By Anoosheh Oskouian, President & CEO and Mike Pawlowski, Senior Technical Sales Engineer, Ship & Shore Environmental, Inc.
Special to The Digest
Most ethanol production facilities at some point must deal with the issue of destroying the byproducts created in processing. Several sources of process gas contain Volatile Organic Compounds (VOCs), and the use of thermal oxidation is often employed to destroy these. What follows is a guide for utilizing thermal oxidation for the effective and efficient destruction of the emissions produced by a typical ethanol facility.
Thermal Oxidation is the process of controlled combustion of process gasses. The process is governed by time, temperature, and turbulence. The overriding condition is temperature. Typically for non-halogenated compounds, 1500°F is required to destroy VOCs when sufficient turbulence is present, and the mixture is retained at temperature for 0.5 seconds or more with the proper volume of oxygen for combustion. Evaluation of the type of thermal oxidizer technology is critical for the economic model of an ethanol plant.
The selection process for thermal oxidizers is driven by the conditions of the incoming stream. Volume and composition of gas to be treated, heat value of the stream (Btu/ft3), and temperature are important factors. Specific to the Ethanol Industry – moisture, particulate, and corrosive constituents drive the materials of construction when designing the proper thermal oxidizer. Special consideration must be given to internal structural elements of the oxidizer to prevent the damage caused by the presence of the corrosive components. A supplier skilled in evaluation of the process should be carefully selected.
A Regenerative Thermal Oxidizer (RTO) is the technology typically employed for situations where the heat content of the incoming stream is not sufficient for self-sustaining direct combustion and where the use of supplemental fuels needs to be minimized. Understanding the operation of a typical RTO is valuable when considering the best equipment for plant operations. The sequence of operation for an RTO follows:
With the shifting of the valves comes a small amount of the incoming stream which does not encounter destruction conditions. The resulting destruction effectiveness is 98-99%. However, the reduction in supplemental fuel is dramatic for low Btu content streams. This is achieved by the high degree of heat recovery (95-97% thermal effectiveness) possible with this technology.
Like any other form of thermal oxidation, a startup burner is employed to raise the temperature of the unit to proper destruction conditions. Once at proper temperature, the process gas can be introduced, blended with the correct amount of dilution/combustion air, and the RTO cycles through the sequence of operations previously pictured. The burner will provide supplemental fuel to maintain the combustion chamber temperature should the Btu content fall below that required for self-sustaining operation.
Example of a 10,000 SCFM Regenerative Thermal Oxidizer (RTO).
The carry-over of organic particulate into the oxidizer is a common problem. It is normally handled with the scheduling of regular periods of “bake out.” This involves raising the temperature of the heat transfer media to levels which will allow the combustion of the particulate to occur. A bake out is similar to a self-cleaning cycle in an oven. Care must be taken to manage this process carefully, as too long between cycles or an aggressive thermal climb can result in a fire, which can damage valves, structural elements, and cause downtime to the facility.
Regenerative Thermal Oxidizers have proven to be the most cost-effective method for the destruction of VOCs in ethanol production facilities. RTOs represent a significant investment in a production facility. However, there are programs in place that can reduce the cost to the end user by design and selection of highly efficient units. Many local and state requirements for energy reduction programs, have resulted in utility companies offering rebates and incentives for the use of energy efficient components and construction of equipment. The use of energy-saving motors and variable frequency drives (VFDs) may qualify for these programs.
By its design, an RTO is an energy-saving device. The use of highly efficient heat exchange beds captures up to 97% of the energy of combustion of the volatile organic compounds. Comparison of the energy consumed by this method of VOC destruction vs. other forms of thermal oxidation can also factor into the qualification for these rebates and incentives. A vendor skilled in applying for these programs can make the process of filing and qualifying for these programs worth the time.
A 50,000 SCFM Regenerative Thermal Oxidizer (RTO)
Ship & Shore Environmental, Inc. is a Long Beach, Calif.-based woman-owned, certified business specializing in air pollution capture and control systems for industrial applications. Ship & Shore helps major manufacturers meet Volatile Organic Compounds (VOC) abatement challenges by providing customized energy-efficient air pollution abatement systems for various industries, resulting in improved operational efficiency and tailored “green” solutions.