Methodological determinants of carbon intensity in the life cycle assessment of beef tallow biodiesel: A critical review

Beef tallow biodiesel is often presented as a low-carbon fuel option, yet the reported carbon intensity of this pathway varies widely across the literature. This review examines the methodological reasons behind that variation by critically analyzing Life Cycle Assessment (LCA) studies focused on biodiesel derived from beef tallow. Rather than comparing results numerically alone, the study investigates how modeling assumptions shape environmental interpretation. The reviewed studies were assessed in terms of feedstock classification, system boundary definition, allocation procedure, land use change inclusion, and reporting basis. The results show that beef tallow has been inconsistently modeled as waste, residue, by-product, or co-product, and that these designations directly influence whether upstream livestock burdens are excluded from or attributed to the biodiesel pathway. Consequently, studies that appear to evaluate the same fuel route often represent materially different systems. Additional divergence arises from differences in boundary start, multifunctionality handling, land-related emissions, and the use of non-equivalent reporting bases. The main contribution of this review is to demonstrate that the variation in reported carbon intensity is driven not only by methodological diversity, but by the recurrent misalignment between feedstock designation, system boundary, and burden attribution. To address this issue, the article advances a consistency-oriented framework to support more transparent interpretation of beef tallow biodiesel LCAs. Overall, the findings indicate that improving LCA practice for this pathway depends not only on better inventory refinement, but also on greater conceptual clarity, methodological coherence, and reporting transparency.