Microwave-induced pyrolysis of sewage sludge

Microwave-induced pyrolysis of sewage sludge

Water Research 36 (2002) 3261–3264 Microwave-induced pyrolysis of sewage sludge J.A. Mene! ndez*, M. Inguanzo, J.J. Pis ! (INCAR) C.S.I.C., Apartado ...

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Water Research 36 (2002) 3261–3264

Microwave-induced pyrolysis of sewage sludge J.A. Mene! ndez*, M. Inguanzo, J.J. Pis ! (INCAR) C.S.I.C., Apartado 73, 33080 Oviedo, Spain Instituto Nacional del Carbon Received 13 July 2001; accepted 17 December 2001

Abstract This paper describes a new method for pyrolyzing sewage sludge using a microwave furnace. It was found that if just the raw wet sludge is treated in the microwave, only drying of the sample takes place. However, if the sludge is mixed with a small amount of a suitable microwave absorber (such as the char produced in the pyrolysis itself) temperatures of up to 9001C can be achieved, so that pyrolysis takes place rather than drying. Microwave treatments were also compared with those carried out in a conventional electric furnace, as well as the characteristics of their respective carbonaceous solid residues. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sewage sludge; Drying; Pyrolysis; Dielectric heating; Microwaves

1. Introduction The disposal of sewage sludge produced by urban and industrial wastewater treatment plants is a matter of great concern [1]. Handling of this waste is not easy and inevitably gives rise to some collateral pollution. The most common alternatives of treatment and/or disposal of sewage sludge are sludge landfill, cropland application, incineration and ocean dumping, none of which are exempt of drawbacks [1]. Another method of disposal which is being investigated at the moment is pyrolysis [2–4]. This technique appears to be less pollutant than incineration, as it concentrates the heavy metals in a solid carbonaceous residue so that the leaching of these metals is not as important as in the ashes from incineration [5]. The pyrolysis of sewage sludge also gives rise to oils and gases, which can be used as fuels. In addition, microwaves are used in various technological and scientific fields in order to heat dielectric materials [6,7]. Microwave heating has also been considered as an alternative to carry out the pyrolysis of biomass [8], coal [9], oil shales [10] and different organic wastes [11]. These are, in general, poor receptors *Corresponding author. Tel.: +34-985-280-800; fax: +34985-297-662. E-mail address: [email protected] (J.A. Men!endez).

of microwave energy, so they cannot be heated directly up to the high temperatures usually required to achieve total pyrolysis. However, microwave-induced pyrolysis is possible if the raw material is mixed with an effective receptor of microwave energy such as carbon [10,11] or certain metal oxides [9]. The aim of this work was to compare sewage sludge pyrolysis using a microwave with that of a conventional electric furnace, and to compare also the characteristics of the chars (carbonaceous residues) resulting from these pyrolysis experiments. The study of the characteristics of the fuel gases and liquids produced in the pyrolysis experiments does not come within the scope of this paper and will be carried out in a future work.

2. Experimental An anaerobic sewage sludge, which was produced in a Spanish urban wastewater treatment plant, was used as starting material. Selected chemical characteristics and the heavy metal content of this material are given in Table 1. Microwave experiments were carried out by placing samples of the wet sludge (ca. 20 g) in a quartz reactor, which in turn was placed inside a multimode resonant microwave cavity. Microwave treatments consisted in

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Table 1 Selected chemical characteristics of the sewage sludge M (wt%) VMd.b. (wt%) Ad.b. (wt%) Cd.b. (wt%) Hd.b. (wt%) Nd.b. (wt%) Od.b. (wt%) Sd.b. (wt%) Calorific value (kcal kg1) 78.1

60.7

36.9

5.8

5.5

13.7

0.8

Metal content of the dry sewage sludge (ppm) Cr Zn Ni Cu

Pb

Hg

Cd

Fe

32

95

1.5

F

8590

1707

18

37.3

179

3974

M: moisture; VM: volatile matter content; A: Ash content; d.b.: dry basis.

