(724f) Catalytic Steam Gasification of Biomass for Hydrogen Production Using Promoted Ni on g-Alumina Catalysts with Low Loads of Metals: Catalyst Stability and Tar Reduction

The depletion of oil poses a big
challenge. Due the big energy demand, it is necessary to look for alternative
energy sources such as hydrogen obtained from biomass gasification. Hydrogen
has a very big heat capacity and biomass is a residual which can be gasified to
obtain hydrogen, a potential fuel and its usage additionally would clean the
environment. However, the gasification of biomass has several drawbacks such as
low efficiency, tar productions and it is necessary to minimize its production
synthesizing accurate stable catalysts.

According to literature the Ni/Al2O₃
catalyst promoted with Ru have avoided the coke deposition and enhanced the
catalyst stability. Unfortunately, Ru is expensive and it is expected that
reducing its load and complementing with Ca or Mg or Mn,
the same or better hydrogen could be produced and lanthanum and cerium have
reported to enhance the activity of nickel based catalysts.

Six different Ni-based fluidizable catalysts were synthesized using an incipient
impregnation technique. 5.0% Ni-based catalysts supported on gamma alumina were
promoted with 2.0wt% La or alternatively with 2wt% Ce.  The
preparation procedure considered, ensured that catalysts were thermally treated
at high temperatures, under free of air atmospheres. Also, 5 % Ni/Al₂O₃ catalysts promoted with couples
of metals: Ru-Ca, Ru-Mg and Ru-Mn with loads in the
range of 0.25-1.0 % w/w were synthesized to remove the tars.

All the catalysts were characterized
using XPS, BET, XRD, SEM, AA, PSD, TPR-D, H₂-chemisorption, and
Raman spectroscopy. TPR and H₂ chemisorption showed good metal
dispersion with 10 nm- 40 nm nickel crystallites and dispersions lesser than 6%. 

Glucose and 2-methoxy-4-methylphenol, surrogates of biomass and toluene,
a surrogate of tar, were selected to study their decompositions and eventual
hydrogen production. The so called rector CREC – Riser Simulator was used to
test all the catalysts and in all the cases, the same products with different
molar fraction were detected: H2, CO, CO₂ and CH₄.

Related to glucose catalytic
gasification, runs were performed using: 
a) 5%Ni/Al₂O₃ (pH4), b) 5%Ni-2%La/g-Al₂O₃ (pH1) and c) 5%Ni-2%Ce/ g Al₂O₃ (pH4).

Comparing with the thermal experiments, the
hydrogen molar fraction augmented 18%. The effect of the promoter is more
notorious at low loads in the case of La than Ce. The catalysts prepared at pH
1 reported bigger hydrogen molar fractions. For the other gases, the pH effect
is significate with different promoter loads. The results are in good agreement
with the thermodynamic equilibrium. See Figure 1.

Figure 1.- Mole Fraction of the compounds with
different catalysts, using air oxidation and hydrogen regeneration.

To test the stability, consecutive experiments
were done in between runs with: a) air regeneration, b) air regeneration
followed by hydrogen pre-treatment, c) no special catalyst reactivation. It was
observed that Ni-based catalysts, which were subjected every run to both regeneration with air and hydrogen pre-treatment displayed
yields in very good agreement with thermodynamic equilibrium data, calculated
with the software Aspen - Hysis. On the other hand,
Ni-based catalysts regenerated with air only, showed the worst hydrogen
productivity, while catalysts under both free of air and free of hydrogen
reactivations gave very acceptable sub chemical equilibrium hydrogen yields. Figure
2 shows hydrogen production for the three series using only Ni catalysts.

Figure 2.- Hydrogen mole fractions for the three
series using Ni based catalysts.

This showed that Ni-based fluidizable
catalysts can perform on-stream for extended periods, requiring limited
periodic reactivation with air and H₂. This indicates that using the
catalysts proposed in this study, without any treatment, is a viable
alternative in a continuous industrial scale.

Related to 2-methoxy-4-methylphenol and
toluene with 5 %Ni/Al₂O₃,the hydrogen molar
fraction was increased in 21.47 % with respect to the thermal experiments and
the promoted catalyst with Ru, 5%Ni-0.25%Ru/Al₂O₃
achieved an additional increment of 5 % until reaching a value of 0.50
hydrogen molar fraction,  which
is in agreement with the thermodynamic equilibrium.  The most efficient catalyst was 5%Ni- 0.25%Ru/Al₂O₃
because it led to the highest hydrogen production  and the lowest CO and CH₄
productions. This catalyst had great interaction of Ni in the support due its
low Ru load (0.25%). This was corroborated by TPR characterization, which
showed that peaks corresponding to Ni and Ru were close each other indicated
that big interaction between then.

 The 0.25% Ru load was selected
to synthesize the catalysts with the different pairs of Ru-Mg, Ru-Ca and Ru-Mn promoters. Reporting that the catalysts 5% Ni-0.25%
Ru-1.0% Mn/Al₂O₃ had the best performance,  increasing
by 9% the mole fraction of hydrogen with respect to the catalyst 5% Ni-0.25%
Ru/Al₂O₃ and this is a big promise to reduce the tars.