WO2010123444A1

WO2010123444A1 - Method and plant for burning solid fuel

Info

Publication number: WO2010123444A1
Application number: PCT/SE2010/050429
Authority: WO
Inventors: Klaus Gärtner
Original assignee: Kappa Sa
Priority date: 2009-04-22
Filing date: 2010-04-20
Publication date: 2010-10-28
Also published as: SE0900536A1; SE533716C2

Abstract

At a method and a plant for burning solid fuel in a rotating furnace (10) and an after-burning chamber (50) the fuel is initially incinerated in an oxygen-deficient environment at a lower temperature interval in the furnace (10) and in a first stage (60) of the after-burning chamber to thereby produce combustible fumes therein. The fumes are thereafter subjected to an increase in velocity by being passed through a contracted passage (70) between the first stage (60) and a second stage (80) of the after-burning chamber (50). At the same time combustion air is blown into the contracted passage (70) for a conclusive combustion at a higher temperature interval of the fumes in the passage (70) and in the second stage (80) of the after-burning chamber (50).

Description

Method and plant for burning solid fuel

TECHNICAL FIELD

This invention relates to a method and a plant for burning solid fuel, compris- ing a rotating furnace and an after-burning chamber connected to the furnace.

BACKGROUND

Prior art inclined rotary kiln incinerators of this kind that are typically utilized for burning various waste material often suffers from more or less imperfect combustion conditions due to insufficient combustion temperatures.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a method and an incineration plant where the fuel can be burnt at high temperatures in order to avoid undesired imperfectly combusted residual products.

In one aspect of the invention, the fuel is initially incinerated in an oxygen- deficient environment at a lower temperature interval in the furnace and in a first stage of the after-burning chamber to thereby produce combustible fumes therein. Means such as a suction blower are provided for drawing the fumes from the rotating furnace and through after-burning chamber. The combustible fumes are subjected to an increase in velocity by being passed through a contracted passage defined by a separating wall between the first stage and a second stage of the after-burning chamber. Combustion air is blown into the contracted passage to thereby obtain a complete combustion of the combustible fumes at a higher temperature interval in the passage and in the second stage of the after-burning chamber.

One aspect forming the basis of the invention, is that in order to achieve a more complete combustion, a highest possible amount of combustion air should interact with a highest possible amount of combustible fumes per time unit. To blow the combustion air into a larger volume of combustible fumes, for example in the first or second stage of the combustion or after-burning chamber, will not result in such an efficient and complete combustion, since then the oxygen atoms of the combustion air and the carbon atoms of the fumes would have less opportunity to interact in the slower flow of the larger volume.

I RECORD COPY-TRΔMSi ώτιnM i In the first stage of the after-burning chamber and in the rotating furnace, an oxygen-depleted gasification environment prevails, resulting in a state of partial pyro- lysis at a sufficient high temperature to disintegrate the fuel into combustible flue-gas particles capable of being combusted at high velocity at the outlet of the contracted flue-gas passage in the second stage of the after-burning chamber. In other words, the after-burning chamber is divided into two stages: a first stage having a lower combustion temperature, for example 950 ⁰C, and a second stage having a higher combustion temperature, for example 1200 ⁰C. The flue-gas passage provides a high increase in velocity for the fumes or flue-gases that together with the extra supply of combustion air in the passage provide for a renewed combustion that substantially burns all non-combusted organic material in the fuel and the gases thereof.

Another aspect is that in order to obtain efficient combustion, a steady state condition should be maintained for the lower and higher temperature intervals in the furnace and the after-burning chamber. The furnace and the after-burning chamber should then have an efficient heat-insulating and heat-accumulating lining, i.e. the lining may need a certain minimum thickness. Such minimum thickness may vary depending on selected lining materials and plant size, but may for a typical waste incineration plant need to be about 1.3 m in the after-burning chamber and about 0.5 m I the rotating furnace. Even temperatures and an efficient combustion are thereby ob- tained in the process. There is thereby also little or no need for additional fuels such as combustible oil or gases to keep the process running.

The preservation of the steady state condition can further be controlled by selectively returning combusted fumes to the furnace from a combusted fumes outlet of the plant. The steady state condition may, however, also be controlled in other ways, for example by adaption of the fuel feed rate to the present type of fuel.

