Estimation of Energy Recovery Efficiency in Solid Recovered Fuel Manufacturing and Use Facilities

Abstract

The importance of waste energy is increasing with increasing emphasis on carbon neutrality. Solid recovered fuels (SRFs) are manufactured to recycle waste into fuel form and can replace fossil fuels by recovering the heat generated by using them. This study calculated the manufacturing and energy recovery efficiency of SRF facilities. The manufacturing efficiency was calculated as the amount of SRFs manufactured compared to the amount of input waste. The energy recovery efficiency was calculated using the R1 method, which is applied to incineration heat energy recovery facilities. The manufacturing efficiency was 69.5%, which varies depending on the combustible material content of input waste. The energy recovery efficiency was 85.4%, which satisfied Korean energy recovery efficiency facility standards. Our study highlights the manufacturing and use of SRFs as one of the options for recycling waste and its potential as a substitute for fossil fuels.

1. Introduction

As environmental problems such as the increase in greenhouse gases and abnormal climate caused by the use of fossil fuels continue to persist, global efforts are being made to solve these problems; one of these efforts is achieving carbon neutrality, where carbon emissions and absorption are equal. Using renewable energy to replace fossil fuels can be effective in achieving carbon neutrality that can reduce greenhouse gas emissions. Among these, there is increasing interest in waste resource energy that recovers energy from waste. In China, incineration heat energy recovery and landfill gas use were investigated as methods that may be applied advantageously under different climate conditions to reduce greenhouse gases [1]. In Kazakhstan, waste management scenarios are being evaluated to mitigate greenhouse gas emissions [2]. In the United Kingdom (UK), the use of biological and heat treatment to recycle municipal solid waste (MSW) for the production of solid recovered fuel (SRF) and incineration to produce electricity and heat were investigated to reduce greenhouse gases [3]. In the Republic of Korea, SRF production from MSW was evaluated as having the highest energy recovery potential among waste resource energy technologies such as landfill gas recovery and waste heat [4]. SRF has added value by recovering discarded waste and using it as fuel. SRF functions as a waste treatment during the manufacturing process and also generates energy during the use process. SRF has the meaning of sustainable waste treatment and energy recovery in carbon neutrality.

Solid recovered fuel, one of the waste resource energies, is a technology that converts solid waste into fuel through processes such as separation, sorting, and crushing. SRF can made from a variety of raw materials. In addition to MSW and industrial waste, research is being conducted on technologies for manufacturing SRF using sewage sludge [5]. It is known that the most important factor influencing the production of solid fuels is the properties of the waste being brought in. If any of the waste materials used in SRF manufacturing potentially contain toxic substances, technology to remove them is essential [6,7]. It is also known that the higher the content of recyclable combustible waste among the wastes being brought into the SRF manufacturing facility, the more SRF production is increased. A study by Nasrullah et al. [8,9] confirmed that paper, cardboard, wood, plastic, and rubber constitute SRF when commercial and industrial wastes are recycled into SRF, and paper, cardboard, plastic, and wood constitute SRF when MSW are recycled into SRF. In addition, the low calorific value of SRF decreases if the moisture content of the imported waste is high. Further, the ash and chlorine content of SRF increases if the raw material is not selected properly [10]. The properties of the waste material and the manufacturing process are expected to play an important role in increasing the SRF manufacturing efficiency.

SRFs are manufactured from waste materials in the form of fuel that meets certain standards [11] and may be used in various fields as a substitute for fossil fuels. First, it was evaluated that it may replace existing fossil fuels in incinerators and power plants [12]. Small- and medium-sized SRF power plants are known to have lower greenhouse gas emission potential compared to coal, natural gas, and electric power plants, as well as standard landfill systems [13]. In addition, SRFs are also utilized in cement manufacturing plants [14,15]. It was reported that utilizing SRFs in the cement manufacturing process can contribute to achieving a higher recycling rate, as SRF ash can be used to manufacture cement clinker [16]. SRF may be used to recover the heat or gases generated after combustion, similarly to waste incineration facilities, through steam production and power generation. A study by De Gisi et al. [17] confirmed that the net energy production efficiency of incinerators, similarly to solid fuel combustion facilities, demonstrated similar ranges of values. The combustion of SRF in a fluidized bed incinerator produces high-quality synthesis gas, which may be used in the chemical industry or as transportation fuel [18].

