Energy Fuels 2009, 23, 4245–4253 Published on Web 08/10/2009
: DOI:10.1021/ef900146e
Mechanisms Behind the Positive Effects on Bed Agglomeration and Deposit Formation Combusting Forest Residue with Peat Additives in Fluidized Beds Linda Pommer,*,† Marcus O¨hman,‡ Dan Bostro¨m,† Jan Burvall,§ Rainer Backman,† Ingemar Olofsson,† and Anders Nordin† † Energy Technology and Thermal Process Chemistry, Umea˚ University, SE-901 87 Umea˚, Sweden, ‡Division of Energy Engineering, Lulea˚ University of Technology, S-971 87 Lulea˚, Sweden, and §Department of Agricultural Research for Northern Sweden, Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, SE-904 03 Umea˚, Sweden
Received February 19, 2009. Revised Manuscript Received July 9, 2009
A compilation was made of the composition of peat from different areas in Sweden, of which a selected set was characterized and co-combusted with forest residue in controlled fluidized-bed agglomeration tests with extensive particle sampling. The variation in ash-forming elements in the different peat samples was large; thus, eight peat samples were selected from the compilation to represent the variation in peat composition in Sweden. These samples were characterized in terms of botanical composition, analyzed for ash-forming elements, and oxidized using a low-temperature ashing procedure, followed by characterization using scanning electron microscopy/electron-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The selected peat samples had in common the presence of a small fraction of crystalline phases, such as quartz, microcline, albite, and calcium sulfate. The controlled fluidized-bed agglomeration tests that co-combusted forest residue with peat resulted in a significant increase in agglomeration temperatures compared to combusting forest residue alone. Plausible explanations for this were an increase of calcium, iron, or aluminum in the bed particle layers and/or the reaction of potassium with clay minerals, which prevented the formation of low-melting bed particle layers. The effects on particle and deposit formation during co-combustion were reduced amounts of fine particles and an increased number of coarse particles. The mechanisms for the positive effects were a transfer and/or removal of potassium in the gas phase to a less reactive particular form via sorption and/or a reaction with the reactive peat ash (SiO2 and CaO), which in most cases formed larger particles (>1 μm) containing calcium silicon and potassium.
Fuels containing alkali are often associated with the formation of problematic deposits on boiler tubes as a result of the condensation of chlorides and sulphates, thus limiting the function of super heater tubes. In addition, high amounts of chlorine in the deposits increase the degree of high-temperature corrosion. The market for biofuels has increased considerably during the last year, and in some regions, there is a shortage of biomass. Sweden has great potential for increasing the production of, e.g., peat, which has recently been shown to be a valuable fuel when used as an additive on account of its reported positive effects in preventing bed agglomeration3-6 and high-temperature corrosion.7 Since the early 1980s, peat has been used to a large extent in Sweden for distant heating and is frequently employed in combustion plants because of its low price, high availability, and positive effects on combustion
Introduction The importance of biofuels for the energy supply will increase because of the coming conversion of the energy system in Sweden. Today, approximately 20% of the total energy consumption is derived from biofuels. New types of biofuel, e.g., logging residue and products from agriculture (e.g., Salix, straw, and Reed canary grass) have a relatively high alkali content, which is expected to lead to operational problems.1,2 The use of several different biofuels and waste in combined power and heating plants has been reported to be associated with ash-related problems, of which bed agglomeration, deposit formation, and high-temperature corrosion have attracted much attention in recent years. Bed agglomeration occurs when the particles in the fluidized bed have been covered by a layer that impairs heat transfer, reduces fluidization quality, and in serious cases, causes total defluidization. The reason for bed agglomeration is that alkali (K and Na) elements introduced into the boiler with the fuel are retained on the particles in the fluidized bed, forming layers of low-melting alkali silicate phases.
(3) Burvall, J.; O¨hman, M. Systemstudie o¨ver askegenskaper i fo¨rbra¨nningsanla¨ggningar vid samfo¨rbra¨nning av torv och biobra¨nslen. Utredning utfo¨rd a˚t Statens Energimyndighet, 2002. (4) Lundholm, K.; Nordin, A.; O¨hman, M.; Bostro¨m, D. Energy Fuels 2005, 19, 2273–2278. (5) Lundholm, K.; Nordin, A.; O¨hman, M.; Burvall, J.; Na¨slund, B. O. Proceedings of 12th European Conference and Technology Exhibition on Biomass for Energy, Industry, and Climate Protection, Amsterdam, The Netherlands, 2002. (6) Bostro¨m, D.; et al. Minskad ba¨ddagglomerering i fluidba¨ddpannor genom sameldning av torv. Nifes Forskningsprogram 2001-2002, 2003; project 13216-1. (7) Manninen, H.; Peltola, K.; Ruuskanen, J. Waste Manage. Res. 1997, 15, 137–147.
