THERMODYNAMIC AND ENVIRONMENTAL COMPARATIVE INVESTIGATION AND OPTIMIZATION OF LANDFILL VS. INCINERATION FOR MUNICIPAL SOLID WASTE: A CASE STUDY IN VARAMIN, IRAN

Waste to energy (WtE) introduces an appropriate solution for municipal solid waste (MSW) disposal and greenhouse gas emission reduction. In this study, for Varamin MSW management, a gas turbine plant with heat recovery unit that is fed by landfill gas (LFG) and combined heat and power (CHP) incineration plant is investigated and compared as two WtE systems to reveal the best plant effectively. Exergy and environmental analyses of two systems are performed. Moreover, the effects of key parameters as decision variables on the energy and exergy efficiencies are identified by sensitive analysis of both systems. Multi-objective optimization of thermal and exergy efficiencies are then done by using Genetic Algorithm (GA) for each studied system. As a result, Furnace in incineration system and Combustion Chamber in landfill system have the most exergy destruction rate. Also, optimization results show that thermal and exergy effectiveness for landfill system are improved by 7.01% and 6.53% respectively; these values for incineration system are calculated to be 45.35% and 92.75% respectively.


227
MSW (Municipal Solid Waste) is considered as a renewable energy source because wastes are continuously produced in the world; hence energy from waste can be a significant way to reduce energy production from non-renewable energy sources such as fossil fuels and GHG (Greenhouse Gas) emissions [7], [8]. The various form of utilization of municipal solid waste in order to energy producing in the developed countries was carried out to indicate the efficient way. The Tokyo that is located in Japan has been considered to a case study to this aim. After assessment of potential scenarios, the best option was selected. In that selected option the energy efficiency will reach 65.95% in 2030. Moreover, in that scenario 6.58 × 105 tons of CO2 emissions will reduce. the article provided a practical guide for founding a more efficient MSW management system for the purpose of a low carbon society [9]. The analyzing of the exergy efficiency of electricity production of the landfill from a landfill that located in Greece was carried out. The main aim in that survey was clarifying this issue that how the extension of the landfill influence power generation. That potential of every form of extensions was compared, and the results were presented [10]. Advanced exergy and environmental analysis of a steam cycle of a municipal solid waste incineration plant were accomplished to determine the thermodynamic performance of the main parts such as exergy annihilation and energy efficiency. To detecting the potential for improvement in the incineration steam cycle, an advanced energetic assessment was done in four categories. The results illustrated that total plant exergy destruction could be decreased by 8.4%, through improvements in efficiency at plant components considering the best available technologies and operating parameters [11]. Thermodynamic evaluation of a municipal solid waste-to-energy boiler for three case studies in Tehran was conducted, and the main reasons of irreversibility within the boiler was detected. Moreover, the energy and exergy analysis were then accomplished for all components, and the entire boiler and results unmasked that the irreversibility in the furnace, with 52,617.3 kJ. s−1 was the primary cause of the exergy annihilation. Besides, the overall energetic and exergetic efficiency of the mentioned boiler were calculated as 78.7 and 16.0 %, respectively [12].
The waste to energy (WtE) conversion systems are used to convert solid waste into fuels, electric power and heat. They are classified as the thermochemical and the biochemical methods [13]. Waste incineration is most commonly used as the thermo-chemical process. Complete oxidation of organic substances in waste materials occurs during the incineration process [14]. Energy recovery is performed by using the heat content of the combustion products. The temperature of products depends on LHV (Low Heating Value), moisture and ash content of MSW. Minimum range of the solid waste LHV that waste can be burnt without auxiliary fuel is 5-7GJ/Mg [15]. Energy productions in this process can be only electricity or combined heat and power (CHP); But CHP is found to perform better in life cycle assessment (LCA) and system efficiency evaluation than only electricity production [16]. One of the biochemical processes in waste to energy systems is Biogas produced from organic waste material in the landfill site. It can be used as fuel directly or produce electricity and heat [17]. Recently biogas-based CHP systems are used widely in the world because of these systems use less fuel than separate power and heat systems [18], [19]. In this research, two waste to energy systems: a landfill plant, which is fed by LFG (Landfill Gas), including a gas turbine associated with a heat recovery unit and a CHP incineration plant are investigated from the standpoints of energy, exergy and environmentally based on the Varamin, Iran data in order to designate the best system efficiently and environmentally. For this purpose, the thermodynamic and exergy efficiencies and total GHG emissions are calculated by utilizing mass, energy and exergy balances to each system using EES software [20]. The effects of key thermodynamic parameters on the energy and exergy efficiencies are then identified by system sensitive analysis. Afterward, both systems are optimized to maximize the thermal