TESTING PROCEDURES FOR CERAMIC REFRACTORY MATERIALS IN BOTTOM GRID OF BIOMASS/WASTE-FIRED CFBS KAROL NICIA 1*, MIKKO HUPA 2, LEENA HUPA 2, EDGARDO CODA ZABETTA 1 1 Foster Wheeler Energia Oy, Varkaus, Finland 2 Process Chemistry Centre, Åbo Akademi University, Turku, Finland *)phone: +48880631021, email: karol.nicia@gmail.com ABSTRACT Refractory ceramic materials are used mainly to limit heat transfer and to protect sensitive components against aggressive environments in combustion devices. However, in some cases a refractory material, used for example to protect the bottom grid of biomass and waste fired CFBs, have failed at conditions where the refractory material should be durable. In this work we have studied whether the observed changes in the performance of the refractory materials depends on their corrosion due to presence of alkali salts in the combustion devices at high temperatures. At first, laboratory analyses and suitable procedures for indentifying the observed underperformance of pre-cast refractory bricks were selected. Then, selected test methods were utilized to evaluate the performance of the pre-cast bricks in laboratory conditions corresponding to typical conditions in CFBs. Finally, observations from the laboratory scale measurements of the most representative from seven different pre-cast materials were compared with samples from boilers. The mineralogical compositions of the refractories were analyzed using Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM-EDXA) and X-Ray powder Diffraction (XRD). For some surfaces also a topographic analysis with spinning disc Confocal Optical Microscope (COM) was utilized. The performance of each refractory was studied by plates (20x20x6mm) with 0.25g of an alkali salt, either pure K 2 CO 3 or a 90/10-mol% mixture of K 2 CO 3 /KCl put on the middle of the plates. The plates with the salt were heat-treated in an electric laboratory furnace at two temperatures, 500 and 700ºC for seven days. After the heat-treatment, changes in the chemical composition of the surface and in the cross-section of the samples were analyzed using SEM-EDX. For some samples, changes in the surface topography were studied using COM. The SEM-EDX results were compared with the analyses of samples from boilers. The SEM-EDX analyses suggested that the observed weakening of the refractories can at least partly be explained by refractory corrosion. In all materials tested, some degree of penetration of potassium was observed. The penetration was found to take place via the matrix phase, preferably through SiO 2 in the refractory. Thus, the results indicated that the laboratory testing method and the equipment used to
analyze the samples can be utilized to establish the performance and the causes of corrosion of ceramic refractory materials in corrosive alkali salt containing environments such as those in CFBs. 1 INTRODUCTION Refractory materials are important components of nearly all combustion devices, used mainly to limit the transfer of heat from the process and to protect more sensitive components from the aggressive combustion environment. For most applications refractory materials are well developed and are of no concern. However, for fuels like biomass, wastes, demolition wood - with their impurities, large inert fractions (stones or metallic debris) and high alkali content - better refractories may be needed to address the demanding corrosion-erosion conditions. As a concrete case, refractory materials used to protect the bottom grid of biomass- and waste-fired circulating fluidized bed combustors (CFBs) have been known to perform occasionally below expectation. In such applications the refractory material consisted of pre-cast bricks of different materials and fabrications. The performance of different bricks had been assessed based on samples from combustors. However, the actual causes for the occasional underperformance of bricks could not be identified with common analytical procedures. The purpose of this work was to: select laboratory analyses and define suitable procedures to identify the causes for the occasional underperformance of pre-cast bricks, develop a test method for the evaluation of pre-cast bricks in laboratory prior their utilization in real combustors, and conduct a first screening on seven selected pre-cast materials, including comparisons of laboratory samples with samples available from real boilers. 2 LABORATORY TESTS In this work we have studied whether the observed changes in the performance of the refractory materials depends on their corrosion due to presence of alkali salts in the combustion devices at high temperatures. At first, laboratory analyses and suitable procedures for indentifying the observed underperformance of pre-cast refractory bricks were selected. Then, selected test methods were utilized to evaluate the performance of the pre-cast bricks in laboratory conditions corresponding to typical conditions in CFBs. Finally, observations from the laboratory scale measurements of the most representative from seven different pre-cast materials were compared with samples from boilers. Seven different materials (coded from A to G ) was include to the experiment. The experiment carried out of corrosion exposure at two different temperatures - to simulate boiler conditions (500ºC and 700ºC) - with two different aggressive salts: K 2 CO 3 and 90/10-mol % K 2 CO 3 /KCl. Exposure time was 7 days. The performance of each refractory was studied by plates (20x20x6mm) with 0.25g of an alkali salt, either pure K 2 CO 3 or a 90/10-mol% mixture of K 2 CO 3 /KCl put on the middle of the plates. Also whole bricks were investigated. Exact tests plan is showed in table 1.
