Progress in Solid Recovered Fuel with an Emphasis on Lignocellulose-Based Biomass—A Mini Review Focused on Japan, South Korea, and Taiwan

Abstract

To reduce greenhouse gas (GHG) emissions, decarbonize coal, and also create a circular economy model, solid recovered fuel (SRF) has been developed as an alternative fuel/energy source in the international community, especially in developed countries with a high dependence on imported energy. This mini review offers updates on the regulatory promotion of the production of SRF, focusing on the reuse of biomass or lignocellulosic waste as a starting material in Japan, South Korea, and Taiwan. In this regard, the status of renewable energy and the policies for bioenergy in Japan, South, and Taiwan are first addressed in this work. It is found that the terms for defining refuse/waste/biomass-derived fuel are different across East Asia. However, SRF is increasingly used for the substitution of fossil fuels in industrial utilities (including boilers, incinerators, and kilns), as well as for steam (heat) utilization and/or power generation. With the international policies of pursuing staged carbon reduction by 2030 and carbon neutrality by 2050, the regulatory promotion of the use of bio-SRF has been actively adopted by these countries or regions. Regarding the quality requirements of SRF and concerns about air pollutant emissions, this work also offers updates on regulatory standards, especially in Taiwan. Finally, prospects for the production of bio-SRF and concerns regarding its use are addressed to support the development of a sustainable and circular society.

1. Introduction

In recent years, the use of solid recovered fuel (SRF) as an auxiliary fuel in industrial utilities has been extensively adopted in some developed countries and regions, such as East Asia and the European Union [1,2,3,4,5,6,7,8]. It should be noted that SRF may be a subset of refuse-derived fuel (RDF), but the former must meet national or international quality standards like ISO 21640 [1,9]. According to the report by Ferronato et al. [7], the authors recommend the use of SRF in cement kilns as a substitute for coke to mitigate greenhouse gas (GHG) emissions and also achieve environmental benefits, based on the results of a life cycle assessment. The starting materials for producing SRF are commonly derived from non-hazardous combustible waste, which can be used as a partial substitute for coal in industrial boilers and kilns. According to the report by the Intergovernmental Panel on Climate Change (IPCC) [10], the use of SRF (or RDF) in the stationary combustion process indicates relatively lower default emission factors compared to coal. Therefore, the substitution can mitigate GHG emissions. Furthermore, the production of SRF and its use not only lessen the environmental impact of waste treatment facilities (i.e., incineration plants and sanitary landfills), but also reduce operation costs in energy-intensive enterprises.

Over the past decade (2015–2024), the 2030 sustainable development goals (SDGs) and the goal of carbon neutrality by 2050 have been adopted by the international community. These policies are important for mitigating the depletion of natural resources and reducing GHG emissions in East Asia, and feature a high dependence on imported energy. In 2022, renewable energy contributed to approximately 10% of the final energy consumption in Japan [11]; around one-third of this renewable energy was derived from biomass and biogenic waste. In the same year (2022), renewables only accounted for a share of around 4.2% of the final energy consumption in South Korea [12]; approximately 50% of this renewable energy was from biomass and renewable waste. In 2023, the dependence on imported energy was 96.19% in Taiwan [13], where biomass and waste only accounted for 0.44% of the total energy consumption. Obviously, the biomass/waste-to-energy pipeline is an available option for solving energy and environmental (waste management) issues, and it is an important energy source with high regional characteristics. As a result, the governments in Japan, South Korea, and Taiwan have adopted regulatory measures for the promotion of SRF production and use in recent years, especially for lignocellulose-based SRF (bio-SRF).

This paper reviews the regulatory promotion of quality standards for SRF, focusing on the reuse of biomass or lignocellulosic waste as a starting material in developed countries and regions in East Asia (i.e., Japan, South Korea, and Taiwan). All of these countries have a large population density and a high dependence on imported energy (>90%). First, the status of renewable energy and the policies for bioenergy in Japan, South, and Taiwan are briefly summarized in this work. Regarding the concerns about air pollutant emissions from the use of SRF, the present study also offers updates on regulatory standards, especially in Taiwan. Finally, prospects for the production of bio-SRF and concerns regarding its use are addressed to support the development of a sustainable and circular society.

