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Occurrence of Zinc and Lead in Aerosols and Deposits in the Fluidized-Bed Combustion of Recovered Waste Wood. Part 2: Thermodynamic Considerations Sonja Enestam,*,†,‡ Kari M€akel€a,† Rainer Backman,§ and Mikko Hupa‡ †
Metso Power Oy, Post Office Box 109, FIN-33101 Tampere, Finland Åbo Akademi University, Biskopsgatan 8, 20500 Åbo, Finland § Energy Technology and Thermal Process Chemistry, Umea University, SE-901 87 Umea, Sweden ‡
ABSTRACT: In the present work, which is the second part in a series of two, multi-phase, multi-component equilibrium calculations were used to study the chemistry and deposition behavior of lead and zinc in the combustion of recovered waste wood (RWW). Particular attention was paid to the deposition behavior in different parts of the boiler under varying flue gas and material temperature conditions. In addition, the influence of fuel composition was considered by studying three different fuel compositions. The results from the calculations were compared to experimental results from two measurement campaigns, whose goal was to experimentally determine the distribution and speciation of zinc and lead compounds in aerosol particles and deposits in the fluidized-bed combustion of RWW. The results from the experimental work are presented in part 1 (10.1021/ef101478n) of this work.
1. INTRODUCTION Recovered waste wood (RWW), comprising packaging materials, demolition wood, timber from building sites, and used wood from residential, industrial, and commercial activities,1 has in recent years become a popular fuel for biofuel boilers. However, RWW is often contaminated with paint, plastic, and metal components, which produce concentrations of elements, such as zinc, lead, chlorine, sodium, and sometimes sulfur, that are elevated relative to those found in virgin wood.24 Zinc and lead originate to a large extent from the surface treatment of the wood, which, according to Krook et al.,5 accounts for 80% of the lead and 70% of the zinc; they also determined that 10% of the lead in waste wood originates from plastic and approximately 14% of the zinc originates from galvanized metal. The impurities in the fuel, mainly lead, zinc, and chlorine, have often led to increased fouling and corrosion of furnace water walls, superheaters, and economizers.3,4,6,7 The corrosivity of the deposits is dependent upon the chemical form (speciation) of zinc and lead. Zinc and lead chlorides have been shown to cause corrosion of superheater tube materials even at low temperatures (230450 °C),813 whereas ZnSO49 and ZnO/PbO11 did not have the same effect. Accelerated corrosion because of lead and zinc has, however, been observed at 600 °C under deposits containing ZnSO4 and PbSO4 in combination with alkali sulfates and calcium sulfate.13 One suggested reason for the high corrosivity of deposits containing zinc or lead chloride is their low melting point.9,1215 The presence of zinc and lead in deposits can lower the first melting point of the deposit to as low as 200 °C,16 but the influence on the melting temperature depends upon the chemical speciation of zinc and lead; chlorides lower the melting point drastically, whereas oxides, for example, do not. Clearly, it is important to know not only if zinc and lead will end up in the deposit but also in which form they will be present. r 2011 American Chemical Society
Part 1 (10.1021/ef101478n) of this work investigated the occurrence and speciation of lead and zinc in deposits and aerosols formed during the combustion of RWW.17 Zinc was identified as ZnSO4 and ZnCl2 in deposits from the superheater area and as ZnO in deposits from the economizer area; moreover, zinc was identified as K2ZnCl4 in the fine aerosol particle fraction in the economizer area. Lead was identified as PbSO4 and PbCl2 in deposits from the superheater area and as Na3Pb2(SO4)3Cl in deposits from the economizer area. In previous studies, speciation of lead and zinc in ashes from combustion of wood, RWW, and municipal solid waste (MSW) have been made. Zinc has been identified as Zn2SiO4, ZnAl2O4, and Zn(OH)2 in bottom ash15,18 and fly ash18 samples from the fluidized-bed combustion of wood and MSW. However, Zn(OH)2 is not stable at combustion temperatures; therefore, it is likely to have formed upon contact with moisture in the air during sample storage or preparation. Moreover, Zn5(OH)6(CO3)215,18 and ZnCl218 have been found in fly ash samples from the combustion of