Corrosive effect of wood ash produced by biomass combustion on refractory materials in a binary Al-Si system

Citation: Ovˇcaˇcíková, H.; Veliˇcka, M.; Vlˇcek, J.; Topinková, M.; Klárová, M.; Burda, J. Corrosive Effect of Wood Ash Produced by Biomass Combustion on Refractory Materials in a Binary Al–Si System. Materials 2022,15, 5796. https://doi.org/ 10.3390/ma15165796 Academic Editor: Panos Tsakiropoulos Received: 31 July 2022 Accepted: 18 August 2022 Published: 22 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Corrosive Effect of Wood Ash Produced by Biomass Combustion on Refractory Materials in a Binary Al–Si System Hana Ovˇcaˇcíková* , Marek Veliˇcka , Jozef Vlˇcek, Michaela Topinková, Miroslava Klárováand JiˇríBurda Department of Thermal Engineering, Faculty of Materials Science and Technology, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic *Correspondence: [email protected]; Tel.: +4-205-9732-1523 Abstract: In terms of its chemical composition, biomass is a very complex type of fuel. Its combustion leads to the formation of materials such as alkaline ash and gases, and there is evidence of the corrosive effect this process has on refractory linings, thus shortening the service life of the combustion unit. This frequently encountered process is known as “alkaline oxidative bursting”. Corrosion is very complex, and it has not been completely described yet. Alkaline corrosion is the most common cause of furnace-lining degradation in aggregates that burn biomass. This article deals with an experiment investigating the corrosion resistance of 2 types of refractory materials in the Al 2 O 3 -SiO 2 binary system, for the following compositions: I. (53 wt.% SiO 2 /42 wt.% Al 2 O 3 ) and II. (28 wt.% SiO 2 / 46 wt.% Al 2 O 3 /12 wt.% SiC). These were exposed to seven types of ash obtained from one biomass combustion company in the Czech Republic. The chemical composition of the ash is a good indicator of the problematic nature of a type of biomass. The ashes were analyzed by X-ray diffraction and X-ray fluorescence. Analysis confirmed that ash composition varies. The experiment also included the calculation of the so-called “slagging/fouling index” (I/C, TA, Sr, B/A, Fu, etc.), which can be used to estimate the probability of slag formation in combustion units. The corrosive effect on refractory materials was evaluated according to the norm ˇ CSN P CEN/TS 15418, and a static corrosion test was used to investigate sample corrosion. Keywords: corrosion; refractory; biomass; thermal processing; wood ash 1. Introduction Worldwide, 80% of electricity is produced using fossil fuels. According to the International Energy Agency (IEA), electricity production reached approximately 25.8 T-kWh in 2020, and an increase to 36.5 T-kWh is expected by 2040 [ 1 ]. As stated by the World Bioenergy Association, 59.2 NPP/year, i.e., 10.3% of the global energy supply, comes from biomass [ 2 ]. Biomass is becoming a popular source of energy which can be used in various ways. Electricity produced from biomass currently corresponds to 493 TWh, which is approximately 2% of the world’s electricity production [ 2 ]. Using biomass as a raw material for powerplants is certainly interesting and useful; however, this technology also has certain disadvantages. The use of biomass in powerplants leads to the formation of residue, called biomass ash. It is estimated that around 480 million tons of ash are produced every year by biomass powerplants worldwide. This is similar to coal ash, with 780 million tons per year [2]. The most frequently burned material is wood (64%), followed by cereals and plant residue from agricultural production. In general, it can be said that the average percentage of ash produced by burning biomass ranges between 1 and 6%; for wood, it is 0.6–1.6%; for bark, it rarely exceeds 3%; straw produces an ash content of around 5%, while grass produces 7%. At the other end of the spectrum, the ash content produced by black coal is significantly higher, reaching 20–30%, and from brown coal, this amount can be even greater [3]. Ash represents a variable composition of mineral and inorganic components. Materials 2022,15, 5796. https://doi.org/10.3390/ma15165796 https://www.mdpi.com/journal/materials Materials 2022,15, 5796 2 of 15 During the combustion process, ash continuously changes its physical and chemical properties, the final product being a molten mixture of original minerals, various eutectics, and elements. Ash causes various problems, especially corrosion, erosion, stickers, etc. [ 4 ] If the melting temperature of ash during combustion is t ash < t flame , then the grate of the hearth can become clogged. Ash layers on the walls of the furnace diffuse into the lining, which then peels off in thin layers. The combustion chamber of the boiler must therefore be structurally