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Article

Pollutant Emissions and Heavy Metal Migration in Co-Combustion of Sewage Sludge and Coal

by
Chunyu Liu
,
Changtao Yue
* and
Yue Ma
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
*
Author to whom correspondence should be addressed.
Energies 2024, 17(11), 2457; https://doi.org/10.3390/en17112457
Submission received: 23 April 2024 / Revised: 19 May 2024 / Accepted: 20 May 2024 / Published: 21 May 2024
(This article belongs to the Section B: Energy and Environment)
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Abstract

:
The treatment of sewage sludge has become a global concern. Large amounts of sewage sludge can be disposed of by burning coal-mixed sludge. Thermogravimetric analysis and lab-scale combustion experiments in a drop tube furnace were utilized to study the combustion characteristics, pollutant emissions, and heavy metal migration during the co-combustion of coal and sewage sludge. The results showed that the blended fuels with a sewage sludge content less than 10 weight percent exhibited coal-like combustion characteristics. Additionally, the additional sewage sludge favored the ignition performance of blended fuels. When sewage sludge was added, the SO2 emissions rose to 76 mg/Nm3 under the 10% sludge condition—nearly three times higher than that of coal alone. While NOx emissions stayed mostly unchanged, HCl and HF emissions were very low. Meanwhile, Cr, Cu, and Ni migrated to the bottom ash, and their concentrations were all reduced with an increase in sewage sludge. Pb, Cd, Cr, Cu, Ni, and Hg migrated to the flue gas, mostly in the form of gaseous components. The results provide crucial information in the co-combustion of sewage sludge and coal, with implications in the development and improvement of large-scale, harmless, and resource-recovering techniques for waste sludge.

