Page 275 - Advances in Eco-Fuels for a Sustainable Environment

238 Advances in Eco-Fuels for a Sustainable Environment

Microalgae have been known as a third-generation biodiesel feedstock, and they
have several advantages such as high growth rate, high lipid content, easy to cultivate,
and nonarable space requirements [15–17]. The yield of microalgae could reach
55tons ha 1 year 1 while the lipid content in microalgae can be as high as 60% under
stress conditions [15]. Microalgae strain and growth conditions affect the structure of
microalgae cell wall, particularly its thickness and chemical composition [18]. The
disruption of cell wall can facilitate lipid extraction from microalgae, but this process
can be a bottleneck, particularly in energy and cost-efficient biodiesel commerciali-
zation [18]. The extraction efficiency and biodiesel conversion can be improved by
using dried microalgae. However, drying is an energy-intensive process that can
increase the cost of biodiesel production considerably [19–21]. Therefore, wet micro-
algae have been a preferred choice for biodiesel feedstock, providing that proper
technologies are applied.
Spent coffee grounds are the waste obtained from coffee brewing processes that
come from the instant coffee industry or domestic sources such as coffee shops
and restaurants. This waste appears as a dark brown solid, normally with high mois-
ture. Studies showed that this waste contained promising amounts of fat usable for
biodiesel production [22, 23]. Based on the global consumption data of coffee beans
released by the International Coffee Organization and the previous assumption of
spent coffee waste generation, 2.8–3 million tons of spent coffee waste were produced
in 2016–17. This fact makes the utilization of spent coffee waste intriguing to produce
various useful products and renewable fuels to save world from the problems caused
by this enormous amount of waste.
Several methods have been developed to convert inedible lipid feedstocks into bio-
diesel, such as the direct use of inedible lipid feedstocks, microemulsion, thermal
cracking, and transesterification. Among these methods, transesterification is the most
common method for biodiesel production due to its simplicity. It is an established
method to convert inedible lipid feedstock into biodiesel [24, 25]. The trans-
esterification can be carried out either using a catalytic (homogeneous or heteroge-
neous) or a noncatalytic process. Although commercial biodiesel production
worldwide uses a homogeneous catalyst, the noncatalytic biodiesel production pro-
cess is an attractive process to develop because it requires no catalyst separation
process.
The concept of an integrated inedible lipid feedstock-based biorefinery to improve
the economic aspect of biodiesel has been explored. Rice bran is a byproduct of the
rice milling process, and it contains various antioxidants that impart beneficial effects
on human health. Among the bioactive compounds, γ-oryzanol, a mixture of ferulic
acid esters and triterpenoid alcohols, is unique to RBO. The beneficial effects of
γ-oryzanol on human health have generated global interest in developing facile
methods for its separation from natural sources such as crude RBO, RBO soap stock,
rice bran acid oil, or biodiesel residue from RBO. An integrated rice bran-based bio-
refinery for the production of biodiesel and γ-oryzanol will reduce the biodiesel price
due to income from γ-oryzanol sales.
The biorefinery concept applied on microalgae is still viable with stepwise optimi-
zation and improvement [26]. Harvesting and drying of biomass and oil extraction