Pollutant Emission and Ash Accumulation Characteristics of Tri-Combustion of Coal, Biomass, and Oil Sludge

Abstract

To study the ash accumulation and pollutant emission characteristics of tri-combustion of coal, biomass, and oil sludge, a fluidized bed and settling furnace system is established for tri-combustion experiments. The effect of blending ratio (the ratio of biomass and oil sludge range from 30% to 50% and 10% to 20%, respectively) and biomass types are examined. The results show that HTB, coal, and oil sludge reach peak NO and NO₂ production at approximately 100 s and 200 s of combustion, respectively, with NO_x levels returning to zero around 300 s. SO₂ peaks around 100 s and then gradually declines. The blending ratio of HTB:oil sludge:coal at 50%:10%:40% demonstrates the most effective control over NO_x and SO₂ emissions, reducing NO, NO₂, and SO₂ production by approximately 33%, 20%, and 50%, respectively. In the ash with a ratio of Hutubi (HTB) + 50% oil sludge, the mass fractions of O, Si, Ca, Al, and Fe are approximately 27%, 23%, 20%, 8%, and 12%, respectively. With the increase in the blending ratio of biomass and oil sludge, the mass fraction of Si in the ash rises, while those of Ca, Al, and Fe decrease.

1. Introduction

The energy supply has an essential effect on the social and economic advancement of various nations [1]. Energy demand is predicted to rise by 50% by 2050 as a result of anticipated urbanization and population expansion [2]. The substantial consumption of energy is accompanied by a surge in CO₂ and pollutant emissions [3]. Globally, the efficient and scientific utilization of energy resources is crucial.

Coal is one of the main traditional energy sources in China and remains an important part of the energy production of China in the coming period [4]. However, the extensive burning of coal emits large amounts of greenhouse gasses and causes environmental pollution. Under the requirements of the “carbon peak and carbon neutrality” goals [5], it is urgent to seek clean and renewable energy to gradually substitute coal. Among many renewable energy sources, biomass has attracted widespread attention due to its abundant reserves and carbon neutrality [6,7]. Currently, the utilization of biomass is primarily based on combustion [8]. However, the direct combustion of biomass poses challenges like ash deposition, slagging, and corrosion [9]. Fortunately, the co-firing of biomass with coal offers an effective approach to address these issues [10]. Moreover, previous studies [11] have demonstrated that the addition of biomass can enhance the combustion characteristics of coal and improve combustion efficiency. Consequently, co-combustion of coal and biomass has garnered significant attention [12,13].

Oil sludge is a common solid hazardous waste in the process of oilfield development and production [14]. It has the characteristics of complex composition, strong volatility, difficult decomposition, and high content of lipid substances [14,15]. Oil sludge can be processed through a variety of techniques, including mechanical separation, solvent extraction, microwave treatment, biological processes, and incineration [16,17]. Incineration is regarded as the most effective disposal measure due to its ability to reduce volume, harness heat energy, and eliminate harmful substances [18]. However, this approach still encounters challenges related to air pollution and inadequate economic benefits [19]. For the incineration of oil sludge, circulating fluidized bed boilers are recommended due to their wide fuel adaptability, excellent mixing characteristics, high combustion efficiency, and low pollutant emissions [20]. The incorporation of sludge in circulating fluidized bed (CFB) boilers could not only realize its resource utilization but the existing flue gas treatment equipment could also control the secondary pollution generated during the sludge incineration process [21]. The biomass resources around the oil field are usually abundant [22]. The clean and efficient treatment of oil sludge and the optimal utilization of biomass resources are crucial for addressing the challenges of fossil fuel scarcity, difficulties in oil sludge disposal, low biomass utilization rates, and severe air pollution. Tri-combusting of coal, biomass, and oil sludge in a CFB boiler facilitates the secure and efficient processing of oil sludge while optimizing the utilization of biomass resources [23]. However, the blending of biomass and oil sludge would affect the pollutant emissions and ash deposition characteristics, which are related to the emission indicators and safe operation of the boiler. Therefore, research on ash accumulation and pollutant emissions from the tri-combustion of coal, biomass, and oil sludge is highly significant for achieving the goals of “peaking carbon dioxide emissions by 2030 and attaining carbon neutrality by 2060 [24] “ as well as for implementing the “conservative, clean, and safe” energy strategy.

