High-Efficiency Cogeneration Systems: the Case of the Paper Industry in Italy

Article High-Efﬁciency Cogeneration Systems: The Case of the Paper Industry in Italy

Marco Gambini, Michela Vellini , Tommaso Stilo *, Michele Manno and Sara Bellocchi Department of Industrial Engineering, University of Rome Tor Vergata, 00133 Rome, Italy; [email protected] (M.G.); [email protected] (M.V.); [email protected] (M.M.); [email protected] (S.B.) * Correspondence: [email protected]; Tel.: +39-06-72597200

Received: 20 December 2018; Accepted: 18 January 2019; Published: 22 January 2019

Abstract: In January 2011, the introduction of high-efficiency cogeneration in Europe radically modified the incentive scheme for combined heat and power (CHP) plants. Since then, the techno-economic feasibility of new cogeneration plants in different areas of application (industry, service, residential, etc.), along with the definition of their optimal operation, have inevitably undergone a radical change. In particular, with reference to the Italian case and according to the most recent ministerial guidelines following the new EU regulation, in the event that cogeneration power plants do not reach an established value in terms of overall efficiency, their operation has to be split into a CHP and a non-CHP portion with incentives proportional to the energy quantities pertaining to the CHP portion only. In the framework of high-efficiency cogeneration, the present study compares different CHP solutions to be coupled with the paper industry that, among all the industrial processes, appears to be the best suited for cogeneration applications. With reference to this particular industrial reality, energy, environmental, and economic performance parameters have been defined, analysed, and compared with the help of GateCycle software. Among the proposed CHP alternatives, results show that gas turbines are the most appropriate technology for paper industry processes.

Keywords: high-efﬁciency cogeneration; primary energy saving; electricity from cogeneration; paper industry; GateCycle; CO2 emissions avoided

1. Introduction The term “cogeneration” refers to the simultaneous production of process heat and electricity from a single energy input. All types of power plants can be employed (e.g., steam power plants, gas turbines, combined cycle power plants, internal combustion engines); the extracted heat largely varies in entity and temperature depending on technology, size, and operation parameters. Such a system has valuable properties to enhance the efficiency of fuel use: the combined production of electricity and heat turns out to be more efficient than the separate production of these two forms of energy. These qualities are a fact, but difficulty in evaluating the efficiency of combined heat and power CHP has been observed since the breakout of this technology. Havelský [1] pointed out the problem back in 1999; a strict regulation of such plants had thus to be created. In the European Union, a well-structured incentive scheme made a contribution to the growth of cogeneration. In 2004, the EC Directive 2004/8 [2] introduced the concept of high-efficiency cogeneration (HEC); the Annexes II and III of the Directive provided it with a quantitative definition establishing that a cogeneration production characterized by primary energy savings (PES) of at least 10% compared to the reference values for separate production (or >0% for small scale cogeneration units, with installed capacity below 1 MWe, and for micro-cogeneration units) is highly efficient and has the right of access to incentives. After the publication of the Directive, the Commission Decision of 19 November 2008 [3] stated that a

Energies 2019, 12, 335; doi:10.3390/en12030335 www.mdpi.com/journal/energies Energies 2019, 12, 335 2 of 21 cogeneration unit operates in full cogeneration mode when the maximum technically possible heat recovery is attained and all produced electricity is thus considered CHP electricity. When a plant does not operate in full cogeneration mode, it is required to distinguish the amount of electricity and heat not produced under cogeneration mode from CHP electricity and heat, which are the only quantities able to receive incentives. The Directive 2017/27/EU [4] repealed Directive 2004/8, confirming all of its points in the Annexes I and II. Italy implemented the Directive by enacting three decrees [5–7], followed by the publication of ministerial guidelines [8]. The authors studied the transposition of the Directive 2004/8 EC in the Italian context, referring to the methodology indicated in the abovementioned guidelines for dealing with cogeneration units in Reference [9]. That paper was an opportunity to better clarify the methodology to evaluate plants in a high-efficiency cogeneration framework and do the groundwork for estimating their primary energy savings, in a similar fashion as the studies carried out by Kanoglu and Dincer [10]. Starting from this, in Reference [11], the authors assessed the performance of Italian cogeneration plants in terms of the effects of the power loss coefficient on CHP electricity and power-to-heat ratio. Plants with a non-zero power loss coefficient display a lower CHP electricity production, in an equivalent total electric output. The calculation of the power loss coefficient and the definition of the reference efficiency are critical aspects of the implementation of the Directive, as verified by Gvozdenac et al. [12]; they studied an actual 150 MW capacity CCGT plant of independent power producers (IPPs) in Thailand, and the investigation showed that the reference value of high-efficiency cogeneration has a large effect on the percentage of CHP electricity output. The impact of the power loss coefficient is not as large, but it influences CHP electricity and CHP fuel energy. These factors are crucial in determining a plant’s profitability, so Gvozdenac et al. [12] proposed a modified procedure for the assessment of a CHP plant’s efficiency in line with the work of Uroševićet al. [13]. Coupling an industrial facility with a combined heat and power system in order to increase the overall efficiency of the plant is now very common practice. For instance, Wang et al. [14] optimized the performance of different cogeneration plants to be implemented in the cement industry in order to recover the available waste heat. The authors reviewed several industry sectors, identifying the most appropriate CHP technology for each of them according to the facility size. The pulp and paper industry is characterized by large electric and heat consumptions, and this means that it lends itself to the implementation of cogeneration.

The Pulp and Paper Industry in Europe: Production and Energy Consumptions Pulp is a cellulose-based material produced by separating lignin and cellulose fibers from wood and it is the basic constituent for the preparation of paper. The process of separation is chemical when pulp is obtained by cooking chipped wood in an autoclave, with the addition of sodium sulphate (Kraft or sulphate pulping process) or magnesium bisulphite (sulphite pulping process); the process is mechanical when fibers are separated from the trunk by means of an abrasive rotary grindstone. The Joint Research Centre (JRC) of the European Commission released the BAT Reference Document for the Production of Pulp, Paper, and Board [15] to make each of the above processes more efficient and CHP systems, based on different thermal power plants, are one of the main available techniques in this respect. According to the data illustrated in this document, Europe is one of the main actors in the pulp and paper industry. The annual production of pulp in Europe was about 41.0 million t/y, constituting 22% of world’s total wood pulp production; this made Europe the second largest pulp producer. Finland and Sweden together covered 57% of European total wood pulp production. Paper and board production totaled 390.9 million t/y worldwide; 25.3% of this amount was produced in Europe. Italy was one of the leading paper and paperboard producers (9.5 million t/y). The number of paper mills in Europe was 887, located mainly in Italy, Germany, France, Spain, and the UK [15]. Energies 2019, 12, 335 3 of 21

In order to clarify how much energy this industry sector consumes, it is possible to refer to the IEA’s “Energy Technology Perspectives” 2017 report [16], which states that Organisation for Economic Co-operation and Development (OECD) Europe’s ﬁnal energy use by pulp, paper, and printing, a quantity adding up all energy supplied to the ﬁnal consumer, was 1.36 EJ in the year 2014, representing 1.7% of the world’s total industry energy consumption. The electric and thermal energy demand by the pulp and paper industry in Europe is bound to increase. Szabó et al. [17] elaborated a world model to reproduce technology and market developments of the pulp and paper industry (PULPSIM) starting from current trends. According to their results, paper demand in Europe is expected to grow up to 120 million t/y by 2030. At the national level, the most recent data regarding the Italian pulp and paper industry come from Assocarta’s “Rapporto ambientale dell’industria cartaria” 2016 report [18] and are dated for 2014. According to this document, 154 paper facilities are present on Italian territory; data concerning consumption and impact are reported in Table1.

Table 1. Energy and environmental impact of the Italian paper industry.

