AU7074594A

AU7074594A - Method of treating solid material at high temperatures

Info

Publication number: AU7074594A
Application number: AU70745/94A
Authority: AU
Inventors: Olli Arpalahti; Matti Hitunen; Kim Westerlund; Kurt Westerlund
Original assignee: Ahlstrom Corp
Current assignee: Ahlstrom Corp
Priority date: 1993-06-24
Filing date: 1994-06-22
Publication date: 1995-01-17
Also published as: NO955252D0; KR960703221A; FI933961A0; CA2164981A1; NO955252L; CN1125980A; WO1995000804A1; EP0705411A1; FI933961A

Description

METHOD OF TREATING SOLID MATERIAL AT HIGH TEMPERATURES

The present invention relates to a method of treating solid material such as wastes and biological sludges at a high temperature and of recovering heat from said process as we.„l as of cooling generated process gases.

Due to the increase of the amount of solid material, such as municipal waste, which refers in this connection in addition to actual municipal waste also to industrial biological and other sludges or the like, the disposal thereof by combusting has become a more and more interesting method of utilizing the energy in the waste, for example, for steam generation. The amount of waste .increases constantly and it is not economically reasonable to indefinetely increase the capacity of the dumping sites. Moreover, for example, large amounts of substances dangerous to the environment may be released from the wastes. The treatment of waste by incineration has thus proved to be an interesting method for disposal and at the same time for utilization of the waste thus generated as efficiently as possible. The incineration of wastes, since the material concerned is very impure, generates flue gases which are rather difficult to treat and which may als release substances which restrict and/or make difficult an efficient heat recovery from the heat being generated in the incineration.

In the treatment of wastes by incineration it is possible to recover energy generated in incineration to a heat exchange med Jam. This is, however, often complicated in the cooling of gases because of the corrosion risk in heat exchange surfaces caused by various substances present, such as chlorine-containing compounds. The heat exchange surfaces, which are in communication with cooling gases containing corroding substances get into very difficult conditions, in which the durability thereof may be very poor.

It has been suggested that the destruction of municipal wastes by incineration would take place in incineration plants utilizing fluidized bed technique. The fluidized bed incineration, however, requires a pre-treat ent of wastes before the incineration, because when using fluidized bed technique the waste must at least be ground to a size fine enough. In most cases this, however, is not enough, but the waste must, for example, be sorted before incineration, because the untreated waste may be very nonhomogeneous both of the content as well size distribution. Also, in fluidized bed incineration, due to its nature the highest temperatures achieved for flue gases are about 1000°C. When the temperatures become higher the bed material may, for example, melt causing disturbances in operation, so usually temperatures lower than that are used.

FI patent 86471 suggests that a fluidized bed boiler were used in connection with the treatment of wastes, in which boiler the incineration fraction being generated in a separate waste pretreatment is incinerated. The biological fraction being generated in the same pretreatment forms biogas as a result of the treatment, which is suggested to be used for the increase of temperature in the flue gas channel subsequent to the fluidized bed boiler. According to said patent by performing this afterburning it is possible to decrease the generation of polyaromatic hydrocarbons harmful to the environment. The described afterburning is carried out, as also the superheating of steam in the flue gas channel subsequent to the combustion chamber.

In this arrangement, however, the problem is the corrosion risk caused by the chlorine compounds in the flue gases, especially when the superheating temperatures of steam will be raised. Chlorine compounds are generated almost always in waste incineration, the presence of which compounds in practice prevents the increase of the superheating temperature high enough, above a certain predetermined temperature.

The incinerat-, _>n of wastes may be carried out also as grate incineration. This is an advantageous method, since it is possible to incinerate untreated waste on the grate. The pretreatment of waste is often a complicated multistage process, whereby the complete elimination thereof brings a significant advantage to the grate incineration compared with the economy of the fluidized bed incineration. In the grate incineration the temperature may be raised to even 1200°C, thus ensuring an efficient incineration. However, also in this case, since the flue gases also contain corroding compounds, such as chlorine, the superheating temperature of steam may not be raised above about 400°C, more precisely 300- 420°C, by grate incineration alone, since the corrosion of superheating surfaces would increase excessively and the operational safety of the plant would substantially decrease.

