WO1995027872A1

WO1995027872A1 - Method of reducing waste-incineration emissions

Info

Publication number: WO1995027872A1
Application number: PCT/EP1995/001308
Authority: WO
Inventors: Herbert MÄRZ
Original assignee: Sbw Sonderabfallentsorgung Baden-Württemberg Gmbh
Priority date: 1994-04-09
Filing date: 1995-04-10
Publication date: 1995-10-19
Also published as: DE59504964D1

Abstract

Described is a method of reducing the emissions produced by the incineration of waste in an incinerator in which part of the exhaust gases produced is recycled to the input to the incinerator, pure oxygen being added to the recycled gases to form synthetic air. The supply pressure to the burner (4a) at the input end (1a) of the rotating tube (1) forming the combustion chamber is adjustable both during start-up under atmospheric conditions and during steady-state operation under synthetic-air conditions, and the ratio of the burner supply pressure under atmospheric conditions to the pressure under synthetic-air conditions is set at 1 : 2.5 to 1:5, preferably 1 : 4.

Description

Title: Process for reducing emissions from waste incineration

DESCRIPTION

The invention relates to a method for reducing the emissions from the incineration of waste in an incineration plant, in which a part of the flue gas produced during the combustion is recirculated to the entrance of the incineration plant, the recirculated flue gas, the pure oxygen required for combustion to form a synthetic air formed from flue gas and oxygen is admixed, the ratio of flue gas and. Oxygen in the synthetic air is 4: 1 to 1: 1.

Such a method is known (see, for example, US Pat. No. 5,179,903). The object of the present invention is a targeted and stable, but above all flexible

Ensure combustion process control. A process control that takes place without flame breaks is called stable.

According to the invention, this object is achieved in that the burner admission pressure of the burner provided at the front end of the rotary tube forming the combustion chamber can be set both in start-up operation under atmospheric conditions or the burner admission pressure in stationary operation under synthetic conditions, and the ratio of the burner admission pressures from atmospheric to synthetic conditions 1: 2.5 to 1: 5 is preferably set to 1: 4.

Advantageous developments of the invention are defined in the subclaims.

An embodiment of the method according to the invention is described below with reference to the accompanying drawing, which shows the process flow.

The special waste, after the pyrolysable portion has been separated beforehand, is introduced into a rotary tube 1, which serves as a combustion chamber. A fluid fuel is supplied as a controllable support energy source via a line 2. The fuel is obtained as a product of an upstream pyrolysis 3 from high-calorific waste. The fuel line 2 branches into a line 2a, which leads to a burner 4a at the front end 1 a of the rotary tube 1, and a line 2b, which leads to a burner 4b in an afterburning chamber 5.

Synthetic air, which is generated in mixers 21, 21 ', 21 "from flue gas and oxygen, passes through the Lines 6, 7, 8 in the combustion process. The supply of synthetic air via line 6 is assigned to the supply of waste via line 60 to the front end 1 a of the rotary tube 1. The supply of synthetic air via line 7 is assigned to the burner 4a at the front end la of the rotary tube 1. The supply of synthetic air via line 8 is assigned to the burner 4b in the afterburning chamber 5. The line 60 is a high-pressure line, via which the waste is pumped into the rotary tube 1 continuously. The mixing chambers 21, 21 ', 21 "are fed flue gas via line 20 and oxygen via lines 25, 25', 25", the latter from an air separation unit 23. The amount of oxygen supplied to the mixing chambers 21, 21 ', 21 "can be adjusted on the controllers 28, 28', 28"; it is important that this adjustability is independent of one another, so that synthetic air with a different ratio of flue gas to oxygen can be supplied via lines 6, 7, 8 at the designated points in the combustion process. The temperature at the front end 1 a of the rotary tube 1 is approximately 700 ° and rises up to approximately 1200 to 1300 ° C. at the rear end 1 b. Subsequently, the flue gas generated during the combustion passes into the afterburning chamber 5 already mentioned, in which the burner 4b is arranged. The burned-out molten slag is drawn off at exit 10 and quenched in a water bath.

