Energy & Fuels 1997, 11, 843-848
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Ash Behavior in a CFB Boiler during Combustion of Salix B.-J. Skrifvars,*,† G. Sfiris,‡ R. Backman,† K. Widegren-Dafgård,‡ and M. Hupa† Åbo Akademi University, Åbo/Turku, Finland, and Vattenfall Utveckling AB, Stockholm, Sweden Received November 18, 1996X
A study on the combustion characteristics of Salix viminalis, a fast growing willow, was conducted at a 12 MW circulating fluidized bed boiler. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed agglomeration or slagging and fouling problems that may occur during the combustion of this type of fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling, and bed agglomeration. Samples from all ingoing (bed material, fuel) and outgoing solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg were collected and analyzed chemically. Selected bed samples and ash samples were also analyzed with a scanning electron microscope (SEM/EDAX). Deposit samples were collected at the cyclone inlet and from two different locations in the convective path using specially designed surface temperature-controlled deposit probes. All collected probe deposits were photographed and characterized visually. Selected samples from both windward (front) side and leeward (back) side of the sampling probes were analyzed chemically as well as with SEM/EDAX. In addition to these samples, the boiler operation was monitored carefully. This included collection of operational data (fuel feed, air distribution, and total air), collection and monitoring of pressure drops in the furnace, flue gas temperature profiles, and emissions. Multicomponent multiphase thermodynamic equilibrium calculations were then performed for predictions of the fly ash thermal characteristics, using the fly ash chemical composition as input data. The thermal characteristics, i.e., the melting behavior, were predicted for the different ash samples and compared with the results from the full scale fouling measurements. The paper discusses the impact of the ash chemistry on the bed agglomeration and fouling tendency found during the combustion tests and draws conclusions about their relevance to the operation of the boiler.
1. Introduction Salix viminalis, a fast growing willow, is for some countries an indigenous fuel that may become a valuable energy resource. Cultivation of Salix for energy production purposes also serves as a possibility of increasing the share of nonfood agricultural production. Combustion of Salix is further attractive from an environmental point of view, since it doesn’t increase the net CO2 emissions into the atmosphere. New combustion techniques, such as fluidized bed combustion, have broadened the possibility of firing Salix in a more feasible way than before. However, little is known about the long term use of Salix in a fluidized bed boiler. In particular, the mineral matter behavior is still to some extent unknown. There are indications from the combustion of other types of biomasses that the mineral matter in those may sometimes cause ash-related problems during combustion.1-4 We wanted to collect some initial data on the ash behavior of the fluidized bed combustion of Salix. A †
Åbo Akademi University. Vattenfall Utveckling AB. Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Dawson, M.; Brown, R., C. Fuel 1991, 71, 585. (2) Baxter, L. Biomass Bioenergy 1993, 4 (2), 89. (3) Nordin, A. Fuel 1995, 74, 615. (4) Nordin, A.; Skrifvars, B.-J.; O ¨ hman, M.; Hupa, M. Agglomeration and defluidization in FBC of biomass fuels - Mechanisms and measures for prevention. Presented at the Engineering Foundation Conference on Applications of advanced technology to ash-related problems in boilers, Waterville Valley, NH, July 1995. ‡
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study was conducted in a 12 MW circulating fluidized bed boiler. The purpose of the study was to increase the understanding of the mineral matter behavior in the boiler and to foresee possible bed agglomeration or slagging and fouling problems that may occur during the combustion of this type of fuel. Special focus was given to the impact of ash chemistry on the slagging, fouling, and bed agglomeration. 2. Field Measurements During a test period of 2 weeks Salix was combusted in a 12 MW circulating fluidized bed (CFB) boiler. The boiler is a semifull scale CFB that produces hot water for the local district heating net. The boiler is also a very suitable research facility and has previously been successfully used when addressing, for example, emission questions of CFBC’s.5,6 The CFB boiler is equipped with a number of operational controlling systems, such as advanced air and flue gas recirculation, bed particle cooler, primary air preheater, and fly ash recirculation possibilities. This enables a wide range of operating options to be tested under controlled conditions in a wide load range (312 MW). The unit is also equipped with a large number of analyzing instruments for continuous monitoring and record(5) Lyngfelt, A.; Åmand, L.-E.; Leckner, B. Low N2O, NO and SO2 emissions from circulating fluidised bed boilers, Proceedings of the 13th ASME International FBC Conference, Orlando FL, 1995; ASME, 1995; pp 1049-1057. (6) Åmand, L.-E. Nitrous oxide emission from circulating fluidized bed combustion. Doctoral Thesis, Chalmers University of Technology, Gothenburg, Sweden, 1994.
