The continual increase in design capacity for ethanol plants has facilitated new designs for production and backend emissions control. As plant capacity, and anticipated production and emission rates, have increased the challenge to provide process and emission control technologies to match these demands has also grown. Standardizing process designs often leads to equipment sizing issues or the inability to accommodate production beyond the design conditions of the emission control system.

Recent advancements in pollution emission control, coupled with increased flexibility and conservation designs, have resulted in significantly increased reliability and compatibility of emissions control systems with various operational or process needs, while reducing the amount of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) released into the atmosphere. These considerations have significantly benefited the ethanol industry.

Regenerative thermal oxidizers (RTOs) are broadly used within the ethanol industry to control emissions from distillers grains dryers, fermentation scrubbers, load out, distillation and various other sources. RTOs control moderate discharges of organic particulate and varying flows of VOC concentrations and moisture content.

Flow and VOC conditions that reflect the anticipated emissions controlled by the RTO system typically come from the process engineer when designing an RTO system. The RTO supplier then sizes the equipment and develops the optimum operating conditions to maximize performance and minimize operating costs. An initial RTO design should be performed to control these conditions as and include a test block capacity allowing for periodic increases above the maximum design (such as increases in system pressure relating to particulate build-up prior to performing a thermal bake-out, which will be discussed later).

Pressure drop increases proportionally by the square of the flow rate. Any increase in flow from the process will have a significant effect on the available horsepower of the RTO system. For modern ethanol plants, these flow conditions vary and, in many cases, are 20 percent higher than the maximum flow conditions provided during the initial design phase. For example, Nestec Inc.'s evaluation of various dryer exhaust data has shown that the flows typically given to the RTO supplier are typically based on gallons of ethanol production rather than the maximum capacity of the dryer. Dryers are normally designed at standard sizes, so for a given capacity the evaporative flow requirements may vary significantly based on ethanol production.


Table 1. Design versus actual operating conditions at the RTO inlet.

Once installed, the flow conditions out of the dryer fluctuate due to actual production rates. If the dryer is sized larger than the rated capacity, the result is higher flows to the RTO. A conservative design that incorporates lower flow velocities within the heat recovery media results in additional available flow beyond the rated test block of the RTO system, while minimizing the particulate buildup (Table 1).

Another area where design conditions differ from actual operation is the centrifuges. In a typical 40 MMgy plant, four centrifuges with a maximum capacity of 120 gallons per minute (gpm) each feed the dryer—three centrifuges continuoually on line, the fourth a spare. However, all four centrifuges are often used in production at 150 gpm each, which is beyond design specifications. The plant runs at a capacity that exceeds other areas of production and faces additional water generation. Both significantly impact the flow being sent to the RTO system.

Keeping Up with Thermal Efficiencies
Another important operating requirement to consider is the RTO burners, which are tasked with trying to meet additional demand while maintaining a regulatory agency temperature requirement of 1,600 degrees Fahrenheit. Although the burner systems typically have an over-fire capacity, operation of the burners in this fashion will result in levels of NOx that are higher than compliance allows and in many cases require additional fuel contribution. Nestec Inc. has developed a supplemental fuel system that enables minimum burner modification while bringing the overall fuel requirements back into normal operating conditions.

For thermal balance, the RTO should provide maximum thermal efficiency and allow for variations in process conditions. For RTO designs using a forced draft design, the burner has even more difficulty since the firing rate of the burner is pressure dependent. Burners that fire into an induced draft will typically yield a higher rated capacity.

Most RTO systems controlling emissions from distillers grains dryers require a feature known as bakeout, a self-cleaning mechanism that enables the RTO system to maintain its original pressure design without adversely affecting its RTO. If bakeout is done on line, the smoke, carbon monoxide, VOC and particulate generated during the process can cause a notice of violation for the plant permit. For safety considerations, and to meet permit requirements, bakeout should be performed off line, since temperatures up to 1,000 degrees Fahrenheit are created at the bottom of the heat recovery beds. The inlet ducting should be isolated and the system's fresh air damper used to provide the flow for the bakeout operation. If done off line, this procedure can be classified as part of the maintenance plan. With a properly designed RTO system, overall system maintenance will also be less, increasing the reliability of the RTO for the plant.

Fermentation Scrubbers
The fermentation scrubber design typically results in the largest variation in Btu value sent to the RTO. This process source, although appearing constant, varies greatly depending on water levels, oxygen content and upstream operation of the fermentors, including the clean in place (CIP) process.

Water level variations depend on how the operators run the fermentation and fermentation scrubbers. By varying water rate to the scrubber, flow and water carryover at the RTO changes, possibly causing high amperage issues for the RTO motor fan.

Oxygen content at the RTO's inlet is primarily controlled by the fresh air damper, but it has been found that the damper position needs to have fixed positions (high and low VOC) to allow for the frequent upstream variations in flow and moisture content. These settings address the minimum oxygen requirements for the RTO, while modulating to a second position based on a high VOC level in the process stream.


Table 2. Emissions from the fermentation tanks as they enter and exit the RTO.

Finally, CIP procedures that occur prior to filling the fermentors send large VOC spikes to the RTO, driving the RTO combustion chamber temperature above 1,700 degrees Fahrenheit and creating a surge in volume that can result an over-amperage for the fan motor. Although some industry experts are looking at regenerative catalytic options for controlling the fermentation scrubbers, most catalysts won't survive long-term exposure at temperatures above 1,500 degrees Fahrenheit (Table 2).

A well-designed RTO system can offer flexibility during ethanol manufacturing, and if designed conservatively, the excess flow can be handled. However, in many ethanol plants, the RTO is operating above the test block. At this capacity, the RTO will exhibit excessive wear and higher operating costs, and may have difficulty meeting performance requirements. As the ethanol industry continues to grow and plant needs change, the greatest success will be seen when involving emission control system suppliers early in the design to coordinate process conditions and their impact on the final system.

James L. Nester is president of Nestec Inc., based in Douglasville, Pa. He holds three patents and has worked in the field of regenerative thermal oxidation for the past 17 years. Reach him at jnester289@aol.com or (610) 323-7670.

The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).