Tar reduction in downdraft biomass gasifier using a primary method

The principle of this new technology is to change theﬂuid dynamic behaviour of the mixture, formed by pyrolysis product and gasiﬁcation agent in combustion zone; allowing a homogeneous t

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Tar reduction in downdraft biomass gasi ﬁer using a primary method

Einara Blanco Machina, Daniel Travieso Pedrosoa,*, Nestor Proenzab, Jose Luz Silveiraa,

Leonetto Contic, Lúcia Bollini Bragaa, Adrian Blanco Machina

a Energy Department, S~ao Paulo State University (UNESP), Guaratingueta, SP, Brazil

b Mechanical Engineering Department, University of Camagüey, Cuba

c Department of Chemistry, University of Sassari, Sassari, Italy

a r t i c l e i n f o

Article history:

Received 19 June 2014

Accepted 30 December 2014

Available online

Keywords:

Biomass

Downdraft gasiﬁer

Gasiﬁcation

Tar

Swirl ﬂow

a b s t r a c t

This work present a novel primary method, for tar reduction in downdraft gasification The principle of this new technology is to change thefluid 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 swirlflow 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 thefinal 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 tech-nology implemented, the tar content obtained in the experiments never overcome 10 mg/Nm3, with a lower heating value of 3.97 MJ/Nm3

1 Introduction

Biomass, mainly in the form of wood, is the oldest form of

en-ergy 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/US-DOE meeting on tar measurement protocol held in Brussels in the

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 com-bustion 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 gasi fica-tion 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 sophisti-cated 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]

* Corresponding author.

E-mail addresses: einara@feg.unesp.br (E.B Machin), traviesocu@gmail.com ,

traviesocu@yahoo.com (D.T Pedroso).

Contents lists available atScienceDirect Renewable Energy

j o u r n a l h o me p a g e : w w w e l s e v i e r c o m/ l o ca t e / r e n e n e

http://dx.doi.org/10.1016/j.renene.2014.12.069

Renewable Energy 78 (2015) 478e483

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The catalytic cracking and electrostaticﬁlters 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

gasiﬁca-tion process Bui et al.[13]developed a multi-stage reactor design

that separates theﬂaming-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

down-draft moving bed gasiﬁer with internal recycle and separate

com-bustion of pyrolysis gas with the aim of reduce a tar content in the

producer gas; in their experiments a minimum of 48 mg/Nm3of tar

was obtained

On this background, the main objective of this work is to

pro-pose 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

un-dergo 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 theﬂuid dynamic behaviour of the mixture

formed by the pyrolysis gases and the gasiﬁcation agent in the

combustion chamber

2.1 The combustion chamber

Swirlﬂows are widely used to intensify the process of heat and

mass transfer between solid particles and airﬂow in vortex

cham-bers, the advantages of swirl ﬂows has been deeply studied by

several authors[16e20] The swirlﬂow 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 90in the Imbert design This modiﬁcation allow that the

circulation G(Equation(1)) of the velocity vector V(ro,t) of any

element of theﬂuid at any positionrs 0 in the plane in which the

nozzles are located, or any other parallel plane below this until the

diaphragm, is different from zero (Gs 0)

G ¼ I

L

The circulation of the vector V (ro, t) combined with the downward movement of thefluid, caused by absorption from the base of the chamber through the diaphragm, generates a swirlflow Thisfluid 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 thefluid 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 1shows a top view of the com-bustion 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 inTable 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 D-3286-91a) 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 1 1 cm The size and shape are very important for the behaviour of biomass in the downdraft gasiﬁer as far as its move-ment, 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 inFig 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

Fig 1 Nozzles inclination in the combustion chamber.

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

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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 H2in 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

conden-sation of the producer gas compounds inside the conducts and the

measurement device The producer gas sample isﬁltered, cooled

and drained, before be analysed in the Gasboard-3100P mobile gas

analyser The temperature are measured by mean of six

thermo-couples (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 inFig 3

3.3 Tar sampling principle

The principle of the test method for gravimetric tar

measure-ment 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 inFig 4, consists of a heated probe (module 1), a heated particle ﬁlter (module 2), a condenser, a series of impinger bottles containing a solvent (iso-propanol) for tar absorption (module 3), and equipment for pres-sure 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 between258 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 Processflow 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 aﬂame 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 inFig 4; each tar sampling takes 45 min

4 Results and discussion Table 2and 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 5shows 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 concentra-tion 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

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 48.06 5.83 0.55 44.03 1.53 9.8 18.74

Olive 46.43 5.63 0.55 44.91 2.48 10.6 17.80

Pine 48.18 5.71 0.15 43.89 2.07 9.0 18.67

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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

ex-periments was 9.10 mg/Nm3for Olive, 4.07 mg/Nm3for Peach and

8.73 mg/Nm3in the case for Pine.Fig 6compares 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/Nm3and HHV between 4.85 and 4.48 MJ/m3 using a multi-staged gasiﬁcation technology

The CO/CO2and 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

Fig 3 Experimental installation setup.

Table 2 Operating parameters.

