Claims:We claim:
1. A waste heat recovery system from exhaust gas of an anode baking furnace, the waste heat recovery system comprising:
at least one secondary duct mounted parallel to a main exhaust duct of the anode baking furnace;
a means for directing a predetermined flow of the exhaust gas from the main exhaust duct to the secondary duct; and
at least one heat exchanger mounted on the secondary duct such that hot flue gas flow from the anode baking furnace circulates in the heat exchanger to heat fluid circulating in the heat exchanger.

2. The waste heat recovery system as claimed in claim 1, wherein the secondary duct is mounted between the furnace and the fume treatment plant.

3. The waste heat recovery system as claimed in claim 1, wherein the means for directing a predetermined flow of the exhaust gas comprises an inlet valve at an inlet connection between the secondary duct and the main duct, and an outlet valve on the secondary duct at an outlet connection and a valve on the main exhaust duct between the inlet connection and outlet connection.

4. The waste heat recovery system as claimed in claim 3, wherein the valves are preferably damper type valves.

5. The waste heat recovery system as claimed in claim 3, wherein the valves of the secondary duct are completely opened when the system is active.

6. The waste heat recovery system as claimed in claim 34, wherein the valve of the main exhaust duct is partially opened to direct the flow of the flue gas into the secondary duct preferably the valve of the main exhaust duct (210) is opened to about 15% to 20% when the system is active.

7. The waste heat recovery system as claimed in one of the preceding claims 1-6, wherein the fluid circulated in the heat exchanger includes furnace oil to preheat at firing temperature.

8. The waste heat recovery system as claimed in one of the preceding claims 1-6, wherein the fluid circulated in the heat exchanger includes water.
, Description:FIELD OF THE INVENTION
The present invention relates generally to furnaces and particularly to a system for recovering waste heat of exhaust flue gases of an anode bake furnace.

BACKGROUND OF THE INVENTION
Anode bake furnaces are used in baking carbon anodes that are required for electrolytic smelting processes for the production of Aluminium. During baking process of the carbon anodes, fumes containing particulate dust, tar, fluorides are released at high temperature. These fumes are termed as ‘flue gas’ and are released into external atmosphere as a waste product. Since the flue gas is at a high temperature, a lot of heat energy is wasted because of letting out the flue gas as the waste product.
Figure 1 shows a process flow diagram of a conventional anode baking furnace (100). As shown in Figure 1, the conventional furnaces for baking carbon anodes use steam from boiler to heat furnace oil (FO). The furnace oil is generally heated to firing temperature in range of 75 – 90°C by means of steam before sending to furnace. Further, high temperature flue gas leaving the furnace is generally cooled in conditioning tower for further treatment of the gas in a fume treatment plant. Usually, the exhaust flue gas from the anode baking has volumetric flow in tune of 90000 – 120000 N/m3h with temperature ranging from 170 – 270°C and corresponding energy of about 16 GJ/h. Such a large amount of energy, though low quality is wasted when the flue gas is released into the atmosphere. Although, the waste heat is abundant from exhaust of open top anode bake furnaces, current thermal integration in primary aluminium facilities remains limited. Moreover, any modification in the exhaust flue gas duct line of the anode bake furnace may affect the performance of the baking furnace. This is due to both, the low quality of waste heat available and the shortage of potential uses within reasonable distance of identified waste heat sources. Further, a large amount of steam and electricity is utilized in the conventional anode baking furnaces to heat and maintain the furnace oil within the desired temperature range. This increases the resource utilization of the anode baking furnaces, thereby increasing the cost of the process.
Therefore, there is a need for a waste heat recovery system that recovers the heat wasted in the fume gases.