The flue gas passes from the afterburning chamber 5 into a radiation part 11, where it is cooled and from there into a waste heat boiler 12 with a heat exchanger 13. The flue gas is used with further cooling to generate steam that is used to generate electricity (not shown here) . The cooled flue gas, which still has a temperature of approximately 300 to 400 ° C., then passes into a two-stage flue gas cleaning system, the first stage of which is designated by 14 and the second stage is designated by 24. In the first stage 14 of the flue gas cleaning system, dust, chlorine compounds (especially HC1), sulfur dioxide and heavy metals are separated.

The part of the flue gas cleaned in the first stage 14 of the flue gas cleaning system, which is set with the control flap 61, is returned via line 20 to the mixing chambers 21, 21 ', 21 "already mentioned.

The control devices 40, 41, 42 are set - likewise independently of one another. The waste is burned in the rotary tube 1. In principle, the proportion of recirculated flue gas in the total amount of pure air created by adding oxygen corresponds to the proportion of nitrogen in normal air (4: 1). However, it can be chosen lower in order to generate an air ratio lambda greater than 1 and thereby to set optimal operating conditions. A working range is a ratio of flue gas to oxygen between 4: 1 and 1: 1, preferably a range 2: 1 to 1.5: 1.

The consequence of this flue gas circuit is that the rotary tube does not, as would result in combustion with pure oxygen, reach temperatures which are too high for conventional combustion technologies. On the other hand, a large part of the flue gas generated during combustion remains in the cycle. It is important that within the circuit in the first stage 14 of the flue gas cleaning system there is a constant washing out of the harmful emissions, in particular of chlorine, Heavy metals and sulfur dioxide. The cleaning in the first stage 14 is thus designed in such a way that there is no enrichment of these components in the circuit, which - in particular HCl - leads to corrosion of the incineration plant or to an increase in the emission of pollutants in the portion of the flue gas that is released outdoors. could lead. This constant cleaning of the flue gas in the circuit also has the consequence that the proportion of harmful exhaust gases in the part of the flue gas which passes through line 19 to the second stage 24 of the flue gas cleaning system and finally via the chimney 27, compared to conventional ones Plants is significantly reduced because the part of the flue gas released into the atmosphere is much lower than that of a conventional plant. The amount of flue gas can be reduced from about 7.5 x 10 ³ m ³ per ton in known processes to about 1.5 x 10 ³ m ³ per ton of special waste.

In particular, there is a ^" reduction in nitrogen oxides in this form of combustion of the waste in the rotary tube 1 with synthetic air by virtue of the fact that the synthetic air hardly contains any more nitrogen, so that nitrogen oxides can no longer be formed in the rotary tube 1 by combustion of the atmospheric nitrogen.

The part of the flue gas which is not returned to the circuit in line 20 passes via line 19 to the second stage 24, in which the residual purification of the flue gas takes place, in particular with regard to residual sulfur dioxide and residual heavy metals. In addition, the cleaning of dioxins and furans is provided in appropriate separators. If necessary, residual nitrogen oxides can also be cleaned there. In order to avoid that false air is drawn in at the sealing points of the rotary tube 1, the nitrogen components of which could lead to the formation of nitrogen oxides in the flue gas, the seals of the rotary tube 1 are provided with chambers 45 at the front end la and 46 at the rear end lb, over which the Lines 51, 52 is also supplied with flue gas. This flue gas serves as a protective gas for the seals and prevents the intake of normal nitrogen-containing air.

As already discussed at the beginning, the combustion capacity of such a system is increased by approximately 1.5 times as a result of the higher heat capacity of the synthetic air compared to conventional systems.

