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Thermodynamic equilibrium study of trace element mobilisation under air blown gasification conditions
Fuel 81 (2002) 75±89
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Thermodynamic equilibrium study of trace element mobilisation under air blown gasi®cation conditions B.B. Argent, D. Thompson* Department of Engineering Materials, University of Shef®eld, Sir Robert Had®eld Building, Mappin Street, Shef®eld S1 3JD, UK Received 22 December 2000; revised 17 May 2001; accepted 13 June 2001
Abstract Thermodynamic equilibria have been examined for several trace elements at levels found in coal that might be used in the air blown gasi®er air (ABG). The results obtained with the factwin suite of programs were used to test the degree to which predictions are consistent with observed partitioning. Conditions of lime-free gasi®cation, gasi®cation with the design limestone addition and limestone addition at a higher fuel-lime ratio have been examined for 20 atm. pressure over a range of temperatures. The effects of varied sulphur and chlorine levels have also been examined. Curtailment of the ABG program means that the range of experimental data does not allow deep comparison of predictions with observations but in general there is broad agreement. It is concluded that the results form the basis of a method of representing the potential for mobilisation (low to standard limestone) from coal particles followed by reincorporation into condensed phases (standard to high limestone) during the in-bed processes. Simulation of cooling of the equilibrium gas phase at 1273 K in isolation by 10 K steps predicts incorporation of a range of elements into a matte, and others into an alkali-based chloride melt. q 2001 Published by Elsevier Science Ltd. Keywords: Thermodynamics; Trace elements; Gasi®cation
1. Introduction A method of predicting trace element mobility under the excess air, atmospheric pressure combustion conditions of the utility boiler has been regarded as highly desirable for many years. Modern gasi®cation systems operate under elevated pressures of 1±2 MPa (10±20 atm), which makes experimental measurements of trace element concentrations inside gasi®ers even more dif®cult than in utility boilers, hence the means to predict trace element mobility is just as desirable as under combustion conditions, if not more so. The air blown gasi®er, which is the subject of this investigation, was designed to operate at 1±2 MPa pressure and supply gas to a turbine [1]. The potential for transfer of even minute amounts of some trace elements to the blades of a turbine over the course of thousands of hours running requires evaluation. The meaningful description of trace element release using kinetic models for as complex a system as coal gasi®cation is not possible. There are too many unknowns. Instead, this work aims to provide thermodynamic equilibria for models of the gasi®er coal±air±lime * Corresponding author. Tel.: 144-114-2225498; fax: 144-1142225943. E-mail address: dennis.thompson@shef®eld.ac.uk (D. Thompson). 0016-2361/02/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0016-236 1(01)00112-0
feed. It should be possible to perturb these models, when suf®ciently complete, to simulate the consequences of, say, addition of the trace element in question to the gasi®er reactor as part of the organic matrix, whereas its main host under high temperature equilibrium is the oxide melt. In this instance a model including trace solution rate is required. In associated papers we have described the use of the fact [2] computational thermodynamic program to examine major coal component distributions in ¯uidised bed combustion [3], the effect of coal mineral content variation on the product ash [4,5], alkali metal partitioning under combustion and gasi®cation conditions [6], the distribution of trace elements in pulverised fuel combustion [7], and their fate in the Pren¯o slagging gasi®er [8]. Delay et al. have also investigated the equilibria for the conditions of the incineration of waste using fact [9]. The importance of inclusion of melts and solid solutions in the models of the systems has been noted, and it has been found that although there are signi®cant gaps in the thermodynamic data, particularly in the representation of solution of trace elements in melts, signi®cant information about the elementary distributions can be obtained from such calculations. fact version 2.1 was used in the studies of ¯uidised bed combustion, the effect of feedstock composition, and alkali
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metal partitioning. It had restrictions on the melt models, in some cases imposed in order to protect the data from alteration or addition (oxide and salt melt data) and in the case of the sulphide melt model, accessible for alteration but too dif®cult to re-model adequately for any attempt at ad hoc extension. The factwin 3.05 version of the program, which has been used in this and the companion study of combustion conditions, allows use of the oxide melt data outside the composition ranges for which it has been fully optimised and the sulphide melt has been extended by the suppliers of the software. These extensions in the modelling facilities have considerable in¯uence on the value of the predictions. 2. Computational model Under ¯uidised bed gasi®cation conditions (in which we refer to the air blown gasi®er concept timescales and temperatures) the time available for particle±gas interaction is greater than under pulverised fuel combustion or slagging gasi®er conditions, whilst the maximum temperature is lower. In both cases substantial inhomogeneities will remain in the products, but the nature of the inhomogeneities will be very different. The most clear-cut inhomogeneity in the case of ¯uidised bed gasi®cation is the presence of limestone particles. We have examined the consequences of this inhomogeneity by considering the equilibrium at bed temperature for standard limestone addition, raised limestone addition and zero limestone addition. In modelling pulverised fuel combustion all minerals with complex structure are assumed to be decomposed, and only the simpler species can be subsequently formed. For the air blown gasi®er the longer residence times and lower peak temperatures suggest this assumption cannot be made, and not only have complex compounds been included in computations but those solid solutions which are most stable have been selected in the course of trial calculations. (The precalculations are necessary because of limitations on the number of solutions, which can be included in any one dataset.) Earlier versions of the fact database included a slag model which includes all major coal mineral oxides, and other models which included one or more trace elements but excluded one or more of the major coal mineral oxides: most seriously, potassium is omitted from all these models. This led to complex computational procedures to adjust for incorrect slag amounts, especially in the lower part of the slag-formation temperature range, and consequent uncertainties in partitioning results. Where the reduced slag model did not predict any slag in the lower temperature range, no predictions for the trace element at all could be made. The fact for Windows revision of the software allows computations using a slag model referred to as SLAG?. This model allows modelling of a slag which includes sets of cations and anions included in the restricted models but for which not all interactions are known. Some uncertainty in the partitioning of the trace
elements concerned is inevitable under these circumstances but the results are likely to be more accurate than for restricted models. Use of the SLAG? model has found to be restricted in practice to the inclusion of 17, or at most, 18 elements in the computational set if immiscibility gaps and the presence of additional anions in the melt are to be examined. If it can be ascertained by trial calculations that the contribution due to additional anions is insigni®cant, and there is no miscibility gap, then up to 23 elements can be included in the database. It has been found during extensive trials that dif®culty in convergence is associated with the number of components involved in the oxide melt. It was found to be possible to model the solution of one trace element in the melt at a time with a miscibility gap and the remainder of the system involving 23 elements. This technique was used in modelling pulverised fuel combustion, where immiscibility is clearly indicated [7]. It was also used in earlier calculations for gasi®cation and it was found that immiscibility was observed when no lime was added. Further calculations have combined with improvements in the mineral solution models provided with fact to indicate there is only a slight tendency to immiscibility at most, and the results presented here use the later model and immiscibility is excluded from consideration. The later version of fact is provided with a revised and more extensive matte model than the previous version, inasmuch as nickel solution is incorporated, but this model does not allow oxygen solution in the matte to be examined. A model of the Fe±S±O±Mg±Mn system (restricted to high FeS content) allows a study of the early stages of the conversion of pyrite to haematite which is considered important, but the oxygen potential permissible is limited. The spinel model provided with fact has been found to give poor agreement with an invariant point for co-existence of an oxide melt with periclase, monticellite, spinel and merwinite in the CaO±Al2O3 ±SiO2 ±MgO system, and a model using the SGTE data for the spinel [10] which has closer agreement is provided. At lower temperatures, trace element solution in any salt melts formed cannot be examined using the existing database, which is protected against modi®cation. The literature provides suf®cient information, and the interactions in the sodium±potassium chloride melt are suf®ciently regular, for us to develop embryonic databases for sodium plus potassium chloride melts with some trace element solution [11] which was used to con®rm signi®cant solution of trace element in the chloride melt under gasi®er conditions. The database includes interaction data for KCl/NaCl, CuCl/ NaCl, ZnCl2/KCl, CaCl2/NaCl, PbCl2/KCl, PbCl2/NaCl and PbCl2/CaCl2. Other interactions were assumed to be ideal. A more extensive database was developed, and is used in a subsidiary cooling study reported in this paper. The actual temperature at which processes are assumed frozen is 1300 K under combustion [4]. The selection of this temperature was made on the basis that the corresponding equilibrium product compositions almost matched those observed in practice and it seems likely that a similar
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
temperature is appropriate for ABG, hence the nominal operating temperature, 10008C 1273 K is used. To allow comparisons with earlier work, calculations were made for a coal corresponding to that used at Eggborough power station and studied in detail by Spears and MartinezTarazona [12±14]. The elements included in the present computations are the organics and other major non-metals (C, H, N, O, S, Cl), major and minor metals and non-metals (Ca, Mg, Na, K, Fe, Al, Si, Ti, P, Mn) and a range of trace elements (Pb, As, Zn, Cu, Ni, Cr). The elemental analyses input to the computer program, for high limestone, low limestone and limestone-free gasi®er conditions (Table 1) included limestone, air and steam injection rates determined on an arbitrary basis to lie within the ranges used in pilot scale operation [3]. Under normal operating conditions it was assumed that limestone was added at a constant molar ratio of calcium to sulphur of 2.133:1, air at an O:C ratio of 0.676 and steam injection at a molar ratio of H2O: (O in the coal 1 O added as air) of 0.293. The high limestone addition corresponded to a Ca:S ratio of 7.5. 2.1. Elemental distributions at temperatures above 1273 K The range of variables affecting the mobility of any element under fuel rich conditions which could affect equilibrium in air-blown gasi®cation conditions are somewhat wider than for pf combustion in a utility boiler. On the other hand, the range of temperature is somewhat more restricted. In re®ning the model, there is a clear need for more emphasis on local equilibrium considerations, because of the larger particulate and lower temperatures involved. Here we examine the effects of temperature and amount of limestone added.
