Co-Combustion Characteristics of Municipal Sewage Sludge and Coal in a Lab-Scale Fluidized Bed Furnace

Abstract

The co-combustion of dry coal and sludge was carried out in a self-built laboratory scale fluidized bed furnace. We used FTIR spectroscopy, high-resolution gas chromatography, high-resolution mass spectrometry, and XRF to capture and dynamically observe the flue gas fly ash and bottom ash. The sludge sample was taken from a municipal sewage treatment plant and its mixing ratio was below 20%. The temperature distribution of the furnace was observed and the emissions of the main gaseous pollutants such as SO₂, NOx, and dioxins were measured in the combustion process. The heavy metal contents of fly ash and bottom ash were later detected to analyze the migration characteristics. The results showed that all of the gaseous pollutants measured increased linearly with the sludge mixing ratio. However, it could also meet the 1 metal content of bottom ash and fly ash did not change much. Through the elaboration of the above points, this study can provide guidance for future research on the co-combustion of coal boiler sludge and suggest favorable conditions for the co-combustion of coal boiler sludge.

1. Introduction

The treatment and disposal of large amounts of sewage sludge are a major social problem in China today [1,2]. The total annual sludge production is increasing year by year with the acceleration of urbanization process. It is well-known that the incineration of sewage sludge has been a widely used solution for its treatment and disposal, especially in Europe, America, Japan, and other developed countries [3,4]. In 2016, the incineration disposal of municipal sludge in Germany accounted for 64.4%, in which the co-combustion of large coal power plants accounted for 26%, and the rest accounted for 10% [5]. It makes sludge harmless, stable, and reduced. However, sludge in China has the characteristics of low volatile content and low heating value compared with other developed countries [6,7,8]. The organic content of sludge in developed countries is approximately 60–70%, while the organic content of sludge from Chinese sewage plants is only 30–60% [9]. In general, the self-sustaining combustion of sludge cannot be achieved when the moisture content is high, so it often needs to be co-burned with coal, natural gas, or other auxiliary fuels. In Germany, sludge is generally dried to 75–90% TS and then co-incinerated with circulating fluidized bed boiler or pulverized coal boiler [10].

Sludge co-combustion disposal refers to the technology where sludge is mixed with coal, domestic garbage, biomass, and other high calorific value fuels in a certain proportion and then sent to the corresponding incineration equipment for combustion [11]. Many researchers have conducted a lot of work on the co-combustion characteristics of sludge and coal by thermogravimetric analysis [12,13,14,15,16,17]. It has been reported that there is no obvious difference between the results of coal and mixtures with a low sludge mixing ratio (<10%). In this range of sludge mixing ratio, coal combustion is absolutely dominant during the co-combustion process. Zhang et al. [18] concluded that the performance of the co-combustion of sewage sludge with coal was sometimes better than the single burning of coal by drop tube furnace experiments. Leckner, B et al. [19] indicated that when sludge energy fractions were less than 25%, co-combustion could be carried out in a circulating fluidized bed plant without exceeding the EU or German emission limits, except for chlorine emissions, which that may have to be reduced by flue gas treatment.

Table 1. Emission standard of air pollutants for incinerators of sludge incineration plants in China [20].

Items	Units	Limited Value
Ashes	mg/m3	80
CO	mg/m3	150
NOX	mg/m3	400
SO2	mg/m3	260
HCl	mg/m3	75
Hg	mg/m3	0.2
Cd	mg/m3	0.1
Pb	mg/m3	1.6
Dioxins	ng TEQ/m3	1.0

shows the discharge standards for the incinerator of a sludge incineration plant in China [20]. The high content of N, S, Cl, P, and ash in sludge and the flue gas generated during its incineration contain a large amount of pollutants such as SO_X, NO_X, HCl, and fly ash particles, which may affect the existing incineration equipment and flue gas cleaning system in long-term co-combustion. Therefore many scholars have studied the pollutant emission characteristics of sludge combustion.

Shimazu et al. [21] found that dried sludge combustion produced higher concentrations of N₂O than normal coal combustion, and that sludge with a higher water content produced lower concentrations of NO after combustion. It was assumed that the FeCl₃ added during sludge pretreatment oxidized to Fe₂O₃ during incineration, thereby catalyzing the production of NO_X. Zhang et al. [22] concluded that NO and SO₂ were mainly produced at the initial stage of combustion and that the SO₂ emissions were reduced when the auxiliary fuel was anthracite. Sun et al. [23] stated that NO_X emissions were due to the decomposition of proteins and aliphatic compounds, while SO₂ emissions were attributed to the decomposition of aromatic sulfur, FeS₂, and FeSO₄. Zhang et al. [24] studied the co-combustion of dried wood sludge and anthracite and found that SO₂ and NO emissions decreased with increasing sludge content, and increased slightly with increasing temperature. Generally speaking, chlorine in sewage sludge is mainly released in the form of HCl and temperature corresponding to the HCl release rate, which is related to the temperature of the maximum CO₂ release rate. Oxygen has a positive effect on HCl release [25].

