Note: Descriptions are shown in the official language in which they were submitted.
~`766S
\
PROCESS AND APPARATUS FOR THE COMBUSTION
OF AMMONIA-CONTAINING WASTE GASES
The invention relates to a process for the
combustion of an ammonia-containing waste gas. The
invention also relates to an apparatus for carrying
out such a process.
Ammonia (NH3)-containing waste gases may
originate, for example, from hydroprocessing crude
mineral oils or products derived from such oils, from
processing coke-oven gas or from coal gasification
processes. Such processes yield liquid or gaseous
product streams containing ammonia, which must be
removed therefrom. ammonia may be removed from such
streams by washing with water at, for example,
elevated pressure and reduced temperature. Washing is
mostly carried out with an abundant quantity of water,
so that dilute, ammonia-containing solutions are
formed. Steam stripping of ammonia-containing
solutions yields water suitable for discharge into
open surface water and an ammonia- and water vapour-
containing waste gas. Such waste gases may also
contain hydrogen sulphide (H2S).
H2S-containiny gases may originate from, for
example, the above hydroprocesses, processing of coke-
oven gas or from coal gasification processes. Examples
'
76~i5
of such hydroprocesses are desulphuriza-tion processes
carried out in a petroleum refinery. Natural gas may
also contain H2S. H2S may be removed from H2S-
containing gases by means of absorption in a regener-
able absorbent. Regeneration of the absorbent yields agas having a higher H2S content than the H2S-
containing starting gas and usually also containing
carbon dioxide (CO2)O Elemental sulphur may be
recovered from the gas yielded by the regeneration by
means of a Claus-type process. In a Claus-type process
the following reactions take place with regard to
sulphur formation:
H S + 3 2 ~S2 + H2O
by which reaction about one third of the H2S is burnt
to SO2 in the so-called Claus thermal stage, followed
by the reaction:
2 H2S SO2 = 3 S + 2 H2O
in the catalytic stages of the process.
In the Claus process gases containing H2S and SO2
are conducted to one or more catalytic zones where
elemental sulphur is formed according to the above
reaction, the molar ratio H2S to SO2 in the gases
~9~66~
-- 3 --
suitably being about 2 : 1. Before introduction into a
catalytic zone the gases are brought to the desired
reaction temperature, suitably between 230C and 280C
and after leaving this zone they are separated by
cooling/condensing into liquid sulphur and, in the
case of the final catalytic zone, H2S-containing waste
gases, also referred to herein as "Claus tail gases".
Claus tail gases also contain sulphur compounds other
than H2S. These H2S-containing waste gases have a
considerably lower H2S content than the H2S-containing
gas introduced into the Claus-type process.
In view of the increasing legislation on environ-
mental pollution abatement NH3-containing waste gases
and H2S-containing waste gases cannot be discharged
into the atmosphere. A suitable method of disposal is
combusting the NH3 in tbe NH3~containing waste gases
to nitrogen and water and combusting the H2S in the
H2S-containing waste gases to SO2 and water; the
combustion gases thus obtained may be passed into the
atmosphere.
British patent specification No. 1,448,085
describes a process for the combustion of Claus tail
gases together with NH3-containing gases in an oven
for NH3 combustion and in the presence of a uel gas
at a temperature between 1000C and 1150C. The gases
formed in the oven pass through a waste heat boiler
76~;
-- 4 --
and are then discharged through a chimney. A disad-
vantage of this process is the relatively high content
of nitrogen oxides (NOX) of the waste gases formed by
the combustion.
It is proposed in the said British patent
specification to recycle the waste gas produced by the
combustion to the oven in which the combustion occurs,
in an amount such that combustion occurs therein with
an oxygen excess in the range O.S to 5.0% by volume of
oxygen. Thus, the waste gases passing to free
atmosphere release less NOX per houx. A disadvantage
of this variant is the reycle of large amounts of gas.
For example, in the experiment described in the said
British patent specification an amount of waste gas
was recycled to the oven, which constituted 95% by
volume of the total of the NH3-containing gas, the
fuel gas and the air supplied to the oven. Hence, the
oven and waste heat boiler must be sized corres-
pondingly larger and a large fan is required to draw
the gases from the line to the chimney and transport
them to the oven.
It is an object of the present invention to avoid
the disadvantage of the above-mentioned known recycle
of waste gases.
7~
-- 5 --
It is a further object of the present invention
to reduce the amount of NOX that is discharged into
the atmosphere.
