A technical and economic evaluation of the pyrolysis of sewage sludge for the production of bio-oil

Available online at www.sciencedirect.com Bioresource Technology 99 (2008) 1409–1416 A technical and economic evaluation of the pyrolysis of sewage sludge for the production of bio-oil Y. Kim, W. Parker * Department of Civil Engineering, University of Waterloo, Waterloo, Canada Received 27 June 2005; received in revised form 22 January 2007; accepted 22 January 2007 Available online 26 March 2007 Abstract Pyrolysis to produce bio-oil from sewage sludge is a promising way, to not only improve the economical value, but also to reduce pollutants associated with sludge. The aim of this study was to evaluate the production of oil from primary, waste activated and digested sludges. The pyrolysis was performed in a laboratory-scale horizontal batch reactor. The operating temperature ranged from 250 °C to 500 °C, while a gas phase residence time of 20 min was maintained with 50 ml/min of nitrogen gas as a purge flow. The maximum oil yield was achieved with primary sludge at 500 °C. Temperature and volatile solids were the most important factors affecting the yield of oil and char, however, sludge type also affected both results. Pre-treatment of sludge with either acids, a base or a catalyst (zeolite) did not improve the quantity of oil produced. The economic values of the oil produced from primary, TWAS, and digested sludges were estimated as 9.9, 5.6, and 6.9 ¢/kg-ds when the value of oil is 32 ¢/kg-oil. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Pyrolysis; Sewage sludge; Bio-oil; Economic estimation 1. Introduction The management of municipal wastewater treatment sludges is a difficult and expensive problem to solve for many utilities. The sludge resulting from domestic waste-water treatment processes consists of a complex hetero-geneous mixture of organic and inorganic materials (Metcalf and Eddy, 2002). With aerobic treatment, gener-ally, 0.5–1 kg of sludge are produced per kilogram of bio-logical oxygen demand (BOD5) treated (Eckenfelder, 2000). The solids typically contain 60–80% organic matter. The organic materials in primary sludge are comprised of 20–30% crude protein, 6–35% fats and 8–15% carbohy-drates (Metcalf and Eddy, 2002). Although sewage sludge contains various valuable materials, it is often disposed of as an undesirable and invaluable substance. Over 7 · 106 tons of dried sewage sludge were produced in 1990 in the US (McGhee, 1991). Canadian municipalities * Corresponding author. E-mail address: wjparker@uwaterloo.ca (W. Parker). spend $12–15 billion annually for sewage sludge treatment (Buberoglu and Duguay, 2004). The common disposal processes for sewage sludge include landfilling, land application and incineration. How-ever, conventional disposal processes have certain limita-tions. Disposal in landfills is still the most frequently chosen alternative for sludge in Europe and the US (Hall and Dalimier, 1994; McGhee, 1991). Landfilling is not always desirable because of limitations in available landfill volume. Land application or the use of sewage sludge as a fertilizer can result in the accumulation of harmful compo-nents, such as toxic metals, in the soil (Vasseur et al., 1999). Incineration is an effective way to reduce the sludge volume and provide stabilization of the organic material in the sludge (Werther and Ogada, 1999). When combined with cogeneration, incineration processes can effectively recover energy from the sludges. However, the air emissions pro-duced from incineration are undesirable and are restricted by regulation. Pyrolysis is the thermal decomposition of organic sub-stances under oxygen-free circumstances. The process 0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.01.056 1410 Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 involves a complex series of chemical reactions to decom-pose organic materials (Muhlen et al., 1989). The products of sludge pyrolysis include oils (organic liquids and tar), gases, char and reaction water. The synthesized oil, char and gas can be used as alternative fuels and temperature has been shown to be an important factor in determining the yields of the various products (Campbell and Bridle, 1989; Lu et al., 1995; Caballero et al., 1997). Generally, lower temperature conversion processes in the temperature range between 275 °C and 500 °C have been used to pro-duce oil from sewage sludge. Pyrolysis is of interest due to the recovery of oil with low emissions of NOx and SOx. It also avoids the formation of toxic