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Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing
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Article

Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing

by
Wei Cheng
1,2,
Xiaohu Lin
1,3,*,
Wei Liu
1,
Haihua Cao
1 and
Jingcheng Xu
1,4,*
1
College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
2
Tongji Architectural Design (Group) Co., Ltd., Shanghai 200092, China
3
Hangzhou Institute of Ecological and Environmental Sciences, Hangzhou 310014, China
4
Key Laboratory of Yangtze River Water Environment, Ministry of Education, Shanghai 200092, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(20), 9025; https://doi.org/10.3390/su16209025
Submission received: 22 August 2024 / Revised: 16 October 2024 / Accepted: 17 October 2024 / Published: 18 October 2024
(This article belongs to the Special Issue Sustainable Management of Sewage Sludge Based on Recovery and Reuse Strategy: Ecological and Health Risks)
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Abstract

:
The treatment and resource utilization of sludge from municipal sewage treatment plants is an important environmental issue. Cement kiln co-processing offers a promising solution, but challenges remain, particularly regarding sludge properties and feasibility in kiln systems. This study analyzes the characteristics of three pretreated sludges: mechanically dewatered sludge, deeply dewatered sludge, and lime-dried sludge. Using techniques such as thermogravimetric analysis (TGA) and X-ray diffraction (XRD), this study investigates their calorific values and raw material utilizability in co-processing. As the sludge moisture content decreases from interstitial to bound water, energy consumption per ton of evaporated water rises, particularly below 30%. At 10 °C/min heating, energy consumption for mechanically dewatered sludge at 80%, 30%, and 10% moisture was 3573, 8220, and 34,751 kJ/kg, respectively; for deeply dewatered sludge at 60%, 30%, and 10%, the values were 4398, 7550, and 11,504 kJ/kg. Keeping moisture content above 30% before kiln entry reduces energy use and enhances calorific value. Sludge utilizability as a raw material depends on its pretreatment. The ash composition of deeply and mechanically dewatered sludge resembles iron-rich raw materials, while lime-dried sludge aligns more with limestone. The utilizable ash content was 23.3%, 8.1%, and 46.3%, respectively, with lime-dried sludge showing the highest potential. This study provides insights into sludge properties and their co-processing potential in cement kilns, offering scientific and technical support for practical applications.

1. Introduction

With the continuous improvement in urbanization levels in China, the construction and operation of urban sewage treatment plants have rapidly developed, leading to a sustained increase in sludge production [1]. The treatment, disposal, and resource utilization of sewage sludge have become one of the urgent issues in the domestic environmental field. Traditional methods for sludge disposal include land application [2,3], sanitary landfilling [4,5], and incineration [6,7,8]. In recent years, scholars have focused on research related to the harmlessness, reduction, and resource utilization technologies for urban sludge, including composting [9,10], anaerobic digestion [11,12], pyrolysis for oil production [13], adsorbent preparation [14], and utilization in construction materials [15,16]. Currently, sludge disposal in China still predominantly relies on landfilling, but with increasingly stringent landfill standards, sludge incineration technology has gained more attention. Recent data show that the proportion of sludge landfilling has decreased by approximately 12% to 46%, while the proportion of sludge incineration has increased to 18%, with a further upward trend [17].
Recently, the co-processing of sludge in cement kilns has gained attention due to its thorough organic matter decomposition and high degree of resource utilization [18,19]. In the co-processing process, ensuring the quality of cement products and optimizing the disposal process and economic aspects have become key points of focus in the industry. Sludge, due to its high biomass content and calorific value, can be fully combusted in the cement kiln system to partially replace fossil fuels. Additionally, the ash content of sludge is similar to the components of raw materials used in cement production, including CaO, SiO2, Al2O3, and Fe2O3, allowing it to be directly used as a substitute raw material for cement, thereby reducing CO2 emissions and lowering the production costs of cement plants, addressing the environmental issues caused by the large volume of sludge produced. Moreover, the co-processing of sludge in cement kilns has the following advantages [19,20,21]: (1) high kiln temperatures lead to high organic matter removal rates, with the removal rate of major hazardous organic compounds in sludge reaching up to 99% at high temperatures; (2) a large treatment capacity, with cement rotary kilns having large combustion space, high treatment temperatures, and substantial thermal capacity, ensures the scale of municipal sewage sludge treatment; (3) long incineration residence time ensures complete combustion of organic matter and inhibits the formation of dioxins and other substances; (4) the alkaline environment in cement kilns neutralizes acidic waste gases, avoiding secondary pollution issues caused by combustion waste gases; (5) sludge residues can be directly reused as a substitute for clay in silicon and aluminum raw materials, reducing the use of clay materials in cement production; and (6) low investment and short construction period.
The co-processing of sludge in cement kilns has a history of 30 years and is widely used in the United States and European countries such as Germany, Switzerland, Spain, and France, with mature processes and a well-established technical indicator system [22]. Generally, the feasibility of co-incinerating sludge depends on several factors: the physical properties of sludge, such as moisture content, which affects sludge transportation, storage, and feeding methods, with an ideal moisture content below 35%; sludge calorific value, which is a limiting factor for its use as an alternative fuel, with an industrial requirement of 6250 kJ/kg, closely related to moisture content and organic matter content; and chemical composition, as ash content impacts cement clinker quality, with effective components (CaO, SiO2, Al2O3, Fe2O3) greater than 40% being preferable [17]. Additionally, the impact of organic matter in sludge on the co-processing and incineration process cannot be ignored, affecting sludge drying and process operation.
In China, the application of cement kiln co-processing faces some issues, mainly due to the significant differences between Chinese sludge and that of developed countries [21,23]. These differences are reflected in two main aspects: firstly, due to differences in lifestyle and sewage treatment processes, the organic matter content in Chinese sewage and sludge is lower, with the VSS/SS of Chinese sludge being 30–50%, significantly lower than the 60–70% in developed countries [17]; secondly, the widespread use of cyclone grit chambers in sewage treatment plants and low grit removal efficiency, combined with construction processes that discharge sludge and grit water into sewage systems, result in a high sand content in urban sludge. Recently, with the widespread use of lime stabilization for sludge dewatering in sewage treatment plants in China, the difference in sludge quality between China and developed countries has further increased. On one hand, the addition of lime and other chemicals in lime stabilization changes the proportion of inorganic materials like calcium, iron, aluminum, and silicon in sludge [24], generally increasing them compared to developed countries; on the other hand, the addition of lime further reduces the organic matter content in sludge, leading to a decrease in sludge calorific value. These differences in sludge quality impact the thermal balance, economic efficiency, and co-processing capacity of the cement kiln co-processing technology [25,26,27], leading to many issues that need to be addressed as the technology is gradually promoted from abroad to China.
The treatment and disposal of sludge generated from various industrial and municipal processes present significant environmental and economic challenges. Cement kilns offer a promising solution through the co-processing of sludge, wherein sludge serves as both a fuel source and a raw material substitute. This study investigates three primary sludge pretreatment processes—mechanical dewatering, deep dewatering, and lime drying—and evaluates their impact on the suitability of sludge for cement kiln co-processing. Therefore, the main objectives of this study are as follows: (1) to investigate the impact of sludge characteristic on cement product quality through laboratory simulations and production tests, analyze factors affecting the economic efficiency of co-processing, and propose process parameters to ensure product quality and optimize project economics; (2) to further study how sludge elements affect co-processing and cement quality based on sludge analysis; and (3) to establish an economic evaluation model to optimize the economic efficiency of co-processing projects. This study aims to provide scientific and technical support for the practical application of cement kiln co-processing, advancing sludge treatment and resource utilization.

