Article pubs.acs.org/EF
Waste Gypsum Board and Ash-Related Problems during Combustion of Biomass. 1. Fluidized Bed Patrycja Piotrowska,† Anders Rebbling,*,† Daniel Lindberg,‡ Rainer Backman,† Marcus Ö hman,§ and Dan Boström† †
Thermochemical Energy Conversion Laboratory, Umeå University, Umeå, Sweden Process Chemistry Centre, Åbo Akademi University, Turku, Finland § Energy Engineering, Division of Energy Science, Luleå University of Technology, Luleå, Sweden ‡
ABSTRACT: This paper is the first in a series of two describing the use of waste gypsum boards as an additive during combustion of biomass. This paper focuses on experiments performed in a bench-scale bubbling fluidized-bed reactor (5 kW). Three biomass fuels were used, i.e., wheat straw (WS), reed canary grass (RC), and spruce bark (SB), with and without addition of shredded waste gypsum board (SWGB). The objective of this work was to determine the effect of SWGB addition on biomass ash transformation reactions during fluidized bed combustion. The combustion was carried out in a bed of quartz sand at 800 or 700 °C for 8 h. After the combustion stage, a controlled fluidized−bed agglomeration test was carried out to determine the defluidization temperature. During combustion experiments, outlet gas composition was continuously measured by means of Fourier transform infrared spectroscopy. At the same place in the flue gas channel, particulate matter was collected with a 13stage Dekati low-pressure impactor. Bottom and cyclone fly ash samples were collected after the combustion tests. In addition, during the combustion tests a 6-h deposit sample was collected with an air-cooled (430 °C) probe. All ash samples were analyzed by means of scanning electron microscopy combined with energy dispersive X-ray spectrometry for elemental composition and with X-ray powder diffraction for the detection of crystalline phases. Decomposition of CaSO4 originating from SWGB was mainly observed during combustion of reed canary grass at 800 °C. The decomposition was observed as doubled SO2 emissions. No significant increase of SO2 during combustion of SB and WS was observed. However, the interaction of SWGB particles with WS and SB ash forming matter, mainly potassium containing compounds, led to the formation of K2Ca2(SO4)3.
■
INTRODUCTION Utilizing biomass for heat and power generation has been significantly increased over the last decades, and therefore problems associated with its combustion have received more attention. Fluidized bed combustion is a fuel flexible technology commonly used for the combustion of biomass. However, biomass ash properties can limit combustor availability due to bed agglomeration, deposit formation, and corrosion. In most cases, it is due to high potassium content, since potassium containing species, e.g., KOH, combine with other ash forming components such as silica and chloride forming compounds with low melting points and/or corrosive properties. Even though countermeasures are suggested in the literature, such as cocombustion, additives, or alternative bed materials, no universal prevention method has been developed. Ash properties differ from fuel to fuel, and so the effect of the countermeasures varies between fuels and conditions. Gypsum board is a common construction material which upon demolition or renovation is landfilled. It consist mainly of calcium sulfate dihydrate (CaSO4 × 2H2O) (>65 wt %), with calcium carbonate (CaCO3) and magnesium carbonate (MgCO3), paper and wood, ferrous metals, fire retardants, and glass fibers comprising the rest.1 Recent studies showed that landfilling of this waste can increase hydrogen sulfide (H2S) generation levels and, therefore, the separation of gypsum board from construction and demolition waste before landfilling has gained importance.2 © XXXX American Chemical Society
It is well-known that CaSO4 is thermodynamically stable in air up to 1300 °C when it decomposes to lime (CaO) and sulfur dioxide/trioxide (SO2/SO3) (reaction 1). However, the decomposition of CaSO4 at lower temperatures under reducing conditions has been reported in the literature.3−13 Graphite, char, hydrogen, and carbon monoxide are all capable of reducing calcium sulfate to SO2, but out of all, carbon monoxide (reactions 2 and 4) is reported to be the most effective CaSO4 reducing agent.8 At CO concentrations higher than 0.1 vol %, CaS or CaO become more stable than CaSO4.9 2CaSO4 (s) ↔ 2CaO(s) + 2SO2 (g) + O2 (g)
