Recover the Dollars Going Up Your Boiler Stack

06/12/2013 | By Eric Woodroof, Ph.D., CEM, CRM

Energy waste can be harnessed to increase efficiency

The exhaust gases from most natural gas-burning boilers are vented to the outside – and quickly forgotten. But there is much value in the heat and water in that waste stream. Recovering these “assets” reduces energy costs and carbon footprint.

In 2012, U.S. industry and commercial buildings consumed approximately 10 trillion cubic feet of natural gas. However, 30 to 50% of this volume is wasted energy going up the flue. And although natural gas is cleaner than coal, every 1 million BTUs of energy recovered means a reduction of approximately 117 pounds of CO2 in the atmosphere.

How to Determine Your Boiler’s Waste
When performing annual boiler/burner tune ups, most maintenance provide a report showing combustion efficiency, which is basically a measure of the fuel-to-air mixture. Although combustion efficiency is important, it does not reflect a boiler's overall efficiency, which includes the amount of energy (post-combustion) that is NOT transferred within the boiler and goes up the flue stack. If you can identify the data points below, you can determine what you are wasting up the flue:

  • What is the exhaust temperature leaving your flue/chimney?
  • How many cubic feet of gas per hour goes into your burner?

Eric A. Woodroof, Ph.D., is the Chairman of the Board for the Certified Carbon Reduction Manager (CRM) program and he has been a board member of the Certified Energy Manager (CEM) Program since 1999. His clients include government agencies, airports, utilities, cities, universities and foreign governments. Private clients include IBM, Pepsi, GM, Verizon, Hertz, Visteon, JP Morgan-Chase, and Lockheed Martin.

Condensing flue gas heat recovery units or “condensing economizers” can help you recover waste heat and use it to preheat air or water going into the boiler or to preheat another fluid. This technology is designed to work with large natural gas appliances used in the commercial building industry as well as the process industry. Any facility with a large boiler, including hospitals and campuses, can benefit from condensing economizers, which typically have paybacks of 2 to 3 years.

In contrast to other exhaust gas heat recovery equipment, condensing equipment is designed to bring the exhaust temperature down below the condensation point, creating a slightly acidic/corrosive “rain” from the exhaust gas. Taking the temperature this low allows the equipment to recover the maximum BTUs. The upside is that natural gas appliances can operate at well over 90% energy efficiency, and even 98% efficiency in some applications. The downside is that the recovery unit requires anti-corrosive materials (stainless steel, aluminum, etc.) to handle the acidity. The condensate has a pH of approximately 4.5, which can easily be neutralized.

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