Elsevier

Energy Conversion and Management

Volume 196, 15 September 2019, Pages 1410-1419
Energy Conversion and Management

Effects of sewage sludge organic and inorganic constituents on the properties of pyrolysis products

https://doi.org/10.1016/j.enconman.2019.06.025Get rights and content

Highlights

  • Effects of organic and inorganic compounds on sludge pyrolysis were investigated.

  • Addition of humic acid to sludge increased the yields of CO and H2.

  • Presence of kaolin in sludge promoted the water-gas shift reaction.

  • CaO in sludge influenced the yield and composition of tar and non-condensable gas.

  • Humic acid and CaO in sludge immobilised heavy metals in the derived char.

Abstract

Pyrolysis of sewage sludge in the presence of humic acid (HA), CaO and kaolin was investigated in order to examine the effects of organic and inorganic constituents on the product properties. The presence of HA in the sludge increased the yields of H2 and CO compared to that from the raw sludge pyrolysis. Water evolved during decomposition of kaolin promoted the water-gas shift reaction resulting in less CO in the gas phase. The presence of CaO in the sludge had a major influence on CO2 sequestration reducing the volume of non-condensable gas by 18% compared to that from the raw sludge pyrolysis. In contrast, CO volume increased due to the induced Boudouard reaction. No significant impact on the distribution of the chemical compounds in the organic or aqueous phase was observed with the addition of HA to the sludge. Among the inorganic additives, CaO catalysed deoxygenation and ring opening reactions leading to an increment of aliphatic compounds in the organic phase of tar while kaolin did not show any significant impact. HA in sludge increased the carbon content in the derived char to 35.2% compared to that in the char derived from the raw sludge (31.0%). In contrast, kaolin and CaO resulted in higher char yields with lower carbon contents due to high ash content in the raw samples. Regarding heavy metals fate, the addition of HA and CaO to the sludge effectively immobilised heavy metals in the derived char while kaolin appeared to be less effective.

Introduction

In the recent years, thermal processing of sewage sludge has received increasing attention as a result of the continuous increase in sludge generation and stringent environmental regulations for the sewage sludge management [1], [2]. Thermal processing allows the extraction of energy from sewage sludge as well as the reduction of sludge volume while destroying pathogens. Incineration and gasification cause heavy metal volatilisation (due to operation above 800 °C) [3], emit large quantities of acidic gases (SOx and NOx) and have ash disposal issues [4]. On the contrary, comparatively low temperatures (i.e. 500–700 °C) used during sewage sludge pyrolysis avoid heavy metal volatilisation while convert the organic fraction of sewage sludge into potential fuel sources, namely tar (also referred to as bio-oil or pyrolysis oil), non-condensable gas (also referred to as pyrolysis gas) and char [5]. Consequently, pyrolysis has been identified as an alternative environmentally friendly technology for sewage sludge valorisation.

Pyrolysis products of sewage sludge are suitable for a wide range of applications. Depending on the reactor configuration, tar obtained from sewage sludge pyrolysis typically contains two phases, namely organic phase and aqueous phase [6]. The organic phase is rich with aromatic and aliphatic compounds (e.g. alkanes, alkenes, benzene derivatives, phenols, ketones, aldehydes and nitriles) and has a higher heating value (HHV) in the range of 22–34 MJ kg−1 on the dry basis [7]. The aqueous phase mainly comprises water and O-containing polar compounds such as carboxylic acids, amides, aldehydes and traces of alcohols [7], [8]. Non-condensable gas yield depends on the pyrolysis temperature and is composed of CO, CO2, H2 and CH4 as well as trace amounts of C2H6, C2H4 and H2S [9]. The derived char has similar carbon content, higher inorganic matter content and larger surface area compared to those of the raw sludge [10]. Due to these properties, char has been explored for various potential applications such as adsorption (i.e. heavy metals and dye) [11], soil conditioning [12] and carbon sequestration [13].

Sewage sludge has a diverse and complex composition resulting in the highly varied yields and properties of the products obtained during its pyrolysis [7], [14]. A vast number of complex reactions among sludge components can take place which are related to sewage sludge origin [10], [15]. In general, the products of sewage sludge pyrolysis are formed via two simultaneous pathways, namely (1) decomposition reactions of the organic compounds present in the sludge [16] and (2) catalytic reactions of the organic products with the inorganic compounds present in the sludge [17], [18]. Thus, it is of great interest to identify the interactions of sewage sludge components and their products during pyrolysis. Two main approaches have been employed to assess the pyrolysis of sewage sludge, namely (i) mathematic modelling and computational simulation [19], and (ii) use of model compounds to represent the constituents in the sludge [20]. Despite the promising information regarding the pyrolysis process (i.e. impact of the operating conditions on the product distribution) using modelling and simulation, there are many uncertainties in predicting and optimising the process conditions (i.e. kinetic modelling of primary and secondary tar cracking reactions) to achieve desired products due to the complexity of the sludge pyrolysis process [21].

