The corrosion effect of fly ash from biomass combustion on andalusite refractory materials
Abstract
The main problem affecting the life of refractory linings in furnaces is alkaline corrosion formed during biomass combustion, especially in systems with SiO2–Al2O3 . This corrosion effect is very intensive compared to using conventional technologies designed for burning traditional fuels. This study focuses on the development of a new type of andalusite refractory material with a higher corrosion resistance to K2CO3 and fly ash after biomass combustion. The original andalusite refractory material is labeled A60PT0, with an oxide content of 60 wt.% Al2O3 and 37 wt.% SiO2 , a compressive strength parameter of 64 MPa, and an apparent porosity of 15%. In the experiment, four mixtures (labeled A60PT1–A60PT4) were modified primarily using the raw materials and granulometry. The fly ash was characterized by an X-ray diffraction analysis with the following phases: quartz, calcite, microcline, leucite, portlandite, and hematite. According to the X-ray fluorescence analysis, the samples contained the following oxides: 47 wt.% CaO, 12 wt.% K2O, 4.6 wt.% SiO2 , 3.5 wt.% MgO, and some minority oxides such as P2O5 , MgO, MnO, and Fe2O3 between 2 and 5 %. The tendency for slagging/fouling of the ash was determined with the help of the indexes B/A, TA, Kt , and Fu. The final material was a shaped andalusite refractory material labeled A60PT4 with a content of 65 wt.% Al2O3 and 36 wt.% SiO2. The properties of the andalusite material were a compressive strength of 106.9 MPa, an apparent porosity of 13%, and the recommended temperature of use up to 1300 ◦C. For corrosion testing, a static crucible test was performed according to the norm CSN CEN/TS 15418 and ˇ the company’s internal regulation. The exposure time of the samples was 2 h and 5 h at temperatures of 1100 ◦C and 1400 ◦C for K2CO3 and ash, respectively. For the evaluation of tested samples, an X-ray powder differential analysis, an X-ray fluorescence analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used.
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Citation: Vlˇcek, J.; Ovˇcaˇcíková, H.; Veliˇcka, M.; Topinková, M.; Burda, J.; Matˇejková, P. The Corrosion Effect of Fly Ash from Biomass Combustion on Andalusite Refractory Materials. Minerals 2023,13, 357. https:// doi.org/10.3390/min13030357 Academic Editor: María Ángeles Martín-Lara Received: 8 February 2023 Revised: 23 February 2023 Accepted: 27 February 2023 Published: 3 March 2023 Copyright: © 2023 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/). minerals Article The Corrosion Effect of Fly Ash from Biomass Combustion on Andalusite Refractory Materials Jozef Vlˇcek 1,2,*, Hana Ovˇcaˇcíková1, Marek Veliˇcka 1,2 , Michaela Topinková1, JiˇríBurda 1,2 and Petra Matˇejková3 1 Department of Thermal Engineering, Faculty of Materials Science and Technology, VSB—Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic 2Institute of Environmental Technology, CEET, VSB—Technical University of Ostrava, 17. listopadu 15/2172, 708 00 Ostrava, Czech Republic 3Centre for Advanced Innovation Technology, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava, Czech Republic *Correspondence: [email protected]; Tel.: +420-59732-1507 Abstract: The main problem affecting the life of refractory linings in furnaces is alkaline corrosion formed during biomass combustion, especially in systems with SiO 2 –Al 2 O 3 . This corrosion effect is very intensive compared to using conventional technologies designed for burning traditional fuels. This study focuses on the development of a new type of andalusite refractory material with a higher corrosion resistance to K 2 CO 3 and fly ash after biomass combustion. The original andalusite refractory material is labeled A60PT0, with an oxide content of 60 wt.% Al 2 O 3 and 37 wt.% SiO 2 , a compressive strength parameter of 64 MPa, and an apparent porosity of 15%. In the experiment, four mixtures (labeled A60PT1–A60PT4) were modified primarily using the raw materials and granulometry. The fly ash was characterized by an X-ray diffraction analysis with the following phases: quartz, calcite, microcline, leucite, portlandite, and hematite. According to the X-ray fluorescence analysis, the samples contained the following oxides: 47 wt.% CaO, 12 wt.% K 2 O, 4.6 wt.% SiO 2 , 3.5 wt.% MgO, and some minority oxides such as P 2 O 5 , MgO, MnO, and Fe 2 O 3 between 2 and 5 %. The tendency for slagging/fouling of the ash was determined with the help of the indexes B/A, TA, K t , and Fu. The final material was a shaped andalusite refractory material labeled A60PT4 with a content of 65 wt.% Al 2 O 3 and 36 wt.% SiO 2. The properties of the andalusite material were a compressive strength of 106.9 MPa, an apparent porosity of 13%, and the recommended temperature of use up to 1300 ◦ C. For corrosion testing, a static crucible test was performed according to the norm ˇ CSN CEN/TS 15418 and the company’s internal regulation. The exposure time of the samples was 2 h and 5 h at temperatures of 1100 ◦ C and 1400 ◦ C for K 2 CO 3 and ash, respectively. For the evaluation of tested samples, an X-ray powder differential analysis, an X-ray fluorescence analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used. Keywords: refractory; fly ash; biomass; corrosion; andalusite 1. Introduction Every year, the world generates 900–1000 million tons of energy waste [ 1 ] as coal, gas, oil, waste (municipal and industrial), and biomass. Today, biomass ranks among the fourth energy source in the world, where biomass accounts for about 14% [ 2 ]. According to the World Bioenergy Association [ 3 ], it makes up about 10% of the global energy supply and it is expected that by 2050, 33 to 50% of the world’s energy reserves could be covered by burning biomass. A total of 476 mil tons of ash is formed from biomass combustion every year [ 4 ]. Depending on the type of combustion and the technology and fuel used, several solid residues are produced, which exhibit different properties. After high-temperature processing, solid residues have different chemical compositions, phase compositions, granulometries, volumes, and quantities, but also dangers. Several solid Minerals 2023,13, 357. https://doi.org/10.3390/min13030357 https://www.mdpi.com/journal/minerals