1. Introduction

Sewage sludge is an inevitable by-product generated in city wastewater treatment processes [1], which has become a serious global concern due to its increasing amount and aggravated damages to human health and ecology system. Based on 80% moisture content calculations, it is anticipated that the European Union produces around 50 million tons of sewage sludge annually [2]. The production of sewage sludge in China has also grown rapidly [3,4], and the fast development of China’s urbanization and industrialization will result in 97.72 million tons of sludge by 2030 [5]. The composition of sewage sludge is complex and it is enriched in various toxic substances, such as organic pollutants, pathogenic micro-organisms, and heavy metals [6,7], which may cause serious pollution to the environment if they are not treated appropriately. In the past few years, sludge treatment and disposal capacity in China have been insufficient and the disposal measures remain relatively backward; how to deal with the large amount of sewage sludge to achieve the purpose of reduction, harmlessness, and recycling has become a topic of concern.
Currently, the main methods to deal with sewage sludge include landfill, agricultural utilization, composting, and thermal treatment [8,9,10]. Disposal of sewage sludge by means of landfilling is forbade by the stringent regulations in some developed countries in terms of the shortage of lands and the increased problem of pollution loads [11]. There are barriers to the agricultural use of sewage sludge due to the fact that organic and inorganic pollutants are harmful to the ecosystem and even enter the food chain [12,13]. However, thermos techniques, such as combustion, gasification, and pyrolysis [14,15,16], are gaining increasing interest because of their advantage of utilizing the energy potential of sewage sludge. Combustion recovers energy in terms of heat and electricity, while gasification and pyrolysis generate energy-rich products refined as fuel [17]. Combustion is a well-established technology with the advantages of large treatment scale and a simple process, and it has proven its use in many cities.
Sewage sludge in its dry form can be regarded as a special type of renewable fuel because of its relative high quantity of flammable substances [18,19]. Due to the unstable ignition of sewage sludge, it is typically co-combusted with coal, wood, and other biomass. By utilizing existing power plants or waste-treatment facilities, sewage sludge co-combustion lowers the cost of new development [20]. According to the research by [21], pulverized coal power plants have an electrical efficiency that is 10–20% higher than that of specialist trash incinerators. Waste sludge treatment in operating power plants should prioritize collaborative treatment with coal due to its energy efficiency benefits. Co-combustion has the potential to lower furnace temperature and combustion efficiency overall [22]. Typically, sewage sludge makes up 10% of the co-combustion ratio [23,24]. Several studies have been performed to explore the co-combustion characteristics of sewage sludge and coal [25,26]. However, the proportion of sludge added therein is often much higher than 10%, which has limited applicability in the case of co-combustion of sludge and coal in coal-fired power plants. Information on the co-combustion properties of sludge added at different proportions less than 10% is limited, and the supplementation of them is greatly needed, especially with the popularization of co-treatment processes. Both the combustion behaviors and the characteristics of coal and sewage sludge, such as volatile, fixed carbon, and ash, are clearly different [27]. Coal and sewage sludge basically maintain their own devolatilization capabilities during co-combustion. According to the research studies by Folgueras et al. and Xu et al. [26,28], the combined effects of both sludge and coal often effect the combustion parameters of blended fuels. Furthermore, sewage sludge’s volatile component releases at a low temperature, aiding in the combined fuel’s ignition [29,30].
Emissions of gaseous pollutants and heavy metals from co-combustion processes are also important problems to constrain the development of co-combustion technology. Zhang et al. [31] indicated that the emission of SO2 and HF rose with the sludge ratio in co-combustion of hazardous sludge and coal, while the NOx and HCl did the opposite. This may be due to the emission of pollutants influenced by the chemical properties of the blended fuels and the combustion conditions. Ma et al. [32] focused on gas emissions during the co-combustion of coal and sludge in an oxygen atmosphere. They discovered that increasing the oxygen content increased NO and SO2 emissions during co-combustion, and that sludge combustion at high oxygen contents might aggravate SO2 and CO emissions. Cheng et al. [20] divided the heavy metals into three groups: non-volatile, semi-volatile, and volatile. According to the simulation results, the non-volatile elements were concentrated in ash, the volatile elements were enriched in gas, and the behaviors of the semi-volatile elements depended on the type of incinerator. Bartoňová et al. [33] focused on the behavior of Cd, due to its toxicity and high volatility, and discussed the possibilities of decreasing Cd emissions. Liu et al. [34] discovered that adding low-rank coals successfully reduced the amount of Cl and Zn combined and encouraged metal minerals to absorb ZnCl2. The conversion of Zn to ZnO·Al2O3·2SiO2 in the co-combustion was aided by the presence of the oxides SiO2, Al2O3, Fe2O3, and CaO. Stable FeCr2O4 and CaCr2O4 can be produced during coal combustion by efficiently capturing Cr vapor with Fe2O3 and CaO. In 2020, the Chinese government published its official standard for pollution control with hazardous waste incineration (GB 18484-2020) [35] to strictly limit the emissions of heavy metals such as mercury, cadmium, lead, chromium, copper, nickel, arsenic, manganese, and their compounds. However, information on pollutant emissions and various heavy metals’ migration during the co-combustion of sludge added at different proportions under 10% is still insufficient.
In this paper, the combustion characteristics of different proportions within 10 wt.% of sewage sludge blended with coal were investigated by thermogravimetric analysis, and the blended fuels were evaluated according to the combustion characteristics’ parameters. The blended fuels were burned in a drop-tube furnace in the laboratory, which has a similar condition to a pulverized coal furnace, and the gaseous pollutant emissions were analyzed to obtain the pollutant emissions’ characteristics of sewage sludge blended with coal. The distribution and migration patterns of heavy metals in the blended fuels in combustion were analyzed, and the existence of heavy metals was further simulated by using FactSage 7.3 to provide a theoretical basis for the industrial application of sewage sludge blended with coal. Our results provide fundamental information on the co-combustion characteristics of sludge and coal at different contents and the corresponding migration behaviors of heavy metals.

2. Material and Methods

2.1. Materials

The sewage sludge used in this experiment was obtained from a wastewater treatment plant in Chengde, Hebei Province. The bituminous coal was collected from the Shenmu region in China, which is a conventional fuel in thermal power plants. Sewage sludge and coal were dried to a constant weight in an oven at 105 °C and pulverized with particle sizes below 74 μm for the experiment in order to be consistent with the particle size of coal powder for a pulverized coal furnace. The basic characteristics of sewage sludge and coal are listed in Table 1. The proximate analysis of both fuels was carried out according to GB/T 212-2008 [36], and the ultimate analysis was measured by a vario EL cube element analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany) according to GB/T 31391-2015 [37]. The heating value was determined by IKA C6000 (IKA GmbH, Staufen, Germany) according to GB/T 213-2008 [38]. The contents of F and Cl were determined by the high-temperature combustion hydrolyzing method according to GB/T 3558-2014 [39] and GB/T 4633-2014 [40]. The ash chemical compositions of coal and sewage sludge were analyzed by the semi-micro analysis method according to GB/T 1574-2007 [41], and the results are provided in Table 2. Table 3 shows the content of heavy metals in coal and sewage sludge. The samples were digested separately with a mixture of HNO3, HF, HCl, and H2O2 and then heated with HClO4. Then, the heavy metal concentrations were determined by the Agilent ICPOES730 inductively coupled plasma optical emission spectrometer (ICP-OES).
In order to investigate the effects of the amount of sewage sludge on gaseous pollutant emissions and heavy mental migration characteristics during the co-combustion process, the dry sewage sludge was blended into coal at a ratio of 0%, 2%, 4%, 6%, 8%, and 10%. The sewage sludge addition ratio (Φ) is defined as the mass of dry sewage sludge to the mass of samples, which is shown in Equation (1).
Φ = M s l u d g e M s l u d g e + M c o a l × 100 %
where Msludge is the mass of dry sewage sludge, g; Mcoal is the mass of dry coal, g.

