EP1327104B1 - Method for the stepped combustion of fuel - Google Patents

Abstract

The invention relates to a method for the combustion of solid, even non-homogeneous, fuel in a furnace. Said method comprises the following steps: (a) solid fuel is subjected to combustion, partial combustion or gasification in the combustion chamber, in an oxygen atmosphere below stoichiometric conditions, (b) additional oxygen is supplied to the combustion space in the transition region between the combustion chamber and the furnace or the freeboard, in a quantity above stoichiometric conditions, and (e) a catalyst is added in order to improve or accelerate the reactions in the region to which the quantity of oxygen above stoichiometric conditions is also supplied, or in a succeeding area.

Description

The present invention relates to a method in which a solid fuel with often heterogeneous properties, preferably waste or biomass, burned under conditions which reduce the formation of nitrogen oxides or suppress.

Nitrogen oxides are in the environment and especially after Combustion processes an undesirable product. Therefore, will usually a considerable procedural effort operated by the in-process nitrogen oxides produced by to prevent a complicated flue gas aftertreatment.

When burning solid fuels, especially waste or biomass, oxygen is needed in excess to the bring about corresponding combustion reactions. in the Contact area between gaseous oxygen and the solid Fuel is usually a (over) stoichiometric Atmosphere adjusted to the burnout of solid fuels to make sure. By the applied (over) stoichiometric Operation is often causes partial Precursors of nitrogen oxides (HCN, amines, oxycyanes), the from the fuel in the degassing and pyrolysis zone arise in the vicinity of the combustion chamber to unwanted Nitrogen oxides (fuel-NOx) are oxidized. Despite this Procedure arise partially zones with large and fast fuel turnover, in which a sub- Near-stoichiometric mixture occurs. There arise especially hot zones that cause the formation of thermal or prompt Favor nitrogen oxides.

In order to reduce the formation of fuel NOx, the methodology of staged combustion has been introduced, especially for liquid, gaseous and dust fuels. It is achieved by air grading or targeted air distribution between primary, secondary and ev tertiary air, that in a first combustion section by substoichiometric operation, the precursors of fuel NOx are reduced to N ₂ (see, for example, US Patent 5,626,085 or US Pat. Patent 4,704,084). The course of the reaction is very complex and not fully understood.

By also present in this area unburned substance CO (product of incomplete combustion) can also NO to N ₂ can be reduced (simplified: 2 CO + 2 NO → 2 CO ₂ + N ₂ ) But it is important to ensure that the reacting Process gases are intimately mixed to allow the reaction.

In the combustion of solid fuels, especially waste, this method has not been successfully used, since the post-combustion of the unburned components (eg CO, C _x H _y ) experience brings great difficulties with it. Large amounts of unburned are not brought to the reaction, since usually only an insufficient mixing with oxygen happens, the temperature is significantly lowered, for example by the injection of secondary air and thus the reaction rate for rapid degradation is no longer sufficient.

The object of the present invention is to provide a method for To provide all solid, even inhomogeneous fuels at the formation of nitrogen oxides largely avoided or at least reduced.

Figur 1 zeigt schematisch die Luftstufung und Katalysatoraufgabe in einem Verbrennungsverfahren gemäß der vorliegenden Erfindung an einer beispielhaften Verbrennungsanlage.

This object is achieved by the provision of a method which comprises a gradation of the air addition to achieve a first sub-stoichiometric and then superstoichiometric reaction atmosphere and the subsequent or accompanying aftertreatment of the flue gases with a solid catalyst which improves and accelerates the running reactions and thus supporting the formation of inert nitrogen.

Figure 1 shows schematically the air staging and catalyst task in a combustion process according to the present invention on an exemplary incinerator.