In fact, although grate incineration is a very flexible arrangement for the capability thereof of incinerating untreated waste, the arrangement, where in steam production the superheating temperature were raised over 420°C, is not possible within the known technique.

Thus the purpose of the present invention is to provide a simple, efficient method of treating waste and other solid material at high temperatures, which is easy to use and in which the generated heat is mainly used for steam production. The purpose of the present invention is also to provide a method of recovering heat in connection with grate inceneration of waste, in which a higher temperature of the superheated steam is achieved than in those according to the prior art.

The purpose of the present invention is to provide a method of treating waste at high temperatures by incinerating them on a grate, whereby generated heat is recovered as superheated steam and in which method, the efficiency of the electricity production of the plant, when used for electricity production gained is higher than that of the prior art.

The basic idea of the method in accordance with the present invention is that wastes may be treated at a high temperature by grate incineration, whereby a complicated waste pretreatment process is unnecessary, but substantially untreated waste may be incinerated and the generated heat may be used for the production of high temperature superheated steam and this again may be further used, for example, for the generation of electricity.

More specifically, it is a characterizing feature of the method in accordance with the present invention that said method comprises at least the following steps of: feeding substantially unpretreated solid material to the incineration chamber for treating solid material at a high temperature; heating or incinerating solid material at the presence of oxygen-containing gas at the temperature of > 1000°C; supplying circulating material into communication with the generated flue gases in such a way that gases and the circulating material are efficiently intermixed; passing the mixture of gases and circulating material further in the process simultaneously recovering heat therefrom; separating circulating material from the gases; passing the gases for further treatment; and passing the circulating material separated from the gases to the cooling process, in which the heat of the circulating material is used for the increase of the temperature of the steam generated in the grate incineration apparatus.

According to a preferred embodiment of the present invention it is possible to carry out the waste treatment in an incineration process, in which wastes, such as municipal waste is incinerated by grate incineration, whereby wastes may be incinerated substantially non- sorted without any pretreatment. Moreover, it is possible to maintain such an incineration temperature that the incineration of used wastes is ensured. The generated process gases may be, if desired, heated to a temperature of more than 1000°C, even more than 1200°C, for example, for destroying compounds harmful to the environment, such as chlorined hydrocarbons. The process gases generated in the grate incineration are brought into contact with the circulating material which absorbs at least a portion of the heat of the flue gases simultaneously warming up. The circulating material brought into contact with the gases and the gases are efficiently intermixed and pass into a separator, which is preferably a cyclone separator. At least a portion of the solid material is separated from the gas and from said separated portion further at least a portion is led to the cooling process of the circulating material which takes place in a cooler, preferably a fluidized bed cooler. One portion of the separated circulating material may be led, if desired, directly back to the process and be brought into communication with the gases generated in the grate incineration. Also yet another portion of the separated circulating material may be completely removed from the process. The separator of the circulating material is preferably a cyclone separator, in which a very efficient gas vortex is generated. Due to said gas vortex it is possible to adjust the air supply to very close to the stoichiometric value, because due to the efficient mixing of the cyclone the unburnt material of the gases, mostly carbon monoxide, comes efficiently into contact with the unreacted oxygen and thus it is possible due to accurate adjustment of the combustion air to decrease unnecessary emissions caused by the nitrogen in the air and at the same time hardly any unburnt oxygen remains in the gases. •Moreover, due to the complete incineration taking place due to the efficient mixing it is possible to shorten the furnace shaft subsequent to the grate incineration, because the separator subsequent to the shaft ensures an efficient, complete incineration.

Heat exchange means are provided in the cooler of the circulating material preferably for superheating steam. The steam to be superheated is preferably generated by steam generation means arranged into connection with the grate incineration apparatus. The steam to be led to the heat exchange means of the cooler may already be presuperheated or also saturated steam. Also heat surfaces for intermediate superheating steam may be located, if required, to the cooler, if the plant is, for example, combined to a two-stage steam turbine. In the cooler, the circulating material cools down superheating the steam flowing in the heat exchange means.

