Considerations for High Pressure Boiler Chemical Treatment – Equipment and Chemistry
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Considerations for High Pressure Boiler Chemical Treatment – Equipment and Chemistry As the ammonia plant design envelope is continually pressed for larger and more efficient plants, the boiler and steam systems become even more critical. In the middle 1960s, the then new single train ammonia units came on-line with 1500 psig (104 bar) steam systems. At that time, experienced ammonia producers said it was like “Operating a power plant with ammonia as the by-product.” Today, such a comparison is still appropriate as heat fluxes, steam pressures, and design production rates have increased. The plant designers and operators have choices when it comes to both the boiler internal treatment and the feed equipment for proper dosing. This paper describes the various systems available and the strengths and weaknesses of each. Daniel M. Setaro GE Water & Process Technologies Chemical Treatment Considerations Thermal Power Systems has published Consensus on Operating Practices for the Control For purposes of this paper, the targeted operating of Feedwater and Boiler Water Chemistry in boiler pressure range is 1500 to 2000 psig (104 to Modern Industrial Boilers.1 Many refer to these 140 bar) as this is the typical HP steam range practices as the ASME guidelines. The found in modern ammonia plants. In this recommended feedwater quality for boiler pressure range, the recommended feedwater systems in the pressure range of concern is quality is quite pure so as to assure the highest shown in Table 1. levels of steam purity. For further conciseness, this paper will address phosphate based internal Table 1: ASME Feedwater Consensus Guidelines for treatment chemistries such as the coordinated 1501 to 2000 psig (104 to 140 bar) systems BOILER FEEDWATER ASME CONSENSUS phosphate and congruent phosphate programs PARAMETER commonly employed in high-pressure industrial Dissolved oxygen ppm < 0.007 systems. In special cases all volatile treatment (mg/l), as O2, measured at programs are employed where there are serious point prior to addition of concerns for phosphate hideout problems. Much oxygen scavenger more stringent demands are placed on boiler Total iron ppm (mg/l), as <0.010 Fe feedwater quality when AVT chemistry is Total copper ppm (mg/l), <0.010 utilized. as Cu Total hardness ppm (mg/l) ND [not detectable] Boiler Feedwater , as CaCO3 pH @ 25oC 8.8 – 9.6 The feedwater quality task group of the industrial Chemical for preboiler Use only volatile alkaline system protection materials upstream of subcommittee of the ASME Research and Technology Committee on Water and Steam in attemperation water source* 2006 279 AMMONIA TECHNICAL MANUAL *As a general rule, the requirements for attemperation scavengers were developed to be a replacement spray water quality are the same as those for steam purity. for hydrazine, as it has suspected carcinogenic In some cases boiler feedwater is suitable. In all cases the spray water should be obtained from a source that is free of properties. Since a detailed treatise of volatile deposit forming and corrosive chemicals such as sodium oxygen scavenging is beyond the scope of this hydroxide, sodium sulfite, sodium phosphate, iron, and paper, let’s leave it that small excess of copper. The suggested limits for spray water quality are: scavenger assures a reducing environment for the <30 ppb (ųg/l) TDS, < 10 ppb (ųg/l) Na, <20 ppb (ųg/l) formation of protective magnetite on the surfaces SiO2, and it should be essentially oxygen free. of the boiler and steam systems. High purity feedwater in all steel systems without dissolved As shown in earlier work presented at the oxygen present is passive and protective at American Power Conference, the release of iron elevated pH levels of 9.0 to 9.6. Minimizing the oxide from steel feedwater heaters is minimized amount of feedwater iron entering the steam as the feedwater pH is increased to the 9.3 to 9.6 drum is a key component of successful internal range.2 In a similar fashion, the feedwater in boiler water treatment. Even a very small route to the steam drum(s) in ammonia plants amount of transported iron oxide over time can should be at elevated pH levels to minimize cause problems in boiler systems. release of iron oxide from the various process coolers and convection section economizers. Data from an ammonia plant study indicated Internal Treatment Chemistry for similar reduction in iron oxide release from the Corrosion Control pre-boiler circuit as pH was increased. The data shown in Table 2 comes from a paper presented Phosphate – a general review at this conference at an earlier date.3 Most high-pressure industrial boilers with high Table 2: Iron release from BFW circuit as function of purity feedwater use a phosphate-based chemical pH treatment program for corrosion control. The Day BFW pH BFW Iron, evolution of phosphate-based boiler chemistries ppb (ųg/l), as Fe* followed improvements in feedwater quality. 