Extensive research [10,25,26,27,28,29,30] has been conducted on the characteristics of pollutant emissions and ash deposition in co-combustion processes. These studies mainly focus on pollutant emissions [25,26,27,28,29] and ash deposition [10,27,30] characteristics of the co-combustion of two fuels. The results show that biomass blending is beneficial in reducing pollutant emissions, but it exacerbates the tendency of ash accumulation and slagging. Thermogravimetric analysis has been employed by several researchers [31,32] to examine the co-combustion characteristic and interactions of various coal and biomass between 20 °C and 900 °C. The results show that adding biomass could enhance the precipitation of volatile matter in coal, which could catalyze the combustion of coal. Some studies [19,33,34] have focused on the co-combustion characteristics of sludge and other fuels. Their findings indicate that the integrated treatment strategy could be more effective for the processing and utilization of oil sludge. Moreover, due to the high calorific value of the blended coal and oil sludge, utilizing this blended fuel for power generation has emerged as a significant development in oil sludge treatment [35]. In addition, some researchers have also conducted research on the blending of the three fuels by thermogravimetric analysis [36,37]. Notably, the pollutant emissions and ash deposition characteristics of the co-combustion of coal and biomass have been extensively studied. However, the study of ternary combustion involving coal, biomass, and oil sludge is a relatively new concept, with limited research on its pollutant emissions and ash deposition characteristics. The effect of blending a third fuel on the pollutant emissions and ash deposition of the blend remains unclear and requires further detailed investigation. Furthermore, the tri-combustion of coal, biomass, and oil sludge has broad application prospects in oil fields. Therefore, it is of great significance to study the pollutant emission and ash deposition characteristics of the co-combustion of coal, biomass, and oil sludge.

In this study, the ash deposition and pollutant emission characteristics of the tri-combustion of coal, biomass, and oil sludge are investigated. A one-dimensional tube furnace and fluidized bed combustion experimental system are set up to conduct ash accumulation and pollutant emission experiments. Scanning electron microscopy–energy dispersive spectrometer (SEM-EDS) and flue gas analyzer are employed to analyze the product obtained after the experiment, the effect of blending ratio and biomass types are discussed in detail. The results of this study could provide a reference for practical applications.

2. Materials and Methods

2.1. Raw Material

In this study, coal, three types of biomasses (collected from Korla (KEL), Hutubi (HTB), and Bayingolin (XM)), and oil sludge are utilized for this investigation. For the convenience of description, the biomass collected from Korla, Hutubi, and Bayingolin are named KEL, HTB, and XM, respectively. Ultimate and proximate analyses of these fuels are shown in

Table 1. Ultimate and proximate analyses of coal, biomass, and oil sludge.

Sample	Ultimate Analysis (ar)					Proximate Analysis (ar)
	C/%	H/%	O */%	N/%	S/%	M/%	A/%	V/%	FC */%
Coal	52.74	2.77	11.18	1.04	0.61	22.00	9.66	25.26	43.08
KEL	40.87	4.94	35.37	0.41	0.06	7.70	10.65	65.66	15.99
HTB	43.29	5.15	37.75	0.46	0.04	6.70	6.61	70.61	16.08
XM	43.45	5.05	37.85	0.47	0.05	7.20	5.93	67.33	19.54
Oil sludge	21.14	3.09	0.16	0.27	0.52	41.60	33.22	22.95	2.23

* By difference.

. The main ash components of these fuels are shown in

Table 2. Main ash components analysis of coal, biomass, and oil sludge.

Sample	Fe2O3/%	Al2O3/%	CaO/%	SiO2/%	TiO2/%	SO3/%	MgO/%	K2O/%	Na2O/%	MnO2/%	P2O5/%
Coal	12.62	13.80	12.12	34.30	0.64	6.39	2.50	1.07	0.85	0.25	2.34
KEL	4.77	9.90	19.43	45.20	0.46	0.66	4.59	5.81	1.93	0.09	0.85
HTB	5.73	12.29	18.25	40.49	0.63	0.69	3.01	7.17	1.83	0.11	1.29
XM	3.65	8.79	23.81	34.94	0.46	1.50	5.73	7.95	1.97	0.08	1.15
Oil sludge	5.44	15.71	4.57	48.00	0.72	4.08	2.56	1.92	2.86	0.07	0.19

.

2.2. Experimental System and Methods

In this study, a fluidized bed combustion system is employed to investigate the characteristics of pollutant release. As shown in Figure 1a, the fluidized bed combustion system is mainly composed of an air supply system, a heating system, a temperature control system, a water-cooling system, and a flue gas collection system. During the experiment, the air supply system is first activated (with an excess air coefficient of 1.2, and the gas flow rate is determined via the actual air demand of the fuel). Once the furnace reaches the set temperature (900 °C) [38], a 1.0 g sample is introduced into the fluidized bed reactor. A reaction time of 10 min is used to ensure a complete reaction. The analyzer performs real-time measurements of N₂O, NO, NO₂, and SO₂ produced during combustion. Through computer software analysis and real-time output of gas relative concentration, the pollutant generation characteristics of the measured sample was obtained.

