WO1989005422A1

WO1989005422A1 - Reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases

Info

Publication number: WO1989005422A1
Application number: PCT/SE1988/000670
Authority: WO
Inventors: Allan Inovius
Original assignee: Allan Inovius
Priority date: 1987-12-11
Filing date: 1988-12-09
Publication date: 1989-06-15
Also published as: JPH03502963A; FI92866C; AU623108B2; NO166461C; IL88588A; SE8704987D0; PT89186B; BR8807842A; NO902557D0; FI92866B; RU2023947C1; IL88588A0; AU2799489A; HUT53956A; LT3829B; ATE91329T1; SE8704987L; CA1319618C; DE3882236T2; EP0395702A1

Abstract

A reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases is in the form of a post-combustion chamber to be connected after a combustion chamber. The reactor has a casing (10) consisting of a generally cylindrical main part (11) passing at the top into a dome-shaped outlet part (12) with an outlet opening (13). Around the casing (10) of the reactor, there is provided a heat-insulated wall (20) whose inner side has substantially the same shape as the casing (10) and which is eccentrically disposed in relation to the casing (10). The casing (10) of the reactor accommodates a partition (15) which is conical and has its apex directed towards the outlet opening (13). A gap (17) is formed between the shell (11) and the partition (15). At the inlet end (14) of the shell, an inlet funnel (18) is provided at a certain distance from the inlet end so as to form a gap (19). A heat exchanger (40) is provided for preheating secondary air supplied through a secondary air intake (25) into the gap (21) between the casing (10) and the wall (20) at a certain distance from the opening (13). At the bottom, the gap (21) is connected to a collecting box (23) and an outlet pipe (24).

Description

REACTOR FOR REDUCING THE CONTENTS OF NITROGEN OXIDES AND SULPHUR OXIDES IN COMBUSTION GASES

The present invention relates to a reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases, which reactor comprises a post-combustion chamber to be connected together with or after a combustion chamber.

The combustion device according to the invention is of the type stated in the accompanying claims and have the features recited therein.

A major problem in the combustion of liquid and solid fuels is the content of sulphur oxides and ni¬ trogen oxides present in the flue gases. Thus, many attempts have been made to reduce this oxide content, both by flue gas cleaning and by catalytic treatment of the exhaust gases. The invention is based on the insight that it is possible to reduce the content of nitrogen oxides and sulphur oxides to a considerable extent if it is ensured that suitable oxidation and temperature con¬ ditions prevail in the passageway between the combus- tion chamber and the chimney.

Swedish Patent 7804761-0 (SE-B-413, 158 ) discloses an apparatus for the combustion of a mixture of gaseous or particulate, combustible material and combustion air. This apparatus is used for combusting various gaseous or particulate materials containing carbon or carbon com¬ pounds, in such a complete manner that the combustion gases emitted are practically free from soot, carbon monoxide and hydrocarbon residues. It is however not stated in the patent specification that the apparatus can be used for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases.

US-A-4, 481 ,889 discloses a method for afterburning flue gases by conducting the impure gases through a bur- ner in an afterburner in which the exhaust gases, by being positively mixed with a combustion gas, are sub-¹ jected to complete combustion. In this process, com¬ bustible gases are thus supplied to bring about after- burning of the flue gases .

DE-A-3,014,590 discloses a pre-co bustion chamber for an oil- or gas-fired, fan-supported burner. This pre-combustion chamber serves to shape the generated flame and to retard it before entering the combustion chamber. This apparatus thus serves as an intermediary between the burner and the combustion chamber, whereas not as a reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases.

The invention will now be described in more .detail hereinbelow with reference to the accompanying drawings illustrating two embodiments of the device according to the invention.

Fig. 1 is a vertical section schematically showing an embodiment of the reactor according to the invention. Fig. 2 is a section taken along the line II-II in Fig. 1. Fig. 3 is a vertical section schematically showing an incineration plant using another embodiment of a reactor according to the invention, and Fig. 4 shows yet another embodiment of a reactor according to the invention. The arrangement shown in Fig. 1 comprises a reactor for reducing the contents of nitrogen oxides and sulphur oxides in combustion gases. The reactor has a casing or wall 10 with a substantially vertical, generally cylin¬ drical shell 11 and a dome-shaped outlet end 12 asso- ciated therewith. The dome-shaped outlet end has a cen¬ tral outlet opening 13. The opposite end of the shell 11 forms an inlet end 14. Inside the casing 10, there is provided a conical partition 15 which has its apex directed towards the outlet end 13 and which is mounted on support members 16 in a manner to define an annular gap 17 between the partition 15 and the casing 10. In¬ stead of an annular gap, the connection between the upper and the lower part of the casing 10 may be in the form of at least two edge recesses distributed around the periphery of the partition, suitably as disclosed in SE-B-413,158 which is included by reference. At the bottom of the reactor, there is provided an inlet funnel 18 which leads the exhaust gases from a combustion cham¬ ber (not shown) into the reactor, so that the exhaust gases will be introduced at a suitably high velocity and directed towards the conical inner side of the par- tition 15. Around the casing 10, there is provided a further casing or wall 20 which has substantially the same shape as the casing 10 but larger dimensions so as to define a gap 21 between the casings 10 and 20. The casing 10 is eccentrically disposed in the casing 20. The casing 20 may consist of a heat-insulating material, but may also be surrounded by such a material. In the illustrated embodiment, an external heat-insula¬ tion 22 is used for the casing 20. The gap 21 between the two casings is connected at the bottom to an annular collecting box 23 connected to an outlet pipe 24, e.g. a chimney.