*To whom correspondence should be addressed. E-mail: linda.
[email protected]. (1) Ohman, M.; Nordin, A.; Skrifvars, B. J.; Backman, R.; Hupa, M. Energy Fuels 2000, 14, 169–178. (2) Ohman, M.; Nordin, A.; Lundholm, K.; Bostrom, D.; Hedman, H.; Lundberg, M. Energy Fuels 2003, 17, 1153–1159. r 2009 American Chemical Society
4245
pubs.acs.org/EF
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
problems. The production of peat for energy has been relatively constant over the past 10 years at approximately 3 TWh/year. Several studies have revealed the positive effects of peat addition, and Lundholm et al. reported an increased bed agglomeration temperature when peat was combusted with bark or logging residue.4 The amount of peat added was 530% of the total content of the ash content. Different bed agglomeration prevention mechanisms have been suggested for the various peat fuels used as additives, involving the effect of the elements Al, S, K, and Ca on the bed particle layers.4 In the literature, a few hypothesizes concerning the positive effects of adding peat to the fuel mixture have been discussed, e.g., increased formation of alkali sulphates that prevents the formation of alkali chlorides and hydroxides, altered composition of the remaining ash in the fluidized bed, and the eroding effect of inorganic peat matter that reduces deposit formation. In some ways, peat is similar to lignite in that the inorganic fraction consists of both distinct minerals and ionic-bound inorganic material. To characterize the inorganic matter, it is necessary to understand its distribution, preservation, and alteration/formation within the peat. Two peats with a similar ash composition can have vastly different physical, chemical, and engineering properties because of the differing constituents of the ash.8 The present authors defined peat as containing 25% or less inorganic material on a dry weight basis.8 Numerous investigations have attempted to characterize the inorganic matter in peat. This knowledge is vital for understanding the ash chemistry when peat is used as a fuel additive.4,9 The inorganic content of peat has large variations, which are dependent upon a number of factors including biological variations in the original biota, moss type, degree of decomposition of the organic matter, position in the moss (lateral or vertical), surrounding soil and bedrock, flow of groundwater, proximity to coastal regions, urbanization, industry, and meteorological conditions.10 An increased understanding of the mechanisms behind the positive effects of using peat as an additive will facilitate the selection of peat sources (i.e., peat with a suitable composition). The objective of the present work was to elucidate the effect of adding peat to forest residue during combustion on bed agglomeration and deposit formation. Data were collected by studying the composition of the layers formed around bed particles, fly ash, aerosols, and deposits, as well as that of the flue gas.
Table 1. Variation of Four Inorganic Elements, Peat Type, and Humification of the Selected Peat Samples peat sample
Al
Si
S
Ca
peat typea
humificationb
1. Brunnsko¨len 2. Ba˚dhus 3. Flobomyren 4. Forellmossen 5. Krypko¨len 6. Norrheden 7. Stentja¨rn A 8. Stentja¨rn D
þ þ þ þ -
þ þ þ þ
0 þ þ þ þ
þ þ þ þ
CS SC-t SC S CS SC-t SC SC
H6 H6 H6 H7-H8 H6 H4-H5 H5 H4-H5
a S represents peat dominated by the plant Sphagnum, and C represents peat dominated by the plant Carex. The last mentioned plant was the dominating species in the peat sample. b H1-H3 represent a low degree of humification; H4-H6 represent a medium degree of humification; and H7-H9 represent a high degree of humification.
