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Energy & Fuels 2002, 16, 956-963
Trace Element Emissions from Co-combustion of Secondary Fuels with Coal: A Comparison of Bench-Scale Experimental Data with Predictions of a Thermodynamic Equilibrium Model B. B. Miller, R. Kandiyoti, and D. R. Dugwell* Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, Prince Consort Road, London SW7 2BY, U.K. Received February 4, 2002
Trace element emissions from the co-combustion of coal with biomass and waste secondary fuels have been measured, under conditions relevant to commercial fluidized bed combustors, using a novel, bench-scale, suspension-firing reactor. Experiments have been conducted using two coals (one Polish, one Colombian), four biomass fuels (wood-bark, straw, pulp sludge, and paper sludge), and three waste fuels (agricultural waste, sewage sludge and plastic waste). Concentrations of eighteen trace elements have been measured in these raw fuels and a variety of combustion and co-combustion ashes, using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), plus an atomic absorption based mercury determination devise. The influence of chlorine and sulfur on trace element release from combustion has been tested also, in the case of wood-bark, by injecting first HCl and then SO2 into the reactor during combustion. Experimental data have been compared with the predictions of a thermodynamic equilibrium model throughout this study. The Metallurgical and Thermochemical Databank (MTDATA) Gibbs free energy minimization software has been used to predict the speciation of individual trace elements. The trace elements have been ranked according to their average retention in combustion ashes, the most volatile being Hg and Se, followed by Cd, Tl, Pb, and As. Potentially problematic trace element emissions have been noted in certain cases, e.g., Cu and Zn from wood-bark, As and Pb from Polish coal, and Cd and Hg from sewage sludge. The injection of HCl served to decrease the retention by ash of the elements Cd, Cu, Zn, Mn, and Ba, while injection of SO2 increased the retention by ash of As and Hg, but decreased that of Cd.
Introduction Combustion of biomass and wastes is an increasingly important option, both as an environmentally friendly means of disposal and as an economically attractive means of raising power or process heat. However, the properties (composition, moisture content, calorific value) of many nonconventional fuels show wide variations; in addition, there are problems with seasonal variability in supply. Thus, coal is proposed as an auxiliary fuel, to stabilize firing conditions, avoid operational problems, and overcome seasonal supply difficulties. Operational and environmental problems encountered during cofiring operations have recently been investigated in several European Union funded research projects.1-3 As a * Corresponding author. E-mail:
[email protected]. Fax: +44 20 7594 5604. (1) European Union Joule III Clean Coal Technology R &D Programme. Development of Improved Solid Fuel Gasification Systems for Cost-effective Power Generation with low Environmental Impact and Advanced Cycle Technologies. Final Report Volume III; 1999 [EUR 19285/III EN]. (2) European Union Joule III Clean Coal Technology R &D Programme. Operational Problems, Trace Emissions and Byproduct Management for Industrial Biomass Co-Combustion. Final Report Volume V; 1999 [EUR 19285/V EN].
result, significant differences have been noted between the behavior patterns of individual trace elements in co-combustion: for instance, the mode of occurrence of the trace element in the fuel has emerged as an important factor, as have related factors such as the levels of concentration of minor elements, such as sodium, potassium, calcium, sulfur, chlorine, and phosphorus. The present study3 has focused on the fate of trace element inventories when coal, plus a range of secondary fuels, have been co-combusted under fluidized conditions (at temperatures of 800 and 900 °C). Experimentation at the bench-scale has been supplemented by thermodynamic equilibrium modeling in the study. The main propose has been to attempt to identify fuel combinations, and firing conditions, most likely to lead to excessive discharge of harmful elements to the atmosphere, plus any potentially synergistic effects. The principal results of this study are summarized here in terms of the propensity for release of 18 individual elements. (3) European Union Joule III Project. Reduction of Toxic Emissions from Co-Combustion of Coal, Biomass and Waste in Fluidised Beds. Final Report; 2001 [JOR 3-CT97-0191].
