Chapter 5 Combustion.pdf

- Bioenergy chapter 5 -

Energy releasing aspects

150

5. ENERGY RELEASING ASPECTS

151

5.1

Heat production, Electricity production, Cogeneration

151

5.2

Constraints to energy intensity set by the raw material

152

5.3

Technical possibilities of increasing the energy intensity

159

5.3.1

Techniques for gasified fuels

159

5.3.2

Techniques for liquefied fuels

161

5.3.3

Techniques for pulverised fuels

162

5.3.4

Techniques for coarse solid fuels

164

5.4

Technical possibilities of limiting the effect of contaminants

167

5.4.1

Contaminants originating from the fuel itself

167

5.4.2

Contaminants originating from incomplete combustion

177

5.4.3

Contaminants originating from extraordinary local conditions

179

5.5

Demands on the fuel set up by the technology used

181

5.5.1

Common techniques for biofuel utilisation

181

5.5.2

Fuel quality variations

184

5.5.3

Ash property variations

187

5.5.4

Fuel feed-rate variations

188

5.5.5

Fuel planning (logistics) influence

189

5.6

Incipient contaminants and their fates during conversion

191

5.7

Ways to influence the ash properties for recycling

195

5.8

Chapter 5 symbols

199

References

200

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- Bioenergy chapter 5 -

Energy releasing aspects

151

5. Energy releasing aspects

The energy conversion plant is the heart in the energy chain from biofuel producer

to end-use customer.

5.1

Heat production, Electricity production, Cogeneration

The end-use customer has different demands depending not only on the climate but

also on the available distribution networks. In Scandinavia a significant part of the

total population is connected to district heating networks. At these latitudes there is

also an obvious need for heating during 8-10 months/year. In more southern

countries, such as those of the Mediterranean region in Europe, the expansion of

district heating has been less successful and individual heating is common. For

safety reasons, electrically-based heating systems are promoted and thus there is a

stronger demand for electricity production in these regions than there is in

Scandinavia. It is also true that the main grids in many rural regions are not

sufficient for todays demands and there is an increasing interest for local, small-

scale co-generation technology based on local fuels.

The thermodynamic constraints shown in the introduction to this book combined

with the ecological constraints described in chapter 2 thus imply that electricity

production in condensing plants based on biofuels is likely to suffer from

economical problems. To reduce the capital cost per produced unit of energy the

plant has to be big. Small-scale electricity production is not yet (late 1990’s) feasible

but there is ongoing technical development in the field. These processes are so far

based on low-temperature steam cycles which limits the attainable efficiency. As

yet, these small-scale processes can only be regarded as complements to heat

production in co-generation processes. A large plant, on the other hand, needs to

be supplied from a large area and transportation becomes a heavy economic burden

on the total system, see chapter 4. To counteract this there is a need for an

increased efficiency in the conversion process. This might be achieved either by

higher steam data in case of a steam turbine or by constructive measures, steam

injection etc., in the case of a gas turbine. However, if the fuel is not upgraded, the

steam data attainable in practice are limited to about 650 o C/40 MPa. If the fuel

quality is upgraded, which may reduce the transportation costs, the fuel price is also

upgraded. For gas turbines fired with bio-fuels the problems are mainly connected

with the ash composition and its content of alkali metals, forming potentially

aggressive species in the flue gases. To summarise: it is not all that simple to

increase the efficiency in a large-scale power plant for electricity production based

on bio-fuels. Partly because of this, large-scale condensing power plants based on

biofuels are not competitive unless they are subsidised or fuel taxes are set so as to

promote bio-fuel use.

Björn Zethræus -

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- Bioenergy chapter 5 -

Energy releasing aspects

152

5.2

Constraints to energy intensity set by the raw material

The thermal load in combustion applications is commonly related either to the

fireplace volume (MW/m 3 ) or to the cross-sectional area (MW/m 2 ). Looking at

coarse fuel particles it is generally true that the combustion rate is limited by the

supply of oxidising agent to the burning surface. Assuming a spherical particle with

diameter d p and density r p , situated in a gas flow with a mean velocity U g from

below and with a gas density r gas , the highest possible gas velocity before the

particle is entrained by the gas (”the terminal velocity”) may be calculated, equations

5-1, 5-2, 5-3:

(

)

d g

0 2 m

max

while

max

(

)

1 14

0 153

1000

max

while

0 43

max

(

)

max

1000 m

The terminal velocity limits the amount of gas throughflow through a grate when

particle entrainment is the operational constraint. This is a normal situation in fixed-

bed combustion of biofuels.

