PATENT
Process
For The Production Of Synthesis Gas By Means Of A Fluid Bed Gasification
Reactor Fed With Carbonaceous Material And Device Suitable For The Purpose
Pub. No.: WO/2012/004001
International Application No.: PCT/EP2011/003441
Publication Date: 12.01.2012
Applicants: ENI S.p.A. [IT/IT]; Piazzale E. Mattei, 1 I-00144 Roma (IT)
Inventors:
BUA, Letizia; (IT).
CARNELLI, Lino; (IT).
CHIODINI, Andrea; (IT)
Abstract:
A process for the production of synthesis gas
which comprises : feeding a carbonaceous material having a particle-size lower
than or equal to 50 mm, to a gasification reactor; feeding an oxidizing gas to
the bottom of the reactor at a flow-rate which is such as to give the solid
mass present in the reactor an aspect of a biphasic bed; filtering the solid
particles entrained by the gas, in a cyclone filter positioned internally at
the top of the reactor; discharging from the cyclone filter a gaseous stream
essentially consisting of synthesis gas and unreacted oxidizing gas; and
recycling the filtered solid particles, discharged at the base of the cyclone,
directly to the biphasic bed, through an inner duct.
DESCRIPTION
The present invention relates to a process for the production of synthesis gas
by means of a fluid-bed gasification reactor fed with carbonaceous material.
More specifically, the present invention relates to a process for the
production of synthesis gas using a fluid-bed gasification reactor fed with
biomass.
Even more specifically, the present invention relates to this process and the
device suitable for this purpose.
Processes for the conversion of carbonaceous materials, for example biomasses,
into energy, can be divided into two main categories: thermochemical , of
interest in the present invention, and biochemical. All conversion processes of
biomasses into energy are based on the extraction of the heat content of the
substance used or storage of this in a different energy carrier which can be
subsequently used at a later moment.
In thermochemical processes, the action of heat allows solid fuel to be
transformed into a gas mainly consisting of hydrogen and carbon monoxide which,
after suitable purification, is suitable for direct use in modern energy
conversion systems, for example gas turbines, for the production of electric
energy or possible transformations into chemical products, for example
hydrocarbons according to the Fischer-Tropsch synthesis. When the solid fuel is
biomass, these processes are applied to products characterized by a humidity on
collection lower than 30% and with a C/N ratio higher than 30.
In particular, gasification is a thermochemical transformation system of a
solid reagent (for example biomasses or carbonaceous material) which is
converted into synthesis gas (mainly H2 and CO, with the presence of
C02, CH4, H20) . This transformation takes
place in an oxygen deficiency, with respect to the quantity necessary for the
complete combustion. The oxidizing agents commonly used are air and oxygen
possibly mixed with steam.
"Equivalence ratio" (ER) is the ratio between the oxygen used and
stoichiometric oxygen, wherein ER is equal to 1 in the case of stoichiometric
combustion, whereas the value ER = 0 corresponds to pyrolysis. Gasification
generally requires an ER equal to 0.25-0.5.
The possible technologies for gasification, in particular in the presence of
biomasses, are:
- fixed bed;
- fluid-bed (bubble, circulating, pressurized)
- entrained flow.
Fixed bed gasifiers are fed with relatively gross particle-sizes (with a maximum
dimension even up to 100 mm) . They are mainly used in medium-small-sized
plants and are characterized by technological simplicity and economical
running. The efficiencies however are not particularly high and they cannot
tolerate excessively fine particles.
Fluid-bed gasifiers have a more efficient thermal exchange and greater control
of the composition of the outgoing gas, but also a delicate fluid dynamics,
with risks of packing and defluidization of the bed itself. The fluid bed
technology is adequate for more reduced dimensions of the particles fed with
respect to the fixed bed technology, preferably lower than 50 mm, but it
tolerates a non-uniform distribution of the particles .