adjusted such that the flame temperature drops below the ash melting temperature, i.e., the temperature on the grate should be lower than the melting temperature of the biomass ash [5]. The major problem from a chemical point of view is corrosion, which comes from the interaction between a refractory and a corrosive medium: gas, molten metals, molten glass, molten salts, or slag. It results in a loss of mass and thickness and in the degradation of the material properties [ 6 ]. The corrosion of refractory materials is a combination of external and internal physical and chemical influences. The process is basically a chemical reaction between the refractory material and the slag or metal. Reactants are transported to the interface of the refractory material, and, in turn, the product reacts and is transported to the liquid phase. The dissolution of refractory materials in the melt is controlled by diffusion. Three types of corrosion have been defined: surface, dimple, and undersurface corrosion [7]. Alkaline corrosion, or “alkaline oxidative bursting”, is extremely common, effective, and particularly harmful to alumina–silicon (Al–Si) lining systems, and it is usually observed in the temperature range of aggregates of 800–1000 ◦ C. During biomass combustion, damage to the refractory lining is observed (Figure 1) as the peeling of surface layers, cracking, the bending of individual parts of the lining, the bulging of entire walls, and eventually their collapse [8,9]. Materials 2022, 15, x FOR PEER REVIEW 2 of 16 significantly higher, reaching 20–30%, and from brown coal, this amount can be even greater [3]. Ash represents a variable composition of mineral and inorganic components. During the combustion process, ash continuously changes its physical and chemical properties, the final product being a molten mixture of original minerals, various eutectics, and elements. Ash causes various problems, especially corrosion, erosion, stickers, etc. [4] If the melting temperature of ash during combustion is tash < tflame, then the grate of the hearth can become clogged. Ash layers on the walls of the furnace diffuse into the lining, which then peels off in thin layers. The combustion chamber of the boiler must therefore be structurally adjusted such that the flame temperature drops below the ash melting temperature, i.e., the temperature on the grate should be lower than the melting temperature of the biomass ash [5]. The major problem from a chemical point of view is corrosion, which comes from the interaction between a refractory and a corrosive medium: gas, molten metals, molten glass, molten salts, or slag. It results in a loss of mass and thickness and in the degradation of the material properties [6]. The corrosion of refractory materials is a combination of external and internal physical and chemical influences. The process is basically a chemical reaction between the refractory material and the slag or metal. Reactants are transported to the interface of the refractory material, and, in turn, the product reacts and is transported to the liquid phase. The dissolution of refractory materials in the melt is controlled by diffusion. Three types of corrosion have been defined: surface, dimple, and undersurface corrosion [7]. Alkaline corrosion, or “alkaline oxidative bursting”, is extremely common, effective, and particularly harmful to alumina–silicon (Al–Si) lining systems, and it is usually observed in the temperature range of aggregates of 800–1000 °C. During biomass combustion, damage to the refractory lining is observed (Figure 1) as the peeling of surface layers, cracking, the bending of individual parts of the lining, the bulging of entire walls, and eventually their collapse [8,9]. Figure 1. Degradation of refractory materials in boilers after combustion of different types of biomasses [8,9]: (1) the damage to refractory materials after 1 year of the combustion of wood chips; (2) the furnace vault after 1.5 years of the combustion of chipboard; (3) the corroded part of refractory samples after 2 years of combustion of plant biomass, and (4–9) the presentation of the defects of the refractory lining after the combustion of biomass for 10 years of operation. The main difference between coal ash and biomass ash is that coal ash contains higher amounts of SiO2 and Al2O3, but it contains lower amounts of K2O and Na2O. The eutectic of Al–Si forming fly ash lies above 1200 °C, while the eutectic of plant fly ash is much lower. Eutectic temperatures for mixtures of alkali metals together with silica or phosphorus have a low melting point: Na2O.2SiO2 (874 °C), K2O.4SiO2 (770 °C), and 2CaO.3P2O5 (774 °C) [10]. Figure 1. Degradation of refractory materials in boilers after combustion of different types of biomasses [ 8 , 9 ]: ( 1 ) the damage to refractory materials after 1 