Minerals 2023,13, 357 2 of 18 residues are presented according to the authors of [ 5 ] in Table 1. The production of ash is, in these processes, unavoidable, and during combustion, changes occur to their physical and chemical properties, size, shape, etc. The result is a melting behavior [ 6 ] of original minerals and eutectics. During combustion, a solid ash phase is formed. Ash is a solid residue that leaves the boiler in the form of slag, cinder, or fly ash [ 7 ]. The fly ash fraction can be formed by coarse particles (particles larger than >1 µ m) formed in a solid bed and fine particles (so-called particles <1 µ m in diameter), which are primarily aerosols. Apart from solid residues, liquid-phase materials (liquid fuel droplets, tar, and water droplets) and gaseous-phase materials (CO, H2, Cl, SO2, SO3, N2, etc.) are formed [8]. Table 1. Chemical and phase composition of selected secondary energy waste (SEW) [5]. Oxides/Type of Ashes CaO MgO SiO2Al2O3Fe2O3K2O Na2O SO3TiO2LOI wt.% A, GB, C 5.5 1.1 33.6 17.8 13.2 3.6 0.23 1.02 1.43 22.2 quartz, mullite, lime, magnetite, periclase, Fe2MgO4 S, GB, C 7.1 1.68 52.5 14.9 15.6 3.2 0.17 0.38 1.37 1.71 quartz, magnetite, hematite, anorthite (CaAl2Si2O8) BA, FB, C 39.6 0.67 20.8 10.5 7.2 1.63 0.12 15.9 1.22 1.5 anhydrite, quartz, lime, hematite, portlandite, gehlenite (Ca2Al2SiO7) FA, FB, C 37.9 0.6 20.4 10.9 8.3 1.57 0.14 6.89 1.42 10.9 quartz, lime, anhydrite, hematite, calcite, CaAlO4 AGF, MW 25.2 1.7 21.2 8.5 5.4 1.8 2.8 22.2 * 9.4 quartz, hematite, halite, sylvan (KCl), anhydrite, basanite (CaSO4·1 2H2O), graphite FA, FB, B 37.9 7.4 16.3 2.5 2.5 12.25 7.05 2.81 0.21 4.2 quartz, calcite, lime, periclase, anhydrite Notes: A—ash, S—slag, BA—bed ash, AGF—ash from grate fireplace, FA—fly ash, GB—granulation boiler, FK—fluid boiler, C—fuel coke, B—fuel biomass, MW—fuel municipal waste and *—not detected The most common thermal disposal of biomass in the Czech Republic is incineration. Combustion is a physical–chemical process where heat is released, and the temperature of the burned materials increases. During this process, gases and other waste products are produced. The most known are reactions (Equations (1)–(3)) occurring during heterogeneous combustion. The oxidation of carbon occurs at higher temperatures. Carbon dioxide will then react with oxygen [ 9 ]. Heterogeneous combustion is a surface process that is affected by particle shape. Biomass particles have different shapes and sizes, and particles are not spherical. Non-spherical biomass particles have a larger surface area as well as significant porosity [ 10 ]. The models of biomass combustion are influenced by the shape of the particles and the surface of the particles during the combustion process [11]. C+O2→CO2(−393 ∆H kJ·mol−1) (1) CO + 1/2O2→CO2(−111 ∆H kJ·mol−1) (2) H2+ 1/2O2→H2O (−242 ∆H kJ·mol−1) (3) Biomass is one of the renewable energy sources (solid biofuels) that is an organic, non-fossil material of biological origin. For example, this includes wood waste, black liquor, bagasse, animal waste, and others [ 12 ]. In the Czech Republic, Wood as biomass is the most common combustion material. It accounts for about 64% of waste from the wood-processing industry, straw, cereals, and plant residues [ 13 ], with an average amount of ash as presented in Figure 1.
Minerals 2023,13, 357 3 of 18 Minerals2023,13,xFORPEERREVIEW3of20 Figure1.TherankingofbiomassburningintheCzechRepublicandtheamount(%)ofashafter combustionofdifferenttypesoffuels. IntheCzechRepublic,grateboilersareusedforburningbiomass.Thesefurnacesare composedofbothmetalandrefractoryparts.Therefractorymaterialsaremulti‐compo‐ nentandheterogeneousceramicswithsixoxides:SiO2,Al2O,MgO,CaO,Cr2O3,andZrO2, aloneorincombinationwithcarbon.Refractorymaterialsareprimarilyusedforprotec‐ tionagainstheatandmechanicalandchemicalcorrosion[14].Aluminosilicatematerials arethemostusedfortheapplicationoflinings.Thesecanbedividedintoshapedorun‐ shapedmaterials. Accordingto[15],aluminasilicarefractorymaterialsisdevidedintogroupsaccord‐ ingtothecontentofAl2O3orSiO2(HA—highalumina,45%−98%Al2O3;FC—fireclay, 30%−45%Al2O3;LF—low‐aluminafireclay,10%−30%Al2O3;SS—siliceous,85%−93%SiO2; andSL—silica,93%SiO2)[15,16].Thelattergroupcanbesubdividedintothreesubgroups accordingtotheAl2O3content:1)mullite,72–80wt.%(3Al2O3∙2SiO2),whichisasolidso‐ lutionwithamolarratioofAl2O3toSiO2withintherangeof3:2to2:1[17];2)bauxite,80– 90wt.%(Al2O3∙2H2O);and3)corundum(Al2O3),withcontents>90wt.%[18].Mulliteand corundumarethemainrefractorymineralgrains[19].Highaluminahasreplacedbetter‐ qualityfirebricksinmanyapplications[17].Andalusite(Al2O3∙SiO2)iscommonlyusedfor preparingcommercialrefractorymaterialswithhighamountsofmullite.Andalusiteand mullitearealuminosilicateswiththegeneralformulaAl4+2xSi2−2xO10−x.Thisgroupin‐ cludessillimanite,andalusite,andkyanite[20].Theheatingofandalusiteleadstothede‐ compositionformingmulliteandSiO2at1100−1600°C[20]. Andalusitematerialscanbedescribedasfollows[19]:themainoxideisAl2O3,com‐ prisinganamountof60%−65%,whichiscombinedwithalowalkalineoxide;themain mineralphaseintheAl2O3–SiO2systemhasvariouscrystalstructuresanddensities.The transformationintomulliteandSiO2ortheglassyphasestartsatapprox.1250°C,andthis stepdependsonthechemicalcompositionandthegrainsize.Thevolumeischangedwith sillimaniteandandalusite(approx.5%−8%),andunfiredandalusiteisused[19].Aspre‐ sentedbytheauthorsof[21],andalusitehasanexcellentresistancetoalkalisduetothe mullitenetworkandthepresenceofamorphousSiO2aftertheabsorptionofthealkaline vapor.Itisknownthatanimprovementinalkaliresistanceresultsinoptimizingthestruc‐ tureofporesandthechemicalcomposition[22]. Theoperatingconditionsinboilerscanbedifferent.Aboveall,thegoalistolimit highcombustiontemperaturesandNOxinthefluegasandtopreventashfromsticking tothechamberwalls,thuseliminatingcorrosionanderosion.Fromadesignpointofview, thecombustionchamberoftheboilershouldbeadaptedsothattheflametemperature dropsbelowthemeltingtemperatureoftheash[23].Heat‐resistantliningsareindirect