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Air-blown gasi®ers operate at the order of 1273 K and temperature variation effects are examined primarily to note the effect of allowing hot-spots to arise but results of 2073 K are included for completeness. The ratio of limestone to fuel is chosen according to the requirements of sulphur adsorption and ¯uidised bed operation, but it is instructive to examine the consequences of equilibration at varied limestone to fuel ratios since inhomogeneities in the bed arise. We have examined the case of no limestone addition as representative of a region of purely coal particles: standard limestone Ð the overall equilibrium case: and `high limestone addition' at a rate corresponding to the equilibrium hydrogen sulphide level in the product gases at the stated operating temperature. This should give an insight into equilibration close to a limestone particle. Results for the gasi®er air addition level but with no additives at one atmosphere pressure are presented in a companion paper [7] where they are used to model an early stage of combustion. Sulphur and chlorine levels in the fuel vary, and in view of the potential for formation of volatile chlorides and sulphides which has been shown to exist for pf combustion and slagging gasi®cation [7,8], a separate study of the in¯uence of these two elements at varied levels has been undertaken. 3. Results 3.1. Effect of temperature and varied limestone addition rates Fig. 1(a)±(c) shows the variation in amount slag formed when no, standard and high limestone addition is used with
Table 1 Input compositions for simulation of no, standard and high limestone addition at high temperatures Element
High lime (mol)
Standard lime (mol)
No lime (mol)
Carbon Hydrogen Sulphur Chlorine Sodium Potassium Calcium Magnesium Iron Aluminium Silicon Oxygen Nitrogen Lead Arsenic Manganese Boron Phosphorus Chromium Titanium Nickel Copper Zinc
5.681426 9.124039 0.0668206 0.020485 0.00436396 0.0117427 5:01466 £ 1021 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 10.200448 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
5.322593 9.124039 0.0668206 0.020485 0.00436396 0.0117427 0.14702572 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 9.1239482 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
5.17996 9.124039 0.0668206 0.020485 0.00436396 0.0117427 0.0043927 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 8.6960500 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
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Fig. 1. Amount of slag formed under conditions of no, standard and high limestone addition.
temperature from 1273 to 2073 K. It is not possible to illustrate the variations in distribution of all the elements in detail, and all major elements except sodium are omitted. The strong variation in sulphur partitioning with amount of
lime added has clear implications for trace partitioning, via volatile sulphide formation and formation of matte, in which some trace elements will dissolve. Sulphur distribution is shown for the three levels of lime addition in Fig. 2.
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Fig. 2. Distribution of sulphur under no, standard and high limestone addition conditions, 1273±2073 K.
Fig. 3 shows sodium distribution, and the distributions of boron, arsenic, lead, zinc, copper and nickel are shown in Figs. 4±9. Manganese mobility is not illustrated. It is immobile under all conditions studied (less than 1% in the gas phase), with the highest mobility at high temperature and with high limestone addition. Its immobilisation at low temperature with no and standard lime addition is in part due to spinel formation and in part due to solution in slag. As might be expected, the total amount of slag increases with limestone addition and increases rapidly with tempera-
ture over 1273±1673 K. Sometimes complex variation in the percentage of the trace elements volatilised as a function of temperature can be understood in terms of the competition between the formation of matte, slag and spinel with the solution of trace elements, and natural tendency for increased volatilisation with temperature. 3.2. Effect of varied chlorine and sulphur addition Studies of the effects of chlorine and sulphur concentration variation under pf combustion conditions both before
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Fig. 3. Distribution of sodium under no, standard and high limestone addition conditions, 1273±2073 K.
and after attainment of the ®nal air level, and for slagging gasi®cation have been presented elsewhere, and show the in¯uence of these elements in the 1500±1600 K temperature range. We have attempted the same exercise for air blown gasi®cation for the temperature range of interest in air blown gasi®cation i.e. 1273 K and slightly above. Sulphur and chlorine levels were varied from zero to twice the basis level used in the main calculations. The results are subject to uncertainties in detail because of the sensitivity of the amount of matte, which controls mobility of several elements, to the exact model of the system. Limited
changes in mobility are noted because almost all elements are either highly mobile or highly immobile under these conditions. Nickel and copper are mainly immobilised because even in the absence of matte, the metal is predicted to be formed. Manganese immobilisation is mainly due to spinel formation, which is independent of the amounts of S and Cl. Lead and zinc are highly mobile, and this mobility is largely due to the formation of the metallic vapour, which is independent of S and Cl amounts. The exception is arsenic. Arsenic is intermediate between copper and nickel, and zinc and
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Fig. 4. Distribution of boron under no, standard and high limestone addition conditions, 1273±2073 K.
lead, in its tendency to become incorporated in matte, but only a small amount of arsenic is dissolved in the oxide melt in the absence of matte. It is therefore normally mobile except where matte is formed, and when a large amount of matte is formed as at 1273 K with standard lime addition, much of the arsenic is normally immobilised. However, when sulphur is absent, because of the small amount of slag formed, arsenic is virtually fully mobilised under these conditions. 3.3. Effect of additional anions in the oxide melt Additional anions including sulphide, sulphate, chloride,
carbonate and hydroxyl group can be included in the oxide melt model. Trial calculations using the fact fully optimised models have shown that for no limestone addition and standard limestone addition compositions the amounts of these anions present in the melt are suf®ciently small for them to be neglected, except in the case of sulphide, for which the contributions are small but not negligible. Typical results obtained using SOLN-SLAG? at standard limestone addition rate, are that at 1623 K, 84.9% copper is dissolved in the slag as Cu2O, and an additional 0.504% is dissolved as Cu2S. Similarly, nickel is dissolved as 97.8% NiO and 0.582% NiS, which becomes 93.9% NiO and 0.181% NiS at 1373 K.
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Fig. 5. Distribution of arsenic under no, standard and high limestone addition conditions, 1273±2073 K.
