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Energy & Fuels 2007, 21, 3180–3188
Effect of Cofiring Coal and Biofuel with Sewage Sludge on Alkali Problems in a Circulating Fluidized Bed Boiler K. O. Davidsson,* L.-E. Åmand, A.-L. Elled, and B. Leckner Department of Energy and EnVironment, Energy Technology, Chalmers UniVersity of Technology, Göteborg, Sweden ReceiVed July 6, 2007. ReVised Manuscript ReceiVed August 31, 2007
Cofiring experiments were performed in a 12 MW circulating fluidized bed boiler. The fuel combinations were biofuel (wood+straw), coal+biofuel, coal+sewage sludge+biofuel, and sewage sludge+biofuel. Limestone or chlorine (PVC) was added in separate experiments. Effects of feed composition on bed ash and fly ash were examined. The composition of flue gas was measured, including on-line measurement of alkali chlorides. Deposits were collected on a probe simulating a superheater tube. It was found that the fuel combination, as well as addition of limestone, has little effect on the alkali fraction in bed ash, while chlorine decreases the alkali fraction in bed ash. Sewage sludge practically eliminates alkali chlorides in flue gas and deposits. Addition of enough limestone to coal and sludge for elimination of the SO2 emission does not change the effect of chlorine. Chlorine addition increases the alkali chloride in flue gas, but no chlorine was found in the deposits with sewage sludge as a cofuel. Cofiring of coal and biofuel lowers the alkali chloride concentration in the flue gas to about a third compared with that of pure biofuel. This is not affected by addition of lime or chlorine. It is concluded that aluminum compounds in coal and sludge are more important than sulfur to reduce the level of KCl in flue gas and deposits.
1. Introduction Using biofuels instead of coal lowers the effective emission of CO2. Biomass includes not only relatively convenient fuels, such as stem wood, but also twigs, branches, bark, agricultural byproducts, and energy crops. These fuels are problematic due to their high content of alkali, especially potassium and possibly also chlorine. When released from the fuel during combustion, these elements may form alkali chlorides and then cause deposits1 and corrosion.2 In fluidized beds alkali may also contribute to bed agglomeration.3 One way to increase the use of biomass is to cofire coal and biomass in existing coal-fired boilers. The coal ash may have positive effects on alkali in biomass since the ash contains aluminum silicates, which are liable to bind alkali in a stable form4,5 similarly to the effect of kaolin.6 Furthermore, coal combustion products include SO2, which has been shown to convert KCl to K2SO4.7 Cofiring of coal and problematic biofuels in a pilot plant has shown that sulfation of alkali chlorides avoids deposits or at least results in a deposit that is less likely to lead to corrosion.8 Other pilotplant studies have shown that coal ash is more important than sulfur to prevent deposition.4 Residuals from modern society can no longer be deposited but should be used as fuel or be incinerated. An example of * To whom correspondence should be addressed. (1) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Biomass Bioenergy 1996, 10, 125–138. (2) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. Sci. 2000, 26, 283–298. (3) Ergudenler, A.; Gahly, A. E. Biomass Bioenergy 1993, 4, 135–147. (4) Aho, M.; Ferrer, E. Fuel 2005, 84, 201–212. (5) Dayton, D. C.; Belle-Oudry, D.; Nordin, A. Energy Fuels 1999, 13, 1203–1211. (6) Steenari, B.-M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67–76. (7) Iisa, K.; Lu, Y.; Salmenoja, K. Energy Fuels 1999, 13, 1184–1190. (8) Robinson, A. L.; Junker, H.; Baxter, L. L. Energy Fuels 2002, 16, 343–355.
such a residual is sewage sludge. To avoid deposition and to reduce the quantity, combustion is an option. If drying is organized in a judicious way, energy recovery is possible, but sewage sludge could be even more interesting for its ashes, as will be explored in the present work. Considering its content of sulfur and aluminum, there may be advantages, analogous to those of coal, with cofiring of sludge together with problematic biofuels. Full-scale experiments have shown that cofiring of wood fuels and sewage sludge strongly reduces the rate of deposition on heat exchanger tubes. This effect seems to be mainly associated with the content of aluminum silicates in the sludge.9 Another possible fuel is municipal waste, which often contains large amounts of chlorine originating from plastics. Chlorine is unfavorable because it easily forms KCl if potassium is present,10 and KCl may be stable under combustion conditions.11 The purpose of the present work is to study alkali-related problems during cofiring of coal and biomass with sewage sludge in a fluidized bed boiler. The synergy effects are then important, namely the interaction of the constituents of two or more fuels burned together in co-combustion, a mode of combustion for which fluidized bed is quite suitable. The constituents of particular interest are sulfur, chlorine, alkali metals, and aluminum silicates, all contained in the fuels. To especially assess the role of chlorine, chlorine was added in the form of straw pellets and PVC (polyvinyl chloride). In this way, the quantity of chlorine could be controlled in relation to significant fuel constituents. The problem in focus is the propensity of alkali-containing biofuels to form deposits on heat (9) Åmand, L.-E.; Leckner, B.; Eskilsson, D.; Tullin, C. Fuel 2006, 85, 1313–1322. (10) Olsson, J. G.; Jäglid, U.; Pettersson, J. B. C.; Hald, P. Energy Fuels 1997, 11, 779–784. (11) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17–46.