contactwiththethermallyprocessedmaterial.Duringtheheatingofthelining,awide temperaturegradientisformed,wherethesurfacetemperatureaffectstherateofthecor‐ rosionreactions. Thecorrosionmechanismisacomplicatedprocess,anditoccursincasesof(a)attack bymelting,(b)thepenetrationoffluegasesintothelining,c)theadhesionofdustparticles tothelining,or(c)aninstabilityoftemperaturesinthecombustionchamber.Themany authorsandapplicationcompaniespreciselyinconnectionwithaluminosilicaterefractory Figure 1. The ranking of biomass burning in the Czech Republic and the amount (%) of ash after combustion of different types of fuels. In the Czech Republic, grate boilers are used for burning biomass. These furnaces are composed of both metal and refractory parts. The refractory materials are multicomponent and heterogeneous ceramics with six oxides: SiO 2 , Al 2 O, MgO, CaO, Cr 2 O 3 , and ZrO 2 , alone or in combination with carbon. Refractory materials are primarily used for protection against heat and mechanical and chemical corrosion [ 14 ]. Aluminosilicate materials are the most used for the application of linings. These can be divided into shaped or unshaped materials. According to [ 15 ], alumina silica refractory materials is devided into groups according to the content of Al 2 O 3 or SiO 2 (HA—high alumina, 45%–98% Al 2 O 3 ; FC—fireclay, 30%– 45% Al 2 O 3 ; LF—low-alumina fireclay, 10%–30% Al 2 O 3 ; SS—siliceous, 85%–93% SiO 2 ; and SL—silica, 93% SiO 2 ) [ 15 , 16 ]. The latter group can be subdivided into three subgroups according to the Al 2 O 3 content: (1) mullite, 72–80 wt.% (3Al 2 O 3· 2SiO 2 ), which is a solid solution with a molar ratio of Al 2 O 3 to SiO 2 within the range of 3:2 to 2:1 [ 17 ]; (2) bauxite, 80– 90 wt.% (Al 2 O 3· 2H 2 O); and (3) corundum (Al 2 O 3 ), with contents >90 wt.% [ 18 ]. Mullite and corundum are the main refractory mineral grains [ 19 ]. High alumina has replaced betterquality firebricks in many applications [ 17 ]. Andalusite (Al 2 O 3· SiO 2 ) is commonly used for preparing commercial refractory materials with high amounts of mullite. Andalusite and mullite are aluminosilicates with the general formula Al 4+2x Si 2−2x O 10−x . This group includes sillimanite, andalusite, and kyanite [ 20 ]. The heating of andalusite leads to the decomposition forming mullite and SiO2at 1100–1600 ◦C [20]. Andalusite materials can be described as follows [ 19 ]: the main oxide is Al 2 O 3 , comprising an amount of 60%–65%, which is combined with a low alkaline oxide; the main mineral phase in the Al 2 O 3 –SiO 2 system has various crystal structures and densities. The transformation into mullite and SiO 2 or the glassy phase starts at approx. 1250 ◦ C, and this step depends on the chemical composition and the grain size. The volume is changed with sillimanite and andalusite (approx. 5%–8%), and unfired andalusite is used [ 19 ]. As presented by the authors of [ 21 ], andalusite has an excellent resistance to alkalis due to the mullite network and the presence of amorphous SiO 2 after the absorption of the alkaline vapor. It is known that an improvement in alkali resistance results in optimizing the structure of pores and the chemical composition [22]. The operating conditions in boilers can be different. Above all, the goal is to limit high combustion temperatures and NOx in the flue gas and to prevent ash from sticking to the chamber walls, thus eliminating corrosion and erosion. From a design point of view, the combustion chamber of the boiler should be adapted so that the flame temperature drops below the melting temperature of the ash [ 23 ]. Heat-resistant linings are in direct contact with the thermally processed material. During the heating of the lining, a wide temperature gradient is formed, where the surface temperature affects the rate of the corrosion reactions. The corrosion mechanism is a complicated process, and it occurs in cases of (a) attack by melting, (b) the penetration of flue gases into the lining, (c) the adhesion of dust particles to the lining, or (d) an instability of temperatures in the combustion chamber. The many authors and application companies precisely in connection with aluminosilicate refractory materials have solved the problem of so-called “alkaline corrosion/alkali-bursting” [ 24 – 26 ].
Minerals 2023,13, 357 4 of 18 The consequences of that negative phenomenon are shown in Figure 2, with photos from different combustion devices after biomass combustion. The high proportion of alkali metal compounds represents a serious problem [ 27 ]. Alkali compounds contained in biomass ash promote the formation of new phases in the A–S system, which is the reason for volume changes and mechanical damage to linings (roof, walls, etc.). Minerals2023,13,xFORPEERREVIEW4of20 materialshavesolvedtheproblemofso‐called“alkalinecorrosion/alkali‐bursting”[24– 26].TheconsequencesofthatnegativephenomenonareshowninFigure2,withphotos fromdifferentcombustiondevicesafterbiomasscombustion.Thehighproportionofal‐ kalimetalcompoundsrepresentsaseriousproblem[27].Alkalicompoundscontainedin biomassashpromotetheformationofnewphasesintheA–Ssystem,whichisthereason forvolumechangesandmechanicaldamagetolinings(roof,walls,etc.). Figure2.Degradationandcorrosionofrefractorymaterialsinboilersafterbiomasscombustion;(A) liningwithcorrodedstickerafter1yearofbiomasscombustion;(B)corrodedpartofandalusite refractorysamplesafter2yearsofcombustionofplantbiomass;(C)brokenrefractoryconcretelin‐ ing;and(D)boilerforuncorking—stickerandcorrodedwalls[28,29]. Fireclayproductscontainlessthan45%Al2O3,asopposedtohigh‐aluminarefractory products,whichcontainmorethan45%Al2O3.High‐aluminarefractorymaterialscontain‐ ingmullite(3Al2O3∙2SiO2asA3S2)haveadifferentprocessofreactionsthanfireclaymate‐ rialswithanAl2O3contentbelow45%.Na‐aluminosilicatephasesareformedbyaNa2O attack.Inmostcases,oxideSiO2isattackedfirstaccordingtothereactionshowninEqua‐ tion(4)intheA–Ssystem[30].Ifitsfreesilicaisconsumedanditsmullite(Al6Si2O13)is attacked,albiteisformed(NaAlSi3O8)bythereactionshowninEquation(5)[30,31].Mul‐ liteisoftenpresentinfireclaymaterialstogetherwithcristobalite(SiO2);itreactswith