3 2phia (solid self-reforming with separation of organic part … · 2011. 6. 30. · processes and...
TRANSCRIPT
S3O2PHIA (Solid Self-Reforming with Separation of Organic Part
from Hetero-Inorganic Atoms) process as novel approach able to
enhance eco-compatibility of thermochemical biomass
degradation processes
P. Giudicianni1, A. Cavaliere
2, R. Ragucci
1
1. Institute of Research on Combustion - C.N.R., Napoli - ITALY
2. Chemical Engineering Department - Università di Napoli - ITALY
1. ABSTRACT
A thermochemical biomass degradation technology is proposed able to enhance the process
eco-compatibility combining the production of a vapor phase highly diluted in steam, to be
used as a fuel in non conventional combustion systems, and the sequestration of C in the soil
by mean of a solid carbon rich residue suitable as a fertilizing or amending agent. In order to
achieve a higher level of eco-compatibility the distribution of inorganic hetero atoms must be
controlled limiting inorganic pollutants release in the vapor phase and enhancing their
availabiity in the solid residue as plants nutrients. To this aim an experimental set-up has been
designed to investigate the effect of the main operating parameters, pressure, heating rate and
final temperature, on the yields and on the chemical and physical properties of products
selecting proper variation ranges of the three parameters, 1-5 bar for the pressure, 10-40
°C/min for the heating rate and 400-700 °C for the final temperature.
2. INTRODUCTION
Nowadays biomass can be considered one of the most promising source of renewable energy,
representing, at present, about 10% of global annual primary energy consumption [1].
Depending on both chemical and physical nature of biomass and on the kind of energy
required different technologies for biomass processing can be applied. Direct combustion
facilities, gasification plants, co-firing systems of municipal solid wastes in coal based plants
and anaerobic digestion devices for biomass with high moisture content for the production of
heat and combined heat and electricity (CHP) are commercially available. Bioethanol and
biodiesel production has been developed in commercial plants while gasification and
fast/flash pyrolysis technologies are operative only in demonstration plants not being yet
technically mature [1]. Consequently, extensive studies have been carried out in order to
optimize the reactor configurations and the operative parameters in dependence on the
chemical and physical nature of biomass, the kind of energy source required and the
environmental restrictions imposed. As for pyrolysis, extensive information is available on
the production of lignocellulosic char [2, 3, 4], products distribution from wood for fast
pyrolysis [5, 6, 7, 8, 9] for the production of liquid fuel and conventional pyrolysis [10, 11,
12, 13, 14, 15, 16] aimed to the minimization of tar content in gasification plants.
In this work a different approach is presented that increases the eco-compatibility of
thermochemical degradation processes combining the reduction of CO2 emission, due to the
use of a renewable energy source, with the sequestration of C in the soil as bio-char ensuring
a proper separation of the inorganic part from the organic one. The aim of the process is a
more efficient exploitation of biomass coupling the production of a vapor phase (gases and
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Processes and Technologies for a Sustainable Energy
condensable volatiles), to be used directly as a fuel in non conventional combustion systems,
and of a solid carbon rich residue (char) able to be used as a soil fertilizing or amending agent
(biochar). Such an approach cannot disregard the behavior of inorganic matter present in
biomass considering both the influence exerted by alkaline metals in biomass degradation
process and the role played by inorganics in solid and gaseous products. Minerals entrapped
in the solid phase are responsible, together with C, of the nutrient properties of biochar; on the
other hand N, S are the main source of pollutant formation in the gas phase.
In this work an experimental set-up, able to demonstrate feasibility and potentials of a
pyrolytic process aimed at the optimal separation of organic/inorganic parts, is introduced and
main design criteria are discussed. The experimental apparatus is designed to investigate the
effect of final temperature, heating rate and pressure on products yields and composition and
on the physical properties solid products. Moreover the influence of these three parameters on
inorganics distribution will be evaluated. Temperature is responsible of their devolatilization
and of the chemical transformation of both inorganics released in gas phase [17, 19] and the
ones retained in the solid phase determining their availability as plant nutrients [20].
In order to gain both a vapor and a solid phase with the desired characteristics, steam has been
used to provide the reacting atmosphere and proper variation ranges of the three above-
mentioned parameters. Finally numerical simulations of reactor fluid dynamics are presented.