organic com-pounds such as dioxins, with low operating costs, as com-pared to incineration (Werther and Ogada, 1999). While pyrolysis of sewage sludges for the production of oils has been of interest for some time, full scale implemen-tation of the technology has been limited (Bridle and Skrypski-Mantele, 2004). Acceptance of the technology has been limited by the low economic value of the pro-duced oil as well as the relative complexity of the process-ing equipment. The economic viability of pyrolysis may be improved if the yield of oil were enhanced or if value-added products such as adsorbents could be produced from the pyrolysis chars. There have been a limited number of studies that have evaluated alternative strategies for enhancing the yield of oils from pyrolysis of wastewater sludges. The use of zeolite as a catalyst to assist in sludge decomposition was found to increase the production of gas and oil versus tar due to cracking of tar (Stammabach et al., 1989). More recently, the use of acidic pre-treatment has been employed to enhance the adsorptive properties of chars that were gener-ated by sludge pyrolysis (Rio et al., 2005). However, the impact of the acid treatment on the yield of oil was not reported. Studies that assess the impact of acid and base treatment on the generation of oils by pyrolysis are lacking. It was hypothesized that pre-treatments may modify the structure of sludge-based organic matter through hydro-lytic mechanisms and that this may result in enhanced oil production during pyrolysis. The objective of this study was to examine the impact of pyrolysis conditions and the use of pre-treatments, that may be considered for adsorbent production, on the gener-ation of oil from a cross-section of wastewater treatment sludges. Pre-treatments that were considered included the use of zeolites, acids and strong bases. An energy-based economic analysis was conducted to identify the sludge source and pyrolysis conditions that were most economi-cally viable for oil generation. 2. Experimental methods 2.1. Sample preparation Two types of sewage sludges and centrifuged anaerobi-cally digested biosolids were collected from the municipal wastewater treatment plant in Ottawa, Canada. Primary sludge was collected from the primary settling tank while thickened waste activated sludge (TWAS) was generated by Alfa Laval (Toronto, Ont.) model 76000 thickening cen-trifuges. Digested sludge was collected as cake after anaer-obic digestion and subsequent centrifugal dewatering by Alfa Laval model 76000DS dewatering centrifuges. In the laboratory, the primary sludge and TWAS were further dewatered using a Thermo Electron model 2349 laboratory centrifuge (Waltham, MA) that was operated at 6000 rpm for 10 min. Each sample was then dried for 24 h at 105 °C in a Fisher Scientific Model 506G laboratory air convection oven (Pittsburgh, PA) and subsequently stored in an airtight container. The primary sludge was pul-verized by hand as it tended to agglomerate during treat-ment. TWAS and digested sludge did not require size reduction. 2.2. Catalyst and pre-treatment of dried sludge samples The use of zeolite as a potential catalyst to enhance pro-duction of bio-oil was investigated. A zeolite (SiO2) with-out alumina was blended with 5 g of dried sludge prior to pyrolysis. Mixing ratios of 0.15, 0.2, 1 and 1.5 g-zeolite/g-ds were evaluated to find the optimum amount of catalyst for the dried sludge. The agents employed for pre-treatment of the dried sludges included 3 M HCl (pH 0.8), 98% and 0.1 M acetic acid (pH 1.0 and 3.5) and 3 M NaOH (pH 13.5) with the pH values measured with a Corning model 320 pH meter (Woburn, MA) before contacting with the sludges. Pre-cisely weighed dried sludge samples (50 g each) were soaked in 200 ml of the agents in a 500 ml flask. The flasks were then placed on a Lab-line Instruments model 3545 orbital shaker (Melrose Park, IL) for 20 min at 150 rpm. After shaking, the sludges were washed several times on fil-ter paper with deionized water to minimize solids loss and subsequently dried at 105 °C for 24 h in an air convection oven. The mass and volatile fraction of each sludge sample was then measured as per standard methods (APHA, 1998) to determine the mass reduction associated with the pre-treatment. 