PARTICLE SIZE DISTRIBUTION OF PRIMARY ASH OF DIFFERENT FOSSIL AND ALTERNATIVE SOLID FUELS

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1 PARTICLE SIZE DISTRIBUTION OF PRIMARY ASH OF DIFFERENT FOSSIL AND ALTERNATIVE SOLID FUELS 1 A. Cammarota, 1 R. Chirone, 2 P. Salatino, 2 M. Urciuolo 1 Istituto di Ricerche sulla Combustione - C.N.R., Naples ITALY 2 Dipartimento di Ingegneria Chimica - Università Federico II, Naples - ITALY ABSTRACT The assessment of the balance between fly versus bottom ash particles during fluidized bed combustion of fossil and non-fossil solid fuels is still an open issue. The paper addresses the characterization of the size distribution of primary ash particles (PAPSD) generated after complete burn off of nine combustible materials: four coals, three wastes and two biomasses. The procedure is reliable for all the solid fuels tested, with the exception of biomass which is characterized by extensive agglomeration of ash and bed material. INTRODUCTION Operating experience with industrial Fluidized Bed Combustors have highlighted that extensive emission of fine particles (in the micronic-submicronic range) during fluidized bed combustion of fossil, waste and biomass fuels, may significantly affect the environmental impact of the operation. On the other hand, the coarser ash particles can substantially affect the inventory and particle size distribution of bed material which, in turn, affect bed hydrodynamics and effectiveness of heat transfer. Continuous bed drain has to be accomplished in order to ensure stable operation of the combustor (1). The aim of the present work is to evaluate the size distribution of primary ash particles generated during burn-out of fossil or alternative fuels by means of a standard procedure (2) and to check its reliability when applied to a new set of alternative fuels, not studied before. The procedure is based on the operation of a 4mm ID bench scale fluidized bed combustor. Elutriated material down to.5µm particle size is collected at the exhaust. Bed material is also drained and analyzed in order to characterize the coarse ash material. Data, which regarded four coals (three sub-bituminous coals and a lignite), three combustible wastes and two biomasses, are worked out to obtain the Primary Ash Particle Size Distribution (PAPSD) of the solid fuels. EXPERIMENTAL APPARATUS AND MATERIALS Figure 1 reports the sketches of the experimental apparatus used in the framework of the present study. The reactor is a 41mm ID stainless steel fluidized bed combustor equipped with a gas pre-heater (6 mm height) and a gas distributor made of several stainless steel nets layered one on the other. The reactor height is.4m to minimize fines loss by adhesion onto reactor walls in the freeboard. Fly ashes are collected by means of a purposely designed device consisting of an impactor (the same device of the first stage of the Andersen probe) followed by a sintered brass filters (average porosity larger than 5µm) and by means of a back up glass fiber filter, finally. The collecting efficiency of this device was tested comparing results obtained using an Andersen probe under the same experimental conditions. The size 1

2 distribution of the fine solids collected to the stack CO TC PT CO 2 O 2 air or nitrogen 1 1. Pre-heater 2. Gas distributor 3. Fluidization column 4. Electric heaters 5. Impactor 6. Brass Filters 7. Glass fiber backup filter 8. Filter 9. Cooler 1. Data acquisition system 1 Fig. 1 Experimental apparatus. on the filter has been characterized by means of a laser particle size analyzer. A data acquisition unit is used to process signals from pressure transducers and on line gas analyzers (O 2, CO, CO 2 ). Electrical heaters keep the reactor at a temperature of 85 C. The bed was made of 18g of quartz in the size range 6 71µm kept at the minimum fluidization at the bed temperature. The main characteristics of the fuels are reported in tables 1.1 and 1.2. Polish III South African Colombian Sulcis Lignite Moisture Volatile Matter Fixed Carbon Ash Carbon Hydrogen Nitrogen Sulfur Chlorine Oxygen Ash LHV, MJ/kg Table I.1 Characteristics of the coals used. The coals and the biomasses were ground and sieved to test particles in the range mm. The wastes are two commercial predried sludges and a selected municipal refuse 2

3 (Refuse Derived Fuel or RDF). The first (Granulated Sludge or GS) resulted from drying and granulation of the raw sludge. The second (Mechanically Dewatered Sludge or MDS) resulted from mechanical dewatering only. Both fuel particles were sieved to select particles in the size range mm. RDF in pellets ( 4mm length and 4 mm of diameter) was made of a mixture of textiles, paper, low density plastics and a small amount of light inert material. Biomasses were cut and sieved to collect fragments in the size range mm. MDS GS RDF Pine Seed Shell Olive Husk Moisture Volatile Matter Fixed Carbon Ash Carbon Hydrogen Nitrogen Sulfur Chlorine Oxygen Ash LHV, MJ/kg Table I.2 Characteristics of the wastes and of the biomasses used. PROCEDURE The Primary Ash Particle Size Distribution (PAPSD) of the fuels was measured by means of a standard procedure (3). The technique is based on batch operation of the bench scale bubbling fluidized bed combustor kept at a temperature of 85 C. Tests were carried out feeding about 15-3g of fuel particles, depending on ash content, in the size range mm. The procedure consists of the following sequence of steps: 1. The fuel batch is loaded into the combustor. Devolatilization takes place under inert conditions for ten minutes. The char is burnt in air until complete burnout. In both stages, gas superficial velocity is kept at the value U 1 (.18m/s), slightly larger than the minimum fluidization velocity of the bed (.15m/s). Collection of fly ashes at the exhaust is performed during this stage, except during the devolatilization time to avoid the condensation of the volatile hydrocarbons in the collecting probe. For this reason the collecting device is connected at the reactor after the devolatilization stage. 2. The gas superficial velocity is increased up to U 2 (.5m/s). In this way particles smaller than 2µm are elutriated. The inert bed material is not elutriated because bed particle have terminal velocities larger by more than one order of magnitude (U t = 5.3m/s). Fly ashes are collected at the exhaust for five minutes in this operative condition. 3. Coarse ash is retrieved from the bed by discharging the bed material (quartz + ash) and separating ash particles from the inert material by sieving. The operating procedure has been optimized considering the following requirements: a. combustion time must be small in order to minimize the occurrence of secondary attrition of ash particles during burn off; b. the coal/char particle temperature must not be high to the point of giving rise to ash sintering. In order to meet constraint a) the bed is fed with the minimum amount of fuel to obtain a 3