NaO2above1000°C,formsanephelinephase(NaS2),andformsα‐Al2O3accordingtothe reactioninEquation(6): Na2SO4+2SiO2=Na2Si2O5+SO2+1⁄2O2(4) Na2Si2O5+2Al6Si2O13=2NaAlSi3O8+5Al2O3(5) 3Al2O3∙2SiO2+Na2O→Na2O∙Al2O3∙2SiO2+2Al2O3(6) Theformationofβ‐aluminaafterthereactionofNa2Owithfreeα‐Al2O3ispresented bygreaterexpansionandinternalstress.ThecompoundNa2O∙11Al2O3isreferredtoin technicalpracticeasβ‐alumina.Fromamineralogicalpointofview,itislistedasdi‐ aoyudaoite[32],whichformsabove1100°C.Theprocessisassociatedwithanincreasein thevolumebyupto18%.TheattackofalkaliK2Ointhealuminosilicatesystemleadsto theformationofK‐aluminosilicatephases.Accordingtotheauthorsof[33]andEquation (7),thisisapresentreactionbetweenpotassiumvaporandarefractorymaterial. Theauthorsof[34]describethepotassiumattackinrefractorymaterialswithahigh contentofAl2O3,inwhichβ‐aluminaandthenpotassiumaluminateariseafterthereaction inEquation(8).ThereactionofSiwithkaliophilitecanthentakeplaceaccordingtoEqua‐ tion(9).ThereactionofpotassiumsilicatewithmulliteisdescribedbyEquation(10), wherecontinuedvolumechangesandexpansionofbetween20and25%occur.Kaliophi‐ lite,KAlSiO4,isformedwithin30minutesatatemperatureof1000°Candcausesflaking Figure 2. Degradation and corrosion of refractory materials in boilers after biomass combustion; ( A ) lining with corroded sticker after 1 year of biomass combustion; ( B ) corroded part of andalusite refractory samples after 2 years of combustion of plant biomass; ( C ) broken refractory concrete lining; and (D) boiler for uncorking—sticker and corroded walls [28,29]. Fireclay products contain less than 45% Al 2 O 3 , as opposed to high-alumina refractory products, which contain more than 45% Al 2 O 3 . High-alumina refractory materials containing mullite (3Al 2 O 3· 2SiO 2 as A 3 S 2 ) have a different process of reactions than fireclay materials with an Al 2 O 3 content below 45%. Na-aluminosilicate phases are formed by a Na 2 O attack. In most cases, oxide SiO 2 is attacked first according to the reaction shown in Equation (4) in the A–S system [ 30 ]. If its free silica is consumed and its mullite (Al 6 Si 2 O 13 ) is attacked, albite is formed (NaAlSi 3 O 8 ) by the reaction shown in Equation (5) [ 30 , 31 ]. Mullite is often present in fireclay materials together with cristobalite (SiO 2 ); it reacts with NaO 2 above 1000 ◦ C, forms a nepheline phase (NaS 2 ), and forms α -Al 2 O 3 according to the reaction in Equation (6): Na2SO4+ 2SiO2= Na2Si2O5+ SO2+ 1/2O2(4) Na2Si2O5+ 2Al6Si2O13 = 2NaAlSi3O8+ 5Al2O3(5) 3Al2O3·2SiO2+ Na2O→Na2O·Al2O3·2SiO2+ 2Al2O3(6) The formation of β -alumina after the reaction of Na 2 O with free α -Al 2 O 3 is presented by greater expansion and internal stress. The compound Na 2 O · 11Al 2 O 3 is referred to in technical practice as β -alumina. From a mineralogical point of view, it is listed as diaoyudaoite [ 32 ], which forms above 1100 ◦ C. The process is associated with an increase in the volume by up to 18%. The attack of alkali K 2 O in the aluminosilicate system leads to the formation of K-aluminosilicate phases. According to the authors of [ 33 ] and Equation (7) , this is a present reaction between potassium vapor and a refractory material. The authors of [ 34 ] describe the potassium attack in refractory materials with a high content of Al 2 O 3 , in which β -alumina and then potassium aluminate arise after the reaction in Equation (8). The reaction of Si with kaliophilite can then take place according to Equation (9). The reaction of potassium silicate with mullite is described by Equation (10), where continued volume changes and expansion of between 20 and 25% occur. Kaliophilite, KAlSiO 4 , is formed within 30 min at a temperature of 1000 ◦ C and causes flaking and the
Minerals 2023,13, 357 5 of 18 expansion of linings. At an Al 2 O 3 content >30%, a new phase of leucite KAS 4 (KAS 4 ) is formed according to Equation (11), which has a melting point of up to 1693 ◦C [32,35–38]. The formation of orthoclase KAS 6 (KAS 6 ) is more favorable at a lower content of Al 2 O 3 (<30%), with an incongruent melting point at 1150 ◦ C. Other products from biomass combustion include water vapor, sulfur oxide, and Cl. Cl significantly shortens the life of the lining, e.g., corrosion from FeCl 2 or ZnCl 2 [ 39 ]. A higher concentration of SO 2 in the flue gas causes alkali sulphation with decreasing flue gas temperatures. 6K + 3Al2O3·2SiO2+ 4SiO2+ 3CO →3(K2O·Al2O3·2SiO2) + 3C (7) Al2O3→K2O·12Al2O3→K2O·Al2O3(8) K2O·Al2O3·2SiO2+ 2SiO2→K2O·Al2O3·4SiO2(9) 3(K2O·2SiO2) + 3Al2O3·2SiO2→3(K2O·Al2O3·2SiO2) + 2SiO2(10) K2O·Al2O3·6SiO2→K2O·Al2O3·4SiO2+ 2SiO2(11) 2. Materials and Methods 2.1. Chemical and Phase Composition of Andalusite Refractory Material Labeled A60PT0 Andalusite refractory material has high compressive strength characteristics, a high load capacity in heat, and good dimensional stability. However, it has a lower density, i.e., a higher apparent porosity of 15% associated with lower corrosion resistance and easier penetration into the refractory material. At first, the original sample labeled A60PT0 was tested. This sample belonged to the group of aluminosilicate refractory samples with a high content of Al 2 O 3 . The chemical composition is shown in Table 2. The parameters measured for this material were the bulk density, 2550 kg/m 3 ; refractories under load (RUL) T 0.5 , 1600 ◦C; cold compressive strength, CS = 64 MPa; and apparent porosity, 15%. Table 2. Chemical composition and properties of andalusite A60PT0 refractory material (wt.%). Raw Materials Al2O3SiO2P2O5MgO CaO K2O Na2O TiO2Cr2O3Fe2O3 60.9 37.3 1.3 0.1 0.1 0.6 0.1 0.4 0.1 1.9 The andalusite refractory material contained 60 wt.% Al 2 O 3 and 37 wt.% SiO 2 . Generally, more impurities such as Na 2 O, K 2 O, CaO, MgO, and Fe 2 O 3 relate to the raw andalusite and the geographical location [ 17 ]. The crystalline phases are recorded in Figure 3. The andalusite sample A60PT0 contained phases such as mullite, andalusite, corundum, and SiO2in form cristobalite. Andalusite and quartz phases were identified too. Minerals2023,13,xFORPEERREVIEW5of20 andtheexpansionoflinings.AtanAl2O3content>30%,anewphaseofleuciteKAS4 (KAS4)isformedaccordingtoEquation(11),whichhasameltingpointofupto1693°C [32,35–38]. TheformationoforthoclaseKAS6(KAS6)ismorefavorableatalowercontentofAl2O3 (<30%),withanincongruentmeltingpointat1150°C.Otherproductsfrombiomasscom‐ bustionincludewatervapor,sulfuroxide,andCl.Clsignificantlyshortensthelifeofthe lining,e.g.,corrosionfromFeCl2orZnCl2[39].AhigherconcentrationofSO2intheflue gascausesalkalisulphationwithdecreasingfluegastemperatures. 