Sulphation characteristics of paper sludge ash.

Roh, Seon Ah ; Kim, Sang Done

INTRODUCTION

Sludge is the largest by-product from waste water treating plants and its disposal is one of the most challenging environmental problems in waste water treating processes. Until now, landfill is the preferred disposal method of sludge; however, landfill is no longer a viable solution because of the decrease in available space, rising tipping fees and the growing concern of their impact on the environment. Thermal utilization of sludge may be an economic and sustainable disposal solution with many advantages, such as reduction of the disposed solid mass

and volume, lower disposal cost and recovery of carbon energy from sludge (Namkung et al., 2004; Roh et al., 2005). For biomass with low heating value, such as sludge, co-combustion with fossil fuel in the existing facilities has been extensively studied. Biomass co-combustion is known to have a significant positive effect for [SO.sub.2] emission that is attributed to the low sulphur content of biomass fuels (i.e., dilution effect) and increased sulphur retention in the ash. The effects of co-firing sewage sludge (Folgueras et al., 2004), paper sludge (Tsai et al., 2002), and agricultural residuals, such as straw, manure and rice husk (Nordin, 1995) as well as especially cultivated energy plants (Hein and Bemtgen, 1998) with coal have been studied. Various biomass fuels contain relatively large amounts of calcium, potassium, magnesium and sodium in reactive forms (Nordin, 1995). These minerals have shown the possibility to reduce sulphur emissions when co-combusting biomass and high sulphur fuels (Nordin, 1995; Hein and Bemtgen, 1998; Tsai et al., 2002; Grubor et al., 2003; Folgueras et al., 2004). The sulphur retention is due to the formation of sulphates, for example [CaSO.sub.4], [K.sub.2][SO.sub.4], and [Na.sub.2][SO.sub.4] (Nordin, 1995; Cheng et al., 2004). Therefore, the amount of fuel-ash-related sulphur sorption increases during co-combustion and sorbents for sulphur reduction may not be necessary if proper control of the biomass feed is maintained (Tsai et al., 2002). However, studies on the sulphation characteristics of biomass ash are comparatively sparse though the possibilities of sulphur retention on biomass ash have been reported in combustors (Nordin, 1995; Hein and Bemtgen, 1998; Tsai et al., 2002; Folgueras et al., 2004). In this study, the sulphation characteristics of calcium-rich paper sludge ash were determined for the application to co-combustion of biomass and coal. The calcium in paper sludge ash originates from the limestone filler used in the manufacturing process to increase the density and whiteness of the paper (Tsai et al., 2002). The effects of sulphation temperature, particle size and [SO.sub.2] concentration on sulphation conversion were determined in a thermobalance reactor and XRD and SEM-EDX are used for the analysis of sulphated ash.

EXPERIMENTAL

The paper sludge employed as a sorbent in this study was from two paper mills in the Chungnam Province, Korea. The proximate and ash analyses of the paper sludge and calcined limestone (Kangwon Province, Korea) used as a reference material are shown in Table 1. As can be seen, most of the combustible is volatile matter for both paper sludge samples and little differences are shown in volatile matter and ash compositions. However, CaO content in the ashes is higher than 30% since they come from limestone, a filler in the paper manufacturing process. Paper sludge ashes and calcined limestone were prepared by heating the samples in a stream of air to 900 [degrees]C at a heating rate 15 [degrees]C/min and soaking for 30 min at 900 [degrees]C in a Lindberg furnace. Five different sizes (107 [micro]m, 327 [micro]m, 750 [micro]m, 1 mm and 2.6 mm) of paper sludge ashes were prepared for the sulphation study.

The sulphation reactions of paper sludge ash and limestone were carried out in a thermobalance reactor (0.055 m-I.D. x 1 m-high). Detailed descriptions of the experimental apparatus and procedure can be found elsewhere (Kang et al., 2000). The reactor was heated to a desired temperature (750-900 [degrees]C) in a mixture of [SO.sub.2] (2000-7000 ppm) and air with a total flow rate of 11 l/min. The ash (0.5 g) was placed in a stainless steel wire mesh basket suspended from an electronic balance and lowered into the reaction zone.

