Energy & Fuels 1999, 13, 359-363
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Articles Predicting Bed Agglomeration Tendencies for Biomass Fuels Fired in FBC Boilers: A Comparison of Three Different Prediction Methods B.-J. Skrifvars,*,† M. O ¨ hman,‡ A. Nordin,† and M. Hupa‡ A° bo Akademi University, FIN-20500-A° bo, Turku, Finland and Energy Technology Center, University of Umea˚ , S-941 28, Pitea˚ , Sweden Received March 9, 1998
In this paper a comparison between three different types of techniques to predict the bed agglomeration tendency of a FBC (fluidized-bed combustor) was performed. The three techniques were the standard ASTM ash fusion test, a compression strength based sintering test and a lab-scale combustion test. The tests were performed on 10 different types of biomasses. The results showed significant differences in the predicted bed agglomeration temperatures depending on which technique was used. The ASTM standard ash fusion test generally showed 50-500 °C higher temperatures than the sintering tests or the lab-scale FBC combustion tests. The sintering test showed, in five cases, 20-40 °C lower sintering temperatures than what was detected as the bed agglomeration temperature with the lab-scale FBC. In two cases, a significantly lower sintering temperature than the bed agglomeration temperature was detected, and in three cases, a significantly higher sintering temperature was detected than the bed agglomeration temperature. The detailed results and their relevance is discussed in the paper.
1. Introduction Bed agglomeration problems during combustion of biomass in fluidized-bed boilers is becoming a wellidentified problem. Today there are a number of reported cases where bed agglomeration has disturbed the combustion process to such an extent that the boiler has been forced to an unscheduled shut down.1-5 Due to the costs related to the unscheduled shut downs, it would be very beneficial to be able to predict possible bed agglomeration risks for a fuel before it is fired in a fluidized-bed boiler. There are a number of ash behavior prediction techniques available. Most of these have, however, been developed for addressing slagging and fouling during pulverized coal firing. In this paper we present the results from a study where we compared three different ways of predicting bed agglomeration. The three prediction methods were A° bo Akademi University. University of Umea˚. (1) Viktoren, A. Report No. 416, Thermal Engineering Research Foundation, 1991. (2) Rudling, L. Report No. 415, Thermal Engineering Research Foundation, 1991. (3) Ergudenler, A.; Gahly, E. A. Biomass Bioenergy 1993, 4, 135. (4) Salour, D.; Jenkins, B. M.; Vafei, M.; Kayhaian, M. Biomass Bioenergy 1993, 4, 117. (5) Nordin, A.; O ¨ hman, M.; Skrifvars, B.-J.; Hupa, M. In Applications of advanced technology to ash-related problems in boilers. Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996; pp 353-366. † ‡
(i) the ASTM fusion test, (ii) a compression strength based sintering test, and (iii) a lab-scale fluidized-bed combustion test. We wanted to compare how well the ASTM ash fusion test and the sintering test could predict the bed agglomeration temperature achieved in the lab-scale furnace and to see what useful data the different prediction techniques could provide. 2. Experimental Section Ten different types of biomasses were chosen for the tests. The biomasses were chosen from a selection of 500 samples so that they would represent a wide range of ash elements typically present in different biomasses. Table 1 shows the fuel and ash analyses of the chosen biomasses. The wheat straw was a Danish straw, very rich in chlorine. The olive flesh was the residue from the olive oil processing stage. The bagasse was a sugar cane type, and the cane trash was mainly the leaves from the sugar cane. The forest residue came from a typical Scandinavian soft wood forest and consisted mainly of those parts of the tree which were not used by the pulping industry, i.e., mainly branches, tops, and needles. The bark came from the barking process of a pulp mill using mainly Scandinavian soft wood as the raw material. The RDF (refusederived fuel) came from source-separated community waste. The chosen biomasses were then subject to the three different tests: (i) the ASTM ash fusion test, (ii) a sintering test based on compression strength measurements of ash pellets, and (iii) a combustion test in a laboratory-scale fluidized-bed furnace in which bed agglomeration was achieved in a controlled manner. 2.1. ASTM Ash Fusion Test. The ash of the tested biomasses, achieved through a laboratory ashing procedure
10.1021/ef980045+ CCC: $18.00 © 1999 American Chemical Society Published on Web 02/09/1999
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Skrifvars et al. Table 1. Fuel Characteristics
wheat straw (Danish)
olive flesh
dry substance ash (wt % db) HHV (MJ/kg db) C (wt % db) H (wt % db) N (wt % db) O (wt % db)
90.3 5.9 15.6 44.8 5.7 0.8 42.6
85.6 9.9 16.0 50.2 6.3 1.4 32.1
SiO2 (wt % ash) Al2O3 (wt % ash) Fe2O3 (wt % ash) CaO (wt % ash) MgO (wt % ash) P2O5 (wt % ash) Na2O (wt % ash) K2O (wt % ash) SO3 (wt % ash) S (wt % ash) Cl (wt % ash) CO2 (wt % ash)
30.6 0.5 0.4 7.9 2.4 4.7 0.7 25.3 4.0 2.9 3.7 5.1
36.2 3.6 4.2 18.2 12.4 4.0 1.7 18.2 3.5 1.4 1.5 n.a.
