U.S. patent application number 10/552119 was filed with the patent office on 2007-11-29 for two-stage plasma process for converting waste into fuel gas and apparatus therefor.
Invention is credited to Pierre Carabin, Peter George Tsantrizos.
Application Number | 20070272131 10/552119 |
Document ID | / |
Family ID | 33035040 |
Filed Date | 2007-11-29 |
United States Patent
Application |
20070272131 |
Kind Code |
A1 |
Carabin; Pierre ; et
al. |
November 29, 2007 |
Two-Stage Plasma Process For Converting Waste Into Fuel Gas And
Apparatus Therefor
Abstract
A two-step gasification process and apparatus for the conversion
of solid or liquid organic waste into clean fuel, suitable for use
in a gas engine or a gas burner, is described. The waste is fed
initially into a primary gasifier, which is a graphite arc furnace.
Within the primary gasifier, the organic components of the waste
are mixed with a predetermined amount of air, oxygen or steam, and
converted into volatiles and soot. The volatiles consist mainly of
carbon monoxide and hydrogen, and may include a variety of other
hydrocarbons and some fly ash. The gas exiting the primary gasifier
first passes through a hot cyclone, where some of the soot and most
of the fly ash is collected and returned to the primary gasifier.
The remaining soot along with the volatile organic compounds is
further treated in a secondary gasifier where the soot and the
volatile compounds mix with a high temperature plasma jet and a
metered amount of air, oxygen or steam, and are converted into a
synthesis gas consisting primarily of carbon monoxide and hydrogen.
The synthesis gas is then quenched and cleaned to form a clean fuel
gas suitable for use in a gas engine or a gas burner. This offers
higher thermal efficiency than conventional technology and produces
a cleaner fuel than other known alternatives.
Inventors: |
Carabin; Pierre; (Montreal,
CA) ; Tsantrizos; Peter George; (Quebec, CA) |
Correspondence
Address: |
LADAS & PARRY LLP
224 SOUTH MICHIGAN AVENUE
SUITE 1600
CHICAGO
IL
60604
US
|
Family ID: |
33035040 |
Appl. No.: |
10/552119 |
Filed: |
April 5, 2004 |
PCT Filed: |
April 5, 2004 |
PCT NO: |
PCT/CA04/00514 |
371 Date: |
October 27, 2006 |
Current U.S.
Class: |
110/250 ;
110/346 |
Current CPC
Class: |
C10J 2300/095 20130101;
C10J 3/18 20130101; C10J 3/721 20130101; C10J 2300/1634 20130101;
C10J 3/466 20130101; C10J 2300/1807 20130101; C10J 3/845 20130101;
C10J 2200/12 20130101; C10J 2300/094 20130101; C10J 3/57 20130101;
C10J 2300/1238 20130101 |
Class at
Publication: |
110/250 ;
110/346 |
International
Class: |
C10J 3/18 20060101
C10J003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2003 |
CA |
2,424,805 |
Claims
1. A two-stage plasma process for converting waste having organic
and inorganic components into fuel gas, which comprises: (a) in the
first stage, vitrifying or melting the inorganic components of the
waste and partially gasifying the organic components; and (b) in
the second stage, completing the gasification of the organic
components so that gas from the first stage of the process entering
the secondary gasifier is exposed to a high temperature such as to
transform essentially all soot present in the gas to CO and to
convert essentially all complex organic molecules to simpler
molecules CO, CO.sub.2 and H.sub.2.
2. A process according to claim 1, in which a dust separation and
removal step is provided between the two stages of the process.
3. A process according to claim 1, in which the fuel gas produced
in the second stage is quenched and cleaned to make it suitable for
use in a gas engine or turbine for production of electricity or in
a gas burner for production of steam or in chemical synthesis
reactions.
4. A process according to claim 1, in which the first stage is
carried out in a plasma arc furnace.
5. A process according to claim 1, in which the second stage is
carried out in a secondary gasifier using a plasma torch with
addition of metered amounts of oxygen, air and/or steam.
6. A process according to claim 4, in which the plasma arc furnace
is a refractory lined, enclosed furnace provided with at least one
direct current graphite electrode adapted to generate a plasma arc
to a bath of liquid inorganic material originating from the waste
itself and located at the bottom of the furnace.
7. A process according to claim 6, in which said liquid inorganic
material comprises a slag layer which is maintained at a
temperature of at least 1500.degree. C.
8. A process according to claim 7, in which said liquid inorganic
material further comprises a metal layer also maintained at a
temperature of at least 1500.degree. C. and located under the slag
layer.