year of the combustion of wood chips; ( 2 ) the furnace vault after 1.5 years of the combustion of chipboard; ( 3 ) the corroded part of refractory samples after 2 years of combustion of plant biomass, and ( 4 – 9 ) the presentation of the defects of the refractory lining after the combustion of biomass for 10 years of operation. The main difference between coal ash and biomass ash is that coal ash contains higher amounts of SiO 2 and Al 2 O 3 , but it contains lower amounts of K 2 O and Na 2 O. The eutectic of Al–Si forming fly ash lies above 1200 ◦ C, while the eutectic of plant fly ash is much lower. Eutectic temperatures for mixtures of alkali metals together with silica or phosphorus have a low melting point: Na 2 O.2SiO 2 (874 ◦ C), K 2 O.4SiO 2 (770 ◦ C), and 2CaO.3P 2 O 5 (774 ◦C) [10]. Aluminosilicate refractories are based on the SiO 2 -Al 2 O 3 system. The equilibrium diagram of this system is given in Figure 2, marking various refractories. The main phase Materials 2022,15, 5796 3 of 15 in the Al–Si binary diagram is mullite (3Al 2 O 3 .2SiO 2 ) [ 11 ], which increases the resistance of the refractory material against the corrosive effects of ash [11]. Materials 2022, 15, x FOR PEER REVIEW 3 of 16 Aluminosilicate refractories are based on the SiO2-Al2O3 system. The equilibrium diagram of this system is given in Figure 2, marking various refractories. The main phase in the Al–Si binary diagram is mullite (3Al2O3.2SiO2) [11], which increases the resistance of the refractory material against the corrosive effects of ash. [11]. Figure 2. Categorization of basic refractory materials in binary diagram of SiO2-Al2O3. Note: * the amount of Al2O3. In the AL–SI system, new phases are often formed as a result of different chemical reactions, gradually degrading the system. The newly formed products have a larger volume than the original material, with expansion being reported between 7 and 30%. This creates compounds in the lining or on its surface that have chemical compositions and physical parameters different from the lining itself [12]. Ternary diagrams of the types Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 also describe the formation of individual phases in the given system (see Figure 3). Figure 3. Ternary diagram of Na2O-SiO2-Al2O3 [13] and K2O-Al2O3-SiO2 [14], marking the individual phases formed during alkaline corrosion. The corrosion mechanism in the Na–Al–Si system includes the formation of albite (NaAlSi3O8), nosean (Na8Al6Si6O28S) [15,16], and natrosilite (Na2Si2O5) by Equation (1), which further reacts with mullite (Al6Si2O13) to form albite (NaAlSi3O8) and aluminum oxide according to Equation (2). Nepheline (NaAlSiO4) can also be formed according to Equation (3). Nosean is rarely reported in the literature as a corrosion product. However, Figure 2. Categorization of basic refractory materials in binary diagram of SiO 2 -Al 2 O 3 . Note: * the amount of Al2O3. In the AL–SI system, new phases are often formed as a result of different chemical reactions, gradually degrading the system. The newly formed products have a larger volume than the original material, with expansion being reported between 7 and 30%. This creates compounds in the lining or on its surface that have chemical compositions and physical parameters different from the lining itself [ 12 ]. Ternary diagrams of the types Na 2 O-Al 2 O 3 -SiO 2 and K 2 O-Al 2 O 3 -SiO 2 also describe the formation of individual phases in the given system (see Figure 3). Materials 2022, 15, x FOR PEER REVIEW 3 of 16 Aluminosilicate refractories are based on the SiO2-Al2O3 system. The equilibrium diagram of this system is given in Figure 2, marking various refractories. The main phase in the Al–Si binary diagram is mullite (3Al2O3.2SiO2) [11], which increases the resistance of the refractory material against the corrosive effects of ash. [11]. Figure 2. Categorization of basic refractory materials in binary diagram of SiO2-Al2O3. Note: * the amount of Al2O3. In the AL–SI system, new phases are often formed as a result of different chemical reactions, gradually degrading the system. The newly formed products have a larger volume than the original material, with expansion being reported between 7 and 30%. This creates compounds in the lining or on its surface that have chemical compositions and physical parameters different from the lining itself [12]. Ternary diagrams of the types Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 also describe the formation of individual phases in the given system (see Figure 3). Figure 3. Ternary diagram of Na2O-SiO2-Al2O3 [13] and K2O-Al2O3-SiO2 [14], marking the individual phases formed during alkaline corrosion. The corrosion mechanism in the Na–Al–Si system includes the formation of albite (NaAlSi3O8), nosean (Na8Al6Si6O28S) [15,16], and natrosilite (Na2Si2O5) by