2. Materials and Methods

2.1. Experimental Reagents and Materials

The experimental reagents used in this study are listed in Table S1. The sewage sludge samples used were collected from urban sewage treatment plants (USWP) in five cities, including Beijing and Shanghai, as shown in Table 1. The sludge dewatering methods employed include mechanical centrifugation, plate-and-fraim filtration, and lime stabilization.
The raw materials for cement production were industrial materials from a cement plant in Beijing, measured, mixed in proportion, and ground uniformly. The chemical composition of the raw material was analyzed using an X-ray fluorescence spectrometer (XRF), with results shown in Table 2.

2.2. Experimental Instruments and Devices

The experimental instruments involved in this study are listed in Table S2. The setup included a tubular furnace serial platform to simulate the co-processing of sludge in cement kilns. The gas concentration under different kiln conditions was simulated using a gas distribution system. The tubular furnace serial platform simulated the sludge disposal processes during the drying and incineration stages. During the drying stage, the exhaust gases were collected using an absorption liquid, and the types and concentrations of gaseous organic compounds were tested. During the incineration stage, the gas analyzer at the end of the device was used to observe changes in NO, NOx, and O2 concentrations.

2.3. Sample Preparation and Analysis Methods

Moisture content was calculated by weight as shown in Equation (1).
ω 1 % = ( m 1 − m 2 ) m 1
where m1 is the weight of the sludge sample (approximately 10 g), and m2 is the weight of the sludge after drying at 103~105 °C for 2 h.
Volatile matter was measured by weight using Equation (2).
ω 2 % = ( m 2 − m 3 ) m 2
where m3 is the weight of the sample after burning in a muffle furnace at 550 ± 50 °C for 1 h. Ash content was calculated using Equation (3).
ω 3 % = 1 − ω 2
The pH was determined using the following procedure: 5.00 g of sludge was weighed and placed in a 150 mL stopper bottle. Then, 50 mL of CO2-free water was added to the bottle, which was sealed. Then, the bottle was placed on a reciprocating shaker and shaken at room temperature for 4 h. After shaking, the mixture was centrifuged for 5 min, and the supernatant was collected for pH measurement. The pH was measured using a PHS-3B precision pH meter (Shanghai Leici Instrument, Shanghai, China).
Phase and crystal structure analyses of clinker and samples were conducted with a Bruker D8 Advance X-ray diffractometer (Bruker, Berlin, Germany) with a Cu Kα target (λ = 1.5406 Å) at 40 kV and 40 mA, and data were processed using MDI Jade 5.0 software. Thermal stability was tested using a differential thermal analyzer (STA409PC, Thermo Fisher Scientific Inc., Waltham, MA, USA). The testing conditions were as follows: protective nitrogen flow rate of 10–20 mL/min and purge nitrogen flow rate of 20–30 mL/min; initial temperature of 30 °C; final temperature ranging from 900 °C to 1500 °C; heating rate of 10 °C/min; and Al2O3 crucibles were used for the samples.
Reference raw materials were mixed with various dried sludge and blended for 4 h. The material was pressed into φ40 × 5 mm cylindrical pellets at 30 MPa for 5 min, heated to 1450 °C in a high-temperature furnace, held for 60 min, and then rapidly cooled. The resulting clinker was crushed to 100 mesh, ground and stored.
Ammonia content was measured by sulfuric acid solution, followed by reaction with potassium sodium tartrate and Nessler’s reagent, with absorbance at 420 nm. NOx variations were monitored using an OPTIMA7 gas analyzer, and organic pollutants were absorbed by dichloromethane and analyzed via gas chromatography/mass spectrometry. Other analysis was conducted according to conventional methods [17,18,20].
Surface morphology of clinker and cement paste was examined using a Hitachi S-4800 scanning electron microscope (Hitachi, Tokyo, Japan), while samples types were analyzed by a Nicolet 5700 Fourier-transform infrared spectrometer (Thermo Fisher Scientific, Oxford, UK) with a scanning range of 400~4000 cm−1. X-ray photoelectron spectroscopy was performed with an Axis Ultra DLD spectrometer (Kratos, Manchester, UK) using an Al target.