10.1021/ef700384c CCC: $37.00 2007 American Chemical Society Published on Web 10/12/2007
Cofiring Coal and Biofuel with Sewage Sludge
Energy & Fuels, Vol. 21, No. 6, 2007 3181 Table 2. Fuel Analyses dried dewatered straw sewage sewage wood coal pellets sludge sludge pellets
Figure 1. Schematic picture of the boiler: 1, combustion chamber; 2, cyclone; 3, loop seal; 4, convection pass; 5, secondary cyclone; 6, bag house filter; 7, stack; 8, ejection of bed material; 9, measurement spot (before convection pass); 10, measurement spot (after convection pass); 11, measurement spot (stack); 12, sampling in the bed; 13, sampling in the cyclone leg; 14, secondary cyclone ash; 15, bag house filter ash. Table 1. Operating Parameters parameter
average
standard deviation
load (MWth) bed temperature, bottom (°C) bed temperature, top (°C) exit temperature in afterburner (°C) temperature in bag house filter pressure drop in furnace (kPa) excess air ratio primary air/total air (%) fluidization velocity (m/s)
5.95 849 863 826 151 7.7 1.19 47 4.6
0.2 3 9 8 4 0.2 0.02 4 0.22
transfer tubes, eventually causing corrosion. Could the formation of deposits be alleviated by a suitable mixture of fuels? 2. Experimental Details 2.1. The Boiler. Figure 1 shows a schematic picture of the 12 MWth circulating fluidized bed (CFB) boiler of Chalmers University of Technology. The boiler is large enough to be representative for a commercial CFB boiler. The combustion chamber (1) has a square cross section of about 2.25 m2 and a height of 13.6 m. Fuels are fed from fuel hoppers to the bottom of the combustion chamber through a fuel chute. Dewatered sewage sludge is fed to the boiler by a pump, which is a developed piston pump, normally used for cement. Make-up bed material and limestone are fed from hoppers. The circulating solids are separated in the primary cyclone (2) and transported through the particle return leg, loop seal (3), and, in the case of coal combustion, also through the external heat exchanger back into the combustion chamber. Primary combustion air is supplied to the wind box below the gas distributor and from there to the bed. Secondary air was added into the combustion chamber 2.2 m above the bottom air nozzles. After passing the exit duct of the cyclone, which acts as an afterburner chamber, the flue gas is cooled to 150 °C in the convection pass (4). At this temperature the fly ashes are separated from the flue gas, first in a secondary cyclone (5) and then in a bag filter (6). An important location is where the deposit probe was situated, and HCl, SO2, and alkali chlorides were continuously measured (9). Here the flue gas is about 800 °C. PVC was added to the fuel chute in some tests, and in these tests, and hydrated lime was supplied to the flue gas pass upstream of the bag filter. Table 1 shows the operating parameters of the boiler. These data were applied in all experimental cases. 2.2. Fuels. The base fuels were a bituminous chlorine-rich coal from the Katowice district in Poland and wood pellets or wood chips. The wood pellets were produced without binding material
proximate analysis (wt %) moisture (as received) 10 10 15 73 ash (dry) 8 8 38 50 combustibles (dry) 92 92 62 50 34 80 92 98 volatiles (dafa) ultimate analysis (wt %, daf) C 82 49 52 52 H 4.8 6.3 7.6 7.7 O 11 43 31 32 S 0.62 0.12 1.9 1.5 N 1.5 0.73 7.4 6.5 Cl 0.32 0.36 0.06 0.1 ash analysis (g/kg dry ash) K 13 110 9.6 14 Na 13 9.0 2.1 7.3 Al 81 6.5 64 76 Si 210 300 90 139 Fe 60 2.1 178 190 Ca 73 52.1 73 38 Mg 38 10.3 12 11 P 1.7 13.5 97 60 Ti 3.7 0.4 9.0 4.8 Ba 1.4 0.4 0.6 0.7 lower heating value (MJ/kg) H (daf) 32.2 18.5 21.1 20.3 H (as received) 26.2 15.1 10.8 0.74 a
8 0.3 100 82
wood chips 42 0.6 99 82
50 50 6.1 6.2 43 44 100% implies accumulation in the boiler. Accumulation of potassium occurs in the C+S+Cl and W+S cases. Accumulation of sulfur occurs in the C+S+L and C+S+Cl cases, and accumulation of chlorine occurs in the C+S+Cl case. A few balances are lower than 100%, which means that more of the element exits the boiler than what enters. This could be caused by memory effects; i.e., elements are released from deposits, etc., already present in the boiler. 3.6. Emissions from the Stack. Figure 7 shows emissions of HCl and SO2. If no additions are made, the emissions are high in the sludge cases as well as in the C+S case. The W+S case gives rise to emission of HCl due to the chlorine content of the straw pellets. Limestone addition eliminates SO2 and lowers HCl. Note that during chlorine addition hydrated lime
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Figure 7. Emissions of HCl and SO2 measured in the stack.