9. A process according to claim 6, in which the waste is introduced
into the furnace on top of the liquid inorganic material and the
organic component in the waste reacts with air, oxygen and/or steam
supplied to the furnace in a predetermined amount adapted to
achieve gasification of organic material in the waste into a
primary synthesis gas containing CO, H.sub.2, CO.sub.2 and N.sub.2
if the waste contains nitrogen or if air is added to the furnace,
and also containing some soot and complex organic molecules.
10. A process according to claim 9, in which the organic material
in the waste is so reacted as to form a layer of partially treated
waste on top of the slag layer and fresh waste is introduced into
the furnace on top of said partially treated waste layer which is
maintained at a temperature of between 700 and 800.degree. C. and
constitutes a cold top for the fresh waste added to the
furnace.
11. A process according to claim 9, in which the primary synthesis
gas is subjected to dust separation and removal in which dust
particles larger than a predetermined size are separated and
removed.
12. A process according to claim 11, in which the removed dust
particles are recycled to the furnace.
13. A process according to claim 5, in which the secondary gasifier
is equipped with a plasma torch fired eductor for exposing the gas
from the first stage of the process entering the secondary gasifier
to a high temperature.
14. A process according to claim 13, in which the high temperature
to which gas from the first stage is exposed in the secondary
gasifier is between 900.degree. C. and 1300.degree. C.
15. A process according to claim 14, in which the high temperature
is achieved mainly by partial oxidation of the gas from the first
stage by injection of predetermined amounts of air, oxygen and/or
steam to the eductor, and the plasma torch provides only a small
fraction of the energy required for maintaining said high
temperature.
16. A process according to claim 13, in which the fuel gas exiting
the secondary gasifier is cooled down very rapidly to a temperature
below 100.degree. C. so as to freeze the thermodynamic equilibrium
of the fuel gas and avoid production of secondary pollutants.
17. A process according to claim 16, in which after cooling, the
fuel gas is subjected to a final cleaning operation to remove any
remaining contaminants.
18. A process according to claim 1, in which the process is carried
out under a negative pressure to preclude exit of toxic fumes or of
flammable materials from any unit operations.
19. A process according to claim 1, in which an oxygen starved
environment in used in the process to preclude dioxin
formation.
20. Apparatus for converting waste having organic and inorganic
components into fuel gas, which includes: (a) a primary gasifier
comprising a refractory lined, enclosed plasma arc furnace provided
with at least one graphite electrode; at least one inlet for
feeding waste into the furnace; means for feeding air, oxygen
and/or steam in metered amounts into the furnace; and a gas take
off port for primary synthesis gas produced in said primary
gasifier; said primary gasifier being adapted to maintain layers of
molten metal and molten slag at the bottom of the furnace and on
top of the molten slag a layer of partially treated waste on top of
which fresh waste is fed; and said at least one graphite electrode
being adapted to generate a plasma arc to the molten slag present
in the furnace during the operation; and (b) a secondary gasifier
to which the primary synthesis gas is fed, said secondary gasifier
being equipped with a plasma-torch fired eductor adapted to expose
the primary synthesis gas entering from the primary gasifier to a
high temperature such as to transform essentially any soot present
in said primary gas into CO and to convert essentially any complex
organic molecule to simpler molecules CO, CO.sub.2 and H.sub.2;
means for supplying metered amounts of air, oxygen and/or steam
into the eductor; said eductor leading to an insulated chamber; and
an outlet being provided in said chamber for the fuel gas resulting
from the operation.
21. Apparatus according to claim 20, in which in the primary
gasifier two graphite electrodes are used creating an arc between
one electrode and the slag during the operation, and creating a
second arc from the slag to the second electrode.
22. Apparatus according to claim 20, in which the eductor provided
in the secondary gasifier is made of a high heat metal alloy or is
refractory lined or water cooled, and is equipped with the plasma
torch at its inlet.
23. Apparatus according to claim 20, further comprising a dust
separator between the primary gasifier and the secondary
gasifier.
24. Apparatus according to claim 20, further comprising a gas
quenching and gas cleaning means following the secondary
gasifier.
25. Apparatus according to claim 20, further comprising an induced
draft fan adapted to operate the apparatus under a negative
pressure.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and apparatus for
a two-stage conversion of organic components contained in solid
and/or liquid waste, at high plasma temperature, into a fuel gas
suitable for use in a gas engine or turbine for the production of
electricity or a gas burner for the production of steam, or in
chemical synthesis reactions.
[0003] 2. Description of the Prior Art
[0004] Numerous methods have been proposed for the conversion of
waste into energy. The most common method is incineration. In
incineration systems, waste is typically introduced in a high
temperature chamber and reacted with large amounts of air. The
process can be one stage or two stages. Whether the incineration
process is one stage or two stages, the process always uses large
amounts of air, resulting in the production of large amounts of hot
off-gas, typically laden with entrained particulates and acid gas
components. Thermal energy is typically extracted from this hot
dust-laden acid gas using a heat recovery boiler.
