Automatic, Variable Ratio, Firing of a Blend of “Free” and Purchased Fuels

-Peter Lavelle, Danbury CT

“Free” fuels are available from many processes: Digester Gas from wastewater treatment plants, Bio-Gas from agricultural and food processing plant waste, Hydrogen from chemical plants, Off-Gases from a Refinery, etc.

The Challenge

Firing these “free” fuels in a boiler, process heater, or engine-generator significantly reduces operating costs and can significantly reduce greenhouse gas emissions. However, the Btu content and flow rate of these “free” fuels vary based on the Processing Plant operations, are frequently insufficient to meet the steam, hot water, or electrical output required from the Utility Plant.

The Solution

A simplistic approach to satisfying the Utility Plant output demand is to run multiple units with some burning only “free” fuel, and some burning only Purchased fuel. Besides the higher capital cost for multiple units, this can also result in units running at lower loads and thus lower efficiencies. Even worse, it can result in units with low turndown capability to continuously cycle on/off –further lowering efficiency, and potentially upsetting the Process Plant operations.

A better approach is to provide a control system and burner that allows multiple fuels to be fired simultaneously in a single unit with a variable Fuel Ratio (“free” fuel versus Purchased fuel). The control logic is designed to burn as much “free” fuel as possible while simultaneously meeting the Plant demand for Steam, Hot Water, or Electrical Output.   

The Fuel Ratio is automatically determined by a level or pressure transmitter that indicates the “free” fuel availability. When “free” fuel availability trends lower, the Fuel Ratio smoothly shifts to burn more Purchased fuel..without shutting down. When “free” fuel availability trends higher, the Fuel Ratio shifts to more “free” fuel –without shutting down.

For both approaches (multiple single fuel units versus blended fuel units), if the Btu content of the “free” fuel varies significantly, then a Methane or Wobbe analyzer may be required to maintain an adequate fuel/air ratio.  

If a burner has multiple fuel injectors, the blending can occur in the furnace. An external fuel blending skid can also be used for any burner, but a blending skid is required for an engine generator.

Our Experience

Preferred Utilities has been manufacturing state-of-the-art combustion controllers and burners for over 40 years. Preferred has provided Variable Ratio, Blended-Fuel systems for over 30 years. Systems have ranged in size from a single 75 Bhp (2.6 klbs/hr) boiler, to multiple 250 klbs/hr boilers, and multiple 3 MWe (continuous) Engine-Generators. “Free” fuel powered Combustion Joy!

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