Open Journal of Metal, 2013, 3, 1-7 Published Online July 2013 (
Behavior of Heavy Metals during the Agro-Industrial
Wastes Gasification
Marcelo Echegaray, Marianela Costante, Alejandra Saffe, Carlos Palacios, Rosa Rodriguez
Instituto de Ingeniería Química, Facultad de Ingeniería, Universidad Nacional de San Juan, San Juan, Argentina
Received May 24, 2013; revised June 27, 2013; accepted July 7, 2013
Copyright © 2013 Marcelo Echegaray et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The characterization analysis of three agro-industrial wastes was performed in order to study its thermal gasification.
Some analyses such as determination of Ca, K and Mg concentration and determination of three representative toxic
metals concentration Cd, Cr and Pb in all its oxidation states and the fundamental state, were carried out. The heavy
metals concentration was also determined in the ashes obtained during the gasification process. The mobility of these
elements was studied through three leaching tests. The behavior of heavy metals, sulfur and chlorine compounds, was
predicted considering the presence of water vapor, syngas, Ca, Mg, K, Si, Al and other ash components. The heavy
metals are not more concentrated in the gasification ash; these pollutants are released during this process. Ca, Mg and K
presence in these residues would promote the pollutants retention. The ash of the studied waste can be disposed in
controlled landfills or used in road construction, according to the obtained results during the leaching test DIN-DEV S4.
The obtained results in the leaching test EPA 1311 TLCP classify these gasification ashes as no toxic waste.
Keywords: Gasification; Agro-Ind ustrial Wastes; Ash
1. Introduction
The agro-industrial sector produces a significant envi-
ronmental impact in specific geographical areas, due to
generated waste, such as the Cuyo Region, Argentina. A
strategy in this sense is to propose an appropriate agro-
industrial waste management in order to minimize the
emitted pollutants, transforming them into high value-
added products or renewable energy source, tending to
During the 2011 harvest, 690,000 tons were used to
produce wine, generating nearly 200,000 kg of stalks
without considering other solid wastes, such as marcs
and wine dregs. The latter are generally used for the by-
products re covery.
However, reuse and/or disposal of exhausted marcs
and wine dregs are a current problem in the region, be-
cause their disposal in landfills is not environmentally
convenient due to they are not fully reused and large
volumes are generated, requiring significant areas of land
for their disposal. The waste from the fruits and vegeta-
bles canning industry have a high water content and, in
many cases, significant amounts of lignocellulose mate-
rials. The final disposal in landfills is also performed in
this region.
Moreover, there is a growing global interest in the
technologies development for the exploitation of renew-
able energy sources because of environmental and eco-
nomic reasons. In particular, due to the continuous in-
crease in the cost of fossil energy resources, biomass is
considered as one of the most promising and viable al-
ternatives. Energy from waste is an important component
of integrated waste management. One of the major limi-
tations in the use of biomass wastes for energy produc-
tion is its availability and moderate calorific value re-
sulting in a low production and high costs compared to
fossil fuels. The reduction of gases emissions, such as
SOx and greenhouse gases; however, is agreed with the
policies of current pollution control [1].
The energy conversion technologies and the biomass-
based systems are the only electricity renewable source
excluding hydro power, a crucial fact for future electric-
ity production. A technology with a great future is the
gasification. After more than 30 years of research, there
is now worldwide interest in the use of H2 as an alterna-
tive transportation fuel [2 ].
The steam gasification of waste is an attractive process
for producing H2-rich gas [3-5]. This process has been
developed to reduce the amount of undesired products
opyright © 2013 SciRes. OJMetal
and the coke formation rate [6]. Furthermore, a vapor
excess can easily be separated by condensation. Regard-
ing existing gasification technologies, the fluidized bed is
attractive because it provides a good contact between gas
and solid, uniform temperatures and high reaction rates,
compared to the fixed bed gasification [7]. Moreover,
fluidized beds hav e a high flexibility in th e feed in terms
of shape, size and composition, as well as a wide range
of operational and safety capabilities [8].
Considering the heavy metals, they are enriched in the
solid waste of gasification (fly ash and bottom), and they
are also released in the gas stream or tar. Their vaporiza-
tion depends on the initial chemical speciation, gasifica-
tion atmosphere, the fluid dynamics, the kinetics of
heavy metals diffusion in the solid particles and reaction
kinetics between the heavy metals and major components
of ash [9,10].
