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Energy Fuels 2009, 23, 5454–5459 Published on Web 10/07/2009

: DOI:10.1021/ef900610k

Sewage Sludge Carbonization for Terra Preta Applications Takuya Yoshida and Michael Jerry Antal, Jr.* Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822 Received June 15, 2009. Revised Manuscript Received September 9, 2009

Sewage sludge is a renewable, negative-cost organic material that is well suited for the production of biocarbons. Due to increasing interest in the use of biocarbons for soil beneficiation and carbon sequestration (i.e., “terra preta” applications), the aim of this paper is an assessment of the production of sewage sludge charcoal for land application. By use of the flash carbonization process we measured sewage sludge charcoal yields near 30 wt % and fixed-carbon yields near 18 wt % from sludges with moisture contents near 7 wt %. Increasing the moisture content of the sludge reduced the charcoal and fixed-carbon yields; nevertheless, sewage sludge with 30 wt % moisture was successfully carbonized. The presence of heavy metals in sewage sludge is an important concern. We found that heavy metals with low boiling points (e.g., As, Cd, Hg, Se) were prone to elution from the carbonization reactor. Because the heavy metal content of the sludge used in this work was not high, the charcoal was found to be acceptable for land application according to U.S. EPA regulation 40 CFR Part 503.

produced from sewage sludge by thermal processes have been foci of recent research.2-14 Biocarbons (i.e., charcoal)15-18 are an attractive alternative to the liquid and solid fuels emphasized by previous researchers, but are seldom considered. Sewage sludge biocarbons are a renewable, “green” coal that can be used to generate electric power in efficient coal fired power plants.19,20 Alternatively, there is now considerable interest in the use of biocarbons for soil beneficiation (“terra preta”) and carbon sequestration.21-24 The remarkable discovery of terra preta do Indio (“Indian dark earth”) in Amazonia highlights the importance of biocarbon as a soil amendment. Prehistoric Amazonians added large amounts of charcoal (15-60 t/ha) to their wet desert (“oxisol”) soil to render it fertile.25 Terra preta soil may occupy 10% of Amazonia (an area the size of France), and it sustained large cities (perhaps as large as the Aztec capital Tenochtitl an) there for 2 millenia.26 Typically, terra preta regions cover 1-5 ha and are 40-60 cm deep. These soils contain up to 70 times as much charcoal as the surrounding oxisol soils, as well as nutrients derived from the addition of excrement and food wastes. Today, terra preta is so valuable that it is mined and sold as potting soil.26 Because biocarbon is stable in the soil for millenia, whereas sewage