The lower temperature interval can extend between about 950 ⁰C and 1100 ⁰C and the higher temperature interval can extend between about 1100 and 1200 ⁰C.

In an embodiment of the invention, non-combusted residual products can be received through an open bottom of the first stage of the after-burning chamber. Non-combusted residual products of the second stage of the after-burning chamber can be received by a particulate bed at a bottom of the second stage.

The lining of the furnace may further have a varying height or thickness to provide spaces between a temporary bottom face of the rotating furnace and the fuel, whereby cool solid fuel is not allowed to form an insulation against the temporary bottom face of the rotating furnace barrel. The hot gases in the furnace may then more easily access all faces of the fuel. If the height or thickness varies in the circumferential direction, the projecting portions of the lining may also more easily carry the fuel around and mix the fuel in the furnace so that all portions of the fuel positively comes into contact with the oxygen-depleted hot gases.

While the furnace is specifically adapted for the incineration of solid fuel, it can also incinerate fluid fuel.

Other features and advantages with the invention may be apparent from the appended claims and the following description of embodiments.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic sectional view of a portion of a plant according to the invention; and FIG. 2 is a sectional view along line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

The diagrammatic view of FIG. 1 shows an incineration plant having a rotational furnace or kiln 10 which is fed from a conduit 32 with fuel 40 in the form of, for example municipal solid waste, by means of a screw feeder 30. As indicated, when starting the plant, and when otherwise needed during use, the furnace can also be fed with gaseous and liquid fuels through fuel lines 34, 36 connected to the furnace 10. Also combusted fumes that have passed the plant can be returned to the furnace 10 through a line 38. As indicated in FIG. 2, the rotationg furnace can have a steel jacket or mantle

20. The diameter of the jacket 20 is typically about 4 m and its length may then be about 17 m. The lining 22 of the furnace comprises refractory blocks 24, 26 (FIG. 2) having an alternating height; the high blocks 26 may typically have a height or thickness d of about 0.5 m. In a manner not shown, particularly the high blocks 26 may also be angled or obliquely oriented relative the rotational axis of the furnace 10 to thereby, in a screw-like manner facilitate the fuel transport through the reclined or inclined furnace 10. In addition to ease access of fumes to the temporary bottom faces of the fuel 40, the higher blocks 26 also serve to carry, spread and mix the solid fuel 40 in the furnace so that the initial incineration process positively reaches every part of the fuel 40. The inclination of the furnace is typically about 4 degrees down to an inlet of an after-burning chamber 50.

On a typical incineration process, the furnace 10 is rotated about 1 rpm by a known drive (not shown), and it will then take about 1 hour for newly fed-in fuel to be gasified to primary fumes / combustive gas and other particles to be introduced in a first stage 60 of two stages 60, 80 in the after-burning chamber 50. Non-gasified solid materials drop down into a water container 62, for example, in the bottom of the first stage 60 and are transported away for further handling by means of a conveyor 64 such as a screw conveyor. According to the invention, the after-burning chamber 50 as well as the furnace 10 has a large mass (typically totaling to 1500 tons), i.e. a thick lining 52, for example of refractory ceramic material, such as brick material or cast compounds, serving as a heat insulation and a heat accumulator, and thereby is able to maintain the high temperatures in a steady state condition with small fluctuations during the operation of the plant. The lining 52 in the after-burning chamber 50 may typically have a thickness D of about 1.3 m. The heat losses through the lining of the afterburning chamber 50 may typically be about 200 kcal/m²/h, while those in the furnace 10 may be about 1700 kcal/m²/h; the furnace 10 has a relatively smaller mass of refractory and insulating material as otherwise the weight of the furnace 10 would be too high.

In order to facilitate the maintenance of the lining 52 in the after-burning chamber 50, the interior surface layer of chamber 50 can be composed of relatively thin refractory blocks 56 having a thickness of about 40 mm, whereby the thick blocks 54 located behind the thin blocks 56 do not need to be replaced when renovating the after-burning chamber 50.

