Measures against Incineration Problems Caused by Clogging of White Smoke Prevention Preheater M. Hayasaka Water Quality Section, Plant Management Division, Tokyo Metropolitan Sewerage Service Corporation, 11F Otemachi Nomura Bldg., 2-1-1, Otemachi, Chiyoda-ku, Tokyo, Japan (E-mail: masa-hayasaka@tgs-sw.co.jp) Abstract An incinerator that we operate and manage exhibited an abnormal increase in pressure and stopped working, which was a serious problem because it significantly affected the sludge treatment plan. When we disassembled the incinerator to inspect it, we found a large quantity of white solids attached to the interior of the white smoke prevention preheater. The pressure in the incinerator increased abnormally because these white solids clogged the exhaust-gas passage in the preheater. We analyzed the white solids and identified them as sodium carbonate and sodium sulfate. The sodium hydroxide solution that was sprayed on the white smoke prevention preheater reacted with the exhaust-gas components and formed the solids. Based on this cause, we took three countermeasures to prevent the formation and deposition of the solids. These countermeasures prevented the deposition of white solids and clogging of the preheater. As a result, continuous stable treatment of sludge was possible. Keywords Incinerator; White smoke prevention preheater; Sodium hydroxide; Sodium carbonate; Sodium sulfate INTRODUCTION & BACKGROUND The sewerage business in Tokyo started with the Kanda sewerage in 1884. Although the sewerage coverage ratio in the Tokyo wards was only 15.6% in 1955, the sewerage system rapidly expanded since the high economic growth period and the coverage ratio reached almost 100% in the area in 1995. The Bureau of Sewerage Tokyo Metropolitan Government (TMG) maintains the living environment through sewage treatment, takes countermeasures against flooding by removing rainwater, maintains the water quality of public water bodies, and thus supports the lives of citizens. TMG, however, faces many challenges including the increasing number of aging sewer facilities, threats of natural disaster (earthquake and local torrential rain) and environmental impact of consuming large amounts of energy. Therefore, Tokyo Metropolitan Sewerage Service Corporation (TMSSC) was founded in 1984 as a partner company that assists and acts on behalf of the TMG. Due to the large amount of work in line with the rapid expansion of sewerage, TMSSC helps TMG to effectively perform sewerage operation without lowering the quality of sewerage service. TMSSC performs the maintenance of sewers and manages the operation and maintenance of sewage and sludge treatment facilities. In addition, based on its extensive experience and advanced technology, TMSSC is widely promoting business together with TMG. Currently in the Tokyo wards area, some 4.48 million m 3 of sewage is treated per day at thirteen
water reclamation centers. The volume of sludge produced from sewage treatment is about 160,000 m 3 per day according to the data for 2015. In the densely populated megacity of Tokyo, it is essential to reduce the sludge quantity and stably treat the sludge because it is difficult to find space to build new landfills for treatment facilities. Accordingly, TMG thickens the produced sludge, dewaters it and finally burns the entire amount. TMSSC is commissioned by TMG to operate and maintain the sludge treatment facilities. This time, in an incinerator operated and maintained by TMSSC, the inner pressure increased abnormally and the incinerator stopped working. This stoppage forced the operator to change the sludge treatment plan and limit the quantity of sludge produced by sewage treatment, which might have significantly impacted the entire sewage treatment process. Therefore, it was essential to identify the cause and take countermeasures as quickly as possible. Accordingly, we disassembled the incinerator to inspect it and found a large quantity of while solids attached to the section between the top and bottom parts in the white smoke prevention preheater. The pressure in the incinerator increased because the white solids clogged the passage for the incineration exhaust gas in the preheater. The sodium hydroxide was sprayed from near the inlet of the preheater when the problem occurred, causing the formation of white solids. However, the formation mechanism was unknown. Therefore, we started our study to clarify the mechanism of solids formation and take countermeasures. OUTLINE OF THE SLUDGE TREATMENT FACILITY This is a fluidized bed incinerator that burns dewatered sludge at about 850 C. The incinerator exhaust gas passes through a dust collector and a treatment tower before being discharged into the atmosphere. A preheater is installed in the former stage of the exhaust gas treatment tower to prevent the generation of white smoke in the exhaust gas discharged from the stack. The white smoke prevention preheater heats the air by the high-temperature exhaust gas through a heat exchanger and injects the heated air directly into the stack. This raises the temperature of the exhaust gas in the stack, which restricts the formation of condensation caused by the steam and prevents the generation of white smoke. Since the interior pressure in the incinerator abnormally increased in August 2015, we stopped the sludge treatment operation and disassembled the incinerator to inspect it. We found a large quantity of white solids attached to the section between the top and bottom parts of the white smoke prevention preheater as shown in Figure 1.When the white solids were formed, it was often found that sulfates, etc. were attached to the stack interior in the exhaust gas treatment tower. Therefore, in this incinerator, an alkaline agent (sodium hydroxide) was sprayed from near the inlet of the preheater in order to preliminarily remove and reduce the acid gases (HCI, SOx, etc.) in the exhaust gas, in the former stage of the exhaust gas treatment tower.