peat
bagasse sugar cane
cane trash
forest residue (soft wood)
reed canary grass
Lucerne grass
bark (Scandinavian soft wood)
RDF community waste
46.8 5.4 n.a. 53.6 6.2 3.2 31.2
93.6 15.2 n.a. 42.4 5.2 0.3 36.8
93.5 5.5 n.a. 47.0 5.9 0.7 40.8
53.3 3.2 n.a. 51.0 6.2 0.4 39.2
90.5 5.7 15.8 46.5 5.7 0.4 41.6
92.1 8.5 16.1 46.7 5.9 3.1 35.6
90.6 3.0 n.a. 51.6 6.0 1600 1540 1220
1190 1200 1200 1480 1480 1220 >1600 >1600 1540 1240
temperature range of 500-1100 °C, indicating that no sintering would have taken place in the samples.
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Skrifvars et al.
Figure 2. Controlled bed agglomeration test for wheat straw. Table 3. Controlled Bed Agglomeration Temperature Determined by the Lab-Scale FBC Rig Taggl (°C) wheat straw olive flesh peat bagasse cane trash forest residue reed canary grass Lucerne bark RDF
Figure 3. Measured Tsint at onset of strength increase detected by the compression strength test versus Taggl determined by the FBC lab rig.
740 930 >1020 >1020 996 982 920-970 670 988 990
In Figure 2 the results from the controlled bed agglomeration test of wheat straw is shown. In the figure the measured bed temperatures are plotted on the left-hand side and the measured bed pressures on the right-hand side as a function of the running time. The bed agglomeration can clearly be detected from the bed pressure plot as a drop in the pressure. The corresponding temperature, the bed agglomeration temperature, is indicated in the figure. A similar plot was received from every run. The corresponding bed agglomeration temperatures are summarized in Table 3. 4. Discussion A comparison between the three different characteristic temperatures achieved with the three test methods is presented in Table 4. The ASTM standard ash fusion test in all cases clearly shows the highest temperature values, indicating that none of the ashes would be problematic at typical FBC bed temperatures of 800-900 °C. Comparing the TID with the Taggl from the controlled bed agglomeration test in the lab-scale FBC furnace, it seems that the ASTM method fails to predict the achieved bed agglomeration in all the cases. The compression strength test indicates a very problematic ash behavior for the ashes of the Lucerne grass,
the wheat straw, and the reed canary grass. In these cases, the sintering temperature, Tsint, is clearly below a typical FBC bed temperature of 800-900 °C. In the case with the ashes from the olive flesh, the RDF, the bagasse, and the cane trash, Tsint is in the range of a typical FBC bed temperature, indicating that some problems may occur. In three cases, with the bark ash, the grot ash, and the peat ash, the compression strength would indicate unproblematic ash behavior since Tsint is higher than typical FBC bed temperatures. Comparing Tsint with Taggl, in five cases a fairly small difference between the two temperatures is detected. This is visualized in Figure 3. The bed agglomeration temperature, Taggl, is 20-50 °C higher than the sintering temperature, Tsint for the ashes of Lucerne, wheat straw, olive flesh, peat, and cane trash. This difference can most likely be explained with the fact that fluidization of the bed in the FB reactor during the controlled bed agglomeration test influences Taggl, while this kind of influence is not included in the sintering test. If we use the temperature at which the ash pellets have reached a strength value of 3 N/mm,2 which in this case would imply that a certain strength needs to be developed between the ash particles before an agglomeration would start, a better fit to the agglomeration temperatures measured in the lab rig is obtained. This is visualized in Figure 4. With the Grot and Bark ashes, Taggl was clearly lower than Tsint. In these cases, it seems that an interaction between the quartz bed material and the ash may have caused the bed agglomeration. Since the compression strength test was done with the pure ash only, these types of possible interactions would not be detected.
