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HomeMy WebLinkAboutBiogas Scrubber Feasibility Study-Technical Memo.pdf I-an Digitally Info:C by Douglas Engineers, In . Contact Info: A109 Engineers,Inc. Dale:2018.03.1109:28:56-OS'00' ORANGE COUNTY SANITATION DISTRICT QpDFESS/p4, SP-125-16 J� y4c BIOGAS SCRUBBER EVALUATION ci m c N0.47513 6 a TECHNICAL MEMORANDUM * * BIOGAS SCRUBBER FEASIBILITY STUDY N�9TP ppl CAU� FINAL March 2016 3150 EMIOLS7REEf,SI17IE 500 COSTAMF A,CAIFFORMA92626 P.714.593.5100 F.714.593.5101 pv//QmbRbcwremnKTewCA4YSD9250BMRMenbba/IM1Oldrca ORANGE COUNTY SANITATION DISTRICT SP-125-16 BIOGAS SCRUBBER EVALUATION TECHNICAL MEMORANDUM BIOGAS SCRUBBER FEASIBILITY STUDY TABLE OF CONTENTS Paae No. 1.0 BACKGROUND.......................................................................................................1 2.0 PURPOSE...............................................................................................................1 3.0 EVALUATION DATA................................................................................................2 4.1 MASS BALANCES.................................................................................................3 4.2 Biogas..........................................................................................................3 4.3 Process Water..............................................................................................6 5.1 STUDIES...............................................................................................................6 5.2 Gas Compressors.........................................................................................6 5.3 Cogeneration Engines..................................................................................7 6.0 CONCEPTUAL DESIGN..........................................................................................7 7.1 SIMPLE PAYBACK COST EVALUATION..............................................................9 7.2 Payback.....................................................................................................12 8.1 LARGE-SCALE TESTING....................................................................................16 8.2 Location .....................................................................................................16 8.3 Schematic...................................................................................................16 8.4 Testing Plan ...............................................................................................16 8.5 Construction Cost Estimate.........................................................................19 9.0 CONCLUSIONS.....................................................................................................19 LIST OF TABLES Table 1 OCSD Plant Data............................................................................................2 Table 2 Pilot Results: December 11, 2013 ...................................................................3 Table 3 Mass Balance: Biogas.....................................................................................5 Table 4 Change In Biogas Quantity..............................................................................6 Table 5 Feasibility-Level Construction Cost Estimate ...................................................9 Table 6 Payback Cost- No Change to CEPT.............................................................12 Table 7 Payback Cost-Alternate CEPT Chemical..................................................... 13 Table 8 Sensitivity Analysis: Increased Recirculation -No Change to CEPT...............15 Table Sensitivity Analysis: Increased Recirculation-Alternate CEPT Chemical.......15 March 2016 i pv\V3mbMme,mKfrm/CHUS�91509N11tEcenbka\IMI I.dou LIST OF FIGURES Figure 1 Carollo Biogas Scrubber Pilot..........................................................................4 Figure 2 Full-Size Conceptual Layout...........................................................................8 Figure 3 Biogas Scrubbing System Schematic............................................................10 Figure 4 Future Site Location......................................................................................11 Figure 5 Large-Scale Pilot Schematic.........................................................................17 March 2016 ii pv\V3mbMme�mKfrm/CHUS�91509N11tEcenbka\IMI I.dou Technical