustion efficiency.

$${\dot{\mathrm{Q}}}_{\mathrm{in}}={\dot{\mathrm{m}}}_{\mathrm{MSW}}\times {\mathrm{LHV}}_{\mathrm{MSW}}\times {\mathrm{\eta }}_{\mathrm{C}}$$

(4)

The power output from the steam power plant is the turbine work which, in the case of more than one stage, is given by Equation (5). In this equation, ‘i’ represents the number of steam turbines, n represents the number of extractions and h_c represents the specific enthalpy at condensation pressure.

$${\dot{\mathrm{w}}}_{\mathrm{out}}={\mathrm{\eta }}_{\mathrm{T}}\left[{\dot{\mathrm{m}}}_{\mathrm{i}}\left({\mathrm{h}}_{\mathrm{i}}-{\dot{\mathrm{m}}}_1\right)+\left({\dot{\mathrm{m}}}_{\mathrm{i}}-{\dot{\mathrm{m}}}_1\right)\left({\mathrm{h}}_1-{\mathrm{h}}_2\right)+\left({\dot{\mathrm{m}}}_{\mathrm{i}}-{\dot{\mathrm{m}}}_1-{\dot{\mathrm{m}}}_{\mathrm{n}}\right)\left({\mathrm{h}}_{\mathrm{n}}-{\mathrm{h}}_{\mathrm{c}}\right)\dots \dots .\right]$$

(5)

The efficiency of the pumps, ${\mathrm{\eta }}_{\mathrm{P}}$, depends on the inlet and outlet enthalpy of the pump divided by the work input to the pump as given by the following equation.

$${\mathrm{\eta }}_{\mathrm{P}}=\frac{\dot{\mathrm{m}}\left({\mathrm{h}}_{\mathrm{i}}-{\mathrm{h}}_{\mathrm{o}}\right)}{{\mathrm{W}}_{\mathrm{pump}}}$$

(6)

$\dot{\mathrm{m}}$ represents the mass flow rate, ${\mathrm{h}}_{\mathrm{i}}$ and ${\mathrm{h}}_{\mathrm{o}}$ represent inlet and outlet specific enthalpy of steam in all the above equations and ${\mathrm{W}}_{\mathrm{pump}}$ represents the pump work.

Exergy is basically classified as physical exergy, chemical exergy, potential exergy and kinetic exergy. In this case, only the physical exergy is considered. Exergy is the maximum possible work based on the ground state of the condition of temperature, which is normally the ambient temperature. The exergy efficiency of a process is the ratio of the total exergy outlet,${\mathrm{Ex}}_{\mathrm{out}}$, divided by the total exergy inlet to the process, ${\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}$, Equation (13). In this case, the total exergy outlet, ${\mathrm{Ex}}_{\mathrm{Total},\mathrm{out}}$, is the sum of the exergy of turbine work, the VAR and the MED, ${\mathrm{W}}_{\mathrm{Total},\mathrm{Turbine}+\mathrm{VAR}+\mathrm{MED}}$, Equation (12). The exergy inlet, ${\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}$, occurs in the FBC boiler,${\mathrm{Ex}}_{\mathrm{th},\mathrm{FBC}}$, the pumps, ${\mathrm{W}}_{\mathrm{Total},\mathrm{pumps}}$, Equation (11) and ${\mathrm{Ex}}_{\mathrm{Destroyed}}$. which is the exergy destroyed, Equation (10). The basic thermodynamic equations for exergy are given in Equations (7) and (8). ${\mathrm{Ex}}_{\mathrm{i}}$ is the exergy inlet at ambient temperature, ${\mathrm{T}}_0$, and the inlet temperature ${\mathrm{T}}_{\mathrm{i}}$. The exergy destroyed is the difference between the total exergy out and the total exergy in, Equation (9).

$${\mathrm{Ex}}_{\mathrm{th},\mathrm{i}}=\left(1-\frac{{\mathrm{T}}_0}{{\mathrm{T}}_{\mathrm{i}}}\right)\times {\mathrm{Q}}_{\mathrm{i}}$$

