As design capacity for ethanol plants continues to increase, new designs for production and backend emissions control continue to evolve as well. Plant capacity and anticipated production and emission rates have increased the challenge to provide process and emission control technologies to match these growing demands. Efforts to standardize process designs often lead to equipment sizing issues or the inability to accommodate production beyond the design conditions of the emission control system.

The combination of recent advancements in pollution emission control technologies along with increased flexibility and conservative system design have resulted in a significant increase in the reliability and compatibility of emission control systems, 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.

The ethanol industry primarily uses regenerative thermal oxidizers (RTOs) to control emissions from distillers grains dryers (DDG), fermentation scrubbers, load out, distillation and various other sources. RTOs also offer the ability to control moderate discharges of organic particulate and varying flows of VOC concentrations and moisture content.

When designing an RTO system, a process engineer typically provides the manufacturer with flow and VOC conditions that reflect the anticipated emissions to be  controlled by the 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 well as to include a test block capacity to allow 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 in the article).

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 being built, these flow conditions vary and, in many cases, are 20% higher than the maximum flow conditions provided during the initial design phase of the RTO system. For example, NESTEC Inc.’s evaluation of various dryer exhaust data has shown that the flows typically given to the RTO supplier are based upon MM 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.

Once installed, the flow conditions out of the dryer fluctuate due to actual production rates, and 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.

Another area where design conditions differ from actual operation is the centrifuges. In a 40 MM gallon plant, four centrifuges typically feed the dryer, with three continuously online and the fourth acting as a spare, with a maximum capacity of 120 gallons per minute (GPM) each. However, all four centrifuges are often used in production at 150 GPM each (beyond design specifications and at maximum output conditions). In this case, the plant runs at a capacity that exceeds other areas of production and is therefore challenged by the additional water generated. Consequently, significantly wetter cake is fed into the dryer resulting in either a higher SCFM condition or a higher evaporative requirement for the dryer. 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 meeting additional demand, while maintaining a regulatory agency temperature requirement of 1600°F. Although the burner systems typically have an over-fire capacity, operation of the burners in this fashion will result in levels of NO_X that are higher than compliance allows, and in many cases, require additional fuel contribution. NESTEC, Inc. has developed a supplemental fuel system that allows minimal 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. In the scenario described above, the burner system becomes maximized, requiring additional fuel. 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 emission from DDG or DDGS dryers require a feature known as bake-out. This process uses a self-cleaning mechanism that enables the RTO system over time to maintain its original pressure design without adversely effecting the operation of the RTO. If bake-out is done online, the smoke, CO, VOCs and particulate generated during the process can cause a notice of violation (NOV) for the plant permit. For safety considerations, and in order to meet permit requirements, the bake-out should be performed offline, since temperatures reach 1,000°F at the bottom of the heat recovery beds. The inlet ducting should be isolated and the system’s fresh air damper should be used to provide the flow for the bake-out operation. If done offline, this procedure can be classified as part of the maintenance plan. With a properly designed RTO system, overall maintenance will also be less; therefore, increasing the reliability of the RTO for the plant.

Fermentation scrubbers

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

Water level variations are primarily associated with how the operators are running the fermentation and fermentation scrubbers. By varying the water rate to the scrubbers, flow at the RTO changes, as does the water carryover, which may also cause high amperage issues for the RTO fan motor.

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 upon a high VOC level in the process stream.

Finally, clean in place (CIP) procedures that occur prior to filling the fermenters send large VOC spikes to the RTO, driving the RTO combustion chamber temperature above 1700°F and creating a surge in volume that can result in an over-amperage condition for the fan motor. Although some industry experts are looking at regenerative catalytic options for controlling the fermentation scrubbers, most catalysts will not survive long-term exposure at elevated temperatures above 1500°F. Table 2 depicts the process conditions and performance of an RTO for this application.

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

A well-designed RTO system can offer flexibility during ethanol manufacturing, and if the RTO is designed conservatively, 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. The NESTEC, Inc. RTO design accounts for these variations so that production and expected RTO performance are met.

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.