was added to the bag filter. This has affected SO2 to some extent, and HCl is strongly decreased. 4. Interpretation and Discussion The experiments can be divided into three groups on the basis of their deposition rates. The highest rate is observed in the W+S case, while in the sludge cases, deposition is close to be eliminated altogether. In the coal cases the deposition rate is in between. These groups are treated separately below. 4.1. Wood as Base Fuel. At the incidence angles of the flue gas of 0° and 50°, the deposit in the W+S case is dominated by potassium and chlorine, and the flue gas has the highest KCl concentration of all cases, while the potassium fraction of the bed material is lowest of all cases. In accordance, the XRD analysis shows that KCl constitutes a major fraction of the deposit. These observations are in line with the material balances, which show accumulation of potassium and chlorine in the boiler. The maximal theoretical concentration of KCl in the flue gas is 164 ppm. The highest measured value is 74 ppm, which shows that a large fraction of potassium and chlorine forms potassium chloride. From a corrosion point of view, this is unfavorable, since both chloride and potassium take part in corrosion reactions.2,16,17 At 180° the fractions of both potassium and chlorine are lower than at the other angles. Furthermore, calcium and silicon at 180° have been transported by other mechanisms than most of the KCl. 4.2. Coal as Base Fuel. In the coal cases the deposition rate is about half of that in the W+S case, and the concentration of KCl in the flue gas is less than a third. Consequently, the deposits are not dominated by potassium and chlorine. The concentration of HCl in the stack is about 50 ppm lower than upstream of the convection pass, which points at conversion of HCl. In the deposit the amount of chlorine is significant only in the C+S+L case, and as there is a marked difference between this case with limestone addition from cases without limestone, chlorine can be suspected to be in the form of CaCl2, which has been suggested in pilot-scale experiments4 and in largescale experminents.13 With limestone addition there is more chlorine in the fly ash than in other cases, which points in the same direction. However, the XRD analysis did not confirm the presence of CaCl2 in the deposit. Instead, it shows that alkali chlorides and CaSO4 are present. Hypothetically, the additional surface, formed by limestone particles, may facilitate condensa(16) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95–108. (17) Pettersson, J.; Asteman, H.; Svensson, J.-E.; Johansson, L.-G. Oxid. Met. 2005, 64, 23–41.
DaVidsson et al.
