Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: Vitrification of the solid residue

Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: Vitrification of the solid residue

J. Anal. Appl. Pyrolysis 74 (2005) 406–412 www.elsevier.com/locate/jaap Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge...

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J. Anal. Appl. Pyrolysis 74 (2005) 406–412 www.elsevier.com/locate/jaap

Microwave-induced drying, pyrolysis and gasification (MWDPG) of sewage sludge: Vitrification of the solid residue J.A. Mene´ndez *, A. Domı´nguez, M. Inguanzo, J.J. Pis Instituto Nacional del Carbo´n (INCAR) C.S.I.C., Apartado 73, 33080 Oviedo, Spain Received 7 June 2004; accepted 14 October 2004 Available online 13 March 2005

Abstract A novel method for sewage sludge treatment based on subjecting the wet sludge to high temperature thermal treatment in a microwave is being investigated at INCAR. Under the appropriate operating conditions, drying, pyrolysis and gasification of the sewage sludge take place, giving rise to a gas with a high CO and H2 content and an oil with a low PAH content. Moreover, due to the high temperatures that can be attained during the process it is possible to obtain a solid residue which is partially vitrified. Unlike other methods aimed at maximizing the porous texture of the solid residue in order to produce adsorbents, the aim of the method proposed in this work is to obtain a solid residue with minimal porous textural development, where the heavy metals present in the residue are occluded in a glassy-like matrix. The advantages of this technique are a substantial volume reduction with respect to the initial sludge and a solid residue that is more resistant to the leaching of organic substances and heavy metals than the char obtained by conventional pyrolysis. # 2005 Elsevier B.V. All rights reserved. Keywords: Sewage sludge; Microwaves; Pyrolysis; Gasification

1. Introduction Sewage sludge, like most organic wastes, contains a great deal of volatile matter and therefore represents a valuable resource, which can be converted into useful products, if it is subjected to suitable treatment. At present, most sewage sludge is disposed of by means of landfill or incineration [1]. As an alternative to these environmentally controversial routes of waste disposal, a novel method based on microwave heating is being investigated at INCAR [2–4]. Unlike other pyrolysis methods, where sewage sludge is previously dried in a separate process, wet sewage sludge (up to 70 wt.% moisture content) is used as the starting material of the process. Thus, during the initial steps of the process the sewage sludge is dried, making full use of the efficiency of microwave drying. By using the appropriate amount of power, temperatures of up to 1000 8C or more can be achieved. High temperatures are preferred in order to maximize the gas fraction yield [5,6], which, in general, is * Corresponding author. E-mail address: [email protected] (J.A. Mene´ndez). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.10.013

easier to handle than the oil fraction. At these temperatures not only does pyrolysis of the sewage sludge take place, but also hydro-gasification [7,4]. Because of the unique characteristics of microwave heating, i.e. direct heating inside the bulk and high heating rates (ca. 200 8C/min) [8], a large amount of steam is formed at the temperatures at which gasification reactions take place. Consequently, drying, pyrolysis and gasification occur in a single and relatively fast process (MWDPG). Table 1 summarizes the most representative results achieved from the MWDPG of a sewage sludge (V) provided by an urban waste water treatment plant. Some of the results related to the oil and gas fractions have already been published [3,4]. The present work, therefore, will focus on the solid fraction. The pyrolysis of different wastes in order to obtain carbonaceous adsorbents from the solid fraction has been the subject of a number of studies [9–11]. However, all these adsorbents have much inferior characteristics to the commercial ones [9–11] (also derived from relatively cheap raw materials). In the particular case of using sewage sludge as raw material to produce adsorbents, an additional problem is leaching, which may occur when a material that contains

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Table 1 Selected characteristics of the products from the MWDPG process of raw sewage sludge Fraction

Gas [4]

Aqueous

Oil [3]

Solid

Yield (wt.%) CV Outstanding characteristics Possible uses

24.2 8361 kJ/m3 H2 = 38.0 vol.%, CO + H2 = 60.3 vol.%

62.7 – –

10.1 5839 kJ/kg Low leaching

Fuel, H2 source, syngas source

Back to the head of the water treatment process

3.0 35818 kJ/kg Low PAH content, high aliphatic content Fuel, source of chemicals