No. 1: Solids at the inlet of the white smoke prevention preheater No. 2: Solids at the top of the white smoke prevention preheater Figure 1: State of deposited solids No. 3: Solids at the bottom of the white smoke prevention preheater METHOD OF INVESTIGATION First, we analyzed the solids attached to the interior of white smoke prevention preheater to identify them. The sections where the samples were taken are shown in Figure 2 and the analysis items in Table 1. In Sample No. 3, we found a difference between the properties of the surface and the interior of the solids, so we analyzed the surface and the interior separately. In the dissolution test, 0.5 g of solids were mixed and stirred in 50 ml of distilled water, and the eluate obtained by filtration was used as analysis samples. Second, we assumed the cause of solids formation based on the analysis results. Lastly, we determined the countermeasures to prevent a recurrence of the problem based on the assumed cause. Inlet of white smoke prevention preheater (No. 1) Stack Top of white smoke prevention preheater (No. 2) Bottom of white smoke prevention preheater (No. 3) White smoke prevention fan White smoke prevention preheater Flue gas treatment tower : Investigation points Figure 2: Locations where samples were
Analysis items Table 1: Details of analysis Instruments used Element analysis O, S, Na, C etc. Energy dispersive X-ray analyzer Dissolution test ph, Na +, NH 4+, SO 4 2- ph meter, ion chromatography analyzer Differential thermal analysis Thermal properties such as melting point Differential thermal analyzer (TG-DTA) Microscopic observation Magnification (1,000 fold) Scanning electron microscope INVESTIGATION RESULTS The analysis results are shown in Table 2 and Figure 3. Solids at the inlet of the white smoke prevention preheater (No. 1) The main components of Solids No. 1 are oxygen, sodium and carbon. According to the analysis results, the weight ratio of the components was C:O:Na = 1:4:3.4. This is almost identical to the composition ratio of the elements in sodium carbonate (Na 2 CO 3 ) (weight ratio C:O:Na = 1:4:3.3). Accordingly, we concluded that Solids No. 1 was sodium carbonate, sodium carbonate hydrate or sodium hydrogen carbonate attached to the stack interior. Meanwhile, the samples were not deliquescent and showed strong alkalinity (ph 11) in the dissolution test. Therefore, the possibility of Solids No. 1 being sodium hydrogen carbonate was very low (when sodium hydrogen carbonate is dissolved, ph becomes weakly alkaline because hydrogen ions are generated). In addition, the solids formed in the section where the ambient temperature was as high as 600 C, and so the possibility of the solids being sodium carbonate hydrate was very low (in hydrate, the hydrated part desorbs at around 100 C). Subsequently, when Solids No. 1 were heated up to 1,000 C (20 C/min) by a differential thermal analyzer (TG-DTA), the thermal characteristics such as melting point were identical to those of a pure reagent of sodium carbonate. Accordingly, almost all of Solids No. 1 was likely to be sodium carbonate. Solids on the top part of the white smoke prevention preheater (No. 2) Main components of Solids No. 2 were oxygen, sodium, carbon and sulfur. For the element structure, the color tone and components were almost the same as those of Solids No. 1, except that Solids No. 2 contained sulfur. In the dissolution test, sulfate ions were detected, but the sodium ion concentration was very high and the ph was strongly alkaline. Accordingly, the impact of sodium sulfate, the ph of which was neutral in the water solution, was judged to be small. Meanwhile, in the component analysis, a large amount of carbon and oxygen was detected as well as sulfur in Solids No. 2. In addition, in the microscopic observation of Solids No. 2, material of the same shape as that of Solids No. 1 was detected and the color tone was also the same. Therefore, the main component of Solids No. 2 was judged to be sodium carbonate, the same as Solids No. 1. Accordingly, the main component of Solids No. 2 was sodium carbonate, and it also contained a small amount of sodium sulfate.
