Husk and straw properties
Properties of Rice Husk and Rice Straw
A. Chemical composition
B. Physical properties
Bulk density; Outer surface area; Porosity; Angle of repose; Equilibrium moisture content; Thermal conductivity; Pressure drop, terminal velocity;
A. Chemical composition
Proximate analysis
Comparison of rice husk, rice straw and wood in % (d.b.)
| Property |
Rice husk *
|
Rice Straw
|
Wood
|
Volatile matter
|
64.7
|
69.7
|
85
|
Fixed carbon
|
15.7
|
11.1
|
13
|
Ash
|
19.6
|
19.2
|
2
|
* average of 11 Authors
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Ultimate analysis
Comparison of rice husk, rice straw and wood in % (d.b.)
Property
|
Rice husk
|
Rice Straw
|
Wood
|
Carbon
|
38.7
|
37.7
|
48
|
Hydrogen
|
5
|
5
|
6.5
|
Oxygen
|
36
|
37.5
|
43
|
Nitrogen
|
0.5
|
0.6
|
0.5
|
Sulphur
|
0.1
|
NA
|
NA
|
Chemical composition of carbon-free rice husk ash
in % d.b.
Chemical composition
|
% d.b.
|
SO2
|
86 - 97.3
|
K2O
|
0.58 - 2.5
|
Na2O
|
0.0 - 1.75
|
CO
|
0.2 - 1.5
|
MO
|
0.12 - 1.96
|
Fe2O3
|
trace - 0.54
|
P2O5
|
0.2 - 2.85
|
SO3
|
0.1 - 1.13
|
Cl
|
trace - 0.42
|
(Source: Houston, 1972)
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B. Physical properties
Bulk density, kg/m3
Property
|
Rice husk
| Rice husk ash |
Rice Straw
|
Wood
|
Loose
| 73-112 | 96-192 | 300-900 | |
Vibrated
| 122-145 | |||
Bricketed or pelleted
| 180 | |||
Ground
| 230-400 |
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Outer surface area
The outer surface area of rice husk is around 4000 m2/m3 and reduced to 300 m2/m3 when the husk is pelleted (Kaupp,1987).
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Porosity
Porosity is the volume occupied by air in relation to the volume of husk itself. The volume of the air occupying the husk pores and the interstices between husk in a rice husk bed is 85 % while a single husk has a porosity of 54 %, with the majority of the pores being closed (Kaupp, 1987). This porosity reflects the low bulk density of the husk.
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Angle of repose
The angle of repose affects the design of all handling components where it is intended that the rice husk will flow under gravity. The angle of repose of rice huks ranges from 35 to 50o depending on moisture content (ARGC, 1968; Arumugam et al., 1981; Chakraverty, 1989; Kaupp, 1987; Pathak et al., 1988). Ground rice husk forms an angle of repose of 40 to 45°, depending on grinding fineness (Nijaguna and Chapgaon, 1989), and is thus similar to the whole husk. But when the husk flows out of a hopper with a side angle of45° it gets jammed due to compaction by gravity (Beagle, 1978). Therefore, loosening devices or steeper angle of walls are required if the husk shall flow under gravity. This complicates the construction of feeding hopper of rice husk furnaces.
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Equilibrium moisture content
Rice is milled at moisture content of 12 to 14 % wet basis (w.b.), producing rice hull at 3 % (w.b.) moisture content after milling. If the husk is stored it absorbs water until the moisture content reaches equilibrium with the ambient temperature and humidity. Under humid tropical conditions the moisture content of rice husk increases up to 15.3 % (w.b.) as shown in Table 2.1, which requires immediate firing of rice hull to avoid energy loss. High moisture content affects the net calorific value, combustion process, free-flowing properties of bulk rice husk, and thus the angle of repose.
Relative humidity (%)
|
Equilibrium moisture content (% w.b.)
| ||
Rice husk
|
Rice straw
|
Wood
| |
10
|
3.7
|
-
|
3.5
|
20
|
5.4
|
-
|
5
|
30
|
6.8
|
-
|
6
|
40
|
7.9-8.1
|
-
|
7.5
|
50
|
9.1 -9.5
|
5.5
|
9
|
60
|
10.1 -10.8
|
6.3
|
10
|
70
|
10.8 - 11.8
|
9.5
|
12
|
80
|
11.6 - 12.9
|
12.5
|
14.5
|
90
|
14 - 15.3
|
21
|
18
|
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Thermal conductivity
Rice husk has a thermal conductivity of around 0.036 W/m °C, which is comparable to most insulation materials has a potential to be used as a building material.
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Pressure drop of air in a rice husk bed and terminal velocity
When air is forced through a rice husk bed the pressure drop increases with increasing air flow rate and is almost directly proportional to the height of the husk or char bed. To prevent fluidization, the maximum air velocity is restricted to 0.2 m/s for rice husk and to 0.08 m/s for charred rice husk (Kaupp, 1984; Nijaguna and Chapgaon, 1989).
Air velocity and pressure drop in rice husk bed (Kaupp, 1984)
If combustion air has a superficial air velocity above 0.08 m/s, the combusting fuel bed will be elutriated and fly ash and sparks will cause contamination or even ignition of the drying products.
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