2. An Overview of Renewable Energy in Japan, South Korea, and Taiwan

Since the late 2010s, Japan, South Korea, and Taiwan have been listed as developed countries or regions in East Asia, with large population densities and a high dependence on imported energy, as summarized in

Table 1. Population, population density, energy-related CO₂ emissions, and shares of renewable energy in Japan, South Korea, and Taiwan [14].

Country	Population (Million, 2023)	Population Density (People/km2, 2023)	Energy-Related CO2 Emissions (Million Tons, 2022)	Share of Renewable Energy (%, 2021) a
Japan	114.39	338	974	8.84
South Korea	51.71	531	549	3.65
Taiwan	23.45	653	270	3.08

^a Compared to final energy consumption.

[14]. In response to the waste-to-energy (or waste-to-fuel) pipeline for reducing GHG emissions, the regulations for the production of SRF and its use in Japan, South Korea, and Taiwan are briefly outlined in this section, focusing on the specifications and standards for producing qualified bio-SRF products.

2.1. Japan

Like other developed countries, Japan has a variety of renewable energy resources, including geothermal, hydropower, wind, ocean, solar energy, and biomass, as well as combustible waste. To mitigate GHG emissions and also create a green society, the Japanese government has steadily expanded its renewable electricity production from these resources in recent years. According to the database in [11], the share of renewable electricity generation in Japan has indicated an increasing trend, from 10% in 2012 to 22% in 2022. Prior to 2012, renewable electricity generation mostly depended on hydropower, which produced between 75 and 90 terawatt hours (TWh) in past decades, contributing to 8% of the power production in Japan. Meanwhile, solar power significantly increased in use from 3.5 TWh in 2010 to 93 TWh in 2022, currently representing more than 9.5% of the power generation in Japan. By contrast, the shares of wind power (9 TWh) and geothermal power in 2022 were relatively low, at around 0.9% and 0.25%, respectively. Regarding biomass/waste-based electricity in Japan, its electricity generation is clearly growing, doubling from 15 TWh in 2015 to 37 TWh in 2022 and contributing to over 4.0% of the power generation in 2022.

2.2. South Korea

Due to a high dependence on imported energy, the South Korean government planned on increasing shares of renewable energy over the past two decades, especially in terms of solar power and wind power. According to the database in [12], the share of renewable electricity generation was below 2% in 2010. However, about 8% of the power generation in South Korea was renewable in 2022 (at 29 TWh). On the other hand, renewable energy sources represented a small share of the total energy supply (at 2.5%), with around 50% being through bioenergy (i.e., solid biofuels, biogas, biodiesel, biogenic waste). The main growth came from solar power, which represented two-thirds of the renewable electricity used in 2022 (at 43 TWh). Wind power and hydropower each contributed approximately 0.6% to power generation (~3 TWh). Biomass/waste-based electricity represents about 1.6% of the electricity generation in South Korea. It should be mentioned that there was a strong increase between 2014 and 2016, particularly in electricity generation from solid biomass (wood pellets) and liquid biofuels. In 2022, over 8.0 TWh of renewable electricity generation was from solid biomass, biogas, liquid biofuels, and municipal solid waste (MSW).

2.3. Taiwan

In recent years, renewable energy development in Taiwan has soared in growth under the promotion of regulatory measures and policies, especially in terms of solar photovoltaics (PV) and wind power. According to statistics [15], in terms of installed capacity in Taiwan by the end of 2016, 2023, and 2024, renewable energy had a total capacity of 4.726, 17.955, and 21.052 gigawatts (GW), respectively, summarized as follows:

• By the end of 2016:

Solar photovoltaics (PV): 1245 gigawatts (MW);

Wind power: 682 MW;

Conventional hydropower: 2089 MW;

Biomass/waste-to-power: 709 MW;

Geothermal power: 0 MW.

2.

By the end of 2023:

Solar photovoltaics (PV): 12,418 MW;

Wind power: 2677 MW;

Conventional hydropower: 2104 MW;

Biomass/waste-to-power: 749 MW;

Geothermal power: 7 MW.

3.