Table 4. Critical Temperatures Determined by the Different Methods ASTM ash fusion wheat straw olive flesh peat bagasse cane trash forest residue reed canary grass Lucerne bark RDF
IDT
ST
HT
FT
compressed strength Tsint
controlled FBC agglomeration Taggl
900 1100 1100 1260 1150 1150 1260 1550 1340 1090
1040 1170 1160 1380 1200 1200 1520 1580 1540 1190
1170 1190 1180 1430 1350 1210 1560 >1600 1540 1220
1190 1200 1200 1480 1480 1220 >1600 >1600 1540 1240
680-700 800-900 1000-1100 850-900 850-950 >1000 680-700 625-650 >1100 800-900
740 930 >1020 >1020 996 982 920-970 670 988 990
Fuels Fired in FBC Boilers
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ized-bed boiler. Three different ways of predicting the bed agglomeration tendency were used: (i) the ASTM standard ash fusion test, (ii) a sintering test based on compression strength measurements of ash pellets, and (iii) a combustion test in a laboratory-scale fluidizedbed furnace in which bed agglomeration was achieved in a controlled manner. The initial deformation temperature TID according to the ASTM test was found in all cases to be higher than the temperature typically found in the bed of a fluidizedbed combustor, hence, indicating no bed agglomeration problems for any of the biomasses.
Figure 4. Estimated Tsint at a 3 N/mm2 average strength in the pellets detected by the compression strength test versus Taggl determined by the FBC lab rig.
In one case, with the reed canary grass, the agglomeration temperature was difficult to determine. The bed pressure difference, one of the main indicators for bed agglomeration in the lab reactor, showed initial signs of bed agglomeration already at 780 °C, but a clear signal for bed agglomeration was not achieved until the bed temperature was raised to 980 °C. Comparing Taggl with Tsint here, we get a difference ranging from 80 to 270 °C. No clear explanation for this difference could be found. The results from these comparative tests indicate that very little useful data can be obtained with the ASTM ash fusion test with respect to bed agglomeration predictions. Many processes in the ash affecting bed agglomeration, such as sintering or melting, seem to have already started at far lower temperatures than what the ash fusion test detects. In this respect, the compression strength test seems to give more accurate results. However, the compression strength test also has limits. One is that this test only detects ash particleto-ash particle or ash particle-to-gas-phase interactions. Interactions with possible bed material are not detected if not the bed material is added to the test pellets in the procedure. Some tests have been performed in this way.7 The results have shown that the added bed material has mainly given a diluting effect on the ash pellets. The controlled bed agglomeration test was determined to give the best predictions, since the test imitated the actual process. However, it should be noticed that the method was not thoroughly compared with any full-scale experiments. Such a comparison remains to be done. 5. Conclusions Ten biomasses were predicted for their tendency to cause bed agglomeration during combustion in a fluid-
In all cases the ASTM test failed to predict the bed agglomeration temperature Taggl, which was achieved with the lab-scale fluidized-bed furnace, indicating that very little useful data in terms of bed agglomeration predictions can be achieved with the ASTM ash fusion test. The sintering test predicted very problematic bed agglomeration in three cases (for the Lucerne ash, the wheat straw ash, and the reed canary grass ash), moderate bed agglomeration in four cases (for the olive flesh ash, the RDF ash, the bagasse ash, and the ash from the crushed leaves of the bagasse plant), and no bed agglomeration in three cases (for the bark ash, the grot ash, and the peat ash). In five cases the sintering temperature Tsint correlated well the agglomeration temperature Taggl. In three cases an interaction between the bed material and the ash seemed to cause failure in the predictions made by the sintering test. In two cases no clear explanation could be given to the failure of the bed agglomeration prediction done by the sintering test. A better bed agglomeration prediction could be achieved with the sintering test than with the ASTM ash fusion test. However, the sintering test seemed to fail at least in those cases when an interaction between the bed material and the ash was assumed to take place. The controlled bed agglomeration test was determined to give accurate predictions of possible bed agglomeration problems caused by the ash. However, since no comparison with full-scale tests were done in this study, it remains uncertain how well bed agglomeration problems in full-scale units compare to the controlled bed agglomeration temperatures measured in the lab-scale rig. Acknowledgment. Financial support from the Swedish National Board for Industrial and Technical Development (NUTEK) and LIEKKI Combustion Research Program in Finland are gratefully acknowledged. EF980045+