In the after-burning chamber 50 there is further provided a separating wall 72 that divides chamber 50 into its both stages 60 and 80.

In the example shown, the vertical separating wall 72 extends also a distance horizontally between the both stages 60 and 80 of the after-burning chamber 50 to define an elongate passage in the shape of a flue-gas channel or a flame port 70 in the top portion of the after-burning chamber 50. The separating wall 72 may, however, also comprise a single vertical or sloping wall layer of refractory material that has a shorter passage 70 (not shown). In the flue-gas channel or passage 70 there is an inlet for combustion air fed by a blower 74 that typically delivers an airflow having a flow rate of about 30 m/s.

By the arrangement described above the plant can be regarded as divided into a primary incineration unit A with an oxygen-depleted, gasifying incineration environ- ment and a secondary incineration unit B with an oxygen-rich, highly combustive combustion environment.

In the example shown, a boiler in the shape of a gas-liquid heat exchanger 90 is connected to an outlet 82 for the smoke gases or fumes from the second stage 80 of the after-burning chamber 50. Steam capable of powering a turbine 92 is gener- ated in the heat exchanger 90. Turbine 92 is in turn capable of driving a generator 94 for producing electrical power.

A suction blower or fan 96 is present at an outlet of the fumes from the boiler 90 for providing a sufficient negative pressure or vacuum in the furnace 10, afterburning chamber 50 and boiler / heat exchanger 90 to draw the fumes or smoke gases through the plant.

In connection to the plant, a plurality of different components (not shown) may further be present, such as a flue-gas purifier, and other components capable of handling solid, liquid and gaseous residual products needed to be taken care of during plant operation. During plant operation a steady state condition is maintained in the rotating kiln/furnace 10 and in the after-burning chamber 50. In order to obtain such steady state condition and a complete combustion process the following measures are taken:

1) In the primary incineration unit A an initial incineration condition prevails in the form of partial pyrolysis in a gasifying, oxygen-depleted environment that produces combustive gas in a substantially constant, lower temperature interval of about 950-1100 °C. Thereby, the fuel 40 can be almost totally gasified so as to leave a minimum amount of residual ash of only about 4-5 % of the original fuel content, which residual ash falls down into the vertical shaft formed by the first stage 60. As a comparison, conventional furnaces of the prior art are expected to leave about 20-30% residual ash, The residual ash may also comprise solid components of a high melting point in the fuel, such as glass and metals, which are prevented from being melted in the lower temperature and may detrimentally adhere to the lining 52.

2) In the secondary incineration unit B a final incineration condition prevails with a substantially constant increased temperature interval of about 1100-1200

⁰C. The increased temperature is obtained in that the combustive gas produced in the incineration unit A is accelerated to a high velocity (typically about 30 m/s) according to the principle of Bernoulli in the reduced passage 70, and in that combustion air is blown across the combustive gas by the blower. As indicated above, very large amounts of O₂ may then effectively interact in the reduced passage with very large amounts of combustible CO in the combustive gas for being transformed into CO₂ in the resulting heated and combusted fumes that leaves the after-burning chamber 50. At entry into the larger volume in the second stage 80 of the after-burning chamber 50, the velocity of the fumes is decreased. Possible remaining solid residual products, such as magma, fall down onto the bottom of the secondary stage 80 in the vicinity of the boiler 41 and are cooled into solid phase. To facilitate removal of such residual products that may form a glassy skin on the bottom, that bottom may have a particle bed layer 84 of clay granules, such as chamotte clay granules.

As diagrammatically indicated in FIG. 1 , a number of temperature sensors 12 may be located at suitable places in the plant to sense the steady state condition in the plant. The temperatures sensed by the sensors 12 are signaled via signal con- nections such as signal lines 14 to a computer 16 such as a control computer for the plant. Computer 16 can then, by means of the temperature information, compare the actual state with a desired steady state in a software application. On deviations between the actual and desired states, the computer can, for example, increase or decrease the return flow of cool fumes through line 38, by controlling a valve 98 in the line 38 via a signal connection 18. If, for example, a larger amount of synthetic resin material having a large energy content (typically about 9-10 Mcal/kg compared to normally about 2,5 Mcal/kg) is temporary present in the fuel, the return flow of cool fumes need to be increased to decrease the mean temperature in the unit A down to at least 1100 ⁰C. It may also be possible to feed water into the plant for cooling pur- poses. If, on the other hand, the temperature sensors 12 signals that the plant runs the risk of being cooled down to a temperature below the steady state condition, the return flow of cool fumes will be throttled down and possibly also the furnace 10 and the after-burning chamber 50 may temporarily be fed by additional fuel via lines 34 and/or 36. These processes are also controlled in a manner known as such by the computer 16 via signal connections 14, 18, as schematically indicated in FIG. 1.