Energy Impact Environmental Impact Consumed Energy Generated Energy Employed Steam Direct Emission Indirect Emission (TWh) (TWh) (TWh) (tCO2) (tCO2) 7.01 5.54 11.94 4.88 0.71

The pulp and paper industry is obviously responsible for the emission of greenhouse gases in the atmosphere; CO2 emissions from energy production are considered direct, those coming from energy purchase are considered indirect. The need for an environmental optimization of paper mills is a long-felt concern, since Thompson’s [19] works on paper mills. Monte et al. [20] analyzed different paper industry production processes, proposing, for each of them, beneficial waste management approaches. Furthermore, considering the issue of emissions from a paper industry facility, Bhander and Jozewicz [21] developed a model to estimate changes in emission when switching fuel, installing air pollution equipment, and implementing energy efficiency measures. Combined heat and power systems make a contribution when it comes to complying with the Kyoto Protocol as well: as observed by Chen et al. [22], the pulp and paper industry is characterized by high fossil energy consumption which is strictly linked to high emission of greenhouse gases. In the wake of this, Boharb et al. [23] outlined a methodology to perform energy audits in the pulp and paper industry. The perception of the need for improved efficiency is now well established in the management of production processes; so, the harsh international competition to which the pulp and paper industry is subjected made energy management a necessary practice for preserving competitiveness. Kong et al. [24] reviewed different energy-efficient technologies to be implemented in the paper industry, creating a data collection useful to assess the most suited ones to each process: CHP systems are perfectly placed in this context, having the ability to improve the global efficiency of the plant. Cogeneration systems, with their improved overall efficiency, can also be a solution to the problems raised by Posch et al. [25]. Returning to the European case, the Joint Research Centre estimates, in its BAT Reference Document for the paper industry, that CHP systems enable paper mills to save about 30% of the energy consumption of a separate production conventional technology and to reduce greenhouse gas emissions. Therefore, the present paper aims at proposing and comparing the most efficient CHP setups which can be employed in the pulp and paper industry, taking into consideration a particular industrial reality. Results can be compared to those achieved by Shabbir et al. [26], taking into account the different production processes considered. Energies 2019, 12, 335 4 of 21

2. Methods

2.1. Quantitative Deﬁnition of the Analysed Process

Table2 displays electric ( Cs,e) and thermal (Cs,t) speciﬁc consumptions for a fully-integrated bark-ﬁred pulp and paper mill, according to what is reported in the above-mentioned JRC BAT Document [15].

Table 2. Electric and heat speciﬁc consumptions of a typical Kraft fully-integrated bark-ﬁred pulp and paper mill.

Process Heat Consumption (kWh/t) Power Consumption (kWh/t) Pulp mill process 5800–31,000 320–640 Paper mill process 4200–21,000 380–760 Total 10,000–52,000 700–1400

While electricity is employed to operate machines, heat, in the form of steam, is deployed in both pulp production and drying. With this regard, this paper proposes a methodology to analyze different CHP technologies potentially suitable for paper mills and evaluated in an HEC framework. According to Reference [18], the paper industry accounts for 154 facilities in Italy with an overall paper production of 56,000 t/y. Supposing a total of 7200 operating hours (equivalent to 300 days a . year), the average hourly production of a single Italian paper mill can be set around 8 t/h (mpaper). The related electric and thermal power requirements were derived from a year-long measurement campaign conducted for a particular Italian industrial reality. Thus, the whole facility operation refers to this particular year. This facility makes use of electricity provided by a power plant, steam from the power plant and natural gas to satisfy the heat demand; therefore, the days for which all three values were able to be measured were the only ones accounted for. About 33% of the thermal demand is satisfied by the thermal power plant steam; the remainder is fulfilled by natural gas burnt in a boiler. It is known that the conditions of the steam provided to the paper mill are a pressure of 8 bar and a temperature of 170 ◦C. The considered Italian mill consists only in a continuous papermaking machine, which has the purpose of forming sheets starting from pulp produced in batches; this machine requires steam at a specific temperature and pressure condition, which implies a single heat extraction. It is also known that the enthalpy of the returned condensate is 417.5 kJ/kg. The sum of these values allows for the calculation of the total amount of heat required by the paper production process. The daily electric load diagram was derived assuming that the electric demand is constant over a period of 1 h. Operating days were divided into “stationary” days (when the average hourly production was around the average value of produced paper in this facility) and “abnormal” days (when the average hourly production deviated from that value, for example because of failures or maintenance). The above measurements allowed for the construction of the typical day consumption diagram, reported in Figure1, where only stationary days were taken into consideration and a paper production of 8 t/h was assumed. The electric and thermal power demand (then used for power plant design) were calculated by averaging over the abovementioned load diagram: Pe = 9.6 MWe and Pt = 32 MWt. Starting from these values it is possible to estimate the specific electric and thermal power consumptions of the plant:

Pt Cs,t = . = 40300 kWh/t mpaper

Pe Cs,e = . = 1200 kWh/t mpaper as power is the product of mass ﬂow rate by the speciﬁc consumption of the paper mill. Falling in the ranges reported in Table2, the obtained values can be considered validated. Table3 sums up the input data common to all plant types. Energies 2019, 12, 335 5 of 21

Energies 2019, 12 FOR PEER REVIEW 5

TheThe following following subsections subsections illustrate illustrate the procedures procedures to tocarry carry out out a design, a design, an HEC an HECand economic and economic analysisanalysis of theof the CHP CHP technologies technologies takentaken into consideration. consideration.

Figure 1. Typical day load diagram. It shows the electric and thermal power demand with values Figure 1. Typical day load diagram. It shows the electric and thermal power demand with values assumed to be constant over a period of 1 h. assumed to be constant over a period of 1 h. Table 3. Input data common to all plant types. Table 3. Input data common to all plant types. Input Data Common to All Plants Input Data Common to All Plants Operating hours (h/y) 7200 Operating hours (h/y) 7200 Daily paper production (t/d) 192 Steam pressureDaily required paper production by user (bar) (t/d) 8 192 Steam temperatureSteam pressure required required by user by (◦ userC) (bar) 170 8 ReturnedSteam condensate temperature enthalpy required (kJ/kg) by user (°C) 417.5 170 ◦ High-efficiency cogenerationReturned reference condensate enthalpy enthalpy (1 bar, 15(kJ/kg)C) (kJ/kg) 63 417.5 Specific thermal consumption (kWh/t) 4000 High-efficiencySpecific cogeneration electric consumption reference (kWh/t) enthalpy (1 bar, 15 °C) (kJ/kg) 1200 63 AnnualSpecific heat thermal demand consumption (MWh/y) (kWh/t) 280,320 4000 Annual self-consumedSpecific electric electric consumption energy (MWh/y) (kWh/t) 69,120 1200 Annual heat demand (MWh/y) 280,320 2.2. Operation Mode andAnnual Simulation self-consumed of the CHP electric Units energy (MWh/y) 69,120

2.2.When Operation designing Mode aand CHP Simulation plant, itof isthe essential CHP Units to set a mode of operation: in fact, a CHP plant can be operated satisfying electric demands or thermal demands. When designing a CHP plant, it is essential to set a mode of operation: in fact, a CHP plant can beA operated plant satisfying satisfying electric demands demands or thermal can cover demands. the user electric demand at full load. In this configuration,A plant however, satisfying the electric heat demanddemands might can cover not bethe fully user covered, electric demand and an auxiliaryat full load. boiler In this is needed to fulfillconfiguration, the remaining however, load. the This heat mode demand of operation might not is be bound fully tocovered, make theand plant an auxiliary self-sufficient boiler inis terms of electricneeded loads, to fulfill avoiding the remaining energy exchanges load. This with mode the of grid. operation is bound to make the plant self-sufficientDepending in on terms power of electric values, loads, the avoiding components energy ofexchanges the system with the are grid. selected from specialized catalogues;Depending starting on from power the values, relevant the operational components parameters of the system indicated, are selected components from specialized are modelled and,catalogues; as a consequence, starting from plant the operation relevant operationa is simulatedl parameters by means indicated, of GateCycle components software. are modelled GateCycle is and, as a consequence, plant operation is simulated by means of GateCycle software. GateCycle is a a PC-based software consisting of a block diagram environment that performs detailed steady-state PC-based software consisting of a block diagram environment that performs detailed steady-state design and off-design analyses of thermal power systems. Simulations run by means of this software design and off-design analyses of thermal power systems. Simulations run by means of this software allowallow the the calculation calculation of of all all the the differentdifferent pressure, temperature, temperature, and and enthalpy enthalpy conditions conditions with respect with respect to theto the working working fluid fluid in in the the system, system, andand as as a a conseq consequence,uence, a quantitative a quantitative evaluation evaluation of the of plant’s the plant’s performance in terms of net power, overall efficiency, primary fuel flow rate, and supplementary fuel flow rate. The variation of the system performance parameters in off-design conditions is evaluated as well, by modelling it in different seasonal situations with GateCycle software. Energies 2019, 12, 335 6 of 21

