1 Introduction

Waste management is among the three most serious problems in the world, along with water scarcity and climate change. As a consequence of the increase of industrial development, population and consumerism, a dramatic increase of municipal solid waste (MSW) has been occurred. MSW includes in addition to household waste other waste, which due to alike nature or composition can be considered as household waste too, such as waste produced by commercial and related activities, office buildings, institutions etc. (Karak et al. 2012). MSW is a highly heterogeneous mixture of materials that depends on living standards, consumer standards and seasonality, whose management is closely linked to the way in which the available resources are used (Sudibyo et al. 2017; Jingxia 2018). The continuously increasing production of excessive amounts of waste is the most serious indication of the unprofitable and excessive use of available resources worldwide.

A sustainable MSW management system has as a top priority the hindrance of MSW production followed by recycling/reuse. Next in the ranking is the energy recovery by thermal treatment of municipal solid waste, which is a key part of the non-hazardous waste management hierarchy. Last in the priority is disposal in landfill (European Commission 2021), which, unfortunately, has been the practice most widely used (Parto et al. 2007; Abdelhamid 2014). Rational waste management requires the choice of the best way of waste valorization, either for the recovery of materials or energy.

In an integrated waste management system, Waste-to-Energy (WtE) is a very significant process because it leads to a considerable decrease of waste mass and volume, it allows the recovery of energy, as well as minerals and chemicals that can then be reused or recycled from MSW, and it destroys harmful contaminants detected in the stream of waste (Watson et al. 2018).

There are two main divisions of thermal treatment technologies, conventional combustion and advanced thermal treatment. Among others, the most established technologies for conventional combustion are mass burn incineration, which is the WtE technology most commonly encountered worldwide, and fluidized bed incineration. However, there is a concern that during the incineration process heavy metals, unburned chemicals and new chemicals are formed, which pollute incinerator ashes, especially the fly ash that is entrained by the gases outside the furnace and is collected before the chemical cleaning of the gases (Vavva et al. 2017), which must be appropriately treated before sent for landfilling.

The main advanced thermal treatment technologies for MSW treatment are pyrolysis, conventional gasification and plasma gasification. Plasma gasification is an ecofriendly technology that transforms wastes into usable by-products (Adi Sesotyo et al. 2019; Ramos et al. 2020). During this thermal process, the high temperatures occurred, combined with an environment poor in oxygen, avoiding incineration reactions, leads to a complete decomposition of the input waste into simple molecules. The temperature at the exit of a plasma gasifier can be above 1100 K, while the plasma torches used in the furnace can reach temperatures above 5000 K (Littlewood 1977; Orr and Maxwell 2000; Leal-Quirós 2004; Galeno et al. 2011; Achinas and Kapetanios 2013; Gray 2014). By high temperatures, it is accomplished a more efficient melting of inorganic materials and a cleaner syngas, while a higher quality syngas is produced due to high efficiency of energy and carbon transformation (Ramos et al. 2020). Ramos et al. (2019) comparing plasma gasification with incineration, concludes that plasma gasification leads to a more efficient waste diminution and control of air pollution residues, as well as to increase of power production. The product of the process is a fuel gas, recognized as synthesis gas (syngas), whose main components are hydrogen (H2) and carbon monoxide (CO). The inorganic components included in the waste are vitrified into inert glass-like slag (Moustakas et al. 2008; Watson et al. 2018).

The GasifEq model proposed by Mountouris et al. (2006) deals with the energetic and exergetic analysis of plasma gasification. The same authors have used GasifEq for the analysis of a waste water treatment plant installed in Greece by Montouris et al. (2008). Achinas and Kapetanios (2013) used a thermodynamic equilibrium model to study a Refuse Derived Fuel (RDF) plasma gasification process in case of Attika Region and proceeded to energy evaluation of the process. Galeno et al. (2011) developed a numerical model (EPJ model) of an Integrated Plasma Gasification/Fuel Cell system so as to evaluate the energy and environmental aspects and to predict the syngas composition and energy balance of the method. Adi Sesotyo et al. (2019) proceeded to an extension of the EPJ model, dealing with the stoichiometric chemical equilibrium, in order to study emissions of the process and other crucial parameters impacting on syngas composition and heating value. Van Rooyen (2018) developed a CFD model to describe the temperature and flow characteristics of a laboratory-scale plasma gasification reactor.

