EP0642380A1

EP0642380A1 - Reducing proceeding and/or a system to intermix and means for the same

Info

Publication number: EP0642380A1
Application number: EP93910538A
Authority: EP
Inventors: Allan Svendssen
Original assignee: GOTHIA TOOL TRADING HB
Current assignee: GOTHIA TOOL TRADING HB
Priority date: 1992-05-18
Filing date: 1993-05-18
Publication date: 1995-03-15
Also published as: SE501418C2; SE9201554L; AU4099493A; WO1993023147A1; SE9201554D0

Abstract

The present invention relates to a process for the admixture of and/or for the achievement of and/or the facilitation of the reduction of undesired compounds, such as soot, hydrocarbons and the oxides of nitrogen and, if the fuel contains other substances, such as sulphur and chlorine, and metal compounds such as mercury and arsenic, for bringing about a reduction in emissions of acidifiying oxides of sulphur and hydrogen chloride, as well as mercury and arsenic compounds, etc., for example in an incineration unit (1n). In accordance with the invention, this is achieved by a special admixture and mixing process, in which suitable reducing agents for nitrogen compounds and/or for other undesired substances, for example oxides of sulphur, are introduced into the incineration unit in direct association with the combustion zone (4n) in conjunction with the supply of gas. This can take place in conjunction with the actual incineration process, or separately from it. The invention also proposes that the utilization of the admixture and mixing process in conjunction with the supply of air (2) or some other suitable gas and/or gas-borne powder or aerosol compound (5) in itself should lead to the formation of low levels of the oxides of nitrogen, but that is does permit the utilization of other processis, either alone or in combination with this process. The invention also relates to an arrangement for the execution of the process.

Description

Reducing proceeding and/or a system to intermix and means for the same

The present invention relates to an admixture and mixing process in an incineration unit intended to bring about the reduction of contaminating substances such as soot, hydrocarbons, oxides of nitrogen and, if the fuel contains other substances such as sulphur and chlorine, and metal compounds such as mercury and arsenic, also to bring about a reduction in emissions of acidifying oxides of sulphur and hydrogen chloride, as well as mercury and arsenic compounds, etc., in accordance with the introduction to Patent Claim 1.

OVER FIRE AIR (OFA)

The prior art includes the so-called OFA method, the principle of which is based on the supply of a proportion of the necessary combustion air to the combustion process via separate air ducts at a later point in time situated after the combustion zone, resulting in the following positive effects:

A) Causes fuel-rich/sub-stoichiometric combustion to take place in the hearth and counteracts the oxidation of the nitrogen present in the air and in the fuel.

B) Causes a lowering of the maximum temperature in the core zones of the flame, resulting in lower N0_X production. The problems associated with the OFA admixture of "cold" combustion air are: to cause the combustion air to take part in combustion, to obtain a final product, of combustion, and to keep emissions of, amongst other things, dust, soot and CO at a low level. In view of this, the OFA method has only been applied to large incineration units which are run under low load conditions.

ROTATING OVER FIRE AIR (ROFA)

The prior art in respect of the reduction of nitrogen in incineration units also includes the so-called ROFA method, which produces the following effects:

A) NO, reduction through an increased degree of rotation and turbulence in the flame of the unit. B) Increased efficiency from a lower flue-gas temperature and the necessary lower excess air.

Flue-gas recycling

The prior art also includes the recycling of the flue-gas into the primary combustion zone and the flame. This method has been found in certain cases to produce a good reduction in NO_x, which can be explained by a reduced oxygen content and an associate-^ reduction in the intensity of combustion.

The negative effects associated with flue-gas recycling are, in the first place, of a practical nature, for example the possibility of condensation associated with corrosion, toxicity in the event of leakage, increased flue- gas flow and a resulting reduction in incineration capacity, higher electricity consumption and reduced efficiency due to an increase in flue-gas temperature.

