Feedstock Quality for Biobased Diesel Isn’t Always What’s Expected or What It Appears

Richard Heiden
24 minutes ago
6 min read

Richard Heiden, principal investigator and owner of R.W. Heiden Associates LLC, performs testing with a gas chromatograph. (Photo: R.W. Heiden Associates LLC)

R.W. Heiden Associates LLC, a Lancaster, Pennsylvania-based provider of laboratory and consulting services to the diesel and biodiesel industry for over 20 years, offers advanced feedstock-testing services that flag impurities common screening tests miss.

Attention biodiesel and renewable diesel producers: The arrival of a feedstock lot is accompanied by expectations of quality that ensures optimum reactor performance and predictable production yields of fatty acid methyl esters (FAME), fatty acid ethyl esters (FAEE), or renewable diesel fuels. Caveat emptor: Feedstocks can contain foreign substances that slow reaction progression or introduce noncompliant components and unsafe impurities, or they can contain substances that compromise eligibility to generate renewable identification number (RIN) credits under the federal Renewable Fuel Standard program. Yet, these feedstocks can still pass common screening tests, suggesting suitability for purpose and the legitimacy of feedstock providers.

This warning applies to all feedstocks in varying degrees. Overall, however, the predictability of virgin seed-oil feedstock compositions is usually quite high, and pretreatment processes that reduce possible impurities are well defined and highly efficient. In contrast, incoming lots of feedstock oils considered used, recycled or from other ill-defined sources can have varying or unexpected properties imparted by foreign substances that fail to be converted to renewable diesel or FAME. Moreover, these feedstocks also can contain significant amounts of discarded FAME from unsuccessful attempts to make fuel that meets quality specifications. Still, despite varying preferences for feedstock that depend on specific feedstock pretreatment and fuel-producing reactions and reactor conditions for conversion to fuel, the feedstock is expected to exclusively contain a collection of free fatty acids, mono-, di- and/or triglycerides. When the organic palette contains substances outside of these classifications, or if the ratios are unacceptable, feedstock can fail to convert to the desired fuel product at expected conversion rates—or fail to undergo conversions to fuel altogether.

In this short, descriptive essay, a newer type of feedstock testing is described. This testing elucidates feedstock authenticity with a detailed scan of the organic constituents and has undergone extensive evaluation as a gauge of feedstock identity and vendor reliability. Importantly, this testing provides a potentially attractive quantitative molecular fingerprint of feedstock composition. Initially used as a screening tool to help identify lots of feedstock contaminated with wasted FAME at levels down to 0.1 percent, this testing approach can help fuel manufacturers identify reliable feedstock sources and minimize reactor downtime while providing verifiable evidence of compositions meeting RIN-generation requirements for feedstock authenticity. As such, the developed tests—based upon modified approaches to common advanced test methods—provide a clear record of feedstock authenticity. Such testing in a confidential setting has assisted fuel producers in vetting feedstock providers for nearly 10 years. Mentioned here are a few examples of discoveries this advanced testing has uncovered.

While feedstock providers vary in their ability to deliver reliable products, in the end the fuel producers potentially suffer from poorly defined feedstocks resulting in unexpectedly low product yields or undetected impurities that compromise RINs eligibility. Fuel producers have varying fatty-acid requirements, depending on the process used. Substances in feedstocks that fall outside of these compositional requirements can result in products that fail to meet specifications or allow entry of substantial percentages of substances that fail to undergo the expected conversion reactions to fuel. Feedstock providers have varying skills and tools needed to assess the purity necessary to meet fuel-production requirements. Up to now, an assessment of feedstock quality mainly rests on the fuel manufacturer, particularly in order to meet RINs requirements for monetary reimbursements.

