Utilizing Flue-Gas Analysis to Improve Combustion-System Performance
This article was originally published on October 6, 2014.
This article represents the third part of a very thorough discussion of combustion-system performance. The other two parts were run as web exclusives on www.industrialheating.com in July and August, and some of the “heavy technical lifting” has already been done. Figures are consecutive as included in the original treatment, so numbering will seem irregular. If you are looking for supporting equations and additional examples, please check out the full online treatment.
Industrial combustion systems employ various methods to control the flow of fuel (primarily natural gas) and oxidizer (air, O2 or combination). The combustion-control system must control the flow of oxidizer in the correct proportion to the fuel flow in order to maintain the desired furnace atmosphere chemistry (oxidizing, reducing or neutral).
Controlling Atmosphere Flows
Fuel, combustion air and/or O2 flows are controlled by adjustable flow-control valves. These gases are supplied under pressure and are “pushed” into the furnace through the burner(s). Flames propagate through the furnace delivering heat, and the hot flue gases exit out the furnace flue, imposing a negative pressure “stack draft” on the furnace. This stack draft can cause significant quantities of infiltration air to be “pulled” through the furnace depending on control of the furnace pressure, which is typically controlled by an adjustable flue damper. Infiltration air is pulled through cracks or gaps in the furnace refractory, door seals, burner mounts or other access ports. The more negative the furnace pressure, the greater the quantity of infiltration air. The result is that the infiltration (dilution) air will cool the furnace and reduce thermal efficiency, thereby increasing fuel costs. Infiltration air will also increase O2 content in the furnace atmosphere, which can be detrimental to many heating/melting process loads, such as aluminum melting or steel billet reheating.
Overly positive furnace pressure can also be detrimental. Under positive furnace pressure, flames and/or hot furnace gases can squeeze out of furnace cracks, gaps or door seals (stingers), causing damage to these areas or causing unsafe conditions for personnel. In some processes, positive furnace pressure can cause smoke or fumes to escape outside of the exhaust-handling system, creating fugitive emissions that can be dangerous and potentially in violation of environmental regulations.
Because of these detrimental effects of positive furnace pressure, many furnaces err on the side of excess negative pressure out of practicality. For example, flue cross sections can be oversized and/or flue dampers maintained in wide-open positions without adjustment.
In order to accurately control furnace pressure and thereby accurately control the real overall oxidizer/fuel ratio and furnace atmosphere composition, the furnace flue damper should ideally be controlled just as carefully as the fuel, air and O2 flow-control valves. In reality, controlling a furnace flue damper can be much more difficult than controlling a gas valve. In many cases, therefore, it is not controlled as carefully. Flue-damper control can be especially important during low firing-rate conditions (holding or idling) and when employing oxy/fuel burners because the reduced combustion-product volumes can cause infiltration air to increase.
Controlling the Combustion System
To properly control any heating or melting furnace combustion system, the following parameters need to be known by the operator:
• What is the oxidizer/fuel ratio?
• Are the indicated flow meter and ratio data accurate?
• Is there significant infiltration air being pulled into the furnace? How much?
• What is the actual furnace chemistry (oxidizing, reducing, neutral)? How do I know? How can I be sure? Can it be quantified?
Typically, oxidizer/fuel ratios in combustion-control systems are set based on the assumption that natural gas is 100% CH4. This is the basis for the commonly employed 2:1 oxy/gas ratio and the 10:1 air/gas ratio (which includes 5% excess air). The actual natural gas composition can be different, however, depending on the source of natural gas or even the time of year, which may dictate adjustments to the air/gas or oxy/gas ratios.
For fuels other than natural gas – such as oil or other liquid fuels, including recycled-waste fuels – the exact composition may not be well known.
Also, flow meters for natural gas, other fuels, combustion air and O2 will typically exhibit some inaccuracy. Flow readings can be off by a small percentage or possibly 10% or more. The accuracy depends on the type of meter, its calibration, maintenance and compensating factors such as temperature, pressure, dust level and humidity. It is especially challenging to accurately measure combustion airflow due to the larger volumes, lower supply pressures, higher dust loadings, variation in ambient humidity and temperature, and difficulty of providing long straight-length piping sections to improve flow-meter accuracy.
