Industrial tests of co-combustion of alternative fuel with hard coal in a stoker boiler

ABSTRACT

This paper presents the results of industrial research on co-combustion of solid recovered fuel (SRF) with hard coal in a stoker boiler type WR-25. The share of SRF in the fuel mixture was 10%. During the co-combustion of SRF, no technological disturbances of the boiler were noted. The obtained SO₂ and NO_X emissions were comparable with coal combustion, but dust emissions increased. During the co-combustion of the coal mixture with 10% of alternative fuel, acceptable standards for co-incineration of waste were exceeded for NOx, dust, CO, HCl, HF, heavy metals, dioxins, and furans. The by-products of waste co-combustion with coal were non-hazardous waste. The obtained results constitute a very important contribution to the process of boiler retrofitting toward a waste co-incineration unit, and to meeting the legislative and environmental requirements.

Implications: Due to some challenges related to waste storage and transportation, combustion in incineration plants and Waste-to-Energy plants is not possible. The adaptation (formal and technical) of medium scale boilers as co-incineration plants reveals high potential. Nevertheless, the lack of experience and investigations of waste co-combustion in real industrial scale grate boilers is observed. Thus, the implication of this article results consists of the investigations using industrial scale mechanical grate boiler (different from incineration type). Moreover, the investigations were carried out in a low-capacity boiler (~50% of nominal capacity). This novel experience is very important because reduced heat dissipation into the grid caused by high ambient temperatures occurs very frequently. These tests are valuable from the point of view of retrofitting the unit to obtain technological and emission parameters that would allow obtaining the status of a waste co-incinerating unit. The results of these investigations are addressed to power plant management board and engineering staff.

Introduction

Within the European Union, waste fuels are widely used in energy-intensive industries such as the production of clinker, paper, electricity, and heat (Chen et al. 2015; Hilber et al. 2007; Kim et al. 2016). The production of fuel from waste is an efficient way of managing municipal waste, unless it is possible to manage this waste in a manner corresponding to a high level of the waste hierarchy (Van Ewijk and Stegemann 2016). Currently, municipal waste is mainly incinerated in dedicated units (municipal waste incineration plants). Having the appropriate infrastructure, these units can carry out the incineration process in an environmentally efficient manner and in accordance with regulations. The recovery of heat and/or electricity allows adopting the status of waste-to-energy (WtE) unit (Malinauskaite et al. 2017). It was estimated that emission of greenhouse gas (GHG) (due to spontaneous emission of CH₄ from landfilling) can be significantly reduced by the application of waste as energy source in WtE units (Ryu 2010). Apart from incinerating waste in dedicated incineration plants, there is also the possibility to co-incinerate of waste in power engineering units, which can achieve the status of waste co-incineration plant after meeting a number of requirements. Waste Incineration Directive (WID 2000/76/EC) makes a distinction between the following types of plant: (a) incineration plants, which are dedicated to the thermal treatment of waste, and may (or may not) recover generated heat; and (b) co-incineration plants, such as cement or lime kilns, steel plants, or power plants. The main purpose of these is energy generation (or the production of material products), in which waste is used as a fuel or thermally treated for the purpose of disposal (Scarlat, Fahl, and Dallemand 2019). The combustion and/or co-combustion of waste may offer potential benefits in terms of heat and/or electricity recovery, although units that combust or co-combust waste have to meet a number of requirements, including, but not limited to the following ones:

• The need to maintain appropriate process parameters (such as the residence time of the flue gases at a temperature above 850°C for at least 2 s).

• The need to adapt the infrastructure allowing both for a thorough control of the amount of waste combusted, and for an intermediate stop of the waste stream directed to the combustion chamber in case of exceeding the emission standards.

• The necessity to meet emission standards. This is related to both tightening emission limits, and to the increasing a number of compounds whose emissions need to be controlled. In addition to pollutants whose emissions are monitored in all occupational power engineering units combusting fossil fuels and/or biomass (such as dust, NO_X, and SO₂), it is also necessary to control emissions of pollutants such as hydrogen chloride (HCl), hydrogen fluoride (HF), heavy metals, dioxins and furans, dust, nitrogen oxides (NOX) and sulfur dioxide (SO2) (Scarlat, Fahl, and Dallemand 2019). The emission standards for these compounds are determined using a mixing rule from Directive 2010/75/EU of the European Parliament and of the Council of 24 November 2010 on industrial emissions (integrated pollution prevention and control).

