Energy & Fuels 2003, 17, 1153-1159
1153
Ash Transformations during Combustion of Meat-, Bonemeal, and RDF in a (bench-scale) Fluidized Bed Combustor Marcus O ¨ hman,* Anders Nordin, Karin Lundholm, and Dan Bostro¨m Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden
Henry Hedman Energy Technology Centre, P.O. Box 726, SE-941 28 Piteå, Sweden
Margareta Lundberg Kvaerner Pulping AB, Power Division, P.O. Box 8734, SE-402 75 Go¨ teborg, Sweden Received November 19, 2002. Revised Manuscript Received May 28, 2003
Following the recent Bovine spongiform encephalopathy (BSE) experiences, thermal treatment of meat- and bonemeal (MBM) in existing fluidized bed combustion (FBC) plants for refusederived fuels (RDFs) has evolved as an interesting disposal and disintegration method. However, only a limited number of studies have previously been performed for combustion of MBM in fluidized beds. The objectives of the present work were, therefore, to determine the bed agglomeration tendencies of these materials during combustion in fluidized beds and to evaluate the effects of dolomite and kaolin addition to the fuel mix, as well as to elucidate the overall ash transformation mechanisms governing the potential bed agglomeration and fouling processes. By controlled agglomeration experiments in a 5 kW bench-scale fluidized bed reactor, the fuelspecific critical agglomeration temperatures in normal quartz bed material were determined for the different fuel/additive mixtures. All collected samples of bed materials, final bed agglomerates, and cyclone ashes were analyzed using SEM/EDS and XRD. The results indicated that the MBM fuels could be expected to be problematic concerning bed agglomeration in normal quartz beds, while kaolin and possibly dolomite addition could be used to reduce this risk to moderate levels. A significant elemental fractionation between the bed material and the cyclone ash was obtained. Apatite (Ca5(PO4)3(OH) or potentially some other calcium phosphates are elutriated from the bed and enriched in the fly ash, while sodium and potassium are enriched in the bed material. The characteristics and the corresponding melting behavior estimations of the necks formed between agglomerated bed particles suggest that silicate melts are responsible for the bed agglomeration. Results from XRD analysis of the fly ash formed from the fuels used in the present study indicated that the risk for melt-related fly ash problems seem relatively small.
Introduction Meat- and bonemeal (MBM) for animal feed was previously produced in rendering plants where animal offal and bones are mixed, crushed, and cooked together. Following the recent Bovine spongiform encephalopathy (BSE) experiences, the use of MBM for animal feeding is now banned1 and different thermal disposal and disintegration methods have been suggested.2 Incineration capacity was urgently required to destroy both the forecasted MBM production each year and the existing stockpiles.3 Here, thermal treatment of MBM in existing fluidized bed combustion (FBC) plants for refuse-derived fuels (RDFs) has evolved as an interesting incineration * Corresponding author. E-mail:
[email protected]. (1) Europe-wide ban on feeding meat and bone meal. Veterinary Record 2000, 147 (24), 670-671. (2) McDonnell, K.; Desmond, J.; Leahy, J. J.; Howard-Hildige, R.; Ward, S. Energy 2001, 26, 81-90.
method. One of the main objectives in combustion of meat byproducts such as MBM is to ensure that any living organism is totally destroyed during the process. FBC is considered a suitable process, as its homogeneous nature and excellent mixing conditions ensure high combustion efficiencies. Because of the moderate temperatures the extents of deposit and thermal NO formations can also be kept to a minimum. However, bed agglomeration could be a potential problem, which can decrease both heat transfer in the bed and fluidization quality, resulting in poor combustion efficiencies and loss of control of bed operational parameters. However, only a limited number of studies have previously been performed for combustion of MBM in fluid(3) Findlay, K.; MacDonald, M. Brighton, U.K., ImechE Conference Transactions 2001, (9, Engineering for Profit from Waste), 53-66. (4) Annual report 2000; Energy Research Centre of The Netherlands: 2000; pp 22-24. (5) O ¨ hman, M.; Nordin, A. Energy Fuels 2000, 14, 618-624. (6) Turn, S. Q.; Kinoshita, C. M.; Ishmura, D. M.; Zhou, J.; Hiraki, T. T.; Masatuni, S. M. J. Inst. Energy 1998, 71, 163-177.
10.1021/ef020273a CCC: $25.00 © 2003 American Chemical Society Published on Web 07/02/2003
O ¨ hman et al.