At high limestone addition level and 1273 K the amount of sulphide in the slag is predicted to be over 29%, very close to the upper limit of con®dence set by the software developers at 30%. At higher temperatures the amount of sulphide falls progressively. 4. Discussion The results show the addition of lime at standard rate has a signi®cant effect on the equilibrium distribution of some elements by comparison with no lime addition, and the
addition of high lime level alters them further. The effects are seen in the composition of the condensed phases present, the distribution of sulphur, and the extent to which various elements are mobilised. 4.1. General variation in condensed phases formed with amount of lime added The addition of limestone increases the amount of oxide melt formed at high and low temperatures (Fig. 1). An exception occurs in the region of 1400 K, where standard limestone addition leads to a slightly lower amount of oxide
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Fig. 6. Distribution of lead under no, standard and high limestone addition conditions, 1273±2073 K.
melt than no lime addition. On going from no limestone to standard and high limestone the minerals formed at low temperatures change from SiO2 (tridymite), KAlSi2O6 (leucite), Fe2Al4Si5O18 (ferrocordite), CaAl2Si2O8 (anorthite), (FeO)(TiO2) (ilmenite), Ca3(PO4)2 and FeCr2O4 to Ca2SiO4 (alpha prime), CaO (lime), Ca3Al2O6, Ca3Ti2O7 and Ca5HO13P3 (hydroxyapatite). At higher temperatures the hydroxyapatite is replaced with calcium phosphate. Mellilite solution B appears at the lowest 300 degrees studied for
standard limestone but not at high limestone addition or with no limestone added. Matte is predicted to form both when no lime is added and when it is added at the standard rate. The maximum amount is formed at 1273 K, and this traps several percent of the sulphur when no lime is added (Fig. 2), and over ten percent at standard lime addition rate. There is a marked change on moving to high lime addition rate, where much more sulphur is trapped in the slag than is present in the gas
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Fig. 7. Distribution of zinc under no, standard and high limestone addition conditions, 1273±2073 K.
phase, in contrast to the 1% trapped at standard lime addition rate, and even less when no lime is added at 1400 K and above. 4.2. Mobility of the elements The elements arsenic, lead and zinc (Figs. 5±7) are highly mobile over the whole range of conditions studied: the limited immobilisation of these elements is dominated by solution in the matte phase at the lowest temperatures. Copper and nickel (Figs. 8 and 9) are not only strongly immobilised by matte at the lowest temperatures, but as the amount of matte diminishes with rising temperature oxide melt takes over this role. At high lime levels the
oxide melt immobilises most of the copper and nickel at 1273 K. The mobility of these two elements at the higher temperatures examined becomes signi®cant. Sodium mobility (Fig. 3) rises with the amount of limestone present, which re¯ects its enhanced activity coef®cient in high calcium melts. The effect of increasing calcium on alkali mobility has been noted elsewhere [15]. Potassium follows the same trend, but in addition its immobilisation in the absence of lime and at low temperatures when standard lime is added re¯ects the formation of signi®cant amounts of leucite. The mobility of boron at no and standard lime addition levels decreases at ®rst as the temperature rises and more is dissolved in the melt, then increases with a further rise in temperature, but the majority is still
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Fig. 8. Distribution of copper under no, standard and high limestone addition conditions, 1273±2073 K.
trapped in the melt at 2100 K with high lime addition (Fig. 4). Phosphorus (not illustrated) is highly volatile without lime addition above 1273 K, but effectively immobilised when lime is added. The remaining elements, aluminium, manganese, calcium, silicon, titanium, iron and chromium, show very limited mobility. 5. Cooling studies Calculation of the equilibrium distribution at 1273 K allows derivation of the gas phase composition which passes to the outlet duct and cools as it travels downstream. The
gases containing trace elements, which leave the combustor or gasi®er contain particulates, which we assume in this study act purely as nuclei for condensation from the gas phase. Calculations are carried out in 10 K steps, which eliminates the possibility that formation of a condensed phase which has a short temperature range of stability is missed. The calculations were carried out with a database including the fact matte (database solution MATT) and alkali salt model (database solution SALT). The SALT model does not include trace element solution. A subsidiary set of calculations has been carried out using a model of alkali chloride with trace element solution, which we have developed. A brief study of the solution of iron and
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Fig. 9. Distribution of nickel under no, standard and high limestone addition conditions, 1273±2073 K.
magnesium using the database solution SOLN-SAL3 of fact 3.05 indicated that at the highest temperature of alkali chloride melt formation 93% of the magnesium was dissolved. The predictions for magnesium deposition at lower temperatures are therefore a result of the limitations of the model. A more extensive study of trace element solution in the chloride melt has been made using a database developed by one of us (DT), which allows solution of Ca, Mg, Mn, Fe, Zn and Pb as chlorides in the alkali chloride host. The input mixture for the case illustrated here was obtained from the standard limestone addition case given in Table 1. The calculations were carried out using the equilibrium gas phase predicted for 1273 K (Table 2). It
was noted that whereas complete equilibration of the gas phase would require deposition of solid carbon, and the formation of methane, because of kinetic hindrance, these equilibria are not attained. Calculations for the standard gasi®cation conditions have been carried out for (i) full equilibrium; (ii) equilibrium in which the carbon deposition indicated is ignored at each stage; (iii) equilibrium in which methane formation is ®xed at the initial level at 1273 K. The exclusion of methane formation requires the assumption that carbon-containing species are frozen at the 1273 K equilibrium level. Results using the three models were for practical purposes, identical. The results of the simulations for standard limestone
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89 Table 2 Composition of gas phase at 1273 K transferred to cooling calculations for standard lime addition Element
Transferred to cooling calculation (mol)
Carbon Hydrogen Oxygen Nitrogen Chlorine Magnesium Sodium Boron Sulphur Iron Copper Nickel Zinc Lead Arsenic Manganese Calcium Potassium
5.322593 9.124039 8.5292 20.263 2.04850 £ 10 22 8.06243 £ 10 210 7.19486 £ 10 24 7.90775 £ 10 24 5.44704 £ 10 22 1.49290 £ 10 25 1.35980 £ 10 210 6.72920 £ 10 213 3.08590 £ 10 25 6.98500 £ 10 26 4.44440 £ 10 27 5.47468 £ 10 26 2.74937 £ 10 22 3.72983 £ 10 23
addition with carbon equilibrated are summarised in Fig. 10(a) and (b). Fig. 10(c) shows a replication of the results for the temperature range where salt melt is formed, using the trace-dissolving melt in place of SOLN-SALT. 5.1. Discussion of results for cooling The early stages of cooling are seen to be dominated by deposition of a matte phase, which dissolves all the nickel and copper carried over in the vapour. A very small amount of zinc and in®nitesimal amounts of lead and arsenic are also incorporated in the matte. At 1173 K matte formation is replaced by FeS deposition and MnS deposition begins at the same time. At 10 K lower, alkali melt deposition begins and then is replaced by a potassium chloride±sodium chloride solid solution at 140 K lower temperature. Deposition of magnesium borate is predicted to commence at 1533 K, but as commented earlier, this is an artefact of the limited model. At 963 K calcium chloride formation is predicted (50 K below single step where a very small amount of calcium borate is predicted to be deposited). Fig. 10 shows that calcium is, like magnesium, strongly retained in an alkali chloride melt and, like the formation of magnesium borate, calcium chloride deposition is a consequence of simpli®cation. Zinc sulphide deposition begins at 1083 K and continues down to the lowest temperature examined, 493 K. Arsenic sulphide deposition begins at 653 K. Lead is deposited, ®rst as the sulphide at 793 K, then as the chloride, with an initial deposition temperature of 593 K. A small amount of iron is deposited as potassium iron chloride below 673 K. The calculations using the alkali chloride melt with trace
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solubility con®rms the rapid loss of magnesium into solution. The amount of calcium in the gas phase also becomes in®nitesimal over a short temperature interval. Manganese is eventually, effectively removed from the gas phase and much of the iron is removed before melt formation ceases. Zinc in the gas phase is reduced to about one third of the original amount, but lead is only slightly dissolved. 5.2. Comparison of computations and experimental observation for gasi®er conditions Comparisons for gasi®cation conditions are constrained by the limited amount of experimental data available. The objective of this study was the simulation of trace element behaviour in the Air Blown Gasi®er (ABG) operated by British Coal Technology Development Division. 1 The speci®cation of the gasi®er called for the use of limestone in the bed, but trials in which trace element measurements were reported and limestone was added were con®ned to biomass-containing combustibles, which have trace element associations far from those of coal. The nearest approach to the speci®ed operating condition of the gasi®er was a nominal bed temperature of 1233 K (9608C), with a peak temperature of 1266 K using Daw Mill coal. The operating pressure was 18 atm, and limestone addition was omitted. This work is not available in the open literature. Results for the partitioning of trace elements between bed char, primary (hot gas, unstated temperature) cyclone ®nes, hot gas ®lter ®nes (853 K) and cleaned fuel gas are reported, and can be converted to relative concentrations of trace element in the three solid phases. Helble et al. [16] have studied a laboratory scale gasi®er in which Illinois No. 6 coal was dropped through a tube furnace with gasi®cation temperature of the order of 1450 K, at atmospheric temperature and with no limestone addition. 5.3. Comparison of observation with prediction for each element Following the approach of Martinez-Tarazona and Spears [12], Querol et al. [17] and Smith [18], in comparing the concentrations of trace elements in pf ash samples, it is noted that the hot gas ®lter ®nes of the ABG contained more than 6 times the concentration of arsenic in the bed char, which is consistent with considerable volatilisation as predicted in this study. It is remarked that the experimental evidence for high mobility of lead inasmuch as it is observed in the cleaned fuel gas of the ABG is very strong, yet the concentration of lead on the hot gas ®lter ®nes is only 7.5 times that in the bed char. Helble's observations support this agreement with the very high mobility predicted for lead in this work. The similarity of arsenic and lead mobilities support the view that arsenic is relatively mobile. Zinc mobility is predicted to be high, but not as high as 1
Con®dential information supplied by CTDD.
