Distribution of Potassium, Chlorine, and Sulfur between Solid and

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Energy & Fuels 2003, 17, 18-28

Distribution of Potassium, Chlorine, and Sulfur between Solid and Vapor Phases during Combustion of Wood Chips and Coal Heije Miettinen Westberg,† Madeleine Bystro¨m,‡ and Bo Leckner*,† Department of Energy Conversion, Chalmers University of Technology, Go¨ teborg, Sweden, and Department of Applied Science, Mid Sweden University, Ha¨ rno¨ sand, Sweden Received March 7, 2002

Results are presented on the distribution of some (inorganic) elements between the solid and vapor phases in a circulating fluidized bed boiler during combustion of wood chips and coal. Samples were taken from all input and output solid material streams as well as from the gas phase at different locations in the boiler. Deposits on a specially designed deposition probe were also measured. An explanation is presented concerning the release and fate of potassium, chlorine, and sulfur during biomass combustion. The explanation is supported by the results from an additional test where hydrogen chloride was added to the combustion chamber. One of the most important factors affecting the reactivity of potassium toward chlorine in the combustion chamber appears to be the moisture content of the fuel.

Introduction Despite the well-known environmental benefits of biomass, technical difficulties related to efficient conversion of some types of biomass remain an obstacle in the exploitation. During heat or electric power generation by thermal conversion, the inorganic components of biomass may have an adverse impact on reactors, furnaces, heat exchangers, turbines, emission control devices, and other equipment. Such impacts are slagging, fouling, and the companion problem of corrosion and, in fluidized beds, agglomeration of bed material. Ash deposition and high-temperature corrosion in combustion facilities cause problems and lead to reduction in heat transfer rates and unscheduled plant shutdowns. Alkali metals, in particular potassium (K), have been identified as key ingredients enhancing these problems. Potassium in combination with other inorganic elements, such as chlorine (Cl), sulfur (S), and silica (Si), is responsible for many undesirable reactions in furnaces.1-7 Ash deposition and corrosion are not unique to biomass combustion systems, but they are often pointed * Corresponding author. Phone +46 31 7721431. Fax +46 31 772 3592. E-mail: [email protected]. † Chalmers University of Technology. ‡ Mid Sweden University. (1) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (2) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17-46. (3) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860-870. (4) Turnbull, J. H. Biomass Bioenergy 1993, 4, 75-84. (5) Nielsen, H., P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. Sci. 2000, 26, 283-298. (6) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 1999, 13, 1114-1121. (7) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Ho¨rlyck, S.; Karlsson, A. Fuel Process. Technol. 2000, 64, 189209.

out as major reasons for operational problems during combustion in general.1-13 Actually, most information concerning this subject comes from investigation of the fate of alkali metals and mineral matter during coal combustion.8,9 Coal and biomass, however, have fundamentally different properties. In general, biomass has less sulfur, fixed carbon, and fuel-bound nitrogen than coal. Biomass also has higher oxygen content and is more reactive than coal. The ash fraction of biomass is typically less than that of coal and the ash has a different elemental composition. The amount of ash can, however, vary widely in different forms of biomass. Coal ash is, to a large extent, composed of mineralogical material, while biomass ash reflects the inorganic material required for plant growth. As a result, the alkali metals in coal tend to be less volatile than those in biomass.1,3,10-12 Various methods can be used to reduce the impact of alkali. Blending biomass with other fuels, like coal, is one strategy to control fouling and slagging problems. Simple leaching with water can remove large fractions of alkali metals and chlorine from the biofuels. Depending on the combustor, sorbents can be added to the fuel (8) Bryers, R. W. Ash deposits and corrosion due to impurities in combustion gases; Hemisphere Publishing Corp.: New York, 1978. (9) Reid, W. T. Coal AshsIts effect on combustion systems. Chemistry of Coal Utilization; Elliot, M. A., Ed.; John Wiley and Sons: New York, 1981; Chapter 21. (10) Dayton, D. C.; Milne, T. A. Laboratory measurements of alkali metal containing vapors released during biomass combustion. Application of Advanced Technologies to Ash-Related Problems in Boilers; Baxter, L., DeSollar, R., Eds.; Plenum Press: New York, 1996; pp 161185. (11) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D. C.; Bryers, R. W.; Oden, L. L. Fuel Process. Technol. 1998, 54, 47-78. (12) Miles, T. R. Alkali deposits found in biomass power plants. NREL/TP-433-8142; National Renewable Energy Laboratory: Golden, CO, 1995; Vol. 1. (13) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. Biomass Bioenergy 1996, 10, 177-200.

10.1021/ef020060l CCC: $25.00 © 2003 American Chemical Society Published on Web 10/17/2002

K, Cl, and S during Combustion of Wood Chips and Coal

Energy & Fuels, Vol. 17, No. 1, 2003 19

Figure 1. The 12 MWth CFB boiler at Chalmers University of Technology. (1) combustion chamber; (2) fuel feed chute; (3) primary air to air plenum; (4) secondary air inlet at 2.1 m; (5) bottom ash removal; (6) hot primary cyclone; (7) particle return leg; (8) particle seal; (9) heat exchanger; (10) cold secondary cyclone; (11) secondary cyclone ash removal; (12) bag house filter; (13) bag house ash removal; (14) probe for flue gas extraction and dust sample collection; (15) flue gas fan; (16) sand bin; (17) lime bin; (18) fuel bunkers; (19) air fan; (20) flue gas recirculation fan. Measurement points: A. Primary cyclone inlet. B. Convection section inlet. C. Convection section outlet. D. Before the secondary cyclone.