The sulphation reaction is expressed as:

CaO + [SO.sub.2] + 1/2[O.sub.2] [right arrow] [CaSO.sub.4] (1)

The sulphation conversions ([X.sub.s]) of the samples (paper sludge ash and calcined limestone) were determined by the mass gain with time recorded in a personal computer from an electronic balance based on the CaO molecule as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2)

where [W.sub.0] is the initial weight of [SO.sub.2] sorbent, [y.sub.c] is the CaO weight fraction in the sample, [M.sub.CaO] and [M.sub.SO3] are the molecular weight of CaO and [SO.sub.3], respectively, and [DELTA] w is the weight gain of the sample at time t (Kang et al., 2000).

The produced sulphur compounds after sulphation were analyzed by X-ray diffraction (XRD) method. The surface and pore variations of the paper sludge ash and limestone due to sulphation were observed by scanning electron microscope (SEM). The specimens were coated with gold in a conventional sputtering system prior to observation under the SEM. Prior to energy dispersive X-ray (EDX) analysis, the samples were embedded in resin, cross-sectioned, polished and coated by carbon and the samples were analyzed for mapping sulphur, calcium and other atoms in the paper sludge ash.

RESULTS AND DISCUSSION

Sulphation Characteristics of Paper Sludge Ash

The sulphation conversion of paper sludge ash A (0.5 g) having the average particle size of 327 [micro]m at [SO.sub.2] partial pressure of 3000 ppm are shown respectively in Figures 1 and 2 as a function of reaction temperature (750-900 [degrees]C) based on the uniform-reaction and unreacted-core models (Kunii and Levenspiel, 1991). Briefly, the kinetic equation of the uniform reaction and unreacted-core models are expressed, respectively, as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4)

where X is the sulphation conversion, k is pre-exponential factor, E is activation energy, T is sulphation temperature, [P.sub.SO2] is [SO.sub.2] partial pressure and n is reaction order.

As can be seen, the uniform-reaction model represents the experimental data better (correlation coefficient, [r.sup.2] > 0.99) than the unreacted-core model (correlation coefficient, [r.sup.2] > 0.96) for sulphation of the paper sludge ash.

An Arrhenius plot for the paper sludge ash A is shown in Figure 3. The activation energy and pre-exponential factor are obtained from the gradient and intersection of the logarithm of the reactivity (ln k) vs. 1/T. The activation energy and the preexponential factor are found to be 33 172 kJ/kmol at 8.88 x [10.sup.-6] [s.sup.-1] [Pa.sup.-1] based on the uniform-reaction model. The obtained activation energy of the sludge ash is somewhat lower than that of the limestone (E [greater than or equal to] 68 880 kJ/kmol) (Kang et al., 2000).

The effect of [SO.sub.2] concentration (2000-7000 ppm) on the sulphation reactivity is shown in Figure 4 where sulphation rate increases with increasing [SO.sub.2] partial pressure. From the slope of the line (ln k vs. ln [SO.sub.2] partial pressure), the reaction order is found to be 1.0 as in the case of calcined limestone particles (Kang et al., 2000).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The predicted sulphation conversion of the paper sludge ash from the uniform-reaction model with the experimental data is shown in Figure 5 where the kinetic model predicts the experimental data very well. The sulphation reactivity and sulphur capture capacity of the paper sludge ash increase with increasing sulphation temperature (Kang et al., 2000) with high sulphation conversions (maximum 70% for 4 h sulphation). This may suggest that the [CaSO.sub.4] formed from the direct sulphation is stable and does not decompose in this temperature range. Hence, the direct sulphation is favourable for high sulphation conversion. Sulphation conversion of the ash is found to be higher than that of the calcined limestone having low (25-45%) reactivity (Kang et al., 2000; Laursen et al., 2003). The [CaSO.sub.4] product in the calcined limestone seals the outer reaches of pore that causes the centre of the particles to remain essentially unsulphated since [CaSO.sub.4] has a larger molar volume than [CaCO.sub.3] and CaO. This behaviour is more pronounced with the larger calcined limestone particles (Howard, 1983; Laursen et al., 2003).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The effect of particle size of the ashes A, B and the calcined limestone on sulphation conversions at 900 [degrees]C at [SO.sub.2] partial pressure of 3000 ppm are shown in Figure 6. Regardless of the ash size (107 [micro] m-2.6 mm), sulphation conversion of the ash A is more than 60%. Also, high sulphation conversion can be obtained with the ash B having particle sizes of 327 [micro]m and 1 mm. But, sulphation conversions of the calcined limestone are 33% with 327 [micro]m particle size and 25% with 1 mm particle size having the larger difference in the initial and the final sulphation conversions.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