© 1997 American Chemical Society
844 Energy & Fuels, Vol. 11, No. 4, 1997
Skrifvars et al. The surface temperature in the tests was set at 450 °C in the sampling locations L1 and L2 to simulate a superheater tube, while the probe in the coldest sampling location L3 was held uncooled. All the probes were photographed and characterized with respect to their collected deposits. Selected samples from both the front (windward) side and back (leeward) side of the sampling probes were analyzed quantitatively for the elements Si, Al, Fe, Ca, Mg, P, Na, K, Cl, and S as well as semiquantitatively with SEM/EDAX.
3. Results and Discussion
Figure 1. Schematic view of the CFB boiler in which the Salix combustion tests were performed. Sampling locations are indicated in the figure. ing of the operation. Numerous instrument tappings enable measurements in many locations in the boiler. A schematic view of the boiler is presented in Figure 1. Three different running modes were used during the tests. The three modes were related to the fuel mixes used. In the first mode, Salix was fired alone, in the two other cases, wood pellets made from forest residue were fired along with Salix to reach higher loads. The air and fuel feed were adjusted so that they corresponded to a gas velocity of approximately 5.5-6 m/s in the furnace. Other boiler operational data such as fuel feed, air distribution and total air feed, pressure drops in the furnace, flue gas temperature profiles, and gaseous emissions were further recorded continuously. Selected data are summarized in Table 1. Samples from all ingoing (bed material and fuel) and outgoing solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg were collected twice a day and analyzed quantitatively for the elements Si, Al, Fe, Ca, Mg, P, Na, K, Cl, and S by wet chemical analyses. Selected bed samples and ash samples were also analyzed semiquantitatively with a scanning electron microscope (SEM/ EDAX). Deposit samples were collected at the cyclone inlet (TFG ) 850 °C) and from two different locations in the convective path (TFG ) 680 and 250 °C, respectively) with a sampling time varying from 15 min to 21 h. The sampling locations are indicated in Figure 1 as L1, L2, and L3. The samplings were done with specially designed surface temperature-controlled deposit probes. The probes were also equipped with a removable ring for later SEM/EDAX analyses.
3.1. Fuel Analyses. The fuel analyses are summarized in Table 2. The Salix moisture content was very high, >50% by weight as received. The fuel was fired within a couple of days from harvesting. No extra drying of the fuel was done between harvesting and firing. The ash content in the Salix was approximately 2% by weight. The corresponding value for the wood pellets, made of forest residue, was 0.5%. Sulfur and chlorine content in Salix was low (0.04% and 0.02% by weight, respectively.). In the wood pellets, the chlorine content was low, 0.01% by weight, and the sulfur content moderate, 0.12% by weight. The main ash element in both fuels was calcium. Salix contained roughly 25 wt % calcium in the ash while the corresponding number for wood pellets was 29%. In Salix also fairly high amounts of potassium and phosphorus were found, 13% and 8% by weight, respectively. Potassium was also found in the wood pellets as the second most abundant element, 6.8% by weight, although in clearly lower amounts than in Salix. The amount of phosphorus in the wood pellets was low, only 0.7% by weight. The bed material used in the combustion tests was quartz sand with a particle size of approximately 500 µm. 3.2. Bed, Primary, Secondary Cyclone and Bag House Ashes. Outgoing ash materials were collected twice a day from four different locations in the boiler: (i) from the bed at the bottom of the furnace, (ii) from the return leg of the primary cyclone, (iii) from the secondary cyclone after the heat exchanger surfaces in the convective part of the flue gas channel, and (iv) from the bag house. These materials were then analyzed quantitatively by wet chemical analyses for the elements Si, Al, Fe, Ca, Mg, P, K, Na, S, Cl, and SO4. Figure 2 summarizes the analyses results. In this figure the analyzed elements are recalculated to their corresponding oxides. A number of trends can be extracted from the figure. One trend is the decreasing share of silicon and the increasing share of major ash elements in the samples as we move from the bed material and primary cyclone ash to the secondary cyclone and bag house ashes. The reason for this trend is obvious; the larger sized quartz bed material is separated at the primary cyclone and stays within the internal loop, furnace-primary cyclone,