Meanwhile, in Korea, there was a negative perception of the use of SRF due to continuous concerns about environmental stability and public complaints during the initial introduction of the SRF system. However, the Ministry of Environment sought to improve the quality standards of SRF and establish a grading system to enhance the reliability of SRF manufacturing and use [19]. In this study, the energy recovery efficiency of SRF manufacturing and use facilities was calculated to expand its utilization. The manufacturing facility calculated the amount of SRF produced from the incoming waste to determine the material recycling rate. The SRF-use facility evaluated the heat recovery capacity of the SRF by comparing it with the amount of steam recovered from the waste incineration facility using the energy recovery efficiency calculation method.

2. Materials and Methods

2.1. SRF Manufacturing Facilities

2.1.1. Target Facilities of Manufacturing

SRF manufacturing facilities in Korea are divided into public and private sectors. These facilities are categorized based on the type of input waste. Our study selected both public and private facilities to examine manufacturing efficiency according to the type of input waste. Municipal waste is brought in standard trash bags in public facilities, and intermediate waste is brought in that has gone through a separation and screening process in private facilities. The input waste was manufactured into SRF through unpacking, shredding, sorting, screening, and molding processes. Sorting processes that are primarily used include wind power, magnetic power, and disk sorters. The manufacturing process and facility characteristics of the target facility are summarized in

Table 1. Operation status of SRF manufacturing facilities.

Facility 1	Type	SRF Manufacture Form	Input Waste Composition (%) 2	Capacity (ton/day)	Manufacturing Process
M-01	public	fluff	MSW (100)	181	Unpacking shredder, sorter, dryer
M-02	public	fluff	MSW (100)	900	Unpacking shredder, screen, sorter
M-03	public	fluff	MSW (100)	130	Unpacking shredder, sorter, screen, dryer
M-04	public	fluff	MSW (100)	400	Unpacking shredder, sorter, dryer
M-05	private	fluff	WSR (100)	147	Shredder, sorter
M-06	private	pellet/fluff	MSW (50), WSR (50)	220	Shredder, sorter, molding machine
M-07	private	pellet	MSW (15), WSR (83), IDW (2)	105	Shredder, sorter, molding machine
M-08	private	fluff	WTR (100)	150	Shredder, sorter

¹ M, manufacturing facility; ² MSW, municipal solid waste; WSR, waste synthetic resins; IDW, industrial waste; WTR, waste tires.

.

2.1.2. Properties Classification of the Waste

A property analysis was conducted to determine the impact of the input waste properties from public and private facilities on manufacturing efficiency. Two public and two private facilities were identified as target facilities. Property classification items were classified into combustible materials and non-combustible materials by referring to the classification standards of the National Waste Statistics Survey [20]. Combustible waste, as defined in the relevant standards, is waste of a combustible nature, and non-combustible waste is waste that does not burn. Combustible waste includes food, paper, plastic, wood, and rubber. Other combustible waste materials are not visually distinguishable and are not suitable as SRF raw materials. Non-combustible waste includes metals, glass, scrap metal, dirt, dried debris, and dried flowers.

2.1.3. Manufacturing Efficiency Calculation

Equation (1) was used to calculate the SRF manufacturing efficiency with the application of two data sets. First, the 2021 waste inflow volume and SRF manufacturing volume were used for all target facilities. Second, two of the target facilities were selected and visited on-site to measure daily data to determine efficiency on the day of manufacturing.

$$Manufacturing efficiency \left(\%\right)={\displaystyle \frac{SRF yield \left(ton/year or day\right)}{Waste input \left(ton/year or day\right)}}\times 100$$

(1)

The annual manufacturing efficiency in 2021 was calculated as the ratio of the amount of the annual waste imported and the annual SRF manufactured. The manufacturing efficiency on the day was calculated as the ratio of the amount of waste imported and the amount of SRF manufactured in one day by selecting one public facility and one private facility (M-04, M-06). Figure 1 presents the manufacturing process of the two facilities. These facilities were selected as facilities capable of measuring the amount of residue from the pretreatment process to measure the daily SRF production amount. The residue output was measured for scrap metal, non-ferrous materials, and impurities. The SRF yield of the facility on the day was calculated by excluding the output from the input on the day of measurement.