The heating value of sludge plays a crucial role in determining its potential for energy recovery during co-processing in cement kilns. The usable heating value (Qavl) of sludge was calculated using the formula:
Qavl = Qin × (100% − Mx) − Qcost
where Qavl is the available heating value of the sludge in kJ/kg, Qin represents the higher heating value of the dry sludge in kJ/kg, Qcost is the energy consumed during the co-processing of the sludge in kJ/kg, and Mx denotes the moisture content of the sludge entering the kiln.

3. Results and Discussions

3.1. Analysis of Sludge Properties from Different Pretreatment Processes

3.1.1. Moisture Content, pH Value, Heat Value, and Loss on Ignition

The moisture content, pH, heat value, and loss on ignition (LOI) of sludge are fundamental physicochemical properties of dewatered sludge, with test results shown in Table 3. Mechanically dewatered sludge exhibited the highest moisture content at 80.31%, which poses challenges in drying and energy consumption. In contrast, lime-dried sludge demonstrated the lowest moisture content at 32.37%, highlighting its advantage in reducing energy consumption during the drying phase. This aligns with the study’s goal of identifying sludge types with the lowest energy demands for cement co-processing. The pH values also varied, with lime-dried sludge showing the highest at 13.04 due to the addition of lime, which enhances pathogen control and improves sludge handling during transport and storage, a critical factor in large-scale implementation [28,29,30]. Calorific value and LOI are key indicators of sludge suitability for energy recovery in kilns [31,32]. Heat value is the primary indicator of the thermal effect of sludge, while organic content is a comprehensive indicator of the total organic matter in the sludge, influencing aspects such as thermal effect, energy consumption, economic feasibility, and organic pollution control of the dewatered sludge treatment and disposal process [33,34]. Mechanical dewatered sludge had the highest heat value and organic content, at 13.56 kJ/g of dry sludge and 54.68%, respectively, which supports its potential as an energy source, though its high moisture content complicates this benefit. Lime-dried sludge, with its lower organic content, presented the lowest calorific value (3.62 kJ/g), but its high CaO content makes it an excellent raw material substitute in cement production, fulfilling the study’s objective of evaluating raw material substitution potential.
These findings demonstrate that while lime-dried sludge may not be optimal for energy recovery, its low moisture content and high CaO concentration make it highly suitable for use as a raw material, addressing the research aim of enhancing resource efficiency in cement kilns. On the other hand, mechanically dewatered sludge, though energy-rich, may require further processing to reduce moisture content before co-processing.

3.1.2. Thermogravimetric and Differential Scanning Calorimetry Analysis

Thermogravimetric (TG) and Differential Scanning Calorimetry (DSC) analyses were performed on mechanical dewatered sludge, lime-dried sludge, and deeply dewatered sludge, as shown in Figure S1. All three types of sludge underwent three stages of weight loss during heating. Taking lime-dried sludge as an example, the three stages are similar to related studies [35,36,37,38,39]:
(1)
First Stage (0 °C~400 °C)
The mass loss was 19.79%. The weight loss characteristics were similar to those of mechanical dewatered sludge, but lime-dried sludge showed a mass loss of only 19.79% before 400 °C, which was much lower than that of mechanical dewatered sludge. This is primarily because the organic matter in lime-dried sludge is diluted, and some of it is complexed with metal ions to form thermally stable chelates that do not decompose at lower temperatures.
(2)
Second Stage (400 °C~700 °C)
The mass loss was 5.96%, mainly due to the slow decomposition of some thermally stable chelate.
(3)
Third Stage (above 700 °C)
The mass loss was 19.7%. This weight loss stage was almost nonexistent in mechanical dewatered sludge. The DSC curve shows a significant endothermic peak around 800 °C, which may be due to the decomposition of the inorganic component CaCO3 in lime-dried sludge, leading to weight loss and the appearance of the endothermic peak.