The invention provides that on the one hand the previously for liquid and other homogeneous fuels used, graded Combustion also for heterogeneous fuels such as waste or Biomass is used, which in addition to the problem, which is predetermined by the inhomogeneity, often very strong are loaded with nitrogenous substances. This should the formation of nitric oxide largely avoided or completely be suppressed. Furthermore, should by the addition of a suitable catalyst in an area of the incinerator, seen in the flow direction not in front of the area of Combustion with excess stoichiometric oxygen is to be achieved, that on the one hand the intended amount of air for the superstoichiometric combustion of an optionally impulse addition in the combustion chamber is independent and on the other hand still enough oxygen to burn out the still unburned substances is present.

By grading the addition of air is first achieved that in the vicinity of the combustion chamber, the solid near to substoichiometric (controlled gasification by near or substoichiometric air addition in the primary air range). The formation of uncontrolled temperature peaks in the area of the combustion chamber is suppressed. Due to the process, larger amounts of unburned substances (eg, CO, C _x H _y , aromatics, etc.) arise. The precursor substances (eg HCN, amines) of the nitrogen oxides also belong to these substances. Already formed NO can react in this area on contact with CO to CO ₂ and N ₂ , since the oxygen affinity of the carbon is greater than that of the nitrogen.

In the following area (the transition of the combustion chamber in which the solid constituents of the fuel burn, to the so-called firebox or freeboard, burn in the above all gases, and possibly in addition to a subsequent flue gas flow direction point) is a feed of further combustion air (secondary - and possibly tertiary air) to afterburn the unburned substances. This creates a superstoichiometric atmosphere. The remaining unburned substances are largely reacted. A significant reduction in the formation of thermal and prompt NO _x is the result of the gradual mode of operation.

In order to intensify or optimize these processes, portions of a catalyst present as a solid, which is preferably finely divided and / or has a high surface area, are additionally added to the flue gas. The catalyst to be used according to the invention is capable of releasing non-gaseous excess oxygen into contact reactions with gaseous substances and subsequently of regenerating it again. The gaseous substances are preferably oxidizable substances, in particular, for example, C _x H _y (hydrocarbons), carbon monoxide, but it can also be oxidized with it difficultly degradable toxins such as PCBs, dioxins or furans.

Suitable for the purposes of the present invention are e.g. ferro-oxide catalysts, e.g. finely dispersed iron oxide. The Addition can be done together with the secondary air and / or additionally de-injected tertiary air. Alternatively, the Catalyst also in other ways, e.g. with recirculated Flue gas or with steam, be blown. An injection in another, further downstream Area of the combustion chamber or boiler at a lower Temperature is also possible. The expert will be the suitable variant readily considering the select given process conditions. It is essential that the Catalyst only after a certain minimum residence time of Flue gases in the combustion chamber or in the firebox or freeboard is used to the above reactions to Nitric oxide reduction not to hinder. The catalyst supports combustion with excess available Oxygen, so that no unburned substances through the Boiler area to move into the flue gas cleaning.

After the combustion chamber and the firebox or freeboard is the Amount of nitrogen oxides when using the inventive Procedure so low that no further additive measures (e.g., SNCR - selective non-catalytic reduction, SCR - selective catalytic reduction) are necessary to the To reduce nitrogen oxides to a level by the permit is prescribed. Another effect is the regeneration of Dioxins and furans suppressed as the corresponding Precursors for the de novo synthesis in a smaller amount incurred or the decomposition of dioxins and furans and the like is made possible catalytically, even in colder regions of the firebox.

In a preferred embodiment of the invention, the flue gas in the combustion chamber additionally imparted a pulse. This can be done for example by the task of recirculated flue gas or tensioned water vapor or similar media through appropriately arranged nozzles. Such process steps are known, for example, from DE 196 19 764 A1 of Hoechst AG or WO 93/07422 of VAW Aluminum AG. As a result of this measure, turbulence is generated independently of the introduction of additional oxygen, as a result of which incompletely burned substances, such as CO and NO, come into better contact with one another and can react to form N ₂ and CO ₂ . The precursor substances of nitrogen oxides are also greatly reduced to N ₂ in this region, which is characterized by a substoichiometric atmosphere. Nevertheless, any resulting nitrogen oxides can be reduced to N ₂ as described above.