By the end of 2024:

Solar photovoltaics (PV): 14,281 MW;

Wind power: 3890 MW;

Conventional hydropower: 2123 MW;

Biomass/waste-to-power: 751 MW;

Geothermal power: 7 MW.

On the other hand, renewable electricity generation in Taiwan also showed an increasing trend in parallel [15]. For the end of 2016, 2023, and 2024, the data (unit: gigawatt hours, abbreviated as GWh) are summarized as follow:

• By the end of 2016 (12,733 GWh):

Solar photovoltaics (PV): 1109 GWh;

Wind power: 1457 GWh;

Conventional hydropower: 6562 GWh;

Biomass/waste-to-power: 3605 GWh;

Geothermal power: 0 GWh.

2.

By the end of 2023 (26,871 GWh):

Solar photovoltaics (PV): 12,909 GWh;

Wind power: 6238 GWh;

Conventional hydropower: 3963 GWh;

Biomass/waste-to-power: 3758 GWh;

Geothermal power: 23 GWh.

3.

By the end of 2024 (33,333 GWh):

Solar photovoltaics (PV): 14,903 GWh;

Wind power: 10,329 GWh;

Conventional hydropower: 4206 GWh;

Biomass/waste-to-power: 3868 GWh;

Geothermal power: 27 GWh.

3. Policies for Promoting Bioenergy in Japan, South Korea, and Taiwan

3.1. Japan

Under the new Strategic Energy Plan, the Japanese government increased its target for its share of renewable electricity to 36–38% of total power generation by 2030 [16], thus reducing GHG emissions by 46% over 2013 levels by 2030. This new policy calls for an increase in installed solar capacity from 79 GW in 2022 to 108 GW by 2030. The newly installed locations include government buildings, corporate buildings, parking garages, public land, and promotional areas. The targeted installation of wind power in Japan focuses on increasing offshore capacity from 0.14 GW in 2022 to 10 GW by 2030. Furthermore, the country intends to become entirely carbon-neutral by 2050. On the other hand, the Japanese government also encouraged the utilization of biomass and waste for producing heat (stream) and/or electricity, because this biomass energy is an important energy source with high regional characteristics. Based on the Biomass Nippon Strategy in 2002, the Basic Act for the Promotion of Biomass Utilization was enacted in 2009 to promote domestic and imported biomass supply [16]. In this regard, biomass/waste-based electricity grew from 15 TWh in 2015 to 37 TWh in 2022, representing about 3.7% of the total power generation in Japan.

3.2. South Korea

Since the Paris Agreement in 2015, the South Korean government has actively adopted a number of renewable energy policy measures under the “Basic Plan on Renewable Energy” [12]. In December 2017, the Korean Government announced the Renewable Energy 3020 Implementation Plan, setting the goal of producing 20% of its energy from renewable sources by 2030. In October 2021, the Korean government announced the goal of carbon-neutrality by 2050, with the aim of reducing the country’s GHG emissions by 40% by 2030 compared to those in 2018. On 13 January 2023, Korea’s Ministry of Trade, Industry and Energy (MOTIE) set out the 10th Basic Plan on Supply and Demand of Electricity, which aims to increase levels from 21.6% by 2030 (at 134.1 TWh) to 30.6% by 2038 (at 204.4 TWh). Concerning the policies promoting the waste/biomass-to-energy pipeline (bioenergy), the Korean government are focused on the production of solid refuse fuel (SRF) from combustible and lignocellulosic waste [17], based on the “Act on the Promotion of Saving and Recycling of Resources Enforcement Regulation” for pursuing circular economy and resource security [18].

3.3. Taiwan

Due to the shortage of self-produced energy and the high dependency (as high as 96–98%) on imported and fossil fuel energy in Taiwan, the government has actively been promoting renewable energy supply and also increasing indigenous energy supply for the energy and industry sectors since the early 2000s [19]. To enhance biomass energy (biofuels, bioenergy, or biopower) diversification, the central competent authorities, including the Ministry of Environment (MOENV), Ministry of Economic Affairs (MOEA), and the Ministry of Agriculture (MOA), set the relevant policies and regulations under the Guidelines on Energy Development [20]. In a previous study [6], the authors summarized the possible amounts of domestic bioresources for biomass energy, and also addressed the regulatory and promotional measures under the authorization of the Renewable Energy Development Act and Waste Management Act. Among them, economic incentives like feed-in tariffs (FITs), installation support (or subsidies), and circular economy have played a vital role over the past fifteen years (2010–2024).

