(220q) Direct Solar-Powered Biomass Gasification Using Low-Temperature Steam

Direct Solar-Powered Biomass
Gasification Using Low-Temperature Steam

Flavio Manenti^*, Zohreh Ravaghi-Ardebili

Politecnico di Milano

Dipartimento di Chimica, Materiali e Ingegneria Chimica
?Giulio Natta?

Piazza Leonardo da Vinci 32

20133 Milano, Italy

*Corresponding author. Phone: +39 (0)2 2399 3287; Fax: +39 (0)2 7063
8173; Email: flavio.manenti@polimi.it

ABSTRACT

This work is aimed at coupling the
concentrating solar technology with the biomass gasification. The main issue is
that the biomass gasification occurs at temperatures of the order of 800-900°C,
whereas the available technology in concentrating solar power plants using
thermal fluids allows to achieve temperatures of 550-600°C. Different elements
are needed to be effective: (1) the detailed characterization of biomass
pyrolysis and the successive gas phase reactions; (2) the selection of
appealing biomass gasification technologies (i.e., traveling grate,
countercurrent); (3) the integration with discontinuous energy sources
(concentrating solar); (4) the definition of proper thermal energy storage; and
(5) the study of effective operational procedures to achieve direct solar
powered biomass gasification.

KEY-WORDS

Biomass gasification; concentrating solar;
biomass pyrolysis; energy integration; thermal energy storage

INTRODUCTION

Biomass is one of the most promising feedstock
able to satisfy the increasing demand for renewable energy, biofuels, and green
chemicals (Cucek et al. 2010, Kleme? et al. 2010, Lam et al. 2010a, Lam et al.
2010b, Vaccari et al. 2005). Unfortunately, biomass conversion to bioproducts
is tough to be industrially scaled-up due to the complexity of chemical and
transport phenomena as well as to be integrated with other renewable sources
that can support for energy and steam generation the gasification process.
Thus, two critical steps are:

1.            the
development of mechanistic models capable of describing transport phenomena and
reaction kinetics for a better understanding of biomass pyrolysis;

2.            the
integration of these models at the process scale to develop novel process
solutions.

For the former one, detailed chemical
mechanisms are needed both for biomass pyrolysis and for the successive gas
phase reactions, since they are still unavailable even for major products
released such as levoglucosan, hydroxymethylfurfural, and phenolic species
(Ranzi et al. 2013). Chemical mechanisms need to be integrated into particle
model accounting for transport phenomena, which are critical in predicting
global reactor performance. Developing these models is challenging because of
the biomass complexity as well as the multi-phase and multi-scale nature of the
conversion process (Mettler et al. 2012). For the latter step, the use of
two-tanks direct thermal energy storage in concentrating solar plant is an
established and appealing technology (Vitte et al. 2012, Manenti and
Ravaghi-Ardebili 2013).

A brief overview of these two main topics is
given hereinafter.

BIOMASS GASIFICATION

Combustion, gasification, and biomass-to-liquid
pyrolysis are some of the main thermo-chemical conversion routes, which can
convert an abundant and well distributed feedstock into energy, syngas,
bio-oil, and chemicals. One of the main problems when studying this type of
feedstock is the intrinsic variability of the biomass composition. As a
consequence, it is necessary to properly characterize the biomass, preferably
on the basis of few lumped components, which are typical for all the possible
feedstock.

The kinetic model adopted is explained in
detailed elsewhere (Ranzi et al. 2013) and is based on a multi-step
devolatilization and decomposition of the three key-biomass reference species:
cellulose, hemicelluloses and lignin. One of the main features of this model is
its ability to provide detailed information on yields composition of gas, tar,
and solid residue. The kinetic model also involves the char gasification and
combustion reactions, with steam and/or air or oxygen, as well as the secondary
homogeneous gas phase reactions of the released gas and tar species. The multistep
kinetic model has been validated on the basis of thermo-gravimetric data of
fine particles, with negligible resistances. Next, the model has been extended
to the reactor scale with the analysis of a countercurrent biomass gasifier.

ARCHIMEDE PLANT

Archimede concentrating solar plant uses linear
parabolic streams and direct two-tanks thermal energy storage where the molten
salt thermal fluid flows inside (Vitte et al. 2012). Energy storage is a major
process design and control issue in concentrated solar plants (Manenti and
Ravaghi-Ardebili 2013). The intrinsic discontinuous nature of solar energy
forces to install units able to store energy under favourable conditions, then
to release energy during night or unfavourable conditions. Accordingly, smoothened
operations and continuous energy production can be guaranteed. Several
technologies are today available for storage; tanks of molten salts, steam
accumulators, high thermal capacity solids are among the most important.

SOLAR GASIFICATION

According to the brief descriptions above, the
idea is to supply the biomass gasification using the steam generated by the
concentrating solar plant. To do so, it would be necessary to produce steam at
900°C, whereas the existing molten-salt solar technology allows to achieve
about 600°C. Nevertheless, it is possible to select specific configuration of
gasifier that may suit the achievement of the solar-powered biomass
gasification. Actually, the use of countercurrent gasifier allows to exploit
the high temperature of ashes slowly moving to the bottom of the unit to
increase the temperature of the steam entering from the bottom (Figure 1). By
doing so, the work is aimed at demonstrating that the lower limit for the inlet
steam temperature to keep the ?hot? condition of the biomass gasifier is not
higher than the steam temperature obtained from the concentrating solar plant,
making it feasible. Oxygen supply is needed to sustain the endothermic reaction
of gasification by means of in situ combustion of a portion of biomass. Specific
operational procedure is needed to operate the integrated plant. Also, the
study will analyse the possibility of cogeneration before supplying the biomass
gasifier to further intensify the solution energetically (Sikos and Kleme?
2010, Zhu et al. 2000). The existing GasDS tool for biomass characterization
and gasification (Ranzi et al. 2013) is adopted for the dynamic simulation of
the countercurrent gasifier and it is integrated in DynSim? suite for process
dynamic simulation by Simulation Science, Invensys.

Figure 1. Qualitative layout.

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 ADDIN EN.REFLIST