Concentration control of silica in water chemical regime for natural circulation high pressure drum boiler unit of thermal power station

Transcription

1 Indian Journal of Chemical Technology Vol. 14, March 2007, pp Concentration control of silica in water chemical regime for natural circulation high pressure drum boiler unit of thermal power station M Azad Sohail a * & A Ismail Mustafa b a Central Chemical Research Laboratory, Ghorasal Thermal Power Station, Palash, Narsingdi 1610, BPDB, Bangladesh b Department of Applied Chemistry and Chemical Technology University of Dhaka, Dhaka 1000, Bangladesh masohail2004@yahoo.com Received 24 January 2006; received revised 26 July 2006; accepted 10 January 2007 The effective relations of silica (SiO 2 ) concentration in boiler drum and generated steam, with respect to ph and pressure for a natural circulation water wall tubes (WWTs) high-pressure (158 and 100 kg f cm -2 ) boiler, (Type TGME-206-COB, Russia) at Ghorasal Thermal Power Station (GTPS) Bangladesh, has been studied. Extensive analyses of different parameters of boiler water chemical regime (WCR) of the plant based experimental results revealed that on maintaining SiO (± 0.15) mgl -1 in boiler drum clean (evaporation) section [at ph 9.1 (± 0.1), temperature 330 C (± 10 C)] of boiler WCR provides excellent results with less corrosion or scale formation in interior surface of boiler WWTs and turbine blades. Such a procedure is effective for high softening of make up water SiO (± 0.01 mgl -1 ) ensuring high degree of purity of feed water cycle of the boiler and appropriate dosing of chemicals in boiler. Keywords: Boiler drum, Silica, Corrosion IPC Codes: C08K3/36, B01B1/00, B01J21/08 Silica is one of the major scale forming elements in boiler water chemical regime (WCR) of thermal power industry. Carryover of silica is also another serious problem, particularly in high pressure and critical pressure boilers, Hence, generally the medium-pressure, high-pressure and critical-pressure boilers are equipped with super heaters and turbine. Silica in steam can result from mechanical carryover of boiler and vapourization of silica 1. The solubility of the silica in the steam is higher than other salts and metal oxides. When steam-containing silica is injected to the turbine blades, it causes reduction of turbine efficiency by deposition on the turbine blades, nozzles etc. and take part in corrosion. It is fact, the colloidal silica is not removed in the demineralization stages but in the boiler, it is converted into ionized form 2 and deposits on the boiler WWTs. The most effective solution of such silica problem is its prevention by accurate control of SiO 2 in boiler WCR. Leakage of raw water into the turbine condensate is another reason for SiO 2 accumulation in boiler. In a view to ascertain the accurate parameters of silica (ions as SiO 2 ) in feed water, generated steam and the drum of boiler unit, the present work was carefully carried out in fossil based (natural gas) boiler unit No. 3 (158 kg f cm -2, 210 MW) and unit No. 2 (100 kg f cm -2, 55 MW) of GTPS, Bangladesh. Experimental Procedure All the chemicals used for analyses were of CP grade (99.5%) procured form BDH (England) and E. Merck (Germany). The trisodium phosphate (Na 3 PO 4 ) and hydrazine (N 2 H 4.H 2 O) used as dosing reagent in WCR of boiler units were made in Germany (99.2% and 32% respectively). Before analysis of phosphate ions in boiler water, the sample water was filtered, because the hydroxyapatite present in water may interfere in the analysis. Concentration of N 2 H 4, NH 3, and SiO 2 in boiler WCR were analyzed very carefully. The analyses were carried out by using an AAS, Model AA6650, Shimadzu (Japan), photoelectric colorimeter, Model KFK-2, (Russia) and Electroconductivity meter, digital, Model-Bibby, (England). ph and ionic strengths were measured with a ph and ion meter, Digital Model , (Russia). Oven, Model N-08-76, (Russia) and furnace, Model Mn-2ym (Russia) were also used.

