feed stock preperation

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BIOMASS FEEDSTOCKS PREPARATION METHODS FOR ENERGY

PRODUCTION AND ITS ECONOMIC EVALUATION

R. Ramos Casado, L.E. Esteban Pascual

CIEMAT - CEDER N-111 Road, 206 km. 42290 Lubia Soria. SPAINPhone +34 975281013 Fax +34 975281051

E-mail: [email protected]

INTRODUCTION

Biomass gasification allows the conversion of different biomass feedstocks to a more

convenient gaseosus fuel that can be used in conventional equipment (e.g. boilers,

engines and turbines) or advanced equipment (e.g. fuel cells) for the generation of heat

and electricity. The conversion to a gaseosus fuel provides a wider choice to

technologies for heat and electricity generation for small to large scale application.

In general, the conversion of biomass to heat and electricity via gasification involves the

following steps: biomass storage and transport, size reduction, drying, feeding,

gasification, fuel gas cleaning and ash disposal and recycling. The former of these

activities are also typical of combustion system in which biomass feedstocks are

valorised.

Biomass includes many different materials, most of them are very heterogeneous and it

has low density. Many operations are involved in pre-treatment of the feedstocks to

obtain materials suitable to be introduced into the feeding system of the combustion or

gasification process. Size distribution and moisture content affecting the flow behaviour

and the process himself, therefore size control and drying operations have are relevant

tasks.Biomass particle size affects gasification reaction rates and gas composition. Since

comminution operations are expensive and energy intensive, there is a trade-off, in

terms of cost and energy, between particle size reduction and yield and characteristics of

the product gas. Besides the biomass particle size is determined by adopted gasification

system. The use of bio fuels in densified forms, like pellets, may be a challenge because

particles of the same size are formed from heterogeneous materials. The costs of pellets

production are high, and only if gasification of pellets optimize the product gas

composition could be considered this option.

In the scope of Spanish project on-cultivos (www.oncultivos.es) about energy crops,

CIEMAT is working on gasification of biomass materials. The gasifier is an

atmospheric circulanting fluidized bed placed in CEDER (Centre of Development ofRenewable Energies).

This work present an evaluation of the energy demanded to prepare this type of biomass

feedstock. Two possibilities are considered: only size reduction and pelletization.

MATERIALS AND METHODS

Rape straw and poplar wood have been chosen as materials to feed the circulating bed

gasifier. Rape (brassica genus) is a crop adapted to unirrigated zones and it is delivered

directly from the land harvested as bales of straw. On the other hand, poplar has been

growed using densities up to 10.000 plants per hectare and harvesting in short rotation

with cycles less than five years. Generally woody materials are crushed into chips onthe land.


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In table 1 the properties of raw materials are shown. The moisture content is very high

in poplar chips, but the natural drying of wood in the storage place during 3 or 4 month

is enough to decrease the moisture until 25%. The low ash content of this type of

materials improves the thermal balance in the process, and reduces the occlusion and

loss of carbon in the residue. The composition of nitrogen in feedstock leads to the

formation of ammonia (NH3). Elements such as sulfur and chlorine lead to theformation of corrosive gas components such as H2S and HCl. In this sense woody

materials have better properties than herbaceous ones.

Table 1.- Chemical characteristics of raw materials

BIOMASS

FEEDSTOCK

Bulk

density

(kg/m3)

Moisture

content

(%)

Ash

content

(% d.b.)

N

(% d.b.)

S

(% d.b.)

Cl

(% d.b.)

RAPE

(Brassica)125 10 - 15 4 - 8 1,4 1,7 0,3 - 0,8 0,1 - 0,4

POPLAR 140 40 - 55 1,5 - 3 0,1- 0,3 0,02-0,03


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Table 2.- Evaluation of milling cost

BIOMASS

FEEDSTOCK

RAW

MATERIALPROCESS

SPECIFIC

MASS FLOW

(Kg/h. kW)

SPECIFIC

ENERGY

(kWh/t)

BULK

DENSITY

(kg/m3)

RAPE Big baleCHOP + MILL

(Sieve 10 mm)6,3 114 185

POPLAR ChipsSHRED + MILL

(Sieve 10 mm)31,9 38 190

The bulk density of both materials is similar, close to 190 kg/m3. Studying the

distribution particle size curves (figures 1 and 2) it can be seen that the amount of

particles less than 1 mm (fine particles) is much higher in the herbaceous materials like

rape straw. A percentage of fine particles close to 60%is produced in rape straw milling.

However this value is less than 30% in wood poplar milling.

RAPE (BRASSICA)

0

10

20

30

40

50

60

70

80

90

100

0,01 0,1 1 10 100

Sieve aperture (mm)

Cumulativepercentageundersize(d.b.)

Figure 1.- Distribution particle size of milled rape straw to 10 mm of sieve aperture.

POPLAR

0

10

20

30

40

50

60

70

80

90

100

0,01 0,1 1 10 100

Sieve aperture (mm)

Cumulativepercentageundersize(d.b.)

Figure 2.- Distribution particle size of milled poplar wood to 10 mm of sieve aperture.


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B- Pelletized biomass

The pellets are produced from the milled material with 4 mm of sieve aperture in both

cases given enough to pelletize with 6 mm opening. The values of specific energy

consumed that are shown in table 3, point out that the total energy demanded to

pelletize the rape (181 kWh/t) is higher than the poplar (136kWh/t). However, in therape straw case more than half of the total energy is inverted in communition (121 of

181 kWh/t ) and in the poplar chips case the energy consumed for size reduction (64

kWh/t) is similar than for pressing process (72 kWh/t).

Table 3.- Evaluation of pelletizing cost

BIOMASS

FEEDSTOCK

INPUT

MATERIALSTAGE

SPECIFIC

MASS

FLOW

(kg/h.kW)

SPECIFIC

ENERGY

(kWh/t)

BULK

DENSITY

(kg/m3)

Big bale

CHOP + MILL

(Sieve 10 mm) 6,3 114 185

Milled to 10 mmMILLING

(Sieve 4 mm)54,8 7 245

RAPE

Milled to 4 mm PELLETIZING 10 60 600

ChipsSHRED + MILL

(Sieve 10 mm)31,9 38 190

Milled to 10 mmMILLING

(Sieve 4 mm)20,7 26 210

POPLAR

Milled to 4 mm PELLETIZING 8 72 670

The table 4 gathers the characteristics of produced pellets. It is worth pointing out that:

- The moisture content of pellets is uniform, close to 10%.- The density of pellets is up to 4,5 times the density of raw materials.- The pellets size is very homogeneous.

Table 4.- Characteristics of pellets.

BIOMASS

FEEDSTOCK

PARTICLE SIZE

(mm)

BULK

DENSITY

(kg/m3)

MOISTURE

CONTENT (%)

HEAT VALUE

LHV

(MJ/kg d.b.)

RAPE

(Brassica)

Diameter: 6

Length : 15-25600 10,4 15.3

POPLARDiameter: 6

Length : 15-25670 9,7 16.8

CONCLUSION

Milling and pelletizing are energy intensive process. Pellets have physical

characteristics as lower moisture, homogeneous size and higher density respect to the

biomass feedstocks that justified the pelletizing energy cost.

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