Dolomites as SO2 Sorbents in Fluid Combustion Technology

Dolomites are not used as SO2 sorbents in fluid combustion technology. The literature data show fundamental discrepancies in the possibility of such use. They mainly concern the role of magnesium in the sorption process of SO2 and the durability of desulfurization products under high-temperature conditions. The article demonstrates that MgO is actively involved in the SO2 binding under fluidized furnace conditions. The resulting products of sulfation contain magnesium in their compositions, and their thermal transformations begin only after the temperature exceeds 1100 °C. It has been shown that dolomites are a potential raw material for the production of SO2 sorbents for fluid combustion technology, and their use is justified due to the higher desulfurization efficiency. Parameters of dolomite descriptions were given, by which it will be possible to predict the effects of flue gas desulfurization before the dolomites’ use in industrial conditions. It has been shown that there are opportunities to expand the domestic raw-material base for the production of SO2 sorbents, based on both dolomite resources present in deposits and dolomite waste accumulated in dumps, as well as generated during the current exploitation and processing of dolomites.


Introduction
Poland is a country rich in dolomites. The national geological balance resources of dolomites as at 31 December 2019 amounted to 498.9 million tonnes, of which 204.8 million tonnes are the resources of exploited deposits (which constitutes 41.0% of the balance resources). The prospective resources are estimated at 504.2 million tonnes. The most economically important are: Precambrian marble dolomites occurring in the Sudety; Middle Devonian and Triassic dolomites in the Silesian-Cracow region; and dolomites of the Middle Devonian in theŚwiętokrzyskie Mountains. The forecast resources are estimated at 504.2 million tons. The mentioned occurrences form a rock series extending over large areas and characterized by a variety of mineralogical and chemical characters. This is reflected in the variability of quality parameters within the documented deposits, imposing the need to separate areas suitable for different applications [1,2].
Dolomites are a typical multi-raw material with various possibilities of use. In the national "balance of mineral resources" [3], deposits of dolomite, depending on the documented use, are separated into industrial (applicable in metallurgy, the glass industry, i.e., the dolomite meal), ceramic, and agricultural (as fertilizer dolomite and dolomite feed) uses, and also for the production of calcined dolomite (used in the fireproof industry), and crushed and block stones (used in construction and road construction as building stone and crushed aggregate).
The share of magnesium in the SO 2 binding reaction is shown. However, the thermodynamic stability of desulfurization products containing magnesium in the structure is questioned under temperature conditions typical for fluidized bed furnaces, thus indicating lower sorption efficiency of dolomites [10,12,14]. 3.
The effective share of MgO in SO 2 binding is confirmed on the basis of the presence of both calcium and magnesium (CaMg 2 (SO 4 ) 3 ), and magnesium sulfates (MgSO 4 ) among the desulfurization products, which indirectly indicates their durability under the experimental temperature conditions (up to 850 • C). The desulfurization efficiency of the dolomites demonstrated in this case is comparable to and even higher than that of the limestone [5,7,8,15].
The dolomites are considered as potential SO 2 sorbents in any case, but the results of the studies show significant discrepancies in both the role of magnesium in the SO 2 binding process and the thermodynamic durability of desulfurization products containing magnesium in their compositions. The research methodology of experimental work concerning the process of sulfation is also diversified. For these purposes, thermogravimetric analysis is commonly used [5,7]. In some cases, specially constructed experimental equipment is used, which approximates the conditions in fluidized furnaces [14]. There are different variants of this type of installation, and the sulfate process itself is carried out under different conditions. For example, the gas composition (in terms of SO 2 or CO 2 concentration) used in the sulfation process is varied. The authors, comparing the SO 2 capture capacity of limestones and dolomites, as a rule ignore the influence of petrographic properties on the sulfur capture capacity [9].
The authors also note that under the conditions of fluidized furnaces, dolomite attrition was extensive compared to limestones [9,16]. They test the attrition of sorbents during the calcination process. The calcination and sulfation processes take place almost simultaneously in real conditions, which can significantly reduce the intensity of the surface abrasion and limit fragmentation of the sorbent grains. They also do not investigate the structural and textural nature of rocks and decarbonation products, which may play a decisive role in this case [17,18].
The aim of the research presented in the paper was to determine the possibility of using dolomites in the form of SO 2 sorbents in fluid combustion technology. During the research, particular attention was paid to the participation of magnesium in the SO 2 binding reaction and the durability of magnesium desulfurization products under high-temperature conditions. The parameters of these rocks responsible for the effectiveness of SO 2 sorption were also studied. The results of the research became the basis for an attempt to expand the domestic resource base with different types of minerals useful for the production of SO 2 sorbents for the needs of fluid combustion technology. As the dolomites exploited in the country represent age-diverse varieties (Precambrian, Paleozoic, Triassic, Silurian) that are also genetically varied (primary dolomites, secondary-resulting from the metasomatosis of limestone, dolomitic marbles, calcium dolomites, dolomitic limestone) thus having a varied mineral composition and structural-textural character, a research methodology was proposed that will allow us to predict the behavior of the sorbent in industrial conditions at the stage of laboratory tests.

