Pyrolysis of sewage sludge in a benchtop fluidized bed reactor: Characteristics of condensates and non-condensable gases
Graphical abstract
Introduction
With growth of urban population and industry development, the production of sewage sludge (SS) has dramatically increased. Currently ∼40 million tonnes of SS (about 80 wt% water content) were produced every year in China [1]. However, 70–80% of SS was just piled up somewhere or not disposed effectively, causing serious environmental problems [2]. SS contains various microorganisms and harmful substances (e.g. pathogens, dioxins, heavy metals) [3], and traditional methods had different limitations in SS treatment and disposal, such as large investment, high operation cost, large footprint and serious secondary pollution [4]. Pyrolysis is an emerging technology for SS treatment and disposal, and much cleaner than incineration in terms of reduced pollutant emissions (e.g. heavy metals, dioxin, nitrogen and sulfur containing pollutants) from pyrolysis processes [5]. Pyrolysis retains considerable amounts of heavy metals in the char depending on properties of raw feedstocks and operation conditions [6], and is considered as a promising technology for SS disposal due to its advantages, such as fast reaction, compact structure and wastes volume reduction, retention and stabilization of heavy metals [7].
Different process parameters (e.g. temperature, residence time, particle size, catalyst) and various types of reactors (e.g. fluidized bed) were investigated for pyrolysis of SS in previous literatures [8]. The industrial implementation of biomass pyrolysis requires suitable reactors that may treat jointly different types of heterogeneous and irregular materials. The fixed bed reactor is very simple in construction and has low operating cost, and the application of fixed bed reactors is the most extensive [9]. However, due to the limitation of fixed beds, the feeding may not be continuous and is usually batch feeding. Pyrolysis takes a long time to start and stop, resulting in energy and efficiency loss. In addition, the quality of pyrolysis products varies considerably and is not even. The rotary kiln bed is less sensitive to the fuel nature and indeed able to accommodate large variations in fuel size, shape and composition, calorific value with minimal pre-treatment [10]. However, heat transfer in rotary reactors has complex mechanisms and it depends on kiln configurations. In most kilns, there is a combination of radiation, convection and conduction in the complex gas solid systems, and heat transfer is further complicated by equipment rotation and particle mixing inside the solid bed [11]. The main advantage of conical spouted beds is the vigorous cyclic characteristics allows handling particles of larger size and irregular shape, sticky solids or fine materials without agglomeration and segregation problems in comparison with common fluidized beds [12]. However, instability problems arise in the scaling-up of the process or operation with fine materials [13], and there is a lack of commercial high-temperature applications. Fluidized bed pyrolysis (FBP), in contrast to fixed bed pyrolysis, is a continuous process and has been widely used, particularly in commercial plants. It is helpful or even essential to provide ‘‘fluidization aids” which promote or enhance the operation and performance of the bed [14]. The favorable features of FBP may be partly offset by the need of fuel pre-treatment to ensure suitable fuel size and composition [15]. However, the advantage of FBP over other types are very obvious, e.g. good gas solid contacting, excellent heat and mass transfer, fast heating rates and reactions, constant temperatures, uniform products properties, no need of moving parts in the hot region, reliable seal between feeding unit and reactor [16]. In general, heat transfer between the bed and fixed surfaces (e.g. heat transfer tubes) are favorable compared with other types of gas-solid contactors (e.g. fixed beds or rotary kilns).
