Elsevier

Bioresource Technology

Volume 99, Issue 5, March 2008, Pages 1409-1416
Bioresource Technology

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

https://doi.org/10.1016/j.biortech.2007.01.056Get rights and content

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.

Introduction

The management of municipal wastewater treatment sludges is a difficult and expensive problem to solve for many utilities. The sludge resulting from domestic wastewater treatment processes consists of a complex heterogeneous mixture of organic and inorganic materials (Metcalf and Eddy, 2002). With aerobic treatment, generally, 0.5–1 kg of sludge are produced per kilogram of biological 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% carbohydrates (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 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. However, conventional disposal processes have certain limitations. 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 components, 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 produced from incineration are undesirable and are restricted by regulation.

Pyrolysis is the thermal decomposition of organic substances under oxygen-free circumstances. The process involves a complex series of chemical reactions to decompose organic materials (Mühlen 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 produce 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 compounds such as dioxins, with low operating costs, as compared 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 implementation 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 produced oil as well as the relative complexity of the processing 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 generated 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 hydrolytic 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 generation 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 economically viable for oil generation.

Section snippets

Sample preparation

Two types of sewage sludges and centrifuged anaerobically 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 centrifuges. Digested sludge was collected as cake after anaerobic digestion and subsequent centrifugal dewatering by Alfa Laval model 76000DS dewatering centrifuges.

In

Summary of analysis results

All subsequent data are expressed as the averages of values that were obtained from replicate measurements that were collected in the replicate pyrolysis tests. At least duplicate 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 replicate tests and replicate

Conclusions

The temperature of pyrolysis and VS content in sludge were the primary factors affecting oil and char yield. The oil yield in all sludge types steeply increased between 250 °C and 400 °C due to VS decomposition in this temperature range. The estimated yields of NCG based upon unrecovered mass were approximately 9–20% and the energy loss was less than 10% over the range of sludge types. The NCG yield increased with temperature for all sludge types.

A catalyst (zeolite) did not improve oil and char

Acknowledgements

The research described in this paper was partially funded by the Natural Sciences and Engineering Research Council of Canada.

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