Secondary Capture of Chlorine and Sulfur during Thermal Conversion

In this work, the interactions of chlorine and sulfur with biomass char during .... The binding energy of chlorine and sulfur in char and char exposed...
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Energy & Fuels 2005, 19, 606-617

Secondary Capture of Chlorine and Sulfur during Thermal Conversion of Biomass Jacob N. Knudsen, Peter A. Jensen,* Weigang Lin, and Kim Dam-Johansen Combustion and Harmful Emission Control (CHEC) Research Centre, Department of Chemical Engineering, Technical University of Denmark, Building 229, 2800 Kgs. Lyngby, Denmark Received May 20, 2004. Revised Manuscript Received November 23, 2004

In larger combustion facilities, secondary reactions between char and chlorine and sulfur may be important. In this work, the interactions of chlorine and sulfur with biomass char during thermal conversion have been experimentally investigated. A laboratory-scale fixed-bed reactor was applied to study the capture of HCl and SO2 by biomass char in the temperature range of 400-950 °C. The observed reaction rate was sufficient for significant recapture of HCl and SO2 to occur under conditions typical for fixed-bed combustors. A maximum in the chlorine and sulfur retention existed at ∼600 °C. Relatively high amounts of chlorine and sulfur could be retained in the char samples over the entire temperature range, compared to the inherent chlorine and sulfur content in the biomass. Spectroscopic and chemical analyses revealed that HCl was mainly captured by the inherent metal species, whereas SO2 was mainly captured by the organic matrix. As a result, the maximum chlorine retention was dictated by the inherent metal content of the biomass. Combustion of the chlorine- and sulfur-laden char samples resulted in high retention values of chlorine and sulfur in the ash at temperatures up to 600 and 800 °C, respectively. At higher combustion temperatures, chlorine and sulfur was gradually released to the gas phase, because of evaporation of KCl and dissociation of sulfates. A larger fixed-bed reactor was applied to simulate the combustion process that occurs in industrial-scale grate-fired boilers. Combustion of wheat straw samples in the large fixed-bed reactor confirmed that higher retention values of both chlorine and sulfur could be obtained, compared to the smaller laboratory reactor, presumably because of secondary capture.

Introduction The relatively high content of inorganic matter of volatile naturesmainly chlorine, potassium, and sulfurs in annual biomass is well-known1-4 to cause severe problems when combusted in boilers for heat and power production. The formation of high mass loadings of aerosols and the immense deposition of potentially corrosive components on heat-transfer surfaces are among the encountered problems. Generation of SO2 and HCl emissions that are on the edge of the allowed emission limits are frequently observed.2 These problems are directly related to the amount of chlorine, potassium, and sulfur that is volatilized from the fuel during conversion. The release of chlorine, potassium, and sulfur to the gas phase has been addressed in a limited number of investigations.5-12 It has been shown that large amounts of HCl and sulfur are released to * Author to whom correspondence should be addressed. Fax: +45 45 88 22 58. E-mail address: [email protected]. (1) Michelsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-08. (2) Sander, B.; Henriksen, N.; Larsen, O. H.; Skriver, A.; Ramsgaard-Nielsen, C.; Jensen, J. N.; Stærkind, K.; Livbjerg, H.; Thellefsen, M.; Dam-Johansen, K.; Frandsen, F. J.; van der Lans, R.; Hansen, J. Emissions, Corrosion and Alkali Chemistry in Straw-Fired Combined Heat and Power Plants, 1st World Conference on Biomass for Energy and Industry, June 2000, Sevilla, Spain. (3) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421-444. (4) Baxter, L. L. The Behavior of Inorganic Material in BiomassFired Power Boilers: Field and Laboratory Experiences. Fuel Process. Technol. 1998, 54, 47-78.

the gas phase during the initial fuel devolatilization at temperatures as low as 200-400 °C.5-12 At higher temperatures, the release of chlorine and sulfur, because of the evaporation of KCl and the evaporation or dissociation of K2SO4, has also been observed.5,6,8,9,11,12 In a study9 on wheat straw pyrolysis, it was found that the fraction of fuel chlorine released as HCl between 200 °C and 400 °C decreased when a larger sample was used, presumably because of interactions between HCl and the char matrix. Similarly, it has been observed11 that SO2 can be captured in wheat char during pyrolysis at 600-800 °C and partially retained in the ash during subsequent combustion. The reaction of carbonaceous materials with different gaseous sulfur compounds (H2S, SO2, COS, CS2) have also been studied13-17 in other (5) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855-865. (6) 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. (7) Bjo¨rkman, E.; Stro¨mberg, B. Energy Fuels 1997, 11, 1026-1032. (8) Olsson, J. G.; Ja¨glid, U.; Pettersson, J. B. C. Energy Fuels 1997, 11, 779-784. (9) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280-1285. (10) Zintl, F.; Stro¨mberg, B.; Bjo¨rkman, E. Release of Chlorine from Biomass at Gasification Conditions. Presented at the 10th European Conference on Biomass for Energy and Industry, Wu¨rzburg, Germany, June 1998. (11) Knudsen, J. N.; Jensen, P. A.; Frandsen, F. J.; Lin, W.; DamJohansen, K. Energy Fuels 2004, 18, 810-819. (12) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385-1399.

