novel polymer technology for boiler deposit control

Transcription

1 Water Technologies & Solutions technical paper novel polymer technology for boiler deposit control Authors: Anthony Rossi and Richard Salazar, SUEZ Presented at the AWT conference of 2015 abstract Waterside scale deposits in steam boilers can have a dramatic impact on fuel efficiency, boiler structural integrity, and maintenance costs for the steam plant operation. In severe cases, waterside scale accumulations can cause boiler tube failures due to overheating, restriction of effective water circulation, as well as under deposit corrosion mechanisms. The primary measures to prevent these failures are effective make-up water pretreatment, internal chemistry control, and effective chemical treatment practices. Regarding the most recent advances in chemical boiler treatment, SUEZ has recently introduced a novel terpolymer chemistry control of waterside scale deposits in steam boilers in the low-to-intermediate pressure ranges up to 900 psig (approximately 60 barg). impact of waterside scale on boiler tube integrity and efficiency Efficient transfer of heat through the boiler tube wall from the combustion to the water side is critical to the integrity and reliability of the tube metallurgy. The process of boiling water and generating steam at the heat transfer surface effectively removes the thermal energy transmitted through the steel tube wall from the combustion process. Steam formation cools and maintains a tolerable temperature profile through the carbon steel tube wall, which is essential to the structural integrity of the boiler tube metallurgy. The left side of Diagram 1 below illustrates a typical and healthy boiler steam generating tube temperature profile through the carbon steel wall. The right side shows a condition where an insulating waterside scale layer is present on the heat transfer surface. Since the boiler operating pressure is fixed, the bulk boiler water temperature is fixed at the steam saturation temperature corresponding to the pressure. figure 1: impact of waterside scale on boiler tube temperature & overheating failures Even a thin layer of insulating scale on the heat transfer surface, as shown, can seriously impede heat transfer through the tube wall, causing elevation of the internal temperature. This is due to the very low thermal conductivity of common boiler scale forming mineral deposits see Diagram 2 and their ability to impede efficient heat transfer. Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. TP1211EN.docx Oct-15

2 figure 2: thermal conductivity of common boiler deposits versus boiler steel figure 3: 600 psig boiler overheating tube failure due to water side scale Common boiler scale deposits, such as the broad categories listed in Diagram 2, occur at the highest temperature surfaces in the boiler, which are the steaming (or evaporative) heat transfer surfaces in the radiant zone. Carbon steel has a very well-defined, crystalline grain structure consisting of almost pure iron grains (referred to as ferrite grains) interspersed with carbon-reinforced iron grains ( pearlite ). It is this unique ferrite-pearlite grain structure that gives carbon steel a much higher tensile strength than pure iron without the carbon component. Long-term elevation of the temperature of the carbon steel tube above approximately 850F begins to degrade the carbon steel grain structure. If such high-temperature conditions persist, destruction of the normal grain structure can reduce the tensile strength of the tube, and its ability to resist boiler pressure. As a result, the boiler tube undergoes deformation (expansion in diameter) in the affected area, and is subject to rupture. This failure mechanism is known to boiler engineers and metallurgists as long-term overheating. Waterside scale deposits are the leading cause of this common failure mechanism. Long-term overheating is consistently among the leading causes of boiler tube failure causing unscheduled outages. An example of a water tube failure due to a hardnessbased waterside deposit from the SUEZ metallurgical laboratory located in The Woodlands, TX is illustrated in Figure 3. In this case, the deposit was composed primarily of calcium phosphate (hydroxyapatite), and the measured deposit weight density on the waterside surfaces exceeded 190 g/ft 2, a very heavy deposit. In addition to long-term overheating, other boiler tube failures can result from excessive waterside scale formation. Under-deposit corrosion (UDC) is one such mechanism, and can be highly destructive in that it is localized and very difficult or impossible to detect via visual inspection methods. Steam generation still occurs under a boiler deposit. A porous or permeable deposit will allow concentration of non-volatile dissolved solids under the deposit, frequently at concentrations far in excess of their bulk boiler water concentrations. Caustic gouging is a localized form of under-deposit corrosion frequently observed in boilers receiving high-purity make-up, such as demineralized or high-quality reverse osmosis pre-treatment. Acidic contaminants, such as chlorides and organic or inorganic acids arising from process contamination can also cause localized UDC failures. In Figure 4, a failure from a 400 psig water tube boiler receiving RO-quality make-up is detailed. In this case, a heavy, iron-dominated deposit formed on the side of the tube facing the hot combustion gases termed the hot side. In this case, both overheating damage and under deposit caustic corrosion contributed to the tube failure. However, the root cause was the waterside iron oxide deposit. Page 2 TP1211EN.docx