[0005] This method of extracting energy from a hot dirty gas is
subject to two main problems. First, heat recovery boilers are
subject to corrosion from the acid gas and fouling from the
particulates, especially above temperatures of 700.degree. C.
Second, the slow cooling of gas in a recovery boiler is the major
cause for the de novo synthesis of dioxins that occurs in the
temperature range of 250-400.degree. C. (c.f. Cernuschi et al.,
"PCDD/F and Trace Metals Balance in a MSW Incineration Full Scale
Plant", Proceeding of the 2000 International Conference on
Incineration and Thermal Treatment Technologies). Thus, energy
cannot be safely recovered at temperatures below 400.degree. C.
because of the risk of forming dioxins. In a typical incinerator,
gases exit the main incineration chamber at 1100.degree. C. and
exit the chimney at 150.degree. C. Of this range, energy can only
be practically and safely recovered in the range from 700 to
400.degree. C., meaning that only about one third of the available
energy can be recovered. Solutions have been proposed to alleviate
some of these problems in incineration. For example, U.S. Pat. No.
5,092,254 of Kubin et al. proposes a process whereby lime is
injected in the incinerator to neutralize the acid gases and reduce
corrosion level inside the incinerator and auxiliary equipment.
U.S. Pat. No. 5,797,336 of Miller et al. discloses a process for
the incineration of waste material whereby waste is first
incinerated in a furnace chamber, then re-burnt in a fluidized bed
afterburning chamber, in order to reduce the number of
particulates, and go to a heat recovery boiler where the gas
temperature is reduced from 700-1100.degree. C. to 100-300.degree.
C. Fundamentally, however, all incineration systems try to extract
energy from a hot dirty gas.
[0006] Also, by using an independent source of heating, such as
plasma, a wide range of waste types can be combusted, independently
of their composition. The plasma also allows reaching high
temperatures that will melt the inorganic components of the waste
into an inert slag and will dissociate them from the organic
components of the waste, which will form a gas.
[0007] A number of methods and apparatus have been proposed for the
decomposition of wastes, hazardous or not, into inert slag and
non-hazardous gases with the use of plasma. Thus, U.S. Pat. No.
4,960,380 of Cheetham describes a two-step process, wherein in the
first step plasma is used to reduce solid waste materials to a
slag-like material from which more harmful constituents have been
removed and to a gaseous effluvium. The effluvium of the plasma
reduction process is scrubbed to remove particulates. The gas is
then processed by additional heating and oxygen addition in order
to convert the carbon monoxide in the gas into carbon dioxide.
Products of incomplete combustion (and/or chemically harmful
constituents) are also eliminated in this step. The oxidized gas is
then suitable for safely exhausting into the atmosphere. In this
system, coherent radiation (laser) is used to generate and sustain
the plasma. This process is targeted at treating low organic
content waste, such as incinerator ash. Moreover, the gas exhausted
from the process being a hot combustion gas, the problems
associated with incineration, described above, also apply to this
process.
[0008] A plasma torch can also be used as an independent source of
heat. For example, U.S. Pat. No. 5,534,659 of Springer et al.
describes a single step method and an apparatus for treating
hazardous and non-hazardous waste materials composed of organic and
inorganic components by subjecting them to high temperature
pyrolysis and controlled gasification of organic materials and
metals recovery and/or vitrification of inorganic materials. The
source of heating for the reactor is a conventional plasma arc
torch.
[0009] U.S. Pat. No. 5,451,738 of Alvi et al. provides a two-step
method for the disposal of waste material, including volatile
components and vitrifiable components, by first heating the waste
to vaporize the hydrocarbon liquids and thereafter feeding to a
primary plasma reactor on the surface of a molten pool where the
vitrifiable components are melted and the volatile components are
volatilized. The reactor is equipped with multiple AC plasma
torches. The torches use copper electrodes, which are water-cooled.
The hydrocarbon liquids and the volatilized components are then fed
to a secondary plasma reactor where they are dissociated into their
elemental components.
[0010] The use of a plasma torch in order to obtain high reaction
temperatures in the gas phase poses some problems. Plasma torches
have relatively low energy efficiency, whereby 30 to 40% of the
electric energy to the torch is typically lost to cool the
electrodes. Moreover, the water-cooled torch presents the risk of
water leaks onto the molten slag inside the reactor, creating an
explosion. By contrast, considerable improvement is produced by
using graphite rods to generate the plasma in an arc furnace, since
graphite can withstand extremely high temperatures (several
thousands of degrees), no water cooling is required and the energy
efficiency of the graphite rod is nearly 100%. Also, the risk of
water leaking into the furnace is eliminated because the graphite
rods need no cooling.
