Using Ski Slopes to Capture and Mix Oxygenated Water to Improve the Efficiency of a Condensate H 2 S Abatement System at Aidlin Power Plant

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1 GRC Transactions, Vol. 40, 2016 Using Ski Slopes to Capture and Mix Oxygenated Water to Improve the Efficiency of a Condensate H 2 S Abatement System at Aidlin Power Plant Brian Benn, Allen Sonneville, Leslie Morrison, and Tom Bahning Calpine Corporation, Geysers Power Plant Keywords Hydrogen sulfide, H 2 S abatement, secondary abatement, condensate, air pollution, iron chelate ABSTRACT A novel retrofit to an existing geothermal condensate hydrogen sulfide (H 2 S) abatement system ( Ski Slopes ) provides the benefits of increasing reaction time, oxygenated water, and ferric iron, to react with and reduce emissions of H 2 S. As a result, reaction rates are improved and less iron chelate abatement solution is required to meet the regulatory H 2 S emission limits. The retrofit captures and redirects oxygenated cooling tower water containing oxidized (ferric) iron chelate as it cascades from the top of the cooling tower. The captured water is redirected to the edge of the cooling tower basin where it is admixed with H 2 S rich condensate from the power plant s surface condenser. After mixing, the combined streams are channeled circuitously through the cooling tower basin to provide increased reaction time between H 2 S and ferric iron prior to completing the cooling circuit at the top of the cooling tower. The development of the retrofit process is described and data are reported documenting a 30% reduction in the iron chelate requirements to date. H 2 S Abatement Chemistry Calpine s Aidlin Power Plant began operation in 1989 under a temporary permit to operate issued by the Northern Sonoma County Air Pollution Control District (NSCAPCD). The permitted H 2 S abatement equipment consisted of a Burner/Scrubber, for NC Gas H 2 S abatement, plus Condensate H 2 S Abatement, wherein the H 2 S dissolved in the condensed steam is removed by reaction with Fe +3 (ferric iron),via Reaction 1. This is accomplished at several plants at The Geysers by the addition of Fe +3 chelate to the cooling tower water. Reaction 1: H 2 S + 2Fe 3+ è S 0 + 2Fe H + (2:1 mole ratio Fe +3 to H 2 S) 1 Reaction 1 stoichiometry shows that, two moles of Fe +3 are required for each mole of H 2 S. Theoretically then, at equilibrium, 100% abatement of the H 2 S dissolved in the condensate is achieved if the H 2 S reacts with 2 moles of iron (Fe +3 ) before being released in the cooling tower, where forced air evaporation will strip and release any unreacted H 2 S. At Aidlin, Reaction 1 had only about 5 seconds of time available from the point that the condensate H 2 S mixes with Fe +3 in the circulating water before being sprayed into the cooling tower (Figure 1). This short reaction time and kinetic limitations means Reaction 1 did not reach equilibrium and required 133% [1] more Fe +3 than that theoretically needed to meet the regulatory permit compliance limit for H 2 S emissions. The ferrous iron (Fe +2 ) that results from Reaction 1 is reoxidized to Fe +3 (Reaction 2) by atmospheric oxygen dissolved in the cooling water as air is drawn through the cooling tower. This is a first order reaction dependent on the concentration of oxygen dissolved in the circulating water. When reoxidized to Fe +3 it is again available for H 2 S abatement on the next pass through the cooling water system. 807

2 Aidlin Power Plant Original Configuration Reaction 2: Fe 2+ + O 2 + 4H + Fe H 2 O (Re-oxidation to Fe 2+ to Fe 3+ ) GEOTHERMAL STEAM FROM WELLS NON- CONDENSABLE GASES + H 2 S Burner/Scrubber TREATED GASES Air + H 2 S About 5 seconds reaction time COOLING TOWER GENERATOR STEAM TURBINE Sulfurous Acid (bi-sulfite HSO 3- ) Air Draft In SURFACE CONDENSER COOLING WATER CIRCULATING WATER Circulating water WATER PUMPS CONDENSATE Hotwell CONDENSATE condensate + H 2 S (l) PUMPS Cooling water blowdown to reinjection Reaction 1: H 2 S + 2Fe 3+ S 0 + 2Fe H + (2:1 mole ratio Fe +3 to H 2 S) Figure 1. At Aidlin, Reaction 1 had only about 5 seconds of time available from the point that the condensate H 2 S mixes with Fe +3 in the circulating water before being sprayed into the cooling tower Reaction 2: Fe 2+ + O 2 + 4H + à Fe H 2 O (Re-oxidation of Fe+ 2 to