Hrycak et al. (2014) examined the effects of power consumption on the rate of material’s conversion into syngas, while Yoon et al. (2012) established a simulation model to examine the impact of material’s location in the reactor on syngas production and composition. Mazzoni et al. (2017b) developed a model implemented in Aspen Plus to study the impact of carious level of air oxygen on syngas composition and lower heating value. Tavares et al. (2019), based on Aspen Plus model, assessed the final syngas composition by studying the impact of three gasification means (air, oxygen and steam) on the equivalence ratio, steam to MSW ratio and gasification temperature.

Ramos et al. (2019) studied a two-stage plasma gasification process in order to analyze the environmental effects (evaluations of 11 different factors) caused and concluded that this procedure is an advanced WtE technology, sustainable and ecofriendly. Munir et al. (2019) examined environmental, energy, cost and service life effects of many conventional procedures for MSW disposal and processing, as well as of plasma gasification aiming at a comparison between them, and concluded that commercialization of plasma gasification is possible to be accomplished by improving the energy losses of the system, its safety, as well as the communication between relevant stakeholders and understanding of this process. Ramos et al. (2020) assessed plasma gasification in terms of socio-economic viewpoint and examined the effect of initial investment, electricity sales price, gate fee and sales price of vitrified slag, on the profitability of the technology.

The research question that tries to answer the present study is whether the plasma gasification process is an energy sustainable and cost-effective process for the treatment of MSW combined with energy recovery, a question not clearly answered yet. The novelty of the present work is that following a procedure based on basic principles of thermodynamics and engineering economics, contributes to answering the above question. For this purpose, the GasifEq model (Mountouris et al. 2006) is modified in order to account for S and Cl containing compounds that were not included in the original model. The modified model, which is named modified GasifEq, is evaluated using experimental data measured in an industrial MSW gasification unit. The model is then applied for a case study of a plasma gasification plant in Greece that treats 750 TPD of MSW. Sensitivity analysis of the process operational parameters is performed, aiming to maximize the net energy produced and minimize the costs of the plant. Finally, a comparison of the overall investment cost of the plasma gasification process with that of the widely used incineration process is presented, based on data compiled from literature.

2 Brief Description of the Plasma Gasification Process

Plasma processing of MSW is an efficient and safe option for disposal of wastes produced by residential, industrial and commercial sectors and activities.

Figure 1 presents a schematic diagram of a plasma gasification process treating MSW. The waste is initially fed to a dryer where some of its initial moisture is removed, aiming to improve the efficiency of the gasification reactions. Usually, the final moisture content of waste does not exceed 10–20% by weight (Molino et al. 2016). Drying is usually accomplished by direct contact of waste with hot air.

Fig. 1
figure 1

Schematic diagram of a plasma gasification process treating MSW

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The sustained high temperatures are encountered in the gasification/vitrification furnace so as to enable organic compounds of the waste stream react completely in a rapid way to form considerable amounts of fuel gases. The high temperatures in the furnace are achieved by thermal plasma generator. In modern applications, plasma torches are used for this purpose, which are devices that transmit electric current to a gas medium such as air through electrical discharges due to the electrical potential difference between two ascending and descending electrodes. This creates a plasma-arc and the gas is converted into plasma (Leal-Quirós 2004). The organic compounds of the waste disassociate completely into individual atoms and recombine as simple gases mainly consisting of CO and H2 (syngas) (Galeno et al. 2011; Watson et al. 2018). At the same time as the organic compounds are transformed into syngas, the inorganic materials are converted into a molten material, consisting of two layers, the bottom one contains the pure metals and the top the metal oxides (Mazzoni et al. 2017a).

The raw syngas passes first from a heat exchanger that exists next to the gasification/vitrification furnace, so as to gain a part of its sensible heat. The temperature of the syngas at the exit of the reactor ranges from 1000 to 1200 °C (Gandhi 2015; Khuriati et al. 2018), and its temperature after the exchanger should be equal to or above 500 °C so as to avoid the production of dioxins (Buekens and Huang 1998). The heat gained from the heat exchanger could be used in a steam turbine for energy production, or for drying the raw waste stream prior to the gasification/vitrification furnace.