According to a preferred embodiment of the present invention, when the cooler of the circulating material is a fluidized bed type cooler, it is possible to supply fluidizing gas of the circulating material to the cooler, so that the heat exchange means arranged to operate as a superheater of steam are in a heat exchange contact with the circulating material. Thus the heat in the circulating material may be recovered in the form of high temperature superheated steam of more than 420°C without a significant corrosion risk in the superheating surfaces. The fluidizing gas to be led to the cooler of the circulating material may be led from the fluidized bed cooler, for example, back to the furnace shaft or to the vicinity of the grate to act as primary, secondary or tertiary air in the waste incineration.

The cooled circulating material is supplied either back into communication with the gases generated in the grate incineration or then it may be either completely or partially removed from the process. A substantial improvement over the prior art technique is that by utilizing the method in accordance with the present invention it is possible to incinerate unpretreated waste by grate incineration and simultaneously to generate superheated steam, the superheating temperature of which may be raised higher than with the methods in accordance with the prior art. By utilizing the method in accordance with the present invention the steam temperature may be raised even to 420-550°C, preferably to 500-550°C, without any corrosion risk in the superheating surfaces. By this arrangement it is possible to significantly raise the temperature of the superheated steam compared with the prior art technique.

By utilizing the cooler of the circulating material it is possible to cool the circulating material, thus also simultaneously to superheat the steam, outside the actual process beyond the re ch of corroding flue gases generated in the grate incineration, whereby the conditions prevailing in the external treatment apparatus do not depend on the conditions of the actual waste treatment process, whereby the superheating temperature of the steam may be raised above 420°C, even to the temperature of 420-550°C, preferably to 500-550°C, because the corroding substances being released in the waste treatment, such as chlorine, are not present. In the cooler, which is preferably a fluidized bed cooler, it is possible to choose a non-corroding substance to act as fluidizing gas. Possible fluidizing gases are, for example, inert gas, such as nitrogen, or if required also oxygen gas may be used as fluidizing gas. Air, however, is preferably used as a fluidizing gas. The temperature of steam to be superheated with circulating material may be raised high in this way without a risk of corroding heat surfaces, for example, due to chlorine.

The gases from the separator of the circulating material are supplied to the further treatment, which may comprise according to a first embodiment cooling of the gas in the waste heat boiler in accordance with the prior art, to ^■which heat surfaces are arranged for heating or steaming of the medium. Further treatment may also comprise cooling of gas in a fluidized bed reactor according to a second embodiment of a method in accordance with the present invention.

For example, following advantages compared with the prior art are gained with the method in accordance with the present invention:

- the pretreatment stage of waste is simplified or it becomes unnecessary;

- a higher superheating temperature is achieved in the steam production without a corrosion risk; - an efficient, complete incineration of combustible material;

- cleaner exhaust gases.

According to a preferred embodiment of the method in accordance with the present invention, the circulating material to be used may contain one or more of the following substances: a substantially inert solid material, a substance binding sulphurous oxides, a catalyte disperging oxides of nitrogen.

The present invention is described below more in detail, by way of example, with reference to the accompanying drawing, in which

Fig 1 schematically illustrates an apparatus for realizing the method in accordance with the present invention.

Fig. 1 illustrates a waste incineration plant 1. The unpretreated waste is preferably introduced into a furnace 2 of the incinerator in the incineration plant through an opening 3. The waste flow along a grate 4 to the lower part of the furnace and incinerate forming flue gases. Air is supplied through the grate to the furnace. The flue gases rise upwards in a furnace shaft 5, to which, if desired, for example, additional burners may be mounted to ensure a complete incineration and/or the dispersion of substances harmful to the environment. Moreover, additional air may be brought to the furnace shaft, such as secondary air so that the incineration in the furnace shaft 5 takes place by staged air supply, whereby the amount of oxygen may be efficiently optimized in view of the incineration. The furnace shaft is preferably formed of a wall construction which may be cooled, such as a so called membrane wall, which is preferably lined, for example, by silicon carbide lining.