2.2. Thermogravimetric Analysis

The thermogravimetric (TG) analysis was carried out using a Shimadzu TGA-50 thermogravimetric analyzer (TGA). An experimental sample of (10 ± 0.2) mg was tiled at the bottom of an alumina crucible with a diameter of 5 mm. Air (79% N2, 21% O2) at a flow rate of 50 mL/min made up the TGA environment. Additionally, a temperature control system was used to raise the temperature by 10 °C every minute to 900 °C.

2.3. Combustion Facility

The combustion experiments were carried out in a drop tube furnace to simulate the combustion environment of a pulverized coal furnace. The schematic diagram of the facility is shown in Figure 1. The equipment consisted of a vertical reactor, a control cabinet, a screw feeder, an air supply system, flue gas and a particle exhaust system, and a cooling water system. The reactor had a diameter of 110 mm and a length of 2 m, with 36 MoSi2 heating rods and 3 thermocouples to control the combustion temperature, which could reach 1500 °C.
In the co-combustion experiments, the mixed fuels were pushed out by the screw feeder at a feed rate of 3 g/min and injected into the furnace together with the primary air. The injected fuels subsequently combusted in the reactor with the secondary air. The combustion temperature was set to 1300 °C, which is basically stable. The excess air ratio was approximately 1.4, with a primary air rate of 0.3 and a secondary air rate of 0.7. The primary and secondary air temperatures were preheated to 120 °C.
During the combustion experiments, the bottom ash was collected in an ash bucket placed at the bottom of the furnace. The flue gas was drawn to the flue gas and particle exhaust system, in which the fly ash particles in the flue gas were separated by a cyclone. The remaining flue gas passed through the probe of the Gasmet DX 4000 portable FTIR flue gas analyzer (Gasmet Technologies Oy, Vantaa, Finland), which could detect the concentration of SO2 and NOx in real time. The flue gas was absorbed by a 0.01 mol/L KOH solution with F− and Cl−. The concentrations of F− and Cl− in solution are detected by Ion chromatography of 883 Basic IC plus (Metrohm AG, Herisau, Switzerland).

2.4. Heavy Metal Analysis

The pure and blended samples and their bottom ashes after combustion were digested separately with a mixture of HNO3, HF, HCl, and H2O2 and then heated with HClO4. Then, the heavy metal concentrations were determined by the Agilent ICPOES730 inductively coupled plasma optical emission spectrometer (Agilent Technologies Co., Ltd., Santa Clara, CA, USA). The contents of heavy metals in flue gas could be obtained by Equation (2).
c = m i c × c i c − m i a × c i a V a i r
where cia (mg/kg) and cic (mg/kg) are the concentrations of elements in bottom ash and fuel, respectively, mia (kg) is the mass of bottom ash, mic (kg) is the mass of fuel, and Vair (Nm3) is the volume of air in combustion.

2.5. Simulation Calculation of Heavy Metals Migration

Simulation calculations of heavy metal compounds were carried out by FactSage 7.3, which is based on the method for Gibbs free energy minimization. As the thermodynamic properties of ingredients in coal and sewage sludge are unconventional in most software, they should be entered in the form of elements. The initial data input for simulation calculations were as follows.
The initial elements in samples were C, H, O, N, S, Cl, P, Si, Al, Fe, Ti, Ca, Mg, K, Na, Cd, Cr, Cu, Hg, Mn, Ni, Pb; the reaction temperature was 1300 °C; the reaction pressure was 1 atm; the excess air coefficient was 1.4; and the sewage sludge proportion was 0%, 2%, 4%, 6%, 8%, 10%.