10.1021/ef049874n CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005

Cl/S Capture during Thermal Conversion of Biomass

Figure 1. Schematic illustration of straw combustion in (a) a continuous moving-bed reactor and (b) a fixed-bed reactor (batch).

contexts. It is generally accepted that highly temperature-stable carbon-sulfur complexes are formed when gaseous sulfur compounds react with carbonaceous materials at elevated temperatures.13-15 In addition, it has been shown that the retention capability of carbon is high and several weight percent of sulfur may be retained.13-15 The interactions of HCl and gaseous sulfur species with char imply that the net volatilization of chlorine and sulfur from biomass, among other things, could be affected by the scale and the contact pattern of the applied combustion device. Thus, the fractional volatilization of chlorine and sulfur measured in small laboratory reactors could, in principle, be different from that observed under similar conditions in large industrialscale furnaces. Moreover, with knowledge of the influence of secondary reactions, the combustion conditions could perhaps be optimized to enhance the secondary capture of chlorine and sulfur for better retention in the bottom ash. In Denmark, annual biomass fuels are frequently combusted in grate boilers ranging from a few megawatts up to 100 MWth. The high amount of fuel and char present on the grate in such systems indicates that secondary reactions of chlorine and sulfur in the bed could be important. Combustion of relatively dry biomass fuels, such as straw, on a moving grate ideally occurs as illustrated in Figure 1a.18,19 A primary reaction front propagates downward from the top of the bed. In this narrow zone, the fuel undergoes heating and drying, followed by devolatilization and partial combustion of volatiles and char. The fractional conversion of volatiles and char in the primary reaction front is highly dependent on the local air-to-fuel stoichiometry in the bed. After the primary reaction front has reached the grate, the residual char is oxidized in a secondary combustion front, which propagates upward from the grate. Because the primary air drives the volatiles through the adjacent char layer, recapture of HCl and gaseous sulfur species contained in the volatiles may occur. However, whether (13) Puri, B. R. Chem. Phys. Carbon 1970, 6, 191-282. (14) Puri, B. R.; Hazra, R. S. Carbon 1971, 9, 123-134. (15) Chang, C. H. Carbon 1981, 19, 175-186. (16) Panagotidis, T.; Richter, E.; Ju¨ntgen, H. Carbon 1988, 26, 8995. (17) Ratcliffe, C. T.; Pap, G. Fuel 1980, 59, 237-243. (18) Van der Lans, R.; Pedersen, L. T.; Jensen, A.; Glarborg, P.; Dam-Johansen, K. Biomass Bioenergy 2000, 19, 199-208. (19) Thunman, H.; Leckner, B. Fuel 2001, 80, 473-481.

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significant recapture occurs during the relatively short residence time of the volatiles in the char layer (a few seconds) will probably be dependent on the reaction chemistry, mass-transfer rate, and thickness of the char layer. In this study, the nature of HCl and SO2 capture in biomass char and the transformations of captured chlorine and sulfur during char burnout have been experimentally investigated. The main objective has been to evaluate the significance of the char matrix in the transformation of fuel chlorine and sulfur during thermal conversion of annual biomass under conditions that resemble grate combustion. Experimental Section Laboratory Reactor. The ability of biomass char to capture HCl and SO2 from a gas stream was investigated using a laboratory scale fixed-bed reactor. A schematic illustration of the experimental setup is shown in Figure 2. The laboratory flow reactor consisted of a ceramic tube placed horizontally in a three-zone electrically heated oven. The reactor was equipped with a water-cooled chamber at one end for quenching of the sample. The primary and secondary gases were supplied to the reactor through a series of mass flow controllers (MFCs). The biomass sample was contained in a cylindrical alumina tube, equipped with a gas-distributor plate in one end. The dimensions of the sample tube are shown in Figure 2. A typical experiment was performed as follows. The reactor was heated to a preselected temperature. The sample tube was loaded with ∼10 g of biomass and inserted into the watercooled chamber of the reactor. The reactor was sealed and 0.5 NL/min of nitrogen was supplied through the distributor plate of the sample tube. A flow of 3.5 NL/min nitrogen was supplied as carrier gas to the water-cooled chamber to prevent the buildup of combustible gases. Secondary air at a feeding rate of 2.0 NL/min was injected through a distributor probe downstream of the reactor to facilitate burnout of the volatiles. After a few minutes of purging, the sample was quickly inserted into the hot zone of the reactor. The pyrolysis process was continued until the preselected temperature was reached (30-40 min) and the CO and CO2 concentrations were 700 °C. The absorption experiment was stopped by switching off the HCl/SO2 supply and flushing the system with pure nitrogen. At this stage, the char sample could either be collected for