From all materials four, most representative refractory types were analyzed under the SEM/EDX and XRD. Also topographic appearance of samples after the tests with corrosive salt mixtures inside the laboratory oven at 500 and 700 C were analyzed with the COM microscopy. All results from exposure, like also original material composition, were compared with XRF test and private laboratory analysis. Moreover materials extend from real boilers were analyzed under the SEM/EDX and results were compared with others. In the end of the experiment erosion test were performed with prior exposed refractory bricks and possible impact of corrosion for abrasion resistance in the refractory material was checked. Tab. 1. Scheme of campaigns in the laboratory furnace Test number Salt Temperature [ C] Exposure time [h] Material tested 1 500 168 2x{A, B C, D} coupons K 2CO 3 A, C bricks 2 700 168 2x{A, B, C, D, E, F} coupons A, C bricks 3 500 168 2x{A, B, C, D, E, F} coupons K 2CO 3/KCl A, C, E, F bricks 4 700 168 2x{A, B, C, D, E, F, G} coupons A, C, E, F, G bricks 3 RESULTS All seven unexposed materials (named thereafter alphabetically from A to G) were analyzed by SEM-EDX and XRD. Selected backscattered SEM images and EDX map analyses of materials B, C, F and G. 3.1 Original material Table 2 summarizes and compares the composition of original (untreated) materials as declared in the commercial certificates, as analyzed in earlier works, and as analyzed during this work. From table 2 is visible, that all material types used for testing can be in general divided into alumina and silicon carbide based materials. High alumina materials consist mainly on mullite, bauxite, corundum and silica (often as quartz or cristobalites).
Table 2. Comparison of refractory material compositions as by material certificates and SEM, XRF and XRD analyses. Type of material A B C D E F G Type of analysis Material certificate Unspecified analysis method XRF (in wt-%) Low cement; high calcined bauxite Al 2O 3-83 SiO 2-11 Fe 2O 3-1.5 Low cement; high calcined bauxite -- Ultra low cement; material based on silicon carbide aggregates with ceramic and organic hard-bond SiC -87 Al 2O 3< 6 SiO 2 < 3 CaO Fe 2O 3 Low cement; high alumina material Al 2O 3-84 SiO 2-8 Fe 2O 3-0.8 CaO - 2.3 Low cement; high alumina material Al 2O 3-43.1 SiO 2-51.7 Fe 2O 3-1.2 TiO 2-1.0 Low cement; low iron; based on silicon carbide material SiC -76.8 Al 2O 3-14.7 SiO 2-5 CaO - 1.3 Fe 2O 3 0.8 Low cement; high alumina material Al 2O 3-81.7 SiO 2-12.2 TiO 2-2.5 Fe 2O 3-0.8 P 2O 5 Earlier analyses XRF -- (in wt-%) XRD -- Al 2O 3 ~69 SiO 2 ~20 CaO ~ 3 Corundum, Mullite, -- -- -- -- -- -- -- -- -- -- Cristobalite Analyses in this work SEM-EDX (in wt-%) Al 2O 3 ~ 63 SiO 2 ~ 21 Fe 2O 3 ~ 2 CaO ~ 2 TiO 2 ~ 1.5 -- SiC ~55 SiO 2 ~36 Si 3N 4~ 7 Al 2O 3<2 CaO ~0.5 Al 2O 3 ~77 SiO 2 ~14 CaO ~ 4 TiO 2 ~ 3 Fe 2O 3 ~ 2 Al 2O 3 ~70 SiO 2 ~23 CaO ~ 4 TiO 2 ~ 2.5 SiO 2 ~ 55 SiC ~37 Al 2O 3~7 CaO ~ 0.5 Al 2O 3 ~78 SiO 2 ~12.5 TiO 2 ~ 3.5 Fe 2O 3 ~ 2 CaO ~ 2
XRD (700 C; 7 days) -- Aluminum oxide, Mullite, Cristobalite Silicon carbide, Silicon oxide, Haxonite, Dolomite, -- -- Aluminum oxide, Silicon carbide, Quartz, Almandine Corundum, Quartz, Calcium aluminum oxide Quartz 3.2 Alkali exposed material SEM imaging and XRD analyses were conducted on selected samples of four different materials after exposure to corrosive material in the laboratory furnace. 