Oscillating Combustion on a High Temperature Forging Furnace

This page updated November 23, 2005.

Gas Technology Institute

Oscillating Combustion on a High Temperature Forging Furnace

CARB Grant Number ICAT99-1


The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Background
The objective of this project was to demonstrate the reduction of emissions and operating costs of a direct-fired industrial forging furnace located in California by improving its combustion performance. The specific objectives were to reduce NOx emissions by 50% and increase thermal efficiency by 5% while maintaining product quality and temperature uniformity. The results of this project will demonstrate the environmental and cost advantages of this new combustion technology, helping to speed its deployment among the California industrial customer base.
Oscillating Combustion, developed by the Gas Technology Institute (GTI), can meet these emissions and efficiency challenges with low-cost, easily-retrofitted modifications to existing industrial burners and controls. Oscillating Combustion is a simple, innovative, low-cost technology that can be applied to a wide range of high-temperature air/gas- or oxygen/gas-fired industrial process such as forging furnaces, glass melters, steel reheat furnaces, aluminum melters, etc. Laboratory results have shown that NOx emissions can be reduced by 65% to 90% while simultaneously increasing heat transfer to the load by as much as 10%. To achieve these results, the technology requires only that a new fuel flow control valve be installed on the fuel line ahead of each burner and the gas supply pressure be adjusted appropriately. A custom valve control system is then used to oscillate the air-fuel ratio above and below the stoichiometric ratio, thereby producing alternating fuel-rich and fuel-lean zones in the flame.
Since combustion under both fuel-rich and fuel-lean conditions produces low levels of NOx, the NOx formed in each zone is significantly lower than that which would occur if the combustion took place without fuel oscillation but at the same overall average fuel flow rate. When the fuel-rich and fuel-lean zones eventually mix in the furnace, after heat has been transferred from the flame to the load and the flame temperature is lower, the resulting burnout of combustible gases occurs with little additional NOx formation. Additionally, the increased flame luminosity resulting from the fuel-rich combustion zones combined with the increased turbulence created by the flow oscillations provides increased heat transfer to the furnace load.
Field Testing Results
In this project, a demonstration of the oscillating combustion technology was carried out on a car bottom forging furnace at Shultz Steel Company in South Gate, California. Oscillating combustion was able to achieve up to 49% reduction in NOx emissions from the forging furnace while keeping the CO emissions averaging less than 100 ppm when the furnace was run in a low (~20%) excess air mode of operation. This amount of NOx reduction essentially meets the emissions goal of the project, though the reduction was somewhat less than the 61% achieved in GTI's laboratory (with a furnace temperature of 2050°F, air preheat temperature of 800°F, and excess air level of 28%) with the same make and model of burner as those on the forging furnace (although with 40% of the capacity).
The NOx emission levels from the forging furnace at the 49% reduction case approached that of low-NOx (50-ppm) burners. For operation at higher (~80%) excess air levels, for which the furnace can generate three or four times as much NOx emissions as for low excess air levels under steady (non-oscillating) conditions, a modest 18% reduction in NOx emissions was achieved with oscillating combustion. This was not unexpected since it was already known from laboratory testing on various burners that little to no reduction in NOx emissions could be achieved with such high excess air levels. The reason for this phenomena is that with high excess air levels, the ability of the oscillating valves to generate fuel-rich conditions within the furnace is severely limited or even eliminated. It should be noted that the magnitude of the NOx reduction in terms of ppm was actually greater for the higher excess air level case. Overall, NOx was reduced by about 60 to 100 ppm regardless of the excess air level.
Since the forging furnace has regenerators, the efficiency of the furnace is not impacted much by the excess air level. The regenerators also tend to cancel out any improvements in heat transfer from the flame to the load within the furnace since the additional heat gained by the load is not available to heat the incoming air. Nevertheless, a fuel savings of up to 3%, dependent on furnace operating conditions, was obtained with oscillating combustion on the forging furnace. For a furnace in continuous use, the fuel savings can be translated into a productivity increase of the same amount, which may have a bigger economic impact. The value of 3% fuel savings added to the value of the 49% NOx reduction would indicate a payback period of 2.3 years.
The SSP valves generated no audible noise in the field. Flame sensing was not a problem at frequencies as low as 0.2 Hz even though no pilot was running. The pulsing of the gas flow did not affect the operation of the gas safety train. The oscillating valve control system itself functioned well during field testing. The only problem observed with general furnace operation with the oscillating combustion system running was the tendency to temporarily overshoot the temperature set point of the furnace zone closest to the door after the door is closed following the removal or insertion of a work piece. The more than expected overshoot with oscillating combustion was likely due to the more effective heat transfer and/or the altered flame shape with oscillating combustion. A simple restriction of the maximum firing rate of that zone, or a small increase of the over-temperature limit, eliminated the potential for the a furnace shutdown due to this phenomenon.
Recommendations
For best performance of oscillating combustion in terms of percentage NOx emission reduction, a furnace (like the car bottom forging furnace that was tested) should be operated with a constant, low excess air level. This is usually also the most thermally efficient means of operation. Essentially, good control of the air-fuel ratio is necessary for the successful implementation of oscillating combustion.
For facilities that might need a varying excess air level (such as for a large turndown of the firing rate or for increasing convective flow within the furnace), a number of air-fuel ratio strategies can be utilized that will minimize the amount of time the excess air level is at less than favorable conditions for oscillating combustion. The simplest strategy would be to have the burners cycle on and off when the firing rate demand is below the burners' turndown capability.
For retrofit installations where the more effective heat transfer or the altered flame shape of oscillating combustion may result in transient over-temperature swings, it is recommended that the over-temperature limits be raised accordingly, the thermocouple for the over-temperature limit be moved to be less sensitive to the effect, or the maximum firing rate be throttled during events when the over-temperature swing could occur.
Laboratory Testing Results
One concern about applying oscillating combustion to the heating of certain metals, such as titanium (one of the major metals forged at Shultz Steel), is that the fuel-rich conditions necessary in oscillating combustion for substantial NOx reduction could generate large quantities of hydrogen in the combustion gases, which is absorbed by titanium. Too much hydrogen absorption would have led to embrittlement of the titanium metal. To alleviate this concern, titanium samples were heated with the Zedtec burner (the same make and model as those on the forging furnace at Shultz Steel, though smaller in capacity) in GTI's test furnace under oscillating combustion with low and high amplitude oscillations. The analysis of the samples found that in all cases, the level of hydrogen in the samples was less than about half of the allowable limit, even for samples directly under the flame with high amplitude oscillations that alternate the flame between very fuel-rich and fuel-lean conditions. The samples downstream of the flame picked up even less hydrogen, and were close in concentration to those samples heated without oscillating combustion.
Retrofit Equipment
The Oscillating Combustion tests above used a valve developed by CeramPhysics, Inc. which is the Solid-State Proportioning (SSP) valve. It has an extremely fast response time, is practically noise free, and has already shown operation for over 110 million cycles without degradation in performance. In this valve, an elastomer disk is sandwiched between the fixed and movable pistons. An actuator in the valve is energized and de-energized at the desired oscillation frequency. The force exerted by the actuator on the moving piston causes the elastomer disk to bulge out and restrict the fuel flow through an annular passage, thus providing the oscillations in the fuel flow rate.
The SSP valves performed well during the test campaign, logging an estimated aggregate one million cycles. No mechanical adjustments were made to the valves during the test campaigns. The SSP valves generated no audible noise in the field.

Funding Source

Funding Amount


ICAT

$161,803

Grantee

$113,025

Sothern California Gas Company

$235,000


Click here for the entire final report.




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