1154 Energy & Fuels, Vol. 17, No. 5, 2003 Table 1. Fuel Characteristics (elemental compositions) meat- and bonemeal pellet dry substance (raw material) dry substance (pellet) ash (wt % db) LHV (MJ/kg wet substance, raw material) C (wt % db) H (wt % db) O (wt % db) N (wt % db) Cl (wt % db) S (wt % db) Si (wt % of ash) Al (wt % of ash) Fe (wt % of ash) Ca (wt % of ash) Mg (wt % of ash) Na (wt % of ash) K (wt % of ash) Mn (wt % of ash) P (wt % of ash) Ti (wt % of ash)
RDF pellet
RDF and meat- and bonemeal pellet
96.7
58.5
81.6 38.8 13.19
88.2 12.5 11.99
94 21.4
51.2 7.3 28.0 0.8 0.61 0.18 18.6 4.45 1.73 14.7 1.26 5.03 2.61 0.126 0.847 1.56
44.9 6.4 23.5 3.5 0.50 0.22 12.3 2.95 1.15 22.3 1.08 3.94 2.11 0.0838 8.13 1.03
32.8 4.6 14.7 8.8 0.29 0.29 < 0.05 < 0.02 0.0319 36.9 0.717 1.82 1.14 0.00253 22.3 0.00120
ized beds.4 Previous work has shown that kaolin and dolomite may be used as mineral additives to prevent ash related problems.5,6 The objectives of the present work were therefore to (i) determine the characteristic and critical bed agglomeration temperatures of MBM, RDF, and mixtures of these materials during combustion in a bench-scale fluidized bed reactor (5 kW); (ii) evaluate the effects of dolomite and kaolin addition to the fuel mixtures on bed agglomeration tendencies; and (iii) elucidate the overall ash transformation mechanisms governing the potential bed agglomeration and fouling processes. Experimental Section Fuels, Additive, and Bed Material Used. Two different raw materials, a MBM mixture (47 wt % meatmeal and 53 wt % bonemeal) and RDF with two different kinds of additives
(dolomite and kaolin) were used in the present study. Five different pellet fuels were made from the raw materials and the additives: 100% meat and bonemeal; 100% RDF; RDF and MBM pellets (2/3 and 1/3 on energy basis, respectively); RDF, MBM, and dolomite (2/3, 1/3 on energy basis and 5%wet of fuel mix basis, respectively); RDF, MBM, and kaolin (2/3, 1/3 on energy basis and 5%wet of fuel mix basis, respectively). The produced pellets had a diameter of 6 mm and length of 10 to 15 mm. The characteristics of the different pellets, additives, and bed material used are summarized in Table 1 and Table 2, respectively. Ashing of the fuel preceding elemental analysis by ICP-AES (all elements except Cl) and wet chemical analysis (Cl) was performed at 550 °C to reduce the loss of volatile ash elements. X-ray diffraction (XRD) analysis of the MBM pellets showed apatite (Ca5(PO4)3(OH,Cl,F)) to be the main crystalline constituent of this fuel. The five different fuels were subjected to combustion tests in a bench-scale fluidized bed reactor in which bed agglomeration was achieved in a controlled manner, resulting in the fuelspecific critical agglomeration temperature.7 The bed material used during the experiments was normal quartz sand (Table 2), initially sieved to a size fraction between 200 and 250 µm. The sand contained more than 98% SiO2 and only small amounts of mineral impurities. The same type of quartz sand is used by a large number of FBC operators in Sweden. Controlled Fluidized Bed Agglomeration Tests. The controlled fluidized bed agglomeration (CFBA) method has previously been described in detail by O ¨ hman and Nordin,7 and only a brief description is given here. The bench-scale reactor (5 kW), is constructed from stainless steel, being 2 m high, 100 mm and 200 mm in bed and freeboard diameters, respectively. The agglomeration tests were initiated by loading the bed with a certain ash-to-bed material ratio, under normal FBC conditions. The excess oxygen concentration was controlled to 6%dry. A fluidization velocity of four times the minimum fluidization velocity was used, and the bed temperature was maintained at 730 °C for all fuels. The reason for the choice of the relatively low bed temperature was to avoid agglomeration during the ashing procedure. At an ash amount corresponding to a theoretical value of about 20 wt % ash in the bed, the fuel feeding was stopped and the operation was switched to external heating. The bed was then heated, at a
Figure 1. Typical illustration of bed variable fluctuations, here for controlled agglomeration test of RDF pellet.
Combustion of MBM and RDF in a Fluidized Bed
Energy & Fuels, Vol. 17, No. 5, 2003 1155
Figure 2. SEM backscatter image of a typical bed agglomerate from the controlled bed agglomeration test of MBM. Table 2. Bed Material and Additive Characteristics bed material (quartz)
dolomite
kaolin
physical properties particle diameter
200-250 µm
