Thermal Destruction of Waste and the Impact of the Presence of Cl and S in Waste on the Emissions of Cu, Ni, and Pb and Their Immobilisation in the Ash Residue

Abstract

This study addresses the pressing environmental problem of the emissions of ecotoxic heavy metals from high-temperature waste combustion processes, including incineration and pyrolysis. Such emissions pose a serious threat to ecosystems and human health. This study investigated the behaviour of the heavy metals Cu, Ni, and Pb during the combustion of various waste materials such as plastic waste, pharmaceutical waste, and pesticide waste. To limit the release of heavy metals into the exhaust gas stream, various additives were used: divanadium pentoxide (V₂O₅), borax (Na₂B₄O₇), and their mixtures with calcium oxide (CaO). Additionally, this study examined the impact of the content of chlorine heteroatoms (Cl) in burned waste materials on the emission levels of Cu, Ni, and Pb. The findings shed light on the mobility of ecotoxic heavy metals in high-temperature waste incineration processes (1273, 1373 K) and offer insight into strategies to improve their immobilisation in grate residues. At a temperature of 1273 K, V₂O₅ with CaO reduced Pb emissions by ~65% for plastic waste and by ~40% for pesticide.

1. Introduction

Many types of waste, e.g., some hazardous waste, including medical waste, animal waste, fuels from waste, and some industrial waste must be neutralised in high-temperature thermal processes—e.g., incineration, plasma processes. The thermal transformation of waste includes waste incineration, alternative technologies to waste incineration and co-incineration of waste with conventional fuels [1,2,3,4,5]. Waste incineration is by far the most widely method used in the world. Waste combustion processes ensure volume and weight reduction, detoxification, and the destruction of hazardous ingredients. Solid post-process products are sanitary, safe, and can be managed or stored safely. At the same time, chemical energy stored in waste is recovered. Another way to use energy from waste is to gasify waste (including biomass waste), and the obtained generator gas (commonly called syngas) is burned or co-burned with coal in power boilers [6]. Unfortunately, waste combustion processes (including fuels used in professional and municipal energy) generate emissions of harmful substances into the atmosphere. Here, we can mention fly ash, dust, acidic gases, and a large spectrum of organic compounds, as well as the aerosols of ecotoxic metals (terms consistent with the UPAC nomenclature), commonly called heavy metals [4,7,8].

Most waste contains ecotoxic metals, even if they do not constitute a morphological component of the waste. The amount of emissions of these metals from waste combustion processes depends, among others, on: the share of ecotoxic metal in the incinerated waste, the physical and chemical properties of the metal, the process temperature, and the type and effectiveness of exhaust gas treatment devices responsible for reducing emissions. Moreover, the low efficiency of the combustion process results in increased emissions of particulate matter (flying dust and, in their stream, soot). Due to their high porosity, these particles have an extensive surface area. They become sorbents for many compounds that are products of incomplete combustion, as well as heavy metals condensing on their surface [9,10]. Therefore, heavy metals present in the fuel as non-flammable components undergo phase changes under the influence of temperature to form vapours and aerosols, and are adsorbed on the surface of fly ash, before entering the environment where they undergo accumulation processes. Ecotoxic metals emitted in this way harm the health of humans and animals, penetrating organisms through aspiration or skin. Accumulating in ecosystems, they harm their health by penetrating through the digestive tract [11,12,13,14].

In 2020, the highest Cd, Pb, and Hg emissions in the EU were recorded in three countries: Germany, Italy, and Poland. These three countries accounted for almost half of the EU’s total emissions [15]. The application of legal regulations (by the EU’s obligations under the Convention on Air Pollutants) has led to a reduction in heavy metal emissions in all EU member states (compared to the levels in 1990). Emissions continued to decline between 2005 and 2020: lead emissions decreased by 49%, mercury emissions by 51%, and cadmium emissions by 39% in all EU-27 member states. In 2020, Germany, Poland, and Italy had the largest share in emissions of ecotoxic (heavy) metals in the EU.