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Knudsen et al.

Figure 2. Schematic illustration of the experimental setup and sample probe. analysis or combusted by supplying an oxygen flow through the sample. In the former case, the char sample was withdrawn from the hot zone and allowed to cool to ambient temperature in the water-cooled chamber, while maintaining a nitrogen atmosphere. In the case of the latter, the oxygen concentration was increased in steps, starting at 3.0 vol % oxygen and ending at 21 vol %. The progression of the combustion was followed by online monitoring of the combustion products CO and CO2. This burn-out procedure was applied to maintain the sample temperature as close as possible to the set-point temperature of the reactor, i.e., to avoid uncontrolled heating of the char particles during burnout. Using a thermocouple placed directly in the char sample, it was verified that the sample temperature never exceeded the set point by more than 60 °C. Approximately 2 h were needed to completely oxidize the char samples. When no further CO or CO2 (600 °C. Because the reaction of SO2 with char involves a change in the oxidation stage of sulfur (reduction), it is

Knudsen et al.

Figure 9. Retention of chlorine and sulfur char in ash after complete burnout of char samples (W1) exposed to HCl and SO2, respectively. HCl and SO2 exposure given in units of mmol/g straw.

likely that it is affected by the local conditions, i.e., oxidizing or reducing. On the other hand, the capture of HCl by metal cations is a form of acid-base reaction, which does not involve a change in the oxidation stage of chlorine; therefore, it is less likely to be dependent on local conditions. The influence of local conditions on the SO2 capture is further investigated in the following section. In summary, the experimental investigation implies that biomass char may capture HCl and SO2 from a gas stream in significant amounts at temperatures relevant for combustion and gasification of biomass in fluidizedand fixed-bed combustors. Because the residence time of the gas in the char sample was chosen to be similar to that of the gas in the bed of grate-fired furnaces, the experiments suggest that recapture of chlorine and sulfur species may occur during the grate firing of annual biomass. However, the results from the laboratory investigation cannot be quantitatively applied to grate-firing, among other things, because of the difference in local conditions, e.g., char produced in pure N2 and exposed to HCl or SO2 in pure N2. Chlorine and Sulfur Retention during Char Burnout. During char burnout, the captured chlorine and sulfur may either be re-released to the gas phase or retained in the ash residue, because of the relatively high content of metals (mainly calcium and potassium) in annual biomass.2,12 In cases where the latter is important, the overall volatilization of chlorine and sulfur from annual biomass could, to some extent, be reduced by the recapture on char. The retention of chlorine and sulfur during the char burnout of char samples exposed to HCl and SO2 has been quantified in the 400-900 °C range, as shown in Figure 9. The absorption and combustion steps have been conducted at the same temperature. Two different quantities of HCl and SO2 were applied to the char samples, corresponding to the metal content of the straw being present in molar excess or deficiency, relative to the anion-forming elements. Figure 9 shows that, if char samples exposed to 0.089 mmol SO2/g straw (150% of straw sulfur) are combusted at 400-800 °C, more than 90% of the char sulfur is retained in the ash, whereas at 900 °C, the sulfur retention decreases to 40%. If the higher load of SO2 (0.45 mmol SO2/g straw, or 750% of

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Table 6. Amount of HCl and SO2 Added to the Char Samples (W1) Prior to Combustion amount of component added (mmol/g straw) +0.089 +0.45 +0.16 +0.4

βa 600 °C

800 °C

Component: SO2 0.68 0.97

0.46 0.53

0.66 0.90

0.39 0.35

Component: HCl

a Degree of saturation. β ) (2S + 3P + Cl) ash/(2Ca + 2Mg + K)fuel, where the subscripts “fuel” and “ash” denote the molar element concentration in the straw and in the straw ash, respectively.

