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Part 2: Oxygen Issues With Energy-Efficient Boilers
By Ryan T. Evans, P.E.
June 2012 -
Another method for increasing efficiency is to apply low-oxygen carbon monoxide technology. Oxygen is required for combustion, but excess oxygen lowers the flame temperature by as much as 500-1,000 degrees in the furnace.
Lower combustion temperatures mean a smaller temperature difference across the heat exchanger, and a smaller temperature difference means less heat transfer. The result is lower efficiency and more fuel waste.
Ideally, oxygen in exhaust gases should be controlled to 3 percent or less across the burner's firing range. When technicians set up a boiler during commissioning, they set the oxygen to 3 percent at high fire, but most burners cannot maintain a stable flame at low-fire conditions with increased excess air. Several burner manufacturers have developed technologies that allow operation at or below 3 percent oxygen across the firing range. A burner retrofit to a high-efficiency burner can result in gas savings up to 20 percent annually.
Besides lowering the flame temperature, higher excess air increases the velocity of the products of combustion, meaning they have less time to transfer their heat before they move through the stack. Low-oxygen technology exists that can result in 3 percent or less oxygen across the entire range of firing rates, which can help efficiency in two ways. It keeps flame temperatures high, and it allows the products of combustion more time to give up their heat to the steam.
Consider the stack losses of a traditional high-pressure steam system with a swirl-type burner and stack temperatures of 350 degrees. It operates at 8 percent oxygen with stack losses of 20.8 percent. By operating at 3 percent oxygen, stack losses drop to 17.9 percent. This results in slowing down the products of combustion, which leads to a typical 30-40 degree temperature drop in the stack. By decreasing the stack temperature from 350 degrees to 310 degrees, the stack loss drops another percent to 16.9 percent — savings of almost 4 percent.
Part 1: Energy-Efficient Boilers Require Proper Maintenance
Part 3: Short-Cycling and Energy-Efficient Boilers