3. Results and Discussion

3.1. TG and DTG Profiles of Coal, Sewage Sludge, and Mixtures

In order to analyze the co-combustion characteristics of coal and sludge, TGA was used to evaluate the combustion behavior of the coal, sewage sludge, and their mixed samples. Figure 2 shows the TG and DTG curves of drying sewage sludge by co-combustion with coal. The combustion behavior of dried sewage sludge was quite different from that of pure coal. The combustion process of sewage sludge could be divided into two stages: release of light volatile substances (first stage: 200–400 °C) and oxidation of complex organic substances (second stage: 400–600 °C), such as char formed in the previous stage [30,42,43]. The pure coal combustion process had only one stage between 300 and 600 °C, which was relative to the cracking of organic substances and combustion of fixed carbon. The thermal weight loss of pure coal was mainly caused by the combustion of fixed carbon, while that of sewage sludge was mainly caused by the devolatilization and combustion of volatile substances. The combustion characteristics of sewage sludge were closely related to its own properties. The thermal weight loss in the sewage sludge combustion process was basically concentrated on the devolatilization stage since the sewage sludge contained a large amount of volatile matter.
The TG curves of the blended samples were located between the two curves of coal and sewage sludge and seemed more similar to that of coal. The maximum weight loss rates of the blended samples were smaller compared with those of the coal, as shown by the DTG curve. It could be seen that the mixed samples had integrative thermal profiles that reflected both coal and sewage sludge; similar phenomena have been shown in other studies [24,44]. As the sewage sludge percentage was small (≤10%) in the blended samples, the influence of sewage sludge on blended fuels was limited.
The combustion parameters of coal, sewage sludge, and their blends are listed in Table 4. With the addition of sewage sludge to the blends, the ignition temperature (Ti) dropped by 1–3 degrees Celsius compared to that of coal burning alone. Ti reflected initial combustion performance and the lower Ti means that it is easier to be ignited. The addition of sewage sludge made ignition easier. This was probably because the sewage sludge contained a lot of volatile substances that could be rapidly released and combusted at a low temperature, making it easier to ignite the fuel blends [24]. Some studies [45,46] suggested that there were some promoting interactions taking place between the individual fuels that could stimulate the devolatilization during the co-combustion process. Sewage sludge with a high H/C ratio might produce large amounts of H and OH radicals via dehydrogenation reactions, which could inhibit secondary reactions such as cross-linking reactions and condensation reactions, and facilitate the thermal cracking of aromatic compounds. Therefore, the synergistic effects favored the ignition behavior, promoting the combustion of the blended fuels at a lower temperature.
In order to evaluate the comprehensive combustion performance of the mixed fuels, the comprehensive combustibility index S is defined in Equation (3).
S = ( d w / d t ) m a x ( d w / d t ) m e a n T i 2 T b
where (dw/dt)max, (dw/dt)mean, Ti and Tb represent the maximum rate of weight loss (%/min), the average rate of weight loss (%/min), ignition temperature (°C), and burnout temperature (°C), respectively [30,42]. S reflects the overall combustion performance of fuels. The larger the value of S, the easier the burning. With the ratio of sewage sludge increasing in blend fuels, S declined. The addition of sewage sludge was unfavorable for co-combustion. This was probably due to the high content of noncombustible materials in sewage sludge, which might increase the energy barrier in the entire combustion process. The ash components from sewage sludge can hinder the oxidation of coal in blends during the co-combustion process. Similar analysis results have been reported in previous research, which pointed out that the ash in paper sludge reduced the combustion efficiency of coal during the co-combustion process [47,48].
Based on the above phenomena, adding sewage sludge to coal favored the ignition behavior, promoting the combustion of blended fuels at low temperatures. However, for the overall combustion process, the addition of sewage sludge hindered the co-combustion behavior and increased the energy barrier. Anyway, at a sewage sludge addition rate of 10% or less when co-combusted with coal, the effect of sewage sludge on the combustion characteristics of mixed fuels was minimal.