2.3. Experimental apparatus A schematic of the laboratory-scale cylindrical batch pyrolysis reactor is shown in Fig. 1. The body of the appa-ratus consisted of a horizontal stainless steel reactor that was 160 mm long and 40 mm in diameter. The reactor was mounted on a 5° slope inside a temperature-controlled chamber for easy oil flow into the separator. The reactor was wrapped with a temperature-controlled heating tape to provide the additional heat input that was required to achieve the pyrolytic temperatures. Nitrogen gas was employed to maintain an oxygen-free environment. The pressure from the gas cylinder was fixed at 10 psig with a regulator and the flow was controlled by a Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 1411 8 Vent T 7 11 16cm 13 6 13 2 5 12 1 3 9 10 4 Fig. 1. Laboratory-scale pyrolysis apparatus. 1. Nitrogen gas cylinder; 2. flowmeter; 3. preheating coil; 4. chamber; 5. reactor; 6. heating tape; 7. thermocouple; 8. temperature controller; 9. separator; 10. vial; 11. condensing coil; 12. connector; 13. lid. rotameter at a value of 50 ml/min. The gas that was ini-tially at room temperature was preheated through a stain-less steel tube using the latent heat in the chamber. The preheating of the nitrogen gas minimized the temperature drop when the gas stream entered the pyrolysis reactor. The pyrolysis time was measured from the time that the temperature controller indicated that the target tempera-ture was achieved. The oil and gases formed as products from sewage sludge pyrolysis and nitrogen gas from the reactor were transferred to a separator through a connect-ing tube (50 mm in length and 0.64 mm OD). The separator (0.3 cm OD SwagelokÒ Tee connector) split the oil and gas flows. The condenser located at the top of the separator allowed the condensable gases to drip back down to the oil collector at ambient temperatures. The mass of sludge that could be processed in a batch varied with the differing sludge sources but was in the range of 25–50. 2.4. Pyrolysis In this study, 5 g of dried sewage sludge was used for each pyrolysis run, even though some of the pretreated sludges had lost mass during treatment. Therefore, the oil and char yields of the pretreated sludge samples required additional consideration in the analysis. After sample prep-aration, 5 g of dried sample was placed in the reactor and it was tightly sealed using caps at both ends. At least dupli-cate runs were conducted for each combination of sludge source, sludge pre-treatment and pyrolysis temperature. Oil and reaction water derived during the pyrolysis was collected in weighed and labeled 2.5 ml vials that were located on the bottom of the separator. The reaction water that was mixed in the collected oil was difficult to separate mechanically in a separatory funnel as the amount of oil was insufficient to apply this method. Hence it was removed using an evaporation method. The oil collector (vial) was connected to the separator with a flexible vinyl tube. Non-condensable gases (NCG) were vented through the condenser. After collecting the oil and reaction water, the vials were placed in a weighed 25 ml glass test tube. The oil residual (mainly tar) was washed from the stainless steel tube between the separator and condenser with ace-tone and collected. The acetone solution was placed in a 50 °C water-bath for 24 h to evaporate the acetone, and the retrieved oil mass was measured. The residual char in the reactor was collected and weighed after cooling and stored in airtight containers. The masses of volatile (VS) and total (TS) solids of the dried feed samples and chars were measured as per stan-dard methods (APHA, 1998). The elemental composition (C, H, N, and S) of all sludge, oil and char samples was measured with an Elementar Americas Vario EL III ele-mental analyzer (Mount Laurel, NJ). A Parr Instrument series 1108 bomb calorimeter (Moline, IL) was employed to measure the calorific values of the three types of dried sludge samples, oils, and chars. Prior to the calorimetry tests, the samples were mixed with diesel fuel because they did not completely combust in the calorimeter. The mea-sured enthalpy of the diesel fuel was 42.5 ± 2 kJ/g. The procedure and calculation employed in the calorimetry tests followed the method provided in the manual from the manufacturer. 