The cleaning system of the raw syngas is a vital part of the process where unreacted particles and harmful substances are removed. Especially, the use of a gas turbine for electricity production in the final step of the process requires high purity gas fuel. A syngas cleaning system may include a cyclone that removes the larger particles, a water quench chamber for fast gas cooling to avoid dioxin reformation, a venturi scrubber for remaining particulate removal, a wet electrostatic precipitator to remove particles at an efficiency of 99%, an absorber for removing water and hydrogen chloride, and a second absorber for removing hydrogen sulfide (Moustakas et al. 2005; Rutberg et al. 2011; Kalogirou 2018; Valderrama Rios et al. 2018).

Finally, for the energy recovery system many alternatives can be used, e.g., steam cycle, gas engine or combined cycle, depending on the calorific value, purity, quantity and composition of the syngas (Mazzoni et al. 2017b; Kalogirou 2018). From all these alternatives, syngas can be utilized in a gas engine that is appropriate for the production of electrical energy from a low calorific value fuel gas like the one generated from the gasification (Achinas and Kapetanios 2012). The electricity produced can be sold to the grid, besides a part of it that is used as input electricity for the plasma torches (Ramos et al. 2020). Moreover, a heat exchanger could utilize the heat of the flue gases of the engine for removing some of the moisture of the original waste.

3 Plasma Gasification Model Development and Evaluation

In the current study, a thermodynamic equilibrium model of plasma gasification process is developed. This model is called modified GasifEq, since it is based on the “GasifEq model” of Mountouris et al. (Mountouris et al. 2006) and is extended aiming to include sulfur and chlorine compounds. It is assumed that the following compounds exist at the outlet stream of the gasifier: hydrogen, carbon monoxide, carbon dioxide, water, methane, nitrogen, sulfur, chlorine, hydrogen sulfide and hydrogen chloride. According to the Gibbs rule, the number of independent reactions at equilibrium is four. The four reactions, which are used in this work, are listed below:

CO + H2O ⇋ CO2 + H2               (Water gas shift)

CH4 + H2O ⇋ CO + 3H2             (Methane decomposition)

H2 + S ⇋ H2S                             (H2S formation)

H2 + Cl2 ⇋ 2HCl                        (HCl formation)

The above analysis does not take into consideration the solid carbon (soot) by the process. This issue has been thoroughly investigated by Mountouris et al. (Mountouris et al. 2006) and this procedure is used in the current study. In all cases examined in this study, it is checked with the model if soot is formed and in all cases no soot formation was found.

For the description of the complete plasma gasification process, four equations from the equilibrium constants, six material balance equations and one total energy balance are solved simultaneously. In addition to this, the Gibbs energies and the enthalpies of formation at 298 along with the temperature dependent heat capacities are used to solve the gasification model. The calculation of these properties is based on the database of the Chemical Properties Handbook (Yaws 1999).

Table 1 presents the MSW ultimate analysis and the basic operational parameters for the Thermoselect process, which is one of the few, large scale, commercially-available MSW combined gasification and melting processes (Hau et al. 2008). The modified GasifEq model predictions are compared with the measured compositions of the gaseous products of the gasifier in Table 2. Also, in Table 2 the results reported by a rather complicated model developed by Hau et al. (2008) are also presented for comparison. The results indicate that the modified GasifEq predictions are in a very good agreement with the data of Hau et al. (2008). Due to lack of experimental data, it is not feasible to evaluate the modified GasifEq model as per HCl composition.

Table 1 Thermoselect Process: MSW ultimate analysis and operational parameters
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Table 2 Gaseous products’ composition (%w/w): Experimental data from Hau et al. (2008), modified Gasifeq and Hau et al. (2008) model
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4 Cost Evaluation of the Plasma Gasification Process

4.1 Overall Investment Cost

For the calculation of the Overall Investment Cost (OIC) of a plasma gasification plant, including costs of mechanical equipment, piping, fluids handling and hardware and control systems, the corresponding data has been selected from the open literature and market, when available. All costs of previous years are converted into current values using CEP indicators (Chemical Engineering Plant Cost Index) (Mignard 2014). Each cost is multiplied by the ratio of CEP for the year 2019 to the CEP for the year the initial cost referred.

