Energy & Fuels 2004, 18, 1093-1103
1093
Trace Element Behavior during Co-Combustion of Sewage Sludge with Polish Coal B. B. Miller, R. Kandiyoti, and D. R. Dugwell* Department of Chemical Engineering & Chemical Technology, Imperial College London, University of London, Prince Consort Road, London SW7 2AZ, U.K. Received February 8, 2004
The co-combustion of sewage sludge (up to 30% by energy content) with coal has been investigated experimentally in a bench-scale suspension-firing reactor, at temperatures representative of commercial fluidized bed combustors. The emphasis has been on the study of the behavior of the potentially most harmful metals present in the raw fuels in trace quantities; data for As, Cd, Cr, Cu, Hg, Ni, Pb, Se, V, and Zn are reported. Metal concentrations have been quantified using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), plus an atomic absorption-based method for Hg. Interpretation of these data has been facilitated by the use of a thermodynamic equilibrium model, based on Gibbs free energy minimization; Metallurgical and Thermochemical Databank (MTDATA). The addition of sewage sludge to coal combustion is seen to lead to increased losses of Cd and Hg from the system, due to the significantly higher content of these elements in the raw sludge and the failure of combustion ashes to retain them in appreciable quantities. However, some benefit has been observed in the cases of As, Pb, and Se, where the addition of sewage sludge is seen to improve retention of these elements in the combustion ash. In the case of the other five elements studies, addition of sludge appears neutral in terms of potential metal loss to the atmosphere.
Introduction Disposal in landfills or at sea has, until recently, been the preferred method of disposal for the residual sewage sludge from wastewater treatment plants, e.g., in 1995, the UK alone dumped some 245 000 tonnes of sewage sludge into the North Sea. However, since 1998 this method of disposal has been banned by the Conference on the North Sea1 due to rising concerns over heavy metal accumulation in fish stocks. Likewise, landfilling of sewage sludge has become more difficult recently because of rising disposal costs, as the limited sites are used up. The low probability of additional capacity being made available in the short term, coupled with recent tightening of European Union legislation (EU Directive 1999/31/EC), makes dumping in landfill an increasingly poor option for the disposal of sewage sludge. The development of novel methods of disposing of sewage sludge, in an economic and environmentally friendly manner, has thus become an issue of prime importance. In principle, the combustion of sewage sludge provides an attractive disposal opportunity. First, the bacterially active organic matter content is completely destroyed; second, the disposal volume is greatly reduced as only the inert combustion ash remains; and finally, there is potential for valuable energy generation to off-set other * Corresponding author. Fax: +44 20 7594 5604. E-mail:
[email protected]. (1) Conference on the North Sea, 1990, The Hague, Netherlands.
processing and ash disposal costs. The latter is possible because the calorific value of the sludge is significantly higher than the energy requirements necessary to dry the sludge (the moisture content of air-dried sludge is usually in the region of 25%). Co-combustion of sewage sludge with coal is a particularly attractive option, since existing combustors can be used with little modification; also, the inherent stability of coal combustion helps to counter the variable quality and calorific value of sewage sludge. Further, the addition of sewage sludge to fluidized bed coal combustors is especially beneficial, since these combustors are readily tolerant of poorquality, high-ash fuels such as sewage sludge and operate at modest temperatures, where problematic ash fusion is less liable to occur (than in suspension-fired [pf] combustors). Thus the current study has focused on a bench-scale experimental simulation of co-combustion of sewage sludge with coal under fluidized bed conditions; it forms part of a larger European Union 4th Framework research initiative. The prime focus has been on the fate of the trace element inventory introduced with the fuels, since the satisfaction of environmental emissions legislation is a key operational issue. Sewage sludge contains appreciably more of certain toxic trace elements, such as Cd, Cu, Hg, and Zn, than the typical coal. Hence the effect of sewage sludge addition to a coal-fired FBC must be fully understood from a heavy metal emissions point of view before the technology can be fully validated.
10.1021/ef040018l CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004
1094 Energy & Fuels, Vol. 18, No. 4, 2004
Miller et al. combustion of finely powdered solid fuels in a circulating fluidized bed combustor. The suspension-firing reactor configuration permits quantification of the release of trace elements from combustion by measurement of the trace element retention in the “bottom ash” recovered from the reactor walls, and the catch-pot, and the finer “sinter-ash” recovered by washing the quartz sinter. The difference between the mass of an element retained in the total ash residue and the mass fed into the reactor in the fuel gives an indication of the maximum amount of that element that may escape the combustor as genuine vapor species, as aerosol, or on fine particulates (approximately less than 5 µm). In addition, comparison of trace element concentrations in the two ash fractions provides qualitative information about the propensity of the more volatile trace elements to recondense on finer ash particles, due to their greater surface area-to-mass ratios, at reactor temperatures. A Polish coal and a sample of municipal sewage sludge of Swedish origin have been selected for co-combustion experiments in the suspension-firing reactor. The aim has been to gain an understanding of the behavior of the trace element inventories of these fuels under different combustion conditions and, particularly, to seek evidence of possible synergistic effects. Initial runs have been conducted with each of these raw fuels fired alone. Subsequently, combustion runs have been conducted with coal-sewage sludge blends containing 10, 20, and then 30% sewage sludge by energy content (equivalent to 25, 43, and 56% sewage sludge by weight). All combustion runs have been conducted at both 800 and 900 °C (although at 800 °C only for the 20% mixture) and with an air/fuel ratio of 1.2:1 throughout.
Trace Element Analyses
Figure 1. The suspension-firing reactor.