In addition, in the transition from the combustion chamber to Firebox or freeboard additional mixing impulse in the Fuel gases are introduced, for example by recirculated flue gas or steam. Also this procedure is known in the art, see e.g. DE 44 02 172 A1.

The addition of primary air can be controlled so that certain temperature targets (e.g., temperature below ash softening point) be respected in or on the fuel bed.

These temperature targets can be determined by suitable temperature measuring devices be monitored, for example with the help of a Infrared camera. In addition, the gas atmosphere and in particular the oxygen content by suitable measuring instruments be monitored.

Alternatively, it is possible, the catalyst also in the further Add the course of the combustion chamber or the boiler to the flue gas. The person skilled in the art will readily find the suitable variant taking into account the given process conditions choose.

The invention will be explained in more detail below with reference to an example and the attached FIG .

It is shown a combustion process in which a fuel as garbage 1 is abandoned. Through a task slide 2 the fuel is pushed into the combustion chamber 3. There will the fuel under supply of primary combustion air. 4 stoichiometrically burned or gasified. A temperature monitor and the control of the air distribution takes place under An aid of an infrared camera 5, for example on the rear wall of the Combustion chamber. The gasification products rising from the bed of solids can be recirculated by the targeted addition of Flue gas 6 or the like. Be intimately mixed. After mixing takes place in the transition from the combustion chamber to the combustion chamber 7 a Injection of further combustion air (secondary air 8 or Tertiary air 9). Here is the afterburning of the unburned Achieved substances from the combustion chamber. By an additional Doping the secondary or tertiary air with a finely divided Catalyst at 8 and / or at 9 will be discussed below Firebox 7 achieved a very good burnout. Alternatively or In addition, the catalyst can also only later the furnace or boiler 10 added to the flue gas when the process conditions appear beneficial to let.

Claims (12)

1. Process for combusting even inhomogeneous, solid fuels in a firing plant, which has the following steps:

   (a) combustion, partial combustion or gasification of the solid fuels in the combustion chamber under below-stoichiometric oxygen atmosphere,

   (b) supply of additional oxygen to the combustion chamber in the transition region from combustion chamber to furnace chamber or freeboard in an above-stoichiometric quantity,

   (c) addition of a catalyst present as a solid for improving or accelerating the proceeding reactions in the region in which the above-stoichiometric quantity of oxygen is also supplied, or in a subsequent region.
2. Process according to claim 1, characterised in that the catalyst is selected from materials which are able to release oxygen in contact reactions on gaseous substances at the prevailing ambient conditions and are able to oxidise in particular hydrocarbons and carbon monoxide.
3. Process according to claim 1 or 2, characterised in that a ferro-oxidic catalyst is used as catalyst, in particular finely dispersed iron oxide.
4. Process according to one of the preceding claims, characterised in that tertiary air is introduced in flue gas direction following the addition of secondary air.
5. Process according to one of the preceding claims, characterised in that the catalyst is introduced together with the secondary air.
6. Process according to one of the preceding claims, characterised in that catalyst is added solely or additionally in a part of the furnace chamber lying further upstream in flue gas direction or in the boiler region of the firing plant.
7. Process according to one of claims 1 to 4, characterised in that a mixing pulse is additionally imposed on the gas situated in the combustion chamber.
8. Process according to claim 7, characterised in that the mixing pulse is imposed by injecting recirculated flue gas or live steam.
9. Process according to claim 7 or 8, characterised in that the mixing pulse delivery takes place in the combustion chamber.
10. Process according to one of claims 7 to 9, characterised in that the mixing pulse delivery takes place in the transition region between combustion chamber and furnace chamber or freeboard or upstream in flue gas direction.
11. Process according to claim 10, characterised in that catalyst is injected together with recirculated flue gas or live steam.
12. Process according to one of the preceding claims, characterised in that the oxygen content in the combustion chamber and/or in the furnace chamber is controlled and regulated.

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