Note: Descriptions are shown in the official language in which they were submitted.
CA 02279821 1999-11-02
Process and Device for Gasification of Waste
,SUMMARY OF INVENTION
It may be that in the future, waste with organic
admixtures will no longer be disposed of in landfills.
Therefore, to an increasing extent, waste is being
subjected to thermal disposal in the form of burning
(refuse incineration) or gasification.
For refuse incineration there are technically
sophisticated processes which, with maximum efficiency
for the generation of thermal and electrical energy,
produce environmentally neutral by-products. This
requires incineration parameters that ensure the
production of slags that are highly resistant to the
having the heavy metals contained therein being leached
out by water. It is also necessary to ensure that dust,
nitrogen oxides, and dioxins/furanes are extensively
scrubbed out of the flue gases. Filter dust and
processing water that are produced also need to be
converted into environmentally neutral products at a
reasonable cost. As a result, the technical costs
involved in the environmentally neutral incineration of
waste products are so high that only units with large
throughput rates are able to work economically on waste.
Large throughput rates in turn require a large area
from which to draw raw material to supply the requisite
amount of waste. This causes the cost of shipping the
waste from the point of origin to the incineration
facility to become a non-negligible part of the overall
cosh .
As an alternative to incineration, waste can also be
gasified with oxygen. Compared to incineration,
gasification has a number of advantages:
~~ j': u.=1:° 1.:1C'_I12?"~t'_O:':, C=.Sl'~lC2t~ 0:1 O~°?'dtF_'S
Wi.t~'1
an oxygen deficit. The main components in the
1
CA 02279821 1999-11-02
gasification flue gas are therefore Hz, CO, and CH4.
Thus, the gasification flue gas can be used as a fuel
gas. Sulfur turns into HzS, which is comparatively easier
to remove from the combustion gas than removing SOZ from
the flue gas of incineration. Also, no NOX is produced.
b) As a rule, gasification takes place at a higher
temperature than incineration. This ensures that organic
oolltitants are more efficiently destroyed, the
dioxin-furane problem is reliably solved, and heavy
metals can be bonded into the slag to form compounds that
cannot be eluted.
c) The amount of fuel gas used in incineration
processes, relative to a standard state, is approximately
one-tenth of the volume amount of flue gas generated by
i5 incineration. When gasification is carried out under
pressure, the volume flow of the fuel gas amounts to even
less than one percent of the volume flow of flue gas.
This means that the equipment required to scrub the gas
is kept relatively small, especially in comparison to
incineration processes.
In making a cost comparison between incineration and
gasification, the oxygen costs for gasification are a
drawback.
Technically, gasification is performed in a
fixed-bed pressure gasifier. This gasifier is combined
with a slag reactor and is characterized by a relatively
low oxygen demand. See, e.g., Thome-Kozmiensky:
Reaktoren zur thermischen Abfallbehandlung, EF-Verlag fur
Energie and Umwelttechnik GmbH, Berlin, 1993. Such a
gasifier has the disadvantage, however, that large pieces
of coal have to be added in order to create a support
structure for the waste. In addition, the
thermodynamically inherently favorable counter-current
type of flow of waste and gasification gas builds up a
pyrolysis zone in the gasifier shaft, so that the flue
gas discharged from the fixed bed gasifier contains
2
CA 02279821 1999-11-02
typical admixtures of a pyrolysis gas (pyrolysis oils,
tars) that require expensive gas scrubbing.
The gasification of waste in an entrained bed is
known as the Noell-KRC process. See, e.g., Gorz, J..
Anforderungen an Einsatzstoffe and Prozei3parameter bei
der Nutzung der Flugstromvergasung fur die Vergasung von
Abfallstoffen, [Requirements for Feedstocks and Process
Parameters in the Use of Flystream Gasification for
Gasification of Wastes). VDI-Seminar 43-40-O1, Freiberg,
20.-21.03.1997. Here, the scrubbing of the gas is
comparatively simple because, except for methane, the gas
does not contain any hydrocarbons. Entrained-bed
gasification requires, however, that the waste be ground
up to a grain size of <0.5 mm.