Introduction In 1998 over 6.9 million dry tons of sewage sludge were produced in the USA.1 The EPA estimated this amount to grow to 8.2 million tons in 2010.1 The disposition of the sludge was as follows: 22% was incinerated, 41% was applied to the land, 12% was subjected to advanced treatments, 17% was sent to landfill, 7% was given to other beneficial uses, and 1% was otherwise disposed. Table 1 displays an elemental (i.e., “ultimate”) analysis of the sewage sludge employed in this work. In an age of increasing fuel scarcity, massive amounts of organic material with composition like that of sewage sludge attract attention as promising feedstocks for the production of renewable biofuels. Both liquid “bio-oil” and “biogas” fuels *To whom correspondence should be addressed. Phone: 808-9567267; Fax: 808-956-2336; E-mail: [email protected]. (1) Anonymous EPA530-R-99-099; U.S. EPA: 1999. (2) Dominguez, A.; Menendez, J. A.; Inguanzo, M.; Pis, J. J. Fuel Proc. Technol. 2005, 86, 1007. (3) Doshi, V. A.; Vuthaluru, H. B.; Bastow, T. Fuel Proc. Technol. 2005, 86, 885. (4) Fonts, I.; Juan, A.; Gea, G.; Murilo, M. B.; Sanchez, J. L. Ind. Eng. Chem. Res. 2008, 47, 5376. (5) Piskorz, J. D.; Scott, D. S.; Westerbeg, I. B. Ind. Eng. Chem. Res. 1986, 25, 265. (6) Stammbach, M. R.; Kraaz, B. Energy Fuels 1989, 3, 255. (7) Karayildirim, T.; Yanik, J.; Yuksel, M. Fuel 2006, 85, 1498. (8) Kim, Y.; Parker, W. Bioresour. Technol. 2008, 99, 1409. (9) Park, E.; Kang, B.; Kim, J. Energy Fuels 2008, 22, 1335. (10) Aznar, M.; Manya, J. J.; Garcia, G.; Sanchez, J. L.; Burillo, M. B. Energy Fuels 2008, 22, 2840–2850. (11) Pinto, F.; Andre, R. N.; Lopes, H. D., M.; Gulyurtlu, I.; CAbrita, I. Energy Fuels 2008, 22, 2314–2325. (12) Shao, J.; Yan, R.; Chen, H.; Yang, H.; Lee, D. H.; Liang, D. T. Energy Fuels 2008, 22, 2278–2283. (13) Pinto, F.; Lopes, H.; Dias, M.; Andre, R. N.; Dias, M.; Gulyurtlu, I.; CAbrita, I. Energy Fuels 2007, 21, 2737–2745. (14) Fonts, I.; Juan, A.; Gea, G.; Murilo, M. B.; Arauzo, J. Ind. Eng. Chem. Res. 2009, 48, 2179–2187. (15) Bourke, J. P.; Manley-Harris, M.; Fushimi, C.; Dowaki, K.; Nunoura, T.; Antal, M. J. Ind. Eng. Chem. Res. 2007, 46, 5954–5967. (16) Meszaros, E.; Jakab, E.; Varhegyi, G.; Bourke, J. P.; Manley-Harris, M.; Nunoura, T.; Antal, M. J. Ind. Eng. Chem. Res. 2007, 46, 5943– 5953. r 2009 American Chemical Society

(17) Mochidzuki, K.; Soutric, F.; Tadokoro, K.; Antal, M. J.; Toth, M.; Zelei, B.; Varhegyi, G. Ind. Eng. Chem. Res. 2003, 42, 5140–5151. (18) Varhegyi, G.; Szabo, P.; Till, F.; Zelei, B.; Antal, M. J.; Dai, X. Energy Fuels 1998, 12, 969–974. (19) Antal, M. J.; Croiset, E.; Dai, X. F.; DeAlmeida, C.; Mok, W. S. L.; Norberg, N.; Richard, J. R.; Majthoub, M. A. Energy Fuels 1996, 10 (3), 652–658. (20) Amari, T.; Tanaka, M.; Koga, Y.; Okuno, S.; Tajima, A. Symp. Environ. Eng. 2006, 16, 151–153. (21) Lehmann, J.; Gaunt, J. J.; Rondon, M. Mitigat. Adapt. Strat. Global Change 2006, 11, 395–419. (22) Marris, E. Nature 2006, 442, 624–626. (23) Laird, D. A. Agron. J. 2008, 100, 178–181. (24) Takemura, S.; Kan, K.; Sugiyama, S.; Sugimoto, F.; Imai, T.; Sato, I.; Sato, H. Shigen-to-Sozai 2006, 122, 368–374. (25) Glaser, B.; Haumaier, L.; Guggenberger, G.; Zech, W. Naturwissenschaften 2001, 88, 37–41. (26) Mann, C. C. Science 2002, 297, 920–923.

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Table 1. Ultimate analysisa of Sewage Sludge Feed SS2 (wt %, dry) C 45.35 a

Table 3. Ash Contentsa of the Feeds

H

O

N

S

ash

feed

ash content (wt %, dry)

6.16

34.2

1.75

0.53

14.71

SS1 SS2 SS3 guava woodchips corncob (l) corncob (d)

12.3 14.6 12.1 2.0 1.5 1.0

Analyzed by Huffman Laboratories, USA.

Table 2. Main Ash Componentsa in Sewage Sludge Feed SS2 (wt %, dry feed basis)

a

Na2O MgO Al2O3 K2O CaO Fe2O3 MnO P2O5 SiO2 TiO2 0.11 a

0.48

2.47

0.06

2.74

1.81

0.02

2.05

4.10

ASTM E 1755-95.

has attained carbon yields that approach the theoretical limits defined by thermochemical equilibrium.33 In this work each sewage sludge sample was carbonized using the FC process. We report findings concerning the effects of process conditions and feedstock characteristics on the yield and properties of the product charcoal. The aim of the present work was to evaluate the suitability of sewage sludge as a feedstock for the production of biocarbons intended for terra preta applications.