3.2.1 SEM results Key results from materials F is illustrated in Figure 1. The black area on the backscattered SEM images shows the epoxy resin used to mold the sample. Based on Figure 1 and other material results (Nicia, 2008), the following observations can be made: Penetration of corrosive salts was detected in every sample, as highlighted by the XRD elemental maps of potassium (K). Grains seemed unaffected by the corrosive salts. Pure aggregates of silicon carbide (SiC) or alumina (Al 2 O 3 ) showed higher resistance to potassium penetration (K) than aggregates with impurities (Nicia, 2008). Potassium penetration occurred in the matrix, preferably but not only along grain boundaries. Potassium penetration was favored through matrix with elevated SiO 2 (e.g. Fig. 1), thus suggesting (consistently with theory) that quartz and cristobalites would be less resistant to chemical attack that other compounds present in refractories Potassium penetration also occurred through mullite (Al 6 Si 2 O 13 ), though less than through SiO 2 (Nicia, 2008).
F (90/10-mol % K 2 CO 3 /KCl; 700ºC; 7 days) Al Si O Ca K Figure 1. SEM backscattered images and corresponding EDX map analyses of F material after exposure to corrosive salts in laboratory heat furnace. Parameters declared nearby the figures: salt composition, furnace temperature, and exposure time. 3.2.2 EDX results EDX line analyses were performed on all investigated materials. Typical line analyses of a selected sample are shown in Figure 2, where the line selected for analyses is indicated on the backscattered image of the sample. In this example, the penetration of potassium (K) correlates well with the silica (Nicia, 2008) in the sample matrix, and deviates after the 400µm mark (Fig. 2), where the silicon carbide grain presents.
C (90/10-mol % K 2 CO 3 /KCl; 700ºC; 7 days) Figure 2. SEM backscattered image and EDX line analysis of material C after 7 days exposure to 90/10- mol % K 2 CO 3 /KCl at 700ºC 3.2.3 Comparisons All data of four (B, C, F, G) analyzed materials were taken and test runs were compared. Below (Tab. 3) comparisons of four test runs from C material is shown and present in the Fig. 3. Table 3. Estimated penetration of potassium in material C after four different tests. Comparison of estimates from EDX elemental maps and EDX line analyses. Test number 1. K 2CO 3; 500ºC 2. K 2CO 3; 700ºC 3. K2CO3+ 90/10-mol % KCl; 500ºC 4. K2CO3+ 90/10-mol % KCl; 700ºC Elemental maps ~ 200 ~300 ~750 ~300 Potassium penetration depth 200 m) Line analyses (K presence ~80 ~600 ~400 to 800)
Potassium penetration depth Depth [µm] 800 700 600 500 400 300 200 100 Elemental analysis Line analysis 0 1 2 3 4 Test number Figure 3. Estimated penetration of potassium in material C after four different tests. Comparison of estimates from EDX elemental maps and EDX line analyses, as by Tab.3. Both estimates from elemental mapping and line analyses agree on that the penetration of potassium was deeper in the tests 3 and 4, i.e. when the salt mix in the samples included potassium as a chloride (KCl). Therefore, it is reasonable to speculate that chlorides are appreciably more aggressive than carbonates, as expected. Unexpectedly, however, both estimates also agree on that in the presence of chlorides the penetration of potassium was deeper at 500ºC (test 3) than at 700ºC (test 4). One explanation could be that at higher temperatures chlorine would volatilize more effectively from the sample. However, other materials showed a more intuitive increase of potassium penetration as a direct function of temperature (Nicia, 2008). Therefore, the surprising behavior shown in Figure 3 is likely an artifact from the sample preparation, which was improved in later tests.