6K+3Al2O3∙2SiO2+4SiO2+3CO→3(K2O∙Al2O3∙2SiO2)+3C(7) Al2O3→K2O∙12Al2O3→K2O∙Al2O3 (8) K2O∙Al2O3∙2SiO2+2SiO2→K2O∙Al2O3∙4SiO2 (9) 3(K2O∙2SiO2)+3Al2O3∙2SiO2→3(K2O∙Al2O3∙2SiO2)+2SiO2 (10) K2O∙Al2O3∙6SiO2→K2O.Al2O3∙4SiO2+2SiO2 (11) 2.MaterialsandMethods 2.1.ChemicalandPhaseCompositionofAndalusiteRefractoryMaterialLabeledA60PT0 Andalusiterefractorymaterialhashighcompressivestrengthcharacteristics,ahigh loadcapacityinheat,andgooddimensionalstability.However,ithasalowerdensity, i.e.,ahigherapparentporosityof15%associatedwithlowercorrosionresistanceandeas‐ ierpenetrationintotherefractorymaterial.Atfirst,theoriginalsamplelabeledA60PT0 wastested. Thissamplebelongedtothegroupofaluminosilicaterefractorysampleswithahigh contentofAl2O3.ThechemicalcompositionisshowninTable2.Theparametersmeasured forthismaterialwerethebulkdensity,2550kg/m3;refractoriesunderload(RUL)T0.5,1600 °C;coldcompressivestrength,CS=64MPa;andapparentporosity,15%. Theandalusiterefractorymaterialcontained60wt.%Al2O3and37wt.%SiO2.Gen‐ erally,moreimpuritiessuchasNa2O,K2O,CaO,MgO,andFe2O3relatetotherawanda‐ lusiteandthegeographicallocation[17].ThecrystallinephasesarerecordedinFigure3. TheandalusitesampleA60PT0containedphasessuchasmullite,andalusite,corundum, andSiO2informcristobalite.Andalusiteandquartzphaseswereidentifiedtoo. Table2.ChemicalcompositionandpropertiesofandalusiteA60PT0refractorymaterial(wt.%). RawMate‐ rials Al2O3SiO2P2O5MgOCaOK2ONa2OTiO2Cr2O3Fe2O3 60.937.31.30.10.10.60.10.40.11.9 Figure3.Phasecompositionoforiginalandalusiterefractorymaterials. Figure 3. Phase composition of original andalusite refractory materials.
Minerals 2023,13, 357 6 of 18 2.2. Characterization of Fly Ash from the Combustion of Woodchips Fly ash was used in the experiment in a heterogenous mixture after biomass combustion as a corrosion agent, as shown in Figure 4. It was obtained from the Czech Republic. During the experiment, fly ash was used in its original form, without mechanical treatment or sieving, primarily for the crucible corrosion test. More chemical and phase information about the ash is present in Table 3and Figure 4. Powdered fly ash has a variable chemical composition and can be considered an unrefined material. The main oxides were CaO, at 47%, and K 2 O, at 12%. The minority oxides were SiO 2 , P 2 O 5 , MgO, MnO, and Fe 2 O 3 , comprising between 2 and 5%. The amount of alkaline material present for corrosion attack/degradation was significant. In this case, the fly ash had a content of 12.2% K2O+N2O. Minerals2023,13,xFORPEERREVIEW6of20 2.2.CharacterizationofFlyAshfromtheCombustionofWoodchips Flyashwasusedintheexperimentinaheterogenousmixtureafterbiomasscombus‐ tionasacorrosionagent,asshowninFigure4.ItwasobtainedfromtheCzechRepublic. Duringtheexperiment,flyashwasusedinitsoriginalform,withoutmechanicaltreat‐ mentorsieving,primarilyforthecruciblecorrosiontest.Morechemicalandphaseinfor‐ mationabouttheashispresentinTable3andFigure4.Powderedflyashhasavariable chemicalcompositionandcanbeconsideredanunrefinedmaterial.Themainoxideswere CaO,at47%,andK2O,at12%.TheminorityoxideswereSiO2,P2O5,MgO,MnO,andFe2O3, comprisingbetween2and5%.Theamountofalkalinematerialpresentforcorrosionat‐ tack/degradationwassignificant.Inthiscase,theflyashhadacontentof12.2%K2O+N2O. Theflyashfrombiomasscombustionhas,inmanycases,lowmeltingtemperatures duetothehighcontentofalkalioxidesandCl.TherewasnoCldetectedinthesample.A corrosionattackbyClreducestheservicelifeofrefractorylinings.Fromthepointofview ofphasecompositions,aspresentedinFigure5,ashwasveryvariable,withsevenphases detected.Onedominantpeakwithahighfrequencywascalcite,followedbymicrocline, portlandite,orthoclase,leucite,hematite,andquartz. Table3.Chemicalcompositionoftestsofflyashafterthecombustionofwoodchips. Oxide wt.% Al2O3SiO2P2O5MgOCaOK2ONa2OTiO2Cr2O3Fe2O3MnOBaOLOI 0.94.62.53.547.612.00.20.10.12.73.50.321 Figure4.Theoriginaltestedflyash;(A)detailsoftheflyashunderamicroscopeand(B)original ashafterwoodchipcombustion. Figure 4. The original tested fly ash; ( A ) details of the fly ash under a microscope and ( B ) original ash after woodchip combustion. Table 3. Chemical composition of tests of fly ash after the combustion of woodchips. Oxide wt.% Al2O3SiO2P2O5MgO CaO K2O Na2O TiO2Cr2O3Fe2O3MnO BaO LOI 0.9 4.6 2.5 3.5 47.6 12.0 0.2 0.1 0.1 2.7 3.5 0.3 21 The fly ash from biomass combustion has, in many cases, low melting temperatures due to the high content of alkali oxides and Cl. There was no Cl detected in the sample. A corrosion attack by Cl reduces the service life of refractory linings. From the point of view of phase compositions, as presented in Figure 5, ash was very variable, with seven phases detected. One dominant peak with a high frequency was calcite, followed by microcline, portlandite, orthoclase, leucite, hematite, and quartz. The analyzed fly ash had a relatively higher LOI (loss on ignition) value, which is caused by the content of the combustible substances or the under-burning of the original material. The size of this proportion of organic substances is mainly influenced by combustion technologies, types of boilers, etc. An LOI above 7 wt.% can also be caused by unregulated or excessively high drafts of flue gases through the chimney when a substantial part of the burnt sawdust or straw also leaves the ash separator. If the ash is examined after all combustible substances have been burned, then the LOI includes the breakdown of the crystalline structure of clays (removal of crystalline water), the breakdown of carbonates (limestone, dolomitic limestone, and dolomite), and any other substances [40]. In the case of ash, it is also possible to determine the fusibility of the ash according to the coefficient of the so-called Tuene number, which is the ratio of acidic and basic oxides. [7].