Additionally, a settling furnace system is applied to conduct ash deposition experiments in this study, as shown in Figure 1b. The settling furnace experimental system consists of a high-temperature settling furnace, a temperature control system, a powder feeding system, an air supply system, and an ash accumulation system. During the experiment, the constant temperature of the furnace is maintained at 900 °C [38], the powder feeding rate is kept at 3.0 g min⁻¹ for 30 min (ensure sufficient ash accumulation), and the air supply system controls the incoming air flow rate at about 1.1 m³ h⁻¹ (actual air). The flue gas temperature at the ash sampling point and the wall temperature are adjusted by changing the depth of the probe in the furnace and the flow rate of the cooling air passed into the probe. After the experiment, the probe is removed from the furnace, and the ash accumulated on the probe surface is collected by mechanical vibration for subsequent comprehensive analysis of the ash characteristics. To ensure reproducibility, all experiments were conducted three times.

2.3. Analysis

To examine the microscopic morphology of the ash surface that has been collected, a scanning electron microscope–energy dispersive spectroscopy (SEM-EDS) (SU3500, Hitachi, Ibaraki Prefecture, Japan) is utilized. A Gasmet infrared flue gas analyzer (Testo 350, Testo SE & Co. KGaA, Schwarzwald, Germany) is applied to measure N₂O, NO, NO₂, and SO₂ generated during the combustion of fuel online and in real time.

3. Results and Discussion

3.1. Pollutants Emission Characteristics

For convenience comparison, different blending ratios of coal, biomass, and oil sludge are numbered. The sample numbers and their compositions are shown in

Table 3. Sample number and composition.

Sample Number	Biomass Type	Biomass Ratio (wt%)	Oil Sludge Ratio (wt%)	Coal Ratio (wt%)
H-01	HTB	30	20	50
H-03	HTB	40	10	50
H-05	HTB	40	15	45
H-07	HTB	40	20	40
H-09	HTB	50	10	40
H-11	HTB	50	15	35
H-08	HTB	45	15	40
K-08	KEL	45	15	40
X-08	XM	45	15	40

. To facilitate description, the coal and oil sludge are labeled as C and OS, separately.

3.1.1. The Effect of Blending Ratio

The variation in pollutant emissions with time during the combustion of different samples is shown in Figure 2. Coal (C), oil sludge (OS), HTB, and H-09, are burned for about 600 s. The concentrations of NO_x and SO₂ tend to have a constant value. At this time, it is considered that the combustion of these four fuels is complete. The NO_x emission characteristics of the four samples are generally similar. At the initial stage of combustion, the NO_x emissions are all relatively low. Subsequently, the NO emission reaches its peak around 100 s, then gradually decreases as the growth rates of N₂O and NO₂ accelerate and reach the peak around 200 s. Around 300 s, various concentrations of NO_x gradually return to zero. The NO emission of HTB is higher compared to C and OS, while its N₂O emission is lower. From the perspective of the generation mechanism of NO and N₂O, the generation of NO mainly comes from volatile nitrogen. When HCN and NH₃ in the volatile encounter oxygen during combustion, a series of homogeneous reactions occur. With the participation of many free radicals, such as OH, O, H, etc., parts of HCN and NH₃ are oxidized to NO. Then, NO is converted to NO₂ and N₂O. After HCN is mainly oxidized to NO; at this time, the reaction of NCO with H, O, and OH slows down, and most of the NCO generates N₂O through Reaction (1) as follows:

$$\mathrm{NCO}+\mathrm{NO}\to {\mathrm{N}}_2\mathrm{O}+\mathrm{CO}$$

(1)

At the beginning of combustion, the lower N and higher O of HTB would lead to a smaller nitrogen–oxygen ratio of HTB. For a unit amount of NCO and HCN, there are more free radicals such as H, O, and OH in HTB, which enable greater consumption of NCO for NO production. Therefore, the remaining ratio of NCO is smaller. Due to the small N in HTB, the remaining NCO is less, making Reaction (1) more difficult to proceed. In addition, the gas–solid heterogeneous reactions are also related to the value of N: O. O is adsorbed on the carbon surface to form C (O), and then O is adsorbed on the C_fas and C (N) to form C (NO). C (N) consumes NO through Reaction (2) as follows:

$$\mathrm{C}\left(\mathrm{N}\right)+\mathrm{C}\left(\mathrm{NO}\right)\to {\mathrm{N}}_2{\mathrm{O}+2\mathrm{C}}_{\mathrm{fas}}$$

(2)

To form N₂O in this process, C (N) is adsorbed on the surface of coke particles, and C (N) forms N₂O through Reaction (3) as follows:

$$\mathrm{C}\left(\mathrm{N}\right)+\mathrm{NO}\to {\mathrm{N}}_2{\mathrm{O}+\mathrm{C}}_{\mathrm{fas}}$$

(3)

Due to the low N levels of HTB, it is difficult to proceed with Reaction (3), resulting in lower N₂O and higher NO emissions from HT.