(1)
CaSO4 (s) + CO(g) ↔ CaO(s) + CO2 (g) + SO2 (g) (2)
CaO(s) + SO2 (g) + 3CO(g) ↔ CaS(s) + 3CO2 (g) (3)
CaSO4 (s) + 4CO(g) ↔ CaS(s) + 4CO2 (g)
(4)
2CaS(s) + 3O2 (g) ↔ 2CaO(s) + 2SO2 (g)
(5)
CaS(s) + 3CaSO4 (s) ↔ 4CaO(s) + 4SO2 (g)
(6)
Received: November 4, 2014 Revised: January 13, 2015
A
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels In fluidized bed combustion, even at an air−fuel ratio of 1.4 and when no air-staging is applied, reducing conditions are dominating in the lower part of the bed.14 This can be a result of the existence of volatile plumes or poor fuel distribution. The total air ratio and the extent of air-staging become important in the view of the large impact of reducing conditions on emission of sulfur from calcium sulfate. During the decomposition of CaSO4, the formation of CaS (reactions 3−4) can take place, which is then oxidized to CaO and SO2 (reaction 5) once oxidizing conditions are prevailing, but at higher temperatures also solid−solid reaction 6 can occur. During the normal combustion process, no significant formation of CaS is expected,4,10 but it is an important intermediate, especially when air staging is applied. The decomposition of calcium sulfate is highly temperature dependent, and only a small portion seems to be emitted under 850 °C.13 It can be, however, enhanced by ferric salt15 or iron oxide,16 which can lower the decomposition temperature to 650 or 870 °C, respectively. The interactions of anhydrite with solid acidic oxides, major components of coal ash, were reported by Brady et al.6 Their experiments of coal combustion in air at approximately 930 °C with CaSO4, with and without addition of a small proportion of silica and alumina, showed that more than 30% of the sulfur was released in all cases. The release of sulfur from anhydrite was also studied with the additives at the same temperature in air without coal combustion, which also led to the decomposition of anhydrite. The authors assign it to the formation of Ca alumino-silicates and point to the influence of solid acidic oxides on the decomposition of CaSO4. Thus, the literature indicates that the decomposition of CaSO4 can take place during fluidized bed combustion. In this case, the decomposition products, i.e., SO2/SO3 and CaO, can be beneficial to ash transformation during the combustion of biomass. The products are commonly known countermeasures against high temperature corrosion of superheater tubes and bed agglomeration during combustion of biomass. However, in none of the cited works the use of waste gypsum boards as a source of CaSO4 and its interference with biomass ash properties was studied. Therefore, the objective of this study was to investigate whether shredded waste gypsum boards (SWGB) can be used as a fuel additive against ash-related operational problems during combustion of biomass in a fluidized bed. The interactions of SWGB with biomass ash were studied by means of thermodynamic calculations and experimentally in bubbling fluidized bed combustion. The fuels used in the study are lean-in-phosphorus-biomass-fuels with varying silicon content, i.e., wheat straw (WS), reed canary grass (RC), and spruce bark (SB). The focus is on the ash composition and decomposition products of anhydrite, i.e., SO2 and CaO.
■
Agricultural Sciences in Umeå, Sweden. SWGB was delivered by RagnSells AB and sieved to a particle size of 0.85−1.4 mm. The fuels and additive samples were analyzed by an external laboratory according to Swedish standards. Elemental analyses were performed with ICP-AES and ICP-SFMS. For this purpose a dry fuel sample was first ashed at 550 °C and digested with LiBO2 and dissolved in HNO3. The fuel analyses are shown in Table 1. The fuels were chosen to represent biomass with a different content of silicon and potassium typical for three different biomass categories: woody biomass, grass, and straw.
