The Fundamental Combustion Process

The co-combustion of sludge and coal is comparable to that of solid fuels. First, there is an initial drying phase. When the fuel reaches a specific temperature, it distorts plastically, releasing volatile substances and forming porous coke. As the temperature rises, volatile matter begins to burn, generating heat that promotes coke ignition and combustion, as well as mineral change and trace element migration. These activities are intricately linked to the stable and efficient burning of fuels in power plant boilers [13].

The organic matter in sludge is mostly composed of lipids, proteins, and sugars. As the temperature rises, these materials undergo successive pyrolysis. The combustion process of sludge is separated into three stages: initial, major, and final. The initial decomposition stage relates to the sludge drying process and often lasts less than 200ºC. The major decomposition stage occurs between 180 and 580ºC, corresponding to a significant release of volatile components from the sludge and oxidation of the coke, which usually occurs in parallel without a clear boundary. The final decomposition step involves breaking down a minor amount of minerals (such as carbonates and sulfates) in sludge [24-28]. The organic structure of coal is primarily composed of aromatic clusters, side chains, fatty bridges, and rings. Compared to sludge, coal combustion is comparatively simple, with a single combustion stage (350-650ºC) that simultaneously releases volatile materials and coke burning. Due to differences in organic and inorganic components, sludge and coal combustion processes differ significantly. In the co-combustion process, sludge combustion is inhibited, and coal combustion is promoted [29]. Sludge combustion is mainly concentrated in the low-temperature zone; the combustion of volatile components is the dominant process of sludge combustion, and the maximum combustion rate usually occurs between 300-400ºC. Due to its low volatile content, coal is less reactive than sludge at lower temperatures, and the combustion of fixed carbon in the high-temperature region dominates the overall combustion reaction, usually reaching a maximum combustion rate at around 500ºC.

The Impact of Fuel Type on Combustion Characteristics

Sludge varies greatly due to regional, seasonal, and treatment process changes, which might impact its combustion properties. Aneta et al. studied the differences in combustion properties of sludge from several wastewater treatment plants, and the sludge utilized in the study was anaerobically digested, dehydrated, and thermally dried. They discovered that, while the overall combustion processes of different sludges were similar, variations in chemical composition caused by different sources resulted in variances in moisture content, initial decomposition temperature, maximum combustion rate, and temperature at which it occurs [30]. Anaerobic treatment can yield more complex organic compounds with greater molecular weights and combustion temperatures in sludge than aerobic treatment. This leads to discrete combustion stages in both low and high-temperature zones. Akdag [31] discovered that adding 10% sludge to coal decreased the combustion efficiency (the ratio of the amount of carbon converted to CO₂ to the total amount of carbon supplied to the fuel mixture) of the blend when compared to coal alone. Under the assumption of using an anaerobic digestion stabilization process, sludge generated by traditional A₂O (Anaerobic-Anoxic-Oxic) wastewater treatment processes had a greater negative impact on combustion efficiency than sludge generated by the University of Cape Town’s (UCT) wastewater treatment process. Furthermore, the degree of coalification substantially influences the co-combustion properties of sludge and coal. Some characteristics of sludge, lignite, and bituminous coal are similar. When co-combustion takes place, the heat released by the sludge after ignition in the low-temperature zone is conducive to the release and combustion of the volatile components in the lignite and bituminous coal and also plays a preheating role for the combustion of fixed carbon. Sludge does not significantly improve the combustion of anthracite coal because of the substantial variations in characteristics between sludge and anthracite coal, which cause their combustion processes to be independent in temperature ranges during co-combustion [32]. According to Zhang et al.’s research [33], sludge performed better when combined with high-rank bituminous coal for co-combustion than with low-rank bituminous coal.

The Impact of the Mixing Ratio on Combustion Properties

One key variable affecting how mixes burn is the ratio of mixing ingredients. Selecting the right amount of fuel to blend with can maximize combustion, improve the synergistic impact of fuels, and reduce issues brought on by a single fuel’s shortcomings. Li Yangyang et al. [34] discovered that when sludge was co-fired with coal, the weight loss during the volatile stage of the blending increased, the weight loss rate accelerated, and the temperature at which the maximum weight loss peak of the coke combustion stage appeared decreased with an increase in the blending ratio. Park [35] investigated the effect of adding up to 20% dry sludge on coal combustion and found that the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the co-mixture were similar to those of the coal; the apparent activation energy of the combustion process of the blends obtained using FWO (Flynn-Wall-Ozawa) is also close to that of the single coal combustion. Additionally, a great deal of research has used combustion characteristic factors to quantify how blending ratios affect certain aspects of combustion. With the sludge ratio as a point of reference, Fig. 4 displays the fluctuation of several combustion characteristic parameters [33]. According to the research, the blend’s ignition and burnout temperatures typically steadily drop as the sludge blending ratio increases, but the stability and combustibility indices rise.