Mean process time (h) 3.80 2.50 3.10 Mean temperature error ± 1.0 K (K)

Flows

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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

bal-ance 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

formation of cool zones inside it and increases the thermal ho-mogenization in this reaction zone was developed This modi ﬁca-tion together with an extension of the reducﬁca-tion 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 generousﬁnancing support to this research References

[1] Giltrap DL, McKibbin R, Barnes GRG A steady state model of gas-char re-actions in a downdraft biomass gasiﬁer Sol Energy 2003;74:85e91 [2] Babu BV, Chaurasia AS Modeling for pyrolysis of solid particle: kinetics and heat transfer effects Energy Convers Manag 2003;44:2251e75

[3] Neeft JPA, Knoef HAM, Onaji P Behaviour of tar in biomass gasiﬁcation sys-tems Tar related problems and their solutions Novem ed 1999 Nederland [4] Devi L, Ptasinski KJ, Janssen Frans JJG A review of the primary measures for tar elimination in biomass gasiﬁcation processes Biomass Bioenergy 2003;24: 125e40

[5] Akay G, Dogru M, Calkan OF Biomass to rescue Chem Eng Lond 2006;786: 55e7

[6] Huang J, Schmidt KG, Bian Z Removal and conversion of tar in syngas from woody biomass gasiﬁcation for power utilization using catalytic hydrocrack-ing Energies 2011;4:1163e77

[7] Jordan CA, Akay G Speciation and distribution of alkali, alkali earth metals and major ash forming elements during gasiﬁcation of fuel cane bagasse Fuel 2012;91:253e63

[8] Abu El-Rub Z, Bramer E, Brem G 2004 review of catalysts for tar elimination in biomass gasiﬁcation Ind Eng Chem Res 2004;43:6911e9

[9] Miskolczi M, Borsodi N, Buyong F, Angyal A, Williams PT Production of py-rolytic oils by catalytic pyrolysis of Malaysian refuse-derived fuels in contin-uously stittred batch reactor Fuel Process Technol 2011;92:925e32 [10] Elbaba IF, Wu C, Williams PT Hydrogen production from the pyrolysis-gasiﬁcation of waste tyres with a nickel/cerium catalysts Int J Hydrogen En-ergy 2011;36:6628e37

[11] Wu C, Wang L, Williams PT, Shi J, Huang J Hydrogen production from biomass gasiﬁcation with Ni/MCM-41 catalysts: inﬂuence of Ni content Appl Catal B Environ 2011;108:6e13

[12] Jordan CA, Akay G Effect of CaO on tar production and dew point depression during gasiﬁcation of fuel cane bagasse in a novel downdraft gasiﬁer Fuel Process Technol 2013;106:654e60

[13] Bui T, Loof R, Bhattacharya SC Multi-stage reactor for thermal gasiﬁcation of wood Energy 1994;19(4):397e404

Table 3

Tests results.

Inputs

Gasiﬁer conditions

Gasiﬁer air (20 C, 1 bar) (kg/h) 6.79 6.20 6.45

Outputs

Water (g/Nm 3 ) 114.5 96.5 102.3

Char e ash (kg/h) 0.160 0.085 0.128

Dry gas analysis

Dry gas HHV (MJ/Nm 3 ) 3.55 3.97 3.65

Gas density (kg/Nm 3 ) 1.183 1.167 1.191

Operating ratios

O 2 /dry biomass 0.45 0.44 0.44

Mass balance and energy efﬁciency

Mass in/mass out 1.01 0.98 0.99

Cold gas efﬁciency 0.61 0.78 0.58

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.

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[14] Susanto H, Beenackers AACM A moving-bed gasiﬁer with internal recycle of

pyrolysis gas Fuel 1996;75:1339e47

[15] Reed TB, Das A Handbook of biomass downdraft gasiﬁer engine systems The

Biomass Energy Fundation Press; 1998

[16] Volchkov EP, Lebedev VP, Lukashov VV The LDA study of ﬂow gas-dynamics

in a vortex chamber Int J Heat Mass Transf 2004;47:35e42

[17] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen 3D numerical simulation of

compressible swirling ﬂow induced by means of tangential inlets Int J Numer

Methods Fluids 2009;59:1285e98

[18] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen 3D tangentially injected

swirling recirculating ﬂow in a nozzle with a slotted-tubedeffects of groove

parameters Int J Numer Methods Fluids 2010;63:1256e69

[19] Syred NJ, Beer M Combustion in swirling ﬂows: a review Combust Flame

1974;23:143e201

[20] Lilley DG Swirl ﬂow in combustion: a review AIAA J 1977;15:1063e78

[21] Catrakis H, Aguirre R, Mason J Physical modeling of turbulent ﬂuid interfaces

and ﬂow regions at large Reynolds numbers World Sci Eng Acad Soc Trans

Mech Eng 2005;2:1e19

[22] Tabak EG, Tal FA Mixing in simple models for turbulent diffusion Comm Pure

Appl Math 2004;57:563e89 http://dx.doi.org/10.1002/cpa.20012

[23] DD CEN/TS 15439 Biomass gasiﬁcation- tar and particles in products gases-sampling and analyze UE 2006

[24] Zainal ZA, Ali R, Lean CH, Seetharamu KN Prediction of performance of a downdraft gasiﬁer using equilibrium modelling for different biomass mate-rials Energy Convers Manag 2001;42:1499e515

[25] Bentzen JD Optimized two-stage gasiﬁer In: Proceedings of ﬁrst world con-ference on biomass for energy and industry; 2000

[26] Bhattacharya SC, Mizanur R, Siddique AHM, Pham HL A study on wood gasiﬁcation for low-tar gas production Energy 1999;24:285e96

[27] Walker M, Jackson G, Peacocke GVC In: Bridgwater AV, editor Small scale biomass gasiﬁcation: development of a gas cleaning system for power gen-eration Progress in thermochemical biomass conversion Oxford, UK: Black-well Scientiﬁc Publications; 2001 p 441e51

[28] Reed TB, Levie B Understanding operating, and testing ﬁxed bed gasiﬁer In: Bioenergy 0 84, Proceeding of World Conference, Goetborg, Sweden, June 21 Elsevier; 1985

[29] Burhenne L, Rochlitz L, Lintner C, Aicher T Technical demonstration of the novel Fraunhofer ISE biomass gasiﬁcation process for the production of a tar-free synthesis gas Fuel Process Technol 2013;106:751e60

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