Combustion Characteristics of Sewage Sludge in a Fluidized Bed

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Combustion Characteristics of Sewage Sludge in a Fluidized Bed Xiangxin Han,* Mengting Niu, Xiumin Jiang, and Jianguo Liu Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: Combustion experiments of Shidongkou sewage sludge from China were carried out by using a small lab-scale fluidized bed, and the effects of moisture content and feed rate of sewage sludge and the transfer of heavy metals were analyzed. Studied sewage sludge with a moisture content of no more than 40% can stably burn in the fluidized bed without any auxiliary fuel input. Enhancing the bed temperature of the dense phase, strengthening the gas−solid mixing of dense phase, and increasing the feed rate of sludge are very necessary for the ignition of sludge with higher moisture content; however, higher feed rate will give rise to an increase of both the incomplete combustion heat loss and the physical absorbing heat amount of new sludge into the fluidized bed, reducing the bed temperature. Gaseous pollutants from the fluidized bed were discussed under different experimental conditions. At last, it was presented that heavy metals except Zn within sewage sludge are mostly concentrated in bottom ashes and bag filter ashes.

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method. Recently, a new integrated “dewatering−drying− incineration” method for sewage sludge has been proposed:7 first, dewatered sludge with high moisture content is sent into a prefurnace drier, heated, and dried by medium-temperature flue gas (400−600 °C); then, the dried sludge with low moisture content is plunged into a fluidized bed to burn and release heat, forming hot flue gas; last, the hot flue gas is introduced into the prefurnace drier and other heaters. Compared with burning dewatered sludge directly,8−10 the technology will only use the thermal heat produced by the combustion of sludge, greatly reducing the consumption of auxiliary fuels. About the integrated dewatering−drying−incineration method for sludge, there are two key technical issues needing to be solved in advance. One issue is what maximum moisture content of sewage sludge is permitted for keeping stable combustion of the sludge fluidized bed without any auxiliary fuel, which is also significant for the design of sludge drying system and the overall heat balance of integrated sludge disposal technology. Another important issue is about the emissions of gaseous pollutants and heavy metals from the sludge fluidized bed. In this paper, combustion experiments of sewage sludge were carried out by a small lab-scale fluidized bed and these two issues were discussed in detail.

INTRODUCTION Sewage sludge is formed during wastewater treatment. Since it has a lot of pathogens and heavy metals, which are sometimes higher than the European Union or United States regulations for the application of sewage sludge in agriculture,1 its disposal is perhaps one of the most complex environmental problems.2 Currently, main disposal methods of sewage sludge include land filling, dumping into sea, recycling in agriculture, and incineration. Although land filling is the simplest disposal method with low cost, it takes up a lot of land resources and contaminates the land and groundwater due to harmful substances within sludge, resulting in secondary pollutions. As a result, some strict standards have been carried out in some western countries to limit land filling for protecting environment. Dumping into sea involves directly dumping sewage sludge into sea without treatment. Though dumping into sea is convenient and economical, the harm is obvious. The complex composition of the sludge will pollute sea, doing harm to marine life, damaging marine ecosystems, and ultimately endangering human safety. Therefore, dumping into sea has been banned by international conventions. Recycling in agriculture is a disposal method in which sewage sludge obtained through a series of processes is usually added to nitrogen, phosphorus, and potassium to produce fertilizer. This disposal method is a way to reuse the sludge resource. However, there is a resistance to the method in Europe these years because of potentially harmful effects of sludge on agricultural products. Because of these defects of land filling, recycling in agriculture, and dumping into sea, the incineration of sewage sludge gradually becomes a more important role in the future.3−6 Compared with the other three disposal methods, the incineration method has more advantages, including a large reduction of sludge volume, relatively low harm to the environment, and resource recycling as energy. Especially for the countries and regions where land resources are in short supply and industrial sewage sludge has high level of toxic substances, incineration has become the only effective disposal © 2012 American Chemical Society

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MATERIALS AND METHODS

Samples. The experimental sewage sludge samples with different moisture contents were obtained from Shidongkou Sewage Disposal Plant, Shanghai, China. Their ultimate and proximate analyses are shown in Table 1. The particles of drier sewage sludge samples were 2.0 mm in average size and were nearly uniform in size distribution. However, the particles of wet sewage sludge, with high moisture content, present agglomerations ranging mostly from 2.0 mm to 5.0 mm.

