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Analysis of deposits from combustion chamber of boiler for dendromass
Fuel 266 (2020) 117069
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Full Length Article
Analysis of deposits from combustion chamber of boiler for dendromass a,⁎
b
a
T
a,c
Beatrice Plešingerová , Bora Derin , Pavol Vadász , Dávid Medveď a b c
Technical University of Košice, Faculty of Materials, Metallurgy and Recycling, Institute of Metallurgy, Letná 9, 042 02 Košice, Slovak Republic Istanbul Technical University, Metallurgical and Materials Engineering Department, Maslak, Istanbul, Turkey Slovak Academy of Sciences, Institute of Material Research SAS, Watsonova 47, 040 01 Košice, Slovak Republic
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Corrosion of linings Boiler furnaces Refractory
This article records the degradation of the refractory alumina-silicate lining in a boiler furnace combusting dendromass after ten years of use. The deterioration of refractory alumina material was evaluated from phase and chemical analyses of accretions. The fusion temperatures of accretion were measured with a high-temperature microscope and the results were compared with the solidus (point of first liquid formation) temperature predicted by FactSage 7.3 thermodynamic simulation software. Content of SiO2 (50–65%) in accretions confirms that fine silicate particles of ash adhere to the lining. These silicates react with subliming alkalis from the dendromass and form aggressive eutectic melts on surface of lining at operating temperatures. Increase in operating temperature, inhomogeneity (porosity) and alkali content in accretions are the main factors influencing accretion viscosity, melt convection and lining corrosion. For this reason accretions on the vertical walls lower down are much thicker than on the walls in the upper part and the arch. The Al2O3 concentration is higher in the arch accretions; there the refractory material corrodes intensively. The fusion temperatures of the glassy accretions (measured at the furnace atmosphere: accretion boundary) are around 1150 °C. However, the calculated temperatures of slag formation stated by FactSage are about 150 °C lower, and these correspond to the operating temperature in the upper section of the combusting chamber. The fusion temperatures increase with the Al2O3 content in accretions closer to the lining. The obtained results will be applied to define the requirements for the development of boiler furnace refractories.
1. Introduction Biomass/dendromass ranks among the fuels in the category of renewable energy and nowadays it often replaces the fossil fuels used in the heating stations preparing heat and hot water for households. Modern boilers using dendromass provide higher efficiency at lower cost. The development of biomass combustion technology is intensive; it is focusing on the enhancement of combustion effectiveness and reduction of emissions, so that energetic biomass utilization should be in compliance with EU norms [1–3]. The refractory materials for boiler linings were not specifically developed for biomass combustion. The same refractory materials (concrete and fire-bricks) are mostly used just as they were applied in older boiler and furnaces [4–6]. The expansion of biomass use in energy and metallurgy forced to focus on their combustion efficiency, ecology and economy. Some researchers worked on the corrosion resistance verification of various types of refractory materials in simulated atmosphere at high temperatures [4,6,7–12]. The studies reveals that the interaction of gas
⁎
with the refractory materials depends on the composition of combustion gas, its flow rate, and operating temperature in boiler, as well as the chemical and phase composition and density of the refractory material. For biomass combustion, the linings should be thermally stable, abrasion and wetting resistant and dense (e.g. refractories with high content of Al2O3 and SiC) [9]. There is a lot of research on biomass related to slagging and fouling of chambers in boilers and furnaces [13–18]. The generated combustion gas is different from that produced in the case of coal combustion, and so the corrosion effect of the gas atmosphere on the lining in boilers is different as well. The gas, ashes and slag produced from biomass combustion contain in addition to carbon dioxide also more water vapour and alkali. The heating capacities of different kinds of biomass are significantly different, as well as the ratio of combustible components to non-combustible residues (ash matter) and the chemical composition of the gases and ashes formed. The various amounts of ash (from 0.1% (soft wood) to 15% (bark)) created from different types of dendromass fuel and the proportion of major contents (SiO2 and CaO) in ashes can vary
Corresponding author. E-mail address: [email protected] (B. Plešingerová).
https://doi.org/10.1016/j.fuel.2020.117069 Received 23 August 2019; Received in revised form 20 December 2019; Accepted 10 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Table 1 Basic characteristics of refractory materials used [31,32]. Material
LCC Concrete Alumina Bricks
Chemical composition (mass %) Al2O3
SiO2
Fe2O3
K2O + Na2O
MgO + CaO
61.5 60
33.5 30–37.5
0.9 1–2
– 0.4 + 0.2
– 0.2 + 0.4
Basic phase material
Apparent density (g.cm−3)
Apparent porosity (%)
Thermal expansion at 1000 °C (%)
Resisting to CO
Max. service temper. (°C)
Mullite –
2.49 2.5–2.55
17 15–20
+0.55 0.5–0.6
Class A –
1 480 1 600
from single figures to high percentages (3–70%) [3,19–22]. An important role is played by humidity of the biomass, vapour and volatile compounds (mainly H2O, carbon oxides, sulphur, phosphorus, chlorine, alkaline and heavy metal compounds), which react together and condense in different parts of the combustion chamber and cooler zones in the heat-exchanger. Only part of the ash remains in the chamber bed. Very fine particles are carried away with hot aggressive combustion gas from the combustion chamber bed to the top and into further parts of the boiler. In the furnace atmosphere the vapours of water, compounds of carbon, sulphur, phosphorus and chlorine react with alkalis (Na, K) and metal alkaline earths (Ca, Mg), which are aggressive to silicon. Together they create the various multi-component compound particles with low eutectic temperature. At the beginning, the aggressive gases and then the low-fusible melts attack the refractory materials. Sticky particles are caught on the hot surface of the refractory lining and gradually cover the walls and arch with glassy deposits. Later on the thick viscose glassy deposits start to flow due to their weight [7,13,14]. In an attempt to monitor these processes, previous researchers measured the fusibility temperature of ashes and slags. Slagging, fouling and sintering indicators were evaluated similarly as in the case of coals [16,17,23–26]. The main difference between coal and biomass ash is that the coal ash contains higher amounts of SiO2 and Al2O3 but lower amounts of K2O and Na2O. The alkalis have decisive role on fusibility of the ashes. Despite these differences, it is possible to use the same methodology for the fusibility characterization of ashes [23]. In addition to the measurement of the fusibility temperature [23,25,27] the indexes for evaluation of ashes fusibility derived from their chemical composition were calculated [17,23]. In the study [23], which focuses on characterization of the biomass, the ashes are divided into classes in relation to the base/ acid index: if B/A 0.2 it indicates a lower deposition tendency of slagging; 0.2–1 a medium tendency and 1 a high tendency. In the study [17], which deals with coal combustion, the intervals of slagging formation are < 0.11; 0.1–0.14 and > 0.14 respectively. The prediction of slag formation start is calculated on the basis of thermodynamic principles and chemical analysis [16]. In the study [13] it was experimentally shown that 15–20% of liquid phase (temperature at T15) makes the particles sticky, and if the deposit material contains more than 70% liquid (T70) the deposit flows down from the vertical walls. The combustion processes in boilers are highly complex. The refractory linings are loaded thermally, mechanically and chemically; simultaneously and repeatedly. The higher the temperature, the more intensive are the thermodynamic interactions between alkali compounds and refractory materials and the low melting-point compounds formed on the hot surface of the lining. Problems can occur during the combustion processes if the type of fuel-biomass is changed. Increasing temperature together with changes in oxide-reduction conditions and abrasive effects of solid particles in the aggregate shorten the life of the lining material [7,13,14,18,28]. For these reasons materials resistant to thermal shock and CO such as moulted refractory alumina-silicate and spinel concrete, which were developed for metallurgical and energy plants using coal combustion or natural gas, are often applied in the boilers [4,5,29,30]. The study investigates the extent of wear on the alumina linings in a dendromass-fuel boiler after 10 years of operation. The inside of the
boiler was not monitored during the operation. The formation of accretions on the refractory lining was evaluated in relation to the composition of the ashes of woodchips combusted at low temperature in laboratory and slags from the grate of boiler. The results of the chemical and thermal analyses of ashes and slags were the basis for modelling the combustion gas composition in combustion chamber and the conditions of accretion formations. The determination of the softening and melting points of the accretions provides information about their formation and the temperature distribution in the chamber. The progress of the accretion melting process and corrosion of the lining and the temperature in the boiler needs to be known in order to establish the economical combustion process and the identification of the proper refractory materials.
2. Experimental part 2.1. Material – corroded lining, slags and ashes The spent refractory linings from the grate boiler (SCHMID AG Holzfeuerungen CH-8360 Eschlikon, type UTSR-3200, 32, 2007) for water heating in flat-building was studied as post-mortem examination. Some measured values for the operating parameters such as fuel batching, time of maximal and minimal performance, and dead time were unavailable. The fuel of the grate boiler was commercially available waste woodchips. The walls of the boiler were made from low cement castable (LCC) with hydraulic bond. The lining in the bottom arch over the grate (position 2 in Fig. 1) was replaced with high-alumina bricks four years ago. The basic characteristics of the originally-used refractory materials (Didurit/Low Cement Bonded Castable and alumina brick [31,32]) are listed in Table 1. Operating temperatures in the furnace grate are 500–900 °C [22]. The difference is caused due to the temperature distribution in combustion chamber of boiler starting from grate to heat exchanger. The maximum temperatures were achieved on the top of the primary combustion part of the boiler where the combustion of the pyrolysis products of biomass occurred. The accretion samples (numbered 1–6) were taken from different parts of the corroded surface of the boiler linings (Fig. 1), after operating for ten years, were taken at the time of general repair of the work lining, and then analyzed and evaluated. Together with the corroded lining materials, the slags and woodchips combusted in the boiler were delivered for analysis. The samples of woodchips and slags to analyse were from the last operating period of boiler. Dry dendromass is characterized by high humidity (10–15%), which is released during combustion in the boiler. The three ashes (PT, PS, PL) were prepared by burning three kinds of industrial woodchips in our laboratory. PT is woodchip from bark; PS and PL are woodchips from softwood. The woodchips were burned on a metal grate and then the ashes were overfired in an electric furnace at a temperature of 500 °C in air atmosphere until a constant weight of ashes was achieved. Major ash- and slag-forming elements (Si, Ca, Al, Fe, Mg, K, Na and Mn) were determined. The chemical compositions of the ashes are compared in Table 2 with the slag conglomerates from the combustion of dendromass.