3.2. SO2 and NOx Emission

SO2 and NOx are the main gaseous pollutants that require treatment in thermal power plants. Because sewage sludge has substantially greater S and N contents than coal, the influence of sewage sludge mixed with coal on SO2 and NOx emissions is noteworthy. Figure 3 shows the SO2 and NOx concentrations under different sludge contents in blended fuels. With an increasing percentage of sewage sludge in mixed fuels, the concentration of SO2 was obviously increased. The concentration of SO2 emissions under the condition of coal alone was 27 mg/Nm3, whereas it increased to 76 mg/Nm3 under the 10 wt.% sludge condition, almost three times larger than that of coal alone. As shown in the ultimate analysis of both fuels, the S content of sewage sludge was nearly two times that of coal; thus, the increased SO2 emissions was due to the rising fuel-S content. Notably, the conversion rate of fuel-S to SO2 exhibited a significant ascending trend as the ratio of sludge rose, from 8% under the coal alone condition to 18% under the 10% sludge condition. In addition, the conversion rate of fuel-S to SO2 was as high as 50% under the sewage sludge alone condition. The primary cause of this phenomenon was that the CaO content of coal was almost twice as high as that of sewage sludge. With an increasing proportion of sewage sludge, the content of CaO in blended fuels was declining. CaO is usually a good capture reagent of SO2 during the combustion process [49]. With the reduction in CaO content in mixed fuels, the SO2 captured was decreased, so the SO2 emissions was increased. The reduction effect of CaO on SO2 emissions was also reported by Zhang et al. [8]. In general, co-combustion of sewage sludge and coal will increase SO2 emissions, which should be taken into full consideration for removal from the flue gas in a real power plant.
For NOx emissions, Figure 3 shows that the concentration of NOx tends to decrease with a proportion of sewage sludge less than 4%, while it shows an uptrend when the proportion of sewage sludge is larger than 4%. This phenomenon was probably due to the following two reasons. First, because of the high content of ash and low calorific value of sewage sludge, the flame temperature in the furnace decreased with the addition of sewage sludge, which hindered the formation of thermal NOx. Similar results have been found in previous studies about the co-combustion of coal and solid recovered fuel [21,50]. Second, as the furnace automatically controlled the temperature at 1300 °C, the temperature drop in the furnace was limited under the condition of a high percentage of sewage sludge, and the composition of the fuel determined the emission of NOx. As shown in ultimate analysis of both the fuels, the N content of the sewage sludge is nearly four times that of the coal. As the proportion of sewage sludge rose, the content of N in blended fuels rose accordingly. Thus, the increased NOx emissions were due to the rising fuel N content. The noticeable effect of mixed fuel N content on NOx emissions was also reported in previous research during co-combustion of coal and other fuels [51,52]. However, the concentrations of NOx ranged from 150 to 300 mg/Nm3 under all the experimental conditions, which should be removed from the flue gas before being released into the atmosphere.

3.3. HCl and HF Emission

As hazardous elements in coal and sewage sludge, Cl and F will be redistributed in combustion products including slag, fly ash, and flue gas after combustion at high temperatures. Chinese standards set strict limits on the emissions of HCl and HF in flue gas in co-combustion of sewage sludge and coal. Figure 4 shows the concentrations of HCl and HF under different sludge contents in blended fuels. The concentration of HCl showed a trend of decreases, then increases, and finally decreases. This phenomenon could be explained in the following two ways. First, combustion temperature was an important factor effecting HCl emissions [53]. The conversion rate of fuel-Cl to HCl increased with increasing temperature. The reason for the decline in HCl emissions was similar to that of NOx, which is that the flame temperature in the furnace reduced with the addition of sewage sludge, which had a negative effect on HCl emissions. Second, the temperature drop in the furnace was limited under the condition of a high percentage of sewage sludge as the furnace was automatically controlled at a high temperature, and the composition of the fuel determined the emission of HCl. As shown in the ultimate analysis of both fuels, the Cl content of sewage sludge was slightly lower than that of coal. Therefore, the HCl emissions of blended fuels were lower than that of coal alone. In summarize, the addition of 10% sewage sludge did not exacerbate HCl emissions during combustion in a real power plant.
Figure 4 illustrates how the concentration of HF emissions tends to go down, then up, and eventually down, following a similar pattern to that of HCl. Additionally, when there was 4–6% sewage sludge present, the HF emissions were at their lowest. The decrease in HF emissions had a similar cause to that of HCl. The volatilization of F increased with the increase in temperature [54], and the addition of sewage sludge led to a reduction in flame temperature in the furnace, which inhibited the release of F. As the percentage of sewage sludge rose, the temperature drop in the furnace was limited as the furnace was automatically controlled at a high temperature, so the fuel-F content determined the emission of HF. As shown in the ultimate analysis, the F content of sewage sludge was slightly lower than that of coal. Therefore, the HF emissions of blended fuels were lower than that of coal alone. Noticeably, the conversion rates of fuel-F to HF were less than 1.5%, which was quite different from an expectation that most of the F would convert to HF at high temperatures due to the fact that F was a very volatile element. This phenomenon might be caused by the following two reasons: First, the occurrence of F in fuels was due to some stable fluoride-bearing minerals, such as fluorophlogopite (KMg3(AlSi3O10)F2) [54], which has a very high decomposition temperature. Second, the Ca compounds exerted a significant effect on fluorine retention and the reduction in fluorine emissions [55]. Al2O3 and SiO2 also played an important role in the fluorine transformations during combustion [56]. As the ash chemical composition shows in Table 2, there is more calcium, silicon, and aluminum than fluorine in the fuels, which could generate fluoride-bearing mineral matters or complex compounds with fluorine, such as CaF2·CaO, CaF2·CaO·Al2O3, and CaF2·CaO·SiO2, and their decomposition temperatures are higher than 1300 °C [54]. As a result, the HF emissions during co-combustion were extremely low and did not cause serious problems.

