Format And Type Fonts

CHEMICAL ENGINEERING TRANSACTIONS

VOL. 39, 2014

A publication of

The Italian Association

of Chemical Engineering

www.aidic.it/cet
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong

ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI:10.3303/CET1439196

Please cite this article as: Diedhiou A., Bensakhria A., Ndiaye L.G., Khelfa A., Sock O., 2014, Study of cashew nut shells

valorisation by gasification, Chemical Engineering Transactions, 39, 1171-1176 DOI:10.3303/CET1439196

1171

Study of Cashew Nut Shells Valorisation by Gasification

Ansoumane Diedhiou
a
, Ammar Bensakhria*

a
, Lat-Grand Ndiaye

b
,

Anissa Khelfa
c
, Omar Sock

a
Université de Technologie de Compiègne, Centre de recherche de Royallieu, EA 4297-TIMR, BP20529 - 60205

Compiègne, France.
b
Université Assane Seck de Ziguinchor, Département de Physique,

BP.523 Ziguinchor, Sénégal.

c
Ecole supérieur de Chimie Organique et Minérale, EA 4297-TIMR, 1 allée du réseau Jean-Marie Buckmaster - 60200

Compiègne, France.

ammar.bensakhria@utc.fr

During these last three years the production of cashew nuts is expanding across West Africa, and

according to the USAID the sub-region produces 85 % of the world harvest. Furthermore, after the tourism,

cashew nuts are one of the pillars of Casamance economy, (In Senegal, cashew nuts yield 35 million CFA

francs a year). Cashew nut shells are a residue obtained from cashew shelling. This biomass residue is

abundant, cheap and generates high energy content. Currently, cashew nuts shells are rejected by

artisanal traders without being valued. They were often burned in open air and cause several socio-

environmental problems. Therefore, the issue of energy recovery by thermochemical process arises to

overcome this cashew nuts shells rejection and open air burning. Nevertheless, gasification technology is

suitable for biomass residues conversion and remains an economical alternative for the valorisation of

cashew nut shells in small scale industries. In order to evaluate the cashew net shells valorisation by

gasification, several experiments were conducted on a char obtained from the pyrolysis of this biomass

and using a fixed bed reactor. As gasifying agents, carbon dioxide, steam and the mixture of carbon

dioxide and steam were used at different temperatures for the gasification of fine particles size (630 m)

and gross particles size (3,000 m) CNS char. In order to compare the effect of each parameter studied,

carbon conversion rate was calculated from gasification experiments results, by carbon mass balance, of

the char carbon content and the produced gas carbon content. Kinetics parameters of CNS char

gasification reaction were also determined, using a volume reaction model, in order to compare reactivity

of the char, by the activation energy and pre-exponential factor comparison. From the results obtained,

temperature has a positive effect on the kinetic of carbon conversion. The results obtained show clearly an

improvement of CNS char reactivity with temperature increasing and using steam as gasifying agent.

However, char particles size has no significant effect on the gasification reactivity.

1. Introduction

The increasing of energy demand coupled with the need to reduce greenhouse gas emissions, and the

threat of exhaustion of oil reserves make us to consider an eventual recourse to the use of biomass waste

as renewable energy source and efforts has been done in order to valorise vegeto-agricultural by-products

such as peanut shells, sugarcane bagasse, nutshells, forest residues and sorghum stems. These raw

materials are carbon neutral and homogenously distributed all over the word, which is a very important

advantage for its utilization as an energetic vector. Therefore, the utilization of biomass energy can provide

dual benefits: it can reduce carbon dioxide (CO2) emission as well as increase fuel security when it is

produced locally. Cashew nut shells are a residue obtained from cashew shelling, an abundant and cheap

biomass residue, and could generate high energy. Currently, cashew nuts shells are rejected by artisanal

traders without being valued. They were often burned in open air and cause several socio-environmental

problems (Tippayawong et al. (2011). Thus, cashew nut shells (CNS), which have a high energetic

content, are currently rejected, in Casamance (Senegal) or burned in open air, and causes several health

problems, as pneumonia, and environmental problems as proliferation bushfire. Among different kinds of

