milling

Husk and straw properties

Attention: open in a new window. PrintE-mail

Properties of Rice Husk and Rice Straw

A. Chemical composition

Proximate analysis; Ultimate analysis;   Chemical composition of RHA

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

Back to top

 

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


Back to top

 

 

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)

Back to top

 

 

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    

 

Back to top

 

 

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).

Back to top

 

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.

Back to top

 

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 of 45° 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.

Back to top

 

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

Back to top

 

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.

Back to top

 

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).

pressure-drop-hull

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.

Back to top