2.2. SRF-Use Facilities

2.2.1. Target Facilities of Use

Facilities using SRFs, like incineration heat recovery facilities, can utilize the heat generated by combustion SRF. Use facilities are also divided into public facilities and private facilities. However, the difference between the two facilities is not significant. Instead, they are classified according to their intended use, such as industrial boilers, power generation, or cogeneration facilities. The incinerator and boiler types at the facilities were categorized into fluid bed/stoker types and separated/combined types. In our study, the processes of the target facilities were classified to compare the energy recovery efficiency of each type of incinerator and boiler. The process characteristics of the target facilities are summarized in

Table 2. The operation status of SRF-use facilities.

Facility	Type	Incinerator Type	Boiler Type	Fuel Type (%)	Capacity (ton/day)	Industry Used	Type of Energy Produced
U-01	Public	Fluid bed	Separated	SRF (100)	500	Electricity generation	Steam, Electricity
U-02	Public	Fluid bed	Separated	SRF (100)	90	Industrial boiler	Steam
U-03	Private	Fluid bed	Separated	SRF (11), Bio-SRF (89)	95	Industrial boiler	Steam
U-04	Private	Fluid bed	Combined	SRF (54), Bio-SRF (46)	144	Paper manufacture	Steam
U-05	Private	Fluid bed	Combined	SRF (88), Bio-SRF (12)	100	Paper manufacture	Steam, Electricity
U-06	Private	Fluid bed	Combined	Bio-SRF (100)	580	Electricity generation	Steam, Electricity
U-07	Private	Fluid bed	Combined	Bio-SRF (100)	750	Electricity generation	Steam, Electricity
U-08	Private	Stoker	Combined	SRF (100)	190	Paper manufacture	Steam
U-09	Private	Stoker	Combined	SRF (100)	140	Industrial boiler	Steam, Electricity

.

2.2.2. LHV Calculation

Different lower heating value (LHV) calculation methods in the R1 formula are used depending on the type of waste incineration facility in Korea [21]. The net calorific value (NCV) method is used for municipal waste incineration facilities, and the heat balance (HB) method is used for industrial waste incineration facilities. Our study compared which of the two methods was more suitable for calculating the LHV of SRF. The LHV of a municipal waste (LHV_w,NCV) incineration facility was calculated using Equation (2).

$${LHV}_{w, NCV}\left(kcal/kg\right)=\left[1.011 \times \left({\displaystyle \frac{m_{stw}}{m_w}}\right)\times h_{\mathit{st}}+1.672{\mathit{t}}_{\mathit{gb}}\right]$$

(2)

where LHV_W (kcal/kg) is the LHV of the waste injected into the target incinerator; m_stw (ton/yr) is the total steam production from the waste in the target incinerator; m_w (ton/yr) is the total waste input into the target incinerator; h_st (kcal/kg) is the net enthalpy of the produced steam; and t_gb (°C) is the average temperature of the exhaust gas at the end of the waste heat boiler. The heat loss correction factors of 1.672 and 1.011 correspond to adjustments for specific factors [21].

The LHV of an industrial waste (LHV_w,HB) incineration facility was calculated using Equation (3). The HB method involves the determination of the LHV by identifying the energy input/output items that fall within the LHV calculation range (specifically limited to energy production equipment). This approach is based on the Law of Energy Conservation.

$${LHV}_{w, HB}\left(kcal/kg\right)=\left(\sum Q_{out}-\sum Q_{in}\right)÷m_w\times 1000$$

(3)

where LHVw (kcal/kg) is the LHV of the waste injected into the target incinerator; ∑Q_out (Gcal/yr) is the total heat output of all heat output items; ∑Q_in (Gcal/yr) is the total heat input of all heat input items except the calorie of the waste; and m_w (ton/yr) is the total waste input into the target incinerator.

Measurement parameters that require on-site measurements were used to calculate the LHV using the HB method, in addition to those measured using instruments including the oxygen and moisture concentrations at the waste heat boiler output, incineration ash ignition loss and water content, and heat losses of the waste heat boiler, incinerator, and residues. These were measured in accordance with the “manual on energy recovery efficiency measurement/calculation methods and procedures [21]”.