4. Regulations for Using Bio-SRF in Japan, South Korea, and Taiwan

In response to the waste-to-energy (or waste-to-fuel) pipeline for reducing GHG emissions, the regulations for the production of SRF and its use in Japan, South Korea, and Taiwan are briefly outlined in this section, focusing on the trends of specifications and standards for producing qualified bio-SRF products due to its significance in net carbon reduction.

4.1. Japan

As mentioned above, the European Union established the quality standard for a type of RDF called “solid recovered fuel (SRF)”, with its calorific value and permissible contents of chlorine and mercury being divided into five classes using the international standard ISO 21640 (“Solid recovered fuels—Specifications and classes”) in 2021 [9]. However, a special waste-derived fuel and its quality standards have been defined by Japanese Industrial Standards (JIS Z 7311) [21]. The term “Refuse derived paper and plastics densified fuel (RPF)” is used in Japan with differentiation from the term SRF. RPF is a pelletized fuel that is produced from paper/pulp waste, wood waste, and plastic waste from industrial activities, for which material recycling is quite difficult or inefficient. Table 1 lists the quality standards of RPF and RPF–coke [1]. In addition, some heavy metals (e.g., mercury) may have harmful effects on human health and the environment. As seen in

Table 2. Quality standards of refuse plastic–paper fuel (RPF) in Japan ¹.

RPF Grade	RPF–Coke	RPF
Grade		A	B	C
Higher heating value 2	≧33 MJ/kg	≧25 MJ/kg	≧25 MJ/kg	≧25 MJ/kg
Moisture	≦3 wt%	≦5 wt%	≦5 wt%	≦5 wt%
Ash	≦5 wt%	≦10 wt%	≦10 wt%	≦10 wt%
Chlorine	≦0.6 wt%	≦0.3 wt%	>0.3 wt%, ≦0.6 wt%	>0.6 wt%, ≦2.0 wt%

¹ JIS Z 7311 (“Refuse Derived Paper and Plastics Densified Fuel”, 2010). ² Also called gross calorific value (GCV).

, the mercury limit was not specified in the JIS. In this regard, the quality standards for RPF are relatively simple compared to those in South Korea and Taiwan (described later). The main materials of RPF are paper/pulp and plastics/resins (except for polyvinyl chloride—PVC, unsaturated polyester resin—FRP, phenol resin), but the mixture of wood scraps, fiber waste, and rubber (or tire) waste is also acceptable if the quality of RPF meets the quality requirements in Table 2. The used paper (or paper waste) may include specialty paper, adhesive tape, rolls of damaged paper, flat damaged paper, and paper container packaging. According to the heating value requirement (≧25 MJ/kg) for RPF, the main composition of Japan’s RPF should contain waste plastics and/or waste tires due to their high calorific value. In general, the lignocellulosic compositions of RPF will be not high because the range of calorific value for dried waste paper is 12–18 MJ/kg [22].

In Japan, RPF is intended mainly as an auxiliary or substitute fuel for coal and other fossil fuels in the energy-intensive industries (i.e., paper, steel, cement) due to its heating properties (e.g., GCV > 25 MJ/kg). The RPF is produced in “pellet” form (diameter 8, 20, or 40 mm) by consolidating wastes at high temperature/pressure. Currently, over 1.2 million metric tons of RPF are produced per year from several operating RPF facilities (85 in 2013 and 227 in 2015) [1]. It should be noted that RPF provides a much superior fuel performance than densified refuse-derived fuel (RDF) based on the quality requirements (JIS Z 7302) for the latter (i.e., NCV >12.5 MJ/kg, moisture content <10%, and ash content < 20%) [23]. Furthermore, a high chlorine content in RPF will cause corrosion in boilers, and thus high-quality RPF has less than 0.3% chlorine content (i.e., RPF Grade-A), which is acceptable for Japanese paper mills [24]. As described above, the Japanese government promulgated the Basic Act for the Promotion of Biomass Utilization in 2009, which was enacted to implement the new national plan for the promotion of domestic and imported biomass supply [16]. In fact, the Act was based on the Biomass Nippon Strategy in 2002 [25].

