The effect of mineral additives on the process of chlorine bonding during combustion and co-combustion of Solid Recovered Fuels

Waste Management 102 (2020) 624–634

Contents lists available at ScienceDirect

Waste Management journal homepage: www.elsevier.com/locate/wasman

The effect of mineral additives on the process of chlorine bonding during combustion and co-combustion of Solid Recovered Fuels Arkadiusz Szydełko ⇑, Wiesław Ferens, Wiesław Rybak Wroclaw University of Science and Technology, Faculty of Mechanical and Power Engineering, Wrocław, 50-370, Poland

a r t i c l e

i n f o

Article history: Received 13 December 2018 Revised 15 October 2019 Accepted 16 October 2019

Keywords: Alternative fuels Halloysite New source of energy Solid recovered fuels (SRF) Sewage sludge

a b s t r a c t The use of solid recovered fuels (SRF) is often associated with an increased risk of chloride corrosion because these fuels can be high in chlorine and alkali. One way to reduce the risk of chloride corrosion is the co-combustion of fuel mixtures in the presence of a mineral additive containing chemical compounds that bind the emitted chlorides. This paper concerns the influence of mineral additives – constituting waste material, on the process of the binding of emitted chlorides. One of these waste materials could be a halloysite, not yet used in power industry. The research has shown its high chloride binding effectiveness, comparable to that of kaolin (known in literature as a mineral additive effectively binding chloride). Moreover, the studies have shown that SRF combustion in the presence of stabilized, dried and granulated sewage sludge, with a 25% mass fraction of sewage sludge, allows reducing chloride emissions. Studies are based on tests in a calorimetric bomb. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Solid recovered fuels (SRF) are produced in order to recover energy and use it for combustion or co-combustion with conventional fuels, such as bituminous coal, brown coal, crude oil or natural gas in existing boiler systems (Andersson and Johnsson, 2005; Autret et al., 2007). The SRF use is beneficial for many reasons: it generates energy from a calorific source such as municipal waste, saves natural resources, and reduces waste landfill. The efficient use of solid recovered fuels in the power industry requires a knowledge of the behaviour of these fuels during combustion and co-combustion with conventional fuels, as well as overcoming the difficulties inherent in the specific properties of these fuels. The utilization of municipal waste can be accompanied by an increased risk of deposit formation due to the presence of large amounts of inorganic substances containing sodium and potassium compounds that affect the reduction of the ash melting temperature (Iacovidou et al., 2018). SRF contain chlorine compounds, which during combustion, accelerate the corrosion of installation. The occurrence of chlorine in alternative fuels is diversified in terms of quantity and quality. Chlorine can occur in various compounds in biomass and plastics – chlorine in the biomass occurs mainly in the form of various compounds of mineral salts, while in plastics (mainly PVC) it is ⇑ Corresponding author. E-mail address: [email protected] (A. Szydełko). https://doi.org/10.1016/j.wasman.2019.10.032 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.

bound in polymers. SRF fuels come from sorted manucipal waste, which includes inter alia, plastic waste (various types of packaging, films, bottles, small items), paper, biological waste (food scraps, kitchen waste), fabrics composite waste and other. Each of these wastes is characterized by a different variable content of chlorine and a form of occurrence of chlorine. The resultant share of chlorine in the fuel combustion, a part of the chlorine is separated in the gas form mainly as HCl (mainly biological waste) and KCl (plastics), NaCl (kitchen waste), some may remain in the residue after burning in a solid form. Studies on finding solutions to reduce the risk of chloride corrosion involve the monitoring of the installation during the process of alternative fuels co-combustion and taking preventive measures. The evaluation of corrosion risk is to be carried out by physical and chemical analyses of fuel burned to determine the chemical composition of fly ash and solid deposits formed on the surfaces of heating tubes and to measure the thickness of the metallographic analyses of pipes. It is also possible to assess the progress of corrosion by continuously measuring the concentration of KCl. This method is very expensive and requires interaction with the KCl neutralization system induced by injecting appropriate additives (Grabke et al., 1995). One of the effective methods that allows reducing the chlorine content in fuel is the segregation of municipal solid waste during the production of SRF by removing materials with a high chlorine content such as PVC. The preventive measures include the use of protective coatings and the use of chemical compounds or powdered minerals that bind chlorides,

A. Szydełko et al. / Waste Management 102 (2020) 624–634

thus enabling KCl to be neutralized. These substances can either be injected into the furnace or added to the fuel, and it is important that they do not reduce the ash melting temperature. The neutralization of chlorides can be accomplished by the sulphation of potassium cations (Henderson 2006) or the binding of chloride anions by compounds containing aluminium, silicon, or calcium (Antonangeli et al., 2017). One of the methods is to use of additives which may reduce the chlorine emission from the combustion system by its binding through various mechanisms depending on the combustion conditions, the form of chlorine compounds and the form of additives. KCl and NaCl can react with aluminosilicates to form solid compounds with the separation of HCl. The binding of HCl is possible in the presence of calcium oxide or support KCl reaction with aluminosilicates. It is currently well known that aluminosilicates, silicas and alkaline aluminosilicates, mainly natural minerals such as kaolinite, are preferred (Ramasamy et al., 2016). New additives that effectively bind chlorides while burning alternative fuels (which are waste materials of various processes) are sought (Wang and Massoudi, 2013). It was observed that one of such additives is sewage sludge (Karlsson et al., 2015) – its use during the co-combustion of biomass with SRF (78% bark pellets + 22% SRF) allows for the reduction of corrosion and the amount of slag formed. Based on this observation it was concluded that in the mineral substance of sewage sludge there may be compounds that effectively bind chlorides. The demonstration of the effectiveness of sewage sludge for binding chlorine would allow identifying a new chloride-binding additive and to demonstrate a new use of sewage sludge of which production is growing each year (in the next 8 years its quantity in Poland will increase by 14.5%). The aims of the present research, carried out in the calorimetric bomb, are:
Examination of the presence of chlorine compounds in alternative fuels and their main morphological ingredients
Assessment of the risk of corrosion occurring during the combustion of alternative fuels
Determination of the chloride binding effectiveness of mineral additives: bentonite, halloysite, longituindal wollastonite (mineral deposits are common in Poland) during the combustion of alternative fuels and the comparison of chloride binding effectiveness by these minerals and minerals known in the literature: dolomite, kaolin, quartz powder and limestone powder.
Determination of the efficiency of chloride binding by sewage sludge during the combustion of solid recovered fuels. The demonstration of this property of sewage sludge would allow using them as fuel additives in the combustion of fuels, which would also help in sewage sludge management.
Proposing a new method in order to determine the effectiveness of chloride binding by mineral additives by co-combustion them with alternative fuels in a calorimetric bomb. 2. Materials 2.1. Investigated fuels To achieve the research objectives, a test material consisting of 10 samples of alternative fuels and 2 coals, was selected. The investigated alternative fuels included:
6 samples of solid recovered fuels (SRF) – from different regions of Poland
Sewage sludge (SS)
3 samples of waste separated from municipal waste which are components of municipal solid waste: – Cardboard waste (CW),

625

– Mixed paper (MP), – Polytetraethylene plastic (PET), The coals, i.e. brown coal (BRW) and bituminous coal (BIT) were the subject of research as reference fuel. The description of the investigated fuels is provided in Table S1. Two coals, used in the power industry, bituminous coal and brown coal, were used in these investigations. The bituminous coal and the brown coal were selected for testing as the representatives of the main conventional fuels combusted in Poland and also to form fuel mixtures with SRFs. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.wasman.2019.10. 032. The main criterion for the choice of alternative fuels for research was the location of their occurrence. The investigated alternative fuels produced from municipal solid waste came from those regions in Poland where, according to data presented in the National Waste Management Plan, the amount of municipal waste available is considerably higher than the processing capacity of municipal waste thermal treatment plants. For economic reasons, it is worth investing in the construction of thermal treatment plants or using alternative fuels to co-fire them with coal in power plants and CHP units. Sewage sludge (SS) was chosen (apart from SRF) to compare physicochemical properties and behaviour during combustion. Sewage sludge is also a waste and an alternative fuel. Three wastes separated from municipal waste were also tested as homogeneous materials: (1) cardboard waste (CW); (2) mixed paper (MP) and (3) plastics (PET) – the main morphological components of SRF, which can be served as components for the production of SRFs with desired physicochemical properties. Waste paper and plastics are also two extremely different fractions of SRF: the biomass-rich fraction (biodegradable fraction) and the non-biomass fraction (nonbiodegradable fraction).