The following ash-forming elements Si, Ca, Al, Fe, K, Na, S, Cl, Mn, and Mg were analyzed in the peat samples using inductively coupled plasma-atomic emission spectrometry (ICP-AES) in accordance with the Swedish standard SS-18 71 71, with the exception of chlorine, which was analyzed by means of X-ray fluorescence (XRF). The peat samples were also characterized in terms of botanical composition using a microscope and visual judgment. The most common peat-forming plants in Sweden are the Sphagnum and Carex species.11 The degree of humification was determined by squeezing the peat by hand.12 Peat with a low degree of humification (H1-H3) only released colored water when squeezed. Peat with a medium degree of humification (H4H6) released troubled water and some mud and mushy mass, while peat with a high degree of humification (H7-H9) released most of the sample through the fingers. To oxidize the organic fraction of the peat and leave the inorganic minerals for the most part unaffected, the samples were ashed in accordance with a low-temperature ashing (LTA) procedure presented by Glusgoter in 1965.13 A total of 1 g of each sample was ashed using a RF Plasma Barrel Etcher from Quorum Technologies in an atmosphere containing oxygen (180 mL/min) and helium (380-400 mL/min). The samples were ashed for 22-36 h at 90 min intervals. The temperature at 2 mm above the sample was below 120 °C during ashing. The forest residue was represented by branches and tree tops from a young population of Norwegian spruce, which was collected directly after cutting to minimize the loss of needles during handling and storage. The samples of forest residue and peat were dried to a moisture content of approximately 20%, ground to a particle size of less than 3 mm, and mixed in proportions of 20% peat and 80% forest residue on a dry basis, which is around double the amount that Lundholm et al.4 demonstrated as having a positive effect on the bed agglomeration temperature. The forest residue and the different mixtures were then pelletized (pellet diameter of 8 mm) in a SPC 300 pellet press with a capacity of 200-300 kg/h. Controlled Fluidized-Bed Combustion (CFBA) Experiments. The fuels were combusted in a 5 kW bench-scale fluidized-bed reactor (Figure 1) built of stainless steel (253MA) with a height of 2.4 m and an inner diameter of 100 mm in the fluidized-bed section and 220 mm in the free-board section. The stainless-steel distribution plate at the bottom of the fluidizing bed had 90 holes (1% open area). The gas flow through the distribution plate was set to 10 times the fluidizing velocity, 10Umf = 80 N L min-1 (0.6 m/s) of primary air. A total of 540 g of quartz sand (>98% SiO2) of the size fraction 200-250 μm was used as bed
Experimental Section Selection and Characterization of Fuels. A compilation of the composition of 83 samples of peat from different areas of Sweden was made to obtain an overview of the variation in ash content and the main ash-forming elements (Si, Ca, Al, Fe, K, Na, S, Cl, Mn, and Mg). In the present study, eight samples were selected from this compilation to represent the variation in the composition of Swedish peat. The selection was based on the concentrations of the four main inorganic elements (Al, Si, S, and Ca) (Tables 1 and 2). (8) Andrejko, M. J.; Fiene, F.; Cohen, A D. ASTM Spec. Tech. Publ. 1983, 820, 5–20. (9) Spedding, P. J. Fuel 1988, 67, 883–899. (10) Wust, R. A. J.; Ward, C. R.; Bustin, R. M.; Hawke, M. I. Int. J. Coal Geol. 2002, 49, 215–249.
(11) Heikurainen, L. Skogsdikning, 1973. (12) Von Post, E. L.; Granlund, G.; von Assarsson, L.; Von Post, E. L.; Granlund, G.; von Assarsson, L. Soedra Sveriges Torvtillga˚ngar, 1992; 91-7192-856-1. (13) Gluskoter, H. J. Fuel 1965, 44, 285–289.
4246
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
Table 2. Fuel Characteristics: Main Ash-Forming Elements ash (% of DS) Si Al Ca Fe K Mg Na P S Cl
forest residue
peat 1
peat 2
peat 3
peat 4
peat 5
peat 6
peat 7
peat 8
3.7 0.68 0.13 0.55 0.09 0.28 0.07 0.04 0.05 0.04 0.04
10.4 3.25 0.34 0.46 0.76 0.08 0.07 0.03 0.07 0.22 0.05
7.9 0.96 0.32 0.92 1.76 0.06 0.08 0.03 0.05 0.28 0.02
4.0 0.52 0.10 0.76 0.23 0.02 0.09 0.01 0.03 0.15 0.03
1.6 0.36 0.09 0.11 0.07 0.03 0.08 0.02 0.03 0.17 0.04
7.5 2.20 0.47 0.40 0.39 0.13 0.04 0.02 0.08 0.19 0.03
4.0 0.45 0.26 0.39 0.69 0.01 0.05 0.01 0.04 0.26
6.9 0.35 0.09 1.05 2.04 0.03 0.07 0.01 0.05 0.35 0.02
6.3 1.41 0.19 0.74 0.50 0.05 0.07 0.02 0.03 0.51 0.03
Figure 1. Fluidized bed reactor: A, fludized-bed section; B, fuel feeding unit; C, free-board section; D, deposit probe; E, cyclone; F, impactor sampling; G, FTIR sampling; H, wet scrubber.