Reburning

Another previously disclosed method involves the supply of additional fuel at the end of the flame, in conjunction with which previously formed NO_* is reduced. In the case of this method, too, the problem is to obtain reliable, controlled admixture in the flame.

Thermal NO_x reduction

A further previously disclosed method is known as thermal NO-, reduction (SNCR) , for example the DeNO-, and N0_x0UT processes. These involve the addition of chemicals such as ammonia (NH₃) and urea ((NH₂)₂C0).

The previously disclosed effects include the reduction of N0_X in the range of temperatures between approximately 900 and 1000°C. It is claimed that this temperature range is capable of being extended by the use of special auxiliary chemicals.

The dosing of the chemical in this case takes place after the actual combustion zone. The result of this dosing is greatly dependent on the reliable and controlled admixture of the chemicals. Furthermore, the stratified flow in the flame chamber has a marked effect on the result, i.e. on the efficiency, on the consumption of chemicals, on the generation of undesired substances as by-products and on the achievable degree of reduction.

The principal object of the present invention is to create the necessary conditions for the reduction of environmentally harmful substances, including in existing incineration units, without the need for major and costly conversions.

The aforementioned object is thus achieved by a method of the kind according to the present invention, which is characterized essentially in that air, flue-gases or residual fuel, etc., and/or reducing agents is/are introduced into the incineration unit in conjunction with the combustion process through asymmetrically positioned ducts, so that rotation of the agents and/or gases which are introduced is achieved in the combustion zone.

A further object of the invention is to find an arrangement capable of application in conjunction with the execution of the method in accordance with the invention.

Said further object is achieved in accordance with the present invention through an arrangement which is characterized essentially in that delivery ducts are situated along the length of the incineration unit at identical, smaller or, preferably, successively increasing relative spacings along the combustion zone through the asymmetrical positioning of the ducts.

The invention is based on the principle of introducing a proportion of the combustion air during the actual incineration process through ducts which are preferably positioned asymmetrically. This takes place at one or more "levels" of the hearth. The asymmetrical positioning of the ducts, in conjunction with their appropriate design and the use of an appropriate air velocity, induces rotation of the flue-gases over the entire cross-section of the hearth and provides the opportunity to increase the degree of rotation and turbulence directly upstream of the point of entry of the combustion gases into the convection section of the boiler, thereby providing, amongst other things, better burn-up. The system offers the opportunity for the admixture of recirculated flue-gases (0- 100%). This results in better admixture, better mixing and better rotation.

Si-umnary

Combustion causes the emission of undesirable compounds. This is true of all combustion of wood, peat and fossil fuels, such as oil, gas and coal, cellulose waste and household refuse, etc.

These substances can be reduced by

- more efficient combustion

- the addition of reducing agents. Previously disclosed methods for the reduction of undesired substances:

OVER FIRE AIR; ROTATING OVER FIRE AIR;

- reburning;

- flue-gas recirculation; - addition of ammonia substances;

- addition of lime products; addition of sodium carbonates.

Problems: uneven conditions during combustion; - poor turbulence - mixing;

- difficulty in achieving reliable admixture; resulting in poor ef iciency and the generation of undesired compounds as by-products. I. The invention relates to a method for achieving admixture and mixing by rotation, e.g. for ROFA.

II. The invention also proposes that the method should give effective admixture, for example of over fire air added by ROFA.

III. The invention also relates to the creation of admixture and/or mixing using this method by the combustion of, for example, the reburning fuel and recirculated flue-gases. IV. The invention also relates to the application of the method for the introduction of reducing agents, e.g. lime, ammonia substances and active carbon, etc., in the gas flow.

V. The invention relates to the application of. the method in accordance with I-IV at all stages of an incineration and cleaning process, not only in the supply air, drying and pyrolysis stages, but also in the primary and secondary combustion and reburning stages, and also subsequently in the flue-gas system. VI. The invention relates to the application of the method in order to permit and/or facilitate (preparation, homogenization) the ability of other methods, e.g. ROFA, to realize their inherent NO., reduction potential (the DeNO.. or N0_X OUT processes) . VII. The invention relates to the application of the method in other contexts, including outside the area of combustion, where there is a requirement for reliable admixture and mixing of substances, primarily in gas flows.