This increase in combustion performance also leads - in relation to the dimensioning of conventionally designed combustion plants - to a change in flow velocities, which may can lead to the flame of the burner 4a being torn off. This is countered according to the invention in that the flow rate of the combustion air in the rotary tube 1 is controlled in accordance with the following considerations:

A distinction is generally made between burners operated with atmospheric air (e.g. for boiler systems, power plants, waste incineration plants, etc.) which are operated with burner admission pressures of 20 to 30 mbar, and burners operated with oxygen (mainly used in the iron and steel industry) can be operated with burner admission pressures of 2 to 3 bar.

With the method of the type mentioned at the outset, it was initially expected that operation with synthetic Air, that is to say air formed by mixing flue gas and oxygen, is also possible with normal burners, such as can be operated with atmospheric ones. It was initially assumed that the probability of a collision of oxygen and fuel molecules was the same both in atmospheric air and in "synthetic" air, ie also composed of flue gas and oxygen. Surprisingly, however, this was not the case. Rather, when using atmospheric burners with the usual burner admission pressures of 20 to 30 mbar, flame instabilities (pulsation) occurred and thus the flame was broken off. There was a risk of deflagration. Stable combustion conditions could not be achieved in partial or full load operation. The reasons for this lay in the changing thermodynamic boundary conditions, which resulted from the combustion process with synthetic air compared to the combustion process with normal air.

According to the invention, the combustion process is now operated with the aid of synthetic air at a - in comparison to the combustion with atmospheric air - reduced flow rate, either by reducing the burner admission pressure of atmospheric burners or, if this does not allow burner admission pressure adjustment due to its use, by Inserting control flaps in front of the burners is realized.

The explanation arises from the consideration of the heat-specific properties of atmospheric combustion air in relation to synthetic combustion air. For this purpose, the formula is based on the volume-specific heat transport capacity C _Pv = Q / (V x <.._ t-T) used. You also sit there

and used it for atmospheric air

75.0 vol.% N ₂ / 77.5 mass% N ₂

13.3% by volume 0 ₂ / 15.7% by mass 0 ₂

11.7% by volume H ₂ 0 / 6.8% by mass H ₂ 0 and for synthetic air

. 75.0% Vol% C0 _2/84 mass C0 ₂₎ is basis

. 13.3 vol% 0 _2/10% by mass 0 ₂₎ the density of the

11.7 vol.% H ₂ 0/6 mass% H ₂ 0) components

1000 ° C

Assuming a temperature of 1000 ° C, a C _pv = 0.36 kJ / m ³ K is obtained for synthetic air and C _pv = 0.53 kJ / m ³ K for atmospheric air.

The heat transport is therefore around 47% better in synthetic air than in normal atmospheric air under otherwise identical conditions.

The following technical consensus results from this difference in heat capacity:

With the same combustion output, less cycle gas is required for heat removal and transmission at the same temperature. From a thermal point of view, the cycle uses less gas.

Based on these considerations, it becomes plausible that when using atmospheric burners, stable combustion - as with atmospheric air - can only be achieved in the case of using synthetic air if the flow rate is reduced accordingly. Hence the ones already mentioned Control devices 40, 41, 42 are provided which make it possible to reduce the flow velocities when using atmospheric burners.

This statement can be made clear from the following comparison at a certain load point (natural gas throughput):

Operation with normal air with synth.

air

Natural gas throughput 50 Ncbm / h 50 Ncbm / h

Air or Synair throughput 610 Ncbm / h 425 Ncbm / h

Temperature rotary tube 1160 ° C 1080 ° C

Burner pre-pressure 20 mbar '8 mbar

Air number lambda 1.28 1.19

From this comparison it becomes clear that when using synthetic air and the same burner output, the amount of combustion air in relation to operation with atmospheric air under stable combustion conditions from 610 Ncbm / h to 425 Ncbm / h, i.e. could be reduced by approx. 30%. As already described, this is due to the higher one

Heat transport capacity attributed to carbon dioxide (flue gas) in relation to nitrogen (atmospheric air).