Sampling. To ensure representative sampling of particles, deposits, and flue gas, the fluidized bed was run for at least 2 h before sampling commenced. Bed Particle Sampling. After 9 h of combustion of the different fuels, a bed particle sample of approximately 20 g was collected from the fluidized bed (point A in Figure 1) prior to the CFBA. Another bed particle sample (bed agglomerate sample) was collected after the CFBA tests. Flue Gas Sampling. The gas sampling was performed after the cyclone in the flue gas channel (point G in Figure 1). The flue gas temperature was about 185 °C. The O2 level in the flue gas was measured using a λ probe. Online analysis of CO, CO2, NO, NO2, SO2, HCl, CH4, NH3, and H2O was performed by means of Fourier transform infrared spectroscopy (FTIR) during approximately 6 h of combustion. Deposit Sampling. Deposits were sampled in the upper part of the free-board section (point D in Figure 1). The air-cooled sampling probe was equipped with a detachable stainless-steel sample ring (SS 2343). The probe was inserted 200 mm from the top of the reactor in the combustion atmosphere after 2 h of
material. After the free-board section, the flue gas was led through a cyclone separator that removed particles with a cut size diameter of >10 μm. The flue gas was then cooled and passed through a wet scrubber before entering the chimney pot. The fuel mixtures were combusted at a bed temperature of 800 °C. The temperature was achieved by a combination of preheated air and the heat from the combustion. Compressed air was used to cool the bed section from the outside at occasional temperature peaks. The flue gas oxygen level was kept at 8-10% during the combustion experiments (9 h), which corresponded to an average of 5.2 kg of forest residue and peat mixtures combusted. The temperature in the free-board section was 800 ( 10 °C, which was controlled by five electric heaters. The combustion experiments were completed by running a CFBA test.14 At the initial stage of bed agglomeration, the fluidizing conditions changed, the pressures over the bed dropped and the bed temperature increased rapidly (Figure 2). (14) O¨hman, M.; Nordin, A. Energy Fuels 1998, 12, 90–94.
4247
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al. Table 3. Input Data and Used Melt Models in the Thermo-Chemical Model Calculations Using FactSage 5.2 element
melt models
C, H, O, N, S, Cl, P, K, Na, Ca, Mg, Fe, Mn, Al salt (Ca, Mg)
salt-liqA (alkali chlorides/ hydroxides/carbonates) liq-K, Ca/CO3, SO4 (-LCSO) s-K, Ca/CO3, SO4 (-SCSO) s-Ca(SO4), Mg(SO4) (-SCMO) liq-Ca, Mg, Na/(SO4) (-LSUL) s-Ca, Mg, Na/(SO4) (-SSUL)
The identified minerals were semi-quantified by classification into four groups with reference to abundance: dominant, major, minor, and trace. SEM/EDS. All particle samples (peat samples, cyclone ash, deposits, and particles) and bed samples were analyzed for morphology and elemental composition using an environmental scanning electron microscope equipped with an energy dispersive spectroscopy detector (Philips XL-30). The particle samples (deposits, cyclone ash, and particles sampled with the impactor) were fixed on double adhesive graphite tape. The bed samples were mounted in epoxy and polished with SiC-sand paper (dry), after which the cross-sections were analyzed. Four bed particles per sample were analyzed, and several point analyzes were performed on each bed particle. An accelerating voltage of 20 kV was used during the analysis. Bed particle samples were analyzed with SEM/EDS to determine both the chemical compositions and the thicknesses of the inner and outer layers formed around the bed particles. This was performed to identify differences in the growth and chemical composition of the layers between the experiments with different fuels mixtures. Deposit samples from both the lee and wind side of the cooled probe were analyzed. Area and spot analysis were made on all samples. Particles collected on the impactor in the sub-micrometer (0.18-0.31 μm) and coarse (3.24-5.44 μm) fractions were selected for chemical analysis and represented the largest particle fractions collected. Prediction of Melt Characteristics of the Fine Fraction of the Particles. Access to thermodynamic data allowed for the possible reaction pathways to be evaluated using the given combustion conditions and components. The model calculations were performed with FactSage 5.2,16 a program based on minimization of Gibb’s free energy for the system studied. In the calculations, data for approximately 500 gas components, as well as for stoichiometric condensed phases, non-ideal solutions (salts and oxide/slag), and non-ideal solid solutions, were used (Table 3). The conditions set in the model calculations represented the conditions (fuel mixtures, temperatures, and oxygen level) in the bench-scale experiments.