2.3. Preliminary Design of CHP Technologies for the Analysed Process

First of all, the ratio Pt/Pe of the specific process was calculated: the result was Pt/Pe = 3.33. Average values of the useful-heat-to-electricity ratio (RH/E) for Italian plants, similar to the ones calculated by the authors in Reference [27], are available from Reference [28]. They represent the current national state of the art of CHP plants in Italy. Such ratios will be recalculated and employed in the following. The CHP technologies selected for the present study are internal combustion engine (ICE), gas turbine (GT), steam power plant with back-pressure turbine (SPP-BPT), steam power plant with condensing turbine (SPP-CT), and combined cycle power plants (CCPP). Renewable energy sources were not taken into consideration. In fact, in Italy, renewable energy sources (RES) are not employed to satisfy the electric demand of industrial facilities, which require also heat. In the industry sector, systems capable of providing both process heat and electricity are installed; the size of such systems is determined with the aim of achieving the values for energy parameters imposed by current regulations to gain access to remuneration. Italian regulations provide for dispatching priority for both RES-based plants and cogeneration units: this means that, for these two plant types, electricity is fed into the grid before the amount of energy produced by fossil-fueled thermal power plants bound to the production of electricity only. However, the electricity produced by cogenerators is remunerated with a price which is not as competitive as that set for RES. If the plant is operated satisfying the electric load, after setting the rated electric power equal to Pe, the rated thermal power is calculated through the value of RH/E corresponding to the chosen technology. If the calculated value is lower than Pt, a supplementary steam flow rate must be estimated. If the plant is operated satisfying the thermal load, after setting the rated thermal power equal to Pt, the rated electric power is calculated through the value of RH/E corresponding to the chosen technology. If the calculated value is lower than Pe, the resulting electric power deficiency must be compensated for by purchasing electricity from the grid; if the calculated value is higher than Pe, the resulting electric power surplus must be released to the grid. An optimal design of plants is carried out bearing in mind two hypotheses:

• the useful heat produced by the CHP unit has to be exploited at its most, granting the cogenerator the achievement of the maximum HCHP/E ratio, with this condition being essential to access the best HEC performances; • electricity has not to be fed into the national grid; this means that the plant size cannot exceed an electric power equal to the electric demand of the process.

In light of this, the calculated value of the process ratio Pt/Pe must be compared to the ratio RH/E related to each CHP technology. As a consequence of these two hypotheses, if:

Pt > RHIE Pe the considered cogeneration technology is able to satisfy the electric demand of the production process, but not the thermal demand; this implies that the CHP unit has to be designed following the electric loads and auxiliary boilers shall be provided; Otherwise, if: Pt < RHIE Pe the considered cogeneration technology is able to satisfy the thermal demand of the production process, but not the electric demand; this implies that the CHP unit has to be designed following the thermal loads and additional electricity shall be drawn from the national grid. An alternative design of CHP technologies (that is, the plants sized according to the electric load will be sized according to the thermal load and vice versa) can be performed as well, in order to Energies 2019, 12 FOR PEER REVIEW 7

the considered cogeneration technology is able to satisfy the thermal demand of the production process, but not the electric demand; this implies that the CHP unit has to be designed following the Energiesthermal2019 loads, 12, 335and additional electricity shall be drawn from the national grid. 7 of 21 An alternative design of CHP technologies (that is, the plants sized according to the electric load will be sized according to the thermal load and vice versa) can be performed as well, in order to corroborate the validity of the design methodology subject of the present paper and to provide a more corroborate the validity of the design methodology subject of the present paper and to provide a complete overview of cogeneration units. more complete overview of cogeneration units. 2.4. HEC Energy Performance Parameters 2.4. HEC Energy Performance Parameters The HEC analysis is conducted by following the guidelines reported in References [7,9,29]. The HEC analysis is conducted by following the guidelines reported in References [7,9,29]. In In short, the overall efficiency ηg must be calculated as follows: short, the overall efficiency ηg must be calculated as follows: E𝐸+𝐻+ HCHP ηg = (1) 𝜂 =F − F (1) 𝐹−𝐹non,-CHP,H where EEis is the the total total electrical/mechanical electrical/mechanical energy, energy,HCHP HCHPis theis the CHP CHP useful useful heat heat energy energy (total (total useful useful heat energyheat energyH reduced H reduced by the by amount the amount of non-combined of non-combined useful useful heat heat energy energyHnon-CHP Hnon-CHP), and), andF − F F−non-CHP Fnon-CHP,H,H is thethe amountamount ofof fuelfuel energyenergy usedused inin cogeneration.cogeneration. If ηg achieves or exceeds the values of 0.80 for combined cycle gas turbines with heatheat recoveryrecovery andand steamsteam condensingcondensing extractionextraction turbine-basedturbine-based plants or 0.75 for other types of cogeneration units,units, the plant does not produce any non-CHP electric energy Enon-CHPnon-CHP. . For lowerlower valuesvalues ofofη ηgg,, thethe amount amount of ofE non-CHPEnon-CHP andand thethe relatedrelatedfuel fuel energy energy ( F(non-CHPFnon-CHP,E)) can be assessed throughthrough the the evaluation evaluation of of the the power power loss loss coefficient coefficientβ representing β representing the electricity the electricity loss caused loss bycaused steam by extraction steam extraction for heat production.for heat production. This parameter This parameter allows for allows the calculation for the calculation of both the of efficiency both the ofefficiency non-combined of non-combined electric energy electric generation energy generationηnon-CHP,E ηandnon-CHP the,E and power-to-heat the power-to-heat ratio Ceff ratio(Cactual Ceff in(C theactual Europeanin the European legislation). legislation).Ceff is required Ceff is required to calculate to calculate CHP electric CHP electric energy EenergyCHP starting ECHP starting from H fromCHP. BeingHCHP. knownBeing knownηnon-CHP ηnon-CHP,E, fuel,E, energyfuel energy for non-CHP for non-CHP electric electric energy energy generation generationFnon-CHP Fnon-CHP,E can,E becan evaluated; be evaluated; fuel energyfuel energy for CHP for CHP electric electric energy energy generation generationFCHP is F thenCHP is determined then determined by residual. by residual. Primary Primary energy energy saving (PES)saving can (PES) be calculatedcan be calculated as follows: as follows:   1 = ⎛ − ⎞ × PES𝑃𝐸𝑆 = 11− CHPH CHPE  × 100 (2) (2) 𝐶𝐻𝑃η 𝐻 + 𝐶𝐻𝑃η 𝐸 Re f Hη + Re f Eη ⎝ 𝑅𝑒𝑓 𝐻 𝑅𝑒𝑓 𝐸 ⎠ where CHPCHP Eη is the electric efficiencyefficiency of thethe CHPCHP portion,portion, CHPCHP H η is the thermal efficiencyefficiency of thethe CHP portion, and Ref Hη and Ref E η areare the the reference reference efficiencies efficiencies for heat and electric production, respectively, providedprovided byby thethe guidelines.guidelines. This parameter represents electric and thermal energy savings inin cogenerationcogeneration arrangement arrangement with with respect respect to to the the case case in in which which electric electric and and thermal thermal energy energy are producedare produced separately. separately. It is It fundamental is fundamental to prove to prove that that a cogeneration a cogeneration unit unit is really is really profitable; profitable; in fact, in Italianfact, Italian regulations regulations establish establish that only that plants only for plants which forPES which≥ 0.1 𝑃𝐸𝑆 (PES > ≥0 0.1 for(𝑃𝐸𝑆 small and>0 for micro-CHPsmall and units)micro-CHP are eligible units) toare access eligible incentives. to access incentives. Figure2 2 shows shows aa schematizedschematized CHPCHP unitunit withwith allall energyenergy flowsflows namednamed accordingaccording toto thethe Directive.Directive.

Figure 2. Scheme of a combined heat and power (CHP) unit with energy ﬂows. The guidelines describe the procedure to split the cogeneration unit in a CHP and a non-CHP part for efﬁciencies below the minimum required threshold.

Energies 2019, 12, 335 8 of 21

2.5. Economic Performance Parameters A preliminary step in evaluating the economic yield of a CHP unit consists in the calculation of the RISP parameter, an expression of the savings fundamental to determine the number of white certiﬁcates (WCs) (the securities attesting the savings, in toe, subjected to the incentive scheme):

ECHP HCHP RISP = + − FCHP (3) ηE,re f ηH,re f

WC = RISP × 0.086 × K (4) where K is the harmonization factor, depending on the size of the plant, and ηE,ref and ηH,ref are the actual reference efficiencies. The economic analysis is performed through the assessment of net present value (NPV) and pay-back period (PBP) for the considered plants, taking into account all positive and negative cash flows. However, when considering a comparison between different technologies, the ratio of the net present value to the total investment (NPV/I) may be better suited to assess the overall plant profitability: it is obvious that a plant for which the profit, after a set period of time, carries more weight than the initial outlay is a solution to be preferred.

