The Orange County Sanitation District (OCSD) was faced with a challenging problem. The South Coast Air Quality Management District (AQMD) mandated OCSD to bring their power generation operation in compliance with the applicable emissions regulation. This meant that 22.5 MW of generated power was at jeopardy unless the regulations were met across the operating range of the plant. Given that OCSD’s power plant operated on fuel produced at the plant, the consequence of buying power from the grid would have been prohibitively expensive. As Orange County has one of the nation’s highest utility rates, OCSD would have incurred utility costs in excess of $30,000 per day if forced to cease their power generation operation. Additionally, the continuous production of digester gas would have had to be flared.
OCSD operates two separate power plants utilizing digester gas as the fuel for eight Cooper Bessemer LSVB reciprocating engines (three 12-cylinder engines at plant 1, five 16-cylinder engines at plant 2) driving generators. The digester gas, fueling the engines, is the byproduct of aerobic bacteria which are a key ingredient of the waste water treatment. In essence the fuel comes at no cost. The exhaust from engine exhaust is routed through boilers to produce steam for heating water and to produce additional electrical power via a steam turbine generator. The hot water is maintained at a precise temperature which in turn heats the waste water through a heat exchanger to an optimum temperature for the bacteria to thrive.
As the daily plant load varied, the engines were not operating at their optimum design; the point which the OEM guaranteed the emissions levels. The engines were originally supplied with OEM standard En-Tronic® FT-100 unit control systems, and two OEM En-Tronic® FT-50s achieved the role of station or Master Control System.
The project requirements were to:
- Reduce emissions – CO by an average of 35% across the load range
- Improved system reliability and availability
- Improved operating range
- Improved safety
The United Controls Group was awarded the contract to supply new engine controls, steam turbine generator controls and a new state of the art master control system. Upgrades by UCG included an all-new PLC controller and all the associated programming and safety systems, including upgrades to the air-fuel ratio controller.The project uses Modicon Quantum PLC controllers, with a Wonderware Archestra-based HMI. The HMI configuration uses common object and blocks across both plants, allowing for rapid deployment of graphical or functional changes with minimal downtime. Redundant servers and desktops, and network switches ensure high availability. The HMI upgrade brings with it full remote-start/stop, live and historical data trending, alarm history and notification (text message and email), and security. The new system also interfaces with the plant’s CEM, Continuous Emission Monitoring System.
Black-start capability was also expanded and improved as a part of the control system upgrade. The facility can be self-started from a total blackout condition in the event of a catastrophic power loss. Additionally, logic was implemented that allows the engine to start on pure digester gas in the event of a gas line disruption from the utility.
Heat energy is recovered from the engine exhaust systems and used for plant heating and cooling, which is controlled by the Master PLC. Upgrades here include a full hot-standby PLC solution, multiple pumps running in lead/lag configuration, and full breaker remote control for the safety of the operators. Energy savings are expected in the form of improved control (PID) of the plant-wide heat loop.
As the first of the upgrades were commissioned in the summer of 2010, the results proved to exceed the expectations of OCSD and AQMD.
The upgrades have resulted in a cleaner-burning, more reliable engine operation, in part due to UCG’s Air Fuel Ratio (AFR) technology. The AFR optimizes the combustion stoichiometry by utilizing a UCG developed algorithm to operate the waste gate valve of the turbo charger based upon continuous fuel flow measurements and gas composition data.
Previously OCSD was only able to operate within in (emissions) compliance between 65% and 100% load, now they can operate within compliance between 45% and 110% load.
The natural gas operational window has been increased by 40%, and
The digester gas CO emissions have been improved by an average of 35% across the load range.
Safety and reliability were improved by modernizing the critical shutdown instruments and providing redundancy in the systems.
UCG has worked closely with Marsh Creek and Black & Veatch throughout the project. The project began in 2009 and will run through 2012.