The emission of ecotoxic (heavy) metals in the waste gas stream from combustion chambers, pyrolysis, and waste gasification is the most difficult type of emission to eliminate. At the same time, this type of emissions is the most dangerous and burdensome for the environment. Therefore, attempts are already made to bind heavy metals in slags or furnace ashes in the combustion chamber [16,17,18].

This work aimed to determine the mobility of the ecotoxic (heavy) metals Cu, Ni, and Pb in the combustion process of plastic waste, pharmaceutical waste, and pesticide waste. Understanding the behaviour of metals in high-temperature processes and the possibility of increasing their immobilisation in the grate residue will allow for reducing their emissions together with dust and fly ash. The research was performed on a laboratory scale.

To limit the removal of metals from the combustion chamber in the exhaust gas stream, additives were used in the form of divanadium pentoxide (V₂O₅) and borax (Na₂B₄O₇) and their mixture with calcium oxide (CaO). The aim of this work was also to analyse the impact of the amount of heteroatom-Cl present in the combustible material of burned waste on the emission levels of Cu, Ni, and Pb.

2. Materials and Methods

The research covered the three following types of waste: mixed contaminated plastic waste, pesticide waste, pharmaceutical waste. At temperatures of 1273 K and 1373 K, the mobility of the ecotoxic metals copper (Cu), nickel (Ni), and lead (Pb) present in these wastes was determined. Attempts were made to reduce Cu, Ni, and Pb emissions using primary methods by binding in the combustion chamber. For this purpose, the additives (V₂O₅, Na₂B₄O₇, and V₂O₅ + CaO, Na₂B₄O₇ + CaO) were added to the waste samples to check whether it is possible to increase the immobilisation of tested metals in the grate residue at the waste combustion temperatures.

2.1. Materials

Plastic waste—mixed, contaminated plastic waste that came from the Municipal Waste Disposal Plant in Białystok (Poland) which segregates municipal waste. These were plastic waste remaining from the stream of municipal plastic waste (from selective collection) after passing through a system of separators/classifiers, from which individual types of plastic wastes were partially segregated.

Pesticide waste—came from the PROEKO plant in Białystok (Poland), which collects hazardous waste. These were mainly waste of expired pesticides.

Pharmaceutical waste—came from the EMKA S.A. plant. in Białystok (Poland), dealing with the collection of medical waste. These were pharmaceutical waste and packaging.

Divanadium pentoxide V₂O₅ p.a. was produced by LOBA Feinchemie, Vienna, Austria.

Borax Na₂B₄O₇ p.a., calcium oxide CaO p.a. was supplied by POCH, Gliwice, Poland.

2.2. Determination of Physicochemical Properties and Concentrations of Cu, Mn, and Pb in the Tested Waste

The waste was crushed in an IKA WERKE electric grinder and dried to constant mass at 378 K. CHN elemental analysis was performed according to PN-EN ISO 16948:2015-07 [19], and chlorine and sulphur were determined according to PN-EN ISO 16994:2016-10: Solid Biofuels—Determination of the Total Sulphur and Chlorine Content [20] and PN-G-04584:2001 [21]. Elemental composition of flammable substance are included in Table 1.

Pesticide and pharmaceutical waste may contain ecotoxic (heavy) metals, which results from their chemical structure or the composite preparation assumptions. In addition, the packaging of such products also contains certain amounts of heavy metals. As for plastic waste, heavy metals in plastics are introduced as additives. These additives may be inorganic compounds insoluble in water or organometallic compounds [22,23]. The presence of heavy metals in plastics may also be a consequence of their residues as post-reaction or catalytic residues in the production of polymers. The concentrations of these metals vary in different materials. This depends on the type of material and the type of additive. Sometimes, such additives can constitute almost 50% of the share of the material [24,25,26].