Table 1. Fuel Properties wheat straw
reed canary grass
spruce bark
[MJ/kg db] [wt % db] [wt % db] [wt % db] [wt % db] 40.3
18.5a 5.8 46.2 5.6 0.9 41.5
19.5 3.1 48.2 6.0 1.1 na
20.1 3.6 na na 0.3 na
na na na na na
[wt % db] [wt % db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db] [mg/kg db]
0.14 0.20 696 10520 96 941 4024 179 6807 26 77 12
0.12 0.02 1300 6800 810 780 3400 89 2300 na 310 na
0.03 0.01 500 1700 98% SiO2) was used, with an initial particle size in the range of 200−250 μm. The minimum fluidization flow was 8 Nl/min at 800 °C. The air flow was kept constant at 80 Nl/ min, with a 50/50 split between primary and secondary air. Secondary air was introduced above the bed. The excess air coefficient (λ) in the bed at 40 Nl/min was on average 0.93 ± 0.06. Ash deposit samples were collected in the freeboard section at the top of the reactor on exchangeable stainless steel rings, which were fitted on an air cooled probe situated in the middle of the flue-gas stream. The surface temperature of the steel rings was set to 450 °C to simulate superheater tubes. The exposure time for the deposition ring was 6 h except for tests with WS. During WS combustion the deposit probe exposure time was 4.5 h due to early defluidization during the test without the additive. After the freeboard section, the flue gas is led through a cyclone separator with a cutoff of approximately >10 μm. The cyclone fly ash was collected after the experiment was finished and the reactor had cooled down to room temperature. The concentration of CO, CO2, H2O, HCl, SO2, and O2 was continuously measured during the combustion stage of each experiment by means of Fourier transform infrared spectroscopy (FTIR). The oxygen content was on average 8.6 ± 2.1% in the dry flue gas for all the experiments except for WS, with and without additive, for which O2 in the flue gas was 7.0 ± 1.3% in dry flue gas. The gas sampling port was placed in the flue gas channel after the cyclone where the temperature was approximately 200 °C. Near the flue gas sampling location the particulate matter was collected by means of a 13-stage low pressure impactor by Dekati, Ltd. (DLPI). Isokinetic sampling was carried out to determine mass size distribution of particle emissions (0.03−10 μm). Aluminum foil was used as a substrate in the impactor which was preheated to approximately 150 °C before and during sampling. The aluminum foils with collected samples were stored in a desiccator for further analyses. After 8 h of combustion, the fuel feed was stopped, and a sample of the bed material was collected. This sample is called “bed material sample after combustion” from here on. Only primary air flow was
Figure 1. Bench scale bubbling fluidized bed reactor with sampling locations. inner diameter of 100 mm in the bed section and of 200 mm in the freeboard section. The maximum temperature that can be reached is 1045 °C (in the bed). A constant temperature is achieved by using preheated primary air, in conjunction with electrical heaters in the freeboard section. A perforated stainless steel plate at the bottom of the fluidized bed with the total open area of 1% functions as an air distributor. In the experiments, the reactor was used as a combustor and subsequently for controlled fluidized bed agglomeration tests. During the agglomeration tests, propane was burned below the air distribution plate to maintain a combustion atmosphere in the reactor. The temperature and pressure drop in the bed were continuously monitored using two thermocouples and two pressure probes. C
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 2. Predicted speciation of ash-forming elements as a function of gypsum proportion for the three different fuels by means of thermodynamic equilibrium calculations; temperature used in calculations was 700 °C for wheat straw and 800 °C for reed canary grass and spruce bark; oxidizing conditions are defined as λ = 1.9 and reducing as λ = 0.9. temperature the bed material was collected. This sample is called “sample after agglomeration test” from here on. Sample Preparation and Analyses. Bed Material Cross Sections. The bed material sample collected after the combustion stage was mainly used to study the formation of layers on silica bed grains. The bed material sample after agglomeration test was used to study the composition of necks formed between silica bed grains. Cross sections of both samples were studied by means of environmental scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (ESEM/EDS). For this purpose, the
kept and was minimized to 37 Nl/min, and combustion of propane gas in a chamber below the primary air distributor plate was initiated. Then, the bed was heated at a constant rate of 3 °C/min until defluidization occurred or the maximum reactor temperature (1045 °C) was reached. The recorded temperature and pressure curves were evaluated to determine the total defluidization temperatures, which refer to the temperature at which no fluidization is observed. It is defined as the first temperature at which the pressure curve reaches its minimum.29 After the temperature in the reactor reached room D
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 3. Gypsum Amount, Fuel Properties, and Defluidization Temperaturesa SWGB feed
combustion temperature
duration of comb stage
DF
fuel
[wt % ar]
[°C]
[h]
[°C]
Si/(Ca + Mg + Na + K)
Si/K
P/Si
Ca/K
Ca/S
S/Cl
wheat straw (WS) WSG2 WSG3 reed canary grass (RC) RCG2 spruce bark (SB) SBG1
0 2 3 0 2 0 1
700 700 700 800 800 800 800
6.5 7.5 8.0 8.0 8.0 8.3 8.0
DC DC DC 1000 996 >1045 992
1.2 0.9 0.8 1.3 0.8 0.2 0.2
2.2 2.2 2.2 4.1 4.2 1.1 1.2
0.1 0.1 0.1 0.2 0.2 0.3 0.3
0.6 1.1 1.5 1.4 3.5 4.8 5.8
2.3 1.4 1.3 2.3 1.3 27.7 5.2
0.8 2.5 3.6 6.6 27.9 3.3 20.8
a
molar ratio
DC: during combustion, DF: defluidization temperature, Comb: combustion, SWGB: shredded waste gypsum board.