The ash disposal conditions as well as their reuse are
established by the trace elements concentration and their
mobility [8].
In view of these aspects, the heavy metals behavior
during agro-industrial waste gasification, their mobility
out the ash matrix and the toxicity determination of gen-
erated solid waste during this process were studied.
2. Experimental
Agro-industrial residues from canning and wine sector
were used: peach pits, stalks and marc, respectively.
These industries are located in the province of San Juan,
In order to obtain the ash, a differential reactor was
used. It is constructed of AISI 316 stainless steel. It is
constituted by a cylinder with 50 mm of diameter and 30
mm of length, heated by an electric resistance with elec-
tronic temperature control. Figure 1 shows a used reactor
According to Kurkela et al. [11] (2006), for feeds with
high alkali content, low gasification temperatures (T =
750˚C - 850˚C) and the steam addition are recommended
to prevent the agglomeration in the fluidized reactor.
Skoulou et al. (2008) [12] studied the effect of tem-
perature (T = 750˚C - 850˚C) and air equivalent ratio (ER
= 0.2 - 0.4) in biomass gasification into a fluidized bed
(ER is the ratio between the air sub-stoichiometric and
the air required for complete combustion amounts). Ex-
perimental results showed that working at 750˚C and ER
equal to 0.2, the H2 optimal content in the syngas is ob-
Taking into account these experimental results ob-
tained by other researchers, the used gasifying agent was
the steam and air mixture. The ER was equal to 0.2 and
the temperature equal to 750˚C. The gasifying agent (air-
steam mixture) entered from the reactor bottom. The
syngas exited at the top.
Figure 1. Used reactor scheme. 1: water tank; 2: pump; 3:
evaporator; 4: mixer; 5: power supply; 6: tempe rature con-
troller; 7: electric resistanc e; 8: reactor ; 9: support steel; 10:
porous metal mesh; 11: agro-industrial wastes; 12: gas out-
put; 13: thermocouple.
For each test, the reactor is loaded with 50 to 60 g of
agro-industrial wastes. The obtained ashes in this reactor
are considered with similar characteristics as the ash bot-
tom obtained in a fluidized bed reactor [13].
2.1. Agro-Industrial Wastes Characterization
The weight loss at 105˚C (ASTM D3173-87, Standard
Test Method for Moisture in the Analysis Sa mple of Coal
and Coke, 1996), the ash and the org anic matter contents
(ASTM D3172-89(2), Standard Practice for Proximate
Analysis of Coal and Coke, 2002), the concentrations of
Cd, Cr and Pb were determined for the studied agro-in-
dustrial wastes. In or de r to d ete r mine the h ea vy me ta l co n-
centrations, first, the samples were digested according to
EPA digestion (US Environmental Protection Agency,
1982). Then, the heavy metals concentrations were de-
termined using a v isible light spectrophotometer (HACH
DR/2010 Spect ro p hotomet er Datal og gin g p ort able).
Cd, Pb and Cr were considered due to their behaviors
during heat treatments which are different. Cd vaporizes
and it is not remained in the ash, Pb shows an intermedi-
ate behavior, and Cr is remained in solid residue of gasi-
fication [1 4].
In order to determine the influence of the presence of
Ca, Mg and K in this biomass gasification, their concen-
tration was determined in the studied agro-industrial
wastes, using the atomic absorption method.
2.2. Heavy Metals Mobility of the Ash Mineral
With the purpose of study the heavy metals mobility of
Copyright © 2013 SciRes. OJMetal
the ash mineral matrix, the Cd, Pb and Cr concentration s
were determined in the solid wastes of gasification, using
the analytical techniques described above. This study is
very important because it determines the final disposal
and/or further use.
Particularly, the use of the gasification ash in different
applications contributes to the sustainability of biomass
use in power generation. Several options are discussed:
use as fertilizer, as a building material or as fuels [15].
The heavy metals mobility was studied by three different
The German test, DIN 38414 part 4 (DEV S4, Ger-
man Standard Procedure for Water, Wastewater and
Sediment Testing, 1984): It is used to classify the
waste. The limits of heavy metals concentrations in
the leaching solution are expressed in mg/l for dis-
posal in landfill (Class 1) and for the ash use in road
constructi o n [ 16].
