Converting Digester Gas to Steam and Electricity at the Blue Plains AWTF

Transcription

1 Converting Digester Gas to and Electricity at the Blue Plains AWTF Michael B. Shafer, PE - Black & Veatch International Company Kent A. Lackey, PE - Black & Veatch International Company ABSTRACT DC is investing hundreds of millions of dollars in overhauling its biosolids management facilities, including a new thermal hydrolysis processing (THP) facility that will generate methane in its digester gas. This paper will discuss the Combined Heat and Power (CHP) portion of the project which will utilize 100 percent of the digester gas to produce steam and electricity. The steam will be returned to the THP facilities and is essential for proper operation. The CHP facilities must continue to produce steam during all anticipated emergency conditions. The electricity will offset a significant portion of the Blue Plains AWTF energy needs. There are significant design challenges in achieving and maintaining the proper gas quality for the combustion turbines that generate the electricity. Heating value, temperature, moisture, and siloxane concentration are all key components that must be balanced throughout the process. This paper will describe the process design used to provide a high quality gas mixture that will minimize maintenance and extend the life of equipment. The Project is being delivered in a Design-Build-Operate (DBO) contract arrangement with Pepco Energy Services as the prime contractor that will operate the facilities for a period of 15 years. Since cogeneration is not familiar to most wastewater treatment plants, the DBO approach allows for experienced staff to operate the facility without impacting the Owner s operations staff. It also allows the Owner to reduce their financial risk by fixing the capital and operating cost over the life of the contract. KEYWORDS Cogeneration, combustion turbines, digester gas, heat recovery steam generators, renewable energy, siloxane removal, thermal hydrolysis INTRODUCTION DC owns and operates the Blue Plains Advanced Wastewater Treatment Facility (AWTF) which treats wastewater generated in the metropolitan area surrounding the nation s capital. With capacity of 370 million gallons per day (mgd), it is touted as the world s largest advanced wastewater treatment plant. The primary biosolids management practice is land application of Class B lime stabilized biosolids, with application fields as far away as Mecklenburg County, Virginia. DC has undertaken and extensive project to upgrade its biosolids management program that will significantly reduce costs for disposal of biosolids and reduce the facility s carbon footprint by generating a significant amount of renewable energy. The project includes four phases, including: 1) Site preparation 2) The Main Process Train (MPT) which includes thermal hydrolysis process (THP) with mesophilic anaerobic digesters 3) Combine Heat and Power (CHP) provides cogeneration of steam and electricity 4) Final Dewatering Facility (FDF)

2 This paper will discuss the CHP portion of the project which will utilize 100 percent of the digester gas to produce steam and electricity. The steam will be returned to the THP facilities and is essential for proper operation, so the CHP facilities must continue to produce steam during all anticipated emergency conditions. The electricity will offset a significant portion of the Blue Plains AWTF energy needs. The project is being delivered in a Design-Build-Operate (DBO) contract arrangement with Pepco Energy Services (PES) as the prime contractor that will operate the facilities for a period of 15 years. Black & Veatch is the engineer of record working as a subcontractor to PES. The construction is being performed by Ulliman Schutte Construction (USC), also working as a subcontractor to PES. The following section will describe each phase of the DBO project in more detail. DESIGN The CHP facility has three main process goals: 1) to consume all digester gas produced in the anaerobic digesters, 2) to provide all necessary steam to the THP process, and 3) to maximize power production. Although an emergency flare is provided on the digester gas supply system, the air permit for the facility only allows the gas to be flared in emergency conditions. Therefore, the CHP facility must be designed to process all digester gas. Table 1 provides the design values for digester gas production as well as the design parameters for gas quality. Since the digesters have not been built, these values are estimates. Table 1 Digester Gas Production and Quality Estimates Design Parameter Digester Gas Design Value Range Flow Rate, scfm 5,333 1,350 to 5,333 Pressure, inches wc to 18 Temperature, o F to 108 Lower Heating Value, Btu/scfh to 620 Moisture Content Saturated Wobbe Index to 611 Digester Gas Concentrations Methane, % by volume (dry) to 65 Carbon Dioxide, % by volume (dry) to 45 Sulfides, ppmv to 100 Siloxanes, mg/m to 100 Note 1: Instantaneous excursion of up to 1,000 ppmv for 24 hours or less. The THP process relies on steam to provide the proper temperature and pressure to provide the thermal hydrolysis that enhances the anaerobic digestion process. Without the steam, the process will not provide adequate treatment of the biosolids. The THP process requires 47,500 mass pounds per hour (lbm/hr) of steam. The other critical output is the power generated in the combustion turbines (CTs). To ensure financial success for the project, the operation will be optimized to provide the highest power output available while still meeting the steam demand. Process Overview The CTs provide the power generation while the heat recovery steam generators (HRSGs) are the primary source of steam for the project. A single CT and HRSG are coupled together with a duct burner installed in the ductwork connecting the equipment, as illustrated in Figure 1.