4.2. South Korea

Prior to 2013, SRF in South Korea was grouped into refused plastic fuel (RPF), similar to RPF in Japan, refuse-derived fuel (RDF), which could be pelletized or not, tire-derived fuel (TDF), and wood chip fuel (WCF) [2]. In 2013, the term SRF (solid recovered fuel, or solid refused fuel) was incorporated into the national legislation (“Act on the Promotion of Saving and Recycling of Resources Enforcement Regulation”) in South Korea. Two types of SRF (i.e., SRF and bio-SRF) are currently classified in response to waste management policies, including waste-to-energy strategies for producing renewable energy in incineration plants.

Table 3. SRF quality standards in South Korea.

Content Standard a	SRF Type a
	General	Bio
NCV b	≧14.6 MJ/kg (Imported SRF ≧ 15.3 MJ/kg)	≧12.5 MJ/kg (Imported bio-SRF ≧ 13.2 MJ/kg)
Chlorine (Cl)	≦0.2 wt%	≦0.5 wt%
Mercury (Hg)	≦1.0 mg/kg	≦0.6 mg/kg
Moisture	≦10 wt% (pellet), ≦25 wt% (non-pellet or fluff)
Biomass	- c	≧95 wt%
Sulfur (S)	≦0.6 wt%	≦0.6 wt%
Ash	≦20 wt%	≦15 wt%
Cadmium (Cd)	≦5 mg/kg	≦5 mg/kg
Lead (Pb)	≦150 mg/kg	≦100 mg/kg
Arsenic (As)	≦13 mg/kg	≦5 mg/kg
Chromium (Cr)	-	≦70.0 mg/kg

^a Pellet shape: diameter ≦ 50 mm, length ≦ 100 mm; non-pellet (fluff) shape: width ≦ 120 mm, length ≦ 120 mm. ^b Net calorific value. ^c Not applicable.

summarizes the quality standards for SRF and bio-SRF in South Korea. As listed in Table 3, content standards, including net calorific value (NHV), shape and size, and chlorine, mercury, moisture, sulfur, ash, cadmium, lead, arsenic, biomass, and chromium content, were adopted to specify SRF quality. It should be noted that the content standard of chromium (Cr) for bio-SRF is based on woody materials/timbers containing preservatives like copper chrome arsenic (CCA).

According to the report in [17], the supplies of SRF and bio-SRF in South Korea in 2019 amounted to around 1.6 million and 2.7 million metric tons, respectively. These supplies were produced from over 250 SRF manufacturing facilities and were used by about 150 SRF consumption facilities (end users), e.g., in industrial boilers and at incineration (waste-to-energy or cogeneration) plants. Almost all supplies of SRF in the form of pellets used combustible fractions (e.g., paper, plastics, wood) of municipal/industrial wastes. The authors also measured the concentrations of dioxins in three bio-SRF incineration plants and four SRF incineration plants [17], showing a 0.02 ng international toxic equivalency quantity (I-TEQ)/Nm³ on average. According to the Persistent Organic Pollutants Control Act in South Korea [18], the limit of dioxin emission is 0.1 ng I-TEQ/Nm³ for incineration plants larger than 2 metric tons/h. The measurement results indicated that all SRF/bio-SRF consumption plants satisfied the emission limit for dioxins.

4.3. Taiwan

To mitigate emissions of GHGs from the energy and industry sectors and also reduce the environmental impact due to the aging and deficiency of MSW incineration plants, the Taiwanese government began to promote the reuse of combustible waste resources from non-hazardous industrial waste as SRF in 2020 [26]. The waste-to-fuel policy is followed by developed countries or regions like the Europe Union, Japan, and South Korea [1]. The central competent authority (i.e., MOENV) promulgated the relevant control systems and standards of the regulation (“Solid Recovered Fuel Manufacturing Guidelines and Quality Standards”) on 1 April 2020. The measures include the necessary requirements to be installed in SRF manufacturing plants to ensure the quality standards of SRF products for end-use facilities. To prevent the emission of air pollutants from industrial boilers in SRF consumption plants, the MOENV also announced amendments to the relevant regulations under the authorization of the Air Pollution Control Act, including “Standards for Co-firing Ratios, Components, and Control Facilities for Fuel Used in Stationary Pollution Sources” on 23 March 2020 and “Emission Standards of Air Pollutants for Boilers” on 8 July 2020. The aims of these amendments were to safeguard ambient air quality and also reduce health risks from the emission of hazardous air pollutants like dioxins and heavy metals (i.e., Pb, Cd, and Hg). Thereafter, the regulations for SRF guidelines and quality standards were revised several times to enhance manufacturing techniques and quality control. To promote the reuse of lignocellulosic resources from non-hazardous industrial waste,