samples were embedded in epoxy and ground with silicon carbide grinding paper having four different grain sizes ranging from 320 to 1200 grit size until a smooth surfaced cross section was obtained. No liquid agent was used during grinding. The ash layers on the silica sand grains and agglomerate necks were investigated at magnifications equal to or higher than 1 kx with spots evenly distributed over the formed layers of agglomerate necks. The elemental composition was studied with a backscatter mode with the beam energy set to 20 keV in lowvacuum mode. Chemical Characterization of Ash Samples. Ash deposits from lee- and windward side of the deposition ring, cyclone fly ash were studied semiquantitatively by means of ESEM/EDS. For this purpose ash samples were mounted on a carbon tape, and then three different area analyses at magnification of 150× were performed for each sample. The results are given as averaged values for the three areas. The elemental composition was studied with Philips XL30 ESEM combined with X-ray detector (EDS) in a backscatter mode with the beam energy set to 20 keV in low-vacuum mode. In addition to elemental analyses by ESEM/EDS, investigation of the crystalline phases in collected ash samples was carried out by means of X-ray diffraction analysis (XRD). Fuel-derived ash particles sieved away with a 100 μm sieve from the bed material after combustion, cyclone fly ash, and the fine fraction, based on the mass size distribution curve, of the PM were subject to XRD analyses. The XRD data collections were performed using a Bruker d8Advance instrument in θ−θ mode, with Cu Kα radiation, and a Våntec-1 detector. Continuous scans were applied and by adding repeated scans, the total data collection time for each sample lasted for at least 4 h. The PDF-2 database,30 together with Bruker software, was used to make initial qualitative identification. The data were further analyzed with the Rietveld technique and crystal structure data from ICSD31 to obtain semiquantitative information on the present crystalline phases.
content, combustors can be put under high risk of superheater corrosion. All three fuels investigated in this work were lean in phosphorus (P/Si ≤ 0.3). Therefore, this work should be interpreted as an investigation of SWGB interactions with biomass ash formed during fluidized bed combustion of phosphorus-lean fuel. The highest potassium and silicon content (0.7 wt %db and 1.1 wt %db, respectively) was observed in the WS. The high concentration of these two elements can lead to a vast formation of K-silicates. The eutectic temperatures for these silicates can be as low as 600 °C, and the formation of molten K-silicates particles during combustion can initiate agglomeration.33−37 RC was also rich in silicon with a molar ratio (Si/ (Ca + Mg + K + Na)) of 1.3 (Table 3). The concentration of potassium in RC amounted to approximately one-third of the corresponding in WS. In addition the calcium content of 0.34 wt %db resulted in the molar ratio Ca/K equal to 1.4. Calcium is beneficial from the perspective of ash related operational problems during biomass combustion. There is a general assumption that the higher the Ca/K ratio the higher the melting temperatures of silicates. However, in the case of the RC it was not high enough to prohibit slag formation in a pellet burner.38 The ash-forming matter of SB on the other hand was dominated by calcium, with the highest Ca/K molar ratio (4.8) and the lowest silicon concentration (0.17 wt %db), leading to Si/(Ca + Mg + K + Na) equal to 0.2. This is in agreement with previous studies.39,40 Substantial proportions of alkali in the bark may evaporate; up to 100 ppm alkali in the flue gas can be expected.39 As illustrated in Table 3, the three fuels showed different properties covering three different groups of biomass: silicon rich with low Ca/K (WS), silicon rich with high Ca/K (RC), and calcium dominated (SB). In addition they all contained different amounts of chlorine with varying S/Cl. WS had the highest concentration of chlorine (0.2 wt %) leading to low S/ Cl ratio (0.8). This could indicate high risk for superheater corrosion. The chlorine concentration in RC and SB was at a similar low level, 0.02 and 0.01, respectively. Prediction of Ash Composition. In Figure 2 the predicted ash composition is shown for all three fuels: WS, SB, and RC with varying proportion of gypsum. The temperatures used in the calculations were the same as during combustion experiments. In the case of WS and SB up to 4 wt %db and 1.5 wt %db of the additive, respectively, showed to have an influence on ash transformations in oxidizing conditions. It seems that the main impact gypsum had on WS and SB ash is on the formation of K2Ca2(SO4)3 (calcium-langbeinite). The higher the gypsum proportion the higher the amount of the
■
RESULTS AND DISCUSSION Fuel Properties. Three different fuels were used during the experiments. Fuel properties (proximate and ultimate analyses) are shown in Table 1. Reed canary grass (RC) and spruce bark (SB) had similar heating values (LHV ≈ 20 MJ/kgdb), while wheat straw (WS) had the lowest, 18 MJ/kgdb. This low value is probably the result of high ash content in WS. Ash content in WS was nearly 2 times higher than in the two other fuels. However, the high content of ash is not as problematic as its composition. The extensive review of biomass composition by Vassilev et al.32 shows that the major ash-forming elements in biomass are silicon and phosphorus with varying proportion between alkali (mainly potassium) and earth alkaline metals (calcium and magnesium). The proportion between alkali and earth alkaline metals often determines the risk of ash deposition and agglomeration/slagging in combustion systems. When a high content of alkali metals in fuel is combined with high chlorine E
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 3. Distribution of potassium as a function of temperature for the three different fuels; the predictions were done for combustion of additivefree fuels and for fuel-gypsum mixtures.