3 Figure 1 Process Overview of Combustion Turbines, Heat Recovery Generators, and Duct Burners Electricity Exhaust High Pressure Digester Gas Combustion Turbines Heat Duct Burner Heat Heat Recovery Generator Medium Pressure Digester Gas The CTs produce electrical power and deliver that energy to the Blue Plains AWTF thru 13.8 kv paralleling switchgear. Each CT has a maximum output of 5 MW, for a total system capacity of 15 MW. That is roughly 1/3 of the power demand at the Blue Plains AWTF. Since there are very few operating scenarios that would allow the CHP system to produce more power than the demand on site, the project does not include an interconnection with the power grid. All power produced will be utilized on site. A CT will produce enough exhaust heat to produce 14,000 lbm/hr of steam in the HRSG. With three units installed, there is insufficient heat to meet the peak steam demand from the THP. The duct burner installed in the ductwork provides additional heat to the CT exhaust duct, allowing the HRSG to provide additional steam, up to 35,000 lbm/hr. With three units operating with the duct burners fully fired, the total steam production exceeds 100,000 lbm/hr. That is well above the maximum steam demand and provides for flexibility in setting the systems up to meet the steam demand while maximizing the power produced in the CTs. The primary source of steam are the HRSGs. However, steam supply is so critical that a fully redundant auxiliary boiler is also provided. The auxiliary boiler is capable of burning digester gas or natural gas, allowing it to provide steam during startup and other potential process upsets that would have minimal gas production. System To produce a reliable, high quality steam supply, there are a number of ancillary facilities required as depicted in Figure 2.

4 Figure 2 Process Schematic Makeup Deaerator Feed Pumps Condensate Pumps Natural/ Digester Gas Auxiliary Boiler Turbine Exhaust HRSG (TYP of 3) Digester Gas Dump Condenser Excess Process To Cambi Makeup water is collected in the deaerator. Pegging steam heats and pressurizes the water prior to the boiler feed water pumps delivering the water to the boilers. is generated in a combination of one or more HRSGs and/or the auxiliary steam boiler. Ideally all digester gas is routed to the CTs, and the heat from the CT exhaust is adequate to meet the steam demand. When the heat is insufficient, some of the digester gas is directed to the duct burners which increases the heat and subsequent steam production. When there is more gas than the CTs can burn, the excess gas is routed to the duct burners or auxiliary boiler which produces excess steam, which is directed to the steam dump condenser and/or sky vent. The 50,000 lbm/hr steam dump condenser cools the steam with plant effluent so that it condenses. The condensate is then returned into the system to recycle the purified water. In extreme design scenarios, it is possible to generate more excess steam than the condenser can handle, so a sky vent is provided to release the steam to the atmosphere before over-pressurizing the steam system. Makeup System To produce high quality steam, a makeup water system is provided to ensure a high quality water meeting the American Boiler Manufacturer Association Boiler 402 requirements. The makeup water must be very low in conductivity, metals, and hardness. A water treatment system will treat potable water through multi-media filters, reverse osmosis (RO) systems, water softener, and chemical feed systems. Redundant RO skids with a 100 gpm capacity provide most of the treatment, including the metals and conductivity removal. The single pass RO systems will also remove much of the hardness, but it is not sufficient to achieve the required 2 ppm standard. Therefore, one polishing softerner will be provided to ensure the hardness standard is achieved. The multi-media filters are provided upstream of the RO skids to reduce the silt density index to a level below 3, which reduces the cleaning frequency of the RO system and improves membrane life. The water treatment system will be provided with two chemical feed systems to ensure proper operation of the RO membrane system. Sodium bisulfite will be fed to remove the chlorine residual prior to contacting the RO membranes. An anti-scalant will be fed to minimize scale formation within the RO treatment system and in the boilers. In addition, space will be provided for additional chemical feed systems including an oxygen scavenger, neutralizing amines, aqueous polymer, and ph adjustment. These chemicals can be fed at the deaerator or directly into the boilers.