The contents of Cu, Ni, and Pb metals in all three types of tested waste were determined using the flame atomic absorption spectrometry (FAAS) method. For this purpose, the waste was digested using the wet method in a Speed four-microwave digester from Berghoff Products + Instruments GmbH. The concentrations of Cu, Ni, and Pb in the minerals were measured using the AAS 9000 spectrometer from EnviSense. An acetylene-air flame (oxidising) was used with a nebulisation efficiency of 5.5 cm³/min. The determinations were made three times (to eliminate gross errors), and the results presented are the arithmetic averages of three determinations. The results are presented in Table 2.

2.3. Experiment Description

The waste for testing was ground in an IKA WERKE electric grinder (prod. IKA WERKE Gmbh, Staufn, Germany) and dried to constant weight at 378 K. Samples of the prepared waste were weighed—100 g each (with an accuracy of 0.0001 g). Waste combustion tests were carried out successively at temperatures of 1273 K and 1373 K on the test stand shown in Figure 1.

Air was supplied to the combustion chamber and the temperature was controlled using a thermocouple. The residence time of the samples in the high-temperature zone was determined to be the limit of measurable, minimal mass losses. All waste sample combustion experiments were repeated three times (to eliminate the gross errors). Metals in the ash residue (amounts of metals that were not carried out in the combustion chamber with exhaust gases) were also determined three times. The results presented are the arithmetic averages of these determinations. Metals were determined using the flame atomic absorption spectrometry (FAAS) method described in Section 2.2. The metal emission level (%) was determined as the difference between the waste metal load (taking this value as 100%) and the amount of metals in the ash residue.

Next, the possibility of increasing the immobilisation (during waste combustion) of copper (Cu), nickel (Ni), and lead (Pb) in the grate, non-combustible ash residue was investigated. For this purpose, the following additives were applied to the tested waste samples (about 1% the mass of the input) in the form of: divanadium pentoxide (V₂O₅), borax (Na₂B₄O₇), and their mixtures with calcium oxide (CaO)—1%V₂O₅ + 1%CaO, 1%Na₂B₄O₇ + 1%CaO). Combustion tests were carried out in the same way as for waste without additives. The amounts of metals retained in the solid post-process residue were determined (as given in Section 2.2).

The measurement equipment described below was used for the tests. This equipment is equipped by the manufacturer with standard software that allows the monitoring of current results and their archiving. The manufacturer does not provide details regarding the description of these tools included in the equipment. The drawings were made using a Microsoft Excel spreadsheet.

3. Results and Discussion

Thermal waste utilisation must be safe for the environment and must not pose a toxicological threat. This, in turn, is a condition for protecting the atmosphere and establishing appropriate legal regulations. A large amount of ecotoxic metals contained in the waste leaves the combustion chamber unchanged and is found in the bottom ash. Fly ash captured is in heat recovery or flue gas purification devices that also contain such metals. Some fractions of metals originally found in waste are also present in the exhaust gases emitted by the systems. Metals contained in the waste subject to incineration may undergo reactions during the process. Generally, the reactions involve the release of oxygen from metal oxides. The new compounds formed are more volatile than the original ones. At the same time, vapours from waste, in the zone of low temperatures and high oxygen concentration, are subject to further reactions, leading back to their original, more complex, and condensed form. Heavy metals leave the combustion chamber in two main ways—the first is the evaporation of the metal, and the second is the entrainment of particles containing non-volatile metals. Some metal compounds found in waste materials are volatile and evaporate in conditions similar to those occurring during waste combustion. Their vapours penetrate the exhaust gas, which carries them through the combustion chamber system. The waste combustion temperature has a large impact on the behaviour of metals through its influence on the effective vapour pressure of the metals undergoing evaporation. The effective vapour pressure is the sum of the pressures of all types of metals present at equilibrium, taking into account their concentrations. During the cooling of the exhaust gas, the vapours condense in two ways—homogeneous (homogeneous), creating new particles, and heterogeneous (heterogeneous), adsorbing on the surface of ash particles. Metals are most often emitted in the form of particles that are smaller than 1 µm. Exhaust gas cleaning devices are less effective at capturing particles this small than they are at capturing larger particles. Metals contained in combustible materials may react in waste incineration installations. These reactions result in the reduction of metal oxides. The created mixtures often evaporate more easily than the original substances [27,28]. These small particles present in the environment can enter the lungs, where toxic metals come into contact with the blood [29]. The distribution of heavy metals in the waste incineration system is related to the type of these metals and their concentration in waste, the composition of the waste, the waste incineration temperature, and the efficiency of the flue gas purification devices. The routes of ecotoxic metals from incinerated waste are shown in Figure 2.