XRD and SEM-EDX Analysis

Fine distribution of CaO and other metal compounds in paper sludge ash comes from the paper manufacturing and waste water treatment processes. In the paper manufacturing process, filler was added through crushing and sieving processes and the filler size became less than several microns. Waste water from this process undergoes biological and chemical treatments and the final residue is sludge. Therefore, CaO and other added chemicals are finely distributed in the sludge.

In addition to CaO, the paper sludge ash contains many other minerals like [SiO.sub.2], [Al.sub.2][O.sub.3], MgO and [Fe.sub.2][O.sub.3], etc. (Table 1). However, these minerals are known to have little sulphur capture ability in the temperature range (750-900 [degrees]C) of this study (Howard, 1983; Nam and Gavalas, 1989; Folgueras et al., 2004). XRD analysis was performed to find other sulphur compounds in the sulphated sludge ash in addition to [CaSO.sub.4] and confirm whether they contributed to the high sulphation capacity or not.

The XRD spectra of the ash A before and after sulphation are shown in Figure 7 where [CaSO.sub.4] (Major, 2 [theta] = 25.5 [degrees], Minor, 2 [theta] = 38.5, 41, 49, 56 [degrees]) is formed from sulphation of the ash and other sulphation compounds that are not exhibited in the XRD spectra.

Scanning electron microscopy (SEM) analyses were performed to characterize the surface and pore properties of the ashes and the calcined limestone before and after sulphation as shown in Figure 8. All of the particles have the average size of 1 mm. Before sulphation, paper sludge ashes (a, c) and the calcined limestone (e) exhibit similar shapes of grains. After 5 h of sulphation, grain size of the calcined limestone increases significantly due to larger molar volume of [CaSO.sub.4], which may cause pore blocking at the outer reaches of the particles (Laursen et al., 2003). However, an SEM image of the sulphated paper sludge ashes A and B reveals that the macropores and surface of the ash are not plugged by sulphation even with increasing grain size.

The EDX analysis was performed to investigate the sulphur adsorption on the inner part of paper sludge ash. The X-ray mappings of the cross-sectioned paper sludge ash A (1 mm) after sulphation are shown in Figure 9. As can be seen, all the components (Ca, Al, Mg, and Si) are evenly distributed and sulphur is uniformly adsorbed on the whole surface of sludge ash from the outer shell to the centre of particles--even the sample size is 1 mm (Laursen et al., 2003).

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

From the SEM-EDX analysis, high sulphation conversion of the paper sludge can be attributed to the uniform sulphation of CaO by the sustained outside pore without blockage since CaO and other metal compounds are evenly distributed in the paper sludge ash as shown in the EDX analysis (Folgueras et al., 2004). Other metal compounds except CaO remain unsulphated as the inert materials and macropores are preserved without blockage by [CaSO.sub.4] formation during sulphation, which provides high sulphation conversion as found in the case of sulphation with limestone (Howard, 1983).

The CaO content in the paper sludge ash is about 1/3 of the limestone but, the sulphation conversion of CaO is about 1.5-2 times higher than that of calcined limestone. Thus, 1 kg of paper sludge ash can substitute about 1/2-2/3 kg of limestone for desulphurization in combustors.