3 figure 4: water wall tube from a 400 psig boiler receiving RO make-up example of metal oxide deposit-induced overheating & under deposit corrosion Excessive scale deposits, particularly brittle forms of deposit, can also fracture and dislodge from tube surfaces, often referred to as chip scale. Large accumulations of chip scale, or large pieces of scale can block or occlude tubes, creating restrictions that result in loss of flow to steam generating tubes. As mentioned, without water flow and steam formation to remove heat, the carbon steel can rapidly overheat and rupture. This mechanism is sometimes referred to a short-term overheating or a thin-lipped burst to distinguish it from long-term overheating. A case of generating tube failure from a 700 psig water tube boiler due to blockage of flow by iron oxide deposit accumulations is illustrated in Figure 5. versus larger, field-erected industrial and utility boilers. The latter commonly have heat absorption circuits downstream of the boiler bank or waterwalls in the combustion path - superheaters and reheaters, feedwater heaters and economizer circuits, which may not be present in package units. These circuits can often absorb the heat which is rejected by the boiler due to the insulating effects of excessive waterside scale. The chart in Figure 6 is from a US Department of Energy bulletin that summarizes the results of a university study detailing the measured fuel energy loss due to several types of waterside scale deposits. In package boiler designs without extensive post-boiler heat recovery circuits, increases in stack gas exit temperatures are normally the most reliable indicator of degraded fuel-to-steam efficiency due to scale formation. figure 6: package boiler efficiency losses due to waterside scale figure 5: 700 psig boiler steam generating bank tube blockage with iron oxide deposit danger of tube failyre due to restricted circulation and short-term overheating rupture impact of waterside deposits on boiler efficiency Excessive waterside scale can also negatively impact boiler fuel-to-steam efficiency. Smaller package watertube and firetube designs are particularly vulnerable due to their limited heating surface area boiler deposit control technology development Acrylic based polymers for boiler treatment have been effective under most conditions, but some have failed under high stress conditions such as high contaminant levels, high pressures, and high heat transfer surface temperatures. The new SUEZ Boiler Terpolymer (BTP) is an evolution of SUEZ boiler organic synthesis. The Terpolymer provides advantages in performance of boilers with pressures up to 900 psig, improving iron oxide scale control, as well as improved hardness and iron contamination transport. Importantly, BTP has also demonstrated the ability to effectively recover from hardness contamination upsets. Extensive testing of BTP against commonly used boiler treatment chemicals such as TP1211EN.docx Page 3