2.2. Sample preparation Fuel samples were taken according to the methodology (Malijonyt et al., 2016) of alternative fuels collection used in the plants from which they were derived. The samples were then transported in sealed plastic containers to the laboratory. For the preparation of representative test samples, material was collected from several sites (this is particularly important due to the heterogeneity of SRF) and then thoroughly mixed. Fuel samples with the consistency of a loose material, such as coal and granular stabilized sewage sludge, were averaged with the recommended quartering technique. After delivering the samples to the laboratory, the content of the containers was poured into specially prepared laboratory trays, weighed and dried in a well-ventilated room. Alternative fuels (Table S1), after sufficiently long periods of time and equilibrium with moisture and transient moisture, were ground (SRF is difficult to grind). Due to the mechanical properties of SRF and PET fuels, resulting from the high content of plastics, the most difficult stage of testing was crushing them. Many types of mills and steps of grinding had to be used to grind alternative fuels. Firstly alternative fuels were pre-milled in a laboratory mill (model SK100, Retsch) and then ground in a cryogenic mill (CryoMill, Retsch) at 196 °C. At sufficiently low temperatures, the plastics contained in alternative fuels change their mechanical and physical properties to become more brittle and fragile. In this way, an alternative fuel sample is crushed and the volatile matter contained in individual materials is retained. Bituminous coal, brown coal and sewage sludge were ground in a laboratory mill (SK100, Retsch) designed for conventional fuel grinding, to a particle size of less than 200 lm and placed in sealed containers.

626

A. Szydełko et al. / Waste Management 102 (2020) 624–634

2.3. Minerals and substances used as additives Minerals differing in chemical composition were studied: (1) aluminosilicates (bentonite, halloysite, kaolin) containing mainly Al2O3 and SiO2; (2) silicate (longitudinal wollastonite) containing SiO2 and CaO; (3) quartz (quartz powder) containing SiO2; and (4) carbonates (limestone, dolomite) minerals rich in CaO and CO2. These minerals came from mines located in Lower Silesia and the Opole region, both of which are rich in natural minerals. So far, in the literature there has been no information about the efficiency of bentonite, halloysite or longitudinal wollastonite used as chloride-binding additives during fuel combustion, hence dolomite and kaolin, whose efficiency is well known, were used as reference minerals. In addition, some pure substances, which are the main constituents of the minerals, were used: calcium carbonate. It allowed checking the influence of these minerals on the chloride binding. Moreover, sewage sludge was used to check its effectiveness of chlorine binding – sewage sludge consists of a combustible substance and contains a significant amount of mineral substance (from 20 to 60% of dry matter) with a various composition resulting from the technology of wastewater treatment and the area from which it comes (agricultural, urban, industrial areas). Additionally, it may contain substances derived from its hygienization (e.g. addition of calcium oxide). Therefore, the composition of sewage sludge may contain mineral compounds found in minerals used as additives in combustion processes. Co-combustion of sewage sludge with biomass or RDF fuels allows its disposal and may additionally reduce the chlorine emission from the combustion of fuels with an increased chlorine content. Sewage sludge also contains chlorine and sulfur and their contribution should be taken into account to assess the impact of sewage sludge on the combustion system. The compositions of used mineral additives and sewage sludge are shown in Table S2.

3. Methodology 3.1. Physicochemical properties of investigated fuels and mineral additives Moisture content was determined according to PN-EN 15414-12-3, and for coals PN-G-04511:1980 was used. Ash content for SRF was determined according to PN-EN 15403, and for other fuels – PN-G-04512:1980/Az1:2002 was used. Volatile fractions of SRFs were determined according to PN-EN 15402, and in the case of other fuels PN-G-04516:1998 was used. The net calorific value (NCV) and gross calorific value (GCV) were determined by the combustion of fuel samples in a calorimetric bomb and the determination of GCV using IKA C2000 calorimeter according to PN-G-04513: 1981. The ultimate analysis of fuels was carried out using Leco TruSpec CHNS automatic analyser. The carbon, nitrogen and hydrogen content of SRF1-6 was determined according to CEN/TS 15407, and for other fuels – to PN-G-04571. The sulphur content in all fuels was determined by PN-ISO 351: 1999, while oxygen was calculated from differences. Chlorine content in SRF was determined/ set in accordance with CEN EN 15408:2008. A portion of 1 g of fuel (particle diameter < 1 mm) was burned in a calorimetric bomb in the presence of oxygen. The gases formed during combustion were dissolved in 10 ml of the absorbing solution previously introduced into the calorimetric bomb. If the percentage of chlorine in the dry matter was greater than 1%, the 0.2 M KOH was used as an absorbent solution, and if the chlorine content was less than 1%, deionized water (H2O) was used. The obtained solution was then subjected to chlorine studies. Chloride content in the obtained solutions was determined by ion chromatography, in a Dionex

ICS-2100 chromatograph equipped with an IonPac AS22 anion exchange column. The carbonate-bicarbonate eluent was used as a mobile phase. Chloride detection was carried out by means of a conductivity detector. The obtained results were calculated and presented as the percentage of chlorides in the sample dry matter by dependence (1):

w¼

ðC  C 0 Þ  V  R  100 m

ð1Þ

where: W C C0 V Rr m

total chlorine content in the dry mass of the sample, % concentration of chlorides in absorbent solution, mg/dm3 concentration of chlorides in the blank sample, mg/dm3 volume of solution, dm3 dilution of solution, – dry sample mass used for studies, mg

The calorimetric bomb method was applied due to the full process control: conditions, amount of fuel, fuel feeding method, limitation of separation of individual fuel fractions during dispensing. The use of a bomb allowed to carry out more tests and to maintain the repeatability of the combustion process. The conditions of the conducted experiment are identical to those for measurements of chlorine determination in a calorimetric bomb (according to PN EN 15408: 2011). During combustion, a significant excess of oxidant is available (over 3 times), the process is conducted at a pressure of 2 MPa. The adiabatic temperature for all analysed fuels was estimated. The calculations were based on the heat balance generated during the combustion of 1 g of fuel. This heat was used to raise the temperature of reagents and combustion products. On the basis of reagents quantity and specific heat, the temperature after the process was determined. The FACTSage program was used for calculations. The calculations showed that for the fuels tested, the adiabatic temperature varied from 224 °C for sewage sludge to 264 °C for SRF2 fuel. Such conditions of the experiment ensure complete combustion and separation of all chlorine compounds in the gas form. The determination of chlorine according to this method is comparable with the results obtained by other methods (Ma et al., 2010). Conducting measurements under the calorimetric bomb conditions ensures repeatability of measurement conditions, tests are carried out for a constant mass of fuel, set pressure and oxidizer temperature, ignition of a fuel sample takes place in an identical manner. The only factor affecting the reproducibility of results is the heterogeneity of RDF fuel samples resulting from its origin. Among the fractions of this fuel there are those with a low chlorine content – e.g. wood waste (<0.1%) as well as high chlorine content (e.g. PVC with a chlorine content exceeding 5%) resulting in averaging chlorine content of about 0.5–1.2%. Small changes in the PVC content in the sample (by several percent) lead to many times greater changes in the chlorine content (up to 10%). The tests were repeated a sufficient number of times to determine the repeatability. The value of the standard deviation related to the average value does not exceed 7% for the tested samples. Transferring the results of the experiment to real conditions is limited due to different combustion conditions in common technical solutions. Repeatable conditions of the experiment allow indicating additives that in the ideal combustion conditions show the highest efficiency (creating a ranking of mineral additives). The use of the additive in the chosen technical solution should be preceded by real-scale tests. According to currently CEN classifications of SRFs the mercury contents in SRFs were also determined (CEN EN 15411:2008) by using atomic absorption spectrometer for mercury determination

A. Szydełko et al. / Waste Management 102 (2020) 624–634

in solid and liquid samples employing amalgamating techniques (LECO AMA254). The measurement of mercury consists of three stages. At the first stage, a sample is dried and then burned in an oxygen stream. At the second stage, the released mercury vapour passes through the catalytic column and is trapped by the amalgamator. In the third step, mercury is released from the amalgamator and measured in both measurement cuvettes by atomic absorption at 254 nm. The determination of the oxide composition of investigated mineral additives was conducted by ICP-OES atomic spectroscopy using ASCRM-010 as a reference substance. Prior to the analysis, the minerals samples were dried, milled and sieved on a <200 lm sieve. The obtained results of the chemical elements were converted to the corresponding oxides to obtain the oxide content of the mineral.