0.53

Analyzed by Huffman Laboratories, USA.

sludge is not stable in the soil, the addition of sewage sludge biocarbon to the soil sequesters carbon and thereby fights climate change. Of course, biocarbons are free of the odor and pathogens that usually accompany sewage sludge.3 The focus of this paper is the production of biocarbons from sewage sludge. Although the C, H, and O content displayed in Table 1 resembles that of many biomass feedstocks employed in the conventional production of biocarbons; the N, S, and ash contents of the sludge are relatively high. The high N content is beneficial as a fertilizer ingredient when the sludge is applied to the soil, but neither N nor S is desirable in a fuel. Likewise, the sludge’s ash content may be desirable for its nutrient value, but the ash reduces the calorific value of the sewage sludge charcoal. Furthermore, although a conventional analysis of the sludge ash composition (see Table 2) reveals nothing disturbing, more detailed analyses uncover the presence of heavy metals (e.g., As, Cd, Hg, Pb, etc.)12 in most sewage sludges; consequently, their application to the soil is regulated by the EPA (40 CFR part 503).27 Also, at elevated temperatures the minerals in the sludge can decompose28 and can be released from the carbonizer into the environment.12 Consequently, the behavior of heavy metals in the sludge and their final disposition following carbonization are of paramount importance.29 This paper presents findings concerning the behavior and disposition of heavy metals in sewage sludge during carbonization. Mankind has carbonized biomass for at least 30 000 years;30 consequently, many technologies are available for charcoal production.31,32 Among these, the flash carbonization (FC) process,33-37 developed in this laboratory, quickly and efficiently converts biomass into charcoal. The FC process has been demonstrated with various biomass feedstocks33-37 and

Apparatus and Experimental Procedures Sewage sludge samples (SS1, SS2, and SS3) were collected at the Ewa sewage sludge station in Hawaii during the months of January, February, and March 2008, respectively. This sludge was derived primarily from domestic wastewater. It was thickened to about 5 wt % concentration and heat treated at 191 °C and 2.4 MPa at the sludge station. This thermal conditioning caused the cell membrane lyses to open and release liquid; thereby facilitating the dewatering of the sludge and solids capture. The sludge retained some odor after this heat treatment; however, the sludge charcoal had no unusual odor. Table 1 displays an ultimate analysis of sludge SS2, and Table 2 presents an analysis of the main ash elements in SS2. The ash contents of the sewage sludge feeds, which were analyzed following ASTM E 1755-95, are listed in Table 3 together with those of the guava (Psidium guajava) woodchips and corncobs that were used as tinder in some runs. We employed two different sources of corncob that had different sizes and densities; consequently we identify these as corncob (l) and corncob (d) (i.e., light corncob and dense corncob), respectively. The samples were dried in air prior to each experiment to reduce their moisture contents. The final moisture contents of the samples depended upon the drying time and room conditions; consequently, the moisture contents of the samples were not identical. In some experiments we attempted to carbonize sewage sludge with a high moisture content that had been dried in air for less than three days. This study employed the same apparatus that was used in our previous work,33-37 but a new ignition procedure. Measured amounts of sewage sludge and tinder (guava woodchips or corncob) were placed inside a cylindrical canister. At the same time, additional samples of the feedstocks were taken for analyses of their moisture contents. Then the canister was loaded into a pressure vessel. The pressure vessel was subsequently pressurized to 1.14 MPa (150 psig) or 2.17 MPa (300 psig) by air. Subsequently, a predetermined, steady air flow was delivered to the FC reactor. Then the ignition heater, which was located in the pressure vessel at the bottom of the canister, was turned on to ignite the tinder. Ignition occurred within a minute, and the pressure within the reactor increased. The valve at the outlet of the reactor was opened somewhat further to lower the pressure to the initial pressure. The flame front moved upward against the downward flow of air; thereby converting the sludge into charcoal. Six minutes after