tion of alkali chlorides and could transport chlorides to the probe’s surface, probably by impaction. A mechanism like impaction explains the lower fraction of chlorine and alkali on the leeward side of the probe. The windward deposits comprise elements from the fuel ash, and they probably, to a large extent, consist of potassium aluminum silicates and CaSO4. This is also supported by the high aluminum flow in the fly ashes in the coal cases compared with the W+S case. Addition of limestone completely removes SO2 from the flue gas but does not affect the concentration of KCl or the deposition rate significantly. Therefore, it seems that the binding of alkali in stable aluminum silicates is the dominant mechanism for reducing KCl in the flue gas, causing to some extent deposits when coal is the base fuel. This was also observed in earlier tests in the same boiler.13,15 Equilibrium calculations on cofiring of coal and biomass have shown that the formation of alkali aluminum silicates may occur at temperatures around 850 °C.18 To further verify the presence of such silicates, an equilibrium calculation was performed corresponding to the W+S+DS case. The results show that potassium aluminum silicates are formed. Laboratory studies have shown that KCl readily reacts with kaolin at 850 °C, forming potassium aluminum silicates6,19 under both reducing and oxidizing conditions.20 Other potassium compounds, likely to be found in the flue gas during coal combustion, are KOH and K2SO4, which also easily react with kaolin.21 The maximal theoretical concentration of KCl is 215 ppm and the measured value is 15–20 ppm, while the concentration of HCl is close to the theoretical maximum. Although relatively low, the concentration of KCl is still above an acceptable level from a deposit point of view in the C+S and C+S+Cl cases. Therefore it is surprising that so little chlorine is found in the deposits. This suggests that if KCl reaches the surface of the deposit probe, it undergoes a fast enough reaction where chlorine is released, probably as HCl. However, in the C+S case, the concentration of HCl is about 50 ppm higher in the hot flue gas than in the stack, so the question why there is no chlorine in the deposits remains open. 4.3. Sludge as an Additional Fuel. In the sludge cases, with exception for the W+S+DS case, the insignificant deposits at 0° and 50° contain no or little potassium. Addition of limestone or chlorine does not alter this, probably because there is no KCl in the flue gas. The W+S+DS case shows increased deposits of potassium. However, there was no KCl in the flue gas, and the potassium found is most likely a consequence of the very small deposit, making analyses difficult. There is potassium in the fly ashes, but it is not collected on the deposit probe. Since K2SO4 is liable to deposit on tubes, potassium must be in another form than K2SO4 in the fly ash. This assumption is also supported by the observation that there is no SO2 in the flue gas in the C+S+DS+L and C+WS+L cases, since it was removed by the limestone. Sewage sludge comprises aluminum silicate structures in the form of zeolites, mainly from detergents, to about 10% of the dry mass.22 Considering the aluminum content of sludge ash (cf. Table 2), it is likely that the sludges (18) Furimsky, E.; Zheng, L. Fuel Process. Technol. 2003, 81, 7–21. (19) Tran, K.-Q.; Iisa, K.; Hagström, M.; Steenari, B.-M.; Lindqvist, O.; Pettersson, J. B. C. Fuel 2004, 83, 807–812. (20) Tran, K.-Q.; Steenari, B.-M.; Iisa, K.; Lindqvist, O. Energy Fuels 2004, 18, 1870–1876. (21) Tran, K.-Q.; Iisa, K.; Steenari, B.-M.; Lindqvist, O. Fuel 2005, 84, 169–175. (22) Kurzendörfer, C. P.; Kuhm, P.; Steber, J. In Surfactant Science Series; Schwuger, M. J., Ed.; Marcel Dekker: New York, 1997; Vol. 65, pp 127–193.
Cofiring Coal and Biofuel with Sewage Sludge
Figure 8. Removal of potassium in the bed ash, secondary cyclone ash, and filter ash versus inflow of fuel aluminum in each case. The straight line is a least-squares fit to the data for secondary cyclone ash.
used in the present study contain a substantial amount of zeolites that form potassium aluminum silicates with potassium from the fuel. There are some observations that support this assumption: (1) The transient effect of sludge addition (cf. Figure 4) shows a period of decreasing concentration of KCl in the flue gas. This illustrates the time needed to build up the concentration of sludge ash in the system, which gradually increases its ability to capture the alkali. (2) This can be compared with the addition of PVC, which does not stay in the system but directly forms HCl upon combustion and gives rise to a sharp increase of KCl, such as seen in Figure 3. These two observations indicate capture of potassium by bed ashes rather than by gases, particularly when sludge is present. This is in direct contrast to wood combustion where the chlorine addition plays the leading role for deposition and formation of KCl. The products of reaction between sludge ash and gaseous alkali compounds are solid at the flue gas temperature and would therefore not deposit easily, unless the surface of the probe is already covered with a sticky layer of other deposits such as KCl. Based on the elemental composition of the deposits in the C+S+DS+L and C+WS+L cases and the absence of SO2 in the flue gas, limestone addition implies formation of CaSO4 and CaCl2. The difference between the deposits at 180° and the windward position is too small for conclusions to be drawn on deposition mechanisms in the sludge cases. 