Obtaining the status of a waste co-combustion plant allows it to obtain benefits related to counting the generated energy (at least in part) as coming from a renewable energy source (RES). The process of becoming a co-incineration plant requires a number of multi-stage tests. One of the first stages is to test the technological capabilities of the unit in terms of co-combustion of waste with traditional fuels, without introducing constructional changes in the boiler. At a later stage, it is planned to conduct research into boiler retrofitting and installation in the entire boiler system (fuel feeding, flue gas monitoring, and purification system) in order to meet the requirements for waste co-incineration plants.

Waste energy in Europe has been used for electricity and/or heat generation for many years. Further, research into co-incineration of waste in industrial-scale power plants has been conducted in Germany, the Netherlands, Italy, Great Britain, Finland, and many other countries of the European Union (Del Zotto et al. 2015). For example, the Weisweiler Power plant site at Weisweiler in Germany, with a total installed capacity of 2060 MW_el, has experience of co-incinerating waste fuels (SRF with a 2% share of energy in fuel) in two 600 MW_el dust boilers (Hilber et al. 2007). In Italy, 5% (share by calorific value) of fuel from waste (RDF supplied by Ecoprogetto) was co-incinerated in a tangential fired dry-bottom boiler in the coal power plant located in Fusina-Venice owned by ENEL (Del Zotto et al. 2015).

Development of fuel-from-waste applications in power plants can be a temporary solution until the prevention of waste production, waste minimization, waste reuse, and recycling have been developed sufficiently to reduce waste landfilling. It is still a problem in European countries where landfilling is the dominant process of municipal solid waste treatment. For example, landfilling share in overall waste treatment is 81% in Greece, 79% in Latvia, 55% in Lithuania, and 55% in Spain (Kuo et al. 2019; Malinauskaite et al. 2017) Moreover, in some countries the total capacity of waste-to-energy plants is not enough to process available waste by incineration. For example, in Poland the capacity of newly opened plants allows to process approximately 9% of the municipal solid waste (MSW) generated in the country (Kuo et al. 2019).

Concerning the co-firing of SRF with fossil fuels in utility boilers, the rule of “10% mass fraction” is suggested in the literature. In other words, the reasonable amount of SRF in fuel mass is up to 10 wt% SRF (Del Zotto et al. 2015; Economopoulos 2010; Kuo et al. 2019). Nevertheless, in some cases the proportion of SRF in used fuel can be significantly higher (Bajamundi et al. 2015). Moreover, co-combustion of waste can increase the deposition rate and the amount of melt found in deposits (Bajamundi et al. 2015). Co-firing of SRF without upgrading the air pollution controls is illegal, even if the applicable emission standards for power plants are met, as this compliance is attained through flue gas dilution (Economopoulos 2010). The low heating value of MSW has forced many operators to co-incinerate with coal in order to keep the WtE business viable (Makarichi, Jutidamrongphan, and Techato 2018).

Benefits from co-incineration of fuels from waste (SRF) in power plants provide an improvement of CO₂ emission indicators, due to the share of biodegradable fraction, biogenic carbon fraction, reduction of fuel costs, in some cases the possibility to reduce emissions of major gaseous pollutants (NO_X, SO₂), and the possibility to obtain benefits related to the classification of fuel as RES. The accompanying technological challenges (such as increased content of K, Na, and Cl) should be emphasized, as it intensifies deposits on heated surfaces and reduces the quality of fly ash as a potential product (Iacovidou et al. 2018). Agraniotis et al. concluded that the substitution of brown coal by SRF in existing boilers was an advantageous practice from environmental, technical, and economic perspectives (Agraniotis et al. 2010).