(7)

$${\mathrm{Ex}}_{\mathrm{in}}=\left({\mathrm{h}}_{\mathrm{i}}-{\mathrm{h}}_0\right)-{\mathrm{T}}_0\left({\mathrm{s}}_{\mathrm{i}}-{\mathrm{s}}_0\right)$$

(8)

$${\mathrm{Ex}}_{\mathrm{Destroyed},}={\mathrm{Ex}}_{\mathrm{out}}-{\mathrm{Ex}}_{\mathrm{in}}$$

(9)

$${\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}={\mathrm{Ex}}_{\mathrm{Total} \mathrm{out}}+{\mathrm{Ex}}_{\mathrm{Destroyed}}$$

(10)

$${\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}={\mathrm{Ex}}_{\mathrm{th},\mathrm{FBC}}+{\mathrm{W}}_{\mathrm{Total},\mathrm{pumps}}$$

(11)

$${\mathrm{Ex}}_{\mathrm{Total},\mathrm{out}}={\mathrm{W}}_{\mathrm{Total},\mathrm{Turbine}+\mathrm{VAR}+\mathrm{MED}}$$

(12)

$${\mathcal{E}}_{\mathrm{MSW}}=\frac{{\mathrm{Ex}}_{\mathrm{out}}}{{\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}}=1-\frac{{\mathrm{Ex}}_{\mathrm{Destroyed},\mathrm{MSW}}}{{\mathrm{Ex}}_{\mathrm{Total} \mathrm{in}}}$$

(13)

Heat produced by the combustion of the fuel is supplied to the water entering at state point 8, state point 10 in Model 2 and state point 6 in Model 3. Energy and exergy balance equations for the three models are given in

Table 1. Energy and exergy balance equations for the models.