To determine the emission of the tested metals, their content in the solid, non-flammable ash residue was determined. The difference between the content of the tested metals Cu, Ni, and Pb in the waste and the amount retained in the ash was the amount of emissions, i.e., the part of the metals that passed into the exhaust gases.

Figure 3, Figure 4 and Figure 5 show the range of the mobility of Cu, Ni, and Pb depending on the combustion temperature (1273 K, 1373 K) of plastic waste, pesticide waste, and pharmaceutical waste.

The highest percentage of emissions in the combustion process of each of the three types of tested waste was recorded for lead Pb, which is consistent with the literature [30]. These values ranged from approx. 85% (pesticide waste) to approx. 91% (plastic waste) when the process of the thermal transformation of waste took place at a temperature of 1273 K. In the case of copper Cu, the emission was ~64% when burning plastic waste, 49% when burning pharmaceutical waste, and 32% when burning pesticide waste. The incineration of pharmaceutical waste and plastic waste generated similar amounts of nickel (Ni) emissions—54–56%, while the emission of pesticide waste was approximately lower by 16% (it was 39%). The process temperature was higher by 100 K (1373 K), resulting in an increase in the emissions of each metal during the combustion of each type of waste. For example, Ni emissions increased by about 30% (plastic waste, pesticide waste), Cu emissions (pesticide waste, pharmaceutical waste), and Ni (pharmaceutical waste) increased by several percent. Pb emissions increased by several percent in each case.

The volatility of metals is the main feature determining the behaviour of metal in waste combustion processes. It is characterised by the volatility temperature. The volatility temperature of a metal is the temperature at which the actual vapour pressure of the metal is 0.1 Pa (1 × 10⁻⁶ atm). Tests have found that over 30% of volatile metals, including 10% of metals with a volatility temperature above 1143 K, are not captured [27]. A rough indication of the relative volatility of particular metals can be found in their melting points. The volatility of individual metals depends on the type of compounds in which the metal occurs. When a metal exhibits this vapour pressure, vaporisation can occur. Therefore, the reason for the increase in metal emissions is the increase in gas pressure resulting from the increase in temperature in the combustion chamber. The vapour pressure of all metals rapidly increases with the increasing temperature [30,31]. The increase in the amount of particles subject to evaporation, their ability to penetrate the lungs, and the toxic properties of metals have an impact on the threat to human health [32].

3.1. The Influence of Chlorine in Combustible Material on the Emission of Cu, Ni, and Pb

Chlorides are more volatile than the corresponding metal oxides or free metals. The effective evaporation pressure of many metals increases with an increasing chlorine concentration and a large amount of individual metals is converted into their chlorides [27,28].

The tested waste had similar values to the main element, i.e., C^daf, in its combustible parts. And so, the values were 71.8%, 70.2%, and 72.5% for the mixed and contaminated plastic waste, pesticide waste, pharmaceutical waste, respectively. However, the amount of the chlorine element C^daf in these wastes varied, with 2.8%, 0.9%, and 1.7%, respectively. Taking into account the properties of metallic elements and chlorine, their chemical forms, behaviour at high temperatures, and literature reports on this topic [33], the emission values of Cu, Ni, and Pb obtained in the experimental studies of waste combustion were compared to Cl/C values. The Cl/C value was 0.0390 for mixed and contaminated plastic waste, 0.0128 for pesticide waste, and 0.0235 for pharmaceutical waste. These correlations are shown in Figure 6, Figure 7 and Figure 8.