2.2.3. Energy Recovery Efficiency Calculation

In our study, the energy recovery efficiency calculation formula for incineration facilities was used to calculate the energy recovery efficiency of SRF facilities. The energy recovery efficiency of the SRF-using facilities was calculated based on the R1 method using Equation (4) [21,22,23].

$$R1\left(\%\right)={\displaystyle \frac{E_p-(E_f+E_i)}{0.97\times (E_w+E_f)}}\times 100$$

(4)

where E_p (Gcal/yr) is the effective energy among energy produced in the form of heat sources or electricity (with a weight of 2.6 for electric power generation and 1.1 for steam or hot water); E_f (Gcal/yr) is the energy supplied from outside that contributes to energy production; E_i (Gcal/yr) is the energy supplied from outside that does not contribute to energy production; E_w (Gcal/yr) is the energy from input waste; and 0.97 is the energy loss coefficient due to heat dissipation.

In Korea, facilities that calculate incineration heat recovery efficiency by applying the R1 method according to related notices are required to submit the calculation data [21]. However, this does not apply to facilities using SRF; therefore, in our study, some data from the target facility were used as alternative data for similar facilities. The scope of alternative data use and the data collection period for the target facility are presented in Table S1. In addition, according to related notices, the period for calculating energy recovery efficiency is one year [21,23]. However, in our study, the recovery efficiency was calculated based on daily data as the operation data collection period of the target facilities differed (see in Table S1), and the details are presented in

Table 3. Measurement data required for application of R1 method to SRF-use facilities.

Index	Data	Measurement Type 1	on Notice	in Our Research
Ew	Waste input	W	Sum of annual	Sum of daily
	Steam production	F	Sum of annual	Sum of daily
		T, P	Avg. of annual	Avg. of daily
	Boiler water supply	F	Sum of annual	Sum of daily
		T	Avg. of annual	Avg. of daily
	Auxiliary fuel input	F	Sum of annual	Sum of daily
	Boiler exhaust gas	T, Conc.Ox, Wt.W	Avg. of annual	Avg. of daily
	Combustion air	F	Sum of annual	Sum of daily
		T	Avg. of annual	Avg. of daily
	Stack exhaust gas	F	Sum of annual	Sum of daily
		Conc.Ox	Avg. of annual	Avg. of daily
	Secondary combustion chamber	T	Avg. of annual	Avg. of daily
	Bottom ash	W	Sum of annual	Sum of daily
		T, Wt.W, LOI	Avg. of annual	Avg. of daily
	Heat loss	C	Avg. of annual	Avg. of daily
Ef	Auxiliary fuel input	F	Sum of annual	Sum of daily
Ei	External power demand	Pwr	Sum of annual	Sum of daily
	Prevention facility auxiliary fuel input	F	Sum of annual	Sum of daily
Ep	Other boiler steam production	F	Sum of annual	Sum of daily
	Electricity supply	Pwr	Sum of annual	Sum of daily
	Steam supply	F	Sum of annual	Sum of daily
		T, P	Avg. of annual	Avg. of daily

¹ W, weight; F, flow; T, temperature; P, pressure; Conc._Ox, concentration of oxygen; Wt._W, water content; LOI, loss on ignition; C, Gcal; Pwr, MWh

.

3. Results and Discussion

3.1. Manufacturing Efficiency Calculations

3.1.1. Waste Properties Analysis

Table 4. Properties analysis of waste brought into SRF manufacturing facilities.

Type	Combustible (wt.%)							Non-Combustible (wt.%)
	Total	Plastic	Vinyl	Paper	Fiber	Rubber	Other 1	Total	Glass	Scrap Iron	Other 2
Public	90.8	5.7	16.2	25.4	11.7	0.6	31.1	9.2	2.2	0.7	6.3
Private	86.4	44.3	29.3	8.8	1.8	1.0	1.1	13.6	4.2	6.3	3.1

¹ Combustion materials not suitable as SRF manufacturing raw materials: food waste, styrofoam, wood, etc. ² Unidentifiable non-combustible materials: dirt, dried debris, etc.

presents the compositional properties of input waste for SRF production. Among the combustible materials, those that could be used as SRF raw materials included plastics, vinyl, paper, fiber, and rubber. Among combustible materials, those that cannot be used as raw materials are classified as others. Non-combustible materials included glass and scrap iron, while unidentifiable materials such as dirt and dried debris were classified as other. In public facilities, the total combustible material content was 90.8%; however, the content of materials suitable for SRF raw materials was 59.7%. In contrast, in private facilities, the content of materials suitable for raw materials was 85.3% out of the total combustible material content of 86.4%. The total combustible material content of public and private facilities differed by 3.6%p.; however, the SRF raw material content differed by 25.6%p. Even if the combustible content of the imported waste is high, if it contains materials that are not suitable for SRF raw materials, it may affect the SRF manufacturing efficiency. The difference in SRF raw material content according to the type of waste brought in was also shown in the study of Nasrullah et al. [8,9]. The combustible material content of MSW and commercial and industrial waste were 78.2% and 81%, respectively, and among them SRF raw materials (plastic-hard, plastic-soft, paper, fiber, rubber) were 66.7% and 74.2%.