2.6. Environmental Performance Parameters

The environmental performance of a CHP unit is evaluated in terms of the avoided amount of CO2 emissions, CO2,a. This parameter is calculated by subtracting CO2 emissions related to a cogeneration plant from those produced by a plant satisfying the same loads in separate production arrangement. The current emission factor attributable to the mix of power plants regularly generating electricity is 330 gCO2/kWh [30] in Italy.

2.7. Total Key Performance Indicator The most interesting contribution of Reference [27] that has been reapplied in this paper is the deﬁnition of the total key performance indicator (TKPI) that may be conveniently used to select, among a variety of possible CHP options, the technology best suited to a speciﬁc application:

PESi PBPmin (NPV/I)i CO2,a,i TKPIi = + + + (5) PESmax PBPi (NPV/I)max CO2,a,max where CO2,a represents the avoided CO2 emissions. With reference to the i-th CHP technology, TKPI adds together four key performance indicators referring to energy savings (PESi/PESmax), economic parameters (PBPmin/PBPi and (NPV/I)i/(NPV/I)max) and environmental impact (CO2a,i/CO2,a,max). Quantities with subscript max (min) refer to the maximum (minimum) value of the considered parameter that can be attained in the whole pool of CHP technologies. Each ratio (key performance parameter) can be equal to 1 at most; this means that total key performance parameter value cannot exceed four. It is a comparative measure of a plant’s performance and particularly useful to make comparisons as it combines all the relevant parameters that must be evaluated to select a technology when an investment has to be made.

3. Results The outcome of the design of a CHP plant for supplying energy to a paper mill, contemplating all plant technologies, will be shown in the following, along with related performance analyses. Energies 2019, 12 FOR PEER REVIEW 9

Energies 2019, 12, 335 9 of 21 3.1. Detailed Optimal Design of Cogeneration Units for a Paper Mill

3.1.3.1.1. Detailed ICE-Based Optimal Cogeneration Design of Cogeneration Unit Units for a Paper Mill The cogeneration unit was designed selecting an internal combustion engine model from a 3.1.1. ICE-Based Cogeneration Unit Wärtsilä catalogue [31]. The chosen model was W 9L46DF, which is a 4-stroke, non-reversible turbochargedThe cogeneration and intercooled unit was engine designed with selectingdirect fuel an injection, internal combustioncharacterized engine by a rated model power from of a Wärtsilä10.305 MW. catalogue According [31]. to Thethe methodology chosen model described was W 9L46DF,in Section which 2.3, such is a a 4-stroke, plant must non-reversible be designed turbochargeddepending on andthe electric intercooled load; engineits scheme with is directshown fuel in Figure injection, 3. This characterized system was by modelled a rated by power means of 10.305of GateCycle MW. According software starting to the methodology from the parameters described reported in Section in 2.3Section, such 2.1. a plant As the must plant be was designed sized dependingaccording to on the the electric electric load, load; itsthe scheme heat production is shown in is Figureinsufficient3. This and system must was be modelled integrated by with means an ofauxiliary GateCycle boiler software in order starting to fully from satisfy the the parameters demand (“non-cogeneration reported in Section heat 2.1. production”). As the plant was sized accordingThe simulated to the electric plant load,displays the an heat electric production efficiency is insufficient of 47.5%, a andthermal must efficiency be integrated of 20.5%, with and an auxiliaryan overall boiler efficiency in order of 68%; to fully the satisfylatter value the demand is lower (“non-cogeneration than the threshold heatefficiency, production”). this meaning that an HECThe simulatedportion must plant be displays extracted an electricfrom the efficiency CHP unit of 47.5%, (β = 0). a thermal The resulting efficiency energy of 20.5%, balance and anis overallillustrated efficiency in Table of 4. 68%; The the CHP latter portion value shows is lower an thanelectric the efficiency threshold of efficiency, 47.5% and this thermal meaning efficiency that an HECof 27.5%. portion On mustthe basis be extracted of the energy from bala the CHPnce, knowing unit (β = from 0). The the resulting guidelines energy that balanceRef Hη = is90% illustrated and Ref inEη Table= 52.5%,4. The the CHP calculated portion PES shows is anequal electric to 17.37%. efficiency In ofTable 47.5% 4, andHCHP thermal and Hnon-CHP efficiency, according of 27.5%. to Onthe theguidelines, basis of are the calculated energy balance, with a knowingreference fromenthalpy the guidelines value for the that returnedRef Hη =condensate 90% and Ref lower Eη =than 52.5%, the theactual calculated one of 417.5 PES iskJ/kg. equal to 17.37%. In Table4, HCHP and Hnon-CHP, according to the guidelines, are calculatedRegarding with aeconomic reference and enthalpy environmental value for theanalysis, returned thecondensate considered lowerinput thandata the are actual reported one ofin 417.5Tables kJ/kg. 5 and 6. Table 5 shows parameters common to all considered technologies. RegardingThe capital economiccost of the andinstalled environmental unit and the analysis, average theO&M considered cost are derived input data from are Reference reported [32]. in TablesFuel was5 and assumed6. Table to5 shows be natural parameters gas in all common cases and to all its considered cost was taken technologies. from Reference [33]; fuel cost in CHPThe arrangement capital costof is thelower installed because unit cogeneration and the average is encouraged O&M cost by are lower derived excise from duties Reference that make [32]. Fuelfuel, wasbasically, assumed discounted. to be natural Fuel gas cost in in all separate cases and production its cost was arrangements taken from Reference is used [to33 ];assess fuel costthe inannual CHP arrangementcost attributable is lower to the because quantity cogeneration of fuel employed is encouraged in the byproduction lower excise of non-CHP duties that energy make fuel,amounts; basically, it is also discounted. used to Fuelevaluate cost the in separate total annual production cost in arrangementsthe reference case is used of separate to assess production the annual costagainst attributable which the to savings the quantity produced of fuel by employed cogeneration in the are production estimated. of Its non-CHP variation energy was evaluated, amounts; itbut is alsothe results used to here evaluate presented the total are annualcalculated cost for in thethe reference values of case 0.0243 of separate€/kWh (separate production production) against which and the0.0193 savings €/kWh produced (CHP arrangement). by cogeneration The are cost estimated. of the electricity Its variation purchased was evaluated, from the but grid the was results derived here presentedfrom Reference are calculated [33] as well; for the calculations values of 0.0243 were performed€/kWh (separate for a cost production) of 0.156 €/kWh. and 0.0193 The€ /kWhresult was (CHP a arrangement).savings of 8.84 TheM€ costper year, of the which electricity produces purchased a PBP fromof 1.12 the years grid and was an derived NPV/I from ratio Reference equal to 3.69. [33] as well;Concerning calculations environmental were performed analys for ais, cost the of ICE-based 0.156 €/kWh. cogeneration The result plant was a avoids savings the of 8.84emission M€ per of year,14,099 which tCO2/y produces with respect a PBP to ofthe 1.12 case years of separate and an production. NPV/I ratio equal to 3.69. Concerning environmental analysis, the ICE-based cogeneration plant avoids the emission of 14,099 tCO2/y with respect to the case of separate production.

Figure 3. Scheme of the internal combustion engine cogeneration unit taken into consideration. Figure 3. Scheme of the internal combustion engine cogeneration unit taken into consideration.

Energies 2019, 12, 335 10 of 21

3.1.2. GT-Based Cogeneration Unit The cogeneration unit was designed selecting a gas turbine model from a Siemens catalogue [34]. The chosen model was SGT-400, which is a twin-shaft engine characterized by a rated power of 9.7 MW. According to the methodology described in Section 2.3, such a plant must be designed depending on the electric load; its scheme is shown in Figure4. Results of the simulations are shown in Table4.

Table 4. Properties and energy balances for the considered cogeneration units (optimal design).

