Environmental Engineering Reference
In-Depth Information
Conventional biofuels
Biochemical
Corn,
Enzymatic
hydrolysis
Glucose,
sucrose
Ethanol,
DDGS
sugar beets
Fermentation
sugar cane
Thermochemical
Triglycerides:
soybean, rape
seed, palm
Biodiesel,
glycerol,
soymeal...
Solvent
extraction
Transester-
ification
Plant bio-oil
Advanced
biofuels
Biochemical
Lignocellulosic
feedstocks
Glucose
xylose
galactose
arabinose
mannose
Enzyme
hydrolysis,
fermentation
Ethanol,
butanol,
hydrocarbons
Chemical
hydrolysis
Thermochemical
Triglycerides:
camelina, algae,
tallow, jatropha
Hydrocarbon
biofuels
Solvent
extraction
Hydrotreatment
Bio-oil
Gasification
Synthesis gas
Fischer-tropsch
Lignocellulosic
feedstocks
Hydrocarbon
biofuels
Pyrolysis,
hydropyrolysis
Py-oil
HydroPy-oil
Hydrotreatment
Biomass
Pretreatment
Intermediates
Conversion
Biofuels
FIGure 8.1
Processing routes for conventional and advanced biofuels.
advanced biofuels. In both cases, biomass feedstock is first converted into an intermediate product
through initial reaction or separation steps. The intermediates are then transformed to final biofuel
product and co-products through additional reaction and separation steps. The vast majority of global
production is conventional biofuels [2008 data, 18.7 billion gallons ethanol/year, 4.4 billion gallons
biodiesel/year (EIA 2010)]. Dry and wet mill corn ethanol facilities produce an intermediate glucose
sugar product and a final product of ethanol plus dry distiller grains solids (DDGS), which is marketed
as an animal feed. The overall fermentation reaction is given by C
6
H
12
O
6
→ 2 C
2
H
5
OH + 2 CO
2
.
Two carbons from the sugar molecule are emitted as carbon dioxide, but the energy content of the
two ethanol molecules is substantially higher than the sugar feedstock. Process energy for corn
ethanol production is typically from natural gas for steam production and electricity is from the local
grid. Because of the importing of these energy resources, corn ethanol has a relatively large fossil
energy demand (ratio of fossil energy required for all processing steps per unit of energy in ethanol
produced) of approximately 0.5-0.8 (Wu et al. 2006; Shapouri et al. 2010). Current (as of 2010)
production rates of corn grain ethanol in the United States are approximately 50% that of global
production at 12 billion gallons/year (RFA 2010), and from sugar cane in Brazil it is approximately 8
billion gallons ethanol/year (EIA 2010). Biodiesel is a methyl ester of fatty acids derived from plant
oils. The biodiesel reaction can be simply described as triglyceride + methanol → 3 fatty acid methyl
esters + glycerol (CH
2
OH-CHOH-CH
2
OH). Methanol is almost always produced from natural gas
(fossil origin), and co-products of biodiesel production include glycerol and a residue from the oil
extraction step (soymeal for example), which is often marketed as animal feed. The key intermediate
is a plant oil obtained from the oil extraction step.
Advanced biofuels take advantage of the most plentiful biomass resource on Earth, lignocellulosic
(woody) feedstocks, and of unconventional plant oils and algae. Processing routes for advanced