In the :~;oel? -YRC process, to act~:al a~trai~ed-bed
gasifier is preceded by a pyrolysis drum. In this
pyrolysis drum, waste that is only coarsely ground is
converted into a pyrolysis gas, as well as an easily
grindable pyrolysis coke. The pyrolysis gas and ground
pyrolysis coke are then further decomposed in the
entrained-bed gasifier. This upstream pyrolysis stage,
the subsequent compression of the pyrolysis gas to the
pressure of the entrained-bed gasifier, and the equipment
required for cooling, grinding, intermediate storage, and
proportioning of the pyrolysis coke are highly
cost-intensive. An aavantage of slag bath gasifiers is
that such upstream pyrolysis drums are not necessary. .
In the Thermoselect process, a pyrolysis stage also
precedes gasification. See, e.g., G. Haf~ler:
THERMOSELECT-Der neue Weg, Restmull umweltgerecht zu
behandeln, Verlag Karl Goerner, Karlsruhe, 1995. In this
case, the costs for preparing the waste for gasification
are very low because the waste is pressea into the
horizontal pyrolysis pit without any special
pre-treatment.
To be sure, however, the gasification process can be
run only at normal pressure because tua pyrolysis pit
3
CA 02279821 1999-11-02
does not ensure reliable sealing of the gas space. This
makes the equipment required for gas scrubbing
comparatively large and expensive.
In addition, the pyrolysis process in the pyrolysis
pit is very incomplete, so that waste with sometimes very
large dimensions drops into the gasification chamber
uncontrolled and floats there on the slag. This makes
the ox~eration of the gasifier very irregular, leading to
large variations in the amount and composition of the
gasification flue gas and/or a highly variable oxygen
demand. These large variations in the gasification flue
gas make it difficult to use the gas. It is also
difficult to compensate for the fluctuating oxygen
demand.
An object of the i~e~ ~i~n is, ,..._r~~io~e, to provide
a process and a device for the gasification of waste that
allow economical operation even at relatively low
throughput rates.
Upon further study of the specification and appended
claims, further objects and advantages of this invention
will become apparent to those skilled in the art.
Or. the process side, these objects are accomplished
according to the invention by virtue of the fact that
gasification takes place in a single stage in a gasifier
with a liquid, rotating slag bath.
This makes it possible to have smaller,
decentralized plants, thereby reducing the costs for
transporting waste.
The process according to the invention is
characterized by a single-stage gasification process by
which feed waste material is converted into a usable fuel
gas and a slag granulate that can be disposed of in a
landfill. Expensive pry-treatment of the feed material
is not required.
The feed material can be fed into the gasifier with
grain sizes of up to 40 mm, so that only coarse crushing
of the waste is required in advance. The mixture is
4
CA 02279821 1999-11-02
divided by screening, for example, into the following
fractions:
d = 0-5 mm
d = 5-40 mm
d > 40 mm.
The overflow from the screen is fed to a mill and then
recycled to the screening machine.
A magnetic separator can be installed upstream of
the gasifier to remove iron components.
10 The liquid slag bath, located in the gasification
zone, performs a number of functions. Mineral components
and heavy metals in the feed material are melted down and
adsorbed. At the same time the slag bath acts as a heat
buffer and reaction mediator and thus ensures an
15 intensive exchange of heat and material.
An important function is the reliable ignition and,
optionally, re-ignition of the burners, to provide the
desired reaction temperature.
According to a preferred embodiment of the
20 invention, excess slag is removed by a slag drain along
with the cracked gas that accumulates during
gasification. The slag drain protrudes above the slag
bath, and the slag flows down into it through a lateral
discharge opening.