This is because when the amount of sludge in the blend increases, so does the amount of volatile matter. Since a lot of volatile matter burns at lower temperatures, the ignition temperature drops. At the same time, the heat released from the combustion of volatile components can provide activation energy for the combustion of fixed carbon, resulting in a lower combustion temperature. While Shao Zhiwei et al. [36] discovered that an increase in sludge ratio would result in a fall in the comprehensive combustion characteristic index, Zhang et al. [33] observed that an increase in sludge ratio progressively raises the fuel’s comprehensive combustion characteristic index as well. The fuel’s mass distribution of volatile and fixed carbon components, as well as the amount of combustible matter it contains, determines the comprehensive combustion characteristics. Sludge can reduce coal’s ignition and burnout temperatures, but it still impairs the coal’s comprehensive combustion feature when combined with coal that has a relatively low ash level and a relatively high combustible matter concentration. When high-ash coal is co-fired, the circumstances are reversed.

Sludge can significantly enhance the blend’s burnout and ignite performance when co-fired with coal [37]. However, the majority of academics feel that blending less than 10% dried sludge is possible without altering the combustion equipment and operation technology of current power plants, as it has a detrimental effect on combustion efficiency, flue gas temperature, and pollutant emissions [38].

Slagging and Ash Deposition

Slagging and ash deposition are key challenges that arise during the co-combustion of sludge and coal. Ash deposition can reduce boiler thermal efficiency and, in severe cases, may even lead to boiler tube explosions [39]. When co-combusted with Zhundong lignite in Xinjiang, interactions between Na-based components in lignite and P-rich and Si/Al-rich minerals in sludge result in the development of high-melting phosphates and aluminosilicates, which increase the retention of Na in the residual ash and reduce the risk of fouling and reduce the risk of slagging when the combustion temperature is below 900ºC [40]. Zhao et al. [41] measured the ash fusion points following co-combustion of sludge and coal blends in varying proportions, observing a trend of lowering and then increasing ash fusion points as the sludge fraction grew from 10% to 50%. Adding a significant amount of CaO from high sludge proportions counteracts the drop in ash fusion points produced by Fe₂O₃ in low sludge proportion blends by interacting with SiO₂ in coal ash to form high-melting Ca₂SiO₄.

In conclusion, slagging behavior during co-combustion of sludge and coal is mostly determined by combustion temperature and mineral content. Mixing sludge with coal typically exacerbates slagging/ ash deposition issues during combustion, whereas interactions between sludge and coal minerals (such as the interaction between P-rich, Si/Al-rich minerals and Na-based components, as well as the interaction between CaO and SiO₂) promote the formation of high-melt compounds. As a result, mineral interactions play an important role in slagging and ash deposition difficulties, although research on the interactions between sludge and coal mineral compositions is still limited.

Fig. 4.

Impact of sludge ratio on factors related to combustion characteristics.

Fuel Interactions

The co-combustion process is rarely a linear superposition of single fuels, and interactions between distinct characteristic fuels can occur, altering combustion performance. Investigating these interactions allows for a better knowledge of fuel combustion processes and establishes the theoretical groundwork for increasing synergistic effects during fuel co-combustion.

Changes in fuel characteristics fundamentally cause interactions. Sludge and coal are two very distinct fuels, with major variances in organic and inorganic components, particle characteristics, etc. Mixing them changes the combustion environment, mass, and heat transfer conditions compared to individual fuel combustion, resulting in interactions. Yang et al. [42] discovered synergistic benefits during the co-combustion of sludge and coal gangue in a tubular furnace, including improved ignition performance, desulfurization, denitrification, and trace element fixation. Thermogravimetric analyses showed significant interactions between the main combustion stages when sludge was co-combusted with bituminous coal. This leads to combustion lags that may originate from complex reaction mechanisms affecting combustion efficiency [43]. Acid washing removed carbonates and silicates from the sludge, shifting the blend’s DTG curve towards the low-temperature zone, implying that ash in sludge inhibits the blend’s combustion [44]. Components such as Ca, K, and Mg in sludge ash promote volatile matter breakdown and fixed carbon combustion [45- 49]. Wang et al. [50] investigated the effects of various pretreatment methods on the interaction of sludge and coal during co-combustion and proposed interaction mechanisms between sludge and coal at various combustion stages, implying that different factors influence the interactions at different stages.

