Tar reduction in downdraft biomass gasifier using a primary method

Renewable Energy 78 (2015) 478e483

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Tar reduction in downdraft biomass gasifier using a primary method
 Luz Silveira a,
Einara Blanco Machin a, Daniel Travieso Pedroso a, *, Nestor Proenza b, Jose
c
a
a
Leonetto Conti , Lúcia Bollini Braga , Adrian Blanco Machin
~o Paulo State University (UNESP), Guaratingueta
, SP, Brazil
Energy Department, Sa
Mechanical Engineering Department, University of Camagüey, Cuba
c
Department of Chemistry, University of Sassari, Sassari, Italy
a

b

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 19 June 2014
Accepted 30 December 2014

Available online

This work present a novel primary method, for tar reduction in downdraft gasification. The principle of
this new technology is to change the fluid dynamic behaviour of the mixture, formed by pyrolysis
product and gasification agent in combustion zone; allowing a homogeneous temperature distribution in
radial direction in this reaction zone. To achieve the change in the fluid dynamic behaviour of the
mixture; the entry of gasification agent to combustion zone is oriented by means of wall nozzles in order
to form a swirl flow. This modification in combination with the extension of the reduction zone, will
allow, to increases the efficiency of the tar thermal cracking inside the gasifier and the extension of the
Boudouard reactions. Consequently, the quantity of tar passing through the combustion zone without
cracking and the concentration of tar in the final gas, decrease significantly in relation with the common
value obtained for this type of reactor, without affecting significantly the heating value of the producer
gas. In this work is presented a new design for 15 kW downdraft gasification reactor, with this technology implemented, the tar content obtained in the experiments never overcome 10 mg/Nm3, with a
lower heating value of 3.97 MJ/Nm3.
© 2015 Elsevier Ltd. All rights reserved.

Keywords:
Biomass
Downdraft gasifier
Gasification
Tar
Swirl flow

1. Introduction
Biomass, mainly in the form of wood, is the oldest form of energy used by humans. Biomass generally means a relatively dry
solid of natural matter that has been specifically grown or has
originated as waste or residue from handling such materials [1].
The thermochemical conversion of biomass (pyrolysis, gasification,
combustion) is one of the most promising non-nuclear forms of
future energy. Biomass is a renewable source of energy and has

many ecological advantages [2]. Gasification is the key technology
of biomass based power generation; is a high-temperature process
(873e1273 K) that decomposes complex biomass hydrocarbons
into gaseous molecules, primarily hydrogen, carbon monoxide, and
carbon dioxide; also are formed some tars, char, methane, water,
and other constituents. Several institutions working on biomass
gasification have given many definitions of tar. In the EU/IEA/USDOE meeting on tar measurement protocol held in Brussels in the
* Corresponding author.
E-mail addresses: (E.B. Machin), ,
(D.T. Pedroso).
/>0960-1481/© 2015 Elsevier Ltd. All rights reserved.

year 1998, it was agreed by a number of experts to define tar as all
organic contaminants [polycyclic aromatic hydrocarbon (PAH)]
with a molecular weight higher than benzene [3]. Tar is undesirable
because of various problems associated with its condensation,
causing problems in the gasification installations as well as in the
equipments that use the producer gas as fuel like internal combustion engines and gas turbines. The required gas quality to fuel
internal combustion engines is normally reached easily in the
modern downdraft gasifiers, except for the content of dust and tar.
Thermal, catalytic or physical processes either within the gasification process (primary methods) or after the process (secondary
methods) can be applied to remove tars. Primary methods have the
advantage that dispenses the use of an expensive cleaning system
for producer gas. In addition, cracking of tars in the reactor could
increases the amount of combustible gases in the producer gas and
therefore, the overall process efficiency. There are some sophisticated options available, which claimed a significantly reduction of
the tar content in the producer gas, however, the method must be
efficient in terms of tar removal, economically feasible, but more
importantly, it should not affect the formation of useful producer
gas components [4].

E.B. Machin et al. / Renewable Energy 78 (2015) 478e483

The catalytic cracking and electrostatic filters are two examples
of the options, that claim a significant tar reduction in the producer
gas, but they increase the cost of the plants, especially in the small
ones. Currently, the preferred option for tar reduction is in the
gasifier itself through process control and the use of primary
measures such as additives and catalysts which modify gasification
conditions [4e12]. Theoretically, producer gas with low tar content
can be obtained if a high-temperature zone can be created, where
the gaseous products of pyrolysis are forced to reside the necessary
time to undergo a secondary gasification. Previous works have been
developed in order to design a downdraft gasifier, able to increase
the efficiency of tar reduction in the producer gas during gasification process. Bui et al. [13] developed a multi-stage reactor design
that separates the flaming-pyrolysis zone from the reduction zone.
In that design, the tar vapours generated in the first zone are
burned or cracked to simple molecules by high temperature in the
second zone, improving the gas quality and conversion efficiency.
The minimum content of gravimetric tar obtained with this design
was 92 mg/Nm3. Susanto and Beenackers [14] developed a downdraft moving bed gasifier with internal recycle and separate combustion of pyrolysis gas with the aim of reduce a tar content in the
producer gas; in their experiments a minimum of 48 mg/Nm3 of tar
was obtained.
On this background, the main objective of this work is to propose a new downdraft gasifier design, able to generate the producer
gas with low tar concentration using a novel primary method
without decreasing significantly the heating value of the producer
gas.