5 Gas Conditioning System Digester gas has many impurities that will have a detrimental effect on the process. Siloxanes are manmade compounds found in wastewater and biogas, which will cause silicon oxides to buildup in the CTs which will significantly reduce the effective life of the equipment. Gas conditioning equipment is provided to remove particulates, moisture, and siloxanes as well as pressurizing the gas to the proper level for combustion in the CTs and other uses. Figure 3 provides a schematic view of the gas conditioning system. Figure 3 Gas Conditioning Schematic Wet Scrubber Flare Digester Gas Particulates Blowers Moisture Removal Condensate Siloxane Removal Static Mixer Natural Gas Blowers Compressors MPDG to HRSG Duct Burner and Auxiliary Boiler HPDG to Combustion Turbines Digester gas is collected from a single main from the anaerobic digesters and routed to the Gas Blower Building. The gas is saturated at temperatures near the operating temperature of the anaerobic digesters, between 90 F and 108 F. A wet scrubber removes foam and particulates from the gas before the centrifugal gas blowers pressurize the gas to approximately 12 psig. A moisture removal system is provided to provide a dry gas for downstream siloxane removal system. The gas is first cooled to a temperature of 75 F to 90 F with plant effluent in shell and tube heat exchangers. The gas is further cooled in a second shell and tube heat exchanger with a chilled water system. As the gas is cooled, the moisture condenses in the heat exchangers and drained to the plant drain system. The result is a gas with a dew point of approximately 45 F. Digester gas from municipal wastewater treatment processes has the potential to contain significant levels of siloxanes, which can lead to harmful buildup of silicon oxides in the combustion turbine recuperators. The average concentration of siloxanes needs to be reduced to 0.5 milligrams per cubic meter (mg/m3) to ensure the siloxanes will not significantly reduce the expected life of the CT equipment. A multi-stage removal process has been designed to ensure effective siloxane removal, including: 1) The main siloxane removal system uses a regenerative, adsorptive desiccant-based media system that adsorbs all forms of volatile organic compounds including siloxanes. 2) The media is regenerated daily by blowing hot air through the vessels, which drives off the volatile organic compounds that have adsorbed on the media. The off-gas is then routed to a siloxane flare to incinerate the organics.