2
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Fig. 1. Scheme of combusting woodchip boiler for heating water: I – batching, II – primary combustion chamber, III – secondary combustion chamber, IV – heat exchanger and off-take. Numbers (1–6) in circles show sampling points.
2.2. Methods – characterization of materials
fusibility) [25,27]. The fusibility characteristics were determined using a high-temperature microscope (Leitz-Wetzlar, Germany) in a resistance furnace with a maximum running temperature of 1500 °C. The sample in ground powder form was pressed into a cylindrical tablet ϕ 3 mm × 3 mm of weight 0.2 g. The tablet was placed on a sintered corundum plate and heated in the furnace at a rate of 10 °C/min in a stationary air atmosphere. The progress was detected with a digital camera simultaneously with the sample temperature. Temperature of fusibility (AF) was calculated according to equation for coal slagging 4DT + HT [23]: AF = , where DT is the initial deformation temperature 5 and HT is the hemisphere melting temperature.
After macroscopic documentation, the specific parts of corroded linings, accretions, slags and ashes were analysed. The samples were analysed by classic chemical analysis of earth; the procedure is based on decomposition of solids by melting. Atom adsorption spectroscopy (AAS; Perkin Elmer 3100) was used for the analyses of elements in leachates from metls. Semi-quantitative energy dispersion spectrometry (EDS; JEOL JSM 7000F, INCA Energy 250 Microanalysis System) was used for local analysis of the elements of accretions. The results of the chemical analyses were used to calculate the base/acid ratio Fe O + CaO + MgO + Na O + K2 O B = 2 3 SiO + Al O + TiO2 , which indicates the propensity for slagA 2 2 3 2 ging or fouling of the material [23]. An X-ray diffractometer (RIGAKU MiniFlex600, Theta-2Theta, CuKα phase) was used for phase analysis of slags and selected accretions. Identification of the individual crystalline phases was performed using the PDXL 2 software package together with the ICCD database of minerals and ceramic phases. The investigations on the thermal characteristics of slags and ashes were obtained by a simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter). The measurements were carried out with 40 mg sample in a Pt-crucible in air atmosphere and heating rate of 10 °C/min up to 1400 °C. The measured data were evaluated with the Netzsch Proteus TA version 6.1 software. The melting process of accretions, ashes and slag was observed, and the softening temperature and fusibility were measured according to the STN ISO 540 standard (Hard coal and coke. Determination of ash
2.3. Analysis of conditions in combustion chamber during the accretion formation The composition of combustion gas in the chamber during forming accretions was simulated using the HSC 9.1 software [33]. The calculation was made for dendromass with 20 wt% humidity and 10% excess air at a pressure of 1 bar, and the estimated content of ash after combustion was 2 wt%. The dendromass composition, 51 wt% C, 5.5% H, 43% O, 0.2% N, 0.02% S and 0.01% Cl, was derived from the previous works [3,21,22] and the composition from the chemical analyses of dendromass ash (PT, PS, PL; Table 2). FactSage 7.3 thermodynamic simulation software which uses the Gibbs energy minimization method was used in the present study to create complex oxide-based phase diagrams and to evaluate the thermal behaviours of some accretions, as shown in Table 7 [34]. The FToxid
Table 2 Chemical analyses of ashes from woodchips and slags. Sample
Abbr.
Chemical analysis after re-calculation based on annealing conditions SiO2
Al2O3
CaO
MgO
Fe2O3
B/A Na2O
K2O
MnO
(wt.%) Woodchips PT Woodchips PS Wood PL Agglomerate slag Powdery slag
PT PS PL S-7 S-8
59.2 23.5 14.1 65.6 63.8
7.8 4.5 9.5 11.2 13.2
26.0 42.2 56.8 12.3 12.6
1.2 7.6 8.5 1.6 2.3
3.2 9.2 5.1 4.8 5.3
L.O.I. – Loss on Ignition, N – undetermined. 3
0.7 0.5 0.1 1.0 0.8
1.7 8.4 5.1 3.5 2.0
0.1 4.1 0.7 N N
L.O.I 2 h//1100 °C
(–)
(wt.%)
0.5 2.4 3.2 0.3 0.3
11 (17) 21 N N
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solution database, which contains oxide solutions evaluated/optimized by the FACT group, was selected in FactSage for the calculations.
Table 3 Evaluation of X-ray analyses – diffraction records of ashes PT, PS, PL and slags 7 and 8.
3. Results
Sample
Abbr.
SiO2
CaCO3
K2Ca (CO3)2
MgCO3
Anorthite/ Anothoclase (Ca,K)–Si–Al–O
Woodchips PT Woodchips PS Wood PL Agglomerate slag Powdery slag
PT PS PL S-7 S-8
+++ ++ + +++ +++
++ ++ +++ – –
Trace – + – –
Trace Trace ++ – –
+ + Trace Trace +
3.1. Ashes and slags The chemical composition of slags (sintered slag – S-7, powdery slag – S-8) is compared in Table 2 with ashes from the woodchips combusted in the laboratory. The content of SiO2 in the slags (S-7, S-8) is significantly higher than in the compared ashes. The ashes (PL, PT and PS) are very fine. PS and PL woodchips at temperature up to 500 °C creates about 0.8–1 wt% ash, however the PT forms ten times more of ash. It is also evident that the compositions of the ashes are markedly different. These results, including the large variance in amounts of two dominant components (CaO, SiO2), correspond to the knowledge from many other studies [3,20,22]. The PT ash from wood bark includes significantly more SiO2 (glass-forming component) than the PL and PS ashes from softwood, which are a source of higher content of alkali metal (K) and alkaline earths (Ca, Mg). The thermal analysis records of ashes and slags (PT, PL and S-7, S-8) are shown in Fig. 2 a, b. Moderate mass loss was recorded in the case of ashes and slags in the temperature region of 25–600 °C, too. The more
Number of + expresses the probability of existence and quantity of solid phase.
alkaline-like ash (PL) shows greater mass loss (2 wt%) than the ash (PT) with high content of SiO2 and the slags (S-7 = 0.5 wt%, S-8 = 1.2 wt %). This mass loss may be due to the decomposition of hydrates and hydroxides, which interact with water vapour and air. The loss in case of the slags could also be due to the trace of unburnt carbon. Sharp mass decrease with endo-effect was detected in ashes in the temperature range 600–800 °C, namely 10 wt% loss in PT and 21 wt% loss in PL ash. In the case of slags there was slight loss with weak endoeffect. The decomposition temperature corresponded to the temperature of initial decomposition of pure CaCO3 powder. The presence of
Fig. 2. TA records of: a) ashes PT and PL; b) slags. Conditions: dose = 40 mg of sample in Pt crucible, heating rate 10 °C/min; atmosphere – air. 4
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Table 4 Evaluation of fusion temperatures of ashes and slags (STN ISO 540) [27]. Sample Abbr.
PT PL S-7 S-8
Fusibility of samples (ashes, slags) in air atmosphere (Tolerance ± 5 °C)
Temperature of fusibility AF (°C)
(Initial) Deformation temperature DT (°C)
(Softening) Spherical temperature ST (°C)
Hemisphere temperature HT (°C)
Fluid temperature FT (°C)
1185 1380 1170 1160
1200 1410 1190 1175
1230 1425 1240 1215
1340 1455 1350 1345
1194 1389 1184 1171
DT – initial deformation temperature; ST – (sphere) softening temperature; HT (H = ½ W) – (hemisphere) melting temperature; FT – (flow) fluid temperature.
The results of X-ray diffraction analyses of glassy samples (1,2 and 4) are summarized in Table 6. Depending on their composition, partial melts crystallized during cooling (Table. 6). Upon solidification of melting the quartz/cristobalite, clino- and orthopyroxens (clinoenstatite, diopsite, hedenbergite) and potassium and calcium feldspar minerals crystallized in a glassy phase. Sample 4-P is of sandy accretions from a cavity created by the flowing of viscous melt on the walls, and therefore SiO2 is dominant there. These results demonstrate that the hot powders adhering to the lining generate glassy layers which on the one hand interact with and corrode the refractory materials, and on the other hand are attacked by aggressive combustion gases and form low-viscosity melts. The temperatures of fusibility of the accretions measured with a high-temperature microscope are shown in Table 7 together with the solidus temperatures calculated by FactSage 7.3 software.
CaCO3 in the ashes was confirmed by means of X-ray diffraction analyses (Table 3). The carbonates are the result of interaction of ultrafine CaO particles with CO2 contained in the combustion gas at lower temperature. Unlike in ashes, the ratio of SiO2 to CaO is higher in slags (Table 2), and therefore the CaO in slag is bound for the most part into silicate and alumina-silicate. This was confirmed by the X-ray analyses (Table 3). The observed moderate and slow mass loss in all samples at temperatures above 800 °C probably relates to the partial decomposition of K, Na compounds. The ultrafine granularity of ashes and higher content of alkali and alkaline earths forms conditions for the formation of melt at lower temperature. Fusibility processes (Table 4) are accompanied with endo-effects, which are observed on the DTA curves. In the case of slags S-7, S-8 the temperature of the initial endo-effect was at 1100 °C, with peaks at 1165 and 1185 °C, and the endo-peak temperature of PT ash was 1204 °C. The endo-effect consists of the sum of all heat processes which take place in the sample, including the heat effect of reactions and melting. The slight mass loss at temperatures above 1200 °C is related to the partial decomposition during melting, possibly of sulphate (CaSO4). The fusibility of ash (PT) and slags is evident from the results in Table 4. Only the DTA curve of PL ash, which has low content of SiO2, does not show more expressive thermal-effect in the range of 1100–1200 °C. The very low content of SiO2 in this ash forms only a small quantity of melt by reaction with CaO, MgO and other K, Na, Fe and Mn compounds. The melts of ashes and slags wetted the corundum plate surface during the fusibility measurement well.