MSW. Aerosol particles formed during the combustion of wood doped with ZnO and HCl have been shown to contain ZnO, ZnCl2, and ZnSO4,19 whereas K2ZnCl4 has been identified in aerosols formed during the combustion of MSW.20 In deposits, zinc has been identified as ZnCl2 and ZnO in the combustion of RWW3,19,21 and as ZnSO415 and K2ZnCl49,13 in the combustion of MSW. Lead has been identified in deposits from RWW combustion as PbO3,21 and PbCl221 and in deposits from waste combustion as PbO,3,15,21 PbCl2,21 PbSO4,15 PbO 3 PbSO4,15 4PbO 3 PbSO4,15 and K2Pb(SO4)2.15 No identification of lead compounds in other ash fractions related to the fuels of interest could be found in the literature. Received: December 29, 2010 Revised: April 5, 2011 Published: April 08, 2011 1970
dx.doi.org/10.1021/ef101761w | Energy Fuels 2011, 25, 1970–1977
Energy & Fuels To enhance the understanding of the chemical behavior of zinc and lead in combustion, several studies have used thermodynamic equilibrium calculations. On the basis of experimental work combined with equilibrium calculations, van Lith et al.22 suggested that the release of zinc and lead from woody fuels takes place through the reduction of zinc and lead compounds in the fuel, which results in the formation of the volatile compounds Zn(g) and PbO(g) during the pyrolysis stage; the release of zinc and lead from the fuel was found to start at around 500 °C and increase to more than 85% at 850 °C. The accumulation and release of trace metals in the fluidized-bed co-combustion of biomass, peat, and refuse-derived fuels (RDFs) was studied experimentally and theoretically using thermodynamic equilibrium calculations by Kouvo et al.23,24 They found that the silica sand bed material captured substantial amounts of lead and zinc, but these elements were released from the bed when the fuel characteristics or process parameters changed. Thermodynamic equilibrium calculations suggested that the release of zinc from the bed material increases strongly with increased chlorine content of the fuel because of the conversion of bed-bound zinc into ZnCl2. The observation that chlorine increases the formation of zinc and lead chloride and, hence, the volatility of zinc and lead has been discussed by several authors.16,2529 Reducing conditions have also been shown to increase the volatility of zinc compared to oxidizing conditions.16,28,30 In addition to the chlorine content of the fuel and the air/fuel ratio, the S, Na, K, and Ca content will influence the behavior of zinc and lead. Increasing the sulfur/chlorine ratio in the fuel shifts the stability of zinc and lead from chlorides toward sulfates.27 However, Spiegel, who studied the stability of zinc and lead chlorides and sulfates under waste incineration conditions, concluded that lead and zinc sulfates are rather unstable at high temperatures; the equilibrium partial pressure of SO2 necessary for their stability is in the range of 100500 ppm at 1000 °C.13 The effect of Cl, K, Na, S, and Ca and the air/fuel ratio on the behavior of zinc and lead in the combustion and gasification of waste-derived fuels was demonstrated by Backman et al.16 Under reducing conditions, in the absence of chlorine, the main volatile species of lead and zinc formed are Pb(g) and Zn(g). Under oxidizing conditions, lead reacts to volatile PbO(g) and zinc reacts mainly to ZnO(s). With high concentrations of HCl in the flue gas (10 000 ppm in the reducing case and 5000 ppm in the oxidizing case), lead and zinc form gaseous chlorides; the formation of chlorides is more extensive under oxidizing conditions than under reducing conditions. If potassium and sodium are present, the major portion of the chlorine is converted into gaseous alkali chlorides. Alkali chlorides are more likely to form than lead and zinc chlorides under combustion conditions; therefore, the net effect is that the formation of lead chloride and zinc chloride is inhibited. The presence of sulfur promotes the formation of alkali sulfates under oxidizing conditions. This inhibits the formation of alkali chlorides, which in turn enhances the formation of gaseous lead chloride and zinc chloride. Under reducing conditions, sulfur forms H2S; therefore, it does not have as strong an influence on the alkali chemistry as it does under oxidizing conditions. Sulfides have been predicted to be the most stable form of zinc and lead at lower temperatures during the gasification of coal (T < 850900 °C)31 and wood (T < 600700 °C).30 Deposit formation via the condensation of chlorides and sulfates in waste incinerators has been studied by Otsuka31 using equilibrium calculations. He concluded that the vapor condensation of