3. Results and discussions 3.1. Summary of analysis results All subsequent data are expressed as the averages of val-ues that were obtained from replicate measurements that were collected in the replicate pyrolysis tests. At least dupli-cate runs were conducted for each experimental condition and at least duplicate measurements were taken for each of the responses reported in this paper. All error bars that are presented in the plots represent the 95% confidence intervals (C.I.) that were estimated on the basis of the rep-licate tests and replicate analytical measurements. In gen-eral, the 95% C.I. of the oil and char yields were estimated to be within ±2.5% of the mean values. The 1412 Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 95% C.I. of the calorific values were found to be within ±10% of the mean values. In the elemental analysis, the 95% C.I. of the oil samples was estimated to be within ±1%, while the char and dried sludge values were within ±5% of the mean values. The pyrolysis conditions and the corresponding charac-teristics of the sewage sludges and products are summa-rized in Table 1. Primary sludge had the highest VS content with an average value of 84%, while digested sludge had the lowest VS content with an average value of 59%. The calorific value of the dried sludges corresponded well with the VS and did not differ significantly between sludge types when expressed on a VS basis with values ranging from 27 to 30 MJ/kg-VS. The calorific value of the pro-duced oils ranged from 36 to 39 MJ/kg-oil and did not seem to be related to the operating temperatures and sludge types. The calorific value of the pyrolyzed chars ranged from 10 to 21 MJ/kg-TS and decreased, as the pyrolysis temperature increased. This agreed with the reduced VS content of the char that was generated at the higher tem-peratures. The calorific values of the oils and chars were similar to that reported by Campbell and Bridle (1986) where values of 32–42 MJ/kg-oil and 7–23 MJ/kg-TS, respectively were observed. The elemental composition of the oil did not vary with operating condition or sludge type. The values of carbon, oxygen, nitrogen, hydrogen, and sulfur concentrations ran-ged between 62 and 74, 8 and 22, 2.7 and 8.5, 9.5 and 9.9 and less than 1%, respectively. Oxygen composition was determined by difference since the elemental analyzer only measured C, H, N and S. Hence, the oxygen values included any uncertainties in the measurements of the other elements. The composition of the char with respect to C, H and O content was a function of the pyrolysis tem-perature and decreased with increasing temperature. Table 1 Summary of results for sludges and pyrolysis products Results Sludge type The energy loss values presented in Table 1 were deter-mined from an energy balance that considered the dried sludges and the pyrolysis products (oil and char) as shown in Eq. (1). Eloss ¼ MdsEds ÿ ðMoilEoil þMcharEcharÞ ð1Þ where, M and E refer to the mass and calorific values and the subscripts ds, oil and char refer to the dried sludge, oil and char, respectively. The calculation was based on the energy per unit mass of dried sludge and the energy that was present in the oil and in the char. It was assumed that the energy loss (Eloss) values quantified energy associated with the gas phase. The energy of the vented gases (non-condensable gases, NCG) was difficult to measure and they were not considered as major products in this study. The energy lost to the NCG as determined from the energy balance was 3–10% (0.5–2.3 MJ/kg-ds) with the exception of TWAS at 300 °C that presented a negative value (energy generation). In the latter case, the calculated energy loss was less than 1% of the calorific value of the dried sludge and hence, was within the variability that was associated with the measurements of the solids and energy content of the samples. The most energy loss was observed with digested sludge at 500 °C with a value of 2.3 MJ/kg-ds. For all cases, the energy loss increased with increasing pyrolysis temperature and was less than 10% of the initial energy content of the dried sludges. In this work, it was assumed that all mass loss was due to NCG. The measured total yields that the dried sludges transferred to the oils, chars and reaction water during pyrolysis ranged from 80% to 91%, hence 9–20% of the mass was estimated to be lost as NCG. Campbell and Bridle (1989) reported slightly different results with NCG yields for digested sludge of 4–12% and included 4–6 MJ/kg-TS at 450 °C in a bench-scale continuous system. They found the NCG yields were pri-marily a function of operating temperature. Caballero et al. (1997) reported the NCG was composed mainly of CO and CO2. Dried sludge VS fraction (wt.%) Calorific value (MJ/kg-TS) TS-based VS-based Oil Yield (wt.%) Calorific value (MJ/kg-oil) Elemental composition C H O N Char Yield (wt.