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Fig. 10. Deposition of condensed phases during cooling from gas phase at 1273 K: (a) chloride solutions and chlorides, (b) matte, sulphides and borates, (c) deposition of chlorides predicted using model alkali chloride melt with trace solution.
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that of lead. Helble's work agrees in predicting 58% mobilisation. However, the results for the ABG are at variance with these ®ndings, with 75% of the zinc retained in the bed char. There is no clear reason for this discrepancy, which when converted to enrichment implies that the bed char concentration of zinc is more than three times that of the primary cyclone and more than four times that of the hot gas ®lter ®nes. Copper is predicted to be enriched in the bed char even more than zinc, with more than ten times the concentration in the char as in the hot gas ®lter ®nes and nearly twice that in the primary cyclone. However, in the case of copper, the predicted mobility is low, so that the results are consistent, although the enrichment in the bed char is outside the scope of this predictive study. Manganese transfer to the hot gas ®lter of the air blown gasi®er translates to slight depletion, and that to the primary cyclone corresponds to even more depletion. Helble considers manganese to be virtually immobile. These results are both consistent with the predictions, which indicate less than 1% of manganese is mobilised. The results for chromium also correspond to depletion in the cyclone ®nes by a factor of three by comparison with the hot gas ®lter ®nes and bed char. Observation of chromium in the fuel gas makes the signi®cance of these results uncertain, since it is considered some contribution due to corrosion of the materials used in constructing the plant may have occurred. Helble considered chromium mobility to be low. This work predicts low mobility in agreement with the experimental ®ndings. The ABG results for nickel are incomplete. Helble observed little mobilisation of nickel, which is consistent with the present predictions. 6. Concluding remarks 1. Modelling of trace element partitioning in ¯uidised beds under gasi®cation conditions requires use of databases which include models of solutions which reproduce trace element dissolution. The predictions indicate that inhomogeneity in the bed leads to signi®cant variation in equilibrium partitioning. 2. The predicted fate of the some of the trace elements carried over in the product gas is markedly altered when the potential for solution in a chloride melt is acknowledged.
Acknowledgements The authors are grateful to BCURA for provision of a grant in support of this work and to Robin Irons (Powergen) and John Sharp (University of Shef®eld) for many helpful discussions.
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References [1] Welford GB, Marshall AR. The pressurised gasi®er in the air blown gasi®cation cycle. Third International Conference on Combustion Technologies for a Clean Environment, paper 32.1, 1995 July 3±6, Lisbon. p. 1±6. [2] Bale CW, Thompson WT, Pelton AD, Erikkson G, Talley P, MelancËon J. Recent developments in the F*A*C*T system, Computer software in chemical and extractive metallurgy. Quebec City: CIM Conference of Metallurgists, 1993. [3] Thompson D, Argent BB. The use of thermodynamic computation packages to predict reactions occurring during combustion and gasi®cation in ¯uidised beds. Third International Conference on Combustion Technologies for a Clean Environment, paper 16.1, 1995, Lisbon. p. 1±10. [4] Spears DA, Sharp JH, Thompson D, Argent BB. Predictions of phases present in ¯y ash, their composition and the in¯uence of these factors on its utility and disposal, Second International Conference on Combustion and Emissions Control. London: Institute of Energy, 4±5 December 1995. p. 71±88. [5] Thompson D, Argent BB. Prediction of coal ash composition as a function of feedstock composition. Fuel 1999;78:539±48. [6] Thompson D, Argent BB. The mobilisation of sodium and potassium during coal combustion and gasi®cation. Fuel 1999;78(14):1679±89. [7] Thompson D, Argent BB. Thermodynamic equilibrium study of trace element mobilisation under pulverised fuel combustion conditions, to be published in Fuel. [8] Thompson D, Argent BB. Prediction of the distribution of trace elements between the product streams of the Prenfo gasi®er and comparison with reported data, to be published in Fuel. [9] Delay I, Swithenbank J, Argent BB. Prediction of the distribution of alkali and trace elements between the condensed and gaseous phases generated during clinical waste incineration. In: Proceedings of the Second International Symposium on Incineration and Flue Gas Treatment Technologies. Flue Gas Treatment, Session 5, 4±6 July 1999; Shef®eld. p. 14 (I Chem E, 1999). [10] Goto K, Argent BB, Lee WE. Corrosion of MgO±MgAl2O4 spinel refractory bricks by calcium aluminosilicate slag. J Am Ceram Soc 1997;80(2):461±71. [11] Lumsden J. Thermodynamics of molten salt mixtures. London: Academic Press, 1966. [12] Martinez-Tarazona MR, Spears DA. The fate of trace elements and bulk minerals in pulverised coal combustion in a power station. Fuel Process Technol 1996;47:79±92. [13] Spears DA, Martinez-Tarazona MR. Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England. Int J Coal Geol 1993;22:1±20. [14] Spears DA, Manzanares-Papayanopoulos LI, Booth CA. The distribution and origin of trace elements in a UK coal; the importance of pyrite. Fuel 1999;78(14):1671±8. [15] Argent BB, Jones K, Kirkbride BJ. Vapours in equilibrium with glass melts, The industrial use of thermochemical data, Sp Pub 34. Chemical Society, 1980. p. 379±90. [16] Helble JJ, Mojtahedi W, Lyyranen J, Jokiniemi J, Kauppinen E. Trace element partitioning during coal gasi®cation. Fuel 1996;75(8):931±9. [17] Querol X, Fernandez-Turiel JL, Lopez-Soler A. Trace elements in coal and their behaviour during combustion in a large power station. Fuel 1995;74(3):331±43. [18] Smith RD. The trace element chemistry of coal during combustion and the emissions from coal-®red plants. Prog Energy Combust Sci 1980;6:53±119.