Internal Steam Power Plant Steam Power Plant Property\Technology Combustion Gas Turbine with Back-Pressure with Condensing Engine Turbine Turbine design electric load electric load thermal load electric load Pe,nom (MW) 10.3 9.7 7.2 10.1 SGaux yes yes no yes Electricity from grid no no yes no ηE 47.5% 34.2% 16.5% 23.1% ηT 27.5% 76.3% 84.7% 63.4% ηg 75.0% >75.0% >75.0% 86.5% F (GWh) 372.9 309.0 312.1 368.5 Funit (GWh) 145.5 201.9 312.1 298.6 FCHP (GWh) 108.3 201.9 312.1 298.6 Fnon-CHP,E (GWh) 37.2 0 0 0 Fnon-CHP,H (GWh) 227.4 107.1 0 69.9 Eprocess (GWh) 69.1 69.1 69.1 69.1 ECHP (GWh) 51.4 69.1 51.6 69.1 Enon-CHP (GWh) 17.7 0 0 0 Eaux (GWh) 0 0 17.5 0 H (GWh) 264.5 264.5 264.5 264.5 HCHP (GWh) 29.8 154.0 264.5 189.3 Hnon-CHP (GWh) 234.7 110.5 0 75.2 Primary energy saving (PES) 17.4% 33.3% 20.4% 12.7% saving (M€) 8.84 10.52 8.71 9.73 EnergiesPpay-back 2019, period12 FOR (PBP) PEER (years) REVIEW 1.12 1.54 1.41 2.00 11 NPV/I 3.69 7.03 9.44 6.37 CO2,a (tCO2/y)Table 6. Input 14,099data for economic 26,751 analysis (different 18,605 technologies). 14,972

Cost\TechnologyTable 5. Input data for economic andICE environmental GT analysisSPP-BPT common to allSPP-CT technologies. CCPP

Unit capital cost (€/Kw)Input Data for Economic 900 and Environmental 1165 1600 Analysis 1800 1000 Average operationFuel cost & maintenance in CHP arrangement (€/kWh) 0.0150–0.0315 cost asFuel fraction cost in of separate the installed production arrangement6 (€/kWh)6 4 0.020–0.0324 4 Grid electricity cost (€/kWh) 0.085–0.170 capital cost (%)Discount rate (%) 6 Emission factor (tCO2/TJ) 55

Figure 4. Scheme of the gas turbine cogeneration unit taken into consideration. Figure 4. Scheme of the gas turbine cogeneration unit taken into consideration.

3.1.3. SPP-BPT-Based Cogeneration Unit A steam power plant with backpressure turbine cogeneration unit was set-up with a rated power of 7.17 MW. According to the methodology described in Section 2.3, such a plant must be designed depending on the thermal load; its scheme is shown in Figure 5. The electricity production is insufficient and must be integrated with an auxiliary amount of energy purchased from the grid (Eaux) to fully satisfy the demand. Results of the simulations are shown in Table 4.

Figure 5. Scheme of the steam power plant with back-pressure turbine cogeneration unit taken into consideration.

Energies 2019, 12 FOR PEER REVIEW 11

Table 6. Input data for economic analysis (different technologies).

Cost\Technology ICE GT SPP-BPT SPP-CT CCPP

Unit capital cost (€/Kw) 900 1165 1600 1800 1000

Average operation & maintenance cost as fraction of the installed 6 6 4 4 4 capital cost (%)

Energies 2019, 12, 335 11 of 21

Table 6. Input data for economic analysis (different technologies).

Cost\Technology ICE GT SPP-BPT SPP-CT CCPP Unit capital cost (€/Kw) 900 1165 1600 1800 1000 Average operation & maintenance cost as 6 6 4 4 4 fraction ofFigure the installed 4. Scheme capital of the cost gas (%) turbine cogeneration unit taken into consideration.

3.1.3.3.1.3. SPP-BPT-Based SPP-BPT-Based Cogeneration Cogeneration Unit Unit AA steam steam power power plant plant with with backpressure backpressure turbine turbine cogeneration cogeneration unit unit was was set-up set-up with with a a rated rated powerpower of of 7.17 7.17 MW. MW. According According to to the the methodology methodology described described in in Section Section 2.3 2.3,, such such a a plant plant must must be be designeddesigned depending depending on on the the thermal thermal load; load; its its scheme scheme is shownis shown in Figurein Figure5. The 5. The electricity electricity production production is insufﬁcientis insufficient and and must must be integrated be integrated with with an auxiliary an auxiliary amount amount of energy of energy purchased purchased from the from grid the (E auxgrid) to(E fullyaux) to satisfy fully satisfy the demand. the demand. Results Results of the simulationsof the simulations are shown are shown in Table in4. Table 4.

Figure 5. Scheme of the steam power plant with back-pressure turbine cogeneration unit taken Figure 5. Scheme of the steam power plant with back-pressure turbine cogeneration unit taken into into consideration. consideration.

3.1.4. SPP-CT-Based Cogeneration Unit A steam power plant with condensing turbine cogeneration unit was set-up with a rated power of 10.1 MW. According to the methodology described in Section 2.3, such a plant must be designed depending on the electric load; its scheme is shown in Figure6. Results of the simulations are in Table4.

3.1.5. CCPP-Based Cogeneration Unit On the basis of the methodology described in Section 2.3, a combined cycle power plant must be designed depending on the electric load; however, the rated electric power, resulting from the typical heat-to-electricity ratio of this technology, would be much lower than the minimum value for such a plant to be viable (that is 20 MW, according to Reference [35]). Therefore, a CC-based cogeneration unit will not be taken into consideration for the application here discussed.

3.2. Detailed Alternative Design of Cogeneration Units for a Paper Mill The aim of this section is to study the alternative design of the previously analyzed CHP technologies in order to corroborate the validity of the design methodology subject of the present paper and to provide a more complete overview of cogeneration units. Energies 2019, 12 FOR PEER REVIEW 12

3.1.4. SPP-CT-Based Cogeneration Unit A steam power plant with condensing turbine cogeneration unit was set-up with a rated power of 10.1 MW. According to the methodology described in Section 2.3, such a plant must be designed Energiesdepending2019, 12 on, 335 the electric load; its scheme is shown in Figure 6. Results of the simulations12 are of 21in Table 4.

Figure 6. Scheme of the steam power plant with condensing turbine cogeneration unit taken Figure 6. Scheme of the steam power plant with condensing turbine cogeneration unit taken into into consideration. consideration. 3.2.1. ICE-Based Cogeneration Unit 3.1.5. CCPP-Based Cogeneration Unit The size of an ICE cogeneration unit designed depending on the thermal load would be too large for thisOn technology, the basis of given the methodology its typical heat-to-electricity described in Section ratio. 2.3, The a combined cogeneration cycle unit power would plant have must to be be composeddesigned ofdepending eight engines on runthe inelectric parallel; load; all engineshowever, would the operaterated electric at maximum power, electric resulting load (infrom order the totypical accomplish heat-to-electricity heat requirements) ratio of andthis atechnology, great part of would electric be powermuch lower would than have the to minimum be delivered value to the for grid.suchTherefore, a plant to since be thisviable situation (that is not20 proﬁtableMW, according in an HEC to Reference context, an [35]). ICE-based Therefore, cogeneration a CC-based unit shallcogeneration not be considered unit will fornot thebe taken application into consideration here discussed. for the application here discussed.

3.2.2.3.2. Detailed GT-Based Alternative Cogeneration Design Unit of Cogeneration Units for a Paper Mill AThe GT-based aim of cogenerationthis section unitis to designedstudy the supplying alternative the design thermal of load the waspreviously modelled analyzed by means CHP of GateCycletechnologies software in order starting to corroborate from the assumptions the validity reportedof the design previously, methodology being known subject at aof rated the present power ofpaper 12.9 MW:and to the provide outcome a more of the complete simulation overview is the energy of cogeneration balance illustrated units. in Table7. With reference to the abovementioned assumptions, this plant displays an overall efﬁciency of 92.9%3.2.1. ICE-Based and a PES Cogeneration equal to 23.4%. Unit This setup shows a negative Eaux: this means that it produces a surplusThe amount size of of an electric ICE cogeneration energy that must unit be designed released depending to the grid. on The the economic thermal analysis load would was carried be too outlarge by for resorting this technology, to the reference given its price typical for purchasedheat-to-electricity electricity ratio. in ItalyThe cogeneration (national unique unit price,would [ 36have]), thatto be is composed 0.042 €/kWh, of eight to price engines the electricity run in parallel; fed into all theengines grid (thiswould trade operate constitutes at maximum an incoming electric cash load ﬂow);(in order however, to accomplish simulations heat in requirements) the cost range 0.042–0.077and a great€ part/kWh of wereelectric also power performed. would Taking have to into be accountdelivered the to values the grid. reported Therefore, in Tables since5 and this6 ,situation and in Section is not profitable3.1.1, the result in an wasHEC a context, savings ofan 11.94ICE-based M € percogeneration year (with unit respect shall to not the be case considered of separate for production), the application which here produces discussed. aPBP of 1.35 years and a NPV/I ratio equal to 8.11. 3.2.2.Concerning GT-Based Cogeneration environmental Unit analysis, it is possible to assume that the release of electricity to the grid causes a decrease in the CO2 emissions related to the mix of power plants regularly generating A GT-based cogeneration unit designed supplying the thermal load was modelled by means of electricity. As a consequence, a GT-based cogeneration plant avoids the emission of 60,647 tCO2/y, GateCycle software starting from the assumptions reported previously, being known at a rated with respect to the case of separate production. power of 12.9 MW: the outcome of the simulation is the energy balance illustrated in Table 7.