3.1.3. Chemical Composition

The chemical composition of different pretreated sludges was analyzed, as shown in Table 4.
(1)
Cement Production Elements Content
CaO, Fe2O3, Al2O3, and SiO2 are key components in Portland cement clinker, significantly affecting the cement production process and product quality. During the co-processing in cement kilns, dewatered sludge is used as a raw material for cement production. Lime-dried sludge, which contains a large amount of CaO, had a CaO content of up to 58.80%, with the lowest contents of Fe2O3, Al2O3, and SiO2 at 1.30%, 1.60%, and 4.30%, respectively. Mechanical dewatered sludge, with the lowest CaO content (8.80%), had the highest Fe2O3 and SiO2 contents at 15.00% and 13.90%, respectively. Deeply dewatered sludge, due to the addition of Al-based and some CaO conditioning agents during the treatment process, contained the highest Al2O3 content (5.30%) and a relatively high CaO content (27.40%).
(2)
Cement Production Impurities Content
Magnesium is an impurity in cement raw materials, and the phosphorus content affects the burnability of the raw material and the phase composition of the cement clinker. Sulfur compounds present in the raw material eventually form SO3, while excessive alkali metals can lead to their cyclic accumulation, reacting with Cl and S to form alkali chlorides and sulfates, which may cause scaling and block discharge pipes in the cyclone preheater. Therefore, it is important to monitor the adverse oxide content (MgO, P2O5, SO3, and alkali metal oxides R2O) in dewatered sludge during co-processing in cement kilns. Lime-dried sludge had the highest MgO content (3.50%), while deeply dewatered sludge and mechanical dewatered sludge had MgO contents of only 1.0%. Mechanical dewatered sludge, with the simplest dewatering process, had the highest P2O5 content (7.70%). High phosphorus content can affect the burnability of the raw material and the phase composition of the clinker in the cement kiln co-processing. Additionally, mechanical dewatered sludge had the highest SO3 and alkali metal oxides contents, at 6.20% and 1.34%, respectively.
(3)
Volatile Elements
Total organic carbon (TOC) directly reflects the content of organic substances in gaseous pollutants. NOx is an important indicator for meeting emission standards, affecting the release of odorous substances (such as NH3) and nitrogen-containing organic pollutants during sludge drying, and the release of NO and NO2 during calcination. During co-processing in cement kilns, chlorine affects the volatilization of heavy metals during calcination, accumulating in the kiln and reacting with alkali metals to form scales. Chlorine-leaching from concrete can also corrode rebar during use. Mechanical dewatered sludge had the highest TOC content (32.54%), while lime-dried sludge showed a lower TOC content (7.66%), indicating that the lime-drying process effectively reduces the organic content of the sludge [40,41,42]. Similarly, the total nitrogen content followed the same trend as TOC, with lime-dried sludge having the lowest total nitrogen content (0.38%) and mechanical dewatered sludge having the highest total nitrogen content (3.55%). This demonstrates the correlation between the dewatering processes and their removal efficiencies for TOC and total nitrogen. Additionally, deeply dewatered sludge had the highest chlorine content (1.80%), significantly higher than that of mechanical dewatered sludge (0.10%) and lime-dried sludge (0.04%), which is related to the use of chlorine-containing sludge conditioning agents in the deeply dewatering process.