When starting a cool plant up to steady state condition, the gaseous or liquid fuels that are supplied through lines 34, 36 can be burned in the furnace 10. To this end, one or more gas or oil burners 58 (only one oil burner is shown in FIG. 1) may additionally be provided in the walls of the after-burning chamber 50.

The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom. Modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit of the invention or the scope of the appended claims.

Claims

1. Method for burning of solid fuel (40) in a rotating furnace (10) and an afterburning chamber (50) connected to the furnace, comprising feeding the fuel into a an axial end of the furnace (10); characetrised by, initial incineration of the fuel (40) in an oxygen-deficient environment at a lower temperature interval in the furnace (10) and in a first stage (60) of the after-burning chamber (50) to thereby produce combustible fumes therein; increasing the velocity of the fumes through a contracted passage (70) between the first stage (60) and a second stage (80) of the after-burning chamber (50); and forced injection of combustion air into the contracted passage (70) to thereby obtain combustion of the combustible fumes at a higher temperature interval in the passage (70) and in the second stage (80) of the after-burning chamber (50).

2. Method according to claim 1 , comprising maintaining a steady state condition for the lower and higher temperature intervals in the furnace (10) and the after-burning chamber (50) by a heat insulating and heat accumulating lining (22,52) of the furnace and the after-burning chamber.

3. Method according to anyone of the preceeding claims, comprising maintaining a steady state condition for the lower and higher temperature intervals by selectively returning combusted fumes to the furnace (10) from a combusted fumes outlet.

4. Method according to anyone of the preceeding claims, comprising maintaining the lower temperature interval between about 950 and 1100 ⁰C and the higher temperature interval between about 1100 and 1200 ⁰C.

5. Method according to anyone of the preceeding claims, comprising receiving residual fuel products through an open bottom of the first stage of (60) the after-burning chamber (50).

6. Method according to anyone of the preceeding claims, comprising receiving residual fuel products on a particle bed (84) at a bottom of the second stage (80) of the after-burning chamber (50).

7. Method according to anyone of the preceeding claims, comprising exposing a temporary bottom face of the fuel (40) to the fumes in the rotating furnace by a varying thickness of a refractory lining (22) of the furnace.

8. Plant for burning solid fuel (40), comprising: a rotary furnace (10); an after-burning chamber (50) connected to the furnace, characterised by, a primary incineration unit (A) comprising the furnace (10) and a first stage of the after-burning chamber (50) for initial incineration of the fuel in an oxygen deficient environment at a lower temperature interval in the furnace (10) and in the first stage (60) for producing combustible fumes therein; a secondary incineration unit (B) comprising a contracted passage (70) for subjecting the fumes to an increase in velocity between the first stage (60) and a second stage (80) of the after-burning chamber (50), and an inlet (72) for forced injection of combustion air into the contracted passage (70) for conclusive combustion of the combustible fumes at a higher temperature interval in the passage (70) and in the second stage (80) of the after-burning chamber (50).

9. Plant according to claim 8, wherein the inlet (72) is oriented radially into the contracted passage (70).

10. Plant according to claim 8 or 9, comprising a blower (74) for injecting the combustion air through the inlet (72).

11. Plant according to claim 8-10, comprising a return conduit (38) for selectively returning combusted fumes to the furnace (10) from a combusted fumes outlet.

12. Plant according to claim 8-11 , comprising an open bottom of the first stage (60) of the after-burning chamber (50) for receiving residual fuel products therethrough.

13. Plant according to claim 8-12, comprising a refractory lining (22) of the furnace (10), the lining (22) having a varying thickness.