Application of Regenerative Thermal Oxidizers (RTO)
Application of Regenerative Thermal Oxidizers for VOC Control and Process Optimization
For more than twenty five years, Regenerative Thermal Oxidizers (RTO) have been used to destroy organic chemical emissions for various industrial applications. Some of the earlier industries included metal coil coating, vinyl flooring manufacturing, heat web offset and other printing applications, specialty chemical, pharmaceutical, and wood products manufacturing. As a result of this broad exposure much was learned about the design and application of the thermal oxidation systems which lend to its use for controlling emissions in the ethanol industry.
The primary emissions from an ethanol dryer include heavy organic condensable material, water, low molecular weight hydrocarbons, and oxygenated organic compounds such as formaldehyde. In addition, As a byproduct of the drying process a significant amount of CO is formed as a result of low O2 in the dryer as well as both thermal and chemical NOx. With such a broad array of constituents to be removed from the process stream the end of pipe abatement system needs to be effective at eliminating each compound but must also be durable and easy to maintain. Furthermore, if the system can be integrated into production whereby the plant can increase uptime in material production the traditional abatement system becomes a more affordable device.
When the ethanol industry emerged the engineering firms were faced with the challenge of not only designing a plant that produced ethanol economically but also met current state and federal air emission standards. Like any industry, process optimization and engineering changes created a moving target for the abatement systems and the initial equipment selections focused upon two scenarios. First, utilization of a thermal oxidation system followed by a waste heat boiler for steam generation, and secondly a regenerative thermal oxidation system to control dryer emissions and detached from the steam generation process.
The primary difference between these two approaches is thermal efficiency and energy consumption. The straight thermal oxidation systems typically have no internal heat recovery capability and utilize primary or secondary heat recovery to capture the energy in the relatively high temperature exhaust (typically 1500°F). In the Ethanol industry, the waste heat boiler uses this hot exhaust gas to generate steam which is integral to the ethanol process and directly links the thermal oxidation system to the process whereby dried material production can only take place when everything is up and running. In the case where a regenerative thermal oxidizer controls the dryer emissions and steam is provided by stand alone boiler systems the high preheat capabilities of the Regenerative design provides up to 95% preheat on the inlet gases. This preheat is accomplished by the use of ceramic material which absorbs and desorbs energy as the process gases are cycled through the ceramic beds of the RTO recovery chambers.
Although both of these designs initially were effective, over time each developed limitations which needed to be addressed. As a result of compliance testing it was found that the voc and co emissions were higher than predicted and aggressive control of these compounds was required to meet the federal and state guidelines. Secondly, depending upon how the dryer was operated large quantities of particulate were being sent to the control equipment causing excessive maintenance and wear. To address the need to increase the removal efficiency of the control devices an operating temperature of 1600°F was required to primarily address CO destruction. 
The problem hear was that the original thermal oxidizer and waste heat boiler systems based their steam requirements upon a 1500°F operating temperature so the steam balance became an operational issue requiring the plants to slow down production. Also, the thermal oxidizers themselves were not sized to run continuously at 1600°F and therefore began to have significant mechanical wear and maintenance issues. On the other side the Regenerative Thermal Oxidizers could typically handle the higher operating temperature but the increase in particulate which ultimately collected in the ceramic heat recovery material became excessive requiring frequent thermal cleaning and plant downtime.
Seeing that there were existing issues which needed to be addressed and a new design was needed to move forward, ICM utilized the RTO technology to supplement the existing thermal oxidizer whereby the process was optimized and total plant efficiency increased while thermally oxidizing 100% of the dryer exhaust. Working with Nestec, a manufacturer of Regenerative Thermal Oxidizer (RTO) systems out of Telford Pennsylvania, the RTO was designed to allow the dryers to send varying process flows to the RTO. The Nestec engineering team, taking experience from controlling similar process conditions (high moisture, particulate, CO, and NOx) from the wood products industry where rotary dryers are used to dry wood wafers, flakes, and fiber, selected the most appropriate ceramic material, valve design, bed velocities and retention time to minimize maintenance and maximize the performance of the RTO. The RTO although still handling the dryer emissions only sees the non-syrup dryer exhaust and therefore much less particulate. Regarding the TO system the remaining dryer exhaust continues to be processed as initially designed but operates at a reduced flow rate to maintain steam balance while being operated at 1600°F to meet emission compliance. Regarding maintenance the RTO still needs to be periodically thermally cleaned. The main difference here is that the plant continues to operate for a short period of time making wet cake or at reduced production rather than having to completely shut down the process. So far, two RTO systems have been supplied for this application. This installation has been running since 2003 and has only required two thermal cleanings per year. As the industry moves forward these types of innovations and the engineering companies such as Nestec Inc. will continue to support the needs of the industry.
James Nester is the President of NESTEC, Inc. and holds multiple patents in the field of Regenerative Thermal Oxidation.