(27) Anonymous, Standards for the Use or Disposal of Sewage Sludge. In Title 40, Part 503, EPA, U. S., Ed. Code of Federal Regulations 1995. (28) Urban, D. L.; Antal, M. J. Fuel 1982, 61, 799–806. (29) van Lith, S. C.; Jensen, P. A.; Frandsen, F. J.; Glarborg, P. Energy Fuels 2008, 22, 1598–1609. (30) Antal, M. J.; Gronli, M. G. Ind. Eng. Chem. Res. 2003, 42, 1619– 1640. (31) Gronli, M. G. Charcoal Production;A Technology Review (in Norwegian), TR F5061; SINTEF: Trondheim, November, 1999. (32) Gronli, M. G.; Antal, M. J.; Schenkel, Y.; Crehay, R. The Science and Technology of Charcoal Production; Norwegian Univ. of Science and Technology: Trondheim, 2003; pp 1-38. (33) Antal, M. J.; Mochidzuki, K.; Paredes, L. S. Ind. Eng. Chem. Res. 2003, 42, 3690–3699. (34) Nunoura, T.; Wade, S. R.; Bourke, J. P.; Antal, M. J. Ind. Eng. Chem. Res. 2006, 45, 585–599. (35) Wade, S. R.; Nunoura, T.; Michael Jerry Antal, J. Ind. Eng. Chem. Res. 2006, 45, 3512–3519. (36) Antal, M. J.; Wade, S. R.; Nunoura, T. J. Anal. Appl. Pyrol. 2007, 79, 86. (37) Yoshida, T.; Turn, S. Q.; Yost, R. S.; Antal, M. J. Ind. Eng. Chem. Res. 2008, 47, 9882–9888.

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NA = not analyzed. b wet basis. c Guava woodchips. d,e 138.7 and 98.2 g of guava woodchips were loaded mixed with sewage sludge feed at the top in run 1 and run 2, respectively. f Air-biomass (wet) ratio. g dry ash free basis. h “Over” means over carbonized. i “Failed” means inhomogeneous carbonization. j “Incomplete” means the amount of air delivered was insufficient to fully carbonize the feed.

10.5 9.6 10.4 18.0 1.20 1.26 1.18 1.52 0.903 0.891 0.905 1.69 2.80 2.54 1.22 1.24 2.17 2.17 1.14 1.14 6.0 7.7 13.1 29.0 9.3 8.9 9.0 420.0 420.0 420.0 corncob (l) corncob (l) corncob (d) 6.0 6.7 15.9 39.1 1078.4 631.4 631.4 827.9 080222 080227 080303 080616 F1 F2 F3 F4

a

failedi failedi incompletej incompletej

22.2 NA 26.5 17.8 19.4 18.9 8.9 19.8 26.8 NA 37.1 29.3 31.4 31.0 15.0 29.8 12.9 12.7 13.5 12.6 12.7 9.9 23.0 11.3 1.28 1.19 1.19 1.20 1.18 1.50 1.55 1.53 0.987 0.893 0.898 1.11 1.11 1.10 2.19 1.11 2.75 2.56 2.44 1.38 1.26 1.27 1.25 1.30 2.17 2.17 2.17 1.14 1.14 1.14 1.14 1.14 7.8 7.1 7.0 8.6 8.2 8.2 23.2 8.7 8.5 7.6 7.7 12.3 10.9 9.9 9.4 8.0 080204 080208 080212 080311 080313 080320 080528 080604 1 2 3 4 5 6 7 8

SS2 SS2 SS2 SS3

g, wet feed

138.7d þ 673.0 guavac 98.2e þ 673.0 guavac 958.0 guavac corncob (l) 420.0 corncob (l) 420.0 corncob (l) 420.0 corncob (d) 420.0 corncob (d) 420.0 6.7 6.5 5.3 6.2 6.4 7.0 30.2 9.1

remark

5456

run

sewage sludge

g, wet

where Mchar and Mfeed represent the dry masses of charcoal and feedstock, respectively; %fC and %feed ash denote the percentage of fixed-carbon contained in the charcoal and the percentage of ash in the feedstock, respectively. Carbonization. In runs 1 and 2, carbonizations were conducted with guava tinder, and the sewage sludge was mixed with guava woodchips. This mixture of woodchips and sludge facilitated carbonization. In these runs, the reactor pressure was held at 2.17 MPa after ignition. Initially we emphasized this relatively high pressure because our previous experience suggested that it might provide superior results. The next three runs (runs 3, F1, and F2) were also conducted at 2.17 MPa, but the sewage sludge was not mixed with woodchips.