Zhundong coal has the advantages of high volatile matter and low ash content, but the content of alkaline earth metals in the coal seams of Zhundong coal far exceeds that of other coal types [26]. Guo et al. believed that the higher the temperature, the more active the chemical properties of the heavy metals Pb and Ni, and the higher the mole fraction in the gas phase. They also found that there was competition between the alkali metal elements and heavy metal elements in the fuel, of which Na was the most significant [27].

Our study aimed at this problem by carrying out combustion experiments using a lab-scale fluidized bed furnace. The blended fuel was made of dried coal and sludge with a different sludge mixing ratio (≤20%). The temperature distribution of the furnace was observed and the emissions of the main gaseous pollutants (SO₂, NO_x, and dioxins) were measured during the combustion process. The heavy metal contents of fly ash and bottom ash were detected to later analyze the migration characteristics.

2. Materials and Methods

2.1. Materials

The sludge sample was collected from the Qige Waste Water Treatment Plant in the city of Hangzhou, China. The sewage plant is the largest one in this city with a capacity of 1.2 million tons per day, mainly dealing with municipal sewage. The coal sample was taken from Zhundong coal mine in Xinjiang Province, China. The combustion characteristic parameters of sludge are very different from that of coal. Due to the high calorific value and low ash content of coal, the maximum combustion rate and average combustion rate of coal are significantly higher than that of sludge. The main reaction temperature range is different, so the temperature corresponding to the maximum reaction rate varies greatly. The temperature of coal is 492.3 °C and that of sludge is about 290 °C. The overall reaction temperature required by coal is higher than that of sludge, so coal can be burned out at a higher temperature, while the burnout temperature of sludge is relatively low. Due to the complex composition and many reactions of sludge, its main reaction temperature range is wider than that of coal, so the combustion time of sludge is significantly higher than that of coal. In this test, the total amount of the samples was 600 g, of which sludge accounted for 20%. All of the samples were dried in an electric drying oven (fan blown type) at 105 °C for 12 h, after which they were milled and meshed to small particles with a size in the range of 0.45 mm to 2 mm.

Before the test, all samples were dried in a 105 °C constant temperature air drying oven for 24 h, and then they were ground with a mortar. Small particles were screened out below 60 meshes to ensure a full reaction during the test. The mixed samples were mixed evenly, according to the mixing ratio under the set working conditions before use. According to the relevant national standards of coal, the industrial analysis, element analysis, and higher heating values of the test samples were determined. The proximate and ultimate analysis results and higher heating values of these samples are shown in

Table 2. Proximate, ultimate analysis results, and higher heating values of the samples.

Samples	Ultimate Analysis (wt%, adb)					Proximate Analysis (wt%, adb)
	C	H	N	S	O	M	V	A	FC
Sludge	16.1	1.49	1.17	0.34	8.28	2.23	24.85	70.39	2.53
Coal	67.99	4.5	1.01	0.88	8.52	2.4	31.63	14.7	51.27

. The volatile content of sludge was 24.85%, much lower than that of European countries. The higher heating value of coal was 5.5 times as much as that of sludge.

2.2. Lab-Scale Fluidized Bed Furnace

The lab-scale fluidized bed furnace used in this study is shown in Figure 1, which was 1360 mm high with an outer diameter of 300 mm. Its internal corundum threaded ceramic tube was 1110 mm high with an inner diameter of 60 mm.

This can be divided to the dense phase area, suspension zone, and simulated flue, respectively, from the bottom to the top. The furnace is heated by resistance wires and a resistance wire with a power of 3 kW is arranged in each of the three sections. The furnace temperature is measured by thermocouples and is controlled by intelligent control cabinet. The outer of the pipe is equipped with a high alumina insulated ceramic tube. Heat preservation cotton was filled between the outer surface of the furnace and the insulated ceramic tube. The air distributor was a 100-mesh steel wire mesh. White sand of 40~60 mesh was used as the fluidized bed material, and a screw feeder was adopted for the feeding of burning material and fluidized bed material. The tail flue was provided with a cyclone separator to collect the fly ash.