Equation (1), which further reacts with mullite (Al6Si2O13) to form albite (NaAlSi3O8) and aluminum oxide according to Equation (2). Nepheline (NaAlSiO4) can also be formed according to Equation (3). Nosean is rarely reported in the literature as a corrosion product. However, Figure 3. Ternary diagram of Na 2 O-SiO 2 -Al 2 O 3 [ 13 ] and K 2 O-Al 2 O 3 -SiO 2 [ 14 ], marking the individual phases formed during alkaline corrosion. The corrosion mechanism in the Na–Al–Si system includes the formation of albite (NaAlSi 3 O 8 ), nosean (Na 8 Al 6 Si 6 O 28 S) [ 15 , 16 ], and natrosilite (Na 2 Si 2 O 5 ) by Equation (1), which further reacts with mullite (Al 6 Si 2 O 13 ) to form albite (NaAlSi 3 O 8 ) and aluminum oxide according to Equation (2). Nepheline (NaAlSiO 4 ) can also be formed according to Equation (3). Nosean is rarely reported in the literature as a corrosion product. However, Materials 2022,15, 5796 4 of 15 due to its structural similarity to nepheline, it can also be expected to produce swelling. The reaction can be described by Equation (4) [17]: Na2SO4+ 2SiO2= Na2Si2O5+ SO2+ 1/2O2(1) Na2Si2O5+ 2Al6Si2O13 = 2NaAlSi3O8+ 5Al2O3(2) 2NaAlSi3O8+ Al6Si2O13 + 3Na2SO4= 8NaAlSiO4+ 3SO2+ 3/2O2(3) 4Na2SO4+ 3Al6Si2O13 = Na8Al6Si6O28S + 6Al2O3+ 3SO2+ 3/2O2(4) In the case of high-alumina refractories (>45% Al 2 O 3 ) containing mullite (A 3 S 2 ) and cristobalite (SiO 2 ), reaction with NaO 2 above 1000 ◦ C forms nepheline (NaS 2 ) and α -Al 2 O 3 according to Equation (5). As can be deduced from the ternary diagram K 2 O-Al 2 O 3 -SiO 2 , at a lower content of Al 2 O 3 < 30%, orthoclase KAS 6 is formed, and at a content of Al2O3> 30% , new phases of leucite (KAS4) are formed according to Equation (6): 3Al2O3·2SiO2+ Na2O→Na2O·Al2O3·2SiO2+ 2Al2O3(5) K2O·Al2O3·6SiO2→K2O·Al2O3·4SiO2+ 2SiO2(6) Since the composition of biomass ash encourages the formation of eutectic melts, it is advisable to use high-alumina refractory materials with an Al 2 O 3 content > 80% or to add silicon carbide for these linings. The compound, aluminosilicate-based materials mainly include products containing oxide-less constituents—graphite and silicon carbide Materials in Al 2 O 3 -SiO 2 -SiC systems combine the high thermal conductivity and chemical inertness of silicon carbide with the chemical and thermal stability of aluminosilicate and corundum. The products are therefore highly resistant to corrosion by liquid metals, as well as to sudden changes in temperature. SiC oxidizes according to Equations (7) and (8) and creates an amorphous SiO 2 film on the surface [18,19]: SiC + 1.5O2→SiO2+ CO (7) SiC + 2O2→SiO2+ CO2(8) To prevent graphite oxidation, firing is carried out without any contact between the fired product and oxygen. The firing temperature is chosen to create a ceramic bond in the products. At present, the process of quick firing is used, ensuring a reducing atmosphere in the kilns at higher temperatures and while cooling the products. K 2 O and Na 2 O, in the form of alkaline vapors, are capable of diffusing into the refractory matrix, and then they react with Al 2 O 3 and SiO 2 components to form K-aluminosilicate and Na-aluminosilicate phases [ 20 ]. In the AL–SI binary system, potassium pairs react according to Equations (9)–(11). The most harmful is the presence of free SiO 2 and Na 2 O, which increase the reaction rate at high temperatures and support the formation of reactive glassy phases, according to Equation (12) [20]: K2O + SiO2→K2O.SiO2(9) 3 (K2O.2SiO2) + 3Al2O3.2SiO2→3 (K2O Al2O3.2SiO2) + 2SiO2(10) K2O.Al2O3.2SiO2+ 2SiO2→K2O.Al2O3.4SiO2(11) 2SiO2+ Na2O→Na2O·2SiO2(12) The so-called slagging/fouling index can be used to estimate the probability of slag formation in combustion units during biomass combustion. Slagging/fouling means the formation of layers (sticky, melted, or soft) of ash particles on heat exchange surfaces. A summary of slagging and fouling indices and their calculation are presented in Table 1. Materials 2022,15, 5796 5 of 15 Table 1. Ash characterization indices [10,21–24]. Index Equation Tendency Slagging/Fouling Low Middle High Ex. High SiO2(%) - <20 20–25 >25 Cl (%) - <0.2 0.2–0.3 0.3–0.5 >0.5 B/A B A=Fe2O3+CaO+MgO+Na2O+K2O SiO2+Al2O3+TiO2<0.5 0.5–1 1–1.75 >1.75 S/A S/A=SiO2 Al2O3<0.31 - 0.3–3 - I/C I C=Fe2O3 CaO <0.31 0.3–3 >3 - Fu Fu =B A·(Na2O+K2O)<0.6 0.6–40 >40 - TA TA =Na2O+K2O <0.3 0.3 < TA < 0.4 >0.4 - Sr Sr =SiO2 SiO2+Fe2O3+CaO+MgO ·100 >72 65–72 <65 The SiO 2 index is often the predominant element in biomass samples and causes the formation of melt, or “stickers”, therefore giving it the characteristic of being slag-forming. The chlorine index Cl acts as an accelerator of the reaction between K and SiO2, which leads to the formation of fused glass deposits and the formation of slag at boiler operating temperatures of 800–900 ◦C [23]. Ash-deposition