627

  wD  100 Ef ¼ 1  wPA

ð2Þ

  wDM  100 Ef ¼ 1  wPA

ð2Þ

where: Ef wDM

wPA

effectiveness of chloride binding by a mineral additive, % chloride concentration with respect to the dry mass of fuel, emitted during fuel combustion in the presence of a mineral additive, % chloride concentration with respect to the dry mass of fuel, emitted during fuel combustion without a mineral additive, %

3.2. Determination of the efficiency of chlorine binding by additives The purpose of the study was to determine the effectiveness of chlorine binding during fuel combustion with mineral additives and sewage sludge. The study was carried out by burning a sample of fuel in the presence and absence of a chlorine-binding additive in a calorimetric bomb filled with oxygen. Analytical samples were made from fuels in the presence of a grounded mineral additive or sewage sludge and without additives. The total mass of the sample was 1 g and additives constituted 5% of the sample mass. The used percentage of SRF in the mixture should not cause problems during the combustion process and operation of the equipment. In addition, increasing the percentage of alternative fuel in a mixture with primary fuel is not recommended, as there may be problems in ensuring sufficient amount of waste generated in the environment of the power plant. Transporting municipal waste from further areas would significantly increase costs, due to the low energy density of alternative fuels from municipal waste, amounting to about 1500 MJ/m3 (for comparison, the energy density of hard coal is about 26,000 MJ/m3), which would make the whole process unprofitable. (1) Combustion in the presence of a mineral additive was subjected to the following materials and fuels in three steps: Studies on the effectiveness of chlorine binding using mineral additives were carried out. PVC, because of its high chloride content (10 times more chlorides than SRFs) and high homogeneity, was combusted in the presence of particular minerals additives. (2) Mineral additives with the highest chloride binding efficiency were used for the combustion of selected Solid Recovered Fuels SRF1-6. (3) Studies on chloride binding efficiency using the sewage sludge chloride during the PVC and SRF1 combustion were conducted. Sewage sludge can be a waste with a high content of aluminium silicate in mineral substance (Hrycaj, 2014). It could potentially be used during the SRF combustion as a material that reduces the amount of emitted chlorides. In addition, sewage sludge is a cheap, readily available material. The possibility of using sewage sludge during the combustion process would also be an effective solution for sewage sludge management. In order to determine the binding efficiency of the chloride compound, emitted during the fuel combustion, a new parameter was determined – the efficiency of chloride binding (Ef). Ef index allows comparing the percentage of chloride binding of minerals with the total chlorides emitted during fuel combustion without the presence of a mineral. Chloride binding efficiency was determined from formula (2):

4. Results and discussion 4.1. Physicochemical properties of the investigated fuels Table 1 shows the values of the parameters determined for fuel samples. Increased moisture content in SRF may be due to a greater content of the bioorganic fraction, as indicated by the relatively high moisture content in mixed paper and cardboard waste. Probably the low moisture content in SRF2 was due to the fact that the fuel was plentiful in plastics. SRF moisture content also depends on the time of year and the region from which the waste came. The granular dried sewage sludge had a similar moisture content to the SRF. Because the sewage sludge was in a stabilized dried form, it contained less moisture than the normally produced sewage sludge (Hrycaj et al., 2007). The amount of ash generated by the combustion of solid recovered fuels (SRF) corresponded to the amount of ash obtained when brown coal was burned (16.1%), with the exception of SRF5 (7.9%), where the amount was two times lower and close to the amount of ash obtained for bituminous coal (8.8%) and mixed paper pulp (9.5%). The combustion of sewage sludge led to as much as 34.9% of ashes, suggesting that ashes can be significantly increased during the co-combustion of conventional fuels with sewage sludge. The lower ash content obtained for SRF2 (12.1%) compared to SRF1 (17.0%) and SRF4 (16.4%) was probably due to the significantly higher content of plastics – low ash content after the combustion of plastics PET (3.4%) also confirms this observation. The content of volatile matter in alternative fuels is much higher than in studied coals. The amount of volatile matter in SRF ranged from 68% to 82% (the lowest in SRF1) and was approximately 2.5 times higher than in bituminous coal (28.7%) due to the presence of high content plastic fractions in SRF (the amount of volatiles for PET is 87.3%). Fixed carbon (FC) in SRF (about 10%) and PET (9%) are significantly lower than for bituminous coal (62%) and brown coal (35%), which results from the high content of volatile matter (Speight, 2005). The NCV and GCV of the SRF were high (up to 28 MJ/kg), close to the value for bituminous coal (32 MJ/kg). Fuel rich in biodegradable fraction of biomass, i.e. mixed paper (16.3 MJ/kg), cardboard waste (16.5 MJ/kg) and sewage sludge (13.8 MJ/kg) have significantly lower GCV values. However, GCV of PET plastics (26.8 MJ/kg), indicates that plastics-rich of SRF would be considerably more caloric than those of SRF fuels which have significant amounts of biomass. The ultimate analysis (Table 1) showed that the carbon content of fuels was in the range of 29% to 65%, with most of the carbon contained in PET (64.7%) and bituminous coal (60.1%). As a conse-

628

A. Szydełko et al. / Waste Management 102 (2020) 624–634

Table 1 Results of proximate analysis and ultimate analysis, and fuel calorific value (analytical state).

* ** ***

Sample

Mtot %

M* %

A %

VM %

FC %

GCV MJ/kg

NCV MJ/kg

C %

H %

N %

S %

O*** %

Cl** %

Hg** mg/kg

SRF1 SRF2 SRF3 SRF4 SRF5 SRF6 CW MP PET SS BRW BIT

26.5 19.1 20.1 30.2 28.7 23.1 4.6 6.0 16.8 43.2 35.1 10.3

3.5 0.9 3.1 4.1 3.9 2.3 3.1 3.4 0.4 2.8 4.4 0.6

17.0 12.1 13.9 16.4 7.9 13.7 9.5 14.5 3.4 34.9 16.1 8.8

68.3 78.6 74.8 71.0 79.2 78.2 74.0 68.9 87.3 51.4 44.4 28.7

11.2 8.4 8.2 8.4 9.0 5.7 13.3 13.2 8.9 10.9 35.0 61.9

22.9 28.1 24.6 22.0 24.4 26.8 16.5 16.3 26.8 13.8 20.0 32.0

21.3 26.6 23.1 20.7 22.6 25.0 15.2 15.0 25.2 12.7 18.9 31.1

51.3 57.5 53.6 39.3 51.4 55.1 41.8 42.6 64.7 55.2 55.2 60.1

6.6 8.4 6.6 5.4 7.6 7.8 5.6 5.5 7.1 4.2 4.5 4.1

0.7 0.4 0.9 0.7 0.3 1.0 0.3 0.2 0.1 0.6 0.6 1.4

0.2 0.4 0.3 0.2 0.1 0.2 0.3 0.1 0.2 1.1 1.8 0.4

20.2 19.7 20.4 33.7 28.7 19.8 39.4 33.6 22.9 1.1 17.3 24.4

0.5 0.6 1.2 0.2 0.1 0.1 0.04 0.05 1.2 0.1 0.1 0.2

1.17 0.32 0.33 0.15 0.05 0.18 0.07 0.06 0.05 0.83 0.21 0.13

As received state. Dried state. The oxygen content determined by difference.