The presence of chlorine in the combustible mass of waste promoted the evaporation of Cu, Ni, and Pb. The higher the share of chlorine in the waste, the higher the Cl/C ratio and, consequently, the greater the amount of metals transferred to the gas phase. This is confirmed by literature data [34].

3.2. Reduction in Emissions of Ecotoxic Metals in the Exhaust Gas Stream

Due to the emission of metals in the waste gas stream, it is necessary to reduce it. The trends in improving combustion processes seek to reduce gas emissions through the appropriate selection of installation operating parameters (primary methods) as well as the use of an appropriate technology for purifying exhaust gases. Primary methods include solutions involving some interference in the process of the thermal transformation of waste at the stage when it has not yet been released. To reduce the emission of metals in the stream of exhaust gases from fuel combustion processes, many researchers have tried to use various sorbents [35,36,37], the task of which was to immobilise metals in the slag by reducing the process of metal evaporation, creating chemical compounds with high thermal stability or sorption on the surface [38,39]. Tests of this type were carried out using bauxite, kaolinite, sodium salts, and limestone. Sorbents were introduced into the combustion chamber in solid or liquid form [36,40,41,42]. Therefore, retaining metals in the grate residue guarantees that their aerosols or submicron dust particles, carrying a load of heavy metals, will enter the atmosphere in smaller quantities.

To limit the mobility of Cu, Ni, and Pb during the combustion of plastic waste, pesticide waste, and pharmaceutical waste, the following additives were used: V₂O₅, V₂O₅ + CaO, Na₂B₄O₇, and Na₂B₄O₇ + CaO (as given in Section 2.3). The temperatures in the combustion chamber were of 1273 K and 1373 K (as in the case of waste only—without additives). The level of metal emissions from the tests of burning waste with additives was determined in the same way as described in Section 2.3. The test results are shown in Figure 9, Figure 10 and Figure 11.

Each of the additives had an immobilising effect on the tested heavy metals Cu, Ni, and Pb during the combustion of each type of waste. However, the magnitude of this impact was different for each of the metals present in each of the wastes burned. This impact depended on the type of additive. It also differed according to the process temperature. As such:

• At a temperature of 1273 K, the additions of borax and V₂O₅ reduced the Cu emissions (during the combustion of each of the tested wastes—plastics, pesticides or pharmaceuticals) by several percent compared to the emission level during waste combustion without adding additives; Ni mobility decreased by several percent when plastic waste and pesticide waste were burned, and by 20–30% when the pharmaceutical waste was burned; with regard to Pb (for each waste), borax turned out to be effective, reducing its emission by several percent, while the addition of V₂O₅ resulted in a reduction in Pb emissions of over 20% in terms of pharmaceutical waste and as much as ~40% in terms of pesticide waste.

• At a temperature of 1373 K, the addition of borax reduced the emission of Cu by several percent when pesticide and pharmaceutical wastes were burned, and reduced the emission of Ni by several percent when plastic and pharmaceutical waste was burned. The emission of Cu reduced by over 20% when the plastic waste was burned, and the emissions of Ni (when pesticide waste was burned) and Pb (when plastic waste was burned) both by ~30%—while the mobility of Pb during the combustion of pesticide and pharmaceutical waste decreased by ~10%. V₂O₅ showed the highest effectiveness in terms of reducing heavy metal emissions during the combustion of plastic waste with regard to Pb and Cu (~50% and ~40%, respectively) and Ni by over 20%; pesticide waste with ~40% Ni, ~30% Pb, ~20% Cu; pharmaceutical waste with ~30% Ni, ~20% Cu, and ~20% Pb.