The US EPA TCLP 1311 test (Toxicity Characteris-
tic Leaching Procedure. Methods for Evaluating Solid
Waste, 1992): It determines the potential leaching of
organic and inorganic material in liquid, solid and
multi-phase, of the residues in contact with ground-
water. This test simulates landfill leaching conditions
and the ash can be classified as toxic or not [17].
The Dutch NEN 7341 test (Determination of the
Leaching Behavior of Granular Materials: Availabil-
ity Test, 1993): This test determines the maximum
proportion of heavy metals leached from different
wastes such as ash. This is achieved by leaching of
finely milled solid (maximizing the contact surface)
and using a high liquid/so lid ratio. The metal fixation
in the solid matrix is predicted with this test [18,19].
The DIN test uses the weaker leaching agent, distilled
water, and the test NEN the stronger, nitric acid.
3. Results and Discussion
The results of the agro-industrial wastes characterization
are shown in Table 1. The highest ash and water content
were found in the stalk. A high water content increases
the energy requirements to carry out the gasification,
decreasing the efficiency of the plant, but on the other
hand, it improves the synthesis gas quality by increasing
the content of CO2, CH4 and H2 [20] and decreasing hy-
drocarbons and tars levels. In order to optimize the gasi-
fier operation, Pfeifer et al. [21] determined the optimum
content equal 20% to 40% by weight at low temperatures
Regarding the ash content, a low percentage of it will
minimize the production of fly and the bottom ash. In
general, these solids contain significant amounts of un-
reacted carbon and sulfur [22].
Cd, Cr and Pb are present in the composition of stud-
ied agro-industrial wastes. The stalks and the peach pits
Table 1. Results of proximate analysis. Determination of
heavy metals and Ca, K, and Mg concentrations in agro-
industrial wastes.
Stalk Marc Peach pits
Weight loss at 105˚C (dry basis %) 73.23 55.06 35.57
Ash (dry basis %) 6.30 5.08 0.73
Organic matter (dry basis %) 93.7 94.92 99.27
Cd (mg/kg dry basis) 1.25 0.02 1.25
Cr (mg/kg dry basis) 25.00 37.50 3.125
Pb (mg/kg dry basis) 75.00 82.92 0.94
Ca (g/kg dry basis) 2.25 2.96 0.02
K (g/kg dr y ba s i s) 19.23 7.38 7.15
Mg (g/kg dry basis) 0.58 0.46 0.44
presented the highest Cd concentrations. With respect to
Cr and Pb, the highest concentrations were found in
winema ki ng waste.
Considering the obtained results b y analyzing the ga si-
fication ash (Table 2), the highest Cd and Pb concentra-
tions were found in the marcs ash. For Cr, the highest
concentrations were found in the stalks ash, in this case
the metal concentrations is more variable (between 3.12
and 15.86 mg Cr/kg of dry weight waste).
Comparing the found heavy metals concentrations in
the stalks and their ash, the Cd concentration variation is
very small and the Cr and Pb concentrations of stalks are
higher than these concentrations in their ash. In the case
of the marcs and their ash, the Cd is more concentrated in
the gasification solid waste, but the Cr and Pb concentra-
tions are higher in the marcs. Comparing the concentra-
tions of three heavy metals found in peach pits and their
ash, a significant variation is not observed.
On this point, it is important to explain the heavy met-
als behavior during the biomass thermal treatment. When
organic matter is consumed during any heat treatment,
heavy metals are exposed to a hot and oxygen-depleted
atmosphere, adjacent to the particle, presenting one of
the following behaviors [9]:
1) They vaporize directly in the initial chemical spe-
2) They react with a compound present in the atmos-
phere and then, they vaporize;
3) They remain unreacted in the mineral matrix.
The vaporized species enter in the gas flow where they
react or condense. The condensed species form new par-
ticles (homogeneous nucleatio n) or they are deposited on
the present particles surfaces (heterogeneous deposition ).
Homogeneous nucleation gas explains the substantial
amount of very fine metal particles (diameter between
0.02 to 1 microns) found in the effluent gases. The het-
erogeneous deposition occurs in larger particles and they
Copyright © 2013 SciRes. OJMetal
Table 2. Heavy metals concentrations in the ash.