15 8.4 40 Prior to the advent of demineralizers, sodium 16 8.5 35 phosphates were used to precipitate calcium to a 17 9.1 11 calcium hydroxyapatite that was a fluid sludge 18 9.1 9 removable by lower drum blowdown. When 19 9.0 <5 demineralized water systems came about, the 20 9.0 6 21 9.1 <5 need for hardness precipitation was replaced by a 22 9.0 <5 need for phosphate buffering. The buffering 23 9.0 <5 action of the phosphate treatment will minimize 24 9.0 6 the potential for acid and caustic based corrosion. *Data from ammonia plant study where deaerator outlet Keeping a clean heat transfer surface is key to iron was in 5 to 10 ppb range, as Fe minimizing under-deposit corrosion cells. Deposit minimization relies on both the Feedwater Oxygen Control feedwater treatment and polymeric dispersants. Besides causing boiler tube overheat failures, To assure operation under a reducing deposits play a role in corrosion. Formation of environment, volatile oxygen scavengers are concentration cells under and within deposits can added to the deaerated boiler feedwater. lead to corrosive conditions at the tube metal Commonly used chemical scavengers include surface. As such, the maintenance of clean carbohydrazide, hydrazine, and hydroquinone. boiler tube surfaces is the most important step Both carbohydrazide and hydroquinone based that one can take to minimize corrosive attack. AMMONIA TECHNICAL MANUAL 280 2006 Coordinated and Congruent Phosphate developed around solubility data. Research data7 showed that pure sodium phosphate solutions at The first use of phosphate chemistry programs 4/5 572oF (300oC) form precipitates at a 2.85 to 1 for corrosion control dates back to the early sodium-to-phosphate ratio. This led researchers8 1940s when Whirl and Purcell developed to propose an upper limit for maintaining coordinated phosphate/pH for control of caustic sodium-to-phosphate congruency. Further embrittlement. The basis of the chemistry is: definition of the chemistry9 found two invariant points, one at 2.85:1 and one at 2.15:1. These NaOH + Na2HPO4 Æ Na3PO4 + H2O boundaries became the upper and lower limits for the congruent control utilized by most industrial As shown above, feedwater caustic will react boiler systems today. with disodium phosphate in the boiler water forming trisodium phosphate. Even though congruent control can maintain pH during minor contaminant ingress and to a minor In a similar fashion feedwater acidic species will extent inhibit calcium deposition during hardness react with trisodium phosphate to form disodium intrusion, its primary purpose is to mitigate phosphate as follows: under-deposit corrosion. It performs this task by maintaining under-deposit sodium phosphate HCl + Na3PO4 Æ Na2HPO4 + NaCl solution chemistry between the congruency limits. By doing this, the concentrated solution By controlling the boiler chemistry to maintain under a boiler deposit does not result in either di-basic phosphate in the boiler (sodium to caustic gouging or acidic phosphate corrosion. phosphate molar ratio less than 3.0 to 1), caustic Figure 1 shows the relative corrosion of carbon (NaOH) cannot concentrate and cause corrosion. steel boiler tubing operating at 590oF (310oC) as This assures that all the sodium is associated a function of pH and concentration of corroding with phosphate and no free NaOH is in solution. species – HCl and NaOH. By maintaining the alkalinity as a captive phosphate alkalinity, under-deposit concentration For years congruent phosphate programs have of caustic is eliminated, thereby minimizing been applied with the assistance of a logarithmic caustic gouging potential. control chart as that shown in Figure 2. Several upper molar ratio boundaries are presented Coordinated phosphate programs were designed depending on the operating pressure of the to prevent caustic gouging, but such problems boiler. These boundaries were set to assure persisted even when coordination was minimal chance of phosphate precipitation at the maintained. The gouging was associated with corresponding temperatures. Staying below the boiler deposits and it suggested that the gouging upper molar ratio limit will minimize the chance was related to the extent of the deposition. 6 It of caustic concentration and gouging of the tube was also noted that phosphate precipitation metal. The lower control boundary of 2.2: 1 occurred at a lower ratio that the 3.0 originally molar ratio is set to minimize the chance of prescribed for coordinated control. acidic phosphate attack. When the boiler pH and phosphate levels stay within the control These observations made researchers re-think the boundaries, the boiler water should be non- sodium phosphate buffer chemistry under the corrosive. The vector diagram indicates the boiler environments where problems were direction a point will go on the phosphate-pH observed. As a result of further studies of diagram if a certain chemical is added to the solution chemistry and two-phase equilibrium, a system.