By a corresponding modification of the atmospheric burner, a continuous regulation of the mass flow rates of atmospheric air can synthetic air and thus the corresponding burner pressure or the corresponding flow rate can be achieved.

Another possibility is the installation of infinitely variable control devices 40, 41, 42, which are positioned in the flue gas lines in front of the burner units.

Such a reduction in the flow rate can be achieved by continuously regulating the corresponding mass flow rate of the synthetic air by appropriately setting the burner admission pressure in the burner. Another possibility is the installation of infinitely variable control devices in the circuit gas lines upstream of the burner systems, as was created in the exemplary embodiment by the control devices 40, 41, 42. In this way, the control of the mass flow rate using synthetic air can be set so that a stable combustion process without flame breaks or pulsations and essentially the same temperatures as when using atmospheric air.

With both variants, the regulation of the mass flow rate of synthetic air can be set in this way be that the combustion process control in synthetic operation can be carried out without interference, ie without pulsation of the flame or flame break and the associated dangerous deflagration.

Essentially, the required process temperatures, as prescribed for conventional combustion, are observed.

The fact that several differently adjustable mixers 21, 21 ', 21 "are provided enables a targeted combustion process control with regard to the special requirements that are specified by the special waste or the combustion process control. Thus, different mixtures of the synthetic air can be obtained in this way , based on the feed material of the waste, the auxiliary firing in the rotary kiln and the synthetic auxiliary firing in the afterburner chamber can be set independently of one another, so that the three controlled systems, auxiliary firing, rotary kiln, afterburner result independently.

If, for example, special waste of high calorific value is incinerated, the support energy required for this is limited accordingly, since the incineration temperature is due to the largely independent combustion process is guaranteed. If, on the other hand, low-calorific value waste is incinerated, the incineration process can only be continued if the supply of support energy is increased accordingly in order to maintain a certain optimal process temperature. When operating with synthetic air, the increase in the amount of support energy can be avoided by increasing the oxygen concentration of the secondary synthetic air so that, in turn, low calorific value waste can be burned with an approximately constant support energy. Or, in other words: the supply and concentration of synthetic combustion air to enrich the waste product can now be regulated depending on the waste determined by the calorific value, as well as by the additional amount of synthetic air. The flue gases, which leave the rotary tube in the direction of the afterburner, are burned there at temperatures between 1200 and 1300 "c. The synthetic air supplied to the burners is only enriched with the stoichiometrically necessary oxygen concentration in the corresponding mixer.

This independent regulation of the composition of the synthetic air in the controlled systems of auxiliary firing, rotary kiln and afterburner ensures targeted and flexible combustion process control.

Claims

P A T E N T A N S P R Ü C H E

Process for reducing the emissions from the incineration of waste in an incineration plant, in which part of the flue gas produced during the incineration is recycled to the entrance of the incineration plant, the recirculated flue gas, the pure oxygen required for combustion to form a flue gas and oxygen formed synthetic air is admixed, the ratio of flue gas and oxygen in the synthetic air being 4: 1 to 1: 1, characterized in that the burner admission pressure of the burner (1) provided at the front end (la) of the combustion chamber forming the combustion chamber ( 4a) can be set both in start-up operation under atmospheric conditions or the burner admission pressure in stationary operation under synthetic conditions, and the ratio of the burner admission pressures from atmospheric to synthetic conditions is set to 1: 2.5 to 1: 5, preferably 1: 4.

A method according to claim 1, characterized in that the burner admission pressure is 5 to 15 mbar, preferably 8 mbar, under synthetic conditions. Method according to Claim 1 or 2, characterized in that the synthetic air formed by flue gas with the addition of pure oxygen can be regulated independently of one another on controlled systems for the auxiliary firing, for the combustion of the waste and for the combustion in the afterburning chamber.