3.4. Heavy Metal Content in Bottom Ash and Flue Gas

The contents of heavy metals in coal and sewage sludge are shown in Table 3. The concentrations of Cd in coal and sewage sludge were 16.05 mg/kg and 9.27 mg/kg, respectively. The concentration of Cd did not match the limits of the Chinese Standard of Soil Environmental Quality Risk Control for Agricultural Land (GB 15618-2018) [57]. Additionally, the Hg in sewage sludge had a concentration of 11.71 mg/kg, and also exceeded the limit of GB 15618-2018. The content of Cr in sewage sludge was 85.42 mg/kg, which did not meet the Chinese Standard of Soil Environmental Quality Risk Control for Construction Land (GB 36600-2018) [58]. The contents of heavy metals in the sewage sludge and coal are generally different, and co-combustion would affect the heavy metal emission behaviors [59] in the flue gas and the ash [60]. The heavy metals exceeding the standards will migrate into ash and flue gas after incineration, which necessitates special attention and suitable disposal methods [4].
The heavy metal concentrations in the bottom ash of coal and blended fuels are shown in Figure 5a. The concentrations of the heavy metals in the bottom ash of coal could be ranked in the order Cr > Cu > Ni, ranging from 183 mg/kg to 264 mg/kg. The content ranking was consistent with that of the initial sample, indicating that the contents of heavy metals in bottom ash were determined by the constituents of the fuel itself. With the additional sewage sludge ratio increasing in blended fuels, the concentrations of Cr, Cu, and Ni in bottom ash all decreased. That was due to the high content of ash in sewage sludge. With the increase in sludge content, the increased rate of ash content after combustion was larger than that of heavy metals in blend fuels, which resulted in the concentrations of heavy metals showing a decreasing trend.
The enrichment factors of elements in the bottom ash of coal and blended fuels are exhibited in Table 5. The enrichment factor [61] reflects the enrichment degree of heavy metals in bottom ash, which is defined as Equation (4).
R E = c i a c i c × A 100
where RE refers to the enrichment factor, A refers to the content of ash in fuel (%), and cia (mg/kg) and cic (mg/kg) are the concentrations of elements in bottom ash and fuel, respectively.
As shown in Table 5, the RE of Cr and Cu showed a downward trend with the increase in sewage sludge proportion, while the RE of Ni was almost constant. It suggested that the addition of sewage sludge reduced Cr and Cu retention in bottom ash and contributed to the transfer of Cr and Cu to the flue gas. However, the addition of sewage sludge had almost no effect on the distribution of Ni in the flue gas and bottom ash. Additionally, the elements Cd, Pb, and Hg were not detected in bottom ash; hence, the concentrations and RE of these heavy metals in bottom ash were 0, which are not listed in Figure 5a or Table 5.
During the combustion process, the heavy metals migrating into flue gas will pollute the atmosphere and threaten human health [62,63]. Figure 5b reveals the heavy metal emissions in the flue gas of coal and blended fuels. The contents of heavy metals in the flue gas of coal were ranked in the order Pb > Cd > Cr > Cu > Ni > Hg, ranging from 0.05 mg/Nm3 to 1.50 mg/ m3. The content ranking was similar to that of the coal sample, but with some differences. For example, the content of Cd was lower than that of Cr and Cu, while this was not the case in flue gas. The appearance illustrated that the contents of heavy metals in flue gas were determined by the constituents of the fuel and the volatility of the element. With the increasing ratio of sewage sludge, the contents of Pb and Cd in flue gas reduced, while the contents of Cr, Cu, Ni, and Hg increased. The difference is made by the constituents of blended fuels. As the contents of Pb and Cd in sewage sludge were smaller than those in coal, blended fuels with a higher ratio of sewage sludge contained less Pb and Cd, which contributed to lower emissions, while the behaviors of Cr, Cu, Ni, and Hg were the opposite.
The elements Cr, Ni, and Cu were distributed in both the flue gas and bottom ash, and this result was consistent with previous research on sewage sludge incineration [20], in which Cr, Ni, and Cu were classified as non-volatiles. Cd, Hg, and Pb existed only in the flue gas due to their easy volatilization. According to China’s standard for pollution control on hazardous waste incineration (GB 18484-2020) [35], the Cd, Cr, Cu, and Ni emissions could meet the national standard under all conditions, while the emissions of Pb and Hg exceeded the limits of 0.5 mg/Nm3 and 0.05 mg/Nm3. According to Directive 2010/75/EU [64] of the European Parliament and of the Council on industrial emissions (integrated pollution prevention and control), the Cd, Hg, and sum emissions of Cr, Cu, Pb, and Ni all exceeded the limits of 0.05 mg/Nm3, 0.05 mg/Nm3, and 0.5 mg/Nm3, and so should be properly handled before the flue gas is released into the atmosphere.

