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Ecofuel conversion technology of inedible lipid feedstocks to renewable fuel 257

9.3.3 Catalytic and noncatalytic biodiesel production from spent
coffee grounds
Biodiesel from spent coffee grounds is obtained through two distinct process: oil
extraction and (trans)esterification of the obtained oil. As seen in Table 3.4, Soxhlet
extraction and simple solid-liquid extraction are the typical techniques to extract oil
from spent coffee grounds. Other advanced extraction methods such as ultrasonication
and microwave-assisted extraction also resulted in similar yields and extract charac-
teristics to that of Soxhlet extraction because all techniques possess similar
solvent-lipid interaction [97]. Hexane was frequently selected due to its compatibility
in extracting lipids [22, 98] while isopropanol was mixed into it to extract the polar
compounds suspected to be FFA. Although the designated purpose of isopropanol is
reasonable, the compound extracted by isopropanol might not be FFA but other
unsaponifiable organic acids. This was proved by the high acid value of the recovered
solvent as well as the smaller higher heating value (HHV) and lower heating value
(LHV) and higher ester value that reflect the smaller average carbon chain length
of the extracted oil than that extracted by hexane [22, 99]. Catalytic biodiesel produc-
tion then proceeds in either one step or two steps. Spent coffee oil contains FFA that
may range from as low as 3.65% to as high as 82.3% [22, 100]. Based on this, base
catalysts may render it ineffective to convert the oil into biodiesel. This is evident by
the higher oil conversion percentage in the summarized processes of single
acid-catalyzed, two-step acid–base catalyzed, and enzyme-catalyzed than the base-
catalyzed processes in Table 9.8.
Modification of the catalyzed process has taken in the concept of integrating oil
extraction and (trans)esterification, namely in situ (trans)esterification. With extrac-
tion and (trans)esterification occurring simultaneously in one reaction pot, the energy
required for solvent evaporation can be eliminated. Park et al. used a methanol-
chloroform mixture (2:1v/v) to extract and transesterify oil from wet spent coffee gro-
unds (60wt% moisture) [104]. With the help of H 2 SO 4 as the catalyst (0.5mL/g spent
coffee grounds), the in situ process involving spent coffee grounds and the chosen sol-
vent system (5mL/g) could produce 15.15wt% of biodiesel on a dried spent coffee
grounds basis at 95°C for 2h. By adding more chloroform (2:2v/v), better oil extrac-
tion was observed and a higher yield was obtained (16.75wt%). The authors attributed
the lipid extraction affinity of chloroform as the cause of this phenomenon. Mean-
while, increasing the methanol amount was found to increase the biodiesel yield by
its role as an acyl acceptor in the (trans)esterification reaction, which agrees with
the result of [105].
Another study about in situ (trans)esterification of spent coffee grounds was carried
out by Liu et al. [23]. The authors evaluated the impregnation of spent coffee grounds
(40.93–50.32wt% moisture) with aqueous H 2 SO 4 as a substituting step for oil extrac-
tion by solvent. This step probably worked due to the hydrolysis of the carbohydrate
shell of spent coffee grounds, as indicated by the increasing biodiesel mass yield with
increasing H 2 SO 4 concentration within 3h of immersion and the same condition of the
transesterification process, which was performed in a Soxhlet apparatus with metha-
nol put in the collecting flask. This research group successfully obtained 17.08wt% of