Table 2 Sample nomenclature, type of treatment and solid fraction yields Sample

Type of treatment

Solid fraction yield (wt%)

Ef-D Mw-D Ef-P Mw-P

Wet Wet Wet Wet

21.2 20.0 7.9 11.7

sludge sludge sludge sludge

dried at 2001C for 10 h in an electric furnace treated in the microwave for 5 min heated at 10001C and 601C min1 in an electric furnace mixed with 5 wt% of char and treated in the microwave for 5 min

subjecting the samples to the microwave action for 5 min. In order to maintain an inert atmosphere during the treatments, a N2 flow of 100 mL min1 was passed through the sample bed for 30 min prior to the commencement of the treatment and also during the treatment and cool-down intervals. The input power of the microwave equipment was set at 1000 W and the microwave frequency used was 2450 MHz. The temperature of the sample during microwave treatment was measured using an infrared optical pyrometer. Details of the microwave device as well as temperature measurements are given elsewhere [12]. For comparative proposes, two samples of the wet sewage sludge were also treated in a conventional electric furnace. The nomenclature used and the type of treatments carried out are summarized in Table 2. The results of the proximate and ultimate analysis were expressed as wt% on a dry basis with no corrections to make up 100%, so the sum of the percentages exceeded 100 in some cases. This is due to different factors: (i) different equipment and procedures were used to carry out the proximate and ultimate analyses; (ii) some of the samples studied in this work contain quite a large amount of ashes (>80 wt%), so the percentage of carbon and heteroatoms was relatively low, which made it difficult to carry out elemental analyses; (iii) a percentage of the sulfur and oxygen determined (directly, not estimated by difference) in the elemental analyses corresponded to the sulfur and oxygen present in the ashes. The pH corresponding to the point of zero charge (pHPZC) of the carbonaceous residues was measured by reverse mass titration follow-

ing a procedure described elsewhere [13]. Apparent density (ra ) was evaluated using mercury picnometry at atmospheric pressure and true density (rHe ) was determined by means of He picnometry.

3. Results and discussion 3.1. Drying The first series of experiments involved heating the sewage sludge as received (78.1 wt% of moisture content) in the microwave. Different periods of time were tested finding that treatment times longer than 5 min did not produce significant differences in the characteristics of the solid residue obtained. The main chemical characteristics of the solid residue of a sample dried in the microwave for 5 min are given in Table 3, and the evolution of the temperature with the treatment time is plotted in Fig. 1. The following observations can be inferred from the results of these experiments: (i) heating in the microwave furnace is quite fast and it takes o90 s to reach the maximum temperature of ca. 2001C; (ii) this temperature remains quite stable throughout the treatment; (iii) as the temperature never reaches values higher than 2001C, in practice it was only possible to dry the sample. In addition, for comparative purposes, a similar sample of sewage sludge was dried under N2 in an electric furnace at 2001C for 10 h (Ef-D). It was observed that the volatile matter content of the residues was lower than that of the sewage sludge. These treatments therefore not only remove moisture but also

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Table 3 Proximate and ultimate analysis, and point of zero charge (pHPZC) of the chars obtained in the different pyrolysis treatments

Ef-D Mw-D Ef-P Mw-P

M (wt%)

VMd.b. (wt%)

Ad.b. (wt%)

Cd.b. (wt%)

Hd.b. (wt%)

Nd.b. (wt%)

O(1)d.b. (wt%)

Sd.b. (wt%)

pHPZC

2.2 2.9 1.1 1.0

48.9 53.9 0.5 0.2

40.7 36.9 80.6 83.8

35.4 36.6 20.9 19.1

4.3 4.8 0.3 0.3

4.0 4.0 0.5 1.0

16.6 18.7 7.0 4.5

0.86 0.89 0.45 0.61

5.9 6.1 8.0 8.9

M: moisture; VM: volatile matter content; A: Ash content; d.b.: dry basis.

1100 1000 900 Temperature (°C)

800 700

Wet sludge mixed with 5wt% of the char obtained in a previous run (repeated three times)

600 500 400 300 200

Wet sludge as received

100 0 0

60

120

180

240

300

Time (s)

Fig. 1. Temperature evolution during microwave treatment of samples of wet sewage as received (Mw-D); and mixed with 5 wt% of the char obtained in a previous run (Mw-P).

some of the volatiles (probably hydrosoluble compounds), which are swept away with the water.