Ash Cd (mg/kg) Cr (mg/kg) Pb (mg/kg)
Stalk 1.22 15.86 14.07
Marc 1.43 11.56 30.27
Peach pits 1.09 3.12 0.63
can be captured by the pollution control systems. To
promote the heterogeneous deposition, it is necessary to
limit the formation of fine metal particles.
The species formation with lower oxidation states than
the initial states are favored by the reducing conditions.
Furthermore, these metals may react with other released
elements, as chlorine or sulfur. These new species are
generally more volatile than the metal species present in
the agro-indus trial waste. The heavy metals vo latilization
during the gasification depends of their speciation and
the gasification atmosphere.
Taking into account the heavy metals partition during
gasification in fluidized bed reactor, the turbulence con-
ditions during its operation cause a significant production
of fly ash with high concentrations of these elements.
The heavy metals partition during heat treatments in flu-
idized bed is governed by the fluid dynamics, the kinetics
of heavy metals diffusion in the ash particles and reaction
kinetics between the heavy metals and the ash compo-
nents [10].
The chemical composition of the mineral matrix has a
great influence on the kinetics of heavy metals vaporiza-
tion; it determines the bonding strength between the
mineral matrix and these elements, as well as the time
required for diffusion out of the particle. Thus, basic sp e-
cies in the matrix (SiO2, Al2O3, CaO) can react with these
metals encapsulating them in the particle center [8]. The
CdO (s), Cr2O3 (s) and PbO (s) may react with HCl, ac-
cording to the following reaction:
 
MeOs2HClgMeClgH Og
The used steam during the gasification phenomenon
affects the reaction equilibrium and the heavy metals
retention as oxides in produced solid waste [23]. A high
water content of feed waste promotes this retention. Then,
during the waste gasification using steam, the reaction is
displaced to the left causing the formation of metal ox-
ides. Notably, Moj tahedi and Salo [24] observ ed the pre-
sence of heavy metal chlorides in volatile p hase when th e
gasification was carried out at high temperature.
According to Park et al. [25], MeO can react with the
syngas according to the Reactions (2) and (3):
MeOs HgMes HOg (2)
MeOs COgMeCs Og (3)
when these reactions occur, the heavy metal gradually
These reactions can be inhibited by the addition of natu-
ral zeolite.
diffuses to the particle surface subsequently vaporized.
et al. [26] observed the augmentation of Cd
heavy metals which remains in the ash-
the bed
er hand, some metals such as Ca inhibit the
hat most of the heavy metals
a fluidized bed
ert and no
shows the principal conditions of lixiviation
ese results, the three metals
lts, all metals had
the highest mobility in the case of the peach pits ash.
d Pb concentrations in the fly ash comparing with bot-
tom ash, during wood gasification in fixed bed; however,
Cr remained in the bottom ash. Pinto et al. [22] detected
higher Pb concentrations in the ash captured by cyclones
comparing with bottom ash, confirming the above men-
tioned studies.
The amount of
creases when the working temperature is high, close to
900˚C, and the synthesis gas quality increases.
According to Wei et al. [27], the sand, used as
aterial, adsorbs heavy metals, decreasing their concen-
trations in exit flow gas. The heavy metals release in-
creases when the adsorption efficiency of this material
On the oth
d material agglomeration, maintaining the fluidization
quality and sand mixed with the biomass to be gasified.
Then, Ca improves the fluidization delaying the heavy
metals release [28,29].
Cui et al. [30] observed t
e enriched in the exit gas flow. The experimental re-
sults are consistent with these observations. Approxi-
mately 70% of trace elements found in the synthesis gas,
including three studied heavy metals, come from the gas-
ified biomass and about 25% from gasification system.
Considering the alkali elements contents in the studie
omass, the Ca concentrations vary in a small range,
except for the peach pits. For K, the found concentration
in the stalk is very high compared to these concentrations
in the marc and peach pits. The Mg concentrations in all
analyzed residues vary in a small range.
If the gasification is carried out into
actor, it is important to consider the biomass tendency
to separate from the bed due to its low density, as well as
the elutriation tendency of C small particle.
On the other hand, the gasification ash are in
volved in chemical equilibrium of the gasification reac-
tions but, it may have a catalytic effect, accelerating the
char gasification reaction with steam, especially when the
ash contains metal oxides as K2O, CaO, MgO, P2O5, etc.