[0011] For example, U.S. Pat. No. 4,431,612 by Bell et al.
describes a single step method and an apparatus for treatment of
solid, liquid and gaseous PCB's as well as other hazardous
materials by introducing them into a chamber and into contact with
a molten bath maintained in such chamber by a DC electric arc,
which maintains the temperature in excess of 1600.degree. C. The
obtained molten bath serves to promote the initial decomposition or
volatilization of PCB's and other hazardous materials, resulting in
a gaseous product that comprises CO, CO.sub.2, H.sub.2, CH.sub.4
and HCl.
[0012] However, Bell et al. do not try to produce fuel gas from
waste. Instead, their objective is to dissociate the waste into
simple molecules. This process of dissociation does not use oxygen
addition and is done in one step. Hence, this process and similar
processes will lead to the production of large amounts of carbon
soot.
[0013] The production of soot under these reducing conditions is
well known as was shown in U.S. Pat. No. 5,451,738 by Alvi et al.
that identified this problem and tried to alleviate it by catching
the carbon black (soot) in a cyclonic scrubber. Similarly, in U.S.
Pat. No. 5,534,659 of Springer et al. the problem of soot formation
is recognized and oxidant injection is used to convert the soot to
carbon monoxide.
SUMMARY OF THE INVENTION
[0014] In order to recover energy from waste in a clean and
efficient way, a technology different than incineration is
proposed. In a gasification system using plasma, waste is converted
to a fuel gas consisting mainly of carbon monoxide and hydrogen, by
heating up the waste in an oxygen-starved atmosphere. The gas
produced is then cleaned of contaminants such as soot, before it
can be used as fuel to produce electricity or steam.
[0015] In a gasification system, most of the energy from the waste
is stored in the form of chemical energy instead of sensible (or
thermal) energy as is the case in an incineration system. The
amount of gas produced by a gasification system is typically four
to five times less than the gas produced in an incineration system.
This gives the possibility of quenching the gas from the
gasification temperatures (800 to 1100.degree. C. depending on
system) down to below saturation using water quenching. This
approach eliminates the problem of dioxin formation, which occurs
in the 250 to 400.degree. C. range.
[0016] The objective of the present invention is to convert
essentially all the waste to fuel gas. For this purpose, in
addition to a primary gasifier, where initial conversion of waste
into fuel gas takes place, there is a need for a second stage
gasifier to convert the carbon soot in the gas to gaseous carbon
monoxide; this second stage includes the addition of metered
amounts of oxygen into the gasifier.
[0017] The energy efficiency is higher when air is added to gasify
the waste, namely by reacting the waste with oxygen, rather than
simply dissociating the waste into simple molecules. The chemical
energy of the products of dissociation is typically much higher
than the chemical energy of the waste being treated. This means
that significant amounts of electrical energy must be used for
dissociation. In the present invention, by adding carefully metered
amounts of oxygen or air and/or steam to the process, it is
possible to limit the amount of electrical (or plasma) energy
required for dissociation. In fact, the amount of oxygen fed can be
increased so that partial combustion of the waste occurs and the
plasma requirements are much reduced. Electrical energy is an
expensive form of energy and it is important to use it as
efficiently as possible.
[0018] By contrast to known plasma waste treatment systems, in the
present invention the plasma energy serves mainly two purposes: 1)
to vitrify (or melt) the inorganic portion of waste in the primary
gasifier while partially gasifying the organic components, and 2)
to provide the activation energy to complete the gasification
reactions in the secondary gasifier.
[0019] In essence, therefore, the present invention provides a
two-stage plasma process for converting waste having organic and
inorganic components into fuel gas, which comprises:
[0020] (a) in the first stage, vitrifying or melting the inorganic
components of the waste and partially gasifying the organic
components; and
[0021] (b) in the second stage, completing the gasification of the
organic components so as to convert them into fuel gas.
[0022] Moreover, a dust separation and removal step is normally
provided between the two stages of the process.
[0023] Furthermore, the fuel gas produced in the second stage is
usually quenched and cleaned to make it suitable for use in a gas
engine or turbine for production of electricity or in a gas burner
for production of steam or in chemical synthesis reactions.
[0024] Generally, the first stage is carried out in a plasma arc
furnace, and the second stage is carried out in a secondary
gasifier using a plasma torch with addition of metered amounts of
oxygen. The plasma arc furnace is preferably a refractory lined,
enclosed furnace provided with at least one direct current graphite
electrode adapted to generate a plasma arc to a bath of liquid
inorganic material originating from the waste itself and located at
the bottom of the furnace. This liquid inorganic material comprises
a slag layer which is maintained at a temperature of at least
1500.degree. C., usually a temperature between 1500.degree. C. and
1650.degree. C., and a metal layer also maintained at such
temperature of at least 1500.degree. C. and is located under the
slag layer.