Considering the combined effects of fuel and oxidizer flow-meter inaccuracies, imperfectly known fuel composition and unknown furnace infiltration air level, the only way to accurately determine actual furnace atmosphere composition (i.e., actual overall oxidizer/fuel ratio inclusive of infiltration air) is by measuring it with gas analyzers.
In many cases, one can determine visually if a flame is oxidizing or reducing based on flame color, shape, etc. But, even with the best visual observation, one cannot quantify the actual flue-gas composition or the extent of oxidizing or reducing conditions. The only way to quantify the furnace atmosphere correctly is by measuring it.
In this paper, furnace flue-gas composition measurements from a copper-melting reverb furnace are presented and discussed. A useful graphical technique (V curve) is employed to compare these measurements to calculated values and to better understand the combustion-control system performance, including calculation of furnace air-infiltration levels.
Measured Flue-Gas Compositions: Field Data from Cu Melting Furnace
Flue-gas composition (O2, CO2, CO, H2) measurements are presented and discussed. Lambda was intentionally varied over a wide range in order to generate a V curve similar to Figure 1. The V curve provides an informative graphical illustration for comparing measured flue-gas compositions to calculated (predicted) values. In this example and those online, the measured data closely follow the V shape of the calculated data.
In other cases, a full V curve “lambda traverse” was not generated, but a plot of O2 versus lambda for even a few data points provides a useful illustration of combustion conditions.
The type of furnace analyzed is a copper reverb batch melt furnace. In the online content, we also discuss a closed-barrel rotary aluminum melt furnace, a well-charged aluminum reverb melt furnace and a glass reverb melt furnace. In all cases, flue-gas compositions were measured with either an empty furnace or during conditions when chemical interaction between the furnace load and the combustion atmosphere is negligible (flat-bath holding for copper).
Throughout the entire article (including online portions), examples of natural gas-fired air/fuel, oxy/fuel and air/oxy/fuel systems are included, and in one air/oxy/fuel example, the furnace was fired with recycled (waste) oil.
Flue-gas compositions are reported as dry basis (H2O removed) because gas analyzers utilize dry flue-gas samples (H2O is removed from the flue-gas sample via chiller/condenser unit).
The presented examples illustrate how flue-gas analysis can provide increased understanding of combustion conditions and control-system operation to improve combustion-system performance. In some of these examples, the flue-gas analysis helped to diagnose and correct problems with the combustion-control system.
Copper Reverb Batch Melt Furnace with Air/Oxy/Gas Firing
Flue-gas compositions were measured from a copper reverb batch melter equipped with a single air/oxy/gas burner. The air/gas ratio controls from the original air/gas burner system were retained and oxy/gas flow ratios for the new air/oxy/gas burner were controlled by a new O2-control system, which was linked to the original air/gas controller.
Flue-gas analysis was utilized to determine if actual furnace atmosphere conditions matched what was predicted by the combustion-ratio settings.
At high fire (5.2 MMBTU/hour with 28% O2 participation), lambda was varied over a wide range to develop a V curve. The actual data followed a V shape, but it was shifted to the right compared to calculated values. It showed that a lambda setpoint value of 1.20 was required to achieve neutral combustion conditions (bottom of the V). The deficiency of total O2 compared to combustion-system settings suggests that there was zero additional infiltration air at high fire (flue damper was in a partially closed position). The furnace also appeared visually to be at positive pressure. Calculated infiltration air at high fire was indeed zero.
Data was also collected for low-fire (holding) conditions. At low fire (1.5 MMBTU/hour with 25% O2 participation), the flue damper was further closed to a nearly closed position. Lambda was again varied to generate a V curve, comparing actual to calculated flue-gas composition. This low-fire data is shown in Figure 5 for setpoint lambda.
Figure 5, with the actual V curve shifted to the left, shows that the furnace is getting more total O2 (O2 plus air) than what would be expected by the lambda setpoints. It indicates that a setpoint lambda of only 0.90 is required to achieve neutral conditions (bottom of the V). At low fire, is the source of “extra air and O2” from air infiltration or from the combustion-control system?
It was noticed that there was a “bias” or offset for this combustion-control system at low fire: Measured lambda values based on gas, air and O2 flow-meter readings were considerably higher than the selected lambda setpoint. The combustion system was delivering more air and O2 than selected at low fire. At high fire, setpoint lambda matched measured lambda more closely, as was the case with the other furnaces studied.