25 The slag bath is preferably caused to rotate by
tangentially feeding in the gasification medium and/or at
least a portion of the waste. It is advantageous for at.
least a portion of the waste to be fed to the gasifier in
the form of pellets via at least one solid burner, with
30 recycled cracked gas as a carrier gas. In this case
waste with a diameter of up to 5 mm is fed into the
gasifier above the slag bath, and a jet of this waste is
formed and aimed at the surface of the slag bath, while
waste with a diameter of 5 to 40 mm is introduced
35 directly into the slag bath.
Preferably, at least one gas burner is used that is
supplied with oxygen as well as with natural gas during
5
CA 02279821 1999-11-02
start-up and with recycled cracked gas during operation.
In addition, it is advantageous for oxygen to be injected
via one or more oxygen lances directly into the slag
bath.
According to an enhancement of the concept of the
invention, sand, lime, and/or other materials are fed
into the slag bath in order to influence the melting
behavior and viscosity of the slag.
Slag that is removed from the slag bath is
preferably allowed to drip into a water bath and is
converted there into a vitreous state that is not subject
to elution.
When the gasifier is started, the slag bath is
preferably formed by a synthetic slag.
An apparatus for implementing the process has means
defining a gasification space, such as a covered reactor,
for gasification of the waste.
On the apparatus side, the objects of the invention
are accomplished by virtue of the fact that the
gasification space has fittings for forming a rotating
slag bath.
The reactor structure defining the gasification
space preferably has an essentially cylindrical shape
with a concentrically arranged slag drain that is lead
through the bottom of the reactor structure.
The rea.ctor jacket should be protected on the inside
by a cooling shield preferably made of finned-tube coils
which are welded together gas-tight and through which
cooling water is forced. On the product (slag bath)
side, the tubes are preferably provided with pins and
packed with a ceramic tamping mass. A layer of slag
cools solidly on this layer and forms a heat-insulating
"slag coat", which protects the cooling shield against
the high operating temperature and against direct attack
by the liquid slag. The thickness of the slag protection
layer depends on the operating conditions (temperature,
sla:.g cor.;posi t~_on) .
6
CA 02279821 1999-11-02
BRIEF DESCRIPTION OF THE DRAWINGS
Various other features and attendant advantages of
the present invention will be more fully appreciated as
the same becomes better understood when considered in
conjunction with the accompanying drawings, in which like
reference characters designate the same or similar parts
throughout the several views, and wherein:
Figure 1 illustrates a cross-section through a
gasifier with a rotating slag bath;
Figure 2 illustrates a longitudinal section through
the gasifier depicted in Figure 1;
Figure 3 illustrates a cross section through the
gasifier showing tangential arrangement of the burners;
and
Figure 4 illustrates the finned-tube coils with pins
and the slag bath.
In the figures the same parts are referred to by the
same reference numbers.
The gasifier comprises a gasification space 1, that
is formed by a reactor jacket 5 and a reactor cover 7.
Reactor jacket 5 is protected by a cooling shield that is
composed of finned-tube hoses which are welded together
gas-tight and through which cooling water is forced.
Since in the upward direction the gasifier is closed
by cover 7, cracked gas that accumulates during
gasyv'_c.~-~o~ :~s o::l y abi a to Tlo~a ou~ of t'~e a=s_=i er,
together with excess slag, through the slag drain formed.
by central tube 6.
The gas outlet in the lower part of the gasifier,
via central tube 6, ensures internal circulation of the
accumulated cracked gas. Swirling of the gas evens out
the dwell time and thus ensures a more complete
establishment of equilibrium. Slag droplets that are
entrained with the swirling cracked gas precipitate, for
the most part, on the gasifier wall and flow off the wall
into slag bath 2. .
7
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To feed the feed material and gasification medium
into gasification space 1, two types of burners, 8 and 9,
are used that are oriented obliquely downward, tangential
to the surface of the slag bath. For example, the
burners can be oriented at an angle of about 30°-60° with
respect to the horizontal. The linear momentum that is
transferred causes the slag to rotate, thus ensuring
thorough mixing of slag bath 2. See Figure 3.