Minerals 2023,13, 357 7 of 18 Minerals2023,13,xFORPEERREVIEW7of20 Figure5.Phasecompositionofflyashafterwoodchipcombustion. TheanalyzedflyashhadarelativelyhigherLOI(lossonignition)value,whichis causedbythecontentofthecombustiblesubstancesortheunder‐burningoftheoriginal material.Thesizeofthisproportionoforganicsubstancesismainlyinfluencedbycom‐ bustiontechnologies,typesofboilers,etc.AnLOIabove7wt.%canalsobecausedby unregulatedorexcessivelyhighdraftsoffluegasesthroughthechimneywhenasubstan‐ tialpartoftheburntsawdustorstrawalsoleavestheashseparator.Iftheashisexamined afterallcombustiblesubstanceshavebeenburned,thentheLOIincludesthebreakdown ofthecrystallinestructureofclays(removalofcrystallinewater),thebreakdownofcar‐ bonates(limestone,dolomiticlimestone,anddolomite),andanyothersubstances[40]. Inthecaseofash,itisalsopossibletodeterminethefusibilityoftheashaccordingto thecoefficientoftheso‐calledTuenenumber,whichistheratioofacidicandbasicoxides. [7]. Duringbiomasscombustion,slagisformed,set,anddepositedincombustionunits. Inthiscase,itispossibletouseindexesforslaggingandfouling.Indexesarecalculated fromtheelementarychemicalcompositionoftheash.Althoughtheseindexesareapplied mainlyforcoalcombustion,manyauthorsalsousethemforbiomassash.Therearethe indexofbased‐acidratio(B/A),theindextotalalkali(TA),and,forexample,thefouling 1‐ calcite/CaCO 3 2‐ hematite/Fe 2 O 3 3‐ quartz/SiO 2 4‐ microcline/KAlSi 3 O 8 5‐ portlandite/Ca(OH) 2 6‐ orthoclase/KAlSi 3 O 8 7‐ leucite/KAlSi 2 O 6 Figure 5. Phase composition of fly ash after woodchip combustion. During biomass combustion, slag is formed, set, and deposited in combustion units. In this case, it is possible to use indexes for slagging and fouling. Indexes are calculated from the elementary chemical composition of the ash. Although these indexes are applied mainly for coal combustion, many authors also use them for biomass ash. There are the index of based-acid ratio (B/A), the index total alkali (TA), and, for example, the fouling index (Fu), which is calculated from a chemical composition according to the formula presented in Table 4[41–43] of the analyzed materials. Table 4. Characterization of fly ash based on Tuene number and indexes of slagging/fouling [ 41 – 43 ]. Tuene Number Index of Base–Acid Ratio Total Alkalis Fouling Index Kt =S+A F+C+MB/A =F+A+C+M+N+K S+A+TTA =N+KFu =B A(N+K) Note: S—SiO2, A—Al2O3, F—Fe2O3, C—CaO, M—MgO, N—Na2O, K—K2O, and T—TiO2. A value of K t = 0.10 is defined as easily fusible ash because the value is less than 2.4, which corresponds to the definition of the K t index as “easily fusible”. With a temperature between 1150 and 1400 ◦ C, it was defined as a “medium” K t , and a value of K t > 2.5 with an ash melting temperature of 1400 ◦C was defined as “hardly fusible” ash. Index B/A (the alkalinity index) is based on the general rule that basic oxides lower the melting temperature of ash while acidic oxides increase their temperature. A B/A value = 11.7 is defined as “extremely high”. A value of B/A < 0.5 is “low”, indicating that the tendency for slagging/fouling of fly ash will be great. As a final result, fly ash is strongly alkaline due to its high CaO content. Index TA (the total alkali index) assesses the formation and ash deposits. For the fly ash, the TA value was 12.2, which is a high value. In general, a value >0.4 is higher, and a value <0.3 is low. Index Fu (the fouling index) expresses the amount of alkali content. Alkalis form a eutectic in combination with SiO 2 . High values (Fu > 40) correspond to higher fouling tendencies. This fly ash achieved a value of Fu = 23.9, which is a medium value. 2.3. Characterization of K2CO3 The second corrosive medium that was used was anhydrous potassium carbonate. This product was from the company Penta s.r.o., Czech Republic. Information regarding
Minerals 2023,13, 357 8 of 18 the chemical and physical properties is declared in its safety data sheet: color—white, state—solid, melting point—891 ◦C, and content in wt.%—>99. 2.4. Characterization Method The chemical composition (XRF) of fly ash was determined using the method of energydispersive X-ray fluorescence spectroscopy (ED-XRF) on a SPECTRO XEPOS (Spectro Analytical Instruments, Kleve, Germany). Powdered samples were shaped and pressed into tablets for the XRD measurement. The mineralogical composition (XRPD) of the samples was evaluated using an X-ray diffraction analysis on the X-ray diffractometer MiniFlex 600 (Rigaku, Japan) equipped with a Co tube and D/teX Ultra 250 detectors. The XRD patterns were recorded over the 5–90 ◦ 2 θ range with a scanning rate of 5 ◦× min. The morphology of particles was studied using the scanning electron microscope QUANTA 450 FEG (FEI, Hillsboro, OR, USA). All images were collected using a secondary electron detector. An accelerating voltage of 25 kV was used. Each sample was Au/Pd-sputtered before the analysis. 2.5. Corrosion Test of Andalusite Refractories and Evaluation The corrosion resistance of andalusite materials was performed by a crucible corrosion test, as shown in Figure 6. K 2 CO 3 and fly ash were used as the corrosion agents. The conditions and evaluation of the experiment proceeded according to the methodology by ˇ CSN P CEN/TS 15418 [ 44 ] and the author of [ 45 ]. Uniform regulation for the corrosion testing of refractory materials does not exist. The shape and test temperatures can be changed depending on the customer’s requirements, the experiment, or the company’s internal regulations [ 45 ]. For the testing, a cube was prepared with the parameters of 5 × 5 × 5 cm and a hole in the middle with a diameter of 2 cm. The corrosion agents were K 2 CO 3 and fly ash. In this study, the following corrosion regimes were applied for the testing of an andalusite refractory material: 1. Start up for 5 h/maximum temperature of 1100 ◦ C/maintain for 5 h/slowly cool/ corrosion agent was 5 g of K2CO3for samples A60PT0 and A60PT4. 