FIELD OF THE INVENTION:
The present disclosure relates to a system for continuous operation of the tail gas generation unit, incinerator and the stack even on malfunction in the waste heat boiler. Particularly, the present disclosure relates to a system having a bypass duct to bypass the waste heat boiler. More particularly, the present invention 5 relates to an air injection system for controlling Flue Gas temperature to stack of sulphur recovery unit. The said bypass duct has air injection nozzles through which air is injected, to control the temperature of the flue gas. The present invention also relates to a system for complete mixing of flue gas and air within a short distance for achieving the desired temperature. The air injection nozzles 10 inject the air which reduces the temperature of the flue gas.
BACKGROUND OF THE INVENTION:
Tail gases are gases that are produced in the refinery, from various processes and that do not require further processing. The tail gases usually contain H2S, COS, 15 CS2. These gases cannot be directly released into the atmosphere. The tail gases are usually incinerated in an incinerator to convert the harmful components such as H2S, COS, CS2 and entrained sulphur into harmless components. The incinerated tail gas is known as flue gas. The temperature of the gas is usually high in the range of above 800oC. Hence, the energy from the flue gas is 20 recovered by a waste heat boiler. A large quantity of heat is generated in the processes involving flue gases and it is recovered in the form of steam in the waste heat boiler. The waste heat boiler frequently encounters tube leakage problems that necessitate shutdown of the incinerator and the tail gas producing process. 25
One of the main process in which tail gas is generated is conventional Claus based sulphur recovery unit process, which converts toxic H2S to elementary sulphur. It is not possible to convert all the H2S in feed streams to sulphur
3
because of equilibrium Claus reaction in sulphur recovery unit reactors. The tail gas from Sulphur Recovery Unit contains unconverted H2S, COS, CS2 and entrained sulphur. This gas is burnt off in incinerator. The incinerator consists of incinerator burner and chamber uses fuel gas and air for combustion reaction of sulphur species to SO2. 5
Flaring of H2S rich gas, is an alternative practice when sulphur recovery unit is not in operation, this results in large quantity of SO2 emission. The flue gas is required to be cooled before its discharge to the stack which is designed for a temperature of 25oC. Inventions of different approaches for flue gas cooling were done in the past. 10
US Patent 8,894,921discloses a gas cooler constructed to cool the hot and raw flue gas. It comprises a gas inlet chamber, a gas outlet chamber and a matrix of spaced-apart, mutually parallel gas cooling tubes. Each cooling tube has an inlet end projecting into the inlet chamber and an outlet end projecting into the outlet chamber. The inlet ends are being bell-shaped and comprising an 15 aerodynamically curved gas-accelerating profile effective to facilitate streamlined flow of hot raw flue gas into the cooling tubes. The improved flue gas cooler makes it possible to connect the flue gas cooler to receive flue gas directly without getting clogged by dust and sublimates present in the flue gas.
US Patent 8,209,985 discloses another invention of heat recovery equipment to 20 recover heat from flue gas. The heat recovery equipment includes a power generation plant that drives a steam turbine by superheated steam produced in a boiler, and an exhaust-gas treatment line that treats flue gas output from the boiler. The exhaust-gas treatment line includes a first air preheater, a heat extractor unit, and a dry electrostatic precipitator. The power generation plant 25 includes a condensed water line. The condensed water line includes a condenser, a condensed water heater, and a low-pressure feedwater heater. The condensed water heater heats water condensed by the condenser with the heat recovered by the heat extractor unit.
4
US Patent 8,007,729 discloses another invention where hydrocarbon feed to a catalytic reactor is heat exchanged with flue gas from a catalyst regenerator. This innovation enables recovery of more energy from flue gas thus resulting in a lower flue gas discharge temperature. As a result, other hot hydrocarbon streams conventionally used to preheat hydrocarbon feed can now be used to generate 5 more high pressure steam.