When the combustion reaction proceeds to about 150 s, NO emissions decrease, while NO₂ and N₂O emissions increase rapidly because NO is gradually converted into N₂O and NO₂. In this process, C (N) is adsorbed on the surface of coke, and then C (N) forms N₂O through Reaction (3). After N₂O emission reaches its peak, its emission concentration begins to gradually decrease to zero. At this stage, N₂O begins to decompose, and N₂O reacts with H and OH radicals to generate N₂. A similar trend is observed with NO₂ emissions. With the consumption of fuel, the relative concentration of O increases, and the conversion of NO and O₂ to NO₂ occurs. However, the concentration of NO₂ at this stage is much lower than the concentration of N₂O, indicating that the reaction between NO and O₂ is not obvious. Most of the NO is converted into N₂O and then decomposed into N₂.

It can be observed that the NO emission curve of HTB declines significantly faster than that of C and OS. The reason is that there is the oxidation of C (N) in the descending stage. As NO formed inside the char diffuses outward, it is further reduced to N₂ by the carbon matrix. However, the C(N) of HTB is much smaller than that of C and OS, so the amount of NO produced by C(N) is much smaller than that of C and OS. As a result, NO emission decreases faster than that of C and OS.

The emission of SO₂ gradually increases after the combustion starts, reaches a peak at around 100 s, and then gradually decreases. Sulfur-containing substances compete with CO and CH for O, increasing the concentration of SO₂. SO₂ consumes H, OH, and O radicals, which reduces the free radicals O and OH required to generate NO. Studies have shown that the generation of SO₂ will reduce the emission of NO. Additionally, the reduction in H radicals hinders its reaction with NO₂, and the concentration of NO decreases while the concentration of NO₂ increases. The NO, NO₂, and SO₂ emissions of coal (C), H-01, H-03, H-05, H-07, and H-09 are shown in the time charts of Figure 3, Figure 4 and Figure 5. It is obvious that the blended fuels have different emission characteristics under different blending ratios. The overall law is similar, but the specific values are different.

From Figure 3, in terms of NO emission, as the biomass blending ratio increases, the peak value of the NO emission curve significantly decreases, and the curve shifts to the right, indicating that the blending of biomass inhibits the generation of NO produced by the reaction of volatile N in coal and oil sludge. There are two sections to the NO concentration curve. Reaction (1) of NO produced during the volatile matter combustion process is mostly responsible for the first peak (from 0 s to 80 s). Reaction (2) and the emission of C(NO) are responsible for the second peak [39], which occurs between 120 s and 160 s. As anticipated, due to their distinct compositions, the distributions of coking NO and volatile NO at various biomass blending ratios differ.

As biomass is incorporated, more volatile N is precipitated in the early stage to produce NO. As biomass is incorporated, the NO emission characteristics first improve and then deteriorate. The early zero emission time of H-07 is the longest, but the peak value is relatively large, which indicates that under the blending ratio of H-07, the NO emission of the blended fuel is uneven, and there will be instantaneous large values. The emission curve of H-11 is relatively narrow, but the peak value is extremely large, and there will also be instantaneous large values. The peak value of H-01 is relatively small, but the emission curve is relatively wide, and the total NO emission is relatively large. Considering the comprehensive peak value and emission curve width, the NO emission characteristic of H-09 is the best. Zhang et al. [40] found a similar occurrence, which is mostly ascribed to the addition of biomass causing the volatile content of the blended fuel to gradually rise. Furthermore, the peak value of the NO production curve tends to move forward as the biomass ratio rises. This might be the result of alkali metals in biomass having a catalytic impact on semi-coke combustion, which encourages semi-coke burnout and causes an early emission of NO in semi-coke [41]. At a high blending ratio, when the biomass blending ratio rises, the NO generation rises to a certain extent, and the NO generation curve shifts to the left, indicating that the pollution generation of biomass begins to dominate.

From Figure 4, in terms of NO₂ emissions, under a low biomass blending ratio, as the blending ratio in biomass increases, NO₂ emissions first decrease rapidly and then stabilize. Under a high biomass blending ratio, the change in biomass blending ratio does not cause obvious NO₂ emission changes. However, with the oil sludge blending ratio rising, the peak value of the NO₂ emission curve increases significantly, and the total emission increases significantly, indicating that some components in oil sludge have a catalytic effect on the chemical reaction that produces NO₂ in the fluidized bed combustion experiment, and can significantly reduce its activation energy, which promotes more NO₂ generation. Therefore, a higher biomass blending ratio and lower oil sludge blending ratio could effectively control the emission and emission rate of NO₂. From Figure 4, H-09 has the longest zero emission time, the smallest peak value, and the narrowest emission curve, consistent with the previous inference; H-09 has the best NO₂ emission characteristics. Therefore, to reduce the generation of NO₂, the blending ratio of H-09 (HTB: sludge: coal = 50%: 10%: 40%) is selected.