and exergy efficiencies simultaneously employing genetic algorithm programming using MATLAB software [21]. Figure 1 shows the configuration of the designed landfill plant. LFG collected from landfill site is used as fuel in this plant. LFG (i.e., stream 3) and compressed air (i.e., stream 2) enter into the combustion chamber and, so, combustion reaction occurs. Then flue gases (i.e., stream 4) go to the gas turbine (G.T) and produce power. After G.T, stream 5 enters to the heat recovery steam generator (HRSG) and its temperature decreases and then goes through stock. The heat of stream five is used to produce steam in HRSG for increasing system efficiency. Figure 2 exhibits the configuration of studied CHP incineration system. Solid wastes are combusted over a moving grate furnace. Heat from flue gas combustion is used in the boiler to produce superheated steam consequently. Then, the flue gas at 155ᵒC goes through the cleaning system and releases to the atmosphere by Journal of Thermal Engineering, Research Article, Vol. 6, No. 6, Special Issue 12, pp. 226-246, December, 2020 228 stock. Superheated steam enters steam turbine (S.T) of a Rankine cycle and produces power. Outlet stream of the turbine is condensed to produce saturated water (stream 9) that is pumped to provide the operating pressure of the Rankine cycle. Then stream six goes to the boiler to complete the cycle. For increasing system efficiency, the heat of stream eight is used to generate domestic hot water (i.e., stream 11) by heat recovery from the condenser component.

229
The thermodynamic modeling of the proposed WtE systems is described in this section. The thermodynamic properties and exergy of the streams are calculated by solving energy and exergy balances via the EES software. The following assumptions are considered for oversimplification of theoretical analysis of the systems: • The process is considered to be steady state.
• The pressure drops in all pipes are neglected.
• Changes in potential and kinetic exergies are neglected.
• Adiabatic situation and equations are assumed to turbines and heat exchangers in this paper • Heat loss from furnace incineration is assumed to be 5% of the MSW low heating value [22].
• Air and combustion products are considered to be ideal gasses.
For the energy analysis of the system, the conservation laws of mass and energy are applied to each component of the framework. Each component of the system can be considered as a control volume. For a component with inlet i and outlet o, mass and energy conversations can be written as below [23]: ∑̇= ∑̇ (1) ∑̇− ∑̇= ∑̇ℎ − ∑̇ℎ (2) Energy efficiency of the landfill cycle is formulated as follows: In the above expression, the terms Ẇ Net,lf and Ẇ S,lf express pure power and steam produced in the landfill cycle respectively. On the other side, the energy efficiency of the incineration cycle is performed as follows: Where, the terms Ẇ Net,inc and Ẇ HW,inc state the net power and hot water produced by the incineration cycle respectively. The maximum work that can be observed from a given form of energy that is utilizing the environmental parameters as the reference state is introduced as exergy [24]. On the other hand, exergy is an attribute of the system and environment together. The exergy rate (Ẋ) of a process stream, in the absence of electrical and magnetic effects, is separated into four terms. Two of them are physical exergy (ẊPH) and chemical exergy (ẊCH) which are defined as follows [25]: Physical exergy is the highest beneficial work available as a system interplays with an equilibrium state. Chemical exergy is dependent on the chemical composition of the process stream (including the mixture or pure) at its state and when it reaches in an equilibrium state with the reference environment [26]. The two other terms, kinetic exergy rate (ẊK N ) and potential exergy rate (ẊP T ), are considered to be negligible as movements are relatively low, and elevation changes are small [27]. Applying the first and the second laws of thermodynamics to the system components, the exergy balance is achieved as follows [28]: Where ẊQ and ẊW determine exergies of the heat transfer and the work which across the boundaries of the control volume. Also, the Dead State situations are considered as T0=298 K and P0=1 bar for the both proposed systems. The feed exergy (ẊF ,k ) and product exergy (ẊP ,k ) of each component in the system is obtained based on the exergy calculated for each inlet/outlet stream of that component. The feed exergy is the required exergy to generate the product of a specified element, and the product exergy is what we desire from a part, regarding exergy [29]. The feed and product exergies for each element of the system are presented in Table 1 and Table 2 for landfill and incineration systems respectively.

231
The difference between feed and product exergies of a component reveals the amount of exergy destructed by that component which is described by the exergy destruction rate parameter (̇, ) as defined by below expression [30]: The exergy efficiency parameter (ε ) is defined as the percentage of the feed exergy procured to the system which is found in the product exergy [31]. So, the corresponding equation is expressed as [32]: Also, the exergy destruction ratio parameter (ẊP ,k ) is calculated to understand which component of the system has the most adverse impact on the system and is determined by the following expression [33]: Where the ẊP ,k is the feed exergy provided to the whole system, and it is equal to Ẋ3 for the landfill system and Ẋ1 for the incineration system based on Figure 1 and 2.