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lead and zinc chlorides is possible with flue gas temperatures above 750 °C and tube surface temperatures below 350 °C. Previous work clearly shows that the occurrence and speciation of lead and zinc in the flue gas, deposits, and ashes are sensitive to fuel composition and combustion conditions. In the present work, which is the second part in a series of two, multi-phase, multi-component equilibrium calculations were used to study the detailed chemistry of lead and zinc in the combustion of RWW. The objective was to identify upon which surfaces in the boiler zinc and lead are likely to be deposited and in what form. Special emphasis was put on the effect of the fuel composition and the location in the boiler. The effect of changes in the fuel composition was tested by comparing three different fuel compositions, and different locations in the boiler were represented by different flue gas and metal temperatures. This study considers deposit formation through the deposition of solid or partially molten particles as well as through the condensation of gaseous compounds. The calculations were compared to experimental results from two measurement campaigns, whose goal was to experimentally determine the distribution and speciation of zinc and lead compounds in aerosol particles and deposits in the fluidizedbed combustion of RWW. The measurement campaigns are described in detail in part 1 (10.1021/ef101478n) of this work.17
2. THERMODYNAMIC EQUILIBRIUM CALCULATIONS The chemistry of lead and zinc was modeled using global multi-phase, multi-component thermodynamic equilibrium calculations in a tailormade calculation routine based on the thermodynamic equilibrium program ChemSheet,32 with thermodynamic data based on the Fact databases.33 The thermodynamic database used includes 16 elements (C, H, N, O, S, Cl, Ca, Mg, Na, K, Al, Si, Fe, P, Pb, and Zn), 1 gas phase (including 163 gas species), 1 liquid solution (salt melt), 2 solid solutions (Na2CO3/K2CO3/Na2SO4/K2SO4 and NaCl/KCl), 18 pure liquid compounds, and 269 pure solid compounds. The data for the liquid solution (salt melt) was tailor-made. The databases available in the literature and in commercial software include data for a liquid solution covering the system (K, Na)(Cl, CO3, SO4), which is commonly present in biofuel ashes. However, this melt can also dissolve salts of zinc and lead, which will significantly lower the first melting point of the system. Solution data for the system (K, Na, Pb, Zn)(Cl, CO3, SO4) are not available in either commercial applications or the literature. For the database used in this work, we added lead and zinc to a previously developed alkali salt model; the parameters will be published elsewhere. The input for the calculations was the chemical composition of the fuel, including all ash-forming elements and the combustion air. Three different fuel compositions were used, corresponding to the fuels in the measurement campaigns in part 1 (10.1021/ef101478n) of this work. The fuel compositions for the three calculated cases are presented in Table 1, together with a comparison to reference RWW samples collected from Swedish power plants. The zinc content of the fuels in the studied cases varies between 74 mg/kg of dry solids (ds) (case 2) and 2200 mg/kg of ds (case 3), ranging from minimum to average values in the reference database. The lead content varies between 7 mg/kg of ds (case 2) and 640 mg/kg of ds (case 3), ranging from minimum to maximum values in the database. The chlorine content is higher in case 1 than in cases 2 and 3, whereas the sulfur content is lower in case 1. With the combination of high chlorine and low sulfur, there is a higher probability that alkali, lead, and zinc chlorides will be formed. The alkali metals and calcium occur in roughly the same amounts in the three cases, corresponding to average values in the reference database. In all of the calculations, atmospheric pressure was used and the air/fuel ratio λ was equal to 1.2. 1971
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Table 1. Fuel Analyses Used as Calculation Inputs Compared to Reference RWW reference RWWa case 1 (full scale)b
case 2 (pilot scale, as-received)b
case 3 (pilot scale, doped)b
minimum
average
maximum
moisture
