Background Art Generally, industrial waste sludge, urban waste, food refuse and sewage sludge are disposed of by land filling and/or incineration. In particular, incineration, by which the amount of waste can be considerably reduced and little secondary contamination occurs, is most recommended for sludge treatment.

Among common waste incineration systems, since 1962, fluid bed incinerators have been used to incinerate waste from urban sewage treatment plants, e. g., wet waste or sludge. The fluid bed incinerator is constructed by a cylindrical, fire-resistant (refractory) wall and has its lower part filled with sand used as a fluid medium. Air is employed to cause material to be incinerated to float and flow in the fluid medium. Here, the air is introduced into the sand bed through an outlet of a panel supporting the sand bed at a pressure of approximately 20 to 34 kPa, causing fluidity of the sand bed. During operation of the incinerator, the sand bed is maintained at a temperature of approximately 800°C or higher. The volume of the fluidized sand is increased by approximately 30 to 60%. The waste is introduced from the lower part of the incinerator. If the flow rate of air is high, some of uncombusted waste in the upper part of the incinerator may be discharged into the air with combustion gas. Thus, the flow rate of air must be carefully controlled. Fluidization maximizes contacts between air and waste, leading to optimal combustion. Since a large amount of the sand used as a fluid medium serves as a heat storage medium, the fluid bed incinerator

is suitably used for combustion of waste having a high moisture content.

However, in some cases, sand particles may form a mass due to the combustion state of waste so that it is not easy to recover heat from the waste, resulting in an excessive increase in operating cost and making the treatment system expensive.

A rotary kiln incineration system, which is most widely used for waste treatment, is also used to incinerate solid waste or sludge. The rotary kiln incinerator has an inner wall with a lining of a refractory material, and rotates about a horizontal axis with a tilt of approximately 2 to 3%. As the kiln rotates, combustion of the waste in the kiln is carried out while the waste is in continuous contact with heat and oxygen in gas flow. The rotation speed of the kiln is in the range of 0.25 to 1.5 rpm, and the movement speed of the outer wall of the incinerator is in the range of 0.3 to 1.5 m/min. The waste directly introduced into the kiln are combusted as the kiln rotates and turned into ash. The ash is trapped at a tank disposed at an end of the kiln. Here, heat is supplied from a furnace positioned at the exit of the kiln. The rotary kiln incineration system has several advantages that the incineration speed of waste can be easily controlled, pre-treatment for incineration is not compulsorily necessary, various kinds of waste can be simultaneously incinerated, and the retention time of waste within the system can be easily controlled. However, according to the rotary kiln incineration system, a separate post-combustion device for burning volatile materials is necessary and keeping uniform conditions for combustion is difficult.

Also, it is quite difficult to establish a perfect seal. Further, since a large amount of heat should be supplied to a combustion chamber, the operating cost is excessively high.

To overcome the disadvantage of the fluid bed incinerator or rotary kiln incinerator from the viewpoint of operating cost, a multiple hearth furnace or incinerator has been proposed and widely used in incinerating sludge from sewage treatment plants. Sludge is introduced into the furnace through an upper inlet of the furnace. The multiple hearth furnace has an overall cylindrical inner wall with a plurality of hearths located one above the other, the hearths being made of refractory bricks. Sludge to be processed by the furnace is introduced

into the uppermost hearth and continuously moves downward to the lowermost one. The multiple hearth furnace includes a lance axis for causing sludge to be stirred for promotion of combustion, and a driving device for rotating the lance axis. Odd-numbered hearth floors, disposed from top to bottom, have small holes between the center shaft and each of the floors, and even-numbered hearth floors have small holes between the furnace wall and each floor, to allow sludge to be moved through the holes by the lancing action. Also, the gas generated due to combustion of sludge is passed to the next upper hearth through the holes to be exhausted. Stirring by the lance axis and downward movement make new faces of the sludge to be exposed to combustible gas. Thus, at the upper hearth of the incinerator, considerable amounts of moisture contained in the sludge are evaporated by high-temperature combustible gas, and the combustible gas exhausted to the air containing the moisture evaporated from the sludge produce unpleasant odor.

Disclosure of the Invention To solve the above-described problems, the present invention provides a regenerative thermal waste incineration system which can cost-effectively incinerate sludge having a high content of moisture and can prevent generation of incomplete combustibles such as unpleasant odor or carbon monoxide that may cause secondary pollution.

Brief Description of the Drawings FIG. 1 shows a regenerative thermal waste incineration system according to the present invention ; FIG. 2 shows a regenerative thermal waste incineration system having a blower at its rear end according to the present invention ; FIG. 3 shows a regenerative thermal waste incineration system having a 3-way, automatic openable valve according to the present invention; and FIG. 4 shows a regenerative thermal waste incineration system having a 4-way, automatic openable valve according to the present invention.