2. Process principle

In the Imbert design of downdraft gasifier, the gasification agent
is fed above a constriction (throat) by nozzles uniformly distributed
on the wall of the combustion chamber, oriented toward the centre
of the circle, that describe the perimeter of the combustion
chamber. In this design, some cool zones are created near to the
nozzles, where the temperature is not sufficiently higher to permit
the thermal cracking of the tar present in the mixture and to undergo its secondary gasification [15]. This is one of the reasons for
the presence of tar in the producer gas. If tarry gas is produced from
this type of gasifier, is common practice reduce the central
constriction area, until a gas with low tar content can be produced.
However, this area dimensions also play an important role in the
gas production rate.
In order to avoid the formation of cool zones, it is proposed in
this work to modify the fluid dynamic behaviour of the mixture
formed by the pyrolysis gases and the gasification agent in the
combustion chamber.

I
G¼

Vðr0 ; tÞdl

(1)

L

The circulation of the vector V (ro, t) combined with the
downward movement of the fluid, caused by absorption from the
base of the chamber through the diaphragm, generates a swirl flow.
This fluid dynamic behaviour would allow to increase the mixing of

the gasifying agent with the pyrolysis gases [21,22]; homogenizing
the temperature inside the combustion chamber, diminishing the
formation of cool areas between the nozzles as main result. In
addition this modification increase the residence time of the gas
inside the combustion chamber; thereby increasing the thermal
cracking of the tar in this zone, minimizing its passage to the
reduction zone, decreasing the tar concentration in the producer
gas. Swirl number S may effectively control the residence time
distribution of the gas mixture, which is function of the fluid entry
angle [18]. The increase of the residence time has the undesirable
effects of decreasing the efficiency and productivity of the gasifier,
as described by Susanto [13]. Fig. 1 shows a top view of the combustion chamber of the reactor, illustrating the inclination of the
inlet nozzles of gasification agent.
3. Experimental approach
3.1. Investigated samples
The gasification tests were performed using three different
woody biomasses, supplied by a wood processing factory. The
biomasses used were Peach (Prunus persica), Olive (Olea europaea)
and Pine (Pinus pinea). The properties of the woody biomass are
shown in Table 1. The elemental compositions were determined
using a CHNS-O Elementar Vario GmbH EL III and the Higher
Heating Value (HHV) using a calorimeter IKA C-5000 (ASTM D3286-91a). The moisture and ash composition were determined
using the ASTM E-871-82 and ASTM D-3174-82. The results were
similar to literature values. For the experiments, the biomasses
were chopped in square-based prism pieces with dimensions of
about 2 Â 1 Â 1 cm. The size and shape are very important for the
behaviour of biomass in the downdraft gasifier as far as its movement, and bridging and channelling formations. In addition, the
height of the oxidation zone and the pressure drop inside the
reactor, depend on these characteristics.
3.2. Experimental setup

The scheme of the downdraft wood gasifier is show in Fig. 2. The
gasifier unit is constituted of two cylindrical coaxial structures
constructed using a mild steel sheet. An insulating material coats
the external one, while the internal cylinder is provided with
additional heat recuperation surfaces to improve the efficiency of

2.1. The combustion chamber
Swirl flows are widely used to intensify the process of heat and
mass transfer between solid particles and airflow in vortex chambers, the advantages of swirl flows has been deeply studied by
several authors [16e20]. The swirl flow of the mixture could be
created changing the entry angle of the gasification agent to the
combustion chamber. The new angle must be different of the
standard 90 in the Imbert design. This modification allow that the
circulation G (Equation (1)) of the velocity vector V(ro,t) of any
element of the fluid at any position r s 0 in the plane in which the
nozzles are located, or any other parallel plane below this until the
diaphragm, is different from zero (G s 0).

479

Fig. 1. Nozzles inclination in the combustion chamber.