Finally, the exergy efficiency of proposed landfill system can be written as: And that for the proposed incineration system will be equal to: GHG emissions are studied to investigate and compare the environmental effects of both processes. Figure  3 shows the emissions structures of landfill and incineration systems. This figure illustrates the considered MSW management scenarios and prediction of the consequences of them. The forms of emissions are different, as it is clear in figure 3, the more useful ways to manage MSW are energy recovery from landfill and incineration systems because of the avoidance potential of the greenhouse gas emissions. GHG emissions are estimated for both operations by Equation 15 and 16 [34], [35] Where GHGLF, GHGLFG, GHGS,LF and GHGER,LF are net GHG emissions from landfill, GHG emissions from landfill CH4, GHG emissions from stack release system and GHG depletions from electricity and heat in landfill respectively. Energy recovery from the landfill can contribute to decreasing fossil fuel resources usage [26].The production of electricity and heat in the landfill system are compared with the electricity and heat emission factor in Iran's gas turbine power plants that are fed by natural gas. The electricity and heat emissions factors are 0.782 kg CO2e per kWh and 0.0624 g CO2e per kJ, respectively [34].

232
Where GHGINC, GHGS,INC and GHGER,INC are net GHG emissions from incineration, GHG emissions from stack discharge system and GHG depletions from electricity and heat in incineration respectively. The energy from MSW incineration system, like the landfill system, reduces energy production from fossil fuel resources [26]. The electricity and heat emission factor in Iran's steam power plants fed by natural gas is 0.633 kg CO2e per kWh and 0.0624 g CO2e per kJ [36].

RESULT AND DISCUSION
As the proposed WtE systems are designed to be used in the Varamin city region, it is needed to determine the specification of fuel for each system according to Varamin city conditions. The flow rate of MSW is considered to be 200 tons/day based on the Varamin daily MSW production, and its chemical composition is obtained experimentally, and the results are described in Table 3. LFG emissions rate in the landfill must be calculated for design landfill CHP plant. Figure 4 illustrates methane, CO2 and total LFG yearly mass rate emissions based on Land GEM model and its input parameters are considered k=0.05 year-1, L0= 173.4 m3/Mg, and methane content volume equals to 60%. LFG rate that is fed to landfill energy system recovery is deemed to be constant equals to 5438.53 tons per year (emissions in 2019). Therefore, the results of the thermodynamic analysis (energy and exergy) of both systems are performed by Table  5 and Table 6 for landfill and incineration systems respectively, using input data shown in Table 4.  Compressor pressure ratio (-) 6 Turbine back pressure (kPa) 100 Low heat value of fuel (kJ/kg) 17685.5 Turbine isentropic efficiency (%) 92 Compressor isentropic efficiency (%) 90 HRSG water inlet pressure(kPa) 100 HRSG water inlet temperature(°C) 25

Incineration system
Dead state temperature(°C) 25 Dead state pressure(kPa) 100 Turbine inlet temperature(°C) 280 Turbine inlet pressure (kPa) 3200 Turbine back pressure (kPa) 10 Turbine isentropic efficiency (%) 85 Pump isentropic efficiency (%) 83 flue gas outlet temperature(C) 155 Low heat value of fuel (kJ/kg) 5247 Cond water inlet pressure(kPa) 100 Cond water inlet temperature(°C) 25    Table 8 and Table 9 based on Table 1 and Table 2 given in the previous section, present the essential exergetic parameters for each component of landfill and incineration systems respectively. According to the Table  5 and 6, the performance of the both studied systems is determined which is described in Table 7.  As it is shown in Table 8 for the landfill system, the G.T and A.C have the highest exergy efficiency in comparison to other components due to their low exergy annihilation. Based on this table, 75% of the whole parts have exergetic efficiencies above 50%, and the HRSG component has the worst exergetic efficiency which equals to 31.5%, so a redesign consideration is needed for this component.