4.1.1 Dryer

The process of waste drying by contact with the high temperature syngas is carried out in a rotary, direct contact dryer of cylindrical dimensions. The energy required for this process mainly depends on the latent heat of the water evaporated, as well as on the sensible heat due to temperature increase of the waste.The cost of a rotary dryer, \({C}_{dryer},\) (in $ for the year 2000) is given by Eq. (1) (Saravacos and Kostaropoulos 2002):

$${C}_{dryer}=1207 {\left({A}_{dryer}*{L}_{dryer}\right)}^{0.69}$$
(1)

where \({\rm A}_{dryer}\): the total surface area of the dryer (m2), which is calculated as follows:

$${\rm A}_{dryer}=\frac{{\dot{Q}}_{drying}}{{U}_{d}\ LMTD}$$
(2)

where \({\dot{Q}}_{drying}\): the drying energy required (kJ/s) calculated as follows:

$${\dot{Q}}_{drying}={\eta }_{dryer} [{\dot{m}}_{vap} {\Delta {\rm H}}_{water}+{\dot{m}}_{water} {Cp}_{water} \left({T}_{dryin{g}_{out}}-{T}_{ambient}\right)]$$
(3)

where \({\eta }_{dryer}=65\%\): thermal efficiency of a rotary dryer, mean value (Peters and Timmerhaus 1991); \({\dot{m}}_{vap}\): total amount of evaporated water (kg/s); \({\Delta {\rm H}}_{water}\)= 2200 kJ/kg: vaporization enthalpy of water; \({\dot{m}}_{water}\): water quantity in waste after drying (kg/s); \({Cp}_{water}\)= 4.18 kJ/(kg K): water heat capacity has been considered constant between 25 and 100 °C; \({T}_{dryin{g}_{out}}\): temperature in oC of waste at the exit of the dryer; \({T}_{ambient}\): ambient temperature (25 °C).\({U}_{d}\) is the overall heat transfer coefficient (\(\frac{kg}{s {m}^{2}})\) calculated by Eq. (4) (Peters and Timmerhaus 1991):

$${U}_{d}=\frac{60\ {G}_{d}^{0.67}}{1000}$$
(4)

where Gd: the mass velocity of drying air, for which a typical value of 5 ksg s−1 m−2 for rotary dryers is used (Peters and Timmerhaus 1991).LMTD is the mean log temperature difference, which is calculated by Eq. (5):

$$LMTD=\frac{max\left({\Delta T}_{\alpha },{\Delta T}_{\beta }\right)-\mathrm{min}({\Delta T}_{\alpha },{\Delta T}_{\beta })}{ln\left(\frac{max\left({\Delta T}_{\alpha },{\Delta T}_{\beta }\right)}{\mathrm{min}({\Delta T}_{\alpha },{\Delta T}_{\beta })}\right)}$$
(5)

where \({\Delta {\rm T}}_{\alpha }={\rm T}_{{h}_{in}}-{T}_{{c}_{out}}\) and \({\Delta {\rm T}}_{\beta }={\rm T}_{{h}_{out}}-{T}_{c, in}\)where \({\rm T}_{{h}_{in}}\): temperature of hot stream at the inlet in oC; \({T}_{{c}_{out}}\): temperature of cold stream at the outlet in oC; \({\rm T}_{{h}_{out}}\): temperature of hot stream at the outlet in oC; \({T}_{c, in}\): temperature of cold stream at the inlet in oC (equal with ambient temperature).From the energy balance, the inlet temperature of air in the dryer (\({\rm T}_{{h}_{in}}\)) in oC is calculated and then by knowing all the temperatures of both streams of the dryer, LMTD is calculated to determine the \({\rm A}_{dryer}\).The length of the dryer \({L}_{dryer}\) (m) calculated based on the surface and diameter of the dryer:

$${L}_{dryer}=\frac{{A}_{dryer}}{3.14\ {D}_{dryer}}$$
(6)

where \({D}_{dryer}\): dryer diameter (m) calculated as follows:

$${D}_{dryer}=\sqrt{\frac{4 {A}_{sa,d}}{\pi }}$$
(7)

where \({A}_{sa,d}\): cross sectional area of rotary dryer (m2) calculated as follows:

$${A}_{sa,d}=\frac{\frac{{V}_{waste}}{{u}_{waste}}}{{n}_{space}}$$
(8)

where \({V}_{waste}\): volume flow of waste (m3/s) calculated below:

$${V}_{waste}=\frac{F}{{\rho }_{waste}}$$
(9)

where \(F\): feed of waste to the plant (kg/s); \({\rho }_{waste}\): waste density, with an average value of 300 kg/m3; \({u}_{waste}\): waste velocity, which is assumed equal to 0.01 m/s; \({n}_{space}\): coverage of dryer by solids with a mean value equal to 0.125 (Peters and Timmerhaus 1991).