www.fuel®rst.com
Thermodynamic equilibrium study of trace element mobilisation under air blown gasi®cation conditions B.B. Argent, D. Thompson* Department of Engineering Materials, University of Shef®eld, Sir Robert Had®eld Building, Mappin Street, Shef®eld S1 3JD, UK Received 22 December 2000; revised 17 May 2001; accepted 13 June 2001
Abstract Thermodynamic equilibria have been examined for several trace elements at levels found in coal that might be used in the air blown gasi®er air (ABG). The results obtained with the factwin suite of programs were used to test the degree to which predictions are consistent with observed partitioning. Conditions of lime-free gasi®cation, gasi®cation with the design limestone addition and limestone addition at a higher fuel-lime ratio have been examined for 20 atm. pressure over a range of temperatures. The effects of varied sulphur and chlorine levels have also been examined. Curtailment of the ABG program means that the range of experimental data does not allow deep comparison of predictions with observations but in general there is broad agreement. It is concluded that the results form the basis of a method of representing the potential for mobilisation (low to standard limestone) from coal particles followed by reincorporation into condensed phases (standard to high limestone) during the in-bed processes. Simulation of cooling of the equilibrium gas phase at 1273 K in isolation by 10 K steps predicts incorporation of a range of elements into a matte, and others into an alkali-based chloride melt. q 2001 Published by Elsevier Science Ltd. Keywords: Thermodynamics; Trace elements; Gasi®cation
1. Introduction A method of predicting trace element mobility under the excess air, atmospheric pressure combustion conditions of the utility boiler has been regarded as highly desirable for many years. Modern gasi®cation systems operate under elevated pressures of 1±2 MPa (10±20 atm), which makes experimental measurements of trace element concentrations inside gasi®ers even more dif®cult than in utility boilers, hence the means to predict trace element mobility is just as desirable as under combustion conditions, if not more so. The air blown gasi®er, which is the subject of this investigation, was designed to operate at 1±2 MPa pressure and supply gas to a turbine [1]. The potential for transfer of even minute amounts of some trace elements to the blades of a turbine over the course of thousands of hours running requires evaluation. The meaningful description of trace element release using kinetic models for as complex a system as coal gasi®cation is not possible. There are too many unknowns. Instead, this work aims to provide thermodynamic equilibria for models of the gasi®er coal±air±lime * Corresponding author. Tel.: 144-114-2225498; fax: 144-1142225943. E-mail address: dennis.thompson@shef®eld.ac.uk (D. Thompson). 0016-2361/02/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0016-236 1(01)00112-0
feed. It should be possible to perturb these models, when suf®ciently complete, to simulate the consequences of, say, addition of the trace element in question to the gasi®er reactor as part of the organic matrix, whereas its main host under high temperature equilibrium is the oxide melt. In this instance a model including trace solution rate is required. In associated papers we have described the use of the fact [2] computational thermodynamic program to examine major coal component distributions in ¯uidised bed combustion [3], the effect of coal mineral content variation on the product ash [4,5], alkali metal partitioning under combustion and gasi®cation conditions [6], the distribution of trace elements in pulverised fuel combustion [7], and their fate in the Pren¯o slagging gasi®er [8]. Delay et al. have also investigated the equilibria for the conditions of the incineration of waste using fact [9]. The importance of inclusion of melts and solid solutions in the models of the systems has been noted, and it has been found that although there are signi®cant gaps in the thermodynamic data, particularly in the representation of solution of trace elements in melts, signi®cant information about the elementary distributions can be obtained from such calculations. fact version 2.1 was used in the studies of ¯uidised bed combustion, the effect of feedstock composition, and alkali
76
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
metal partitioning. It had restrictions on the melt models, in some cases imposed in order to protect the data from alteration or addition (oxide and salt melt data) and in the case of the sulphide melt model, accessible for alteration but too dif®cult to re-model adequately for any attempt at ad hoc extension. The factwin 3.05 version of the program, which has been used in this and the companion study of combustion conditions, allows use of the oxide melt data outside the composition ranges for which it has been fully optimised and the sulphide melt has been extended by the suppliers of the software. These extensions in the modelling facilities have considerable in¯uence on the value of the predictions. 2. Computational model Under ¯uidised bed gasi®cation conditions (in which we refer to the air blown gasi®er concept timescales and temperatures) the time available for particle±gas interaction is greater than under pulverised fuel combustion or slagging gasi®er conditions, whilst the maximum temperature is lower. In both cases substantial inhomogeneities will remain in the products, but the nature of the inhomogeneities will be very different. The most clear-cut inhomogeneity in the case of ¯uidised bed gasi®cation is the presence of limestone particles. We have examined the consequences of this inhomogeneity by considering the equilibrium at bed temperature for standard limestone addition, raised limestone addition and zero limestone addition. In modelling pulverised fuel combustion all minerals with complex structure are assumed to be decomposed, and only the simpler species can be subsequently formed. For the air blown gasi®er the longer residence times and lower peak temperatures suggest this assumption cannot be made, and not only have complex compounds been included in computations but those solid solutions which are most stable have been selected in the course of trial calculations. (The precalculations are necessary because of limitations on the number of solutions, which can be included in any one dataset.) Earlier versions of the fact database included a slag model which includes all major coal mineral oxides, and other models which included one or more trace elements but excluded one or more of the major coal mineral oxides: most seriously, potassium is omitted from all these models. This led to complex computational procedures to adjust for incorrect slag amounts, especially in the lower part of the slag-formation temperature range, and consequent uncertainties in partitioning results. Where the reduced slag model did not predict any slag in the lower temperature range, no predictions for the trace element at all could be made. The fact for Windows revision of the software allows computations using a slag model referred to as SLAG?. This model allows modelling of a slag which includes sets of cations and anions included in the restricted models but for which not all interactions are known. Some uncertainty in the partitioning of the trace
elements concerned is inevitable under these circumstances but the results are likely to be more accurate than for restricted models. Use of the SLAG? model has found to be restricted in practice to the inclusion of 17, or at most, 18 elements in the computational set if immiscibility gaps and the presence of additional anions in the melt are to be examined. If it can be ascertained by trial calculations that the contribution due to additional anions is insigni®cant, and there is no miscibility gap, then up to 23 elements can be included in the database. It has been found during extensive trials that dif®culty in convergence is associated with the number of components involved in the oxide melt. It was found to be possible to model the solution of one trace element in the melt at a time with a miscibility gap and the remainder of the system involving 23 elements. This technique was used in modelling pulverised fuel combustion, where immiscibility is clearly indicated [7]. It was also used in earlier calculations for gasi®cation and it was found that immiscibility was observed when no lime was added. Further calculations have combined with improvements in the mineral solution models provided with fact to indicate there is only a slight tendency to immiscibility at most, and the results presented here use the later model and immiscibility is excluded from consideration. The later version of fact is provided with a revised and more extensive matte model than the previous version, inasmuch as nickel solution is incorporated, but this model does not allow oxygen solution in the matte to be examined. A model of the Fe±S±O±Mg±Mn system (restricted to high FeS content) allows a study of the early stages of the conversion of pyrite to haematite which is considered important, but the oxygen potential permissible is limited. The spinel model provided with fact has been found to give poor agreement with an invariant point for co-existence of an oxide melt with periclase, monticellite, spinel and merwinite in the CaO±Al2O3 ±SiO2 ±MgO system, and a model using the SGTE data for the spinel [10] which has closer agreement is provided. At lower temperatures, trace element solution in any salt melts formed cannot be examined using the existing database, which is protected against modi®cation. The literature provides suf®cient information, and the interactions in the sodium±potassium chloride melt are suf®ciently regular, for us to develop embryonic databases for sodium plus potassium chloride melts with some trace element solution [11] which was used to con®rm signi®cant solution of trace element in the chloride melt under gasi®er conditions. The database includes interaction data for KCl/NaCl, CuCl/ NaCl, ZnCl2/KCl, CaCl2/NaCl, PbCl2/KCl, PbCl2/NaCl and PbCl2/CaCl2. Other interactions were assumed to be ideal. A more extensive database was developed, and is used in a subsidiary cooling study reported in this paper. The actual temperature at which processes are assumed frozen is 1300 K under combustion [4]. The selection of this temperature was made on the basis that the corresponding equilibrium product compositions almost matched those observed in practice and it seems likely that a similar
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
temperature is appropriate for ABG, hence the nominal operating temperature, 10008C 1273 K is used. To allow comparisons with earlier work, calculations were made for a coal corresponding to that used at Eggborough power station and studied in detail by Spears and MartinezTarazona [12±14]. The elements included in the present computations are the organics and other major non-metals (C, H, N, O, S, Cl), major and minor metals and non-metals (Ca, Mg, Na, K, Fe, Al, Si, Ti, P, Mn) and a range of trace elements (Pb, As, Zn, Cu, Ni, Cr). The elemental analyses input to the computer program, for high limestone, low limestone and limestone-free gasi®er conditions (Table 1) included limestone, air and steam injection rates determined on an arbitrary basis to lie within the ranges used in pilot scale operation [3]. Under normal operating conditions it was assumed that limestone was added at a constant molar ratio of calcium to sulphur of 2.133:1, air at an O:C ratio of 0.676 and steam injection at a molar ratio of H2O: (O in the coal 1 O added as air) of 0.293. The high limestone addition corresponded to a Ca:S ratio of 7.5. 2.1. Elemental distributions at temperatures above 1273 K The range of variables affecting the mobility of any element under fuel rich conditions which could affect equilibrium in air-blown gasi®cation conditions are somewhat wider than for pf combustion in a utility boiler. On the other hand, the range of temperature is somewhat more restricted. In re®ning the model, there is a clear need for more emphasis on local equilibrium considerations, because of the larger particulate and lower temperatures involved. Here we examine the effects of temperature and amount of limestone added.