From Figure 5, in terms of SO₂ emission, at a low biomass blending ratio, increasing the biomass blending ratio would increase the emission peak of SO₂, while the total emission amount remains almost unchanged. At a high biomass blending ratio, increasing the biomass blending ratio could effectively reduce the emission peak and total emission amount of SO₂. This may be due to the low sulfur concentration of biomass and the sulfur-fixing action of alkali metals in biomass [42]. In addition, as the biomass ratio increases, the first emission timing of SO₂ is progressively advanced. Zhang et al. [40] consistently found the same change trend, suggesting that this may be because adding biomass may encourage burning, advance the process of coke combustion, and increase sulfur emission [43]. From the complete combustion process of 600 s, the SO₂ generation characteristics of H-09 and H-11 with higher biomass blending ratios are significantly lower in peak height and average height than the SO₂ generation characteristic of blended fuels with other blending ratios. Therefore, H-11 has the best SO₂ generation characteristic among the measured blending ratios. In addition, the SO₂ generation characteristic of H-09 is slightly worse than that of H-11 and significantly stronger than the SO₂ generation characteristics of other blending ratios. By reducing the SO₂ generation from blended fuels, H-11 and H-09 will be significantly superior to other samples in controlling SO₂, and both are good choices. In the case of only considering the generation characteristics of SO₂, the tri-combustion ratio of H-11 (HTB:oil sludge:coal = 50%:15%:35%) should be adopted. In the case of considering the generation characteristics of multiple pollutants, the tri-combustion ratio of H-09 (HTB: oil sludge:coal = 50%:10%:40%) can also be considered.

Compared with the combustion of coal, the blending ratio of H-09 has the best control effect on the generation of pollutants. It could reduce the generation of NO by about 20%, the generation of NO₂ by about 38%, and the generation of SO₂ by about 50%. This is similar to the findings of Yan et al. [44]. The research results indicate that biomass contains fewer N and S compared to coal, and the emissions of NO_x and SO₂ continue to decrease as the blending ratio of biomass increases.

3.1.2. The Effect of Biomass Type

Three different types of biomasses are blended in a circulating fluidized bed according to the ratio of biomass:oil sludge:coal = 45%:15%:40%, and the NO_x generation of the three blended fuels is detected. The results are shown in Figure 6.

As shown in Figure 6, the NO_x and SO₂ generation laws of the three biomass-blended fuels are roughly similar. It can be seen from Figure 6a that the initial emission time of N₂O from the blended fuel of XM is significantly earlier, and the amount of N₂O generated is significantly greater than that of the blended HTB or KEL. The reason is that the flammability index of XM blended is greater than that of HTB or KEL, and its reactivity ability in the early stage of combustion is stronger. Therefore, the blended XM promotes the precipitation of volatile N, accelerates the combustion process, promotes the precipitation of C(N), and then produces and emits N₂O faster. However, the blended HTB has the worst combustion characteristics, so the precipitation of N is the slowest and the least sufficient, resulting in the slowest generation of N₂O.

As shown in Figure 6b, the initial NO₂ emission time of the blended fuel blended with XM is earlier due to its better early combustion characteristics. The blended fuel with HTB has a larger NO₂ emission peak when the amount of NO generated in the preliminary reaction product is less. It may be the higher O:N ratio that causes NO to be more easily oxidized to NO₂ in an oxidizing atmosphere.

As shown in Figure 6c, the SO₂ generation of the blended fuels blended with three different biomasses is not very different in the peak size and the initial emission time. It is speculated that this phenomenon occurs because the sulfur content of the three biomass is similar. Among them, the initial emission time of the blended XM is slightly faster than that of the other two samples, mainly due to its better early combustion characteristics. The initial emission time of SO₂ in the blended KEL is later than that of the other two samples where the S in KEL is higher than that of XM and HTB, and the total amount of SO₂ generated is also significantly smaller than that of the other two samples. The reason is that KEL has a higher alkali metal content (The K content in the dry basis of KEL is 5348.8 mg·kg⁻¹, which is greater than 4066.0 mg·kg⁻¹ of HTB and 3982.5 mg·kg⁻¹ of XM. The sodium content is 1588.0 mg·kg⁻¹, which is greater than 926.8 mg·kg⁻¹ of HTB and 879.8 mg·kg⁻¹ of XM.), and alkali metals have sulfur fixation effects, so the emission of sulfur element in KEL is inhibited to a greater extent.