Fluidized beds have been employed in industries due to their flexibility to convert low quality solid wastes to value-added products with a high efficiency and decreased pollutant emissions (e.g. NOx and SOx) [17]. The homogeneous and low reaction temperature of fluidized beds limits the emissions of NOx, whereas sorbent bed materials can be used for in-situ sulfur capture. Fast pyrolysis in a fluidized bed is attractive because it offers fast heating rate, high mass and heat transfer rate, rapid de-volatilization, ease of control, manageable product collection, etc. [18]. Moreover, fast pyrolysis in fluidized beds is one of the most efficient processes in converting biomass into liquid products, and remarkably decomposes biomass into clean and high-quality bio-oil [19]. Bed temperature is considered to be the most influential parameter in FBP, thus some authors focused their research on analyzing its effect on the liquid yield. Jaramillo-Arango et al. [20] reached the maximum condensate yield of 40 wt% at 600 °C from FBP of SS. Fonts et al. [21] obtained a maximum liquid production of 40 wt% at 550 °C. Sun et al. [22] studied pyrolysis of SS in a wide range of temperatures between 300 and 900 °C, showing that the maximum liquid yield was achieved at moderate temperatures of around 550 °C. It was reported that FBP of SS released volatile matters, especially lipids, during pyrolysis, which promoted oil production [23]. The effect of residence time of the pyrolysis vapors on bio-oil production has also been studied for SS pyrolysis in previous studies [24], showing an increase of liquid yields with decreasing residence time. To date, many studies have been carried out on adding catalysts to SS pyrolysis to improve the quality and quantity of biofuel (bio-oil and bio-gas) generated [25]. However, limited studies regarding the influence of aluminosilicate compounds on sludge pyrolysis were reported [26]. Literatures disclosed that catalyst with silica and alumina composition (e.g. kaolin and Ca-bentonite) and oxides of metal (e.g. CaO) increased the yield and improved fuel properties by decreasing oxygen content of bio-oil [27]. Aluminosilicate clay would act as solid acid catalyst in the cracking reactions of the organic components of sludge and improve the yield of volatile products. As natural ores, kaolin and Ca-bentonite had some merits of good catalytic performance and low price.
The goal of the present study was to gain more insights on influences of different pyrolysis conditions (e.g. temperature and catalyst) on non-condensable gas properties and condensate characteristics for FBP of SS. This work is expected to provide useful information and promising methods for fluidized bed application and SS pyrolysis.
Section snippets
Raw materials
The SS used in this study was collected from Gaobeidian city, Hebei province. The original SS had a water content of up to 76.25 wt%. After drying in an oven (WHL-25AB, Taisite) at 105 °C for 12 h, the SS was crushed, then proximate, ultimate and XRF (X-ray fluorescence) analyses are performed as shown in Table 1. The dried SS was sieved to particle size of 0.105–1 mm for experiments. Dichloromethane (analytical reagent, mass fraction ≥99.5%) and ethanol (analytical reagent, mass fraction
Influences of temperature on products yields
FC, condensate and non-condensable gases were obtained after pyrolysis of the SS feedstock. The effects of pyrolysis temperatures on products yields are shown in Fig. 3a. As reported by other authors [29], an increase in pyrolysis temperature led to a reduction in FC yield and an increase in non-condensable gas yield. With the temperature increasing from 450 °C to 950 °C, the yield of FC decreased from 74.33 wt% to 58.66 wt% whereas gas yield rose from 4.08 wt% to 11.75 wt%. Atienza-Martínez [30
Conclusions
This paper studied the effects of temperature and catalysts on SS pyrolysis in FBP. The maximum condensate yield (15.83 wt%) was observed at 750 °C without additives. Ca-bentonite resulted in an increase in condensate yields and CaO decreased condensate yields. The yields of non-condensable gases rose significantly with the addition of catalysts (especially kaolin). FTIR spectra indicated the presence of alcohols, carboxylic acid, alkanes, alkenes, amines, phenols, ketones and other compounds
CRediT authorship contribution statement
Yang Liu: Conceptualization, Investigation, Writing - original draft. Chunmei Ran: Methodology, Investigation. Azka Rizwana Siddiqui: Writing - review & editing. Polina Chtaeva: Formal analysis, Investigation. Asif Ali Siyal: Methodology. Yongmeng Song: Investigation. Jianjun Dai: Project administration, Supervision, Funding acquisition, Writing - review & editing. Zeyu Deng: Resources. Jie Fu: Data curation. Wenya Ao: Validation. Zhihui Jiang: Writing - review & editing. Tianhao Zhang:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported financially by the Ministry of Science and Technology of the People’s Republic of China [grant number 2017YFE0124800].
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