Biomedical Engineering Reference
In-Depth Information
Table 1.5
Chemical differences of wheat straw and wood [
13
]
Wheat straw
Wood
Lignin
Phenolic hydroxyl (C9
phenyl propane unit)
0.4-0.45/C9
0.1-0.3/C9
Phenolic hydroxyl in lignin
44/100 units
9-13/100 units of birch
Carboxyl in lignin
13.8/100 units
Carboxyl in lignin
0.1-0.2/C9
-
Proportion of ester in lignin
2.60 % ester bond
1-2.8 %
Almost absence of ester
bond
'
Ferulic acid in lignin
-Ether bond
-
Molecular weight of kraft
lignin
7,500
18,200 (birch)
Polydispersity
Proportion of lignin unit
(V:S:H)
1:0.77:0.31
1:1.2:0.05 (hardwood)
1:0.02:0.009
(softwood)
Lignin carbohydrate
complexes (LCCs)
Accounting for 60 % of
water-soluble lignin
Without LCC
Others
Polymerization degree of
hemicellulose
84
120
Structural composition of
hemicellulose
90 % of xylan
80-90 % of xylan
(hardwood)
60-70 % of mannose
(softwood)
Crystallinity of cellulose
40-60 %
60-80 %
Organic extracts
Less than 1 %
4 % for softwood
Less than 1 % for
hardwood
Ashes
8 % (60 % are SiO
2
)
Less than 1 % (mainly
contains CaO, K
2
O,
Na
2
O)
Characteristics of fiber
1.0-1.5 mm (length)
5 mm (length)
Microfibrils and hybrid fiber
39 %
5 % (softwood)
25 % (hardwood)
1.2.2.3
Morphological Differences of Cellulose
Straw has a relatively high content of microfibrils and hybrid cell (about 40-50 %)
and a low content of fiber cells (40-70 %). Wood has a low hybrid cell content
and a high fiber cell content. For example, the fiber cell contents of hardwood and
softwood are 60-80 and 90-95 %, respectively. Table
1.5
indicates that wheat straw
and wood have obvious differences not only in chemical composition but also in the
morphological structure of lignin and hemicellulose.
In sum, there is significant difference between straw and wood. Extensive
research is necessary concerning the inhomogeneous composition and structure of
straw and to separate the components of straw on the basis of their characteristics.