Received: Revised: Accepted: Published: 10565

January 21, 2012 July 12, 2012 July 26, 2012 July 26, 2012 dx.doi.org/10.1021/ie3014988 | Ind. Eng. Chem. Res. 2012, 51, 10565−10570

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Table 1. Ultimate and Proximate Analyses of Shidongkou Sludge with Different Moisture Contents proximate analysis, wt %

ultimate analysis, wt %

moisture, wt %

Mar

Var

Aar

FCar

Qad.net , kJ/kg

Car

Har

Oar

Nar

Sar

7.29 15 25 30 35 40

7.29 15 25 30 35 40

42.74 39.19 34.58 32.27 29.97 27.66

44.58 40.87 36.06 33.66 31.26 28.85

5.39 4.94 4.36 4.07 3.77 3.49

7846.0 6985.60 5869.65 5311.67 4753.69 4195.72

24.83 22.77 20.09 18.75 17.41 16.07

3.31 3.03 2.68 2.50 2.32 2.14

14.39 13.19 11.64 10.87 10.10 9.31

4.47 4.10 3.62 3.37 3.12 2.90

1.13 1.04 0.91 0.85 0.79 0.73

feed system, a temperature control system, and a ventilation system. The main part of the fluidized bed includes an electrical air heater, a dense phase region, a dilute phase region, a cyclone, and a bag filter. The electrical air heater, the dense phase, and the dilute phase were equipped with electrical resistance wires. The fluidized bed furnace can be heated up to 850 °C under the control of the temperature control system. The inner diameter of the dense phase is Φ51 mm and gradually expands upward to Φ82 mm at the dilute phase. The whole height of the furnace is about 800 mm, and the height of sludge feeding entrance point was 350 mm away from the air distributor. The fluidizing air distributor between the dense phase and the electrical air heater was made of two layers of stainless steel mesh with 200-mesh. The electrical air heater was designed using packed bed technology, making the experimental system compact and easy to operate. The main part of the fluidized bed was wrapped with aluminum silicate fibers for reducing heat loss. Quartzite particles of 0.28−0.45 mm were used as bed materials with a static height of 90 mm. Experimental Methods. The combustion characteristics of Shidongkou sewage sludge were investigated in the hot fluidized bed according to the following procedures. (1) At the beginning of each experiment, the fluidizing air was adjusted to a specified air flow rate for making the fluidized bed in a stable fluidization state. It was calculated that the minimum fluidization velocity Umf was 0.322 m/s at the condition of ambient temperature, and so, the minimum air flow rate Qmf was 2.13 N m3/h. For keeping a good fluidization state in the dense phase during the heating, the specified air flow rate Q0 was adjusted to about 4.2 N m3/h, and consequently, the superficial fluidization velocity in the dense phase was 0.643 m/s. (2) Then, starting electrical heaters to heat the air heater, dense phase and dilute phase. In this process, the fluidizing air temperature was maintained between 300− 400 °C, and the furnace temperature was set at the temperature point of 700 °C.

Prior to combustion experiments of the small lab-scale fluidized bed, ash fusibility of the studied sewage sludge had been investigated by using a LEITZ II-A thermal microscope. The deformation, softening, and flow temperatures are 1169, 1214, and 1267 °C, respectively. Since the bed temperature of a fluidized bed boiler is usually kept below 1000 °C, the clinker problems will not occur for the studied sewage sludge. Experimental System. The schematic diagram of a small experimental combustion system for sewage sludge is shown in Figure 1. The experimental system consists of a fluidized bed, a

Figure 1. Schematic diagram of the experimental combustion system of sewage sludge. (1) air compressor, (2) manometers, (3) rotameter, (4) pressure regulator, (5) voltage regulator, (6) electrical air heater, (7) air distributor, (8) solids discharge pipe, (9) thermocouple, (10) feeding entrance, (11) temperature controller, (12) riser, (13) cyclone, (14) cooler, (15) bag filters, (16) valves, (17) vacuum pump, (18) chimney.

Figure 2. Flame and temperature distribution of the fluidized bed of sewage sludge. 10566

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(3) When the fluidized bed temperature reached the set temperature of 700 °C and was maintained for a period of time, the air supply was increased and sludge samples began to be fed into the dense phase region, and subsequently, the electric heating controllers of dense phase and dilute phase were switched off. The hot gas produced in the furnace would, in turn, pass a cyclone, a bag filter, a vacuum pump, and a chimney, and finally, it would be released into the ambient air. (4) When the sludge combusted stably in the furnace, flue gases were sampled from the outlet of the cyclone and analyzed online by a Varios Plus industrial flue gas analyzer, which can detect CHs, O2, CO, NO, NO2, SO2, and other gases. According to the National Standards of China (GB13271-2001), the measured concentration of NOX and SO2 are converted to the value under the standard condition with the excess air coefficient of 1.4. In addition, all the feed rates of sludge given in the following text have been converted to those of dry sludge with zero moisture content.