	Model 1	Model 2	Model 3
Energy
Boiler	$({\mathrm{m}}_{\mathrm{f}}\times \mathrm{LHV}\times {\mathrm{\eta }}_{\mathrm{C}})+{\mathrm{m}}_8{\mathrm{h}}_8={\mathrm{m}}_1{\mathrm{h}}_1$	$({\mathrm{m}}_{\mathrm{f}}\times \mathrm{LHV}\times {\mathrm{\eta }}_{\mathrm{C}})+{\mathrm{m}}_{10}{\mathrm{h}}_{10}={\mathrm{m}}_1{\mathrm{h}}_1+{\mathrm{m}}_4{\mathrm{h}}_4$	$({\mathrm{m}}_{\mathrm{f}}\times \mathrm{LHV}\times {\mathrm{\eta }}_{\mathrm{C}})+{\mathrm{m}}_6{\mathrm{h}}_6={\mathrm{m}}_1{\mathrm{h}}_1$
Turbine	${\mathrm{m}}_1{\mathrm{h}}_1={\mathrm{m}}_2{\mathrm{h}}_2+{\mathrm{m}}_3{\mathrm{h}}_3+{\mathrm{W}}_{\mathrm{T}}$	${\mathrm{m}}_1{\mathrm{h}}_1={\mathrm{m}}_2{\mathrm{h}}_2+{\mathrm{m}}_3{\mathrm{h}}_3+{\mathrm{W}}_{\mathrm{T},\mathrm{HP}}$ ${\mathrm{m}}_4{\mathrm{h}}_4={\mathrm{m}}_5{\mathrm{h}}_5+{ \mathrm{W}}_{\mathrm{T},\mathrm{LP}}$	${\mathrm{m}}_1{\mathrm{h}}_1={\mathrm{m}}_2{\mathrm{h}}_2+{\mathrm{W}}_{\mathrm{T}}$
VAR	${\mathrm{m}}_2{\mathrm{h}}_2={\mathrm{m}}_4{\mathrm{h}}_4+{\mathrm{Q}}_{\mathrm{c}}$	${\mathrm{m}}_2{\mathrm{h}}_2={\mathrm{m}}_7{\mathrm{h}}_7+{\mathrm{Q}}_{\mathrm{c}}$	${\mathrm{m}}_2{\mathrm{h}}_2={\mathrm{m}}_3{\mathrm{h}}_3+{\mathrm{Q}}_{\mathrm{c}}$
MED	${\mathrm{m}}_3{\mathrm{h}}_3={\mathrm{m}}_5{\mathrm{h}}_5+{\mathrm{Q}}_{\mathrm{W}}$	${\mathrm{m}}_5{\mathrm{h}}_5={\mathrm{m}}_6{\mathrm{h}}_6+{\mathrm{Q}}_{\mathrm{W}}$	${\mathrm{m}}_3{\mathrm{h}}_3={\mathrm{m}}_4{\mathrm{h}}_4+{\mathrm{Q}}_{\mathrm{W}}$
OFWH	${\mathrm{m}}_4{\mathrm{h}}_4+{\mathrm{m}}_6{\mathrm{h}}_6={\mathrm{m}}_7{\mathrm{h}}_7$	${\mathrm{m}}_7{\mathrm{h}}_7+{\mathrm{m}}_8{\mathrm{h}}_8={\mathrm{m}}_9{\mathrm{h}}_9$	-
Pump	${\mathrm{m}}_5{\mathrm{h}}_5+{\mathrm{W}}_{\mathrm{p}1}={\mathrm{m}}_6{\mathrm{h}}_6$ ${\mathrm{m}}_7{\mathrm{h}}_7+{\mathrm{W}}_{\mathrm{p}2}={\mathrm{m}}_8{\mathrm{h}}_8$	${\mathrm{m}}_6{\mathrm{h}}_6+{\mathrm{W}}_{\mathrm{p}1}={\mathrm{m}}_8{\mathrm{h}}_8$ ${\mathrm{m}}_9{\mathrm{h}}_9+{\mathrm{W}}_{\mathrm{p}2}={\mathrm{m}}_{10}{\mathrm{h}}_{10}$	${\mathrm{m}}_4{\mathrm{h}}_4={\mathrm{m}}_5{\mathrm{h}}_5$+${\mathrm{W}}_{\mathrm{p}}$
Exergy
Boiler	${\mathrm{Ex}}_{\mathrm{th},\mathrm{FBC}} \dot{+{ \mathrm{Ex}}_8 }={\mathrm{Ex}}_1$	${\mathrm{Ex}}_{\mathrm{th},\mathrm{FBC}}\dot{+{ \mathrm{Ex}}_3 }+{\mathrm{Ex}}_{10}={\mathrm{Ex}}_1+{\mathrm{Ex}}_4$	${\mathrm{Ex}}_{\mathrm{th},\mathrm{FBC}}+{\mathrm{Ex}}_5={\mathrm{Ex}}_1$
Turbine	${\mathrm{Ex}}_1={\mathrm{Ex}}_2+{\mathrm{Ex}}_3+{\mathrm{Ex}}_{\mathrm{th},\mathrm{T}}$	${\mathrm{Ex}}_1={\mathrm{Ex}}_2+{\mathrm{Ex}}_{\mathrm{th},\mathrm{HPT}}$ ${\mathrm{Ex}}_4={\mathrm{Ex}}_5+{\mathrm{Ex}}_{\mathrm{th},\mathrm{LPT}}$	${\mathrm{Ex}}_1={\mathrm{Ex}}_2+{\mathrm{Ex}}_{\mathrm{th},\mathrm{T}}$
VAR	${\mathrm{Ex}}_2={\mathrm{Ex}}_4+{\mathrm{Ex}}_{\mathrm{th},\mathrm{VAR}}$	${\mathrm{Ex}}_2={\mathrm{Ex}}_7+{\mathrm{Ex}}_{\mathrm{th},\mathrm{VAR}}$	${\mathrm{Ex}}_2={\mathrm{Ex}}_3+{\mathrm{Ex}}_{\mathrm{th},\mathrm{VAR}}$
MED	${\mathrm{Ex}}_3={\mathrm{Ex}}_5+{\mathrm{Ex}}_{\mathrm{th},\mathrm{MED}}$	${\mathrm{Ex}}_5={\mathrm{Ex}}_6+{\mathrm{Ex}}_{\mathrm{th},\mathrm{MED}}$	${\mathrm{Ex}}_3={\mathrm{Ex}}_4+{\mathrm{Ex}}_{\mathrm{th},\mathrm{MED}}$
OFWH	${\mathrm{Ex}}_4+{\mathrm{Ex}}_6={\mathrm{Ex}}_7$	${\mathrm{Ex}}_7+{\mathrm{Ex}}_8={\mathrm{Ex}}_9$	-
Pump	${\mathrm{Ex}}_5+{\mathrm{W}}_{\mathrm{pump} 1}={\mathrm{Ex}}_6$ ${\mathrm{Ex}}_7+{\mathrm{W}}_{\mathrm{pump} 2}={\mathrm{Ex}}_8$	${\mathrm{Ex}}_6+{\mathrm{W}}_{\mathrm{pump} 1}={\mathrm{Ex}}_8$ ${\mathrm{Ex}}_9+{\mathrm{W}}_{\mathrm{pump} 2}={\mathrm{Ex}}_{10}$	${\mathrm{Ex}}_4+{\mathrm{W}}_{\mathrm{pump} }={\mathrm{Ex}}_5$