482.5 527.3 424.0 631.4 631.4 631.4 827.9 773.0

yfC ychar (%, dry) (%, d.a.f.g) max ABRf of press delivered flow rate reaction (MPa) air (kg/kg) (L/s, std) time (min) moist. content (%, wbb)

tinder feedstock

We conducted 12 sewage sludge carbonization runs that are summarized in Table 4. The results of proximate analyses of the charcoals are shown in Table 5. As in our previous work, we define the charcoal yield ychar and the fixed-carbon yield yfC as follows: M char ychar ¼ ð2Þ M feed   %fC ð3Þ yfC ¼ ychar 100 - %feed ash

moist. overall content moisture (%, wbb) content (%, wbb)

Results and Discussion

initial air load press (MPa)

Table 4. Experimental Conditions and Resultsa

where Ci, is the concentration (mg/kg) of element i in the feed sample or the charcoal sample and Ci,ICP is the ICP result (μg/L) for element i. In addition, the oven-dried sewage sludge (SS2) and some of the charcoal samples were subjected to the bomb combustion method for Hg, As, and Se analyses by Hazen Research Inc. We did not quantify the N and S content of the charcoal: this is a subject of future work.

SS1 SS1 SS1 SS2 SS2 SS2 SS3 SS3

the ignition heater was turned on, it was turned off. When the desired amount of air was delivered, the air flow was halted. The oxygen concentration in the effluent gas from the FC reactor was monitored by an oxygen analyzer (Bacharach, Inc., model OXORII) and recorded. The amount of delivered air was calculated from the pressure levels of the accumulator before and after the experiment. The reactor was depressurized and cooled, and finally, the products of carbonization were removed for analysis. The charcoal product was usually divided into three sections and equilibrated in the open air prior to analysis. The charcoal in each section was weighed and a representative sample from each section was subjected to proximate analysis according to ASTM D1762-84. Some of the feed samples and the charcoal samples were ashed and subjected to analysis. The sewage sludge feed (SS2) ash was analyzed at Huffman Laboratories Inc. (Na, Mg, Al, K Ca, Fe, Mn, P, and Si) and the University of Waikato (Li, Cr, Co, Ni, Cu, and Zn) by ICP-MS. Some of the charcoals were also ashed and analyzed by ICP-MS at the University of Waikato. The ICP-MS data reported by the University of Waikato were the raw measurements of the ICP analysis (i.e., the concentration (μg/L) of each element in 500 mL of solution into which 1 g of ashed sample was dissolved). Thus, for the ICP data, the concentration of each element (in mg/kg) in the sewage sludge feed sample or the charcoal sample was calculated as: 500 %ash ð1Þ C i ¼ C i, ICP 1000 100

good good good good good good overh good

Yoshida and Antal

date (yymmdd)

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: DOI:10.1021/ef900610k

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Table 5. Proximate Analyses of the Sewage Sludge, Guava, and Corncob Charcoals run

fixC (wt %, dry)

VM (wt %, dry)

ash (wt %, dry)

1

overall top (sewage sludge) top (guava) bottom (guava)

79.1 56.3 87.9 83.9

9.5 3.4 3.0 12.8

11.4 40.3 9.1 3.2

3

overall top (sewage sludge) bottom (guava)

67.7 42.3 74.0

20.3 6.4 23.8

12.0 51.3 2.2

4

overall top (sewage sludge) middle (sewage sludge) bottom (corncob)

55.9 38.3 37.1 80.3

10.4 4.5 8.4 15.9

33.7 57.2 54.5 3.8

5

overall top (sewage sludge) middle (sewage sludge) bottom (corncob)

56.9 40.2 40.7 83.9

11.6 8.9 14.3 11.8

31.5 50.9 45.0 4.3

6

overall top (sewage sludge) middle (sewage sludge) bottom (corncob)

56.3 44.9 42.1 74.8

14.6 9.1 12.6 19.9

29.2 46.0 45.3 5.3

7

overall top (sewage sludge) middle (sewage sludge) bottom (corncob)