Historically, the use of energy from waste in Poland has been minimal, despite the undoubted potential economic and environmental benefits of such activities. Under national conditions, energy recovery from large-scale waste is almost exclusively linked to the co-incineration of alternative fuels in the cement industry (Karwat, Głowiński, and Stańczyk 2014). If the system solutions of waste management are changed, and there is insufficient processing capacity in the cement industry, there is an oversupply of energy fractions generated in the regional municipal waste treatment plants (RIPOK) in the market. In this situation, it becomes necessary to look for additional solutions for the collection of this combustible waste. The growing domestic production potential of fuels from waste looks to the energy sector with the hope of new sales directions (Wasielewski 2017). Currently in Poland, waste fuels are not co-combusted in existing power plants, or combined heat and power plants, due to formal, legal, technical, and economic problems. The co-incineration plant has the status of a waste co-incineration plant and must therefore comply with the requirements for thermal treatment plants, including strict emission standards (Krawczyk and Szczygieł 2013). Therefore, it is necessary to distinguish between new WtE plants (they have very specific Flue Gas Treatment [FGT] system) and the power plants that can obtain status of being a co-incineration plant after formal (legislative) and technological processes of adaptation. It is known that very complex FGT systems are applied in the WtE, to guarantee very low emissions at the stack (Kuo et al. 2019). For many years, industrial research tests undertaken in Poland have not yet led to commercial recovery of energy from waste in existing boiler installations. The Institute for Chemical Processing of Coal (Agraniotis et al. 2010) actively participated in many of these research projects. Research experience in co-incineration of waste in power engineering units focuses mainly on dust boilers (Agraniotis et al. 2010; Škobalj et al. 2017) and fluidized bed boilers (Bajamundi et al. 2015; Wagland et al. 2011). The use of waste for energy purposes is also widespread in cement plants (Herrero and Vilella 2018; Pomberger and Sarc 2014). However, it has recently caused considerable controversy that old-type cement plants may emit significant amounts of pollutant, due to the use of waste as a fuel (Herrero and Vilella 2018).

Investigations of co-incineration grate (stoker) type power boilers (excluding specially adapted grate furnaces for waste incineration plants) are not common (Wasielewski, Głód, and Telenga-Kopyczyńska 2018). Typically, testing of grate boilers has been conducted in terms of gaseous pollutant emissions and dust during combustion of fossil fuels (Wojdyga, Chorzelski, and Rozycka-Wronska 2014), or by comparing the emission of grate boilers (dedicated to the combustion of waste) and grate boilers used in the power industry (considering the very common grate boiler WR-25) (Wielgosiński, Namiecińska, and Czerwińska 2018). Grate furnaces are successfully used in units in which SRF are dedicated fuels (incinerators and waste-to-energy plants WtE). It is estimated that 87%–90% of furnaces installed in incineration plants are grate furnaces (De Gisi et al. 2018). Grate firing combustion plants are normally units with a scale not exceeding 100 MW in fuel (Kleinhans et al. 2018). Grate boilers type WR-25 are the most common units of medium-capacity power plants and combined heat and power (CHP) plants in Poland. It is estimated that over 190 units of this type are used in Poland (Krawczyk and Badyda 2014; Wielgosiński, Namiecińska, and Czerwińska 2018). Usually, the co-incineration process is carried out in fluidized beds and pulverized combustors. Unlike the experience with incineration plants, there is little research into the co-incineration of waste in grate boilers (which could potentially become waste co-incineration plants).

This article presents the results of one of the industrial co-combustion tests of compacted alternative fuel (non-hazardous waste, classified under code 191210) with hard coal, which was carried out in a boiler with a mechanical grate type WR-25. The investigations were carried out in a low-capacity boiler (approximately 50% of nominal capacity). Such investigations have been not carried out previously. As already mentioned above, such boiler installations are commonly used in domestic heating. The aim of the research was to determine the influence of co-incineration of the waste with hard coal on the emission of pollutants into the air, and the quality of solid by-products of co-incineration in a unit not adapted for co-incineration of waste. This research is the first stage in the process of retrofitting the boiler (and the whole system) to achieve the requirements for waste co-incineration plants. From this perspective, it is possible to adapt WR-25 boilers for this purpose.

Materials and Methods

Characteristic of the tested boiler installation

The boiler used for testing was type WR-25, manufactured by SEFACO S.A. This is a high-temperature water-tube boiler equipped with a furnace and movable mechanical grate. The boiler operates in conditions of forced draught and blowing, using two air fans and two flue gas fans. The basic fuel for this boiler is finely sized hard coal.

Basic technical data of the WR-25 boiler is as follows:

• design pressure: 2.45 MPa

• heating surface: 1,063 m²

• boiler volume capacity: 13.6 m³

• maximum thermal input: 35 MW

• feed water temperature: 70°C

• maximum permissible water temperature: 150°C.