%) Calorific value (MJ/kg-TS) TS-based VS-based Energy loss (MJ/kg-ds) Primary 84 23 27 8–42 36–38 62–74 2.7–8.5 8–22 9.5–9.9 33–85 17–21 32 0–1.6 TWAS 69 19 27 12–33 37 63–65 9.4–9.7 17–20 6.8–8.5 43–77 13–20 32–35 ÿ0.16–1.9 Digested 59 17 30 4–26 38–39 69–74 9.7–9.9 8.4–15 5.6–6.3 53–87 10–16 34–36 0.5–2.3 3.2. Effect of temperature on oil and char yields Oil yields versus pyrolysis temperature on the basis of both TS and VS are illustrated Fig. 2. The oil yields based on the TS of primary, TWAS and digested sludge ranged between 8 and 42, 12 and 33 and 4 and 26 wt.%, respec-tively. On the basis of TS, the primary and digested sludge oil yields continuously increased with temperature, while TWAS had an optimum yield of oil at around 400 °C. The results from primary sludge were 10–20 wt.% higher than the other sludge types over most of the temperature range with the exception of 250 °C. The oil yields of pri-mary and digested sludge increased more steeply over the temperature range between 250 °C and 350 °C, while TWAS increased steadily up to 400 °C. These results indi-cate that most of the oil from sludge came at temperatures under 400 °C for all sludges. Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 1413 60 50 40 30 20 Primary Sludge 10 TWAS Digested Sludge 0 200 250 300 350 400 450 500 550 Temperature, oC Closed Markers: Based on TS Open Markers: Based on VS Fig. 2. Effect of temperature on oil yield for three sludge sypes. The oil generated in pyrolysis would be expected to orig-inate with the material described by the VS (Bridle, 1982). Hence, the data were also normalized on a VS basis (Fig. 2). The trend of yields with temperatures did not change compared to the TS basis. However, the VS-based yields from primary sludge and TWAS were generally sim-ilar, while those from digested sludge were slightly lower than the other sludges. Work by Campbell and Bridle (1989) indicated that the VS in primary sludge and TWAS contained more oil precursors and these were destroyed during the sludge digestion process. Char yields for the different sludges versus pyrolysis temperature are presented Fig. 3. The char yields decreased with increasing temperature as increased quantities of VS were converted to oil and NCG. Digested sludge produced higher char yields than the other sludges over all tempera-tures as the digested sludge contained less VS than the other sludges. The char yields from primary, TWAS, and 100 digested sludges ranged between 33 and 85, 43 and 77 and 53 and 87 wt.%, respectively. The use of the chars as adsorbent materials has been reported by other researchers (Lu et al., 1995; Tay et al., 2001; Chen et al., 2002). For the purpose of this study, however, the value of the chars was not considered. 3.3. Effect of catalyst and chemical pre-treatment The impact of incorporating zeolite into the sludge matrices was evaluated to determine if the yield of oil might be enhanced in the pyrolysis process. The yields of oil and char, based on TS, from primary sludge are presented ver-sus the ratio of catalyst to sludge solids masses in Fig. 4 when the samples were pyrolyzed at 500 °C. It can be seen that above a ratio of 0.2 g/g the addition of catalyst resulted in lower char yields but did not substantially impact the yield of oil. These results suggest that addition of the catalyst resulted in increased conversion of VS to gas. Hence, it was concluded that while catalysts may be useful for generating pyrolysis gases, the production of oils was not enhanced. Acid pre-treatment of wastewater sludges has been eval-uated in previous studies to enhance the adsorptive proper-ties of chars that are generated by pyrolysis. Adsorbents may represent an alternative value-added product of pyro-lysis that would improve the economic viability of pyro-lysis, if both oil and adsorbents could be generated simultaneously. It was hypothesized that either strong acids or strong bases may hydrolyze complex organics and hence enhance their conversion to oil in the subsequent pyrolysis. The impacts of acid and base addition on the mass of solids during pre-treatment and the yield of oil dur-ing pyrolysis are presented in Figs. 5a–5c. Pre-treatment of the sludges was observed to modify the total mass and the VS fraction of the sludges. Total mass reductions of primary, TWAS and digested sludge with acid treatment ranged from 15 to 38, 9 to 26 and 8 to 26 50 90 80 70 60 50 Primary Sludge TWAS 45 Digested Sludge 40 35 30 Oil Char 40 25 30 200 250 300 350 400 450 500 550 Temperature, oC 20 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Ratio (Zeolite/ds), g/g Fig. 3. Effect of temperature on char yield for three sludge types. Fig. 4. Effect of catalyst on yields of oil and char. ... - tailieumienphi.vn