. ${\mathrm{m}}_{\mathrm{f}}$ is the fuel mass flow rate in kg/s, $\mathrm{LHV}$ is the lower heating value of fuel in kWh/kg and ${\mathrm{\eta }}_{\mathrm{C}}$ is the overall efficiency of the boiler. ‘m’ represents the water or steam mass flow rates and ‘h’ represents the corresponding enthalpies. The subscripts represent the different state points for the three models.

The three different models are compared in terms of their thermal efficiency (η_th) and exergy efficiency (η_ex) for the following operating conditions:

• FBC operation at different pressures
• VAR for single effect and double effect
• MED for 3 stage and 5 stage

3.2. Operation of MSW Boiler

Running hours of the models are calculated after consideration of the forced outage factor (FOF) and the planned outage factor (POF), taken as 109.5 h and 664.88 h as per the best industrial practices [18]. Equation (14) is used to determine the annual running hours.

$$\mathrm{Running} \mathrm{hour}=24\frac{\mathrm{h}}{\mathrm{day}}\times 365\frac{\mathrm{day}}{\mathrm{year}}-\mathrm{FOH}-\mathrm{POH}=7985.62 \mathrm{h}$$

(14)

Heat generated from MSW combustion is transferred by radiation and convection to the water/steam tubes and is further taken to the emission control equipment for removal of fly ash and harmful gases. The radiation and convection losses from the outer walls of the boiler are negligible. The bottom ash is periodically removed from the bottom of the combustion chamber along with bed material and make-up material is added periodically. The FBC boiler efficiency (η_B) is mainly based on the flue gas losses and other minor losses are ignored [14], it is calculated using Equation (15). The temperature of the flue gas is determined by how low it can be depending on the acid dew point temperature.

$${\mathrm{\eta }}_{\mathrm{B}}=1-\left[\frac{ {\dot{\mathrm{m}}}_{\mathrm{fg}}\times {\mathrm{C}}_{\mathrm{pfg}}\times \left({\mathrm{T}}_{\mathrm{fg}}-{\mathrm{T}}_0\right)}{{\dot{\mathrm{m}}}_{\mathrm{f}}\times {\mathrm{LHV}}_{\mathrm{f}}}\right]$$

(15)

In the above equation, ${\dot{\mathrm{m}}}_{\mathrm{fg}}$ is the flue gas mass flow rate, ${\mathrm{C}}_{\mathrm{pfg}}$ is the specific heat of the flue gas, ${\mathrm{T}}_{\mathrm{fg}}$ is the temperature of flue gas, ${\mathrm{T}}_0$ is the ambient temperature, ${\dot{\mathrm{m}}}_{\mathrm{f}}$ is the fuel mass flow rate and ${\mathrm{LHV}}_{\mathrm{f}}$ represents the mass flow rate of the fuel. MSW incineration models are designed based on the MSW generation rate. The fuel supply rate (${\dot{\mathrm{m}}}_{\mathrm{f}}$) and the lower heating value of the fuel (${\mathrm{LHV}}_{\mathrm{f}}$) are obtained from data collected at the location.