mixture to sequester alkali metals. Another possibility to reduce the amount of alkali vapor is hot gas cleanup. These solutions to fouling and slagging would greatly benefit from an understanding of the mechanisms of alkali metal release from biomass combustion. Clearly, it is important to know the forms and amounts of alkali metals in biomass, to anticipate the types of alkali metal transport to be expected, and to anticipate the forms of alkali metal-containing species that will be transported.1,3,10,13-15 This is why the present study aims at describing the distribution of inorganic ash elements in biomass and coal between gas and solid phases (flue gases and ashes) at different temperatures related to circulating fluidized bed (CFB) combustion, but it also aims at a better understanding of potassium release from biofuels in general. To further investigate the role of chlorine, HCl was added to the combustoion chamber in a case with 100% wood. The amount of chlorine added was about the same as that introduced with 100% coal. In parallel to this work, an ash deposition test program was accomplished, and some results from this program will also be presented here. First, the measurement results will be presented; then, in a second part of the paper, an effort will be made to explain the potassium release mechanisms and their consequences for deposits. Experimental Section The Boiler. The tests were run in a 12 MWth CFB boiler at Chalmers University of Technology, Figure 1. The combustion chamber (1) is made up of membrane tube walls with a height of 14 m and a cross section of about 2.5 m2. Fuel is fed (14) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M. Biomass Bioenergy 1997, 12, 241-252. (15) Dayton, D. C.; Deidre, B.-O.; Nordin, A. Energy Fuels 1999, 13, 1203-1211.

to the bottom of the combustion chamber through a fuel chute (2). Primary air is introduced through nozzles in the bottom plate (3), and secondary air is injected through the nozzle register (4). The entrained bed material is separated from the gases in the hot refractory-lined cyclone (6) and returned to the combustion chamber through the return leg (7) and particle seal (8). The bed temperature is controlled by heat transfer to the membrane-tube walls, by a heat exchanger located in the particle seal (9), and by recycling of flue gases that are mixed with the primary air in the air plenum (3) before entering the combustion chamber (1). A secondary cyclone (10) is located upstream of the bag filter (12). The boiler load was maintained at about 9 MWth during all tests, according to the measurements shown in Table 1. The operating conditions, especially bottom bed temperature and total excess air, were kept as constant as possible during the tests, and the fuel feed rates were adjusted accordingly. The fuels were wood chips from fir, wood pellets, and bituminous coal, run as pure wood or coal and as mixtures of wood and coal. In one case of pure wood, HCl was added with the fuel. The fuel analyses are given in Table 2. The bed material was silica sand, and no limestone was added during the tests. As seen in Table 1, only a minor quantity of pellets was added to allow the use of moist wood chips while maintaining the bed temperature. Solid samples were taken from the bottom bed (5), the return leg (7), the secondary cyclone ash (11), and from the bag filter ash (13), while gas samples were taken from the ports A, B, C, and D in all cases examined. The collected solid samples were analyzed by an external laboratory (Alcontrol, Sweden). X-ray powder diffractometry (XRD) was applied for identification of crystalline compounds in the solid samples. A Siemens D5000 diffractometer with Cu KR radiation and a scintillation detector was used. Identification of compounds was made by the DIFFRAC AT software, with JCP database as a source of reference data. In the present tests, two sets of on-line conventional fluegas analyzers were used for continuous monitoring of O2, CO, and SO2 in the stack and O2, CO, CO2, and SO2 in the combustion chamber and the gas paths. The latter system

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Energy & Fuels, Vol. 17, No. 1, 2003

Westberg et al. Table 1. Operating Conditions

wood only

HCl-add

coal only

coal and wood chips

coal and wood chips

90 10 0 0 0 9.1 1.22 6.7 56.2 851 878 885 315 150 150

90 10 0 14.5 4.1 9.3 1.22 6.7 55.2 849 878 884 342 150 150

0 0 100 0 0 9.5 1.20 6.3 46.9 854 874 855 293 150 150

60 0 40 0 0 9.0 1.21 6.5 47.6 853 882 872 302 150 150

25 0 75 0 0 9.5 1.19 6.4 48.8 853 873 850 294 150 150

wood chips [% of total energy input] pellets [% of total energy input] coal [% of total energy input] HCl-dosage [kg/h] dosage time [h] load [MW] excess air ratio [-] fluidizing velocity [m/s] primary air flow/total air flow [%] bed temp.sbottom [°C] temp.sinlet of hot cyclone [°C] temp.sconvection inlet [°C] temp.sconvection outlet [°C] temp.s inlet of sec cyclone [°C] temp.s inlet of bag filter [°C] Table 2. Fuel Analysis

proximate analysis [wt-%] moisture ash combustibles ultimate analysis [wt-%, daf] C H O S N Cl lower heating value Hu [MJ/kg, daf] ash analysis [mg/kg dry fuel] K Na Al Si Fe Ca Mg P Ti S

wood chips

wood pellets

41.0 0.4 58.6

10.5 0.4 89.1

9.2 14.0 76.8

49.8 6.2 43.8 0.014 0.14 0.019 17.8

49.7 6.4 43.8 0.018 0.11 0.022 19.7

68.2 4.2 25.8 0.57 1.15 0.046 32.2

1142 32 67 380 49 1342 230 73 1.5 82

407 24 27 406 56 1149 161 77 1.1 50

1184 231 10062 52886 5782 4859 2454 259 68 1950

coal

includes an FTIR (Bomem M110) for on-line measurement of H2O, CO2, SO2, CO, and HCl. The conventional analyzers were calibrated before use every day. HCl and Cl2 were measured with a wet chemical method. The combustion gas was transported from the gas-sampling probe to the wet chemical setup through heated gas sampling lines, as shown in Figure 2. The combustion gas passed through five wash bottles with absorption solutions. The first three wash bottles contained a 0.05 M H2SO4 solution in which HCl and organically bound chlorides dissolved. The last two bottles contained a 0.01 M NaOH solution in which Cl2 dissolved. Ion chromatography was used to analyze the trapper solutions. Deposition Probe. The air-cooled deposition probe was used in the central position of the flue gas channel in the inlet to the convection path downstream of the cyclone and upstream of the convection heat exchangers. The probe consists