Figure 2. Illustration of typical fluctuations of the bed temperature and difference pressures during experiments when forest residue was co-combusted with peat 3.
combustion, and deposits were collected for 6 h in the free-board section (800 °C). The surface temperature of the cooled probe ring was set to 440 °C to simulate the deposit formations on super heater tubes. The temperature was controlled by adjusting the flow of pressurized air inside the probe. The residence time of the flue gas from the bed section to the deposit sampling point was 12 s with the above settings. The cyclone ash and particles were collected within 1 s after particle sampling. The deposits on the cooled probe were separated into two fractions: a fine fraction on the lee side of the probe and a coarse fraction on the wind side of the probe. The fractions were not completely separated, because there was a degree of overlap. Cyclone Ash. The cyclone was constructed to have a flue gas cut size larger than 10 μm. These particles were collected at the bottom of the cyclone (point E in Figure 1) (680-720 °C). Particle Sampling. Particles with a diameter of 10 μm or less were sampled from the beginning of the flue gas channel (point F in Figure 1). The sampling of particles was isokinetic to ensure a representative sample. A 13-stage Berner-type low-pressure impactor (Dekati Ltd.) sampled particles of 10-0.3 μm. The precyclone used prior to the impactor had a cut size of approximately 11 μm depending upon the conditions of each experiment. In the impactor, aluminum sample foils were used to collect the particles. The impactor and the precyclone were preheated to 140 °C. The particle samples were extracted at a flue gas temperature of 280-290 °C. The air flow through the impactor during the 3-8 min sampling period was 10 N L min-1. A total of 30-80 N L flue gas was sampled in the different experiments. To obtain a representative average of the flue gas during the experiments, the sampling was carried out in short intervals of 30-60 s distributed over at least 30 min. After sampling, the aluminum foils were dried in a desiccator for at least 24 h, weighed, and analyzed by means of SEM/EDS and XRD. Analysis Methods. XRD. The particle samples were analyzed to identify crystalline phases using powder XRD. Ashed peat samples and particles from the cooled probe ring (deposits), cyclone ash, and the impactor (stages 4 or 5 and 10 or 11) were mounted on a Si low-background sample holder and analyzed. The XRD investigations were performed using a Bruker d8Advance instrument in θ-θ mode, with an optical configuration that involved primary and secondary Go¨bel mirrors. Continuous scans at a rate of 1°/min were applied. Analyses of the diffraction patterns were performed using a PDF-2 databank.15
Results and Discussion A broad variation in ash-forming elements and composition of the peat from different areas of Sweden was observed (Figure 3). According to Figure 3, the average alkali content of forest residue may be captured in the combustion process by active elements in peat, such as sulfur, or be incorporated in compounds containing silicon and calcium. Selection and Characterization of Peat Samples. The composition of forest residue and the selected peat samples are (16) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melancon, J.; Pelton, A. D.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189–228.
(15) International Center for Diffraction Data (ICDD). Newtown Square, PA, 2004.
4248
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
Figure 3. Variation in composition of the main ash components for 83 different peat samples.
becomes covered with ash, which in some conditions can limit further ashing. This implied that particle size can influence the ashing rate and highlighted the importance of intermediate stirring and grinding of the samples during the ashing procedure.17 The results of the present study indicate that higher humificated peat was oxidized to a greater degree than lower humificated peat. Peats 4 and 5 had a weight loss of 96 and 80%, respectively, with a degree of humification of 7-8 and 6 (Table 1). XRD analysis of the ash fraction after low-temperature ashing demonstrated that only a small fraction of crystalline phases was present in the peat samples (Table 4). Quartz, microcline, and albite were the minerals detected in the main part of the samples together with CaSO4 3 0.67H2O. In 12 other peat samples,6 minerals were found to a corresponding degree (Table 5). Notable was the fact that clay minerals were only detected in one of the eight samples. Clay minerals in peat are frequently suggested to be behind the positive effects when co-combusting peat with biomass. Because the level of oxygen during the low-temperature ashing influenced the amount of CaSO4 3 0.67H2O, this phase was probably formed during the ashing procedure because of, e.g., oxidation of pyrite, and is probably not present in the natural peat in any significant amounts. The XRD analysis indicated that the peat ash contained high amounts of amorphous material and only small amounts of crystalline phases. In leaching studies, more than 70% of calcium was leachable in water and ammonium acetate,18,19 implying that calcium was mainly present in a reactive form. Neither silicon nor aluminum was leached to any great extent using water and ammonium acetate. The results of all of the studied peat samples with the sole exception of peat 1 indicated that the majority of silicon was bound as quartz and that only a minor part is reactive. A large amount of potassium and sodium in the studied peat samples was bound to feldspar. Aluminum was partly bound to feldspar, and the remaining parts could potentially be complex bound to humus, which can be more reactive than feldspar.