3.2. Ash Accumulation Characteristics of Co-Combustion of Coal and Biomass

Figure 7 and Figure S1 (see Supplementary Materials) show the microscopic morphology of the collected ash under the conditions of different biomass blending ratios at 100 and 500 times, respectively. The microscopic morphology and removability of the deposited ash reflect the ash deposition characteristics of the fuel to a certain extent. The deposited ash has quite a loose structure, and it is easy to remove, which proves that the sedimentation of the burned ash is weak and vice versa. Biomass and coal contain different components, so the ash deposition characteristics are also very different during combustion. As illustrated in Figure 7, it occurs when the ratio of biomass is small, and the ash slag is mainly in large blocks and a few spherical shapes. At the same time, there are also some finely fragmented ash slag diffusely distributed. The ash particles are relatively isolated from each other, and the deposited ash structure is relatively loose. There are obvious gaps between the particles, and no obvious sintering and melting phenomena are observed. Pure biomass ash is mainly spherical, and the particles of deposited ash are relatively small; the structure is relatively dense; the gap between the particles is not obvious, and some parts of the ash accumulation have no gaps, indicating that the pure biomass ash has obvious fusions and adhesions. As the biomass blending ratio rises, the overall color of the ash gradually deepens, indicating that the degree of sintering gradually deepens. When the biomass blending ratio is at 40%, the structure of the sedimentation ash particles is more compact than that of pure coal sedimentation ash particles, indicating that at the blending ratio of 40%, the sedimentation ash appears slightly melted due to the blending of biomass. When the blending ratio of biomass increases to 60%, more dense structures appear in the sedimentation ash, but the gap between each part is relatively obvious. When the blending ratio of biomass continues to increase to 100%, the sedimentation ash appears with obvious agglomeration phenomenon, so many molten agglomerates can be seen in Figure 7d.

Figure 8 presents the energy spectrum analysis results of ash samples at various biomass blending ratios under 900 °C. As the biomass blending ratio rises, the content of Ca on the ash sample surface gradually increases, and the Na also shows a weak increasing trend, while the content of Al and Si gradually decreases, and the tendency of ash deposition and slagging becomes greater. When the biomass co-combustion ratio is low, there is a large amount of Si and Al s in the deposited ash, and the content of Ca is relatively small. When the biomass blending ratio is at 100%, the content of Al and Si in the deposited ash is significantly reduced, while the content of Ca is significantly increased. When a large ratio of biomass is blended, coal ash gradually transforms into pure biomass ash. Therefore, when a high ratio of biomass is co-combustion, serious ash deposition and slagging phenomena would occur.

Based on the above analysis, when the flue gas temperature is at 900 °C, there will be serious ash accumulation and slag formation when the biomass blending ratio is high, and low-melting-point compounds containing calcium will aggravate the melting and adhesion of ash particles, resulting in ash accumulation that is difficult to remove.

3.3. Ash Accumulation Characteristics of Tri-Combustion of Coal, Biomass, and Oil Sludge

Under an electron microscope at 500-times magnification, the microscopic morphologies of combustion ash samples of HTB biomass and sludge with varying blending ratios at a flue gas temperature of 450 °C and a wall temperature of 190 °C are displayed in Figure 9 and Figure 10. Under an electron microscope at 100-times magnification, the microscopic morphologies of combustion ash samples of HTB biomass and sludge with varying blending ratios at a flue gas temperature of 450 °C and a wall temperature of 190 °C are displayed in Figures S2 and S3 (see Supplementary Materials). The microscopic appearance characteristics and removability of deposited ash reflect the ash deposition characteristics of the fuel to a certain extent. If the structure of the deposited ash is relatively loose and easy to remove, it proves that its deposition property during combustion to form ash is weak. Conversely, it is strong. Biomass and coal contain different components, so their ash deposition characteristics are very different during the burning process. From the figures, it is evident that the ash samples are generally spherical and lumpy, and the ash particles are independent of each other. When the mixing ratio of biomass and oil sludge is low, the particles are mostly spherical, small, and loose in position. Each ash particle can be clearly distinguished, and no bonding phenomenon occurs. As the ratio of biomass and oil sludge rises, the structure of the ash will be tighter, and the volume of the ash particles will gradually become larger. Partial bonding phenomenon will occur, and part of it will be lumpy, and the arrangement will no longer be neat. This shows that when the ratio of biomass and oil sludge rises, the ash particles after biomass and oil sludge combustion will melt and sinter, which is due to the occurrence of the chemical reaction previously speculated. It can be found that as the oil–sludge ratio rises, the number of large-volume ash particles in the figure increases significantly. It is speculated that the oil sludge combustion increases the volume of ash particles, and as the oil–sludge ratio rises, the melting and sintering phenomenon is deepened. It can be speculated that a higher biomass ratio encourages the fusion and adhesion of ash samples.