• The additions of both additives mixed with CaO in the waste reduced the mobility of the metals to a greater extent: at a temperature of 1273 K, the V₂O₅ with CaO reduced Pb emissions by ~65% when plastic waste was burned; and by ~40% when pesticide or pharmaceutical waste was burned; it reduced the Ni emissions (when burning pesticide waste) and Cu emissions (when burning plastics and pharmaceuticals) both by ~20%; it reduced the Cu emissions by several percent (when pesticide waste was burned) and Ni emissions (when plastic waste was burned) both by several percent; it reduced Ni emissions (when pharmaceutical waste was burned) by over 30%; borax with CaO reduced the mobility of Cu by ~20% during the combustion of plastic or pharmaceutical waste and by several percent during the combustion of pesticide waste and reduced the emissions of Ni by several percent during the combustion of plastic waste; the emissions of Ni (when pesticide waste was burned) and Pb (when plastic, pesticide, or pharmaceutical waste was burned) decreased by over 20%; borax with CaO reduced Ni emissions by ~30% during the combustion of pharmaceutical waste.

• At a temperature of 1373 K, doping borax with CaO or doping V₂O₅ with CaO reduced Cu emissions during the combustion of pharmaceutical waste at the level of values analogous to those at 1273 K, and Ni emission by ~50% during the combustion of pesticide waste; borax with CaO reduced Cu emissions by ~40% when pesticide waste was burned and by ~20% when plastic waste was burned; reduced Ni emissions by over 30% when plastic waste was burned and by ~20% when pharmaceutical waste was burned; and reduced Pb emissions by ~20% when pesticide and pharmaceutical wastes were burned and by ~40% when plastic waste was burned. In the case of the use of the V2O5 additive with CaO, its highest effectiveness in reducing Pb emissions during the combustion of plastic waste was recorded—namely by ~70% and by ~50% when burning pesticide or pharmaceutical waste, respectively; it also reduced Cu emissions when burning plastic waste and Ni emissions when burning pesticide waste both by ~50%. While burning plastic or pharmaceutical waste, Ni emissions were reduced by ~40%, and the mobility of Cu was reduced by ~30% during pesticide waste combustion.

The impact of the additives used to minimise Cu, Ni, and Pb emissions during waste combustion varied in nature. At high temperatures, V₂O₅ forms thermally stable compounds with heavy metals—vanadates. Borax acts as a flux, while CaO chemically sorbs chlorine, thereby limiting the formation of chlorides of the tested ecotoxic metals, which are more volatile than their other forms.

3.3. The Influence of S/Cl (Chlorine and Sulphur in Combustible Waste Material) on the Emission Levels of Cu, Ni, and Pb

If there is chlorine in the flammable parts of waste (which are subjected to thermal destruction during the combustion process), then ecotoxic metals (heavy metals found in the waste) form chlorides in the combustion chamber. Metal chlorides are more volatile forms than, for example, oxides or sulphates. If the combustible waste material also contains sulphur, it is released into its gaseous form SO₂ during combustion. Then, reactions with metal chlorides take place in the gas phase:

MeCl₂ + SO₂ + H₂O + 1/2O₂ → MeSO₄ + 2HCl(1)

where Me means metal.

The resulting sulphates, being less volatile, may be less mobile than chlorides. For each type of waste, S/Cl values were calculated based on the sulphur and chlorine content (Table 1 and Table 3). The highest S/Cl value—2.1110—was recorded for pesticide waste, with a values that were 3.4 times lower for pharmaceutical waste and 8.4 times lower for plastic waste.

The influence of the S/Cl ratio on the immobilisation of metals in the solid grate residue was determined, as shown in Figure 12.

Higher S/Cl values in the waste, about each metal, resulted in lower metal emission values, and therefore higher percent contents in the grate ash. At higher S/Cl (pesticide and pharmaceutical waste), lead immobilisation values were higher by several percent at both temperatures compared to the lowest value (plastic waste). In the case of Cu, its immobilisation values were slightly different, although the trend was identical to that for Pb. At the waste combustion temperature of 1273 K, when the S/Cl ratio was 0.25 (plastic waste), Cu was retained in the grate ash in an amount of over 30%. With a 2.6 times higher S/Cl value (pharmaceutical waste), the amount of Cu remaining in the ash was higher by several percent, with another that was 3.4 times higher than the S/Cl ratio (pesticide waste), and even higher amount was recorded by ~20%. At a temperature of 1373 K, while maintaining the trend as at 1273 K, the Cu immobilisation values were as follows: several percent, then ~25% more, and then another ~15% more. As for Ni, differences in the trends in the influence of S/Cl on the behaviour of this element vary depending on temperature. Thus, at a temperature of 1273 K, the S/Cl value was 0.6471 (pesticide waste) and the nickel in the ash was only a few percent higher than when the S/Cl value was 0.2500 (plastic waste). However, at a temperature of 1373 K, it was over 10%. The opposite was true for the next higher S/Cl value, namely 2.1110 (pharmaceutical waste). At a temperature of 1273 K, an almost 25% higher Ni immobilisation value was recorded, and at a temperature of 1373 K, this value increased by a few percent.