quence, alternative fuels rich in plastics exhibited higher carbon content (especially the SRF2). The carbon content of the SRF samples (except SRF4) remained at the same level as in brown coal (51–58%). Alternative fuels SRF4 and biomass fuels (mixed paper, cardboard waste and sewage sludge) have a lower carbon content (40–55%). Solid recovered fuels turned out to be fuels richer in hydrogen than other fuels. Hydrogen content in alternative fuels (7%) and PET (7%) was higher than in coal (4.5%), sewage sludge (4.2%) and higher than waste paper (about 5%). This is due to the presence of plastic fractions containing long chain polymers. The exception was SRF4 (5.5%) which contained significantly less hydrogen. Oxygen content in SRF samples (20–34%) was higher than that of brown coal (17.4%) and sewage sludge (17.8%). In other fuels, the oxygen content was similar to that in alternative fuels. The nitrogen content of SRFs (approx. 0.5%) was much smaller, even three times lower than in bituminous coal (1.4%). The lowest nitrogen content compounds were found in PET plastics (0.1%) and the largest in sewage sludge (5.3%). Solid recovered fuels contained even 4 times less sulphur (0.1%) than bituminous coal (0.4%), 17 times less than brown coal (1.8%) and 12 times less than sewage sludge (1.24%). Based on these observations, it can be stated that the amount of emitted sulphur compounds during the cocombustion of coals with alternative fuels would be lower than that of pure coal. The level of chlorides in PET plastics (1.2%) and some SRFs, i.e. SRF1 (0.5%), SRF2 (0.6%) and SRF3 (1.2%), was significantly higher than in bituminous coal (0.2%), brown coal (0.1%) or sewage sludge (0.1%). Due to the low content of chlorides in alternative fuels, SRF4 (0.2%), SRF5 (0.1%) and SRF6 (0.1%), it was concluded that solid recovered fuels do not always have to possess high chlorides content. The highest content of mercury was found in SRF1 (1.2 mg/ kg) and sewage sludge (0.8 mg/kg). In other alternative fuels, the amount of mercury did not exceed 0.35 mg/kg. The high content of mercury in SRF1 compared to the other alternative fuels indicated that this fuel could be contaminated with mercury as a result of occurrence wastes containing high level of mercury (electronic elements, thermometers, fibres, compact fluorescent lamps).

4.1.1. Classification of studied SRF fuels The current SRF fuel classification system consists in giving fuels codes that take into account the calorific value of fuel and the content of chlorine and mercury compounds in it. This is to enable contact between fuel producers and their potential recipients. Classification of fuels produced from municipal waste in accordance with EN-15359: 2012 allows creating of 125 different codes.

The methods for determining the content of mercury and chlorine in the fuels are described in 3. Methodology. The calorific value (NCV) was determined by burning fuel samples in a calorimetric bomb and determining the heat of combustion using the IKA C2000 calorimeter according to PN-G-04513: 1981. Based on the obtained results, presented in Table 1, the SRFs were classified as shown in Table 2. The studied SRF in terms of calorific value belonged to the 1st or 2nd class of fuels, which indicates by their high calorific value. In terms of chloride content, they also belonged to the 1st or 2nd class (with the exception of SRF4 belonging to the 4th class), which means quite low chloride levels (<0.6%). The mercury content in the tested SRFs was also very low (<0.02 mg/MJ), which corresponds to the first fuel class – the exception was SRF1, which contained much more mercury (class 3). Based on the obtained results, it was found that produced SRFs was a fuel with very high energy values, low mercury content and a relatively low level of chlorides. 4.2. Chemical composition of minerals and substances used as additives Determination of the composition of the oxide mineral is one of the basic tests for the determination of the level of compounds that can bind chlorides. Based on the spectrometric analysis, the elemental compositions of the examined minerals and sewage sludge were determined and then their contents were calculated into the percentage of their respective oxides. The elemental composition of the examined minerals and sewage sludge is shown in Fig. S1. The obtained results showed that bentonite contained mainly SiO2 (68.6%) and Al2O3 (14.9%) and a considerable amount of Na2O and K2O. Halloysite also consists mainly of SiO2 (42.1%), Al2O3 (33.2%) and also contains a significant amount of Fe2O3 (8.1%). Wollastonite longitudinal consists mainly of SiO2 (50.2%) and CaO (45.4%), dolomite – CaO (32.5%), MgO (19.5%) and CO2 (35.2%), whereas kaolin of SiO2 (52.1%) and Al2O3 (33.4%). The composition of sewage sludge ash was similarly to composition of minerals additives. Sewage sludge contained mainly SiO2 (46%), Al2O3 Table 2 Classification of studied SRFs. SRF

Class of SRF

SRF1 SRF2 SRF3 SRF4 SRF5 SRF6

NCV2; NCV1; NCV2; NCV2; NCV2; NCV1;

Cl2; Cl2; Cl4; Cl1; Cl1; Cl1;

Hg3 Hg1 Hg1 Hg1 Hg1 Hg1

629

A. Szydełko et al. / Waste Management 102 (2020) 624–634

(11.8%), CaO (9.2%). Ash of sewage sludge, in contrast to mineral additives, showed high content of P2O5 (12.8%), Fe2O3 (5.3%) and SO3 (3.3%). 4.3. Chlorine compounds and their behaviour during combustion Alternative fuels co-combusted with fossil fuels (Athanasiou et al., 2007; Baxter, 2005; Król, 2010) are typically characterized by significantly higher chlorides than commercial used coals. Chlorine can be present in SRF in both: plastics (mainly PVC) and biological materials. In the plastics fraction chlorine is present in chlorinated polymers, i.e. polyvinyl chloride (PVC) and Teflon. Chlorine contained in PVC represents up to 56% of the total weight of PVC. Chlorine contained in the biogenic fraction occurs mainly in mineral compounds in the form of soluble chlorides, mainly sodium, potassium, magnesium and calcium, and in small amounts in organic compounds. Alkaline chlorides contained in the biomass come from the soil solution are absorbed by the plants from the soil. Depending on the type of biomass, chlorine is present in up to 0.05% of wood biomass and up to 1% in the biomass of annual plants (grass, straw and rapeseed straw, etc.), which is related to the application of potassium fertilizers such as potassium chloride (KCl). Replacing potassium chloride with sulphate (K2SO4) can cause almost a fivefold reduction in chlorine content in biomass. Determining the content and forms of chlorine compounds is of particular importance for the use of fuels. The classification according to EN 15359:2012 of solid recovered fuels (SRF) produced from municipal solid waste encompasses the determination of the chlorine content in these fuels. The potential of a given fuel to generate threats associated with the corrosive consumption of boiler surfaces, HCl emissions, and the efficiency of wet desulfurization plants is determined by the content and forms of chlorine compounds. The determination of chlorine content in fuels is also important for predicting the amount of mercury emissions and its speciation during combustion (Urbanek et al., 2014). In order to estimate the proportion of chlorine compounds present in the form of organic polymers (contained in plastics) and in the form of salts, such as sodium chloride, potassium chloride, magnesium chloride and calcium chloride (chlorine contained in the biomass fraction), a sample of the fuel was separated by manual sorting for plastics and biomass fractions. Detailed particle analysis of fractions is shown in Table 3. Based on the obtained results it can be concluded that chlorine present in the polymers of the SRF1 made 63% of the total amount of chlorine compounds, while in the case of SRF2 as much as 80%. Chlorine contained in inorganic salt represented 37% of the total

Table 3 Content of Cl, Hg and gross calorific value (GCV) SRF1 and SRF2 and selected morphological fractions of these fuels. Fuel/fractions

