3 2PHIA (Solid Self-Reforming with Separation of Organic Part … · 2011. 6. 30. · Processes and Technologies for a Sustainable Energy. condensable volatiles), to be used directly

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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