US Patent 8,042,497 discloses a method and apparatus that effectively bypasses flue gas through or around selected boiler convection heat transfer tube banks within a new or existing boiler flue. Heat transfer tubes are removed, or omitted in the design of a new boiler flue, forming one or more voids at one or more 10 locations within the tube banks. The bypass flue or conduit is formed within each void, for example using steel plates, along with existing flue walls, or using an integral sleeve. A wall of the bypass flue includes water or steam-cooled tubes. Dampers are installed at either end of or within the bypass flue to control the amount of flue gas directed through each bypass flue. 15
US Patent 7,637,233 discloses a control system for maintaining a desired heat exchanger outlet flue gas temperature across a range of boiler loads. The heat exchanger includes a plurality of tubular configurations in heat exchange contact with the flue gas with each tubular configuration having a separate feed water inlet. Flue gas temperature control is achieved by modulating the feed water flow 20 rates through the tubular configurations. In the system having two tubular configurations, the overall heat transfer capacity of the heat exchanger is reduced to maintain the desired heat exchanger outlet flue gas temperature by reducing feed water flow through one tubular configuration and overflowing the other, while maintaining the total flow of feed water through the heat exchanger 25 substantially constant.
US Patent 6,748,880 discloses a flue gas passage arrangement for a steam generator which permits adjustment of the heat transfer effectiveness of a final bank of heat exchanger surface to control temperature of the flue gas flowing
5
through and exiting from the flue gas passage. Economizer heating surface is located within the flue gas passage, and a baffle plate extends through the flue gas passage and creates two flue gas paths there through. The economizer heating surface may be a single, common bank having a section located in both gas paths, or in only one gas path. A variable position damper is provided in one gas path, 5 either below or above one section. The variable position damper is selectively opened or closed to permit or restrict the flow of flue gases through the second section to control the effectiveness of the economizer heating surface to maintain the temperature of the flue gas at the minimum operating temperature.
Although the prior arts disclose various means to recover heat from the flue gas, 10 none of the prior art discloses the process for continuous operation of the incinerator and tail gas generation process when the means for recovering heat (such as waste heat boiler) is not operational or malfunctions. Industry need is to operate both incinerator and sulphur recovery unit on a continuous basis.
CN201740408U discloses a system of having a bypass duct in case of emergency 15 or shutdown of the waste heat boiler. The bypass duct of the flue gas is passed through the cooling tower before it is sent to stack. The cooling tower configuration of CN201740408U involves an extra equipment which involves additional cost.
In view of the above discussion, the inventors of the present disclosure felt a need 20 to develop a system and a process which operates continuously even when the waste heat boiler does not operate. The system of the present invention is cost effective in comparison to the prior art.
25
6
OBJECTIVE OF THE INVENTION:
It is the aim of the inventors to provide a system for continuous operation of the tail gas generation unit, incinerator and stack irrespective of the waste heat boiler operating or not.
Yet another aim of the inventors is to provide a process and system to control the 5 temperature of the flue gas from the incinerator of the tail gas generation process such as sulphur recovery unit.
Yet another aim of the present disclosure is to provide a control system for monitoring flue gas temperature after mixing the raw and hot flue gas with air. The present invention overcomes the shortcomings of the prior art by bypassing 10 the hot flue gas and quenching the gas by air beyond the known feasible limits of the prior art.
Yet another aim of the present disclosure is to provide a low-cost process for smooth operation of tail gas generation unit (sulphur recovery unit) when incinerator waste boiler placed at downstream of incinerator is not in operation. 15
Yet another aim of the inventors is to provide a system for complete mixing of flue gas and air at a short distance for achieving the desired temperature.
Yet another aim of the inventors is to control the flue gas temperature by injecting air through a specially designed mixing system.
20
SUMMARY OF THE INVENTION:
The present disclosure relates to a system for continuous operation of the tail gas generation unit, incinerator and the stack. Particularly, the present disclosure relates to a system having a bypass duct to bypass the waste heat boiler. The bypass duct connects the flue gas from the incinerator directly to the stack, which 25 lets the flue gas in the atmosphere. The tail gas from the tail gas generation unit is
7
incinerated in the incinerator to produce flue gas and further, the waste heat boiler reduces the temperatures of the flue gas which is then passed to the stack. In the event of failure of waste heat boiler, the tail gas production unit and the incinerator have to be stopped. The present invention provides a system having an alternate bypass conduit which does not require the tail gas production process 5 and the incinerator to be stopped in case of failure in WHB. More particularly, the present invention relates to the system for reducing the flue gas temperature in the bypass duct. Since, the flue gas does not pass through the WHB the temperature of the flue gas is usually high in the range of above 800oC. The bypass stream (flue gas) cannot be passed directly into the stack as the stack is 10 designed for a maximum of 400oC. The temperature of the flue gas needs to be reduced before passing the bypass stream to the stack. More particularly, the present invention relates to an air injection system for controlling Flue Gas temperature to stack of sulphur recovery unit. The said bypass duct has air injection nozzles through which air is injected, to control the temperature of the 15 flue gas. The present invention discloses the system having a bypass duct, wherein the bypass duct has air injected through nozzles. The present invention also relates to a system for complete mixing of flue gas and air at a short distance for achieving the desired temperature. The air injection nozzles inject the air which reduces the temperature of the tail gas. 20
In a further aspect of the disclosure the present disclosure also relates to the process of continuous operation of the tail gas stack. The process involves passing the tail gas to the incinerator and the flue gas produced in the incinerator is let out to the atmosphere after reducing its temperature.