Solids on the bottom part of the white smoke prevention preheater (No. 3-1 and 3-2) In the outer appearance of Solids No. 3, there was a difference between the surface (No. 3-1: about 1 mm thick) and the interior (No. 3-2). The surface color was ivory while the interior color was white. The main components of both were oxygen, sodium, sulfur and carbon. However, compared to the interior of the solids, the surface contained three times as much sulfur and about half as much carbon. In the microscopic observation, polyhedral crystal-like particles were detected on the surface (Fig. 3) while spherical particles were detected in the interior. Thus, there was a clear difference in shape between the two parts. In the interior, the results of components analysis, microscopic observation, color tone and differential thermal analysis (TG-DTA) were identical to the results for the top part of the white smoke prevention preheater (No. 2). Accordingly, the main component of the interior (No. 3-2) was judged to be sodium carbonate, the same as Solids No. 2. Based on the results of the component analysis and differential thermal analysis, the component of the surface (No. 3-1) was judged to be a mixture of sodium carbonate and sodium sulfate. However, the surface (No. 3-1) contained more sodium sulfate compared to Solids No. 1. Based on all the analysis results, Solids No.3 were considered to be a mixture of sodium carbonate, which is the main component, and sodium sulfate. In addition, it was found that sodium sulfate existed in a large quantity in the surface part of the solids. Analysis items Table 2: Results of analysis Solids in white smoke prevention preheater Unit Inlet (No. 1) Top part (No. 2) Surface (No. 3-1) Bottom part Interior (No. 3-2) Color White White Ivory White Element analysis Dissolution test O 47.7 47.7 46.4 48.5 S 0.0 6.5 18.4 5.6 Na 40.2 35.7 30.0 35.3 Wt.% C 11.9 9.4 4.6 9.6 Others 0.2 0.7 0.6 1.0 Total 100 100 100 100 ph 11.4 11.4 11.2 Na + 41.2 38.7 35.8 NH 4 + Wt.% Not detected Not detected Not detected SO 4 2- * 0.5 ( 0.2) 14.6 (4.8) 40.1 (13.4) *Values in parentheses represent the sulfur content in sulfate ions.
Surface 20 μm Interior Inlet (No. 1) Bottom part (No. 3) Bottom part (No. 3) Cross-sectional area 20 μm Different shape Top part (No. 2) Bottom part Surface (No. 3-1) Bottom part Interior (No. 3-2) Figure3: Results of microscopic observation (Solids in white smoke prevention preheater) DISCUSSION Likely cause of solids formation Based on the operating conditions and the analysis results, we believe that the solids formed through the following mechanism. The concentrations of carbon dioxide (CO 2 ) and sulfur oxide (SOx) in the incineration exhaust gas are 9 vol. % and 1,000 ppm, respectively, which are the usual design values. Therefore, carbon dioxide existed at a high concentration, 90 times higher than that of sulfur oxide. The sodium hydroxide solution that was sprayed in the preheater reacted with the highly concentrated carbon dioxide and formed a large quantity of sodium carbonate. The sodium carbonate formed suddenly in a large quantity, and therefore was not transferred by the exhaust gas flow to the latter stage; instead it was deposited in the white smoke prevention preheater. Subsequently, the deposited sodium carbonate clogged the passage for the incineration exhaust gas (Solids No. 1 and 2). The surface of the deposited sodium carbonate was exposed to the exhaust gas and reacted with the sulfur oxide. As a result, the surface of Solids No. 2 gradually changed to sodium sulfate. Meanwhile, the sodium carbonate remained unchanged in the interior of Solids No. 3 because the interior did not contact the exhaust gas. Accordingly, the properties of Solids No. 3 were different between the surface and the interior.