Figure 4. Ash compositions of forest residue and eight selected peat samples.
presented in Figure 4. From these data, it can be seen that the potassium levels in forest residue were significantly higher than in peat. A tendency toward higher chlorine levels in forest residue was also observed. The levels of sulfur and iron were lower in forest residue than in the peat samples. The variation in composition between the peat samples was relatively large, which was the reason for the screening and selection process. The peat samples were collected from different areas of Sweden and classified as either Sphagnum peat or Carex peat (see Table 1). The degree of humification varied between H4 and H8. Low-Temperature Ashing. In a previous unpublished study, low-temperature ashing of 12 peat samples revealed that 68-97% of the sample was volatilized when ashed for 180-1080 min. The efficiency (weight loss) of the lowtemperature ashing method was dependent upon both the composition of the peat and the grain size. The weight loss of the peat samples with high levels of iron was generally higher than that of the other peat samples. A study by Shirazi and Lindqvist demonstrated that, in low-temperature ashing, the particles are oxidized from the surface inward.17 The surface
(18) Steenari, B. M.; Schelander, S.; Lindqvist, O. Fuel 1999, 78, 249– 258. (19) Zevenhoven-Onderwater, M.; Blomquist, J. P.; Skrifvars, B. J.; Backman, R.; Hupa, M. Fuel 2000, 79, 1353–1361.
(17) Shirazi, A. R.; Lindqvist, O. Fuel 1993, 72, 125–131.
4249
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
Table 4. Identified Crystalline Compounds in Ash after Low-Temperature Ashing Using XRDa name
formula
peat 1
peat 2
peat 3
peat 4
peat 5
peat 6
peat 7
peat 8
quartz/Chris
SiO2 CaSO4 3 0.67H2O KAlSi3O8 NaAlSi3O8 Al2Si2O5(OH)4
**** *** ** **
**** *** ** **
**** *** ** ***
**** *** * **
**** * ** *
****
**** ** * *
**** *** * *
microcline albite hallyosite a
** **
////, dominant; ///, major; //, minor; /, trace.
Table 5. Identified Crystalline Compounds in Ash after Low-Temperature Ashing Using XRD from an Earlier Study Using the Same Method6a name
formula
peat A
peat B
peat C
peat D
peat E
peat F
peat G
peat H
peat I
peat J
peat K
quartz/Chris
SiO2 CaSO4 3 0.67H2O KAlSi3O8 NaAlSi3O8 Al2Si2O5(OH)4
**** **
**** * * *
****
****
**** **
*** **
****
* *
**** *** ** ***
**** ** ** ***
****
* **
**** ** ** ***
microcline albite hallyosite a
**
*
***
* **
////, dominant; ///, major; //, minor; /, trace.
Figure 6. Average element composition and standard deviation of the outer layer formed around bed particles during combustion of fuel mixtures containing forest residue (FR) and peat. Presented on a silica- and oxygen-free basis.
Figure 5. Bed agglomeration temperatures determined from CFBA tests co-combusting forest residue (FR) with peat.
Bed Agglomeration. The agglomeration temperature when combusting forest residue was 970 ( 4 °C (n = 3) and, hence, indicated agglomeration tendencies at temperatures in the range of fluidized-bed operation. The agglomeration temperatures for forest residue/peat mixtures all showed a significant increase (Figure 5). No agglomeration occurred below 1080 °C when mixing peat 2, 7, and 8 with forest residue. The agglomeration temperature was dependent upon the level of calcium and silicon in the peat. High calcium and low silicon levels increased the agglomeration temperature in most cases. The ash retained in the bed had formed layers around the bed particles, and the changed composition of the ash upon combustion with peat resulted in a higher initial melting temperature of the bed particle layers.5,6 Because the composition of the different peat samples varied, there may be several reasons for the increased melting temperature of the layers that formed around the bed particles. Plausible explanations for the increased bed agglomeration temperature included (1) a reaction of potassium with sulfur (in sulfur-rich peat) to potassium sulphates, (2) an increased calcium, iron, or aluminum content because of the admixture of peat, causing a higher melting temperature in the bed particle layers, and (3) a reaction of potassium with clay minerals (e.g., kaolinite) to potassium alumino-silicates, thus preventing potassium from reacting with and lowering the melting temperatures of the bed particle layers.