3.3. MSW Generation Rate and Its Components

The assessment of waste quantity was produced within the urban limits of Riyadh, Saudi Arabia. Six different samples were collected from dumping sites as well as dust collection bins from different types of locations. Manual sorting was conducted to segregate the different components. The generation rate was estimated to be 11,232 tons per day or 4,100,000 tons per year, which were dumped in a centralized dumping yard where 94% of the waste was dumped with the help of 1400 trucks [19] using the relationship given in Equation (16) [20].

$${\mathrm{MSW}}_{\mathrm{gen}}={{\displaystyle \sum }}_{\mathrm{j}=1}^{365}{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{C}}_{\mathrm{i}}{\mathrm{V}}_{\mathrm{i}}{\mathrm{d}}_{\mathrm{i}}{\mathrm{t}}_{\mathrm{ij}}$$

(16)

In the above equation, m represents the total number of vehicles used, ${\mathrm{C}}_{\mathrm{i}}$ represents the volumetric capacity of truck i, ${\mathrm{V}}_{\mathrm{i}}$ indicates the loading ratio of truck i, ${\mathrm{d}}_{\mathrm{i}}$ indicates the density of MSW, and ${\mathrm{t}}_{\mathrm{ij}}$ represents the trips by a truck i on day j.

Waste has several constituents like paper, plastic, glass, wood, textiles and organics (food waste). Constituents of the waste were determined as per D5231-92 standard [21] and the results are given in

Table 2. Components of MSW.

Components	LHV kWh/kg	%	LHV per kg	Contents of the Components
Paper	3.75	28.5	1.03	Wasted Papers, cardboard, box board, bags, magazines, tissue, newspapers, tissues
Plastic	9.72	5.2	0.60	Disposable glass, spoons, plates, wrapping films, wrapping film, plastic bottles, polythene
Glass	0.00	4.6	0.00	Bottles, glassware, bulbs, ceramics, etc.
Wood	4.72	8	0.38	Bottles, glassware, bulbs, ceramics, etc.
Textiles	5.23	6.4	0.39	Clothes, diapers, etc.
Organics	1.55	37	0.56	Food stuff, fruits and vegetable refuse, peel, etc.
Others	3.36	10.3	0.35	Leathers, rubber, fibers, rubber, yard waste, soils, tire, appliances, electronics

. The heating values of the different components are calculated from standard data for higher heating values obtained from the bomb calorimeter [22]. The lower heating value of component waste is determined by summing up the individual values using Equation (17).

$$\mathrm{LHV}={{\displaystyle \sum }}_1^{\mathrm{j}}({\mathrm{W}}_{\mathrm{j}}\times {\mathrm{HHV}}_{\mathrm{j}})$$

(17)

In the above equation, n is the number of components, ${\mathrm{W}}_{\mathrm{j}}$ is the weight fraction of component j and ${\mathrm{HHV}}_{\mathrm{j}}$ is the higher heating value of component j obtained by bomb calorimeter using Dulong’s formula from the ultimate analysis [23].

The recycling process involves separation of the paper, plastic, wood and textiles for useful end uses. The lower heating value before recycling is 3.31 kWh/kg (11,916 kJ/kg) and after recycling is 1.297 kWh/kg (4669.2 kJ/kg).

3.4. Variants within the Three Models

VAR systems can operate with either single (as in Figure 2) or double generators, depending on availability of the quantity and temperature of heat. Double effect systems with two generators operate at a higher COP of the order of 1.2 compared to single effect ones with one generator that has about 0.7 COP [24]. Proportionately, the cooling provided is also higher. Although triple effect systems are also possible if higher order of heat and temperature is available, the analysis here is confined to single effect and double effect, since preference for higher turbine power is given in the three models. Performance of MED systems is proportional to the number of effects used, as in Figure 3 [25]. This depends on the top brine temperature TBT, which is the temperature to which the brackish water is heated, by the condensate from the turbine in the initial stage [26]. Considering the temperatures of the order of 70 °C at the turbine exit, three effect and five effect MED have been considered here on an arbitrary basis. The four variants were analyzed for above models working under the following types of operation.

• Variant A: Single effect VAR+3 effect MED
• Variant B: Single effect VAR + 5 effect MED
• Variant C: Double effect VAR + 3 effect MED
• Variant D: Double effect VAR + 5 effect MED

4. Results and Discussion

Values of the operating parameters at different points for the three models 1, 2 and 3 are given in

Table 3. Operating parameters for Model 1, Model 2 and Model 3.

