CARBON the total thermal energy used by many ethanol plants has probably decreased since 1998. Dryer energy use is reduced at many plants in a variety of ways: improved dryer efficiency; drying DDGS to higher moisture content; reducing the water content of the wet cake or condensed distillers solubles (syrup); process improvements; centrifuge design alterations; etc.
Calculating Energy Savings
All of the above could cause actual energy use associated with drying DDGS to be lower than 9,900 Btu per gallon at a specific ethanol mill. If the actual energy use is less, then the savings resulting from installing COES might be lower. In order to accurately evaluate the savings, we must establish the difference in thermal and electrical energy consumption between periods when drying is occurring and when it is not. We must also have “before COES” and “after COES” scenarios for dryer energy usage with all other parameters being held constant. The scenarios must be of reasonable duration and sufficiently representative of both present and future normal operating conditions. CARB prefers plant-specific empirical data on the energy required to dry DDGS to average or specific levels. According to the latest guidance, the simplest and most straightforward operational data is validated and verifiable thermal and electrical energy consumption data from meters or from a plant’s automated datalogging system. If that data can definitively establish the difference in thermal and electrical energy consumption between periods when drying is and is not occurring, no further plant information would be necessary. CARB
anticipates that plant data logging will allow for a two-year analysis of all drying and energy use scenarios. A producer would then calculate the average drying energy for all drying modes including wet, dry and modified.
If such data is unavailable, the applicant must propose an approach to estimating dryer energy usage based on plant-specific information that is available. This gets tricky because there is no prescribed approach to do this. According to CARB, a dryness energy estimation approach must, at minimum, consider the following and include them in the life cycle analysis (LCA) report: • Initial wet cake water content. • Final DDGS (or DG, if syrup is sold separately) water content. • Initial thin stillage (solubles) water content. • Final solubles (syrup) water content. • Mass of corn oil extracted. • Use of enhanced stillage dewatering (enzymatic or other). • Dryer efficiency. Applicants may also apply for a single CI for all distillers grains dryness levels, as long as it is based on two years of data and will be valid for foreseeable future plant operations. Applicants must demonstrate in their LCA reports that the mass and energy balances are approximately close and explain any discrepancy that prevents complete closure. Once drying energy has been determined, that value should be used to calculate DDGS dryness-based CIs. The calculations must be
done on a dry-matter basis, and must properly account for extracted corn oil and syrup on a dry matter basis. Syrup and corn oil must be allocated proportionally (on a dry-matter basis) to all distillers grains dryness categories when CIs are calculated for the coproduct credit. Finally, when the CIs are certified, the plant must allocate ethanol gallons produced to the dryness categories after converting all distillers grains masses to a dry-matter basis. If there are three drying modes, the producer would take the drying energy into the CA-GREET model to generate three distinct CIs to be used to calculate LCFS credits. The LCFS is clear about placing the responsibility of compliance on regulated parties who use the credits. If plant operational data does not match the inputs in the CAGREET model that is the basis of the CI value, then the credits generated as a result of combusting the final fuel can potentially be disputed. Most regulated parties who take the LCFS or RIN credit for corn oil biodiesel don't trace the origin of the corn oil when they purchase biodiesel. They probably should. Author: James M. Ramm, PE Senior Engineer, EcoEngineers 515-344-9715 email@example.com Contributing Author: Shashi Menon, managing partner, EcoEngineers
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