Environmental Engineering Reference
In-Depth Information
groups are removed. It is low cost and uses inherently safe materials, but it requires extremely long
reaction times.
8.4.2.3 solvent Pretreatments
8.4.2.3.1 Organic Solvents
Biomass is treated with an ethanol:water or methanol:water solvent to remove 70-90% of the
lignin in the aqueous phase. The solvent is then recovered by distillation, and the condensed
black liquor is diluted with water to precipitate lignin. The solvent can also contain a catalyst
composed of acids, bases, or mineral salts. Solvents are easy to recover, and lignin is isolated
as a solid, whereas carbohydrates are produced as syrup. However, pretreated solids must be
washed with solvent before water washing, requiring complicated washing arrangements and
higher energy costs. Organic solvents also have to be tightly controlled because of fire and
explosion hazards.
8.4.2.3.2 Ionic Liquids
Ionic liquids are salts in which the ions are only loosely coordinated, resulting in a liquid at or
near room temperature. Some ionic liquids have been shown to be a good solvent for cellulose,
disrupting the crystalline structure, then precipitating out amorphous cellulose after the addition
of an antisolvent. Although expensive, ionic liquids have many advantages such as recoverability,
low vapor pressure, and nonflammability. Research into ionic liquid pretreatments is still at an
introductory stage.
An economic analysis performed by Eggeman et al. (2005) found that major economic effects on
plant operations from pretreatment include yield of five- and six-carbon sugars, solids concentration
(and subsequent ethanol concentration), enzyme loading, and hemicellulase activity. There is little
overall economic differentiation among the pretreatment technologies; low-cost pretreatment
reactors were often counterbalanced by higher costs associated with catalyst or product recovery.
The process was modeled using ASPEN Plus 10 and implemented in four parts: (1) capital cost
estimate, (2) operating cost estimate (in which feed pricing was assumed to be $35/dry t, enzyme
pricing is assumed to be $0.15/gallons of ethanol), (3) revenue (from ethanol sales and electricity
sales at a price of $0.04/kW-h), (4) and discounted cash flow (2.5 years construction, 0.5 years
start-up, and 20 years operation with cash flows discounted at 10%/year). Ethanol pricing is done on
a rational pricing basis in which the minimum ethanol selling price (MESP) to achieve a zero net
present value is determined. The MESP for the listed pretreatment technologies ranges from $1.34
(dilute acid) to $1.67 (hot water).
8.4.3 E
nzymatic
h
ydrolySiS
of
c
ElluloSE
and
h
EmicElluloSE
After pretreatment, a solid residue remains that contains cellulose, plus varying amounts
hemicellulose and lignin depending on the pretreatment strategy used. Cellulose, and any
hemicellulose that may be present, is then hydrolyzed to monomeric sugars for fermentation. The
enzymatic hydrolysis step provides the greatest opportunity for biomass ethanol production to be
cost-competitive with that of other liquid biofuels (Mosier et al. 2005). SHF is more costly than SSF
for several reasons, an important one being that using SSF eliminates the cost of a reaction vessel
and reduces the enzyme loading necessary because fermentation consumes the glucose product,
which is a strong inhibitor of further enzymatic hydrolysis.
8.4.3.1 cellulases
Cellulose degrading enzymes are classified by sequence homology into families 1, 3, 5-9, 12,
44, 45, 48, 61, and 74 of the glycoside hydrolases. On the basis of modes of action, they can be
classified into three groups: exo-1,4-β-d-glucanases (cellobiohydrolases, EC 3.2.1.91), endo-1,4-β-d-
glucanases (endoglucanases, EC 3.2.1.4), and β-glucosidases (EC 3.2.1.21) (McFarland et al. 2007).