Fe +3 ) By introducing added oxygen into the process to re-oxidize Fe +2 to Fe +3 the amount of ferric iron required is cut in half as shown by combining Reaction 1 with Reaction 2 to get Reaction 3 [2]. Reaction 3: H 2 S + Fe 3+ + ½O 2 à Fe 3+ + S 0 + H 2 O (1:1 mole ratio Fe +3 to H 2 S) It would appear that Fe +3 is catalytic in this case, but in actuality the iron has been reduced and then reoxidized. The reoxidation of Fe +2 to Fe +3 is the slower (limiting) step in this reaction. Aidlin Power Plant Permit to Operate When permitting the Aidlin plant, the NSCAPCD wrote operating permit conditions that required Reaction 1 stoichiometry (2:1 molar Fe to H 2 S) to ensure that the H 2 S emission limit (2.25 lb/hr) was met. For the amount of H 2 S supplied to the plant, this required that the cooling tower water contain at least 16 ppm Fe +3. Due to the short reaction time in the circulating water mentioned above, the actual amount of iron required to meet the compliance emission limit was 133% [1] of stoichiometric. To increase reaction time, the plant was modified in 1992 by relocating the H 2 S laden condensate discharge, first to the circulating pump suction (Condensate Reroute), and then to the far end of the cooling tower basin (Cold Water Basin Reroute) [2] (Figure 2). This modification extended the reaction time from the original 5 seconds to over 10 minutes. After rerouting the condensate to the Cold Water Basin (now termed Reroute Extension) the H 2 S emissions were near zero when circulating water iron was at the permit required 2:1 mole ratio (Fe +3 :H 2 S). The cost of the Fe +3 chelate product to maintain the mandated 16 ppm Fe +3 concentration at Aidlin was about $205,000 dollars/year. In May of 2015, modification of the Aidlin Permit to Operate allowed setting the Fe +3 concentration requirement based on actual H 2 S emissions, rather than fixing the Fe +3 concentration at 16 ppm. This permit modification provided an opportunity to reduce the iron chelate consumption cost, if Reaction 3 stoichiometry could be applied. 808

3 Aidlin Power Plant - Condensate Reroute Extension Reaction 2: Fe 2+ + O 2 + 4H + Fe H 2 O (Re-oxidation to Fe 2+ to Fe 3+ ) GEOTHERMAL STEAM FROM WELLS NON- CONDENSABLE GASES + H 2 S Burner/Scrubber TREATED GASES Air + H 2 S Extended Reroute: Increased reaction time by >10 minutes COOLING TOWER GENERATOR STEAM TURBINE Sulfurous Acid (bi-sulfite HSO 3- ) Air Draft In WATER SURFACE CONDENSER Circulating water CONDENSATE CONDENSATE Hotwell condensate + H 2 S (l) PUMPS Cooling water blowdown to reinjection Reaction 3: H 2 S + 2Fe /2O 2 2Fe 3+ + S 0 + 2H + (1:1 mole ratio Fe +3 to H 2 S) Figure 2. To increase reaction time, the plant was modified in 1992 by relocating the H 2 S laden condensate discharge, first to the circulating pump suction (Condensate Reroute), and then to the far end of the cooling tower basin (Cold Water Basin Reroute) [2] Aidlin Power Plant Special Case Application Aidlin is a unique case at The Geysers because it utilizes a surface condenser in combination with a Burner/Scrubber, while all other Burner/Scrubber applications utilize direct contact condensers. This means that, like other surface condenser equipped plants, the H 2 S rich condensate can be re-routed to the cooling tower basin to maximize reaction time, and provide oxygenated water to reoxidize Fe +2. However, in the case of Aidlin, the basin cooling water is very oxygen limited. Very little re-oxidation of the Fe +2 occurs in the cooling tower basin (Reaction 2) except at the cooling tower water surface where oxygenated water cascading through the cooling tower lands. The cooling tower water beneath the surface is limited in oxygen as a result of a sort of inorganic chemical oxygen demand. Sulfides (HS -, S -2 ) and sulfite (HSO 3 ) quickly consume any oxygen dissolved in the water. Sulfite (HSO 3 ) for example, is a common oxygen scavenger used to remove oxygen from boiler feed water. The presence of sulfite in the Aidlin plant water is the result of the Burner/ Scrubber operation, wherein vent gas H 2 S is thermally oxidized to make sulfur dioxide (SO 2 ), which is then scrubbed with water to form HSO 3 via: Reaction 4: H 2 S +3/2 O 2 àso 2 + H 2 O Thermal Oxidation and Reaction 5: SO 2 + H 2 O àhso 3 + H + Scrubbing For this reason, Aidlin Condensate Reroute Extension (Figure 2) improved the efficiency of the H 2 S abatement reaction through increased residence (reaction) time for Reaction 1, but very little Reaction 2 reoxidation improvement in efficiency was seen (and therefore little Reaction 3). Note that sulfite production by the Burner/Scrubber serves an important function in the Aidlin cooling tower water by dissolving elemental sulfur (S o or S 8 ) that is produced by Reaction 1. If not dissolved, the sulfur would build up in the cooling tower basin at a rate of about 2,500 lbs/day. Dissolving sulfur is accomplished by: Reaction 6: S 0 + HSO 3 à S 2 O H + Thiosulfate production 809