Figure 2. Wet chemical setup.

of concentric tubes that allow the passage of air for temperature control. Part of the outer tip of the probe is an exchangeable metal ring, the test ring, for deposit collection. The metal temperature is measured with three thermocouples placed in the 12, 3, and 6 o’clock positions next to the test ring, where 12 o’clock is directed toward the gas stream. The temperature on the windward side of the probe was automatically kept at the desired level by a temperature controller connected to a mass flow controller for the cooling air. Before the tests the metal rings were weighed and stored in bottles in a desiccator with silica gel as drying agent. In all cases investigated, the surface temperature on the windward side of the deposition probe was 550 °C and the exposure time was about 4 h. Directly after exposure an ocular inspection was made of the probe and the rings, and thereafter the rings were again stored in the desiccator until weighing and analyses. Identification of crystalline compounds in the deposits was carried out in the same way as for the ashes.

Results Input and Output Flows of Ashes. Figure 3 shows the input of seven ash components via the fuel in the case of wood, coal and wood, and coal. The fuel flow is highest for pure wood (0% coal) and decreases with increasing share of coal, i.e., with increasing heating value. The inputs of S, Si, and Al substantially increase with the share of coal as explained by the higher concentration of these elements in coal than in wood: the main part of the coal ash consists of Si and Al. The input of potassium decreases only slightly with increasing share of coal, while the inputs of Cl, Na, and Ca increase somewhat. The output of ashes from the boiler is dominated by the flow from the secondary cyclone as seen in Figure 4, which also reflects the increase of ash flow as a consequence of the high ash content in the coal, 14% compared to less than 1% for wood. Because of the low ash content there is no flow of bottom ash in the wood case. The small amount of ash formed is not accumulated in the bed but entrained with the gas and separated in the secondary cyclone and in the bag filter, as confirmed by the constant pressure drop between top and bottom of the bed. In the wood case, the ash flows from the secondary cyclone and the bag filter were calculated theoretically, because the flows were too small to be accurately measured. In this calculation, all ash elements entering with the fuel into the furnace were assumed to leave as secondary cyclone or bag filter ash. The ratio of the two ash flows was taken from an earlier test where reliable values were obtained for the same fuel and boiler conditions. Figure 5 shows the



Figure 3. Input of ash elements in fuel mixtures from 0% to 100% coal. (a) K, Na, S, and Cl; (b) Si, Al, and Ca.

Figure 4. Output flows of boiler ashes.

Figure 5. Ash balances.

overall mass balances calculated as the ratio of input of ash with the fuel to output of bed and fly ash from secondary cyclone and bag filter. Only in the wood case does the theoretically calculated mass balance yield 100%, but a closure of about 80% is acceptable for the present purpose. Gas Compounds and Temperature in Flue Gas. The composition and temperature of the combustion gas will be shown in three (ABC) of the four measuring points (ABCD). The results in point D are similar to those in point C, and point D serves only as a check of accuracy. As seen in Figure 6a, the gas temperature in the cyclone inlet is about 875-880 °C in all cases, while the concentration of water vapor decreases from 20% for pure wood to 8% for pure coal. The CO concentration is also highest for wood, 2100 ppm, and decreases with

Figure 6. Gas composition (left scale) and temperature (right scale, °C). (a) cyclone inlet, (b) inlet of the convection path, (c) outlet of the convection path.

increasing share of coal to 650 ppm. The SO2 concentration increases substantially with the share of coal from

22


about 5 ppm in the wood case to 520 ppm in the pure coal case. The HCl concentration also increases with the share of coal from about 14 to 76 ppm. In the inlet to the convection path (downstream of the hot cyclone) the gas temperature decreases with increasing share of coal from 885 to 850-855 °C, Figure 6b. The evaporation of fuel moisture and most of the conversion have already occurred in the bottom part of the combustor and there is only a slight difference in water vapor concentration between entrance and exit of the cyclone, caused by a small combustion in the cyclone and its outlet. For the same reason, the CO concentration in the wood case dropped from 2100 ppm in the cyclone inlet to less than 100 ppm, while in the coal case there is only a slight decrease. The CO concentration becomes higher in the inlet of the convection path with increasing share of coal. The HCl concentration was similar to that in the cyclone inlet, but the SO2 concentration was slightly higher in the convection path inlet than in the cyclone inlet. Figure 6c shows the flue gas temperature and composition in the outlet of the convection path. Here, the gas temperature decreases with increasing share of coal from 326 to 293 °C. The gas concentrations are about the same in the inlet and the outlet of the convection path, except for HCl that is 1 to 50 ppm lower in the outlet. No Cl2 could be detected in any of the measurement points. The behavior described in Figures 6 can be summarized as follows. The thermal conditions in the combustion chamber were controlled to yield the same temperature in all cases. Hence, the gas temperature in the inlet of the cyclone is practically constant (Figure 6a). Despite refractory lining there is a certain cooling of the gases in the cyclone. However, there is also some combustion, more for the volatile fuel than for coal, as is obvious from the declining temperature in the gas downstream of the cyclone with increasing fraction of coal (Figure 6b). This temperature profile is still seen after the gases have passed the heat exchanger surfaces in the convection path (Figure 6c). As a consequence of the combustion mentioned, the CO concentration decreases in the cyclone for the high-volatile wood (Figure 6a,b) but not so much in the case of coal, most likely because of additional production of CO during combustion of residual coal char in the cyclone. With this description, and considering that cyclone combustion only is a small part of the total combustion, the behavior of the remaining gas constituents is readily interpreted based on the fuel composition in Table 2. Elements in Boiler Ashes. Figures 7-9 show concentrations of solids in the ashes from the bed, the secondary cyclone, and the bag filter. The concentrations in the particle return leg ((7) Figure 1) were similar to those in the bed, and only the bed analyses are shown. (This similarity is interesting in itself, since it shows that the particles in the circulating fluidized bed system are well mixed.) The silica content in the bed was 95% because the original bed material was silica sand. The aluminum concentration increases with increasing coal share, while the calcium concentration is almost the same in all cases. Although all other elements are diluted compared to silica, a reduction of the potassium concentration with increasing share of coal is noticeable,