One or two layers were formed around the bed particles of all fuel mixtures combusted in the fluidized bed. The thickest bed particle layers consisting of an inner homogeneous layer (5-15 μm) and an outer more heterogeneous layer (5-10 μm) were obtained upon combustion of unmixed forest residue. These layers were found on 90-95% of the bed particles. A difference when combusting fuel mixtures containing peat was that the inner layer was less frequently found. The presence of inner layers also varied depending upon the type of admixed peat. Moreover, 80% of the particles were only partly covered by layers. The layers formed were typically thicker in the concave parts of the particle compared to the convex parts. In all of the experiments, the spot analysis (SEM/EDS) on the outer layer demonstrated that its composition was similar to that of the fuel ash but with a slightly lower potassium level (Figure 6). The results are on an oxygenand silicon-free basis, because oxygen and silicon from the quartz grains interact with the analysis of the layers around the bed particles. When unmixed forest residue was combusted, the composition of the inner bed particle layer was dominated by potassium, silicon, and oxygen. In this case, the level of potassium was higher in the inner than in the outer layers. The addition of peat to the fuel mixture decreased the level of potassium in the inner layer and increased the calcium level 4250
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
Figure 7. Average element composition and standard deviation of the inner layer formed around bed particles during combustion of fuel mixtures containing forest residue (FR) and peat. Presented on a silica- and oxygen-free basis.
(Figure 7). SEM/EDS analysis of agglomerates formed during combustion of forest residue and mixtures of forest residue and peat showed that the composition of the “necks” gluing the bed particles together was similar to the composition of the inner layers. This further strengthens the hypothesis that the composition of the inner layer has an important role in bed agglomeration, which is in accordance with the hypothesis by Nuutinen et al., where the innermost layer was also found to be similar to the adhesive material in agglomerates.20 Moreover, Nuutinen et al. observed a difference in composition between the inner and outer layers concerning potassium and calcium combusting, e.g., bark, wood, and peat.20 Combusting peat alone resulted in a single bed particle layer mainly consisting of iron, silicon, and calcium.20 The tendency toward an increased iron, silicon, and calcium content and a lower amount of the inner layer when peat was added to forest residue was also observed in the present study. A quantitative comparison of the layer formation between the different fuel mixtures combusted was not possible, because the fraction of covered bed particles, the amount of covered surface of the bed particles, and the thickness of the layers varied within and between bed particles. Therefore, the bed material was analyzed for total elemental composition (ICP-AES). Tendencies toward higher calcium levels and lower potassium levels were found when peat was used in the fuel mixture compared to forest residue combusted alone. No distinction in composition was found between the fuel mixtures to which peat was added. The addition of peat to the fuel mixture increased the amount of calcium and reduced the level of potassium from the fuel captured in the fluidized bed. The assumption that calcium is a factor with a positive influence on bed agglomeration was also supported by a regression model fitted to the agglomeration temperature containing the main ash-forming elements of the peat samples. Aluminum was also found to be a variable with a positive influence on the agglomeration temperature, although this has not yet been verified in experiments. The increased concentration of calcium and reduction in the concentration of potassium detected in the inner layer when peat was added to the fuel mixture resulted in a
significant increase in the melting temperature. This is of great interest, because it has been demonstrated that the melting behavior of the bed particle layers strongly influences the agglomeration tendency.1,21 The most likely explanation for the reduced potassium level in the inner layer is (1) transference of potassium in the gas phase to a less reactive particular form via sorption and/or reaction with peat ash (containing calcium and silicon) and/or (2) formation of solid sulfur-rich compounds when peat with high sulfur content was used. Because almost no clay minerals were detected in the peat, the removal of potassium in the gas phase via sorption and reaction with clay minerals to less problematic potassium alumino-silicates were considered to be of less importance. Aluminum, originally bound to humic substances in the peat, may possibly react with potassium to form potassium aluminates. However, this needs to be further investigated. Particle and Deposit Characterization. The results of the particle sampling demonstrated that the particle distribution was bimodal (0.030-10 μm) (Figure 8). Upon combustion of forest residue alone, the main particle mass mainly consisted of fine particles (1 μm) increased markedly and the fraction of fine particles (10 μm) were rich in calcium, silicon, and potassium and also contained 5-10 mol % of magnesium, aluminum, sulfur, and iron. The addition of peat to the fuel mixture increased the levels of silicon, iron, and sulfur and decreased the levels of chlorine, potassium, and calcium in the ash, which was in accordance with a study concerning fly ash characteristics during combustion of wood with peat additives.23 In XRD analysis of the cyclone ash, the following crystalline phases were identified: quartz (dominant), potassium chloride, calcium sulfate, calcium carbonate/-oxide, feldspars (microcline and albite), maghemite (Fe2O3), merwinite (Ca3Mg(SiO4)2), biotite (KMg3(Si3Al)O10(OH)2), and apatite. When peat was added, the amount of calcium sulfate in the particles generally increased and potassium chloride decreased, all according to XRD measurements. This pattern of crystalline phases was to a large extent also found by Steenari et al.23 The fact that there was no difference in the composition of crystalline phases in combusted forest residue alone and when peat was added may be explained with the fact that the addition of peat did not change the particle formation
Figure 9. Composition and standard deviation of the fine particle fraction (0.2-0.3 μm) for the different fuel mixtures. FR = forest residue.