4.4. The Fate of Fuel Alkali. The removal of potassium from the boiler by the ashes is correlated with elements in the fuels. Figure 8 shows, for each experiment, the relative flow (mass out/mass in) of available potassium in the ashes versus inflow of aluminum. The higher the aluminum input of the fuel, the higher the potassium removal as secondary cyclone ash. This is a further argument for the importance of aluminum–silicon compounds for capture of potassium. Interestingly, neither capture of potassium in the bed nor removal of potassium as filter ash is strongly correlated with inflow of aluminum. This suggests that, after fuel conversion, aluminum compounds are transported as fly ash particles, of a size predominantly captured by the secondary cyclone, which react with gaseous potassium in a form not liable to deposit, presumably a potassium aluminum silicate. A weaker correlation (R2 ) 0.50) was found between potassium in the secondary cyclone ash and fuel inflow of sulfur, suggesting that sulfation of potassium occurs but is less important under the present circumstances. The correlation (R2 ) 0.61) between removal of potassium in secondary cyclone ash and total input of ash was also weaker than that of the aluminum compounds. The bed chemistry is affected by the fraction of the potassium input that stays in the bed in all cases. There may
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be two major potassium compounds in the bed. Potassium released from the fuel can react with quartz in the bed sand or with silicon compounds from the fuel forming potassium silicates.3 Potassium may also react with aluminum silicates from the fuel and form potassium aluminum silicates.4,5 Potassium in coal is likely to be bound in this manner already before conversion.23 Addition of chlorine, as in the C+S+Cl case, decreases the alkali content of the bed material. Chlorine has been shown to easily react with alkali/potassium,10,11 and formation of KCl is therefore likely to hinder, to some extent, formation of alkali silicates in the bed. The sintering temperature of the bed material was not analyzed in this work, but if one considers calcium24 and especially aluminum6 to be elements that favor high sintering temperature, it can be argued that the sintering temperature of the bed material would be lowest in the W+S case. However, in this case the potassium level is lowest. It is therefore difficult to establish in which case the risk for bed agglomeration is largest. The smallest risk is probably in the C+WS case; i.e., cofiring with sewage sludge has a positive effect on plant operation by lowering the risk for defluidisation caused by bed agglomeration. 4.5. Summary of the Discussion. To summarize, firing alkali-rich biofuels alone, as in the W+S case, is likely to induce both bed agglomeration and deposits. Cofiring of biofuels and coal, and even more with sewage sludge, decreases the risk for agglomeration and deposits, and the combustion gas will contain less KCl. Consequently, in such cases the deposits do not contain chlorine, mainly due to formation of potassium aluminum silicates that are less prone to deposit on tubes. Limestone can be added to eliminate SO2 emissions without causing KCl deposits. Addition of chlorine shows that it is possible to cofire high-chlorine fuels, such as municipal waste, with biomass and coal, without causing deposits that contain chlorine. However, it is even more favorable, from an alkali perspective, to cofire only sewage sludge and biofuels, since this completely eliminates KCl and deposits containing chlorine. Also in the latter case, limestone can be added without disturbing the neutralizing effect on alkali by the sludge. 5. Conclusions • Cofiring of coal and an alkali-rich biofuel decreases the risk for agglomeration and lowers the concentration of KCl in the combustion gas compared with firing only biofuel. Deposits do not contain chlorine. • Cofiring of coal and/or sewage sludge with an alkali-rich biofuel completely eliminates KCl in the combustion gas and deposits. • Limestone can be added to a fuel mixture of an alkali-rich biofuel and a coal without disturbing the neutralizing effects of coal on alkali. The fraction of chlorine in the deposit increases, pointing at the formation of CaCl2, but this was not confirmed by XRD analysis, which showed that alkali chlorides were formed. The presence of alkali chlorides may be explained by the fact that the limestone particles constitute an additional surface, on which gaseous alkali chlorides can condense and thereby be transported to the probe surface. • Limestone can be added to a fuel mixture of an alkali-rich biofuel, sewage sludge, and/or coal without disturbing the neutralizing effects on alkali of coal and sludge. (23) Raask, E. Prog. Energy Combust. Sci. 1985, 11, 97–118. (24) Öhman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R.; Hupa, M. Energy Fuels 2000, 14, 169–178.
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• The neutralizing effect on alkali probably occurs because of reaction between potassium and aluminum silicates in the coal ash and in the sludge ash. However, sulfation of KCl cannot be ruled out as a parallel mechanism. Acknowledgment. This work was financially supported mainly through the EU contract No. SES6-CT-2004. Additional support was received from the Swedish Energy Administration (STEM). Deposit steel rings were supplied by Sandvik Materials Technology. Dried and dewatered sewage sludges were supplied by
DaVidsson et al. SYVAB (Himmerfjärdsverket) and Gryaab (Ryaverket), respectively. The practical performance of the tests was carried out with heavy support from the operating staff of Akademiska Hus AB and researchers belonging to the Department of Energy and Environment Division of Energy Technology, which is gratefully acknowledged. The analysis of the deposit rings by SEM–EDX was carried out by Jan Höglund (Vattenfall Utveckling AB), who is also acknowledged. EF700384C
