Ultimate Analysis of Different Solid Biofuels (Dry Basis) [5, 6, 7].
Fuel
C
%
H
%
N
%
S
%
O
%
Cl
%
Walnut shell
52.80
5.60
1.40
0.04
39.25
0.10
Almond shell
47.80
6.00
1.10
0.06
44.94
0.10
Coconut shell
51.30
5.40
0.10
0.10
43.10
–
Hazelnut shell
50.80
5.60
1.00
–
42.40
0.20
Pistachio shell
49.70
5.90
–
–
44.40
–
Sunflower seed husk
47.40
5.80
1.40
0.05
45.25
0.10
rice husk
47.80
5.10
0.10
–
47.00
–
cottonseed hull
42.80
5.40
1.40
0.50
49.90
–
Coffee husk
45.40
4.90
1.10
0.35
48.30
–
Soya husk
45.40
6.70
0.90
0.10
46.90
–
Mustard husk
45.80
9.20
0.40
0.20
44.40
Barley straw
39.92
5.27
1.25
–
53.56
–
Wheat straw
43.20
5.00
0.62
0.11
49.57
1.50
Corn stover
42.50
5.04
0.75
0.18
51.53
Corncob
42.50
5.00
0.80
0.20
51.50
-
Tobacco stalk
49.50
5.70
0.80
–
43.80
0.20
Tobacco leaf
48.30
4.90
1.20
–
45.30
0.30
alfalfa stem
45.40
5.80
2.10
0.09
46.61
-
Sugarcane bagasse
44.80
5.40
0.40
0.01
49.3
-
Cotton residues after fiber extraction
42.80
5.40
1.40
0.5
49.9
-
Olive residue after oil extraction
49.90
6.20
1.60
0.05
42.05
0.20
Peach pit
53.00
5.90
0.30
0.05
40.75
-
Plum pit
49.90
6.70
0.90
0.08
42.40
0.02
Apricot stone
52.38
6.57
1.07
0.15
38.78
-
Switchgrass
46.70
5.90
0.80
0.19
46.41
-
Bamboo whole
52.00
5.10
0.40
0.04
41.50
0.06
Kenaf grass
48.40
6.00
1.00
0.15
44.30
0.15
Sawdust
46.90
5.20
0.10
0.04
47.76
-
Poplar wood
48.40
5.90
0.40
0.01
45.29
-
Spruce wood
51.40
6.10
0.30
–
41.20
0.10
Beech wood
49.50
6.20
0.40
–
43.90
–
Fir wood
51.90
6.10
0.30
–
41.70
–
Red oak wood
50.00
6.00
0.30
–
43.70
–
Eucalyptus
50.00
6.00
1.30
–
42.70
–
Willow
49.80
6.10
0.60
0.10
43.40
0.10
Olive wood
49.00
5.40
0.70
–
44.60
0.30
tea waste
48.90
5.50
0.50
0.50
44.50
0.10
Sewage sludge
50.10
7.30
6.10
2.30
34.10
0.10
Proximate Analysis of Solid Biofuels (Dry Basis) [5, 6, 7].
Fuel |
Ash % |
Volatile matter % |
Fixed carbon % |
Walnut shell |
2.80 |
59.30 |
37.90 |
Almond shells |
3.30 |
74.00 |
22.70 |
Coconut shell |
0.71 |
77.19 |
22.10 |
Hazelnut shell |
2.50 |
76.30 |
21.20 |
rice hull |
22.30 |
61.00 |
16.70 |
Sunflower seed husk |
4.00 |
76.20 |
19.80 |
Cotton refuse |
6.60 |
81.00 |
12.40 |
Cotton stalk |
17.30 |
62.90 |
19.90 |
Coffee husk |
2.80 |
76.50 |
20.70 |
Soya husk |
5.40 |
74.30 |
20.30 |
Barley straw |
4.30 |
82.41 |
13.29 |
Wheat straw |
13.70 |
66.30 |
20.00 |
Corncob |
1.10 |
87.40 |
11.50 |
Corn stover |
5.10 |
84.00 |
10.90 |
alfalfa stem |
6.50 |
76.10 |
17.40 |
Rapeseed |
6.50 |
78.1 |
15.40 |
Sugar cane bagasse |
11.30 |
73.7 |
15.00 |
Olive residue after oil extraction |
4.10 |
77.50 |
18.40 |
Cotton residues after fiber extraction |
6.60 |
81.00 |
12.40 |
Beech wood |
0.50 |
82.50 |
17.00 |
Fir wood |
1.70 |
80.20 |
18.10 |
oak wood |
0.50 |
77.60 |
21.90 |
Poplar wood |
1.30 |
82.3 |
16.40 |
Spruce wood |
1.50 |
70.20 |
29.30 |
Willow wood |
1.71 |
82.22 |
16.07 |
Beech bark |
5.70 |
65.00 |
29.30 |
Sawdust |
2.80 |
82.20 |
15.00 |
Peach pit |
1.00 |
79.10 |
19.90 |
Switchgrass |
8.90 |
76.70 |
14.40 |
Kenaf grass |
3.60 |
79.40 |
17.00 |
Sudan grass |
8.65 |
72.75 |
18.60 |
Red canary grass |
8.90 |
73.40 |
17.70 |
Tea waste |
1.50 |
85.50 |
13.00 |
Chiken litter |
37.80 |
47.80 |
14.40 |
Also, the chlorine content of solid biofuels may exceed coal level. Chlorine facilitates mobility of many inorganic compounds, in particular potassium.
Higher moisture and ash content can cause issues for ignition and combustion. Due to the lower calorific value, the biomass combustion can be accompanied by flame instability. Biomass co-combustion with high quality coal avoids flame instability problems and reduces the corrosion effect.
Particle size or granulation depends on the presentation form of biomass. Due to the lower calorific value, biomass should be processed mechanically by compacting, pressing and briquetting. Therefore, the biomass particle size can vary from a few millimetres for sawdust up to a maximum 80 cm for logs. In fig. 4 different samples obtained by biomass compaction are shown. It is recommended that solid biofuels to be used in domestic, commercial and industrial installations to be subjected to pre-treatment processes such as washing, drying, size reducing and compaction to achieve a more uniform quality, to facilitate the handling and to reduce the moisture content to an acceptable level.
Figure 4. Samples of compacted biomass.
Wood is the most used solid biofuel. The raw material can take the following forms: logs, twigs, stems, leaves and needles from the forest, bark, sawdust, shavings and sawdust from the wood industry and wood recovered from construction. This may be used directly as fuel or can be transformed in forms that are much more easily transported, stored and burned, such as pellets, briquettes and powder.
Firewood is a wood fuel in the form of tree stem that can be or not treated. For easier handling, the stems are pressed together in order to obtain bundles of equal size, like a log. Pellets are produced by grinding of chips, shavings or bark and pressing the obtained dust into a mold. The heat resulting from friction is enough to soften the lignin. By cooling, the lignin becomes rigid and acts as a binder. pellets have cylindrical or spherical form with a diameter less than 25 mm. Briquettes have rectangular or cylindrical form and are obtained by pressing together of sawdust, chips, shavings or bark in a piston or screw press. The energy content of pellets and briquettes is about 17 GJ / ton, the moisture content is 10%, and density of about 600-700 kg/m3. Table VIII gives the characteristics of the compacted biomass.