480

E.B. Machin et al. / Renewable Energy 78 (2015) 478e483

Table 1
Elemental composition and HHV of the studied biomasses.
Biomass

C
%wt db

H
%wt db

N
%wt db

O
%wt db

Ash
%wt db

Moisture
%wt

HHV
MJ/kg

Peach
Olive
Pine

48.06
46.43
48.18

5.83
5.63
5.71

0.55
0.55
0.15

44.03
44.91
43.89

1.53
2.48
2.07

9.8
10.6
9.0

18.74
17.80
18.67

the gasification process (Fig. 2). The internal capacity is 0.452 m3,
the height of the gasifier is 1.02 m and the internal radius at the
drying e pyrolysis zone is 0.30 m. The dimensions of reduction
zone are enlarged to boost the rate of the Boudouard and the
wateregas reactions, in order to increase the concentration of CO
and H2 in the producer gas and also decrease the gas temperature.

The gasification agent for the experiments (air) is supplied using an
electric blower with control valve, capable of supply the required
air for the gasification process.
The lines are heated up to 453 K in order to prevent condensation of the producer gas compounds inside the conducts and the
measurement device. The producer gas sample is filtered, cooled
and drained, before be analysed in the Gasboard-3100P mobile gas
analyser. The temperature are measured by mean of six thermocouples (type K) located at different height of the reactor bed. Air
and gas flows are measured with an orifice and differential
manometer. All the experimental data is recorded by data logger in
5 min intervals. The simplified experimental setup for the test of
the modified reactor is presented in Fig. 3.
3.3. Tar sampling principle
The principle of the test method for gravimetric tar measurement is based on the continuous sampling of a gas stream,

containing particles and organic compounds (tar) under isokinetic
conditions; according to the methodology described in DD CEN/TS
15439:2006 [23].
The determination is carried out in two steps: sampling and
analysis. The equipment for sampling shown in Fig. 4, consists of a
heated probe (module 1), a heated particle filter (module 2), a
condenser, a series of impinger bottles containing a solvent (isopropanol) for tar absorption (module 3), and equipment for pressure and flow rate adjustment and measurement (module 4).
Upstream of the condenser, the tubes connecting these parts are
heated in order to prevent tar condensation. Temperatures of the
condenser and the impingers were properly selected to ensure
quantitative collection of the tars (1, 2, and 4 is between 308 and
313 K, and 3, 5 and 6 is between À258 and 253 K). Tar collection
occurs both by condensation and by absorption in the condenser, in
the impinges, and by capturing of aerosols in glass frits. The analysis
of the samples is carried out according to the methodology
described in Ref. [23].

3.4. Process flow description
The gasifier system was run nine times, for periods between 2.5
and 4 h. To start the gasifier, initially the fuel biomass is loaded up
to the reactor maximum capacity and is closed. Subsequently is
introduced a propane gas duct by the air entrance to the reactor, to
create a flame inside the combustion chamber, then the vacuum
pump was turned on and the propane gas feed is removed. In less
than 15 min or when the temperature in pyrolysis zone (TC 2 and
TC 3) reaches 573 K the ignition step is completed and the record of
the profile of reactor temperatures and the gases flow starts. The
producer gas analysis starts when the preset temperature profile in
the reactor is reached, due to the high concentration of condensable
gases in the producer gas composition during the ignition process.
The tar sampling process starts at the same time of the producer
gas analysis, with the installation shown in Fig. 4; each tar sampling
takes 45 min.
4. Results and discussion

Fig. 2. Reactor's scheme.

Table 2 and Table 3 shown the performance of the biomass
gasifier system and the composition of the producer gas during the
experiments, at regular intervals of 5 min.
Fig. 5 shows a typical behaviour of the temperature profile in the
reactor during the experiments. As it is observed, there are an
oscillation of the temperature value in all the bed section during all
the experiments, with the exception of the temperature of the
producer gas, where the temperature remain more stable. The main
reason of this variation is biomass movement inside the reactor
during the gasification process. The temperature of the producer

gas remains in the range of 410e430 K, lower than the typical range
of 700e720 K reported for this type of reactor.
The HHV of the producer gas is calculated from the concentration of the combustible components. For all the experiments, the
HHV obtained was higher to 3.50 MJ/Nm3, and the higher values
were obtained in the experiments using Peach as fuel, where the
mean value was 3.97 MJ/Nm3.
These values are lower than the theoretical and experimental
results reported in the literature; Zainal et al. [14] report 4.72 and
4.85 MJ/Nm3 respectively for same capacity and type downdraft
gasifier.
These results are because the medium content of H2, CO and CH4
in the producer gas obtained in the experiments with the tested
reactor was slightly lower than the typical composition of the
producer gas reported by several authors [2,3,13,14,24,25]. The O2
concentration has the same behaviour, showing an increase in the


E.B. Machin et al. / Renewable Energy 78 (2015) 478e483

481

Fig. 3. Experimental installation setup.