3.2. Pyrolysis In order to carry out the pyrolysis of the sewage sludge using microwave energy, higher temperatures than those employed for treating the raw sludge (drying) were necessary. Hence, on the basis of earlier reports that describe pyrolysis of poor microwave absorbers [8– 11], it was decided to mix the sewage sludge with a dielectric material able to absorb the microwave energy to a larger extent than wet sewage sludge. Thus, in the next series of experiments, the carbonaceous residue (char) produced in the pyrolysis of the sewage sludge was used as microwave receptor. Using the char obtained in different experiments, the minimum amount needed for blending with the wet sludge to achieve a sufficiently high and steady temperature was determined (note that for the first experiment a char obtained in the electric furnace was used and then the char obtained in subsequent experiments in the microwave). A blend containing ca. 5 wt% of char-sized particles of about 1 mm, and uniformly mixed with the sludge, was found

to perform adequately. The use of this char has the advantage that there is no need to add any compound from a different source other than the sewage sludge itself. A plot of the temperature evolution of a series of three different experiments, each of which was performed using the char produced in the previous experiment, is shown in Fig. 1. The three experiments show similar patterns. A maximum temperature of ca. 9001C is reached at about 2 min after the beginning of the experiment. It then remains more or less stable for two more minutes before finally undergoing a slight decrease at the end of the experiment. The chemical characteristics of the solid residues produced by these treatments are summarized in Table 3. The most remarkable feature is the high ash content of the pyrolyzed samples (>80 wt%) along with the almost negligible volatile matter content, indicating that pyrolysis was practically complete. Moreover, microwave pyrolysis seems to be slightly more thorough than the pyrolysis carried out in the electric furnace. Pyrolyzed samples are also of a basic nature, unlike dried samples which are slightly acidic. Another interesting feature, in order to dispose of residues by means of land filling, is the important volume reduction that can be achieved with these treatments. Thus, the volume reduction with respect to the sample loaded in the reactor (wet sewage sludge), was calculated using the following expression:   mc ds % volume reduction ¼ 1  100; ms rac where: ms is the mass of wet sewage sludge; ds the density of wet sewage sludge=1.06 g cm1 (determined experimentally); mc the mass of the char; and rac the apparent density of the char. With respect to drying, no relevant differences were found between the two treatments: Ef-D and Mw-D, which resulted in volume reductions of 84% and 85%, respectively. In pyrolysis, the treatments not only remove water but also most of the volatile matter, resulting in volume reductions of 93% and 89% for samples EF-D and Ef-P, respectively. This reduction in volume is only a few points higher than in the case of drying because the carbonaceous residues produced by pyrolysis have higher porosity: 50.3% and 54.0% for

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Table 4 Selected textural properties of the sludge and carbonaceous residues Sample

Ef-D

Mw-D

Ef-P

Mw-P

rHe a (g cm3) ra b (g cm3) Porosity (%)

1.63 1.42 12.4

1.57 1.41 10.5

2.58 1.19 54.0

2.38 1.18 50.3

a b

True density, determined with He. Apparent density determined with Hg.

Mw-P and Ef-P, respectively (Table 4). Furthermore, pyrolysis in the microwave was performed using a blend of sewage sludge and char, which do not undergo any volume reduction. This would explain why the volume reduction (and the solid fraction yield) of the sample Mw-P was slightly lower than in the case of the sample pyrolyzed in the electric furnace (unmixed). Nevertheless, all of these treatments give rise to a significant volume reduction i.e. >84%. The pyrolysis of sewage sludge is not only valuable from the point of view of volume reduction. The potential financial rewards from the fuel gases and liquids produced are also attractive. Moreover, as it has already been suggested, the carbonaceous residues obtained by the pyrolysis of sewage sludge could be used as adsorbents [14]. Thus, the basic nature and the textural characteristics of the carbonaceous residues obtained with this method would make them, at least in principle, suitable for use as ‘cheap’ adsorbents of acidic compounds or other pollutants generated in wastewater treatment processes (e.g. H2S, CH3SH, phenols, etc.).

4. Conclusions By means of microwave energy, wet sewage sludge can be rapidly and efficiently pyrolyzed if some of the carbonaceous residue, which acts as a microwave receptor, is mixed with the raw material. On the other hand, if just the wet sludge is subjected to microwave action, only drying of the sludge takes place. Compared with conventional heating, microwave heating saves considerable time and energy for a similar degree of drying or pyrolysis. The microwave treatment of sewage sludge makes it possible to achieve a volume reduction of more than 80%, obtaining a porous carbonaceous residue of basic nature and providing a source of fuel gases and liquids.

Acknowledgements The authors thank the Spanish Ministry of Science and Technology (Research Project PPQ2001-2083-C0201) and the European Commission (Research Project 1FD97-0394-C02-02) for financial support.

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