Materials and Methods
The study used dolomites mined from deposits located in Poland: Lower Silesia-Romanowo Górne, Rędziny, Lesser Poland-Żelatowa, Upper Silesia-Chruszczobród II (Table 1, Figure 1). In the case of dolomites from the Romanowo Dolne deposit, the product of the fine regrind was the subject of the study. In other cases, rocks material derived from trials of operating walls or drilling cores were tested. For comparative studies we also covered industrial sorbent, made of limestone. The sulfation experiment was carried out based on the guidelines developed by Ahlstrom Development Laboratory [20]. This method is based on determining two indicators: the reactivity (RI index) and absolute sorption (CI-capacity index). The reactivity index determines the ratio of the calcium content (in the case of dolomite studies, magnesium was additionally included) in the sample to the amount of sulfur after the sorption process (Ca/S moles). The absolute sorption index CI, in turn, determines the amount of sulfur sorbed by 1000 g of the sorbent (g S/1000 g of the sorbent). The SO2 sorption studies were carried out using a material with a particle size of 0.125-0.250 mm. The sulfation experiment was carried out on the basis of an instance designed on the basis of a gas-tight retort furnace acting as a fixed bed. As required, the samples were subjected to a decarbonation process at 850 °C for 30 min prior to the sulfation. A sample of the sorbent mass of 150 mg was placed inside the combustion chamber, on a perforated ceramic plate, in such a way that the individual grains of the sorbent were not in contact with each other. In this way, free access of gases to individual sorbent grains was ensured during the experiment. The first, synthetic, air containing 80% N2 and 20% O2 was passed through it. Then, a gas containing 1780 ppm of SO2, 3% of O2, 16% of CO2, and N2 making up the rest was passed through the samples with a speed of 950 mL/s for another 30 min. In the next stage, the content of absorbed sulfur was determined using an elemental analysis apparatus for carbon, hydrogen, nitrogen, and sulfur (Series 628) by LECO. The results of the research became the basis for calculating the values of reactivity (RI) and absolute sorption (CI) indicators according to the formula: (2) Figure 1. The locations of dolomite deposits from which materials were collected for research on the background the main tectonic units on the sub-Cenozoic surface of the Polish [19]. Explanations: 1-Rędziny deposit; 2-Romanowo Górne deposit; 3-Żelatowa deposit; 4-Chruszczobród II deposit.
The sulfation experiment was carried out based on the guidelines developed by Ahlstrom Development Laboratory [20]. This method is based on determining two indicators: the reactivity (RI index) and absolute sorption (CI-capacity index). The reactivity index determines the ratio of the calcium content (in the case of dolomite studies, magnesium was additionally included) in the sample to the amount of sulfur after the sorption process (Ca/S moles). The absolute sorption index CI, in turn, determines the amount of sulfur sorbed by 1000 g of the sorbent (g S/1000 g of the sorbent). The SO 2 sorption studies were carried out using a material with a particle size of 0.125-0.250 mm. The sulfation experiment was carried out on the basis of an instance designed on the basis of a gas-tight retort furnace acting as a fixed bed. As required, the samples were subjected to a decarbonation process at 850 • C for 30 min prior to the sulfation. A sample of the sorbent mass of 150 mg was placed inside the combustion chamber, on a perforated ceramic plate, in such a way that the individual grains of the sorbent were not in contact with each other. In this way, free access of gases to individual sorbent grains was ensured during the experiment. The first, synthetic, air containing 80% N 2 and 20% O 2 was passed through it. Then, a gas containing 1780 ppm of SO 2 , 3% of O 2 , 16% of CO 2 , and N 2 making up the rest was passed through the samples with a speed of 950 mL/s for another 30 min. In the next stage, the content of absorbed sulfur was determined using an elemental analysis apparatus for carbon, hydrogen, nitrogen, and sulfur (Series 628) by LECO. The results of the research became the basis for calculating the values of reactivity (RI) and absolute sorption (CI) indicators according to the formula: Resources 2020, 9, 121 5 of 21 Explanations: x Ca , x C p , x S p , x C b , x S b -percentages of calcium in the sorbent, carbon in the sorbent after the sulfating process, sulfur after the sulfating process, carbon before the sulfating process, and sulfur before the sulfating process, respectively (%); M S , M Ca , M C , M CO 2 , M SO 3 -molar masses of sulfur, calcium, carbon, carbon dioxide, and sulfur trioxide, respectively (kg/kmol).
To evaluate the sorption capacity, the five-level scale proposed by Ahlstrom Development Laboratory was used (Table 2). Table 2. The reference values of the reactivity (RI) (Ca moles/S moles) and the absolute sorption (CI) (g S/1000 g of the sorbent) [20]. In the next stage of the study, an attempt to define the sorbent's parameters affecting its reactivity was made. The analyses were aimed at: Temperature and decomposition degree and the thermal dissociation course of dolomites process (STA 449 F3 Jupiter + QMS 403C Aelos, Netzsch, Selb, Germany). The research was carried out using thermogravimetry (TG) and differential thermal analysis (DTA). The samples were analyzed in the temperature range of 20-1000 • C. The heating rate was 20 • C/min. The measurements were made in the air atmosphere. -A porous texture analysis was performed using mercury porosimetry. The following porous texture parameters were determined: 1.
The coefficient of effective porosity, that is the ratio of pore volume to the total volume of the sample [21][22][23][24]: Explanations: φ-the coefficient of effective porosity (%); V tot -the total volume of mercury in the pores (mL); V b -external volume (mL); V s -skeleton volume (mL); ρ b -bulk density (g/mL); ρ s -skeletal density (g/mL).

2.
The specific surface area of porous space, that is, the pore area in relation to the sample unit mass. This parameter characterizes the flow resistance of reservoir media in the porous medium. The specific surface area, assuming the reversibility of the injection process is determined on the basis of the obtained pore volume according to the following equation [21,24]: Explanations: A-the total surface area of porous space (m 2 /g); dV-partial pore volume corresponding to the given capillary pressure (m 3 ); P-capillary pressure (psi); γ-surface tension of mercury (dyna/cm); θ-contact angle ( • ).