wt %
17
14
9.8
6.5
9.5
27
C
wt % dsc
50
50
50
46
48
50
H
wt % ds
5.6
5.6
5.5
5.4
5.8
6.3
O
wt % ds
39
40
41
34
40
44
N
wt % ds
1.7
1.7
1.0
0.3
1.0
2.5
S
wt % ds
0.05
0.11
0.14
0.01
0.07
0.13
Cl
wt % ds
0.17
0.03
0.01d
0.01
0.07
0.26
ashe Al
wt % ds mg/kg of ds
3.1 1200
2.3 1200
2.3 630
1.0 100
3.6 1500
13.1 11000 13000
Ca
mg/kg of ds
2900
2500
3500
1900
4200
Fe
mg/kg of ds
930
860
360
100
1500
11000
K
mg/kg of ds
930
600
910
400
870
2300
Mg
mg/kg of ds
580
330
1200
300
630
2000
Mn
mg/kg of ds
130
72
68
100
110
200
Na
mg/kg of ds
690
520
640
300
830
1800
P Si
mg/kg of ds mg/kg of ds
79 6200
90 4600
54 2200
36 500
170 5600
540 28000
Ti
mg/kg of ds
1100
610
640
150
1200
4500
Pb
mg/kg of ds
17
7
640
13
160
630
Zn
mg/kg of ds
260
74
2200
100
1800
12000
a
On the basis of 13 RWW analyses. b Average values from three analyses. c ds = dry solids. d The analyzed chlorine content was below the detection limit of 0.01 wt %. e At 550 °C. The low temperature was used to prevent the loss of easily volatilized compounds. The output of the calculations was the chemical composition of the gas phase as well as the amount and composition of the condensed phases as functions of the temperature. The temperature range investigated was 2501100 °C; the hottest temperature represents the flue gas temperature in the furnace, after which the temperature decreases as the flue gas passes the heat exchanger surfaces in the flue gas channel. In the interpretation of the results, gaseous compounds at a given temperature (which represents a certain location in the boiler/flue gas channel) were considered to be potential condensable matter that can condense on surfaces colder than the dew point of the gas compound in question. Condensed matter present at the temperature of the flue gas may be deposited as solid, molten, or partially molten particles. The tendency of the particles to stick to surfaces is greater when they contain molten material; 15% molten phase has been defined as the point at which salt particles become sticky.34
3. EXPERIMENTAL RESULTS USED FOR THE EQUILIBRIUM STUDY 3.1. Description of the Measurement Campaigns. The
experimental results discussed in this work originate from two measurement campaigns performed in a full-scale bubbling fluidized-bed (BFB) hot-water boiler (20 MWth) burning 100% RWW and in a pilot-scale BFB boiler (2 MWth) burning as-received RWW and RWW doped with lead and zinc. The measurement campaign in the full-scale boiler lasted for 10 days, during which fuel samples, deposit samples, and aerosol particle samples were collected. The fuel was the as-received RWW normally used at the power plant. Average values from fuel analyses on 3 of the measurement days are presented in Table 1 (case 1). An air-cooled gradient probe with three detachable stainless-steel sampling rings was used to collect deposit samples
Figure 1. Schematic diagram of the full-scale test boiler with the fuel, particle, and deposit sampling locations indicated. The white boxes in the third pass represent convective heat exchanger surfaces, and the black boxes represent district heating economizers.
at two locations in the boiler, MP1 and MP2. The location labeled MP1 in Figure 1 is in the upper part of the furnace, where the flue gas temperature was approximately 850 °C; the location labeled MP2 is in the third pass in the area of the convective heat exchangers, where the flue gas temperature was approximately 380 °C. The sampling times were 2 h for MP1 and 2, 48, and 120 h for MP2. The metal temperatures of the probe rings were set to 400, 500, and 600 °C in MP1 and 300, 350, and 380 °C in MP2. The temperature was measured with type-K thermocouples. The flue gas and metal temperatures of MP1 were chosen to simulate typical secondary/tertiary superheater conditions; MP1 is hereafter referred to as the hot superheater area. The flue gas and metal temperatures of MP2 were chosen to simulate conditions typical of the economizer area; MP2 is hereafter referred to as the 1972
dx.doi.org/10.1021/ef101761w |Energy Fuels 2011, 25, 1970–1977