The invention is described below as a number of preferred illustrative embodiments, in conjunction with which reference is made to the attached drawings, in which:

Figs. 1-5 show examples of various types of incineration installations; and

Figs. 6-8 show sections through incineration installations with an arrangement of gas delivery openings.

In accordance with the present invention, one or more separate techniques are used simultaneously in respect of NO-, reduction, for example. A very good combined result is achieved in this way. In addition to the reduction of N0_X, rotating over fire air in itself leads to more effective combustion, resulting in better efficiency. Chemicals such as ammonia substances, introduced via the rotating over fire air, flue-gas or similar, produce more effective admixture, lower chemical consumption and a broader area of application, thanks to the temperature-reducing effect of the method through the increased absorption of heat in the hearth.

The method in accordance with the present invention for bringing about the reduction of, for example, the oxides of nitrogen in an incineration unit 1, l¹, l², l³, 1* ... lⁿ of the desired type, to which air 2 or some other gas is supplied, takes place, in the first place, by the introduction of agents 3 which exhibit a nitrogen-reducing, f nction into the combustion zone 4, 4¹, 4², 4³, 4* ... 4ⁿ of the incineration unit in question, or at a later stage in conjunction with said gas supply during the actual combustion process. It is also possible to introduce agents 3 which bring about the reduction of sulphur or the reduction of some other substance into said zone 4-4ⁿ- or at a later stage.

The reducing agent 3 should preferably be introduced together with air 2, flue-gas 5, residual fuel 6 or the like, so that the desired penetration and impulse for rotation and mixing are achieved. The introduction performed in this way is of a nature such that rotation of the introduced reducing agent 3 and the gas 2 , 5, 6 is achieved in the combustion zone 4- 4ⁿ, preferably by the introduction of reducing agent 3 and gas 2, 5, 6 asymmetrically on opposite sides 7-8; 9-10 of the combustion zone 4-4*, causing deflection of and turbulence in the mixture of gaseous substances 2, 5, 6, resulting in effective admixture through rotation.

The introduction of gas in the form of at least air 2 preferably takes place from one or more points 11 at identical, smaller or, preferably, successively increasing relative spacings, which act at a successively greater distance A, B, C, etc. , along the combustion zone 4-4ⁿ, preferably via ducts 11, for . example of a previously disclosed kind. Said ducts 11 can be positioned on essentially opposite sides 7-8; 9-10 of the incineration plant 1-1" in question, for example on two, three or four walls or similar, having a circular cross-section, as shown in Fig. 8, or having a cross-section of some other form.

In order to utilize the invention to its full extent, a reducing agent 3 is added preferably in the form of a suitable chemical which brings about the desired reduction of the oxides of nitrogen, or a reduction in sulphur, or pwerforms some other function, for example one of the following: ammonia (NH₃), urea ((NH₂)₂C0), lime products, sodium carbonates and active carbon, and in one or more of the following forms: liquid, spray or powder.

The quantity or level of the reducing chemical 3 required in order to be able to meet and achieve the desired degree of reduction is appropriately metered into preferably rotating over fire air and/or gas via the supply of supply air and/or gas and/or the supply of residual fuel 3; 5; 6.

An arrangement intended to permit the execution of the process in accordance with the present invention in order to bring about the reduction of, for example, oxides of nitrogen in an incineration unit 1-1", into which air 2 or one or more suitable gases is/are introduced in accordance with the foregoing, appropriately comprises a number of delivery openings 11 arranged along the length of the incineration unit when viewed along the successively increasing distance in the direction away from a combustion zone 4-4" of the kind in question.