4.1.2 Heat Exchanger

Through a plate heat exchanger of gas–gas, the syngas transfers its sensible heat into the drying air. A typical efficiency for a commercial plate heat exchanger is 80% (Peters and Timmerhaus 1991; Karellas et al. 2012; Wang et al. 2014).The cost of a plate heat exchanger, \({C}_{heat exchanger}\), (in $ for the year 2011) is given by Eq. (10) (Quoilin et al. 2011):

$${C}_{heat exchanger}=1.4\ {f}_{material}\ (190+310 {\rm A}_{HEX})$$
(10)

where \({\rm A}_{HEX}\): the total heat exchanging surface (m2), which is calculated by Eq. (11):

$${\rm A}_{HEX}=\frac{{\dot{Q}}_{he}}{{U}_{he}\ LMTD}$$
(11)

where \({U}_{he}\): the total heat transfer coefficient for plate heat exchangers. For \({U}_{he}\), a mean value of 40 W/(m2 K) is used (Edwards 2008); \({\dot{Q}}_{he}\): the actual energy transferred (kJ/s). For its calculation, the method of efficiency factor is used:

$${\dot{Q}}_{he}=\varepsilon {\dot{Q}}_{max}$$
(12)

where \(\varepsilon\): efficiency factor, equal to the ratio of actual heat transfer to maximum possible transferred heat, defined as:

$${\dot{Q}}_{max}\left(\frac{kJ}{\mathrm{s}}\right)={C}_{min} ({T}_{hot,in}-{T}_{cold,in})$$
(13)

where \({T}_{hot,in}\): temperature in oC of syngas in the inlet of the heat exchanger;\({T}_{cold,in}\): ambient temperature (25 °C);\({C}_{min}\): the minimum heat capacity between air and syngas in kJ/(s*K), as follows:

\({C}_{min}={\dot{m}}_{water} {Cp}_{water}\) if \({\dot{m}}_{water} {Cp}_{water}<{\dot{m}}_{syngas} {Cp}_{syngas}\)

\({C}_{min}={\dot{m}}_{syngas} {Cp}_{syngas}\) if \({\dot{m}}_{water} {Cp}_{water}>{\dot{m}}_{syngas} {Cp}_{syngas}\)and \({f}_{material}\): material factor, equal to 12 (Guthrie and Grace 1969). Due to high temperatures in the gasification reactor, materials of heat exchanger should be resistant to corrosion of acid gases that are included in product syngas.

4.1.3 Gasification/Vitrification Furnace

The cost of the gasification/vitrification furnace, \({C}_{furnace}\), (in $ for the year 2007) was calculated from Eq. (14) (Peters and Timmerhaus 1991):

$${C}_{furnace}={(0.3+f}_{material})\ {e}^{(11.808+0.81 \mathrm{ln}\left[Q\right])}$$
(14)

where \({f}_{material}\): material factor, equal to 12, because the furnace must be constructed from a ceramic refractory material resistant to high temperatures (Guthrie and Grace 1969); \(Q\): the energy (MW) supplied by the plasma torches.

4.1.4 Plasma Torches

Some of the main manufacturers of plasma torches are Westinghouse Plasma Corporation, Plasma Energy Applied Technology and Phoenix Solution (Galeno et al. 2011). Dimensioning of plasma torches results from the energy consumed, which is calculated by the energy analysis assuming an 85% torch efficiency. Based on market research, typical torches have an input power of 1.2 MW per torch (Leal-Quirós 2004; Minutillo et al. 2009). Taking this into account, the total number of torches (\(NoT\)) required for this process is estimated. The cost of an 1.2 MW torch (\({C}_{torch}\)) is equal to 1,275,000 $ for the year 2011 (Phoenix Solution Company 2019). Consequently, the total cost of the plasma torches is easily calculated by Eq. (15):

$${C}_{total, torches}=NoT\ {C}_{torch}$$
(15)

4.1.5 Clean–up Systems

The clean-up systems considered for the plasma gasification plant and the relevant investment costs are presented in Table 3 (Rezaiyan and Cheremisinoff 2005).

Table 3 Fixed Installation Costs for clean-up systems
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The cyclone is located after the reactor and before the heat exchanger, and thus, its cost should be adjusted for materials resistant to high temperatures (\({f}_{material}=12\)).