Westberg et al.

see Figure 7a. The sodium concentration increases slightly, while the sulfur concentration clearly increases with increasing share of coal. The chlorine content in the bed samples is low in all cases. The particles collected by the secondary cyclone are much smaller than those in the bed. The silica concentration has decreased to half of the value in the bed and the concentration of the other elements has increased substantially. Still, silica is the most abundant element, except oxygen, in the secondary cyclone ash as well as in the bed. In Figure 8a the concentrations of potassium, sodium, sulfur, and chlorine are shown to increase with increasing wood share. The sulfur concentration in the secondary cyclone ash is higher than the chlorine concentration in all samples. The silica and the calcium concentrations decrease, while the aluminum concentration increases with the addition of coal. Though the calcium content in coal is higher than in wood, it is clear from Figure 8b that the calcium concentration decreases with increasing coal share. Trends similar to those for the secondary cyclone ash were obtained in the bag filter for potassium, sodium, sulfur, and chlorine, although the elemental concentrations are much higher, Figure 9a. The most noteworthy result is the considerable reduction of chlorine concentration in the ash when coal is added. The ash elements from the fuels are least diluted by sand in the bag filter ash, and the concentration of all elements, except silica and alumina in the wood case, is highest in this ash, Figure 9b. The increase of aluminum and the decrease of calcium when coal was added are similar to what was seen in the secondary cyclone ash. The silica concentration increases with increasing coal share, contrary to the trend in the secondary cyclone ash. In the bag filter ash the concentration of silica is less than 30% of that in the secondary cyclone ash. For the other species it is higher in the bag filter ash than in the secondary cyclone ash. In all cases, except the wood case, silica is the main component in the ash. In the wood case calcium is the main component. The ash behavior, illustrated by Figures 7-9, can be summarized as follows: the bed material/ash flow rate is sequentially reduced as the material passes the primary cyclone, the secondary cyclone, and the bag filter. At the same time, the particle size is reduced in the various separators. The figures show an enrichment of K, S, Cl, and to a minor extent also of Na and Ca, whereas the bulk material Si tended to remain in the bed and only minor attrited fragments and ash constituents end up in the final separator, the bag filter. The strong preferential enrichment of K, S, and Cl in the wood ash is also clearly seen. HCl Addition. HCl was added by sprinkling diluted hydrogen chloride acid over the fuel particles in the fuel feed chute just before the inlet to the furnace in the case of pure wood. The input of potassium and chlorine are shown in Table 3: in the HCl addition case, chlorine increased substantially, from 13 to 77 mol/h, which is about the same as in the coal case, whereas the potassium input was constant, about 54 mol/h. The table also presents the composition of the combustion gas in the cyclone inlet with and without HCl addition. The HCl concentration increased slightly, the CO concentration doubled and the SO2 concentration increased ap-



Figure 7. (a and b) Concentration of ash elements in bed samples.

Figure 8. (a and b) Concentration of ash elements in secondary cyclone ash.

Figure 9. (a and b) Concentration of ash elements in bag filter ash. Table 3. Input of Potassium and Chlorine and Gas Concentration at Cyclone Inlet input component

wood

potassium [mol/h] chlorine [mol/h]

53 13

HCl addition 55 77

gas concentration at cyclone inlet

wood

HCl addition

HCl [ppm] SO2 [ppm] CO [ppm] H2O [%]

14 5 2092 20

22 20 4352 20

proximately four times, but the emission of SO2 was about the same as before addition. In Figure 10a the potassium and chlorine contents in the ashes sampled with and without HCl addition are compared. There is a considerable increase of both potassium and chlorine in the secondary cyclone and bag filter ashes when HCl was injected. The rise in chlorine concentration was most pronounced in the secondary cyclone ash. Figure

10b shows the sodium and sulfur contents in the ashes. No increase in sodium concentration was detected, while the sulfur content increased in the bag filter ash with addition of HCl to the furnace. Crystalline phases identified by XRD in some of the fly ashes are summarized in Table 4. Deposits. In all five cases investigated, the exposure time, the surface temperature on the windward side of the deposition probe, and the exposure location in the boiler were the same. The gas temperature in the measurement point varied between 850 and 885 °C (see Table 1, convection path inlet). Figure 11 presents a schematic picture of the deposition rings in each case as a result of ocular inspections of the rings. In the wood case, the deposit had an even structure of condensed material, somewhat thicker on the windward side than on the leeward side. With coal admixture to the wood chips, the most obvious change was the disappearance of the deposit on the leeward side and that the con-

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Westberg et al.

Figure 10. (a) Concentrations of potassium and chlorine in boiler ashes during combustion of wood chips and pellets with and without HCl addition. (b) Concentrations of sodium and sulfur in boiler ashes.