The arrangement is preferably executed in such a way that air ducts 11 are so arranged as to act at mutually separate levels 12, 13, 14, 15, 16, 17 on the mutually opposing walls 7-8; 9-10 of an incineration unit 1-1" and/or are displaced laterally in pairs in relation to one another.

Illustrated as examples of an appropriate incineration unit to which the invention is applicable are a soda recovery boiler in Fig. 1; an oil-gas-powder boiler with a top-mounted vertically acting burner in Fig. 2; a solid fuel boiler for wood fuels or household refuse in Fig. 3; an oil-gas-powder boiler with one or more side-mounted burners in Fig. 4; and a CFB boiler, i.e. a boiler with a circulating fluidized bed, in Fig. 5, although other types of boiler and designs are suitable to utilize the present invention. If the reduction obtained by ROFA is insufficient to satisfy external requirements or to meet the desired degree of reduction, an appropriate level of ammonia (concentrated or diluted) or an ammonia-based preparation, such as urea or lime products, or sodium carbonates for sulphur reduction, or other suitable chemicals, can be introduced into the rotating over fire air in powder, spray or liquid form.

The admixture of chemical substances is thus also permitted during and in conjunction with the actual combustion process.

The necessary quantity of an ammonia substance, etc. , is metered by a control system via the intended number of ducts. The process is suitable for installations of all types, both small and large, for example of the following types: grate-fired/solid fuel; liquid gas-fuelled via burners; fluidized bed; circulating fluidized bed; soda recovery; engines; gas turbines, and afterburning in cyclones, etc.

The process is applicable to different types of fuels, such as heating oil; fuel oil; natural gas; household refuse; bio-fuels; powder fuels, and cellulose waste, etc. The process is also applicable to the controlled supply and admixture of reburning fuel.

The design of the ducts 11 can be executed in various ways, for example as ordinary openings, nozzles, sprinklers, and vaporizers, etc., and preferably circular or rectangular in form. The supply of chemicals, etc., can also take place in various ways, for example by being mixed into the air 2, flue-gas 5 or equivalent (e.g. steam) before it is introduced into the incineration plant 1-1", or by being introduced separately from the supply of air or gas to the plant 1-1" and not mixed with the air until it is inside the actual combustion area 4-4", or as a combination of these. Combustion air 2 or recirculated flue-gas 5 (carrier gas), a reducing agent 3 and, where appropriate, residual fuel 6 are introduced for combustion via asymmetrically positioned air ducts 11 in the walls 7-10 of the boiler. The air ducts 11 are dimensioned with reference to, amongst other things, the cross-sectional area of the hearth, so that sufficient penetration and impulse for the desired admixture, mixing and rotation are achieved.

The underlying principle of the flow is that air 2 or flue-gas 5 or residual fuel 6 are introduced into the hearth in the form of jets via ducts 11. The jets are deflected on the opposite side (in the case of rectangular section - 90 degrees). This deflection is followed by strong turbulence resulting in thorough admixture. In addition to this admixture, rotation 18 (corkscrew movement) of the entire cross-section is obtained, resulting in higher unit volume velocity. The creation of turbulence is a physical consequence of the change in direction.

The turbulence of the air 2 and the chemical 3 and their admixture in the combustion zone 4-4" can be appreciated from the drawings, in particular Figures 6-8, from which it must be understood that the recycled flue gas and the residual fuel are subjected to the same admixture and mixing as the aforementioned air 2 which is introduced. An increased area of application, for example for

N0_X reduction, is thus now achieved with the help of ammonia or some other suitable substance due, amongst other things, to the lower flame temperature and the larger area at an appropriate temperature, i.e. through the displacement of the thermal balance towards the hearth.

The admixture of NH₃ and (NH₂)₂CO in the flame was not previously possible, and these substances were burned up to form more N0_X. A lower flame temperature now permits the addition of NH₃ and (NH₂)₂CO in direct association with and at the final combustion stage through effective admixture, mixing and combustion.

The invention should now have been appreciated from the foregoing, although the invention is not restricted to what is described and illustrated above, but may be varied within the scope of the Patent Claims without departing from the idea of invention.