4. Discussion The damage of to the furnace alumina lining after 10 years of operating of grate boiler, the creation of stalactites, flow down and thickness of melt accretions and dead cavities are evident from Fig. 3. The conditions under which the accretions in the combustion chamber were formed were simulated using the HSC 9.1 software [33]. The combusting dendromass generates combustion gas with 65–70 vol % N2, 15–17% CO2 and comparable volume of H2O and ~2% O2. The calculated composition of the gas atmosphere was similar to that in study [7]. In this atmosphere, if the ash contains a little SiO2 and the major component is Ca, then the Ca(HCO3)2 is stable at temperatures below 600 °C, but as the temperature increases, the generated CaCO3 decomposes into CaO and partially reacts with silicon oxides (Table 6). During cooling and at high partial pressure of CO2, the CaO has a tendency to react with CO2 and form CaCO3. Our thermal analyses and X-ray diffraction analyses confirmed the presence of CaCO3 in the ashes (Table 3). Trace amounts of sulphur and chlorine combine together with water vapour to form aggressive hot gases. The presence of thermodynamically stable KOH is very presumable at temperatures above 500 °C. The sulphates of alkali metals (K, Na) are more stable at 700 °C, but above this temperature CaSO4 is more stable. CaSO4 decomposes only above the temperature of 1100 °C, and then the concentration of SO2 increases in the combustion gas. Chlorine reacts primarily with K and Na, creating melts of chlorides which evaporate above 750–800 °C. Alkaline compounds react with silicon oxide from the dendromass (3–18 wt% SiO2 in ash) and create low-temperature melting glassy phases and mixed silicate compounds. The sticky silicate particles adhering to the lining create a glassy melt which corrodes the lining. The corrosion process intensifies, the higher the temperature rises in the combustion chamber and the lower the viscosity of melts becomes. The compositions of the analysed slags (S), ashes (P), accretions (number) and refractory material from lining (T) were plotted in the ternary and pseudo-ternary phase diagrams of SiO2-CaO-Al2O3 (Fig. 4 A and B). It can be seen here that overload operating temperatures (above
3.2. Corroded refractory linings from boiler Fig. 3(a–f) documents the lining degradation in different parts of the boiler (SCHMID UTSR-3200 32) after 10 years of working. The number of the samples in Fig. 3 identifies the position of collection of the sample displayed in Fig. 1. Samples 1–5 have glassy appearance, and they are heterogeneous and porous. In contrast the sample from position 6 has the character of powder. The chemical analyses results of accretions and refractory materials are given in Table 5. The letter A, B, C or P denotes the repeated analyses from different parts of the samples 1–6. The T samples characterizing the lining materials were picked from the used refractory materials, from places at least 5 cm under accretions: T-2 – arch over the grate, taken from alumina bricks built in four years ago (position 2 in Fig. 1); T-3 – top arch of the combustion chamber (position 3) and T6 – side wall at the grate (position 6). The powdery fouling from position 6 and glassy accretions of different colour, bubbling and thickness from position 4 were analysed with EDX. The spot-analyses of several glassy samples (4) confirmed, in addition to the predominant elements Si, Al, Ca, Mg, K, Na and Fe, also sporadic distribution of Cu, Ba, Cr, Zr and trace amounts of Mn, Ni, Ti and P. In sample 6, small quantities of Ni and Cr elements were found only randomly, but Mn, P and Ti elements were always detected together with the elements found in the chemical analysis (Table 5). 5
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Fig. 3. Photo-documentation of accretions in the combustion chamber presented in Fig. 1.: a) 1 – frontal part of top arch; b) 2 – frontal arch dividing the combustion chamber; c) 3 – rear of top arch (before exchanger) and detail of stalactite and accretion on refractory material; d) 4 – front wall and detail of accretion/slagging; e) 5 – sidewall; f) 6 – wall below bottom arch in combustion chamber.
and slags (5–5.3) indicate that more CaO than SiO2 is taken away by the combustion gases to cooler parts of the boiler. This can be seen in Fig. 4a too. The ratio SiO2/CaO in the slags is comparable to that in the accretions. The base/acid ratio values for the PL, PS and PT ashes, 3.2; 2.43 and 0.49 respectively (Table 2), point to their great tendency to form slagging in the combustion chamber, according to the classification mentioned in the study [23] (B/A = 0.2–1 indicates medium and > 1 high deposition tendency). Lower content of SiO2 is the reason for the extremely high B/A values for PL and PS and the high fusibility
1000 °C) can be already very destructive to thermal-resistant corundum-spinel materials if the K2O (Na2O) and SiO2 are together present in furnace gases. K2O and Na2O with SiO2 forms low melting temperature phases which wets and dissolves the lining material slowly. Then, the eutectic melts containing about 12 – 15 wt% of Al2O3 form on the surface alumina lining. The resistance of corundum and mullite and other refractory materials by aggressive alkaline gas were studied [8–10] and the results confirmed a wetting of surface and slow infiltration of melts into alumina refractories and their dissolution. The differences between the ratio of SiO2/CaO in ashes (0.25–2.28) 6
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Table 5 Chemical analyses of accretions from walls and arches of boiler grate. Sample
Abbr.
Chemical analysis (dominant oxides) SiO2
Al2O3
B/A
CaO
MgO
Fe2O3
Na2O
K2O
MnO
[wt.%] Sticker on arch Tip of brown stalactite Glass layer on brick Surface of glassy layer Sticker on arch Brown glass Blue glass Blue glass Sandy sticker Brown glass Blue glass Dusty deposit Top dusty deposit Lining material Lining material Lining material
41.1 49.9 54.3 56.3 45.8 66.1 66.2 66.0 68.9 66.4 63.9 41.9 35.8 35.8 32.2 39.0
1-A 1-B 2-A 2-B 3-A 4-A 4-B 4-C 4-P 5-A 5-B 6-A 6-B T-2 T-3 T-6
[–] 34.6 14.7 28.0 17.9 34.3 11.2 11.6 9.2 9.8 11.6 11.6 46.7 34.5 58.8 59.5 53.5
13.9 20.5 9.6 13.1 12.0 10.6 11.6 13.3 8.8 10.8 12.0 6.1 13.1 2.3 3.9 3.5
2.4 3.2 1.6 2.1 1.8 1.6 1.7 1.8 1.5 1.5 1.6 0.5 2.9 0.3 0.2 0.3
2.8 4.9 3.8 5.5 3.2 6.0 4.8 5.1 4.8 5.8 6.5 3.1 4.9 2.6 3.7 3.0
0.4 0.9 0.4 1.6 0.4 1.0 0.9 1.7 2.6 1.1 1.0 0.4 2.9 0 0.1 0.4
4.6 4.7 2.2 3.0 2.4 3.6 3.2 2.4 3.1 3.0 3.5 1.0 5.3 0 0.2 0.2
N 0.7 N 0.4 N N N 3.1 N N N N 0.6 N N N
0.32 0.53 0.21 0.34 0.25 0.29 0.29 0.32 0.26 0.28 0.33 0.13 0.41 – – –
(sample 4 and 5 contained same traces of Cu, Zr and Ni); N – undetermined. Table 6 Evaluation of X-ray diffraction records of selected accretions. Sample – accretion
Abbr
Amorphous phase
Quartz/ Cristobalite (Si-O)
Wollastonite (CaSi-O)
Clino/Orthopyroxens (Ca/Mg/ Fe-Si-O)
Anorthite Gehlenite(Ca-AlSi-O)
Mullite*, Corundum(Al-SiO)
Orthoclase Microcline Leucite* (K-Al-Si-O)
Brown stalactite Brown glass Brown glass Blue glass Sandy sticker in cavity
1-B 2-B 4-A 4-B 4-P
+ +++ +++ ++ –
– (+) + (+) ++
++ (+) – – –
+ + – – +
(+) ++ ++ (+) +
+, +* ++* – – –
++* + – – –
Above the grate, in the bottom of the combustion chamber, the accretion/deposit on the walls (sample 6-B) is powdery, even though the alkali content is high (Table 2). In these types of boilers, the temperatures above the grate are not higher than 850 °C. The temperature of ash particles decreases due to the secondary air supply needed for the complete combustion of fuel. The ash annealed at this temperature creates a coarser powder, which is in correlation with measured data of slags and ash fusibility (Table 4). Above the arch dividing the combustion chamber, the walls and upper arch are glassy accretions (samples 1 – 5). The accretions on the lower walls are several times thicker than on the arch. Varicoloured layers in the accretions reveal the variability of conditions in the
temperature of PL. The PT ash with high SiO2 content has similar B/A values to those for the accretions. Temperature and Na, K, Fe concentrations are determining factors for silicate particle slagging. Sticky particles cover the lining surface with a layer of glassy melt, which subsequently protects the lining from direct attack by hot aggressive gases. On the other hand, the refractory lining is then corroded by the melt, which slowly releases and dissolves the resistant particles into the melt. The composition, temperature and flow of gases in the chamber, which carry the fine particles, are important for the growth of accretions (Fig. 3). The viscosity and basicity of the accretions are decisive for the corrosion kinetics of the aluminasilicate refractory lining.