3.5. Simulation Calculation of Heavy Metals Migration

Some heavy metal contents in bottom ash and flue gas exceed the limits of Chinese standards and should be disposed of appropriately. Understanding the existence form of heavy metals will help to remove them accurately. For this purpose, simulation calculations of heavy metal migration were carried out by FactSage 7.3, and the existence form of heavy metals is shown in Figure 6. In the product after complete combustion at 1300 °C, the heavy metals of Cr, Cu, Ni, Pb, Cd, and Hg existed completely in gaseous form. CrO2(OH)2, CrO3, and CrO2OH were the main forms of elemental Cr, and a small amount of CrO(OH)3, CrO(OH)2, CrOOH, and CrO2 was also generated. As the content of sewage sludge increased, the mole fraction of CrO2(OH)2 increased, the mole fraction of CrO3 decreased, and the mole fraction of CrO2OH was almost constant. Ni(OH)2 was the main form of elemental Ni presented with a mole fraction of 99.17–99.30%. Small amounts of NiCl2, NiCl, NiO, and Ni vapor were also generated. CuCl and copper vapor were the main forms of elemental Cu; in addition, a small amount of CuO was also generated. As the content of sewage sludge increased, the CuCl mole fraction decreased and then increased, while the copper vapor mole fraction increased and then decreased.
PbO was the main form of elemental Pb presented with a mole fraction of 99.33–99.39%; in addition, small amounts of PbCl2, PbCl and Pb vapor were generated. Cd existed mainly in the form of Cd vapor with a mole fraction of 97.10–97.13%; in addition, a small amount of CdO was generated with a mole fraction of 2.73–2.76%. Hg existed mainly in the form of Hg vapor with a mole fraction of 99.08~99.10%; in addition, a small amount of HgO was generated with a mole fraction of 0.90~0.92%.
The distribution of heavy metals such as Cr, Cu, and Ni in the simulation results was not exactly the same as in the experimental results. This difference is probably because the simulation results of FactSage were theoretical values under the condition of complete combustion at 1300 °C, whereas in the actual combustion process, the blended fuel probably did not complete combustion because its residence time in the drop tube furnace was short. This led to a certain difference between the simulation values and the experimental values. For volatile heavy metals such as Pb, Cd, and Hg, the simulation results of FactSage were in good agreement with the experimental results.

4. Conclusions

Co-combustion with coal is a feasible method for disposing of sewage sludge, which can achieve the goals of energy recovery and environmental benefits. In this work, the co-combustion characteristics of sewage sludge and coal in different contents have been quantitatively investigated using thermogravimetric analysis and element analysis. The results showed that the influence of sewage sludge on blended fuels was limited with an additional ratio under 10 wt.%, and the combustion characteristics of mixed fuels were similar to those of coal. Adding sewage sludge to coal with a low content favored the ignition behavior, promoting blended fuel combustion at a lower temperature. The co-combustion processes of sewage sludge and coal would increase SO2 emissions, while the NOx emissions did not change significantly. Emissions of HCl and HF were extremely low. The bottom ash of blended fuels contained heavy metals such as Cr, Cu, and Ni. With the increasing percentage of sewage sludge, the concentrations of Cr, Cu, and Ni in bottom ash were decreased, but heavy metals such as Pb, Cd, Cr, Cu, Ni, and Hg were discovered in flue gas. The simulation results of FactSage 7.3 showed that after complete combustion at 1300 °C, Cr existed in the form of CrO2(OH)2, CrO3, and CrO2OH, while Cu, Ni, and Pb existed as CuCl and copper vapor, Ni(OH)2, and PbO, respectively. Cd and Hg turned into simple substances. It should be noted that the emissions of Pb and Hg in flue gas exceeded the limits of the Chinese standard and should be properly handled before being released into the atmosphere. Furthermore, the catalyst in the flue gas treatment system will be deactivated with the increasing content of heavy metals. The existence forms for heavy metals in the combustion products serve as a reference for removing them from flue gas. Our work not only provides insights into the fundamental investigation of co-combustion processes and mechanisms of sewage sludge and coal but also facilitates the development of large-scale, harmless, and energy-harvesting treatment techniques for both city and industry wastes.