calcium-langbeinite. On the other hand, in reducing condition the formation of calcium-langbeinite did not take place, but other effects can be observed. The composition of WS ash has changed from dominated by low temperature melting K2Si4O9 toward dominated by K2Si2O5 and CaSiO3, which have higher melting points than K2Si4O9. The composition of SB ash, however, was not significantly affected by the addition of gypsum in reducing conditions. The only influence that was observed was on the formation of CaS, which was increasing proportionally to the amount of used gypsum starting at approximately 0.5 wt % of gypsum. In the case of RC at oxidizing conditions, no influence of gypsum was predicted on ash composition. The amount of anhydrite was increasing proportionally to the amount of added
gypsum, and the other ash components remain stable, with slag (Slaga) and hydroxyapatite being the dominant phases. In reducing condition, however, with increasing proportion of gypsum the amount of calcium silicates was increasing. The amount of formed silicate slag (Slaga) was reduced to almost 1/ 3 of the initial amount. In addition, the calculations for all three fuels with and without the addition of gypsum and varying temperature were carried out (Figure 3) at oxidizing conditions. The results are shown as the molar distribution of K-species. The prediction of WS ash shows that during combustion of WS the main problem can be related to the formation of Ksilicates, as indicated by the formation of K2Si4O9, and the consequent slag formation. When gypsum was added the ash F
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
the biomass ash/SWGB mixtures. In addition, the high S/Cl molar ratio can imply a reduced potential for superheater corrosion when SWGB is used as a fuel additive during combustion. However, the actual decomposition of CaSO4 to solid CaO and gaseous SO2/SO3 is not known, and, therefore, the molar ratios can be misleading in this particular case. The influence of SWGB on combustion was significant in the case of WS. The addition of SWGB led to longer combustion, and smoother operation. However, defluidization took place before the agglomeration test was carried out, after 7.5 and 8 h of combustion. For the two other fuels (RC and SB) no significant influence on the agglomeration was observed. The defluidization temperatures in the case of RC, with and without SWGB addition, were the same in both cases, approximately 1000 °C. Thermodynamic calculations (Figure 3) point at reduced amount of slag at 1000 °C for RC/gypsum mixture. However, for both cases they point at lower defluidization temperature, nevertheless the experimentally determined defluidization temperature for both cases was similar. In the case of SB the defluidization temperature without gypsum was above 1045 °C but when 1 wt %ar SWGB was added defluidization occurred at approximately 1000 °C. The agglomeration at approximately 1000 °C can be explained with thermodynamic calculations (Figure 3) and high amount of salt melt which formed. Composition of Cyclone Fly Ash. Elemental composition of the fly ash collected in the cyclone (dp > 10 μm) is shown in Figure 4 and the summary of molar ratios based on the EDS values is presented in Table 4. The crystalline phases detected by XRD are shown in Table 5.