Table 4. Waste categories used as feedstock for producing bio-SRF in Taiwan.

Waste Type	Item Name a	Reporting Code a
Paper	Mixture containing paper	D-0609
	Waste paper	R-0601
	Pulp/paper residue	R-0604 b
Wood	Waste wooden pallet	D-0701
	Mixture containing wood	D-0799
	Waste wood	R-0701
Sludge	Pulp sludge	R-0904
	Texture sludge	R-0906
Animal/plant-derived waste	Sugarcane bagasse	R-0102
	Spent mushroom compost	R-2401
	Plant-based residue	R-0120
	Plant-based residue	D-0120 b

^a D-type codes denote general industrial waste without recyclable value; R-type codes denote industrial waste with recyclable value, as announced by the MOEA. ^b Newly coded.

lists the waste categories used as materials for producing bio-SRF in Taiwan based on these regulations.

Table 5. Reported generation amounts of waste categories used as feedstock for producing bio-SRF in Taiwan ^a.

Lignocellulosic Waste Categories (Waste Code)	2019	2020	2021	2022	2023
Mixture containing paper (D-0699)	209,097	204,647	232,907	219,739	218,419
Waste paper (R-0601)	4120	3325	3476	5242	3659
Pulp/paper residue (R-0604)	- b	-	-	-	-
Waste wooden pallet (D-0701)	2312	2204	2005	1741	1346
Mixture containing wood (D-0799)	12,872	14,891	23,483	13,428	14,325
Waste wood (R-0701)	64,329	71,922	96,919	107,400	112,641
Pulp sludge (R-0904)	398,836	402,126	462,711	448,896	551,204
Textile sludge (R-0906)	53,837	53,734	61,243	52,788	51,031
Sugarcane bagasse (R-0102)	15,993	19,718	23,554	19,065	18,473
Spent mushroom compost (R-2401)	145	35	0	0	1084
Plant-based residue (R-0120)	51,039	59,699	67,178	68,010	78,013
Plant-based residue (D-0120)	- b	-	-	-	-

^a Source [27]. ^b Not yet reported.

further summarizes the generation amounts of waste categories used as materials for producing bio-SRF in Taiwan [27]. Industrial waste codes must be used by the generators to report their generation amounts and treatment methods using the online system. D-type and R-type codes denote general industrial waste without recyclable value and renewable resources announced by the MOENV, respectively. The data listed in Table 5 do not reflect the actual situation in Taiwan. Taking spent mushroom compost as an example, annual generation has amounted to 105–225 thousand metric tons since 2010 in Taiwan [28]. However, spent mushroom compost may be directly reused or dumped without being reported by mushroom-raising enterprises.

Over four years, to develop the SRF industry in Taiwan to the annual production of approximately 300,000 metric tons, the MOENV conducted field surveys (or onsite inspections) of 66 enterprises, including 48 manufacturing plants and 18 end-use plants, during the period of June–August 2024. According to the checkup report [29], the results indicated that most SRF enterprises were in accordance with relevant regulations, but some SRF enterprises had problems regarding waste treatment technology, waste operation and management, air pollution prevention and control, and environmental testing. To strengthen the regulations concerning the production and use of SRF in the industry sector, the MOENV passed and/or revised relevant regulations in January 2025, including “Emission Standards of Air Pollutants for Boilers”, “Standards for Co-firing Ratios, Components, and Control Facilities for Fuel Used in Stationary Pollution Sources”, “Management regulations for Continuous Automated Monitoring Facilities of Air Pollutants from Stationary Pollution Sources”, “Stationary Pollution Sources that Should Be Equipped with Continuous Automated Monitoring Facilities and Connected to the Competent Authorities”, “Stationary Pollution Sources that Should Be Regularly Tested and Reported”, and “Management Regulations on the Reuse of Commonly Industrial Waste as Materials for Manufacturing Solid Recovered Fuel” (new regulation). It should be noted that the regulation “Management Regulations on the Reuse of Commonly Industrial Waste as Materials for Manufacturing Solid Recovered Fuel” is a new regulation based on “Solid Recovered Fuel Manufacturing Guidelines and Quality Standards” and other updated guidelines and specifications like ISO 21640 (“Solid Recovered Fuels—Specifications and Classes”).

