Environmental Engineering Reference
In-Depth Information
15.6.2.1 acid-catalyzed steam explosion
Steam pretreatment was derived from a failed steam explosion pulping process (Kokta and Ahmed
1998). In acid-catalyzed steam explosion, SO
2
(De Bari et al. 2007) or sulfuric acid (Ballesteros et
al. 2006; Sassner et al. 2008) has been used as a catalyst. Wood chips are often first impregnated
with acid catalyst, either in gas phase with SO
2
or in the aqueous phase with sulfuric acid, before
steam pretreatment. The further size reduction to fiber or fiber bundle level is accomplished through
steam explosion. Acid-catalyzed steam pretreatment is actually another form of dilute acid pre-
treatment in which the pretreatment is carried out in the vapor phase rather in the aqueous phase.
The explosion feature has now been used by many dilute acid operations for further size reduction.
Therefore, the difference between dilute acid and acid-catalyzed steam pretreatment is becoming
less clear. Catalyzed steam pretreatment works well with hardwood when pretreatment is conducted
at an elevated temperature of around 210°C (De Bari et al. 2007; Sassner et al. 2008). The effective-
ness on hardwood is achieved at the expense of the large amount of energy consumption in steam
explosion. Typical energy consumption for the pretreatment at 210°C is about 1.8 MJ/kg oven-dried
wood, even after accounting for low-quality steam recovery. The conversion of softwood cellulose
is less satisfactory than that of hardwood (Duff and Murray 1996). Furthermore, total hemicellu-
lose and glucose yield from pretreatment and enzymatic hydrolysis is about 70% (Gable and Zacchi
2002). Typical pretreatment conditions for wood are temperatures around 210°C and SO
2
or sul-
furic acid charge of 1-2% on oven-dried wood. Pretreatment time varies from 3 to 10 min. Steam
explosion can produce a relatively concentrated hemicellulose sugar stream from the pretreatment
hydrolysate when the washing water is limited to the minimum (e.g., less than two times the bio-
mass solids). Just like dilute acid pretreatment, steam explosion has a relatively low hemicellulose
recovery of about 65% (Lynd 1996). The scalability of the process has not yet been addressed for
commercialization.
15.6.2.2 organosolv Pretreatment
The development of organosolv pretreatment technology is directly related to organosolv pulping
(Kleinert 1974; Aziz and Sarkanen 1989; Paszner and Cho 1989). The chemistry of organosolv
pulping is fairly well understood (McDonough 1993). Early work using organosolv pretreatment for
fermentable sugar production was mostly conducted in the 1980s, with some success (Holtzapple
and Humphrey 1984; Chum et al. 1988). The ethanol organosolv process was originally designed
to produce clean biofuel for gas turbine combustors and was further developed into the Alcell
R
process for pulp production from hardwood (Williamson 1988; Pye and Lora 1991; Stockburger
1993). Ethanol is now the preferred solvent in organosolv process for biomass fractionation and
pretreatment (Pan et al. 2005). The main advantages of the ethanol organosolv process are that 1)
it can be directly applied to wood chips to produce a readily digestible cellulose substrate from
almost all kinds of feedstock, including softwood and hardwood (Pan et al. 2005, 2006), therefore
it eliminated the need for wood size reduction, and 2) it also produces very high purity and qual-
ity lignin with the potential of high-value applications (Kadla et al. 2002). Typical pretreatment
conditions for woody biomass are temperatures of 175-195°C, pretreatment time around 60 min,
ethanol concentration in pretreatment liquor of 50%, pretreatment liquor pH 2-3, and liquid to
biomass solid ratio of 4-7. In pretreating poplar wood (Pan et al. 2006), about 70% of the lignin
was removed from the substrate and recovered as high-purity lignin. Approximately 80% of the
xylan was separated from the substrate, with 50% recovered as monomeric xylose in the soluble
stream. About 88% of the glucan was retained in the substrate, and almost all of it was converted to
glucose. Despite the excellent cellulose conversion, the xylose recovery rate was low. Furthermore,
the relatively high liquor-to-solid ratio used in pretreatment produces a lower hemicellulose sugar
concentration in the pretreatment hydrolysate. It also increases thermal energy consumption in
pretreatment. Because the recovery of solvent ethanol is also expensive, the organosolv process is
expensive.