The furnace shaft is provided within a distance from the grate with means 6, 17, by means of which circulating material is brought into contact with gases being •released from the grate incineration so that the gases and the circulating material are efficiently intermixed and heat exchange takes place between the gases and the circulating material cooling the gas and heating the circulating material. Also material exchange may take place. Members 17 comprise according to the invention means for facilitating the maintenance of the suspension formed of the circulation material, preferably comprising at least the zone of the furnace shaft, in which the cross-sectional flow area is decreased. An efficient mixing may be provided by arranging a spouted bed of circulating material above the construction.

The mixture of gases and circulating material flows upwards in the furnace shaft and simultaneously it may release heat for evaporating the medium flowing on the wall of the furnace shaft. For the sake of clarity this has, however, not been illustrated in the drawing. The upper portion of the furnace shaft is connected to the separator 7, in which solid material is separated from the gases. The separator of the circulating material is preferably a cyclone separator, in which a very efficient gas vortex is generated. Due to said gas vortex the air supply to the incineration grate and furnace shaft may be adjusted to very close to the stoichiometric value, because due to the efficient mixing of the cyclone the unburnt material of the gases, mostly carbon monoxide, efficiently comes into contact with the unreacted oxygen and thus it is possible, due to the accurate adjustment of the combustion air to decrease unnecessary emissions caused by the nitrogen in the air and at the same time hardly any unburnt oxygen remains in the gases. Moreover, due to the complete incineration taking place due to said efficient mixing it is possible to shorten the furnace shaft subsequent to the furnace, because the separator subsequent to the shaft ensures an efficient, complete incineration.

The circulating material separated from the gases in the separator, or at least a portion thereof, is introduced into a cooling apparatus 8 via a channel 10. One portion of the separated circulating material may be supplied, if required directly back into contact with the gases generated in the grate incineration through a channel 9. Said transfer path of the circulating material is schematically illustrated as a separate channel, but it may also be an integrated portion of the cooling apparatus. Additionally, one portion of the separated circulating material may be completely removed from the process. Said flow of the circulating material is not illustrated in the drawing, but the circulating material may be removed from the desired place, preferably after it has cooled down in the cooling apparatus 8. Heat exchange means 11 are provided in the cooler of the cooling apparatus, said heat exchange means being provided with conduits 14, 15 for introducing steam and further for the discharge thereof. Saturated steam is preferably introduced into heat exchange means for superheating it, but in some cases also reheating of already superheated steam may come into question.

The circulating material led thereto in the cooler cools the steam flowing in the heat exchange means while it is superheated to a temperature of above 420°C, preferably 500-550°C. At least a portion of the cooled circulating material is led back to the furnace shaft into contact with the gases generated in the furnace incineration by means of means 12 and 6. A portion of the circulating material may also be removed from the process. Additionally, fluidizing gas is supplied by means of means 13 to the cooling apparatus, which is preferably a fluidized bed cooler. Fluidizing gas may be led from the fluidized bed cooler, for example, to the back to the furnace shaft along channel 16 to the vicinity of the grate to operate as primary, secondary or tertiary air in the waste incineration. Said gas may, if required, be led also to the vicinity of means 6, 17. In this connection it must be understood that means 6 and 12 as well as 9 and 10 may as well be means integrated in the cooling apparatus or furnace shaft and they do not have to be separate, as is illustrated for clarity in Fig. 1. Additionally, the whole cooling apparatus may be integrated in the grate incineration furnace.

The gases from the separator 7 of the circulating material are led to further treatment by means of conduit 14.

The arrangements illustrated in the drawing are, of course, exemplary and may be realized in a manner suitable for each case.

The method in accordance with the present invention is compared in the following by means of examples with the arrangement in accordance with prior art. According to the examples the heat being released from the gases is used for the generation of superheated steam when the superheating surfaces are covered, according to the invention, from the untreated process gases containing corroding substances. The superheated steam is used in a conventional steam turbine process in the same way as in a condense power plant to generate only electricity .(Example 1) or as a district heating plant for a combined generation of electricity and heat (Example 2) .