Figure 9 and Figure 10 show the SEM images at 500× magnification of ash samples from low-ratio co-combustion of HTB biomass and oil sludge. When the blending ratio of biomass and oil sludge is small, the particles are mainly spherical, and the diameters of most spherical particles are between 15 and 45 μm. There are fewer irregular ash particles, which do not stick to each other. As the blending ratio of biomass and oil sludge gradually rises, the number of irregular-shaped ash particles gradually increases, and the bonding between ash particles is enhanced. In high ratios of biomass and oil sludge fuels, the agglomeration phenomenon is very obvious, and the volume of ash is significantly increased; this is due to the high ratio of biomass containing more K and Na.

Figure 11 and Figure S4 (see Supplementary Materials) show the microscopic morphologies of combustion ash samples of HTB biomass and sludge with a high blending ratio at a flue gas temperature of 650 °C and a wall temperature of 380 °C under an electron microscope at 500- and 100-times magnifications. The ash sample at high temperature presents a blocky-shaped large ash particle that is broken into blocky fragments. At the same time, the surface of ash particles is rough, and the melt encapsulates many fine particles, increasing the viscosity of ash particles. When the flue gas and wall temperature rise from 450 °C to 650 °C and 190 °C to 380 °C, its viscosity increases, reducing the melting and agglomeration of minerals. The thermal decomposition of minerals such as calcite (CaCO₃) and anhydrite (CaSO₄) weaken the melting polymerization effect and finally makes the ash sample present with a broken porous structure. Fe₂O₃ acts as a binder at high temperatures, forming a rough morphology.

Comparing the electron microscope images of ash samples under low-temperature and high-temperature conditions, the ash samples form loose flocculent slag at low temperatures. As the temperature rises, it melts and shrinks into a spherical shape. This is because as the temperature rises, the content of CaO and Fe₂O₃ in the mixed ash increases, promoting the formation of anorthite, albite-anorthite, etc., thus leading to severe slagging.

Table 4. EDS analysis data of fuel ash samples with different blending ratios.

Samples	O	Na	Mg	Al	Si	P	S	K	Ca	Fe
30% biomass + 20% oil sludge	23.99	1.39	4.41	7.2	15.25	1.61	3.31	3.71	23.22	14.57
35% biomass + 15% oil sludge	28.07	1.41	2.48	8.2	21.85	0.77	1.56	3.75	19.52	12.39
40% biomass + 10% oil sludge	28.7	1.84	2.21	7.55	21.34	0.51	2.42	3.92	16.1	9.88
35% biomass + 20% oil sludge	27.25	1.42	2.16	7.9	21.07	0.89	1.87	3.5	18.85	10.15
40% biomass + 15% oil sludge	28.27	1.67	2.55	7.6	20.39	0.91	3.64	4.75	20.25	9.98
45% biomass + 10% oil sludge	25.36	1.18	2.4	7.75	20.56	0.87	2.28	4.05	20.28	10.17
40% biomass + 20% oil sludge	27.68	1.89	2.29	7.82	21.34	0.67	2.76	4.19	18.1	11.45
45% biomass + 15% oil sludge	28.15	1.34	2.16	8.05	23.73	0.56	1.85	3.8	17.83	11.5
50% biomass + 10% oil sludge	28.15	1.73	2.1	7.9	21.1	0.88	3.39	4.1	18.87	11.17
45% biomass + 20%oil sludge	27.58	1.46	2.21	7.75	23.56	0.64	1.69	4.34	16.75	9.45

presents the EDS analysis data of fuel ash with different blending ratios. According to the EDS analysis results, in the ash sample with a ratio of 30% HTB + 50% oil sludge, the mass fraction of O is approximately 23.99%; the mass fraction of Si is about 15.25%; the mass fraction of Ca is around 23.22%; the mass fraction of Al is approximately 7.2%, and the mass fraction of Fe is about 14.57%. Among them, most Si and Al exist in the form of oxides, and Ca exists in the form of silicate. As the blending ratio of biomass and sludge rises, the mass fraction of Si in the ash sample increases; the mass fraction of Ca decreases, and the mass fractions of Al and Fe decrease slightly. The mixed ash melts first during the heating process. Low-proportion biomass ash has little influence on the structure of the mixed ash due to melting and bonding in the ash, and the slagging tendency is weak. As the biomass blending ratio rises, the content of SiO₂ and Al₂O₃ in the mixed ash decreases, and the content of CaO and Fe₂O₃ increases. In the samples, ash particle density increases; bridge structures become more pronounced; the number of bonded granular particles rises, and the surface becomes rougher. This may result from chemical reactions between compounds like SiO₂, CaO, and Fe₂O₃, forming eutectic compounds such as CaO(Al₂O₃)₂ and Ca(Al₂SiO₈). As the proportion of oil sludge increases, these reactions become more complete, leading to a higher tendency for slagging and making ash residues more difficult to clean. The current results were in accordance with the studies of Lv et al. [45]. At the same time, due to an isomorphism reaction, a small amount of Na is mixed into anorthite and aluminosilicate, resulting in severe slagging.

