Page 14 - Biofuels for a More Sustainable Future

Biofuels technologies: An overview of feedstocks, processes, and technologies 7

biomass (including wood pellets, hog fuel, and wood chips) utilized for elec-
tricity and heat production in BTU energy units is larger than bioenergy
from conventional biofuels. In the United States alone, wood biomass con-
sumption amounted to 2.04 quadrillion BTU in 2015, while it totaled
11 million tons in wood pellet capacity.

2.2.1 Cellulosic ethanol (second generation biofuels)
Cellulosic ethanol can be produced from any material containing cellulose
and lignocellulose. The main feedstock sources for cellulosic ethanol pro-
duction can be divided as follows:
(a) Energy crops grown specifically for the purpose of conversion into
biofuels (e.g., switchgrass, miscanthus, wheat straw, poplar, willow,
jatropha).
(b) Green waste used as a by-product of other production processes (e.g.,
corn stover and other field residue, e.g., stalks and stubble (stems),
leaves, seed pods, as well as forest/park residues).
According to Chen et al. (2010), 40%–70% of hemicellulose and 72%–90%
of cellulose in corn cobs could be converted to ethanol using different bac-
teria and fungi. Also application of more unconventional feedstocks contain-
ing cellulose or lignin (e.g., kapok fiber, pineapple waste, waste papers, and
coffee residue waste for bioethanol production) has recently been investi-
gated (Dutta et al., 2014; Choi et al., 2012; Ruangviriyachai et al., 2010;
Chen et al., 2010).
The question of economic efficiency of the second generation biofuels
remains open due to high costs related to breaking down cellulose, making it
a lesser competitive feedstock and biofuel in general compared to fossil fuels.
Although many industrial and laboratory attempts have been undertaken in
the past decade to lower the production costs of cellulosic ethanol, the
experiments were not as successful as initially anticipated, with the average
price for cellulosic ethanol still not being competitive enough with tradi-
tional gasoline. As of 2010, production costs of cellulosic ethanol equaled
to $2.65/gal of fuel (Coyle, 2010), which was $1 more than costs of corn
ethanol. The more recent research studies and scenarios by the National
Renewable Energy Laboratory (NREL) have proven that cellulosic ethanol
could be cost competitive at $2.15/gal (NREL, 2013). Due to this eco-
nomic limitation determining the market access, most studies in this area
are focused on improving technological processes of cellulose decomposi-
tion and breakdown (Liu and Bao, 2017; Gao et al., 2018; Shadbahr
et al., 2018; Song et al., 2018). Many studies attempted to provide solutions