2.3. Test Condition and Analysis Instruments

The combustion texts with different sludge mixing ratios were carried out at the temperature of 800 °C, and each combustion process took about 30 min. The primary air flow was 3 m³/h, and the mixed fuel feeding amount was controlled to 20 g/min. Yue et al. [28] showed that the air flow rate could totally eliminate the influence of external diffusion on the results. A total of 300 g of bed material was sent to the furnace before each experiment. In total, five different text conditions were set up with the sludge mixing ratio of 0%, 5%, 10%, 15%, and 20%, respectively.

As shown in Figure 2, the schematic diagram of the experimental method used in this paper. The temperatures of the three-section furnace were measured by thermocouples and displayed on the control panel. The flue gas was continuously monitored by a Gasmet DX-4000 FTIR gas analyzer (Gasmet Technologies Oy, Vantaa, Finland). The collected FTIR spectra were processed by Calcmet Software 2005. The flue gas sample for PCDD/Fs analysis was collected by using a glass adsorption bin that was filled with XAD-2. The PCDD/Fs were analyzed by using a high resolution gas chromatograph coupled with a high-resolution mass spectrometer (HRGC/HRMS) (JEOL JMS-800D, Japan). A DB-5 ms (60 m × 0.25 mm × 0.25 μm) capillary column was used for separation of the PCDD/Fs congeners. All gas data were reported with reference to the standardized conditions (i.e., per normal Nm³ dry gas with 11% of O₂). The heavy metal content of the fuels, bottom ash, and fly ash were measured by X-ray fluorescence equipment (XRF, Thermo Scientific ARL ADVANT’X IntelliPowerTM 4200, Waltham, MA, USA).

3. Results and Discussion

3.1. Temperature Distribution and Combustion Efficiency

When the combustion conditions were stable, we recorded the temperature of the three-stage furnace every minute and calculated the average value. The furnace temperature distribution of the sludge and coal co-combustion test under different sludge contents is shown in

Table 3. Temperature distribution of the furnace in the stable condition.

Test Number	Sludge Mixing Ratio/%	Bottom Temperature/°C	Middle Temperature/°C	Top Temperature/°C
1	0	900	890	840
2	5	895	895	830
3	10	890	900	825
4	15	880	905	820
5	20	870	910	810

. The temperature in the middle and bottom of the furnace was about 900 °C, which was much higher than the initially set temperature (800 °C). It should be noted that the heat generated by burning the fuel was sufficient to heat the primary air and compensate for the heat loss. The reaction of sludge combustion was mainly the decomposition of organic matter and the combustion of volatile matter. Therefore, with the increase in the sludge content, the furnace bottom temperature decreased, and the middle temperature increased. This was a strong criterion for the main combustion zone to move slightly toward the middle of the furnace. As the calorific value of sludge was far lower than that of coal, the increase in the sludge content would reduce the total heat entering the furnace, and eventually lead to a slight drop in the temperature of the flue gas outlet or furnace top. From the comprehensive view of temperature change, with the increase in the sludge content, the temperature tended to decrease as a whole. The temperature at the top and bottom decreased about 30 °C, and the heat release decreased, which led to the decrease in the power generation efficiency.

With the addition of sludge, the temperature distribution of the furnace tended to be nonuniform, which was due to the differences between the combustion characteristics of coal and sludge. Therefore, it is necessary to select a suitable additional combustion amount when co-burning sludge in the coal-fired boiler. In this way, a correspondingly uniform temperature distribution of the furnace will be realized to facilitate the stable operation of the boiler.

As shown in

Table 4. The CO₂, CO, and CH₄ concentrations of the flue gas with different sludge mixing ratios.

Test Number	Sludge Mixing Ratio/%	CO2/%	CO/%	CH4/%
1	0	2.16	1.986	0.0310
2	5	4.83	1.814	0.0283
3	10	6.81	1.241	0.0164
4	15	4.00	1.152	0.0149
5	20	5.08	0.795	0.0101

, the content of CO and CH₄ in the flue gas decreased gradually with the increase in the sludge mixing ratio. The calorific value of sludge was lower than that of coal as well as the combustion oxygen demand. In the case of the constant primary air flow rate, the excess air factor of the furnace was greater with a higher sludge mixing ratio. Therefore, the incomplete combustion loss of gas decreased and the combustion efficiency became higher. However, it should be noted that under the fixed condition of 800 °C, when the sludge content was 15% and 20%, the CO₂ release ratio decreased. According to the actual data, we considered that it might be related to the increase in the total gas emissions, as there was no interference with the above views.