SUMMARY OF THE INVENTION
Accordingly, the present invention provides a waste heat recovery system to recover heat from flue gases of anode bake furnace, comprising at least one secondary duct mounted parallel to a main exhaust duct of the anode baking furnace, a means for directing a predetermined flow of the hot flue gas from the main exhaust duct to the secondary duct and at least one heat exchanger mounted on the secondary duct such that hot flue gas flow from the anode baking furnace passes through the heat exchanger to heat fluid circulating in the heat exchanger.
According to an embodiment of the invention, the secondary duct is mounted between the furnace and the fume treatment plant.
According to another embodiment of the invention, the means for directing a predetermined flow of the exhaust gas comprises an inlet valve and an outlet valve on the secondary duct and a valve on the main exhaust duct. Preferably, the valves are damper type valves.
According to a further embodiment of the present invention, the fluid circulated in the heat exchanger includes furnace oil to preheat at firing temperature. Alternatively, the fluid circulated in the heat exchanger includes water.
The waste heat recovery system of the present invention can be installed without affecting operational parameters of Baking Furnace. Further, the waste heat recovery system ensures smooth operation of Fume Treatment Plant during normal operation of the anode baking furnace as well as maintenance of the heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
Figure 1 shows an existing process flow diagram of an anode baking system;
Figure 2 shows a process flow diagram of a waste heat recovery system with the anode baking system according to an embodiment of the present invention; and
Figure 3 shows a perspective view of a waste heat recovery system mounted on a main exhaust system of anode baking system according to an embodiment of the present invention.

Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
The present invention in general provides a waste heat recovery system to recover heat from flue gases of anode bake furnace. The waste heat recovery system comprises at least one secondary duct mounted parallel to a main exhaust duct of the anode baking furnace and a means for directing a predetermined flow of the hot flue gas from the main exhaust duct to the secondary duct. According to present invention, at least one heat exchanger is mounted on the secondary duct wherein the hot flue gas flow from the anode baking furnace passes through the heat exchanger to heat fluid circulating in the heat exchanger.
The embodiments of the invention shown and discussed herein are merely illustrative of modes of application of the present invention. References to details in this discussion are not intended to limit the scope of the claims to the said details, or to the figures used to illustrate the invention.
Referring Figures 2 and 3 show a waste heat recovery system (200) in an anode baking furnace (not shown) according to a preferable embodiment of the present invention. As shown in figures 2 and 3, the waste heat recovery system (200) includes a single heat exchanger (204), a pump (206), and a single secondary duct (220) mounted on a main exhaust duct (210) of the anode bake furnace (not shown).
The secondary duct (220) is adapted parallel to the main exhaust duct (210) of anode bake furnace (not shown). Alternatively, a plurality of secondary ducts can be mounted on the main exhaust duct. Advantageously, the secondary duct (220) is mounted between the furnace (not shown) and the fume treatment plant (FTP).
As shown in Figures 2 and 3, the heat exchanger (204) is mounted on the secondary duct (220) so that the flow of hot flue gas (212) from the anode baking furnace continuously circulates in the heat exchanger (204). Alternatively, a plurality of the heat exchangers can be mounted on the secondary duct(s). The pump (206) pumps the furnace oil from the storage tank into the heat exchanger (204). Alternatively, the furnace oil may be allowed to flow under gravity. The heat exchanger (204) preferably used is a shell and tube type heat exchanger wherein furnace oil flows in the tube and flue gas flows in the shell of the heat exchanger.
As shown in Figures 2 and 3, the secondary duct (220) has an inlet valve (302) and an outlet valve (304). The main exhaust duct (210) also has a valve (306) placed between inlet and outlet connections of the secondary duct (220) with the main exhaust duct (10). According to the preferable embodiment of the invention, the valves (302, 304, and 306) include damper type valves which can be operated manually or automatically to control the diversion of the flow to the secondary duct (220). The opening of the valves (302, 304, and 306) are controlled in such a way that a part of the flue gases flowing through the exhaust duct (210) are directed to flow through the secondary duct (210). In an example, the inlet valve (302) and outlet valve (304) of the secondary duct (220) are 100% opened and the valve (306) of the main exhaust duct (210) is opened to about 15% to 20% when the anode baking furnace is in the operating state and the waste heat recovery system (200) is active. Alternatively, the valve on the main exhaust duct may be closed for maximum recovery of the heat or as per the requirement, the opening and closing of the valves can be operated.
In operation, the inlet valve (302) and the outlet valve (304) of the secondary duct (220) are completely opened and the valve (306) of the main exhaust duct (210) is partially opened during the operation of the waste heat recovery system (200) for directing the flow of hot flue gas from the main exhaust duct (210) through the secondary duct (220). These hot flue gases flow through the heat exchanger (204) which is mounted on the secondary duct (220) and exchange heat with the furnace oil flowing through the heat exchanger such that the furnace oil is heated to a predetermined temperature. The heated furnace oil is fed to the anode baking furnace or storage tank. During the down time of the waste heat recovery system (200) or maintenance of the waste heat recovery system, the total volume of the flue gas (212) can be allowed to flow through the main exhaust duct (210) by closing the inlet valve (302) and the outlet valve (304) of the secondary duct (220) and full opening the valve (306) of the main exhaust duct (210) without disturbing the operation of the anode baking furnace. In a test taken for 24 hours and temperatures measured at various intervals, the furnace oil at the inlet of the heat exchanger (204) was fed at 4.5 m3/hr to the heat exchanger at a room temperature and it was found that the temperature of the furnace oil at the outlet of the heat exchanger (204) was between 75-90 degrees Celsius i.e. at about firing temperature when the flue gas supplied to the heat exchanger was at 150-2300C. The dampers of the secondary duct were kept fully opened and the damper of the main exhaust duct was kept open 17%. It was observed that the thermal efficiency of the anode baking furnace was increased and the waste heat recovery system had increased the overall thermal efficiency of the anode baking furnace by about 30%.
Advantageously, the waste heat recovery system eliminates the usage of steam or electricity for heating the furnace oil to a firing temperature. The waste heat recovery system also reduces load on conditioning towers to drop down the temperature of the hot exhaust flue gases before treatment as the flue gas loses the heat to the furnace oil in the heat exchanger in the secondary duct and mixes with the hot flue gas in the main exhaust duct which reduces the temperature of the flue gas before FTP.
The waste heat recovery system also reduces maintenance costs by recovering the waste heat of the flue gases. The waste heat recovery system requires less space for mounting. The application of the waste heat recovery system can also be extended to generate hot water and/or steam. Further, the modified waste heat recovery system may be used to generate electricity.
Moreover, since the waste heat recovery system is positioned on the secondary duct and not on the exhaust duct, failure of the waste heat recovery system does not hinder the operation of the anode baking furnace as well as quality of the anode baking process.
Thus, the waste heat recovery system of the present invention provides a compact solution for the shortcomings of the conventional anode bake furnaces. The waste heat recovery system reuses the heat exhausted from anode bake furnaces in form of hot flue gases, which is otherwise wasted.
Further, the waste heat recovery system can be adapted in the plant/industry wherein the flue gas temperature is more than 1500C or where cooling of the flue gas is required or FTP is required or where the flue gas temperature. The waste heat recovery system works without affecting operational parameters of anode baking furnace and allows smooth operation of Fume Treatment Plant. Moreover, the waste heat recovery system is safe as it is mounted on a secondary duct which can be closed at any time without hampering the operation of the anode backing furnace. Further, a plurality of systems can be installed on the flue gas exhaust duct depending on the temperature of the flue gas required at stack or FTP.
While the preferred embodiment of the invention has been illustrated and described herein, it is to be understood that the invention is not limited to the precise construction herein disclosed, and the right is reserved to all changes and modifications coming within the scope of the invention.
Although there has been shown and described the preferred embodiment of the present invention, it will be readily apparent to those skilled in the art that modifications may be made there to which do not exceed the scope of the appended claims. Therefore, the scope of the invention is only to be limited by the following claims.