2. Start up for 5 h/maximum temperature of 1100 ◦ C/maintain for 2 h/slowly cool/ corrosion agent was 20 g of K2CO3/tested samples A60PT1, A60PT, and A60PT3. 3. Start up for 5 h/maximum temperature of 1400 ◦ C/maintain for 5 h/slowly cool/ corrosion agent was 5 g of fly ash/tested samples A60PT0 and A60PT4. Minerals2023,13,xFORPEERREVIEW9of20 K2CO3andflyash.Inthisstudy,thefollowingcorrosionregimeswereappliedforthe testingofanandalusiterefractorymaterial: 1. Startupfor5hours/maximumtemperatureof1100°C/maintainfor5hours/slowly cool/corrosionagentwas5gofK2CO3forsamplesA60PT0andA60PT4. 2. Startupfor5hours/maximumtemperatureof1100°C/maintainfor2hours/slowly cool/corrosionagentwas20gofK2CO3/testedsamplesA60PT1,A60PT,andA60PT3. 3. Startupfor5hours/maximumtemperatureof1400°C/maintainfor5hours/slowly cool/corrosionagentwas5gofflyash/testedsamplesA60PT0andA60PT4. Figure6.Crucibletestofandalusiterefractorymaterial. Aftercooling,thecruciblewasverticallycutandthepenetrationofrefractorymate‐ rialwasmeasured.Theaffectedareawasotherwiseevaluated.Theadvantagesofthetest areitssimplicityandquickresults.Theevaluationofcorrosiontestscanoftenbesubjec‐ tive,andoftendependsonavisualevaluationandoperatororresearchexperience.This studywasevaluatedaccordingtoanorm[44]withindividualclassesdescribedinTable 5andaninternalevaluationinTable6.Bothregulationswereimplementedonthetested materials. Table5.CorrosionevaluationofrefractorymaterialsafterČSNPCEN/TS15418[44]. DescriptionofCorrosionClassificationTestClass unaffected/novisibleattackU lightlyattacked/minorattackLA attacked/clearlyattackedA corroded/completelycorrodedC Table6.Corrosionevaluationofrefractorymaterialsafterinternalregulation[45]. CorrosionAttack/InfiltrationCracksClass nochangesnoA +<6mmcorrosionand/orinfiltrationnoB ++>7mmcorrosionand/orinfiltrationslightC +++>9mmcorrosionand/orinfiltrationlarge,visibleD 3.ResultsandDiscussion 3.1.ExperimentI:ModificationofOriginalMixtureofAndalusiteRefractoryMaterialA60PT0 andPreparingFourNewMixtures TheexpectedmodificationsoftheoriginalandalusitematerialA60PT0wereadjust‐ mentstoitsproperties.Sometimes,thismaterialisusedinafurnacewherebiomassis burnedwithnooptimalorlongtimeeffect.Theaimwastoincreasethecorrosionre‐ sistance,namely,toincreasetheamountofAl2O3≥60%,reducetheapparentporosity(AP) valueto≤12%,andbestableduringtheapplicationatatemperatureof1300°C.InTable 7,fourrecipesofmodifiedmixturesareshown(labeledA60PT1–T4).Thechemicalcom‐ positionsoftheseadjustedmixturesarealsoindicatedinTable8.Theoriginalsamplefor developingthefournewcompounds(labeledA60PT1–T4)wasA60PT0. Figure 6. Crucible test of andalusite refractory material. After cooling, the crucible was vertically cut and the penetration of refractory material was measured. The affected area was otherwise evaluated. The advantages of the test are its simplicity and quick results. The evaluation of corrosion tests can often be subjective, and often depends on a visual evaluation and operator or research experience. This study was evaluated according to a norm [ 44 ] with individual classes described in Table 5and an internal evaluation in Table 6. Both regulations were implemented on the tested materials.
Minerals 2023,13, 357 9 of 18 Table 5. Corrosion evaluation of refractory materials after ˇ CSN P CEN/TS 15418 [44]. Description of Corrosion Classification Test Class unaffected/no visible attack U lightly attacked/minor attack LA attacked/clearly attacked A corroded/completely corroded C Table 6. Corrosion evaluation of refractory materials after internal regulation [45]. Corrosion Attack/Infiltration Cracks Class no changes no A + <6 mm corrosion and/or infiltration no B ++ >7 mm corrosion and/or infiltration slight C +++ >9 mm corrosion and/or infiltration large, visible D 3. Results and Discussion 3.1. Experiment I: Modification of Original Mixture of Andalusite Refractory Material A60PT0 and Preparing Four New Mixtures The expected modifications of the original andalusite material A60PT0 were adjustments to its properties. Sometimes, this material is used in a furnace where biomass is burned with no optimal or longtime effect. The aim was to increase the corrosion resistance, namely, to increase the amount of Al 2 O 3≥ 60%, reduce the apparent porosity (AP) value to ≤ 12%, and be stable during the application at a temperature of 1300 ◦ C. In Table 7, four recipes of modified mixtures are shown (labeled A60PT1–T4). The chemical compositions of these adjusted mixtures are also indicated in Table 8. The original sample for developing the four new compounds (labeled A60PT1–T4) was A60PT0. Table 7. Raw composition of andalusite refractory mixtures A60PT1, A60PT2, A60PT3, and A60PT4. Mixtures EC BB AN RA C H3PO4P AP BD CS (%) (%) (kg/m3)(MPa) A60PT1 - - x x x x - 16.2 2518 61 A60PT2 x x x - x x - 12.6 2612 70.6 A60PT3 x x x - x x - 13.7 2656 87.5 A60P74 - - x - x - x 11.7 2679 106.9 Note: EK—electro fused corundum, BB—burnt bauxite, AN—andalusite, RA—reactive Al 2 O 3 , C—clay, P— plasticizer, AP—apparent porosity, BD—bulk density, and CS—compressive strength. Table 8. Chemical composition of andalusite refractory mixtures A60PT1, A60PT2, A60PT3, and A60PT4. Mixtures SiO2Al2O3TiO2Fe2O3CaO MgO K2O Na2O (%) A60PT1 34.36 63.23 0.29 0.81 0.1 0.16 0.48 0.20 A60PT2 34.05 63.33 0.43 1.38 0.24 0.15 0.48 0,.1 A60PT3 30.00 67.66 0.35 1.52 0.24 0.13 0.45 0.21 A60PT4 32.65 65.06 0.34 1.38 0.21 0.14 0.42 0.25 The first step was to modify the granulometry to increase the density of the refractory materials. The new granulometries of the compositions were a combination of several fractions: the first change was the addition of sub-mesh fractions under 0.09 mm (~30%); the second change was an increase in the over-mesh with grains of 1 mm (~37%) and an increase in the coarse fraction of andalusite with grain sizes of 1–4 mm.