Countermeasures to Prevent the Deposition of Solids We took three countermeasures to prevent the formation and deposition of solids based on the cause of solids formation. Through these countermeasures, the deposition of solids and clogging of the white smoke prevention preheater were prevented, and continuous stable treatment of sludge was possible. Change of spray position and method of sodium hydroxide. Conventionally, sodium hydroxide was sprayed at the inlet of the white smoke prevention preheater. However, heat-transfer pipes are closely arranged in the middle stage of the preheater, making it difficult to remove the deposited solids. Therefore, the spray position was changed to the stack located at the bottom of the white smoke prevention preheater where solids are much less likely to deposit, thus preventing an abnormal increase of pressure in the incinerator caused by the clogging of the exhaust gas passage. In addition, the installation position of the spray nozzle was moved to the center of the stack s cross-section area to improve the contact efficiency between the sprayed sodium hydroxide and the exhaust gas. Change of sodium hydroxide concentration. At the time when the solids had formed, the concentration of sprayed sodium hydroxide was controlled at 48% during operation. However, the highly concentrated sodium hydroxide used in the spray reacted quickly with the carbon dioxide in the exhaust gas, facilitating the formation of sodium carbonate solids. Therefore, there was a potential risk of clogging the spray nozzle piping. Accordingly, we investigated different settings for the concentration of sodium hydroxide to prevent the pressure increase in the incinerator caused by sodium carbonate and to reduce the sulfate generated in the stack of the exhaust gas treatment tower at the same time. As a result, it was found that the best concentration of sprayed sodium hydroxide was 2% to 4%. Accordingly, we could simultaneously reduce the amount of formed sodium carbonate solids (white smoke prevention preheater) and that of sulfate (stack of the exhaust gas treatment tower) by changing the setting concentration. Modification of nozzle. The conventional spray nozzle could only spray sodium hydroxide. Therefore, if the nozzle piping is clogged, sodium hydroxide could not be sprayed, reducing the removal efficiency of acid gas. Accordingly, we added air purge and water-washing functions to the spray nozzle. As a result, we could prevent clogging of the nozzle caused by the solids and stably spray the sodium hydroxide. In addition, we installed a pressure gauge on the spray nozzle to constantly monitor whether the sodium hydroxide was properly sprayed.
CONCLUSIONS We investigated the incineration problems caused by clogging of white smoke prevention preheater and obtained the following results. The formed substances were sodium carbonate and sodium sulfate. The solids were formed as follows. The sodium hydroxide sprayed at the inlet of the white smoke prevention preheater reacted with the carbon dioxide in the exhaust gas to form sodium carbonate. The formed sodium carbonate was deposited gradually from the top part of the preheater. Meanwhile, at the bottom part of the preheater, the deposited sodium carbonate reacted with the exhaust gas containing sulfur oxide and changed to sodium sulfate on the surface part only. To prevent the formation of solids, the position of the sodium hydroxide spray was changed from the top part to the bottom part of the white smoke prevention preheater and the spray conditions were changed. As a result, the formation of solids was prevented and the incinerator stably operated. REFERENCES The Chemical Society of Japan, 2013, Chemical handbook Basic course Revise 5th Tokyo, Maruzen publishing, co. Page I-262, 266 (ISBN 978-4-621-07341-4 C 3543)