Dry state Procentage %

Cl %

Hg ppm

GCV MJ/kg

SRF1 /paper /fiber /biomass /plastics /residue

– 21.1 7.2 3.4 50.1 18.3

0.5 0.6 0.2 0.6 1.3 0.5

1.2 1.9 2.8 0.9 1.7 2.8

22.9 15.5 21.8 17.7 29.9 13.1

SRF2 /paper /fiber /biomass /plastics /residue

– 2.2 14.0 14.0 62.5 7.3

0.6 0.2 0.8 0.2 0.4 0.5

0.3 0.2 1.4 0.2 0.1 0.9

28.2 16.5 27.9 22.7 32.7 20.1

chlorine content of SRF1 and 20% of the total chlorine of SRF2. Chlorine emitted during the co-combustion of solid recovered fuels can also be derived from the co-combustion of conventional fuel. In bituminous coal, chlorine is present in the form of chloride anions Cl binding in salts (sodium, potassium, calcium, magnesium and iron chlorides) or in organic chlorine compounds. During cocombustion of fuels, chlorine can be bound by aromatic organic compounds, leading to the formation of dioxins. Chlorine, contained in water filling spaces of carbon particles, forms free ions. These ions interact with carbonaceous materials and are more stable than water, thus they remain in the coal at higher temperatures than water molecules. Chloride ions can be attached to the carbon surface by weak bonds and then are released as HCl during pyrolysis (about 300 °C) or strong bonds, i.e. in alkali metal chlorides that crystallize at high temperatures and often evaporate only at temperatures significantly higher than the melting point of minerals. The content of chlorine in coal varies according to the geographic region in which the deposit is located (Vassilev et al., 2000). The content of chlorine in Polish bituminous coals ranges up to 0.4% and its amount increases proportionally to the salinity degree of groundwater and its degree of porosity. Chlorine derived from organic polymers or from mineral compounds in high temperature reacts with gaseous alkali oxides (i.e. Na2O(g), K2O(g)) and creates NaCl or KCl, molecular Cl2(g), and then in secondary reactions with water forms HCl(g). Alkali metal chlorides (sodium, potassium, calcium) can condense on fly ash particles or on heating surfaces of the boiler. This condensation phenomenon affects about 40–85% of total chlorine and depends on the used precipitator and on the alkali metal concentration. The remaining chlorine is emitted as gaseous hydrogen chloride (HCl(g)). In the case of the combustion process in a fluidized bed reactor, chlorides associated with fly ash are capable of interacting with the compounds of a fluidized bed material and create eutectics with them. The melting point of eutectics is significantly lower than the melting temperature of the bed (Grabke et al., 1995). This phenomenon can lead to bed agglomeration. In addition, during combustion chlorine can react with elemental mercury and form mercury chlorides that are bound by ash particles, which reduces the emission of elemental mercury in the exhaust gas (Acuna-Caro et al., 2006, Hrycaj et al., 2007, Urbanek et al., 2014). At low temperatures chlorides can form various polycyclic polychlorinated organic compounds, such as dioxins and benzofurans which are hazardous to human health and the environment. The corrosion risk assessment is performed by determining the chlorine, potassium and sulphur content of the alternative fuel presented in Table 1. Currently there are two ratings: Blomberg index (Af) (Blomberg, 2006) and Born index (PWk) (Born, 2005). The Blomberg index (called free alkali index) determines the amount of moles of alkali metals (sodium and potassium) excess in relation to the moles of sulfur and chlorine present in the fuel related to the calorific value of fuel. The fuel effluent index of Blomberg (Af) is determined in accordance with Eq. (3), however, the hazard criteria for chloride corrosion depending on (Af) are shown in Table S3.

Af ¼

Naþ þ K þ  2  S2  Cl GCV



½mol=MJ

ð3Þ

The Born chloride corrosion index (PWk) also includes chlorides, sulphur and alkali content in the fuel, however, it is not dependent on fuel mass. The criteria for determining the PWk index are presented in Table S3. Corrosion hazard indices (Af and PWk) were determined for the studied fuels and listed in Table 4.

630

A. Szydełko et al. / Waste Management 102 (2020) 624–634

Table 4 Classification of the risk of chloride corrosion according to Af and PWk indicators. Blomberg index Af [mol/MJ]

Chloride corrosion hazard

S/Cl

Na+

K+

PWk

Chloride corrosion hazard

0.012 0.008 0.009 0.014 0.007 0.007 0.004 0.005 0.011 0.006 0.001 0.006

Very high High High Very high High High Low Low Very high Low Very low Low

0.42 0.84 0.33 1.59 0.60 1.58 2.88 8.10 0.18 18.21 27.22 48.81

5.62 5.22 4.80 6.56 3.34 3.30 1.22 1.86 7.04 2.08 0.46 1.96

2.42 1.36 2.52 2.38 1.50 2.42 1.24 1.16 1.62 4.80 0.20 5.58

4 3 3 3 3 3 1 1 4 1 1 1

Very high High High High High High Very low Very low Very high Very low Very low Very low

According to the determined values of Af and PWk indexes, it can be stated that the risk of intense chloride corrosion during the combustion of SRF1-6 and PET is high or very high compared to the risk of corrosion during the combustion of conventional fuels. It is therefore necessary to seek solutions to minimize the occurrence of corrosion during the burning of solid recovered fuels. 4.4. Effect of mineral additives on chloride binding Mineral additives that can be used in the combustion of solid recovered fuels should not only have high chlorine binding efficiencies but also they should not reduce the ash fusing temperature (AFT). Such properties characterise such refractory minerals as quartz, metakaolitnite, mullite and rutile (Wang and Massoudi, 2013). Although such minerals as anhydrite, calcium silicate, hematite reduce chlorides, they probably may affect the reduction of the ash melting temperature, which may increase the intensity of slag formation. Currently, waste materials from various production processes are being sought, which confirms the effectiveness of chloride binding. Combustion in the presence of a mineral additive was subjected to the following materials and fuels. 4.4.1. Determination of the effectiveness of chlorine binding during the combustion of PVC in the presence of mineral additives In order to determine the effectiveness of chloride binding by the investigated mineral additives, the Cl rich material (PVC) was combusted. This material is homogeneous and contains more chlorine compounds than SRF (Szydełko, 2017). PVC combustion was carried out in the presence of: (1) mineral additives which ability to bind chlorides during the combustion of alternative fuels has been not demonstrated yet: bentonite (BT), halloysite (HAL), longitudinal wollastonite (LW); (2) mineral additives known in the literature for their ability to bind Cl: kaolin (KLN) and dolomite (DOL) and (3) pure chemical substances: calcium carbonate CaCO3 (CC) and ammonium sulfate (NH4)2SO4 (AS). The results of determination of chloride content in dry matter and the determination of chloride depletion in relation to PVC resulting from the combustion of PVC with mineral additives are shown in Fig. 1. According to the results, the effect of the used mineral additives during the PVC combustion on the level of emitted chlorides was observed. The highest effectiveness (33.5%) of chloride binding by minerals was observed during PVC combustion in the presence of halloysite. Using halloysite during combustion of PVC allowed reducing the Cl amount (by 43%) compared to Cl emitted during the combustion of pure PVC. The chloride binding effectiveness (Ef) of calcium carbonate was 39% during combustion of PVC. As a result of the PVC combustion in the presence of bentonite, the ability of this mineral to bind Cl was not demonstrated. Although this

mineral contained more alumina Al2O3 (15%) than longitudinal wollastonite (0.1%), due to the high levels of sodium oxide Na2O and potassium K2O (3%) and decrease of the AFT, the determined amount of emitted chlorides may be incorrect. According to obtained results, it can be concluded that the content of alkali metal oxides significantly influences the determination of the amount of emitted Cl. In these studies it was also shown that neutralization of KCl potassium chloride with calcium carbonate is more effective than in the reaction of its sulfonation or use of silicon oxides, as evidenced by the amount of Cl emitted during PVC combustion in the presence of pure calcium carbonate (39%), limestone powder (25%), quartz meal (6.5%) and ammonium sulphate (2.9%). However, aluminosilicate proved to be much better binding chlorides than calcium carbonate – the proof of this is a much higher value of Cl binding efficiency when burning PCV in the presence of halloysite (33.5%) and kaolin (22.4%) than in the presence of dolomite (13.7%). 4.4.2. Determination of the effectiveness of chlorine binding during the combustion of SRF1-6 in the presence of halloysite and kaolin Due to the fact that among the minerals, the highest chloride binding efficiency was observed for the combustion of PVC in the presence of halloysite, this mineral was selected for the further investigation of the chloride binding effectiveness during the combustion of SRF1-6. Another aluminosilicates – kaolinaluminosilicate-binding clay (known in literature as mineral binding chlorides during co-combustion processes (Xu et al., 2014)) was also tested. In order to compare the effectiveness of halloysite and kaolin for chloride binding emitted during the combustion of various alternative fuels formed from municipal solid waste, studies

Ef [%]

SRF1 SRF2 SRF3 SRF4 SRF5 SRF6 MP CP PET SS BRW BIT

Born index

45 40

35 30 25 20 15

10 5 0 -5 -10

HAL

KLN

BT

QM

LW

DOL

LM

SA

CC

kind of addives

Fig. 1. Effectiveness of chloride binding by mineral additives during PVC combustion.