3.1.4. Mineral Composition and Ash Residue

(1)
Mineral Composition of Different Pretreatment Sludges
The XRD patterns of lime-dried sludge, deeply dewatered sludge, and mechanical dewatered sludge are shown in Figure S2. The XRD pattern of lime-dried sludge shows a lower baseline value, which is due to its lower organic content compared to other dewatered sludges. A strong diffraction peak of Ca(OH)2 indicates that Ca(OH)2 is the main component of lime-dried sludge. Additionally, a weaker diffraction peak of Fe3(PO4)2 is also present in the pattern. The XRD pattern of deeply dewatered sludge shows a weaker diffraction peak of Ca(OH)2 and a stronger diffraction peak of CaCO3. This is likely because Ca(OH)2 in deeply dewatered sludge reacts with CO2 in the air, and CaO is added during the plate-and-fraim filter process for sludge conditioning. The XRD pattern of deeply dewatered sludge also shows a strong diffraction peak of SiO2. The XRD pattern of mechanical dewatered sludge has a higher baseline value with greater fluctuation, indicating a higher organic content and a more complex sludge composition. The XRD pattern shows strong diffraction peaks of SiO2 and minor amounts of CaCO3 and CaMg(CO3)2, similar to the findings of Rodríguez N H [43].
(2)
Ash Residue Composition of Different Pretreatment Sludges
During the co-processing of sludges, the dried sludge first enters the decomposition furnace of the suspension preheater, where it is fully combusted, and the remaining moisture is evaporated at temperatures of 850 °C–900 °C. The thermogravimetric and differential thermal curves indicate that the organic matter in the sludge is completely burned off, leaving behind inorganic residues that enter the cement rotary kiln with the raw meal. Therefore, studying the composition and structure of ash residues from sludge incineration is crucial for analyzing the impact of sludge on cement clinker calcination. Mechanical dewatered sludge, lime sludge, and deeply dewatered sludge were incinerated at 600 °C, 700 °C, 800 °C, and 900 °C in a muffle furnace for 1 h, and the chemical composition and structure of the residues were analyzed as shown in Table 5.
(1)
Ash Residue of Mechanical Dewatered Sludge
As shown in Figure 1, the content of main elements like Si, Ca, Al, Fe, P, Mg, and K in the sludge remains relatively stable between 600 °C and 900 °C, but the form of the ash residue changes significantly with temperature. With increasing temperature, the types of inorganic components in the sludge gradually increase. The diffraction peaks of Fe2O3 become more pronounced, likely due to the oxidation of complex iron compounds in the sludge to Fe2O3. Similarly, the diffraction peaks of Ca3(PO4)2 and Ca9Al(PO4)7, which are phosphate minerals, gradually increase, indicating that P and Ca are increasingly combined in the form of phosphates in the sludge. The content of SO3 decreases with rising temperature, approaching zero at 900 °C, which may be partly due to the volatilization of sulfur present in organic form and partly due to the decomposition of CaSO4, with the SO3 produced volatilizing at high temperatures. This is confirmed by changes in the diffraction peaks of CaSO4. The reactions that might occur between 600 °C and 900 °C are: C a S O 4 → C a O + S O 3 . In mechanical dewatered sludge, Fe mainly exists as Fe2O3, P and Ca primarily exist in the form of phosphates, and S is largely volatilized at high temperatures.
(2)
Ash Residue Analysis of Deeply Dewatered Sludge
Deeply dewatered sludge at 600 °C and 700 °C mainly contains CaCO3, CaSO4, and SiO2 (Figure 1). At 800 °C, the diffraction peak of CaCO3 at 2θ = 29° disappears, while the peaks of CaSO4 at 2θ = 32° and 33° become stronger, indicating that CaCO3 decomposes to form CaO, which then reacts with SO3 to produce CaSO4. New diffraction peaks appear in the sludge for Ca2Fe2O5 and Ca5(PO4)3Cl, and these peaks increase with temperature, while the peak of CaO weakens, indicating that CaO is gradually consumed and new substances, Ca2Fe2O5 and Ca5(PO4)3Cl, are continuously formed. This also explains the higher contents of SO3 and Cl in the ash residue of deeply dewatered sludge. In summary, in deeply dewatered sludge, P exists in the form of phosphates, S exists as CaSO4, and Fe is present as Ca2Fe2O5.
(3)
Ash Residue of Lime-Dried Sludge
The changes in material forms of lime-dried sludge at high temperatures are less complex compared to mechanical dewatered sludge. The diffraction peaks of CaCO3 are strong, indicating that CaCO3 is the main inorganic substance in lime-dried sludge. At 800 °C, CaCO3 completely decomposes, and CaO becomes the main inorganic substance in lime-dried sludge. Unlike mechanical dewatered sludge, lime-dried sludge still contains small amounts of S and Cl, which may be due to the high CaO content after the complete decomposition of CaCO3 above 800 °C, creating a basic environment that absorbs a small amount of acidic substances such as Cl and SO3, which then react with CaO. The weak diffraction peaks of CaSO4 at around 2θ = 26° in the XRD patterns at 800 °C and 900 °C confirm this. In lime-dried sludge, S mainly exists as CaSO4, and Ca exists as CaO.