54.7 44.5 12.5 93.4

2.6 2.7 1.9 3.2

42.6 52.8 85.6 3.4

8

overall top (sewage sludge) middle (sewage sludge) bottom (corncob)

61.0 52.3 45.6 83.6

12.0 8.3 13.2 13.6

27.0 39.4 41.2 2.7

Although we successfully carbonized the entire bed in run 3, the pure sewage sludge was not fully carbonized in runs F1 and F2. In other words, the bed of charcoal was inhomogeneous, containing both charcoal and partially carbonized feed (i.e., “brands”) throughout. Note that the amount of sewage sludge in run 3 was 424 g and the tinder guava woodchips was more than twice that of sewage sludge (958 g); whereas in runs F1 and F2 we loaded more sewage sludge and less tinder into the canister. Moreover, the oxygen concentrations (not displayed in Table 4) of runs F1 and F2 were relatively high all through the run (more than 2%) and rose to 5% when the air flow was terminated, indicating that the volatile matter was not effectively burned during carbonization. The results of these three runs imply that sewage sludge cannot be carbonized via self-sustained combustion under the conditions employed in these runs. In view of our previous positive experiences with high pressure carbonizations, this is a surprising conclusion. It appears that sewage sludge volatile matter has unusual combustion properties and does not burn well during carbonization at 2.17 MPa. The remaining carbonization runs were performed at 1.14 MPa. In the case of runs F3 and F4, the sewage sludge bed was not fully carbonized because it was relatively wet and we delivered less air than required for complete carbonization. Nevertheless, in these two runs the charcoal portion of the bed was homogeneous (i.e., it contained no brands). In runs 4-8 the pure sewage sludge beds were fully carbonized. Effects of Moisture Content. We also studied the relationship between the moisture content of the sewage sludge and the required air/biomass ratio, with 24-h-air-dry (run 7), 72-h-air-dry (run F4), and 120-h-air-dry (run 8) sludges. Because we neglected to standardize the sludge bed height

Figure 1. Relation between overall moisture content and air-biomass ratio.

during the air-drying process, the moisture content of the sewage sludge used in run F4 (72-h-air-dry) was higher than that used in run 7 (24-h-air-dry). Figure 1 displays the measured air-biomass ratio (ABR, the weight ratio of air fed to the system to the loaded biomass) as a function of the moisture content. In Figure 1 the required ABRs in runs F3 and F4 were estimated from the weights of the carbonized portion and the uncarbonized portion after the experiments and the respective amounts of air delivered by calculating the additional air amount needed to carbonize the brands. It is remarkable that the FC process successfully carbonized sewage sludge with high moisture content up to 30.2 wt % on a wet basis (23.2 wt % inclusive of sludge and tinder). The results indicate that sewage sludge with higher moisture content requires higher ABR due to the higher heat demand to vaporize the moisture. However, we remark that the very 5457

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charcoal products of runs 5 and 6 were much lower than those of the char bed residue studied by Pinto et al.13 Similarly, the concentrations of Cr, Co, Ni, Cu, Zn, Cd, and Pb in the charcoal products of runs 5 and 6 were much lower than those of the bottom ash and fly ash samples studied by Shao et al.12 In addition to concern about the addition of heavy metals to the soil when sewage sludge charcoal is used for terra preta applications, there is also concern about the release of gas from the carbonizer into the environment. This gas could be laden with heavy metals. In order to quantify this release, we define a depletion ratio of an element during carbonization as: P j M char, j C char, i, j ð4Þ depletion ratio of element i ¼ 1 M feed C feed, i