3. SET-UP OF THE EXPERIMENTAL APPARATUS
In the set-up of the experimental apparatus the main design steps have been the choice of the
pyrolysing agent, the definition of the main operative parameters to be controlled and the
corresponding variation ranges to be examined and the best reactor configuration allowing to
control the above mentioned operative variables. In the following sections a detailed
description of the experimental apparatus and of its design criteria is given.
3.1. Definition of the pyrolysing agent
Pyrolysis experiments are generally carried out in an inert environment (nitrogen or helium)
while an oxygenated gas (steam or CO2) can be used as pyrolyising or gasifying agent in
dependence on the established thermal conditions. Previous studies dealing with the
production of char based activated carbon show the positive effect of steam rather than N2 and
CO2 on the liquid quality and physical properties of char [18]. In a flow of steam, the yields of
water soluble liquid products increase at the expense of gaseous and solid products given the
ability of steam to perform a more efficient penetration of solid matter enhancing desorption,
distillation and removal of volatiles. On the contrary during pyrolysis in a flow of nitrogen,
higher char yields are obtained with lower porosity due to the deposition of carbonaceous
material inside char pores [21]. A vapor phase (gas and liquid) produced in steam pyrolysis
seems suitable to be burned in non conventional combustion systems operated in MILD
conditions [22, 23]. These considerations have induced to select steam as pyrolysing agent.
3.2. Definition of controlled operative variables
The main operative parameters that influence a pyrolysis process are final temperature,
sample heating rate, pressure, feedstock properties and flow rate of the pyrolyising agent. The
latter is strictly linked to residence time of vapor phase in the reaction environment [24].
Variation ranges of temperature, heating rate and pressure, have been selected on the basis of
their effect on both products yields and composition and morphology of residual char in order
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Ischia, June, 27-30 - 2010
to set the optimal conditions to obtain both a solid and a vapor phase (gas and liquid) with the
desired characteristics.
Temperature variation range has been set at 673-973 K. At 673 K char yield obtained in a
pyrolysis process varies between 15% and 45% wt (on dry biomass basis) [5, 7, 8, 25. 26, 5,
27, 28], depending on heating rate and gas residence time. Char yield decreases with
increasing temperature that, in turn, produces an increase of gas and liquid yields, even
though the yield of liquid has a maximum at temperature between 700 and 950 K at the onset
of secondary degradation of liquid primary products [24]. Even if at temperatures lower then
673 K char yield reach its maximum, proximate analysis of biomass char obtained from
processes for the production of activated carbon shows that its volatile matter content is still
high determining low values of specific surface area, while an increase in carbonizing
temperature has a positive effect on this property [29].
Although high temperatures enhance adsorption properties of solid residue in presence of
steam too much severe thermal conditions promote gasification reactions between gasifying
agent and char and the equilibrium shift of the water gas-shift reactions that increase gas
yields at the expence of char [30, 31, 32]. Therefore temperature is fundamental in the
determination of gas composition being responsible of the activation of secondary
degradation of primary pyrolysis products and of gasification reactions. CO and H2 content in
the gas phase increases with temperature, mostly during fast pyrolysis heating conditions,
enhancing the energetic value of gas phase [24], while chemical composition of liquid shift to
higher content of low molecular weight aromatic compounds (phenols), precursors of
condensed tertiary products.
Consequently, even if higher temperature produces higher gas yields and a more valuable
syngas, in order to avoid an excessive consumption of char due to the onset of the
heterogeneous gasification reactions the upper limit of temperature has been set at 973 K,
temperature at which the gasification reactivity of char can be considered quite low [33, 34,
35, 36].