The influence of S/Cl in waste on Cu, Ni, Pb emissions during the combustion of waste with additives (at temperatures of 1273 K and 1373 K) is shown in Figure 13, Figure 14 and Figure 15.

Analysing the trend of the influence of S/Cl values in waste on the behaviour of the tested ecotoxic metals during waste combustion (to which metal-absorbing additives were added), it was found that this varies depending on the type of metal and the process temperature. With regard to Cu, it was observed that the trend related to the percentage of its immobilisation is identical, i.e., higher growth- in the S/Cl values, and a higher amount of Cu in the ash. The trend is the same regardless of the temperature of the waste incineration process. The only difference is between the amount of copper retained in the ash when the S/Cl value was the lowest—0.25, and the next S/Cl 0.6471—an increase of up to 10%, and between S/Cl 0.6471 and the next S/Cl 2.1110—an increase of even more than 20%. The upward trend is also maintained in the case of Ni (regardless of the temperature). A decrease in immobilisation was only noted between S/Cl at 0.25 and S/Cl at 0.6471 when CaO was not added to the additives (Na₂B₄O₇ and V₂O₅). However, when S/Cl was 0.6471 and another was 2.1110, there was an increase in Ni immobilisation. Lead behaved completely differently at different waste loads of sulphur and chlorine S/Cl. Here, in each case, the trend of the negative impact of a higher S/Cl value (between the lowest 0.2500 and the next 0.6471) on the retention of Pb in the ash is observed. This content is clearly smaller. Then, the trend changes to an upward one, but the percent of Pb immobilisation at S/Cl 2.1110 is still smaller than that when S/Cl was 0.2500. In the latter context, the exception is the sample with the addition of Na₂B₄O₇, burned at a temperature of 1273 K. The amount of Pb in the ash at the S/Cl value is lower than when S/Cl was 2.1110.

4. Conclusions

Laboratory tests were carried out by burning three types of waste: plastic waste (mixed and contaminated), pesticide waste, and pharmaceutical waste. The combustion processes were carried out at temperatures of 1273 K and 1373 K. The behaviours of the ecotoxic metals Cu, Ni, and Pb, which were present in the waste materials, were determined. Elements such as Cl and S were present in the combustible parts of the waste. The influence of Cl as Cl/C and S/Cl on the emissions of these metals was examined. To limit the mobility of Cu, Ni, and Pb during waste combustion, the following additives were used: V₂O₅, V₂O₅ + CaO, Na₂B₄O₇, and Na₂B₄O₇ + CaO. The amount of Cu, Ni, and Pb emissions from the waste combustion chamber depended on the type of waste, the type of metal, and its concentration in the waste, as well as the amount of Cl heteroatom and the S/Cl correlation in the waste and the process temperature. At a temperature of 1273 K, Pb was the most mobile (during the combustion of any type of waste). Lower emission values concerned Cu and Ni. An increase in the process temperature to 1373 K increased the emissions of all tested metals. The mobility of metals was influenced by the presence of Cl in the waste. The higher the share of the element chlorine in the waste, the greater the amounts of the metals Cu, Ni, and Pb transferred to the gas phase. Each additive had an immobilising effect on the tested heavy metals Cu, Ni, and Pb during the combustion of each type of waste and at each temperature. V₂O₅ showed the greatest effectiveness in retaining metals in the grate residue. Doping with CaO additives reduced the metal emissions (by binding Cl released during waste combustion) compared to the value when CaO was not added. The transformations of ecotoxic metals into their volatile chlorides occurred to a lesser extent. The mobility of metals (apart from Cl) was also influenced by the share of S in the waste. Heavy metal sulphates are less volatile than their chlorides. Therefore, higher S/Cl values in waste, for each metal, resulted in lower metal emission values, and therefore higher percent contents in the grate ash. However, the influence of S/Cl values in waste on the behaviour of Cu, Ni, and Pb during the combustion of waste with additives varied depending on the type of metal and the process temperature. Experimental results have shown that the use of immobilising additives in waste that is neutralised in the combustion process allows the reduction in the emissions of Cu, Ni, of Pb in the flue gas stream. This gives perspectives for further research regarding