State Points	T (°C)	P (kPa)	x (%)	H (kJ/kg)	m (kg/s)	Exergy (kJ/kg)
(a) Model 1
1	250	1200	100	2936	422	903.7
2	120	190	100	2707	211	579.4
3	90	70.18	0.99	2659	211	434.6
4	118.6	190	0	497.9	211	51.39
5	90	70.18	0	377	211	25.95
6	90.01	190	0	377.2	211	26.07
7	104.3	190	0	437.5	422	37.83
8	104.5	1200	0	438.8	422	38.92
(b) Model 2
1	250	1200	100	2936	422	903.6
2	120	198.7	1	2589	84.4	556.7
3	120	198.7	1	2589	337.6	556.7
4	250	198.7	100	2971	337.6	676.3
5	120	198	1	2970	337.6	675.5
6	119.9	198	0	503.4	337.6	52.72
7	120	198.7	0	503.8	84.4	52.83
8	119.9	198.7	0	503.4	337.6	52.72
9	119.9	198.7	0	503.5	422	52.74
10	120	1200	0	504.7	422	53.85
(c) Model 3
1	250	1200	100	2936	422	903.5
2	120	198.7	1	2589	422	557.4
3	100	80	0.6	1756	422	283.4
4	100	80	0	391.7	422	28.64
5	100.4	1200	0	421.7	422	35.4

a–c. Values of the enthalpy and exergy are determined from the basic thermodynamic equations using the EES program at different points in the models. In the case of Model 1, the steam is superheated at the turbine inlet and the bleeding point at 2. The outlet from the turbine is saturated at 3 and in liquid state at the other points in this case. The calculations are performed with boiler outlet condition at 1200 kPa, 250 °C. In the case of Model 2, steam is superheated at points 1 and 4 and is saturated vapor at the turbine outlets 2, 3 and 5. It remains in a liquid state at all other points down stream. In the case of Model 3, steam is superheated at point 1 and expands to saturated vapor at turbine exit at point 2. It is liquid for the rest of the cycle.

Results of the performance of components show interesting outputs in

Table 4. Performance of the different components of the three models.

Output @ P = 1200 (kPa) T = 250 °C	Units	Model 1				Model 2				Model 3
		A	B	C	D	A	B	C	D	A	B	C	D
Turbine output Power (×103)	kW	106.5	106.5	106.5	106.5	146.6	146.6	146.6	146.6	146.1	146.1	146.1	146.1
VAR Inlet (×103)	kW	456.1	456.1	456.6	456.1	176.0	176.0	176.0	176.0	351.9	351.9	351.9	351.9
MED Inlet (×103)	kW	481.6	481.6	481.8	481.6	832.9	832.9	832.9	832.9	575.6	575.6	575.6	575.6
Cooling rate (×103)	kW	319.3	319.3	315.0	314.7	320.3	320.3	592.0	592.0	320.3	320.3	419.8	419.8
Distilled water output	kg/s	357	639.5	357.2	639.8	805.8	1388	806	1388	477.2	839.8	477.2	839.8

at a boiler output pressure of 1200 kPa and temperature of 250 °C. Both Model 2 and Model 3 show maximum and similar performance in terms of power available from the turbine. Model 1 shows maximum performance for cooling (VAR) and Model 2 shows maximum performance for desalination. The values of power output from turbines for the three models show relatively lower output from Model 1 compared to Models 2 and 3, which are almost similar in performance. This is because of the bleeding of 50% of superheated steam for the VAR in Model 1, as seen in the VAR inlet condition data. Power available at the VAR inlet is minimal in the case of Model 2 and compared to Models 1 and 3. This is due to the fact that, unlike Models 1 and 3, a low mass flow rate occurs in Model 2 due to the reheat system involved. The power available for MED is seen to be maximum for Model 2. This is explained by the fact that in Model 1, the mass flow rate of the fluid is low and in Model 3, the heat available in fluid is used up in the VAR and reheating helps to improve the heat input into the MED system. The cooling rate depends on the COP of the VAR used, which is different for single effect and double effect systems. The fresh water production rate is also based on the number of effects involved in the desalination process.