Example 1. Pure electricity generation. Case A illustrates waste incineration in accordance with the prior art, in which steam may be superheated only to a temperature of about 420°C and this is compared with Case B, in which in a process in accordance with the present invention it is possible to raise the superheating temperature of steam, for example, to 520°C.

The process values of the example are given in Table 1. Table 1.

Case A. Case B.

Steam Temperature °C 420 520

Steam Pressure bar 35 80

Steam Amount kg/s 25 23.4

Feed Water Temp. °C 170 170

Cooling Water Temp. °C 20 20

Power MW 63.7 63.7

Net Production of El. MW 19.2 21.6

In this condense power plant the net production of electricity is 12% higher with the process in accordance with the present invention than with a convention cooling process of gases generated in the waste incineration due to a higher superheating temperature gained.

Example 2. Combined electricity and district heat production. The cases of the example correspond to the cases of Example 1. The process values are given in Table 2.

Table 2.

Case A. Case B.

Steam Temperature °C 420 520 Steam Pressure bar 35 80 Steam Amount kg/s 25 23.4 Feed Water Temp. °C 170 170 Cooling Water Temp. °C 75 75

Power MW 63.7 63.7

Net Production of El. MW 14.5 17.1

In a combined production of electricity and district heat the net production of electricity rises 18% higher with the method in accordance with the present invention than with a conventional process.

It is appreciated from the above description that the beneficial method of treating waste in accordance with the present invention is significantly more efficient and advantageous than the methods of the prior art. The method in accordance with the present invention offers an especially advantageous solution in cases where the steam turbine may be chosen to endure superheating temperatures of the steam to be generated by the method in accordance •with the present invention, in other words in new waste treatment plants, or in cases where the already existing turbine allows the use at temperatures above 420°C.

The invention is not intended to be restricted to the above mentioned embodiments, but it may be varied and adjusted within the accompanying patent claims, which alone determine the inventive concept and the scope of invention.

Claims

1. A method of treating solid material, such as waste and/or biological sludges at a high temperature by a grate incineration apparatus and for the recovery of heat in steam production, characterized in that said method comprises at least the following steps of:

- feeding substantially unpretreated solid material to the incineration chamber for treating sclid material at a high temperature;

- heating or incinerating solid material at the presence of oxygen-containing gas at the temperature of > 1000^βC;

- supplying circulating material into communication with the generated flue gases in such a way that gases and the circulating material are efficiently intermixed;

- passing the mixture of gases and circulating material further in the process simultaneously recovering heat therefrom;

- separating circulating material from the gases; - supplying the gases to further treatment; and

- supplying the circulating material separated from the gases to the cooling process, in which the heat of the circulating material is used for the increase of the temperature of the steam generated in the grate incineration apparatus.

2. Method in accordance with claim 1, characterized in that in the cooling process circulating material is fluidized by means of fluidizing gas supplie thereto in such a way that the heat exchange means arranged to operate as superheaters of steam are in heat exchange contact with the circulating material.

3. Method in accordance with claim 1 or 2, characterized in that the temperature of steam is raised above 420°C.

4. Method in accordance with claim 3, characterized in that the temperature of steam is raised above 450°C.

5. Method in accordance with claim 3 or 4, characterized in that the temperature of steam is raised above 500°C.

6. Method in accordance with claim 2, characterized in that cooled circulating material is recirculated into contact with generated flue gases so that the gases and the circulating material are efficiently intermixed.

7. Method in accordance with claim 1, characterized in that afterburning of the unburnt substances takes place in the separation stage of the circulating material.

8. Method in accordance with claim 1, characterized in that the further treatment of gases comprises heat exchange for steam generation or superheating.

9. Method in accordance with claim 1, characterized in that the flow velocity of the generated flue gases is increased before the circulating material is brought into contact with the flue gases.

10. Method in accordance with claim 2, characterized in that the fluidizing gas to be led to the cooling process is led from the cooling process into contact with the flue gases generating in the furnace incineration.

11. Method in accordance with claim 9, characterized in that spouted bed of circulating material is provided by increasing the velocity.