77
Air-blown gasi®ers operate at the order of 1273 K and temperature variation effects are examined primarily to note the effect of allowing hot-spots to arise but results of 2073 K are included for completeness. The ratio of limestone to fuel is chosen according to the requirements of sulphur adsorption and ¯uidised bed operation, but it is instructive to examine the consequences of equilibration at varied limestone to fuel ratios since inhomogeneities in the bed arise. We have examined the case of no limestone addition as representative of a region of purely coal particles: standard limestone Ð the overall equilibrium case: and `high limestone addition' at a rate corresponding to the equilibrium hydrogen sulphide level in the product gases at the stated operating temperature. This should give an insight into equilibration close to a limestone particle. Results for the gasi®er air addition level but with no additives at one atmosphere pressure are presented in a companion paper [7] where they are used to model an early stage of combustion. Sulphur and chlorine levels in the fuel vary, and in view of the potential for formation of volatile chlorides and sulphides which has been shown to exist for pf combustion and slagging gasi®cation [7,8], a separate study of the in¯uence of these two elements at varied levels has been undertaken. 3. Results 3.1. Effect of temperature and varied limestone addition rates Fig. 1(a)±(c) shows the variation in amount slag formed when no, standard and high limestone addition is used with
Table 1 Input compositions for simulation of no, standard and high limestone addition at high temperatures Element
High lime (mol)
Standard lime (mol)
No lime (mol)
Carbon Hydrogen Sulphur Chlorine Sodium Potassium Calcium Magnesium Iron Aluminium Silicon Oxygen Nitrogen Lead Arsenic Manganese Boron Phosphorus Chromium Titanium Nickel Copper Zinc
5.681426 9.124039 0.0668206 0.020485 0.00436396 0.0117427 5:01466 £ 1021 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 10.200448 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
5.322593 9.124039 0.0668206 0.020485 0.00436396 0.0117427 0.14702572 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 9.1239482 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
5.17996 9.124039 0.0668206 0.020485 0.00436396 0.0117427 0.0043927 6:71869 £ 1023 2:75834 £ 1022 8:28876 £ 1022 1:38742 £ 1021 8.6960500 20.263384 9:44889 £ 1026 1:30657 £ 1026 2:21839 £ 1024 1:00000 £ 1023 5:05752 £ 1024 6:27552 £ 1025 1:91942 £ 1023 4:86395 £ 1025 6:70103 £ 1025 3:49360 £ 1025
78
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
Fig. 1. Amount of slag formed under conditions of no, standard and high limestone addition.
temperature from 1273 to 2073 K. It is not possible to illustrate the variations in distribution of all the elements in detail, and all major elements except sodium are omitted. The strong variation in sulphur partitioning with amount of
lime added has clear implications for trace partitioning, via volatile sulphide formation and formation of matte, in which some trace elements will dissolve. Sulphur distribution is shown for the three levels of lime addition in Fig. 2.
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
79
Fig. 2. Distribution of sulphur under no, standard and high limestone addition conditions, 1273±2073 K.
Fig. 3 shows sodium distribution, and the distributions of boron, arsenic, lead, zinc, copper and nickel are shown in Figs. 4±9. Manganese mobility is not illustrated. It is immobile under all conditions studied (less than 1% in the gas phase), with the highest mobility at high temperature and with high limestone addition. Its immobilisation at low temperature with no and standard lime addition is in part due to spinel formation and in part due to solution in slag. As might be expected, the total amount of slag increases with limestone addition and increases rapidly with tempera-
ture over 1273±1673 K. Sometimes complex variation in the percentage of the trace elements volatilised as a function of temperature can be understood in terms of the competition between the formation of matte, slag and spinel with the solution of trace elements, and natural tendency for increased volatilisation with temperature. 3.2. Effect of varied chlorine and sulphur addition Studies of the effects of chlorine and sulphur concentration variation under pf combustion conditions both before
80
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Fig. 3. Distribution of sodium under no, standard and high limestone addition conditions, 1273±2073 K.
and after attainment of the ®nal air level, and for slagging gasi®cation have been presented elsewhere, and show the in¯uence of these elements in the 1500±1600 K temperature range. We have attempted the same exercise for air blown gasi®cation for the temperature range of interest in air blown gasi®cation i.e. 1273 K and slightly above. Sulphur and chlorine levels were varied from zero to twice the basis level used in the main calculations. The results are subject to uncertainties in detail because of the sensitivity of the amount of matte, which controls mobility of several elements, to the exact model of the system. Limited
changes in mobility are noted because almost all elements are either highly mobile or highly immobile under these conditions. Nickel and copper are mainly immobilised because even in the absence of matte, the metal is predicted to be formed. Manganese immobilisation is mainly due to spinel formation, which is independent of the amounts of S and Cl. Lead and zinc are highly mobile, and this mobility is largely due to the formation of the metallic vapour, which is independent of S and Cl amounts. The exception is arsenic. Arsenic is intermediate between copper and nickel, and zinc and
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
81
Fig. 4. Distribution of boron under no, standard and high limestone addition conditions, 1273±2073 K.
lead, in its tendency to become incorporated in matte, but only a small amount of arsenic is dissolved in the oxide melt in the absence of matte. It is therefore normally mobile except where matte is formed, and when a large amount of matte is formed as at 1273 K with standard lime addition, much of the arsenic is normally immobilised. However, when sulphur is absent, because of the small amount of slag formed, arsenic is virtually fully mobilised under these conditions. 3.3. Effect of additional anions in the oxide melt Additional anions including sulphide, sulphate, chloride,
carbonate and hydroxyl group can be included in the oxide melt model. Trial calculations using the fact fully optimised models have shown that for no limestone addition and standard limestone addition compositions the amounts of these anions present in the melt are suf®ciently small for them to be neglected, except in the case of sulphide, for which the contributions are small but not negligible. Typical results obtained using SOLN-SLAG? at standard limestone addition rate, are that at 1623 K, 84.9% copper is dissolved in the slag as Cu2O, and an additional 0.504% is dissolved as Cu2S. Similarly, nickel is dissolved as 97.8% NiO and 0.582% NiS, which becomes 93.9% NiO and 0.181% NiS at 1373 K.
82
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Fig. 5. Distribution of arsenic under no, standard and high limestone addition conditions, 1273±2073 K.