Best mode for carrying out the Invention FIGS. 1 and 2 show a regenerative thermal waste incineration system according to the present invention. The regenerative thermal waste incineration system according to the present invention includes a two-bed regenerative incineration furnace having first and second ceramic layers 105 and 106 for recovering heat from high-temperature gas, third and fourth ceramic layers 107 and 108 for compensating for the temperature of the waste sharply decreasing in the course of drying the waste to keep the incineration temperature at a constant range and accumulating high-temperature heat after incineration, first and second waste inlet valves 109 and 110 installed at spaces between the ceramic layers 105 and 107 and between the ceramic layers 106 and 108, the two-bed regenerative furnace 113 having a space enough for complete combustion of combustibles between the ceramic layers 107 and 108 and operating by means of automatically openable valves 101,102,103 and 104 so as to be capable of accumulating and regenerating heat, a blower 100 for supplying combustion air, and a cyclone 111 and a bag filter 112 for trapping incineration residues. The blower 100 may be installed at the front end of the incineration system, as shown in FIG. 1, or at the rear end of the incineration system, as shown in FIG. 2. The automatically openable valves 101,102,103 and 104 can be replaced with 3-way valves 114 and 115, as shown in FIG. 3, or with a 4-way valve 116, as shown in FIG. 4.

The regenerative thermal waste incineration system having the configuration as shown in FIG. 1 operates as follows.

Combustion air is supplied to the heat-accumulating ceramic layer 105 through the valve 101 by means of the blower 100 and pre-heated to a high temperature of 800 to 1200°C. Here, the valves 101 and 104 are open whereas the valves 102 and 103 are closed. The wet waste having a high moisture content is supplied to a space between the ceramic layer 105 and the ceramic layer 107 through the first waste inlet valve 109. At this time, the valve 110 for introducing the waste is in a closed state. The wet waste is separated into vapor

and dried waste by hot air passed through the ceramic layer 105. Although the temperature of the combustion air is sharply dropped due to supply of evaporation heat, the temperatures of the combustion air, vapor and waste are raised again while passing through the heat-accumulating, high-temperature ceramic layer 107.

The dried waste is burnt and the combustion heat further increases the temperature of exhaust gas, compared to the case of passing through the ceramic layer 107.

While passing through the ceramic layers 108 and 106, the exhaust gas discharges most of sensible heat into the ceramic layers 108 and 106 cooled during previous operation. The temperature of the exhaust gas varies according to the proportion of moisture contained in the waste. Thus, heat is supplied from the furnace 113 to allow the ceramic layers 108 and 106 to have heat enough for subsequent operation.

After operating for a predetermined time in the above-described manner, the first waste inlet valve 109 is closed and untreated waste at a space between the ceramic layer 105 and the ceramic layer 107 are completely combusted using hot air passed through the ceramic layer 105 and ash remaining after combustion is allowed to escape. This operation is called a forward operation. After the forward operation, the valves 101 and 104 are closed and the valves 102 and 103 are opened to switch air flow. Then, the second waste inlet valve 110 is opened to introduce the waste into a space between the ceramic layer 106 and the ceramic layer 108. At this time, the first waste inlet valve 109 is closed and a backward operation is performed in the same manner as the forward operation.

The forward and backward operations are repeated.

During a forward operation, the ash generated due to incineration is filtered at a cyclone 111 through the valve 104 via the ceramic layers 107,108 and 106.

During a backward operation, the ash is filtered at the cyclone 111 through the valve 103 via the ceramic layers 108,107 and 105. The remaining ash unfiltered at the cyclone 111 is trapped at the bag filter 112. The exhaust gas is purified to be radiated into the air.

The regenerative thermal waste incineration system according to the present invention has a very high heat recovery efficiency by directly heating ceramic packing materials using high-temperature exhaust gas, bringing room-temperature air into contact with high-temperature air to thus regenerate heat, and then drying and incinerating the waste having a high moisture content using the high-temperature air. Therefore, extra fuel expenses necessary for incinerating the waste having a high moisture content can be noticeably reduced.

Also, since the exhaust gas is not brought into direct contact with raw waste after incineration, there is no emission of unpleasant odor from the exhaust gas and secondary polluants due to incomplete combustion.

An example illustrating calculation of the amount of fuel required in the regenerative thermal waste incineration system according to the present invention and the conventional fluid bed incinerator is described below.

Example 1 Wet sludge introduced into the regenerative thermal waste incineration system according to the present invention includes 87% of moisture, 3% of ash and 10% of organic matter represented by CsHB02CIo., N, So.,. The standard conditions of incineration include 1 tone/hr in feed rate, 850°C in combustion temperature and 5% in radiation loss. The amount of total emissions from the wet sludge is 5,880 kcal/kg.

The amount of air introduced into the regenerative thermal waste incineration system is 5,000 Nm3/hr, a temperature difference (AT) between the inlet and outlet of the regenerative thermal waste incineration system is 80°C, and a total amount of exhaust gas is 6,149 Nm3/hr. Here, the total heat capacity needed is 754,000 kcal/hr, and sludge's self-radiation, i. e., 588,000 kcal/hr, is subtracted therefrom to yield the heat capacity actually needed, i. e., 166,000 kcal/hr.

Extra heat capacity in the conventional fluid bed incinerator is calculated as follows. The amount of air introduced into the conventional fluid bed incinerator is 1,600 Nm3/hr in the case of 1.3 in air ratio, and a total amount of exhaust gas is 2,793 Nm3/hr. In the case where the combustion air introduced to the

conventional fluid bed incinerator is preheated to 350°C using exhaust gas, the total heat capacity needed is 1,125,000 kcal/hr, and sludge's self-radiation, i. e., 588,000 kcal/hr, is subtracted therefrom to yield the heat capacity actually needed, i. e., 537,000 kcal/hr.

Industrial Applicability The use of the regenerative thermal waste incineration system according to the present invention increases an energy saving effect compared to the case of using the conventional fluid bed incinerator.