3.
The average pore diameter (D av ) is expressed with the weight of the pore size for the entire pore diameter range in the sample and is calculated using the following equation [21,23]: Explanations: D av -the average pore diameter (µm); V tot -the total volume of mercury in the pores (mL); A-total specific surface area of the pore space (m 2 /g).
In order to characterize the decarbonation and SO 2 sorption processes, the mentioned texture parameters were determined for both the decarbonizated and sulfated samples.
-Phase composition and textural nature of the resulting desulfurization products. X-ray diffractometer (Rigaku MiniFlex 600) and a scanning microscope (FEI Quanta 200 FEG) were used for the tests.
An attempt was made to demonstrate the stability of desulfurization products under high-temperature conditions based on DTA and TG (TGA / DSC 3+, Mettler Toledo, Greifensee, Switzerland) in the temperature range up to 1200 • C. For this purpose, the characteristic melting temperatures of desulfurization products in both oxidizing (air) and reducing atmosphere (mixture of CO and CO 2 in a volume ratio of 3:2) were also determined based on PN-G-04535: 1982 [25]. The observations were conducted by heating the test material to a temperature of 1500 • C.
The research was carried out with the use of research equipment of the AGH University of Science and Technology in Cracow with the support of the Energy Center AGH.

Phase and Chemical Composition of the Dolomites Tested
The material used in the studies represents high-quality dolomites with a CaCO 3 and MgCO 3 at the level of 48-57% wt. and 40-49% wt. respectively ( Table 3). The dolomite is by far the dominant mineral component in relation to both calcite and other minerals.
Dolomites from the Rędziny deposit represent dolomitic marbles with a characteristic granoblastic structure and a clearly marked parallel texture. The main component of the rock is dolomite, which forms crystals with a clearly elongated structure, showing the typical multiple-twins of this mineral ( Figure 2a). In addition, scattered in the rock are a few, small enclaves filled with fibrous serpentine mineral, probably chrysotile ( Figure 2b). Additionally, a few talcum crystals of a lamellar shape were identified between the dolomite crystals. The total share of non-carbonate minerals was about 5% by volume of rocks [26]. Dolomites from the Rędziny deposit represent dolomitic marbles with a characteristic granoblastic structure and a clearly marked parallel texture. The main component of the rock is dolomite, which forms crystals with a clearly elongated structure, showing the typical multiple-twins of this mineral ( Figure 2a). In addition, scattered in the rock are a few, small enclaves filled with fibrous serpentine mineral, probably chrysotile ( Figure 2b). Additionally, a few talcum crystals of a lamellar shape were identified between the dolomite crystals. The total share of non-carbonate minerals was about 5% by volume of rocks [26]. In the mineral composition of dolomites from the Romanowo Upper deposit, in addition to the dolomite, the presence of calcite and small amounts of quartz, albite, vermiculite, and flogopite were found ( Figure 3). The mineral composition and, above all, the presence of vermiculite and flogopite indicates that these dolomites are also the result of poorly advanced metamorphic transformations. In the mineral composition of dolomites from the Romanowo Upper deposit, in addition to the dolomite, the presence of calcite and small amounts of quartz, albite, vermiculite, and flogopite were found ( Figure 3). The mineral composition and, above all, the presence of vermiculite and flogopite indicates that these dolomites are also the result of poorly advanced metamorphic transformations.
In the mineral composition of dolomites from the Romanowo Upper deposit, in addition to the dolomite, the presence of calcite and small amounts of quartz, albite, vermiculite, and flogopite were found ( Figure 3). The mineral composition and, above all, the presence of vermiculite and flogopite indicates that these dolomites are also the result of poorly advanced metamorphic transformations.  In the dolomites from the Chruszczobród II deposit, the dolomite that builds the rock forms two generations of crystals. The first of these are microcrystalline crystals, occurring in spherical shapes clusters, corresponding to the relics of ooid or oncolytic forms, characteristic of limestone ( Figure 4a). The second generation is crystals that are clearly larger, often with the likes of rhomboedric (Figure 5b), showing zonal and skeletal structure. It should be assumed that these are secondary (metasomatic) dolomites resulting from the limestone dolomitization process. The process of calcite dolomitization took place under the influence of magnesium-rich waters circulating through crevices, which caused dissolution of calcium carbonate, partial discharge of Ca 2+ ions, and their replacement with Mg 2+ ions. In the final stage of the dolomitization process, the water was additionally rich in iron, which resulted in the formation of iron-enriched dolomite-Ca(Mg,Fe)(CO 3 ) 2 ( Figure 6). In the dolomites from the Chruszczobród II deposit, the dolomite that builds the rock forms two generations of crystals. The first of these are microcrystalline crystals, occurring in spherical shapes clusters, corresponding to the relics of ooid or oncolytic forms, characteristic of limestone ( Figure 4a). The second generation is crystals that are clearly larger, often with the likes of rhomboedric ( Figure  5b), showing zonal and skeletal structure. It should be assumed that these are secondary (metasomatic) dolomites resulting from the limestone dolomitization process. The process of calcite dolomitization took place under the influence of magnesium-rich waters circulating through crevices, which caused dissolution of calcium carbonate, partial discharge of Ca 2+ ions, and their replacement with Mg 2+ ions. In the final stage of the dolomitization process, the water was additionally rich in iron, which resulted in the formation of iron-enriched dolomite-Ca(Mg,Fe)(CO3)2 ( Figure 6).
In addition to the dolomite, opal and chalcedon were found in small quantities, filling rock pores mainly (Figure 4b), less commonly in the form of concretions or bioclasts-sponge needles ( Figure  5a). The pyrite framboidal occurrences, bearing clear traces of oxidation and concentration of kryptocrystalline iron compounds, found in the pores and surfaces of dolomite crystals were also identified ( Figure 5a). The described mineral phases indicate the presence of secondary mineralization processes, mainly silification and pyritization [27].      The calcite has been identified in the dolomites from the Żelatowa deposit, apart from the dolomite. The content of dolomite relative to the calcite is variable and these rocks can be classified as dolomites, calcareous dolomites, and even dolomitic limestones. Microscopic observations in cathodoluminescence (CL) revealed the presence of allochem components characteristic of limestones, such as ooids and pellets, which recrystallized during diagenesis. (Figure 7a,b). In their mineral composition, calcite occurs next to dolomite. Between them there is an abundant block cement, made of dolomite. The calcite and dolomite cleavage fissures and crystal surfaces are covered iron hydroxide compounds, with a characteristic brown color [26]. Dolomites from the Żelatowa deposit, like the dolomite from the Chruszczobród II deposit, were also formed as a result of metasomatic transformations of limestones. The diversified advancement of the dolomitization process is the result of the coexistence of limestones and dolomites in the Żelatowa deposit, as well as transition links between these rocks-calcareous dolomites and dolomitic limestones. In addition to the dolomite, opal and chalcedon were found in small quantities, filling rock pores mainly (Figure 4b), less commonly in the form of concretions or bioclasts-sponge needles (Figure 5a). The pyrite framboidal occurrences, bearing clear traces of oxidation and concentration of kryptocrystalline iron compounds, found in the pores and surfaces of dolomite crystals were also identified ( Figure 5a). The described mineral phases indicate the presence of secondary mineralization processes, mainly silification and pyritization [27].
The calcite has been identified in the dolomites from theŻelatowa deposit, apart from the dolomite. The content of dolomite relative to the calcite is variable and these rocks can be classified as dolomites, calcareous dolomites, and even dolomitic limestones. Microscopic observations in cathodoluminescence (CL) revealed the presence of allochem components characteristic of limestones, such as ooids and pellets, which recrystallized during diagenesis. (Figure 7a,b). In their mineral composition, calcite occurs next to dolomite. Between them there is an abundant block cement, made of dolomite. The calcite and dolomite cleavage fissures and crystal surfaces are covered iron hydroxide compounds, with a characteristic brown color [26]. Dolomites from theŻelatowa deposit, like the dolomite from the Chruszczobród II deposit, were also formed as a result of metasomatic transformations of limestones. The diversified advancement of the dolomitization process is the result of the coexistence of limestones and dolomites in theŻelatowa deposit, as well as transition links between these rocks-calcareous dolomites and dolomitic limestones.
The calcite has been identified in the dolomites from the Żelatowa deposit, apart from the dolomite. The content of dolomite relative to the calcite is variable and these rocks can be classified as dolomites, calcareous dolomites, and even dolomitic limestones. Microscopic observations in cathodoluminescence (CL) revealed the presence of allochem components characteristic of limestones, such as ooids and pellets, which recrystallized during diagenesis. (Figure 7a,b). In their mineral composition, calcite occurs next to dolomite. Between them there is an abundant block cement, made of dolomite. The calcite and dolomite cleavage fissures and crystal surfaces are covered iron hydroxide compounds, with a characteristic brown color [26]. Dolomites from the Żelatowa deposit, like the dolomite from the Chruszczobród II deposit, were also formed as a result of metasomatic transformations of limestones. The diversified advancement of the dolomitization process is the result of the coexistence of limestones and dolomites in the Żelatowa deposit, as well as transition links between these rocks-calcareous dolomites and dolomitic limestones.