4 Aidlin is also unique at The Geysers because of its counterflow cooling tower, instead of the ubiquitous crossflow towers. The counterflow tower does not have a hot water basin to distribute the hot condensate and H 2 S throughout the cooling tower before cascading through splash-fill heat transfer surfaces. In the counterflow tower the circulating water is sprayed through an array of nozzles onto high efficiency packing material. There is no opportunity to inject oxidizing air like that done at other Geysers plants [2], nor is there an open channel area where turbulence can be used to oxygenate the water [3]. Challenges and Opportunities of Increased Reaction Time and Re-oxidation at Aidlin Strategy Experience at other Geysers plants has proven that extending reaction time [2] and by aeration to re-oxidize Fe +2 to Fe +3 [3], the amount of Fe +3 required can be reduced to even less than the 1:1 mole ratio suggested by Reaction 3. The recent changes in the operating permit offered an opportunity to reduce H 2 S abatement costs if Reaction 3 stoichiometry could be applied to Aidlin. Iron product costs were approximately $205,000 per year. Increased reaction time by condensate re-route (Figure 2) had already been applied at Aidlin, but not to the full extent possible and not with oxygenated water. The condensate re-route had reduced the Fe +3 requirements from 130% of stoichiometric to 100%. A further challenge for improving abatement efficiency at Aidlin was to accomplish it without affecting the cooling tower efficiency, or adding substantial mechanical equipment like pumps, tanks, and compressors. The modified system had to continue to control cooling tower sulfur solids through the Burner/Scrubber sulfite and thiosulfate production. Retrofit work requires a plant outage to affect the changes. There is no opportunity to test the new process. The modification has to work at least as well as the status quo or another costly plant outage would be required to make adjustments. The strategy employed for achieving Reaction 3 stoichiometry was to build a structure above the surface of the cooling tower water which would collect oxygenated cooling water rain containing Fe +3, and then mix it with the H 2 S laden condensate at a location in the cooling tower as distant as possible from the circulating water pump inlet. The location must also allow timely mixing with the HSO 3 laden Burner/Scrubber water to provide good solids control. Implementation To capture aerated cooling tower rain containing Fe +3, wooden troughs (nicknamed Ski Slopes ) were constructed in the vacant area beneath the cooling tower packing, above the cooling water surface. The troughs extend halfway across the cooling tower basin, and are sloped about one foot from top to bottom (see Figure 3, next page). Approximately 4500 gallons per minute of freshly oxygenated water runs down each slope and into the side of the cooling tower basin, for a total of 18,000 GPM out of a 36,000 GPM circulating water rate. H 2 S laden condensate from the plant condenser is distributed into the cooling tower basin through a perforated pipe located immediately below the end of the ski slopes so the Fe +3, H 2 S, and dissolved O 2 are all present together in a confined zone for mixing. Inside of the cooling tower basin, mixing baffles have been constructed which force the condensate/cooling water mixture, with accompanying reactants, to traverse a circuitous (baffled) route across the tower basin and back before being drawn into the circulating water pumps, then through the condenser (tube side) and then to the top of the cooling tower to be sprayed for evaporative cooling. Finally, to avoid the sulfur solids