10.1021/ef0200065 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002
Co-combustion of Secondary Fuels with Coal
Energy & Fuels, Vol. 16, No. 4, 2002 957 potential interference from trace elements contained in any bed solids added. A detailed description of this reactor is available elsewhere.4,5 Trace element analyses of fuel and ash samples have been done using inductively coupled plasma mass spectrometry (ICP-MS). Additional measurements have been made by inductively coupled plasma-atomic emission spectroscopy (ICPAES), mainly for Mn, V, and Zn. Details of trace element analysis procedures, particularly the sample preparation methods, have been presented elsewhere.6,7 Assessment of the Hg content of ash residues has been accomplished separately using an atomic absorption based devise.8 Measurements of the percentage retention of trace elements in ash were in general subject to experimental error levels of at least (25%. Variations above or below this error range were encountered: the actual experimental error varied with the raw fuel combusted and the trace element measured. Experiments have been conducted using two coals (one Polish, one Colombian), four biomass fuels (wood-bark, straw, pulp sludge, and paper sludge) and three waste fuels (agricultural waste, sewage sludge, and plastic waste). Fine particles of fuel (100-200 µm) have been fed continuously to the quartz suspension-firing reactor which has been operated under atmospheric pressure, with an air:fuel ratio of approximately 1.2, at two different temperatures levels, 800 and 900 °C.
Thermodynamic Modeling
Figure 1. The suspension-firing reactor.
Experimental Technique The experimental study has used a novel suspension-firing reactor to simulate conditions within a small pocket of a fluidized bed combustor. The reactor design is based on a vertical quartz reactor tube, 1260 mm in length and 48 mm internal diameter, in which reacting solid particles (injected axially from the top) are suspended in an upward flow of hot reactant gas. The particles are constrained to circulate within a limited section of the quartz reactor tube by a quartz-sintered disk, providing an upper boundary, and a conical restriction, providing a lower boundary. The design is shown in outline in Figure 1. The reactor tube is located axially within a two zone electric furnace. The lower zone, 2 kW, heater serves to preheat the incoming reactant gas in the lower section of the quartz tube, beneath the conical constriction, to a prescribed reaction temperature. The upper, 1 kW, heater serves to off-set heat losses and maintain approximately isothermal conditions in the reaction section above the conical constraint. The reactor is fed with a monitored flow of cylinder gas through the conical base of the quartz tube; exhaust gases leave through a sidearm at the top end of the tube, above the quartz sintered disk which acts as a filter for particulates. Termination of the gas flow permits residual circulating solids to fall, by gravity, through the conical constriction, and down into the ash-pot. In the suspension-fired reactor, fuel samples may thus be combusted “in suspension” in an upward flow of air at a controlled temperature, thereby simulating the combustion of finely powdered solid fuels. In a typical bubbling fluidized bed combustor the fuel constitutes only a small proportion of the bed inventory (1-3%), the greater part being sand or sulfur sorbent (limestone or dolomite), plus fuel ash. One important feature of the suspension-firing reactor is the capability of attaining fuel particle densities comparable to those in fluidized bed combustion, without the carrying the inert bed solids loading. When the stated aim of the experiment is to determine trace element releases from combusting fuel, the absence of bed solids is a critical advantage, since it entirely eliminates
Thermodynamic equilibrium modeling of trace element partitioning behavior has been carried out using the MTDATA computer code developed by the National Physical Laboratory, Teddington, London. This code is based on the principle of Gibbs free energy minimization to calculate equilibrium species compositions for a variety of physical situations. Two particular modules have been used viz., ACCESS, the database search program, and MULTIPHASE, the module that performs Gibbs free energy minimization calculations for systems containing mixtures of condensed phases and gases. Several thermodynamic databases are supplied with MTDATA but only the Scientific Thermodata Group Europe (STGE) database has been used in this study. To reduce computer storage requirements, a subset of data for elements of interest for particular simulations has been transferred into the ACCESS module: all likely compounds of the individual elements of interest have been included each time. The MULTIPHASE module has then been run to simulate conditions over the temperature range from 900 to 1400 K in 20 K steps, at constant, atmospheric pressure with various system compositions. It is customary to conduct thermodynamic equilibrium studies of trace element behavior during combustion on the basis of one element at a time, to keep computing requirements to manageable proportions. Furthermore, in previous studies, each trace element of interest has been studied in conjunction with only the major elements present in the parent fuel: C, H, N, O, and S. However, some minor elements (which include, typically, Na, K, Ca, Mg, Fe, Al, Si, Cl, and P) are known to influence trace element behavior significantly by chemical interaction, e.g., chlorine often increases mobility of metals through formation of volatile chloride species. The transfer of all the data on major and minor elements into ACCESS, in (4) Miller, B. B. Ph.D. Thesis, University of London, London, 2001. (5) Miller, B. B.; Dugwell, D. R.; Kandiyoti, R. Fuel 2002, 81, 159171. (6) Richaud, R.; Lachas, H.; Healey, A. E.; Reed, G. P.; Haines, J.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 2000, 79, 1077-1087. (7) Lachas, H.; Richaud, R.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Analyst 1999, 124, 177-184. (8) Richaud, R.; Lachas, H.; Collot, A.-G.; Mannerings, A. G.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 359-368.