Public facilities receive municipal waste in volume-based bags that have not undergone any preprocessing, while private facilities receive intermediate waste that has undergone a separation and sorting process. According to Korea’s waste disposal policy, municipal waste generators must separate recyclable materials. Separate disposal includes recyclable plastics, vinyl, and paper, while non-recyclable municipal waste is disposed of in standard trash bags. Therefore, public facilities that must manufacture SRF bags for standard trash bag disposal have no choice but to bring in waste in separate conditions compared to private facilities. Private facilities primarily import intermediate waste that has gone through a separation and sorting process to manufacture SRFs. Intermediate waste is waste that has been made easy to recycle, and private facilities importing it have relatively advantageous conditions. Intermediate processing wastes brought into private facilities are mainly wastes under the extended producer responsibility (EPR) system. The EPR system means that product manufacturers are responsible for everything from production to recycling. Waste brought into private facilities includes composite synthetic resins among EPR wastes. Composite synthetic resins are manufactured into SRF because material recycling is more difficult than single-material synthetic resins [24]. If private facilities bring in EPR wastes, they can not only improve the quality of SRF but also generate revenue for the facility by utilizing EPR subsidies [25].

3.1.2. The SRF Manufacturing Efficiency

Figure 2 presents the annual manufacturing efficiency of the target facilities in 2021. The average manufacturing efficiency of the public was 49.7%, and that of the private was 89.3%. The manufacturing efficiency was higher in private than in public, which appears to be because the SRF raw material count among the input waste was higher in private facilities. Figure 3 presents the residue discharge rate and SRF production rate on the day of manufacturing. The same-day SRF manufacturing rate of the M-04 facility was 61.3%, which is similar to the public facility SRF raw material content in Table 3. The M-06 facility had a molding process of 98.2% and a non-molding process of 95.6%. The facility has a process in which waste is brought in, sorted for non-combustibles, and then goes through a primary crushing process, where some are made into molded SRFs and the rest are made into non-molded SRFs. The manufacturing efficiency was different due to the difference in the separation and sorting processes between the molding and non-molding processes; however, both processes demonstrated a high manufacturing efficiency of over 90%. This is similar to the annual manufacturing efficiency of private facilities, and also because the content of SRF raw materials was high in terms of the properties of the waste being brought in. The relationship between the properties of input waste and SRF manufacturing efficiency was also confirmed by Nasrullah et al. [26]. In this study, the SRF manufacturing efficiency was higher in municipal waste with a higher SRF raw material content when comparing waste, industrial waste, and construction waste.

Figure 1 presents the manufacturing process of the same-day manufacturing efficiency measurement facilities. The M-04 facility had an unpacking shredder and various types of sorting machines. The residues were automatically collected through a conveyor belt and were divided into scrap metal, non-ferrous metal, and impurities. The M-06 facility has a molding process and a non-molding process. In the molding process, scrap and non-ferrous metal were classified through magnetic separation after crushing, and SRF was manufactured through a molding machine. In the non-molding process, impurities were blown away through a wind-powered sorter, and scrap metal and non-ferrous metal were sorted through magnetic sorting.

The manufacturing process of the M-04 facility had more sorters and fewer crushers compared to that of the M-06 facility. It is generally known that the properties of the input waste have a significant impact on the SRF manufacturing efficiency. However, some studies have confirmed that this may be overcome by supplementing the manufacturing process. Jo et al. [27] improved the manufacturing efficiency from 21.7% to 30.9% by modifying the process of shredder, trommel screen, and dryer. The manufacturing efficiency was improved by modifying the shredder to efficiently cut small-sized MSW and collect missed combustible materials. Modifying the shredding process affects the manufacturing efficiency. Cutting large-sized combustible materials such as paper, cardboard, wood, and textiles into small pieces to help sort the waste can also improve manufacturing efficiency [26]. Therefore, the manufacturing efficiency of public facilities is expected to be increased by modifying the primary crushing or shredder to enable efficient sorting.

