To overcome these drawbacks, two-step biodiesel production
methods have been developed:firstly FFAs are esterified catalyzed by
acid as a pretreatment, and then the acylglycerols are transesterified
using a homogeneous alkali catalyst (typically NaOH). For some
methods using high acid value oils as feedstock (such as mahua oil,
rubber seed oil, and tobacco oil), esterification of FFA and methanol is
applied. However, among these methods, the methanol esterification
step is time-consuming and requires large amounts of methanol and
acid catalyst. It has been suggested that for high acid value feedstock,
the molar ratio of methanol to oil needs to be up to 18:1 to achieve a
higher conversion yield of FFA to FAME[16]