Memorandum BIOGAS SCRUBBER FEASIBILITY STUDY 1.0 BACKGROUND The Orange County Sanitation District(OCSD)utilizes digester gas as a fuel source to offset utility costs for the operation of both Plant No. 1 and Plant No. 2. One of the biggest challenges with the use of biogas is the removal of contaminants. These contaminants are not only harmful to equipment, but are also regulated by authorities. Currently, OCSD is embarking on extensive biogas cleaning in order to provide a fuel quality commensurate with the needs of their existing cogeneration engines, which are being modified to meet new South Coast Air Quality Management District standards. The Carollo Engineers, Inc. (Carollo)Biogas Scrubber takes advantage of the treatment plants' abundant supply of water to provide a convenient and effective method for cleaning biogas. A simple venturi is used, in combination with a gas/water separator,to force contaminants into solution and produce a high-quality biogas. Hydrogen sulfide(H2S), carbon dioxide(CO2), and siloxanes can all be removed by this system, and, due to the relatively low solubility of methane in water, very little methane is lost during scrubbing. The Carollo Biogas Scrubber was pilot tested for proof-of-concept at OCSD's Plant No. 1 on June 12, 2013, and again on December 11, 2013.The results were excellent for removal of the aforementioned contaminants,with potential for significant benefits to OCSD's cogeneration program using internal combustion engines. 2.0 PURPOSE Based on successful pilot testing, OCSD decided to move forward with this second phase of evaluation of the Carollo Biogas Scrubber. The overall goal for this Phase 2 evaluation is to determine the feasibility of implementation of the Carollo Biogas Scrubber at Plant No. 1 and Plant No. 2. This phase included work in the following areas: • Data Collection. • Mass Balances. • Engine and Compressor Studies. • Identification of Discharge Water Treatment. • Conceptual Design. • Site Visit. • Payback Cost Evaluation. March 2016 1 pv\V3mbMme,mKfrm/CHUS�91509N11tEcenbka\IMI I.dou • Sensitivity Study. • Large-Scale Testing Plan. This report summarizes the results of the work performed in Phase 2 as well as recommended next steps for OCSD. 3.0 EVALUATION DATA OCSD provided information found in Table 1 in order to complete the feasibility study in Phase 2. Table 7 OCSD Plant Data SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Information Item Plant No. t Plant No.2 Data Source 1. Quantity of digester gas 2,420,000 old(low)to 3,633,000 ofd SP-14101 available to be treated (cfd) (high)per plant 2. Gas Quality: • Methane(%) 62 62 Jeff Brown • Carbon Dioxide (%) 36.5 36.5 Jeff Brown • Hydrogen Sulfide(ppm): — Current Operation(�) 30 30 Jeff Brown — No 112S Control with 3,000 3,000 Jeff Brown and Rudy Kilian Ferric Chloride(3) • Total Siloxanes (ppm) 7.514t 7.514t SP-141 3. Ferric Chloride Dosage(gallons/month): • Headworks:Total for CEPT approx. 116,000 approx. 97,500 Typical for 2013-Current Headworks-Solely for H2S None None Jeff Brown Control • Digesters approx. 17,50015I None Typical for 2013-Current 4. Femic Chloride Cost($/dry ton) $540161 $540161 5. O&M Costs for Activated $19,000(7) $19,000i7) Carbon($/exchange) 6. Power Cost($/kilowatt-hour) $0.087 $0.087 Jeff Brown 7. O&M Cost for Discharge Water Based on the cost of pumping from Treatment PEPS to AS Plant Notes: (1) The range of available gas was determined by averaging high and low values for the projected gas production in the year 2020 from SP-141. (2) Current operation:adding ferric chloride to the digesters as needed with no change to current CEPT dosing. (3) There is no recorded history of uncontrolled H2S at OCSD available.This concentration is typical for facilities that do not control H2S with similar processes. (4) Actual siloxane concentrations at OCSD have been observed at much lower levels than the value used from SP-141. (5) Ferric chloride used at digesters was recently reduced for Plant No. 1,and may be eliminated in the future. (6) Cost of ferric chloride ranges from$502.00 to$579.00 per dry ton among the suppliers contracted to OCSD. (7) This cost is all-inclusive for a single carbon change-out. Change-out is required for every 184 to 200 million cubic feet of biogas cleaned.Approximately six changes per plant will be required each year by 2020. March 2016 2 pv\V3mbMme,mKfrm/CHUSn91509N11tEcenbka\IMI I.dou 4.0 MASS BALANCES The