The WR-25 boiler is equipped with a single-stage flue gas dedusting system, which consists of a cyclone battery located outside the heating plant building. The solid by-products of combustion are slag and fly ash. Fly ash is introduced into the boiler slag traps, and together with slag is discharged to a slag dump. More specific information about this boiler is described in the literature (Wasielewski and Głód 2017; Wasielewski, Głód, and Telenga-Kopyczyńska 2018).

Testing methodology

Two series of comparative tests (full industrial scale) were carried out as follows:

• a baseline test of hard coal combustion;

• a comparative test of the combustion of a mixture of hard coal with 10% mass share of alternative fuel.

The fuel mixture was prepared at the storage yard, using a loader and weighed portions of hard coal (45 Mg) and alternative fuel (5 Mg). The coal and fuel mixture were both fed to the boiler bunkers using the existing feeding system, and then to the grate with the same layer thickness. During both tests, the boiler operated at a similar output. This was approximately 50% of the nominal capacity, due to reduced heat dissipation into the grid caused by high ambient temperatures. More particular descriptions of experimental methods are presented in Supplementary Materials.

Results and discussion

Characteristics of raw materials

Figure 1 presents a photograph showing the appearance of the tested alternative fuel (waste code: 191210). The material was compact (pieces of less than 40 mm). The compacting of the material was intended to reduce dust during transport and storage operations. The SRF constituted the mixture of fractions: plastics (approx. 30%), wood (approx. 30%), paper and cardboard (approx. 20%), fabric (approx. 10%), and rubber (approx.10%) that came from the municipal waste sorting installation. The information on the morphological composition of SRF was obtained from the SRF manufacturer. Aluminum foil inclusions were visible in the structure of the material. According to the information obtained from the manufacturer–-the SRF manufacturing process involved crushing of a waste material for granulation size < 20 mm and compacting without additional binder using a prototype extruder. Compacting of SRF was aimed at improving its transport characteristics. The material had a compact form (fragments below 40 mm).

Figure 1. Alternative fuel

The results of physicochemical properties of all the materials combusted are presented in Supplementary Materials. The results of the determination of ash composition of combusted materials are presented in Table 1.

Table 1. Chemical composition of ash from materials combusted during the tests

Compound	Unit	Hard coal	Alternative fuel	Mixture of coal and 10% of alternative fuel
SiO2	%	56.87	51.14	>55.20
Al2O3	%	22.40	16.33	16.02
Fe2O3	%	6.25	6.42	7.89
CaO	%	2.86	10.28	6.02
MgO	%	2.51	2.61	2.93
P4O10	%	0.40	0.51	0.43
SO3	%	2.42	3.70	3.49
Mn3O4	%	0.10	0.15	0.15
TiO2	%	1.02	1.98	1.13
BaO	%	0.13	0.14	0.12
SrO	%	0.06	0.05	0.05
Na2O	%	0.84	2.24	1.62
K2O	%	3.00	2.16	2.23

The data presented in Table 1 shows that the chemical composition of ash from alternative fuel is similar to coal in terms of the content of the main components (silicon dioxide SiO₂ and aluminum trioxide Al₂O₃), whereas ash from this waste contains much more calcium oxide (CaO). This might have a beneficial effect on the emission of SO₂ during co-combustion of the mixture of this material with hard coal. There is less iron trioxide Fe₂O₃ in the ash from the alternative fuel. The total content of alkaline oxides (Na₂O and K₂O) in ash is similar, although the proportions are slightly different. Coal contains more potassium dioxide (K₂O). In the samples of burnt fuels, the content of heavy metals was also determined. The results of these determinations as well as characteristics of by-products of incineration/co-incineration and combustion performance are presented in Supplementary Materials.

Emissions of pollutants into the air

Table 2 displays a comparison of emissions from coal combustion and co-incineration of coal mixture with 10% proportion of alternative fuel with emission standards to be met during the combustion of hard coal and co-incineration of waste in a WR-25 boiler.