In the gap 21 between the two casings, there may be provided a heat exchanger (not shown in more detail) for preheating secondary air. In the embodiment accord- ing to Fig. 1, secondary air is however supplied through an annular space 40 formed between the casing 20 and the external heat-insulation 22. The preheated secondary air is fed through a secondary air intake 25 into the space between the two casings at some distance from the outlet opening 13.

Between the lower edge of the inlet end 14 of the inner casing 10 and the inlet funnel 18, there is de¬ fined an annular gap 19 for the separation of ash par¬ ticles which have been separated in the post-combustion chamber 10 or formed during the combustion therein. When using the arrangement according to Figs. 1 and 2 , it is advantageous to have the exhaust gases from the combustion chamber arrive in the inlet funnel - 18 at a maximum velocity of 2 m/s. By the conical shape-., of the inlet funnel, the gas velocity is increased and. . the gases are directed towards the inner side of the conical partition 15. As a result of the intense turbu¬ lence in the space below the conical partition, residual carbon monoxide will oxidise into carbon dioxide, and this oxidation will proceed in the space above the par¬ tition. From the outlet opening 13, the flue gases enter into the gap between the casings 10 and 20 where afterburning and treatment of sulphur oxides and nitro¬ gen oxides is performed under the action of the preheated secondary air which is supplied through the secondary air intake 25 and preferably heated to a temperature of about 700°C. By the eccentric arrangement, intense mix¬ ing is achieved as well as compression alternating with expansion of the flue gases which are moving helically downwards to the collecting box 23 before passing out to the outlet pipe or chimney 24 at a temperature of about 900°C.

The principle of the inventive device is based .on experiments with ideal turbulence for final oxidation of all hydrocarbon materials with a controlled low partial pressure in the gas phase to achieve a suffi- cient contact time with hot catalytic contact surfaces. The hot contact surfaces initially consist of the mate¬ rial in the partition 15. Behind this concave partition, there is thus a slower turbulence in a reducing atmos¬ phere in order to obtain the necessary production of carbon monoxide for the process, e.g. for reducing the sulphur content in the combustion gases. In stoichio- metric combustion and according to the following formulae, sulphur deposits by more than 90% as sulphur droplets which have been sublimated during the cooling. Since the post-combustion chamber is vertically mounted, the sublimated sulphur, together with other particles, will automatically pass to the ash bed through the gap be¬ tween the inlet funnel 18 and the inlet end 14.

When the post-combustion chamber is used in large- scale plants, the formula 2CO+S0₂ ' __*■ S+2C0₂ applies. For plants with over-stoichiometric combustion, formulae CO+O- ._■ CO+CO- and S0₂+CO+H₂0 *" H₂S+CO and S0₂+H₂S <- • ^ S+H₂0 apply.

If the gases entering the post-combustion chamber have a temperature of 900°C and a flow velocity of at most 2 m/s, it is possible to obtain substantially soot- and particle-free exhaust gases when a catalys¬ ing surface exists on the conical partition 15 and on other contact surfaces affecting the combustion gases. The different formulae relating to the combustion chamber appear from the following.

The device according to the invention as illustrat¬ ed in Fig. 3 has substantially the same design as that in Fig. 1. The device in Fig. 3 is shown together with an incineration plant of the type disclosed in Swedish Patent 7804761-0 (SE-B-413,158) . For a more detailed description of this arrangement, reference is thus made to said patent specification which is included by refe¬ rence. The device in Fig. 3 is generally designated 30. After this incineration device, there is a further com- bustion chamber 31 in which noxious waste or solid fuels, for instance, can be combusted. From this combustion chamber or furnace 31, the combustion gases flow through a gap 32 up to the inlet funnel 18 and into the post- combustion chamber according to the invention. The gap 32 is formed between the incineration device 30 and a heat-insulated furnace wall 33. At the lower end of the space defined by the furnace wall 33, there is an ash outlet 34. Since the post-combustion chamber or reactor in Fig. 3 is essentially designed as in Fig. 1, equiva- lent parts have been given the same reference numerals. In the embodiment shown in Fig. 3, the partition 15 extends as far as the inner side of the cylindrical shell surface 11, and edge recesses are provided which extend obliquely through the partition 15 adjacent the . shell surface, such that the passage between the space: *. below the partition and the space above' it imparts a " . helical motion to the flue gases when entering the .upper chamber above the partition 15.