Carollo Biogas Scrubber pilot unit consists of a gas/water contactor with a gas release valve and a venturi. The scrubber uses the Venturi to pull a vacuum and draw biogas into the system.Through pressure and water contact, contaminants that are highly soluble in water leave the biogas with the discharge water stream, and scrubbed biogas is produced with minimal remaining contaminants. See Figure 1 for a photo of the Carollo Biogas Scrubber during pilot testing at OCSD. During pilot testing, samples of biogas were taken at the inlet and outlet of the Carollo Biogas Scrubber and tested at AtmAA Inc. and at OCSD's Plant No. 1 lab facility. The results from this pilot showed that this system not only removes H2S and CO2, but also significantly removes siloxanes from biogas. It was also discovered that some nitrogen and oxygen, contained within the plant water used, left solution and was added to the scrubbed biogas. The average results from the December 11, 2013, pilot testing are shown below in Table 2. Table 2 Pilot Results: December 11,2013 SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Component Inlet Gas Quality0) Outlet Gas Quality0) Removal Methane(CH4)(4) 61.7% 58.6% 4.9% Carbon Dioxide(CO2)(4) 36.5% 2.2% 94.1% Hydrogen Sulfide(H2S)t5l 7.7 ppm 0.00(2) 100% Siloxanestsl 3.3 ppm 0.09 ppm 97.3% Nitrogen(N2)(4) 1.2% 30.0% - Oxygen(02)(4) 0.3% 8.7% - Notes: (1) Values based on average of samples analyzed from pilot test by AtmAA and OCSD. (2) H2S levels were below the detection limit of testing equipment for both OCSD and AtmAA. (3) Nitrogen and oxygen gained during the scrubbing pilot came from dissolved air leaving plant water and transferring into the biogas.This impact may change based on the water supply used for biogas scrubbing and the number of effective cycles used in the scrubbing process. (4) Values reported are percent by volume. (5) Values reported are parts per million by mass. 4.1 Biogas The pilot testing used a single pass of water through the Carollo Biogas Scrubber to simplify operation, which eliminates the concern of saturating the process water with contaminants and halting the driving force of contaminant removal. Operating a full-scale system on a single pass would require a significant amount of plant water and discharge water to be sent downstream for treatment. For the current study, recirculation of water is considered in order to minimize the plant water requirements and flow of water requiring treatment. March 2016 3 pv\V3mbMme,mKfrm/CHUS�92509N11tEcenbka\IMI I.dou t Opp, ,, Feed Gas Collection Discharge Gas Collection Gas Separating Valve _ Gas Contactor S Feed Gas Collection R ? u CAROLLO BIOGAS a SCRUBBER PILOT FIGURE ORANGE COUNTY Engineers wo e wol d�f,wnn vi tsr^ SANITATION DISTRICT The biggest obstacle when recirculating water is that, as concentrations of contaminants approach their solubility limits,the driving force to remove contaminants decreases. Currently, sufficient data does not exist to determine the solubility limits of siloxanes and their impact on the driving force in the removal of all biogas contaminants. CO2 is expected to be the controlling contaminant due to its relatively low solubility compared to 1-12S and far greater concentrations in biogas than both H2S at 3,000 ppm and siloxanes at 7.5 ppm. Based on CO2solubility, potential recirculation rates of two and four effective cycles were used to evaluate potential payback for this process at OCSD, because these rates may bracket the amount of recirculation possible for effective CO2 removal. This needs to be determined through additional pilot testing. Table 3 shows the inlet biogas quality and projected outlet biogas quality based on the pilot test results and both effective recirculation cycles of process water. Not only will water recirculation reduce the flow of water to treatment, but it may minimize the amount of nitrogen and oxygen added to the scrubbed biogas as it is depleted further from the process water with each cycle. Similarly, methane removal (loss)from the gas will be reduced with water recirculation as the water becomes saturated with the less soluble methane.The ratio of nitrogen and oxygen added to, and the methane removed from,the scrubbed biogas is assumed to be reduced from the concentrations measured during pilot testing based on the number of effective cycles. Further pilot testing is required to determine the extent recirculation can be