4 polymethacralate and acrylic acid was conducted on research boilers. The testing of BTP chemistry in the research boilers was conducted with relative steam flow, residence times, and heat fluxes equivalent to field boilers. This experimental procedure has developed many water treatment programs. Utilizing research boilers provided the opportunity to rapidly simulate a wide variety of pressures and contaminant levels. Feedwater contaminant was varied by pressure with 150 psig (10 barg) tests utilizing 8 ppm Ca (as CaCO 3 ), 2 ppm Mg (as CaCO 3 ) and 1 ppm iron. Between 300 and 600 psig (21 and 42 barg), 4 ppm Ca (as CaCO 3 ), 1 ppm Mg (as CaCO 3 ) and 1 ppm iron was used. Feed water silica was fixed at 6 ppm (as SiO 2 ) for all pressures. Contaminate transport efficiency was measured by comparing the level of a specific contaminant. In this case we evaluated calcium, magnesium, iron and silica, in the feedwater during a run to the level in the research boiler blowdown, while considering the degree of dissolved solids concentration due to cycles of concentration in the boiler. This can be expressed by the equation below: Throughout the testing, BTP demonstrated the ability to control deposit formation on heat transfer surfaces equal or better than current industry standards at equivalent polymer dosages or below. Relative to standard All-Polymer dispersants, BTP was able to maintain comparable calcium transport with significant improvements to magnesium and iron transport. Figure 7 illustrates the ion transport efficiency for various contaminants and chemistries. Based on transport efficiency for hardness, silica and iron, the Acrylic Acid/Sulfonate/Sulfonated ethoxylate (AA/S/EOS) boiler terpolymer showed significant improvements compared to commonly used benchmark chemistries. figure 8: Ion Transport Comparison at 300 psig (21 bar) field performance Research testing results were duplicated and validated with field trials. A Northeastern US college was operating a simple cycle heat recovery steam generator producing 160 psig steam from the exhaust gas from a combustion turbine. This unit received a combination of softened make-up and zeolite-polished condensate as feedwater. During the period of late 2012 into 2014, the main condensate return line from campus was intermittently out-of-service for excavation and repairs. Hardness levels in the feedwater during this period were elevated and highly variable, averaging approximately 3 ppm (as CaCO3). There were frequent excursions between 5 and 30 ppm (as CaCO3). The intermittent and highly variable pattern of hardness contamination resulted in hardness scale formation in the steam generator. Prior to the introduction of BTP, the previous chemical treatment was an earlier generation All- Polymer that provided good results when the hardness contaminant loading was lower. However, with the poor quality of condensate return with high hardness levels in the feedwater, there was significant progressive build-up of hardness scale. Upon inspection of the HRSG in the spring of 2013, scale consisting of calcium, magnesium, silica, and iron were present throughout the boiler. Figure 8 below shows the condition of the boiler during the inspection. Page 4 TP1211EN.docx

5 figure 8: 2013 inspection of 160 psig HRSG figure 9: BTP performance under varying feedrates Based on research testing, the application of the BTP all-polymer program would allow the flexibility to continue to use the higher hardness make-up water while protecting the internal heat transfer surfaces from deposition and scale, while minimizing the amount of contaminants building up in the system. The SUEZ Boiler Terpolymer (BTP) was introduced in early spring 2013 a few weeks prior to the inspection. During the initial trial of BTP, we gradually increased the feedrate of the BTP. This temporarily resulted in a significant underfeed of the treatment during the initial phase. As shown in Figure 9, during the initial startup of the trial, BTP was fed at less than 10% of feedwater demand due to the intermittent hardness excursions. During this period, transport levels were maintained in the 40 to 50% range for hardness, iron, and silica. Once the treatment dosage was reset based on feedwater demand, the contamination rejection rates increased dramatically. As shown in Figure 9, the calcium rejection measured had been maintained for more than 6 weeks above 250%. With continued hardness excursions and the proper BTP chemical treatment dosage based on feedwater demand, the inspection the following year showed great improvement. Figure 10 shown below was taken in May This photo is from the same HRSG inspection as Figure 8, and shows the tubes and drum are visibly cleaner with much less deposition than in figure 10: 2014 inspection of 160 psig HRSG A second field evaluation was performed in a D-type watertube package boiler with a rated capacity of 28,000 pounds of steam per hour, operating at approximately 170 psig and receiving deaerated feedwater comprised of a combination of 20% softened municipal make-up water and 80% returned condensate. TP1211EN.docx Page 5