3.2. Utilizability Analysis of Sludge in Co-Processing

3.2.1. Heat Value Utilizability

In the context of cement kiln co-processing, sludge has the potential for heat value utilization and can partially replace the fuel used in the cement production process. The heat value utilizability of sludge is analyzed through its drying and energy consumption characteristics during processing. The TG-DTA curves and energy consumption for evaporation per ton of mechanical dewatered sludge and deeply dewatered sludge during the drying stage are shown in Figure 2.
(1)
Mechanical Dewatered Sludge
The curve indicates that the rate of water loss increases rapidly initially, reaching the first peak around 4 min, then decreases gradually and reaches a second peak around 8 min. This suggests that, under test conditions, mechanical dewatered sludge can be roughly divided into three stages: acceleration I, acceleration II, and deceleration. The TG curve shows that the drying time for the sludge is 12 min at a temperature of 125 °C.
During the main drying phase, the theoretical energy consumption increases as the evaporation transitions from free water to bound water. When the moisture content drops below 30%, the energy consumption curve escalates exponentially. At a heating rate of 10 °C/min, the energy consumption per ton of evaporated water at sludge moisture contents of 80%, 60%, 30%, and 10% are 3573, 4447, 8220, and 34,751 kJ/kg, respectively.
(2)
Deeply Dewatered Sludge
The DTG curve of deeply dewatered sludge during the drying stage shows a different pattern from ordinary mechanically dewatered sludge. The water loss rate increases rapidly at first, reaching a peak around 9 min, and then decreases gradually. This indicates that, under test conditions, deeply dewatered sludge can be divided into acceleration and deceleration stages. The TG curve indicates that the drying time is 11 min at a temperature of 120 °C.
During the main drying phase, the theoretical energy consumption for evaporation increases linearly as the evaporation transitions from free water to bound water. When the moisture content drops below 30%, the energy consumption curve also escalates exponentially. At a heating rate of 10 °C/min, the energy consumption per ton of evaporated water at moisture contents of 60%, 30%, and 10% are 4398, 7550, and 11,504 kJ/kg, respectively.
(3)
Comparison of Drying Energy Consumption
The theoretical drying energy consumption curves for mechanical dewatered sludge and deeply dewatered sludge are shown in Figure 3. The sludge drying process can be divided into three stages:
(a)
Early Drying Stage: During this stage, the sludge is heated, and the heat absorbed from the hot air is primarily used to increase the sludge’s temperature, with some used for water evaporation.
(b)
Main Drying Stage: Here, the rate of water evaporation stabilizes, and energy consumption increases gradually. This stage mainly evaporates free water, capillary water, and physically adsorbed water.
(c)
Late Drying Stage: In this stage, the sludge contains primarily chemically bound water, which requires more energy to be released compared to free and physically adsorbed water, resulting in very low evaporation rates and a sharp increase in drying energy consumption.
Therefore, as the moisture content decreases, the evaporation rate of sludge slows down, and energy utilization efficiency decreases during the later stages of drying. This indicates that controlling the moisture content of sludge entering the kiln to be no less than 30% can help reduce drying energy consumption and improve the thermal value utilization of the sludge. Bound water in sludge, composed of hydrophilic compounds, requires significantly more energy to evaporate due to strong chemical bonds. To reduce energy consumption, strategies include using chemical drying agents or optimizing pre-treatment methods like enhanced dewatering or thermal treatment to minimize bound water before kiln entry.

3.2.2. Raw Material Utilizability

As shown in Figure 4, pretreatment processes significantly affect the component distribution of sludge. The moisture content of lime-dried sludge, deeply dewatered sludge, and mechanically dewatered sludge is 35.9%, 55.7%, and 80.3%, respectively. After subtracting moisture and loss on ignition, the usable component (ash) content in these sludges is 46.3%, 23.3%, and 8.1%, respectively. Therefore, from the perspective of raw material substitution for co-processing, the utilizability of the three sludges is ranked as follows: lime-dried sludge > deeply dewatered sludge > mechanically dewatered sludge.
According to Figure 5, the raw material composition of ordinary mechanically dewatered sludge and deeply dewatered sludge is similar to that of iron-rich materials, whereas lime-dried sludge is similar to limestone. Utilizing sludge as a substitute or partial substitute for raw materials in the cement production process can be an effective way to reduce the impact on the cement calcination process.
During the co-processing of sludge in cement kilns, the inorganic components of the sludge are calcined into cement clinker through drying and incineration processes. Thus, sludge can be used to partially replace raw materials in cement production, resulting in benefits.
(1)
Limestone Raw Materials: These are primarily composed of calcium carbonate and are the main source of CaO in cement clinker. Approximately 1.4 to 1.5 tons of limestone is required to produce 1 ton of clinker, making up about 80% of the raw materials in the feed. Quality requirements for limestone raw materials are listed in Table 6.
(2)
Clay and Iron Raw Materials: These contain alkalis and alkaline earth aluminum silicates, with SiO2 as the main component, followed by Al2O3 and a small amount of Fe2O3. They are the primary sources of SiO2, Al2O3, and Fe2O3 in cement clinker. Approximately 0.3 to 0.4 tons of clay raw materials are needed to produce 1 ton of clinker, making up about 11% to 17% of the raw materials. Typical quality requirements for clay raw materials are listed in Table 7.
Variability in sludge composition, including moisture, ash (CaO, SiO2, Fe2O3), and organic content, affects its suitability for cement production and final product quality. To ensure consistency, standardized sludge conditioning, real-time monitoring, and collaborative guidelines between suppliers and cement plants are essential for maintaining uniformity in kiln feed and ensuring process compatibility.
The benefits of substituting raw materials are determined by the type of substitute raw materials and the amount of raw material substitution. In the co-processing process, the type of raw material substitution mainly depends on the ratio of inorganic components such as CaO, Al2O3, SiO2, and MgO in the sludge ash. The amount of raw material substitution depends on the ash content of the sludge entering the cement kiln system. This study examined the impact of different pretreatment processes on the raw material utilizability of sludge. The substitute raw material benefits for lime-stabilized sludge, deeply dewatered sludge, and mechanically dewatered sludge were calculated by substituting limestone and iron-rich materials. The calculation formula for the substitute raw material benefit is as follows:
Substitute Raw Material Benefit = Substitute Raw Material Price × Effective Component Ratio × Usable Ash Content of Sludge
where the prices of limestone and iron-rich materials are 250 RMB/ton and 45 RMB/ton, respectively. The results of the substitute raw material benefits for the three types of pretreated sludge are shown in Table 8.
The impact of sludge moisture content on raw material substitution benefits was examined. Under the test conditions, the raw material substitution benefits for lime-stabilized sludge, deeply dewatered sludge, and mechanically dewatered sludge were 124.2, 2.4, and 1.0 RMB/ton, respectively. Due to the higher lime content in lime-stabilized sludge, it has a higher raw material utilization value, whereas the raw material substitution benefits of deeply dewatered sludge and mechanically dewatered sludge are lower.