According to Table 9 for the proposed incineration system, Pump and S.T components have the highest exergetic efficiency respectively in the order and 60% of the cycle's parts have an exergetic efficiency below 50%. The lowest value of exergy efficiency is related to the furnace component, Frnc, in the system due to its highest value of exergy destruction, 13.9 MW. The combustion reaction in this component results in a high amount of irreversibilities and, so, the high amount of exergy destruction rate. For a comparison between different components of the landfill system, the values of exergy destruction ratio are performed by Figure 5 As it is seen in this figure, C.C and HRSG have the significant rule in destructing the total exergy input to the system (about 96% is destructed by these components). Irreversibility in heat exchanger (HRSG) occurs because of transferring heat through finite temperature difference. The significant quantity of entropy is produced in the combustion chamber (C.C) due to combustion and, so, resulting in a significant amount of annihilation. 236 Figure 6 shows the exergy destruction ratio of various components for the incineration system. The exergy destruction ratio value is the most for the French among the other ingredients (83% approximately). The main reason of irreversibility in the furnace, Frnc, component is related to the chemical reaction; Therefore, a notable reduction in exergy destruction cannot be expected. Still reducing the air-fuel ratio and preheating the combustion air are proposals for improving the performance of the mentioned component. Also, it is realized from this figure that the Pump component has the lowest value of exergy destruction ratio in the system because of its small consumed power.  A sensitive analysis is performed for both proposed systems, i.e., landfill and incineration systems; each system is analyzed in two methods: one method considers the system without any heat recovery, like a common landfill or incineration system, and, so, the only production of the system will be the electric power. The other method includes a combined heating and power landfill or incineration framework which generates the electric power and the steam/hot water simultaneously as the productions of the system. Figure 9 shows the effect of LFG mass flow rate, ̇3, on the energy and exergy efficiencies of the landfill system in both common (right hand Yaxis) and CHP (left hand Y-axis) methods. According to this figure, the effectiveness of the CHP landfill system (i.e., with heat recovery) is more than that of the landfill system without heat recovery because of generation of steam beside the primary production, electric power. Also, it can be perceived that the total energy efficiency of the common landfill system, η If,C , declines when the LFG flow rate increases; it is because of increment of LFG flow rate more than increment of produced electric power by the G.T. On the other hand, the total energy efficiency of the CHP landfill system, η lf,CHP , increases by LFG flow rate increasing due to the extra steam produced in the system as another production of CHP system. The trend for the exergy efficiency of each method of the landfill system is similar to the energy efficiency for them because of the definition of the exergy efficiency. 238 Figure 10 demonstrates the effect of the compressor pressure ratio (i.e., P2/P1) on the energy and exergy efficiencies of the landfill system in both conventional and CHP methods. As it has arisen from this figure, the total energy efficiency of the universal landfill system, η lf,C , has a maximum value in particular value of P2/P1 because of the compressor consumed power increment as its pressure ratio increases. The decrement of η lf,CHP with a compressor pressure ratio increase is due to the decrement of combustion products temperature and, so, the decrement of steam flow rate produced by the HRSG component. Considering the mentioned explanation for energy efficiency, the trend attained for exergy efficiency variation is expected according to the efficiency definition. Figure 10. Effect of compressor pressure ratio on the energy and exergy efficiencies in common and CHP methods of the landfill system The effects of inlet air flow rate, ̇1, on the energy and exergy efficiencies of the landfill system in both conventional and CHP methods are demonstrated by Figure 11 Based on this figure, as the combustion air flow rate increases, the energy and exergy efficiencies of the CHP landfill system decrease. 239 Figure 12 indicates the effects of combustion products temperature, T4, on the energy and exergy efficiencies of the incineration system. According to this figure, as the combustion products temperature increases, the energy and exergy efficiencies of the cycle in both methods increase; the reason is the increment of the steam flow rate produced in the Blr and, so, the increment of power provided by S.T in the cycle. Figure 12. Effect of combustion products temperature on the energy and exergy efficiencies in common and CHP methods of the incineration system The effect of steam turbine outlet pressure, P8, on the energy and exergy efficiencies of the incineration system is presented in Figure 13 for both considered methods. This figure shows that as the outlet pressure of the S.T increases, the effectiveness of the system decreases because of decrement of the power produced by the turbine. Figure 13. Effect of turbine back pressure on the energy and exergy efficiencies in common and CHP methods of the incineration system As it is seen in Figure 14, the effect of steam turbine inlet pressure, P7, on the energy and exergy efficiencies of the incineration system in common and CHP methods is exhibited. An increase in S.T inlet pressure, based on Figure 14, increases system efficiency because of electric power generation increment in the turbine, as the primary production of incineration system in each