produced by Reaction 3, the process stream exiting the Burner/Scrubber and comprising the products of Reaction 5 (HSO 3, bisulfite), are added to the cooling tower basin through another perforated pipe located just downstream of the mixing baffles to encourage the resulting Reaction 6; soluble thiosulfate (S 2 O 3= ) production. The thiosulfate produced is ionically balanced in the water with sodium and ammonium ions, and the resultant dissolved salt (sodium or ammonium thiosulfate) is removed via cooling tower blowdown water injected into the Geysers steam reservoir. Construction of the water diversion troughs as well as relocation of the condensate, and Burner/Scrubber water, was completed during a two week plant outage in May,

5 Cooling tower stack H 2 S (emitted) Fe +2 and residual H 2 S Fan Cooling spray distribution nozzles Circulating water from condenser Thiosulfate (S 2 O 3= ) Zone Oxygenated cooling water Fe +3 O 2 Fe +3 O 2 O 2 Fe +3 Ski Slope troughs HSO 3 from Burner/Scrubber O 2 S 2 O = S 3 2 O = 3 O 2 Fe +3 HSO Fe +3 3 HSO 3 Fe +3 O 2 HSO 3 Mixing Baffles H2S laden condensate from condenser shell side H 2 S + Fe 3+ + ½O 2 Fe 3+ + S 0 + H 2 O H 2 S H 2 S H 2 S H 2 S To Circulating Pumps intake Reaction 3 Zone Figure 3. To capture aerated cooling tower rain containing Fe+3, wooden troughs (nicknamed Ski Slopes ) were constructed in the vacant area beneath the cooling tower packing, above the cooling water surface. The troughs extend halfway across the cooling tower basin, and are sloped about one foot from top to bottom Evaluating the Retrofit Benefit As mentioned above, the H 2 S emissions from Aidlin were nearly zero when the Fe +3 concentration in the circulating water equaled 16 ppm, as required by permit. This restriction meant that Aidlin was over-abated, and that something less than a 2:1 Fe +3 :H 2 S mole ratio was achievable. A theoretical determination of the required Fe +3 concentration can be made utilizing a reaction kinetics model, as follows: The H 2 S abatement rate equation is assumed to be: Equation 1. d[h 2 S]/dt = [H 2 S]* {k(o 2,T) * [O 2 ] + K(Fe/H 2 S, T) * [Fe +3 ]} [4] (1) where k is the oxygen kinetic rate constant and K is the iron kinetic rate constant. Assuming that oxygen concentration and iron concentration remain constant in the process, integrating gives: Equation 2. ln{[h 2 S]/([H 2 S],initial)} = - {k*[o 2 ] + K*[Fe +3 ]} * (Time) (2) K and k can be determined by substituting measured parameters (time, [H 2 S], Fe +3 ) into Equation 2 to determine an overall H 2 S abatement reaction rate constant k. Once k is known, new operating parameters can be inserted into Equation 2, to calculate the H 2 S emissions under new conditions. This kinetic rate calculation for Aidlin after 1992 re-route extension (extended reaction time) predicted that 10 ppm Fe +3 would keep H 2 S emissions below the allowable limit of 2.25 lb/hr. Subsequent source testing conducted under a permit variance in 2007, demonstrated that an Fe +3 concentration of 9.5 ppm produced emissions of 2.25 lb/hr H 2 S. This validates the kinetic model. For the purposes of evaluating the Ski Slopes retrofit, the baseline Fe +3 concentration is assumed to be 9.5 ppm. Utilizing the rate equation to predict the needed Fe +3 concentration given the additional reaction time, and additional dissolved oxygen provided by the new retrofit, returns a minimum Fe +3 of 4.5 ppm, which is an Fe +3 :H 2 S mole ratio of 0.64:1 (at current supplied steam H 2 S concentrations). The anticipated benefit from the new retrofit 811

6 Table 1. After the addition of the sloped troughs and changes in the air permit, iron concentrations were gradually reduced. Date is the cost of Fe +3 Chelate equivalent to 5 ppm Fe +3 (9.5 ppm 4.5 ppm) in the cooling tower water, a savings of approximately $70,000 per year. Test Results 8/5/2015 8/11/ % 6% After the addition of the sloped troughs and changes in the air permit, iron concentrations were 8/13/ % 5 gradually reduced (Table 1). The lowest Fe +3 concentration achieved, as of October 2015, by the Ski 8/14/ % 6 Slope modification is 6.5 ppm. This concentration 8/18/ % 5 however, does not represent the concentration that 8/20/2015 8/25/2015 9/3/ /5/ % 25% 35% 44% produced H 2 S emissions at the plant limit of 2.25 lb/hr, but represents the Fe +3 concentration below which