Table 7 Measured fusion temperatures of accretions from combustion chambers (STN ISO 540) and the solidus temperatures calculated by FactSage 7.3 software*. Sample Abbr.
Fusibility of samples in air atmosphere (Tolerance ± 5 °C) Deformation temperature DT (°C)
Softening temperature ST (°C)
Hemisphere temperature HT (°C)
Fluid temperature FT (°C)
Note
1-A 2-A
1270 1255
1280 1270
1295 1330
1335 1375
1-B 2-B 4-A 4-B 4-C 4-P
1145 1175 1150 1135 1120 1180
1165 1200 1175 1155 1130 1200
1195 1285 1235 1220 1210 1290
1265 1350 1365 1340 1345 1370
6-B
1240
1250
1290
1310
From boundary of refractory materials – accretion Boundary of accretionatmosphere – Sintering from 1050 °C – Expanding volume and foaming Expanding volume due to gas at 1240 °C
7
Temperature of fusibility AF (°C)
Calculated solidus temperature (°C)*
1275 1270
1204 1098
1155 1197 1167 1152 1138 1202
1040 1028 1007 1020 983 968
1250
1173
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Fig. 4. Calculated (FactSage 7.3) phase diagrams (wt%) of (a) SiO2-CaO-Al2O3 and (b) SiO2-CaO-Al2O3 -K2O (5 wt%)-FeO (5 wt%) with plots of sample compositions.
the gas determine the viscosity of the glassy accretions, which corrodes the refractory materials. The 1-B sample from the boundary phase of furnace atmosphere-accretion has lower fusibility temperature of about 100 °C (Table 7) than that of the 1-A sample from accretion-refractory lining boundary. The eutectic melt with 13–16 wt% Al2O3 forms on the surface accretion (Table 5, Fig. 4 b). Increasing content of Al2O3 from 0 to ~15 wt% decreases the temperature of slag formation shown in the ternary diagram in Fig. 4. Further increase in Al2O3 content (> 15 wt%) has reverse effect. The higher temperature and lower viscosity of melts support their dropping and flow, which causes the corrosion of refractory materials to accelerate. Part of the melt from the upper arch dropped onto the lower arch (position 2, which was repaired four years ago). Just as in
combustion chamber (temperature, composition of gas). The variability of dendromass composition with different heating capacity could be the cause of this. Blue-brown coloured accretions can be formed due to the variable ratio of Fe2+/Fe3+ induced by instable atmosphere composition (mainly gaseous compounds of S, Cl, K, Na, and oxide-redox conditions: ratio of CO/CO2) [35]. The higher content of Al2O3 in accretions on the arch (samples 1 and 3) is evidently the result of melting of the refractory material and continuous dropping of glassy slag. In fact, in this zone there are the highest temperatures in the boiler. The refractory material here is attacked more intensively than that of the walls. The glassy layer of accretion is only several centimetres thick, and stalactites are porous (Fig. 3, samples 1 and 3). Temperature and concentration of alkali in 8
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higher Al2O3 content and heterogeneity show higher fusibility temperature. If the Al2O3 content in the refractory materials increases, the amount of silicate slag decreases, which partially inhibits the corrosion process and retards the accretion creep.
the case of samples from position 1-B and 2-B, the fusibility temperatures (Table 7) are lower than 1-A and 2-A (~100 °C). The surface compositions of accretions converge to eutectic melt with melting temperature ~1104 °C (Fig. 4b, point 26). This composition predicts that the wollastonite, feldspar and amorphous glass phases should form during cooling of the eutectic melt. This corresponds with the results of our X-ray diffraction analyses (Table 6). The crystalline phases are a result of sticky ashes interacting with the lining materials and crystallisation of melt accretions during their cooling process. The closer the composition of accretions approach the eutectic point, the more melt is created (Fig. 4). The composition of porous accretions (samples 4, 5) from the walls correlate with the compositions of the slags (S-7,-8). The accretions on the walls contain more than 66 wt% of SiO2 and therefore they form more amorphous phases after solidification (Table 6). Depending on their position in the combustion chamber (temperature distribution, gas circulation), the surfaces of the walls are covered with accretions nonuniformly. The accretions on the top parts of the chamber wall, as well as on arch, are only 2–3 cm thick and their surface is glassy. However, accretion thicknesses at the bottom of the chamber are even more than 10 cm thick, and these surfaces are glassy in places and covered with sintered brown sand in other positions. Under the sandy shell in glassy accretions there are single-direction prolonged macro- and micro-pores (Fig. 3; samples 4, 5). The ashes covered with glassy melt probably react together, forming gases. Changes in the temperature and oxideredox conditions initiate the generating of pores, as well. This porosity contributes to the accretions’ lability, unequal flows and cavity formation. The 4-P sample characterizes the sintered sandy content from cavities containing mainly SiO2 cristobalite (Table 6). If the presence of pores and weight of accretions on the walls are taken into the account, then the visco-elastic flow along the walls can become significant. On the other hand, the increasing presence of undissolved particles and Al2O3 concentration change at the liquid/lining boundary tend to retard viscous flow. It is notable that the Factsage-calculated solidus temperatures are lower (about 100–150 °C) than the measured fusibility temperatures of the accretions (Table 7). The increasing SiO2 and CaO concentrations towards the surface of accretions and the reverse concentration gradient of Al2O3 lead to clear lower temperature of slag formation starting. According to our calculations the accretion surface starts to melt at temperatures around 1000 °C. These temperatures are the limit for boiler operation, so probably such high temperature could be rarely achieved on the accretion surface in the upper part of the combustion chamber, even at high performance levels of the boiler. The maximum service temperatures of the refractory materials used in this boiler (Table 1) are much higher [31,32]. The corrosion progress of the refractory lining is a function of the temperature and composition of the melt and refractory material, its granularity and matrix density [36]. Moreover, if the components of refractory materials are able to increase the melt viscosity, then they are more resistant to corrosion. Increases in operating temperature and alkali content in the atmosphere are the main factors influencing the viscosity of accretions, which is responsible for corrosion of the boiler lining. Small amounts of Al2O3 cause a decrease in melting temperature of the CaO-SiO2 system (Fig. 4b, tridymite and wollastonite areas), but not as intensively as Na2O or K2O. If a change in the coordination number of Al from 4 to 6 occurs, the viscosity of alkali glass with high content of Al2O3 can decrease (even though the alkali content is low). The structural changes caused by the composition of glass melt change its properties markedly. It very negatively affects the corrosion of refractory materials (the eutectic curve linking point 25 with 24 in Fig. 4b). Higher amounts of Al2O3 affect it to the contrary. The more substitution of SiO2 for Al2O3 increases the melting temperature and viscosity of melts, retards the penetration-diffusion processes in the melt and improves the inclination toward crystallisation (at content above 5%) [35]. The accretions (1-A, 2-A) sampled deeper below the surface with
5. Conclusion Variability in the composition of the combusting dendromass (woodchips) in boilers affects the temperature and oxidation-reduction conditions in the combustion chamber, and consequently also the amount and quality of slag, flying ashes, combustion gases and accretion forming. A great proportion of the ash melt stays in the slag, and then a smaller proportion of it creates accretions on the combustion chamber walls and the rest is carried away by the hot gases. The SiO2 content in the slag and accretions is dominant and comparable. The speed of accretion formation has significant impact on the performance and life of boilers. The impact on refractory materials of long-term thermal and chemical effects of combustion gases varies in different zones of the combustion chamber. The temperature over the grate is not high and therefore the refractory lining in this part is covered in a thin layer of powdery deposit. A proportion of the ash particles carried by hot gases towards the upper part of the chamber adhere to the hot walls and arch and create a eutectic melt with lining material containing up to 60–65% of SiO2, ~ (9–15)% Al2O3: (8–20) % CaO, ~(2–5)% K2O and ~5% of Fe-oxides, a mixture which very slowly corrodes the lining and creates heterogeneous layers with inclusions, i.e. partially-soluble particles of refractory material and pores in the accretions. The concentration of Na, K and Ca oxides in accretions towards the melt/furnace atmosphere boundary significantly increases, but Al2O3 concentration decreases. This has an important effect on the melt content and its viscosity. Decreasing viscosity supports the flow of the surface layers of accretions. The composition of the surface accretions converge to eutectic melts with eutectic temperature of 1104–1168 °C, while temperatures around 1050 °C become critical for boiler operation. The accretion layer is thinner in the arch and upper parts of the walls, in zones of higher temperature. Here the more intensive (viscoelastic) flow and dripping of accretions occur, which accelerates the corrosion of refractory material, and then there is higher Al2O3 concentration in the accretions. The result of long-term thermal loading conditions is uneven downward flow on the lining walls, and the accretions are several centimeters thick in the lower parts of the chamber. The porosity of accretions significantly supports this as well. Thicker layers of accretion partly protect the lining from the progress of chemical corrosion; however they strain it mechanically instead. The measured fusibility temperatures (AF) of surface accretions from the arch and walls were approximately 1150 °C. These are lower by about 100 °C than for the subsurface accretions and the sandy deposit in cavities with higher content of Al2O3 and SiO2, respectively. The solidus temperatures of accretions calculated by FactSage 7.3 software are approximately 1000 °C, and only 968 °C for the sandy deposits. These temperatures correspond to the level of lining boiler deterioration, and our analyses of the surface accretions show that the operating temperature of the boiler was not often overreached. During the ten years of operation of this boiler, only the refractory materials of the lower arch were changed after six operating years, on which accretions tended to drip from the upper arch. Monolithic linings made from alumina materials are able to resist the aggressive combustion gases and ashes from biomass combustion. Obtained knowledge on the corrosion of corundum and mullite–corundum linings will become the basis for corrosion model tests and design of the composition of lining refractory material for boilers operating on dendromass combustion.