composition changed. K2Ca2(SO4)3 (calcium-langbeinite) was formed instead of K-silicates indicating that calcium-langbeinite is more stable than K-silicates. There was no K participating in silicates formation at the conditions used in calculations up to 950 °C. Potassium containing slag started to form above 950 °C. This indicates that during combustion of WS the temperature of initial slag formation was moved from approximately 760 °C to above 950 °C when gypsum was added. At approximately 850 °C K2SO4-melt formation took place. The prediction of SB ash composition indicates that there should be no problem related to bed agglomeration during combustion of this fuel. Only a small amount of salt melt was predicted to form. Most of potassium between 850−1200 °C was in the gaseous form as KOH. The volatile potassium hydroxide can react with bed material in fluidized bed. If the bed material is quartz based, this can lead to the formation of molten K-silicates and consequently to agglomeration. However, the high content of Ca in SB (Table 1) can prevent this by the formation of Ca-silicates instead of the K-silicates. When gypsum was added the amount of KOH was reduced and K2Ca2(SO4)3 was formed instead. In addition no K-carbonates were predicted. The calculations imply that calcium-langbeinite was formed by the reaction of KOH with CaSO4. The K2SO4rich melt formed at 800 °C as K2SO4 (Hexa) started to melt at this temperature. At approximately 900 °C calcium-langbeinite started to contribute to this melt. The prediction of RC ash composition indicates that the formation of K-rich slag at relatively low combustion temperatures, i.e., 720 °C, may be the main problem during combustion of this fuel. At lower temperatures, below 800 °C, K2Ca2(SO4)3 was predicted to form. Above 800 °C the potassium was almost entirely present as slag (Slaga). When 2 wt % of gypsum was added the amount of K which formed K2Ca2(SO4)3 increased from around 30% to more than 50%. However, as in the additive-free case calcium-langbeinite was not stable above 800 °C. Addition of gypsum did affect the amount of formed slag. The highest reduction in slag (Slaga) took place between 900 and 1000 °C. Combustion Experiments. The differences between the fuels had influence on their combustion behavior and resulted in different defluidization temperatures (DF). Combustion of WS led to defluidization after 6.5 h of combustion at 700 °C, which is in accordance with previous experiences and wellknown properties of WS, and could be correlated to the high Si content combined with a low Ca/K ratio resulting in the formation of low temperature melting silicates, which has been indicated by the thermodynamic calculations. In the case of RC the defluidization temperature was much higher compared to WS, approximately 1000 °C. It can be correlated to the higher Ca/K molar ratio which indicates less problems with low temperature melts. However, it is not in agreement with thermodynamic calculations, which predicted high amount of slag formation at temperatures as low as 800 °C. In the case of SB defluidization did not occur and the reactor was turned off when the reactor’s temperature limit (1045 °C) was reached. This in accordance with the thermodynamic calculations, small amount of melt formed, and by high calcium content in the fuel. In Table 3 the influence of SWGB on the overall composition of the fuels is shown in the form of molar ratios. The addition of SWGB resulted in a significant increase of Ca/ K molar ratio indicating an increase of melting temperatures for
Figure 4. Elemental composition on C-, O-free basis of the cyclone fly ash (dp >10 μm).
In the case of WS (combustion experiments at 700 °C), the addition of SWGB did not significantly affect the composition of the cyclone fly ash. The additive caused a slight increase of calcium and silicon (Figure 4) in the cyclone fly ash, leading to an increase of Ca/K from 0.7 to 0.9 and of Si/K from 1.6 to 2.0 (Table 4). Sulfur concentration remained at the same level as for the test without SWGB. Nevertheless, the composition of the crystalline part of the cyclone fly ash (Table 5) indicates that the formation of Ca-containing phases was favored upon the addition of SWGB was fed during combustion, i.e., Ca2SiO4 and CaSO4. In addition, different proportion of SWGB seemed to have a different effect on the ash composition (Figure 4, Table 5). Higher chlorine concentration was found in the cyclone fly ash sample when a smaller proportion of SWGB was G
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