The Sorption of Efficiency SO 2
The values of the CI and RI indexes, presented in Table 4, allowed us to assess the sorption properties of the studied dolomites as excellent in the scale presented in Table 2 (RI < 2.5; CI > 120). These values suggest that the dolomites can be treated as a potential raw material for the production of SO 2 used in fluid combustion technology. Comparing the RI and CI values of dolomites with industrial sorbent, which is the best product of this type on the domestic market, it can be concluded that sorbents produced from dolomites will be characterized by a higher SO 2 binding efficiency compared to the limestone milling products available on the market. The RI and CI values of the studied dolomites are much more favorable than the analyzed industrial sorbent. In order to confirm the high efficiency of SO 2 binding by the studied dolomites, the degree of individual dolomite grains conversion was compared with the industrial sorbent. In the photographs, sorbent grains were switched in cross-section after the sulfate process (Figure 8a,b). The interiors of dolomite grains have been evenly sulfated (Figure 8a), in contrast to the industrial sorbent. In their case, only the outer part of the grains are characterized by a high degree of conversion; the inside of the sorbet doesn't react with SO 2 (Figure 8b).
In order to confirm the high efficiency of SO2 binding by the studied dolomites, the degree of individual dolomite grains conversion was compared with the industrial sorbent. In the photographs, sorbent grains were switched in cross-section after the sulfate process (Figure 8a,b). The interiors of dolomite grains have been evenly sulfated (Figure 8a), in contrast to the industrial sorbent. In their case, only the outer part of the grains are characterized by a high degree of conversion; the inside of the sorbet doesn't react with SO2 (Figure 8b).

The Dolomite Parameters Responsible for the Efficiency of SO2 Sorption
The influence on the efficiency of SO2 sorption in fluidized bed boilers, in addition to the chemical and phase composition having: - The course and temperature of calcite/dolomite thermal dissociation; -the structural and textural nature of the sorbent, and above all its porosity, which shapes the course of the sulfation process, both in laboratory and industrial conditions [28].
The dolomite decarbonation process, regardless of the origin of this mineral, is in two stages (Table 5, Figure 9). The first step involves the decomposition of CaMg (CO3)2 into CaO and MgO and the recombination of some gaseous CO2 with CaO to form secondary CaCO3 (calcite). In the second stage, the calcite formed is decomposed into CaO and CO2.