During the start-up phase, natural gas is burned in
gas burners 8. During operation, recycled cracked gas
with oxygen (if necessary, with the addition of steam) is
burned in gas burners 8. Steam can be added to provide
adjustment of process temperature.
In solid burners 9, the fine-grain fraction (d < 5
mm) of the feed waste material is burned with oxygen, in
which process recycled cracked gas acts as a carrier gas.
Small particles are converted in the gas space above slag
bath 2 in an entrained-bed gasification process. Because
of the longer reaction time that is required, larger
particles have time to hit the slag and sink into it.
The coarse-grain fraction (d = 5-40 mm) of the feed
material is fed directly into slag bath 2 by means of a
proportioning screw via a radially arranged nozzle 10.
The intensive transfer of heat and material ensures
the reliable gasification of the organic components,
rl.~.~.i.~i..'~. t:.iC ~i~.~.=.c:- CO,urC:°_T?t8 c:=c ti:'_,.=ca
COY:_'1 a:':G ai~SCi.'Dc''CI
by the slag.
Only a portion of the required oxygen is fed with
the burners. The rest passes through tangentially
arranged oxygen lances 11 directly into slag bath 2; this
offers a number of advantages.
The direct injection ensures intensive and thorough
mixing of the slag bath since, on the one hand, the
linear momentum is transferred better and, on the other
hand, the rising oxygen bubbles provide additional
turbulence.
8
CA 02279821 1999-11-02
In addition, the oxygen makes it possible to gasify
in the slag bath the organic components that are
introduced into the slag, thereby, on the one hand,
accelerating the gasification reaction and, on the other,
reducing the number of crystal nuclei, which increase the
viscosity of the slag.
When the device is started up, the slag bath should
be formed by a synthetic slag (Ca0 + SiOz + A1203) . To
this are added lime and sand at a lime:sand ratio of
approximately 0.8 to approximately 1.2, as well as a
smaller proportion of A1203 (approximately 10 wt% based on
the total weight), whereby the latter are dumped into the
reactor. During start-up, the mixture is melted down by
the combustion of the natural gas is injected into the
burners and is thereby brought up to operating
temperature.
While the gasifier is in operation, the slag bath is
constantly replenished with mineral components that are
brought in with the waste.
The properties of the slag (melting point,
viscosity) are determined by its composition. Main
components of the slag are CaO, Si02, and A1203. Other
slag components are metals and their oxides that are
brought in with the waste. Together with the slag
components, they form eutectics whose melting points lie
considerably below those of the individual components
(see Pawlek; Metallhuttenkunde [Metal Foundry Practice],~
Walter de Gruyter (1983)).
An important parameter of the operation of the slag
bath gasifier is the viscosity of the slag. Silicic acid
is formed by Si0° tetrahedra whose centers'contain a Si
atom surrounded by four O atoms. Because of common
oxygen atoms, these tetrahedra form space lattices which
also remain in the liquid state as coherent complexes.
The limited mobility of these large structures ensures
high viscosity. The A13' cations can substitute the Si°+
CatlOnS ciIlG i0~~ iil~q tt''.t=c:~'1~::1'i:~:.. 1.~.'_S, ~i~~g haS ci:'1
9
CA 02279821 1999-11-02
effect on the viscosity of slag that is similar to that
of SiOz . SiOz and A1203 are so-called network co-formers
(see Kozakevitch, Urgain; Viscositat and Gefiige von
fliissigen Schlacken [Viscosity and Structure of Liquid
Slags], Metz 1954).
So-called network co-formers, such as Ca0 and MgO,
are capable of breaking the tetrahedral bonds of the
oxygen atoms and thus cause a reduction in the viscosity
of the slag.
In the range of Ca0/SiOz = 0.8-1.2, the Ca0-Si02
system is sufficiently liquid at temperatures above
1450°C. Via a radially arranged nozzle 12 above slag bath
2, materials such as sand and/or lime can be added to the
slag, so that the melting and viscosity behavior of the
slag can be influenced within certain limits.