Energies 2019, 12, 335 13 of 21

Table 7. Properties and energy balances for the considered cogeneration units (alternative design).

Steam Power Plant with Combined Cycle Property\Technology Gas Turbine Back-Pressure Turbine Power Plant design thermal load thermal load thermal load Pe,nom (MW) 12.9 13.33 59.5 SGaux no no no electricity from grid no no no ηE 34.5% 23.1% 43.6% ηT 58.5% 63.8% 36.4% ηg 92.9% 87.0% 80.0% F (GWh) 452.3 414.4 934.7 Funit (GWh) 452.3 414.4 934.7 FCHP (GWh) 452.3 414.4 727.5 Fnon-CHP,E (GWh)] 0 0 207.2 Fnon-CHP,H (GWh) 0 0 0 Eprocess (GWh) 69.1 69.1 69.1 ECHP (GWh) 155.9 95.9 317.5 Enon-CHP (GWh) 0 0 104.1 Eaux (GWh) −86.8 −26.8 −352.5 H (GWh) 264.5 264.5 264.5 HCHP (GWh) 264.5 264.5 264.5 Hnon-CHP (GWh) 0 0 0 PES 23.4% 13.1% 19.1% saving (M€) 11.94 10.09 11.28 PBP (years) 1.35 2.64 6.53 NPV/I 8.11 4.79 0.84 CO2,a (tCO2/y) 60,647 25,122 155,743

3.2.3. SPP-BPT-Based Cogeneration Unit In Section 3.1.3, it was noted that a SPP-BPT-based cogeneration unit requires the purchase of electricity from the grid if designed supplying the thermal load: this means that a design of this kind of plant based on the electric demand would produce a huge waste of exhaust heat, which is pointless in a CHP context. Therefore, a SPP-BPT-based cogeneration unit shall not be considered for the application here discussed.

3.2.4. SPP-CT-Based Cogeneration Unit A SPP-CT-based cogeneration unit designed supplying the thermal load was modelled by means of GateCycle software starting from the assumptions reported previously: the outcome of the simulation is the energy balance illustrated in Table7.

3.2.5. CCPP-Based Cogeneration Unit A CCPP-based cogeneration unit designed supplying the thermal load was modelled by means of GateCycle software starting from the assumptions reported previously: its scheme is shown in Figure7 (rated electric power equal to 59.5 MW). The simulated plant displays an overall efﬁciency of 73.4%: this value is lower than the threshold efﬁciency, therefore an HEC portion must be extracted from the CHP unit (β = 0.1813). The resulting energy balance and other results of the simulations are illustrated in Table7. On the basis of the energy balance, the calculated PES is equal to 19.05%. Energies 2019, 12 FOR PEER REVIEW 14

3.2.5. CCPP-Based Cogeneration Unit A CCPP-based cogeneration unit designed supplying the thermal load was modelled by means of GateCycle software starting from the assumptions reported previously: its scheme is shown in Figure 7 (rated electric power equal to 59.5 MW). The simulated plant displays an overall efficiency of 73.4%: this value is lower than the threshold efficiency, therefore an HEC portion must be extracted from the CHP unit (β = 0.1813). The resulting energy balance and other results of the simulations are illustrated in Table 7. On the basis of the energy balance, the calculated PES is equal to 19.05%. Energies 2019, 12, 335 14 of 21

Figure 7. Scheme of the combined cycle cogeneration unit taken into consideration. Figure 7. Scheme of the combined cycle cogeneration unit taken into consideration. 4. Discussion 4. DiscussionIn this section, TKPI is evaluated for each of the examined technologies from the performance parametersIn this assessed section, inTKPI the previousis evaluated section. for each Results of the presented examined in thistechnologies section are from obtained the performance assuming theparameters following assessed values in for the costs: previous 0.0243 section.€/kWh Results (fuel presented cost in separatein this section production are obtained arrangement), assuming 0.0193the following€/kWh (fuel values cost for in CHPcosts: arrangement), 0.0243 €/kWh 0.156 (fuel€ /kWhcost in (cost separate of purchased production electricity), arrangement), 0.042 €/kWh 0.0193 (cost€/kWh of electricity(fuel cost in fed CHP into arrangement), the grid). The 0.156 maximum €/kWh (minimum)(cost of purchased reference electricity), values for 0.042 the €/kWh indicators (cost appearingof electricity in TKPI fed into formula the aregrid). selected The maximum considering (minimum) the two sizingreference methodologies values for (optimalthe indicators and alternative)appearing separately.in TKPI formula are selected considering the two sizing methodologies (optimal and alternative)With respect separately. to the optimal design option, Figure8 shows the resulting total key performance indicatorWith values, respect broken to the down optimal into design their different option, Figu contributions.re 8 shows the resulting total key performance indicatorWith a values, TKPI equal broken to 3.47,down gas into turbine their different turns out contributions. to be the most performing technology, followed by steamWith power a TKPI plants equal with to 3.47, backpressure gas turbine turbine turns out (TKPI to be equal the tomost 3.10). performing Internal combustiontechnology, enginesfollowed areby disadvantagedsteam power plants by the with impossibility backpressure of exploiting turbine (T allKPI heat equal produced; to 3.10). Internal in fact, thecombustion amount ofengines heat deliveredare disadvantaged by cooling waterby the jacket impossibility and oil system of exploiting cannot be all harnessed heat produced; being at in much fact, lowerthe amount temperature of heat levelsdelivered than whatby cooling is required water by jacket the paper and mill’soil system processes. cannot Steam be harnessed power plants being with at condensingmuch lower turbinestemperature are the levels least than profitable what units is required for the applicationby the paper here mill’s discussed, processes. showing Steam a TKPIpower of plants 2.16. with condensingRegarding turbines the alternative are the least design, profitable the breakdown units for of the the application resulting total here key discussed, performance showing indicators a TKPI isof displayed 2.16. in Figure9. As for optimal design methodology, gas turbines and steam power plants with condensing turbine rank respectively first and last among the proposed solutions. Combined cycles are extremely favored by their excellent environmental performance, improved by the huge quantity of electricity fed into the grid, which contributes to the reduction of CO2 emissions in the model utilized in the present analysis; the environmental indicator plays a major role on the total key performance indicator for this technology that would be otherwise dramatically affected by its poor economic performance, consequence of the very high initial investment (CC size is 4–5 times larger than those of the other technologies). Considering also that the optimal design is not feasible for CC-based cogeneration (see Section 3.1), it can be inferred that this kind of plant is not viable for being coupled with paper mills. However, where the two design methodologies can be compared, it is clear that the optimal design must be preferred, for the same technology, to higher TKPI values (TKPIs in Figure9 would be smaller if calculated by using reference values of TKPIs in Figure8). This statement reaffirms the validity of the sizing methodology proposed by the authors. EnergiesEnergies 2019 2019, 12, 12 FOR FOR PEER PEER REVIEW REVIEW 1515

RegardingRegarding thethe alternativealternative design,design, thethe breakdownbreakdown ofof thethe resultingresulting totaltotal keykey performanceperformance indicatorsindicators is is displayed displayed in in Figure Figure 9. 9. As As for for op optimaltimal design design methodology, methodology, gas gas turbines turbines and and steam steam powerpower plants plants with with condensing condensing turbine turbine rank rank respecti respectivelyvely first first and and last last among among the the proposed proposed solutions. solutions. CombinedCombined cycles cycles are are extremely extremely favored favored by by their their ex excellentcellent environmental environmental performance, performance, improved improved by by 2 thethe huge huge quantity quantity of of electricity electricity fed fed into into the the grid, grid, which which contributes contributes to to the the reduction reduction of of CO CO 2emissions emissions inin the the model model utilized utilized in in the the present present analysis; analysis; the the environmental environmental indicator indicator plays plays a a major major role role on on the the totaltotal key key performance performance indicator indicator for for this this technology technology that that would would be be otherwise otherwise dramatically dramatically affected affected by by itsits poor poor economic economic performance, performance, consequence consequence of of the the very very high high initial initial investment investment (CC (CC size size is is 4–5 4–5 times times largerlarger than than those those of of the the other other technologies). technologies). Consid Consideringering also also that that the the optimal optimal design design is is not not feasible feasible forfor CC-based CC-based cogeneration cogeneration (see (see Section Section 3.1), 3.1), it it can can be be inferred inferred that that this this kind kind of of plant plant is is not not viable viable for for beingbeing coupled coupled with with paper paper mills. mills. However,However, where where the the two two design design methodologies methodologies ca cann be be compared, compared, it it is is clear clear that that the the optimal optimal designdesign must must be be preferred, preferred, for for the the same same technology, technology, to to higher higher TKPI TKPI values values (TKPIs (TKPIs in in Figure Figure 9 9 would would bebe smaller smaller if if calculated calculated by by using using reference reference values values of of TKPIs TKPIs in in Figure Figure 8). 8). This This statement statement reaffirms reaffirms the the Energies 2019, 12, 335 15 of 21 validityvalidity of of the the sizing sizing methodol methodologyogy proposed proposed by by the the authors. authors.