Fig. 4 shows a further embodiment of a reactor according to the present invention. Corresponding parts have been given the same reference numerals. The esserc- tial difference between the embodiments of Fig. 1 and Fig. 4 is the way of supplying secondary air through a secondary air intake 45. In this embodiment, the secondary air intake 45 consists of a gap between two conical walls 40, 41. This gap is fed with secondary air which may have been preheated in any suitable man¬ ner. The air is either blown through the gap 45 or sucked therethrough as a result of the ejector effect . produced by the exhaust gases entering the reactor through the inlet funnel 18. In the embodiment of Fig. 4, the conicial partition 15 has been designed in the manner shown in the above- mentioned SE-B-413,158, which means that there are pro- - vided at least two through passages 17 formed of edge recesses distributed around the circumference of the partition and extending obliquely therethrough so as to impart a turbulent effect to the flue gases when passing between the inlet chamber and the outlet cham¬ ber.

The reactor according to the invention may advan— tageously be used also in incineration plants operating with a fluidised fuel bed. INCOMPLETE COMBUSTION

(CH₂)n n 0₂ n CO + n H₂0 oil, gas oxygen carbon water monoxide vapour

II CONVERSION

CO H₂0 _---. co₂ H₂ carbon water carbon hydrogen monoxide vapour dioxide gas

METHANISING

CO 3H_ — Λ CH₄

2 -sr H₂0 carbon hydrogen methane water monoxide gas vapour

III COMPLETE COMBUSTION

(CH₂)n

2 ⁿ °2 oil, gas oxygen

CO _-___.

\ °2 v carbon oxygen monoxide

H₂ _

\ °2 hydrogen oxygen gas

CH₄ 30₂ methane oxygen gas

S + sulphur oxygen

2 CO + SO. 2 CO.

"sr

Alternative reaction with excess of 0_ and H-O:

C+0₂ ^ CO+C0₂ S0₂+CO+H₂0 -^ H₂S+C0₂ S0₂+H₂S ^ S+H₂0

Claims

1. Reactor for reducing the contents o_f nitrogea oxides and sulphur oxides in combustion gases, compris¬ ing a post-combustion chamber to be connected together with or after a combustion chamber, c h a r a c t e r i s e d in that the reactor has a casing (10) with a substantially vertical, generally cylindrical shell (11) having an inlet end (18) connected to the combus¬ tion chamber, and an opposite outlet end (12) which is generally dome-shaped and has a central outlet opening (13), said casing (10) further having a parti¬ tion (15) provided with a through passage (17) and dividing the interior of the casing into an inlet cham¬ ber and an outlet chamber, that the central portion of said partition (15) is designed as a conical shell the outer side of which is facing said outlet chamber and the inner side of which is facing said inlet chamber, that the through passage (17) of the partition (15) either is in the form of a gap (17) between the parti¬ tion and the cylindrical shell (11) or is in the form of at least two edge recesses distributed around the - periphery of the partition, said reactor further com¬ prising a heat-insulated wall (20) enclosing said cas¬ ing (10) and spaced apart therefrom, and a secondary air intake (25, 45) opening either into the space (21) between said heat-insulated wall (20) and said casing (10) at a distance from the central outlet opening (13) of the outlet chamber, or into the inlet chambe of the casing (10), and that the space (21) between the casing (10) and the heat-insulated wall (20) is connected in the area of the inlet end of the casing (10) to an outlet pipe (24),_and that the connection between the combustion chamber and the inlet chamber of the reactor is in the form of a conically tapering inlet funnel (18) projecting into the inlet chamber, an annular gap being defined between said inlet funnel and the inlet end (14) of the casing (10) for separat¬ ing ash particles.

2. Reactor as claimed in claim 1, c h a r a c - t e r i s e d in that the edge recesses of the parti¬ tion (15) extend obliquely through the partition adja¬ cent said cylindrical shell.

3. Apparatus as claimed in claim 1 or 2, c h a ¬ r a c t e r i s e d in that at least one of the cy- lindrical shell (11), the dome-shaped outlet end (12) and the partition (15) contains a material having the ability to catalyse the oxidation of carbon and cartron compounds.

4. Reactor as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that a secondary air heater (40) is provided for preheating the secondary air supplied to the secondary air intake.

5. Reactor as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the secondary air intake (45) opens at the mouth of the conically taper¬ ing inlet funnel (18) of the inlet chamber.