At high limestone addition level and 1273 K the amount of sulphide in the slag is predicted to be over 29%, very close to the upper limit of con®dence set by the software developers at 30%. At higher temperatures the amount of sulphide falls progressively. 4. Discussion The results show the addition of lime at standard rate has a signi®cant effect on the equilibrium distribution of some elements by comparison with no lime addition, and the
addition of high lime level alters them further. The effects are seen in the composition of the condensed phases present, the distribution of sulphur, and the extent to which various elements are mobilised. 4.1. General variation in condensed phases formed with amount of lime added The addition of limestone increases the amount of oxide melt formed at high and low temperatures (Fig. 1). An exception occurs in the region of 1400 K, where standard limestone addition leads to a slightly lower amount of oxide
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
83
Fig. 6. Distribution of lead under no, standard and high limestone addition conditions, 1273±2073 K.
melt than no lime addition. On going from no limestone to standard and high limestone the minerals formed at low temperatures change from SiO2 (tridymite), KAlSi2O6 (leucite), Fe2Al4Si5O18 (ferrocordite), CaAl2Si2O8 (anorthite), (FeO)(TiO2) (ilmenite), Ca3(PO4)2 and FeCr2O4 to Ca2SiO4 (alpha prime), CaO (lime), Ca3Al2O6, Ca3Ti2O7 and Ca5HO13P3 (hydroxyapatite). At higher temperatures the hydroxyapatite is replaced with calcium phosphate. Mellilite solution B appears at the lowest 300 degrees studied for
standard limestone but not at high limestone addition or with no limestone added. Matte is predicted to form both when no lime is added and when it is added at the standard rate. The maximum amount is formed at 1273 K, and this traps several percent of the sulphur when no lime is added (Fig. 2), and over ten percent at standard lime addition rate. There is a marked change on moving to high lime addition rate, where much more sulphur is trapped in the slag than is present in the gas
84
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
Fig. 7. Distribution of zinc under no, standard and high limestone addition conditions, 1273±2073 K.
phase, in contrast to the 1% trapped at standard lime addition rate, and even less when no lime is added at 1400 K and above. 4.2. Mobility of the elements The elements arsenic, lead and zinc (Figs. 5±7) are highly mobile over the whole range of conditions studied: the limited immobilisation of these elements is dominated by solution in the matte phase at the lowest temperatures. Copper and nickel (Figs. 8 and 9) are not only strongly immobilised by matte at the lowest temperatures, but as the amount of matte diminishes with rising temperature oxide melt takes over this role. At high lime levels the
oxide melt immobilises most of the copper and nickel at 1273 K. The mobility of these two elements at the higher temperatures examined becomes signi®cant. Sodium mobility (Fig. 3) rises with the amount of limestone present, which re¯ects its enhanced activity coef®cient in high calcium melts. The effect of increasing calcium on alkali mobility has been noted elsewhere [15]. Potassium follows the same trend, but in addition its immobilisation in the absence of lime and at low temperatures when standard lime is added re¯ects the formation of signi®cant amounts of leucite. The mobility of boron at no and standard lime addition levels decreases at ®rst as the temperature rises and more is dissolved in the melt, then increases with a further rise in temperature, but the majority is still
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
85
Fig. 8. Distribution of copper under no, standard and high limestone addition conditions, 1273±2073 K.
trapped in the melt at 2100 K with high lime addition (Fig. 4). Phosphorus (not illustrated) is highly volatile without lime addition above 1273 K, but effectively immobilised when lime is added. The remaining elements, aluminium, manganese, calcium, silicon, titanium, iron and chromium, show very limited mobility. 5. Cooling studies Calculation of the equilibrium distribution at 1273 K allows derivation of the gas phase composition which passes to the outlet duct and cools as it travels downstream. The
gases containing trace elements, which leave the combustor or gasi®er contain particulates, which we assume in this study act purely as nuclei for condensation from the gas phase. Calculations are carried out in 10 K steps, which eliminates the possibility that formation of a condensed phase which has a short temperature range of stability is missed. The calculations were carried out with a database including the fact matte (database solution MATT) and alkali salt model (database solution SALT). The SALT model does not include trace element solution. A subsidiary set of calculations has been carried out using a model of alkali chloride with trace element solution, which we have developed. A brief study of the solution of iron and
86
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
Fig. 9. Distribution of nickel under no, standard and high limestone addition conditions, 1273±2073 K.
magnesium using the database solution SOLN-SAL3 of fact 3.05 indicated that at the highest temperature of alkali chloride melt formation 93% of the magnesium was dissolved. The predictions for magnesium deposition at lower temperatures are therefore a result of the limitations of the model. A more extensive study of trace element solution in the chloride melt has been made using a database developed by one of us (DT), which allows solution of Ca, Mg, Mn, Fe, Zn and Pb as chlorides in the alkali chloride host. The input mixture for the case illustrated here was obtained from the standard limestone addition case given in Table 1. The calculations were carried out using the equilibrium gas phase predicted for 1273 K (Table 2). It
was noted that whereas complete equilibration of the gas phase would require deposition of solid carbon, and the formation of methane, because of kinetic hindrance, these equilibria are not attained. Calculations for the standard gasi®cation conditions have been carried out for (i) full equilibrium; (ii) equilibrium in which the carbon deposition indicated is ignored at each stage; (iii) equilibrium in which methane formation is ®xed at the initial level at 1273 K. The exclusion of methane formation requires the assumption that carbon-containing species are frozen at the 1273 K equilibrium level. Results using the three models were for practical purposes, identical. The results of the simulations for standard limestone
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89 Table 2 Composition of gas phase at 1273 K transferred to cooling calculations for standard lime addition Element
Transferred to cooling calculation (mol)
Carbon Hydrogen Oxygen Nitrogen Chlorine Magnesium Sodium Boron Sulphur Iron Copper Nickel Zinc Lead Arsenic Manganese Calcium Potassium
5.322593 9.124039 8.5292 20.263 2.04850 £ 10 22 8.06243 £ 10 210 7.19486 £ 10 24 7.90775 £ 10 24 5.44704 £ 10 22 1.49290 £ 10 25 1.35980 £ 10 210 6.72920 £ 10 213 3.08590 £ 10 25 6.98500 £ 10 26 4.44440 £ 10 27 5.47468 £ 10 26 2.74937 £ 10 22 3.72983 £ 10 23
addition with carbon equilibrated are summarised in Fig. 10(a) and (b). Fig. 10(c) shows a replication of the results for the temperature range where salt melt is formed, using the trace-dissolving melt in place of SOLN-SALT. 5.1. Discussion of results for cooling The early stages of cooling are seen to be dominated by deposition of a matte phase, which dissolves all the nickel and copper carried over in the vapour. A very small amount of zinc and in®nitesimal amounts of lead and arsenic are also incorporated in the matte. At 1173 K matte formation is replaced by FeS deposition and MnS deposition begins at the same time. At 10 K lower, alkali melt deposition begins and then is replaced by a potassium chloride±sodium chloride solid solution at 140 K lower temperature. Deposition of magnesium borate is predicted to commence at 1533 K, but as commented earlier, this is an artefact of the limited model. At 963 K calcium chloride formation is predicted (50 K below single step where a very small amount of calcium borate is predicted to be deposited). Fig. 10 shows that calcium is, like magnesium, strongly retained in an alkali chloride melt and, like the formation of magnesium borate, calcium chloride deposition is a consequence of simpli®cation. Zinc sulphide deposition begins at 1083 K and continues down to the lowest temperature examined, 493 K. Arsenic sulphide deposition begins at 653 K. Lead is deposited, ®rst as the sulphide at 793 K, then as the chloride, with an initial deposition temperature of 593 K. A small amount of iron is deposited as potassium iron chloride below 673 K. The calculations using the alkali chloride melt with trace
87
solubility con®rms the rapid loss of magnesium into solution. The amount of calcium in the gas phase also becomes in®nitesimal over a short temperature interval. Manganese is eventually, effectively removed from the gas phase and much of the iron is removed before melt formation ceases. Zinc in the gas phase is reduced to about one third of the original amount, but lead is only slightly dissolved. 5.2. Comparison of computations and experimental observation for gasi®er conditions Comparisons for gasi®cation conditions are constrained by the limited amount of experimental data available. The objective of this study was the simulation of trace element behaviour in the Air Blown Gasi®er (ABG) operated by British Coal Technology Development Division. 1 The speci®cation of the gasi®er called for the use of limestone in the bed, but trials in which trace element measurements were reported and limestone was added were con®ned to biomass-containing combustibles, which have trace element associations far from those of coal. The nearest approach to the speci®ed operating condition of the gasi®er was a nominal bed temperature of 1233 K (9608C), with a peak temperature of 1266 K using Daw Mill coal. The operating pressure was 18 atm, and limestone addition was omitted. This work is not available in the open literature. Results for the partitioning of trace elements between bed char, primary (hot gas, unstated temperature) cyclone ®nes, hot gas ®lter ®nes (853 K) and cleaned fuel gas are reported, and can be converted to relative concentrations of trace element in the three solid phases. Helble et al. [16] have studied a laboratory scale gasi®er in which Illinois No. 6 coal was dropped through a tube furnace with gasi®cation temperature of the order of 1450 K, at atmospheric temperature and with no limestone addition. 5.3. Comparison of observation with prediction for each element Following the approach of Martinez-Tarazona and Spears [12], Querol et al. [17] and Smith [18], in comparing the concentrations of trace elements in pf ash samples, it is noted that the hot gas ®lter ®nes of the ABG contained more than 6 times the concentration of arsenic in the bed char, which is consistent with considerable volatilisation as predicted in this study. It is remarked that the experimental evidence for high mobility of lead inasmuch as it is observed in the cleaned fuel gas of the ABG is very strong, yet the concentration of lead on the hot gas ®lter ®nes is only 7.5 times that in the bed char. Helble's observations support this agreement with the very high mobility predicted for lead in this work. The similarity of arsenic and lead mobilities support the view that arsenic is relatively mobile. Zinc mobility is predicted to be high, but not as high as 1
Con®dential information supplied by CTDD.