FigureFigureFigure 8. 8.8. TotalTotalTotal keykey performance performance indicator indicator (TKPI) (TKPI) va valuesvalueslues and and breakdown breakdownbreakdown for forfor the thethe considered consideredconsidered cogeneration technologies (optimal design). cogenerationcogeneration technologies technologies (optimal (optimal design). design).

Figure 9. TKPI values and breakdown for the considered cogeneration technologies (alternative design). FigureFigure 9. 9. TKPI TKPI values values and and breakdown breakdown for for the the considered considered cogeneration cogeneration technologies technologies (alternative (alternative design).Thedesign). choice of considering two different design criteria responds to the intent to set two separate possible scenarios. If the optimal methodology is deployed, the plant size is defined so as not to produce a surplus amount of electricity with respect to process requirements, possibly employing auxiliary boilers or purchasing electricity from the grid if cogeneration thermal or electric power are lower than the required ones. The alternative approach is based, instead, on the admissibility of feeding electricity into the grid. Previous studies developed by the authors, as well as the present one, reveal that cogeneration plants not releasing electricity are today more profitable (in contrast with the situation established by the previous Italian legislation). In fact, results of the present analysis confirm that plants shifted towards to production of heat (satisfaction of the thermal demand) make more sense in an HEC context—it is evident that the optimal design indeed respects the condition of avoided release of electric energy; it was also possible to analyze plants excluded from the optimal design case and to theoretically study contexts remunerating the release of electricity to the grid. The validity of the optimal design criterion is confirmed by the diagram in Figure 10. Energies 2019, 12 FOR PEER REVIEW 16

The choice of considering two different design criteria responds to the intent to set two separate possible scenarios. If the optimal methodology is deployed, the plant size is defined so as not to produce a surplus amount of electricity with respect to process requirements, possibly employing auxiliary boilers or purchasing electricity from the grid if cogeneration thermal or electric power are lower than the required ones. The alternative approach is based, instead, on the admissibility of feeding electricity into the grid. Previous studies developed by the authors, as well as the present one, reveal that cogeneration plants not releasing electricity are today more profitable (in contrast with the situation established by the previous Italian legislation). In fact, results of the present analysis confirm that plants shifted towards to production of heat (satisfaction of the thermal demand) make more sense in an HEC context—it is evident that the optimal design indeed respects the condition of avoided release of electric energy; it was also possible to analyze plants excluded from the optimal design case and to theoretically study contexts remunerating the release of electricity to the grid. The validity of the optimal design criterion is confirmed by the diagram in Figure 10. Energies 2019, 12, 335 16 of 21

Figure 10. TKPI values and breakdown for the considered cogeneration technologies with white certificates taken into account (combining optimal design and alternative design). Figure 10. TKPI values and breakdown for the considered cogeneration technologies with white Thiscertificates diagram taken was into realized account by (combining considering optimal the design maximum and alternative (minimum) design). values of PES, PBP, NPV/I, and CO ,a in a set composed of all seven designed units (four optimal and three alternative), thus not 2This diagram was realized by considering the maximum (minimum) values of PES, PBP, NPV/I, separating the optimally designed technologies from the alternatively designed ones. Gas turbines and CO2,a in a set composed of all seven designed units (four optimal and three alternative), thus not designedseparating according the optimally to the designed alternative technologies criterion fr showom the the alternatively highest TKPI; designed however, ones. givenGas turbines the strong assumptionsdesigned according under which to the the alternative environmental criterion contribution show the highest was evaluated TKPI; however, (plants given releasing the strong electricity to theassumptions grid much under benefit which from the it), environmental such a parameter contribu cantion be was reasonably evaluated disregarded (plants releasing and electricity the optimally designedto the grid gas much turbine benefit is thus from confirmed it), such a parame to be theter winningcan be reasonably technical disregarded solution (TKPI and the = optimally 2.469 against TKPIdesigned = 2.390, gas excluding turbine is the thus environmental confirmed to contribution).be the winning technical solution (TKPI = 2.469 against TKPIIn the = 2.390, analysis excluding conducted the environmental thus far, white contribution). certificates are not taken into account in the economic analysis.In White the analysis certificates conducted are marketable thus far, white securities certific whichates are attest not taken primary into account energy savingsin the economic achieved in energyanalysis. final White uses through certificates efficiency are marketable improvement securities measures which attest (including primary cogeneration). energy savings Such achieved securities arein dispensed energy final by Gestore uses through dei Servizi efficiency Energetici improv (GSE,ement Energy measures Service (including Manager) cogeneration). and are a real Such income securities are dispensed by Gestore dei Servizi Energetici (GSE, Energy Service Manager) and are a for plants. This means that they indirectly affect the results of the economic calculations here presented, real income for plants. This means that they indirectly affect the results of the economic calculations representing a positive value to be added to cash flows. According to the GSE annual report on white here presented, representing a positive value to be added to cash flows. According to the GSE certificatesannualEnergies report [201937],, 12 inon FOR Italy, white PEER the certificatesREVIEW average value[37], in of Italy, incentive the average securities value is of equal incentive to 267 securities€/toe for is the equal year to17 2017; on the267 basis€/toe for of thisthe year value, 2017; it is on possible the basis to of price this energyvalue, it savings is possible for to each price technology energy savings (RISP for parameter), each generatingtechnology a further (RISP positive parameter), cash genera flow. Theting TKPIa further parameters positive cash can beflow. therefore The TKPI recalculated: parameters the can results be are reportedtherefore in recalculated: Figures 11 theand results 12. are reported in Figures 11 and 12.

Figure 11. TKPI values and breakdown for the considered cogeneration technologies with white certiﬁcatesFigure taken11. TKPI into values account and (optimal breakdown design). for the considered cogeneration technologies with white certificates taken into account (optimal design).

Figure 12. TKPI values and breakdown for the considered cogeneration technologies with white certificates taken into account (alternative design).

The difference from the previous case is slight: since all technologies benefit from incentives, the rankings established in the previous case are maintained. Gas turbines, whose TKPI increases to 3.63, are confirmed to be the most suitable technology; combined cycle economic performance is enhanced but not enough to make it a winning solution. Besides, it must be noted that white certificates are subject to certain variability in the individual national context and that incentives vary in different legislative realities: this is an element that can be taken into consideration by carrying out a parametric analysis as shown in Figures 13 and 14. It is clear that the change in terms of performance of the individual technology is overall negligible, precisely because performance slightly improves for GT and SPP-BPT, and stays constant for SPP-CT and even somewhat worsens for ICE: this is caused by the growth of the maximum NPV/I ratio, which is faster than that of the NPV/I ratio for the individual technology, with increasing incentives. In any case, gas turbines are hereby confirmed to be the winning

Energies 2019, 12 FOR PEER REVIEW 17

technology (RISP parameter), generating a further positive cash flow. The TKPI parameters can be therefore recalculated: the results are reported in Figures 11 and 12.