3.2. Calculation of Energy Recovery Efficiency

3.2.1. LHV Calculation Results

In

Table 5. Energy recovery efficiency calculation results for SRF-use facilities.

Facility	LHV (kcal/kg)		Calculation Factor (Gcal/day)					Energy Recovery (%)
	HB	NCV	Ew_HB	Ew_NCV	Ef	Ei	Ep	HB	NCV
U-01	3541	3693	1756.73	1832.23	0.17	7.62	1254.89	73.2	70.2
U-02	2719	2971	126.34	138.08	10.08	28.98	148.47	82.7	76.1
U-03	3316	3709	292.26	326.92	0.09	23.31	258.05	82.7	74.0
U-04	4796	5442	522.92	593.39	1.04	11.56	501.26	96.1	84.8
U-05	4219	6222	306.15	451.57	0.13	9.32	319.97	104.5	70.9
U-06	2884	3300	1593.93	1824.07	5.78	3.89	1674.73	107.3	93.8
U-07	3019	3206	2205.63	2341.84	5.67	19.91	2158.56	99.4	93.7
U-08	4080	4755	342.92	399.61	0.15	21.74	300.49	83.7	71.8
U-09	3382	3339	398.04	393	0.51	20.54	351.52	85.5	86.6

, the average LHV was estimated to be 3551 kcal/kg using the HB method and 4071 kcal/kg using the NCV method. The LHV was calculated using the NCV method, which was approximately 12.8% higher than that calculated using the HB method. The HB method is based on the Law of Energy Conservation. The diversity of incineration equipment and waste properties may be considered when the LHV is calculated through the HB method. Measurements other than the instrument must be performed to apply the HB method. The results from the target facilities are presented in

Table 6. Analysis results of measurements other than instruments of SRF-use facilities.

Facility	Heat Loss 1 (Gcal/day)	Bottom Ash Properties (%)		Boiler Exhaust Gas Properties (%)
		Loss on Ignition	Water Content	Oxygen Content	Water Content
U-01	14.13	0.06	0.07	9.26	13.98
U-02	28.72	0.2	0.04	9.35	9.79
U-03	6.49	0.45	0.02	5.94	16.21
U-04	2.97	0.1	0.13	6.43	15.39
U-05	3.65	0.03	0.04	9.47	14.14
U-06	7.74	0.05	0.03	5.26	18.86
U-07	14.45	0.08	0.04	5.93	22.17
U-08	19.91	3.32	11.82	4.8	18.26
U-09	10.31	12.67	34.71	10.22	13.84

¹ Heat loss measurement range: incinerator, boiler (primary, secondary), economize.

. Heat loss analysis (incinerator, boiler, and economizer) revealed that the average heat loss was 12.04 Gcal/L. The average heat loss of the fluid bed type was approximately 3.95 Gcal/day, lower than that of the stoker type. Low heat loss indicates high combustion efficiency of the fluidized bed incinerator [28]. The average loss on ignition of the bottom ash of the stoker type was 8.00%, and the moisture content was 23.27%. The average loss on ignition of the bottom ash of the fluid bed type was 0.14%, and the moisture content was 0.05%. This appears to be related to the well-known combustion characteristics of the fluid bed type. Compared to the stoker type, the unburned carbon (measured by loss on ignition) of the ash is known to be lower in the fluidized bed type and higher in the stoker type. It is also known that the water content of the ash can increase the loss on ignition content [29]. There was no difference in the properties of the exhaust gas depending on the type of incineration, with an average oxygen concentration of 7.41% and an average moisture content of 15.85%.

The NCV method applied in this study is a simple formula for calculating the LHV of Korea’s municipal waste incineration facilities. This simple formula is a modification involving the addition of the European correction factor to the Korean factor by calculating the boiler efficiency, flue gas loss, and other losses of domestic municipal waste incineration facilities. The data required to derive the correction factor applied in the simple formula are heat output items (Q_out) and heat input items (Q_in). Nevertheless, the NCV simple formula currently used in Korea needs to update its heat loss correction coefficient to reflect the operating characteristics of incinerators and boilers, as presented in the [30]. Therefore, if all heat input and output items can be measured in the future under the recommendation of installing measuring instruments for energy recovery efficiency calculation in SRF-use facilities, it will be possible to derive a simple formula that reflects its characteristics.

