Optimum Combustion for Reduced Greenhouse Emissions

Last week, I highlighted some of the greenhouse gas regulatory reporting changes happening here in the U.S. I checked in with Emerson’s Doug Simmers, a global product manager in the Rosemount Analytical Combustion Gas business. Although alternative sources of energy continue to grow as a percentage of energy consumed, combustion of oil, gas, and coal is forecasted to be the dominant source of the world’s growing energy needs for the next several decades.

Carbon Dioxide (CO2) is a natural byproduct of combustion. Unless a technological breakthrough in usable energy production occurs, combustion and its greenhouse gas byproduct will be with us for quite a while. Doug stressed to me that combustion flue gas analysis is key in reducing greenhouse gas emissions. Some sources of energy such as natural gas with its high hydrogen (H2) content produce much less CO2 than other sources such as coal, which has a higher carbon content.

For process manufacturers with utility boilers, industrial boilers, kilns, process heaters, catalyst regenerators, incinerators, etc. maximizing combustion efficiency minimizes greenhouse gas production.

Combustion Efficiency
The relationship between excess air, carbon monoxide (CO), CO2, and oxygen (O2) is shown in the graph. Doug notes the counterintuitive point that maximum combustion efficiency actually occurs at the point of maximum CO2 production. Less energy is produced as you move away from this point, which in turns causes the combustion process to run longer to produce the same amount of energy. The extra time required actually means more CO2 is produced.

To operate in the chart’s “blue box” area of maximum combustion efficiency, it’s important to continuously analyze the flue gas and close the fuel/air ratio control loop. Unfortunately, the flue gas does not always have a homogeneous distribution, especially when multiple burners are involved.

Doug described how a stratification profile is developed using an array of Oxymitter transmitters of varying lengths. This array is used for balancing the burners, detecting burner fouling, discovering poor fuel distribution and spotting variations among units. He noted how the oxygen probes made with zirconium dioxide (ZrO2) “fuel cell” technology were well suited since they operate well at elevated temperatures, which permit an in situ (in place) design.

The output from these ZrO2 sensors is inverse, and logarithmic. This means that the signal increases at the low O2 levels commonly experienced in combustion processes. The accuracy is stated as a “percent of reading”, as opposed to the conventional “percent of full scale”. The accuracy actually improves at lower O2 levels. The life of these O2 cells can exceed 3-5 years, depending on sulfur levels, which shorten the cell’s life.

Given that fossil fuel combustion will be around for a while, maximizing the efficiency of this combustion helps minimize greenhouse emissions. The place to begin is with better flue gas analysis to maximize heat rate, balance la rge furnaces, and diagnose operational problems.

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