Furthermore, residual energy fromsl

Furthermore, residual energy fromsludge gasification can be
also obtained.
Nowadays biogas represents the highest electricity capability
production among renewables scenario. It is a remarkable
fact that biogas production in Europe by 2011 represented
10.1 Mtoe, this means 35.9 TW h of electricity generated [1].
The biogas produced in anaerobic digestion has been used
frequently in spark ignition gas engines or dual fuel engines
with an efficiency range in terms of chemical energy conversion
to electricity of 35e42% [2]. However, the high concentration
of CO2 presented (40e50%) limits energy efficiency and
can produce ignition problems in this type of engines. From
this point of view, biogas valorisation into hydrogen, represents
an alternative and environmental friendly option when
fuel cells are selected as a high efficient energy conversion
system [3e6].
Biogas streams can be converted directly into hydrogen
using the CO2 present in biogas streams as an oxidant to
perform the reforming reaction [7e11]. Among technical
options for biogas valorisation into hydrogen, the direct
reforming of methane with carbon dioxide with an external
biogas combustion to generate the heat necessary to support
the reaction enthalpy, is the optimum strategy to
minimize the cost of hydrogen production [12]. Taking into
account the state of the art, the development of catalysts
able to overcome deactivation processes mainly due to coke
deposition, is the key for applying this emergent technology
[13]. Many papers have been the published concerning to
the development of catalysts resistant to poisoning by carbon
formation, publishing several reviews of the state of the
art [14e16].
Nickel, iron and cobalt-based catalysts are quite active in
methane decomposition reaction. Tokunaga et al. [17] performed
the initial researches studying the activity of the
Group VIII (Fe, Co and Ni) first transition metals series using
silica and alumina as support, resulting Ni and Co the most
active. The problem to overcome is to control activity due to
this type of catalysts are capable of activating CeC and CeH
bonds to form coke [18,19]. The coke produced blocks active
sites of catalysts leading to its deactivation. However, the
catalyst active phase is not only the key factor to design an
active and stable catalyst. The properties of the support
strongly affect the catalyst activity due to the variation in
surface area and acidebase properties. From this point of
view, the adsorption and dissociation of CO2 is favoured by
basic supports. Gadalla et al. [20] studied Ni catalysts on
different supports, concluding that Al2O3, Al2
O3eMgOeCaO
and Al2O3 have a higher activity than Al2O3eSiO2 (experiencing
rapid deactivation), and that the supports containing
MgO and CaO were stable.
Taking into consideration the increase of anaerobic
digestion processes in waste water treatment plants, the
production of sewage sludge (by-product) has increased
worldwide in recent years, so environmentally friendly
treatment of sewage sludge has become an important issue
[21]. Energy valorisation of sewage sludge by gasification
reduces the waste volume while a part of the energy contained
in it is recovered. The main drawback for this technology
is the high tar content in the gasification gas streams.
Tar are defined in the Technical Specification CEN/TS 15439
as all organic compounds presented in the gasification
product gas excluding gaseous hydrocarbons (C1eC6). Tar
can condense when the temperature is below the dew point
or can be polymerize or adsorbed in fine particles producing
soot. All these processes produce dirty, corrosion and plugging
in pipes, filters and engines with great costs in maintenance
as well as loss of energy efficiency which limits
seriously gasification application [22]. Current removal
methods, based on physical (filtration) o chemical processes
(solvents treatment) are not satisfactory enough for tar
removal [23]. On onehand, thesemethods simply transfer tar
compounds from gas stream to the solvent or to the solids
used as filtration system, producing highly pollutant and
hazardous waste, which must usually be managed as a toxic
and dangerous waste. On the other hand, these methods
operate at low temperatures, and require the gas to be cooled
down which implies a decrease in energy efficiency of the
integrated system.
From this point of view, tar cracking and reforming
methods are an attractive choice to reduce the concentration
of tar in the gasification product gas stream because of tar
can be destroyed through reactions at high temperature
immediately downstream the gasifier without creating a
waste liquid stream [24,25]. Among the processes for tar
removal at high temperature, is the tar catalytic cracking,
process carried out at temperatures above the gasifier operation
temperature (>1000
C), the most used, being rapid
deactivation of catalysts due to the formation of coke the
main limitation. These poly-aromatic species (tar) block
catalytic active sites. The limiting stage for the development
of gasification technology is synthesizing stable catalysts
resistant to poisoning by coke and minor components produced
in biomass gasification such as H2S, capable of
ensuring a long-term operation, and which maximize selective
conversion of tar towards valuable products in synthesis
gas production, primarily CO and H2.
Many efforts have been done to develop catalysts with
enough activity and stability to perform tar reforming
based on natural minerals such as olivine [26,27], dolomite
[28,29], or synthetic catalysts supported on alumina, silica
using different active phase i.e. nickel, cobalt and precious
metals.
Zhang et al. [29] studied the performance of nickel catalysts
supported on silica for catalytic cracking of tars at 550
C. The
catalyst activity was maintained for a short period due to the
formation of large amounts of coke on the catalyst surface,
phenomenon also described by Aznar [30] and Baker [31].
Lamacz et al. [32,33] tested Ni catalysts supported on CeeZr
mixed oxide, using toluene and 1-methyl naphthalene as tar
model compound. The catalyst activity was very good in the
case of toluene and worse in the case of 1-methyl naphthalene
to become starting at 500
C, reaching a conversion of 99% at
800
C in the case of toluene, and 88% in for 1-methyl naphthalene.
The catalyst performance key focuses on the ability
of the catalyst to supply oxygen network.
Alumina supported catalysts have been extensively studied
using precious metals such as Pt as active phase and
transition metals such as Ni [34,35].