Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 1 1 335694
R~FINING OF RAW GAS
This invention relates to a process for the refining
of a raw gas produced from a carbonacQous material by means
of a gasification process in which the refing takes place in
a secondary stage separated from the gasifier of the gasifi-
cation process.
A raw gas produced from different kinds of biofuels
and used as a fuslgas is a valuable oil substitute for
demanding applications in which the process dsmands make
direct solid fuel fireing impossible, e.g. fireing of lime
kilns or conversion of existing oil fired boilers.
For other types of applications, 8 .g. so-called coge-
neration (of electrical power and heat) by ue of diesel en-
gines, very high demands on the gas purity concerning prima-
rily tars and dust are set. Moreover, environmental aspects
often lead to demands on low concentrations of compounds
which when combustsd form harmful emissions, ~uch as NOX, SOx
and various chlorinated compounds. The last mentioned is
valid especially for a gas produced from refuse derived fuel,
RDF. These demands on the gas purity can be satisfied by the
raw gas being refined by an appropriate method.
Gasification of RDF with subsequent refining of the
raw gas means an environmentally favourable method for energy
recovery from wastes by utilization of refined gas in exist-
ing boilers or for cogeneration in diesel engines and/or
boilers.
Besides, utilization of raw gas often is connected
with other technical problems.
At temperatures below 1200C tar is always present in
a raw gas produced by gasification of a carbonaceous mate-
rial, e.g. coal, peat, bark, wood or RDF, which limits the
utilization to combustion of hot gas in direct or close con-
nection to the gasifier. Operational disturbances caused by
tarcoating on apparatuses and armatures are a great problem
'S~
1 335694
which limits the availability. During combustion of hot gas,
nitrogen and in certain cases also sulphur ~e.g. from peat)
bound in tars, as well as ammonia, H2S (peat) or HCl (from
RDF), furthermore give rise to emissions which are harmful to
the environment (N0x, Sx and HCl, respectively, and chlori-
nated hydrocarbons, i.a. dioxines).
Despite extensive research concerning tar and ammonia
conversion, 80 far no process which in an industrial scale
can achieve sufficisntly far-reaching raw gas refining has
been developed. The traditional way of reducing tar contents
in a raw gas is by means of wet scrubbing, but aerosol for-
mation in the scrubber makes the tar removal inefficient.
Furthermore, a proce~s water wit~h high contQnts of organic
compounds and ammonia is obtained. Consequently, this water
in its turn must be cleaned before being discharged to a
sewerage. When gasifying RDF the process water also contains
high concentrations of dissolved hydrochloric acid and/or
ammonium chloride. When gasifying more sulphur rich fuels,
e.g. peat or coal, the raw gas also has to be purified to
remove hydrogen sulphide.
The object of the presented invention is to provide a
raw gas refining process, by means of which the above men-
tioned problems will be solved to a great extent.
This object is achieved by the process according to
the invention having the features defined in the enclosed
claims.
The invention thus concerns a process for the refining
of a tar and ammonia containing raw gas, in special cases
also containing considerable quantitiQs of hydrogen chloride,
the gas being produced by means of an arbitrary gasification
proc~ss from a carbonaceous material, e.g. bark, wood, psat
or Refuse Derived Fuel, RDF, wherein in a secondary stage
conversion takes place in contact with an appropriate activs
(catalytic and possibly absorbing) mat~rial, e.g. dolomite,
of the tar and ammonia presQnt in the raw gas, preferably to
1 33569~
such a level that the remaining contents are below 500 and
300 mg/Nm3 respectively. In special cases absorption of hy-
drogen chloride to almost thermodynamic equilibrium simul-
tanQously takes place. The secondary 8tage consists of a Cir-
culating Fast Fluidized Bed (CFB~ with a bed material at
least mainly in form of an active material, e.g. dolomite.
With this arrangement the secondary stage also could be in-
tegrated with an arbitrary CFB-gasifiQr, only preceded by a
primary particle separator, or another type of gasifier.