[0025] The waste is introduced into the furnace on top of the
liquid inorganic material and the organic component in the waste
reacts with air, oxygen and/or steam supplied to the furnace in a
predetermined amount adapted to achieve gasification of the
organics in the waste into a primary synthesis gas containing CO,
H.sub.2, CO.sub.2 and N.sub.2 if the waste contains nitrogen or if
air is added to the furnace, and also containing some soot, fly ash
and complex organic molecules. The organic material in the waste is
preferably reacted in the furnace so as to form a layer of
partially treated waste on top of the slag layer and fresh waste is
introduced into the furnace on top of said partially treated waste
layer which is maintained at a temperature of between 700 and
800.degree. C. and constitutes a cold top for the fresh waste added
to the furnace. The primary synthesis gas exiting from the furnace
is subjected to dust separation and removal in which dust particles
larger than a predetermined size are separated and removed. These
dust particles are then normally recycled to the furnace, while the
remainder of the gas is fed to the secondary gasifier.
[0026] The secondary gasifier is preferably equipped with a plasma
torch fired eductor which ensures that the gas from the first stage
of the process entering the secondary gasifier is exposed to a high
temperature such as to transform essentially all soot present in
the gas to CO and to convert essentially all complex organic
molecules to simpler molecules CO, CO.sub.2 and H.sub.2. This high
temperature to which the gas from the first stage is exposed in the
secondary gasifier is usually between 900.degree. C. and
1300.degree. C., preferably around 1100.degree. C., and it is
achieved mainly by partial oxidation of the gas from the first
stage by injection of predetermined amounts of air, oxygen and/or
steam to the eductor, while the plasma torch provides only a small
fraction of the energy required for maintaining said high
temperature.
[0027] The fuel gas exiting the secondary gasifier is normally
cooled down very rapidly to a temperature below 100.degree. C. so
as to freeze the thermodynamic equilibrium of the gas and avoid
production of secondary pollutants, and after such cooling, the
fuel gas may be subjected to a final cleaning operation to remove
any remaining contaminants.
[0028] The entire process is preferably carried out under a
negative pressure to preclude exit of toxic fumes or of flammable
materials from any unit operations. Also, an oxygen starved
environment is used in the process to preclude dioxin
formation.
[0029] The present invention also provides for an apparatus for
converting waste having organic and inorganic components into fuel
gas, which normally includes:
[0030] (a) a primary gasifier comprising a refractory lined,
enclosed plasma arc furnace provided with at least one graphite
electrode; at least one inlet for feeding waste into the furnace;
means for feeding air, oxygen and/or a stem in metered amounts into
the furnace; and a gas take off port for primary synthesis gas
produced in said primary gasifier; said primary gasifier being
adapted to maintain layers of molten metal and molten slag at the
bottom of the furnace and on top of the molten slag a layer of
partially treated waste over which fresh waste is fed; and said at
least one graphite electrode is positioned so as to generate a
plasma arc to the molten slag present in the furnace during the
operation; and
[0031] (b) a secondary gasifier to which the primary synthesis gas
is fed, said secondary gasifier being equipped with a plasma-torch
fired eductor which ensures that the primary synthesis gas entering
from the primary gasifier is exposed to a high temperature such as
to transform any soot present in said primary gas into CO and to
convert any complex organic molecules to simpler molecules CO,
CO.sub.2 and H.sub.2; means for supplying metered amounts of air,
oxygen and/or steam into the eductor; said eductor leading to an
insulated chamber with a minimal heat loss; and an outlet being
provided in said chamber for the fuel gas resulting from the
operation.
[0032] Preferably, the primary gasifier has two graphite
electrodes, one of which creates an arc between one electrode and
the slag during the operation, and a second arc is created from the
slag to the second electrode.
[0033] The eductor provided in the secondary gasifier is preferably
made of a high heat metal alloy or is refractory lined or water
cooled, and is equipped with a plasma torch at its inlet.