As illustrated in Fig. 5, it is widely assumed that fuel interactions are mostly due to non-catalytic and catalytic mechanisms: (1) Interaction of volatile substances. For instance, during thermal breakdown, sludge with a high H/C ratio produces a large amount of H and OH radicals, which, while promoting the breakdown of coal molecules, also prevents radical recombination and cross-linking. (2) Variations in physicochemical qualities lead to modifications in mass and heat transfer conditions. Because sludge includes a high concentration of low-boiling-point organic compounds, there is a greater pressure between the particles in the low-temperature zone, which prevents the volatilization of volatile stuff in coal. Concurrently, a rise in pressure during the first phase could lessen the oxygen’s chemical adsorption on the coal surface, hastening the blend’s weight loss. Furthermore, after combining big particle-size sludge and coal, the blend experiences notable temperature gradients due to the sludge’s high hardness and difficulty grinding, which delays the volatilization of volatile materials. (3) Interactions of heat. While the heat from burning coal’s fixed carbon can help the remaining materials in sludge decompose, the heat released by burning the volatile matter in sludge in the low-temperature zone can also preheat the coal’s fixed carbon. Volatile materials are generally released in the blend when non-catalytic mechanisms mostly show their influence. The term “catalytic mechanisms” primarily describes how inorganic sludge constituents catalyze coal combustion processes. On the one hand, elements such as Ca, Mg, and K in sludge can catalyze the decomposition of volatile matter at lower temperatures, improving the ignition performance of the blend; on the other hand, they stimulate the breakdown of volatile materials at lower temperatures, hence enhancing the blend’s ignition capability. However, alkali (earth) metals can efficiently catalyze the burning of fixed carbon. During the catalytic process, metal elements act as oxygen transporters, speeding up the oxygen’s passage to the coke’s surface and enabling the blend to burn more quickly.

Fig. 5.

Mechanism of interaction between sludge and coal slurry during co-combustion.

In actuality, the fuel interaction mechanism during co-combustion has a great deal of complexity. The interaction’s final effect results from several interaction mechanisms working together. While all fuels have the ability to create interactions, the degree and way in which these interactions manifest – as inhibition or synergy – depend on a number of variables, including the fuel’s physicochemical qualities, mixing ratio, and combustion conditions. To increase the synergy in the combustion of sludge and coal, more studies on the interaction processes during the co-combustion of sludge and coal are required to elucidate the contributions of each interaction mechanism.

NO_x Emissions

Sludge has a nitrogen concentration that is much higher than that of coal. The nitrogen content of sludge can reach 2.5 to 9.0%, much higher than the 0.5 to 2.5% in coal [51]. Additionally, sludge’s inorganic and organic components differ from coal’s, meaning the two fuel sources have different generation and emission properties. Therefore, the main area of concern during the co-combustion of coal and sludge is NO_x’s emission and control technology.

Blending sludge with coal rich in volatile matter and nitrogen concentrations results in reduced NO emissions. Adding sludge to the combustion of lignite, bituminous coal, and lean coal increases NO emissions during the volatile combustion phase but decreases them during the coke combustion phase [52]. This phenomenon was attributed to reduced coke content and the catalytic effect of minerals in the sludge on NO reduction. A synergistic effect is produced when sludge and gangue are co-combusted at 800ºC. The denitrification reaction consumes the CaO in the blend, thus reducing the oxidation of N-containing substances by CaO, ultimately leading to a 10% reduction in NO emissions relative to the theoretical value [42]. Conversely, mixing less than 5% sludge lowers NO emissions and flame temperature, while blending more than 5% sludge increases the mixture’s nitrogen concentration and NO emissions [53]. In addition, fuel properties, combustion temperatures, blending ratios, boiler type, and pretreatment techniques influence NO emissions [54]. For example, compared to bubbling fluidized beds, the deposition of sludge ash in circulating fluidized beds has less of an impact on NO_x levels [55]. Additionally, CaO during sludge dewatering pretreatment catalyzes the reduction of N₂O, which is more uniformly distributed in the sludge and more effective in reducing NO_x generation [56].