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economizer area. The operating principle of the deposit probe is described in detail elsewhere.35 Aerosol particles were sampled isokinetically in the economizer area (MP2). The samples were collected with a 13-step lowpressure cascade impactor from Dekati, Ltd. (DLPI) that classifies particles according to aerodynamic diameter in the range of 0.0310 μm; the aerodynamic diameter is the diameter of a sphere of unit density (g/cm3), which has the same gravitational settling velocity as the particle in question. A pre-separating cyclone with a cut-point (D50) of 10.5 μm was used before the impactor. No dilution was applied, and the equipment (probe, impactor, and cyclone) was heated to 120 °C during sampling. The flue gas temperature at the sampling location was approximately 380 °C. The elemental composition of the deposit and aerosol samples collected during the full-scale campaign was analyzed using a scanning electron microscope (SEM) with an EDAX energy-dispersive X-ray spectroscopy (EDS) detector, and the phase composition of the deposit and impactor samples was analyzed using powder X-ray diffraction (XRD) to identify crystalline phases. The amounts of lead (17 mg/kg of ds) and zinc (260 mg/kg of ds) in the fuels used in the full-scale tests were moderate compared to the reference concentrations detected in RWW samples from Swedish power plants (up to 630 mg/kg of ds lead and 12 000 mg/kg of ds zinc). Therefore, pilot-scale tests were performed to assess the influence of very large amounts of lead and zinc in the fuel. The tests were performed with as-received RWW (case 2) and RWW doped with zinc and lead (case 3); the doped RWW was prepared from as-received RWW by spraying it with paint to which ZnO and PbCO3 had been added. The
Figure 2. Schematic diagram of the pilot-scale boiler with the locations of the deposit probes indicated.
compositions of the fuels are presented in Table 1, and a schematic diagram of the pilot-scale boiler is presented in Figure 2. The boiler, which is equipped with a cyclone and a return leg, can be used in either circulating fluidized-bed (CFB) or BFB mode. For the pilot-scale tests, the BFB mode was used. Deposits were collected on air-cooled probes with detachable rings at two locations in the upper part of the furnace (Figure 2). The flue gas temperatures at the sampling locations were 930 °C (MP1) and 680 °C (MP2). The probe temperature was set to 500 °C at the hotter flue gas location (MP1) and to 350 °C at the cooler flue gas location (MP2). The flue gas and metal temperatures of MP1 are typical of the hot superheater area (secondary/ tertiary superheaters) in a BFB boiler, whereas the temperatures of MP2 represent typical conditions at the primary superheater. The sampling locations will hereafter be referred to as the hot superheater area (MP1) and the primary superheater area (MP2). The sampling time for each probe test was 2 h. The operating principle of the deposit probe is described in detail elsewhere.35 The elemental composition of the deposit samples from the pilot-scale tests was analyzed by SEMEDS. The phase composition of the deposit samples was analyzed by time-of-flightsecondary ion mass spectrometry (TOFSIMS), a surface-sensitive analysis method, which measures the weight of the ions emitted from the sample surface when it is sputtered with Bi3þ ions. While this technique is not quantitative, it can be used to draw conclusions about the chemical compounds present in the sample and about the relative amounts of species in different samples. Sj€ovall et al.21 described TOFSIMS and its application in deposit analysis. In this work, the TOFSIMS instrument was calibrated to facilitate the identification of chlorides and sulfates of K, Na, Zn, and Pb. A more detailed description of the experimental procedure along with the analyses of the samples is presented in part 1 (10.1021/ef101478n) of this work.17 3.2. Results from the Measurement Campaigns. Table 2 presents a summary of the occurrence and speciation of lead and zinc in aerosols and deposits in the two measurement campaigns. The portions of lead and zinc in the deposits/particle fractions are given as atomic percentage of the total ash-forming elements Na, K, Ca, Mg, Al, Si, P, Ti, Zn, Pb, Cl, and S. The following elements were excluded: Fe, Cr, Ni, Mn, C, and O. While C and O were excluded because of inaccuracies in the analysis, Fe, Cr, Ni, and Mn were excluded because the probe ring material contained these metals; therefore, some of the metals detected in the samples
Table 2. Summary of Zinc and Lead in Deposits and Aerosol Particles in the Test Campaigns17 a case 1 (full scale) deposits: hot superheater area (Tfg = 850950 °C; Tmet = 400600 °C)b Zn 57% [not identified]
deposits: economizer area (Tfg = 380 °C; Tmet = 300380 °C)
case 3 (pilot scale, doped)
14% [not analyzed]
2738% [ZnSO4]