Figure 11. Schematic picture of the deposition ring in each case. Table 4. Crystalline Compounds Identified in Fly Ashes, in Order of Importance case

secondary cyclone ash

bag filter ash

wood SiO2, CaCO3, KCl, MgO KCl, CaCO3, Ca-silicate HCl-add KCl, CaCO3, SiO2, MgO KCl, CaCO3, MgO, CaSO4 coal SiO2, CaCO3, silicates, MgO SiO2, KCl, Fe2O3, MgO

densed deposit on the windward side became thinner, while on each side of the ring (approximately at 120°) an ash bank started to build up. With increasing coal admixture the ash bank grew thicker, while the condensed layer on the windward side diminished until it fully disappeared in the pure coal case. When HCl was added to the wood chips, the deposit had the same appearance as in the wood case, but it was much thicker and the surface structure reminded of an orange peel. The deposit formation rate (DFR) in g/(m2,h) is presented together with the results from the XRDanalyses in Table 5. The rates were of the same order of magnitude in all cases except for the HCl-addition case when it was 10-20 times higher. The XRDanalyses identified crystalline phases, and noncrystalline phases possibly present in the deposits could not be analyzed by this method. The compounds are listed in order of importance. In the wood case no distinction of the deposit on the windward side and on the flank could be made due to the very small amount of deposit. Therefore the windward side and the flank deposits

were analyzed together to obtain a more reliable result. The XRD-analyses show that KCl and K2SO4 were the main compounds containing potassium both in the windward side (including the flank deposit) and in the leeward side deposits, although also a trace of K2Ca2(SO4) 3 was found in the former. Some compounds containing calcium were also found in these deposits. With 40% coal admixture to the wood chips there was no deposit on the leeward side of the deposition ring, and the composition of the deposits on the windward side and on the flank were more complex with elements typical for coal ash. In the case with 75% coal and 25% wood chips, the deposit on the windward side was so thin that there was not enough material for a reliable analysis, so the windward side and the flank deposits were analyzed together, as in the wood case. The results were similar to the case with 40% coal and 60% wood chips showing that the deposits consist of typical coal ash compounds. There were no compounds containing potassium in the case with 75% coal and 25% wood chips. In the pure coal case, there were no deposits at all, and the ash banks observed on the flanks consisted of typical coal ash compounds. Again, no compounds containing potassium were observed. The XRD analyses from the HCl-addition case show that potassium chloride is the main compound in all the deposits examined. Traces of SiO2, CaCO3, and CaSO4 were also found in the deposits. Interpretation General. Alkali metals comprise both potassium and sodium, but potassium is the dominant source of alkali in most biomass fuels. It is contained in the major alkali compounds of concern when firing the problematic kinds of these fuels. The sodium content in the fuels studied was low, and since sodium plays a minor role compared to potassium as a nutrient for a plant, the following

Table 5. Deposit Formation Rates (DFR) in g/(m2,h) and Crystalline Phases Identified by XRD Analyses DFR g/m2-h

crystalline phases on windward side

crystalline phases on flank

crystalline phases on leeward side

wood

2.0 1.7

analyzed together with the windward side SiO2, CaSO4, K2Ca2(SO4) 3, MgO, K2O‚Al2O3‚SiO2 SiO2, CaSO4, MgFe2O4, Fe2O3 SiO2, CaSO4, Fe2O3 KCl, (SiO2, CaCO3)

KCl, CaCO3, K2SO4, MgO

40% coal + 60% wood 75% coal + 25% wood coal HCl-add

KCl, K2SO4, CaCO3, K2Ca2(SO4) 3, CaO CaO, K0.2Na0.8Cl, Ca(Fe,Mg)(CO3)2 analyzed together with the flank no deposit found KCl, (SiO2, CaCO3)

case

0.9 1.2 20.8

no deposit found no deposit found no deposit found KCl, (CaCO3, CaSO4)



description is focused on potassium, but potassium can be exchanged with sodium in most of the reactions presented. A comparison between potassium in woody biomass and potassium in coal gives a better understanding of the differences in behavior of potassium during combustion of these two types of fuel. Potassium in Woody Material. Potassium is an important nutrient and constitutes an essential part of the plants’ enzyme activity both for respiration and photosynthesis. The potassium ion is also important for the formation of proteins and starch. Potassium ions, as well as chlorine and calcium ions, are taken up in the root system and are thereafter transported to the various parts of the plant by the transport channels (the xylem). Unlike other nutrients, potassium does not appear to be structurally bound in the plant, but is highly mobile like nitrogen and phosphorus. Thus, during the plant’s period of growth, potassium is found as an ion in the sap and in the cell fluids. The potassium ions, together with the chlorine ions, regulate the osmotic pressure in the plant cells. There is also a concomitant transport of potassium and chlorine ions to equalize the electrochemical potential (over the plasma membranes) in the plant. To summarize, potassium exists in the plant to a high extent as a dissolved ion, surrounded by water molecules in the same manner as the chlorine ion.16-18 When a tree is cut down the ongoing metabolic processes stop. The wood starts to dry and the water content decreases. Depending on how far the drying process reaches, some, or all, of the potassium ions in the transport channels precipitate as, for example, potassium hydroxide, potassium carbonate, or potassium chloride. If the drying process has not reached far, some of the potassium ions will still remain dissolved in the water phase. To some extent, potassium can also be bound to the organic matrix in wood. Rydholm19 means that alkali metals occur in wood as oxalates and carbonates or as metal ions attached to the carboxyl groups of carbohydrates. Further, Wornat et al.20 have pointed out that the dominant fraction of the inherent inorganic material in many biomass fuels is most likely associated with oxygen-containing functional groups within the organic matrix. These groups provide sites for inorganic elements to become incorporated in the fuel matrix as, for example, cations. The potential amount of this type of inorganic material is greater in biomass than in most coals, mostly due to the higher oxygen content in biomass chars. Nevertheless, considering the high temperature environment in a combustion chamber, the potassium-containing compounds in wood, regardless of the drying status, are not chemically very stable. Therefore, under combustion conditions, the alkali metals are susceptible to vaporization. It is not straightforward to predict the actual melting temperatures for mixtures. The melting temperature of, for