high ash content and very low VM content of the middle section of the sewage sludge charcoal from run 7 indicate that too much air was delivered; consequently, the charcoal was over carbonized. Ash Components and Trace Elements in the Charcoals. The ash contents of the Ewa sludges employed in this work (see Tables 1 and 3) were much less than those studied by Urban and Antal (i.e., 29-55 wt %),28 Aznar et al. (41 wt %),10 Shao et al. (i.e., 21-33 wt %),12 Pinto et al. (i.e., 35 wt %),11,13 and Fonts et al. (i.e., 40-41 wt %).4,14 Likewise, the wt % contents of the main ash components in the Ewa sludge (see Table 2) were much lower than those studied by Shao et al.12 and Pinto et al.11,13 These attractive features of the Ewa sludge probably reflect its source (i.e., domestic wastewater as opposed to industrial sources). Table 6 shows analyses of the main ash components in the charcoal from run 3. Because of pyrolytic weight loss during carbonization, the concentrations of most of the ash components in the charcoal increased relative to the feed. Concerning the use of sewage sludge for land application in the USA: 40 CFR Part 50327 regulates the concentrations of As, Cd, Cu, Hg, Ni, Pb, Se, and Zn, (Table 7) which are all trace elements in sewage sludge. Consequently, we obtained trace element analyses by ICP-MS and the bomb combustion method for Hg, As, and Se; the results of which are shown in Table 8 for both the sewage sludge feed SS2 and the charcoal products of runs 5 and 6. The concentrations of all the regulated elements in the sewage sludge charcoals were lower than the stipulated limits; therefore, these sewage sludge charcoals can be used for soil beneficiation (i.e., terra preta). We remark that the concentrations of Hg, Cr, Ni, Cu, Zn, Cd, and Pb of the Ewa sewage sludge were much lower than those of the sewage sludge employed by Pinto et al.13 Likewise, the concentrations of Cu, Zn, and Pb in the

where Cfeed,i is the concentration (mg/kg) of the ith element in the feed, Mchar,j is the dry mass of the charcoal (kg) in the jth section (top or middle), and the Cchar,i,j is the concentration (mg/kg) of the ith element in the jth section of the charcoal. Table 9 lists the calculated depletion ratios of each trace element for runs 5 and 6. The depletion ratios of Zn and Pb displayed in Table 9 may be compared with careful measurements reported by van Lith et al.29 of the release of these elements into the gas phase as a function of combustion temperature during wood combustion. Van Lith et al. found that the release of both Zn and Pb began at about 500 °C and

Table 6. Main Ash Compnentsa of the Sewage Sludge Charcoal (wt %, dry feed basis) Na2O MgO Al2O3 K2O CaO Fe2O3 MnO P2O5 SiO2 TiO2 0.34 a

1.70

8.03

0.27

9.15

5.63

0.08

7.60 13.18 1.98

Analyzed by Huffman Laboratories, USA.

Table 7. 40 CFR Part 503 Regulationa of Sewage Sludge for Land Application (mg/kg) As

Cd

Cu

Hg

Ni

Pb

Se

Zn

41

39

1500

17

420

300

100

2800

a

Figure 2. Depletion ratio (average) of element as a function of boiling point.

Unrestricted use if sludge contains less than these concentrations.

Table 8. Concentrations of Trace Elements in the Sewage Sludge Feed and Charcoals (mg/kg)a As

Se

Hg

Li

Cr

Co

Ni

Cu

Zn

Sr

Ag

Cd

In

Ba

Pb

Bi

U

SS2 (feed)

0.46

2.9

0.64

1.0

7.4

0.6

5.6

62.0

141.3

39.9

1.5

0.40

0.05

35.9

3.9

4.5

0.27

run 5 charcoal, top run 5 charcoal, middle

0.33 0.24

1.92 1.36

0.07 0.07

1.7 1.2

14.4 13.8

3.4 1.2

13.6 11.2

149.5 137.6

302.0 429.9

107.7 95.2

5.1 3.4

0.33 0.61

0.11 0.10

90.9 81.3

11.4 12.3

7.6 10.6

0.74 0.80

run 6 charcoal, top run 6 charcoal, middle

0.31 0.16

4.77 0.95

0.04 0.08

2.4 1.9

20.6 18.1

1.9 1.7

16.6 15.6

194.3 192.2

389.3 489.7

145.4 130.8

4.8 4.8

0.37 0.63

0.18 0.14

129.4 102.9

14.3 13.7

8.6 12.7

1.1 0.95

a As, Se, and Hg concentrations are the data of bomb combustion method conducted by Hazen Research, Inc. Other data are calculated from the ICP-MS data provided by The University of Waikato.

