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  1. Composition of Biomass Ash [5, 13].

Biomass

Substance (% mas)

SiO2

Al2O3

TiO2

Fe2O3

O

MgO

Na2O

K2O

SO3

P2O5

cl

Red oak wood

48.2

9.5

-

8.5

17.5

1.1

0.5

9.5

2.6

1.8

0.8

Wheat straw

48.0

3.5

-

0.5

3.7

1.8

13.5

20.0

1.9

3.5

3.6

Walnut shell

23.7

2.4

0.1

1.5

16.6

13.4

1.0

32.8

2.2

6.2

0.1

Almond shell

23.5

2.5

0.1

2.8

10.5

5.2

1.6

48.5

0.6

4.5

0.2

Sunflower shell

29.3

2.9

0.1

2.1

15.8

6.4

1.5

35.6

1.3

4.8

0.2

Olive husk

32.5

8.4

0.3

6.3

14.5

4.2

26.2

4.3

0.6

2.5

0.2

Olive kernel

67.7

20.3

0.05

0.05

0.5

0.05

11.2

0.15

-

-

-

Hazelnut shell

33.7

3.1

0.1

3.8

15.4

7.8

1.3

30.4

1.1

3.2

0.1

Swithgrass

66.25

2.22

0.28

1.36

10.21

4.71

0.58

9.64

0.83

3.92

-

Rice straw

77.20

0.55

0.50

2.46

2.71

1.79

12.59

-

-

-

Boilers with circulating fluidized bed are suitable for biomass combustion, because they are flexible to changing in fuel quality and produce ash with unburned fuel. The high content of unburned carbon in ash reduces chemical stability of ash and increases ash volume, leading to increased cost of ash handling, transportation and storage. To mitigate these effects, the ash should be recycled to be re-burned or the combustion process has to be improved. Ash re-burning reduces NOx emissions by 20%, and increases CO emissions at about 100-140ppm. The quantity of re-circulated ash amount must be reduced in order to avoid an increased corrosion tendency in boiler. Economic benefits of boilers with circulating fluidized bed are limited to low cost of fuel and ash handling. The ashes from these boilers can be recycled and used as a nutrient for forest land. Fly ash from combustion of biomass in the fixed bed has a high content (50%) of unburned carbon and is therefore not suitable to direct recycling.

Low-temperature corrosion of biomass boiler is produced by hygroscopic species (in particular iron chloride) present in ash deposit that are subject to wide temperature variations caused by boiler intermittent operation. Compared with deposits generated during coal combustion, deposits resulting from biomass combustion are denser and difficult to remove. In conclusion it can be said that the corrosion phenomenon depends on the content of Cl, alkali metal and S in the fuel.

Biomass co-combustion

Co-firing of biomass with coal leads to lower total emissions relative to unit of generated energy due to very different compositions. It also may leads to reduced costs of fuel and less wastes. Technical issues are solved by experiments and tests.

In co-combustion plants an increased deposit formation may occur. Also the use of ash in building materials should be limited due to constituents in biomass, especially alkali metals. Changes of ash composition lead to limitation of biomass use to about 10% of the fuel input.

Co-combustion of biomass technique is used also to substitute fossil fuels and to reduce CO2 emissions.

Combustion technologies

Biomass combustion is used for heat production in systems such as: district heating systems (0.5 MW - 50 MW); combined heat and power systems (CHP) with steam turbines, Stirling engines (10 kWe to 100 kWe) and gas turbines. Biomass can be used also in large power stations (>100 MWe) to substitute maximum 10% of total input fuel.

the most frequently used furnace types for biomass combustion are shown in Table XI [14].

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