Ca compounds are commonly present in the sewage sludge [15] due to the use of lime (CaO) in wastewater treatment and are known to benefit pyrolysis. For instance, CaO was found to adsorb CO2 from non-condensable gases [22] and facilitate the retention of sulphur [23] in the derived char during pyrolysis of CaO conditioned sludge. Deoxygenation effect of CaO resulted in increased hydrocarbon content in the tar [24]. Furthermore, volatilisation of heavy metals decreased [25] and the formation of inorganic phosphorous was increased [26]. Contradictory observations were also reported. For instance, Zhang et al.[25] demonstrated that the catalytic activities of CaO in CaO-conditioned sludge could result in slightly lower char yields while Liu et al. [23] reported higher char yields due to increased ash contents after CaO addition in the sludge. Furthermore, H2 yield increment with the increase in temperature for CaO-conditioned sludge as reported by Ma et al. [24] contradicts the results of other studies [25]. Varying tar compositions were also reported in previous studies indicating that the influence of CaO is specific to the sludge properties as well as to the operating conditions [23], [24].

There are limited studies regarding the influence of aluminosilicate compounds which are commonly found in sewage sludge, on the sludge pyrolysis process [10]. Ischia et al. [27] reported that the release of crystalline water from clay could result in an increase in CH4 content. Fixation of heavy metals into the derived char by the addition of clay in the sludge was also demonstrated [25]. However, the impact of aluminosilicate compounds on the pyrolysis product distribution and properties has not been clearly determined. Aluminosilicate compounds are commonly used for heavy metal immobilisation during thermal treatment of hazardous wastes (i.e. sintering and vitrification) [28]. Thus, the presence of these compounds on heavy metals immobilisation during sludge pyrolysis needs to be further examined. To date, a few studies have been conducted to highlight the impact of the organic compounds present in sewage sludge during thermal processing. In a recent study, humic acid (HA) (representing the humic substances of sewage sludge) was used as a model sludge for catalytic sub- and super-critical water gasification [20]. It was found that the recalcitrant and condensed chemical structure of HA led to inferior carbon conversion even at extreme reaction conditions. In addition, HA commonly forms various complexations with heavy metals [29]. Thus, the fate of these complexes during pyrolysis of sludge should be further examined.

The aim of this study was to investigate the impacts of organic and inorganic constituents in sewage sludge on the pyrolysis products. HA was used to represent the organic substances while CaO and kaolin were used as representatives of Ca and aluminosilicate compounds, respectively. First, the changes in the properties of the raw sludge after the addition of these compounds were examined, and then the variations in the decomposition of the mixtures and their impact on the yields and properties of pyrolysis products (i.e. char, tar and non-condensable gas) were studied. Furthermore, the influence of the additives on the immobilisation of heavy metals during pyrolysis was investigated in order to evaluate the environmental acceptability of the derived char.

Section snippets

Materials and experimental procedure

Anaerobically digested and mechanically dewatered municipal sludge (MS) was collected from Ulu Pandan water reclamation plant located in Singapore. Prior to experiments, MS was dried at ≤105 °C at least for 24 h and grinded to particle sizes between 0.25 and 1.18 mm. Calcium oxide (CaO, technical grade 97%) and humic acid (HA in sodium salt, technical grade) were acquired from Acros Organics, Thermo Fisher Scientific. Kaolin was purchased from Kaolin (Malaysia) Sdn Bhd. Kaolin samples were

Sample properties

MS, MS-H, MS-K had initial moisture contents of 2, 5 and 3%, respectively, while MS-C showed moisture content <0.1% probably due to the formation of Ca(OH)2. Fig. 2 shows the properties of MS and the mixtures on a dry basis. According to Fig. 2a, MS contained 65% VM, 7.5% FC and 27.5% ash. The addition of inorganic compounds (kaolin and CaO) increased the ash contents of MS-K and MS-C to 38 and 43%, respectively, while the ash content of MS-H remained comparable with that of MS. These results

Conclusions

The impact of the organic and inorganic constituents on the sludge pyrolysis was studied using HA, kaolin and CaO as their model compounds in the raw sewage sludge. Sludge was mixed with the additives and pyrolyzed at 700 °C. Pyrolysis processing of the mixtures and product properties were examined and discussed. The following conclusions are obtained:

  • The presence of HA in the sludge increased the yields of H2 and CO in the non-condensable gas. The addition of kaolin decreased the content of

CRediT authorship contribution statement

W.D. Chanaka Udayanga: Conceptualization, Data curation, Formal analysis, Writing - original draft. Andrei Veksha: Conceptualization, Data curation, Writing - review & editing. Apostolos Giannis: Conceptualization, Project administration, Writing - review & editing. Grzegorz Lisak: Project administration, Resources. Teik-Thye Lim: Conceptualization, Project administration, Resources, Writing - review & editing, Supervision.

Declaration of Competing Interest

None.

Acknowledgement

W.D. Chanaka Udayanga acknowledges Interdisciplinary Graduate Program of Nanyang Technological University, Singapore for the PhD scholarship. Author would also like to thank Ms. Koh Danyu and Ms. Elvy Riani Wanjaya of NEWRI analytical team for their kind support during sample characterisation.

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