SYMBIOSYS EMISSIONS PHILOSOPHY
In its most simple definition, emissions are materials that escape to the atmosphere. Many emissions can be materials that are useful as inputs for another process or operation. It is often both environmentally and economically desirable to minimize the escape of these materials into the environment. The SymbiosysTM system attempts to efficiently utilize all onsite materials with minimal waste.
Effective resource management is key to both overall energy efficiency and economic performance. It is also related to emissions. Emissions are considered to be wasted resources. In the SymbiosysTM system, emissions and every other byproduct are considered as potentially valuable assets, to be recovered, reused, or sold.
TYPICAL ETHANOL PLANT VOC & ODOR EMISSIONS VS. SYMBIOSYS
There are two primary emissions of concern from a typical ethanol plant: volatile organic compounds (VOCs) and odor. More than 75% of the potential VOC emissions from a typical ethanol plant originates in the DWG drying process.
With Symbiosys there is no DWG dryer system. The potential VOCs are not generated and odors typically associated with the drying operation are not produced. The fossil fuel combustion (for dryer and thermal oxidizer operation) and its associated emissions are also eliminated.
FERMENTATION EMISSIONS
A second potential source of ethanol plant odors is fermenter emissions. Most of these are ethanol, therefore efficient recovery is economically and environmentally desirable. In SymbiosysTM, potential fermenter emissions are recovered with a wet scrubber, a device that uses plain water to capture the ethanol and other compounds. The ethanol-rich scrubber water is sent back into the process to assure maximum ethanol recovery. This approach also captures compounds that have offending smells, recovering them before they escape to the air. As a result, BBA expects to nearly eliminate fermenter odor.
Even the emissions from minor process tanks are sent to a wet scrubber. To assure that all possible process materials are recovered and emissions are minimized, wet scrubber water is returned to the process for further use. Although wet scrubbers are used as product recovery devices in the SymbiosysTM system, it is interesting to note that the USEPA considers a wet scrubber to be the Best Available Control Technology (BACT) for ethanol plant applications.
EMISSION CONTROL COUPLED WITH ENERGY MANAGEMENT
In a stand-alone ethanol plant, one or more large fossil-fueled boilers are needed to provide heat for key process steps. For a 50 million gallon/year stand-alone facility, approximately 200 million BTU/hr of fossil-fueled heating capacity would typically be required. The BBA plant includes a 120 million BTU/hr natural gas-fired boiler for emergencies only, as all of the steam needed is produced by a high-temperature combustion system that operates primarily on biofuel. Iti is expected that natural gas will be combusted only in situations where the biofuel cannot maintain the necessary temperatures (1400 F). The high-temperature combustion system is expected to be over 99% efficient at eliminating organic compounds (both volatile and nonvolatile) and is a preferred technology by both state and federal environmental agencies because of its clean combustion.
CONTROLLING VAPORS WHILE TRANSFERRING FUEL TO TRUCKS
As the denatured ethanol is loaded into tanker trucks, VOC vapors that have been displaced by the incoming liquid may escape. BBA plans to capture these vapors and burn them in a flare system. Although the flare is considered to be an insignificant source of combustion-generated emissions (it averages less than 8 ounces of VOC emissions per hour of operation), it is the only source of combustion-generated emissions in this ethanol plant. This flare is a control device that is considered by U.S. EPA to be the Best Available Control Technology (BACT) for this ethanol plant application
CONTROLLING PARTICULATE EMISSIONS
The primary sources of particulate emissions from an ethanol plant are grain unloading, handling, processing, and storage. However, in the SymbiosysTM system, potential dust emissions are reduced by at least 99.8% through the use of fabric filters. During the latter stages of grain handling/processing, at least 99.8% of the particles generated during the grain processing are expected to be recaptured by fabric filters and reintroduced for use in the ethanol process. As a result, the maximum projected particulate emissions rate from this process is expected to be less than 7 ounces per hour.
CURRENT STATUS OF BBA'S ENGINEERING PLANS
As of October 15, 2005, final process engineering data regarding expected ethanol plant emissions is still being developed. The final emissions levels may be even lower than the levels described above because the final development phase includes by-product recovery optimization.
ENERGY EFFICIENCY OF ETHANOL PRODUCTION & THE SYMBIOSYS DIFFERENCE
A 2002 USDA report concludes that "corn ethanol is energy efficient, in that it yields 34 percent more energy than it takes to produce it, including growing the corn, harvesting it, transporting it, and distilling it into ethanol." Most researchers agree with this conclusion as shown in the attached NEV table.
The USDA results are based on standard ethanol plants, but the SymbiosysTM design is substantially more efficient. In SymbiosysTM, most of the energy used in the ethanol plant is derived from what would otherwise be waste products of the other production processes in the system. For instance, major energy users in a typical ethanol plant are the dryer and its associated control device, the thermal oxidizer, which are not present in SymbiosysTM. The BBA ethanol plant is expected to perform far better from both a fossil fuels conservation standpoint and from an overall energy balance perspective.