Minerals 2023,13, 357 16 of 18 Minerals2023,13,xFORPEERREVIEW17of20 pores.Thiswascausedbyaflyashcoatingthatfilledthemicroscopicallyvisiblepores andformedalayeronthesurfaceoftheA60SPT4refractorymaterial. Figure16.SEM/EDSanalysisofandalusiterefractorymateriallabeledA60PT4aftercorrosiontest withflyashat1400°C. 4.Conclusions Duringbiomasscombustion,itisveryproblematictomaintainthequalityofbiomass andthedeclaredtemperatureregimeinthefurnace.Thefinalproductsafterburningbi‐ omassaresolidproductssuchasflyashandslagwithaheterogeneouscomposition. Thesecanmeltorformdepositsanddegradematerialssuchassteelandrefractorymate‐ rialsinthedevice.Theseby‐productsinfluencethecorrosionoftheworkingliningoffur‐ naceunits.Alkalinecorrosionisabigproblemwithaluminumsilicarefractorymaterials. Toassessthedegreeofcorrosionandtheinfluenceofdegradation,thefollowingmustbe known:(a)thepropertiesofproductscreatedduringthecombustionofbiomassand(b) theoperatingconditionsoffurnaceaggregates. Furnaceaggregatesforburningbiomasshaveoperatingtemperaturesofupto1200 °C,atwhichtemperatureameltmaynotform.However,itoftenhappensthatthereisa short‐termexceedingoftheoperatingtemperatureupto1400°Ccausedbyfluctuations inthecalorificvalueofthebiofuel.Forthisreason,itisrecommendedtouserefractory materialswithahighercontentofAl2O3(above60%).Themodificationofthemixture compositionwasmainlyaimedatachievingahigherdensityandreducingtheporosity fromtheoriginalvalueof15%to11%,whichwouldensurelessinfiltrationofunwanted oxides. OtherparametersofsampleA60PT4wereavolumetricweightof2679kg/m3anda compressivestrengthofCS=107MPa.ThenewlypreparedmaterialA60PT4contained mainlyoxides,32%SiO2,and65%Al2O3andwasresistanttocorrosionbyK2CO3andthe testedflyashobtainedfrombiomasscombustion.Accordingtotheregulationsandinter‐ nalstandards,thisisanewmaterialadaptedtoresisttheenvironmentofbiomasscom‐ bustion.ForsampleA60PT4,newphaseswerenotdetectedafterthereactionofflyash andK2CO3,asdeterminedbyanX‐rayanalysis. Figure 16. SEM/EDS analysis of andalusite refractory material labeled A60PT4 after corrosion test with fly ash at 1400 ◦C. Points 12 and 13 were were very similar with an elementary amount. For the last point, 15 (Figures 15 and 16D), the potassium content decreased to a value of 7.2%, and the same trends were observed for the element Al, with 17.2%. The results of the scanning electron microscope (Figure 16A,C) corresponded with points 1 and 6; the structure of the samples was not compact and hard to identify in shape. On the contrary, points 13 and 15 (Figure 16B,D) stood out with a compact sintered surface with visible microcracks and pores. This was caused by a fly ash coating that filled the microscopically visible pores and formed a layer on the surface of the A60SPT4 refractory material. 4. Conclusions During biomass combustion, it is very problematic to maintain the quality of biomass and the declared temperature regime in the furnace. The final products after burning biomass are solid products such as fly ash and slag with a heterogeneous composition. These can melt or form deposits and degrade materials such as steel and refractory materials in the device. These by-products influence the corrosion of the working lining of furnace units. Alkaline corrosion is a big problem with aluminum silica refractory materials. To assess the degree of corrosion and the influence of degradation, the following must be known: (a) the properties of products created during the combustion of biomass and (b) the operating conditions of furnace aggregates. Furnace aggregates for burning biomass have operating temperatures of up to 1200 ◦ C, at which temperature a melt may not form. However, it often happens that there is a shortterm exceeding of the operating temperature up to 1400 ◦ C caused by fluctuations in the calorific value of the biofuel. For this reason, it is recommended to use refractory materials with a higher content of Al 2 O 3 (above 60%). The modification of the mixture composition was mainly aimed at achieving a higher density and reducing the porosity from the original value of 15% to 11%, which would ensure less infiltration of unwanted oxides. Other parameters of sample A60PT4 were a volumetric weight of 2679 kg/m 3 and a compressive strength of CS = 107 MPa. The newly prepared material A60PT4 contained mainly oxides, 32% SiO 2 , and 65% Al 2 O 3 and was resistant to corrosion by K 2 CO 3 and
Minerals 2023,13, 357 17 of 18 the tested fly ash obtained from biomass combustion. According to the regulations and internal standards, this is a new material adapted to resist the environment of biomass combustion. For sample A60PT4, new phases were not detected after the reaction of fly ash and K2CO3, as determined by an X-ray analysis. Author Contributions: Conceptualization, H.O.; methodology, M.T. and J.V.; investigation, M.T., J.B. and P.M.; writing—original draft preparation, H.O. and J.V.; writing—review and editing, H.O. and J.V.; visualization, H.O.; supervision, M.V.; project administration, M.V.; funding acquisition, M.V. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education, Youth and Sports of the Czech Republic via the Research and Development of Multifunctional Materials for Sustainable, grant number SP2023/034, and the Research on the Management of Waste, Materials and Other Products of Metallurgy and Related Sectors, grant number CZ.02.1.01/0.0/0.0/17_049/0008426. Data Availability Statement: The data presented in this study are available from the corresponding author upon request. Conflicts of Interest: The authors declare no conflict of interest. References 1. St˛epie´n, M.; Białecka, B. Inwentaryzacja Innowacyjnych Technologii Odzysku Odpadów Energetycznych. Syst. Support. Prod. Eng. 2017,6, 108–123. 2. Demirbas, A. Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues. Prog. Energy Combust. Sci. 2005,31, 171–192. [CrossRef] 3. Bioenergy, I.E.A. 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