Entrained flow gasifiers allow a high power to be reached (hundreds of MW) but
require very fine particles (=*100-400 μπι) . They are mainly used with liquid
fuels or with coal. For biomasses, a previous torrefaction or alternative co-
firing with coal is considered. The technology which is currently most
promising, especially for the use of biomasses, is the fluid-bed technology.
Fluid-bed gasification has various basic principles which make it differ
considerably from other types of gasification :the primary solid fuel (for
example, biomass) is immersed in a bed of fluidized inert material, generally
silica sand. In the case of fuels with a high ash content, the ash itself of
the fuel can act as bed. There are also studies in which the inert bed is
substituted with catalytically active material (for example, dolomite, olivine,
etc.) as it favours reforming and water gas shift reactions between the gases
gradually formed, modifying the composition of the gas phase directly in the
gasifier itself and thus raising the H2/CO ratio.
- the solid particles of biomass are kept in suspension (fluidization) by means
of the flow of gaseous reagents injected under pressure from below (ascending
stream) . The gaseous reagents consist of oxidizing agent (which can be air or
oxygen) and possibly steam.
The turbulences typical of the fluid bed create a series of positive effects,
for example:
- the mixing between solid particles and gas is excellent, the transfer of heat
and matter between the flows involved in the gasification is high and efficient
;
- the reaction rates are higher with respect to fixed-bed reactors (updraft and
downdraft) and consequently the residence time of the reagents is shorter, i.e.
resulting in a lower volume of the reactor;
The fluid bed is also characterized by a uniform temperature distribution
(isotherm) in the gasification zone, with temperatures typically within the
range of 800÷900°C. This temperature range is obviously a compromise between
the various requirements;
- resistance of the materials;
- the risk of sintering of the powders and slagging of the ashes, which
dramatically increases if the temperature of the bed exceeds the melting point
of the ashes. The agglomeration of the fine particles inside the bed should be
avoided as it leads to a loss in the fluidization of the bed;
- the necessity of raising the conversion of the carbon and avoiding the
formation of tar, for which an increase in the temperature of the bed would be
desirable .
In conclusion, a fluid bed is considered as being a bed of solid particles
situated in a duct at whose base gas is introduced with a certain surface rate
(i.e. referring to the empty duct) defined as Ug. In addition to this
parameter, there are a further two parameters which are fundamental in the
fluid-dynamic analysis of the regimes of a fluid bed:
the vacuum degree (ε) : volumetric fraction occupied by the gas with respect to
the overall volume used by the solid bed; and
- average slip rate (Usl) : difference between the effective average rates of
the gas and solid, assumed positive if directed upwards.
Fluid-bed gasification envisages two basic types of reactor:
- Bubbling Fluidized Bed (BFB) ;
- Circulating Fluidized Bed (CFB) .
In the first type (BFB) , the reactor is composed of two sections having a
different diameter, the upper part (larger diameter) allows a slowing-down of
the gaseous stream and consequently the reflux of most of the solid particulate
(gasification bed, non-combusted particles, powders, etc.); normally a cyclone
is envisaged downstream, for separating any possible entrained fine products.
In the second configuration (CFB) , the reactor has a larger quantity of
entrained solids, which are separated in a cyclone situated downstream and
reinserted into the gasifier through an outer pipe, complicating the plant
design.
The Applicant has now found a new plant configuration of a biphasic fluid-bed
gasification reactor which simplifies the production process of synthesis gas,
essentially based on CO and H2, starting from carbonaceous material
.
An object of the present invention, as better described also in the enclosed
claims, therefore relates to a process for the production of synthesis gas
which comprises:
a. feeding a carbonaceous material having a particle- size lower than or equal
to 50 mm, to a gasification reactor;
b. feeding an oxidizing gas to the bottom of the reactor at a flow-rate which
is such as to give the solid mass present in the reactor an aspect of a
biphasic fluid-bed comprising a dense phase, consisting of the solid and
interstitial gas, and a diluted phase consisting of gas bubbles which rise
along said bed;
c. filtering the solid particles entrained by the gas, in a cyclone filter
positioned internally at the top of the reactor;
d. discharging from the cyclone filter a gaseous stream essentially consisting
of synthesis gas and unreacted oxidizing gas; and
e. recycling the filtered solid particles, discharged at the base of the
cyclone, directly to the biphasic fluid bed, through an inner duct.