example, K2CO3 is quite high (898 °C), and the impression might be that the compound is stable at high temperatures. This is, however, only true for the pure crystalline compound but not for impure substances. Potassium in Coal. The aging of coal is characterized by increasing concentration of carbon and decreasing concentrations of oxygen and hydrogen. The types of minerals found in coal depend on the location of the coal stratum. Often, geological changes have resulted in coal layers alternating between freshwater and saltwater sediments. The mineral matter in coal, including potassium, can be divided into three categories: 1. Inorganic species remaining in organic, mainly carboxylic, associations. Potassium is found in simple salts as potassium hydroxide, potassium carbonate, and potassium chloride. 2. So-called inherent minerals formed at the same time as the coal and finely divided throughout the coal matrix in horizontal assemblages, for example, kaolinite (Al2O3‚SiO2‚2H2O) and muscovite (K2O‚3Al2O3‚6SiO2‚ 2H2O). 3. Minerals formed by precipitation from various water solutions penetrating the coal layer, such as surface water with dissolved salts being transported downward or hydro-thermal solutions rising in pores and cracks. These minerals are often located in vertical formations in the coal seam, for example, calcite (CaCO3), siderite (FeCO3), pyrite (FeS2), halite (NaCl), gypsum (CaSO4‚2H2O), and sylvite (KCl). The most abundant forms are the alumino-silicate clay minerals together with quartz.21,22 Potassium bound in muscovite, category 2, is chemically very stable, while the stability is lower for category 1 and 3 compounds. In a combustion chamber, potassium from categories 1 and 3 takes part in chemical reactions, while the potassium in category 2 stays quite unreactive. Minerals from category 2, in the form of kaolinite, even sequester reactive potassium and turn it into unreactive chemical compounds.15 Thus, only a part of the potassium in coal is chemically reactive, and, in addition, some minerals in coal have a potential to capture potassium present in the combustion chamber. Combustion of Wood Chips. A fuel particle, in this case a wood chip, entering a combustion chamber heats. When the temperature reaches 100 °C the water evaporates. The water vapor diffuses out, while the drying front moves toward the center of the particle. After some time, the surface temperature has increased enough for the pyrolysis reactions to start, simultaneously with the evaporation of water inside the particle. The pyrolysis front moves toward the center of the particle, and when the surface temperature is high enough the surface ignites. Some char combustion may take place during the devolatilization phase (large particles), but the final char combustion proceeds while the particle is dry. Because of the stirred character of a fluidized bed the char particle will be exposed to an average release of water vapor from surrounding drying fuel particles. Since the wood chips may have a high moisture content, 40-50 wt %, a very humid and therefore extreme

(16) Weier, T. E.; Barbour, M. G.; Stockning, R. C.; Rost, T. L. Botany: An introduction to plant biology; John Wiley and Sons: New York, 1982; Chapters 5, 11. (17) Hopkins, W. G. Introduction to plant physiology; John Wiley and Sons: New York, 1995; Chapter 1, 2, 4, 5, and 8. (18) Bjo¨rkman, E.; Stro¨mberg, B. Energy Fuels 1997, 11, 1026-1032. (19) Rydholm S. A. Pulping Processes; Interscience: New York 1965. (20) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131-143.

(21) Raask, E. Mineral impurities in coal combustion; Hemisphere Publishing Corp.: New York, 1985. (22) Steenari, B.-M. Chemical properties of FBC ashes. Ph.D. Thesis, ISBN: 91-7197-618-3, Department of Environmental Inorganic Chemistry, CTH, Go¨teborg, Sweden, 1998.

26


Westberg et al.

environment prevails inside and around the particle while pyrolysis takes place. Even the combustion of char will be exposed to moist surroundings. As mentioned above, potassium in wood is mainly found dissolved in the water phase, or as, for example, potassium hydroxide, chloride, or carbonate. If potassium is dissolved in the water phase, it most probably enters the gas phase as a hydroxide. The same applies if it already has precipitated as a hydroxide. In the case of high water vapor concentration in the gas phase, potassium hydroxide is relatively stable and survives until it reacts to form an even more stable compound. The final product depends on the surrounding reactants. High water vapor concentration in the gas phase also stabilizes another component, HCl.1 The simultaneous presence of KOH and HCl in the gas phase increases the probability of reaction and formation of KCl. The fate of potassium hydroxide depends on the chemical environment, and it is difficult to give a complete list of possible reactions. The following merely indicates a set of plausible reaction paths that could take place at the low combustion chamber temperatures considered

KOH + HCl f KCl + H2O

(1)

KOH + SO2 f KHSO3

(2a)

KHSO3 + KOH f K2SO3 + H2O

(2b)

(ox.)

K2SO3 98 f K2SO4

(2c)

KOH + CO2 f KHCO3

(3a)

KOH + KHCO3 f K2CO3 + H2O

(3b)

Reactions 1 and 2 depend on the local HCl and SO2 concentrations in the gas phase. Reaction 3 gives an example of reactants competing with HCl and SO2 for reaction with KOH. The chlorine content in wood chips is 5-10 times lower than the potassium content. If the potassium in wood already has precipitated as KCl or as K2CO3, these are the most likely compounds to enter the gas phase. If potassium is bound to some carboxylic group in a hydrocarbon, it will probably enter the gas phase as a hydroxide, since the chemical bond between the potassium and the oxygen in the carboxylic group is quite weak. The bond breaks when the temperature increases, and at high surrounding water vapor concentration is potassium hydroxide the most plausible product. Some of the potassium in the wood will not enter the gas phase and some will enter the gas phase just for a short time, depending on the properties of the surroundings. If the fuel particle is moist, consists of large pieces, and has high density, the necessary prerequisites for an extreme environment in and around the particle during combustion prevail. If the fuel particles contain less moisture, are smaller, and have a lower density, as in the case of, for example, wheat straw with approximately 10 wt % of moisture, the environment in and around the fuel particle is not extreme with respect to water vapor. Dayton et al.,10 who studied combustion of small and relatively dry biomass particles, suggest that an overall high potassium content of a given