4.1.6 Power Generation System

In our case study, gas engines are the most suitable choice for the power generation system, because their main advantage is the low fuel quality requirement such as syngas. According to market research, a typical range of gas engine capacity is from 250 kW to 4.4 MW (Jenbacher Engines 2019). It was assumed that each engine will generate 3 MW.

The cost is 4,639,000 $ (for 2011) per engine (Wu et al. 2016):

$$C_{Eng.Total}=4,639,000\;NoE$$
(16)

where \(NoE\): the number of engines required, calculated by Eq. (17):

$$NoE=\frac{{P}_{Eng,out}}{Eng\_Output}$$
(17)

where \({P}_{Eng,out}\): the energy (MW) produced by the gas engines based on the input energy of syngas in the engines (according to syngas’ heating value) and the efficiency of the engines, which is considered equal to 40% (Achinas and Kapetanios 2012; Molino et al. 2016; Jenbacher Engines 2019); \(Eng\_Output\) =3 MW as described above.

4.1.7 Other Costs

Concerning the OIC, it is essential to specify the following costs (Quoilin et al. 2011):

  • The cost of required piping, that is considered equal to 10% of the equipment’s cost.

  • A hardware and a control system in combination with the measuring instruments ensure the safe and proper operation of the system. The total cost of Hardware, measurement and control system is considered to be the 7% of the equipment’s cost.

  • An additional cost of 13% due to fluid handling and other essential processes is considered.

4.2 Operation and Maintenance Costs

Annual Operation and Maintenance (O&M) costs of a plasma gasification system include the costs of labor, maintenance, consumables and others. Table 4 includes all O&M costs considered in this study, which leads to an annual operating cost equal to 11% of the initial investment cost (Clark and Rogoff 2010).

Table 4 Annual O&M costs of Plasma Gasification Plant
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The above cost analysis does not take into account the interest expenses of the loan, as well as the depreciation of investment, which will be taken into consideration during the financial evaluation.

4.3 Income

The annual income of the plant includes the revenues from the net electricity sales to the grid and the revenues from waste disposal. The net electricity sold to the grid is the gross electricity produced minus the electricity required by the plasma torches.

The 2009/28/EC European Directive (European Commission 2019) defines biomass as “the biodegradable fraction of products, waste and residues of biological origin from agriculture, forestry and related industries, including fisheries and the aquaculture, as well as the biodegradable fraction of industrial and municipal wastes”. It is assumed that 50% of the MSW entering the plant is biodegradable and thus this is considered as biomass (Molino et al. 2016; Georgiopoulou and Lyberatos 2017; Vasileiadou et al. 2020).

Therefore, regarding sale of power to the grid, 50% of generated power can be sold at a price of 87.85 €/MWh, which is the selling price of the electricity produced by “energy recovery plants of biodegradable fraction of municipal waste” in Greece (LAGIE 2019). The remaining 50% of the energy generated will be sold at the conventional selling price in Greece, which is 65 €/MWh (average pool system marginal price for 2019 (LAGIE 2019)).

5 Case Study

As a case study, a plasma gasification plant in Greece that treats 750 TPD of MSW is examined. The composition of a typical MSW in Greece is given in Table 5. Corresponding data of Greek MSW composition is given by Komilis et al. (Komilis et al. 2012) and Gidarakos et al. (Gidarakos et al. 2006). The main operating factors of the model are the gasification temperature, the moisture content of waste that enters the furnace and the oxygen supplied to the furnace.

Table 5 Typical composition of MSW in Greece
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5.1 Parametric Analysis—Optimization of Process Parameters

According to previous studies (Mountouris et al. 2006, 2008; Huang and Tang 2007; Byun et al. 2012; Gandhi 2015; Indrawan et al. 2019), the gasification temperature was set equal to 1273 K. The synthesis gas, after the cyclone, enters the heat exchanger, transferring a part of its sensible heat to air for the waste drying before its gasification. In order for the plant to be self-sustained as per the heat demand, the energy required for the waste drying should be covered by the thermal energy produced by the system. So, the sensible heat of syngas should be greater than the required thermal energy for waste drying. This is one of the limitations we took into account in the analysis and optimization of the system. Also, for electricity production, the use of gas engines has been chosen because of the low quality requirement of the fuel used. Thus, an additional limitation is the lower heating value (LHV) of the syngas, which should be greater than 1.25 kWh/Nm3 for good performance of the engines (Achinas and Kapetanios 2012).