elevated H 2 S concentrations measured by the continuous monitor in the Burner/Scrubber exhaust 10/8/ % 8 gas (the treated gas) began to approach the limit of 17 ppm. The reason for this phenomena is that at 10/8/ % 8 the relatively low Fe +3 concentrations, some H 2 S 10/15/ % 10 survives the added oxygen and extended reaction time in the cooling tower basin, and is drawn into the Burner/Scrubber water supply stream. That H 2 S is liberated in the SO2 scrubbers where the ph drops to below 3.0. The flue gas H 2 S concentration limit is 17 ppmv and the concentration in the treated gas increases to near this limit as the iron concentration in the cooling tower water decreases. Given the treated gas H 2 S concentration limitation, Fe +3 concentration reduction testing has been suspended until a new Burner/Scrubber water supply can be established. Conclusions The amount of Fe +3 Chelate required by geothermal plants to meet the permitted cooling tower H 2 S emission limit can be greatly reduced through capture of oxygenated cooling tower rain water; admixing this water with the H 2 S laden condensate stream, and providing as much reaction time for the two mixed streams as physical constraints allow. Fe +3 concentration requirements were shown to be reduced from the stoichiometric required Fe +3 :H 2 S ratio of 2:1, down to as low as 0.64:1 before unrelated H 2 S abatement system trouble caused Fe +3 reduction testing to cease pending Burner/Scrubber water supply changes. Implementation to date has reduced iron chelate concentration requirement by 30%. Once fully implemented, the expected reduction will be 75% lower than the original permit requirement and 50% lower than the expected consumption without the Ski Slopes. Chemical Cost Savings Approximately $205,000 in annual chemical usage was required prior to the permit modification and implementation of the Ski Slopes (see Chart). Much of this (40%) was the result of the permit condition requiring 16 ppm Fe +3 in the cooling tower water. The permit change and the Ski Slope modifications were implemented simultaneously, so the benefit from the Ski Slopes is determined by subtracting the predicted benefit applied to the permit modification. After installation of the Ski Slopes iron chelate feed reduction testing began in July of Permit to operate limitations required a very gradual decrease (1 ppm/week) and the Fe +3 concentration was reduced from 16 ppm down to 6.5 ppm by the end of October, saving approximately $57,000 in When fully implemented the expected Fe +3 concentration is 4.5 ppm and the annual chemical cost for iron chelate product will be approximately $72,000. Future Work Circ. Water [Fe] ppm H 2 S Emissions Lbs/Hr % of Allowable Emissions (2.25 Lbs/Hr) Treated Gas [H 2 S] (Limit 17 ppm) 7/30/ % 2 Fe +3 concentration reduction efforts will continue following changes to the Burner/Scrubber water supply. The next step anticipated for improving the H 2 S abatement efficiency at Aidlin will be to add the H 2 S laden condensate at the top of the Ski Slopes such that the oxygenated cooling tower rainwater will directly impact and mix with the condensate H 2 S prior to entering the cooling tower basin. 812

7 References [1] Dorrity, Charlotte M. 1995, Utilizing the Ammonia in Geothermal Steam for H2S Abatement, Geothermal Resources Council Transactions, Vol. 19. p [2] Ballantine, Daniel B.; Benn, Brian J., 1993, Optimization of Condensate H2S Abatement at The Geysers, Geothermal Resources Council Transactions, v.17, p [3] Hillard, Catherine; Benn, Brian, 2014, Improvements in Residence Time and Air Oxidation For H2S Abatement in The Geysers Geothermal Power Plants With Direct-Contact Condensers and Crossflow Cooling Towers, Geothermal Resources Council Transactions, p [4] Chemical Reaction Engineering, Second Edition, Levelspiel, Octave, pp 44-46, Integral Method of Analysis of Data. Assumes irreversible unimolecular first-order reactions. 1 For clarity, the reactions have been simplified, e.g. sulfur actually forms S 8, which would therefore require 8H 2 S + 16 Fe +3 è S Fe H +. The 2:1 mole ratio remains. Cations to balance the ions in solution are generally Na + and NH4 + and are not important in this context. Ionic iron species are chelated with HEDTA, which is not shown. 813

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