We have found that sufficient conversion of tars and
ammonia and in special cases simultaneous absorption of hy-
drogen chloride can be achiQvsd, by first separating the tar
containing gas from pyrolysing larger fuel particles in the
gasifying stage and then in a separate secondary stage in the
form of a circulating fast fluidized bed contacting the gas
with a suitable active material, such as dolomite, at
suitable proces~ paramsters.
If the carbonaceous material also contains sulphur in
considQrable amounts, which e.g. is the ca~e for peat,
absorption of hydrogen sulphide on the catalytic and absorb-
ing material will of course also take place.
The amount of active material which is required in
relation to the raw gas amount is determined by the required
space-velocity for catalytic conversion of tars and ammonia
and depends on several parameters such as ths temperature,
the residence time of the gas, the particle size of the ac-
tive material, the partial pressure of reactants and the de-
gree of deactivation of the active material. Too low tempera-
turs and/or C02 partial pressure can result in the tar con-
version causing carbon deposition on the active surface,
which results in deactivation. If this occurs the material
can be activated by treatment with an oxidizing gas, e.g. air
and~or steam. Absorption of HCl (and/or H2S) takes place ~o
rapidly at the temperatures of intersst that these reactions
become almost determined by the equilibrium and result in a
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consumption of active material corresponding to the formed
solid chloride Cand sulphide resp.).
We have thus found that absorption of chloride Cand in
certain casQs also of hydrogen sulphide) on an active mate-
rial such as dolomite is a rapid reaction and rQquires pre-
sence of a considerably less amount of active material in
relation to the gas flow than catalytic conversion of tars
and ammonia.
Utilization of a secondary stage in the form of a fast
circulating fluidized bed (CFB) means considerable advan-
tages.
Such a bed is able to handle dust entrained from the
gasifier, gives very uniform temperatures in the reaction
zone and also gives a homogeneous contact between gas and bed
material, that is to say little risk for variations in con-
version/absorption degree. Further, the particle size can be
varied downward~ to a great extent, for those cases in which
this is needed to give increased conversion at a given tempe-
rature and space-velocity. Considerable erosion of the bed
material also results in increased accessible active surface.
Also, a secondary stage designed as a CFB with advantage can
be integrated with an arbitrary CFB gasifier, which merely
has a primary particle separator, or another type of gasi-
fier. One also achieves relatively small diameters when
scaling up, since the gas velocities can be kept relatively
high, up to about 10 m/s, preferably up to 6 m/s.
In case the gasifier consists of a CFB gasifier, a
connection directly after primary dust separation can thus be
made. If an active material is used as a bed material in the
CFB gasifier, the secondary stage can in an advantageous
manner be integrated with the gasifier, e.g. 80 that dust
from a secondary particle separator after the secondary stage F
is totally or partly recycled to the gasifier. In this way,
the total losses of bed material also become lower, and one
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also obtains the advantage of using only one type of bed
material.
Ths necessary amount of active material in the reactor
shaft of the secondary stage for sufficient catalytic conver-
sion of tar and ammonia is controlled by the totally added
amound and by controlled recirculation of bed material. Re-
quired conversion determines suitable combination of tempera-
ture, particle ~ize and amount of active material. Because of
abrasion, deactivation and/or absorption of HCl (and possibly
H2S) consumed active material is replaced by adding corre-
sponding amount3 of fresh active material and/or activated
such material. The residence time of the gas can be controll-
ed by the combination diameter~hei~ht abovs the ~as inlet.
In those special cases, when HCl is present in the raw
gas in considerable amounts, the active material entrained by
the outlet gas from the secondary stage means that the HCl
absorption is improved, since thermodynamically it becomes
more far-reaching at lower temperatures, undsr the condition
that the refined gas is cooled down to an essentially lower
temperature before final dust removal.
In the following the invention will be described by
way of a non-limiting embodiment while referring to the en-
closed drawing, which schematically shows a gasification and
gas refining system which embodies the present invention.