Table 1. Running Conditions of the Boiler during the Salix Combustion Tests temperatures fuel start
end
bed
cyc inl
cyc outl
flue gases conv hot
conv cold
CO
SO2
O2
Salix wood min max min max min max min max min max min max min max min max kg/s kg/s °C °C °C °C °C °C °C °C °C °C ppm ppm % % % %
A 13.12.1994 9:00 14.12.1994 18:55 1.11 B 15.12.1994 10:50 18.12.1994 8:45 0.81 C 19.12.1994 12:00 21.12.1994 7:35 0.82
0.00 0.14 0.17
770 784 812 820 803 830 595 630 283 325 1400 3800 6.0 831 843 854 858 830 837 605 618 307 332 2200 3900 5.3 840 844 860 866 848 857 636 642 358 363 3600 6100 5.1
6.1 6.2 5.5
0.0 0.0 0.0
0.0 0.0 0.0
Ash Behavior
Energy & Fuels, Vol. 11, No. 4, 1997 845
Figure 2. Quantitative wet chemical analyses of bed, primary cyclone, secondary cyclone, and bag house samples taken during three different running modes A, B, and C of the Salix combustion tests. The elements are expressed as wt % of their corresponding oxides except for Cl. Table 2. Analyses of the Fuels Used during Test Salix Tests fuel analyses
ash analyses
Salix wood moist C H N S Cl
wt % wt % (db) wt % (db) wt % (db) wt % (db) wt % (db)
55.0 49.7 6.2 0.6 0.0 0.0
7.0 50.9 6.3 0.1 0.1 0.0
HHV LHV
MJ/kg (db) MJ/kg (db)
19.8 18.4
20.4 19.0
Figure 3. SEM image of a bed sample collected from the CFB boiler during running condition C of the Salix combustion tests. Magnification is ×100. Numbers in the image indicate EDAX point analyses presented in Table 3.
Salix wood ash Si Al Ca Fe K Mg Mn Na P
wt % (db) wt % (ash) wt % (ash) wt % (ash) wt % (ash) wt % (ash) wt % (ash) wt % (ash) wt % (ash) wt % (ash)
2.1 2.8 0.3 27.3 0.9 14.7 2.3 0.3 0.4 4.3
0.5 2.9 0.7 29.6 0.7 6.8 2.8 2.0 0.6 0.7
while smaller sized particles, such as condensed ash particles and small fragments of the bed, and all gases work their way out to the convective part of the flue gas channel. A second trend is the decrease in the total amount of analyzed elements in the different samples as we move from the bed and primary cyclone samples to the secondary cyclone and bag house ashes. In the bed and primary cyclone samples, the analyzed elements calculated to include their corresponding oxides add up to approximately 100%, which shows that most of the elements in the samples are accounted for. In the secondary cyclone and bag house ashes, however, the sum of oxides remain clearly below 100%, indicating some missing element. Assuming the analyzed calcium to be calcium carbonate and estimating the amount of carbon as CO2, one can correct for the missing share. A third noteworthy trend is in the shares of sulfur and chlorine in the different samples. In the bed and primary cyclone samples the sulfur content is low but increases as we move out to the secondary cyclone and bag house samples. Chlorine is missing completely in the bed, primary cyclone, and secondary cyclone samples but turns up in the bag house samples. It seems obvious that chlorine escapes through the primary cyclone and is enriched in the fly ash fraction collected by the bag house. One can also see shifts in the running conditions in these analyses. For example, the share of silicon in the bed samples taken during condition A is somewhat higher than those of the samples taken during conditions B or C. The trend is even more clear in the primary cyclone samples. Obviously, a higher share of fuel ash elements was present in the bed during conditions B and C, maybe because of the higher temperature in the bed and furnace, which would enhance the reaction between the bed material and ash elements.