combustion rate of the fuel gas in the reactor as negative effect of
the modifications implemented.
The mean tar content of the producer gas obtained in the experiments was 9.10 mg/Nm3 for Olive, 4.07 mg/Nm3 for Peach and
8.73 mg/Nm3 in the case for Pine. Fig. 6 compares the tar content in
the producer gas obtained by several authors 19e35 mg/Nm3 [26],
5 mg/Nm3 [25], 97 mg/Nm3 [27], 50 mg/Nm3 [28] and 10 mg/Nm3
[29]; with the content obtained in the studied reactor. The gas

quality is comparable with the obtained in experiments with the
optimized two stages gasifier, developed by Bentzen [25] (5 mg/
Nm3), but with higher HHV. Burhenne et al. [29] reported similar

gas quality, with a minimum tar content of 10 mg/Nm3 and HHV
between 4.85 and 4.48 MJ/m3 using a multi-staged gasification
technology.
The CO/CO2 and H2/CO ratios are constant; the heating value of
the gas is a direct consequence of its chemical composition, which
depends on the reaction conditions, rather than the heating value
of the entering biomass, equal for all those experienced.
The increase of the residence time of the gas mixture in reactor
as consequence of the modification in the combustion chamber
also has the undesirable effects of decreasing the efficiency and

Table 2
Operating parameters.
Biomass

Fig. 4. Modular sampling train of tar.

Mean process time
Mean temperature error ± 1.0 K
T1
SD
T2
SD
T3
SD
T4

SD
T5
SD
T6
SD
Biomass fed
Flows
Air
Gas

Olive
(h)
(K)

(kg)
(Nm3/h)

3.80

Peach
2.50

513
18
531
49
880
30
1193
60

1123
68
417
7
8.74

473
20
491
21
780
25
1173
65
1153
73
425
9
7.6

5.74
28.9

5.3
18.4

Pine
3.10
503
22

521
18
853
22
1143
61
1103
62
408
5
7.75
5.4
21.3


482

E.B. Machin et al. / Renewable Energy 78 (2015) 478e483

Table 3
Tests results.
Biomass
Inputs
Gasifier conditions
Feed
(kg/h)
Gasifier air (20  C, 1 bar)
(kg/h)
Outputs
Dry gas

(kg/h)
Water
(g/Nm3)
Char e ash
(kg/h)
Tar
(mg/Nm3)
Error
±0.01
SD
Dry gas analysis
CO
(% vol.)
H2
(% vol.)
CO2
(% vol.)
CH4
(% vol.)
O2
(% vol.)
N2
(% vol.)
Dry gas HHV
(MJ/Nm3)
Gas density
(kg/Nm3)
Operating ratios
O2/dry biomass
CO/CO2

H2/CO
Mass balance and energy efficiency
Mass in/mass out
Cold gas efficiency

Olive

Peach

Pine

3.3
6.79

3.05
6.20

2.5
6.45

9.02
114.5
0.160
9.10

8.60
96.5
0.085
4.07

8.17
102.3
0.128
8.73

0.19

0.19

0.19

17.4
13.2
12.4
0.8
1.3
54.9
3.55
1.183

17.7
15.0
13.5
1.2
0.9
51.7
3.97
1.167

16.0

12.1
11.4
0.2
0.9
59.4
3.65
1.191

0.45
1.40
0.76

0.44
1.31
0.85

0.44
1.40
0.76

1.01
0.61

0.98
0.78

0.99
0.58

productivity of the gasifier; that is why these parameters are lower

than in commercial gasifiers. According to this, more experiments
are required to determinate the optimum angle to achieve a balance between all these effects in order to obtain a clean gas without
diminish significantly the overall efficiency of the gasification
process. Furthermore the small size of experimental model and its
proportionally higher heat loss, influences in the overall process
efficiency.
These results have been obtained applying additionally, a
cleaning system truly simple and inexpensive, for particles
removing.
5. Conclusions
A clean producer gas was obtained with a novel downdraft
gasifier. A modified combustion chamber that prevents the

Fig. 5. Temperature profile along the reactor height in the 3rd experimental test using
Olive.

Fig. 6. Comparison between the gas quality obtained by different authors and the
present study.

formation of cool zones inside it and increases the thermal homogenization in this reaction zone was developed. This modification together with an extension of the reduction zone allows
diminishing the tar content in the producer gas. The mean values of
this parameter in all the experimental tests were lower than 10 mg/
Nm3. The low tar and particle content makes the producer gas
obtained in this reactor suitable to the use in cycle Otto engines.
Acknowledgement
We are grateful to the Coordination for the Improvement of
Higher Education Personnel (CAPES) (process 5993105), from the
Brazilian Ministry of Education (MEC) and to the National Council
for Scientific and Technological Development (CNPq) (process
162633/2013-0) from the Ministry of Science and Technology

(MCT) for their generous financing support to this research.
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