Author Contributions

All authors contributed to the study conception and design. Formal analysis, Investigation, Visualization, Writing—Original draft were performed by C.L. Funding acquisition, Project administration, Supervision, Writing—Reviewing and Editing were performed by C.Y. Conceptualization, Methodology, Resources, Validation were performed by Y.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science Foundation of China University of Petroleum, Beijing (Grant number 2462022YXZZ001), and the National key research and development program (2019YFC1906300).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors have no relevant financial or non-financial interest to disclose.

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Figure 1. Schematic diagram of drop tube furnace.
Figure 1. Schematic diagram of drop tube furnace.
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Figure 2. (a) TG curves and (b) DTG curves of sewage sludge co-combustion with coal.
Figure 2. (a) TG curves and (b) DTG curves of sewage sludge co-combustion with coal.
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Figure 3. SO2 and NOx emissions under different sludge contents in blended fuels.
Figure 3. SO2 and NOx emissions under different sludge contents in blended fuels.
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Figure 4. HF and HCl emissions under different sludge contents in blended fuels.
Figure 4. HF and HCl emissions under different sludge contents in blended fuels.
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Figure 5. Heavy metal concentrations in (a) bottom ash and (b) flue gas.
Figure 5. Heavy metal concentrations in (a) bottom ash and (b) flue gas.
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Figure 6. Existence form of (a) Cr, (b) Ni, (c) Cu, (d) Pb, (e) Cd, (f) Hg in combustion products.
Figure 6. Existence form of (a) Cr, (b) Ni, (c) Cu, (d) Pb, (e) Cd, (f) Hg in combustion products.
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Table 1. Proximate and ultimate analysis of coal and sewage sludge (dry basis).
Table 1. Proximate and ultimate analysis of coal and sewage sludge (dry basis).
SampleProximate Analysis (wt.%)Ultimate Analysis (wt.%)Content (mg/kg)HHV (MJ/kg)
AVFCCHNOSFCl
Coal3.6332.7563.6174.114.130.8716.620.64212.37899.5035.11
Sludge41.6851.087.2436.235.213.8811.621.38177.78632.0916.97
A: ash; V: volatile; FC: fixed carbon; C: carbon; H: hydrogen; O: oxygen; N: nitrogen; S: sulfur; F: fluorine; Cl: chlorine; HHVs: higher heating values.
Table 2. Ash chemical composition of coal and sewage sludge (dry basis).
Table 2. Ash chemical composition of coal and sewage sludge (dry basis).
SampleChemical Composition (wt.%)
SiO2Al2O3Fe2O3TiO2CaOMgOK2ONa2OMnO2SO3P2O5
Coal14.4610.963.760.1254.420.340.172.131.298.480.02
Sludge21.616.2037.100.798.653.541.820.970.207.579.80
Table 3. Heavy metal content of coal and sewage sludge (dry basis).
Table 3. Heavy metal content of coal and sewage sludge (dry basis).
SampleHeavy Metal Content (mg/kg)
CrCuHgNiCdPb
Coal 20.0618.052.0113.0416.0559.18
Sludge 85.42107.3811.7120.019.2730.75
Table 4. Combustion parameters for coal, sewage sludge and their blends.
Table 4. Combustion parameters for coal, sewage sludge and their blends.
SampleTi
(°C)		Tb
(°C)		S × 108
(%2/°C3 min2)
Coal3855306.75
2% sludge3845306.62
4% sludge3845306.39
6% sludge3845306.22
8% sludge3835306.03
10% sludge3825295.96
Sludge2356411.19
Ti: ignition temperature, Tb: burnout temperature, S: combustibility index.
Table 5. Enrichment factors of elements in bottom ash.
Table 5. Enrichment factors of elements in bottom ash.
ElementCoal2% Sludge4% Sludge6% Sludge8% Sludge10% Sludge
Cr0.480.440.430.420.410.30
Cu0.520.450.420.320.260.21
Ni0.510.520.520.510.510.52
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Liu, C.; Yue, C.; Ma, Y. Pollutant Emissions and Heavy Metal Migration in Co-Combustion of Sewage Sludge and Coal. Energies 2024, 17, 2457. https://doi.org/10.3390/en17112457

AMA Style

Liu C, Yue C, Ma Y. Pollutant Emissions and Heavy Metal Migration in Co-Combustion of Sewage Sludge and Coal. Energies. 2024; 17(11):2457. https://doi.org/10.3390/en17112457

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Liu, Chunyu, Changtao Yue, and Yue Ma. 2024. "Pollutant Emissions and Heavy Metal Migration in Co-Combustion of Sewage Sludge and Coal" Energies 17, no. 11: 2457. https://doi.org/10.3390/en17112457

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