Safe disposal

This oven works at a frequency of 2450 MHz and the power delivered can be regulated up to 2000 W. The sample is placed in the centre of a microwave guide which directs the microwaves from a magnetron to the sample. The guide terminates in a water sink, which absorbs the radiation not absorbed by the sample (transmitted power). The reflected power can be minimized by means of a multistub matching waveguide. The input power was set at 1000 W at the beginning of the experiment in order to ensure rapid heating. However, once the temperature had reached 1000 8C only 600 W was necessary to keep the temperature constant. The experiment was carried out by placing the sample of wet sludge (ca. 15 g) in a quartz reactor. The microwave treatment consisted in subjecting the samples to microwave action for 15 min, at the end of which no appreciable gas evolution was observed. In order to ensure an inert atmosphere during the treatments, a He flow of 100 mL min 1 was passed through the sample bed for 30 min prior to the commencement of the experiment, this being reduced during the experiment to 10 mL min 1. It should be noted that wet sewage sludge cannot be heated above 200 8C unless it is mixed with an appropriate microwave receptor [2]. In order to overcome this problem and achieve the high temperatures required for pyrolysis, 5 g of the carbonaceous solid residue resulting from the pyrolysis of the sewage sludge (obtained in previous experiments) was used as microwave receptor. The receptor was homogeneously blended with the sewage sludge, and the mixture was subjected to microwave action. The temperature of the sample during the experiments was monitored by means of an infrared optical pyrometer. A plot of the evolution of temperature with time is shown in Fig. 1. This temperature corresponds to the external surface, the

different amounts of heavy metals and organic substances is used under the harsh conditions at which the industrial application of adsorbents usually takes place. With the MWDPG method a completely different approach is applied. Thus, a partially vitrified material of relatively low porous textural development can be obtained by subjecting the solid residue to high temperatures. This is possible since the carbonaceous residue obtained is a very good microwave absorber that can easily be heated [2] (merely by increasing the microwave power) to temperatures above 1000 8C (this value corresponds to the external temperature measured with an optical pyrometer but the internal temperature is presumably higher). The objective of this work was to explore the possibilities offered by this method for obtaining a solid residue with the above mentioned properties and determine the physical and chemical characteristics of the resulting material.

2. Experimental 2.1. Sewage sludge An aerobically digested sewage sludge (V) from an urban waste water treatment plant was used. This sludge has a moisture content of 71% and an ash content of 29.3% (dry basis). Table 2 summarizes the main chemical characteristics of the sewage sludge and its calorific value. 2.2. Microwave-assisted pyrolysis The MWDPG of the sewage sludges was carried out in a one process, using a single-mode microwave cavity oven [8]. Table 2 Selected chemical properties of the solid residues Moist (wt.%)

V Vmw Vef a b c d e

71 1.8 0.6

Ashes (dba) (wt.%)

29.3 84.5 80.4

Dry basis. Volatile matter. Calorific value. Calculated by difference. Not determined.

VMb (wt.%)

67.6 2.8 5.0

Organic fraction (db) C (wt.%)

H (wt.%)

N (wt.%)

S (wt.%)

Od (wt.%)

38.5 15.4 17.1

2.9 0.0 0.6

5.9 0.4 0.9

0.6 0.4 0.4

22.8 0.0 0.6

pH

CVc (db) (kcal/kg)

n.d.e 11.7 11.6

3954 1397 1485

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Fig. 1. Temperature profile during treatment in the microwave.

temperature in the sample bulk presumably being higher. In a previous work [12] it was estimated that the temperature inside the bulk was about 100 8C higher.

determine the heavy metal content (trace elements) 100 mg of the sample was digested in a mixture of HCl (5 mL, 60%) and HNO3 (5 mL, 60%). The attack was carried out in pressurized containers, which were heated in a microwave oven at 600 W. After diluting the solution up to 50 mL, the metal content was determined using an ICP-MS (inductively coupled plasma mass spectrometer, HP4500 model). It should be pointed out that despite the harsh conditions used to dissolve the samples, the chars were not totally dissolved, there remaining a small residue after the treatment, so the values obtained probably underestimate the full metal content of the chars. A test to evaluate the lixiviation of heavy metals from the samples was also carried out as follows: 5 g of the dry solid was slurried into 100 mL of an acetic solution at pH 2.8 and shaken for 18 h. The metal content of the solution was then measured with the same equipment as mentioned above, and the results expressed as milligrams per liter of the lixiviated metals.