In Figure 6, the same V curve is repeated, but this time the compositions are plotted versus actual measured lambda (from flow-meter values) instead of setpoint lambda. Comparing the two curves shows that measured lambda is 15% higher than setpoint lambda at low fire. Because of this “bias or offset,” the combustion system was delivering 15% more air and O2 than selected. Considering the measured lambda instead of set-point lambda, the V curve is now shifted to the right. This indicates that the furnace is getting about 5% less total O2 than calculated (assuming 100% CH4, per equations in Appendix). Calculation of furnace infiltration-air level, at low fire, indicates zero infiltration air.
All of the low-fire data was with the flue damper in a “nearly closed” position. The damper was then pulled back to the “partially closed” position utilized at high fire. For the same low-firing conditions (1.5 MMBTU/hour with 25% O2), higher flue-gas O2 levels were measured over a range of lambda values with the damper in this more-open position. This data is shown in Figure 7. This suggested higher infiltration air levels with the more-open damper setting.
Calculated infiltration-air levels at low fire with this partially closed damper position ranged from one to two times the natural gas flow in SCFH (using the equations from the Appendix). Infiltration air was calculated to be zero with the nearly closed damper position. Estimated annual natural gas and O2 savings through this low-fire damper adjustment were $5,200/year, assuming low fire for 40% of the time. The reduced furnace atmosphere O2 level during holding can also provide product quality benefits.
Finally, at low fire the burner system was temporarily increased to 75% O2 but with the damper returned to the “nearly closed” position. Flue gas O2 was very similar to the 25% O2 “partially closed” case, and calculated air infiltration was slightly less, about 0.75-1.5 times the gas flow.
In summary, flue-gas analysis provided the following benefits for this furnace:
• Calibration of combustion-control system settings vs. actual furnace atmosphere
• Determined the “offset” in the combustion oxidizer/fuel ratio control
• Calculated the infiltration-air quantity at low fire
• Aided in adjusting damper position between low and high fire
As illustrated in several examples from fuel-fired industrial melting furnaces, flue-gas analysis can be utilized to provide the following benefits:
• Calibrate the combustion-control-system settings to provide the desired furnace atmosphere (oxidizing or reducing)
• Accurately quantify the extent of oxidizing or reducing conditions
• Diagnose combustion-system equipment problems or control-system mis-adjustments, offsets or inaccuracies
• Measure the quantity of furnace infiltration air. This is especially useful when comparing different firing rates and/or varying O2 participation levels.
• Aid in adjusting the flue damper for improved control of infiltration air (furnace pressure) and excess O2 level in the furnace atmosphere to provide energy cost savings and product quality and/or yield improvement.
Graphical techniques are presented that help to illustrate and quantify the combustion conditions (V curve) and provide insight when comparing measured flue-gas composition to calculated (predicted) values.
These beneficial flue-gas analysis techniques are applicable to any fuel-fired heating or melting furnace utilizing natural gas or other gaseous or liquid fuels and employing air/fuel, oxy/fuel or air/oxy/fuel combustion systems. IH
For more information: Contact Stewart Jepson, senior applications specialist, Air Liquide Industrial US LP, tel: 713-896-2332; web: www.us.airliquide.com
Flue-Gas Measurement Techniques
A fair level of technical expertise is required in order to accurately and reliably measure flue-gas compositions from combustion-gas samples. Accurate gas analyzers must be used, and the appropriate type of analyzer should be selected based on whether the process data is to be taken as a “snapshot” or continuously recorded with time. It is important to clean and dry the combustion-gas sample before sending it to the analyzer(s) since gas analyzers typically will become fouled if sent wet gas samples containing H2O vapor or liquid (H2O is always present in hydrocarbon combustion gases).
Typically, a small initial crude or gravimetric water drop-out chamber is employed because much of the water (and dirt) will first drop out in this chamber. Downstream, the sample can then be more finely dried with a refrigerated condenser unit, or “sample chiller.” In many combustion processes, especially scrap melting furnaces, the combustion gases can be quite heavily dust-laden, even containing sticky slag-like particles. This dirt must be filtered out of the sample before sending it to the analyzer. The sample probe material and size and location in the flue are also very important considerations. Often, the sample probe will clog, and it can periodically need to be blown out with compressed air or replaced entirely. Water-cooled sample probes may be desirable at very high temperatures.
Parts 1 and 2 can be viewed online as can the appendix, which is referenced in this article. Read each by using www.industrialheating.com/FG1 or /FG2.