Claims

Patent Claims

1. A process in an incineration unit (1, l¹, l², ³ , 1") intended to create the conditions inside the incineration unit for the reduction of environmentally harmful substances through the admixture and mixing of combustion air, fuel and/or reducing agents such as lime, ammonia substances or active carbon, characterized in that air (2), flue-gases (5) or residual fuel (6), etc., and/or reducing agents (3) are introduced into the incineration unit in conjunction with the combustion process via asymmetrically positioned ducts (11), so that rotation (18) of the introduced agent (3) and/or gas (2, 5, 6) is achieved in -the combustion zone (4-4").

2. Process in accordance with Patent Claim 1, characterized in that air (2) , flue-gas (5) or residual fuel (6), etc., is introduced into the incineration unit in the form of jets via ducts (11), so that the jets are deflected in such a way as to produce turbulence and resulting admixture over the entire cross-section of the incineration unit (1-1").

3. Process in accordance with Patent Claims 1-2, characterized in that the reducing agent (3) or the residual fuel (6), etc., are introduced together with air (2) or flue- gas (5), etc., in such a way that the desired penetration and impulse for mixing are achieved.

4. Process in accordance with Patent Claims 1-3, characterized in that the introduction of the reducing agent (3) and the gas (2, 5, 6) takes place at opposite sides (7- 8; 9-10) of the combustion zone (4-4⁴), so that the deflection_.of and the creation of turbulence in the mixture of gaseous substances (2, 5, 6) takes place with resulting admixture and mixing during rotation (18).

5. Process in accordance with either of the Patent Claims 3-4, characterized in that the introduction of gas preferably in the form of air (2) takes place from a number of points (11), which act at successively increasing relative spacings along t.he length of the combustion zone (4-4⁴).

6. Process in accordance with any of the foregoing Patent Claims, characterized in that said introduction takes place via ducts (11) positioned on essentially mutually opposite sides (7-8; 9-10), for example on two, three or four walls or similar of circular cross-section or of cross- section of some other form.

7. Process in accordance with any of the foregoing Patent Claims, characterized in that a reducing agent (3) is introduced in the form of a suitable chemical for the intended reduction, for example at least one of the following: ammonia (NH₃), urea ((NH₂)₂C0), a lime product, sodium carbonate or active carbon, and in liquid, spray or powder form, for example.

8. Process in accordance with any of the foregoing Patent Claims, characterized in that admixture and/or mixing is also created in conjunction with the combustion of the reburning fuel and recirculating flue-gases.

9. Process in accordance with any of the foregoing Patent Claims, characterized in that the introduction in question takes place at every stage of an incineration and cleaning process, not only in the supply air, drying and pyrolysis stages, but also in the primary and secondary combustion and reburning stages, and also subsequently in the flue-gas system.

10. Process in accordance with any of Patent Claims 1-8, characterized in that the introduction in question also takes place outside the area of combustion, where there is a requirement for reliable admixture and mixing of substances, primarily in gas flows.

11. Arrangement for the execution of the process in an incineration unit (1, -A, l², l³, 1*, 1") in order to create the necessary conditions for the reduction of environmentally harmful substances through the admixture and mixing or combustion air, fuel and/or a reducing agent such as lime, ammonia-based substances or active carbon, in accordance with any of the foregoing Patent Claims, characterized in that delivery ducts (11) are situated along the length of the incineration unit at identical, smaller or, preferably, successively increasing relative spacings along the combustion zone (4-4") through the asymmetrical positioning of the ducts (11).

12. Arrangement in accordance with Patent Claim 11, characterized in that ducts (11) are positioned on essentially opposite sides (7-8; 9-10) of the incineration plant (1-1") in question.

13. Arrangement in accordance with any of the Patent Claims 11-12, characterized in that the ducts (11) are executed as openings, nozzles, sprinklers and vaporizers, etc. , preferably of circular or rectangular form.