Table 2. Comparison of pollutant emissions during combustion of hard coal and coal mixture with 10% proportion of alternative fuel

		Measured emission		Emission limit a
Contaminant	Unit	Hard coal combustion	Mixture of coal and 10% of alternative fuel combustion	Hard coal combustion	Mixture of coal and 10% of alternative fuel combustion
SO2	mg/m^3u	1 267.5	1 070.1	1,500	1,309.9
NOX	mg/m^3u	397.2	471.4	400	367.3
Dust	mg/m^3u	97.1	192.8	100	44.5
CO	mg/m^3u	37.7	89.4	-	37.6
HCl	mg/m^3u	239	538.4	-	208.9
HF	mg/m^3u	0.14	13.88	-	0.2
TOC	mg/m^3u	26.6	70.5	-	24.1
Cd+Tl	mg/m^3u	0.0008	0.0010	-	0.05
Hg	mg/m^3u	0.0003	0.0002	-	0.05
Sb+As+Pb+Cr+Co+Cu+Mn+Ni+V	mg/m^3u	0.1336	0.3765	-	0.5
PCDD/DF	ng/m^3u	0.0384	7.99	-	0.1

Notes. *Concentration of substances in the flue gases under contractual conditions, converted/calculated to 6% oxygen content in the flue gases

Emission standards for both fuel combustion and waste incineration/co-incineration are specified in the Regulation of the Minister of the Environment of March 1, 2018 on emission standards for certain types of installations, fuel combustion sources, and waste incineration or co-incineration facilities (Journal of Laws of 2018, item 680). The emission limits for co-incineration were determined using the mixing rule (Directive 2010/75/EU of the European Parliament and of the Council of November 24, 2010 on industrial emissions (integrated pollution prevention and control)). Emission standards for the combustion of hard coal for the tested WR 25 boiler power plant, based on the above regulation, only apply to three types of pollutants (SO₂, NO_X, and dust), whereas the scope of co-incineration of waste is much broader (additionally: CO, HCl and HF, TOC, heavy metals, and polychlorinated dioxins and furans).

Comparing the results of pollutant emission measurements with the permissible values for co-incineration of waste, it should be stated that the following concentrations were exceeded during co-incineration of alternative fuel with coal:

• nitrogen oxides NO_X (significant),

• carbon monoxide CO (more than twice),

• hydrogen chloride HCl (almost two and a half times),

• hydrogen fluoride HF (almost 70 times),

• dust (more than four times),

• TOC (more than twice),

• polychlorinated dioxins and furans (almost 80 times).

The very high emission levels of polychlorinated dioxins and furans results from both the large amount of their precursors (HCL and volatile organic substances expressed as TOC) in the flue gases, and the lack of technical possibilities for the tested WR 25 boiler to meet the process requirements for thermal waste processing, sufficient for the thermal destruction of these compounds. This applies in particular to the residence time of the exhaust gas at temperatures above 850°C for at least 2 s, as found in many other similar studies (Wasielewski, Głód, and Telenga-Kopyczyńska 2018). At the same time, it should be noted that the emission of SO₂ from co-combustion of alternative fuel with coal was lower than the emission of this pollutant during combustion of hard coal itself, which confirms the beneficial effect of increased calcium dioxide content in the ash of alternative fuel. It should also be noted that despite the high content of heavy metals in both the alternative fuel and the fuel mixture with its share, the required emission standards for these pollutants were met. The selected concentrations of gaseous compounds (mg/m_n³ dry at 6 v% O₂) in flue gas from co-firing units are presented in Table 3. Besides of 13.8 MWth industrial steam boiler (moving stoker) (Skodras et al. 2002), the concentration of CO (if available) is comparable in all cases. Despite some differences, the NO_x concentrations are in the same order of magnitude for all cases. The range of SO₂ concentration is more spread, starting from the level of 50 mg/m_n³ up to the level of 1,523 mg/m_n³. It can be noticed that the most comparable results to this study was obtained by the Nadziakiewicz and Kozioł (2003). In this case, emission of SO₂, NO_x are almost the same values comparing to this study. The tendency of NO_x emission is also comparable. Namely, the increase of waste share in fuel mix caused the increase of NO_x emission.