method, as P7 increases. Multi-Objective optimization is conducted for the landfill and incineration systems separately by using genetic algorithm programming in MATLAB. The decision variables and their variation range, which are used for the optimization purpose, are determined by sensitive analysis discussed in the previous section. The selected decision variables and their reasonable range for the landfill system are: • Compressor pressure ratio, P cpr : 2 ≤ P cpr ≤ 9 • LFG mass flow rate, ̇3: 0.172 ≤̇3 ≤ 0.305 (in kg/s) • Air inlet flow rate, ̇1:: 2.14 ≤̇1 ≤ 3.30 (in kg/s) Where, in fact, P cpr : equals to P2/P1. Subsequently for the decision variables of the incineration system we have: • Combustion products temperature, T 4 : 897.85 ≤ T 4 ≤ 1200 (in °C) • Steam turbine outlet pressure, P 8 : 10 ≤ P 8 ≤ 100 (in kPa) • Steam turbine inlet pressure, P 7 : 3200 ≤ P 7 ≤ 4500 (in kPa) Considering the maximization of the energy and exergy efficiencies of the whole system as the objectives of optimization, the objective functions for the landfill system are expressed as follows: The maximum error for the above equation is computed to be 0.225% approximately. Therefore, the final results of landfill system optimization are reported in Table 10. Based on this table, the final-optimal point performs a total energy efficiency of 82.15% and total exergy efficiency of 78.27% for the CHP landfill system. Table 10 also indicates the number of outputs of the system at the optimum point and the corresponding optimum thermodynamic parameters of the system (i.e., decision variables). As presented in this table, in the final optimum point which both energy and exergy efficiencies of the whole process are simultaneously in their possible maximum values, energy and exergy efficiencies improve by 7.01% and 6.53% respectively as compared to the non-optimized system. On the other hand, for the aim of incineration system optimization, considering Genetic Algorithm parameters as: 270 generations, with a search population size of 350 individuals, gene mutation probability of 0.4, a crossover probability of 0.5, a tournament size of 5 and a function set of: Times, Minus, Plus, Divide, Exp, Log, the output based on the mentioned decision variables for the energy efficiency function is attained as follows: The maximum error for the above equation is then determined to be 3.586% approximately. The final optimization results of the incineration system are indicated in Table 11. As vented in this table, for the CHP incineration system, the final-optimal point indicates the total energy efficiency of 81.34% and total exergy efficiency of 69.12%. This table also presents the outputs of the optimized system and the corresponding thermodynamic parameters of the system. In the final optimum state, which both total energy and exergy efficiencies are simultaneously in the maximum values, they increase by 45.35% and 92.75% respectively compared to the base case.

CONCLUSION
In this paper, two WtE systems, a CHP landfill plant and a CHP incineration system, are investigated from the viewpoint of exergy and environmentally based on Varamin city data to determine the best order conclusively. For this purpose, energy and exergy performances and, also, total GHG emissions calculations are conducted using EES software. Then, the effects of key parameters as decision variables on the energy and exergy efficiencies are identified by sensitive analysis of both systems. Afterward, each system is optimized to thermal and exergy efficiencies maximization simultaneously employing a genetic algorithm method using MATLAB software. Several significant outcomes can be drawn from this study: • In the landfill system, C.C and HRSG have the significant rule in annihilating the total exergy input to the system as they destruct 96% of total exergy fed to the cycle; so, improving the exergetic efficiency of these components is necessary. Preheating the combustion air by radiation zone of C.C and avoiding mismatched heat capacity rates of the two streams in the HRSG are proposals to improve the performance of these components.
• The exergy annihilation ratio is the most for the French in the incineration framework among the other components. Reducing the air-fuel ratio and preheating the combustion air can decrease the irreversibilities of chemical reaction occurred in France and, so, improve the performance of the mentioned component. • Environmental analysis based on GHG emissions shows that landfill system emitted less GHG than incineration system. Increment of recovery rate for LFG in landfill system and improvement of electricity and heat production and, so, energy and exergy efficiencies in both systems can reduce the GHG emissions. • An increase in LFG flow rate of CHP landfill system causes an increase in the total energy and exergy efficiencies of the system. Also, as the combustion products temperature climb, the energy and exergy efficiencies of the CHP incineration system increase. • The multi-objective optimization of the landfill system utilizing GA performs 7.01% and 6.53% improvement in total energy and exergy efficiencies respectively as compared to a non-optimized system. Additionally, the overall electric power produced is diminished by 39.90%, and the steam production is extremely enhanced by 110.31% at the optimum conditions. • For the incineration system, the multi-objective optimization using GA indicates a significant improvement of 45.35% and 92.75% in total energy and exergy efficiencies respectively as compared to the base system. Moreover, the net electric power and the hot water produced are improved by 52.69% and 57.50% at the optimal-point of the system. • Finally, according to the lower GHG emissions and higher energy and exergy efficiencies achieved by landfill system, using the proposed CHP landfill plant is energetically and environmentally more appropriate than using the incineration plant for Varamin city.