• The emission and immobilisation of other ecotoxic (heavy) metals during the combustion of various wastes;

• The search for substances (or mixtures of various substances) that will effectively retain ecotoxic metals in the solid residue of the grate;

• The optimisation of the amount of additives used;

• The mathematical modelling of the immobilisation process of ecotoxic (heavy) metals on the grate.

Author Contributions

Conceptualization, D.K., S.P. and P.M.; formal analysis, D.K., S.P. and P.M.; methodology, D.K., S.P. and P.M.; software, P.M.; validation, S.Ł. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

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Figures and Tables

Figure 1. Diagram of the test stand.

Figure 2. Pathways of ecotoxic metals from incinerated waste.

Figure 3. Cu emissions during waste combustion at temperatures of 1273 K and 1373 K.

Figure 4. Ni emission during waste combustion at temperatures of 1273 K and 1373 K.

Figure 5. Pb emission during waste combustion at temperatures of 1273 K and 1373 K.

Figure 6. The influence of the amount of chlorine (Cl/C) in waste on Cu emissions during waste combustion at temperatures of 1273 K and 1373 K.

Figure 7. The influence of the amount of chlorine (Cl/C) in waste on Ni emission during waste combustion at temperatures of 1273 K and 1373 K.

Figure 8. The influence of the amount of chlorine (Cl/C) in waste on Pb emission during waste combustion at temperatures of 1273 K and 1373 K.

Figure 9. The impact of additives on Cu emissions during waste combustion at temperatures of 1273 K and 1373 K.

Figure 10. The impact of additives on Ni emission during waste combustion at temperatures of 1273 K and 1373 K.

Figure 11. The impact of additives on Pb emissions during waste combustion at temperatures of 1273 K and 1373 K.

Figure 12. The influence of S/Cl in waste on Cu, Ni, and Pb emissions during waste combustion at temperatures of 1273 K and 1373 K.

Figure 13. The influence of S/Cl in waste on Cu emissions during the combustion of waste with additives (at temperatures of 1273 K and 1373 K).

Figure 14. The influence of S/Cl in waste on Ni emission during the combustion of waste with additives (at temperatures of 1273 K and 1373 K).

Figure 15. The influence of S/Cl in waste on Pb emissions during the combustion of waste with additives (at temperatures of 1273 K and 1373 K).

Table 1

Elemental composition of flammable substance.

Type of Waste	Carbon cdaf	Hydrogen hdaf	Nitrogen ndaf	Sulphursdaf	Chlorinecldaf	Oxygenodaf
Plastic waste	71.8	8.4	0.4	0.7	2.8	15.9
Pesticide waste	70.2	7.1	2.2	1.9	0.9	17.7
Pharmaceutical waste	72.5	6.6	1.3	1.1	1.7	16.8

Table 2

Concentrations of metals in waste.

Type of Waste	Cu (ppmm)	Ni (ppmm)	Pb (ppmm)
Plastic waste	29.9	23.5	5392
Pesticide waste	57,451.0	224	178.3
Pharmaceutical waste	151.6	377.8	11.1

Table 3

S/Cl ratio values.

	Type of Waste
	Plastic Waste	Pesticide Waste	Pharmaceutical Waste
S/Cl	0.2500	2.1110	0.6471

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