EP4036471B1

EP4036471B1 - Waste incinerator

Info

Publication number: EP4036471B1
Application number: EP21153944.0A
Authority: EP
Inventors: Stanislav Paluda
Original assignee: Individual
Current assignee: Individual
Priority date: 2021-01-28
Filing date: 2021-01-28
Publication date: 2023-09-13
Anticipated expiration: 2041-01-28
Also published as: EP4036471A1

Description

Field of Art

The present invention relates to improvements in waste incinerator plants.

Background Art

Waste is currently treated by various technologies, including thermal treatment such as incineration. Competing technologies include recycling or landfilling. Recycling is not suitable for all types of waste, especially not for hazardous waste or for waste for which the cost of recycling is significantly higher than the cost of production. Landfilling is unsuitable for hazardous waste, and the European Union wishes to limit landfilling to minimum in the course of the next decade. Waste incineration is thus becoming a necessary and developing technology. To-day incinerators use a burner or furnace which is connected to a boiler, followed by a heat exchanger and a scrubber. The ashes and other solid residues are usually landfilled, and the gases are let out into the environment, after further cleaning when necessary.
Current incinerators use various types of furnaces, including rotary kilns. The co-current rotary kiln is prevalent in the currently used incinerators, since the counter current set up of the kiln involves a significant temperature gradient across the inlet door, thus requiring specially engineered designs of the fire-resistant inlet door.
In the scrubber, the flue gas is subjected to treatment by scrubbing in order to remove chemical pollution and fine ashes. The scrubbing may be dry scrubbing or wet scrubbing or a combination thereof. Dry scrubbing is more commonly used. Wet scrubbing produces a large amount of contaminated water. Semi-wet scrubbing using lime milk is a compromise between wet scrubbing and dry scrubbing.
The incinerators known in the art are typically adapted to a certain type of waste, require a significant amount of fuel, and produce emissions, which are usually within the set emission limits, but considering the amount of the solids and gases produced, it is desirable to further decrease the amount of emissions, i.e. emitted solids and hazardous gases. Furthermore, it is desirable to provide a stable working regimen without any problematic peak values of emissions or temperatures.
US 4,768,448 discloses a grate incinerator with a wet scrubber system provided with a filter press. US 4,922,841 discloses a counter current drum incinerator with a dry gas cleaning system.

Disclosure of the Invention

The present invention provides a waste incinerator which contains a rotary kiln the outlet of which is connected to secondary combustion chamber(s) which are in turn connected to a boiler connected to a heat exchanger which is further attached to a wet scrubber, and flue gas outlet of the wet scrubber is connected to a fan and a chimney, wherein

the rotary kiln is a counter current rotary kiln,
the wet scrubber has closed liquid circulation provided with at least two decanting tanks and a filter press, and
the flue gas outlet of the wet scrubber is connected to the fan and the chimney through the heat exchanger.