According to the present invention, the biphasic fluid-bed reactor essentially
consists of a vertical cylindrical structure suitable for containing the
carbonaceous material to be gasified. The vertical "structure
can be a cylindrical body with a substantially constant transversal section or
it can consist of two superimposed cylindrical bodies, with a different
transversal section.
The body with the larger transversal section is preferably positioned above the
body with the smaller transversal section, whereas a tapered element serves as
connection means between the two bodies.
The cylindrical structure with two different bodies is preferred, when the
carbonaceous material to be gasified comprises a relatively large
particle-size. In this case, in fact, as there is a slowing-down of the gaseous
flow rate in the upper part of the reactor, there is consequently a
slowing-down of the solid part entrained, with reflux, at least of the larger
particle-size, in the fluid bed.
Feeding inlets of the oxidizing gas and carbonaceous material are envisaged at
the bottom of the reactor. The oxidizing gas can consist of oxygen, air, or any
other 02/N2 mixture in which the oxygen content is higher
than 21% by volume, calculated with respect to the dry air. The oxidizing gas
can also comprise steam in such quantities as to give an 02/steam
ratio by volume ranging from 0.5 to 3.
The carbonaceous material can consist of coal, or solid oil residues (for
example, PET coke) , in powder having a particle-size of 1-50 μπι, biomasses or
mixtures thereof . Preferred carbonaceous material consists of biomass.
The term "biomass" , as used in the present description and claims,
essentially refers to a product of a biological origin which can be used for
energy purposes. Biomasses are therefore all products of agricultural crops and
forestation, comprising residues of agricultural processing and silviculture,
agro-food waste products destined for human nutrition or zootechnics, residues,
not chemically treated, of wood processing and the paper industry, all organic
products deriving from the biological activity of animals and human beings,
such as those contained in urban waste (waste "organic fraction").
The average chemical composition of a long-stemmed biomass consists of about
25-30% of lignin and 75% of carbohydrates, i.e. molecules of sugar joined to
form long polymeric chains . The two most important carbohydrates are cellulose
and hemicellulose . The long cellulose polymers form the fibres which give
plants their resistance, whereas lignin acts as a glue for keeping these fibres
joined. Hemicellulose basically has the role of keeping cellulose and lignin
associated.
Other constituents present in biomasses in variable quantities are so-called
extracts, compounds with a low molecular weight which can be separated by means
of organic solvents (terpenes, fats, waxes, phenols) , or with hot water
(tannins and inorganic salts) . The inorganic material, present as a phase
closely distributed in the fuel, comprises salts of Si, K, Na, S, CI, P, Ca, Mg
and Fe, which produce the ash deposit.
The oxidizing gas is introduced into the gasification reactor in a lower
position with respect to the feeding point of the carbonaceous material . In
particular, the feeding point of the oxidizing gas is in correspondence with a
volume at the bottom of the reactor, called "plenum", from which the
oxidizing gas starts its ascent along the height of the reactor. The plenum is
separated from the rest of the volume of the reactor by means of a perforated
septum on which the material to be gasified at the start-up, rests.
Once the oxidizing gas reaches a sufficient pressure in the plenum to exceed
the weight of the overlying mass of carbonaceous material, it begins to flow
with a Ug rate.
With a Ug null, there is no fluid bed: the biomass remains accumulated as in a
fixed bed.
By gradually increasing the Ug, a progressive increase in the thrust of the gas
on the solid is registered, and consequently an increase in the pressure drop
(Δρ) through the bed. The solid remains stationary under the initial
"fixed bed" condition until it reaches the minimum fluidization rate
(Umf) , in correspondence with which the thrust exerted by the gas on the solid
completely balances the weight of the bed.