The determination of the optimum values for the final waste moisture content, after drying, along with the feed of air/oxygen to the gasification/vitrification furnace was based on the energy and economic analysis of the system.

Figure 2 displays the impact of the moisture content of the waste and the oxygen feed to the gasifier on the net electricity produced and Fig. 3 their effect on the LHV of the synthesis gas. The results show that increase of the oxygen in the gasifier leads to higher production of net electricity, while increase of the waste moisture content results in increase of energy consumed by the plasma torches, and thus, in decrease of net power output. Moreover, the low heating value of synthesis gas decreases as moisture content increases and as the oxygen feed increases.

Fig. 2
figure 2

Impact of moisture content and oxygen on power produced

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Fig. 3
figure 3

Impact of moisture content and oxygen on low heating value of syngas

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Also, as shown in Fig. 4, the maximum fixed cost corresponds to maximum moisture content and minimum oxygen feed. Combination of the results presented in Fig. 2 and Fig. 5 comes to the conclusion that the Net Present Value (NPV) of the investment is directly related to the net power produced. The NPV is calculated as a function of the fixed cost and the annual profit of the investment, which depends on the net electrical energy sold to the grid.

Fig. 4
figure 4

Impact of moisture content and oxygen on overall investment cost

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Fig. 5
figure 5

Impact of moisture content and oxygen on NPV

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Taking all the above into consideration, the maximum NPV for a twenty-year investment is 29.80 M€, which results for an oxygen feed of 0.434 kmol/kmol dry ash free waste and 11% waste moisture content (after drying). The LHV of the syngas produced is 1.26 kWh/Nm3. It should be noted that an indicative gate fee of 50 € per tonne of waste has been considered (Watson et al. 2018; Ramos et al. 2020).

The composition of the syngas for the optimum case is described in Table 6, while Table 7 presents the energy results. The sensible heat of the produced syngas is recovered in a heat exchanger, working with an efficiency of 80% and an outlet syngas temperature of 500 °C, which is adequate for the MSW drying to 11% moisture content without need for an external heat source, as described in Sects. 2 and 4.1.2.

Table 6 Chemical composition of synthesis gas
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Table 7 Output variables of model
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As it is presented in Table 7, the calculated process’ electrical efficiency is 23.3% that is higher than a typical efficiency of a traditional mass burning incinerator of the same capacity, ranging between 18–22% (Kleis and Dalager 2004; Qiu 2012; Kalogirou 2018).

The electrical efficiency is defined by Eq. (18):

$${\eta }_{e}=\frac{{P}_{net} \ 24}{F\ {LHV}_{MSW}}$$
(18)

where \({P}_{net}\): the net power produced in MW, after removing the power consumed by plasma generator, which is presented in the Table 7; \(F\): the feed of waste to the plant (TPD); \({LHV}_{MSW}\): the LHV of the MSW (MWh/tonne). A typical value of LHV for the Greek MWS is 2.78 MWh/tonne, which has been estimated by the Dulong equation (Cho et al. 1995).

Table 8 describes the OIC of a plasma gasification plant that treats 750 TPD of MSW for the year 2019. In the calculations, an indicative gate fee of 50 € per ton of waste has been considered.

Table 8 Financial results for a plasma gasification plant in Greece that treats 750 TPD of MSW
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The Levelized Cost of Electricity (\(LCOE\)) in €/MWh reflects the net present value per energy produced for a period of time regarding a power production plant and is calculated by Eq. (19) (Finance Corporate Institute 2021):

$$LCOE=\frac{\sum_{t=1}^{n}\frac{{I}_{t}+{M}_{t}+{F}_{t}}{{(1+r)}^{t}}}{\sum_{t=1}^{n}\frac{{E}_{t}}{{(1+r)}^{t}}}$$
(19)

where \({I}_{t}\): Overall investment cost in € at the period of time t; \({M}_{t}\): Annual operational cost in € at the period of time t; \({F}_{t}\): Fuel’s cost in € at the period of time t. In our case study, \({F}_{t}\) is equal to 0 € since no external energy is used to the process; \({E}_{t}\): Net energy in MWh produced at the period of time t; \(r\): Inflation, considered equal to 1.5%; \(n\): Timelife of the investment, considered equal to 20 years.