Measurements indicate, as also stressed in chapter 2, that the ultimate analyses of

the dry, ash free substance of various biofuels are very much alike:

Table 5:1a Sample ultimate analysis of different biofuels - Wood fuels

Wt-%, dry ash-free substance

O(diff)

Ash in dry subst

Miscellaneous

Forest residues

(Finnish)

52.0

6.2

0.41

41.4

1.33

Forest residues

(Swedish, high ash)

53.2

6.0

0.94

39.8

4.05

Chips (stem)

52.1

6.1

0.30

41.4

0.60

Peat (highly decomp)

57.0

5.8

0.95

36.0

Mean value:

53.6

6.0

0.65

39.6

Std. deviation:

2.3

0.2

0.34

2.5

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Softwoods, stem wood

Saw dust, pine

51.0

6.0

0.08

42.9

0.08

Douglas fir

58.0

7.0

0.11

44.9

0.80

Pitch pine

59.7

7.3

33.1

1.13

Western hemlock

51.5

5.9

0.10

42.3

2.20

White cedar

50.0

6.4

44.7

0.37

Redwood

53.6

5.9

0.10

40.4

0.20

Mean value:

54.0

6.4

0.10

41.4

Std. deviation:

4.0

0.6

0.01

4.4

Hardwoods, stem wood

Poplar

51.9

6.3

41.8

0.65

White ash

49.8

6.9

43.1

0.30

White oak

50.5

6.6

42.8

0.24

Mean value:

50.7

6.6

42.6

Std. deviation:

1.1

0.3

0.7

Bark

Spruce bark

51.1

6.0

0.41

42.4

2.34

Pine bark

(Finnish)

53.4

5.8

0.41

40.6

1.72

Pine bark

55.0

5.8

0.10

39.0

2.90

Oak bark

41.5

5.7

0.21

52.5

5.30

Redwood bark

52.1

5.1

0.10

42.6

0.40

Mean value:

50.6

5.7

0.25

43.4

Std. deviation:

5.3

0.3

0.15

5.3

Summary, wood fuels:

Mean value:

52.4

6.1

0.32

41.8

Std. deviation:

3.9

0.5

0.30

3.9

na = not separately analysed

Table 5:1b Sample ultimate analysis of different biofuels - Agricultural fuels

Wt-%, dry ash-free substance

O(diff)

Ash in dry subst

Ref

Wheat straw (Den)

49.6

6.2

0.61

43.5

4.71

Wheat straw

50.5

6.2

1.05

42.2

4.67

Barley straw (Fin)

49.1

6.1

0.64

44.1

5.88

Rapeseed

49.5

6.1

0.82

43.4

2.86

Flax (whole straw)

50.6

6.3

1.34

41.7

2.93

Flax (shive)

51.2

6.2

0.61

41.9

1.81

Reed canary grass

49.4

6.3

1.54

42.7

8.85

Mean value:

50.0

6.2

0.94

42.8

Std. deviation:

0.8

0.1

0.38

0.9

na = not separately analysed

Countries: Den = Denmark, Fin = Finland

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Table 5:1c Sample ultimate analysis of different biofuels - European agrofuels

Wt-%, dry ash-free substance

O(diff)

Ash in dry subst

Sweet sorghum (It)

49.7

6.1

0.42

43.7

4.74

Kenaf (It)

48.4

6.0

1.04

44.4

3.63

Miscanthus (It)

49.5

6.2

0.62

43.1

3.31

Sugar cane (It)

49.5

6.2

0.52

43.6

3.70

Miscanthus (Ger)

49.7

6.1

0.31

43.7

2.30

Mean value:

49.4

6.1

0.58

43.7

Std. deviation:

0.5

0.1

0.28

0.5

Countries: Ger = Germany, It = Italy

From the tables we may thus see that the dry, ash-free substance in wood fuels

contains 52.4 + 3.9 % carbon while agricultural fuels contain 50.0 + 0.8 % carbon

and agrofuels, specifically from the European continent, contain 49.4 + 0.5 %

carbon. Statistically we may not distinguish between these fuels based on the

carbon content. For the hydrogen content the corresponding number pairs become

6.1+0.5, 6.2+0.1 and 6.1+0.1. Once again the fuels may not be distinguished based

on the hydrogen content. Finally, for the oxygen, the numbers are 41.8+3.9,

42.8+0.9 and 43.7+0.5 which gives rise to the same conclusion. For the nitrogen

there seems to be a difference between wood fuels and agrofuels in favour, lower

nitrogen content, of the former. However, the differences reported in this table are

not very significant with mean values and standard deviations 0.32+0.30, 0.94+0.38

and 0.58+0.28, respectively.

From a purely stoichiometric viewpoint, we may thus assume that the dry ash-free

substance of most any biofuel consists of 51.3 + 3.0 % C, 42.4 + 3.0 % O, 6.1 +

0.5 % H and 0.5 + 0.3 % N. Combustion of 1 kg of this substance to CO 2 and H 2 O

requires an external supply of exactly 44.75 moles of oxygen equivalent to 1.08 m 3

at 101325 Pa, 20 °C. The lower heating value for the dry ash-free fuel becomes 19.2

MJ/kg.

Assuming a fuel particle diameter of 1 cm, a dry substance density of 400 kg/m 3 , a

gas density of 1.20 kg/m 3 and a gas viscosity of 18.2 10 -6 Pa s, typical values for

wood and air at ambient conditions, then yields U g max » 29.5 m/s. This is equivalent

to a volume flow of 29.5 m 3 /s m 2 or an oxygen flow (21 % O 2 in air) of 6.2 m 3 O 2 /s

m 2 . Thus it is possible to combust completely 5.76 kg fuel/m 2 s, provided there is

perfect mass transfer of oxygen from gas phase to solid phase. This is equivalent to

110 MW/m 2 which thus constitutes an upper limit for the combustion intensity of 1

cm diameter biofuel particles in a fixed-bed boiler.

If it is assumed that the particle maintains its density during the whole combustion

process and only loses mass by a successive diameter decrease and if the condition

is that only 1 % of the fuel may be entrained by the gas flow, then the terminal

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