9
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Acknowledgements
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Full Length Article
Analysis of deposits from combustion chamber of boiler for dendromass a,⁎
b
a
T
a,c
Beatrice Plešingerová , Bora Derin , Pavol Vadász , Dávid Medveď a b c
Technical University of Košice, Faculty of Materials, Metallurgy and Recycling, Institute of Metallurgy, Letná 9, 042 02 Košice, Slovak Republic Istanbul Technical University, Metallurgical and Materials Engineering Department, Maslak, Istanbul, Turkey Slovak Academy of Sciences, Institute of Material Research SAS, Watsonova 47, 040 01 Košice, Slovak Republic
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass Corrosion of linings Boiler furnaces Refractory
This article records the degradation of the refractory alumina-silicate lining in a boiler furnace combusting dendromass after ten years of use. The deterioration of refractory alumina material was evaluated from phase and chemical analyses of accretions. The fusion temperatures of accretion were measured with a high-temperature microscope and the results were compared with the solidus (point of first liquid formation) temperature predicted by FactSage 7.3 thermodynamic simulation software. Content of SiO2 (50–65%) in accretions confirms that fine silicate particles of ash adhere to the lining. These silicates react with subliming alkalis from the dendromass and form aggressive eutectic melts on surface of lining at operating temperatures. Increase in operating temperature, inhomogeneity (porosity) and alkali content in accretions are the main factors influencing accretion viscosity, melt convection and lining corrosion. For this reason accretions on the vertical walls lower down are much thicker than on the walls in the upper part and the arch. The Al2O3 concentration is higher in the arch accretions; there the refractory material corrodes intensively. The fusion temperatures of the glassy accretions (measured at the furnace atmosphere: accretion boundary) are around 1150 °C. However, the calculated temperatures of slag formation stated by FactSage are about 150 °C lower, and these correspond to the operating temperature in the upper section of the combusting chamber. The fusion temperatures increase with the Al2O3 content in accretions closer to the lining. The obtained results will be applied to define the requirements for the development of boiler furnace refractories.
1. Introduction Biomass/dendromass ranks among the fuels in the category of renewable energy and nowadays it often replaces the fossil fuels used in the heating stations preparing heat and hot water for households. Modern boilers using dendromass provide higher efficiency at lower cost. The development of biomass combustion technology is intensive; it is focusing on the enhancement of combustion effectiveness and reduction of emissions, so that energetic biomass utilization should be in compliance with EU norms [1–3]. The refractory materials for boiler linings were not specifically developed for biomass combustion. The same refractory materials (concrete and fire-bricks) are mostly used just as they were applied in older boiler and furnaces [4–6]. The expansion of biomass use in energy and metallurgy forced to focus on their combustion efficiency, ecology and economy. Some researchers worked on the corrosion resistance verification of various types of refractory materials in simulated atmosphere at high temperatures [4,6,7–12]. The studies reveals that the interaction of gas
⁎
with the refractory materials depends on the composition of combustion gas, its flow rate, and operating temperature in boiler, as well as the chemical and phase composition and density of the refractory material. For biomass combustion, the linings should be thermally stable, abrasion and wetting resistant and dense (e.g. refractories with high content of Al2O3 and SiC) [9]. There is a lot of research on biomass related to slagging and fouling of chambers in boilers and furnaces [13–18]. The generated combustion gas is different from that produced in the case of coal combustion, and so the corrosion effect of the gas atmosphere on the lining in boilers is different as well. The gas, ashes and slag produced from biomass combustion contain in addition to carbon dioxide also more water vapour and alkali. The heating capacities of different kinds of biomass are significantly different, as well as the ratio of combustible components to non-combustible residues (ash matter) and the chemical composition of the gases and ashes formed. The various amounts of ash (from 0.1% (soft wood) to 15% (bark)) created from different types of dendromass fuel and the proportion of major contents (SiO2 and CaO) in ashes can vary
Corresponding author. E-mail address: [email protected] (B. Plešingerová).
https://doi.org/10.1016/j.fuel.2020.117069 Received 23 August 2019; Received in revised form 20 December 2019; Accepted 10 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Table 1 Basic characteristics of refractory materials used [31,32]. Material
LCC Concrete Alumina Bricks
Chemical composition (mass %) Al2O3
SiO2
Fe2O3
K2O + Na2O
MgO + CaO
61.5 60
33.5 30–37.5
0.9 1–2
– 0.4 + 0.2
– 0.2 + 0.4
Basic phase material
Apparent density (g.cm−3)
Apparent porosity (%)
Thermal expansion at 1000 °C (%)
Resisting to CO
Max. service temper. (°C)
Mullite –
2.49 2.5–2.55
17 15–20
+0.55 0.5–0.6
Class A –
1 480 1 600
from single figures to high percentages (3–70%) [3,19–22]. An important role is played by humidity of the biomass, vapour and volatile compounds (mainly H2O, carbon oxides, sulphur, phosphorus, chlorine, alkaline and heavy metal compounds), which react together and condense in different parts of the combustion chamber and cooler zones in the heat-exchanger. Only part of the ash remains in the chamber bed. Very fine particles are carried away with hot aggressive combustion gas from the combustion chamber bed to the top and into further parts of the boiler. In the furnace atmosphere the vapours of water, compounds of carbon, sulphur, phosphorus and chlorine react with alkalis (Na, K) and metal alkaline earths (Ca, Mg), which are aggressive to silicon. Together they create the various multi-component compound particles with low eutectic temperature. At the beginning, the aggressive gases and then the low-fusible melts attack the refractory materials. Sticky particles are caught on the hot surface of the refractory lining and gradually cover the walls and arch with glassy deposits. Later on the thick viscose glassy deposits start to flow due to their weight [7,13,14]. In an attempt to monitor these processes, previous researchers measured the fusibility temperature of ashes and slags. Slagging, fouling and sintering indicators were evaluated similarly as in the case of coals [16,17,23–26]. The main difference between coal and biomass ash is that the coal ash contains higher amounts of SiO2 and Al2O3 but lower amounts of K2O and Na2O. The alkalis have decisive role on fusibility of the ashes. Despite these differences, it is possible to use the same methodology for the fusibility characterization of ashes [23]. In addition to the measurement of the fusibility temperature [23,25,27] the indexes for evaluation of ashes fusibility derived from their chemical composition were calculated [17,23]. In the study [23], which focuses on characterization of the biomass, the ashes are divided into classes in relation to the base/ acid index: if B/A 0.2 it indicates a lower deposition tendency of slagging; 0.2–1 a medium tendency and 1 a high tendency. In the study [17], which deals with coal combustion, the intervals of slagging formation are < 0.11; 0.1–0.14 and > 0.14 respectively. The prediction of slag formation start is calculated on the basis of thermodynamic principles and chemical analysis [16]. In the study [13] it was experimentally shown that 15–20% of liquid phase (temperature at T15) makes the particles sticky, and if the deposit material contains more than 70% liquid (T70) the deposit flows down from the vertical walls. The combustion processes in boilers are highly complex. The refractory linings are loaded thermally, mechanically and chemically; simultaneously and repeatedly. The higher the temperature, the more intensive are the thermodynamic interactions between alkali compounds and refractory materials and the low melting-point compounds formed on the hot surface of the lining. Problems can occur during the combustion processes if the type of fuel-biomass is changed. Increasing temperature together with changes in oxide-reduction conditions and abrasive effects of solid particles in the aggregate shorten the life of the lining material [7,13,14,18,28]. For these reasons materials resistant to thermal shock and CO such as moulted refractory alumina-silicate and spinel concrete, which were developed for metallurgical and energy plants using coal combustion or natural gas, are often applied in the boilers [4,5,29,30]. The study investigates the extent of wear on the alumina linings in a dendromass-fuel boiler after 10 years of operation. The inside of the
boiler was not monitored during the operation. The formation of accretions on the refractory lining was evaluated in relation to the composition of the ashes of woodchips combusted at low temperature in laboratory and slags from the grate of boiler. The results of the chemical and thermal analyses of ashes and slags were the basis for modelling the combustion gas composition in combustion chamber and the conditions of accretion formations. The determination of the softening and melting points of the accretions provides information about their formation and the temperature distribution in the chamber. The progress of the accretion melting process and corrosion of the lining and the temperature in the boiler needs to be known in order to establish the economical combustion process and the identification of the proper refractory materials.
2. Experimental part 2.1. Material – corroded lining, slags and ashes The spent refractory linings from the grate boiler (SCHMID AG Holzfeuerungen CH-8360 Eschlikon, type UTSR-3200, 32, 2007) for water heating in flat-building was studied as post-mortem examination. Some measured values for the operating parameters such as fuel batching, time of maximal and minimal performance, and dead time were unavailable. The fuel of the grate boiler was commercially available waste woodchips. The walls of the boiler were made from low cement castable (LCC) with hydraulic bond. The lining in the bottom arch over the grate (position 2 in Fig. 1) was replaced with high-alumina bricks four years ago. The basic characteristics of the originally-used refractory materials (Didurit/Low Cement Bonded Castable and alumina brick [31,32]) are listed in Table 1. Operating temperatures in the furnace grate are 500–900 °C [22]. The difference is caused due to the temperature distribution in combustion chamber of boiler starting from grate to heat exchanger. The maximum temperatures were achieved on the top of the primary combustion part of the boiler where the combustion of the pyrolysis products of biomass occurred. The accretion samples (numbered 1–6) were taken from different parts of the corroded surface of the boiler linings (Fig. 1), after operating for ten years, were taken at the time of general repair of the work lining, and then analyzed and evaluated. Together with the corroded lining materials, the slags and woodchips combusted in the boiler were delivered for analysis. The samples of woodchips and slags to analyse were from the last operating period of boiler. Dry dendromass is characterized by high humidity (10–15%), which is released during combustion in the boiler. The three ashes (PT, PS, PL) were prepared by burning three kinds of industrial woodchips in our laboratory. PT is woodchip from bark; PS and PL are woodchips from softwood. The woodchips were burned on a metal grate and then the ashes were overfired in an electric furnace at a temperature of 500 °C in air atmosphere until a constant weight of ashes was achieved. Major ash- and slag-forming elements (Si, Ca, Al, Fe, Mg, K, Na and Mn) were determined. The chemical compositions of the ashes are compared in Table 2 with the slag conglomerates from the combustion of dendromass.