4 significant amount of ash samples. Constraint b) was satisfied by operating the combustor in air at 85 C under conditions that did not bring about ash sintering. This was confirmed by the steadiness of fluidization parameters and by the particle analysis of the ashes discharged from the bed. Ash particles cumulatively collected were characterized from the standpoint of particle size distribution, assumed equal to the PAPSD of the fuel. EXPERIMENTAL RESULTS AND DISCUSSION The biomasses tested (tab. 1.2), like young plants, have a rather limited ash content, which, however, is composed to a large extent by low-melting compounds. This feature resulted in the extensive formation of bed-ash agglomerates during fluidized bed combustion of such fuels. Figure 2 shows a SEM photograph of an aggregate in which is possible to recognize quartz particles immersed in a continuous matrix of molten material. Of course, this occurrence prevented characterization of bottom ash in this case. Fig. 2 A typical aggregate found in the bed Bottom Ash Fly Ash ash out / ash in, % Colombian Coal South African Coal Sulcis Lignite Polish Coal MDS GS RDF Fig. 3 Fractional ash reporting to bottom and fly ash Figure 3 shows the fractional ash reporting to bottom and fly ash, as obtained in experiments with the various fuels. 4

5 Colombian, Sulcis and Polish coals are characterized by a nearly equal, and rather large, fractional bottom ashes (d p > 2µm): 72%, 79% and 75% respectively. Much smaller is the content of bottom ashes of the South African coal ( 34%). Both sludges give rise to a rather limited formation of fine primary ash particles whereas a large fraction has size close to that of the parent fuel particles (4). As expected on account of its constituent materials, Refuse Derived Fuel gives rise to an extremely large fraction of fine particles ( 9%) and to a negligible amount of coarse particles. Collection of ash was effective to the point that ash balance was accurate to ±1% Colombian South African Sulcis lignite Polish III A cyclone B MDS GS RDF E cyclone F cumulative distribution, % C bottom brass filter G bottom brass filter 8 D H 6 PAPSD PAPSD d p, µm d p, µm Fig. 4 Cumulative size distribution of the ashes. Figure 4 reports the comparison between PAPSD of ash material collected for the four coals and the three wastes investigated. Details of the particle size distributions are shown in the frames A, B, C and E, F, G for coals and wastes respectively. Analysis of cumulative 5

6 distributions of primary ash particle size distribution (frames D and H) highlights that the contribution of fine particles is significant for South African Coal and Refuse Derived Fuel. The particles whose size is less than 5µm (of the order of a typical cyclone cut-off) account for about 5-6% of the whole ashes for these fuels and for less than 1% for the others. GS and MDS have about 65% and 8% particles coarser than 1mm, respectively. CONCLUSIONS 1. An upgrade of the procedure for the characterization of the extent and particle size distribution of ash generated upon solid fuel burn-off in fluidized beds has been developed and applied to test four coals, three wastes and two biomasses. In particular, the procedure failed in the case of biomass because of extensive formation of ash/sand agglomerates. 2. The procedure, based on the operation of a 4mm ID bench scale fluidized bed combustor with optimized design of the fine particle collection at the exhaust, enables the characterization of elutriated material down to.5µm particle size. 3. The coals tested (with the exception of South African) present a large amount of bottom ashes. The size distributions of primary ash particles remaining after complete carbon burn off closely resembled those of the parent fuels for both the sludges. Combustion of RDF yielded practically only fine ash particles, entirely reporting to fly ash. REFERENCES 1. Cammarota, A., Chirone, R., Marzocchella, A., Salatino, P.: 7th International Conference on Circulating Fluidized Beds, Niagara Falls, Ontario, Canada, May, p. 661 (22). 2. Cammarota, A., Chirone, R., Marzocchella, A., Salatino, P.: 16 th International Conference on Fluidized Bed Combustion, Reno, Nevada (USA), May, 78, (21). 3. Urciuolo, M., Cammarota, A., Chirone, R., Salatino, P.: Joint Meeting of The Scandinavian-Nordic and Italian Sections of The Combustion Institute, ISBN , (23). 4. Cammarota, A., Chirone, R., Salatino, P., Scala, F., Urciuolo, M.: Proc. XXX Symp. (Int.) on Combustion, The Combustion Institute, Pittsburgh (PA) 24, in press. 6