The Dolomite Parameters Responsible for the Efficiency of SO 2 Sorption
The influence on the efficiency of SO 2 sorption in fluidized bed boilers, in addition to the chemical and phase composition having: - The course and temperature of calcite/dolomite thermal dissociation; -the structural and textural nature of the sorbent, and above all its porosity, which shapes the course of the sulfation process, both in laboratory and industrial conditions [28].
The dolomite decarbonation process, regardless of the origin of this mineral, is in two stages (Table 5, Figure 9). The first step involves the decomposition of CaMg (CO 3 ) 2 into CaO and MgO and the recombination of some gaseous CO 2 with CaO to form secondary CaCO 3 (calcite). In the second stage, the calcite formed is decomposed into CaO and CO 2 .  In the case of the studied dolomites, the decomposition of CaMg(CO3)2, related to the loss of CO2 from the crystal structure, begins after exceeding the temperature of 600 °C. At the temperature of 850 °C, the degree of decarbonation is high and amounts to 39.51-45.58% wt. Increasing the temperature to 1000 °C doesn't cause significant changes in mass, and the loss at the level of 3.0-3.5% wt. is caused by the decomposition of calcite, both originally present in the mineral composition of the rock, and secondary, formed as a result of recombination of CaO and CO2 resulting from the decomposition of the dolomite (Table 6, Figure 10).  In the case of the studied dolomites, the decomposition of CaMg(CO 3 ) 2 , related to the loss of CO 2 from the crystal structure, begins after exceeding the temperature of 600 • C. At the temperature of 850 • C, the degree of decarbonation is high and amounts to 39.51-45.58% wt. Increasing the temperature to 1000 • C doesn't cause significant changes in mass, and the loss at the level of 3.0-3.5% wt. is caused by the decomposition of calcite, both originally present in the mineral composition of the rock, and secondary, formed as a result of recombination of CaO and CO 2 resulting from the decomposition of the dolomite (Table 6, Figure 10). In the case of the studied dolomites, the decomposition of CaMg(CO3)2, related to the loss of CO2 from the crystal structure, begins after exceeding the temperature of 600 °C. At the temperature of 850 °C, the degree of decarbonation is high and amounts to 39.51-45.58% wt. Increasing the temperature to 1000 °C doesn't cause significant changes in mass, and the loss at the level of 3.0-3.5% wt. is caused by the decomposition of calcite, both originally present in the mineral composition of the rock, and secondary, formed as a result of recombination of CaO and CO2 resulting from the decomposition of the dolomite (Table 6, Figure 10).  The two-stage process of decarbonation has a decisive impact on the dolomite porosity development. Table 7 presents exemplary parameters of the porous texture of dolomites (based on the example of the dolomite from the Chruszczobród II deposit) in relation to the industrial sorbent produced from limestone, determined using the mercury porosimetry. Using this method, pores with diameters greater than 5 nm are measured, excluding the range of micropores and some of the mesopores.
Differences can also be observed in the average pore diameter between the dolomite and the industrial sorbent. The detailed information in this case is provided by the pore volume distributions as a function of the diameters presented in Figures 11 and 12. The secondary porosity of dolomite (on the example of dolomite from the Chruszczobród II deposit- Figure 12), important for the SO 2 binding process, is based on a wider range of pore diameters (from approx. 0.03-11 µm) compared to industrial sorbent, which develops porosity mainly based on pores with diameters in the narrow range of 0.3-6.0 µm (Figure 11). Differences can also be observed in the average pore diameter between the dolomite and the industrial sorbent. The detailed information in this case is provided by the pore volume distributions as a function of the diameters presented in Figures 11 and 12. The secondary porosity of dolomite (on the example of dolomite from the Chruszczobród II deposit- Figure 12), important for the SO2 binding process, is based on a wider range of pore diameters (from approx. 0.03-11 μm) compared to industrial sorbent, which develops porosity mainly based on pores with diameters in the narrow range of 0.3-6.0 μm ( Figure 11). The sorbent porosity, which is formed as a result of the release of CO2 from the calcite/dolomite structure (the so-called secondary porosity), is directly responsible for the desulfurization efficiency, due to the fact that the SO2 sorption process takes place on the internal surface of the pores formed during thermal dissociation [29]. Because the molar volume of the resulting desulfurization products in the form of CaSO4, in the case of dolomite, also CaMg2(SO4)3, is much greater with respect to CaCO3 and CaMg(CO3)2, the reactivity of the sorbent will be determined by the specific surface area capable of reacting with SO2. The surface capable of reacting with SO2 should be considered one that has been developed through pores of sufficiently large diameters, on the border of meso-and macropores (division according to International Union of Pure and Applied Chemistry).
In industrial conditions, where the processes of decarbonation and SO2 sorption occur almost simultaneously, the primary porosity of the sorbent, which is an individual feature of the rock, shaped by processes occurring during sedimentation, diagenesis, and rock epigenesis, will also be important from the sorption properties point of view. These types of pores create diffusion channels The sorbent porosity, which is formed as a result of the release of CO 2 from the calcite/dolomite structure (the so-called secondary porosity), is directly responsible for the desulfurization efficiency, due to the fact that the SO 2 sorption process takes place on the internal surface of the pores formed during thermal dissociation [29]. Because the molar volume of the resulting desulfurization products in the form of CaSO 4 , in the case of dolomite, also CaMg 2 (SO 4 ) 3 , is much greater with respect to CaCO 3 and CaMg(CO 3 ) 2 , the reactivity of the sorbent will be determined by the specific surface area capable of reacting with SO 2 . The surface capable of reacting with SO 2 should be considered one that has been developed through pores of sufficiently large diameters, on the border of meso-and macropores (division according to International Union of Pure and Applied Chemistry).
In industrial conditions, where the processes of decarbonation and SO 2 sorption occur almost simultaneously, the primary porosity of the sorbent, which is an individual feature of the rock, shaped by processes occurring during sedimentation, diagenesis, and rock epigenesis, will also be important from the sorption properties point of view. These types of pores create diffusion channels of CO 2 from the inside and SO 2 to the inside of the sorbent grains, intensifying the processes of SO 2 decarbonation and sorption. Also, in this case, dolomites are characterized by better development of both specific surface and porosity ( Table 7).
The parameters of the porous texture presented in Table 7 and the surface morphology of the sorbent grains presented in Figure 13a,b and Figure 14a,b show that the thermal dissociation process in the case of both dolomite and industrial sorbent leads to the expansion of the parameters of the porous texture, while both the specific surface and porosity of the dolomite are definitely better developed. The specific surface area of the decarbonated dolomite reaches the value of 18.57 m 2 /g, and the industrial sorbent the value of3.14 m 2 /g. In the case of effective porosity, the differences are visible in particular for the range of pore diameters from 0.01 to 10 µm, considered as sorption pores [27]: dolomite-42.76 m 2 /g; industrial sorbent-28.16 m 2 /g. The sorbent porosity, which is formed as a result of the release of CO2 from the calcite/dolomite structure (the so-called secondary porosity), is directly responsible for the desulfurization efficiency, due to the fact that the SO2 sorption process takes place on the internal surface of the pores formed during thermal dissociation [29]. Because the molar volume of the resulting desulfurization products in the form of CaSO4, in the case of dolomite, also CaMg2(SO4)3, is much greater with respect to CaCO3 and CaMg(CO3)2, the reactivity of the sorbent will be determined by the specific surface area capable of reacting with SO2. The surface capable of reacting with SO2 should be considered one that has been developed through pores of sufficiently large diameters, on the border of meso-and macropores (division according to International Union of Pure and Applied Chemistry).
In industrial conditions, where the processes of decarbonation and SO2 sorption occur almost simultaneously, the primary porosity of the sorbent, which is an individual feature of the rock, shaped by processes occurring during sedimentation, diagenesis, and rock epigenesis, will also be important from the sorption properties point of view. These types of pores create diffusion channels of CO2 from the inside and SO2 to the inside of the sorbent grains, intensifying the processes of SO2 decarbonation and sorption. Also, in this case, dolomites are characterized by better development of both specific surface and porosity ( Table 7).
The parameters of the porous texture presented in Table 7 and the surface morphology of the sorbent grains presented in Figure 13a,b and Figure 14a,b show that the thermal dissociation process in the case of both dolomite and industrial sorbent leads to the expansion of the parameters of the porous texture, while both the specific surface and porosity of the dolomite are definitely better developed. The specific surface area of the decarbonated dolomite reaches the value of 18.57 m 2 /g, and the industrial sorbent the value of3.14 m 2 /g. In the case of effective porosity, the differences are visible in particular for the range of pore diameters from 0.01 to 10 μm, considered as sorption pores [27]: dolomite-42.76 m 2 /g; industrial sorbent-28.16 m 2 /g. After the SO 2 sorption process, the surface value decreased to 0.21 m 2 /g-in the case of dolomite and 0.52 m 2 /g-industrial sorbent ( Table 7), suggesting that the dolomite surface was used more effectively (covered of sulfate). The effective porosity of the sulfated samples is high, especially of dolomite-55.39%. The value of this parameter, in combination with the size of the average pore diameter, is very favorable, due to both the decarbonation process (CO 2 release from the calcite structure) and SO 2 sorption, especially in industrial conditions, when these processes occur almost at the same time. These dependencies are illustrated by photographs showing the surface morphology of the sulfated sorbent grains (Figure 15a,b). After the SO2 sorption process, the surface value decreased to 0.21 m 2 /g-in the case of dolomite and 0.52 m 2 /g-industrial sorbent ( Table 7), suggesting that the dolomite surface was used more effectively (covered of sulfate). The effective porosity of the sulfated samples is high, especially of dolomite-55.39%. The value of this parameter, in combination with the size of the average pore diameter, is very favorable, due to both the decarbonation process (CO2 release from the calcite structure) and SO2 sorption, especially in industrial conditions, when these processes occur almost at the same time. These dependencies are illustrated by photographs showing the surface morphology of the sulfated sorbent grains (Figure 15a,b).   After the SO2 sorption process, the surface value decreased to 0.21 m 2 /g-in the case of dolomite and 0.52 m 2 /g-industrial sorbent ( Table 7), suggesting that the dolomite surface was used more effectively (covered of sulfate). The effective porosity of the sulfated samples is high, especially of dolomite-55.39%. The value of this parameter, in combination with the size of the average pore diameter, is very favorable, due to both the decarbonation process (CO2 release from the calcite structure) and SO2 sorption, especially in industrial conditions, when these processes occur almost at the same time. These dependencies are illustrated by photographs showing the surface morphology of the sulfated sorbent grains (Figure 15a  The dolomite grains were covered with sulfate with a clearly porous texture (Figure 15a). The pores are open, and their diameters reach sizes of up to several micrometers. On the industrial sorbent grains' surfaces, sulfate crusts were formed, with fewer open pores and a much smaller diameter (Figure 15b). The porous texture of the calcium sulfates produced on the sorbent grains' surface ( Figure 15a) guarantees the free flow of SO 2 , ensuring a uniform sulfation process of the grains [17]. If a sulfate with a limited porosity is formed on the sorbent grains' surface (Figure 15b), the diffusion of SO 2 in the initial desulfurization stage will be stopped, which will result in a low degree of sorbent utilization, manifested by the presence of unreacted particles (Figure 8), and under industrial conditions also with undissociated inner parts of the sorbent grains [30,31].