Heating rate variation range has been set at 10-40 K/min (slow pyrolysis). This choice has
been made in order to maximize char production and optimize the chemical and physical
properties of char at the expense of gas and liquid yields. In fact, low heating rates favor char
formation reducing biomass devolatilization and the activity of secondary degradation
reactions due to lower average temperatures experienced by the sample during the process
[24]. Moreover low heating rates (below 10 K/min) allow for a slow release of volatile
compounds determining a final porosity that resembles the original porous structure of
biomass enhancing micropores fraction, fundamental for the attainment of a high specific
surface area [37, 38]. Nevertheless at so much low heating rate char is characterized by a
relatively high volatiles content compared with the one obtained under more severe heating
conditions [37, 38]. Moreover the closer internal structure of char produced at low heating
rates does not allow an easy escape of volatiles from the char particle. Consequently the
increased residence time of volatiles inside the char particle favors their polimerization to
form secondary char and consequent pores occlusion. This effect is limited in more severe
heating conditions [39]. These considerations have led to explore heating rates higher then 10
K/min, although typical of slow pyrolysis, setting the limit at 40 K/min.
Pressure variation range has been set at 1-5 bar. The effect of pressure on product distribution
is not easily predictable due to the formation of tars that prevent the attainment of
thermodynamical equilibrium of pyrolysis reactions [2]. Few and contradictory data are
presented in literature about the effect of pressure on pyrolysis product yields [40, 41, 42, 43,
44, 45]. However from previous studies focused on cellulose pyrolysis, it can be deduced that
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Processes and Technologies for a Sustainable Energy
an increase in pressure, generally linked to higher gas residence time in reaction environment,
determines higher char yields together with the production of lighter volatiles [2]. On the
other hand pressure values higher than 5 bar determine the formation of fused intermediates
on char surface that favor agglomeration of close char particles reducing the specific surface
area of the solid residue, although this effect becomes more significant at heating conditions
typical of fast pyrolysis [46].
3.3. Definition of reactor configuration
In order to allow an effective control of the thermal conditions in the reaction chamber,
sample size (d<600 m), mass (m=6 g) and reaction configuration, depicted in Fig.1, have been
properly selected. The biomass is grinded finely in order to limit the intra-particle thermal
gradient during the process and placed on a multiplate sample tray in the way of a monolayer
to avoid heat transfer resistance related to a packed bed configuration. In the most severe
heating conditions particle heating rate has been estimated to be ten times higher than the
fastest heating rate imposed to the steam flow (40 K/min). Biomass sample is spread over the
first four plates. The role of the the fifth plate is to make more uniform the velocity profile
inside the reactor chamber as will be explained in the numerical simulation section.
Sample tray plates are placed uniformly along the rectangular cross-section (width=0.04 m,
height=0.052 m) of the reaction chamber (lenght=0.024 m). To limit external heat loss the
reactor chamber is jacketed so that the steam flows in the jacket, equipped with baffles to
allow a uniform air distribution, before reversing its flow to enter the reaction environment
through a ceramic flow straightener. The external jacket is surrounded by heating panels
covered by a layer of insulating panels (not reported in fig. 1). A thermocouple is placed in
the reactor jacket, just before the flow straightener, to measure temperature and control heat
flux to the steam super heater. At the exit of the reaction chamber temperature and pressure
are monitored.
The experimental set-up, showed in Fig.2, consists in a steam generator followed by a super
heater equipped with a programmable controller to control steam heating rate, a jacketed
reactor, a condensation device, a liquid collection system and a gas sampling station. The
steam produced by the steam generator at a pressure, set by a regulating valve, at the exit of
steam generator passes through the super heater where it is heated at a controlled heating rate
and then enters the reactor jacket. Pressure is continuously monitored by means of a pressure
transmitter at the jacket inlet.
Biomass sample (1.5 g spread on each sample tray plate) is invested tangentially by the steam
flow. In order to carry out the process under controlled thermal conditions the intensity of the
applied heat flux to the super heater is used as the adjustable variable of the programmable
controller and the temperature of steam at the end of the reactor jacket is monitored. At the
exit of the reactor a regulating valve allow to impose a constant steam rate along the sample
tray plates (0.225 m/s.) The effluent gas passes through a condensation device made up of a
jacketed coil where condensable volatiles cool down and condense. At the exit of the
condenser a system consisting in two catch pots immersed in a thermostatic bath at 273 K
hosts the condensed volatiles for off-line chemical characterization, while permanent gases
flow in a silica gel trap in order to reduce their moisture content before being sampled.
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Ischia, June, 27-30 - 2010
Fig. 1 Longitudinal section of the reactor for the study of S3O
2PHIA process.
Fig. 2 Experimental apparatus for the study of S3O
2PHIA process.