By increasing the gas rate over Umf, the pressure drop through the bed remains
substantially constant whereas the bed begins to expand and the slip rate
increases together with the vacuum degree. Under these conditions, the system
has a biphasic appearance: together with a dense phase consisting of the solid
and interstitial gas, there is a diluted phase consisting of gas bubbles which
rise along the bed. This is the fluid-dynamic regime in which fluid-bed reactors
operate in a boiling configuration.
By further increasing the air introduction rate, the bed configuration
gradually changes, reducing its biphasic nature and shifting towards a
turbulent state, characterized by a reasonable homogeneity of the distribution
of the solid in the gaseous stream: the solid no longer forms a continuous
phase, but is dispersed in clusters and in long strips of particles: there is
no net upward flow of solid in the bed. The bed remains in turbulent regime
until the transport rate (Utr) , or blowout rate is reached, close to which a
brusque increase in the quantity of particles entrained by the gaseous stream
is registered, which would rapidly lead to the emptying of the bed. To maintain
the continuity of the process, the entrained particles must be refed from the
bottom of the reactor, through their recirculation by means of a specific
device. This is the overall fluid-dynamic regime in which circulating fluid-bed
reactors operate.
According to the present invention, the preferred fluid-dynamic regime for
maintaining the carbonaceous material under stable biphasic fluid-bed
conditions can be represented by the following equation, which allows the
minimum fluidization rate (Umf) to be calculated:
ε \d pJ dp ε
wherein the densities of the solid matter and of the gas ( ps and pf
) respectively, the viscosity of the gas
( JUf ) , the vacuum degree in the bed ( ε ) and the average diameter of the
solid particles ( d'p ) are taken into consideration.
This type of equation is substantially a balance between the force of
gravitation and the force which exerts the fluid on the solid. It is in
relation to ε, which represents the vacuum degree inside the fluid bed, and is
valid for all values of the Reynolds number .
The gas which leaves the bed of carbonaceous material, essentially consisting
of synthesis gas and residual non-reacted oxidizing gas, entrains solid
particles which are filtered by means of a cyclone arranged internally at the
head of the reactor. This arrangement surprisingly not only simplifies the
plant configuration but also shows an improved abatement efficiency of the
entrained powders.
Various fluid-dynamic tests were carried out for studying the behaviour of a
silica bed inside a glass prototype which simulates the geometry of the
gasification reactor which is to be used in the process, object of the present
invention. A study was effected of the quantity of entrained solid, varying
some of the parameters among which:
• the gas flow-rate
• the geometry of the reactor
· the diameter of the particles
so as to be able to effect various evaluations, such as :
• the possibility of seeing the behaviour of the bed;
· the systems to be used for reducing the entrainment of the solid;
• measuring the pressure drops in relation to the type of geometry and in
relation to the gas flow-rate.
The prototype has a geometry very similar to that of a gasification reactor.
The quartz column, which simulates the reactor, has an internal diameter of 25
mm and a height of 42 cm, and is equipped with a sintered septum whose function
is to uniformly distribute the air and support the material forming the bed.
Air of the supply network is used as gasifying agent, whose pressure and
flow-rate are regulated by a pressure reducer and a rotameter (0-2000 1/h) .
The inlet and outlet pressure are also kept in constant control by means of a
differential pressure gauge to determine the pressure drops of the system.
The collection system is obtained by the application of a joint which
discharges the entrained solid into a flask. The abatement system of the
powders was then obtained with:
a widening having a diameter equal to 58 mm so as to reduce the gas rate and
consequently the entrainment of the solid;
- a cyclone inside the widening ;
- an outer cyclone.
Using various geometries, it was therefore possible to verify the most suitable
area, considering two fundamental aspects such as solid entrainment and
pressure drops.
The gas flow-rate used in all tests is 640 Nl/h.
Free Full Text Source: http://www.wipo.int/patentscope/search/en/WO2012004001
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