3.2. Emission Characteristics of Gaseous Pollutants

The influence of the sludge mixing ratio on SO₂ emissions is shown in Figure 3. The concentration of SO₂ in the flue gas gradually increased when the sludge mixing ratio increased, which might be due to the high S content of the sludge and its promotional effect on SO₂ release, and the SO₂ in the flue gas might also originate from the cracking of pyrite in the Zhundong coal. Meanwhile, the Ca inherent in ZDC can also play a role in sulfur fixation [29]. The experimental results demonstrate that the addition of sludge exacerbates SO₂ emissions due to the fact that the elemental sulfur in sludge was mostly in an organic form, whereas in coal, it was in the form of sulfate, and therefore, the sulfur in sludge was more easily decomposed and oxidized to SO₂. However, the sulfur contents of both sludge and coal were quite low, as shown in Table 1. In the absence of desulfurization devices and measures on the test furnace, even when the sludge mixing ratio reached 20%, the SO₂ concentration in the flue gas was only 84 ppm, which is below the limit of the Chinese national pollution control standard for waste incineration [30]. Therefore, the co-combustion of small amounts of sludge in a coal-fired boiler will not cause excessive emissions of SO₂.

Generally speaking, the combustion characteristics of basic fuel accounts for the main influence on the co-combustion behavior. It is generally believed that when the sludge content is less than 25%, the co-combustion behavior will be dominated by coal [19]. As shown in Figure 4, when the sludge mixing ratio increased, the concentration of NO and N₂O increased, while the concentration of NO₂ decreased. The NO_x emissions increased with the addition of sludge due to the abundant organic matter in it. The NO_x concentration in the flue gas was 520 ppm when the sludge mixing ratio reached 20%. If the 80% denitrification efficiency of a general coal-fired power plant is calculated, the final NO_x emission concentration would be 104 ppm, which is also lower than the limit of the national MSW incineration pollution control standard (300 ppm/mean value in an hour). Therefore, the emissions of NO_x could be easily controlled with existing denitrification devices when co-burning a small amount of sludge in a coal-fired boiler.

The I-TEQ values of the PCDD/Fs in the flue gas samples with different sludge mixing ratios are shown in

Table 5. The I-TEQ values in the flue gas samples with different sludge mixing ratios.

	Sludge Mixing Ratio (ng I-TEQ/Nm3)
	0%	5%	10%	15%	20%
2,3,7,8-TCDD	0.01186	0.04471	0.05447	0.06378	0.06236
1,2,3,7,8-PeCDD	0.01011	/	0.02164	0.02587	0.05884
1,2,3,4,7,8-HxCDD	/	0.00282	0.00268	/	0.00310
1,2,3,6,7,8-HxCDD	0.00200	0.00334	0.00428	0.00557	0.00579
1,2,3,7,8,9-HxCDD	0.00192	0.00260	0.00379	0.00561	0.00473
1,2,3,4,6,7,8-HpCDD	0.00135	0.00158	0.00180	0.00150	0.00191
OCDD	0.00130	0.00073	0.00084	0.00099	0.00044
2,3,7,8-TCDF	0.00475	0.01159	0.01566	0.01925	0.03328
1,2,3,7,8-PeCDF	0.00183	0.00452	0.00752	0.00623	0.01726
2,3,4,7,8-PeCDF	0.03657	0.05927	0.08749	0.08921	0.12987
1,2,3,4,7,8-HxCDF	0.00520	0.00821	0.01146	0.01468	0.01290
1,2,3,6,7,8-HxCDF	0.00510	0.00918	0.01437	0.01380	0.02951
2,3,4,6,7,8-HxCDF	/	0.00506	0.00295	0.00805	0.00580
1,2,3,7,8,9-HxCDF	0.00499	0.01205	0.01657	0.02062	0.01917
1,2,3,4,6,7,8-HpCDF	0.00232	0.00334	0.00404	0.00512	0.00635
1,2,3,4,7,8,9-HpCDF	0.00040	0.00074	0.00076	0.00114	0.00058
OCDF	0.00108	0.00057	0.00062	0.00079	0.00060
Total I-TEQ	0.0908	0.1703	0.2510	0.2822	0.3925

. The emissions of PCDD/Fs for the incineration of coal only was below the value of the Chinese limit (0.1 ng I-TEQ/Nm³). With the addition of sludge, the concentration of PCDD/Fs in the flue gas increased rapidly. When the sludge mixing ratio reached 20%, the emissions of PCDD/Fs were 0.3925 ng I-TEQ/Nm³, more than four times that of the coal only test condition. This may be caused by the high content of chlorine in sludge, which helped the generation of dioxins. As the XRF results of dry sludge and coal show, the content of the chlorine element in sludge was 0.182% higher than that of coal (0.104%). The PCDD/Fs emissions of all the sludge co-burning test conditions also exceeded the Chinese emission limit. However, in the actual industrial process, the combustion temperature is a little higher, which will reduce the generation of dioxins, and the dioxin removal efficiency of the flue gas treatment facilities was more than 90%. Therefore, the final emissions of dioxins will be lower than the limits.