6 During the summer of 2014, this boiler was treated with SUEZ Boiler Terpolymer (BTP) technology. An upset in the condensate returned to the feedwater tank caused a temporary excessive amount of hardness in the condensate and feedwater. The hardness levels for the condensate and feedwater ranged from 5 to 100 ppm (as CaCO3) from 8/10/14 thru 9/20/14 shown in figure 11. figure 13: 2014 inspection of steam drum figure 11: hardness in feedwater of package boiler The last field trial of the terpolymer was undertaken to evaluate performance in a system with high levels of hardness contaminant, where effective deposit was critical and challenging. This metals refining facility operates a series of large firetube boilers at 90 psig, producing approximately one million pounds of steam per day for the refining process. Inspection photos from Fall 2013 and Fall 2014 are shown in Figures 12 and 13, respectively. The distinguishing feature of the inspection photos show that despite the elevated hardness in condensate and feedwater, the steam drum surface in the 2014 inspection was essentially clean except for some slight trace deposits. The feedwater consists of an average of 30% deaerated, softened make-up and 70% returned condensate. Due to the process, the condensate is consistently contaminated with high levels of hardness, which averaged slightly above 5 ppm (as CaCO 3 ) during the trial period, as shown in Figure 14. figure 14: northeast metal refiner boiler feedwater contaminant levels averages for Oct Sep figure 12: 2013 inspection of steam drum Page 6 TP1211EN.docx

7 The incumbent treatment program consisted of an All- Polymer treatment based on a synthetic carboxylated polymer, and for additional deposit control, was augmented with a chelant/polymer/phosphate program. As shown in Figures 15 and 16, although there was significant waterside deposition, this previous program maintained the boilers in operational condition despite the high and continuous influx of hardness. From this data, time-averaged levels of transport of these contaminants through the boiler to the blowdown were calculated. Data was collected over a period of almost two years for both programs, and the summary is presented in Figure 17. figure 17: northeast metal refiner average contaminant transport levels feedwater to boiler blowdown figure 15: firetube boiler 4 May 2013 prior to Solus AP program During the Solus AP (BTP) trial period (without the supplemental chelant/polymer/phosphate product), the new terpolymer provided significantly improved transport of magnesium and silica, and slightly enhanced transport of iron and calcium. figure 16: boiler 4 May 2013 prior to Solus AP program The Solus* AP All-Polymer was initiated following the May 2013 outage and inspection. In order to track and compare performance to the previous program, feedwater and boiler water samples were collected on both programs on a regular basis for analysis at the SUEZ customer service laboratory in the Woodlands, TX. Hardness, iron and silica levels were tracked, along with average boiler cycles of concentration. Two photos of the waterside surfaces of Boiler 4 taken during the May 2014 inspection, 12 months after start of the Solus AP program, are shown in Figures 18 and 19. This is the same boiler shown in Figures 15 and 16 from the May 2013 prior to the Solus AP program implementation. Due to the high level of incoming hardness, accurately judging boiler cleanliness after approximately one year on the new program is somewhat subjective. There was, however, visible evidence of reduced overall deposition. As illustrated in Figure 19, there was consistent evidence of partial removal of the layered scale accumulations from heavily encrusted lower firetube passes. The remaining scale observed in the lower pass tubes was relatively soft and nonadherent, and much of it could be removed easily by hand. TP1211EN.docx Page 7

8 figure 18: firetube boiler 4 May months after start of Solus AP program figure 19: firetube boiler 4 May months after start of Solus AP program summary The results of field trials of the patented SUEZ Boiler Terpolymer (BTP) program have been very successful, and correlated well with the results of the research-scale laboratory boiler evaluations. The new All-Polymer program provided not only improved control of iron oxide and hardness deposition versus the benchmark programs against which its performance was compared, but importantly, improved tolerance to very high stress conditions, including elevated hardness levels for extended periods. Solus AP (BTP) met both the performance and economic expectations of the customers, and is now in continuous use at each of the trial sites discussed. * Solus is a trademark of SUEZ. May be registered in one or more countries. references 1. D. Meskers, A. Rossi, K. Person Evaluation of Terpolymer for Deposit Control in Hardness and Iron Dominated Boiler Systems 2. D. Meskers, A. Rossi, K. Person Solus* AP Advanced Boiler All-Polymer Technology 3. D. Meskers, et. al., U.S. Patent 8,728,324 Page 8 TP1211EN.docx