Figure 11. TKPI values and breakdown for the considered cogeneration technologies with white Energiescertificates2019, 12, 335 taken into account (optimal design). 17 of 21

Figure 12. TKPI values and breakdown for the considered cogeneration technologies with white Figure 12. TKPI values and breakdown for the considered cogeneration technologies with white certificates taken into account (alternative design). certificates taken into account (alternative design). The difference from the previous case is slight: since all technologies benefit from incentives, the rankingsThe establisheddifference from in the the previous previous case case are is maintained.slight: since all Gas technologies turbines, whose benefit TKPI from increases incentives, to 3.63, the arerankings confirmed established to be the in most the suitableprevious technology; case are maintained. combined cycleGas economicturbines, whose performance TKPI isincreases enhanced to but3.63, not are enough confirmed to make to be it athe winning most suitable solution. technology; combined cycle economic performance is enhancedBesides, but it not must enough be noted to make that whiteit a winning certificates solution. are subject to certain variability in the individual nationalBesides, context it andmust that be incentives noted that vary white in different certificates legislative are realities:subject to this certain is an elementvariability that canin the be takenindividual into consideration national context by carrying and that out incentives a parametric vary analysis in different as shown legislative in Figures realities:13 and 14 .this It is is clear an thatelement the changethat can in be terms taken of performanceinto consideration of the individualby carrying technology out a parametric is overall analysis negligible, as shown precisely in becauseFigures performance13 and 14. It is slightly clear that improves the change for GT in and terms SPP-BPT, of performance and stays of constant the individual for SPP-CT technology and even is somewhatoverall negligible, worsens precisely for ICE: this because is caused performance by the growth slightly of theimproves maximum for GT NPV/I and ratio,SPP-BPT, which and is fasterstays Energies 2019, 12 FOR PEER REVIEW 18 thanconstant that for of theSPP-CT NPV/I and ratio even for thesomewhat individual worsens technology, for ICE: with this increasing is caused incentives. by the growth In any of case, the gasmaximum turbines NPV/I are hereby ratio, confirmedwhich is faster to be thethan winning that of technicalthe NPV/I solution ratio for for the a CHPindividual unit coupled technology, with technical solution for a CHP unit coupled with a pulp and paper mill and this statement is in line awith pulp increasing and paper incentives. mill and this In statementany case, isgas in lineturbines with are the outcomehereby confirmed of the research to be conducted the winning by with the outcome of the research conducted by Shabbir et al. [26]. Shabbir et al. [26].

Figure 13. TKPI sensitivity analysis for varying incentive values (optimal design). Figure 13. TKPI sensitivity analysis for varying incentive values (optimal design).

Figure 14. TKPI sensitivity analysis for varying incentive values (alternative design).

Moreover, the entire range of variation for the costs of fuel (cogeneration and non-cogeneration arrangements) and electricity (purchased and sold to the grid), as indicated in Table 5 and in Section 3.2.2, has been explored leading to a variety of scenarios; in all of them gas turbine proved to be the best suited technology. In the different analyses conducted by the authors, technology hierarchy results were confirmed and so was the role of gas turbines as the preferable technology among the possible CHP options to be deployed in the pulp and paper industry.

4. Conclusions The present paper examines a variety of possible cogeneration technologies to be coupled with the pulp and paper mill industry in the context of high-efficiency cogeneration. The analysis was based on the actual production amounts and load diagrams for a particular industrial reality in Italy and was carried out from a thermodynamic, economic, and environmental perspective, overviewing

Energies 2019, 12 FOR PEER REVIEW 18 technical solution for a CHP unit coupled with a pulp and paper mill and this statement is in line with the outcome of the research conducted by Shabbir et al. [26].

Energies 2019, 12Figure, 335 13. TKPI sensitivity analysis for varying incentive values (optimal design). 18 of 21

Figure 14. TKPI sensitivity analysis for varying incentive values (alternative design). Figure 14. TKPI sensitivity analysis for varying incentive values (alternative design). Moreover, the entire range of variation for the costs of fuel (cogeneration and non-cogeneration arrangements)Moreover, the and entire electricity range of (purchased variation for and the sold costs to of the fuel grid), (cogeneration as indicated and innon-cogeneration Table5 and in arrangements)Section 3.2.2, has and been electricity explored (purchased leading toand a variety sold to ofthe scenarios; grid), as inindicated all of them in Table gas turbine 5 and in proved Section to 3.2.2,be the has best been suited explored technology. leading In to the a differentvariety of analyses scenarios; conducted in all of by them the gas authors, turbine technology proved to hierarchy be the bestresults suited were technology. confirmed In and the so different was the roleanalyses of gas conducted turbines as by the the preferable authors, technologytechnology amonghierarchy the resultspossible were CHP confirmed options to and be deployedso was the in role the of pulp gas and turbines paper as industry. the preferable technology among the possible CHP options to be deployed in the pulp and paper industry. 5. Conclusions 4. ConclusionsThe present paper examines a variety of possible cogeneration technologies to be coupled with the pulpThe present and paper paper mill examines industry ina variety the context of possible of high-efficiency cogeneration cogeneration. technologies The to analysis be coupled was basedwith theon thepulp actual and productionpaper mill amountsindustry andin the load context diagrams of high-efficiency for a particular cogeneration. industrial reality The in analysis Italy and was was basedcarried on out the from actual a thermodynamic,production amounts economic, and load and diagrams environmental for a particular perspective, industrial overviewing reality in internal Italy andcombustion was carried engines, out from gas turbines,a thermodynamic, steam power econom plantsic, withand environmental backpressure turbine, perspective, steam overviewing power plants with condensing turbine and combined cycle power as possible CHP solutions. The plant was sized and modelled according to the available data. Thermodynamic simulations were performed by means of GateCycle software and the energy balances of the considered CHP plants were derived accordingly. The economic analysis was conducted in accordance with the principles of high-efficiency cogeneration (HEC), thus following the guidelines issued by the European Commission and making use of the previously obtained energy balances. As a result, energy savings related to CHP plants were properly estimated and economically valued with respect to separate energy production. Results of the different analysis were quantified through energy, economic, and environmental parameters, added together in a total key performance indicator (TKPI) thus allowing an objective comparison among the different cogeneration technologies taken into account. The calculations indicate gas turbines as the winning technical solution, with a TKPI of 3.47, that can be increased up to 3.63 when white certificates are included in the analysis. The main achievement of the present study is to provide a solid methodology for a simple and robust evaluation of CHP performances allowing different types of power plants to be compared. The approach illustrated in this paper applies specifically to paper mills; however, being based on general evaluation criteria, the research can be potentially extended and successfully applied to other industry sectors as well.

Author Contributions: Conceptualization, M.G. and M.V.; Methodology M.G. and M.V.; Software, M.V. and T.S.; Validation, M.M. and S.B.; Investigation, M.G., M.V. and T.S.; Writing—Original Draft Preparation, T.S.; Writing—Review & Editing, T.S., M.V. and S.B.; Supervision, M.G. Energies 2019, 12, 335 19 of 21

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

CC combined cycle CCPP combined cycle power plants with condensing turbine Ceff power-to-heat ratio from the Italian legislation CHP combined heat and power CHP Eη electric efficiency of the CHP portion CHP Hη thermal efficiency of the CHP portion CO&M operation and maintenance costs CO2,a CO2 emissions avoided Cs,e specific electric demand Cs,t specific thermal demand E electricity Eaux auxiliary electricity ECHP electricity from cogeneration Enon-CHP electricity not from cogeneration Eprocess electricity required by the process F fuel input in a CHP system FCHP fuel input to produce useful heat and electricity from cogeneration Fnon-CHP,E fuel input to produce electricity not from cogeneration Fnon-CHP,H fuel input to produce heat not from cogeneration GT gas turbine H heat h specific enthalpy HCHP useful heat from cogeneration HEC high-efficiency cogeneration Hnon-CHP useful heat not from cogeneration I total investment ICE internal combustion engine K harmonization factor . mpaper produced paper mass flow rate NPV net present value PBP pay-back period Pe electric power Pe,nom nominal electric power PES primary energy saving Pt thermal power Ref Eη reference efficiency for electric production according to the guidelines Ref Hη reference efficiency for heat production according to the guidelines RH/E useful-heat-to-electricity ratio RISP parameter expressing energy savings in the Italian legislation SGaux auxiliary steam generator SPP-BPT steam power plants with backpressure turbine SPP-CT steam power plants with condensing turbine TKPI total key performance indicator WC white certificates Energies 2019, 12, 335 20 of 21

Greek Letters β power loss factor by a heat extraction at a steam turbine ηCHP cogeneration efficiency ηE electric efficiency ηE,EQ electric equivalent efficiency ηE,ref actual reference electric efficiency ηg overall efficiency ηH,ref actual reference thermal efficiency ηnon-CHP,E efficiency of non-combined electrical/mechanical energy generation ηT thermal efficiency ηthreshold threshold efficiency

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