The variation of total power available at the turbine outlet with different boiler pressures are given in Figure 7a for the three models. Power output from the turbine in Model 1 is less due to extraction of steam for VAR. There is a proportionate increase in the output at higher pressures. Model 2 and Model 3 show almost equal values of power output at lower pressures but tend to be different at higher operating pressures. This is because of the higher temperature and exergy at higher pressures. Figure 7b gives the power available at the inlet of the VAR for the three models. Actual cooling obtained depends on the COP of the VAR machine used. Model 1 shows the highest power available due to the fact that 50% of the steam quantity is extracted at superheated conditions from the turbine for the VAR, which is also the reason for low power output from the turbine in Figure 7a. This type of model is appropriate for situations where a high cooling load exists, especially in summer conditions. The VAR power available drops at higher pressures in all three cases because at higher pressures, and expansion in the turbine produces more power output as explained earlier. Model 2 shows the highest power availability for MED, which is attributed to better heating fluid quality due to the reheating process of the steam. The power availability decreases with the pressure in all the three cases. Figure 7c gives the power available at the inlet of the MED system for freshwater production. Actual freshwater production depends on the specific energy consumption of the MED plant.

The cooling rate achieved is a function of the COP of the VAR system, which is determined by the combined performance of the generator and condenser. The effect of the steam pressure of 1200 kPa and 1400 kPa on the four variants A, B, C and D are given in the bar chart in Figure 8a,b. An increase in steam pressure results in a decrease in the cooling rate in all the cases both for single effect and double effect VAR systems due to decrease in the COP. Fresh water production rates by the different models for 3 effects and 5 effects for two different steam pressures of 1200 kPa and 1400 kPA are shown in Figure 9a,b. Notably, 5 effect MED gives better performance compared to 3 effect, which is in agreement with the concept of a higher production rate for higher number of effects. Also, the production rates are higher in all the three models for higher steam pressures.

Figure 10 gives the comparison of energy efficiency of the models at two temperatures of 1200 kPa and 1400 kPa. Energy efficiency is marginally improved in the case of Models 2 and 3 but only a marginal improvement in the case of Model 1. This is attributed to the fact that higher pressures have more impact on the reheat cycle of Model 2 and the series arrangement of components in Model 3. Figure 11 gives the effect of pressure rise on overall exergy efficiency of the three models and it is seen that only a marginal improvement in exergy is shown by Models 2 and 3 but considerable improvement in the case of Model 1. This is due to the fact that higher temperatures and pressure increases which results in higher exergy availability to the functional components of Model 1. Considering the three models in terms of both energy and exergy efficiencies, shows marked improvement in performance in all cases.

Different Climate Zones-Implementation Case Study of the Models

Three different urban cities were selected based on their diversity of requirements for electrical power, cooling energy and water. Riyadh, Saudi Arabia, lies in the arid desert climate zone as per the adopted Koppen climatic classification (BWh) which is a popular climate classification methodology. This location has a very high cooling energy demand of 33,934 GWh per year which is more than double that of the winter demand [27]. The energy for fresh water generation from desalination plants is also high at 722 GWh per year, because of non-availability of natural fresh water resources [28]. Data on waste generation and composition was also obtained. New Delhi, India, is located in the humid sub-tropical semi-arid climate zone (Cwa and Bsh) and has a cooling energy demand during the summer. Fresh water supply is mainly from natural sources and desalination requirement are low. Electrical power demand is very high, about 26,319 GWh per year and is supplied by conventional power generation [29]. London, United Kingdom, has a temperate oceanic climate zone (Cfb) and has an electricity demand of 26,319 GWh per year [30]. Cooling energy requirement and freshwater requirement from desalination are considerably less compared to the previous two cities due to the favorable climate and availability of natural resources. The best model for implementation from among the above three for each of the above cities was selected based on economic benefits achieved using results obtained in

Table 5. Comparison of utility requirements supplied by MSW incineration in three cities worldwide.

