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In-Depth Information
also has low formation of hydroxymethylfurfural (HMF) and furfural, two of the main inhibitors to
fermentation. The combined severity factor under optimal SPORL pretreatment conditions ranges from
1.3 to 1.7. When SPORL was applied to spruce, a softwood, under the conditions for optimal cellulose
conversion (Zhu et al. 2009), HMF and furfural formation levels were about 7 and 3 mg/g oven-dried
wood, respectively. This is an order of magnitude lower than the 50 and 25 mg/g produced using steam-
catalyzed pretreatment when glucose yield was maximized at a combined severity factor between 3 and
3.4 (Larsson et al. 1999). SPORL has an overall glucose recovery of 93% from spruce. SPORL also
has excellent hemicellulose recovery. Major saccharide—arabinose, galactose, xylose, and mannose—
yields are about 56, 86, 76, and 88%, respectively, for spruce (Zhu et al. 2009).
Because ethanol organosolv pretreatment is the most robust process in terms of removing lig-
nocellulose recalcitrance, Figure 3.6 a and b compare cellulose conversion between SPORL and
organosolv pretreated softwood (Zhu et al. 2009) and hardwood (Wang et al. 2009). Very similar
enzymatic hydrolysis conditions were used (i.e., 2% solid consistency, enzyme loadings of about 20
FPU cellulase and 30 CBU ˜β-glucosidase/g cellulose). SPORL effectively removed lignocellulose
recalcitrance and achieved cellulose conversion rates that match those of ethanol organosolv pre-
treatment with equivalent glucose yield.
Because SPORL is developed based on a wood pulping process, it has excellent process scal-
ability, which is one of the key challenges in commercialization of lignocellulosic ethanol technolo-
gies. Most processes have not been demonstrated at commercial scales. Capital equipment required
for commercial demonstrations of steam explosion and organosolv processes does not exist. On
the other hand, the pulp and paper industry has the capability of handling biomass on the scale
of 1000 tons/day, equivalent to the scale of future cellulosic ethanol production of 10
8
L per year.
The SPORL process can make full use of the capital equipment, process technologies, and human
capital in the pulp and paper industry, which can significantly reduce technological and environ-
mental barriers for commercialization. Specifically, a pulping digester can be used for the sulfite
pretreatment, and a disk refiner can be used for size reduction after the pretreatment. Furthermore,
fermentation of SSL is a mature technology and has been practiced in the pulp and paper industry
for many decades. With the good performance of a newly developed yeast strain on SSL (Helle et al.
2004, 2008), the prospect of achieving good ethanol yield through fermenting SPORL pretreatment
hydrolysate is excellent. Therefore, SPORL is one of the most promising pretreatment processes for
lignocellulosic ethanol production from forest biomass.
15.7 conclusIons
Eucalyptus
,
Pinus
,
Populus
, and other promising tree species have considerable current and poten-
tial use for bioenergy worldwide.
Eucalyptus
, the most widely planted hardwood with 19.6 million
ha, has many bioenergy applications in Brazil, China, India, Australia, and other tropical to freeze-
infrequent countries. In the next decade, genetic and genomics studies will likely identify most
genes regulating heritable variation of biomass productivity and wood property traits in
Eucalyptus.
Pinus
species have similar bioenergy potential worldwide, especially loblolly and slash pines in the
southeastern United States and
P
.
radiata
in New Zealand, Chile, and other temperate countries.
Populus
species are very promising for temperate regions of North America, Europe, and Asia.
Biotechnology may also enhance the bioenergy production and qualities of
Pinus
and
Populus
species. SPORL pretreatment of forest biomass from eucalypts, pines, poplars, and other species
promises to increase bioethanol production efficiency.
acknoWledGments
J. G. Isebrands gratefully acknowledges the clerical and moral support of Sharon K. O'Leary in
preparing this manuscript. D. L. Rockwood appreciatively recognizes the patience and support of
Joanne Rockwood in this chapter's development.