2.3. Electric furnace pyrolysis

3. Results and discussion

For comparative purposes sample V was also pyrolyzed in an electric furnace (ef) equipped with a similar quartz reactor. The reaction proceeded in a helium atmosphere at a flow rate of 10 mL min 1. The pyrolysis temperature used was 1000 8C and the soaking time was 10 min. The sample was heated to 1000 8C in approximately 14 min by applying the maximum available power of the furnace.

Fig. 2 shows the yields of the different fractions achieved by treating the sewage sludge in the microwave and electric furnaces. The differences in gas and oil fractions and the fact that the aqueous fraction was lower than the initial moisture content of the sewage sludge (indicating that hydrogasification reactions took place) have been discussed elsewhere [3,4]. Regarding the solid residue, the differences in the yield of this fraction due to the pyrolysis method are negligible. Consequently at the relatively high temperatures applied in this work (which ensures almost total pyrolysis) the use of microwave heating has no influence on the solid fraction yield.

2.4. Sample characterization The porous texture of the solid fraction was characterized by the physical adsorption of N2 and CO2 at 77 and 273 K, respectively. The adsorption isotherms of N2 and CO2 were determined using an automatic ASAP 2000M and a Gemini 2375, respectively, both from Micromeritics. Mercury porosimetry was also carried out employing a Micromeritics Autopore-IV-9500 apparatus, which operates at pressures between 0.005 and 225 MPa, corresponding to pore diameters of 250,000 and 5.5 nm, respectively. In addition a scanning electron microscope DSM 942 from Zeiss equipped with an EDX detector (OXFORD LINK-ISIS) was used to study the texture and chemical elements present on the surface of the samples. A proximate analysis (moisture, ash and volatile matter content) and elemental analysis (C, H, O, N, S) of the organic fraction of the sludge and solid residues were carried out in a LECO TGA-601 thermobalance and in a LECO CNHS-932 apparatus, respectively. The oxygen content was calculated by difference. To evaluate the degree of leaching of the organic substances from the dry sewage sludge and the solid pyrolysis residues, a slurry of 5 g of 1–3 mm size was prepared from the materials in 100 mL of distilled water. The slurry was shaken for18 h and then BOD5 (biological oxygen demand) and COD (chemical oxygen demand) standard tests were performed on the liquid obtained after the solids had been filtered. To

3.1. Physical characterization Some of the main parameters that characterize the porous texture of the solids obtained in the electric (Vef) and microwave ovens (Vmw) are summarized in Table 3. As can

Fig. 2. Yields (wet basis) of the different fractions obtained in the microwave and electrical furnace treatments.

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Table 3 Textural properties of solid residues

Vmw Vef a b c d

Dr (He) (g/cm3)

Daa (Hg) (g/cm3)

BETb (m2/g)

Microporec (cm3/g)

Mesoporeb,d (cm3/g)

2.53 2.54

1.42 1.03

22 50

0.016 0.020

0.005 0.004

From Hg porosimetry at 1 atm of pressure i.e. Dp < 12 mm. From the nitrogen isotherm (77 K). Obtained by applying the Dubinin–Radushkevich equation to the CO2 isotherm (273 K). DFT method.

Fig. 3. Plots of mercury porosimetry for samples Vef and Vmw. Variation in the specific pore volume with pore diameter: accumulative volume (dots), derivative (lines).