Table 3
sts and relative ratio of the studied heavy metals found
in the leaching solution.
Taking into account th
ere detected in leaching tests DIN-DEVS4, except to
peach pits ash. Cd had higher mobility in leaching test
DIN-DEVS4 for the marc ash. The Pb and Cr showed the
highest mobility for the peach pits ash.
Considering the EPA-TLCP test resu
Copyright © 2013 SciRes. OJMetal
Copyright © 2013 SciRes. OJMetal
Tatio of the studied heavy metals found in the leaching solution. able 3. Main characteristics of the leac hing tests. Relative r
Lixiviation test Leaching solution Time (h) pH final value Cd (%) Cr (%) Pb (%)
DIN-DEV S4 Distilled water 24 Final pH: 9
Stalk ash
Marc ash 66.64 1.50 2.75
Peach pits ash 15.62
0.00 0. 33
85 5. 03
Acetic acid 18 Final pH: 4.5
E PA-TLCP 1311
Stalk ash
Marc ash 0. 0025,00 3, 01
Peach pits ash 15.62
100 6. 67
32.10 4. 04
NEN 7341 Nitric acid 3 (s tep 1)7 (s tep 1)
3 (step 2) 4 (step 2)
Stalk ash
Marc ash 33.36 24.00 3. 86
Peach pits ash 0. 00
0.00 3. 01
100 5. 52
Reg 7341 test results, Cd was not released
rom the peach pits and marcs ash, the Cr and Pb had
005 mg/l to
, 5 and 5 mg/l for
lution for each tests.
sification ash, in general, have a sig-
arding NEN
highest mobility from the peach pits ash. Heavy metals
mobility did not vary with the pH variation.
Established limit concentrations in the leachate solu-
tion for Cd by DIN 38 414 test are 0.05 and 0.
placed in a landfill or used in the road construction,
respectively. For Pb, these limit concentrations are 0.2
and 0.05 mg/l, respectively. The Cr is not regulated in
this test. Taking into account the Cd and Pb concentra-
tions in leachate solution from the studied wastes ash, it
is concluded that these solid residues can be disposed in
landfills or used in road construction.
The limit concentrations in the leachate solution, ac-
cording to EPA test-TLCP 1311 are 1
Figure 2. Relative ratio of the studied heavy metals found in
the leaching solution for stalk ash.
4. Conclusions
ount previous research,
be dried before the gasification in
he process yield. The residues ana-
ention of heavy metals.
r parameters set by this test must be
d, Cr and Pb, respectively.
Figure 2 show the relative ratio of the studied heavy
metals found in the leaching so
The studied agro-industrial wastes have higher water
content than 20%. Taking into acc
these wastes should
order to optimize t
In order to analyze the ash reuse as fertilizer, it is im-
portant to consider that the ash can be only K solyzed have low ash contents. This aspect will have a sig-
nificant impact on the obtained amount of fly and bottom
ash from the gasification process.
The studied heavy metals are not more concentrated in
the ash; therefore, they are released as gas, fly ash (ho-
mogeneous or heter ogeneous nucleation) or ta r.
The Ca content improves the ret
cause they do not contain nitrogen and the phosphorus
is content in forms with very poor solubility. The Mg and
Ca content can improve quality especially in soil pH
control. Considering this aspect and the retention of these
elements in the solid matrix, it is concluded that the gasi-
fication ash from the stalks are most suitable for this use.
The ash from fluidized bed has constituent material of
the bed (sand) and can be reused in road construction or e improvement of the fluidization conditions produces
their retention.
The results obtained in the leaching test DIN-
ncrete; however, its con tent of carbon, alkali and chlo -
rine does not make it appropriate to be used as a con-
struction material.
Considering the reuse as fuel, it is important to em-
phasize that the ga
ggest that the ash of this waste can be disposed in con-
trolled landfills or used in road construction. It should be
noted that the othe
alyzed. The results obtained in the leaching test EPA
1311 TLCP are smaller than the limits set by the test.
The gasification ash can be reused for power genera-
tion or as fertilizers. In both cases, it will be necessary to
perform an economic evaluation. The power generation
ficant amount of unburned carbon. This reuse is obvi-
ously the best choice because it has the same purpose as
the original material: power generation, however, the
heavy metals behavior must be considered [15].
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