In the system shown in the drawing carbonaceous mate-
rial 1 is conveyed to a gasifier 3, which consists of a cir-
culating fast fluidized bed (CFB). This comprises a reactor
51, a primary separator 52 and recirculation means 53 for bed
material separated in the primary separator. The bed material
consists of an active catalytic and absorbing material, pre-
ferably in the form of dolomite, mixed with ungasified car-
bonaceous material, char. The primary separator 52 is a
mechanical separator of non-centrifugal type, suitably a
U-beam separator, in accordance with what is described in our
~ 1 335694
European Patent EP 0 103 613, relating to a CFB boiler and
hereby referred to.
The hot raw gas 2 produced in the gasifier 3 is with-
drawn directly from the primary separator 52 and is fed di-
~ 5 rectly to a gas cleaning secondary stage 25 without any addi-
tional dust removal. The secondary stage 25 i8 designed as a
circulating fast fluidized bed ~CFB) 26 and has the same kind
of active bed material ~8 the gasifier 3.
The raw gas 2 is supplied to the secondary stage 25 8c
that it constitutes a fluidizing gas.
The secondary stage 25 is designed with a long and
narrow reactor shaft with arbitrary cross section (e.g. cir-
cular or square). Bed material which follows with the gas
stream out form tha top of ths reactor shaft is separated to
a major part in a primary particle separator 27, preferably a
U-beam separator of the same ~ind as the U-beam separator of
the gasifier, followed by a secondary separator 28, preferab-
ly a cyclone. The material 30 separated in the primary par-
ticle separator i8 recycled to the lower part of the circu-
lating bed 26 through a recirculation facility. ThG material
29 separated in the secondary particle separator 28 is added
mainly to the lower part of the gasifier 3, stream 31. When
needed, a part of the material stream 29 also can be supplied
to the lower part of the circulating bed 26, stream 34,
and/or be discharged out of the system, stream 43.
For feeding fresh catalytic and absorbing material 14
to the secondary stage 25 a side feeding device 15 located on
a suitable height is used. Consumed andfor deactivated mate-
rial 35 is discharged by means of a discharging device 36 3
located in connection with the bottom of the secondary stage
25.
The active material used in the secondary stage in
this example consists of a calcium-magnesium carbonate con-
taining material, preferably dolomite, with a particle size
smaller than 2 mm, preferably smaller than 1 mm, which in
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1 335694
combination with the passing gas forms the circulating
fluidized bed 26.
The ga~ velocity in the upper section of the reactor
shaft, calculated on the free cross section, is adjusted 80
that it is below 10 m/s, preferably not above 6 m/s.
The fluidizing gas of the fast circulating bed 26 con-
sist~ of the raw gas 2 and added oxidizing gas 13, e.g. air.
When needed additional oxidizing gas 33 can be added to the
secondary stage 25 on one or on several other suitable,
higher located levels.
Conversion of tar and ammonia contained in the raw gas
2 and absorption of chloride containsd in the raw gas take
place by means of contact with the catalytic and absorbing
material in the circulating bed 26 within a tsmperature in-
terval of 600-1000C, preferably 700-900C or most preferably
850-950C. The required temperature level is maintained by
burning combustibls gas components insidQ the sQcondary stagQ
25, which is controlled by adjustment of the amount of added
oxidizing gas, strQams 13 and 33.
The average suspension density in the reactor shaft of
the seconday stage 25 is maintained within an interval of
20-300 kg/m3, preferably within an interval of 80-250 kg/m3,
80 that a neces~ary contact between the passing gas and the
active material is obtained. This is achieved by adjusting
the total amount of circulating material in combination with
controlling the flow rate of recycled material 30 and 34.
The residence time of the gas in the reactor shaft,
calculated on an empty reactor shaft, is maintained within an
interval of 0.2-20 8, preferably within an interval of 0.5_7
8.
When needed, activation of deactivated catalytic and
absorbing material can be performed by adding oxidizing gas
32, e.g. air, to the material which is recycled to the lower
part of the circulating bed, streams 30 and 34. The amount of
added oxidizing gas 32 is controlled 80 that the activation
8 1 335694
takQs place within a tQmperature interval of 600-1000C, pre-
ferably within an interval of 750-900C.