In counter current rotary kiln, the flue gases flow in the opposite direction than the direction of movement of the waste, against the inclination of the kiln. Solids move by the rotary motion and by gravity from the high end of the kiln to the low end of the kiln.
The use of counter current rotary kiln results in a more complete burning of the ash and less than half of the total amount of fly ash, compared to the commonly used co-current rotary kiln. Counter current rotary kiln is also more suitable for burning trash comprising very varied materials.
The waste is incinerated in the counter current rotary kiln at temperature of at least 900 °C. In a stationary chamber of the rotary kiln, the temperatures may even exceed 1100 °C.
Preferably, the counter current rotary kiln is provided with a slag and ash outlet in its lower part. Prior art waste incinerators are typically provided with the slag and ash outlet at the bottom of secondary combustion chamber. This prior art solution results in the content of unburnt carbon being about 4 % or more, due to lack of available oxygen. In this preferred embodiment of the invention, sufficient supply of oxygen may be provided and the waste is burnt more completely, down to the content of the unburnt carbon well below 2 %.
Secondary combustion chamber(s) ensure complete burning of the remaining particles and matter in the flue gases. The flue gas exiting the secondary retention chamber(s) is chemically stabilized.
Within the framework of the invention, it was found that a sufficient volume must be provided for secondary combustion. The minimum total volume of secondary combustion chamber(s) is 1.8 times the volume of the rotary kiln. More preferably, the volume of secondary combustion chamber(s) is 1.8-3 times the volume of the rotary kiln. Most preferably, the volume of secondary combustion chamber(s) is 2-2.5 times the volume of the rotary kiln. Sufficient volume of secondary combustion chambers results in thorough burning of the waste and reduction of load on the following components of the incinerator plant. This leads to economically and environmentally effective operation of the plant.
It is strongly preferred to include at least two secondary combustion chambers, in a more preferred embodiment three secondary combusion chambers are provided. In prior art incinerators, the necessary retention time of the flue gas in the secondary combusion chamber of 2 seconds was achieved by causing rotation of the flue gases using a suitably directed burner. However, such solution causes unnecessary use of energy (gas) by the burner, and especially causes a peak increase of undesirable emissions upon the start of the burner, and are not needed in the present invention. The use of at least two secondary combusion chambers having a sufficient total volume allows to achieve the retention time of significantly more than 2 seconds for the flue gas in a simple and effective way.
The flue gas must be maintained at a temperature of at least 850 °C in the secondary combustion chamber(s) for at least 2 seconds, as required by technological norms. The herein described volume and optionally number of the secondary combustion chambers allows to increase the retention time to at least 5 seconds, preferably to at least 7 seconds. Preferably, the temperature of burning in the secondary combustion chambers is at least 1100 °C.
The boiler uses the heat of the flue gas to convert water into steam, the steam may be used as an energy resource, e.g., for heating or for producing electricity.
The heat exchanger allows to further decrease the temperature of the flue gas before it enters the wet scrubber. To this end, the heat exchanger is configured to bring into thermal contact the flue gas entering the heat exchanger from the boiler or from the secondary combustion chamber(s) with the flue gas entering the heat exchanger from the wet scrubber. A typical temperature of the hot flue gas (from the boiler or even directly from the secondary combustion chamber(s)) is about 400 to 800 °C, while a typical temperature of the cold flue gas (from the wet scrubber) is about 70 to 100 °C.
The heat exchanger is preferably configured to maintain the volume flow ratio of the hot flue gas to the cold flue gas from 2:1 to 1:2, more preferably about 1:1.
In some embodiments, the heat exchanger contains substantially concentric tubes wherein the hot flue gas passess through the inner tube and the cold flue gas passes through the outer tube.
Wet scrubber according to the invention has a closed liquid circulation. This overcomes the usual problem of wet scrubbers which consists in the production of a large amount of contaminated water. According to the invention, the closed liquid circulation includes at least two decanting tanks in which the solids are decanted, the decanted solid portion is moved to a filter press which, by pressing the decanted portion, produces a filter cake and returns the liquid back to the decanting tank or to the wet scrubber. The liquid from the last decanting tank is returned to the wet scrubber to be used repeatedly for scrubbing the flue gas.
The wet scrubber may be configured to use water or aqueous hydroxide or lime solution to remove in particular acidic compounds (HCl, HF etc.) and/or sulfur oxides from the flue gas. Furthermore, the wet scrubber may be configured to utilize carriers (e.g., ring-shaped carriers) for deposition and removal of salts produced by wet scrubbing.
The liquid in the wet scrubber preferably contains an alkali metal or an alkaline earth metal hydroxide, such as NaOH. This allows to remove acidic contaminants such as hydrochloric, hydrofluoric, sulfuric, nitric acids and acidic oxides. As the hydroxide is consumed in the chemical reactions, the closed liquid circuit preferably contains a stock tank for storing hydroxide solution and for adding it to the wet scrubber liquid in a controlled manner.
In preferred embodiments, the volume of wet scrubber is about 0.8 to 1.5 times the volume of the rotary kiln, more preferably the volume of wet scrubber is about the same as the volume of the rotary kiln.
There must be at least two vertically oriented decanting tanks in the closed liquid circulation, which are connected by a tube in their upper half, preferably in their upper third. The first decanting tank may or may not be connected to the filter press, while the second (and further) decanting tank is provided with a bottom outlet for solids or suspensions, said bottom outlet being connected to the filter press. This ensures that majority of the solids remains in the first decanting tank, and the second (or further) decanting tank(s) thus achieve(s) an additional fine cleaning, resulting in an increased purity of water which is returned to the wet scrubber.
Furthermore, the filter press functions more effectively especially in the arrangements in which the first decanting tank is not connected to the filter press - such an arrangement results in a lower load and thus an increased effectiveness of the filter press. The overall effects of this arrangement include less frequent need for outages and less need for cleaning of the components of the wet scrubbing assembly.
In preferred embodiments, the total volume of the decanting tanks is about 2-4 times the volume of the rotary kiln, more preferably about 3 times the volume of the rotary kiln.
The scrubbed flue gas existing from the wet scrubber then passes through the heat exchanger where it is employed to cool down the flue gas exiting the boiler or the secondary combustion chamber before the flue gas enters the wet scrubber.
The wet scrubber may be followed by a filter to remove the remaining solid and ash particles from the flue gas, and/or by sorbent columns for adsorbing remaining gaseous contaminants. Such filters and sorbent columns are known to a person skilled in the art and are commonly used in waste incinerators. In some embodiments, the filters located between the heat exchanger and the fan may include a dioxin filter or a mechanical filter for filtering off solids.
The fan ensures the movement of flue gas through the device. The low pressure (vacuum) generated by the fan draws the flue gas from the rotary kiln and through the device.
The pipes connecting the individual components are typically made of metals such as steel. However, such metallic tubes are mechanically rigid and tend to collect deposits of solid particles and ashes in any bends or elbows, thus requiring regular cleaning which requires outages. Within the framework of the invention, it was found that it is very advantageous to use metallic straight parts of the piping and plastic elbows or bends. The plastic elbows or bends are flexible and tend to slightly vibrate, thus not collecting the solid deposits to a significant extent. This further increases time between maintenance outages. The plastic material must be selected so that it withstands the relevant temperatures - this information is available from producers of various plastics.
The present invention further encompasses a method of incineration of waste, comprising the steps of:

incinerating waste in counter current rotary kiln to produce flue gas,
leading the flue gas from the rotary kiln via secondary combustion chamber(s) to a boiler or to a heat exchanger, wherein the flue gas is retained in the secondary combustion chamber(s) for at least 5 seconds at a temperature of at least 850 °C;
in the heat exchanger, transfering energy from the flue gas coming from the boiler or from the secondary combustion chamber(s) to flue gas coming from wet scrubber,
wet scrubbing the flue gas coming from the heat exchanger in the wet scrubber, and recycling the scrubbing water through at least two decanting tanks in a closed liquid circuit,
leading the flue gas, optionally via filters, to a fan and a chimney, wherein the fan causes the movement of the flue gas through incinerator components.

The present invention thus provides improvements in waste incinerators which lead to surprisingly significant decrease in emissions, and which result in significantly less frequent need for maintenance outages.

Brief description of drawings

Figure 1 schematically shows an embodiment according to the present invention.
Figure 2 schematically shows a comparative embodiment, used for comparing the effectiveness of certain features of the invention.

Examples of carrying out the Invention

Example 1

An example embodiment of the present invention is schematically shown in Figure 1. The arrows in the figure show the direction of the flow of the waste and of the flue gas.
The waste incinerator has a waste inlet 1 through which the waste to be incinerated enters the system and is led to a counter current rotary kiln 2. The rotary kiln 2 is provided with a slag and ash outlet 3 at its lower part. The waste is burnt in the rotary kiln 2 and converted into flue gas. The flue gas exits the rotary kiln via outlet 4 and enters secondary combustion chambers 51 and 52, one after another. The flue gas exiting from the secondary combustion chamber 52 may enter the boiler 6, or may by-pass the boiler. In the boiler 6, the flue gas transfers energy to water, thus producing steam which can be used in downstream technologies or heating. The water from the boiler 6 or from the by-pass (i.e., directly from the secondary combustion chamber 52) enters a heat exchanger 7 in which it is brought into thermal contact with cold flue gas from wet scrubbing. The cooled flue gas from the heat exchanger 7 enters a wet scrubber 8. The wet scrubber 8 is provided with two decanting tanks 91 and 92. The decanting tanks 91 and 92 are connected by a tube in approximately the upper third of their height. Water from the wet scubber 8 enters the the decanting tank 92 where larger solid particles precipitate. Water then passes into the decanting tank 91 via the connecting tube which is in a height which ensures that only fine suspension enters the decanting tank 91. From the tank 91 water is led back to the wet scrubber 8.
The scrubbed flue gas exiting the wet scrubber 8 is led back to the heat exchanger 7 to cool down the flue gas to be scrubbed. The exit of the cold flue gas from the heat exchanger 7 leads the flue gas to a filter 11 and dioxin filter 12, and then via a fan into a chimney 13.
Replacing metallic bends in the piping connecting the components by plastic bends resulted in a much lower frequency of maintenance outages.
Secondary combustion chambers 51 and 52 have a total volume of about 2 times the volume of the rotary kiln 2. The decanting tanks 91 and 92 have a total volume of about 3 times the volume of the rotary kiln 2, and the ratio of volumes of the tanks 91 and 92 is about 1:1. The wet scrubber 8 has about the same volume as the rotary kiln.

Example 2

The embodiment described in Example 1 was compared with a comparative embodiment shown in Figure 2 which contained a waste inlet 101, a counter current rotary kiln 102 equipped with a slag and ash outlet 103 in its lower part. Outlet 104 is connected to secondary combustion chambers 151, 152, which are in turn connected to a boiler 106 (which can be by-passed) from which the flue gas is led to a heat exchanger 107 followed by a wet scrubber 108. The wet scrubber is provided with one decanting tank 109 provided with a filter press 110. The flue gas from the wet scrubber 108 is led directly into a filter 111 and dioxin filter 112, followed by a fan and a chimney 113.
The differences of the embodiment of Figure 2 from the embodiment shown in Figure 1 are: smaller volume of the secondary combustion chambers (by about 30 %), only one decanting tank, and the flue gas from the wet scrubber not being led via the heat exchanger and not cooling the hot flue gas before entering the wet scrubber.
The amounts of contaminants and emissions were measured in samples drawn from a sampling spot located in the chimney, downstream from the fan.
The content of solid contaminants (TZL) was measured gravimetrically, SO₂ was measured using UV spectrometry, CO was measured using infrared spectrometry, NO_x was measure unsing chemiluminescence, O₂ was measured using paramagnetic measurements, total organic carbon (TOC) was measured using by gas chromatography with flame ionization detection. Content of solid particles of size fractions PM₁₀ and PM_2.5 were calculated as 85% and 60% proportion of TZL. The measurements and calculations were carried out by a certified laboratory service.