mailto:ammar.bensakhria@utc.fr

1172

biomass, CNS was studied for bio-oil production by flash pyrolysis (Melzer et al. 2013) but its gasification

was not really investigated. The most gasification studies was investigated on the charcoal, obtained from

the forest residues biomasse pyrolysis. The most parameters studied concern the gasification reactivity of

biomass char and the effects of the operating parameters as reaction temperature, particle size, gasifying

agent (carbon dioxide, steam, air or oxygene) as it was reported by several authors. The effects of the

gasifying agent and the particles size on the char reactivity, was investigated by Guizani et al. (2013),

using thermogravimetric analysis. High pressure gasification was investigated by Fermoso et al. (2009),

using a pressurized thermo-gravimetric apparatus. Coetzee et al. (2013) studied the reactivity of large coal

particle during steam gasification using also TGA analysis and various kinetic models such as the volume

reaction model (VRM) was applied for their results. Dong et al. (2010) investigated the reactivity of

biomass with CO2, as gasifying agent, using also volume reaction model (VRM) and compared to shrinking

core model and random pore model for their experimental data interpretation. Hyo et al. (2014) studied

gasification of two types of char at high temperature at lab-scale tube furnace and used the several

kinetics models to obtain the reaction rate constant.

This study aims to investigate the effects of gasification atmosphere, temperature and char particles size

on the reactivity of cashew nut shells char gasification. Gasification tests were carried out on two char

particles size (630 and 3,000 mm), at three reaction temperatures (950, 1,000 and 1,050 °C), using three

gasifying agents (carbon dioxide, steam and the mixture of carbon dioxide and steam).

2. Materials and methods

Cashew net shells (CNS) used in this study come from Casamance region in Senegal. In order to prepare

char samples for gasification tests, CNS was pyrolysed, using a muffle oven, during 15 min at 450 °C

under inert atmosphere. The char obtained was ground and sieved into two fractions: a fine fraction with

particles size < 630 m and gross fraction with particles size < 3,000m. The proximate and ultimate

analysis of the CNS used and the char obtained are listed in Table 1.

CNS char gasification tests were conducted using a tubular fixed bed reactor (36 mm internal diameter

and 350 mm height) and equipped with a porous plate for bed support. Figure 1 shows a flow diagram of

the system used.

Table 1: Characteristics, on dry basis, of the cashew nut shells (CNS) and char

Proximate analysis (wt.%) Ultimate analysis (wt.%)

VM FC Ash C H O N Cl S

CNS 81.6 15.8 2.6 58.1 7.3 34.4 0.62 <0.1 0.01

CNS Char 27.2 65.5 7.5 83.4 4.07 11.6 0.96 <0.1 0.03

Figure 1: Simplified representation of the system the fixed bed reactor

Massflow
controllers

Electrical

Oven

Gas analyser and
data acquisition

données

Reactif gas preheating section

Char

fixed bed

gas condensation
and cleaning

Valves

N2 CO2

Water

µGC

Reaction temperature

measurement and

control

Rotametre

1173

After reactor preheating, 15 g of char is mixed with 70 g of sand and charged in the reactor, under a

nitrogen atmosphere, until the desired temperature. Sand was used in order to improve heat transfer

inside bed particles and for minimizing the preferential gas passage.

The reactor temperature is controlled by means of a thermocouple, in contact with the sample bed and

connected to a temperature controller. The gasification tests were carried out isothermally at 950, 1,000

and 1,050 °C, using carbon dioxide (90 nL/h) , steam (90 nL/h) and the mixture of steam and CO2 (45 nL/h

of each) and carried in an inert flow of 10 nL/h of nitrogen.

Flow rates of CO2 and N2 were fixed by the use of mass flow controllers while the flow rate of water was

adjusted by an HPLC piston pump. The composition of the produced gas is obtained by online gas

analysis, using an SRA-Instruments gas analyzer (GC), after gas condensation and cleaning.

