外文翻译循环流化床锅炉中燃烧煤泥炭或木材时灰特性

英文原文Ash behaviour in a CFB boiler duringcombustion of coal, peat or woodBengt-Johan Skrifvarsa, Rainer Backmana, Mikko Hupaa,George Sfirisb, Tomas Abyhammarb and Anders LyngfeltCAbo Akademi University, Turku, FinlandVattenfall AB, Stockholm, SwedenChalmers University of Technology, Gothenburg, Sweden(Received 19 November 1996； revised 16 June 1997AbstractThis paper presents selected results from an extensive on-site measurement campaign where the ash behaviors in a 12 MW CFB boiler was studied during firing of coal, peat and wood. Samples were taken from all in-going (bed material, fuel) and out-going solid material streams (secondary cyclone and bag filter) as well as from the bed and the return leg. Deposit samples were further collected from the cyclone inlet and from two different locations in the convective path. In addition, the boiler operation was monitored, including collection of operational data, flue gas temperature profiles and emissions. The paper discusses the differences in the ash chemistry that were detected between the three different combustion cases and draws conclusions on the impact of the chemistry on the bed agglomeration and fouling tendency for each fuel.(Keywords: ash behaviors；CFB combustion；deposit formation).INTRODUCTIONCirculating fluidized bed combustion is occasionally connected with fuel mineral matter related problems. The released fuel mineral matter, the ash, may cause both bed agglomeration in the furnace of the boiler as well as deposits in other locations. The heat exchanger surfaces, the cyclone and the return leg are typically sensitive areas where deposits may cause extensive operational problems. In addition to physical parameters such as fluidization characteristics and mixing, the ash chemistry plays an important role in the ash behaviour. This paper presents results from an extensive on-site measurement campaign where the ash behaviour in a 12 MW CFB boiler was studied during the firing of coal, peat and wood. Focus is on the impact of ash chemistry on slagging, fouling and bed agglomeration.EXPERIMENTALThe boiler in which the measurements were conducted, is a semi-full scale CFB, that produces hot water for local district heating. The boiler is also a very suitable research facility and has previously been successfully used when addressing, for example, emission questions from CFB. The CFB boiler is equipped with a number of operational control1 systems, such as an advanced air distribution and possibilities for 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 (3-12 MW). The unit is also equipped with analytical instrumentation for continuous monitoring and recordingof the operation. Numerous instrument tappings enable measurements in many locations in the boiler. A schematic view of the boiler is presented in Figure I.During the combustion tests, samples from all in-going (bed material, fuel) and out-going solid material streams(secondary cyclone and bag filter) as well as from the bed and the return leg were collected twice a day and analysed quantitatively for the elements Si, Al, Fe, Ca, Mg, P, Na, K, Cl and S by wet chemical analyses. Deposit samples were collected at the cyclone inlet (TFG = 850C) and from one location in the convective path (TN = 680C) with a sampling time varying from 15 min to 21 h. The sampling locations are indicated in Figure I as Lot. 1 and 2. The sampling was carried out using specially designed surface temperature controlled deposit probes. The probes were also equipped with a removable ring for later SEM/EDAX analyses.The probe surface temperature in the tests was set at 450C to simulate a superheater tube. All the probe samples were photographed and the collected depositis were characterized with respect to their collected deposits. Selected samples from both front (windward) side and back (leeward) side of the sampling probes were analysed quantitatively for the elements Si, Al, Fe, Ca, Mg, P, Na, K, Cl and S.In addition, the boiler operation was monitored. This included collection of operational data (fuel and sorbent feed, air distribution and total air), flue gas temperature profiles and emissions (CO, CO2, SO2, NO, and N2O). The data relevant for slagging and fouling evaluations are summarized in Table I.RESULTSFuel analysesThe fuel analyses are summarized in Table 2. The ash content in the wood was approximately 0.8 wt%. The corresponding values were 4.5 wt% for peat and 11.9 wt% for coal. The sulphur content in the wood was low, 0.03 wt%. The corresponding value for peat was 0.41 wt%. The fired coal was a high sulphur coal with 1.9 wt% sulphur. The chlorine content in wood is generally low, typically 0.01-0.05 wt%. A typical value for coals of the kind that was used in these tests is 0.1-0.5 wt%. In peat the chlorine was analysed to be 0.04 wt%.The main ash element in wood was calcium (23.2 wt% in ash) followed by potassium (12.8 wt% in ash) and magnesium (3.5 wt% in ash). In peat the main element was silicon (16.6 wt% in ash) followed by calcium (11.8 wt% in ash) and aluminium (7.9 wt% in ash). The bed material used during the combustion of wood and peat was a quartz sand with a particle size of approximately 350 mm. During the coal combustion tests calcium carbonate was added to the bed in the Ca:S-ratio of 2.7.Figure 1 A schematic view of the CFB boiler in which the combustion tests were preformed. Sampling locations are indicated in the figure.Table 1 A summary of the running conditions in the CFB during the combustion testsTable 2 Fuel and ash analyses of the coal, peat and wood, used during the combustion tests.Ash and deposit analysesNo heavy slagging or fouling was experienced during the tests. Generally the deposits were thin, 0. l- 1 mm. The thickest deposits were roughly 3 mm in thickness after a sampling time of 12 