2 196 INDIAN J. CHEM. TECHNOL., MARCH 2007 Table 1 Supplied (Russian) parameters for the natural circulation high pressure (158 kg f. cm -2 ) drum boiler unit TGME-206-COB (Russia) of Ghorasal Power Station, Bangladesh Parameters ph E.C.* Alk. p/m Hd. SiO 2 N 2 H 4 NH 3 Cl - Cu Fe DO Na P DemiWater Feedwater 9.1± Condensate 9.1± Boilerdrum (clean) Salt section (blow down) upto 40 Steam 9.1±0.1 - EC*-μ Scm -1, Other units-mgl -1 00/ / / Trace upto Measurement of deposits The quantity of deposits obtained in boiler WWTs were determined 3 by rubbing out deposits from a definite surface area (3 3 cm 2 ) of WWTs followed by its weighing and is expressed in gm -2. Composition of deposits (as oxide) was determined by gravimetric 4, titrimetric, atomic absorption spectrometric and colourimetric 4-6 analyses. For each case, multiple analyses were made and their average value is presented in the tables. The concentration of phosphate, N 2 H 4, NH 3, and SiO 2 were measured by colourimetric analyses 4. Results and Discussion The effects of SiO 2 in WCR of a natural circulation high-pressure drum boiler units has been studied. Ghorasal Power Station is one of the largest thermal power station in Bangladesh. The plant consists of six boiler units of total electricity generating capacity of 950 MW (4 210 MW MW). The boiler (TGME-206-COB, Russia) of unit No. 3, (210 MW) GTPS is being operated for the last eight years ( ) maintaining the parameter in WCR shown in Table 1. During overhauling (1994) of the unit, inspection revealed deposits and scale on the boiler WWTs (Table 2). The average value of deposit attained on the fireside WWTs was 900 gm -2. This value is found to be too high in comparison with the Russian 4 and Japanese 8 cleaning limits of 400 gm -2 and 500 gm -2 respectively. The qualitative analytical 9 results of deposit expose that it contains highest amount of Fe 2 O 3 (46.6%) Table 2 Quantitative range of deposits (gm -2 ) in boiler WWTs after eight years operation of unit No. 3 (maintaining as Table 1) Location Insulation side Fire side Back wall Front wall Right wall Left wall followed by SiO 2 (19.4%), P 2 O 5 (19.0%) and CuO (5.6%) respectively (Table 3). The higher amount of SiO 2, P 2 O 5 and Fe 2 O 3 in deposit on WWTs might be due to excessive dose of phosphate compound (Na 3 PO 4 ) as well as administration of higher range of SiO 2 in boiler WCR. The acceptable limit of silica for high-pressure boiler units at GTP plant is 0.04 mgl -1 in feed water and mgl -1 in the boiler drum clean (evaporation) section. These appear to be too high in comparison to Japanese 8 practice (for 130 kg f cm -2 boiler; mgl -1 and 0.3 mgl -1 respectively). The most important parameter is the concentration of silica in boiler drum evaporation section with respect to concentration of silica in the steam is shown graphically in Fig. 1, for the investigated mentioned boiler unit No. 3 and boiler unit No. 2 respectively. Thus, it is clear that on increasing the silica concentration in boiler drum (evaporation section), increases the carryover of SiO 2 in steam. In practice phosphates do not prevent silica scale formation in WWTs but the caustic formed during dissociation does and thereby, increase the ph of the boiler water.

3 SOHAIL & MUSTAFA: CONCENTRATION CONTROL OF SILICA IN WATER 197 Table 3 Chemical composition (%) of deposits at different location of boiler after eight year operation of unit No. 3 (SiO 2 range in BDCS: mgl -1 ) Identity of tubes Fe 2 O 3 SiO 2 Al 2 O 3 CaO MgO P 2 O 5 CuO Front wall P*-4, T* Back wall P*-3, T* Front wall P*-4, T* Trace Back wall P*-4, T* Average composition (%) on mixing P*-Panel, T*-Tube, BDCS-Boiler drum clean section. Fig. 1 Concentration of SiO 2 in boiler drum versus concentration of SiO 2 carry over in steam Fig. 2 Relation among ph of boiler water (evaporation section), maximum allowable SiO 2 concentration in boiler water (evaporation section) and when SiO 2 concentration in the steam is (± 0.005) mgl -1 The ph of the boiler mainly is responsible for the solubility of silica in steam. The higher the allowable ph [(upto a certain limits 9.6 (±0.1)] in boiler drum clean section the less the solubility of the silica (±0.005) mgl -1 in steam (Fig. 2). But, at high ph phosphate deposition or corrosion may occur for higher limit of phosphate dose in boiler WCR. Silica scale is mostly responsible for bulging and bursting of WWTs and super heater tubes, because of its low 1 thermal conductivity Kcal.m -1.h. C. It is apparent from the Fig. 2 for such a type of high-pressure boiler (100 kg f cm -2 and 158 kg f cm -2 ) that the concentration of silica in boiler drum evaporation section should be minimized (SiO mgl 1 and SiO mgl 1 respectively) to attain SiO 2 in steam (±0.005) mgl -1. During investigation, ph of the boiler drum evaporation section (DES) was reduced to 9.1 (±0.1) by minimizing phosphate dosing [1.25 (± 0.25) mgl -1 PO 3-4 ] in boiler DES. The SiO 2 in the feed water allowed 0.02 (±0.1) mgl -1 and continuous blow down remains open partially while SiO 2 in steam attained (±0.005) mgl -1. The relations between boiler pressure with respect to concentration of SiO 2 in boiler drum is shown in Fig. 2. It was also observed that when boiler pressure reduces then the phosphate ions in boiler DES increases but ph reduces. During investigation it was also observed that there is a close relationship between ph and p-alkalinity of boiler drum (evaporation section) (Fig. 3); and p-alkalinity and m-alkalinity of boiler drum (evaporation section) (Fig. 4) respectively. During overhauling of unit No. 3 (1994) a little quantity of (10-30 gm -2 ) soft powdery substance (white to very light brown) with a thin layer of deposit was observed on turbine blades, but there was no corrosion (Table 4). The turbine (unit No. 3)

