Two-stage liquefaction of sewage sludge in methanol-water mixed solvents with low-medium temperature

Highlights

• Low-medium temperature continuous HTL of sludge in methanol-water co-solvent.

• Prolonging the reaction time at low temperature can increase the yield of bio-oil.

• Two-stage liquefaction reduces energy consumption and increases energy recovery.

• Two-stage HTL can promote the accumulation of organic matter from aqueous to bio-oil.

• Migration and transformation of nitrogen in two-stage HTL process.

Abstract

Hydrothermal liquefaction could be a promising technology to convert sewage sludge into bio-oil for energy recovery. In this study, in order to obtain the maximum yield of bio-oil and improve the energy recovery of sludge, the influence of temperature and residence time on the multi-temperature continuously liquefaction of sludge in methanol-water mixed solvents was investigated. The results showed that the two-stage liquefaction significantly affected the yield, elemental composition, higher heating value, energy recovery and chemical composition of bio-oil. By optimizing the reaction conditions of hydrothermal liquefaction, the maximum utilization of sludge resources was realized. The maximum yield (42.2 %) and ester content (41.8 %) of bio-oil were obtained when the reaction conditions were 260 °C, 40 min for the first stage and 340 °C, 40 min for the second stage. The migration and transformation of nitrogen indicated that nitrogen elements were mainly concentrated in the liquid product and the N-heterocyclic compounds are the main forms of nitrogen in bio-oils.

Introduction

Serious challenges are posed to the green sustainable development by the massive consumption and the environmental pollution of fossil fuel. As a clean and green alternative energy of fossil fuel, biofuels have attracted the attention of scholars. Together with other renewable energy sources, biofuels have the potential to completely replace the current conventional energy sources, reinforcing energy security to reduce the emissions of the greenhouse gases [1]. Bio-oil is one of the most promising biofuels because of its low CO₂ emission. At present, China has more than 3800 sewage treatment plants, and more than 6 million ton of sewage sludge (SS) was produced annually [2]. The volume of SS in China continues to increase with the rapid socioeconomic development and the growth of population [3]. As an important component of biomass resources, a large amount of organic matter is contained in the SS, mainly including proteins, polysaccharide, sugars, lipids, nucleic acids, etc. [4]. In addition, due to its high energy density, it is a potential raw material for preparing liquid biofuels. In recent years, the resourceful utilizations of sludge as a sustainable energy preparation method has been vigorously developed. However, the high content of moisture (above 80 %), harmful and toxic substances, especially heavy metals, bring huge issues to its disposal and resource utilization. Traditional treatment methods, including landfill, land use and incineration, are difficult to meet the concept of economic and environmental development [5]. Therefore, it is urgent to seek an environmentally friendly treatment technology. As a low temperature thermochemical conversion technology, hydrothermal liquefaction (HTL) can sidestep the energetically costly drying step by directly utilizing wet biomass as a feedstock to produce bio-oil. In the process of HTL of SS, macromolecular organics are hydrolyzed and depolymerized to form small molecular compounds, then dehydrated, decarboxylated and deamination to form water-soluble monomers. With the increase of reaction time, the monomer was polymerized and reconstituted to produce bio-oil and bio-char [6].

More recently, researchers have been trying to optimize feedstock characteristics and reaction conditions (e.g., catalysts, temperature and residence time) in an attempt to increase the yield of bio-oil [7]. Kapusta [8] found that the yield of bio-oil was highest at 320 °C and 340 °C, and decreased when the temperature was higher than 340 °C. Li et al. [9] compared the liquefaction behavior of sludge in methanol-water and n-hexane-water mixed solvents, and the maximum bio-oil yield in methanol-water mixed solvents was 46.5 wt%. With the continuous expansion of the application scope of SS liquefaction, many scholars have systematically investigated the influence of different reaction parameters on the energy consumption in the process of SS liquefaction [10]. Chen [11] found that the total energy recovery of bio-oil and solid residue reached its maximum at 350 °C. Wang et al. [2] found that the yield of bio-oil was increased and the nitrogen and sulfur contents in bio-oil were decreased when CuSO₄ was used as catalyst.

Since a large number of organic components such as proteins, polysaccharides and lipids in the SS are mainly contained in the extracellular polymer (EPS), the pretreatment technology was considered by many scholars to lyse EPS. The cell wall and cell membrane of the EPS of SS were hydrolyzed and destroyed in subcritical water environment, the lipid in the EPS was released, and the yield of bio-oil was improved. By destroying cell walls and hydrolyzing cell membranes, subcritical water released lipids in EPS of SS [12], which could increase the yield of bio-oil. Yang et al. [13] used subcritical water and mixed surfactant CTAB-AEO₉ to nearly double the yield of the bio-oil and increase the ester content in the bio-oil. Jazrawi et al. [14] conducted preliminary treatment of microalgae at a temperature of less than 200 °C and hydrothermal liquefaction of the remaining solids from 250 °C to 350 °C, and the N content in algal biocrude could be reduced. Kim et al. [15] combined pretreatment of sludge by alkali and ultrasonic method could enhance the decomposition effect of SS.

In addition, the bio-oil produced by HTL of SS has high nitrogen content, which limits the subsequent utilization of bio-oil. Therefore, the migration and transformation of nitrogen is also a research hotspot in the process of SS HTL. Model compounds were used to studied the transformation of nitrogen in thermochemical transformation process due to the complex structure of SS [16]. Shanmugam et al. [17] found that the phosphorus and nitrogen elements in the aqueous product could be recovered for the preparation of magnesium ammonium phosphate during HTL of algae. Leng et al. [18] recycled the hydro-phase by-products obtained by HTL of microalgae as nutrients for microalgae cultivation.

However, to the best of our knowledge, present publications regarding SS HTL mainly focus on adding chemical reagents to improve the yield of bio-oil. Few scholars have studied to improve the yield of bio-oil by adjusting the reaction conditions in the HTL process. Optimizing the reaction conditions of the organic matter hydrolysis process in the SS is of positive significance for improving the yield of bio-oil.

No scholars have studied the influence of the reaction conditions in the hydrolysis stage of organic matter in sludge on the yield of bio-oil and energy recovery of SS. Moreover, previous work on HTL of SS was mostly carried out under high temperature and over extended reaction time conditions likely to be uneconomical in large-scale production plants. Therefore, in this current study, we proposed a method for preparing bio-oil by two-stage continuous HTL of SS at medium and low temperature. The objective of the study is: (i) to systematically investigate the effects of low-medium temperature two-stage liquefaction on all products of SS HTL, (ii) to identify the chemical structural and key compounds present in the bio-oil and element composition in each phase product, (iii) to describe the nitrogen migration and transformation pathway and distribution between the products phase.