Combustion

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 (NO_X) 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 NO_X 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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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.

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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If you're in the coal-fired power generation business, you may know that Coal-Gen 2009 is going on this week. During the conference, a Midwest power producer and the Emerson Power & Water Solutions team gave a joint presentation on combustion optimization.

Emerson's Jeff Williams, one of the presenters, was kind enough to send me a copy so I could relay a few highlights in this post. The presenters discussed how they were able to optimize the combustion process to reduce NO_X levels beyond the guarantee level.

Coal-fired power plants are impacted by many dynamic factors including source fuel type & quality, market deregulation, tightened emission standards to name a few. Costs for NO_X and SO₂ credits have increased over the last few years.

There are many pre- and post-combustion technologies available to reduce NO_X and SO₂ emissions, each with its own cost-benefit ratio--investment cost of the technology vs. the %NO_X reduction.

For the project described in the presentation, the team benchmarked pre-project NO_X, O₂, and steam temperature levels and burner tilt performance. Two improvements were identified, the addition of separated OverFire air (SOFA) dampers & tilts and combustion optimization in the plant's Ovation control system.

The OverFire air process redistributes air within the boiler combustion zone and injects additional air above the combustion zone to complete the combustion process. Decreasing the air within the burner zone lowers stoichiometry, which lowers the flame temperature and reduces thermal NO_X. This also reduces the tendency of fuel-bound nitrogen to oxidize to nitrous oxides.

To compensate for temperature excursions caused by rapid changes in SOFA positions, advanced control strategies were developed. These control strategies were based on an advanced non-linear, fuzzy-neural NARMAX (FNM) algorithm.

The team followed a multi-step process, which included a study of the current combustion process, DCS control improvements, parametric testing, model development, open-loop testing, closed-loop testing, and commissioning.

For this project's optimization model NO_X and CO were the control variables. Manipulated variables included the OFA and SOFA dampers, SOFA tilts, O₂ trim, auxiliary air dampers, window-to-furnace differential pressure, fuel air dampers, and feeders. The disturbance variables included load, ambient temperature, total air flow, and burner tilts demand.

Over the multi-year process that included the combustion optimization, followed by the SOFA equipment, followed by the advanced control optimization of the SOFA equipment, the plant reduced annual NO_X output from over 1400 tons to under 600 tons.

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Last week at the ISA Expo in Houston, I sat in on a great session featuring Emerson's Ed Bailey, as well as folks from Siemens, Ametek and a private consultant with years of experience with Dow Corning. The session was entitled, Energy Management Issues for Process Optimization, and it had the following description:

Subjects open for discussion in this session include nearly anything relevant to this topic, not just process control and instrumentation. Expect discussions regarding process maintenance, process modifications, maybe whole new processes that were less cost effective under the old energy cost structure but now are more cost effective.

Ed leads the technology development efforts for the Rosemount Analytical Gas measurement products. He kicked off the panel discussion showing the forecasted growth of energy production. From an ExxonMobil outlook study, most of the world's growing energy needs will continue to be met by the combustion of oil, gas, and coal.

To help manage the carbon emissions, to deal with the increases in fuel costs over their historical averages, and to operate in an environment with increasing governmental regulations, process manufacturers have an ever-increasing need for improved combustion flue gas analysis. The best way to minimize carbon dioxide (CO₂) emissions is to operate existing combustion processes at their maximum efficiency.

Ed described some of the existing industry practices like averaging the output of a few analyzers as not providing enough insight to diagnose and optimize the burners. Burner differences and stratification are normal conditions that this averaging strategy does not well address. Instead, Ed recommended a mix of oxygen (O₂) and carbon monoxide (CO) measurements be used combined with neural network strategies that enable more complex models to be created to maximize efficiency versus the load/fuel variations--and to minimize mono-nitrogen oxide compounds (NO_x). The key point is that more discrete measurement points, which in turn feed more sophisticated control algorithms, will drive efficiency.

One of the discussion points during the session was the use of zirconium oxide (ZrO₂) oxygen analyzers to measure the residual oxygen remaining in the flue gases from any combustion process. Ed mentioned the Rosemount Analytical in-situ oxygen transmitter as an example of a zirconium oxide oxygen analyzer to help provide data to better control and optimize the combustion process.

An interesting question came into the panel about the safety considerations of running the combustion process right on the edge at its most efficient but potentially dangerous point. The panel had good thoughts that you need to separate the control aspects from the safety instrumented system burner management aspects. Like any process with safety risks, a risk analysis and risk mitigation strategy per the IEC 61511 international safety standard is critical.

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