Cl- [%]

A. Szydełko et al. / Waste Management 102 (2020) 624–634 0.9

0.8 0.7

2NaClðsÞ + SO2 + O2 + H2 OðgÞ ! Na2 SO4ðsÞ + 2HCl

ð6Þ

2NaClðsÞ + H2 OðgÞ + mSiO2 ! Na2 OmSiO2 + 2HCl

ð7Þ

In the presence of mineral additives, some of the chlorine can be bound to compounds found in solid form. Calcium carbonate at temperatures above 860 °C decomposes into CaO and CO2. At lower temperatures it can react directly with HCl to form CaCl2 (8):

0.6 0.5 0.4 0.3

CaCO3 + 2HCl ! CaCl2 + H2 O

0.2

ð8Þ

CaO can react with HCl by reaction (9):

0.1

CaO + 2HCl ! CaCl2ðsÞ + H2 OðgÞ

0 SRF1

SRF2

SRF3

SRF4

SRF5

100% SRF

10% KLN

CaOðsÞ + SO2ðgÞ + O2ðgÞ ! CaSO4ðsÞ

10% HAL

Fig. 2a. The amount of chlorides emitted during combustion of SRF1-6 in the presence of 10% of halloysite (HAL) or 10% of kaolin (KLN).

in which SRF1-6 was subjected to combustion in the presence of 10% halloysite or 10% kaolin were conducted. The results are shown in Figs. 2a and 2b. According to the obtained results it can be concluded that during the combustion of SRF chloride binding effectiveness is comparable to that of kaolin. In the combustion of all SRFs in the presence of halloysite the level of emitted chlorides was reduced as compared to the SRF combustion without the mineral additive. The largest loss of chloride level was observed for the SRF1. 4.4.3. The attempt to explain the chloride binding occurring during the use of mineral additives Chlorine binding can take place via various reactions depending on the temperature, the original form of chlorine compounds and the amount of reagents and the presence of other compounds (e.g. steam, sulfur). As a result of burning RDF type, chlorine passes into a gas form in the form of KCl, NaCl or HCl – for example, the decomposition of PVC leads to the release of chlorine by reaction (4):

(CH2 CHCl)n ! nHCl + nC2 H2

ð4Þ

KCl (or NaCl) can react with aluminosilicates or sulfur oxide to form solid compounds with HCl evolution by reaction (5)–(7):

Al2 O3 + 2NaClðgÞ + H2 OðgÞ ! 2NaAlO2ðsÞ + 2HClðgÞ

ð9Þ

SO2 can also bind CaO leading to the formation of CaSO4 (10):

SRF6

kinds of SRFs combustion with/or without KLN or HAL

ð5Þ

40

ð10Þ

High temperature of combustion process caused thermal degradation of materials contained in SRFs i.e. PCV, PET, biomass. Products of the degradation of plastics and biomass are respectively chloride anions (which forms i.e. HCl), and hydroxyl anions and cations of sodium/potassium (Cao et al., 2016; Sun, 2004). These ions can react with another compounds and be transported with volatile parts to different parts of the installation where they cause corrosion. Trapping chlorides anion and sodium/potassium cations by minerals additives allows remaining them in ash (Kassman, 2012) (11).

(CH2 CHCl)n ! nHCl + nC2 H2

ð11Þ

KCl (or NaCl) can react with aluminosilicates or sulfur oxide to form solid compounds with HCl evolution by reaction (12)–(14):

Al2 O3 + 2NaClðgÞ + H2 OðgÞ ! 2NaAlO2ðsÞ + 2HClðgÞ

ð12Þ

2NaClðsÞ + SO2 + O2 + H2 OðgÞ ! Na2 SO4ðsÞ + 2HCl

ð13Þ

2NaClðsÞ + H2 OðgÞ + mSiO2 ! Na2 OmSiO2 + 2HCl

ð14Þ

In the presence of mineral additives, some of the chlorine can be bound to compounds found in solid form. Calcium carbonate at temperatures above 860 °C decomposes into CaO and CO2. At lower temperatures it can react directly with HCl to form CaCl2 (15):

CaCO3 + 2HCl ! CaCl2 + H2 O

ð15Þ

CaO can react with HCl by reaction (16):

CaO + 2HCl ! CaCl2ðsÞ + H2 OðgÞ Ef [%]

631

ð16Þ

SO2 can also bind CaO leading to the formation of CaSO4 (17):

35

CaOðsÞ + SO2ðgÞ + O2ðgÞ ! CaSO4ðsÞ

30

Dolomite and limestone powder contain up to 90% of calcium carbonates and magnesium carbonates. These carbonates can react with both: chloride anions (Poskrobko et al., 2012) and metal cations (Kloprogge and Wood, 2015). Summary of multistep reaction: carbonates react with two chloride anions by binding them to calcium/magnesium chloride and with two sodium cations, binding them to sodium oxide. These products remain in the ash, what reduces the emission of sodium and chloride ions (18a) and (18b):

25 20 15 10 5 0 SRF1

SRF2

SRF3

SRF4

SRF5

SRF6

kinds of SRFs combustion with KLN or HAL +10%KLN

+10%HAL

Fig. 2b. Chloride binding effectiveness of 10% halloysite (HAL) and 10% kaolin (KLN) addition during SRF1-6 combustion.

ð17Þ

CaCO3 + 2NaCl ! CaCl2 # + Na2 O + CO2 "

ð18aÞ

MgCO3 + 2NaCl ! MgCl2 # + Na2 O + CO2 "

ð18bÞ

A. Szydełko et al. / Waste Management 102 (2020) 624–634

Halloysite and kaoline contain about 80% of hydrated aluminosilicates: Al2[Si2O5](OH)45H2O (Poskrobko et al., 2012), wherein water is completely disconnected at about 400 °C. Aluminosilicates of 650–1000 °C are capable of binding both Na+ and Cl (released during phase transitions of various materials) according to reactions (19) (Sanchez-Segado et al., 2015) and (20) (Sharp et al., 1989). Ions are remained in ash in a form of aluminosilicate complex (Na8Al6Si6O24Cl2).

Al2 Si2 O5 (OH)4 + 2NaOH ! 2NaAlSiO4 # + 3H2 O,

ð19Þ

6NaAlSiO4 + 2NaCl ! Na8 Al6 Si6 O24 Cl2

ð20Þ

Moreover, Al2Si2O5(OH)4, at high temperatures, passes into Al2O32SiO2 – a complex that binds K+/Na+ as shown in reaction (21) (Sanchez-Segado et al., 2015):

Al2 O3 2SiO2 + 2KCl + H2 O ! K2 OAl2 O3 2SiO2 # + 2HCl

ð21Þ

In addition, halloysite contains iron oxide (Fe2O3) which is also present in the metal protective layer, reacting with Cl and also contributing to lowering the chloride emissions (22) (RodríguezDiaz1 et al., 2013):

4KCl + 2Fe2 O3 + O2 ! 2K2 Fe2 O4 + 2Cl2 "

ð22Þ

The high effectiveness of the chloride binding was also found in the longitudinal wollastonite, a mineral containing about 96% of calcium silicate (CaSiO3) in a reaction (23) (Newton and Manning, 2006):

CaSiO3 + 2NaCl + 2H2 O ! CaClOH + H3 NaSiO4 #

ð23Þ

The above reaction leads to the formation of CaClOH and H3NaSiO4 precipitate that remain in ash. However, the sodium silicate has a low melting point, which may intensify the slag in the furnace (Wei et al., 2005), therefore, the use of silicon compounds may be unfavourable. Quartz powder (SiO2) is able to react with sodium cations and hydroxyl anions and form sodium silicate that remain in ash (24) (Smirnov et al., 2012).

2SiO2 + 4NaOH ! 2Na2 SiO3 # + 2H2 O

ð24Þ

Alkali metal cations can also be bonded through (NH4)2SO4 in reactions (25) and (26) (Tillman et al., 2009).