straw sulfur) is applied, only 60% of the char sulfur is retained in the ash during combustion at 500 °C and the sulfur retention decreases even further, up to 800 °C. Figure 9 furthermore shows that the retention of chlorine is equally as high as that for sulfur at 400 and 600 °C, even if a relatively high load of HCl is applied to the char (0.40 mmol/g straw, or 240% of straw chlorine). However, at >600 °C, the chlorine retention decreases radically and only 10%-25% is retained at 800 °C. The decrease in chlorine and sulfur retention when HCl and SO2 are added in excess to the metal constituents of the sample clearly indicates that the chlorine and sulfur retention during burnout is limited by the available amount of metals, mainly calcium and potassium. The degree of saturation of the ash (β) can be expressed as the molar ratio, (Cl + 3P + 2S)ash/(2Ca + 2Mg + K)fuel, if it is assumed that all of the metals in the fuel is completely retained in the ash. After combustion of the samples exposed to the higher amount of HCl and SO2 at 600 °C, Table 6 reveals that the degree of saturation β is close to unity. This indicates that virtually all of the metals are combined with the added chlorine and sulfur, i.e., the ash is saturated and no more chlorine and sulfur can be retained. In contrast, after combustion at 800 °C, Table 6 shows that the β values are 0.35 and 0.53 for the HCl- and SO2-treated samples (high load), respectively. This indicates that, at 800 °C, chlorine and sulfur cannot be retained in stoichiometric amounts, relative to the calcium and potassium content of the fuel, even though excess HCl and SO2 are supplied. It has been documented12 that the evaporation of KCl during combustion of annual biomass primarily occurs at 700-800 °C. Above 700 °C, chlorine will also be largely released to the gas phase from chlorides of calcium and magnesium, because of increasing equilibrium vapor pressure.27,28 Thus, the decreasing chlorine retention during the burn-out stage at temperatures of >700 °C observed in Figure 9 is caused by the increased volatility of the metal chlorides in the system. It was furthermore shown in Figure 9 that the sulfur retention during char burnout decreased notably at ∼800 °C. The volatility of K2SO4 and CaSO4 in the temperature range of 600-900 °C is low;11,12,29 hence, (27) Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H. Ind. Eng. Chem. Res. 1992, 31, 164-171. (28) Shemwell, B.; Levendis, Y. A.; Simons, G. A. Chemosphere 2001, 42, 785-796.

Figure 10. Chlorine content in ash, relative to the original chlorine content of the straw (W1). HCl exposure given as a percentage of the chlorine content of W1 straw. Reference has no HCl.

Figure 11. Sulfur content in ash, relative to the original sulfur content of the straw (W1). SO2 exposure given as a percentage of the sulfur content of W1 straw. Reference has no SO2.

the fact that sulfur cannot be retained in almostequivalent amounts to the calcium and potassium content in the fuel at 800 °C (see Table 6) cannot be attributed to the volatilization of sulfates. This suggests that potassium and calcium are consumed by another source. Earlier work9,11,12 has shown that, at temperatures above ∼700-800 °C and in the presence of silicate, calcium and potassium are gradually incorporated into silicate structures, which implies that less calcium and potassium is available to capture the sulfur that is released from the char matrix. Consequently, less sulfur can be retained in the ash than that indicated by the calcium and potassium content in the fuel. A fraction of the potassium in the fuel may also be released to the gas phase at 800 °C, because of the straw’s inherent chlorine content. In Figures 10 and 11, the chlorine and sulfur content in the ash, relative to the original chlorine and sulfur content of the straw, is depicted for untreated char, char + HCl, and char + SO2, respectively. It seems that, in the entire investigated temperature range, additional chlorine and sulfur is retained in the ash, compared to the untreated samples (reference). However, it is also seen that, at temperatures above 700 and 800 °C, (29) Cheng, J.; Zhou, J.; Liu, J.; Zhou, Z.; Huang, Z.; Cao, X.; Zhao, X.; Cen, K. Progr. Energy Combust. Sci. 2003, 29, 381-405.


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Figure 12. Influence of char conversion on the SO2 capture ability. Sulfur retention when SO2 is added in N2 prior to combustion (0% char conversion) is defined as an index value of 100. Settings: 700 °C, 10 g straw (W1), +0.089 mmol SO2/g straw.

respectively, less chlorine and sulfur than contained in the fuel itself (i.e.,