For the financial evaluation of the investment, the NPV is calculated by Eq. (20):

$$NPV=-Fixed Cost+\sum_{t=1}^{20}\frac{NCF}{{(1+r)}^{t}}$$
(20)

where NCF = Annual income – Operating cost – Tax rate × (Annual Profit) – Amortization; Annual Profit = Annual income – Operating cost – Depreciation – Loan Interest; Amortization = Interest + Repayment; r: the inflation rate.

For the financial evaluation, we considered a lifetime of investment of 20 years. Also, we assumed that 40% of overall investment cost is financed by loan at an interest rate of 4.5% and a paid off period of 10 years. The tax rate is estimated equal to 30% and the inflation rate equal to 1.5%, based on present data.

The NPV for a 20-years investment is calculated to be 29.80 M€, which is positive after 16 years. The internal rate of return (IRR) is equal to 4%, which is greater than the sum of interest rate of a savings bank (~ 1%) and a risk-based increment of 2%. Based on these, we can conclude that the investment is profitable.

Figure 6 shows how the NPV of the investment changes for different gate fees. For a gate fee of 40 €/MSWtonne, the NPV is negative even after 20 years, while for 60 €/MSWtonne the investment depreciation time is reduced to 13 years. Ramos et al. (2020) states that landfill fees can vary from 10 € to more than 100 € per tonne of treated waste, according to the country.

Fig. 6
figure 6

Net present value of plasma gasification investment for different gate fees

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6 Cost Comparison between Plasma Gasification and Incineration

In this section, cost data for incineration and plasma gasification compiled from the literature are presented in order to compare these two processes using a financial point of view and evaluate the financial results of this case study, presented in the Table 8. Figure 7 displays the OIC of incineration and plasma gasification plants versus the capacity of the plant, and Fig. 8 shows the OIC per year-tonne of MSW as a function of plant capacity.

Fig. 7
figure 7

Overall Investement Cost vs capacity for incineration and plasma gasification plants. (Data for incineration were taken from: Porteous 1967; Alam and Challis 1992; Koe et al. 2001; Porteous et al. 2003; Karagiannidis 2008; Fitzgerald 2009; Rodríguez 2011; Qiu 2012; Athanasiou et al. 2015; Sudibyo et al. 2017; Kalogirou 2018. Data for plasma gasification were taken from: Yassin et al. 2009; Clark and Rogoff 2010; Themelis and Arsova 2010; Galeno et al. 2011; Li et al. 2016; Ducharme 2017; Kalogirou 2018; Ramos et al. 2020)

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Fig. 8
figure 8

Overall Investement Cost per MSW tonne per year vs capacity for incineration and plasma gasification plants

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Although a more thorough investigation is needed, the numbers presented in Fig. 7 and Fig. 8 indicate that the investment cost of a plasma gasification plant is comparable to that of an incineration plant. It should be noted that the OIC of 134.5 M€ calculated in the present study (Table 8) for a plasma gasification plant of 750 TPD capacity (225,000 tonne/year) agrees very well with the value estimated according to the literature data (Fig. 7) that is between 130 and 140 M€.

7 Conclusions

In the first part of this work, a thermodynamic framework that deals with a complete thermodynamic analysis of the plasma gasification process is presented. This analysis aims to: (a) predict the concentrations of the main gaseous components produced (hydrogen, carbon monoxide, carbon dioxide, water, methane and nitrogen); (b) predict the concentrations of minor components in the final synthesis gas (sulfur and chlorine compounds); and (c) perform a detailed energetic analysis. The predictions of the modified GasifEq model were compared with experimental data measured in the Thermoselect gasification process that is one of the few large scale, commercial, combined gasification and melting MSW processes. The GasifEq model predictions agree very well with the real process data. In the second part of this work, a preliminary design, optimization and cost analysis for a plasma gasification plant in Greece that treats 750 tonne/day of MSW is presented. The results indicate that the investment yields a positive net present value (NPV) after 16 years when a gate fee of 50 €/MSWtonne is assumed, while for a gate fee of 60 €/MSWtonne the NPV of the investment is positive after 13 years. The comparison of the plasma gasification process of MSW with the widely used incineration of MSW, as per their overall investment cost, using data from the literature, reveals that the investment cost of a plasma gasification plant is comparable to that of an incineration plant.