3.2.2. Energy Recovery Efficiency Calculation Results

For incineration heat recovery facilities, the data collection period for the factors for calculating energy recovery efficiency must be one year. However, among the target facilities, there were facilities with data collection periods of less than one year (see Table S1). Therefore, in our study, the recovery efficiency calculation factor was calculated as one day. Table 5 presents the recovery efficiency of the target facilities. The average of the stoker type (MU-03, M-03) was 84.6% when calculated using the HB method and 79.2% when calculated using the NCV method. The average fluid bed type was 92.3% for the HB method and 80.5% for the NCV method. The reason the recovery efficiency varies depending on the LHV calculation method is that the LHV is applied to calculate the E_w factor, indicating the input heat amount of waste. Differences in the LHV calculation method are explained in detail in the previous sections.

The stoker type is favorable for incinerating a large amount of non-homogeneous waste, and the fluid bed type is favorable for generating stable energy by burning certain fuels, such as fossil fuels [31]. SRF has the same shape and size when produced as fuel based on quality standards, and it is standardized. Therefore, most target facilities are designed and operated as the fluid bed type, which is favorable for burning certain fuels. The stoker-type incinerators are used in the facilities to change the type of target waste. It is generally known that the combustion conditions of the fluid bed type are more favorable than those of the stoker type [28]. Favorable combustion conditions can also affect the results of energy recovery efficiency calculations.

In Korea, the incineration heat energy recovery facility must have an energy recovery efficiency standard of 75% or higher. As presented in Table 5, all facilities except the U-01 facility satisfied the incineration heat recovery facility standard when the HB method was applied. Meanwhile, as per the R1 standard in Europe, facilities with efficiencies above 60% are classified as recovery facilities, and those with less than 60% are classified as treatment facilities. According to the standards for Korean incineration heat recovery facilities, all facilities except the U-01 facility appear to be able to meet the heat recovery standards. Additionally, as per the R1 standard, all target facilities are considered recovery facilities. The result of applying the R1 standard to SRF-use facilities varied across countries. SRF-use facilities in southern Italy were classified as treatment facilities under the R1 standard [17]. This appears to be due to the characteristics of the R1 method. The R1 method assigns different weights depending on the type of energy being recovered. It assigns 1.1 to steam energy and 2.6 to electric energy. The reason for applying the weights is to apply the average production efficiency of heat and electric energy produced in traditional power plants. There are several advantages and disadvantages to the R1 method. A study by Grosso et al. [32] showed that a disadvantage is that the recovery efficiency is affected by the type of recovered energy, incineration capacity, and location conditions. For example, there is criticism that large incinerators in cold regions with high heat consumption are advantageous in calculating the recovery efficiency. An advantage of the R1 method is that it contains the concept of comparing the energy recovery efficiency of traditional power plants and the energy recovery efficiency of waste incineration facilities. Consequently, the R1 method is expected to be an appropriate method in that it applies the same calculation standards to calculate the energy recovery efficiency of SRF-use facilities compared to waste incineration facilities.

4. Conclusions

In this study, energy recovery efficiency was calculated for SRF manufacturing, and facilities in Korea were used to expand the utilization measures of SRF, which is a type of waste resource energy. For SRF manufacturing facilities, manufacturing efficiency was calculated using the amount of SRFs produced compared to the input waste. Manufacturing efficiency differed depending on the proportions of the input waste. The average manufacturing efficiency of public facilities in 2021 was 49.7% and that of private facilities was 89.3%. Compared to public facilities, the efficiency of private facilities that brought in industrial waste with a higher content of SRF raw materials was higher. To increase the efficiency of public facilities, an appropriate pretreatment process, such as a primary shredder, appears to be necessary. In the case of SRF-use facilities, the energy recovery efficiency of the stoker type was 79.2–84.6% depending on the lower heating value calculation method (NCV or HB), and that of the fluidized bed type was 80.5–92.3%. It is necessary to develop a simple calculation formula exclusively for SRF combustion through future research to more accurately and easily calculate the LHV of SRF. Energy recovery efficiency was calculated using the R1 method, which is used in waste incineration facilities. The efficiency was calculated to be over 60% for all target facilities, and they were classified as energy recovery facilities according to the R1 standard. The findings of this study will be used to prepare support systems for waste resource energy facilities in Korea in the future.