(NH4 )2 SO4 ! 2NH3 + SO3 + H2 O

ð25Þ

SO3 + H2 O + 2KCl ! 2HCl + K2 SO4 #

ð26Þ

In this two-step reaction, only potassium cations are bound to sulphate anions to form insoluble potassium sulphate (K2SO4), while chlorides remain in an ionized form (HCl). Hence the low efficiency of chloride binding by ammonium sulphate. 4.5. Determination of the effectiveness of chlorine binding during fuel combustion in the presence of sewage sludge The combustion of alternative fuels produced from municipal waste was carried out in the presence of different mass fractions of sewage sludge – a waste which, due to the high content of mineral rich in aluminosilicate, can show chloride affinity. The demonstration of chloride sewage sludge affinity for chlorides could be important in the management of sewage sludge. Aluminosilicates can be found in the mineral matter of sewage sludge coming from a sewage treatment plant to which a sewerage network is connected and in which there is no separation of sewage from storms. The amount and composition of aluminosilicates contained in sewage sludge depend primarily on their content and composition in the native land on which the sewage network is built. As a result

of rainfall, these compounds are washed out of the soil into sewerage. 4.5.1. Determination of the effectiveness of chlorine binding during PVC combustion in the presence of sewage sludge In order to investigate the effectiveness of chlorine binding by sewage sludge, PVC was combusted in the presence of 10% (wt.%) of the SS. Because of the presence of aluminosilicates and calcium oxides in the mineral matter, some of the chlorides emitted during PVC combustion would be bound. For the comparison of chloride binding effectiveness by SS, PVC was also combusted in the presence of 10% HAL and 10% KLN. The obtained results of the effectiveness of the used additives are shown in Fig. 3. According to some studies (Chouafa et al., 2015; Liao et al., 2015, Tillman et al., 2009) it can be concluded that the values of the chloride binding effectiveness of halloysite during the combustion of PVC were comparable to the chloride binding effectiveness by kaolin. This was consistent with the observations made during the determination of the chloride binding effectiveness of both minerals during SRF1-6 combustion. The effectiveness of chloride binding by sewage sludge was more than two-times lower than effectiveness of chloride binding by halloysite or kaolin. In assessing the effectiveness of chlorine binding by kaolin, halloysite and sludge, the amount of compounds and minerals (conducive to chlorine binding) contained in the compared additives should be taken into account. The sludge under study contains mainly a combustible substance and the mineral part constitutes 35% of the mass. Increasing the proportion of sewage sludge in the mixture to be combusted up to 30 percent increases the proportion of chlorine-binding mineral to a similar size as in the case of the addition of kaolin and halloysite and comparable binding efficiency. The increase in SS percentage during co-combustion with alternative fuels would allow for better chlorine biding by sewage sludge. 4.5.2. Determination of the effectiveness of chlorine binding during the co-combustion of SRF1 and SRF2 in the presence of different percentages of sewage sludge In the presented work, the influence of extruded sewage sludge on the amount of chlorides emitted during the combustion of SRF with average chloride content (SRF1 and SRF2 selected) was investigated. There were five mixtures in which the sewage sludge content (wt.%) was: 0%, 25%, 50%, 75% and 100%, respectively. For the preparation of SRF/SS fuel mixtures, the applied sewage sludge was stabilized, dried and granulated. SRF1 and SRF2 were milled with a CryoMill before mixing them with the SS. Then, in the fuel mixtures: GCV, mercury and chlorine content were determined according to the methods described in the Methodology. Since the increase in the SS percentage in the mixture with SRF reduces

Ef [%]

632

40 35 30 25 20 15 10 5 0 HAL

KLN

SS

mineral additves combusted with PVC Fig. 3. Chloride binding effectiveness of 10% halloysite (HAL), 10% kaolin (KLN) and 10% sewage sludge during PVC combustion.

Ef [%]

A. Szydełko et al. / Waste Management 102 (2020) 624–634

60 50 40 30 20 10 0 25% SS

50% SS

75% SS

part of SS combusted with SRF1 and SRF2 SRF1

SRF2

Fig. 4. The effectiveness of chlorine binding during combustion of SRF1 and SRF2 in the presence of 25%, 50% and 75% of sewage sludge.

the calorific value and can help to increase the mercury content at the first stage of research, the values of both parameters were also determined in these mixtures. The results of the determination of GCV, mercury and chlorine content in SRF1/SS and SRF2/SS mixtures are presented in Table S4. Based on the results obtained, it can be concluded that with the increase in the percentage of sewage sludge in mixtures with the SRF1 and with the SRF2 the Hg content of the combusted mixture proportionally increases (for SRF1 from 0.79 ppm to 1.25 ppm, for SRF2 from 0.6 to 1.25 ppm). GCV and Cl content in the dry mass of the mixture decrease (the GCV decreases proportionally to SS content, from 23.2 MJ/kg to 13.7 MJ/kg). The high calorific value of alternative fuels (approx. 25 MJ/kg, as shown in Table 1) is due to the presence of the non-biodegradable organic fraction, while the low calorific value of sewage sludge (about 14 MJ/kg, as shown in Table 1) is due to biodegradable, moisture and ash content. The content of Hg in SRF depends on the components of the waste stream, especially on the presence of electronic residue components, fluorescent tubes and batteries from an inadequate segregation system, and in sewage sludge on the presence of industrial wastewater (Urbanek et al., 2014). The Hg amount in the dry mass of the fuel increases along with the increase in the SS percentage (increased linearly). Mercury content in SRF1 was 60% lower than in SS. However, the use of 25% of sewage sludge results in a slight increase in mercury content in both SRF1 (10%) and SRF2 (12%). The content of chlorides in alternative fuels SRF1 or SRF2 is almost 8-times higher than in sewage sludge. Fig. 4 shows the effectiveness of chloride binding during combustions of mixtures SRF1 and SRF2 with 25%, 50% and 75% sewage sludge. The experience shows that during co-combustion with chlorine-rich alternative fuels, sewage sludge is capable of retaining chlorides in ash and thus exhibits anticorrosive properties. The use of sewage sludge reduced the level of chlorides on average by 40%.

5. Conclusions Fuels produced from municipal waste contain a significant level of chlorine compounds, thereby increasing the risk of chloride corrosion. Chlorine in SRF is contained in both plastic fractions (mainly in PVC) and in the biomass fraction in which chlorine is bound in the form of salts, i.e. sodium, potassium, calcium and magnesium chloride. Chlorine present in plastic fractions may account for 60–80% of total chlorine of fuel.

633

The use of certain mineral additives during the burning of alternative fuels reduces the amount of emitted chlorides. Chlorine can be effectively bound by mineral compounds found in minerals such as calcium compounds found in dolomite and limestone or aluminosilicates in kaolins and halloysite. The studies have shown that halloysite has a high chloride binding effectiveness, comparable to that of kaolin (about 30%) (Yu et al., 2016). These minerals in the form of dust are waste material from the process of processing and obtaining aggregates. Their use for binding chlorine in combustion chambers is also a way of their management. Consequently, the use of waste fractions during the cocombustion of alternative fuels contributes to the development of usage of mineral waste. The combustion of alternative fuels produced from municipal waste SRF in the presence of stabilized, dried and granulated sewage sludge, allows reducing chloride emissions even more than 50%. The calcium and aluminosilicates present in the sewage sludge bind chlorides emitted during the combustion of alternative fuels. Sewage sludge can be a cheap, readily available material that effectively reduces chlorine emissions during the burning of alternative fuels. The use of sewage sludge as a chloride-binding additive during the combustion process can be a new solution for the management of sewage sludge, which is problematic. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was supported by National Science Center (Poland); grant No. 2013/11/N/ST8/01906 (A. Szydełko). References Acuna-Caro, C., Thorwarth, H., Scheffknecht, G., 2006. A thermodynamic study on the effects of individual flue gas components on mercury speciation. Power Plant Chem. 8, 374–381. Andersson, K., Johnsson, F., 2005. Process evaluation of an 865MWe lignite fired O2/ CO2 power plant. Energy Convers. Manage. 47, 2487–2488. https://doi.org/ 10.1016/j.enconman.2005.10.017. Antonangeli, J.A., Neto, J.F., Costa Crusciol, C.A., Ferracciú Alleoni, L.R., 2017. Lime and calcium-magnesium silicate in the ionic speciation of an Oxisol. Sci. Agric. 74 (4), 317–333. https://doi.org/10.1590/1678-992X-2016-0372. Athanasiou, C., Coutelieris, F., Vakouftsi, E., Skoulou, V., Antonakou, E., Marnellos, G., Zabaniotou, A., 2007. From biomass to electricity through integrated gasification/SOFC system-optimization and energy balance. Int. J. Hydrogen Energy 32, 337–342. https://doi.org/10.1016/j.ijhydene.2006.06.048. Autret, E., Berthier, F., Luszezanec, A., Nicolas, F., 2007. Incineration of municipal and assimilated wastes in France: assessment of latest energy and material recovery performances. J. Hazard. Mater. B 139, 569–574. https://doi.org/ 10.1016/j.jhazmat.2006.02.065. Baxter, L., 2005. Biomass-coal co-combustion: opportunity for affordable renewable energy. Fuel 1295, 84. Blomberg, T., 2006. Which are the right test conditions for the simulation of high temperature alkali corrosion in biomass combustion. Mater. Corros. 57, 170– 175. https://doi.org/10.1002/maco.200503905. Born, M., 2005. Cause and risk evaluation for high-temperature chloride corrosion. VGB Power. Tech 5, 107–111. Cao, W., Jun Li1, J., Leo, L., Lue, L., Zhang, X., 2016. Release of alkali metals during biomass thermal conversion. Arch. Ind. Biotechnol. 1, 1–3. Chouafa, M., Idres, A., Bouhedja, A., Talhi, K., 2015. Chemical treatment of kaolin. Case study of kaolin from the tamazert– jijel mine. Min. Sci. 22, 171–180. https://doi.org/10.5277/msc152214. Grabke, H.J., Reese, E., Spiegel, M., 1995. The effect of chlorides, hydrogen chloride and sulfur dioxide in the oxidation of steels below deposits. Corros. Sci. 37, 1023–1043. Henderson, P., 2006. Reducing superheater corrosion in wood-fired boilers. Mater. Corros. 57 (2), 128–134. https://doi.org/10.1002/maco.200503899. Hrycaj, G., 2014. Characteristics of Combustion and Co-Combustion of Coal with Sewage Sludge. PhD thesis Wrocław University of Science and Technology. Hrycaj, G., Król, K., Płaza, P., Rybak, W., 2007. Thermal decomposition of sewage sludge and biomass sludge blends, proceedings of ‘‘Success and visions for bioenergy. Thermal processing of biomass for bioenergy, biofuels and bioproducts”, 22–23 Salzburg, Austria.