Figure 10. Emitted amounts (mol N-1 m-3) of measured elements in the particle fraction of 0.2-0.3 μm.
fine particle fraction (Figure 10). Adding sulfur-rich peat (such as peats 7 and 8) resulted in the enrichment of sulfur in the particles. The crystalline fraction in the fine particles did not exhibit any significant differences when peat was added to the fuel. Crystalline species found in the fine particle fraction were KCl and K2SO4/K3Na(SO4)2. Thermo-chemical calculations were performed to evaluate the melt behavior of the fine particle fraction. The amount of melt formed when the particles were heated decreased when peat was added to the fuel (Figure 11). This, together with the observed reduction in fine particles, suggested the formation of a decreased amount of melt. As a consequence, the
(23) Steenari, B. M.; Lindqvist, O. Fuel 1999, 78, 479–488.
4252
Energy Fuels 2009, 23, 4245–4253
: DOI:10.1021/ef900146e
Pommer et al.
Figure 12. Composition of the coarse particle fraction (3-5 μm) for the different fuel mixtures. FR = forest residue.
The deposits formed on the wind side of the cooled probe had a similar composition to the coarse fraction from the impactor sampling. Conclusion Peat from different areas had a large variation in ash composition and contained a low amount of crystalline phases and a high level of amorphous phases. Clay minerals were only detected in one of the eight peat samples. Combustion of forest residue and peat fuel mixtures in controlled experiments in a bench-scale fluidized bed including extensive sampling and analysis revealed the following effects of adding peat to the fuel mixture: (1) an increased bed agglomeration temperature, (2) a decreased concentration of potassium and an increased concentration of calcium in the inner bed particle layer and tendencies of an increased level of calcium of the bed material, (3) a reduced amount of fine particles, and (4) an increased amount of coarse particles. The mechanisms for the positive effects were the transfer and/or removal of potassium in the gas phase to a less reactive particular form via sorption and/or the reaction with the reactive peat ash (SiO2 and CaO), which in most cases formed larger particles (>1 μm) containing calcium silicon and potassium.
Figure 13. Emitted amounts (mol N-1 m-3) of measured elements in the particle fraction of 3-5 μm. FR = forest residue.
chemistry but instead changed the amount of released/ volatilized compounds during the combustion process in the fluidized bed, as demonstrated. Deposit Characteristics. The composition of the particles on the lee side was similar to that of the fine particle fraction sampled using the impactor, because both had the same elevated levels of potassium, chlorine, and calcium. Adding peat to the fuel decreased the levels of potassium and chlorine and increased those of silicon, sulfur, calcium, and iron. The thermo-chemical calculations supported these results, because they illustrated that a decreased amount of melt should be present. A reduction in the amount of potassium and chlorine in the deposits suggested a decreased deposit formation and less chlorine-induced corrosion. The dominant crystalline phases identified by means of XRD were potassium chloride and calcium sulfate. Adding peat to the fuel did not affect the composition of the crystalline phases in either the fine or coarse particle fraction.
Acknowledgment. The Swedish Energy Agency is acknowledged for financial support. The authors thank the peat suppliers for providing peat samples and Mikael Thyrel and Magnus Rudolfsson at the Swedish University of Agricultural Sciences, Biomass Technology and Chemistry in Umea˚, for their help with drying, mixing, and pelletizing the various fuels. The authors also express their thanks to Andreij Schukarev for the XPS analysis.
4253