3. Results and discussions

The effect of the main operation variables as temperature, particle of the char size and nature of the

gasifying agent on the CNS char gasification was studied, by the evaluation and the comparison of the

char carbon conversion. The carbon conversion, X, (Eq(1)) was defined as the total carbon contained in

the produced gas (CO, CO2 and CH4), with respect to the total carbon contained in the char fixed bed. The

amount of gas generated during gasification tests was calculated from nitrogen balance, since the amount

of nitrogen fed in and the composition of nitrogen evolved are known.

ashinitial

tinitial

t
mm

mm
X




 (1)

Where Xi, minitial , mt and mash are respectively the carbon conversion rate, the initial mass of carbon of the

sample used, the carbon mass of the produced gas and the mass of the sample ash.

3.1 Effect of temperature on the coal gasification
Regarding the endothermic effect, reaction temperature is one of the most important operating parameters

affecting the performance of gasification. In order to evaluate the effect of this parameter on the reactivity

of the char, several tests were carried out on two particles size of the char particles (<630 m and < 3,000

m). Three gasifying agent were tested (CO2, steam and the mixture of the two gasifying) agent at three

reaction temperatures (950, 1,000 and 1,050 °C). The results obtained from these tests, traduced by the

variation of the carbon conversion (Eq(1)) versus time, are summarized on Figure 2.

Figure 2: Influence of temperature on reactivity of CNS char with CO2 (a and b), steam (c and d) and with

CO2 and steam mixture (e and f)

1174

Figure 3: Influence of particle size on reactivity (a=CO2, b=H2O, and c= CO2/H2O)

From the results obtained, one can observe clearly that the carbon conversion of CNS char during its

gasification with steam, CO2 or with the mixture of the two gasifying agent is improved with the reaction

temperature increasing, traducing an enhancement of the char reactivity. High temperatures improve

reaction kinetics parameters and so the enhancement of the char conversion rate as it was also observed

by several authors. Ye et al. (1998) point out that the gasification rate increases with increasing reaction

temperature for H2O and CO2 reaction with char. The results obtained by Dong et al. (2010) showed an

increase of CO production with the temperature increasing, traducing that higher temperature favoured the

reaction. Quinglong et al. (2012) observed also that reaction temperature is an important factor with regard

to the composition of final syngas. Coetzee et al. (2013) indicated that the reactivity was found to be a

temperature sensitive. And, Fermoso et al. (2009) explained that temperature is one of the most important

operating parameters affecting the performance of gasification.

3.2 Effect of particle size on rate of carbon conversion

In order to evaluate the effect of the particles size on the gasification reactivity of the CNS char, several

tests were carried out on two char particles size (<630 m and 3,000 m). The results obtained from these

tests, traduced by the evolution of the carbon conversion rate versus time, for each reacting agent at the

three temperatures and the two char particles size tested are given on Figure 3. From these results, one

can observe that there is no significant difference between the results obtained on the two particles size

CNS char tested, especially when carbon dioxide or steam were used alone, for the three temperatures

tested. This result was also obtained by Ye et al. (1998) for the South Australian coal gasification with

carbon dioxide and steam, which means that the gasification reactions occur homogeneously throughout

the particle and are controlled by chemical kinetics. The same observations, concerning the char particles

size effect on the steam gasification reactivity, were also obtained by Hanson et al. (2002) for char

particles size of 0.5 and 2.8 mm and Coetzee al. (2013) for the two chars particles size 5 et 10 mm.

CNS char particles size could have an effect on the gasification reactivity when a mixture of carbon dioxide

and steam is used as the gasifying agent, especially at low temperature (950 °C). This effect could be

explained by the char reactive surface development during gasification which improves the conversion rate

of carbon.

3.3 Kinetics parameters determination

In order to better evaluate the effect of the gasifying agents and the char particles size on the gasification

reactivity, kinetics parameters of these reactions were calculated for each situation, using the volume

reaction model (VRM), according to the relation 2, for the determination of the rate constant (kVRM) for each

temperature tested. The pre-exponential factor (k0) and activation energy (Ea) were obtained from

ArrheniusEq(3), from the plot of ln(kVRM) versus 1/T.

 Xk
dt

dX
VRM

 1 (2)

VRM
kk



 exp
0

(3)

The Arrhenius equation plot results are given in Figure 4, however the pre-exponential factor and the

activation energy obtained from these plots, for each test conditions, are regrouped in Table 2.