biomass feedstock does not necessarily correspond to high release of alkali metal vapor during combustion, and that the chlorine concentration in the fuel often dictates the amount of alkali found in the vapor phase, even more than the alkali concentration. This is an effect of low water vapor concentration in the gas phase, which has the consequence that only KCl, already precipitated in the fuel particle before combustion or during the first stage of combustion, survives and reaches the gas phase. Since KCl is among the most stable high-temperature gas-phase alkali-containing species, it will survive, but less stable compounds will not reach the gas phase. For sulfur, Dayton et al.10 did not find a correlation similar to that between gas-phase alkali and Cl-content in the fuel. There was just a weak correlation between alkali release and sulfur content, consistent with the work of Baxter et al.11 The most extensive studies of the mechanisms of alkali metal release during combustion of biomass were carried out by Dayton et al.1,10 This work consisted of laboratory measurements of alkali metal vapors released from several biomass feedstocks under different combustion conditions (5% to 20% O2 in He, at 800 °C and 1100 °C), using molecular beam mass spectrometry. At both reactor temperatures, only a minor part of the alkali metals was released during the devolatilization phase compared to the char combustion phase. Pyrolysis experiments by Jensen et al.23 and Olsson et al.24 confirm that the major part of the alkali metal release takes place at temperatures above 500-700 °C. Dayton1 showed that about 23 wt % of the potassium left a sample of switch grass during combustion in 5 to 20% O2 at 1100 °C, mostly in the form of potassium chloride, almost independent of the inlet oxygen concentration. If 20% water vapor was added to the inlet gas (at 10% O2 and 1100 °C), the alkali metal release remained approximately constant, but the potassium in the vapor phase shifted from KCl to KOH and the share of KCl decreased from 23% to 15 wt % of the potassium in the switch grass. Obviously, the surrounding water vapor concentration affected the form in which alkali metal was found in the gas phase. Jensen et al.23 showed that chlorine leaves the fuel particles in two steps, about 60% when the temperature increased from 200 to 400 °C, while most of the residual chlorine is released between 700 and 900 °C. Also Dayton et al.1 found that HCl is released primarily during the devolatilization phase. Furnace temperature and oxygen concentration have little or no effect on the formation of HCl, but the addition of water vapor enhances the formation of HCl during the char combustion phase. It is obvious that a major part of the chlorine in biomass fuels is not released at the same time as the alkali metals, or not even during the same combustion phase. Still some KCl is formed. Application to the Present Test Results. When there is chlorine in the fuel, potassium can be released as KCl to the gas phase during conversion. However, if the potassium is in a chemically reactive form, i.e., such as it exists in biomass and if the water vapor concentration in and around the fuel particle is high, then the (23) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (24) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. Energy Fuels 1997, 11, 779-784.



potassium may be released as potassium hydroxide. If the water vapor concentration is high enough to stabilize the KOH until it meets other reactants in the surroundings, whose reactivity and concentration are sufficient, especially HCl and SO2, KCl, K2SO4, and of course also K2CO3, can be formed according to reaction paths 1-3. By changing the fuel composition from pure wood to that of co-combustion of wood and coal, and to pure coal, the combustion conditions were changed to give an opportunity to study the effects of potassium reactivity and water vapor concentration. The measured concentration of water vapor in the flue gases decreased with increasing share of coal from 20% to 8% (Figure 6), but the local, not measured, water vapor concentration around the wood particles in the combustion chamber could be higher than these average values and indeed higher than for coal particles. The potassium input, on the other hand, did not change much during the tests (Figure 3). Except for KCl, the only plausible gas-phase compounds containing chlorine at high temperatures are HCl and Cl2, but no Cl2 was detected during the present tests. The inputs of chlorine, potassium, and sodium did not vary much for the different fuel mixes (Figure 3a), but still the HCl concentration increased substantially with the coal share (Figure 6). In each case, the HCl concentration remained constant between the cyclone inlet and the inlet of the convection path (Figure 6a,b). The concentration decreased only in the outlet of the convection path (Figure 6c), where the temperature is lower and the probability of formation of ash products such as CaCl2 increases. Obviously, HCl is released/ formed early in the combustion process and reactions involving HCl already have taken place in the combustion chamber upstream of the cyclone. Due to dilution by sand and low chlorine and potassium concentrations in the bed (Figure 7a), the results related to the bed will not be further discussed. More interesting are the ash flows from the secondary cyclone and the bag filter. The K, S, and Cl concentrations increase (especially in the wood case) with decreasing particle size in the fly ash, that is, from the cyclone to the filter (Figures 4,8a,9a). The concentrations in the figures are given in mmol/kg-dry ash, and it is directly seen that with coal admixture the potassium concentration is at least five times higher than the chlorine concentration, but this difference decreases with increasing wood share (Figures 8a,9a), and for pure wood the molar ratio of potassium to chlorine is unity in the filter ash. This information, further supported by the ash constituents from Table 4, indicates that the potassium was found as KCl in the wood case, and that KCl was only present to a minor extent in the coal case. The input of potassium and chlorine was about the same in all cases (Figure 3), and obviously the reactivity of potassium toward chlorine decreases with increasing coal share. The reason KCl ends up in the fly ash, and especially in the bag filter ash, can be sought in the stability of gaseous KCl at high temperatures, and also in the particle size of this ash. As a result of the chemical stability of KCl, once formed it is not very reactive (however, the influence of SO2 will be discussed below), and consequently most KCl survives in the gas phase until the temperature drops and homogeneous