4. PRODUCTS ANALYSIS
Chemical analysis of gas phase is carried out by a micro-Gas Chromatograph equipped with a
Thermal Conductivity Detector (Agilent 3000 Quad) directly at the sampling point. It is made
up of two independent channels each one equipped with a specific capillary column to allow
the simultaneous detection of all the species of interest. Each channel is equipped with a
TCD. detector, sensitive enough to detect ppm-level concentration of target analytes.
Liquid phase is analyzed off-line after a preliminary fractionation because of its complex
chemical nature [5, 9, 16, 47, 48]. Three fractions will be collected and analyzed separately:
Liquid condensed on the walls of heat exchanger coil;
Non-polar fraction of liquid collected in the catch pots;
Polar fraction of liquid collected in the catch pots highly diluted in condensed water.
The three fractions will be analyzed with different analytical techniques on the basis of their
own chemical nature: UV-Visible Spectroscopy, to evaluate the condensing degree of
aromatic functionality, Size Exclusion Chromatograpy to detect species with molecular
weight in the range 100-1E10 amu, Atmospheric Pressure Ionization with Laser
Ionization/Maldi/ Electrospray source to analyze species of molecular wight up to 4000 amu,
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Processes and Technologies for a Sustainable Energy
Mass Spectrometry/Gas chromatograpy to detect compounds of molecular weight up to 300
amu.
Chemical and physical properties of solid residue will be studied by mean of Scanning
Electron Microscopy to analyze bulk and surface composition and adsorption techniques in
order to determine porosity and specific surface area.
5. NUMERICAL SIMULATION
A preliminary study of reactor fluid dynamics has been carried out in order to simulate the
velocity field distribution along the reactor jacket and the reaction chamber. A uniform
velocity field in the cross section of the reaction chamber and along the plates of the sample
tray is required to obtain an effective control of the heating conditions experienced by the
sample during the process.
To study the momentum transport the following conditions have been set:
The presence of biomass on the plates has been neglected;
The process is isothermal at T varying in the range 673-973 K;
Pressure has been varied in the range 1-5 bar;
Steam rate along the plates of the sample tray is 0.225 m/s;
The flow inside the reactor has been considered laminar (Re<100).
In Fig.3 is shown the velocity component along the z-coordinate at the reactor center at
different values of z.
Fig. 3 Velocity component along longitudinal direction at reactor centre at different values of
z. Operative condition in the test run are P=1 atm and T=973K.
The variation of the z-velocity component maximum along the plates is less than 13.3 %
while along each plate the variation range of z-velocity component maximum range between
0.4 and 31%. Along each plate z-velocity component increases with z being affected by the
increase of the cross sectional area of the reaction chamber at plate end.
Numerical simulation performed at different operative conditions (T variable in the range
673-973 K and P variable in the range 1-5 bar) show similar trend allowing to conclude that
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Ischia, June, 27-30 - 2010
biomass sample spread over the plates of the sample tray is exposed to uniform fluid
dynamical conditions in the whole temperature and pressure range examined.
6. CONCLUSIONS
In this work a novel approach to the exploitation of biomass has been proposed with the aim
to enhance the eco-compatibility of biomass thermochemical degradation process. Biomass,
exposed to a mild heating treatment in presence of steam, allows to produce a vapor phase
suitable to be burned directly in combustion system operated in MILD conditions and a solid
residue with the characteristics of an activated carbon capable to be used as amending and
fertilizing agent and to allow CO2 sequestration in the soil. To enhance the eco-compatibility
of the degradation process the distribution of inorganic hetero atoms has to be studied in order
to reduce as much as possible inorganic pollutants release in the vapor phase and to limit their
chemical transformations in the solid residue responsible of their unavailability as plants
nutrients. To this aim an experimental apparatus has been set up in order to evaluate the
influence of some operative parameters, final temperature, heating rate and pressure on the
chemico-physical characteristics of the products. Reactor configuration has been designed to
allow an effective control of the thermal conditions inside the reaction chamber. Preliminary
numerical simulations have been performed showing a uniform distribution of velocity field
inside the reactor that is a fundamental requirement for a proper control of thermal conditions
experienced by the biomass sample during the process.
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