used while maintaining the driving force required to remove contaminants and the impact on methane, nitrogen, and oxygen recirculation will have. Table 3 Mass Balance: Biogas SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District No. of Cycles - - 2 2 4 4 Inlet Gas Inlet Gas Mass Outlet Gas Outlet Gas Outlet Gas Outlet Gas Component %Volume (moles/f 3) %Volume (moles/f 3)t2t %Volume (moles/f 3)t2t Methane(CH4) 62.00 0.73 80.3 0.71 87.2 0.72 Carbon Dioxide 36.50 0.43 2.9 0.03 3.12 0.03 (CO2) Hydrogen 0.30 0.0036 0.0 0.0 0.0 0.0 Sulfide(H2S) Siloxanes Negligible 7.5 ppm Negligible 0.225 ppm Negligible 0.225 ppm Nitrogen(N2) 0.00 - 15.00) 0.12 7.5 0.05 Oxygen (02) 0.00 - 4.40t 0.04 2.2 0.02 Notes: (1) Nitrogen and oxygen gained during the scrubbing pilot came from dissolved air leaving plant water and transferring into the biogas.This impact may change based on the water supply used for biogas scrubbing and the number of effective cycles used in the scrubbing process. (2) Per cubic foot of inlet gas volume. March 2016 5 pv\V3mbMme,mKfrmK'AKKS�91509N11tEcenbka\IMI I.dou The high rate of CO2 removal from the biogas reduces the total volume of gas leaving the scrubber.Table 4 shows the reduction in mass for 1 cubic foot of biogas after being cleaned as well as the resulting change in total biogas flow. Table 4 Change In Biogas Quantity SPA 25-16 Biogas Scrubber Evaluation Orange County Sanitation District Unit Mass Flow Inlet Gas 1.17 moles/ft3 2,100 cfm Outlet Gas(two cycles) 0.90 moles/ft3 1,631 cfm Change(two cycles) 23.0% 22.3% Outlet Gas(four cycles) 0.82 moles/ft3 1,474 cfm Change(four cycles) 29.7% 29.8% 4.2 Process Water Assuming plant water is to be used for the process, discharge water will be relatively clean with very low biochemical oxygen demand (BOD)and will contain only the additions of sulfides and CO2 from the scrubbed biogas. It is estimated for a full-scale system that only 800 pounds of suedes per day will be added to the total loading of the plant, which represents a small fraction of total suedes typically found at a wastewater treatment facility. If CO2 is assumed to stay in solution, this would add 32,000 pounds of CO2 to the process water per day, but much of the CO2 is expected to leave solution as it is released into the biogas scrubber water recirculation tank. There may be other impacts, such as corrosion or scaling within the system, that are currently unknown. 5.0 STUDIES The scrubbed biogas will impact downstream equipment such as the gas compressors and cogeneration engines. 5.1 Gas Compressors The volumetric flow of biogas out of the Carollo Biogas Scrubber is reduced, largely due to the high removal of CO2.This translates into a decrease in required compression since a large volume of inert gas is no longer being compressed with the usable methane. Currently, raw biogas is sent to the compressors. 1­12S and siloxanes deteriorate compressor components and, in turn, produce further wear on the equipment.With the proposed location of a full-scale system upstream of the compressors, only scrubbed gas will pass through the compressors;this has potential for significant maintenance savings. March 2016 6 pv\V3mbMme,mKfrm/CHUS�91509N11tEcenbka\IMI I.dou 5.2 Cogeneration Engines Engine manufacturers confirmed that the projected biogas quality could require changes to the current cogeneration engine operation. The higher available British thermal unit(BTU), reduced flow, reduced CO2, and additional oxygen in the scrubbed biogas probably would require initial adjustments to the engines for proper operation. In addition, periodic changes may be needed. These changes would be minimized if a consistent Flow and quality of gas can be provided to the cogeneration engines, but further testing is needed to determine this consistency under actual operating conditions. Similarly, engine emissions would be expected to vary from current operations, but the raw emissions and emission treatment impacts are unknown and should be determined through additional testing. 