GB2116531A

GB2116531A - Process and apparatus for the combustion of ammonia-containing waste gases

Info

Publication number: GB2116531A
Application number: GB08207141A
Authority: GB
Inventors: Hendrikus Johannes An Hasenack; Jan Waller
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1982-03-11
Filing date: 1982-03-11
Publication date: 1983-09-28
Also published as: NL8300576A; AU555492B2; DE3308406A1; DE3308406C2; JPS58164922A; BE896111A; FR2522983A1; GB2116531B; ZA831619B; FR2522983B1; AU1217783A; JPH0377408B2; CA1197665A

Abstract

Simultaneous disposal of NH3- containing waste gas and combustible sulphur compounds-containing waste gas by:- a) combusting the former waste gas (1) in the presence of fuel gas (2) with a first, sub-stoichiometric amount of free oxygen (5), b) mixing the combustion gases from step a) with a second amount of free oxygen (9), the total of the first and second amount being super- stoichiometric and c) mixing the gases from step b) with the latter waste gas (11) and combusting the resulting mixture with a third, super-stoichiometric amount of free oxygen (12). <IMAGE>

Description

SPECIFICATION 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-containing gases may originate from, for example, the above hydroprocesses, processing of coke-oven gas or from coal gasification processes. Examples of such hydroprocesses are desulphurization 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 regenerable absorbent.

Regeneration of the absorbent yields a gas having a higher H2S content than the H2S-containing starting gas and usually also containing carbon dioxide (CO2). 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: 3 H2S+ ~ 2 4 S 2 + H20 2 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 5 + 2 H20 in the catalytic stages of the process.

In the Claus process gases containing H25 and SO2 are conducted to one or more catalytic zones where elemental sulphur is formed according to the above reaction, the molar ratio H25 to SO2 in the gases suitably being about 2:1. Before introduction into a catalytic zone the gases are brought to the desired reaction temperature, suitably between 2300C and 280cm and after leaving this zone they are separated by cooling/condensing into liquid sulphur and, in the case of the final catalytic zone, H2Scontaining 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 H25 content than the H25-containing gas introduced into the Claus-type process.

In view of the increasing legislation on environmental pollution abatement NH3-containing waste gases and H25-containing waste gases cannot be discharged into the atmosphere. A suitable method of disposal is combusting the NH, in the NH3-containing waste gases to nitrogen and water and combusting the H25 in the H25-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 fuel gas at a temperature between 10000C and 1 500C. The gases formed in the oven pass through a waste heat boiler and are then discharged through a chimney. A disadvantage of this process is the relatively high content of nitrogen oxides (NO,) 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 0.5 to 5.0% by volume of oxygen. Thus, the waste gases passing to free atmosphere release less NO, per hour. A disadvantage of this variant is the recycle 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 45% 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 correspondingly 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.

It is a further object of the present invention to reduce the amount of NO, that is discharged into the atmosphere.

Accordingly, the invention provides a process for the combustion of an ammonia-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 oxygencontaining 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 compoundscontaining waste gas and a third amount of a free oxygen-containing gas, the third amount of free oxygen being super-stoichiometric, 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; H2S-containing waste gases and particularly Claus tail gases, are very suitable.

In order to reduce air pollution, various procedures have 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 compounds and elemental sulphur from Claus tail gases. Such a process is described in, for example, British patent specification No. 1,356,289.

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 1 4000C to 16000 C.

The mean residence time of the flue gases in step (a) is suitably in the range of from 0.2 sec. to 2 sec.

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 smaller than that of the combustion zone in which step (a) is carried out. Mixing suitably takes place during a time in the range of from 0.010 sec. to 0.020 sec.

The total amount of free oxygen used in steps (a) and (b) is preferably between 100% and 130% of the stoichiometric amount.

In step (c) 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 oxygencontaining 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 sec.

Step (c) is suitably carried out at a temperature in the range of from 8000C to 1 0000C. 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 400% 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 oxygencontaining 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 chambers may have any suitable shape and are preferably substantially 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 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.