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Table 1. Trace Element Concentrations in the Raw Fuels fuel wood-bark straw Colombian coal Polish coal sewage sludge paper sludge agricultural waste pulp sludge plastic waste
less than 1 ppm As, Be, Cd, Co, Hg, Se, Sb, Tl As, Be, Cd, Co, Hg, Ni, Mo, Sb, Tl, V Be, Cd, Hg, Sb, Tl Cd, Hg, Mo, Tl Be, Hg, Tl As, Be, Cd, Co, Hg, Se, Tl As, Be, Cd, Co, Hg, Pb, Sb, Se, Tl As, Be, Co, Hg, Mo, Sb, Se, Tl As, Be, Hg, Se, Tl
1 to 10 ppm
10-100 ppm
greater than 100 ppm
Ni, Mo, Pb, V
Cr, Cu, Sr
Cr, Cu, Pb
Ba, Mn, Zn
As, Co, Cu, Ni, Mo, Pb, Se
Ba, Cr, Mn, Sr, V, Zn
As, Be, Co, Sb, Se As, Cd, Co, Sb
Cr, Cu, Ni, Pb, Sr, V, Zn Ni, Mo, Pb, V
Ni, Mo, Sb, V
Cr, Cu, Mn, Pb, Sr
Ba, Mn Ba, Cr, Cu, Mn, Sr, Zn Ba, Zn
Cr, Mo, V
Ba, Ni, Sr, Cu
Mn, Zn
Cd, Pb, V
Ba, Cr, Cu, Ni, Sr
Mn, Zn
Cd, Co, Ni, Mo, Sb, V
Cr, Cu, Mn, Pb, Sr
Ba, Zn
addition to that of the particular trace element under study, results in an extremely large and unwieldy datafile. Fortunately, many of the possible compounds contained in the datafile are unlikely to form, or at least are unlikely to exist in appreciable quantities, under the combustion conditions modeled, so that the size of the datafile may be limited by judicious removal of many compounds. Thus any compound predicted to exist at concentrations four, or more, orders of magnitude below that of the most significant compound of each major element has been rejected. Adoption of this criterion has led to the rejection of approximately two hundred compounds from the system comprising the elements C, H, N, O, Cl, and S only. A further thirty compounds may be excluded when silicon is included in the set. Elemental species have been included throughout, even where not predicted to be present in significant quantities, to simplify entry of the fuel composition into the model. Some limitations have been encountered with the model where, in certain cases, the upper temperature limit of validity of the available thermodynamic data is significantly below the temperature region of interest. This is particularly problematic when phosphorus is included, e.g., potassium phosphate, sodium phosphate, and potassium hydrogen phosphate have valid upper temperature limits of only 1000, 900, and 593 K, respectively. Carefully selected datafiles have thus been prepared for systems containing the major elements [C, H, N, O, S, and Cl], the minor elements [Na, K, Mg, Ca, Al, Fe, Si, and P] and up to five trace elements of interest. Thermodynamic equilibrium models have then been run to simulate the combustion of the main fuels (wood-bark, coal, or coal/straw), 70% main fuel 30% auxiliary fuel blends and the auxiliary fuel alone [where appropriate]. The system composition for thermodynamic equilibrium calculation is entered into the MULTIPHASE module on a molar basis. Here a standard fuel weight of 100kg has been taken, rather than the actual suspension reactor inventory (maximum 10 g) in order to avoid very small input values and facilitate solution convergence. Modeling of a full system containing five trace elements, plus majors and minors, over the temperature range from 900 to 1400 K took around 2 h per run using a 750 MHz personal computer. Analysis of the output data generated from the modeling may be further refined by careful scrutiny. The predicted formation of any compounds between trace and minor elements may exceed that occurring in reality, due to the low probability of these elements coming into contact during the short residence times available in a combustor. Where such species have been predicted to occur, the model has been rerun excluding them. Interactions between trace elements are even more unlikely to occur, as are compounds containing multiple atoms of the same element, such as V4O10 and Sb4O6. Both categories have thus been excluded from the results.