The Share of Magnesium in the Process of SO 2 Binding and Durability of Desulfurization Products Under High-Temperature Conditions
During analyzing the possibility of using dolomites as SO 2 sorbents in the conditions of fluid combustion technology, the most controversial is the thermodynamic stability of desulfurization products containing magnesium under high-temperature conditions [11,13,14]. The share of magnesium in the SO 2 binding process is also controversial, and the magnesium oxide (MgO) formed during the thermal dissociation of dolomite is treated as a non-reactive ballast [5,13].
The share of MgO in the SO 2 capture process was confirmed by X-ray analysis of desulfurization products obtained as a result of dolomite sulfation. The phase composition of the experimentally sulfated dolomite in the gas-tight retort furnace system presented in the diffractogram clearly indicates the presence of both calcium sulfate-CaSO 4 (anhydrite) and calcium and magnesium double sulfate-CaMg 2 (SO 4 ) 3 ( Figure 16).
( Figure 15b). The porous texture of the calcium sulfates produced on the sorbent grains' surface ( Figure 15a) guarantees the free flow of SO2, ensuring a uniform sulfation process of the grains [17]. If a sulfate with a limited porosity is formed on the sorbent grains' surface (Figure 15b), the diffusion of SO2 in the initial desulfurization stage will be stopped, which will result in a low degree of sorbent utilization, manifested by the presence of unreacted particles (Figure 8), and under industrial conditions also with undissociated inner parts of the sorbent grains [30,31].