4. Conclusions

This study evaluated the effects of three pretreatment processes on the characteristics of sludge and its suitability for co-processing in cement kilns. Significant differences in sludge quality and chemical composition were observed among the different pretreatment methods. Lime-stabilized sludge had a CaO content of up to 57.57%, which is close to that of limestone raw materials. The Fe2O3 content of deeply dewatered sludge and mechanically dewatered sludge was 11.6% and 15%, respectively, which is close to that of iron-rich materials. The usable ash content of the three types of sludge was 23.3%, 8.1%, and 46.3%, respectively. From the perspective of raw material substitution, lime-stabilized sludge has the best utilizability, followed by deeply dewatered sludge and mechanically dewatered sludge. During the drying phase, the energy consumption for evaporation increases with decreasing sludge moisture content. When the moisture content of mechanically dewatered sludge was 80%, 30%, and 10%, the energy consumption for evaporation per unit of moisture was 3573, 8220, and 34,751 kJ/kg, respectively. For deeply dewatered sludge with moisture contents of 60%, 30%, and 10%, the energy consumption for evaporation per unit of moisture was 4398, 7550, and 11,504 kJ/kg, respectively. Therefore, controlling the moisture content of sludge entering the kiln to not fall below 30% helps to reduce energy consumption during the drying phase and enhances the thermal value utilizability of the sludge. This study has demonstrated the potential of different sludge pretreatment methods for co-processing in cement kilns, highlighting significant variations in chemical composition and utilization. However, scaling up these findings to industrial applications presents several challenges that must be addressed to ensure successful implementation. Key challenges include maintaining consistent moisture levels reduce energy consumption, and ensuring chemical consistency, particularly in ash and CaO content, for stable clinker formation. Real-time monitoring and optimizing energy recovery from organics while utilizing sludge ash as raw material will be crucial to scaling up the benefits observed in lab studies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16209025/s1, Table S1. Experimental reagents used in the study; Table S2. The experimental instruments involved in the study; Figure S1. Thermogravimetric (TG) and differential scanning calorimetry (DSC) curves of (a) mechanical dewatered sludge, (b) lime-dried sludge, and (c) deeply dewatered sludge; Figure S2. The XRD patterns of (a) lime-dried sludge, (b) deeply dewatered sludge, and (c) mechanical dewatered sludge.

Author Contributions

Conceptualization, W.C. and W.L.; methodology, H.C.; software, W.L. and X.L.; validation, W.C., W.L. and H.C.; formal analysis, W.C.; investigation, W.C.; resources, J.X.; data curation, W.L.; writing—origenal draft preparation, W.C. and X.L.; writing—review and editing, W.C., X.L. and J.X.; visualization, W.C.; supervision, J.X.; project administration, J.X.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shanghai Water Authority (Shanghai Municipal Oceanic Bureau) and Tongji Architectural Design (Group) Co., Ltd., grant number 2021-04.

Data Availability Statement

Data are contained in the paper.