Accordingly, the invention provides a process for
the combustion of an a~monia-containing waste gas,
which process comprises the following steps:
a) combusting the ammonia-containing waste gas in
the presence of fuel gas with a first amount of a
free oxygen-containing gas, the first amount of
free oxygen being sub~stoichiometric, calculated
on combustion of NH3 to N2 and H2O and of the
fuel gas to CO2 and H2O,
b) mixing the combustion gases formed in step (a)
with a second amount of a free oxygen-containing
gas, the total of the first and second amount of
free oxygen being superstoichiometric with
respect to the said combustion, and
c) mixing and combusting the gases formed in step
(b) with a combustible sulphur
compounds-containing waste gas and a third amount
of a free oxygen-containing gas, the third amount
of free oxygen being super-stoichiometrlc,
calculated on combustion of the combustible
compounds in the combustible sulphur compounds-
containing waste gas to SO2, CO2 and H2O.
The free oxygen-containing gas to be supplied in
the process according to the invention may be any
gaseous stream containing significant quantities of
free oxygen and other components which do not
interfere significantly with the combustion. The free
oxygen-containing gas is preferably air, but the use
of, for example, oxygen-enriched air or pure oxygen,
is not excluded.
Any suitable combustible sulphur compounds-
containing waste gas may be used in the process
according to the present invention; ~2S~containing
waste gases and particularly Claus tail gases, are
very suitable.
In order to reduce air pollution, various
procedures haze been developed to remove sulphur
compounds and elemental sulphur from Claus tail gases.
Such procedures considerably reduce the amounts of
SO2, SO3 and water to be released into the free
atmosphere. In this light, the combustible sulphur
compounds-containing waste gas used according to the
present invention is suitably the off-gas of a process
for the removal of sulphur compouncls and elemental
sulphur from Claus tail gases. Such a process is
described in, for example, British patent specifi-
cation No. 1,356,289.
376~i5
The amount of free oxygen used in step (a) is
suitably in the range of from 65% to 99% and is
preferably in the range of from 70 to 90% of the
stoichiometric amount for combustion of NH3 to N2 and
H2O and of the fuel gas to CO2 and H2O.
The temperature used in step (a) is suitably
maintained at a value in the range of from 1400C to
1600C.
The mean residence time of the flue gases in step
(a) is suitably in the range of from 0.2 s to 2 s.
The NH3-containing gas and the fuel gas should be
combusted in step (a) in the presence of each other.
For this purpose, the two gases may be introduced
separately into the contact zone, using one or more
burners for the NH3-containing gas only and one or
more burners for the fuel gas only. It is preferred,
however, to mix the NH3-containing gas and the fuel
gas and to combust in step (a) the mixture thus
formed.
At the end of step (a) the flue gases contain as
main components N2, CO2, H2O, NH3, HCN, NO, CO and H2.
The purpose of step (b) is to cause thorough mixing of
the flue gases with a second amount of free oxygen-
containing gas in a very short time. This purpose may
be attained in any suitable mixing chamber, for
example in a cylindrical tube with an inside diameter
~7~
-- 8 --
smaller than that of the combustion zone in which step
(a) is carried out. Mixing suitably takes place during
a time in the xange of from 0.010 s to 0.020 s.
The total amount of free oxygen used in steps (a)
and (b) is preferably between 100% and 130~ of the
stoichiometric amount.
In step a the flue gases from step (b) are
suitably first thoroughly mixed in a very short time
with the combustible sulphur compounds-containing
waste gas and the third amount of free oxygen-
containing gas. In step (c) a residence time of the
mixture is allowed sufficient to attain complete
combustion of the combustible sulphur compounds
usually, this residence time is in the range of from
0.2 to 1 s.
Step (c) is suitably carried out at a temperature
in the range of from 800C to 1000C. This temperature
can be adjusted by regulating the amount of fuel gas
supplied to step (a).
The amount of free oxygen supplied in step (c) is
suitably between 100% and ~00% and preferably of from
200% to 300% of the stoichiometric amount.
The flue gases leaving step (c) may be passed
through a waste heat boiler in which they are cooled
by indirect heat exchange with water, simultaneously
generating steam.
The invention further provides an apparatus
suitable for use in the process according to the
present invention, which apparatus comprises:
a) a combustion chamber, provided at the front end
with inlet means for gaseous fuel and oxygen-
containing gas,
b) a mixing chamber in open connection with the rear
end of the combustion chamber and provided with
inlet means for oxygen-containing gas, and
c) a quenching chamber in open connection with the
mixing chamber and provided with inlet means for
combustible gases and oxygen-containing gas.
