Analyzing Flue Gas Emissions

Back in January, I shared some thoughts from Emerson’s Doug Simmers on improving combustion in a post, Optimum Combustion for Reduced Greenhouse Emissions. He’ll be presenting later this year on flue gas analysis and I was able to get my hands on a rough draft. I’ll highlight a few of his early ideas and hope to have more later this year once the paper has been presented. For those without combustion-related processes, flue gas is defined:

…gas that exits to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator.

Plants have long monitored their combustion flue gas for excess oxygen and carbon monoxide in order to operate at the best efficiency, or heat rate and lowest carbon dioxide (CO2) and nitrogen oxide (NOX) point. In plant operations, furnaces never achieve ideal conditions and vary with changing loads or firing rates.

Doug describes a curve with % excess O2 on the y-axis and % steam flow (or % fuel flow) on the x-axis. The ideal O2 level varies along a curve with the firing rate. Over time, the burner wears out and the curve needs to be reestablished. Curves established for natural gas and light oil fuels may remain valid for years while heaver fuels such as coal, petroleum coke, solid biofuels, etc. may plug the burners and cause the need for new optimum O2 curves to be reestablished.

He notes the other goals may dictate the point of best combustion efficiency. One example might be minimizing thermal NOX produced. Control strategies include starving the burner of air to reduce the temperature of the fuel/air mixture at the burner and adding overfire air to complete the combustion. He notes that neural networks are often used to determine the optimum air required. Another approach is to recirculate the flue gas to mix with the combustion air. Oxygen probes can measure and control the O2 going to the burner.

Some the technologies used in flue gas analysis includes zirconium oxide (ZrO2) fuel cell oxygen analyzers to measure flue gas oxygen levels. Infrared spectroscopy measures carbon monoxide levels. It’s usually desired that levels be below 200 PPM. If the CO levels start climbing, it’s often an indication that the fuel/air mixture is becoming too rich.

In the paper, Doug highlights an emerging trend to use the flue gas analyzers as a diagnostic tool for detecting furnace problems such as fouled classifiers and burners, coal roping, leaks in air heaters and duct seals, and excessive slagging. I hope to share more about these and their impact on flue gas emissions in a future post.

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