Feedstock cleanliness and authenticity are essential prerequisites for smooth reactor performance including maximum yields and uncompromised fuel quality. This is true whether the feedstock is for renewable diesel or FAME, or whether the fuel-production process is batch or continuous. Acceptable feedstock quality is very much married to the processes used to make fuel. As such, this can vary considerably. Many classical tests can be feedstock specific, are developed for food-grade oils or fats, and yet fail to provide sufficient detail suited for fuel manufacturing. Some are useful, such as free fatty acid content, in gauging the quality of feedstock needed for some reactors. Despite major process variations and differences, both renewable diesel and FAME production require the organic components of feedstock to be comprised of various forms of fatty acids, mono-, di- and triglycerides, and in some cases, even free fatty acids that are convertible to fuel. Each process for making fuel has its own specific requirements for feedstock fatty acids. While the processes for conversions to fuels and the chemical composition of the end products differ, the certainty of feedstock composition prior to manufacturing the fuel is critical for predictable product yields and hydrocarbon or fatty-acid distributions that define physical and combustion properties of the fuel.

Using sophisticated modifications of chromatographic test methods, impurities that compromise feedstock organic compositions—at levels above 0.1 percent mass per mass (m/m)—can be sought. Also, undesirable components can be flagged and the quality of feedstock sources ascertained. When impurity levels appear sufficiently high to alter feedstock purity or compromise RINs eligibility or reaction progression, foreign substances can be identified or quantified by more advanced analysis techniques, such as gas chromatography-mass spectrometry (GC-MS). The same concerns apply to small batch reactors or large continuous processes. Both are susceptible to yield losses from contaminants but, in reactors requiring large volumes of feedstock, significant dollars can be lost due to downtime when the produced fuel unexpectedly fails to meet tough fuel-product specifications, or when it contains undesirable impurities—or even those undetected by common wet-chemistry methods—that compromise fuel properties. An appropriate approach to identify the key fatty-acid components and quantify potentially troublesome lots of feedstock is clearly required to reduce effects on product quality and help fuel manufacturers find reliable feedstock vendors. In addition to FAME contamination of feedstock at levels well above 0.5 percent m/m, unexpected foreign components have frequently appeared at remarkable concentrations.

Using such methods, feedstocks for fuel manufacturing have undergone detailed scrutiny and startling examples are revealed and discussed below. While numerous low-level organic impurities in feedstock are likely innocuous or have little significance, some fail to participate in reactions that produce fuel and pose as important diluents that are potentially illegal if allowed into the feedstock stream. The range of impurities detected is considerable and often, as hoped, is below 1 percent m/m. In summary, UCO feedstocks frequently can contain high levels of lubricants, hydrocarbons of various types, and fatty acids at times outside the molecular-weight range needed to produce compliant fuels. Also detected were FAEE and FAME at low (less than 1 percent m/m) to high (in excess of 10 percent m/m) percentages. Feedstocks containing the FAEE components were associated with corn oil derived from ethanol plants and are likely considered legitimate byproducts of ethanol manufacturing—but they are also variable in percentages, occasionally reaching levels of over 10 percent in corn-oil feedstock products. Such high levels of ethyl esters might influence feedstock conversions to FAME, for instance, by altering the amounts of reactants needed for conversions.

FAME, on the other hand, is also a common feedstock contaminant but of suspicious origin. These unsuspected contaminants are possibly sourced from FAME reactors that for whatever reason fail to complete the conversion of feedstock to fuel. FAME contaminants in feedstock are of questionable origin at best and potentially invalidate calculations for RINs reimbursements. These FAME contaminants can be difficult to detect at low percentages in feedstock and require more sophisticated testing than commonly used today. The impurities from illegitimate feedstock sources also should be considered when calculating RINs reimbursement dollars.

In another example of unusual feedstock adulteration at a FAME plant, a liquid visually resembling feedstock left handlers at the fuel factory itchy, some requiring medical attention. An initial screening by a specially developed gas chromatography f lame ionization detector (GC-FID) method revealed substantial percentages of a substance compositionally different from expected feedstock components. Detailed follow up analyses including GC-MS confirmed that the active (itchy) components were similar or identical to those found in poison ivy. Follow-up investigations revealed the feedstock supplier was provided liquid waste from a pharma company and the provided liquid “looked pretty good” to unsuspecting observers.