6.0 CONCEPTUAL DESIGN For conceptual design of a full-scale system that recirculates water with two effective recirculation cycles,the following design criteria were used: • Average Digester Gas Flow: 2,100 cubic feet per minute (cfm)per plant. • Total Process Water Flow(including recirculation): 12,400 gallons per minute(gpm). • Makeup Water Flow/Discharge to Treatment Water Flow: 6,200 gpm. • Biogas Scrubber Units: — Venturi Size: 8-inch unit. — Quantity: 6 duty+ 1 standby. • Biogas Scrubber Inlet Water Pressure: 70 pounds per square inch gauge(psig). • Contact Chamber Pressure:4 psig. • Water Recirculation Pumps: — Type: Horizontal End Suction Centrifugal. — Quantity: 2 duty+ 1 standby. — Flow: 3,100 gpm per pump. — Horsepower: 170 brake horsepower(hp) per pump. A conceptual full-scale system designed to treat an average of 2,100 cfm of biogas can be seen in Figure 2. Six duty biogas scrubbing units are required to provide sufficient treatment capacity,with a seventh unit in place as a standby.With six 8-inch units at full capacity, receiving 2,060 gpm each, total digester gas suction capacity is 2,160 standard cubic feet per minute(scfm). Six 8-inch units are expected to provide sufficient turndown capability to cover the full range of expected digester gas flows. March 2016 7 pv\V3mbMme,mKfrm/CHUS�91509N11tEcenbka\IMI I.dou i Met Gas Make-up Water Recirculation Pump (Typ-3) Biogas Scrubber CONCEPTUALLAYOUT ORANGE COUNTY -_- DISTRICT Water from the scrubbers will discharge into an aboveground recirculation tank, which allows wastewater to passively flow over a weir to a drain as makeup water is added to the system. Makeup water is introduced downstream of the recirculation pump discharge to utilize all of the makeup water to remove contaminants from biogas before it is wasted/drained. A submerged baffle wall provides energy dissipation prior to flow entering the recirculation pump suction line. Controls for this system can vary widely depending on the level of automation OCSD desires. A sample process flow/control diagram is shown in Figure 3 with water and gas flowmeters and a makeup water flowmeter. Pumping rates could be adjusted with variable frequency drives(VFDs) based on incoming gas available for treatment, controlled by digester gas pressure. After the conference meeting on May 21, 2015, OCSD led a site visit of Plant No. 1, and it was determined that the best potential location for this system would be at the existing Dewaterng Building C,which will be demolished upon start-up of the new dewatering centrifuges for Plant No. 1. See Figure 4 to see this location. 7.0 SIMPLE PAYBACK COST EVALUATION The basis for the feasibility-level construction cost estimate is as follows: • There is sufficient plant water(6,200 gpm)available for the process. • Gravity flow would be used to transport discharge water to the Primary Effluent Pump Station (PEPS)for pumping to the aeration basins for treatment. The feasibility-level construction cost estimate for a full-scale system is$3.6 million; the breakdown of the estimate can be seen in Table 5. Table 5 Feasibility-Level Construction Cost Estimate SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Area Cost(USD)111 Site Work $300,000 Mechanical Piping and Equipment $2,700,000 EI&C $600,000 Total(2) $3,600,000 Notes: (1) Cost is calculated in 2015 dollars with no escalation. (2) Total cost does not include engineering,administration, and OCSD project costs. March 2016 9 ��r�ow�mKuwcaocsuvssoeoa¢mereu�mui mc. Raw Digester Gas from Digesters Scrubbed ,as Digester Gas Rotameter Release to LPDGH Valve M Make-Up Water M Venturi Gas Gas Scrubber Scrubber No. 1 No.2 Recirculation Recirculation Recirculation Pump No.1 Pump No.2 Pump No.3 Recirculation Sump J BIOGAS SCRUBBING Process Slowdown torreatment SYSTEM SCHEMATIC CA w„A FIGURE3 ^r 1 ORANGE COUNTY Engineers._work,,Warmers Wnn Water® SANITATION DISTRICT OV CCC 1 Oil r ^ •� \- fry N ..'� 7.1 Payback Payback for each plant was determined by taking into account the potential savings and operational costs of a full-scale biogas scrubbing system when weighed against the feasibility-level construction cost estimate. The following create the basis for each facility. • There would be no cost savings in reducing the amount of ferric chloride used for chemically enhanced primary treatment(CEPT)at both plants.A change in ferric chloride dosing would negatively impact effluent quality, i.e., there is no"overdosing" for CEPT that is being used for H2S control at the digesters. • Through discussions with OCSD, it has been determined that there would be no cost savings for eliminating ferric chloride dosing at Plant No. 1 and Plant No. 2 digesters, because digester dosing has been (or will be)eliminated. 