h. These deposits were collected from sampling location 2 during the wood combustion tests. The coal and peat firing caused hardly any deposits in the sample locations of the boiler.Figure 2 summarizes the chemical analyses for the ashes and deposits. In this figure the analysed elements are recalculated to their corresponding oxides. No bag house samples could be collected during the wood combustiontests since the amount of the ash reaching the bag house was too low. Neither could any quantitative wet chemical analyses be performed on the deposit samples collected at the cyclone inlet, or during the coal and peat combustion tests, on the front side deposit in the convective part of the flue gas channel, because of the small amount of sample. Some trends can be extracted from the figure that are interesting from the ash behaviour point of view.One trend is the decreasing share of silicon and the increasing proportion of ash elements in the samples collected from the wood and peat combustion tests from the bed material and primary cyclone ash to the secondary cyclone and bag house ashes. The reason for this trend is that the larger sized quartz bed material is separated at the primary cyclone and remains within the internal loop, furnace-primary cyclone, while smaller sized particles, such as condensed ash particles, small fragments of the bed and all gases work their way out to the convective part of the flue gas channel. For the samples collected from the coal firing tests the proportion of the sulphur absorbent calcium is clearly seen in the bed and primary cyclone samples. To distinguish between elements originating from the bed and from the fuel in the secondary cyclone and bag housesamples is, however, very difficult, since both contained calcium and silicon.A second trend is the decrease in the total amount of analysed 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 analysed elements calculated to their corresponding oxides add up to approximately l00%, which shows that most of the elements in the samples are taken into account. However, in the secondary cyclone ash as well as in the deposit samples collected during the wood combustion tests, the sum of oxides remain below l00%, indicating some missing component. This can also be seen to some extent in the corresponding samples from the peat combustion test. Assuming part of the analysed calcium to be calcium carbonate and estimating the amount of carbon as CO2, a correction can be made for the missing fraction. The secondary cyclone and bag house samples from the coal combustion tests contained a high amount of unburnt carbon, making it less valid to make a similar correction.A third noteworthy trend is the proportion of sulphur andchlorine in the different samples. In the bed and primary cyclone samples collected during the wood and peat combustion tests both values are very low. In the wood combustion tests chlorine appears to be enriched in the deposit samples, while the peat combustion tests do not show any similar trend. In the coal combustion tests sulphur is found to a very large extent in the bed and primary cyclone samples, and to a lesser extent in the deposits, secondary cyclone and bag house ashes and in the flue gas as SO2(g). Chlorine is absent from all the samples collected during the coal combustion runs. The results indicate that chlorine escapes out through the primary cyclone in all the cases tested. In the case of wood combustion, chlorine appeared to be strongly enriched in the deposits, while for peat, to some extent ( 0.2 wt%), it was found in the bag house samples. For coal, chlorine also escaped the bag house.DISCUSSIONBased on the quantitative wet chemical analyses, the amount of the different compounds in the deposits was evaluated by stoichiometrical analyses. It was assumed that all analysed sulphur and chlorine was associated with the alkali. If there was any remaining sulphur or chlorine this was assumed to be associated with calcium. If, again, remaining alkalis were present then this was assumed to be associated with carbonates. These components were classed up to a fraction called alkali salts. If excess of alkali was still present then these this was assumed to be associated with the silicon, classed as a silicate fraction as oxides. All phosphorus was assumed to be associated with the calcium, giving the inert Ca3(PO4 )2 and the remaining calcium with carbonate, classing the CaSO4 and CaCO3 as other salts.Excess of calcium was assumed to be associated with silicates, as well as the remaining oxides. The evaluation is visualized in Figure 3.The analyses indicate clearly the differences in ash chemistry depending on which of the fuels, coal, peat or wood is fired. In coal and peat firing the main ash components appear to be silicates while in wood combustion the main components are alkali and alkaline earth salts. A further implication from these evaluations is that alkali salts seem to be present in clearly larger proportions in the deposits when firing wood than when firing coal or peat,even if the wood itself contained low amounts of chlorine and sulphur. In the cases with peat and coal, the sulphur formed sulfates with the alkalis, but chlorine appeared to escape the fly ash and deposits completely. The stoichiometric evaluations show that in both the coal and peat firing cases sulphur is found in excess to alkalis in all analysed deposits. SO2 was further detected in the flue gases during these tests, while the SO2 content in the flue gases during the wood firing tests was below the detection level for the analysers. It has been earlier shown that SO2 in the flue gas may drive out chlorine from a deposit6. Obviously this was also the case during the coal and peat combustion tests. The melting behaviour or stickiness of the deposits in the different