A New Approach to Maximize the Potential of Reciprocating Engines Operating on Bio-Fuel Energy

Since the Industrial Revolution one of the oldest and “greenest” bio-fuel energy sources has been the byproduct of sewage and landfill. These biogases also known as Land Fill Gas or Digester Gas can be used as a fuel in an internal combustion engine, the clear choice for their efficiency in heat recovery and utility as a prime mover. The problem with bio-fuels is their unpredictable and varying fuel heating values which creates a challenge for maintaining air fuel ratio (AFR). If AFR is not controlled this can lead to engine instability and an increase in NOx, CO and THC emissions. With today’s ever increasing scrutiny of combustion pollutants this could spell the end of these types of fuels in combustion engines. AETC has embraced this challenge to provide a system that addresses the seasonal fuel gas quality, Low Heating Value (LHV) fluctuation to operate engines at best achievable emissions. This case study focuses on two Caterpillar 3516 Generator Engines rated 1000VA, at 1200 rpm, lean burn gas and turbocharged, running on renewable energy source supplementing power to a waste water treatment facility in California. The engines operate on wide range of fuel mixture including landfill, digester gas and air blended natural gas over a heating value range from 350–650 BTU. The fuel gas LHV constantly varies depending on fuel availability controlled by pressure switches within the individual fuel headers. Determining fuel heating values by using a gas calorimeter is not a viable option due to its high cost and poor reliability when operating in the environment of unfiltered Digester and landfill gas. AETC installed their Advanced Monitoring System (AMS) to utilize the engine as a calorimeter and to determine the fuels LHV. As part of the AMS functionality, the system acquired all the existing AFRC parameters such as kilo-Watt, RPM, Fuel Flow, Air Manifold Pressure and Temperature to determine the combustion performance. This simple approach offers surprisingly good performance while tying together basic thermodynamics, combustion performance and emissions. The system can also be used to parametrically determine engine emissions, based on the calculated combustion pressure without installing pressure sensors. The AMS monitors and determines emissions based on Trapped Equivalence Ratio, Effective Bulk Temperature or Pressure Ratio on single or multiple fuels providing a green/red light as an indicator of in/out of compliance accurately meeting today’s most stringent regulatory conditions.