SUMMARY AND CONCLUSIONS

The effects of [SO.sub.2] concentration and particle size on sulphation of paper sludge ash were determined in a thermobalance reactor. The activation energy and the pre-exponential factor are found to be 33 172 kJ/kmol at 8.88 x [10.sup.-6] [s.sup.-1] [Pa.sup.-1] based on the uniform-reaction model. The reaction order is found to be 1.0 at [SO.sub.2] partial pressures of 2000-7000 ppm. Compared with limestone, high sulphation conversion of the paper sludge ashes can be obtained regardless of their particle size.

From the X-ray diffraction analysis, most of the sulphation compounds are observed to be [CaSO.sub.4]. The outer pore of the sludge ash is not blocked by [CaSO.sub.4] and the sulphation occurs uniformly throughout the ash. The uniform distributions of CaO and other inert minerals in the ash provide uniform sulphation with good penetration of [SO.sub.2] into pores of the sludge ash without pore blocking during sulphation of CaO. The CaO content in the paper sludge ash is about 1/3 of the limestone with high CaO content but, the sulphation conversion is about 1.5-2 times higher than that of the calcined limestone.

ACKNOWLEDGMENT

The authors acknowledge a grant-in-aid of research to S. D. Kim from the Ministry of Science and Technology, Korea.

NOMENCLATURE

E activation energy (kJ/mol)
k reaction rate (1/s)
[M.sub.CaO] molecular weight of CaO (g)
[MSO.sub.3]] molecular weight of [SO.sub.2] (g)
[r.sup.2] correlation coefficient (-)
R universal gas constant (8.314 J/mol * K)
T temperature ([degrees]C)
t time (s)
[W.sub.0] initial weight of sample (g)
[DELTA]w weight gain of the sample at time t (g)
X sulphation conversion (-)
[y.sub.c] CaO weight fraction in sample (-)

Greek Symbol

[theta] scan axis degree of XRD ([degrees])

Manuscript received April 12, 2006; revised manuscript received July 31, 2006; accepted for publication October 6, 2006.

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* Author to whom correspondence may be addressed. E-mail address: kimsd@kaist.ac.kr

Seon Ah Roh [1] and Sang Done Kim [2] *

[1.] Environmental Systems Research Center, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea

[2.] Department of Chemical and Biomolecular Engineering and Energy and Environment Research Center, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea

Table 1. Proximate and ash analyses of the paper sludges and calcined
limestone

 Proximate analysis: wt.% Ash analysis (wt.%):
 (ASTM 3173-73:30) (X-ray fluorescence
 spectrometry)

Paper Volatile matter 57.5 Si[O.sub.2]: 35.8,
sludge A Fixed carbon 1.05 [Al.sub.2][O.sub.3]: 19.1,
 Ash 36.97 [Fe.sub.2][O.sub.3]: 3.77,
 Moisture 4.48 CaO: 33.8, MgO: 4.4,
 Heating value (kJ/kg) 10 674 S[O.sub.3]: 2.06,
 [K.sub.2]O: 0.53,
 [Na.sub.2]O: 0.2,
 Ti[O.sub.2]: 0.34

Paper Volatile matter 44.8 Si[O.sub.2]: 36.42,
sludge B Fixed carbon 2.74 [Al.sub.2][O.sub.3]:
 Ash 46.9 21.93, [Fe.sub.2]
 Moisture 5.56 [O.sub.3]: 3.88, CaO:
 Heating value (kJ/kg) 12 466 32.30, MgO: 3.48,
 S[O.sub.3]: 1.33,
 [K.sub.2]O: 0.55,
 [Na.sub.2]O: 0.11

Calcined Si[O.sub.2]: 0.51, [Al.sub.2][O.sub.3]: 0.34,
limestone [Fe.sub.2][O.sub.3]: 0.17, CaO: 93.65, MgO: 5.33