7.1.1 No Chanaes to Current CEPT Table 6 shows a breakdown of the annual savings and expenditures for the case where ferric chloride is still used for CEPT at both plants and two effective recirculation cycles are used. This case is not economically feasible as there is no savings at Plant No. 1 or Plant No. 2. Table 6 Payback Cost- No Change to CEPT SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Plant No. 1 Plant No. 2 Annual Savings Ferric Chloride at Digesters $0 $0 Iron Removed from Solids $0 $0 Carbon $106,400 $106,400 Compressor Energy(22.3%)(�) $155,900 $81,900 Compressor O&M (22.3%)(3) $117,100 $46,800 Annual Costs Plant Water Pumping -$274,100 -$274,100 Recirculation Water Pumping -$191,800 -$191,800 PEPS Pumping -$35,600 -$35,600 Methane Loss(4) -$122,100 -$122,100 March 2016 12 pv\V3mbMme,mYfrmK'/tlCS�91509N11tEcenbka\IMll.dou Table 6 Payback Cost- No Change to CEPT SPA 25-16 Biogas Scrubber Evaluation Orange County Sanitation District Plant No. 1 Plant No. 2 Yearly Savings -$244,200 -$388,500 Simple Paybackl'I No Payback No Payback Notes: (1) Based on feasibility-level construction cost estimate. Does not include engineering, administration, and OCSD project costs. (2) The electrical cost of operating the gas compressors is reduced by 22.3%of the reported annual cost from OCSD due to the reduction in gas volume needing compression. (3) O&M savings for gas compression is anticipated due to the removal of H2S and siloxanes from the digester gas prior to compression.The impact of this benefit is not definitively known, so the benefit was calculated as a percent reduction in reported annual O&M costs equal to the improved compressor efficiency(22.3%). (4) Based on the loss of power that would have been produced by CenGen with the methane removed by the scrubbing system.This cost assumes a 33% engine efficiency. 7.1.2 Alternate Chemical Used for CEPT This case takes into account that,with the implementation of this biogas scrubbing system, control of H2S with CEPT is no longer needed. This would allow OCSD to move to an alternate chemical. This case looks at the annual savings required in order to produce a 10-year payback for Plant No. 1, by changing to an alternate chemical for CEPT. If annual CEPT costs are reduced by$600,000 at Plant No. 1, there is approximately a 10-year payback for a full-scale biogas scrubbing system. For the same reduction at Plant No. 2, there are savings, but no reasonable payback. Table 7 summarizes the payback cost associated with this case. Changing to an alternate chemical for CEPT may provide potential savings but may also negatively impact other processes benefitting from the current use of ferric chloride, such as volatile solids reduction and dewaterability of solids. Careful consideration should be given to any changes in current CEPT operations as it will influence more than just H2S control. Table 7 Payback Cost-Alternate CEPT Chemical SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Plant No. 1 Annual Plant No. 2 Annual Savings Savings Annual Savings Ferric Chloride at Digesters $0 $0 Carbon $106,400 $106,400 Compressor Energy(22.3%)('I $165,900 $81,900 March 2016 13 pv\V3mbMme,mYfrmK'/tlCS�91509N11tEcenbka\IMll.dou Table 7 Payback Cost-Alternate CEPT Chemical SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Plant No. 1 Annual Plant No. 2 Annual Savings Savings Compressor O&M(22.3%)t�1 $117,100 $46,800 Reduced CEPT Cost(3) $600,000 $600,000 Annual Costs Plant Water Pumping -$274,100 -$274,100 Recirculation Water Pumping -$191,800 -$191,800 Discharge Water Treatment -$35,600 -$35,600 Methane Loss -$122,100 -$122,100 Yearly Savings $355,800 $211,500 Simple Payback(4) 10.1 years 17.0 years Notes: (1) The electrical cost of operating the gas compressors is reduced by 22.3%of the reported annual cost from OCSD due to the reduction in gas volume needing compression. (2) O&M savings for gas compression is anticipated due to the removal of H2S and siloxanes from the digester gas prior to compression.The impact of this benefit is unknown.The benefit was calculated as a percent reduction in reported annual O&M costs equal to the improved compressor efficiency(22.3%). (3) This cost is based on the net required annual savings needed from changing CEPT chemicals in order to produce reasonable payback at Plant No. 1. $600,000 is approximately 10% of the total chemical cost of ferric chloride for 2015 at Plant No. 1. It is 20% of the total ferric chloride cost at Plant No. 2. (4) Based on feasibility-level construction cost estimate. Does not include engineering, administration,and OCSD project costs. 