2
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Fig. 1. Scheme of combusting woodchip boiler for heating water: I – batching, II – primary combustion chamber, III – secondary combustion chamber, IV – heat exchanger and off-take. Numbers (1–6) in circles show sampling points.
2.2. Methods – characterization of materials
fusibility) [25,27]. The fusibility characteristics were determined using a high-temperature microscope (Leitz-Wetzlar, Germany) in a resistance furnace with a maximum running temperature of 1500 °C. The sample in ground powder form was pressed into a cylindrical tablet ϕ 3 mm × 3 mm of weight 0.2 g. The tablet was placed on a sintered corundum plate and heated in the furnace at a rate of 10 °C/min in a stationary air atmosphere. The progress was detected with a digital camera simultaneously with the sample temperature. Temperature of fusibility (AF) was calculated according to equation for coal slagging 4DT + HT [23]: AF = , where DT is the initial deformation temperature 5 and HT is the hemisphere melting temperature.
After macroscopic documentation, the specific parts of corroded linings, accretions, slags and ashes were analysed. The samples were analysed by classic chemical analysis of earth; the procedure is based on decomposition of solids by melting. Atom adsorption spectroscopy (AAS; Perkin Elmer 3100) was used for the analyses of elements in leachates from metls. Semi-quantitative energy dispersion spectrometry (EDS; JEOL JSM 7000F, INCA Energy 250 Microanalysis System) was used for local analysis of the elements of accretions. The results of the chemical analyses were used to calculate the base/acid ratio Fe O + CaO + MgO + Na O + K2 O B = 2 3 SiO + Al O + TiO2 , which indicates the propensity for slagA 2 2 3 2 ging or fouling of the material [23]. An X-ray diffractometer (RIGAKU MiniFlex600, Theta-2Theta, CuKα phase) was used for phase analysis of slags and selected accretions. Identification of the individual crystalline phases was performed using the PDXL 2 software package together with the ICCD database of minerals and ceramic phases. The investigations on the thermal characteristics of slags and ashes were obtained by a simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter). The measurements were carried out with 40 mg sample in a Pt-crucible in air atmosphere and heating rate of 10 °C/min up to 1400 °C. The measured data were evaluated with the Netzsch Proteus TA version 6.1 software. The melting process of accretions, ashes and slag was observed, and the softening temperature and fusibility were measured according to the STN ISO 540 standard (Hard coal and coke. Determination of ash
2.3. Analysis of conditions in combustion chamber during the accretion formation The composition of combustion gas in the chamber during forming accretions was simulated using the HSC 9.1 software [33]. The calculation was made for dendromass with 20 wt% humidity and 10% excess air at a pressure of 1 bar, and the estimated content of ash after combustion was 2 wt%. The dendromass composition, 51 wt% C, 5.5% H, 43% O, 0.2% N, 0.02% S and 0.01% Cl, was derived from the previous works [3,21,22] and the composition from the chemical analyses of dendromass ash (PT, PS, PL; Table 2). FactSage 7.3 thermodynamic simulation software which uses the Gibbs energy minimization method was used in the present study to create complex oxide-based phase diagrams and to evaluate the thermal behaviours of some accretions, as shown in Table 7 [34]. The FToxid
Table 2 Chemical analyses of ashes from woodchips and slags. Sample
Abbr.
Chemical analysis after re-calculation based on annealing conditions SiO2
Al2O3
CaO
MgO
Fe2O3
B/A Na2O
K2O
MnO
(wt.%) Woodchips PT Woodchips PS Wood PL Agglomerate slag Powdery slag
PT PS PL S-7 S-8
59.2 23.5 14.1 65.6 63.8
7.8 4.5 9.5 11.2 13.2
26.0 42.2 56.8 12.3 12.6
1.2 7.6 8.5 1.6 2.3
3.2 9.2 5.1 4.8 5.3
L.O.I. – Loss on Ignition, N – undetermined. 3
0.7 0.5 0.1 1.0 0.8
1.7 8.4 5.1 3.5 2.0
0.1 4.1 0.7 N N
L.O.I 2 h//1100 °C
(–)
(wt.%)
0.5 2.4 3.2 0.3 0.3
11 (17) 21 N N
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solution database, which contains oxide solutions evaluated/optimized by the FACT group, was selected in FactSage for the calculations.
Table 3 Evaluation of X-ray analyses – diffraction records of ashes PT, PS, PL and slags 7 and 8.
3. Results
Sample
Abbr.
SiO2
CaCO3
K2Ca (CO3)2
MgCO3
Anorthite/ Anothoclase (Ca,K)–Si–Al–O
Woodchips PT Woodchips PS Wood PL Agglomerate slag Powdery slag
PT PS PL S-7 S-8
+++ ++ + +++ +++
++ ++ +++ – –
Trace – + – –
Trace Trace ++ – –
+ + Trace Trace +
3.1. Ashes and slags The chemical composition of slags (sintered slag – S-7, powdery slag – S-8) is compared in Table 2 with ashes from the woodchips combusted in the laboratory. The content of SiO2 in the slags (S-7, S-8) is significantly higher than in the compared ashes. The ashes (PL, PT and PS) are very fine. PS and PL woodchips at temperature up to 500 °C creates about 0.8–1 wt% ash, however the PT forms ten times more of ash. It is also evident that the compositions of the ashes are markedly different. These results, including the large variance in amounts of two dominant components (CaO, SiO2), correspond to the knowledge from many other studies [3,20,22]. The PT ash from wood bark includes significantly more SiO2 (glass-forming component) than the PL and PS ashes from softwood, which are a source of higher content of alkali metal (K) and alkaline earths (Ca, Mg). The thermal analysis records of ashes and slags (PT, PL and S-7, S-8) are shown in Fig. 2 a, b. Moderate mass loss was recorded in the case of ashes and slags in the temperature region of 25–600 °C, too. The more
Number of + expresses the probability of existence and quantity of solid phase.
alkaline-like ash (PL) shows greater mass loss (2 wt%) than the ash (PT) with high content of SiO2 and the slags (S-7 = 0.5 wt%, S-8 = 1.2 wt %). This mass loss may be due to the decomposition of hydrates and hydroxides, which interact with water vapour and air. The loss in case of the slags could also be due to the trace of unburnt carbon. Sharp mass decrease with endo-effect was detected in ashes in the temperature range 600–800 °C, namely 10 wt% loss in PT and 21 wt% loss in PL ash. In the case of slags there was slight loss with weak endoeffect. The decomposition temperature corresponded to the temperature of initial decomposition of pure CaCO3 powder. The presence of
Fig. 2. TA records of: a) ashes PT and PL; b) slags. Conditions: dose = 40 mg of sample in Pt crucible, heating rate 10 °C/min; atmosphere – air. 4
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Table 4 Evaluation of fusion temperatures of ashes and slags (STN ISO 540) [27]. Sample Abbr.
PT PL S-7 S-8
Fusibility of samples (ashes, slags) in air atmosphere (Tolerance ± 5 °C)
Temperature of fusibility AF (°C)
(Initial) Deformation temperature DT (°C)
(Softening) Spherical temperature ST (°C)
Hemisphere temperature HT (°C)
Fluid temperature FT (°C)
1185 1380 1170 1160
1200 1410 1190 1175
1230 1425 1240 1215
1340 1455 1350 1345
1194 1389 1184 1171
DT – initial deformation temperature; ST – (sphere) softening temperature; HT (H = ½ W) – (hemisphere) melting temperature; FT – (flow) fluid temperature.
The results of X-ray diffraction analyses of glassy samples (1,2 and 4) are summarized in Table 6. Depending on their composition, partial melts crystallized during cooling (Table. 6). Upon solidification of melting the quartz/cristobalite, clino- and orthopyroxens (clinoenstatite, diopsite, hedenbergite) and potassium and calcium feldspar minerals crystallized in a glassy phase. Sample 4-P is of sandy accretions from a cavity created by the flowing of viscous melt on the walls, and therefore SiO2 is dominant there. These results demonstrate that the hot powders adhering to the lining generate glassy layers which on the one hand interact with and corrode the refractory materials, and on the other hand are attacked by aggressive combustion gases and form low-viscosity melts. The temperatures of fusibility of the accretions measured with a high-temperature microscope are shown in Table 7 together with the solidus temperatures calculated by FactSage 7.3 software.
CaCO3 in the ashes was confirmed by means of X-ray diffraction analyses (Table 3). The carbonates are the result of interaction of ultrafine CaO particles with CO2 contained in the combustion gas at lower temperature. Unlike in ashes, the ratio of SiO2 to CaO is higher in slags (Table 2), and therefore the CaO in slag is bound for the most part into silicate and alumina-silicate. This was confirmed by the X-ray analyses (Table 3). The observed moderate and slow mass loss in all samples at temperatures above 800 °C probably relates to the partial decomposition of K, Na compounds. The ultrafine granularity of ashes and higher content of alkali and alkaline earths forms conditions for the formation of melt at lower temperature. Fusibility processes (Table 4) are accompanied with endo-effects, which are observed on the DTA curves. In the case of slags S-7, S-8 the temperature of the initial endo-effect was at 1100 °C, with peaks at 1165 and 1185 °C, and the endo-peak temperature of PT ash was 1204 °C. The endo-effect consists of the sum of all heat processes which take place in the sample, including the heat effect of reactions and melting. The slight mass loss at temperatures above 1200 °C is related to the partial decomposition during melting, possibly of sulphate (CaSO4). The fusibility of ash (PT) and slags is evident from the results in Table 4. Only the DTA curve of PL ash, which has low content of SiO2, does not show more expressive thermal-effect in the range of 1100–1200 °C. The very low content of SiO2 in this ash forms only a small quantity of melt by reaction with CaO, MgO and other K, Na, Fe and Mn compounds. The melts of ashes and slags wetted the corundum plate surface during the fusibility measurement well.