Experimental Method Experiments have been carried out in a suspension-firing reactor, which has been described fully elsewhere.2,3 Briefly, this reactor is based on a vertical quartz reactor tube, 1260 mm in length and 48 mm internal diameter, in which reacting solid particles are suspended in an upward flow of hot reactant gas (Figure 1). Fuel particles are fed continuously to the reactor through a rotary valve and a water-cooled injection tube with the aid of a carrier gas; an injection rate of 1 g/min is typical. These 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 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 offset 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-firing reactor, fuel samples may thus be combusted “in suspension” in an upward flow of air at a controlled temperature, thereby simulating the (2) Miller, B. B. Ph.D. Thesis, University of London, 2001. (3) Miller, B. B.; Dugwell, D. R.; Kandiyoti, R. Fuel 2002, 81, 159171.
Trace element quantification has been accomplished by inductively coupled plasma-mass spectrometry (ICPMS) with some additional measurements by inductively coupled plasma-atomic emission spectroscopy (ICPAES) for Zn in combustion ashes. The concentration of a wide range of trace elements can be quantified by these techniques, but particular attention has been given to those elements of most concern in combustion applications because of their toxicity, viz., As, Cd, Cr, Cu, Hg, Ni, Pb, Se, V, and Zn. Details of the ICP-based trace element analysis procedures have been presented elsewhere,4,5 and only a brief outline is presented here. Mercury, which is also of prime concern, cannot be quantified by the ICP-based techniques because it is volatilized during the sample preparation procedure. Assessment of the Hg content of ash residues has been accomplished separately using a LECO amalgamator, an atomic absorption-based instrument.6 The raw fuels, plus all the ashes from both the raw fuel and blends combustion experiments, were prepared for analysis by the ICP-based techniques by both wet ashing and microwave digestion methods. “Wet ashing” breaks down and digests the entire sample, thereby theoretically releasing all of the trace elements into solution. However, the contact with strong acids, and subsequent heating to dryness, causes some of the more volatile elements (e.g., As, Se, and Hg) to be partially/ (4) Lachas, H.; Richaud, R.; Jarvis, K. E.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. The Analyst 1999, 124, 177-184. (5) Richaud, R.; Lachas, H.; Healey, A. E.; Reed, G. P.; Haines, J.; Mason, P.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 2000, 79, 1077-1087. (6) Richaud, R.; Lachas, H.; Collot, A.-G.; Mannerings, A. G.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 359-368.
Co-Combustion of Sewage Sludge with Polish Coal
Energy & Fuels, Vol. 18, No. 4, 2004 1095
Table 1. Fuel Ultimate Analyses and Minor Element Contents Polish coal
Table 2. Trace Element Contents of Polish Coal and Sewage Sludgea,b
sewage sludge
elemental analysis C H N O (by diff) S Cl P
85.0 4.81 1.24 7.9 0.84 0.11 0.02
52.5 6.61 7.23 26.3 2.91 0.06 4.32
above values, % daf ash, % as received moisture, as above
16.2 2.9
41.4 11.0
minor elements, g/kg Na K Mg Ca Al Fe Si
0.67 2.89 3.14 4.94 16.1 7.52 33.4a
2.21 3.90 3.17 18.8 24.6 77.6 61.6a
lower heating value
28.6b
8.4b
a Si contents and blower heating values (in MJ/kg) by IVD Stuttgart, Germany.
totally lost by volatilization. Furthermore, the inclusion of perchloric acid causes chloride ions to be present in the final solution. Chloride ions can lead to the formation of polyatomic ions in the ICP-MS plasma, creating interference with the measurement of As and Se. To avoid some of these problems, a “softer” microwave digestion method has been used for As and Se determinations. Here the sample is placed inside a small PTFE vessel with 1 mL of nitric acid; the vessel is then placed inside a larger PTFE vessel with 10 mL of nitric acid and locked into a microwave carousel. The acid, heat, and subsequent pressure serve to leach trace elements from the organic substrate and break down some of the ash constituents. However, in the absence of hydrofluoric acid, any silicates in the ash remain undissolved, so that not all of the trace elements associated with the mineral matter may be released. However, the method has been found to be quantitative for As and Se. Fuel Characterization The coal selected is a blend from seams around Krakow, Poland. The sewage sludge sample came from the wastewater treatment plant at Himmerfja¨rdsverket, Sweden, which treats wastewater from So¨derta¨lje and South-West Stockholm. Both raw fuels have been dried, ground, and sieved to 100-200 µm prior to combustion in the suspension-firing reactor; their major and minor elemental analyses are presented in Table 1. The carbon, hydrogen, and nitrogen contents have been measured simultaneously using an Exeter CE440 elemental analyzer; the sulfur content has been measured in a LECO 132 analyzer; and the chlorine and phosphorus contents have been measured by X-ray fluorescence. In addition, the Na, K, Ca, Mg, Fe, and Al concentrations in the samples have been measured by ICP-AES after first digesting the samples using the “wet ashing” procedure. The estimated accuracies of major and minor element determinations are (1% and (2%, respectively.
Asc Ba Be Cd Co Cr Cu Hgd Ni Mn Mo Pb Sb Sec Sr Ti Tl V Zn
Polish coal
sewage sludge
1.83 ( 0.54 376 ( 74 1.30 ( 0.12 0.21 ( 0.07 6.0 ( 1.1 24 ( 2 25 ( 3 0.058 0.010 17.3 ( 3.0 176 ( 9 0.98 ( 0.09 34 ( 4 1.67 ( 0.38 2.4 ( 0.8 85 ( 7 780 ( 18 0.33 ( 0.08 24 ( 1 43 ( 10
3.8 ( 0.5 266 ( 49 0.49 ( 0.08 2.3 ( 1.7 9.6 ( 2.3 143 ( 30 283 ( 45 0.85 ( 0.13 51 ( 14 396 ( 66 18.7 ( 4.1 32 ( 8 3.3 ( 1.0