7.1.3 Sensitivity Analysis-Additional Recirculation Cycles Both payback cost cases above were reevaluated for the case where the effective number of recirculation cycles could be doubled to four for the biogas scrubbing system. The basis for this sensitivity analysis is: • CO2 will be allowed to reach theoretical saturation, but release to atmosphere in the recirculation tank would be expected to reduce the concentration. This was observed during pilot testing, but the amount of CO2 leaving solution is currently unknown. • CO2 concentration in water flowing into the scrubbing units is assumed not to exceed 50 percent of saturation. Makeup water flow would then be reduced to 3,100 gpm as would the associated operations and maintenance(O&M)cost of pumping makeup water. March 2016 14 pv\V3mbMme,mYfrmK'/tlCS�91509N11tEcenbka\IMll.dou • Recirculation pumps would be increased in capacity to 4,650 gpm to maintain required total process flow of 12,400 gpm. • Recirculation pump motors would increase to 255 brake hp to accommodate additional flow. Table 8 compares the reevaluated payback period for each plant with no changes to CEPT. Table 8 Sensitivity Analysis: Increased Recirculation -No Change to CEPT SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Payback Case Plant No. 1 Annual Savings Plant No. 2 Annual Savings Payback(two cycles)(') No Payback No Payback Payback(four cycles)(2) No Payback No Payback Notes: (1) Based on a feasibility-level construction cost of$3.6 million. (2) Based on a feasibility-level construction cost of$3.8 million. Table 9 compares the reevaluated payback period for each plant with an alternate chemical used for CEPT that would reduce annual cost by$600,000. Table Sensitivity Analysis: Increased Recirculation-AlternateCEPT Chemical SP-125-16 Biogas Scrubber Evaluation Orange County Sanitation District Payback Case Plant No. 1 Annual Savings Plant No. 2 Annual Savings Payback(two cycles)(') 10.1 years 17.0 years Payback(four cycles)(2) 6.7 years 10.1 years Notes: (1) Based on a feasibility-level construction cost of$3.6 million. (2) Based on a feasibility-level construction cost of$3.8 million. Through doubling the effective recirculation cycles, the payback periods for both plants could be reduced by several years, if$600,000 can be saved annually on CEPT-related costs by using an alternative chemical.This value,though high, may merit further evaluation to determine the feasibility of this savings. If savings potential exists,four recirculation cycles may allow for reasonable savings and payback at both plants. 7.1.4 Sensitivity Analysis-Larger Biogas Scrubbing Units Payback cost cases were reevaluated for the case where 4 duty+ 1 standby 12-inch scrubbing units are used instead of the 6 duty+ 1 standby 8-inch units for the biogas scrubbing system. The manufacturer estimates the cost of a single 12-inch unit to be more than double the cost of an 8-inch unit. Since five 12-inch scrubbing units are required, the March 2016 15 rv\V3mbMme,mYfrmK'/tlCS�91509N11tEcenbka\IMll.dou additional capital cost of the biogas scrubbing units would outweigh the savings in reduced electrical, instrumentation, and control (EI&C)for this alternative. Furthermore,to provide adequate suction to treat 2100 cfm, the total water flow through the system would also increase, going from 12,400 gpm to 17,600 gpm. This additional flow further adds cost to O&M and capital investment to provide recirculation pumping. For these reasons, it is not viable to consider 12-inch units for this system. 8.0 LARGE-SCALE TESTING One goal of Phase 2 of evaluation for the Carollo Biogas Scrubber is to determine if further pilot testing of the equipment is merited.This larger-scale testing would incorporate a larger pilot unit than previously tested at OCSD, more controls, and testing the process to determine operational boundaries. 8.1 Location After a site walk of Plant No. 1 on May 21, 2015, and subsequent discussion, it was determined that the best location for pilot testing would be the basement of the Gas Compressor Building.This location has ample room for the pilot without inhibiting operations staff.This location also conveniently provides all utility connections required for pilot testing without requiring extensive additional piping. Pilot testing would require short trips within the basement to collect samples and log data. Long-term stays within the Gas Compressor Building would not be required. Two fans will be used during pilot testing—one near the equipment and one near the basement door. These fans will provide continual mixing of air, preventing buildup Of CO2 near the pilot that may occur as CO2 leaves solution. 