The Share of Magnesium in the Process of SO2 Binding and Durability of Desulfurization Products Under High-Temperature Conditions
During analyzing the possibility of using dolomites as SO2 sorbents in the conditions of fluid combustion technology, the most controversial is the thermodynamic stability of desulfurization products containing magnesium under high-temperature conditions [11,13,14]. The share of magnesium in the SO2 binding process is also controversial, and the magnesium oxide (MgO) formed during the thermal dissociation of dolomite is treated as a non-reactive ballast [5,13].
The share of MgO in the SO2 capture process was confirmed by X-ray analysis of desulfurization products obtained as a result of dolomite sulfation. The phase composition of the experimentally sulfated dolomite in the gas-tight retort furnace system presented in the diffractogram clearly indicates the presence of both calcium sulfate-CaSO4 (anhydrite) and calcium and magnesium double sulfate-CaMg2(SO4)3 ( Figure 16). The dolomite desulfurization products' durability under high-temperature conditions was tested using thermogravimetric analysis and on the basis of experimentally determined characteristic temperatures of sintering, softening, melting, and flowing. The research results are presented on the example of dolomites from the Chruszczobród II deposit.
The characteristic sintering, softening, melting, and flowing temperatures of dolomite sulfation products indicate their durability under high-temperature conditions, both in oxidizing and reducing atmosphere ( Table 8). The spontaneous sintering of the tested material, manifested by the change from loose to weakly bound forms, took place after the temperature exceeded 1100 °C (1150 °Coxidizing atmosphere and 1130 °C-reducing atmosphere). The softening process, which was The dolomite desulfurization products' durability under high-temperature conditions was tested using thermogravimetric analysis and on the basis of experimentally determined characteristic temperatures of sintering, softening, melting, and flowing. The research results are presented on the example of dolomites from the Chruszczobród II deposit.
The characteristic sintering, softening, melting, and flowing temperatures of dolomite sulfation products indicate their durability under high-temperature conditions, both in oxidizing and reducing atmosphere ( Table 8). The spontaneous sintering of the tested material, manifested by the change from loose to weakly bound forms, took place after the temperature exceeded 1100 • C (1150 • C-oxidizing atmosphere and 1130 • C-reducing atmosphere). The softening process, which was accompanied by the transition to a plastic form, was observed under oxidative conditions at the temperature of 1390 • C. The changes up to 1500 • C weren't observed under reducing conditions. Similarly, in the case of melting and flowing points, characteristic changes for this type of process weren't observed in the studied temperature range. Table 8. The characteristic sintering (T S ), softening (T A ), melting (T B ), and flowing (T C ) temperatures of dolomite sulfation products presented on the example of dolomites from the Chruszczobród II deposit ( • C).

Characteristic Temperature Atmosphere
Oxidizing Reducing The durability of desulfurization products containing magnesium in high-temperature conditions is also indicated by the results of the thermogravimetric analysis. In the temperature range 920-1020 • C with a maximum at 994 • C, a exothermic effect is visible, associated with a weight loss of 11.9% wt. (of which the decomposition of calcite alone accounts for 6.7% wt.- Figure 17). This effect should be associated with the decomposition of calcite, which marked its presence on the diffractograms of dolomite samples after the decarbonation process. The decomposition of CaMg 2 (SO 4 ) 3 begins at about 1110 • C ( Figure 17). The weight loss associated with the decomposition of this sulfate at 1250 • C is 21.5% wt.
Resources 2020, 9, x FOR PEER REVIEW 17 of 20 accompanied by the transition to a plastic form, was observed under oxidative conditions at the temperature of 1390 °C. The changes up to 1500 °C weren't observed under reducing conditions. Similarly, in the case of melting and flowing points, characteristic changes for this type of process weren't observed in the studied temperature range. The durability of desulfurization products containing magnesium in high-temperature conditions is also indicated by the results of the thermogravimetric analysis. In the temperature range 920-1020 °C with a maximum at 994 °C, a exothermic effect is visible, associated with a weight loss of 11.9% wt. (of which the decomposition of calcite alone accounts for 6.7% wt.- Figure 17). This effect should be associated with the decomposition of calcite, which marked its presence on the diffractograms of dolomite samples after the decarbonation process. The decomposition of CaMg2(SO4)3 begins at about 1110 °C ( Figure 17). The weight loss associated with the decomposition of this sulfate at 1250 °C is 21.5% wt.