potential may be evaluated in terms of base-to-acid (B/A). The basicity index B/A (base/acid ratio) is based on the general rule that basic oxide compounds lower the melting point, and acidic compounds raise it. The B/A ratio is an indication of the fusion and slagging potential of ash. I/C (iron/calcium ratio) stands for Fe 2 O 3 /CaO, e.g., ash with a ratio of Fe2O3/CaO = 0.3/3.0 containing eutectics that increase slag formation. The Fouling index Fu (fouling index) is the B/A ratio, also taking into account the alkali content (Na 2 O + K 2 O). Fouling refers to the dry deposition of ash particles or the condensation of volatile inorganic components on heat transfer surfaces. The normal percentage of alkali in biomass ash is between 25 and 35%, and it forms a eutectic in combination with silica. Ash has a high viscosity (Sr) value, slag viscosity index Sr [ 24 ], so it will have a low tendency to slag. The TA (total alkali) index assesses the fuel’s ability to form ash layers. Values of individual ash samples, defined based on the above-mentioned indices, are summarized in Section 3.2. The chemical composition of ash is a good indicator of the problematic nature of biomass. For biomass fuels, massive slagging of heat exchange surfaces of boilers occurs during combustion. Ash composition and atmosphere in a combustion chamber influence the ash-melting temperature [ 10 ]. Indicators tell us of the characteristics of ash in terms of their influence on the formation of the glassy phase, and thus their tendency to slag and clog linings, heat exchange surfaces, and gas flow routes. These indices are based on chemical composition of biomass and its combustion. The equations are mainly based on fuel evaluation. However, since there is no specific index for biomass, it is possible to apply these indices to this type of fuel as well. 2. Materials and Methods 2.1. Ashes from Wood Biomass Combustion Seven different types of ash from different types of wood biomass were used for the experimental portion of our study. All of these were obtained from the Czech Republic, mainly from the Moravian–Silesian Region, but one was from the Central Bohemian Region. Ashes utilized during the experimental portion were used in the original form for the crucible test. the granulometry was not adjusted. More information about the ash samples is presented in Table 2. Materials 2022,15, 5796 6 of 15 Table 2. Characterization of wood ash used for experiment. Type of Wood Biomass Disposal Method Labeled Spruce pellets combustion P019 Woodchips combustion P020 Woodchips combustion P031 Woodchips, woodbark, sawdust, pellets, scraps combustion P032 Woodchips, woodbark, sawdust, pellets, scraps combustion P033 Woodchips gassification P059, P060 Materials 2022, 15, x FOR PEER REVIEW 6 of 16 the crucible test. the granulometry was not adjusted. More information about the ash samples is presented in Table 2. Table 2. Characterization of wood ash used for experiment. Type of Wood Biomass Disposal Method Labeled Spruce pellets combustion P019 Woodchips combustion P020 Woodchips combustion P031 Woodchips, woodbark, sawdust, pellets, scraps combustion P032 Woodchips, woodbark, sawdust, pellets, scraps combustion P033 Woodchips gassification P059, P060 2.2. Refractory Materials Tested refractory materials were manufactured by one of the largest producers and suppliers of refractory products and raw materials in the Czech Republic. Two types of shaped refractory materials, belonging to the silica–aluminum group, were selected for the corrosion experiment. The first type was quality labeled as STV. It is a shaped refractory material classified as standard fire clay. The second type was quality labeled as ARS60N and is classified high alumina. The parameters of the mentioned tested materials with their properties are shown in Table 3. Table 3. Chemical composition and properties of refractory materials. Oxides wt.% STV ARS60N SiO2 53.5 28.40 Al2O3 40.5 46.60 TiO2 2.1 - Fe2O3 2.1 0.88 CaO 0.3 0.2 MgO 0.3 0.27 K2O+ Na2O 0.8 + 0.2 0.5 SiC - 13.2 Bulk density (kg/m3) 2150 2700 Apparent porosity (%) 18.0 15 Cold crushing strength (MPa) 30 70 Refractory qualities under load (RUL) T0.5 (°C) 1360 >1500 2.3. Corrosion Crucible Test and Evaluation Method The crucible test gives only approximate results. The refractory cube was filled with corrodent and heated to the testing temperature for a specified period. The testing conditions (temperature and corrodent composition) may reflect the expected service conditions, but in some situations, a more aggressive corrodent and/or high temperature may be used to speed up the attack to determine the resistance of the refractory to the corrosive liquid in a relatively short time. The crucible test is described step by step in Figure 4. The refractory cuboid sample with a cylindrical hole in the central portion was filled with cor2.2. Refractory Materials Tested refractory materials were manufactured by one of the largest producers and suppliers of refractory products and raw materials in the Czech Republic. Two types of shaped refractory materials, belonging to the silica–aluminum group, were selected for the corrosion experiment. The first type was quality labeled as STV. It is a shaped refractory material classified as standard fire clay. The second type was quality labeled as ARS60N and is classified high alumina. The parameters of the mentioned tested materials with their properties are shown in Table 3. Table 3. Chemical composition and properties of refractory materials. Oxides wt.