Ba, Mn, Zn
Results and Discussion Trace Element Contents of the Raw Fuels. Sewage sludge had the highest trace element content of the fuels studied (Table 1). It contained the highest concentrations of As, Co, Cr, Cu, Hg, Ni, Mo, Sr, and Zn. Straw had the lowest trace element content, with concentrations of As, Be, Cd, and Se all falling below the limits of quantification. Behavior of Trace Elements during Cocombustion. Potential release of trace elements to the atmosphere has been inferred from measurements of retention and relative enrichment of individual elements. Retention is defined as the percentage of an element introduced with the fuel that is recovered with the total combustion ash. Relative enrichment is defined as the ratio of the concentration of an element in the “sinterash” relative to that in the “bottom-ash” recovered from the reactor. Values greater than unity imply concentration of the element by recondensation of volatile species on the finer “sinter-ash”. This approach has led to the generation of a series of diagrams of the type illustrated in Figures 2-5. These particular figures show the retention (Figures 2 and 4) and relative enrichment (Figures 3 and 5) of 10 elements (As, Cd, Hg, Se, and Pb plus Cr, Cu, Ni, V, and Zn), at two temperatures (800 °C in upper sector, 900 °C in lower sector) for the combustion of four fuels separately (Polish coal, Columbian coal, Columbian coal/straw, wood bark). This paper sets out to summarize the findings in terms of the percentage retention in ash of 18 key elements. These elements have been divided into four groups based on their volatility and toxicity: elements of principal concern [As, Cd, Hg, Se, and Pb], elements of intermediate concern [Cr, Cu, Ni, V, and Zn], elements of moderate concern [Be, Mn, Mo, and Tl], and elements of minor concern [Ba, Co, Sb, and Sr]. In each case, experimental results are contrasted with the prediction of the thermodynamic equilibrium model, MTDATA. However, prior to considering elements individually, three general comments may be made regarding the experimentally observed behavior of trace elements during combustion and the accompanying thermodynamic model predictions. First, for most of the trace elements investigated in this study, the difference between the proportion of trace element retained in ash after combustion at 800 °C, and that at 900 °C, was less than experimental error. Thus, no significant influence
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Figure 2. Percentage retention of As, Cd, Hg, Se, and Pb.
Figure 3. Enrichment of As, Cd, Hg, Se, and Pb on sinter ash.
of combustion temperature on percentage retention could be detected within the experimental range employed here. However, in a few cases the 100 °C temperature difference was important; these are mentioned in the discussion of the behavior of the particular trace element, below. Second, for some trace elements the degree of volatilization predicted by thermodynamic calculations exceed significantly that found by experiment. In principle, any of the trace elements studied could have been bound, or partially bound, in the original fuel in a manner that prevented complete release from combustion ash. Certain mineral matter components may not transform and release their trace element inventories at the experiment temperatures used here. Alternatively, the limitation may be kinetic, with insufficient time available for the original or intermediate form to decompose. The mode of occurrence in the fuel is therefore an important factor in the behavior of almost all trace elements. The possible modes of occurrence of most trace elements are numerous and complex, and the determination of these modes
in fuels is an important topic for further study. It must be noted that not all possible modes of occurrence are available in the database used for modeling. Third, some elements were volatilized during combustion, contrary to the prediction of the thermodynamic equilibrium model. This phenomenon is likely to occur in cases where the trace element is partly held in the organic matrix of the raw fuel. During combustion, the organic matrix would burn away releasing trace elements dispersed within it into the gas phase. The trace element so released may recondense immediately on ash particles, if the formation of a solid phase is thermodynamically favorable. Recondensation, however, appears to be kinetically constrained and, since there is only a short time (