6 3) The main siloxane removal system cannot achieve the required siloxane concentrations throughout the 18 month life of the desiccant media. Therefore, two siloxane polisher vessels with carbon media will treat the digester gas following the main siloxane removal units. 4) Pre-filters will be provided prior to the main siloxane removal vessels to remove any remaining water and particulates following the moisture removal system. 5) Post-filters are provided following the polishing units to remove any particulates that may have been suspended in the gas from the treatment vessels. Following siloxane removal, the gas is pressurized to the appropriate levels for downstream combustion. Four 450 hp rotary screw compressors will deliver 200 psig gas to the CTs. A gas blending system is provided upstream of the compressors to supplement the digester gas with up to 30% natural gas. The duct burners and auxiliary boilers have a much lower pressure requirement. Two 50 hp centrifugal blowers are provided to pressurize the medium pressure digester gas to approximately 13 psig for those applications. BUILD One of the most recognized benefits of design build contracts is the accelerated schedules that can be accomplished. The CHP project has to be designed and constructed within 26 months, which is a very aggressive schedule for a project of this magnitude. There are many variations of design-build contracts that have different schedule implications. Some allow the design-build team a great deal of freedom in coordination between design and construction, while others require the design to progress at certain intervals before construction can begin. The amount of flexibility with regards to the design-build contractor is dictated by the Owner in the design build proposal documents. Owner s can provide very detailed documents with a high level of specificity and detailed design requirements similar to what might normally be experienced in a traditional design-bid-build project, or Owner s can provide more limited detailed design information and allow the design-build contractor design flexibility. The latter is more of a performance based design-build configuration whereas the former is more of a prescribed design approach. It is important to understand that higher levels of flexibility allow the design-build contractor to lower cost with more creative designs. The DC CHP project utilized a highly prescribed design-build approach with significant design reviews. The design-build proposal documents included significant technical specifications and drawings for which the design-builder was not authorized to change. While this approach limited the design-build flexibility and potential for cost savings due to creativity, it ensured DC would receive high quality facilities that utilize all the standard materials and construction techniques established in their design standards. Additionally, comprehensive review packages were required at both the 60 percent and 90 percent design stage. Thus the design-build approach for the CHP project followed a more traditional design-bid-build approach. However, two key design-build elements were essential to meeting the project schedule. An early foundation design package was developed to allow the construction team to begin work before the final design was completed. Also, early procurement of the CTs and HRSGs was performed to allow these major equipment items to be on-site at critical points in the construction process. In a traditional design-bid-build approach, the structural design is one of the last elements of design to be completed, once all of the loads are fully understood. However, the construction of the foundations is typically one of the earliest construction activities. For the CHP project, the structural loads were conservatively estimated at the beginning of the design to allow a separate foundation package to be

7 issued. That design package received a full review from DC and also had to be permitted through the DC Department of Consumer and Regulatory Affairs (DCRA). Even with the extensive review process, construction of the 179 auger cast-in-place piles began over four months prior to final design completion. The CHP project includes complex equipment that has very long lead times. The CTs and HRSGs both had lead times that were over one year. These large pieces of equipment also need to be set relatively early in the construction sequence to allow all the ancillary equipment, piping, and wiring to be installed. For this project, the CTs and HRSGs were procured in the first month based on the bridging documents provided in the request for proposals. Other equipment was procured at the 90 percent design stage. This allowed for an expedited major equipment review and fabrication schedule allowing for installation as soon as the foundations were completed. OPERATE The operational component of the contract brings several benefits to DC. DC staff are very accustomed to wastewater treatment systems and operation of wastewater process, but have no experience operating power generating equipment of the type and size being provided by the CHP project. CTs, HRSGSs, and 13.8 kv electrical switchgear are not commonly found at wastewater treatment facilities. By contracting with a firm experienced in power generation, DC can focus on its core business of wastewater treatment and biosolids management. Another key benefit is the reduction in long-term financial risk for DC. Power generation markets have fluctuated significantly in recent years and the contract structure has been set up to fix most of the capital and operating and maintenance costs to the maximum extent practical. Thus, DC has, in essence, a guaranteed rate of return for the CHP system at the outset of the project. The project includes a lump sum of $81 million for the design and construction of the facilities. In addition, PES has signed a 15-year operational agreement worth $89 million to operate and maintain the CHP facilities. The operations contract includes a series of performance guarantees that stipulate the amount of steam and electricity to be produced under a wide range of operational parameters, including digester gas quality, ambient temperatures, and operational hours of the equipment. There are significant penalties for the DBO team should the system performance not be met. These performance guarantees allow DC to establish a financial model that reliably predicts the long-term cost of the energy generation and the overall CHP system. Finally, a long term operations contract allows the cost of CHP O&M to be projected and guaranteed by the contract operator based on their experience which likely leads to reduced overall O&M cost due to their experience.