% STV ARS60N SiO253.5 28.40 Al2O340.5 46.60 TiO22.1 - Fe2O32.1 0.88 CaO 0.3 0.2 MgO 0.3 0.27 K2O + Na2O 0.8 + 0.2 0.5 SiC - 13.2 Bulk density (kg/m3)2150 2700 Apparent porosity (%) 18.0 15 Cold crushing strength (MPa) 30 70 Refractory qualities under load (RUL) T0.5 (◦C) 1360 >1500 2.3. Corrosion Crucible Test and Evaluation Method The crucible test gives only approximate results. The refractory cube was filled with corrodent and heated to the testing temperature for a specified period. The testing conditions (temperature and corrodent composition) may reflect the expected service conditions, but in some situations, a more aggressive corrodent and/or high temperature may be used to speed up the attack to determine the resistance of the refractory to the corrosive liquid in a relatively short time. The crucible test is described step by step in Figure 4. The refractory cuboid sample with a cylindrical hole in the central portion was filled with corrosive, medium/powdered ash with a heating temperature of 1200 ◦ C for 2 h. After cooling, the tested sample was cut through along the vertical axis, and the corroded portion was measured. Materials 2022,15, 5796 7 of 15 Materials 2022, 15, x FOR PEER REVIEW 7 of 16 rosive, medium/powdered ash with a heating temperature of 1200 °C for 2 h. After cooling, the tested sample was cut through along the vertical axis, and the corroded portion was measured. Figure 4. Schematic diagram of the crucible test of refractory materials. After the corrosion test, samples were visually checked for compactness, potential cracks, and holes in the sample and walls. The ČSN P CEN/TS 15418 method [25] and the internal regulation method of P-D Refractories CZ a.s. [26] were used for test evaluation. The classification used for reporting the condition of the crucible with defined parameters [25] U: unaffected/no visible attack; LA: lightly attacked/minor attack; A: attacked/clearly attacked and C: corroded/completely corroded. In addition to the abovementioned evaluation regulations, another internal regulation method of P-D Refractories CZ was also used [26]. Table 4 shows the parameters of the classification after the corrosion test. Two evaluation methods may sometimes be requested by a customer or company testing laboratory, and the parameters can be used for comparison. Table 4. Alkali test classification after internal regulation of P-D Refractories CZ [26]. Class Classification Corrosion Infiltration Cracks A not attacked no corrosion and/or infiltration No B slight attack <6 mm corrosion and/or infiltration No C distinctive attack >7 mm corrosion and/or infiltration Slight D severe attack >9 mm corrosion and/or infiltration large, clearly visible cracks 2.4. Characterization Methods The chemical composition (XRF) of the ash was determined by energy-dispersive XRay fluorescence spectroscopy (ED-XRF) on the SPECTRO XEPOS (Spectro Analytical Instruments, Kleve, Germany). Powdered samples were shaped/pressed into tablets for XRD measurement. The mineralogical composition (XRPD) of the samples was evaluated using X-Ray diffraction analysis on the X-Ray diffractometer MiniFlex 600 (Rigaku, Tokyo, Japan) equipped with a 0Co tube and a D/teX Ultra 250 detector. XRD patterns were recorded in a 5–90° 2θ range with a scanning rate of 5° min−1. 3. Results and Discussion 3.1. Ash Characterization Chemical analysis is a good indicator for determining the problematic nature of biomass. The chemical composition of all of the ash types is presented in Figure 5. Biomass Figure 4. Schematic diagram of the crucible test of refractory materials. After the corrosion test, samples were visually checked for compactness, potential cracks, and holes in the sample and walls. The ˇ CSN P CEN/TS 15418 method [ 25 ] and the internal regulation method of P-D Refractories CZ a.s. [26] were used for test evaluation. The classification used for reporting the condition of the crucible with defined parameters [ 25 ] U: unaffected/no visible attack; LA: lightly attacked/minor attack; A: attacked/clearly attacked and C: corroded/completely corroded. In addition to the