be observed, both solids have a low meso- and micropore volume, as a result of which their BET specific surface area is very small, especially if compared with carbon adsorbents. Despite the poor development of their microand mesoporous texture, these materials have a relatively high porosity 53.7 and 43.9%—calculated from the helium (real) and mercury (apparent) densities (Table 3)—for Vef and Vmw, respectively. Mercury porosimetry was carried out in order to study the pore size distribution in the range of 250 mm to 5.5 nm. The corresponding plots are shown in Fig. 3. It can be seen that the solid residue obtained in the microwave oven has a lower pore volume than that of the electric furnace over the whole range of pore sizes covered by the porosimeter. From the derivatives of the accumulative curves it can be inferred that, for both solids pores larger than 10 mm, which can be attributed either to very large macropores or to inter-particle interstices, represent a significant proportion of the macropores. Another maximum is observed in the 300–400 nm range, which also corresponds to relatively large macropores (micro-cracks, fissures or small holes in the particles). However, it should be noted that while sample Vef has a third maximum in the mesopore range (5.5 nm), sample Vmw does not have these type of pores. It should also be noted that these mesopores

Fig. 4. Variation in the BET surface area, meso- and micropore volume with the burn-off degree of the solid residue obtained (ash-free basis) by electrical furnace pyrolysis and activation with CO2 at 850 8C.

are larger than those detected by the N2 isotherms, which are practically inexistent (Table 3). In sum, both samples present relatively large pores in the macropore range and a low porosity in the meso- and micropore range, the sample obtained in the electric furnace having a greater pore volume (over all pore size ranges, but especially the smaller ones) than its counterpart in the microwave. Despite the poor results obtained for the porous texture, an attempt to develop more porosity and a higher specific surface area was made. Thus, the sample from the electric furnace, which, in principle, showed better characteristics, was activated with CO2 (850 8C, 50 mL min 1) at different degrees of burn-off, ranging from 20 to 75 wt.% (ash-free basis). The results corresponding to the BET surface area, and meso- and micropore volume are presented in Fig. 4. Physical activation with CO2 gave rise to an increase in mesoporosity, while the microporosity remained more or less unchanged. In any case these values are still very low compared with those of commercial activated carbons. From these experiments it can be concluded that the high temperature pyrolysis of sewage sludge produces solids

Table 4 Selected heavy metals present in the dry sewage sludge and solid residues obtained from pyrolysis

V Vmw Vef

Fe (ppm)

Zn (ppm)

Pb (ppm)

Mn (ppm)

Cu (ppm)

Cr (ppm)

Ni (ppm)

Co (ppm)

Cd (ppb)

Hg (ppb)

9900 22600 25400

662 571 147

246 507 524

214 393 419

143 233 251

121 142 123

13 36 29

4 7 7

2906 1080 1030

919 90 143

410

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Fig. 5. Percentage of volume reduction with respect to the wet sewage sludge of the: Vd dried sludge (included here for comparative proposes), Vef solid residue obtained from electric furnace treatment, Vmw: solid residue obtained from microwave treatment. These values were calculated from the solid yields and the Hg apparent densities.

with a very poor pore textural development, i.e. poor characteristics which make them unsuitable for use as adsorbents and which are not very much improved by physical activation. Nevertheless, Nagano et al. [10] suggested that similar materials, which also have relatively large pores, could be used to adsorb dioxins. But they used solid wastes as raw materials that upon pyrolysis gave rise to a much higher carbon content in the solid residue than those obtained from sewage sludge (Table 2). Moreover, their best results were obtained by chemical activation with acids. Other authors also claim to have produced activated carbons (with a very poor pore textural development) by chemical activation of sewage sludge with acids [9,11]. However, due to the presence of heavy metals in the sewage sludge (Table 4), using acids to activate it may lead to an acid, which is highly contaminated with the metals and the various organic substances present in the sludge. In other words, chemical activation of the sewage sludge may simply

Fig. 6. SEM microphotographs of the solid residues obtained in the electric (Vef) and microwave furnaces (Vmw).