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Figure 3. Effect of moisture content on CO emission.

RESULTS AND DISCUSSION

Moisture Content of Sludge. Figure 2 shows the effect of moisture content of sludge on the flame and temperature distribution in the fluidized bed under the same feed rate of dry sludge (1.65 ± 0.1 kg/h) and the same fluidizing air rate (5.79 N m3/h). The combustion phenomena were observed through a hole at the top of the furnace. Obviously, increasing moisture content causes flame color to become dark and the furnace temperature to drop. Similarly, M. Urciuolo et al.11 also found that a sewage sludge particle looks darker than the bed during drying and devolatilization, attributed to the large amount of moisture present in the sewage sludge particles reducing both particle temperature during devolatilization and the heating value of the released volatile matter. In this work, the phenomena were analyzed further: (1) for reaching the same feed rate of dry sludge, the feed rate of sludge would increase with increasing the moisture content of sludge. The ignition heat of new sludge into the dense phase region increased with increasing moisture content, resulting in more heat consumption of the bed in both the temperature rise of sludge and vaporization of moisture within sludge and the subsequent temperature drop of the dense phase and the change of the flame color gradually from light yellow into dark red; (2) with the increase of moisture content, the resistance of oxygen diffusion onto the particle surface of sludge increased, and more combustible matter was brought into the upper region of the furnace with hot gases, resulting in an increase of incomplete combustion loss of CO, as shown in Figure 3. When the moisture content of sewage sludge exceeded 30%, sewage sludge could not stably burn in the furnace under the same combustion conditions of sludge with low moisture content. To maintain the furnace temperature high enough to stably burn sewage sludge with higher moisture contents of 35% and 40%, one effective method is increasing the feed rate. Figure 4 shows the change of the furnace temperature of the fluidized bed of sewage sludge with the moisture content of 35% after the feed rate was increased from 1.65 kg/h to 2.93 kg/h. Figure 5 shows the flame and temperature distribution of fluidized bed of sewage sludge with the moisture content of 40% when the feed rate of dry sludge was increased to 2.70 kg/ h. From Figures 4 and 5, it can be seen that the sludge can burn

Figure 4. Change of furnace temperature for sewage sludge with a moisture content of 35%.

Figure 5. Flame and temperature distribution of fluidized bed of sewage sludge with a moisture content of 40%.

stably. However, though the feed rate of sewage sludge with higher moisture content was much higher than that of other sludge samples with lower moisture content, the furnace temperature was low. The reason is attributed to higher feed rate increasing the ignition heat and incomplete combustion loss. For keeping combustible matter burning completely in the fluidized bed, the amount of air into the furnace should be properly increased with increasing the feed rate. However, there is usually an upper limit of fluidizing air, and excessive fluidizing air will also reduce the bed temperature and combustion efficiency. So, it is necessary for the fluidized bed burning sewage sludge with high moisture content to be equipped with a secondary air system to supply extra air. At the same time, the air staging can realize very low levels of NO emissions.12 It was suggested that the best distribution ratio of air ratio conditions was when the primary air ratio to secondary air ratio was 1.1:0.2.13 Sewage sludge has a low temperature to start releasing volatile matters,14 creating a good condition for homogeneous ignition mechanism. According to the analysis of combustion 10567

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Figure 6. Effect of moisture content on NOX and SO2 emissions from sewage sludge fluidized bed.

Figure 7. Effect of feed rate on the furnace temperature (fluidizing air of 6.75 N m3/h).

Figure 8. Flame and temperature distribution of the fluidized bed of sewage sludge with moisture content of 7.29% (fluidizing air of 5.31 N m3/h).