Conclusions
The dolomites can be treated as a potential raw material for the production of SO2 absorbents used in fluidized bed furnaces. The determined values of the reactivity indexes (RI) and absolute sorption (CI) of dolomites are characterized by higher reactivity (RI: 1.8 kmol Ca/kmol S; CI: 174 gS/1 kg sorbent) compared to sorbents produced from limestone (RI: 2.35 kmol Ca/kmol S; CI: 130 gS/1 kg sorbent). The study results showed that the porous textural parameters of the sorbent, produced during the thermal dissociation process, had a decisive influence on the efficiency of SO2 sorption.
The results of the specific surface areas and porous study of samples in their natural state, after the decarbonation and sulfation processes, made by mercury porosimetry, supported by observations using scanning microscopy, gave an excellent image of the texture of the tested dolomites and industrial sorbent, and illustrated the course of the desulfurization process. Dolomites, in comparison to limestones, have more favorable parameters of the porous texture, thanks to which they are able

Conclusions
The dolomites can be treated as a potential raw material for the production of SO 2 absorbents used in fluidized bed furnaces. The determined values of the reactivity indexes (RI) and absolute sorption (CI) of dolomites are characterized by higher reactivity (RI: 1.8 kmol Ca/kmol S; CI: 174 gS/1 kg sorbent) compared to sorbents produced from limestone (RI: 2.35 kmol Ca/kmol S; CI: 130 gS/1 kg sorbent). The study results showed that the porous textural parameters of the sorbent, produced during the thermal dissociation process, had a decisive influence on the efficiency of SO 2 sorption.
The results of the specific surface areas and porous study of samples in their natural state, after the decarbonation and sulfation processes, made by mercury porosimetry, supported by observations using scanning microscopy, gave an excellent image of the texture of the tested dolomites and industrial sorbent, and illustrated the course of the desulfurization process. Dolomites, in comparison to limestones, have more favorable parameters of the porous texture, thanks to which they are able to bind SO 2 more effectively. The efficiency of SO 2 binding in this case is determined by the size of the pores, both primary and secondary. The primary pores play the role of CO 2 and SO 2 diffusion channels, which is important especially in real conditions, where the processes of decarbonation and sorption occur simultaneously. The binding of SO 2 takes place on the inner surface of the pores formed during the thermal dissociation of the sorbent, with the production of a layer of calcium sulfates (CaSO 4 ) and calcium and magnesium sulfates (CaMg 2 (SO 4 ) 3 ) with a molar volume greater than the molar volume of carbonates (CaCO 3 , CaMg(CO 3 ) 2 ) and oxides (CaO, MgO). Consequently, this may cause blockage of pores having smaller diameters; as a result, part of the sorbent surface is excluded from the SO 2 sorption if the pores have a very small diameter, as seen in the samples of the limestone industrial sorbent. The secondary porosity is determined by the course of the decarbonation process. In the case of dolomites, this process is a two-stage course with the formation of the CaCO 3 transitional phase, which is important during the expansion of the porous texture. Another parameter, which affects the SO 2 binding efficiency, is the decarbonation temperature. Dolomites compared with limestones are characterized by a significantly lower temperature calcination. Dolomite decomposes completely below the temperature of 800 • C. The calcite decarbonation temperature often exceeds 900 • C, and practical experience shows that to obtain 100% decomposition of CaCO 3 into CaO and CO 2 , the temperature should be kept at 1000 • C. Under the fluidized bed furnaces conditions, it results in lower desulfurization efficiency. The presented research results clearly show that MgO is involved in the binding of SO 2 to form a double calcium and magnesium sulfate CaMg 2 (SO 4 ) 3 . It wasn't possible to precisely determine the Ca: Mg ratio in the resulting sulfate, due to too fine a formation of crystals and a significant share of calcium sulfate in the sulfation products. It was shown, that the desulfurization products containing magnesium in their composition are stable in temperature conditions typical for fluidized bed furnaces. This is indicated both by the sintering, softening, melting, and flowing temperatures (determined under oxidizing and reducing conditions), which are characteristic for the products of dolomite sulfation and the results of thermogravimetric analysis. Thermal transformations of magnesium-containing sulfation products begin after the temperature exceeds 1100 • C.

The Possibilities of Expanding the Domestic Base of Raw Materials for the Production of SO 2 Sorbents Based on Dolomites
The studied dolomites came from both documented deposits in the industrial category: Chruszczobród II and Rędziny, as well as block and broken stones: Romanowo Górne,Żelatowa. It should be assumed that potentially each documented deposit of dolomite in these categories may constitute a base for the production of SO 2 sorbents used in fluid combustion technology. The research results indicate that dolomites with a low calcite content will be predisposed for use as SO 2 sorbents. This is indicated by the studies of dolomites from theŻelatowa deposit, which are characterized by a variable content of calcite and can be classified both as dolomites, limestone dolomites, and even dolomitic limestones. In the case of these dolomites, the values of the reactivity index (RI) were characterized by significant differences from 1.82 to 2.45 kmol Ca/kmol S.
It should be assumed that there is also scope to expand the national raw material base for the production of SO 2 sorbents based on dolomite waste. The research results on dolomite waste from landfills located in the Rędziny mine (RI: 1.66 kmol Ca/kmol S) iŻelatowa (RI: 2.01 kmol Ca/kmol S), as well as waste dolomites from the Sieroszowice copper mine (RI: 1.58 mol Ca/mol S) indicate that they can be considered as a potential raw material for the production of this type of sorbents [26]. Predisposed for use as this type of sorbents will also be thicker grain fractions coming from the milling of dolomite for the production of dolomite flour, especially since the mill is using industrial dolomites with the highest quality parameters.
Preliminary results of the research on the sorption properties of dolomite waste after the flotation of Zn and Pb and Cu ores, indicate the possibility of their use as SO 2 sorbents in fluidized bed furnaces. In this case, Zn-Pb ore flotation waste collected in the Trzebionka sediment pond in Trzebinia was investigated (RI: 2.29 kmol Ca/kmol S), as well as that from the current production of the Copper Ore Enrichment Plant of KGHM Polska Miedź S.A. (RI: 2.12 kmol Ca/kmol S). However, the problem of metal contamination remains unresolved, which at the moment disqualifies these wastes from the possibility of such use.