Table 6. Quality standards of solid recovered fuel (SRF) in Taiwan.

Quality Item	Sample Basis	Testing Method d	Limit/Promulgation Day
			17 January 2025	1 January 2026
Net calorific value a	As received b	NIEA M216 [30], ISO 21654 [31]	≧10.0 MJ/kg	≧10.0 MJ/kg
Chlorine (Cl)	db c	NIEA M217 [32], EN 15408 [33]	≦3 wt%	≦1.5 wt%
Mercury (Hg)	As received b	NIEA M360 [34], EN 15411 [35]	≦0.15 mg/MJ	≦0.10 mg/MJ
Lead (Pb)	db	NIEA M360, EN 15411	≦150 mg/kg	≦150 mg/kg
Cadmium (Cd)	db	NIEA M360, EN 15411	≦5 mg/kg	≦5 mg/kg

^a Lower heating value. ^b Using wind-dried sample or moisture-constant sample. ^c Dry basis. ^d ISO 21654: Solid recovered fuels—Determination of calorific value; EN 15408: Solid recovered fuels—Methods for the determination of sulfur (S), chlorine (Cl), fluorine (F), and bromine (Br) content; EN 15411: Solid recovered fuels—Methods for the determination of the content of trace elements (As, Ba, Be, Cd, Co, Cr, Cu, Hg, Mo, Mn, Ni, Pb, Sb, Se, Tl, V, and Zn).

and

Table 7. Solid recovered fuel (SRF) classification system in Taiwan ^a.

Classification Properties b	Classes
	1	2	3	4
NCV (MJ/kg)	≧25	≧20	≧15	≧10
Chlorine (Cl, wt%)	≦0.2	≦0.6	≦1.0	≦1.5
Mercury (Hg, mg/MJ)	≦0.02	≦0.03	≦0.05	≦0.10

^a The class “5” for Cl (≦3.0 wt%) and Hg (≦0.10 mg/MJ) will be not applicable from 16 January 2026. ^b The statistical measure is based on the average. The measurements of Hg and NCV (net calorific value, also called lower heating value) are based on received basis, but the Cl measurement is based on dry basis.

list the quality standards of SRF and its classification system in Taiwan, respectively. Obviously, the MOENV tightened the SRF specifications to protect air pollution in the coming year. For instance, the contents of Cl and Hg in SRF products were lowered to 1.5 wt% and 0.10 mg/MJ, respectively. To guarantee that SRF quality meets the limits in Table 6, several standard methods (coded as NIEA) have been established by the central competent authority (i.e., National Environmental Research Academy under the MOENV) since 2021.

Furthermore, the MOENV also set up the most stringent emission standards for heavy metals and dioxins (i.e., 0.1 ng I-TEQ/Nm³) in boilers (

Table 8. Emission standards for air pollutants from boilers.

Item No.	Air Pollutant	Emission Standard a	Date of Enforcement b
			Existing Source	New Source
1	Particulate	30 mg/Nm3	2 January 2025
2	Sulfur oxides	50 ppm	2 January 2025
3	Nitrogen oxides	100 ppm	2 January 2025
4	Lead (Pb) and its compounds	0.2 mg/Nm3	2 January 2025	1 January 2026
5	Cadmium (Cd) and its compounds	0.02 mg/Nm3	2 January 2025	1 January 2026
6	Mercury (Hg) and its compounds	0.05 mg/Nm3	2 January 2025	1 January 2026
7	Dioxins	0.1 ng-TEQ/Nm3	2 January 2025	1 January 2026

^a Nm³ means a cubic meter of air at a temperature of 273 K (Kelvin) and atmospheric pressure. TEQ (Toxicity Equivalency Quantity) is the method for calculating the toxicity weighting of dioxin concentrations based on the TEQ value (1.00) of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TCDD). ^b Boilers established before 3 January 2025 are considered existing sources. Boilers established after 4 January 2025 are considered new sources.