Table 3. The comparison of the emission of selected gaseous pollutant, mg/m_n³ at 6%O₂, dry

Type of furnace	SO2	NOx	CO	Ref.
13.8 MWth industrial steam boiler (moving stoker), Co-combustion of Greek lignite from Ptolemais reserve, natural waste wood, MDF residues, and power poles. Natural wood–lignite; 80/20 (wt%) Nitrogen content 1.3 wt% (dry base) for lignite and 0.1 wt% (dry base) for natural wood; combustible sulfur content 1.1 wt% (dry base) for lignite and 0.2 wt% (dry base) for natural wood	60	550	250	Skodras et al. (2002)
As above, MDF–lignite, 80/20 (wt%). Nitrogen content 1.3 wt% (dry base) for lignite and 2.4 wt% (dry base) for MDF; combustible sulfur content 1.1 wt% (dry base) for lignite and 0.3 wt% (dry base) for MDF	50	610	80	Skodras et al. (2002)
28-ton-per-hour traveling grate utility boiler. Combustion of biomass blend (forestry waste, such as cortex and wood blocks) in coal-fired grate boiler. No flue gas cleaning method exists, except for using a multitubular cyclone. Emissions at coal combustion is not available	84	257	n/a	Li et al. (2009)
Woody residues in a 4 MW moving grate boiler, decreased boiler capacity (2.3 MW). Nitrogen content 0.19 wt% (dry base)	n/a	177	65	Razmjoo, Sefidari, and Strand (2016)
29 MW traveling grate boiler of a heating plant in China. Nitrogen content 0.68% (as received), total sulfur content 0.95 (as received) (re-calculated, NOx was obtained as a sum of NO and NO2 as follows NOx = NO2 + NO×46/30, calculated using flue gas temperature 125.4°C and O2 = 13.9 v%)	792	518	n/a	Chen et al. (2017)
Co-combustion of coal and sewage sludge (0–25 wt%) in a stoker-fired boiler, capacity was not provided. Nevertheless, according to the authors, results were comparable with industrial heating-boiler of thermal power about 16 MW. Nitrogen content 0.24 wt% (as received) for coal and 0.45 wt% (as received) for sludge, combustible sulfur content 0.8 wt% (as received) for coal and 0.9 wt% (as received) for sludge				Nadziakiewicz and Kozioł (2003)
As above, 0 wt% sewage sludge	1,275	253	n/a	Nadziakiewicz and Kozioł (2003)
As above, 10 wt% sewage sludge	1,395	299	n/a	Nadziakiewicz and Kozioł (2003)
As above, 20 wt% sewage sludge	1,523	403	n/a	Nadziakiewicz and Kozioł (2003)
Co-firing of “densified refuse derived fuel” (RDF-5) with bituminous coal, paper sludge, and waste tires 130 ton/h steam circulating fluidized bed co-generation boiler (thermal output of 103 MWth), furnace temperature 850°C. Flue gas treatment unit was not described. Nitrogen content (as received) 1.27 wt% for coal, 0.36 wt% for waste tires, 0.35 wt% for paper sludge, and 0.25 wt% for RDF-5; combustible sulfur content (as received) 0.9 wt% for coal, 2.17 wt% for waste tires, 0.22 wt% for paper sludge, and 1.08 wt% for RDF-5				Wan et al. (2008)
As above, mass share of co-firing fuels: coal 55 wt%, waste tires 23 wt%, paper sludge 22 wt%, RDF-5 0 wt%	478	159	57	Wan et al. (2008)
As above, mass share of co-firing fuels: coal 39 wt%, waste tires 23 wt%, paper sludge 22 wt %, RDF-5 16 wt%	473	132	86	Wan et al. (2008)
This study, hard coal only	1268	397	38	This study
This study, hard coal + 10 wt% of alternative fuel	1070	471	89	This study

The gaseous pollutant emission values obtained from the tests occurred because the WR-25 boiler installation (in which the tests were carried out) does not have a flue gas cleaning system adjusted to the emission requirements for incineration/co-incineration of waste. Therefore, the potential retrofitting of the boiler should also take into account the flue gas cleaning system.

During previous investigations (Wasielewski, Głód, and Telenga-Kopyczyńska 2018), the experiments were carried out at higher capacity of boiler, i.e., 17.352 MW_t and 18.835 MW_t for hard coal and coal + waste (10% by weight). The type of boiler (i.e., WR-25 but in different localization) and the share of waste in fuel mixture (i.e., 10% by weight) were similar. Despite some similarity between the experiments presented in literature (Wasielewski, Głód, and Telenga-Kopyczyńska 2018) and in this study, the comparison of the results from these investigations is limited. The characterization of waste in the compared studies (Wasielewski, Głód, and Telenga-Kopyczyńska 2018) is not the same. For example, chlorine content in the mixture of coal and waste in previous study (Wasielewski, Głód, and Telenga-Kopyczyńska 2018) was 0.39 w%, whereas in this study chlorine content is 0.556 w% (see Table S1 in Supplementary Material). Nevertheless, we try to compare the mentioned results using specific co-efficients. More specific analysis is in terms of emissions of pollutants into the air as well as the guidelines for boiler retrofitting are presented in the Supplementary Material.