nucleation or condensation of KCl takes place predominantly on small particles. The sulfur input (Figure 3) and the SO2 concentration in the flue gas (Figure 6) increased with coal share, but the rise in the exit flow of sulfur with the fly ashes was not dramatic (Figures 4 and 9). In the secondary cyclone ash, the S, Na, Si, or Al concentrations did not change much, except in the case of pure wood, while the K, Ca, and Cl concentrations decreased with increasing coal share (Figure 8). Because of the high ash content in coal (Figure 4), the actual exit fly ash flow of all elements, except chlorine, increased with coal share. No crystalline phases containing CaSO4 or K2SO4 were identified by XRD, but still it is most probable that calcium or potassium had captured the sulfur found in the ashes. To summarize, the reactivity of potassium toward chlorine decreases with coal admixture for at least three reasons: higher SO2 concentrations, less ability of potassium in coal to form KOH, and the lower concentration of water vapor around the coal particle to stabilize KOH formed. As a result of the many changing parameters and the complex situation, no clear trend could be seen for the potassium-sulfur chemistry. In the pure wood case, the input of potassium was six times higher than that of chlorine and fourteen times higher than the input of sulfur (Figure 3a), so even if all the chlorine and sulfur reacted with potassium, there would still be potassium left over. If the potassium is present in the form of potassium hydroxide, lacking chlorine and sulfur, it reacts with what is present, for example CO2, and with what it can form a stable compound. If this idea is correct, a higher input of chlorine, while keeping all other parameters constant, would increase the potassium and chlorine concentrations in the fly ashes. This is what happened: the added chlorine in the HCl addition test reacted with the potassium hydroxide present according to reaction 1 and formed KCl, which ended up in the fly ashes (Figure 10a) and on cold surfaces. The experiences from the analyses of the composition of the ashes just explained are quite similar to the observations regarding the deposits on the deposition probe: a substantial deposit with a high content of KCl was found for pure wood, different from the case of coal (Figure 11 and Table 5). During addition of HCl the deposition rate was 10 to 20 times higher, mostly containing KCl. It was observed that the composition of the combustion gas in the cyclone inlet changed with the addition of HCl. Although the input flow of chlorine increased almost six times, the concentration of HCl in the combustion chamber barely doubled, while the CO concentration doubled and the SO2 concentration was 4-fold (Table 3). Most of the chlorine supplied obviously reacted and formed KCl to a great extent but no other gaseous chlorine compounds were detected. The rise of the CO concentration can be explained by the radical inhibition effect of halogens.25,26 Halogens are wellknown flame inhibitors, and by quenching of radicals, the CO oxidation to CO2 decreases. The rise of the SO2 concentration is most likely a consequence of a competi(25) Goulakrishnan, P.; Lawrence, A. D. Combust. Flame 1999, 116, 640-652. (26) Casias, C. R.; McKinnon, J. T. Combust. Sci. Technol. 1996, 116-117, 289-315.

28


tion between SO2 and HCl for the same reactant, i.e., KOH. When the HCl input increases, the probability decreases for SO2 to react with KOH. However, the sulfur concentration increased in the bag filter ash during HCl addition (Figure 10b). Iisa et al.27 studied the sulfation of KCl in both vapor and molten phases under combustion conditions (with an extremely high SO2 concentration, 1-4%), and found that the sulfation of KCl was significantly faster in the vapor phase than in the molten phase. The authors concluded that most of the in-flight KCl sulfation in a boiler takes place in the gas phase and that the sulfation of condensed KCl aerosol particles is limited. In the cyclone inlet the temperature is still high (>870 °C), so most KCl formed is in the vapor phase and could thus be sulfated. The degree of sulfation depends on the concentrations of SO2 and KCl in the gas phase and both were increased by the addition of HCl. The higher concentrations of sulfur and potassium found in the bag filter ash indicate that the sulfates formed ended up in the smaller particles during addition of HCl. This is another indication of formation of sulfates from KCl rather than from KOH in the combustion chamber. Conclusions If potassium has a chemically reactive form, such as it has in biomass fuels, and if there is a high concentration of water vapor in and around the fuel particle, potassium could be released and stabilized in the form of potassium hydroxide. The final potassium-containing product is then determined by the reactivity and concentration of the other reactants in the surrounding gas phase, especially HCl and SO2. Depending on the availability of chlorine in the fuel particle, potassium can also be released as KCl, but in the case of pure wood investigated here the low concentrations of chlorine and sulfur in the fuel limit the formation of KCl and K2SO4, both in the fuel particle and in the gas phase. (27) Iisa, K.; Lu, Y.; Salmenoja, K. Energy Fuels 1999, 13, 11841190.

Westberg et al.

The addition of HCl to the combustion chamber in the pure wood case substantially increased both the potassium and the chlorine yields in the fly ashes and on the deposition probe. KCl was shown to be a dominant constituent, which proves the existence of reactive KOH in the combustion chamber forming KCl according to

KOH + HCl f KCl + H2O During wood combustion, the chlorine concentration in the combustion chamber is the limiting factor for formation of KCl and deposit growth. The initial deposit formation rates were the same for coal, wood, and their mixtures (0.9 to 2.0 g/(m2 h)), but during addition of HCl the rate was 10 to 20 times higher. The composition of the deposits was completely different for coal and wood. The same amount of potassium was supplied with coal and wood and the supply of chlorine with coal was the same as in the case of addition of HCl to wood, but with coal admixture to the fuel the reactivity of potassium toward chlorine decreases. Only minor quantities of KCl were traced in the ashes and there were no alkali deposits on the deposition probe, neither on the front side nor on the rear side. (If there had been deposits only on the rear side one could have suspected abrasion because of the higher ash flow in the coal case). There are at least three reasons for the behavior during coal combustion: higher SO2 concentration, less ability of potassium in coal to form KOH, and lower water vapor concentration around the coal particle. The water is needed to stabilize the KOH formed. Due to the many changing parameters and the complex situation, no clear trends could be confirmed for the potassium-sulfur chemistry. Acknowledgment. The work has been supported financially by a grant from Mid Swedish University (M.B.) and from Swedish National Energy Administration (H.M.W.). The authors gratefully acknowledge helpful opinions received from Professor Mikko Hupa, A° bo Akademi University. EF020060L

Distribution of Potassium, Chlorine, and Sulfur between Solid and

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