abovementioned evaluation regulations, another internal regulation method of P-D Refractories CZ was also used [26]. Table 4shows the parameters of the classification after the corrosion test. Two evaluation methods may sometimes be requested by a customer or company testing laboratory, and the parameters can be used for comparison. Table 4. Alkali test classification after internal regulation of P-D Refractories CZ [26]. Class Classification Corrosion Infiltration Cracks A not attacked no corrosion and/or infiltration No B slight attack <6 mm corrosion and/or infiltration No C distinctive attack >7 mm corrosion and/or infiltration Slight D severe attack >9 mm corrosion and/or infiltration large, clearly visible cracks 2.4. Characterization Methods The chemical composition (XRF) of the ash was determined by energy-dispersive X-ray fluorescence spectroscopy (ED-XRF) on the SPECTRO XEPOS (Spectro Analytical Instruments, Kleve, Germany). Powdered samples were shaped/pressed into tablets for XRD measurement. The mineralogical composition (XRPD) of the samples was evaluated using X-ray diffraction analysis on the X-ray diffractometer MiniFlex 600 (Rigaku, Tokyo, Japan) equipped with a 0Co tube and a D/teX Ultra 250 detector. XRD patterns were recorded in a 5–90◦2θrange with a scanning rate of 5◦min−1. 3. Results and Discussion 3.1. Ash Characterization Chemical analysis is a good indicator for determining the problematic nature of biomass. The chemical composition of all of the ash types is presented in Figure 5. Biomass ash almost always contains carbonates, especially calcite, and very often portlandite, as well as a proportion of organic carbon. Materials 2022,15, 5796 8 of 15 Materials 2022, 15, x FOR PEER REVIEW 8 of 16 ash almost always contains carbonates, especially calcite, and very often portlandite, as well as a proportion of organic carbon. Figure 5. Concentrations of major elements in ash after wood biomass combustion. Oxides in biomass ash can be divided into acidic (SiO2, Al2O3, TiO2, etc.) and basic (K2O, CaO, MgO, Na2O, Fe2O3, P2O5, etc.). Acidic oxides increase the melting point of ash. The higher the content of acidic oxides, the higher the melting point. On the other hand, basic oxides lower the melting point of the ash. The predominant oxides are SiO2 and CaO. A high level of CaO is typical for wood. The higher the content of basic oxides, the lower the melting point. SiO2 plays an important role as a glass-forming oxide, while CaO and K2O reduce the viscosity of the resulting glass-forming melt. The nature of the oxides and their representation determines the formation of other compounds and the behavior of the refractory material in contact with the corrosive agent. Ash was analyzed by XRDF, and this showed variable sample composition. The percentage of single oxides is as follows: SiO2 9.13–55.17 wt.%, CaO 16.33–41.79 wt.%, Al2O3 0.98–10,14 wt.%, Fe2O3 1.80–13.16 wt.%. For alkali oxides it is Na2O 0.38–12.23 wt.% and K2O 6.11–19.17 wt.%. The amount of Cl is around 0.6 wt.%. In terms of chemical composition, ash resembles low-melting glass. The variability of chemical composition complicates accurate representation in a ternary diagram. An approximate composition based on the largest content of wt.% of oxides is shown in the diagram. Four ash types, labeled P020, P033, P059 and P060, are marked in the CaO-Al2O3-SiO2 ternary diagram, and two types, labeled P031 and P019 are marked in the K2O-SiO2-CaO system, as presented in Figure 6. Figure 5. Concentrations of major elements in ash after wood biomass combustion. Oxides in biomass ash can be divided into acidic (SiO 2 , Al 2 O 3 , TiO 2 , etc.) and basic (K 2 O, CaO, MgO, Na 2 O, Fe 2 O 3 , P 2 O 5 , etc.). Acidic oxides increase the melting point of ash. The higher the content of acidic oxides, the higher the melting point. On the other hand, basic oxides lower the melting point of the ash. The predominant oxides are SiO 2 and CaO. A high level of CaO is typical for wood. The higher the content of basic oxides, the lower the melting point. SiO 2 plays an important role as a glass-forming oxide, while CaO and K 2 O reduce the viscosity of the resulting glassforming melt. The nature of the oxides and their representation determines the formation of other compounds and the behavior of the refractory material in contact with the corrosive agent. Ash was analyzed by XRDF, and this showed variable sample composition. The percentage of single oxides is as follows: SiO 2 9.13–55.17 wt.%, CaO 16.33–41.79 wt.%, Al 2 O 3 0.98–10.14 wt.%, Fe 2 O 3 1.80–13.16 wt.%. For alkali oxides it is Na 2 O 0.38–12.23 wt.% and K2O 6.11–19.17 wt.%. The amount of Cl is around 0.6 wt.%. In terms of chemical composition, ash resembles low-melting glass. The variability of chemical composition complicates accurate representation in a ternary diagram. An approximate composition based on the largest content of wt.