From the data analytics in Table 4, as the blending ratio of biomass and oil sludge rises, the content of O, Si, K, and Na s gradually increases, and the content of Mg, P, Ca, and Fe s gradually decreases. This phenomenon is due to the high content of Si and Na contained in biomass, while the content of Fe, Ca, and others contained in coal samples is higher. Since Si can form a high-melting-point polymer with Na and K, it can be inferred that when the mixing ratio of biomass and oil sludge is large, the furnace is more prone to slagging and becomes difficult to clean, which is also an inevitable defect of biomass, oil sludge, and coal blending.

In summary, when the ratio of oil sludge to biomass mixing rises, the ash sample structure will be more compact; the volume of ash particles will gradually increase, and part of the bonding phenomenon will appear, and part of it will be lumpy, and the arrangement will no longer be neat. This shows that when the ratio of oil sludge to biomass mixing rises, the ash particles after the combustion of biomass and oil sludge have melted and sintered. At low temperatures, the samples with a higher blending ratio of biomass and oil sludge can be observed. When the blending ratio rises, the density of ash particles in the sample increases; the bridge structure increases significantly; the number of bonded bulk particles increases, and the surface is rougher. In addition, when the blending ratio of biomass and oil sludge is large, the furnace is more prone to slag and becomes difficult to clean.

This experiment simulates an actual furnace combustion, setting the flue gas temperature (high temperature/low temperature) to 650/450 °C and the wall temperature (high temperature/low temperature) to 380/190 °C. By calculating the air volume and adjusting the fan to provide the required combustion air, performed by connecting the air compressor at one end of the dust accumulation probe and adjusting the air volume to make the probe reach the required wall temperature, we achieve the simulated effect in real conditions.

3.4. Suggestions for Industrial Application

The tri-combustion of coal, biomass, and oil sludge could not only reduce the dependence on fossil fuels and save energy but also realize the recycling of waste. Regarding pollutant emissions, blending biomass could effectively reduce SO₂ and NO_x emissions [46] and decrease the harmful content of oil sludge through combustion [33]. For industrial applications, it is recommended to dry, crush, and pelletize biomass to improve its density and energy density. For oil sludge, dehydration, deoiling, and purification treatments should be carried out to reduce the content of harmful substances. In addition, the oil sludge and biomass can also be pelletized, taking advantage of the adhesiveness of oil sludge which makes it more conducive to pelletizing. By sending coal and oil sludge biomass into the furnace, respectively, biomass could effectively reduce the ignition temperature and make it more conducive to increasing the combustion speed of oil sludge.

4. Conclusions

In this study, coal, biomass, and oil sludge are employed for investigation. Pollutant emission and ash accumulation characteristics of tri-combustion of coal, biomass, and oil sludge are discussed. The main conclusions can be drawn as follows:

(1)

With the increase in the biomass blending ratio, the generation amount of NO first decreases and then increases; the generation amount of NO₂ gradually decreases and tends to be stable, and the generation amount of SO₂ first increases and then decreases. Among the measured blending ratios, the H-09 with a blending ratio of HTB: oil sludge: coal = 50%:10%:40% has the best control effect on the generation of pollutants under the flue gas temperature of 900 °C. Its generation characteristics are the best, which could reduce the generation of NO, NO₂, and SO₂ by about 20%, 38%, and 50%, respectively. Previous studies [40,42,44] have shown that biomass contains fewer N and S compared to coal, and the emissions of NO_x and SO₂ continue to decrease as the blending ratio of biomass increases;

(2)

With the increase in biomass blending ratio, the content of Ca on the surface of the ash sample gradually increases. The content of Na also has a slight increasing trend, while the content of Al and Si gradually decreases, and the tendency of ash accumulation and slag becomes larger. When the biomass blending ratio is low, there are a lot of Si and Al in the deposition ash, and the content of Ca is relatively small. When the biomass blending ratio is high, there was serious ash accumulation and slag phenomenon. The current results were in accordance with the previous study [45];

(3)

In the ash sample with a 50% proportion of biomass HTB and oil sludge, the mass fraction of O, Si, Ca Al, and Fe are approximately 27%, 23%, 20%, 8%, and 12%, respectively. As the blending ratio of biomass and oil sludge increases, the mass fraction of Si in the ash sample increases, the mass fraction of Ca decreases, and the mass fractions of Al and Fe both decrease to a small extent. In industrial applications, further research is required to comprehensively understand pollutant emissions and ash deposition associated with the tri-combustion of coal, biomass, and oil sludge.