Physical Parameters

The moisture content of green biomass can be quite high and can adversely affect the combustion process. If the moisture content is excessive, the combus­tion process may not be self-sustaining and supplemental fuel must be used, which could defeat the objective of producing energy by biomass combustion for captive use or market. High moisture can also cause incomplete combustion, low overall thermal efficiencies, excessive emissions, and the formation of products such as tars that interfere with operation of the system. Predrying of the fuel or blending it with dry fuel to reduce the equivalent moisture content before combustion may be necessary in these cases. Woody biomass fuels containing 10 to 20 wt % moisture are generally preferred for conventional biomass combustion systems. This moisture content range permits a close approach to complete combustion without incurring the costs of further bio­mass drying and allows temperatures in the combustion chamber to reach 750 to 1000°C. As already mentioned, lower moisture contents in biomass fuels can facilitate attainmant of even higher combustion temperatures.

Another factor in biomass combustion is fuel particle size and particle size distribution. The furnace design often determines the optimum ranges of these parameters. But in general, the smaller the fuel particles, the more rapid and complete the combustion process. The larger particles require longer residence times in the combustion chamber at a given temperature. In commercial sys­tems, the capital and operating costs of fuel particle size reduction and predry-

Parameter

Pine wood

Kentucky bluegrass

Feedlot manure

RDF

Bituminous coal

Anthracite coal

Coke

Moisture, wt %

15.0

15.0

15.0

15.0

3.1

5.2

0.8

Higher heating value, MJ/kg

18.05

15.92

11.36

12.51

32.61

29.47

29.50

C/H wt ratio

8.2

7.8

6.6

7.5

16.0

33.6

106

Air/fuel wt ratio

5.37

5.51

3.97

4.25

10.81

9.92

10.09

Product C02, wt/wt fuel

1.90

1.68

1.29

1.51

2.94

2.96

3.12

Product H20, wt/wt fuel

0.56

0.53

0.47

0.49

0.49

0.22

0.07

Nj from air, wt/wt fuel

4.85

4.97

3.58

3.83

8.26

7.58

7.73

C02 in dry flue gas, mol %

19.9

17.4

18.4

20.0

18.5

19.9

20.4

N02 in dry flue gas, mol %

0.032

1.55

1.13

0.21

0.0

0.0

0.266

S02 in dry (lue gas, mol %

0.0

0.054

0.075

0.035

0.086

0.101

0.089

aThe data for pine wood, Kentucky bluegrass, feedlot manure, and RDF were calculated from empirical formulas derived from the data in Table 3.5. Each biomass fuel was assumed to contain 15.0 wt % moisture. The data for the coals and coke are from Reed (1983), except for the data on NO2 and SO; in the dry flue gas, which were calculated from the empirical formulas derived from the elemental analyses in Reed (1983). The overall assumptions are that combustion is complete, the ash and nitrogen in air are inert, and all organic nitrogen and sulfur are oxidized to N02 or S02.

ing are weighed against their beneficial effects on combustion and furnace design and costs.

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