combustion cases were also evaluated in the temperature range 500-900C. Here it was assumed that the alkali salt fraction was subject to melting, the remainder being in a solid state. Inorganic mixtures such as those found in fuel ashes do not melt at one specific temperature, but have a wide temperature range where both solid and liquid phases are present. If the sample, for example a bed sample, a fly ash or a deposit, contains a high enough portion of molten phase, it will cause problems. In the bed this would be seen as bed agglomeration, in a deposit or a fly ash as a growing deposit. In certain types of boilers it has been found that a value of approximately l0-20% melt in a fly ash will make it stickys9. For silicate melts one has also to take into consideration the viscosity of the silicate melt”. It is recognized that the process of ash sintering and agglomeration is complex and to relate the percentage of molten ash in an ash particle to the stickiness of the ash particle is, at best, only a coarse approximation.Nevertheless, it is considered that this kind of engineering approach provides useful information about ash behaviour.Figure 2 quantitive wet chemical analyses of ash and deposit samples collected during the firing of wood (top), coal (middle) and peat (bottom) in the CFBC. The elements are calculated to wt% of their corresponding oxides except for chlorine. The deposits collected from location 2 in the convective flue gas channel, before the heat exchangers.In Figure 4 three melting behaviour calculations are presented, one with the composition of the deposit in location 2 found during the wood combustion tests, another with the composition of the deposit found from the same location during the peat combustion tests, and a third with the composition of the deposit found during the coal combustion tests, also in location 2. This was achieved by assuming equilibrium bulk behaviour, using a commercial thermodynamic multi-component, multi-phase equilibrium calculations routine.In the first two cases with the deposits from the wood combustion tests a first melting point. To, is already reached at 564C in the front side deposit and 582C in the back side deposits. The alkali salt part of the deposit is completely molten at 724C on the front side and at 754C on the back side, indicating that roughly 20% of the deposit would be molten. In the case of the deposit from the coal firing test, the alkali part starts to melt also at a fairly low temperature（523C）but the amount of molten phase in the alkali portion remains very low up to 749C and is not completely molten until a temperature of 858C is reached. Since the proportion of alkali salts in the deposit already originally was low, roughly 5 wt% of the total deposit, the amount of molten phase in the total deposit thus remains low, even if all the alkali salts are in a molten phase. For the deposit found during the peat combustion tests the melting behaviour is similar to that in the coal fired tests, i.e. a low amount of melt throughout the calculated temperature range, with a temperature of 523C ,a low of 866C and a total amount of alkali salt melt in the deposit not exceeding 5 wt%.The implication from this figure for the full-scale combustion is that when alkali salts are present in the deposit, they may make the deposit sticky at a fairly low temperature, leading to extensive deposit formation.It seems probable that deposits from wood combustion could be more sticky than those experienced in coal or peat firing.Figure 4 melting behaviour of the L2 deposits, collected from the CFB during combustion of coal, peat or wood. The amount of melt expressed on the -axis as wt% in total deposit and the temperature expressed on thex-axis in “C”. The alkali salt fraction is subject to melting-the rest is assumed to be inert.CONCLUSIONS(1) The chemistry in the flue gas and fly ash of the CFB was shown to shift when the fuel was changed from coal or peat to wood.(2) In coal and peat combustion the chemistry suggests that the main components in the ash samples were mainly silicates. Sulphur was most likely associated with calcium and excess sulphur was found in the flue gas as SO2(g). Chlorine escaped almost completely the fly ash and deposits. This gave fairly non-sticky deposits in temperatures, typical for the post-cyclone flue gas channel in the CFB boiler (Tro 850C).(3) When shifting the fuel to wood chips, alkali salts appeared in the secondary cyclone samples and the deposits. Even if the fuel contained low amounts of chlorine and sulphur, these two elements were very effectively enriched in the deposits and in the fly ash present in the convective part of the flue gas channel.These types of deposits were predicted to be already sticky at temperatures around 730-750C.(4) The implication for the boiler is that even if the coal and peat contained an order of magnitude higher amount of ash forming elements in the fuel, the resulting ash entering the flue gas channel was a fairly non-sticky,well-behaved ash, unlikely to cause any ash-related problems.(5) For the wood combustion the results suggest the opposite.The fly ash entering the flue gas channel was predicted to be fairly problematic due to its high proportion of alkali salts.(6) ACKNOWLEDGEMENTSThe combustion tests were performed at the circulating fluidized bed boiler located at the Technical University of Chalmers, Gothenburg, Sweden. The technical support from the operators at the boiler as well as from the scientists at the Department of Energy Conversion is gratefully acknowledged. This work was supported by the Vattenfall AE3, the Nordic Energy Research Program and the Finnish National Combustion and Gasification Research Program, LIEKKI-2.REFERENCES1 Anthony, E. J., Progress in Energy Combustion Science,1995, 21, 239.2 Skrifv