Before starting operation of the process heating of
the secondary stage 25 including its bed material takes place
by means of combustion of LP gas 24 therein.
The refined gas strQam 4 leaving the secondary separa-
tor 28 of the secondary stage 25 is relieved from entrained
finely divided bed material and stsam in the subsequent gas
treatment stages.
The gas passes through two heat exchangers. In the
first heat exchanger 37 heat exchange takes place with oxi-
dizing gas, str~am 10, intended for both the ~Tasifier 3 and
the secondary stage 25, 80 that preheated oxidizing gas 11 at
the outlet from the heat exchanger 37 has a suitable tempera-
ture, preferably about 400C. The preheatQd oxidizing gas 11
is used both in the gasifier 3 (among others as fluidizing
gas~, stream 12, and in the sQcondary stagQ 25, streams 13,
32 and 33.
In the subsQquent second heat exchanger 38 the tempe-
rature of the gas 5 is lowered to a level which permits the
outlet gas 6 to be further cleaned by using e.g. standard
textile filters or a cyclone for further dust removal, at 39,
i.e. preferably down to 150-300C . The removQd dust 18 is
withdrawn from the dust removal stage 39.
As mQntioned bQfore, the gas stream 4 contains en-
trained finely divided active material which follows with the
gas stream out of the secondary separator 28. In spQcial
cases, e.g. in connection with gasification of RFD, the raw
gas 2 from the gasifiQr contains considQrable amounts of HCl.
Since absorption of HCl on calcareous materials, such as do-
lomite, is favoured by sinking temperature, thQ gas cooling
in the heat exchangers 37 and 38 contributss to increase the
degree of absorption of residual HCl on ths entrained mate-
rial.
1 335694
The almost dust-free gas 7, which leaves the dust rs-
moval stage 39, is fed to a scrubber 40, in which it is re-
lieved from moi~ture and other water soluble components. In
the scrubber 40 both moistening of the gas stream 7 and con-
densation of steam take place. At the current conditions also
precipitation of almost all of the residual fines and absorp-
tion of water soluble gas components, e.g. ~H3, HCl and/or
NH4Cl, take place.
The water stream 20 leaving~the scrubber 40 i8 recir-
culated by a pump 41, whereby it i8 cooled in a heat ex-
changer 42, 80 that the temperature of the water 19 recycled
to the scrubber 40 is kept within the interval 15-20C. Ex-
cess water 21 i8 drained from the water circuit.
The gas 8 leaving the scrubber can for industrial app-
lications be regarded as pure, i.e. it i8 almost free from
tars, ammonia, dust, HCl and H2S. However, at the present
outlet temperatures ~about 30C) it is uaturated with steam.
Depending on the application, in order to decrease the rela-
tive humidity, the gasstream 8 can be preheated or passed
through an additional drying stage in order to rsduce its
moisture content. The pure gas satiesfies the requirements
for engine operation, e.g. by means of turbocharged diesel
engines, and can be burned without any subsequent exhaust gas
cleaning.
For more simple applications, e.g. heat generation in
boilers, the scrubber 40 can be omitted, 80 that the refined
gas can be utilized either directly after the heat exchanger
37, stream 22, or after the dust separator 39, stream 23.
In the describQd example the secondary stage 25 has
been integrated with a gasifier 3 based on CFB technology.
The gasifier 3 can produce the raw gas 2 from several dif-
fsrent kinds of fusls, e.g. coarse bark, peat or refuse de-
rived fuels ~DF. As bed material in the circulating bed of
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the gasifier 3 it is, as mentioned, convenient to use a cata-
lytic and absorbing material of the same type as in the
secondary stage 25.
The total pressure drop of the oxidizing gas supplied,
e.g. air, at the passage through ths production loop, is
slightly above 1 bar. This sets requirements on using a com-
pressor 16, which increases the oxidizing gas pressure in
stream 9 to the pressure level in stream 10 necessary in view
of the purpose involved.