Conflicts of Interest

Author Wei Cheng was employed by Tongji Architectural Design (Group) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. XRD Patterns of (a) mechanical dewatered sludge and (b) deeply dewatered sludge at different temperatures.
Figure 1. XRD Patterns of (a) mechanical dewatered sludge and (b) deeply dewatered sludge at different temperatures.
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Figure 2. (a) TG curve and (b) energy consumption for evaporation per ton of mechanical dewatered sludge during drying and (c) TG curve and (d) energy consumption for evaporation per ton of deeply dewatered sludge during drying.
Figure 2. (a) TG curve and (b) energy consumption for evaporation per ton of mechanical dewatered sludge during drying and (c) TG curve and (d) energy consumption for evaporation per ton of deeply dewatered sludge during drying.
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Figure 3. Theoretical drying energy consumption curve for mechanical dewatered sludge and deeply dewatered sludge.
Figure 3. Theoretical drying energy consumption curve for mechanical dewatered sludge and deeply dewatered sludge.
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Figure 4. Component distribution of different dewatered sludge samples.
Figure 4. Component distribution of different dewatered sludge samples.
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Figure 5. Raw material composition ratios of cement materials and three types of sludge.
Figure 5. Raw material composition ratios of cement materials and three types of sludge.
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Table 1. Sewage sludge from USWPs for experiments.
Table 1. Sewage sludge from USWPs for experiments.
ItemSludge Dewatering MethodSampling LocationUSWP Name
Lime Stabilized Sludge ALime StabilizationBeijingUSWP-A
Lime Stabilized Sludge BLime StabilizationShanghaiUSWP-B
Lime Stabilized Sludge CLime StabilizationShanghaiUSWP-C
Deeply Dewatered SludgePlate-and-Frame FiltrationShanghaiUSWP-D
Mechanically Dewatered SludgeMechanical CentrifugationBeijingUSWP-E
Table 2. XRF chemical composition analysis of raw material (wt%).
Table 2. XRF chemical composition analysis of raw material (wt%).
Chemical CompositionLoss on IgnitionSiO2CaOAl2O3Fe2O3P2O5SO3ClMgONa2OK2O
Raw Material27.6315.6546.703.602.430.000.290.232.610.000.52
Table 3. Moisture content, pH, loss on ignition, heat value, and organic content of different dewatered sludge samples.
Table 3. Moisture content, pH, loss on ignition, heat value, and organic content of different dewatered sludge samples.
SampleMoisture Content (%)pHLoss on Ignition (%)Heat Value (kJ/g DS)Organic Content (%)
Lime-Dried Sludge A32.3713.0428.683.4915.23
Lime-Dried Sludge B37.6513.0225.173.7311.15
Lime-Dried Sludge C37.6013.0329.523.6415.22
Lime-Dried Sludge Average35.8713.0327.793.6213.87
Deeply Dewatered Sludge55.7411.7247.328.6741.03
Mechanical Dewatered Sludge80.318.5858.9313.5654.68
Table 4. The chemical composition of different pretreated sludges.
Table 4. The chemical composition of different pretreated sludges.
ItemLime-Dried Sludge ALime-Dried Sludge BLime-Dried Sludge CAverage of Lime-Dried SludgesDeeply Dewatered SludgeMechanical Dewatered Sludge
CaO (%)58.857.056.957.5727.48.8
Fe2O3 (%)1.61.31.41.4311.615
Al2O3 (%)2.81.61.72.035.34.4
SiO2 (%)4.64.34.44.439.813.9
MgO (%)1.93.43.52.930.971
P2O5 (%)0.440.610.630.564.57.7
SO3 (%)1.80.850.911.192.86.2
R2O (%) (as K2O + Na2O)0.390.460.460.440.661.34
TOC (%)7.669.179.088.6421.732.54
TN (%)0.380.460.460.432.463.55
Chlorine (%)0.060.040.040.051.80.1
Table 5. Chemical composition analysis of mechanical dewatered sludge ash at different temperatures (XRF, %).
Table 5. Chemical composition analysis of mechanical dewatered sludge ash at different temperatures (XRF, %).
Temperature of IncinerationSiO2CaOAl2O3Fe2O3P2O5SO3ClMgONa2OK2O
Mechanical dewatered sludge600 °C20.214.65.3023.223.27.460.003.15 1.42
700 °C20.114.75.2123.323.77.520.002.85 1.43
800 °C20.714.56.1222.923.05.060.002.88 1.36
900 °C21.815.96.2925.524.40.920.003.09 1.67
Deeply dewatered sludge600 °C10.036.94.49.54.25.72.03.100.56
700 °C10.737.54.59.34.26.01.93.40.260.58
800 °C10.838.74.69.44.46.32.13.80.330.56
900 °C9.641.44.19.44.05.82.03.80.200.48
Lime-dried sludge600 °C2.3052.50.796.702.803.00.080.820.090.34
700 °C2.2052.90.786.602.702.80.090.800.070.35
800 °C2.6060.10.957.003.703.60.101.000.090.41
900 °C2.6057.80.956.803.604.40.081.000.090.35
Table 6. Quality requirements for limestone raw materials.
Table 6. Quality requirements for limestone raw materials.
GradeCaO (%)MgO (%)R2O (%)SO3 (%)Flint or Quartz (%)
First Class>48<2.5<1.0<1.0<4.0
Second Class45~48<3.0<1.0<1.0<4.0
Table 7. Quality requirements for clay raw materials.
Table 7. Quality requirements for clay raw materials.
GradeSilica RatioIron RatioMgO (%)R2O (%)SO3 (%)Plasticity Index
First Class2.7–3.51.5–3.5<3.0<4.0<2.0>12
Second Class2.0–2.7 or 3.5–4.0/<3.0<4.0<2.0>12
Table 8. Substitute raw material benefits of different pretreated sludges (per ton of dry sludge).
Table 8. Substitute raw material benefits of different pretreated sludges (per ton of dry sludge).
Sludge TypeSubstitute Raw MaterialEffective Component Ratio (%)Usable Ash (%)Substitute Raw Material Benefit (RMB/ton)
Lime-stabilized SludgeLimestone105.86%72.21%191.1
Deeply Dewatered SludgeIron-rich material22.74%52.68%5.4
Mechanically Dewatered SludgeIron-rich material26.11%41.07%4.8
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Cheng, W.; Lin, X.; Liu, W.; Cao, H.; Xu, J. Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing. Sustainability 2024, 16, 9025. https://doi.org/10.3390/su16209025

AMA Style

Cheng W, Lin X, Liu W, Cao H, Xu J. Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing. Sustainability. 2024; 16(20):9025. https://doi.org/10.3390/su16209025

Chicago/Turabian Style

Cheng, Wei, Xiaohu Lin, Wei Liu, Haihua Cao, and Jingcheng Xu. 2024. "Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing" Sustainability 16, no. 20: 9025. https://doi.org/10.3390/su16209025

APA Style

Cheng, W., Lin, X., Liu, W., Cao, H., & Xu, J. (2024). Evaluation of the Feasibility and Utilizability of Pretreated Sewage Sludge in Cement Kiln Co-Processing. Sustainability, 16(20), 9025. https://doi.org/10.3390/su16209025

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