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transfer the problem of the presence of heavy metals and other pollutants from the solid to the acid. In the light of what has been said above, the high temperature pyrolysis of sewage sludges generates solid residues, which are difficult to upgrade as adsorbents. Nevertheless in addition to the benefits of the oil and gas fractions [3,4], the use of severe pyrolysis conditions leads to a considerable reduction in the volume of the residue. As can be observed in Fig. 5, while the mere drying of the sewage sludge (Vd) reduces the volume by 77.7% with respect to the wet sludge, this value is as high as 89% in the case of electric furnace pyrolysis and 92% in the case of MWDPG. Taking into account the poor characteristics of this product, a substantial decrease in volume may have considerable economic benefits as a result of a reduction in the cost of disposal. Another issue that must be considered is how safe it is to dispose of the solid waste by the landfill method. This material contains residual organic compounds (see below) and heavy metals (Table 4), which eventually, under certain conditions, could leach out from the solid. A reduction in the pore network of these solids would inhibit, at least in principle, the lixiviation of these substances, since the surface area exposed to the lixiviating agent would also be reduced. As shown above, microwave treatment produces a solid residue, which is less porous than the solid produced in the electric furnace. Furthermore, SEM microphotographs (Fig. 6) reveal that the solid produced in the microwave has a vitreous-like texture (very different from the porous texture of the solid produced in the electric furnace), and so the heavy metals are occluded inside the vitreous matrix. 3.2. Chemical characterization Table 2 shows some of the chemical characteristics of the sewage sludge and chars. These chars contain more than 80 wt.% of ashes i.e. inorganic matter, indicating that most of the organic fraction of the raw sludge has been transformed into gases and oils during the treatment. However, despite the harsh conditions used in the pyrolysis of the sewage sludge, the solid residues still contain some residual volatile matter. This is probably due to the uneven pyrolysis of the bulk of the sample. Interestingly, this residual volatile matter is lower in the case of the sample treated in the microwave. Accordingly, the microwave char also presents an organic fraction with a lower heteroatom content. This is an important point since an additional problem that may arise from the presence of volatile

411

Fig. 7. Amount of lixiviated metals from chars obtained in the electric furnace (Vef) and microwave (Vmw).

substances in these materials is that organic compounds may leach upon disposal. We tested for this possibility by performing standard BOD5 and COD tests of the dry sludge and chars, as described in the experimental section. The results of the test are summarized in Table 5. As expected, the solid residues obtained after the pyrolysis treatments exhibited a considerably lower leaching of organic substances than the sewage sludge. Moreover, the char obtained in the microwave process has a lower COD than the one obtained by conventional pyrolysis. This is a further confirmation that the microwave process gives rise to a solid residue that is more resistant to leaching than the solid from conventional pyrolysis. In relation with the heavy metal content, the samples were analyzed following the procedure described in the experimental section and the results are presented in Table 4. It should be noted that the procedure adopted in order to attack and dissolve the chars was not enough to dissolve them completely, so the values probably underestimate the actual metal content of the chars. Nevertheless, from the results it can be inferred that most of the Hg, Cd, and probably some of the other heavy metals were volatilized during pyrolysis. In order to determine the degree to which the heavy metals resisted leaching, a lixiviation test was performed (see experimental section) on the residues, the results of which are presented for comparison in Fig. 7. It is remarkable that in all cases the amount of metals lixiviated from sample Vmw is lower that of sample Vef. The results also suggest that the solids produced in the microwave are more resistant to leaching than those obtained in the electric furnace.

4. Conclusions Table 5 BOD5 and COD of a slurry of the sewage sludge and solid residues obtained from pyrolysis

V Vef Vmw

BOD5 (mg/L)

COD (mg/L)

6255 0 0

10100 2200 500

The MWDPG of wet sewage sludge at high temperatures produces a solid residue of a very low micro- and mesopore volume. This residue may partially vitrify if a sufficiently high temperature is reached during the process. The characteristics of solids obtained by pyrolyzing sewage sludge are generally too poor for them to be upgraded as

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adsorbents, i.e., low development of porous texture with low micropore volumes and small surface areas. These characteristics are not greatly improved by physical activation of the chars. A comparison of the microwave method with conventional pyrolysis in an electrical furnace showed that the former is more efficient (i.e. it produces solids with less residual volatile matter) and that it gives rise to a slightly higher reduction in the volume of the solid residue. Upon comparing the characteristics of the solids obtained by MWDPG with those obtained by conventional pyrolysis, it was found that the char obtained in the microwave is less porous and more resistant to the lixiviation of organic substances and heavy metals than that produced by conventional pyrolysis.

Acknowledgments The authors thank the Spanish Ministry of Science and Technology (research project PPQ2001-2083-C02-01) and

FEDER for financial support. A.D. is also grateful to FICYT (Asturias) for supporting his research.

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