[0034] The apparatus may further comprise a dust separator, such as
a hot cyclone, between the primary gasifier and the secondary
gasifier, and a gas quenching and gas cleaning means following the
secondary gasifier. It may also be equipped with an induced draft
fan adapted to operate the apparatus under a negative pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] A preferred, non-limitative embodiment of the invention will
now be described with reference to the accompanying drawings, in
which:
[0036] FIG. 1 is a diagrammatic representation of a preferred
embodiment of the present invention;
[0037] FIG. 2 is an elevation section view of a preferred
embodiment of the primary gasifier used within the process and
apparatus of the present invention; and
[0038] FIG. 3 is an elevation section view of a preferred
embodiment of the secondary gasifier used within the process and
apparatus of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0039] The process of the present invention can be used to process
various types of industrial, hazardous or domestic waste in the
form of liquids or solids. The solid wastes can be hospital waste,
mixed plastics waste, municipal solid waste, automobile shredder
residue or the like. The liquid wastes can be spent solvents, used
oils, petroleum sludge, municipal water treatment sludge, de-inking
sludge or similar liquids. Normally, the waste will comprise
organic and inorganic constituents and in most cases, it will be
rich in organic materials. When the waste comprises a combination
of solids and liquids, the liquid portion should normally not
exceed about 30% by weight of the total.
[0040] As shown in FIG. 1, the waste is first introduced into a
primary gasifier (12) which is a plasma furnace. This plasma
furnace is normally a refractory lined, enclosed, graphite arc
furnace, where the plasma is generated by one or several direct
current electrodes forming an electric arc, generally as shown in
FIG. 2. The plasma is generated by the electricity 16 flowing
through graphite rods to a bath of liquid inorganic material,
usually slag originating from the waste itself. This slag is
maintained at a temperature of 1500.degree. C. or more. Any metal
(i.e. non oxidized inorganic material) present in the waste forms a
distinct layer below the slag layer. This metal layer is also
maintained at high temperature of 1500.degree. C. or more. When
starting the system, the slag can be formed from a previous run or
be a common inorganic material such as sand or clay.
[0041] The organic material present in the waste reacts with
primary air, oxygen and/or steam 14 that is added to the furnace
using lances. This process is called gasification. The net result
of the gasification process is the production of a combustible gas
called primary synthesis gas 18, containing CO, H.sub.2, CO.sub.2
and N.sub.2 if the waste contains nitrogen or when the gasifier is
fed with air, since air contains 21% O.sub.2 and 79% N.sub.2 by
volume. The primary synthesis gas also contains soot and some
complex organic molecules.
[0042] Gasification occurs as the results of a series of complex
chemical reactions that can be simplified as follows; [0043]
C+O.sub.2->CO.sub.2 (exothermic) [0044]
C+H.sub.2O->CO+H.sub.2 (endothermic) [0045] C+CO.sub.2->2 CO
(endothermic) [0046] CO+H.sub.2O->CO.sub.2+H.sub.2
(exothermic)
[0047] Some of the reactions are endothermic and some reactions are
exothermic. The amount of oxygen, air and/or steam fed to the
gasifier can be adjusted to balance the exothermic and endothermic
reactions so as to minimize the amount of electric energy required
in the furnace. Contrary to dissociation, gasification with metered
amounts of oxygen, air and/or steam requires minimal amounts of
electrical energy to produce the synthesis gas.
[0048] The slag in the primary gasifier 12 is covered with
untreated and partially treated waste, also called a cold top. This
cold top serves two purposes. First, since the slag is covered with
the relatively cold partially treated waste, the furnace roof and
spool are not exposed to the high radiative heat from the slag,
reducing heat losses in the furnace and increasing refractory life.
Second, the cold top favours the condensation of heavy metals onto
the partially treated waste and their subsequent fusion into the
slag. The slag 20 is periodically removed from the primary gasifier
when required.
[0049] However, due to its relatively cold temperatures (700 to
800.degree. C.), the cold top favours the production of complex
organic molecules and soot (carbon) in the primary gasifier 12.
[0050] In order to trap the large soot particles, a dust separator
22 is installed at the gas outlet of the primary gasifier 12. Dust
24 that is removed by the dust separator 22 is normally returned to
the primary gasifier 12 for further processing.
[0051] The gas exits the dust separator 22, cleaned of large
particulates (generally larger than 10 microns). However, it still
contains fine soot particulates and complex organic molecules. A
secondary gasifier 26 is used to convert the soot and complex
organic molecules to CO, H.sub.2 and CO.sub.2. The secondary
gasifier 26 operates using electricity 28 in the form of a plasma
torch at a higher temperature than the cold top, namely between 900
and 1300.degree. C. and preferably around 1100.degree. C. At this
elevated temperature, the thermodynamic equilibrium between C, CO,
CO.sub.2, H.sub.2 and H.sub.2O, favours the formation of CO rather
than the formation of C (or soot). Also, at this high temperature,
complex organic molecules are converted to simpler molecules CO,
CO.sub.2 and H.sub.2. Complex organic molecules such as products of
incomplete combustion (PIC) are well known pollutants and could be
difficult to burn at lower temperatures. The secondary gasifier 26
ensures that they are converted to the inoffensive CO and H.sub.2
form.