carbonation and was captured in the cyclone with particles ≥10 μm. With the addition of SWGB the amount of CaSO4 and Ca(OH)2 increased in the cyclone fly ash during SB combustion (Table 5). The influence of the additive on the sulfur concentration in the cyclone fly ash was observed in the case of RC. The doubled concentration of sulfur in the ash (Figure 4) resulted in a lower Ca/S ratio (Table 4, Table 5). The dominant and major composition of the crystalline part of the ash (Table 5) shifted from the phosphate−silicate system to the phosphate−sulfate system. According to calculations (Figure 2) hydroxyapatite was next to silicate slag the dominant phase of RC ash. However, there was no apatite in the bottom ash, indicating that small apatite particles formed from the fuel during combustion in the bed were elutriated into the flue gas and were the dominant phase during RC combustion. The shift toward sulfates during SWGB addition can be explained by the diluting effect of the additive. Characterization of Bed Particles. The main bed grain layer formation mechanisms during FB-combustion of biomass fuels in quartz bed are summarized in the literature.29,35,37,42−44 The mechanisms include direct reaction of K-compounds in gaseous or aerosol phase with the bed particles, forming an inner (reaction) layer. This low temperature melting silicate layer can capture fuel ash particles, forming an outer (coating) layer. Depending on the ash properties, this can be accompanied by diffusion or dissolving of calcium into the
Table 4. Molar ratios characterizing the composition of the cyclone fly ash based on concentrations determined by ESEM/EDS (Figure 4) molar ratio fuel
Si/(Ca + Mg + Na + K)
Si/K
Ca/K
Ca/S
S/Cl
0.8
1.6
0.7
3.5
0.6
0.7 0.8 1.2
1.4 2.0 5.2
0.6 0.9 2.1
3.1 4.3 3.1
0.4 0.6 47.0
1.0 0.1
4.2 3.3
2.2 17.9
2.1 21.5
22.1 2.3
0.1
2.8
16.8
9.6
6.7
wheat straw (WS) WSG2 WSG3 reed canary grass (RC) RCG2 spruce bark (SB) SBG1
used, WSG2. When a higher proportion of SWGB was added (WSG3), chlorine concentration in the cyclone fly ash was at the same level as during WS combustion without the additive. In the case of SB the dominant phase detected by XRD was CaCO3 for both tests, with and without, the additive. Werkelin et al.41 reported that in bark and forest residues 70% of the total calcium is present as calcium oxalate minerals; i.e., CaC2O4 × H2O which upon combustion form small crystals of CaO. In the present experiments, CaO, which formed first, underwent Table 5. XRD Analyses of the Cyclone Fly Asha fuel
dominant crystalline phase (>50 wt %)
major crystalline phase (50−10 wt %)
minor crystalline phase (5−10 wt %)
WS
K2SO4 KCl Ca5(PO4)3(OH)
(23%) (29%) (16%)
Q Ca2MgSi2O7 K2Ca2(SO4)3
(6%) (5%) (5%)
WSG2
K2SO4 KCl Ca5(PO4)3(OH)
(17%) (36%) (13%)
WSG3
K2SO4 KCl Ca5(PO4)3(OH)
(15%) (27%) (16%)
(Na,Ca)(Al,Si)4O8
(13%)
CaSO4 K2Ca2(SO4)3 Ca3(PO4)2
(16%) (10%) (45%)
Q Ca2MgSi2O7 CaSO4 Ca3Mg(SiO4)2 Q Ca2SiO4 CaSO4 CaCO3 Q CaSO4 K2SO4 K2Ca2(SO4)3 Ca5(PO4)3(OH) C
(6%) (6%) (5%) (6%) (5%) (8%) (8%) (5%) (5%) (6%) (5%) (7%) (5%) (5%)
RC Ca3(PO4)2
(51%)
RCG2
a
SB
CaCO3
(65%)
CaO
(10%)
Ca(OH)2 Ca5(PO4)3(OH)
(5%) (8%)
SBG1
CaCO3
(53%)
CaO Ca(OH)2 CaSO4
(10%) (10%) (10%)
Ca5(PO4)3(OH)
(6%)
trace crystalline phase ( 10 μm. Q: quartz; C: cristobalite. H
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 5. Elemental composition on C-, O-, Si-, Al-free basis of the inner (reaction) layer and outer layer (coating); approximately 40 spots were analyzed for each sample collected during combustion of RC, SB and approximately 30 for each sample collected during combustion of WS.
potassium-silicate melt.45 The second mechanism of layer formation is related to a direct adhesion of partly molten fuel derived ash particles on the surface of bed grains, and is the main mechanism behind the outer (coating) layer formation. In the present study the composition of the layers was studied, and the results are shown in Figure 5. The composition of the inner (reaction) layer was separated from the outer (coating) layer. The cross sections of typical bed particles with both inner and outer layer found in the bed sample after combustion of all three fuels with and without SWGB addition are shown in Figure 6. The combustion of WS resulted in the formation of both inner reaction layer and outer (coating) layer; however, the latter one dominated. The individual ash particles were found in the bed, and some of them were found glued to the surface of the sand grains (Figure 11 top). The layers were not uniformly spread around the silica sand grains. The thickness of the layer varied from 2 to 10 μm. In the case of RC hardly any layers could be distinguished and the total layer thickness varied from 1 to 3 μm. Some individual ash particles were observed in the bed materiel sample. The combustion of SB, on the other hand, resulted in the formation of thick and uniform inner reaction layer on the bed grains. The heterogeneous outer layer was also present but not to the same extent as the inner reaction layer. The total layer thickness was approximately 4−5 μm. During SB combustion, based on thermodynamic calculations, there is a surplus of available potassium that can react with quartz grains. The high amount of calcium in SB may dissolve into the melt and solidify resulting in the thick and uniform layer around quartz grains. The thickness of the layers in case of RC and WS was much thinner or even did not exist. That can