characteristics of Shidongkou sludge,15 the ignition temperature of sludge with the moisture content of 7.29% is only 228 °C and the ignition is homogeneous, making the sludge ignite easily in the fluidized bed furnace. In this experiment, the sludge was plunged into the dense phase where the fluidizing air was also introduced through the air distributor. Consequently, in the dense phase with sufficient O2 supplies and high temperature, the sludge can easily burn to release heat. The ignition time t of sludge may be simply described as follows: t = t1 + t 2

make more sludge particles and volatiles be taken into the dilute phase to burn, which results in the gradual decrease of the temperature difference between the dense phase and the dilute phase, as shown in Figure 2. Besides, the amount of flue gas will increase with increasing the moisture content, which will also take more heat from the dense phase to the dilute phase. Figure 6 shows the effect of moisture content on the emissions of NOX and SO2 of sewage sludge when keeping the same feed rate of dry sludge (1.65 ± 0.1 kg/h) at the fluidizing air of 5.79 N m3/h and 6.75 N m3/h, respectively. With increasing the moisture content of sludge, NOX emission increases slightly while SO2 emission decreases significantly. A possible cause of the sharp decrease of SO2 is that the increase of the moisture content of sludge will make the water vapor of flue gas easily react with SO2 to H2SO3, which will be erosive for metal tubes of heaters in the low-temperature zone of the fluidized bed. In addition, the increase of the moisture content of sewage sludge will cause the following gasification reactions occur, forming a weak reductive atmosphere.

(1)

Here, t1 is the time spent on the heating of sludge particle and the evaporation of moisture, in seconds, and t2 is the time lasting from devolatilization to ignition, in seconds. For the sludge with low moisture content, t1 is shorter and the sludge particle is easier to ignite in the dense phase, and thus, the heat taken to the dilute phase is smaller, resulting in a big temperature difference between the dense phase and the dilute phase. With the increase of moisture content, t1 gradually lengthens; and the diffusion resistance of O2 increases, which also causes an increase of t2. Because of strong turbulence characteristics of the fluidized bed, the increase of t1 and t2 will

C + H 2O → CO + H 2 10568

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Figure 9. Effect of feed rate on NOX and SO2 emissions of sewage sludge fluidized bed (fluidizing air of 5.31 N m3/h).

CO + H 2O → CO2 + H 2

Table 2. Analysis of Heavy Metals of Sewage Sludge and Its Ashes (mg/kg)

In such a reductive atmosphere, CaO from the thermal decomposition of CaCO3 of sludge may react with H2S as follows:

bottom ash

item

sewage sludge

Mash

Css

Mash

Css

metal recovery efficiency, %

Cd Pb Cr As B Cu Zn Ni

12.65 172.90 772.95 11.61 19.64 1141.19 2151.87 40.46

18.79 224.7 1006.0 19.80 36.27 1516.0 2416.0 69.09

7.23 86.44 386.99 7.62 13.95 583.18 929.40 25.91

11.61 141.0 630.8 12.44 30.59 867.0 1755.0 64.67

1.12 13.56 60.66 1.20 2.94 83.38 168.78 6.84

66.01 57.84 57.91 75.97 86.00 58.41 51.03 80.94

CaO + H 2S → CaS + H 2O

When the formed CaS goes into the oxidation zone, the following reactions will occur further. The second reaction is minor,16 which will result in a decrease of SO2 emissions. CaS + 2O2 → CaSO4

CaS + (3/2)O2 → CaO + SO2

Feed Rate of Sludge. Figures 7 and 8 show the effect of feed rate on the flame and temperature distribution of the fluidized bed when keeping the fluidizing air constant. The bed temperature increases gradually and the flame becomes brighter with increasing the feed rate. Figure 9 shows the effect of feed rate on the emissions of NOX and SO2 from the fluidized bed. With the increase of feed rate, SO2 emission gradually increases for sewage sludge with low moisture content. However, SO2 emission is very low for the sewage sludge with high moisture content of 25%. When the feed rate increases from 1.36 kg/h to 2.2 kg/h, NOX emission changes slightly for the three samples with different moisture contents. Especially after the feed rate exceeds 1.65 kg/h, NOX emission seems unchangeable. The increase of feed rate can bring more combustible nitrogen elements into the furnace, which is active for producing NOX.17 In the other side, because the fluidizing air was kept constant in this experiment, the increase of feed rate will produce an increasing reductive atmosphere, which has a negative effect on the formation of NOX. As a result, NOX emissions present a little change with increasing the feed rate. Transfer of Heavy Metals. Table 2 shows the analysis of heavy metals of sewage sludge with the moisture content of 7.29% and its ashes. The measurement was performed by using an ICP-AES (Iris Advantage 1000). The conversion value Css,

bag filter ash

which reflects how many heavy metals were transferred from sewage sludge into ash, is defined according to the following equation: Css =