[0052] The secondary gasifier 26 is equipped with a plasma-torch
fired eductor as shown in FIG. 3. This eductor ensures that all the
gas entering the secondary gasifier 26 is exposed to the high heat
and the high intensity radiation of the plasma flame. This ensures
essentially complete conversion of all or substantially all the
components of the synthesis gas entering the secondary gasifier 26
into simple gaseous molecules of CO, CO.sub.2, H.sub.2 and
H.sub.2O.
[0053] Two measures are taken in order to ensure high energy
efficiency of the secondary gasifier 26. First, the plasma torch 28
provides the activation energy for the conversion reactions, while
small metered amount of secondary oxygen, air and/or steam 30 is
added, so that the energy required to increase the gas temperature
from 800 to 1100.degree. C. is provided mainly by the partial
oxidation of the primary synthesis gas 18. Second, the secondary
gasifier 26 chamber is insulated with a material such as ceramic
wool, in order to ensure minimal heat loss from the chamber.
[0054] The synthesis gas 32 exiting the secondary gasifier 26 is
then cooled by cooling water using a water quench 34. In the water
quench, the gas is cooled very rapidly, in a few milliseconds, from
1100.degree. C. to below100.degree. C. This rapid cooling allows to
freeze the thermodynamic equilibrium of the gas and, hence, to
avoid the production of secondary pollutants such as dioxins and
furans. Dioxins and furans are mainly formed from the recombination
of chlorine and carbonated compounds (such as CO and CO.sub.2) in
the gas. By cooling the gas quickly, this recombination does not
have time to occur. The gas is then subjected to gas cleaning 36
which may be a series of known unit operations that will remove
remaining contaminants from the gas such as: fine dust, heavy
metals, acid gases (hydrogen chloride and hydrogen sulphide),
etc.
[0055] The whole system is kept under a negative pressure by the
use of an induced draft fan 38. This ensures that no toxic fumes
can exit the system and that the flammable H.sub.2 and CO stay
inside the system, limiting the dangers of fires or explosions. The
fan can be of turbine or positive displacement type, depending on
gas composition. Gas composition will be a function of operating
conditions and type of waste being processed.
[0056] The output of the system is clean combustible fuel gas,
which can be used for different applications. First, it can be
burned in a gas engine or gas turbine 40 for the production of
electricity. In that case, cogeneration is also possible: the waste
heat from the engine or turbine can be used to produce steam and/or
hot water. Depending on system size and waste type, the electricity
produced by the engine or turbine may be enough to run the plasma
arcs of the primary gasifier 12 and/or the plasma torch of the
secondary gasifier 26. The gas can also be used as a source of heat
for a boiler 42. In that case, the gas is burned in a standard
burner, just as any other commercial gas such as natural gas or
liquid petroleum gas (LPG). It can also be used for chemical
synthesis 44 as a reaction gas. In all these cases, since the fuel
gas has been cleaned essentially of all contaminants, the emissions
from the burning or processing of this gas will also be clean of
any contaminants.
[0057] FIG. 2 illustrates the preferred embodiment of the primary
gasifier 12. The solid and liquid wastes are introduced into the
primary gasifier 12 as a waste mixture through an isolation valve
46 and into one or multiple feed chutes 48. Alternatively, liquid
waste may be fed trough an injection nozzle 50 into partially
treated waste 52 inside the furnace. By feeding the liquid waste
into relatively cold zones of partially treated waste 52, one
ensures that the gasification reactions of the liquid waste are
progressive, rather than violent and sudden, which would occur if
liquid waste were fed directly on top of the hot slag 20.
[0058] The waste is laid over a pool of slag 20 and molten metal
21. The slag and metal are maintained in a liquid state at a
temperature of 1500.degree. C. or more by the use of plasma arcs 54
and resistive heating (not shown). The plasma arcs 54 are generated
by one or more graphite electrodes 56 that carry DC electric
current. Current typically flows from one electrode to the other
when more than one electrode 56 is used, creating an arc between
one electrode tip 57 and the slag 20, then passing through the
highly electrically conductive hot slag 20 and molten metal 21 and
creating a second arc from the slag 20 to the second electrode tip
57. The electrodes are typically submerged in waste 52, and the
plasma arcs 54 are typically covered by waste 52. This favours the
passage of current inside the hot slag 20 and molten metal 21,
rather than through gas, directly from one electrode to the other.
The slag 20 is covered with partially treated waste 52 also
referred to as a cold top. Fresh waste 51 is continuously or
intermittently added as the gasification reactions in the furnace
reduce the volume of waste 52 present.
[0059] Waste 52 is heated by plasma arcs 54, which favour the
conversion of the organic components of the waste into CO and
H.sub.2. This process is referred to as the gasification reactions.
