Environmental Engineering Reference
In-Depth Information
Lignin is believed to impede enzymatic hydrolysis of cellulose by interact-
ing with biomass surfaces and either blocking the path of processive hydrolases
(e.g. Cel7A), preventing enzymatic access to specific binding sites, or through non-
specific binding of cellulolytic enzymes [34-36] to lignin. Several low-temperature
pretreatment protocols, such as alkaline peroxide [37, 38] or lime and oxygen [39],
address these issues by removing substantial amounts of lignin. Although these pro-
cesses are highly relevant to the pulp and paper industry, the fate of lignin and its
impact on enzymatic digestibility after high-temperature acidic or neutral pretreat-
ments has largely been neglected until recently [40-42]. Recent observations show
that lignin undergoes significant structural changes during high temperature pre-
treatments. These changes cause it to both mobilize during elevated temperatures
and then coalesce upon cooling, both within the cell wall matrix and on the biomass
surfaces [40]. This mobilized processed lignin, when redeposited onto cellulose sur-
faces, can impede enzymatic digestion presumably due to the occlusion of substrate
binding sites [42]. All of these transport limitations during lignocellulosic con-
version to ethanol impact the overall process performance and thus warrant more
detailed further investigation.
2 Macroscopic Transport Through Plant Tissues
In a large-scale process, pre-impregnation of catalyst into large pieces of biomass
(>1 cm) is often overlooked; however, milling biomass to reduce this problem can
incur large energy and equipment costs [1, 14, 15]. This problem is compounded
by the widespread use of process irrelevant biomass sizes for laboratory exper-
iments. Most laboratory studies on biomass to ethanol conversion processes use
finely milled materials (20-80 mesh is standard) where the effects of macroscopic
transport processes are not easily observed or are masked altogether [43-45]. In
larger pilot studies using compression screw feeders, these transport effects can be
further masked by the high-shear feeder causing biomass size reduction [6, 8]. Often
this size reduction occurs after catalyst impregnation, limiting catalyst effectiveness
on pretreatment. A further complication is that compression of the feed stock may
cause biomass pore structure collapse, leading to uneven heat and mass transfer dur-
ing pretreatment [10, 13] as well as limitation of catalyst access to the interior of the
biomass.
Before larger biomass particles containing intact tissues are used in processing,
it is essential to understand the catalyst transport processes and pathways and the
limitations associated with them (Fig. 1). In living plants, vascular tissues such
as xylem and phloem are the primary routes for transport of water and nutrients
along the length of the plant stem and leaves. Additional transport within tissues and
between adjacent cells is carried out through (1) the pits, areas of thin primary cell
wall devoid of secondary cell wall between adjacent cells and (2) the apoplast, the
contiguous intercellular space exterior to the cell membranes [46]. In dry senesced
plants, studies with dyes to visualize fluid movement through tissues showed that
the apoplastic space is the major catalyst carrier route, with limited fluid movement
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