Generation and conversion of carbonaceous fine particles during bubbling
fluidised bed gasification of a biomass fuel
F. Miccio
a,
*
, O. Moersch
b
, H. Spliethoff
b
, K.R.G. Hein
b
a
Istituto Ricerche Combustione CNR, via Metastasio 17, 80125 Napoli, Italy
b
Institut fu
¨
r Verfahrenstechnik und Dampfkesselwesen IVD-Universita
¨
t Stuttgart, Pfaffenwaldring 23, 70550 Stuttgart, Germany
Received 2 June 1998; received in revised form 25 February 1999; accepted 25 February 1999
Abstract
This article presents an experimental research concerning the biomass gasification in bubbling fluidised bed. The attention is focused on
the part played by comminution phenomena during the conversion of fuel particles, and on the occurrence of post-conversion of elutriated
chars in the freeboard. The aim is pursued by means of measurements of bed carbon load, elutriation rate as well as particulate concentration
at various heights of the freeboard in a laboratory scale facility equipped with an isokinetic probe. The results of experiments show that fines
generation is relevant during gasification, and post-conversion of particles takes place after their release from the bed. The carbon post-
conversion in the freeboard achieves a maximum value of 70%, even when the operating conditions are unfavourable for gasification. The
results are also confirmed by measurements of gas composition in the freeboard. The laboratory analyses of samples taken during experi-
ments could confirm that a percolative fragmentation takes place in parallel with carbon post-conversion. ᭧1999 Elsevier Science Ltd. All
rights reserved.
Keywords: Biomass; Gasification; Fluidised bed
Nomenclature
C
i
concentration (%, g m
Ϫ3
)
C
P
particle concentration (g m
Ϫ3
)
d particle size (mm)
d
C
particle size of coarse particles (m)
E
C
carbon elutriation rate (mol h
Ϫ1
)
E
C,0
carbon elutriation rate at bed surface (mol h
Ϫ1
)
F
CP
mass flow rate of gasified fixed carbon (kg h
Ϫ1
)
F
A
mass flow rate of the fluidising air (kg h
Ϫ1
)
F
F
mass flow rate of the fuel (kg h
Ϫ1
)
F
P
mass flow rate of fine particles (kg h
Ϫ1
)
k
perc
percolative fragmentation constant
k
a
attrition constant
n
i
molar flow rate (mol h
Ϫ1
)
n
i
ء
calculated molar flow rate (mol h
Ϫ1
)
N
C
bed carbon load (mol)
Q
g
volumetric flow rate of the gas (m
3
h
Ϫ1
)
T temperature (ЊC)
U fluidisation velocity (m s
Ϫ1
)
U
mf
minimum fluidisation velocity (m s
Ϫ1
)
X
C,P
carbon content of particles
X
FC,F
content of fixed carbon in the fuel
X
db
relative percentage of sieved fractions (%)
X
dC
percent mass ratio of the carbon in sieved fractions
(%)
z reactor elevation from the air distributor (m)
Greek symbols
e
percentage error (%)
h
C
conversion efficiency of fixed carbon (%)
h
C,post
post-conversion efficiency of fixed carbon in the
freeboard (%)
h
g
gasification efficiency
l
equivalence ratio
j
i
conversion degree
r
g
gas density (kg m
Ϫ3
)
1. Introduction
Gasification of biomass fuels offers a series of advantages
compared with the combustion for power generation, first of
all the higher global efficiency, which can be achieved in a
process based on a gasification step coupled with a motor or
turbine [1]. At present, the research concerning the biomass
gasification is mainly devoted to improve the quality of the
producer gas, because the high concentration of tar and fine
Fuel 78 (1999) 1473–1481
0016-2361/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(99)00044-7
www.elsevier.com/locate/fuel
* Corresponding author. Tel.: ϩ 39-0815935379; fax: ϩ 39-
0815931567.