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Air/Fuel Ratio Control for Landfill Gas Engines


PROJECT OVERVIEW:

GCS has supplied an air-fuel ratio control and actuation system for a power generation plant at a landfill facility. The plant utilizes three Caterpillar G3516LE lean burn gas engine-generators each rated for 850 kW @ 1200 RPM. The plant is run in parallel to the local utility in base load operation. The company operates multiple landfill sites across the state with each site utilizing different quantities of the same engine model.

TYPE OF FACILITY
  • • Landfill Power Generation
TYPE OF PLANT
  • • Base Load Utility Paralleled
PRIME MOVERS
  • • (3x) Caterpillar G3516LE Lean Burn Gas Engine • Generators Fuel Pressure: 1-5 psi Fuel System: lmpco • Carbureted Fuel Type: Variable Quality Landfill
  • • Ignition System: Caterpillar EIS 4160 V, 850 kW, 60 Hz@ 1200 rpm
CONTROL TECHNOLOGIES
  • Woodward E3
  • • Integrated gas engine control system
    • Versions for rich burn and lean burn engines
    • Closed-loop on kW feedback. No Oxygen sensors required
    • System covers a variety of applications including pipeline natural gas through fluctuating low-quality fuels
    • System is scalable to a variety of engine sizes
Woodward 2301 D Woodward Flotech Woodward ProAct ISC
• Digital Speed Control
• Load Sharing Device
• Throttle Bodies with Integrated Electric Actuators
• On-Board Drivers
• Digital Electric Actuator
• On-Board Driver

The new air-fuel ratio control system was implemented for multiple reasons. First was the desire to keep the engine emissions in control as the engines did not previously have emissions control systems. Second was the poor and inconsistent engine starting performance the operators were experiencing.

The control system features the Woodward E3 System Platform with dual Flotech throttle bodies used for fuel control. Although this air-fuel ratio controller is available with integrated speed control in the same unit, the decision was made to utilize the version without speed control. The various plants across the state utilize the same 2301 D governing system across the board, and the desire was to keep them the same. As part of this project, GCS interfaced the E3 control with the existing 2301 D controls, however the on-engine actuation was changed. The engines originally had hydro mechanical Woodward EG3P actuators that were changed to electric ProAct actuators. To support this upgrade, GCS installed a standard kit developed in-house specifically for this purpose.




EXCLUSIVE TECHNOLOGY: GAS QUALITY CLOSED LOOP OPERATION

Prior to installing the new E3 system, the operators had to manually throttle the fuel back with hand valves in the fuel lines and constantly nurse the fuel to the engines until they started. Even then, the process was never consistent from one start to another.


APPLICATION ISSUE: INCONSISTENT STARTING ON LANDFILL GAS

In a generator application, the E3 Control System can be operated in two different closed-loop modes: UEGO Closed Loop or Gas Quality Closed Loop (GQCL).

UEGO Closed Loop utilizes a traditional oxygen sensor in the engine exhaust. The drawback to this mode of operation is the requirement for an oxygen sensor installed in the engine exhaust to be used in the system at all times. Since standard oxygen sensors on the market are designed for automotive-style gasoline engines, not gas engines, both the accuracy and lifetime of traditional sensors in operation on industrial gas engines are typically issues.

Gas Quality Closed Loop (GQCL) is a mode of operation that is built into every Woodward E3 control. This allows the engine to run closed loop not on an oxygen sensor, but on kW load from the generator.

GQCL Block Diagram

 

Gas Quality Closed Loop mode uses the measured generator power (kWe), manifold pressure (MAP), and manifold temperature (MAT) signals to infer changes in gas quality.
The main advantages of this mode of operation over UEGO closed loop are:

1. The oxygen sensor is not required for normal operation. The oxygen sensor is used only for initial setup and commissioning of the system and is no longer required. This eliminates costly replacement of sensors over the lifetime of the system and the dependence on unreliable devices.

2. The system can compensate for larger changes in gas quality. GQCL mode results in better overall performance due to the engine being able to more accurately control the air-fuel ratio over larger swings in gas quality.

GQCL operation is based on algorithms that are exclusive to Woodward. The only required addition to the system is a kW load feedback signal into the E3 control. In the case of this project, the existing 2301 D contained kW load measuring functionality. An analog output proportional to generator load was programmed and sent to the E3 control to enable the use of GQCL.

REPLACEMENT ACTUATION

The Caterpillar G3500A Series engines were commonly supplied from the factory with Woodward EG3P hydraulic actuators and 2301A speed controls. Although this system was and still is widely used, it does present a few problems. First, the actuator itself is dependent on a clean engine oil supply for its internal pump to function. Second, the original engine design from Caterpillar includes an inconvenient linkage design between the actuator and throttle body that has multiple pivot points and long linkage rods. Each joint introduces another opportunity for shaft play which results in unstable control. In recent years, the Caterpillar factory has changed this throttle actuator design on certain G3500A series engines as a result of these issues. GCS has designed an actuation package that replicates this new factory engine design utilizing the Woodward ProAct ISC actuator.

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