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B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
Fig. 10. Deposition of condensed phases during cooling from gas phase at 1273 K: (a) chloride solutions and chlorides, (b) matte, sulphides and borates, (c) deposition of chlorides predicted using model alkali chloride melt with trace solution.
B.B. Argent, D. Thompson / Fuel 81 (2002) 75±89
that of lead. Helble's work agrees in predicting 58% mobilisation. However, the results for the ABG are at variance with these ®ndings, with 75% of the zinc retained in the bed char. There is no clear reason for this discrepancy, which when converted to enrichment implies that the bed char concentration of zinc is more than three times that of the primary cyclone and more than four times that of the hot gas ®lter ®nes. Copper is predicted to be enriched in the bed char even more than zinc, with more than ten times the concentration in the char as in the hot gas ®lter ®nes and nearly twice that in the primary cyclone. However, in the case of copper, the predicted mobility is low, so that the results are consistent, although the enrichment in the bed char is outside the scope of this predictive study. Manganese transfer to the hot gas ®lter of the air blown gasi®er translates to slight depletion, and that to the primary cyclone corresponds to even more depletion. Helble considers manganese to be virtually immobile. These results are both consistent with the predictions, which indicate less than 1% of manganese is mobilised. The results for chromium also correspond to depletion in the cyclone ®nes by a factor of three by comparison with the hot gas ®lter ®nes and bed char. Observation of chromium in the fuel gas makes the signi®cance of these results uncertain, since it is considered some contribution due to corrosion of the materials used in constructing the plant may have occurred. Helble considered chromium mobility to be low. This work predicts low mobility in agreement with the experimental ®ndings. The ABG results for nickel are incomplete. Helble observed little mobilisation of nickel, which is consistent with the present predictions. 6. Concluding remarks 1. Modelling of trace element partitioning in ¯uidised beds under gasi®cation conditions requires use of databases which include models of solutions which reproduce trace element dissolution. The predictions indicate that inhomogeneity in the bed leads to signi®cant variation in equilibrium partitioning. 2. The predicted fate of the some of the trace elements carried over in the product gas is markedly altered when the potential for solution in a chloride melt is acknowledged.
Acknowledgements The authors are grateful to BCURA for provision of a grant in support of this work and to Robin Irons (Powergen) and John Sharp (University of Shef®eld) for many helpful discussions.
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References [1] Welford GB, Marshall AR. The pressurised gasi®er in the air blown gasi®cation cycle. Third International Conference on Combustion Technologies for a Clean Environment, paper 32.1, 1995 July 3±6, Lisbon. p. 1±6. [2] Bale CW, Thompson WT, Pelton AD, Erikkson G, Talley P, MelancËon J. Recent developments in the F*A*C*T system, Computer software in chemical and extractive metallurgy. Quebec City: CIM Conference of Metallurgists, 1993. [3] Thompson D, Argent BB. The use of thermodynamic computation packages to predict reactions occurring during combustion and gasi®cation in ¯uidised beds. Third International Conference on Combustion Technologies for a Clean Environment, paper 16.1, 1995, Lisbon. p. 1±10. [4] Spears DA, Sharp JH, Thompson D, Argent BB. Predictions of phases present in ¯y ash, their composition and the in¯uence of these factors on its utility and disposal, Second International Conference on Combustion and Emissions Control. London: Institute of Energy, 4±5 December 1995. p. 71±88. [5] Thompson D, Argent BB. Prediction of coal ash composition as a function of feedstock composition. Fuel 1999;78:539±48. [6] Thompson D, Argent BB. The mobilisation of sodium and potassium during coal combustion and gasi®cation. Fuel 1999;78(14):1679±89. [7] Thompson D, Argent BB. Thermodynamic equilibrium study of trace element mobilisation under pulverised fuel combustion conditions, to be published in Fuel. [8] Thompson D, Argent BB. Prediction of the distribution of trace elements between the product streams of the Prenfo gasi®er and comparison with reported data, to be published in Fuel. [9] Delay I, Swithenbank J, Argent BB. Prediction of the distribution of alkali and trace elements between the condensed and gaseous phases generated during clinical waste incineration. In: Proceedings of the Second International Symposium on Incineration and Flue Gas Treatment Technologies. Flue Gas Treatment, Session 5, 4±6 July 1999; Shef®eld. p. 14 (I Chem E, 1999). [10] Goto K, Argent BB, Lee WE. Corrosion of MgO±MgAl2O4 spinel refractory bricks by calcium aluminosilicate slag. J Am Ceram Soc 1997;80(2):461±71. [11] Lumsden J. Thermodynamics of molten salt mixtures. London: Academic Press, 1966. [12] Martinez-Tarazona MR, Spears DA. The fate of trace elements and bulk minerals in pulverised coal combustion in a power station. Fuel Process Technol 1996;47:79±92. [13] Spears DA, Martinez-Tarazona MR. Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England. Int J Coal Geol 1993;22:1±20. [14] Spears DA, Manzanares-Papayanopoulos LI, Booth CA. The distribution and origin of trace elements in a UK coal; the importance of pyrite. Fuel 1999;78(14):1671±8. [15] Argent BB, Jones K, Kirkbride BJ. Vapours in equilibrium with glass melts, The industrial use of thermochemical data, Sp Pub 34. Chemical Society, 1980. p. 379±90. [16] Helble JJ, Mojtahedi W, Lyyranen J, Jokiniemi J, Kauppinen E. Trace element partitioning during coal gasi®cation. Fuel 1996;75(8):931±9. [17] Querol X, Fernandez-Turiel JL, Lopez-Soler A. Trace elements in coal and their behaviour during combustion in a large power station. Fuel 1995;74(3):331±43. [18] Smith RD. The trace element chemistry of coal during combustion and the emissions from coal-®red plants. Prog Energy Combust Sci 1980;6:53±119.