Name of the city		New Delhi [31]	Riyadh [32]	London [33]
Climatic zone		Humid subtropical	Arid desert	Temperate oceanic
MSW generation rate (million tons/year)		4.13	3.4	7
Composition	Paper	11%	11.6	15%
	Plastic	13.5%	13.0	15%
	Glass	6%	3.0	11%
	Wood	6%	6.60	5%
	Textiles	1.5	2.90	5%
	Organics	50–55%	57.20	18%
	Others	6.3%	3.36	28%
LHV (kWh/kg)		2.06	2.51	2.77
Electrical power requirement (GWh/y)		26,319	48,478	37,800
Cooling energy requirement, GWh/y)		1350	33,934	0.018
Desalination energy requirement (GWh/y)		1040	722	816,000
Availability from MSW using Model 1 (% replacement)	Electrical power, GWh/y	3718.76 (14.3%)	3061.45 (6.32%)	26,319 (23.95%)
	Cooling, GWh/y	7963.04 (589.85%)	6555.53 (19.32%)	1350 (999.75%)
	Water, GWh/y	378.37 (36.38%)	311.49 (42.56%)	1040 (61.66%)
Availability from MSW using Model 2 (% replacement)	Electrical power, GWh/y	5118.96 (19.45%)	4214.16 (8.69%)	26,319 (32.97%)
	Cooling, GWh/y	3072.78 (227.65)	2529.65 (7.45%)	1350 (385.79%)
	Water, GWh/y	654.37 (62.92%)	538.71 (73.60%)	1040 (106.65%)
Availability from MSW using Model 3 (% replacement)	Electrical power, GWh/y	5101.51 (19.38%)	4199.79 (8.66%)	26,319 (32.85%)
	Cooling, GWh/y	4300.67 (318.57%)	3540.50 (10.43%)	1350 (539.95%)
	Water, GWh/y	452.22 (43.48%)	372.29 (50.87%)	1040 (73.70%)

.

Application of the three models to the three cities gives interesting results. Power generation from turbines is taken as such for electricity production. The cooling produced by the VAR process is calculated after considering the COP of 0.7 for the single effect system and the desalination output from MED is calculated after considering the desalination thermal power requirement of 310 kJ/L of water and electrical power requirement of 2 Watt hour per liter [34]. Model 2 gives the best results in terms of overall gain in energy for Riyadh. The desalination energy requirement can be substituted by about 73.6% of the required 722 GWh per year. Electrical energy is replaced from 6.32% to 32.97% in all cases for the three cities. Cooling energy availability is much greater than the requirement in the case of New Delhi and London due to the low cooling demand.

5. Conclusions

A comparison of three different models for simultaneous production of power, cooling and fresh water using municipal solid waste as fuel with two variants in the VAR cooling system and two variants in the MED freshwater production was analyzed in this study. The models were considered for power generation using the conventional Rankine cycle and performance was considered at two different operating pressures. The composition of waste sample collected was used to determine the heating value, which was 3.31 kWh/kg under non-recycling scenario with a waste generation rate of 11,232 tons per day. Significant difference in heating value occurs due to the removal of combustibles like plastics and wood waste.

Power output from the three systems at 1200 kPa boiler outlet pressure, the cooling rate produced by the VAR system and the freshwater production by the MED system were determined under the different conditions with the help of Engineering Equations Solver. The maximum value of turbine power output obtained was 146,600 kW from Model 2, the maximum cooling power available was 456,100 kW from Model 1 and the maximum desalination power available was 832,900 kW from Model 2. Energy and exergy efficiency were maximum for Model 2 at 93% and 97% at 1200 kPa and both these values were reduced at higher pressures. It was seen that higher pressures of steam produced by the FBC boiler results in higher power availability in the turbine but reduced production from the cooling system and freshwater production. Energy efficiency is slightly improved in the case of Models 2 and 3 but no significant change in the case of Model 1. Exergy efficiency showed an increase in the case of Model 1 and no significant effect in the case of Models 2 and 3.

Case studies of simultaneous MSW conversion to energy, cooling and freshwater production were performed for three different cities in diverse climate zones. Waste generation data and waste composition data of Riyadh, New Delhi and London were tested for their outputs and Model 2 gives the best result in terms of electricity production, cooling and freshwater production for Riyadh. Cooling produced was more than the requirement for New Delhi and London due to low cooling loads, which suggests modifications to the energy utilization model. Hence, further studies with variations in models have good scope for other locations. Arid climate conditions have maximum benefit due to the simultaneous requirement for power, cooling energy needs and desalinated water. All these studies were performed under a non-recycling scenario. Recycling would result in reduced heating values, which could be explored further.