To the same extent as slag-forming components are
fed to slag bath 2, excess slag flows off via slag drain
6. According to the invention, the drain tube protrudes
above slag bath 2 and has a drain opening at the desired
height. Compared to a design where the slag overflow has
a drip edge, this design according to the invention
creates a concentrated, thicker jet of slag, thereby
keeping strands from forming. Slag drain 6 is built as
the crucible structure composed of finned-tube coils that
are welded together and cooled with pressurized water.
These tubes are provided with pins on both sides and are
packed with a ceramic tamping mass. A layer of slag
cools solidly on this layer and protects the material
aaainst the high operating temperature and against direct
attack by the chemically aggressive slag. See Figure 4.
Because slag and hot cracked gas are tapped off
together, the slag is kept liquid by the high
temperatures of the gas.
The rest of the removal process takes place via the
post-reaction space 13. This space may be designed as,
e.g., a pot furnace or, as shown in Figure 1, as a
cyclonic zur :ace. T:_ ti:e lat''_ ~i-:e slGr i5 rei~-ned, so
CA 02279821 1999-11-02
that any foaming that may occur will not impede removal.
In the event that temperatures in cyclonic furnace 3 are
not sufficient to allow the slag to flow freely, a burner
14, shown in Figure 2, that is operated with recycled
cracked gas and oxygen may be installed.
Before the product gas leaves the gasifier,
carbon-containing particles that are not converted in the
gasification zone may undergo further conversion in the
post-gasification zone.
In the case of the cyclonic-type design, slag
droplets and solid particles that are entrained with the
cracked gas are deposited on the walls, thus
significantly reducing the removal of entrained flue
dust.
As shown in Figure 2, a water bath 4 is attached to
the post-reaction space to granulate the slag. The slag
granulate cannot be eluted and may be disposed of in
landfills without restrictions.
If enough is spent on hardware, it is possible to
operate the gasifier according to the invention under
elevated pressure.
The gasifier according to the invention can be used
advantageously for a broad range of waste materials.
Below two sample applications are described in
greater detail.
The working temperature of the slag bath gasifier is
set at 1600°C. Waste gasification is carried out as an
autothermal process, in which case the amount of heat
that is required to cleave the waste as well as to melt
the mineral components is generated by partial oxidation
of the combustible components with oxygen.
In the foregoing and in the following examples, all
temperatures are set forth uncorrected in degrees
Celcius; and, unless otherwise indicated, all parts and
percentages are by weight.
The entire disclosure of all applications, patents
and publications, cit~cc~.':O-~c: cicQ ~~:~;:low, and of
11
CA 02279821 1999-11-02
corresponding German application no. 19735153.0 filed
August 13, 1997 is hereby incorporated by reference.
F~MPLES
Example 1: Refuse IncinPrarinn
Table 1 shows the composition of a standard refuse
according to the Land Environmental Department of North
Rhine-Westphalia.
The pre-treatment of the refuse is limited to the
coarse crushing of the feed material to a grain size of
less than 40 mm. Iron may also be removed by magnetic
separation.
The 0-5 mm grain fraction is fed into the
gasification space via the solid burners, while the 5-40
mm grain fraction is fed in via a nozzle by means of feed
screws.
Table 1: Composition of a Standard Refuse According to
the Land Environmental Department of North Rhine-
Westphalia.
Refuse Mass-% Ash Component kg/t of Refuse
Components
C 27.16 Si02 110
H 3 . 45 A1203 34
O 18.39 Ca0 31
N 0.3 Fe 30
25- S 0.5 Na20 15.205
Cl 0 . 5 Fe203 15
i :v-~~.s pure :? ~ r,c~ i ~ !
(Hz0)
Ash 25 A1 4
K20 3
As Table 3 indicates, 357m3i.N. [i.N. - in normal
state] of oxygen (96 vol-% of Oz) is used for the
autothermal gasification of 1.0 ton of refuse.