As above-mentioned, when co-burning sludge in a coal-fired furnace, the emissions of all the main gaseous pollutants such as SO₂, NO_x, and dioxins increased linearly with the sludge mixing ratio. It can be easily seen that the pollutant emissions will be serious when burning sludge alone, which will hardly meet the national MSW incineration pollution control standard in China. However, the pollutant emissions in the experiments were all lower than the limits when considering the removal efficiency in an actual industrial process. This shows that co-burning a small amount of sludge in a coal-fired boiler can not only use the existing equipment to save investment costs, but also can meet the emissions standard. Therefore, this could be a feasible and economic way for sludge treatment and disposal in China.

3.3. Migration Characteristics of Heavy Metals

The high risk of heavy metals released to the environment is a major problem restricting sludge utilization in China. The contents of the main heavy metals contained in the sludge are listed in

Table 6. Heavy metal contents in sludge.

Heavy Metals	Contents (mg/kg Dry Weight Basis)
	Sludge	Chinese Standard (GB 18918-2002)
Cr	209 ± 10	600
Pb	72 ± 10	300
Ni	116 ± 6	100
Cu	292 ± 15	800
Zn	1080 ± 50	2000

. Among these five heavy metals, the content of Ni was a little larger than the limit of the regulation of the Chinese standard (GB 18918-2002) for sludge agricultural utilization (acid soil), which directly prohibits farming disposal of the sludge. Chinese sludge has a low volatile content and heating value compared with other developed countries. Self-sustaining combustion cannot be achieved when the moisture content is high. Therefore, a large amount of sludge in China needs to be disposed of by co-burning in a coal-fired boiler.

The amount of sludge added in the coal-fired boiler will affect not only the total amount but also the migration characteristics of heavy metals. During the combustion process, volatile heavy metals in flue gas are partially absorbed and enriched in fly ash, while the non-vaporized heavy metals remain in the bottom ash. The concentration of heavy metals in the bottom and fly ash with different sludge mixing ratios is shown in Figure 5. When the sludge was added to the fuel, the content of heavy metals except Pb in the bottom ash greatly increased compared with the pure coal condition. The content of Zn in the bottom ash gradually increased with the increase in the sludge mixing ratio. However, other heavy metal contents with different sludge mixing ratios did not show much difference. For the heavy metal content in fly ash, the content of Ni substantially dropped when the sludge was co-burned. According to the comparison between the heavy metal contents of the bottom ash and fly ash, Cr and Ni were easier for volatilization and to enrich in fly ash. The content of Zn in the bottom ash was larger. The concentrations of Cu and Pb in the bottom ash and fly ash were not very different. This indicates that the original heavy metal emission control facilities in the flue of a coal-fired boiler can meet the emission standard when co-burning a small amount of sludge. However, the disposal method of bottom ash should be adjusted due to the increase in the heavy metal content.

4. Conclusions

In this study, the co-combustion of coal and a small amount of sludge was conducted. (1) When the sludge mixing ratio was 0~20%, the furnace temperature distribution and combustion efficiency changed little. (2) When the sludge mixing ratio was lower than 20%, the emission of gaseous pollutants such as SO₂, NO_X and dioxin increased linearly with the sludge mixing ratio, but it could also meet the national standard for the pollution control of municipal solid waste incineration in China. (3) When the sludge mixing ratio reached 20%, due to the high chlorine content in the sludge, the discharge of PCDD/Fs was 0.3925 ng I-TEQ/Nm³, which was more than four times that of the pure coal test conditions. (4) Among the main heavy metals contained in sludge and coal, chromium and nickel were more likely to be enriched in fly ash; zinc content in the bottom ash was higher; and the concentration of Cu and Pb was not different to that of fly ash. (5) This research was based on the co-combustion of coal and sludge under the condition of coal-fired boilers, and the original emission control facilities in the flue of coal-fired boilers can help the pollutants meet the emission standards while saving on the costs of pollutant treatment. Therefore, according to the situation studied in this paper, the co-combustion of a small amount of sludge in coal-fired boilers is a feasible approach to sludge treatment and disposal in China.