The following table shows the obtained data for the two embodiments shown in the figures:

Contaminant	Emissions in tons/year Figure 2 (comparative)	Emissions in tons/year Figure 1 (invention)	Decrease of emissions in %
TZL	0.043	0.009	79.1
NO_x	2.624	1.858	29.2
SO₂	0.276	0.125	54.7
TOC	0.078	0.049	37.2
PM₁₀	0.0363	0.0008	97.8
PM_2.5	0.0256	0.0005	98.0

Claims

Waste incinerator which contains a rotary kiln (2) the outlet (4) of which is connected to secondary combustion chamber(s) (51, 52) which are in turn connected to a boiler (6) connected to a heat exchanger (7) which is further attached to a wet scrubber (8), and flue gas outlet of the wet scrubber (8) is connected to a fan and a chimney (13),
characterized in that
- the rotary kiln (2) is a counter current rotary kiln,

- the wet scrubber (8) has a closed liquid circulation provided with at least two decanting tanks (91, 92) and a filter press (10), and

- the flue gas outlet of the wet scrubber (8) is connected to the fan and the chimney (13) through the heat exchanger (7).
The waste incinerator according to claim 1, wherein the counter current rotary kiln (2) is provided with a slag and ash outlet (3) in its lower part.
The waste incinerator according to claim 1 or 2, wherein minimum volume of the secondary combustion chamber(s) (51, 52) is 1.8 times the volume of the rotary kiln (2).
The waste incinerator according to any one of claims 1 to 3, wherein at least two secondary combustion chambers (51, 52) are provided.
The waste incinerator according to any one of claims 1 to 4, wherein the heat exchanger (7) is configured to bring into thermal contact hot flue gas entering the heat exchanger (7) from the boiler (6) or from the secondary combustion chamber(s) (51, 52) with cold flue gas entering the heat exchanger (7) from the wet scrubber (8), and to maintain the volume flow ratio of the hot flue gas to the cold flue gas from 2:1 to 1:2, more preferably about 1:1.
The waste incinerator according to any one of claims 1 to 5, wherein the heat exchanger (7) contains substantially concentric tubes wherein the hot flue gas passess through the inner tube and the cold flue gas passes through the outer tube.
The waste incinerator according to any one of claims 1 to 6, wherein the volume of the wet scrubber (8) is 0.8 to 1.5 times the volume of the rotary kiln (2).
The waste incinerator according to any one of claims 1 to 7, wherein the volume of the decanting tanks (91, 92) is about 2-4 times the volume of the rotary kiln (2).
The waste incinerator according to any one of claims 1 to 8, wherein the at least two decanting tanks (91, 92) are vertically oriented and connected by a tube in their upper half, preferably in their upper third.
The waste incinerator according to any one of claims 1 to 9, wherein the first decanting tank (92) is not connected to the filter press (10), while the second decanting tank (91) is provided with a bottom outlet for solids or suspensions, said bottom outlet being connected to the filter press (10).
The waste incinerator according to any one of claims 1 to 10, wherein the straight portions of the pipes connecting the individual components are of metal such as steel, and at least some elbows or bends of the pipes are made of plastic.
A method of incineration of waste, comprising the steps of:
- incinerating waste in counter current rotary kiln (2) to produce flue gas,

- leading the flue gas from the rotary kiln (2) via secondary combustion chamber(s) (51, 52) to a boiler (6) or to a heat exchanger (7), wherein the flue gas is retained in the secondary combustion chamber(s) (51, 52) for at least 5 seconds at a temperature of at least 850 °C;

- in the heat exchanger (7), transfering energy from the flue gas coming from the boiler (6) or from the secondary combustion chamber(s) (51, 52) to flue gas coming from wet scrubber (8),

- wet scrubbing the flue gas coming from the heat exchanger (7) in the wet scrubber (8), and recycling the scrubbing water through at least two decanting tanks (91, 92) in a closed liquid circuit,

- leading the flue gas, optionally via filters (11, 12), to a fan and a chimney (13), wherein the fan causes the movement of the flue gas through incinerator components.