4 198 INDIAN J. CHEM. TECHNOL., MARCH 2007 Table 4 Quantitative range of deposits (gm -2 ) attained in different stages of turbine blade during overhauling (1994) of unit after eight years of operation (Parameters as Table 1) Unit No. Pressure boiler HP Cylinder MP kg f cm -2 Cylinder LP Cylinder * *Single stage turbine Table 5 Chemical composition (%) of deposits at different stages of turbine blade after eight years operation (Parameters as Table 1) of units Unit No. Turbine stage Fe 2 O 3 SiO 2 P 2 O 5 Al 2 O 3 CuO Fig. 3 Relation between ph and p-alkalinity. (CaCO 3 mg L -1 ) in boiler water Fig. 4 p-alkalinity (CaCO 3 mg L -1 ) verses m-alkalinity (CaCO 3 mg L -1 ) curve for boiler water deposit contains highest amount of SiO 2 (79.8% in LPC) followed by Fe 2 O 3 and CuO. This type of deposition on turbine blades is due to the carryover of SiO 2 as well as metallic oxides into the steam (Table 5). After overhauling of the depicted power plant boiler unit (No. 3), it was acid cleaned 9 (1995). Inspection revealed g of residual deposits remains per square meter of surface in WWTs after 3 HPC MPC LPC * HPC *Single stage turbine cleaning operation. The residual deposit contained only trace amount of silica (1.2%). The deposits on the turbine blades of unit No. 3 were also cleaned manually by mechanical techniques (in 1995). Considering the adverse effects of SiO 2 on boiler WWTs and turbine units of both the No. 3 and No. 2, a water treatment plant, has been installed (capacity 140 M 3 h -1, SiO 2 <0.015 mgl -1 ) in GTPS including newly added sophisticated mixed bed ion exchanger unit. The boiler unit No. 3 was then again operated for more than seven years maintaining SiO 2 level of 0.35 (± 0.15 mgl -1 ) in boiler drum evaporation section (ph 9.1 ± 0.1). While SiO 2 in feed water was 0.02 (± 0.01) mgl -1, keeping other chemical parameters unchanged except phosphate dosing 1.25 (± 0.25) mgl -1 in boiler drum evaporation section. Similar investigations were also carried out on boiler unit No.2. In 2003, the boiler unit No. 3 was farther shut down for inspection and deposit analyses were performed very carefully. The analytical results of deposits attained on boiler WWTs (2003) show, it contains maximum amount of Fe 2 O 3 (88.4%) followed by SiO 2 (6.7%), P 2 O 5 (2.4%), CuO (1.05%) and Al 2 O 3 (0.52%) respectively. The overall results obtained were excellent. There was small amount of deposit (45-60 gm -2 ) in boiler WWTs and turbine blades were found very clean.

5 SOHAIL & MUSTAFA: CONCENTRATION CONTROL OF SILICA IN WATER 199 Conclusion The concentration of silica (SiO 2 ) in boiler feed water 0.02 (± 0.01) mgl -1 and boiler drum evaporation section 0.35 (± 0.15) mgl -1 for a natural circulation high pressure drum boiler ( kg f. cm -2 ), should be controlled strictly (at ph 9.1 ± 0.1) to maintain minimum silica level (SiO 2 <0.015 mgl -1 ) in steam. Carryover of SiO 2 in steam due to faulty operation should be avoided by maintaining accurate boiler drum level. Load should be increased gradually (stepwise) avoiding overloading, steam separator should be efficient, priming and foaming should be eliminated in low-pressure boiler. Periodical and continuous blow down should be controlled strictly to maintain minimum level of TDS in boiler water. Acknowledgement The authors are indebted to Bangladesh Power Development Board (BPDB) for providing scope to carry out this work at the Ghorasal Thermal Power Station, Bangladesh. The authors are also thankful to G. Zachinsky, Main Expert (Water treatment) Moscow Energy center, UL. N. Krasnoselskaya-6, Moscow, Russia, for his suggestion. References 1 Kurita Handbook of Water Treatment, Kurita Water Industries Ltd. Japan (1985) Schroeder C D, Solution to Boiler and Cooling Water Problems (Van Nostrand Reinhold, New York, USA), 1991, Sohail M A & Mustafa A I, Indian J Chem Technol, 8 (2001) Sediments at Thermal Power Facilities (Energia, Russia), 1967, 3. 5 Kudesia V P, Water Pollut, India (1980) Operational Manual, GTPS, BPDB, Bangladesh, Jeffery G H, Bassett J, Mendham J & Denney R C, Vogels Text Book of Qualitative Chemical Analysis, 5th Edn (London), 1989, Chemistry Control of Thermal Power Plants Training Materials, Thermal Power Development, TEPCO, JICA, Japan (1987) Sohail M A, Bulavco A & Kostikov S, J Bang Chem Soc, 10 (1) (1997) 1.