Coal NO_x abatement technology states that two types of NO_x abatement technology can be used with sludge combustion: flue gas treatment technology, which includes selective catalytic reduction and selective non-catalytic reduction, may not be as effective in treating NO_x in the flue gas due to the higher NO_x generated by sludge combustion than by coal. The other NO_x abatement technologies, which include flame cooling, air-graded combustion, low-oxygen dilution combustion, and flue gas recirculation, mostly focus on reducing NO_x generation within the furnace chamber [12]. Air-graded combustion and low-oxygen dilution combustion provide notable benefits over flue gas treatment systems in terms of technology and cost [57]. In air-graded combustion technology, air is fed into the burner in two stages: during the early stage of combustion, the fuel in the fuel-rich zone is combusted in an oxygen-deficient state, and the HCN and NH₃ produced during sludge combustion reduce the generated NO to N₂ in a reducing atmosphere; in the burnout zone, the fuel is combusted in an oxygen-rich state, and NO_x generation is limited due to the low flame temperature [58]. It has been shown that regulating the main air excess component may reduce 50-80% of the NO_x emissions [59]. This lowers the chamber’s peak temperature and effectively inhibits the production of thermal nitrogen oxides. Furthermore, optimizing the combustor geometry and operating parameters reduces NO_x emissions from sludge combustion to less than 75 ppm when low-oxygen combustion technology is applied in a horizontal cyclone combustor [60]. These findings suggest that low-oxygen combustion technology is a potentially useful technique for lowering NO_x emissions from sludge combustion processes. The key to reducing NO_x emissions during sludge-coal co-combustion is to determine the burner geometry and operating parameters, such as setting parameters such as the primary air ratio and excess air coefficient in air-graded combustion technology, as well as designing the burner and optimizing the operating conditions in low-oxygen dilution combustion technology. To improve the combustion process, the influence of each parameter on NO_x emissions must be evaluated under specific fuel circumstances. This allows for tuning the combustion process, increasing its efficiency.

SO₂ Emissions

Together, the sulfur content and mineral sulfur-fixing capacity are the two most important parameters that determine SO₂ emissions from sludge and coal combustion. As the sludge proportion increases in the co-combustion process, the SO₂ emission tends to rise linearly [61]. The ratio of primary and secondary air has a significant effect on SO₂ emissions [62]. As the combustion temperature increases, SO₂ and HCl emissions increase [63].

The presence of extracellular polymeric molecules in sludge can inhibit the release of bound water, making dehydration more difficult. Inorganic conditioning agents such as CaO, FeCl₃, and coal ash are widely utilized to improve sludge dewatering efficiency while lowering storage, transportation, and subsequent processing costs. These conditioning chemicals also help to reduce SO₂ emissions from coal burning. Specifically, as earlier research has shown, CaO in sludge can react with SO₂ to generate CaSO₄, lowering SO₂ emissions. However, FeCl₃ can increase the volatilization of Na and K during co-combustion, lowering the sulfur-fixing rate of alkali metals. Furthermore, synthesizing calcium ferrite from Fe₂O₃ and CaO might impede SO₂ reduction efforts. As a result, Fe has a bidirectional effect on SO₂ emissions. Zhang et al. [56] discovered a CaO conditioner could collect practically all SO₂ when sludge is combusted alone, lowering SO₂ emissions by 15.8-48.4% when co-combusted with coal. The optimum desulfurization temperature for sludge combustion was 800ºC in an oxygen atmosphere [64]. Varying the doping ratio of sludge to coal results in differing levels of reduction in polluting gas emissions [65].

In conclusion, the S content of sludge plays an important role in determining SO₂ emissions when sludge and coal are co-combusted. When the S content in sludge is significantly higher than that in coal, blending sludge will increase SO₂ emissions during coal combustion; when the S content in sludge and coal is comparable, the sulfur fixation of minerals in the sludge can effectively reduce SO₂ emissions. Furthermore, the conditioning agent added to the sludge dewatering pretreatment process can efficiently collect SO₂ during the combustion process, reducing SO₂ emissions.