The combustion, mixing and quenching chambars may
have any suitable shape and are preferably sub-
stantially cylindrically shaped.
The inlet means in the combustion chamber may
have any suitable shape. Preferably, the inlet means
for gaseous fuel of the combustion chamber is provided
with openings substantially equally distributed over
the cross-section of the combustion chamber. Injecting
the gaseous fuel radially into the flow of
oxygen-containing gas creates an intensively mixed
turbulent diffusion flame.
The cross-sectional area of the mixing chamber is
preferably in the range of from 5% to 50% of that of
~97665
- 10 -
the combustion chamber, thus promoting intensive
mixing.
Circle-cylindrically shaped mixing chambers are
preferred and in this case the mixing chamber
preferably has a length to diameter ratio in the range
of from 5 : 1 to 1 : 1.
The quenching chamber is preferably provided with
means for outwardly deflecting the gases from the
mixing chamber, thus providing for thorough and rapid
mixing with the combustible gases and oxygen-
containing gas also supplied to the quenching chamber.
The invention is further elucidated by reference
to the accompanying drawing and Example.
EXA~lPLE
Thus, referring to the drawing, a gas consisting
of NH3 (5.28 kmol/h) and water vapour (0.09 kmol/h)
and having a temperature of 40C is supplied via a
line 1. A fuel gas (7.75 kmol/h) consisting of 85% by
volume of methane and 15% by volume of nitrogen is
supplied via a line 2 at a temperature of 15C. The
NH3-containing gas and the fuel gas are combined in a
line 3 and the mixture obtained is supplied to the
burner of a circle-cylindrically shaped combustion
chamber 4. Air (1513 Nm3/h of which 30g Nm3/h free
oxygen, temperature 50C) is supplied to the
~9~s~
combustion chamber 4 via a line 5, the amount of free
oxygen being 80% of the stoichiometric amount required
for complete combustion of NH3 to N2 and H2O and of
methane to CO2 and H2O. The notion "Nm3" refers to lm3
of gas having a temperature of 0C and a pressure of
1.01 bar. The mixture supplied via the line 3 is
injected radially into the air flow via a ring-shaped
tune provided with openings along the inner circum-
ference, the openings being equally distributed over
the cross-section oE the combustion chamber 4, thus
creating a turbulent diffusion flame and combusting
NH3 and fuel gas at the same rate. The mean residence
time of the flue gases in the combustion chamber 4 is
0.7 s and the temperature therein 1500C. The com-
bustion chamber 4 has an inside diameter and length of
100 and 300 cm, respectively, the length being
measured from the ring-shaped tube to the rear end.
The flue gases pass from the combustion chamber 4
into a circle-cylindrically shaped mixing chamber 7
(inside diameter 30 cm, length 50 cm), in which they
are mixed with a second amount of air (568 Nm3/h of
which 116 Nm3/h free oxygen, temperature 50C)
supplied via the line 6, a line 8 and a line 9, the
total amount of free oxygen supplied via the lines 5
and 9 being 110% of the stoichiometric amount and the
~76Ç~5
- 12 -
cross-sectional area of the mixing chamber being 9~ of
that of the combustion chamber.
The flue gases pass from the mixing chamber 7
into a circle-cylindrically shaped quenching chamber
10 (inside diameter 100 cm, length 450 cm) provided
with a baffle 13 for outwardly deflecting the flue
gases irom the mixing chamber 7. The flue gases are
mixed in the quenching chamber 10 with Claus tail gas
(261.54 kmol/h, temperature 154C) supplied via a line
11 and a third amount of air (1230 Nm3/h of which 251
Nm3/h free oxygen, temperature 50C) supplied via a
line 12, the amount of free oxygen being equivalent to
250~ of the stoichiometric amount requlred for
complete combustion of the combustible compounds in
the Claus tail gas to SO2, CO2 and H2O. The mean
residence time of the flue gases.in the quenching
chamber 10 is 0.3 s. The Claus tail gas has the
following composition (amounts in moth
6~i5
.
- 13 -
Com- Com- Com-
~onent Amount ponent Amount ponent Amount
N2144.65 H2 1.57 COS 0.16
H2O67.25 H2S 1.03 Selemental
C243.28 SO2 0.52
CO2.52 2 0.41
The flue gases leaving the outlet of the
quenching chamber 10 have a temperature of 900C and
contain less than 150 parts per million (vol.) of NOX.
