Page 69 - Lignocellulosic Biomass to Liquid Biofuels

48 Lignocellulosic Biomass to Liquid Biofuels

pretreatment process depending on the feedstocks. Since a large number
of enzymatic hydrolysis and fermentation inhibitors can be formed under
severe SE conditions, particularly at high temperature, some additional
acids, such as SO 2 ,H 3 PO 4 , and H 2 SO 4 , can decrease temperature needed
[1]. Tooyserkani et al. studied the effects of SO 2 -catalyzed SE pretreat-
ment on softwood pellets. They found that the addition of SO 2 substan-
tially reduced the particle size of the woodchips and eliminated further
grinding [181]. The addition of acid catalysts in SE process can increase
the recovery of hemicellulose sugars, decrease the formation of inhibitors,
and further improve the enzymatic hydrolysis of the pretreated solid resi-
dues [175]. External H 2 SO 4 addition can promote the solubilization of
hemicelluloses, lower pretreatment temperature, but also partially hydro-
lyze cellulose especially the amorphous region [182].

2.5.2 Supercritical CO 2 pretreatment

Supercritical CO 2 (SC-CO 2 ) can diffuse like a gas into the interspaces of
biomass and dissolve like a liquid as a part of biomass materials [1]. SC-
CO 2 pretreatment is also usually termed CO 2 explosion pretreatment
which has a similar mechanism to that of SP. In the operation of the
process, SC-CO 2 is fed into a high-pressure vessel containing biomass fol-
lowed by heating to the required temperature and keeping for several
minutes at high temperatures. In this process, CO 2 diffuses into the bio-
mass and forms carbonic acid to hydrolyze hemicelluloses. The pressure is
then suddenly released and the biomass structure is disrupted thus increas-
ing cellulose accessibility.
SC-CO 2 pretreatment has a couple of advantages, including low cost,
nontoxicity, nonflammability, easy recovery, and low environmental
impact [7]. Compared with thermal treatments, SC-CO 2 can be operated
at lower temperatures (but higher pressure), which reduced the formation
of inhibitors from hemicelluloses and lignin degradation. Moreover, SC-
CO 2 has higher diffusivity and low viscosity so that it may work more
efficiently for wet biomass [183]. It was found that the enzymatic digest-
ibility of SC-CO 2 pretreated aspen and southern yellow pine increased as
the moisture content of the raw biomass increased; but for absolutely
dried aspen and southern yellow pine, SC-CO 2 showed no significant
improvement [183]. SC-CO 2 also can be combined with other pretreat-
ment to further improve cellulose digestibility. For SC-CO 2 pretreated
sugarcane bagasse the glucose yield of enzymatic hydrolysis could reach to