1175

Figure 4: Arrhenius plots of CNS char gasification reaction with the various gasifying agents

Table 2: Kinetic parameters of CNS char gasification

Gasifying

agent

Ea(kJ/mol) K0 (s
-1

) R
2

Particles size

CO2 144.91 2.64E+3 0.9937

630 m

Steam 187.17 9.90E+3 0.9988

Mixture (CO2/steam) 164.79 5.37E+3 0.9979

CO2 142.51 1.95E+3 0.9984

3,000 m Steam 182.28 1.11E+4 0.9959

Mixture (CO2/steam) 174.40 4.77E+3 0.9845

From these results, we can notice that the experimental data were very well represented by the Volume

Reaction Model (VRM) with high regression coefficients (R
2
> 0.99) and so this model could describe the

evolution of char particles along the gasification process.

The activation energy obtained for the CNS char gasification is comprised between 145 and 185 kJ/mol,

function of the gasifying agents and char particles size, which is in concordance with the results obtained

by Dong et al. (2010). From the results obtained and regarding kinetics parameters, one can observe

clearly that CNS char reactivity is improved by steam gasification, traduced by a high pre-exponential

factor and low activation energy, compared with gasification with carbon dioxide. The mixture gives an

intermediate reactivity between the gasification with carbon dioxide and steam.

4. Conclusions

The reactivity of cashew nut shells char gasification with the reacting agents: carbon dioxide, steam and

the mixture of CO2 and steam were evaluated at various temperatures (950 - 1,050 °C) and for two

different particles size (630 and 3,000 m). Temperature increasing showed an improvement of CNS char

carbon conversion for the three reacting agents and the two chars particles size used. High temperatures

improve syngas production, mainly carbon oxide and/or hydrogen, with high carbon conversion rate.

The results obtained showed also that the char particles size has no effect on the carbon conversion rate,

during its gasification with carbon dioxide or with steam, for the two particles size studied. However, an

effect of the char particles size was observed when a mixture of carbon dioxide and steam was used as

gasifying agent, especially at low temperature. Finally, reactions kinetics parameters showed a best

reactivity of CNS char with steam, compared with its reactivity with carbon dioxide or with the mixture of

the two gasifying agents.

1176

References

Coetzee S., Neomagus H.W.J.P., Bunt J.R., Everson R.C., 2013, Improved reactivity of large coal particles

by K2CO3 addition during steam gasification, Fuel Processing Technology, 114, 75–80.

Dong K.S., Sun K.L., Min W.K., Jungho H., Tae-U.Y., 2010, Gasification reactivity of biomass chars with

CO2, Biomass and Bioenergy, 34, 1946–1953.

Fermoso J., Arias B., Plaza M.G., Pevida C., Rubiera F., Pis J.J., García-Peña F., Casero P., 2009, High-

pressure co-gasification of coal with biomass and petroleum coke, Fuel Processing Technology, 90,

926–932.

Guizani C., Escudero Sanz F.G., Salvador S., 2013, The gasification reactivity of high-heating-rate chars

in single and mixed atmospheres of H2O and CO2, Fuel, 108, 812–823.

Hanson S., Patrick J.W., Walker A., 2002, The effect of coal particle size on pyrolysis and steam

gasification, Fuel, 81 531-537.

Hyo J.J., Sang S.P., Jungho H., 2014, Co-gasification of coal–biomass blended char with CO2 at

temperatures of 900–1100°C, Fuel, 116, 465- 470.

Melzer M., Blin J., Bensakhria A., Valette J., Broust F., 2013, Pyrolysis of extractive rich agro-industrial

residues, Journal of Analytical and Applied Pyrolysis, 104, 448–460

Tippayawong N., Chaichana C., Promwangkwa A., Rerkkriangkrai P., 2011, Gasification of cashew nut

shells for thermal application in local food processing factory, Energy for Sustainable Development, 15,

69–72.

Ye D.P., Agnew J.B., Zhang D.K., 1998, Gasification of a South Australian low-rank coal with carbon

dioxide and steam: kinetics and reactivity studies, Fuel, 77, 1209–1219.