Method for extracting biodiesel based on waste coffee grounds

The invention discloses a method for extracting biodiesel based on waste coffee grounds. The method comprises the following steps: I, (1) boiling coffee grounds with boiled water in a coffee pot, filtering off the liquid, and drying residual coffee grounds; (2) adding a n-heptane solvent into the coffee grounds to extract coffee oil; (3) distilling an extract, recycling the n-heptane solvent, and drying to obtain the coffee oil; II, (1) adding water into the coffee oil for medium-temperature hydration; (2) heating, adding an alkali liquid, heating for temperature rise, separating out oil and nigre, washing with water, settling, separating, and drying; III, (1) adding an excessive amount of methanol into the refined coffee oil, under the catalytic function of NaOH, performing transesterification on grease and methanol at normal pressure; (2) after the reaction is completed, performing normal-pressure distillation so as to recycle the excessive methanol; (3) filtering and recycling the catalyst, putting the fil