Digester Gas to Biomethane--Design, Construction and Commissioning Lessons Learned

INTRODUCTION The digester gas that is produced at Metro Vancouver's existing Lulu Island Wastewater Treatment Plant (LIWWTP) mesophilic digesters and digested sludge storage tank (DSST) is collected and burned in the plant's boilers to provide process and building heat. Surplus digester gas flows to the waste gas burners and is flared. Metro Vancouver desired to upgrade the surplus digester gas produced from the digesters and deliver pipeline quality biomethane (renewable natural gas, RNG) to the natural gas utility instead of flaring. The LIWWTP is a secondary wastewater treatment plant in Richmond, BC with effluent discharge to the main arm of the Fraser River. The plant was originally commissioned in 1973 and serves a population of approximately 200,000 people in the Richmond area. The liquid stream process includes coarse screening, primary treatment (pre-aeration grit removal and sedimentation), and secondary treatment (trickling filters, solids contact tanks and secondary clarifiers). The solids stream process includes gravity thickening for primary sludge, dissolved air flotation thickening for secondary sludge, anaerobic digestion, and digested sludge dewatering using centrifuges. Metro Vancouver engaged AECOM Canada Ltd. to provide design and construction engineering services at the LIWWTP to upgrade the surplus digester gas produced from the digesters to pipeline quality biomethane gas and deliver to the natural gas utility. The Sludge Gas Treatment (SGT) system is sized to upgrade surplus digester gas, which is defined as the portion of total digester gas not used by boilers for plant process and building heating needs, for a range covering the present surplus digester gas flow through to the design year of 2035. By upgrading the surplus digester gas, wasting events (flaring) will be minimized and greenhouse gas (GHG) emissions from the LIWWTP will be reduced. Construction of the LIWWTP SGT system was completed in September 2021 and it is currently in operation. DISCUSSION The focus of this discussion is on the challenges faced during the design and construction of the SGT system, including technology selection, fluctuating surplus digester gas flow, process design and control considerations, and lessons learned from project construction and operation. Technology Selection In order to permit the injection of digester gas that is produced at LIWWTP into the natural gas grid, it must be cleaned and upgraded to pipeline quality biomethane. Purification of digester gas involves removal of particulates, water, carbon dioxide, contaminants such as hydrogen sulphide and other trace contaminants. One of the main considerations for the evaluation of the available digester gas upgrading technologies was whether the manufacturer had proven installations with equipment manufacturers' support in North America. After review and consultation with equipment manufacturers of digester gas cleanup technologies, four technologies were considered for detailed evaluation: -water scrubbing -membrane separation -pressure swing adsorption -amine scrubbing. Some of the factors considered in the evaluation of the four technologies included: -Ability to upgrade digester gas quality to be in compliance with the natural has utility Biomethane Acceptance Specifications -Capacity -Turndown and expandability -Capital cost and life cycle cost -Carbon dioxide (CO2) waste gas stream disposal -Operating and maintenance costs -Ease of operation and maintenance -Noise level -Greenhouse gas emissions generation. Ultimately, water scrubbing was selected as the preferred digester gas upgrading technology for this project. Digester Gas Production The LIWWTP SGT system is sized to operate effectively at the design flows expected over the next 10-15 years. The surplus digester gas projections were based on digester gas production data (surplus flow to the waste gas burners) provided in process control reports from 2002 to 2015. The process control reports reveal that there are frequent changes in flow rate from the existing gasholder to the waste gas burners. The monthly average daily flow rate of digester gas over a 2-year period is presented in Figure 1. The significant flow variations would be below the minimum turndown of the SGT system, which would interrupt operation and significantly reduce the production of treated digester gas. Several options to attenuate the fluctuations were considered including running the boilers at steady state, using the existing gasholder, providing a new gasholder, increasing the size of the SGT system, and adding a small new boiler. The preferred approach was determined to be adding a new membrane gasholder to attenuate the surplus gas flow and decouple the SGT system from the existing gas management system. The new gasholder, shown in Figure 2, also acts as a control device for determining the digester gas feed rate to the SGT system. Process Design & Control Considerations The existing gasholder, which is connected to the digester gas header, maintains the digester gas at a constant pressure; the gasholder has a weighted piston which rides up and down as digester gas accumulates or is released from the gasholder. Primary control of the gas management system at LIWWTP is based on the piston level in the existing gasholder. When the piston level exceeds a specific setpoint, level control valves modulate open to allow surplus digester gas to the new SGT gasholder or to the waste gas burners. To provide a more stable flow for the SGT system, a number of control methods and gasholder storage volumes were reviewed, including: -time averaged flow control -constant flow based on surplus digester gas average daily flow -constant flow, fill and draw operation, -PID control operation, and -flow to level ratio control. Flow to level ratio control was selected as it provides a reasonably stable flow to the SGT system with low flaring and the lowest required storage volume. LESSONS LEARNED The lessons learned from the design and construction of the LIWWTP SGT project are summarized below: Fluctuating gas production can be managed through storage -Pressure conditions of the gas supply need to be measured and documented in detail in the early stages -Finding a balance between footprint and piping/equipment layout requirements -Programming/integration with existing facilities needs careful consideration -Changing weather conditions require a review of heat tracing requirements for piping and tanks -Options to recycle off-spec purified gas can further reduce wasting CONCLUSIONS The LIWWTP SGT System upgrades surplus digester gas to pipeline-quality biomethane (RNG), significantly reducing digester gas wasting events. In 2022, on average, the LIWWTP has seen a 78% reduction in wasting events when compared with previous years with no SGT system in operation. During summer months, the wasting events were reduced by as much as 99%. The system has been consistently producing greater than 98% methane and is projected to generate 60,000 gigajoules of RNG for the natural gas utility, enough to supply 660 typical homes.