Air, oxygen and/or steam are added through a lance 58, in order to
favour the gasification reactions in the highest temperature zones
of the primary gasifier 12.
[0060] The inorganic components of the waste melt and form two
distinct layers: a bottom layer of the denser metal 21 and a top
layer of the lighter slag 20. Once cooled, this slag 20 becomes a
glassy rock, which can be used for construction or other purposes.
The rock is non-leaching in nature and allows to trap heavy metals
and other contaminants into a glass matrix. Slag 20 and metal 21
can be extracted separately from the furnace through two distinct
tap holes 60 and 62.
[0061] In the primary gasifier 12, the organic molecules in the
waste react with sub-stoichiometric amounts of oxygen, air and/or
steam (i.e. less than the oxygen required for complete oxidation of
the waste) to form the primary synthesis gas 18. Steam used in the
primary gasifier can come from water already present in the waste
or be added separately.
[0062] The primary synthesis gas 18 is normally composed of
combustible CO, H.sub.2 and of non-combustible CO.sub.2 and
N.sub.2. Since the slag is covered by partially treated waste or
cold top 52, the gases exit the primary gasifier at a relatively
low temperature (800.degree. C.). Because of the relatively low
temperatures involved in cold top operation, the primary synthesis
gas 18 also contains soot and complex organic molecules (such as
ethylene, acetylene and aromatic compounds).
[0063] The advantage of cold top operation is higher energy
efficiency for two reasons: 1) the furnace spool 64 (top section)
is kept at a low temperature and 2) the primary synthesis gas 18
exiting the furnace has a lower temperature.
[0064] By keeping the spool 64 cold, the radiative heat losses to
the roof are much reduced. The radiative heat losses are a function
of temperature to the 4.sup.th power
(q=.epsilon..sigma.(T.sub.1.sup.4-T.sub.Surr.sup.4)). In
consequence, the effect of covering the slag by partially treated
waste and reducing its temperature from 1500.degree. C. to
800.degree. C. produces a reduction in radiative heat loss of about
10 times.
[0065] Reducing the temperature of the primary synthesis gas 18
also reduces the sensible heat of the gas exiting the furnace and,
therefore, the sensible heat carried out of the furnace.
[0066] Another advantage of the cold top operation is to limit
entrainment of particulates. Because the fresh waste 51 falls on a
relatively cold surface of the waste 52 being processed, the
gasification reactions are less violent and happen in stages as the
waste progresses down from cold top temperature to reaction
temperature of 1500.degree. C. at the slag 20 surface.
[0067] A still further advantage of cold top operation is to
minimize the volatilization of metals, volatilized metals at the
high slag temperature condense on the cold waste particles and have
a better chance of being trapped in the slag.
[0068] Due to the lower temperatures on the top of the reactor,
some waste will exit the reactor unreacted or partially reacted.
For example, some oil waste will vaporize before being completely
dissociated into CO and H.sub.2. The thermodynamic equilibrium
under the reducing conditions of the furnace favour the production
of carbon soot at the relatively low temperature at the outlet of
the furnace (800.degree. C.). A secondary gasifier 26 working at
around 1100.degree. C. is used to convert any remaining complex
organics in the primary syngas to CO and H.sub.2. It is shown in
FIG. 3 of the drawings. The carbon soot is converted to CO by the
addition of oxygen, air and/or steam to the secondary gasifier. At
1100.degree. C., thermodynamic equilibrium, under reducing
conditions, favours the production of CO, rather than soot (C).
[0069] The use of the secondary gasifier 26 also gives the option
of controlling the chemistry of the fuel gas or secondary synthesis
gas 32 produced by the system, without affecting the operation of
the primary gasifier 12 (dust entrainment, electrode erosion, slag
volatilisation). For example, adding steam into the secondary
gasifier 26 will tend to increase the amount of hydrogen present in
the secondary synthesis gas 32, while reducing the amount of carbon
soot and carbon monoxide.
[0070] The secondary gasifier 26 includes a high temperature
chamber 66, equipped with a gas mixer or eductor 68 at the chamber
inlet. The inside walls of the eductor 68 can have different
construction: refractory-lined, water-cooled, or high heat metal
alloy. The eductor is equipped with a plasma torch 70 at the inlet.
The eductor 68 provides a suction effect on the primary synthesis
gas and favours intimate contact of the soot particles and complex
organic molecules with the plasma flame in the eductor throat 69.
The high temperature chamber is insulated with insulation 67 in
order to ensure minimal heat loss from the chamber.
[0071] The present invention is not limited to the specific
embodiments described above, but may comprise various modifications
obvious to those skilled in the art without departing from the
invention and the scope of the following claims.
* * * * *