8.2 Schematic Figure 5 provides a sample pilot schematic, offering manual control of water recirculation, gas flow, and outlet gas pressure. Flow and pressure gauges are provided to allow complete recordkeeping of the process conditions. 8.3 Testing Plan Testing will consist of three stages, each stage lasting 2 weeks. During testing, the following matrix elements will be recorded and evaluated: • Water and Gas: — CO2. — Siloxanes. — Oxygen. — Nitrogen. March 2016 16 pv\V3mbMme,mYfrmK'/tlCS�91509N11tEcenbka\IMll.dou Sample Line Raw Digester Gas Sample FI PI Scrubbed Digester Gas FI Gas Relief Valve FI Make-Up Water X Venturi a PI Gas FI Scrubber PI Recirculation Tank Recirculation Pump LARGE-SCALE Process Blowdown PILOT SCHEMATIC to Treatment - A FIGURE �^r ORANGE COUNTY Engineers._Workrng Wonders wun warer® SANITATION DISTRICT • Gas Only: — Methane. — H2S. • Optimal Flow through Venturi, including turndown. • Optimal Pressure in Contact Chamber. The following equipment will be required: • One 2-inch venturi biogas scrubber. • One 500-gallon water recirculation tank. • One water recirculation pump. • Instrumentation per Figure 5. • Equipment skid. 8.3.1 First Staae - Sinale Pass • Run the system with a single pass of water scrubbing the gas, i.e., no water recirculation. • After start-up,the system shall be allowed to run continuously for an hour at stable conditions prior to sampling. • Take six inlet and outlet samples of gas and water at six evenly spaced intervals each day. • Record water and gas pressure and flow into and out of the scrubber at time of sampling. 8.3.2 Second Staae-Two Effective Water Recirculation Cycles • Set makeup water flow and outlet water flow to allow two effective recirculation cycles in the system. • Once proper flow is achieved, the system shall be allowed to run continuously for an hour at stable conditions prior to sampling. • Take six inlet and outlet samples of gas and water at six evenly spaced intervals each day. • Record water and gas pressure and flow into and out of the scrubber at time of sampling. March 2016 18 pv\V3mbMme,mKfrm/CHUSa91509N11tEcenbka\IMI I.dou 8.3.3 Third Staae - Maximum Effective Water Recirculation Cycles • Reduce makeup water flow and outlet water flow to 100 gpm less than that required for recirculation at two effective cycles. Continue to reduce by 100 gpm at each sampling interval until 10 effective recirculation cycles are reached. • Once proper flow is achieved, the system shall be allowed to run continuously for an hour at stable conditions prior to sampling. • Take six inlet and outlet samples of gas and water at six evenly spaced intervals each day,for two days, at each flow setting. • Record water and gas pressure and flow into and out of the scrubber at time of sampling. • Repeat the above process and reduce makeup water and outlet water flow by an additional 100 gpm. Repeat until the loop reaches 10 effective water recirculation cycles. 8.4 Construction Cost Estimate The feasibility-level construction cost estimate for the pilot is$350,000, which includes a $50,000 allowance for instrumentation. 9.0 CONCLUSIONS If OCSD continues to use ferric chloride for CEPT, reasonable payback is not expected for either plant. Reasonable savings are not expected for the construction of a biogas scrubbing system with two effective recirculation cycles, unless an alternative chemical can be used for CEPT with an associated annual savings of$600,000 at Plant No. 1. Reasonable savings at Plant No. 2 are not expected in this case. If an alternate chemical is used and recirculation is doubled, more reasonable payback may be possible for both plants. Initial findings show higher potential for reasonable payback at Plant No. 1, which merits consideration of potential savings by changing chemicals used for CEPT at Plant No. 1. If potential savings exist with an alternate CEPT chemical, further pilot testing at Plant No. 1 should be performed in order to better define the operational limits and consequences of the technology. March 2016 19 pv\V3mbMme,mKfrm/CHUS�91509N11tEcenbka\IMI I.dou