% of oxides is shown in the diagram. Four ash types, labeled P 020 , P 033 , P 059 and P 060 , are marked in the CaOAl 2 O 3 -SiO 2 ternary diagram, and two types, labeled P 031 and P 019 are marked in the K2O-SiO2-CaO system, as presented in Figure 6. Materials 2022, 15, x FOR PEER REVIEW 9 of 16 Figure 6. Approximate position of ash types P020, P033, P059, P060, in ternary diagram CaO–Al2O3–SiO2 [27] and P031 and P019 in ternary diagram K2O–Al2O3–SiO2 [28]. The next method of ash characterization was X-ray powder diffraction phase analysis (XRPD). The samples were compared to the reference diffractogram database published by ICDD (PDF-2) in the range of 5–90° 2theta. The results for the analyzed samples are presented in Table 5, where there is an overview of the phases in the samples. Table 5. Phase composition of analyzed biomass ash samples. Phase Composition Labeled of Sample P019 P020 P031 P032 P033 P059 P060 quartz (SiO2) x x x x x calcite (CaCO3) x x X x x x X graphite C x CaO x x X x magnesite (MgCO3) X MgO X x anorthite (CaAl2Si2O8) x microcline (KAlSi3O8) x x x x arcanite (K2SO4) X anhydrite (CaSO4) x anorthoclase x leucite (KAlSi2O6) x x orthoclase (KAlSi3O8) x sylvite (KCl) x portlandite Ca(OH)2 x x hematite (Fe2O3) x x mullite (Al4.59Si1.41O0.97) x analcime (NaAlSi2O6) x As confirmed by the analysis, the most frequently recurring phases are quartz, anorthite, calcium silicate, hematite, anhydrite, and microcline. In ash samples P059 and P060, there were seven phases identified as portlandite; microcline, leucite, and portlandite occur in both. Samples P019 and P032, were especially rich in the glass phase. 3.2. Calculation of the Slagging and Fouling Indices To predict slagging/fouling in a combustion furnace, it is possible to use indices for the SiO2, basic/acid ratio, silica/alumina ratio, fouling, iron/calcium ratio, and total alkalis, Figure 6. Approximate position of ash types P 020 , P 033 , P 059 , P 060 , in ternary diagram CaO-Al 2 O 3 - SiO2[27] and P031 and P019 in ternary diagram K2O-Al2O3-SiO2[28]. Materials 2022,15, 5796 9 of 15 The next method of ash characterization was X-ray powder diffraction phase analysis (XRPD). The samples were compared to the reference diffractogram database published by ICDD (PDF-2) in the range of 5–90 ◦ 2theta. The results for the analyzed samples are presented in Table 5, where there is an overview of the phases in the samples. Table 5. Phase composition of analyzed biomass ash samples. Phase Composition Labeled of Sample P019 P020 P031 P032 P033 P059 P060 quartz (SiO2) x x x x x calcite (CaCO3) x x X x x x X graphite C x CaO x x X x magnesite (MgCO3) X MgO X x anorthite (CaAl2Si2O8) x microcline (KAlSi3O8) x x x x arcanite (K2SO4) X anhydrite (CaSO4) x anorthoclase x leucite (KAlSi2O6) x x orthoclase (KAlSi3O8) x sylvite (KCl) x portlandite Ca(OH)2x x hematite (Fe2O3) x x mullite (Al4.59Si1.41O0.97) x analcime (NaAlSi2O6) x As confirmed by the analysis, the most frequently recurring phases are quartz, anorthite, calcium silicate, hematite, anhydrite, and microcline. In ash samples P 059 and P 060 , there were seven phases identified as portlandite; microcline, leucite, and portlandite occur in both. Samples P019 and P032, were especially rich in the glass phase. 3.2. Calculation of the Slagging and Fouling Indices To predict slagging/fouling in a combustion furnace, it is possible to use indices for the SiO 2 , basic/acid ratio, silica/alumina ratio, fouling, iron/calcium ratio, and total alkalis, as summarized in Table 6. A special index only for biomass does not exist, but many authors have calculated these indices with regard to the probability of slag forming in combustion units. Table 6. Calculation of slagging and fouling indices for individual ash types. Ash Index SiO2(%) Cl (%) B/A S/A I/C Fu TA Sr P019 9.1 l0.21 s8.1 ex 9.3 h0.0 l240.1 h29.7 h15.8 h P020 46.5 h0.41 h0.7 m5.6 h0.2 l7.7 m11.5 h67.1 m P031 16.2 l0.16 l3.7 ex 6.4 h0.1 l72.1 h19.3 h25.3 h P032 19.1 l1.74 ex 2.3 ex 3.3 h0.1 l46.4 h20.0 h35.2 h P033 55.1 h0.10 l0.5 l5.4 h0.4 m3.6 m7.6 h71.7 m P059 41.7 h-1.2 h11.6 h0.1 l10.2 m8.05 h 49.8 h P060 33.1 h-1.3 h3.4 h0.5 m6.9 m5.3 h41.0 h Note: X l: low value; X m: middle value; X h: high value; X ex: extreme value. In the case of SiO 2 content in P 020 , P 033 , P 059 and P 060, they have a high inclination towards slagging. The high levels of silica in wood biomass ashes may have been caused by contamination with different elements (clay, sand, etc.); also, each part of the wood plant may contain different amounts of oxides. According to chloride content, extremely high fouling inclinations were observed in samples P 032 = 1.74 and P 020 = 0.47, while a low fouling inclination with a value > 0.2 was calculated for P033 = 0.1.