Conclusion

This research has shown that in boiler installations equipped with the WR-25 grate boiler, which is the most frequently used unit in the domestic heating industry, energy recovery from waste cannot be carried out in an environmentally safe manner without a modernization of the boiler structure and a flue gas cleaning system. Nevertheless, it is worth considering the retrofitting of the boiler and the entire flue gas cleaning system. The reduction of emissions of main pollutants and dust could be achieved by using the simultaneous method of removing NO_X and SO₂ in a wet electrostatic precipitator. Emission reductions can also be obtained by the simple decrease of waste share, and waste thermal processing (such as by torrefaction). However, for the last option, it would be necessary to build an independent installation for waste processing, with all the environmental requirements.

Co-combustion of alternative fuel with hard coal should be carried out in a boiler installation dedicated to co-combustion of waste, with structural and technological solutions ensuring compliance with process and emission requirements for thermal waste processing. The research presented herein is a reflection of experience in co-incineration of municipal waste at full scale. Currently, many WR-25 type boilers are operated in Poland and abroad, which may be used in plants applying for the status of waste co-incineration plants after undergoing certain modifications. This research was carried out with the use of a full industrial scale boiler, with its full integration in a system supplying heat to individual consumers. These tests are valuable from the point of view of retrofitting the unit to obtain technological and emission parameters that would permit obtaining the status of a waste co-incinerating unit. Due to quite high total capacity of WR-25 boilers (190 × 35 MWth units in different localizations) and the status of “distributed energy sources” these boilers would be used as a very efficient way to accomplish the “waste-to-energy” aim and to reduce waste landfilling. This research is the first stage in the process of retrofitting the boiler (and the whole system) to achieve the requirements for waste co-incineration plants.

Acknowledgment

The authors gratefully acknowledge the National Centre for Research and Development and the National Fund for Environmental Protection and Water Management, agreement No. GEKON/O5/268002/17/2015; Ministry of Science and Technology, Taiwan, as well as National Centre for Research and Development, Poland, agreement No. PL-TW IV/4/2017; the Ministry of Science and Higher Education (IChPW no. 11.20.011).

Disclosure statement

The authors declare no conflict of interest.

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website.

Additional information

Funding

The presented research was conducted within the framework of the research project GEKON/O5/268002/17/2015, which was financed by the National Centre for Research and Development and the National Fund for Environmental Protection and Water Management: “EkoRDF—an innovative technology of producing alternative fuels from municipal waste for power plants and combined heat and power plants—a key element of the waste management system in Poland.” The research was also supported by the project, “Utrzymanie potencjału badawczego Centrum Badań Technologicznych” (IChPW no. 11.20.011), financed by the Ministry of Science and Higher Education, as well as by the project entitled “Towards the enhancement of an application of municipal solid waste (MSW) in energy sector” National Centre for Research and Development, Poland, agreement No. PL-TW IV/4/2017.

Notes on contributors

Ryszard Wasielewski

Ryszard Wasielewski Ph.D. in environmental engineering, Silesian University of Technology (2012) Expert in the field of waste management and thermochemical conversion of waste. The author of over 200 publications and 10 patents.

Krzysztof Głód

Krzysztof Głód M.Sc. in Innovative Processes and Energy Management, the Silesian University of Technology (2000) Head of teams carrying out research projects in the field of fuel combustion, co-firing processes and waste co-incineration.

Janusz Lasek

Janusz Lasek Ph.D. in metallurgy, Silesian University of Technology (2009), Habilitation (D.Sc.) in Engineering and Technology / Environmental Engineering, Mining and Energy (2020). Over 15 year experience of scientific activity including combustion and co-combustion of biomass, waste and fossil fuels as well as methods to reduce NOx emission from the combustion processes carried out in stationary furnaces.