Table 4. Estimated penetration of potassium in four materials exposed to salt mix 90/10-mol % K 2 CO 3 /KCl at 700ºC. Comparison of estimates from EDX elemental maps and EDX line analyses. Material type B C F G Potassium penetration depth m) Elemental maps ~400 ~300 ~1000 ~200 Line analyses ~330 ~430 ~1500 --- Potassium penetration depth in four materials from test 4 1600 Depth [µm] 1400 1200 1000 800 600 400 200 Elemental analysis Line analys is 0 B C F G Material type Figure 4. Estimated penetration of potassium in four materials exposed to salt mix 90/10-mol % K 2 CO 3 /KCl at 700ºC. Comparison of estimates from EDX elemental maps and EDX line analyses, as by Tab.4. Both estimates from elemental mapping and line analyses agree on that the penetration of potassium was significantly deeper in material F than in any other tested material. Both analyses also agree on than the penetration in materials B and C is similar. This is an unexpected result, because the two materials are know to have different performance in real combustors. Thus, the technique would need further improvements to achieve superior discrimination capabilities. Furthermore, it appears
that material G could perform better than any other material. However, this result must be still validated with samples from boilers. 4 CONCLUSIONS Results from experiment show that is possible to induct corrosion in the refractory material and see changes due to chemical attack in laboratory conditions. In all of investigated materials corrosion was observed during experiment study, as penetration of alkali (potassium in this study) inside the refractory material. In all materials tested, some degree of penetration of potassium was observed. The penetration was found to take place via the matrix phase, preferably through SiO 2 in the refractory. Thus, the results indicated that the laboratory testing method and the equipment used to analyze the samples can be utilized to establish the performance and the causes of corrosion of ceramic refractory materials in corrosive alkali salt containing environments such as those in CFBs. Results proved that corrosion impact in the refractory depend mainly on material composition and laboratory conditions, in which experiment was carried out. Laboratory condition which have an impact of potassium penetration, were mainly temperature and salt composition. Corrosion inside the refractory pre-casts carried out inside the material matrix, while aggregates were not attack by the salt. From different matrix composition the most suffered for corrosion consist mainly of silica (SiO 2 ) thus suggesting (consistently with theory) that quartz and cristobalites would be less resistant to chemical attack that other compounds present in refractories. Potassium penetration also occurred through mullite (Al 6 Si 2 O 13 ), though less than through SiO 2 Opposite to matrix behavior, material aggregates (grains) were corrosion resistant and no penetration was observed inside the grains. Pure aggregates of silicon carbide (SiC) or alumina (Al 2 O 3 ) showed higher resistance to potassium penetration (K) than aggregates with impurities Analysis of used material from real CFB boiler show biggest impact of calcium and sulphure than potassium, what should be reconsider for a salt composition for future corrosion studies. In the end of the experiment erosion test were done with A and C material in the industry laboratory. Both materials appeared to be extremely resistant against erosion, and considering the accuracy of the method such resistance did not changed significantly by either aggressive salts or temperature. Thus, during this study was observed, no significant effect has corrosion attack to abrasion resistance. Results from the same material after different condition exposure show that exposure at 700ºC during 7 days with the 90/10-mol % K 2 CO 3 /KCl present most valuable test results.
Further work is advisable in the following areas: investigate further the role of calcium and sulfur penetrating the refractory material, improve the laboratory procedures and test methods towards a superior capability to discriminate and rank bricks according to their tested performance, find conclusive proves that chemical aggression and erosion by hot bed material in CFBs can be decoupled without artifacts; or alternatively develop a simple, compact, and easily reproducible test rig that combines all these effects, and test Electron Spectroscopy for Chemical Analysis (ESCA) and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) as alternative or complementary techniques to SEM-EDX and XRD REFERENCES Nicia K., 2008, Testing procedures for refractory material in bottom grid of biomass/waste-fired CFBs MSc Thesis, Varkaus, Finland