**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 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 The Directive 2017/27/EU [4] repealed Directive 2004/8, confirming all of its points in the Annexes I and II.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 Uroševi´c et 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].

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 Table 1.



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.

#### **2. Methods**

#### *2.1. Quantitative Definition of the Analysed Process*

Table 2 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.


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 Figure 1, 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:

$$C\_{s,t} = \frac{P\_t}{\dot{m}\_{paper}} = 40300 \text{ kWh/t}$$

$$P\_p$$

$$C\_{s,c} = \frac{P\_c}{\dot{m}\_{papcr}} = 1200 \text{ kWh/t}$$

as power is the product of mass flow rate by the specific consumption of the paper mill.

Falling in the ranges reported in Table 2, the obtained values can be considered validated. Table 3 sums up the input data common to all plant types.

The following subsections illustrate the procedures to carry out a design, an HEC and economic analysis of the CHP technologies taken into consideration.

**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.


**Table 3.** Input data common to all plant types.