4. Discussion The damage of to the furnace alumina lining after 10 years of operating of grate boiler, the creation of stalactites, flow down and thickness of melt accretions and dead cavities are evident from Fig. 3. The conditions under which the accretions in the combustion chamber were formed were simulated using the HSC 9.1 software [33]. The combusting dendromass generates combustion gas with 65–70 vol % N2, 15–17% CO2 and comparable volume of H2O and ~2% O2. The calculated composition of the gas atmosphere was similar to that in study [7]. In this atmosphere, if the ash contains a little SiO2 and the major component is Ca, then the Ca(HCO3)2 is stable at temperatures below 600 °C, but as the temperature increases, the generated CaCO3 decomposes into CaO and partially reacts with silicon oxides (Table 6). During cooling and at high partial pressure of CO2, the CaO has a tendency to react with CO2 and form CaCO3. Our thermal analyses and X-ray diffraction analyses confirmed the presence of CaCO3 in the ashes (Table 3). Trace amounts of sulphur and chlorine combine together with water vapour to form aggressive hot gases. The presence of thermodynamically stable KOH is very presumable at temperatures above 500 °C. The sulphates of alkali metals (K, Na) are more stable at 700 °C, but above this temperature CaSO4 is more stable. CaSO4 decomposes only above the temperature of 1100 °C, and then the concentration of SO2 increases in the combustion gas. Chlorine reacts primarily with K and Na, creating melts of chlorides which evaporate above 750–800 °C. Alkaline compounds react with silicon oxide from the dendromass (3–18 wt% SiO2 in ash) and create low-temperature melting glassy phases and mixed silicate compounds. The sticky silicate particles adhering to the lining create a glassy melt which corrodes the lining. The corrosion process intensifies, the higher the temperature rises in the combustion chamber and the lower the viscosity of melts becomes. The compositions of the analysed slags (S), ashes (P), accretions (number) and refractory material from lining (T) were plotted in the ternary and pseudo-ternary phase diagrams of SiO2-CaO-Al2O3 (Fig. 4 A and B). It can be seen here that overload operating temperatures (above
3.2. Corroded refractory linings from boiler Fig. 3(a–f) documents the lining degradation in different parts of the boiler (SCHMID UTSR-3200 32) after 10 years of working. The number of the samples in Fig. 3 identifies the position of collection of the sample displayed in Fig. 1. Samples 1–5 have glassy appearance, and they are heterogeneous and porous. In contrast the sample from position 6 has the character of powder. The chemical analyses results of accretions and refractory materials are given in Table 5. The letter A, B, C or P denotes the repeated analyses from different parts of the samples 1–6. The T samples characterizing the lining materials were picked from the used refractory materials, from places at least 5 cm under accretions: T-2 – arch over the grate, taken from alumina bricks built in four years ago (position 2 in Fig. 1); T-3 – top arch of the combustion chamber (position 3) and T6 – side wall at the grate (position 6). The powdery fouling from position 6 and glassy accretions of different colour, bubbling and thickness from position 4 were analysed with EDX. The spot-analyses of several glassy samples (4) confirmed, in addition to the predominant elements Si, Al, Ca, Mg, K, Na and Fe, also sporadic distribution of Cu, Ba, Cr, Zr and trace amounts of Mn, Ni, Ti and P. In sample 6, small quantities of Ni and Cr elements were found only randomly, but Mn, P and Ti elements were always detected together with the elements found in the chemical analysis (Table 5). 5
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Fig. 3. Photo-documentation of accretions in the combustion chamber presented in Fig. 1.: a) 1 – frontal part of top arch; b) 2 – frontal arch dividing the combustion chamber; c) 3 – rear of top arch (before exchanger) and detail of stalactite and accretion on refractory material; d) 4 – front wall and detail of accretion/slagging; e) 5 – sidewall; f) 6 – wall below bottom arch in combustion chamber.
and slags (5–5.3) indicate that more CaO than SiO2 is taken away by the combustion gases to cooler parts of the boiler. This can be seen in Fig. 4a too. The ratio SiO2/CaO in the slags is comparable to that in the accretions. The base/acid ratio values for the PL, PS and PT ashes, 3.2; 2.43 and 0.49 respectively (Table 2), point to their great tendency to form slagging in the combustion chamber, according to the classification mentioned in the study [23] (B/A = 0.2–1 indicates medium and > 1 high deposition tendency). Lower content of SiO2 is the reason for the extremely high B/A values for PL and PS and the high fusibility
1000 °C) can be already very destructive to thermal-resistant corundum-spinel materials if the K2O (Na2O) and SiO2 are together present in furnace gases. K2O and Na2O with SiO2 forms low melting temperature phases which wets and dissolves the lining material slowly. Then, the eutectic melts containing about 12 – 15 wt% of Al2O3 form on the surface alumina lining. The resistance of corundum and mullite and other refractory materials by aggressive alkaline gas were studied [8–10] and the results confirmed a wetting of surface and slow infiltration of melts into alumina refractories and their dissolution. The differences between the ratio of SiO2/CaO in ashes (0.25–2.28) 6
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Table 5 Chemical analyses of accretions from walls and arches of boiler grate. Sample
Abbr.
Chemical analysis (dominant oxides) SiO2
Al2O3
B/A
CaO
MgO
Fe2O3
Na2O
K2O
MnO
[wt.%] Sticker on arch Tip of brown stalactite Glass layer on brick Surface of glassy layer Sticker on arch Brown glass Blue glass Blue glass Sandy sticker Brown glass Blue glass Dusty deposit Top dusty deposit Lining material Lining material Lining material
41.1 49.9 54.3 56.3 45.8 66.1 66.2 66.0 68.9 66.4 63.9 41.9 35.8 35.8 32.2 39.0
1-A 1-B 2-A 2-B 3-A 4-A 4-B 4-C 4-P 5-A 5-B 6-A 6-B T-2 T-3 T-6
[–] 34.6 14.7 28.0 17.9 34.3 11.2 11.6 9.2 9.8 11.6 11.6 46.7 34.5 58.8 59.5 53.5
13.9 20.5 9.6 13.1 12.0 10.6 11.6 13.3 8.8 10.8 12.0 6.1 13.1 2.3 3.9 3.5
2.4 3.2 1.6 2.1 1.8 1.6 1.7 1.8 1.5 1.5 1.6 0.5 2.9 0.3 0.2 0.3
2.8 4.9 3.8 5.5 3.2 6.0 4.8 5.1 4.8 5.8 6.5 3.1 4.9 2.6 3.7 3.0
0.4 0.9 0.4 1.6 0.4 1.0 0.9 1.7 2.6 1.1 1.0 0.4 2.9 0 0.1 0.4
4.6 4.7 2.2 3.0 2.4 3.6 3.2 2.4 3.1 3.0 3.5 1.0 5.3 0 0.2 0.2
N 0.7 N 0.4 N N N 3.1 N N N N 0.6 N N N
0.32 0.53 0.21 0.34 0.25 0.29 0.29 0.32 0.26 0.28 0.33 0.13 0.41 – – –
(sample 4 and 5 contained same traces of Cu, Zr and Ni); N – undetermined. Table 6 Evaluation of X-ray diffraction records of selected accretions. Sample – accretion
Abbr
Amorphous phase
Quartz/ Cristobalite (Si-O)
Wollastonite (CaSi-O)
Clino/Orthopyroxens (Ca/Mg/ Fe-Si-O)
Anorthite Gehlenite(Ca-AlSi-O)
Mullite*, Corundum(Al-SiO)
Orthoclase Microcline Leucite* (K-Al-Si-O)
Brown stalactite Brown glass Brown glass Blue glass Sandy sticker in cavity
1-B 2-B 4-A 4-B 4-P
+ +++ +++ ++ –
– (+) + (+) ++
++ (+) – – –
+ + – – +
(+) ++ ++ (+) +
+, +* ++* – – –
++* + – – –
Above the grate, in the bottom of the combustion chamber, the accretion/deposit on the walls (sample 6-B) is powdery, even though the alkali content is high (Table 2). In these types of boilers, the temperatures above the grate are not higher than 850 °C. The temperature of ash particles decreases due to the secondary air supply needed for the complete combustion of fuel. The ash annealed at this temperature creates a coarser powder, which is in correlation with measured data of slags and ash fusibility (Table 4). Above the arch dividing the combustion chamber, the walls and upper arch are glassy accretions (samples 1 – 5). The accretions on the lower walls are several times thicker than on the arch. Varicoloured layers in the accretions reveal the variability of conditions in the
temperature of PL. The PT ash with high SiO2 content has similar B/A values to those for the accretions. Temperature and Na, K, Fe concentrations are determining factors for silicate particle slagging. Sticky particles cover the lining surface with a layer of glassy melt, which subsequently protects the lining from direct attack by hot aggressive gases. On the other hand, the refractory lining is then corroded by the melt, which slowly releases and dissolves the resistant particles into the melt. The composition, temperature and flow of gases in the chamber, which carry the fine particles, are important for the growth of accretions (Fig. 3). The viscosity and basicity of the accretions are decisive for the corrosion kinetics of the aluminasilicate refractory lining.