EXAMPLE 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 400C is supplied via a line 1. fuel gas (7.75 kmol/h) consisting of 85%v of methane and 1 5% of nitrogen is supplied via a line 2 at a temperature of 1 50C.

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 309 Nm3/h free oxygen, temperature 500C) is supplied to the 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 mixture supplied via the line 3 is injected radially into the air flow via a ring-shaped tube provided with openings along the inner circumference, the openings being equally distributed over the cross-section of 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 sec. and the temperature therein 1 5000C.The combustion 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 11 6 Nm3/h free oxygen, temperature 500C) 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 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 from the mixing chamber 7. The flue gases are mixed in the quenching chamber 10 with Claus tail gas (261.54 kmol/h, temperature 1 540C) supplied via a line 11 and a third amount of air (1230 Nm3/h of which 251 Nm3/h free oxygen, temperature 500C) supplied via a line 12, the amount of free oxygen being equivalent to 250% of the stoichiometric amount required 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 sec. The Claus tail gas has the following composition (amounts in kmol/h): Component Amount Component Amount Component Amount N2 144.65 H2 1.57 COS 0.16 H20 67.25 H2S 1.03 Selemental 0.151 CO2 43.28 SO2 0.52 CO 2.52 O2 0.41 The flue gases leaving the outlet of the quenching chamber 10 have a temperature of 9000C and contain less than 1 50 parts per million (vol.) of NOX.

Claims

1. A process for the combustion of an ammonia-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 H20 and of the fuel gas to CO2 and H2O, b) mixing the combustion gases formed in step (a) with a second amount of free oxygencontaining gas, the total of the first and second amount of free oxygen being super-stoichiometric with respect to the said combustion, and c) mixing and combusting the gases formed in step (b) with a combustible sulphur compoundscontaining waste gas and a third amount of a free oxygen-containing gas, the third amount of free oxygen being super-stoichiometric, calculated on combustion of the combustible compounds in the combustible sulphur compounds-containing waste gas to SO2, CO2 and H2O.

2. A process as claimed in claim 1, in which the free oxygen-containing gas is air.

3. A process as claimed in claim 1 or 2, in which the combustible sulphur compounds-containing waste gas is a Claus tail gas.

4. A process as claimed in any one of the preceding claims, in which the amount of free oxygen used in step (a) is in the range of from 65% to 99% of the stoichiometric amount.

5. A process as claimed in claim 4, in which the amount of free oxygen used in step (a) is in the range of from 70% to 90% of the said stoichiometric amount.

6. A process as claimed in any one of the preceding claims, in which step (a) is carried out at a temperature in the range of from 1 4000C to 1 6000C.

7. A process as claimed in any one of the preceding claims, in which the total amount of free oxygen supplied in steps (a) and (b) is between 100% and 130% of the stoichiometric amount.

8. A process as claimed in any one of the preceding claims, in which the amount of free oxygen, supplied in step (c) is between 100% and 400% of the stoichiometric amount.

9. A process as claimed in any one of the preceding claims, in which step (c) is carried out at a temperature in the range of from 8000C to 1 O000C.

10. A process as claimed in claim 1, substantially as hereinbefore described, with reference to the Example.

11. An apparatus suitable for use in a process as claimed in any one of the preceding claims, which apparatus comprises: a) a combustion chamber, provided at the front end with inlet means for gaseous fuel and oxygencontaining 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.

12. An apparatus as claimed in claim 11, in which the combustion mixing and quenching chambers are substantially cylindrically shaped.

13. An apparatus as claimed in claim 11 or 12, in which 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.

14. An apparatus as claimed in claim 12 or 13, in which the cross-sectional area of the mixing chamber is in the range of from 5% to 50% of that of the combustion chamber.

15. An apparatus as claimed in any one of claims 12 to 14, in which the mixing chamber is circle cylindrically shaped.

16. An apparatus as claimed in claim 15, in which the mixing chamber has a length to diameter ratio in the range of from 5 :1 to 1:1.

17. An apparatus as claimed in any one of claims 11 to 16, in which the quenching chamber is provided with means for outwardly deflecting the gases from the mixing chamber.

18. An apparatus as claimed in claim 11, substantially as hereinbefore described with reference to the drawing.