be explained by higher content of reactive silicon in the fuel which can bind potassium as K-silicate in fuel ash rather than reacting with the bed material. On the basis of the SEM pictures (Figure 6), the addition of SWGB did not have any significant influence on the appearance of layers, but the chemical composition of the layers changed (Figure 5). The effect of the addition of the SWGB on the composition of the inner (reaction) layer (Figure 5) was mainly observed during combustion of RC, with tripled calcium concentration in the inner reaction layer for the test with the additive compared to the test without. The higher Ca concentration must have been the result of reaction between CaSO4 and biomass asheither directly on a bed grain surface within the formed layer or with the fuel ash. In the case of SB,
there was no significant difference in the composition of the inner (reaction) layer between the test with and without the additive. On the other hand, during WS combustion somewhat higher sulfur concentration was observed in the layer when the SWGB was added. The composition of the outer (coating) layer is shown in Figure 5. In the case of RC, an increased calcium concentration could be observed during the addition of the SWGB. For the tests with WS and SB, the influence of the additive was more pronounced. An increase of potassium and sulfur, and of calcium and sulfur, was observed in the outer layer, for SB and WS, respectively. The formation of an outer (coating) layer is the result of fine fuel ash particles depositing on the bed grain surface. Therefore, it can be assumed that the additive influenced the fuel ash properties by changing the composition of the outer (coating) layer. In Table 6 the composition of the fuel ash particles sieved from the bed material is shown. It can be noticed that during combustion of both, WS and SB, the formation of K2Ca2(SO4)3 (calcium-langbeinite) took place when SWGB was added. The formation of solid ash particles containing calcium-langbeinite could explain the differences observed in the composition of the outer (coating) layer on the bed grains for tests when the additive was used. This is in agreement with thermodynamic calculations (Figure 3). However, the mechanisms leading to the formation of this compound are not clear. One potential explanation can be, as noted in thermodynamic prediction chapter in the case of SB ash, the reaction of volatile KOH with anhydrite originating from SWGB and leading to the formation of calciumlangbeinite. However, this hypothesis would demand a certain vapor pressure of SO3. In the case of RC combustion, no significant formation of calcium-langbeinite was indicated. This leads to the conclusion that RC ash chemistry with the addition of SWGB is not favoring the formation of calcium-langbeinite at 800 °C in a fluidized quartz bed. The interaction of potassium species (e.g., KOH(g), KCl(l,g), K2CO3(g)) with anhydrite as the mechanism to form calcium-langbeinite in fluidized bed combustion of biomass was supported by ESEM/EDS results presented in Figure 7. In this figure SWGB particles found in the bed material sample after combustion (8 h of combustion) with the additive are shown together with the spot analyses. In addition, a particle from a reference test (REF) is shown. During the reference test, silica sand and SWGB were heated up in a combustion atmosphere (i.e., propane burning) to 900 °C in a I
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 6. Scanning electron microscopy (SEM) images of typical cross sections of bed particles with formed layers/coatings analyzed further for their chemical composition with energy dispersive X-ray spectroscopy (EDS).
fluidized bed reactor. In superficial parts of the SWGB particles from the tests with WS and SB approximately 20 at% of potassium was found. For the reaction to take place, however, a certain vapor pressure of SO2/SO3 would be necessary. This Kcapturing effect of SWGB addition could be beneficial from the perspective of ash behavior and operational problems during biomass combustion. The potassium incorporated into the
SWGB particles most probably will not contribute to the reaction with bed material and certainly not to the formation of chlorides. Agglomerates. Agglomerates found in the sample of bed material after agglomeration test were analyzed by means of SEM/EDS. The formed necks are shown in Figure 8, and their composition is shown in Figure 9. The necks (Figure 8) J
DOI: 10.1021/ef5024753 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 6. XRD Analyses of the Bottom Asha fuels
dominant crystalline phase (>50 wt %)
WS
WSG2
WSG3
RC
RCG2
SB SBG1
a
Q
(69%)
major crystalline phase (50−10 wt %) Q MCL PCL K2Ca2(SO4)3 CaSO4 Q PCL K2Ca2(SO4)3 CaSO4 Q PCL Q PCL MCL CaSO4 Q PCL MCL PCL MCL CaSO4 PCL MCL Q
minor phase (5−10 wt %)
(24%) (14%) (50%) (12%) (31%) (16%) (20%) (18%) (47%) (11%) (12%) (22%) (43%) (31%) (40%) (27%) (18%) (13%) (12%) (19%) (10%) (27%) (18%) (40%)
trace crystalline phase (
