Gasification of pyrolysis oil was studied in a fluidized bed over a wide temperature range (523−914 °C)
with and without the use of nickel-based catalysts. Noncatalytically, a typical fuel gas was produced. Both a
special designed fluid bed catalyst and a crushed commercial fixed bed catalyst showed an initial activity for
syngas (H2 and CO) production at T > 700 °C. However, these catalysts lost activity irreversibly and elutriation
from the fluid bed occurred. The equilibrium catalytic activity suffered from incomplete reforming of
hydrocarbons (CH4). In all the experiments the carbon to gas conversion was incomplete, which was mainly
caused by the formation of deposits and the slip of microcarbonaceous particles. A two-stage reactor concept,
which consisted of a sand fluidized bed followed by a fixed catalytic bed, was proposed and tested. This
system uncouples the atomization/cracking of the oil and the catalytic conditioning of the produced gases,
enabling protection of the catalyst and creating opportunities for energy efficiency improvements. In a bench
scale unit of this reactor (0.5 kg oil/h), methane and C2−C3 free syngas (2.1 Nm3 CO + H2/kg dry oil, H2/CO
= 2.6) with a low tar content (0.2 g/Nm3; dry, N2 free gas) was produced in a long duration test (11 h).
The liquefaction of lignocellulosic biomass is studied for the production of liquid (transportation) fuels. The process concept uses a product recycle as a liquefaction medium and produces a bio-oil that can be co-processed in a conventional oil refinery. This all is done at medium temperature (≈ 300 °C) and pressure (≈ 60 bar). Solvent-screening experiments showed that oxygenated solvents are preferred as they allow high oil (up to 93% on carbon basis) and low solid yields (≈ 1-2% on carbon basis) and thereby outperform the liquefaction of biomass in compressed water and biomass pyrolysis. The following solvent ranking was obtained: guaiacol>hexanoic acid ≫ n-undecane. The use of wet biomass results in higher oil yields than dry biomass. However, it also results in a higher operating pressure, which would make the process more expensive. Refill experiments were also performed to evaluate the possibility to recycle the oil as the liquefaction medium. The recycled oil appeared to be very effective to liquefy the biomass and even surpassed the start-up solvent guaiacol, but became increasingly heavy and more viscous after each refill and eventually showed a molecular weight distribution that resembles that of refinery vacuum residue.
Lignocellulosic feedstock can be
converted to bio-oil by direct
liquefaction in a phenolic solvent such as guaiacol with an oil yield
of >90 C% at 300–350 °C without the assistance of catalyst
or reactive atmosphere. Despite good initial performance, the liquefaction
was rapidly hindered by the formation of heavy components (molecular
weight > 1000 Da), which increased the viscosity of the bio-oil
upon
recycling the bio-oil or a fraction of it as a liquefaction solvent.
This paper explores the possibility to minimize the production of
this undesirably heavy fraction by optimizing the process parameters
such as temperature, heating rate, reaction time, and concentration
of water. This study allowed us to find a compromise between maximizing
the bio-oil yield and minimizing its heavy fraction. It also provides
insight onto the reaction network of the liquefaction reaction, showing
for instance that all product fractions, including the heaviest products
and the char, are mainly direct liquefaction products rather than
secondary reaction products, e.g. from bio-oil recondensation. However,
the resulting heavy fraction is still too high to allow effective
recycling of the bio-oil. Complementary approaches need to be investigated.
This paper proposes and examines an alternative thermo-chemical process for biomethane production from lignocellulosic biomass, termed self-gasification. Self-gasification of biomass is envisaged to utilize a high-pressure steam gasifier (30−80 bar) at temperatures of 600−900 °C and to use the alkali metals in biomass as gasification and methanation catalysts. The concept was studied by performing preliminary process simulations and by several screening experiments with wood. The simulations gave insight into the effect of gasifier operating conditions on methane yield. After screening tests of different alkali metals, KOH was chosen as a model compound for biomass-derived ash for further experimentation. It improves char reactivity by more than an order of magnitude, and thermogravimetric data interpreted by a first-order reaction model showed that it accelerates the pyrolysis reaction reducing the activation energy from E
a = 143 kJ/mol to E
a = 65 kJ/mol. Methane amounts higher than dictated by equilibrium are produced with and without impregnated KOH.
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