Page 281 - Advances in Eco-Fuels for a Sustainable Environment

244 Advances in Eco-Fuels for a Sustainable Environment

The complicated structure of the microalgae wall inhibits oil extraction. The thickness
of the microalgae cell wall depends on the growth medium. The selection of species
and the proper culture medium resulting in a fragile cell wall should be considered due
to ease of cell disruption pretreatment [18, 39]. In addition, operational parameters
such as temperature, pressure, and biomass conditions need to be considered to obtain
higher cell disruption and oil extraction efficiency. Biomass conditions, particularly
biomass concentration, dried/wet, and the growth stage of cultivation are some factors
that need to be observed first.
The drying step after harvesting is carried out to reduce water content from more
than 60% to less than 10% [43]. Lipids extraction of dried microalgae give higher
yield than wet microalgae because the cell wall in dried microalgae can be destroyed
easily. [44]. However, the drying step consumes large amounts of energy, which can
be up to about 80% of the total energy consumption [20, 43]. Alternatively, wet alga
containing 80% water was utilized directly after harvesting and was found difficult in
releasing oil from the microalgae cell [45]. High water content is the limiting factor
and hinders the high efficiency of biodiesel conversion [43]. Eliminating the drying
step reduces not only the energy consumption but also the total production cost. The
challenge is to select advanced and sensible technology to convert biodiesel from wet
microalgae and still acquire high efficiency.
Regardless of high energy consumption and high production cost, microalgae are
still worth considering as a new alternative feedstock in biodiesel production because
of sustainability and the abundance of biomass compared to other feedstocks. To make
microalgae-based biodiesel commercialization feasible, selecting the proper micro-
algae species and culturing conditions and developing new technologies need to be
considered.

9.2.3 Spent coffee grounds

Spent coffee grounds are abundant as the waste of the food and beverage industries. In
the brewing process, generated spent coffee waste is about 60% of the mass of the
brewed coffee beans, by which 50% of the ground coffee beans are processed [46,
47]. Based on the global consumption data of coffee beans released by the Interna-
tional Coffee Organization and the previous assumption of spent coffee waste gener-
ation, 2.8–3 million tons of spent coffee waste was produced in 2016–17. This fact
makes the utilization of spent coffee waste intriguing to produce various useful prod-
ucts and renewable fuels to save the world from the problem that can be caused by this
enormous amount of waste.
Spent coffee grounds are often found as a mixture of the Arabica and Robusta cof-
fee ground waste, though pure Arabica or Robusta coffee powder is also available [28,
48, 49]. Those two varieties are popular coffee cultivars in various coffee-producing
countries. The mixture typically consists of 60%–80% of Arabica and the rest is
Robusta coffee [50]. The mixture was designed in order to maintain a certain strength
of flavor contributed by the Arabica and tailor the caffeine balance, in which the
Robusta often contains more than the Arabica [51]. The compositional range of the