Owing to the moisture content of the feed material,
t':e cracked gas that is obtainec: ~as a lance ~ro::or tion
of vapor. In addition, the cracked gas has large CO and
12
CA 02279821 1999-11-02
Hz contents, so that sufficient energy reserves are
available to cover any higher heat losses that may occur.
The ash from the refuse has high contents of Si02 and
A1Z03, which make the slag highly viscous. If this causes
operating problems, the viscosity of the slag can be
reduced by feeding in lime via a nozzle.
E~s.~mole 2 : Ga ' f i c-afi i nn Of Old PVC''
An advantageous application of the slag bath
gasifier according to the invention is the gasification
of.waste PVC since, in addition to refuse disposal, the
HC1 contained in the PVC can be recovered and used as HC1
gas in oxychlorination and ultimately to produce PVC
again.
Table 2 presents the composition of a PVC-containing
waste mixture.
Table 2: Waste Mixture with a High PVC Content
Component Mass-%
Pure PVC 61
Softener 20
Chalk g,6
Combustible waste 6.4
Non-combustible waste ~ 4
In addition to screening with corresponding crushing
of the waste PVC to a grain size of d < 40 mm and the
magnetic separator for removing iron, an additional air
separator (zigzag separator) should be pro-.-=ded for the
pre-treatment process. In the latter separator, heavy
non-ferrous metals are separated which, in the slag bath',
are converted into metal chloride for the most part and
thus would reduce the HC1 yield. By contrast, the
silicates and light metals (A1, Mg) are desired slag
formers .
The option of using relatively coarse-grain feed
ma~~~:iai is especially economical in the case of ~'JC
13
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because this means that crushing in a cutting mill is
sufficient and it is no longer necessary to carry out
low-temperature grinding, which is cumbersome and very
cost-intensive.
The d = 0-5 mm grain fraction is fed into the
gasification space via the solid burners, while the d =
5-40 mm grain fraction is fed in via a nozzle by means of
feed screws.
Table 3 shows that an oxygen demand of 420 m3i.N. Per
ton of waste PVC is used for autothermal gasification of
waste PVC.
An almost 100% HC1 yield is obtained. The HC1 is
recovered from the cracked gas via subsequent absorption
and distillation and sent for further processing. The
formation of metal chlorides in the slag bath may reduce
the HC1 yield. Injecting oxygen directly into the slag
bath creates an oxygen surplus in the slag; this causes
the formation of metal chlorides by the slag components
to be suppressed or metal chlorides to be oxidized with
C12 cleavage, to the extent that the tendency of the
elements toward oxidation predominates over that towards
chlorination (compare free reaction enthalpy).
MeCl2 + ~ OZ ~ Me0 + ~ C12
The HCl-free cracked gas is rich in CO and HZ and can
be used to produce electrical energy and process steam.
The chalk that is contained in the old PVC is
cleaved into COZ and Ca0 in the slag bath, so that it may
be necessary t~c add sand to the slag.
14
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Table 3: Balancing for Cracked Gas of Waste Materials in
the Slag Bath Gasifier (T = 1600°C, Q" = 0) .
Waste Oxygen Amount Composition
of
Cracked
Gas
Demand of HC1
HZ
H20
CO
COZ
N2
(96 Cracked
vol-% Gas
of O)
3 /t 3 /t
m i.N m I.N VOl
.
-%
Re- 357 1230 0 13.7 42.6 23.6 17.5 1.3
fuse
(NRW) 422 1531 14. 27.1 1.4 55.2 0.7 1.1
Old 4
PVC
The two examples given with wastes of very different
compositions show that, from the standpoint of the energy
budget, gasification with oxygen in the slag bath can act
on a large variety of types of waste without major
problems.
The preceding examples can be repeated with similar
success by substituting the generically or specifically
described reactants and/or operating conditions of this
invention for those used in the preceding examples.
From the foregoing description, one skilled in the
art can easily ascertain the essential characteristics of
this invention, and without departing from the spirit and
scope thereof, can make various changes and modifications
of the invention to adapt it to various usages and
conditions.