Table 7 Measured fusion temperatures of accretions from combustion chambers (STN ISO 540) and the solidus temperatures calculated by FactSage 7.3 software*. Sample Abbr.
Fusibility of samples in air atmosphere (Tolerance ± 5 °C) Deformation temperature DT (°C)
Softening temperature ST (°C)
Hemisphere temperature HT (°C)
Fluid temperature FT (°C)
Note
1-A 2-A
1270 1255
1280 1270
1295 1330
1335 1375
1-B 2-B 4-A 4-B 4-C 4-P
1145 1175 1150 1135 1120 1180
1165 1200 1175 1155 1130 1200
1195 1285 1235 1220 1210 1290
1265 1350 1365 1340 1345 1370
6-B
1240
1250
1290
1310
From boundary of refractory materials – accretion Boundary of accretionatmosphere – Sintering from 1050 °C – Expanding volume and foaming Expanding volume due to gas at 1240 °C
7
Temperature of fusibility AF (°C)
Calculated solidus temperature (°C)*
1275 1270
1204 1098
1155 1197 1167 1152 1138 1202
1040 1028 1007 1020 983 968
1250
1173
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Fig. 4. Calculated (FactSage 7.3) phase diagrams (wt%) of (a) SiO2-CaO-Al2O3 and (b) SiO2-CaO-Al2O3 -K2O (5 wt%)-FeO (5 wt%) with plots of sample compositions.
the gas determine the viscosity of the glassy accretions, which corrodes the refractory materials. The 1-B sample from the boundary phase of furnace atmosphere-accretion has lower fusibility temperature of about 100 °C (Table 7) than that of the 1-A sample from accretion-refractory lining boundary. The eutectic melt with 13–16 wt% Al2O3 forms on the surface accretion (Table 5, Fig. 4 b). Increasing content of Al2O3 from 0 to ~15 wt% decreases the temperature of slag formation shown in the ternary diagram in Fig. 4. Further increase in Al2O3 content (> 15 wt%) has reverse effect. The higher temperature and lower viscosity of melts support their dropping and flow, which causes the corrosion of refractory materials to accelerate. Part of the melt from the upper arch dropped onto the lower arch (position 2, which was repaired four years ago). Just as in
combustion chamber (temperature, composition of gas). The variability of dendromass composition with different heating capacity could be the cause of this. Blue-brown coloured accretions can be formed due to the variable ratio of Fe2+/Fe3+ induced by instable atmosphere composition (mainly gaseous compounds of S, Cl, K, Na, and oxide-redox conditions: ratio of CO/CO2) [35]. The higher content of Al2O3 in accretions on the arch (samples 1 and 3) is evidently the result of melting of the refractory material and continuous dropping of glassy slag. In fact, in this zone there are the highest temperatures in the boiler. The refractory material here is attacked more intensively than that of the walls. The glassy layer of accretion is only several centimetres thick, and stalactites are porous (Fig. 3, samples 1 and 3). Temperature and concentration of alkali in 8
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higher Al2O3 content and heterogeneity show higher fusibility temperature. If the Al2O3 content in the refractory materials increases, the amount of silicate slag decreases, which partially inhibits the corrosion process and retards the accretion creep.
the case of samples from position 1-B and 2-B, the fusibility temperatures (Table 7) are lower than 1-A and 2-A (~100 °C). The surface compositions of accretions converge to eutectic melt with melting temperature ~1104 °C (Fig. 4b, point 26). This composition predicts that the wollastonite, feldspar and amorphous glass phases should form during cooling of the eutectic melt. This corresponds with the results of our X-ray diffraction analyses (Table 6). The crystalline phases are a result of sticky ashes interacting with the lining materials and crystallisation of melt accretions during their cooling process. The closer the composition of accretions approach the eutectic point, the more melt is created (Fig. 4). The composition of porous accretions (samples 4, 5) from the walls correlate with the compositions of the slags (S-7,-8). The accretions on the walls contain more than 66 wt% of SiO2 and therefore they form more amorphous phases after solidification (Table 6). Depending on their position in the combustion chamber (temperature distribution, gas circulation), the surfaces of the walls are covered with accretions nonuniformly. The accretions on the top parts of the chamber wall, as well as on arch, are only 2–3 cm thick and their surface is glassy. However, accretion thicknesses at the bottom of the chamber are even more than 10 cm thick, and these surfaces are glassy in places and covered with sintered brown sand in other positions. Under the sandy shell in glassy accretions there are single-direction prolonged macro- and micro-pores (Fig. 3; samples 4, 5). The ashes covered with glassy melt probably react together, forming gases. Changes in the temperature and oxideredox conditions initiate the generating of pores, as well. This porosity contributes to the accretions’ lability, unequal flows and cavity formation. The 4-P sample characterizes the sintered sandy content from cavities containing mainly SiO2 cristobalite (Table 6). If the presence of pores and weight of accretions on the walls are taken into the account, then the visco-elastic flow along the walls can become significant. On the other hand, the increasing presence of undissolved particles and Al2O3 concentration change at the liquid/lining boundary tend to retard viscous flow. It is notable that the Factsage-calculated solidus temperatures are lower (about 100–150 °C) than the measured fusibility temperatures of the accretions (Table 7). The increasing SiO2 and CaO concentrations towards the surface of accretions and the reverse concentration gradient of Al2O3 lead to clear lower temperature of slag formation starting. According to our calculations the accretion surface starts to melt at temperatures around 1000 °C. These temperatures are the limit for boiler operation, so probably such high temperature could be rarely achieved on the accretion surface in the upper part of the combustion chamber, even at high performance levels of the boiler. The maximum service temperatures of the refractory materials used in this boiler (Table 1) are much higher [31,32]. The corrosion progress of the refractory lining is a function of the temperature and composition of the melt and refractory material, its granularity and matrix density [36]. Moreover, if the components of refractory materials are able to increase the melt viscosity, then they are more resistant to corrosion. Increases in operating temperature and alkali content in the atmosphere are the main factors influencing the viscosity of accretions, which is responsible for corrosion of the boiler lining. Small amounts of Al2O3 cause a decrease in melting temperature of the CaO-SiO2 system (Fig. 4b, tridymite and wollastonite areas), but not as intensively as Na2O or K2O. If a change in the coordination number of Al from 4 to 6 occurs, the viscosity of alkali glass with high content of Al2O3 can decrease (even though the alkali content is low). The structural changes caused by the composition of glass melt change its properties markedly. It very negatively affects the corrosion of refractory materials (the eutectic curve linking point 25 with 24 in Fig. 4b). Higher amounts of Al2O3 affect it to the contrary. The more substitution of SiO2 for Al2O3 increases the melting temperature and viscosity of melts, retards the penetration-diffusion processes in the melt and improves the inclination toward crystallisation (at content above 5%) [35]. The accretions (1-A, 2-A) sampled deeper below the surface with
5. Conclusion Variability in the composition of the combusting dendromass (woodchips) in boilers affects the temperature and oxidation-reduction conditions in the combustion chamber, and consequently also the amount and quality of slag, flying ashes, combustion gases and accretion forming. A great proportion of the ash melt stays in the slag, and then a smaller proportion of it creates accretions on the combustion chamber walls and the rest is carried away by the hot gases. The SiO2 content in the slag and accretions is dominant and comparable. The speed of accretion formation has significant impact on the performance and life of boilers. The impact on refractory materials of long-term thermal and chemical effects of combustion gases varies in different zones of the combustion chamber. The temperature over the grate is not high and therefore the refractory lining in this part is covered in a thin layer of powdery deposit. A proportion of the ash particles carried by hot gases towards the upper part of the chamber adhere to the hot walls and arch and create a eutectic melt with lining material containing up to 60–65% of SiO2, ~ (9–15)% Al2O3: (8–20) % CaO, ~(2–5)% K2O and ~5% of Fe-oxides, a mixture which very slowly corrodes the lining and creates heterogeneous layers with inclusions, i.e. partially-soluble particles of refractory material and pores in the accretions. The concentration of Na, K and Ca oxides in accretions towards the melt/furnace atmosphere boundary significantly increases, but Al2O3 concentration decreases. This has an important effect on the melt content and its viscosity. Decreasing viscosity supports the flow of the surface layers of accretions. The composition of the surface accretions converge to eutectic melts with eutectic temperature of 1104–1168 °C, while temperatures around 1050 °C become critical for boiler operation. The accretion layer is thinner in the arch and upper parts of the walls, in zones of higher temperature. Here the more intensive (viscoelastic) flow and dripping of accretions occur, which accelerates the corrosion of refractory material, and then there is higher Al2O3 concentration in the accretions. The result of long-term thermal loading conditions is uneven downward flow on the lining walls, and the accretions are several centimeters thick in the lower parts of the chamber. The porosity of accretions significantly supports this as well. Thicker layers of accretion partly protect the lining from the progress of chemical corrosion; however they strain it mechanically instead. The measured fusibility temperatures (AF) of surface accretions from the arch and walls were approximately 1150 °C. These are lower by about 100 °C than for the subsurface accretions and the sandy deposit in cavities with higher content of Al2O3 and SiO2, respectively. The solidus temperatures of accretions calculated by FactSage 7.3 software are approximately 1000 °C, and only 968 °C for the sandy deposits. These temperatures correspond to the level of lining boiler deterioration, and our analyses of the surface accretions show that the operating temperature of the boiler was not often overreached. During the ten years of operation of this boiler, only the refractory materials of the lower arch were changed after six operating years, on which accretions tended to drip from the upper arch. Monolithic linings made from alumina materials are able to resist the aggressive combustion gases and ashes from biomass combustion. Obtained knowledge on the corrosion of corundum and mullite–corundum linings will become the basis for corrosion model tests and design of the composition of lining refractory material for boilers operating on dendromass combustion.
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Acknowledgements
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