Figure 4. Quantitative wet chemical analyses of front and back side deposits collected on the deposit probes from sampling locations 2 and 3 in CFB boiler during the Salix combustion tests. The elements are expressed as wt % of their corresponding oxides except for Cl. The deposit sampling time is indicated on the x-axis.
SEM/E Å DAX analyses were also performed on selected samples. Figure 3 presents an image of a bed sample taken during the running condition C. The image is taken with a ×100 magnification. The numbers indicated in the image are EDAX point analyses performed on that specific location in the sample. These analyses are shown in Table 3. The SEM/EDAX analyses clearly indicate the heterogeneity of the bed. It consisted of original bed particles typically consisting of silicon alone but with occasional particles of potassium, calcium, and aluminum silicates (points 1, 4, and 5 in the SEM image). Most of the bed particles seemed to be coated with elements originating from the fuel ash (points 2 and 6 in the SEM image). Individual fuel ash particles seemed to be missing. 3.3. Deposit Samples. The quantitative wet chemical analyses of the deposit samples collected during the Salix combustion tests are summarized in Figure 4. The deposits collected in the three different locations in the boiler (L1, furnace at cyclone inlet; L2, convective part before the heat exchangers; L3, convective part after the heat exchangers) were generally not very thick. One can, however, distinguish among four different types. (1) One type consists of very thin, shell-type deposits with a thickness below 0.1 mm. This type of deposit was mainly found on the front and back side of the probe in the sampling location L1 in the upper part of the furnace at the primary cyclone inlet at a flue gas
846 Energy & Fuels, Vol. 11, No. 4, 1997
Skrifvars et al.
Table 3. SEM/EDAX Point Analyses of Bed Samples Taken during Condition C of the Salix Testsa point no.
Si wt %
Al wt %
Ca wt %
Fe wt %
K wt %
Mg wt %
Mn wt %
Na wt %
P wt %
S wt %
Cl wt %
1 2 3 4 5 6
32.8 0.8 1.5 14.5 26.6 0.3
7.2 0.0 0.1 0.0 6.3 0.1
0.0 22.3 33.5 40.9 5.4 32.8
0.0 0.7 0.2 0.0 0.0 0.2
13.4 12.0 1.5 3.9 19.7 15.1
0.0 9.9 9.0 0.0 0.0 3.8
0.0 1.1 0.6 0.0 0.0 0.9
0.0 0.6 0.1 0.0 0.0 0.5
0.0 8.5 12.9 1.8 0.0 4.2
0.0 5.4 0.8 1.2 0.0 6.8
0.0 0.0 0.0 0.0 0.0 0.0
a
Analyses are points indicated with corresponding numbers in the image in Figure 3.
temperature of approximately 810-870 °C. The deposits did not grow very much even when the sampling time was increased from 15 min to 21 h. The deposits were, however, very hard to remove from the sampling probes. Hence, no deposit samples could be taken for quantitative wet chemical analyses. The semiquantitative SEM/ EDAX analyses on the probe rings detected the following elements in the front and back side deposits in amounts as shown below: front side amount (ave 3 analyses) wt % Ca K P Mg S Cl Si Al Na
32.8 11.3 7.3 4.7 3.9 0.1 1.3 0.2 0.1
back side amount (ave 3 analyses) wt % Ca K P Mg S Cl Si Al Na
21.5 10.9 4.8 3.6 3.1 1.7 0.6 0.1 0.8
(2) A second type of deposit was a moderately thin, brittle, shell-type deposit with a thickness between 0.5 and 5 mm. This type was found on the front side of the probe in the sampling location L2 in the convective part of the flue gas channel at a flue gas temperature of approximately 600-640 °C. This deposit grew slowly but continuously as the sampling time was increased from 15 min to 21 h. The deposit was fairly easy to remove from the probe. The semiquantitative SEM/ EDAX analyses of the probe rings (average of three overall analyses) detected the following elements as the six main ones in the deposit. For comparison the quantitative wet chemical analyses of the same deposit is shown in the right column: L2, front side amount SEM/EDAX (ave 3 analyses) wt % Ca K P Mg S Cl Si Al Na
17.5 26.7 5.2 4.1 4.4 8.8