634

A. Szydełko et al. / Waste Management 102 (2020) 624–634

Iacovidou, E., Hahladakis, J., Deans, I., Costas, V., Purnell, P., 2018. Technical properties of biomass and solid recovered fuel (SRF) co-fired with coal: impact on multi-dimensional resource recovery value. Waste Manage. 73, 535–1454. https://doi.org/10.1016/j.wasman.2017.07.001. Karlsson, S., Åmand, L.-E., Liske, J., 2015. Reducing high-temperature corrosion on high-alloyed stainless steel superheaters by co-combustion of municipal sewage sludge in a fluidised bed boiler. Fuel 139, 482–493. https://doi.org/ 10.1016/j.fuel.2014.09.007. Kassman, H., 2012. Strategies to Reduce Gaseous KCl and Chlorine in Deposits during Combustion of Biomass in Fluidised Bed Boilers ISBN 978-91-7385-6676. Department of Energy and Environment Division of Energy Technology Chalmers University of Technology, Göteborg, Sweden. Kloprogge, T., Wood, B., 2015. Chemical bonding and electronic structures of the Al2Si2O5(OH)4 polymorphs kaolinite, dickite, nacrite and halloysite by X-ray photoelectron spectroscopy. Clay Sci. 19, 39–44. Król, K., 2010. Impact of Solid Biofuels on Coal Behavior During Co-Combustion PhD thesis. Wrocław University of Science and Technology. Liao, Y., Wu, S., Chen, T., Cao, Y., Ma, X., 2015. The alkali metal characteristic during biomass combustion with additives. Energy Proc. 75, 124–129. https://doi.org/ 10.1016/j.egypro.2015.07.209. Ma, W., Hoffmann, G., Schirmer, M., Chen, G., Rotter, V.S., 2010. Chlorine characterization and thermal behaviour in MSW and RDF. J. Hazard. Mater. 178 (2010), 489–498. Malijonyt, V., Dace, E., Romagnoli, F., Gedrovics, M., 2016. Methodology for determining the mixing ratio of selected solid recovered fuels. Agron. Res. 14 (Sp.1), 1169–1179. Newton, R.C., Manning, C.E., 2006. Solubilities of corundum, wollastonite and quartz in H2O–NaCl solutions at 800°C and 10 kbar: interaction of simple minerals with brines at high pressure and temperature. Geochim. Cosmochim. Acta 70, 5571–5582. https://doi.org/10.1016/j.gca.2006.08.012. Sun, S.F., 2004. Physical Chemistry of Macromolecules. Basic Principles and Issues. John Wiley & Sons Inc.. Poskrobko, S., Król, D., Łach, J., 2012. Hydrogen chloride bonding with calcium hydroxide in combustion and two-stage combustion of fuels from waste. Energy Fuels 26, 842–853. https://doi.org/10.1021/ef2016599. Ramasamy, S., Hussin, K., Al Bakri Abdullah, M.M., Ruzaidi Ghazali, C.M., Binhussain, M., Sandu, A.V., 2016. Interrelationship of Kaolin, Alkaline Liquid Ratio and Strength of Kaolin Geopolymer. Mater. Sci. Eng. 133, 012004. https://doi.org/ 10.1088/1757-899X/133/1/012004.

Rodríguez-Diaz1, R.A., Uruchurtu-chavarín, J., Molina-Ocampo, A., PorcayoCalderon, J., Mendoza, M.E., Valdez, S., Juárez-Islas, J, 2013. Hot corrosion behavior of feal intermetallic compound modified with silver in molten salt mixture. Int. J. Electrochem. Sci. 8, 11877–11895. Sanchez-Segado, S., Makanyire, T., Escudero-Castejon, L., Hara, Y., Jha, A., 2015. Reclamation of reactive metal oxides from complex minerals using alkali roasting and leaching – an improved approach to process engineering. Green Chem. 17, 2059–2080. https://doi.org/10.1039/C4GC02360A. Sharp, Z.D., Helffrich, G.R., Bohlen, S.R., Essene, J., 1989. The stability of sodalite in the system NaAlSiO4-NaCl. Geochim. Cosmochim. ACM 53, 1943–1954. Smirnov, S.Z., Thomas, S.G., Kamenetsky, V.S., Kozmenko, O.A., Large, R.R., 2012. Hydrosilicate liquids in the system Na2O–SiO2–H2O with NaF, NaCl and Ta: evaluation of their role in ore and mineral formation at high T and P1. Petrology 20 (3), 271–285. https://doi.org/10.1134/S0869591112020063. Speight, G.J., 2005. Handbook of Coal Analysis. John Wiley & Sons, Hoboken, pp. 56– 60. Szydełko, A., 2017. Combustion and co-combustion of alternative fuels. PhD thesis, Wrocław University of Science and Technology. Tillman, D.A., Duong, D., Miller, B., 2009. Chlorine in solid fuels fired in pulverized fuel boilers - sources, forms, reactions and consequences: a literature review. Energy Fuels 23, 3379–3391. Urbanek, B., Szydełko, A., Moron´, W., 2014. Mercury content in ash of solid fuels. Chall. Mod. Technol. 5 (2), 39–43. Vassilev, S.V., Eskenazy, G.M., Vassileva, C.G., 2000. Contents, modes of occurrence and origin of chlorine and bromine in coal. Fuel 79 (8), 903–921. Wang, P., Massoudi, M., 2013. Slag behavior in gasifiers. Part I: influence of coal properties and gasification conditions. Energies 6, 784–806. https://doi.org/ 10.3390/en6020784. Wei, X., Schnell, U., Hein, K.R.G., 2005. Behavior of gaseous chlorine and alkali metals during biomass thermal utilization. Fuel 84, 841–848. https://doi.org/ 10.1016/j.fuel.2004.11.022. Xu, L., Liu, J., Kang, Y., Miao, Y., Ren, W., Wang, T., 2014. Safely burning high alkali coal with kaolin additive in a pulverized fuel boiler. Energy Fuels 28 (9), 5640– 5648. https://doi.org/10.1021/ef501160f. Yu, J., Lushi Sun, L., Ma, Ch., Qiao, Y., Yao, H., 2016. Thermal degradation of PVC: a review. Waste Manage. 48, 300–314. https://doi.org/10.1016/j. wasman.2015.11.041.