), in line with domestic regulations (e.g., “Control and Emission standards of Dioxins from Waste Incineration Plant”) and international emission regulations, such as those of South Korea (i.e., “Persistent Organic Pollutants Control Act”) [18]. As shown in

Table 9. Content standards for SRF used in stationary pollution sources.

Content Standard a	SRF
	Type I	Type II
Chlorine (Cl)	≦0.2 wt%	≦3.0 wt%
Lead (Pb)	≦150 mg/kg	≦150 mg/kg
Cadmium (Cd)	≦5 mg/kg	≦5 mg/kg
Mercury (Hg)	≦0.02 mg/kg	≦0.15 mg/kg
NCV b	≧25 MJ/kg	≧10 MJ/kg

^a The standards of Hg and NCV are based on received basis. The others are based on dry basis. ^b Net calorific value.

, the MOENV further revised regulations (i.e., “Standards for Co-firing Ratios, Components, and Control Facilities for Fuel Used in Stationary Pollution Sources”) where SRF is classified into two types (Type I and Type II) to encourage the use of cleaner SRF (SRF Type I) with a higher heating value (≧14.5 MJ/kg) and lower contents of chlorine (≦0.2 wt%) and mercury (≦0.02 wt%). As referenced in Table 7, SRF Type I is identical to Class 1. To promote the use of lignocellulosic resources from the agriculture sector as auxiliary fuels (or fuels, or fuel raw materials) in industrial utilities (e.g., boilers, furnaces, incinerators, kilns), solid biomass fuels, i.e., agricultural and forestry plants, bagasse, wood, and their residues (not subjected chemical treatment, bonding, or surface coating processes), were also grouped into the regulation. The content standards for solid biomass fuels include net calorific value (≧14.5 MJ/kg), chlorine (≦0.3 wt%), sulfur (≦0.3 wt%), mercury (≦0.1 wt%), lead (≦20 wt%), and cadmium (≦1 wt%).

5. Conclusions and Future Prospects

In recent years, the term “solid recovered fuel” (SRF) has been extensively used in the international community because it is considered an available and auxiliary fuel for reducing GHG emissions in the energy and industry sectors. Therefore, its quality standards and production requirements have been regulated by some developed countries or regions, like the EU, Japan, and South Korea. Due to the international policies for pursuing staged carbon reduction by 2030 and carbon neutrality by 2050, this mini review paper thus focused on the quality standards for bio-SRF in Japan, South Korea, and Taiwan. It was found that bioenergy (including general SRF and bio-SRF) is increasingly used for the substitution of fossil fuels in industrial utilities for steam (heat) utilization and/or power generation. To reduce the health risks posed by SRF consumption to end-users, quality standards and emission standards for heavy metals and dioxins—produced through vent gas when co-fired with fossil fuels in industrial utilities—have been enhanced in recent years.

To support the development of a sustainable and circular society in the near future, the prospects for the production of bio-SRF and the concerns regarding its use are addressed as follows:

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Expanding the available biomass sources, including woody remains and bamboo-based residues, disaster-derived woody waste, and building-derived woody waste, which are in accordance with the relevant regulations, especially regarding the limits of arsenic (As), copper (Cu), chromium (Cr), and lead (Pb), will be beneficial.

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The quality standards of bio-SRF could be checked by adding low-melting elements like sodium (Na) and potassium (K).

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Co-firing bio-SRF in industrial facilities could be implemented to reduce GHG emissions.

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Surveying the chlorine concentrations (or levels) of bio-SRF, especially for herbaceous biomass, can prevent the emission of hydrogen chloride and dioxins from fuel discharge.

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Increasing the feed-in tariff (FIT) rates of bio-SRF for power generation is recommended due to its lower heating value and increased costs during transportation and pretreatment.