Heavy Metal Emissions

The level of heavy metal content in sludge is considerable, and the issue of heavy metal emission during co-combustion warrants attention. Heavy metals in sludge are generally classified into three groups based on their volatility: volatile elements (Hg and Se), semi-volatile elements (As, Sb, Pb, Cd, and Zn), and non-volatile elements (Cr, Ni, Cu, and Mn). The fuel properties and combustion parameters largely influence the emission of heavy metals during co-combustion. The two components, CI and S, have an impact on the volatility of heavy metals. Research has demonstrated that Cl in sludge reacts with heavy metals at high temperatures to produce low-boiling chlorides, accelerating the movement of heavy metals to flue gas. Among chlorides, CaCl₂ can significantly decrease the volatilization temperature of heavy metals such as As and Cu, while FeCl₃ can promote the volatilization of Cu, Zn, and Pb [66, 67]. The influence of NaCl on the volatilization of heavy metals is in the order of Pb>Zn>Cr>Cu>Mn>Ni. The removal of most heavy metals was efficient in the presence of CaCl₂ and MgCl₂ among the different types of combustors [68]. S mainly controls the emission of heavy metals in two ways: first, sulfur-containing minerals have a certain fixed effect on heavy metals; second, some heavy metals can combine with SO₂ produced during the combustion process to form sulfates [69]. Additionally, moisture and ash can affect the volatilization of heavy metals. The release rates of As, Se, and Pb increase with the temperature rise, and the final release rate is Se>As>Pb [70]. It is proposed that during the co-combustion process of sludge and slime, the interaction between minerals poses a barrier to the migration of heavy metals into the gas phase. A combination of physical and chemical effects shapes this phenomenon. In addition to fuel characteristics, combustion temperature is a crucial factor affecting the volatility of heavy metals. For instance, as the temperature rises from 850ºC to 1000℃, the volatilization rate of Pb and Zn doubles, and the volatilization rate of Cr also increases from 65.7% to 74.9%. However, the volatilization rate of Mn, Cu, and Ni remains insensitive to temperatures in the same temperature range [71].

Although some metals are highly volatile during combustion, most are retained in the fly ash and captured by the dust collector in practical power plant applications. Thus, it is essential to pay special attention to the problem of heavy metal pollution in the ash reuse process following sludgy-coal co-combustion.

Dioxin Emissions

Sludge combustion generates a significant amount of unconventional pollutants, including dioxins. Dioxins are two types of tricyclic aromatic organic compounds, namely PCDDs (Polychlorinated dibenzo-p-dioxins) and PCDFs (Polychlorinated dibenzofurans). These compounds are highly toxic and capable of causing teratogenic, carcinogenic, and mutagenic effects.

There are four primary sources of dioxins in the sludge combustion process: (1) inherent dioxins present in the sludge, (2) de novo synthesis of residual carbon and chlorine macromolecules in fly ash at low temperatures [72], (3) chlorinated precursors such as chlorobenzene and chlorophenol in the gas phase that are pyrolyzed and rearranged at 500-800ºC, and (4) incomplete combustion or heterogeneous catalytic reactions that synthesize dioxin-like precursors. The formation of dioxins can be inhibited based on their formation mechanism, mainly by sulfur compounds, CaO, and nitrogen compounds. Sulfur-containing compounds produce SO₂, which deactivates the catalyst and inhibits the formation of dioxin-like precursors in the combustion process.

The study of the impact of S and CaO on dioxin formation during co-combustion of coal and organic solid waste has revealed that both substances can limit dioxin production [73]. Additionally, sludge combustion has been found to reduce dioxin emissions, and nitrogen-containing compounds have been shown to have an inhibitory effect on dioxin formation [74]. The mechanism behind this is primarily due to nitrogen compounds inhibiting the formation of dioxins by poisoning metal catalysts, reacting with chlorine to decrease chlorine sources, and releasing free radicals to replace CI or H [75].

In conclusion, optimal combustion conditions such as more complete combustion, a reducing atmosphere, lower chlorine content in the fuel, and less catalytic metal can help reduce dioxin formation. Furthermore, the interaction between S/N/CI and the catalytic metal may also affect the pathway of dioxin formation. Despite receiving extensive attention, dioxin’s formation mechanism and emission characteristics are still unclear, and more in-depth research is needed. A suitable reduction technology should be sought to reduce dioxin emissions during sludge and coal co-combustion in coal-fired power plants. A comparison of some of the literature data is shown in Table 1.

For the pollutants generated by the co-combustion of sludge and coal, emission reduction can be achieved through the following comprehensive measures: In terms of NOₓ control, low-temperature combustion is adopted, and iron sludge (Fe₂O₃) is added. This way, fuel-nitrogen can be catalyzed to convert into N₂, reducing NO. For SO₂ emission reduction, the calcium-based component (CaO) in sludge is used for co-combustion with coal gangue to generate CaSO₄ for sulfur fixation. At the same time, the ratio and temperature are optimized to achieve the purpose of emission reduction. In terms of heavy metal pollution prevention and control, a multi-stage dust removal system is used to capture fly ash, landfills are carried out according to the heavy metal concentration classification, and CaO or high-sulfur coal is added to form sulfides to reduce the migration of heavy metals. The incineration temperature is controlled above 600 ºC for dioxin pollutants to decompose dioxins. High-temperature cyclone dust removal is combined to block its resynthesis path, and then activated carbon or sulfur/iodine adsorbents are used to remove residues.