Table 9. Depletion Ratios of Trace Elements in Sewage Sludge (g/g) run 5 run 6

As

Se

Hg

Li

Cr

Co

Ni

Cu

Zn

Sr

Ag

Cd

In

Ba

Pb

Bi

U

0.80 0.85

0.82 0.70

0.96 0.97

0.52 0.35

0.39 0.21

-0.24 0.13

0.29 0.13

0.26 0.06

0.17 0.06

0.18 -0.04

0.05 0.01

0.63 0.62

0.32 0.03

0.23 0.03

0.02 -0.09

0.35 0.27

0.07 -0.17

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Energy Fuels 2009, 23, 5454–5459

: DOI:10.1021/ef900610k

Yoshida and Antal

increased sharply to more than 85% at 850 °C. We did not measure the peak temperature reached by the sewage sludge feed during carbonization; nevertheless, we can estimate this temperature from values of the charcoal’s volatile matter content. In his Ph.D. thesis Schenkel30,38 showed that the volatile matter content of a charcoal is well correlated with its peak carbonization temperature. The volatile matter contents of charcoals from runs 5 and 6 ranged from 9 to 14 wt % (see Table 5), indicating peak carbonization temperatures above 550 °C.30 The depletion ratios of Zn and Pb listed in Table 8 are somewhat less than values indicated in Figure 3 of van Lith et al.29 This may indicate an ability of the charcoal bed to retain (by adsorption or other means) some of the heavy metals and restrict their release into the environment of the carbonizer. On the other hand, the high depletion ratios of As, Cd, Hg, and Se displayed in Table 9 indicate the need for a scrubber or other means of capture at the exhaust of the carbonizer. In Table 9, most of the depletion ratios (except As, Hg, and Co) of run 5 are higher than those of run 6. Regarding the experimental conditions, run 5 employed a lower airflow rate and a longer reaction time. Considering these conditions, normally the longer reaction time contributes to an increase in the depletion ratio, while the lower flow rate can diminish the depletion of heavy metals in the charcoals. Thus, it is likely that the longer reaction time increased the depletion ratios in run 5, while the lower airflow rate had less effect. Moreover, the reaction temperature itself also should affect the migration of the ash elements. Therefore, we investigated the relation between the boiling temperature of the elements and their depletion ratios (average values of run 5 and run 6), which is shown in 29

Figure 2. We see that lower boiling point elements tend to have high depletion ratios. It should be noted that the depletion ratio, which expresses the extent to which each element is released during carbonization, can be correlated with the boiling point of each pure element, even though the ash elements may exist as some kind of oxide or salt in the sewage sludge. Conclusion (1) Sewage sludge beds with moisture contents as high as 30 wt % can be carbonized by the FC process, but the yield of biocarbon decreases as the moisture content increases. (2) Heavy metals characterized by low boiling points (e.g., As, Hg, Cd, Se) are prone to elution from the FC reactor during carbonization; whereas heavy metals with high boiling points (e.g., Pb, Co, Ni, Cu, Zn, Sr) are prone to retention in the charcoal. (3) The sewage sludge charcoal produced in this work is acceptable for land application (i.e., terra preta) according to EPA regulation 40 CFR Part 503.27 Such land applications would sequester carbon and thereby fight climate change. (4) Future work should give attention to the disposition of N and S during the carbonization process, together with continuing emphasis on the disposition of the heavy metals present in sewage sludge. Acknowledgment. We thank the following sponsors of our work for their support: the Consortium for Plant Biotechnology Research, Inc. (Dorin Schumacher); the Office of Naval Research under the Hawaii Energy and Environmental Technologies (HEET) initiative, the Coral Industries Endowment of the University of Hawaii; and the University of Hawaii. We thank Nicanor G. Musico (CC of Honolulu) for technical information concerning the Ewa sludge, Dr. Steve Cameron (University of Waikato) for ICP ash analyses, and Lloyd Paredes (Hawaii Natural Energy Institute) for his assistance with the experimental work. Also, we thank two anonymous reviewers for their helpful comments and constructive criticisms.

(38) Schenkel, Y. Modelisation des Flux Massiques et Energetiques dans la Carbonisation du Bois en Four Cornue. Universitaire des Sciences Agronomiques de Gembloux (Belgique), Ph.D. Dissertation, Gembloux, Belgium, 1999.

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