MashA ar Cash 1 − Mar

(2)

where Css is the conversion content of heavy metals in dry sludge, mg/kg, Mash is the measured content of heavy metals within ash, mg/kg, Aar and Mar describe the ash content and moisture content of sewage sludge, respectively, %, as received basis, and Cash is the mass percentage of bottom ash and bag filter ash from sewage sludge ash, %. In this work, Cash is 80% for bottom ash, and 20% for bag filter ash. It can be seen that the concentrations of heavy metals in bottom ash are higher than those in bag filter ash, and the concentration of heavy metals follows this sequence: Zn > Cu > Cr > Pb > Ni > B > As > Cd, similar to the results of Shao et al.18 In addition, heavy metals except Zn within sewage sludge were mostly transformed to the bottom ash and bag filter ash, which is helpful for processing heavy metals further. 10569

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(8) Xiao, H. M.; Liu, K. Co-combustion kinetics of sewage sludge with coal and coal gangue under different atmospheres. Energy Convers. Manage. 2010, 51, 1976. (9) Otero, M.; Sanchez, M.; Garcia, A.; Moran, A. Simultaneous thermogravimetric-mass spectrometric study on the co-combustion of coal and sewage sludges. J. Therm. Anal. Calorim. 2006, 86, 489. (10) Leckner, B.; Amand, L. E.; Lucke, K.; Werther, J. Gaseous emissions from co-combustion of sewage sludge and coal/wood in a fluidized bed. Fuel 2004, 83, 477. (11) Urciuolo, M.; Solimene, R.; Chirone, R.; Salatino, P. Fluidized bed combustion and fragmentation of wet sewage sludge. Exp. Therm. Fluid Sci. 2012, http://dx.doi.org/10.1016/j.expthermflusci.2012.03. 019. (12) Lopes, M. H.; Gulyurtlu, I.; Cabrita, I. Control of pollutants during FBC combustion of sewage sludge. Ind. Eng. Chem. Res. 2004, 43, 5540. (13) Takahashi, K.; Miyamoto, H.; Ito, T.; Suyari, M.; Suzuki, T. Combustion improvement in fluidized-bed sewage sludge incinerator. Clean Air 2002, 3, 233. (14) Kim, J.; Lee, H. Investigation on the combustion possibility of dry sewage sludge as a pulverized fuel of thermal power plant. J. Ind. Eng. Chem. 2010, 16, 510. (15) Ji, P.; Han, X. X.; Jiang, X. M. Study of combustion characteristics of dried sewage sludge. J. Eng. Therm. Energy Power 2009, 24, 533 (in Chinese). (16) Lyngfelt, A.; Leckner, B. SO2 capture in fluidized-bed boilers: Re-emission of SO2 due to reduction of CaSO4. Chem. Eng. Sci. 1989, 44, 207. (17) Lee, D. H.; Yan, R.; Shao, J.; Liang, D. T. Combustion characteristics of sewage sludge in a bench-scale fluidized bed reactor. Energy Fuels 2008, 22, 2. (18) Shao, J.; Yan, R.; Chen, H.; Yang, H.; Lee, D. H.; Liang, D. T. Emission characteristics of heavy metals and organic pollutants from the combustion of sewage sludge in a fluidized bed combustor. Energy Fuels 2008, 22, 2278.

CONCLUSIONS The moisture content of sewage sludge and the emissions of pollutants are two important parameters for the fluidized bed combustion technology of sewage sludge. Using a small labscale fluidized bed, combustion experiments of Shidongkou sewage sludge from China were carried out, and the effects of the moisture content and feed rate of sewage sludge and the transfer of heavy metals were analyzed. The main conclusions and recommendations are given as follows: (1) Shidongkou sewage sludge with a moisture content of less than 40% can stably burn in the fluidized bed without any auxiliary fuel input. However, higher feed rate will bring about the increase of incomplete combustion heat loss and physical absorbing heat amount of new sludge into the fluidized bed, decreasing the bed temperature. It was recommended that the fluidized bed should be equipped with a secondary air system to supply extra air when burning the sewage sludge with high moisture content. (2) The increase of moisture content of sewage sludge will make SO2 react easily with H2O to H2SO3, reducing SO2 emission. The low-temperature erosion issue of H2SO3 should be considered for designing an industrial fluidized bed boiler of sewage sludge. (3) For sewage sludge with low moisture content, SO2 emission gradually increases with the increase of feed rate. However, NOX emissions present a little change, attributed to the cooperative effects of increasing combustible nitrogen elements into the furnace and strengthening the reductive atmosphere. (4) Heavy metals except Zn within studied sewage sludge were mostly transformed to the bottom ash and bag filter ash, which is helpful for processing heavy metals further.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-34205521. Fax: +86-21-34205521. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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