Corn Oil Removal Impacts Carbon Accounting

Managing carbon intensity values under California’s LCFS.
By James Ramm | April 16, 2014

California’s Low Carbon Fuel Standard allows an ethanol plant to submit a Method 2B petition to the California Air Resource Board requesting a lower carbon intensity (CI) number. Jan. 8, CARB issued guidance for ethanol producers seeking a lower CI number based on distillers grains dryness levels.

The new guidance makes clear that use of default drying energy values is no longer acceptable because it may overestimate CI reductions. Going forward, petitions must be based on plant-specific, operational information on drying or overall plant energy. The proliferation of advanced technologies for corn oil extraction systems (COES) and the increasing use of nonfood-grade (NFG) corn oil for biodiesel production contributed to the latest guidance. CARB also states it cannot prescribe a single approach because drying systems and processing parameters can vary widely from one plant to the next.

Over the past two years, the use of NFG corn oil for biodiesel production has grown tremendously. Corn oil biodiesel is eligible for a D4 RIN (renewable identification number) under the renewable fuels standard and the LCFS has assigned it a CI value of 4 grams of CO2 equivalent per megajoule of biodiesel produced. Biodiesel made from NFG corn oil is eligible for the CI value of 4 because the energy required to dry the DDGS at the ethanol plant after the corn oil is extracted is considerably lower. This energy saving is credited to the corn oil and subsequently to the biodiesel made from it.
The low CI value for NFG corn oil biodiesel assumes the DDGS is dried fully at the ethanol plant, for if the distillers grains were not dried, there would not be any energy savings to be allocated to the corn oil. Second, it assumes that the ethanol plant selling the corn oil is not submitting a Method 2 pathway and lowering the CI of its ethanol based on lower energy consumption, as this would be double counting. Third, it assumes that the actual energy savings from installing COES is 3,070 Btu per gallon of ethanol.

Default Values
A regulated party using the CI value of 4 for corn oil biodiesel must demonstrate that the net energy savings from the extraction of corn oil at the ethanol facility is greater than or equal to the value used in the California- adapted model for greenhouse gases, regulated emissions, and energy use in transportation (CA-GREET) calculation: 3,070 Btu per gallon of ethanol. If the net energy savings at the ethanol plant from corn oil production is less than this value, the biodiesel producer must use the Method 2A/2B pathway option of the LCFS.

In the CA-GREET default scenario for a dry mill corn ethanol plant, CARB assumes a default drying energy of 9,900 Btu per gallon of ethanol. This number is calculated from a 1998 study, which stated that the total energy consumption at an average dry mill plant is 36,000 Btu per gal of thermal energy, and 27.5 percent of that is used for drying DDGS. The assumptions that go into using the default value include:

•    Sensible heat effects are ignored.
•    The wet cake must contain approximately 65 percent water.
•    The syrup (solubles) must contain
     approximately 65 percent water.
•    The dry DDGS must contain
     approximately 10 percent water.
•    The drying process must be
     approximately 91 percent efficient.

In the latest guidance, CARB admits that 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 data-logging 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.

Estimated Savings
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
•    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 CA-GREET 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
[email protected]

Contributing Author: Shashi Menon, managing partner, EcoEngineers