Another aspect of the present disclosure relates to a process to monitor the flue 25 gas temperature to stack in Claus sulphur recovery plant. The tail gas from the plant is burnt in incinerator by air and fuel gas for converting all sulphur compounds to SO2. The flue gas generated from the incinerator is released to atmosphere by the stack. Since the flue gas produced in the incinerator at very high temperature can be used to generate steam, the downstream of the 30
8
incinerator uses incinerator waste heat boiler and then the stack. Medium or high pressure steam is produced in the waste heat boiler. It is found that because of the highly corrosive nature of SO2, tubes in these equipment frequently develop leaks, resulting in shutdown of the incinerator and the Claus sulphur plant (tail gas generating units) instantly. 5
BRIEF DESCRIPTION OF FIGURES
The above and other aspects and advantages of the present invention will become apparent from the following detailed description embodiments, taken in 10 conjunction with drawings, wherein
Figure 1 is schematic representation of the Claus type sulphur plant with the incinerator waste heat boiler bypass system.
Figure 2 is schematic representation of control scheme of the incinerator waste heat boiler bypass system 15
Figure 3 is schematic graphical representation of the Incinerator temperature and the ratio of flue gas to quench air mass flow to maintain flue gas temperature below 325 deg C.
Figures 4(1) and 4(2) are the schematic representations of top and front view of the mixing duct 20
Figure 5 Shows a schematic graphical representation of the development of the velocity profile in the flue gas flow till 10-14 meters
Figure 6 Shows a schematic graphical representation of the development of the temperature profile in the flue gas flow till 10-14 meters.
Figure 7 is a schematic graphical representation of the contours of the velocity 25 magnitude and pressure profile in the flue gas flow.
9
DETAILED DESCRIPTION OF INVENTION
While the disclosure is susceptible to various modifications and alternative forms, specific aspects thereof have been shown by way of examples and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the 5 invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention. The applicant would like to mention that the examples and comparative studies are mentioned to show only those specific details that are pertinent to understanding the aspects of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those 10 of ordinary skill in the art having benefit of the description herein.
The present disclosure relates to a system for continuous operation of the tail gas generation unit, incinerator and the stack. More particularly, the present disclosure relates to a system having a bypass duct to bypass the waste heat boiler. The bypass duct connects the flue gas from the incinerator directly to the 15 stack, which lets out the flue gas into the atmosphere. The tail gas from the tail gas generation unit is incinerated in the incinerator to produce flue gas and further, the waste heat boiler reduces the temperatures of the flue gas which is then passed to the stack. In the event of failure of waste heat boiler, the tail gas production unit and the incinerator have to be stopped. The present invention 20 provides a system having an alternate bypass conduit which does not require the tail gas production process and the incinerator to be stopped in case of failure in WHB. More particularly, the present invention relates to the system for reducing the tail gas temperature in the bypass duct. Since, the tail gas does not pass through the WHB, the temperature of the flue gas is usually high in the range of 25 above 800oC. The bypass stream (flue gas) cannot be passed directly into the stack as the stack is designed for a maximum of 400oC. The temperature of the flue gas needs to be reduced before passing the bypass stream to the stack. The present invention discloses the system having a bypass duct ,wherein the bypass duct has air injected through nozzles. The present invention also relates to a 30
10
system for complete mixing of flue gas and quench air at a short distance for achieving the desired temperature. The air injection nozzles injects the air which reduces the temperature of the flue gas.
In a further aspect of the disclosure the present disclosure also relates to the process of continuous operation of the flue gas stack. The process involves 5 passing the tail gas to the incinerator and the flue gas produced in the incinerator is let out to the atmosphere after reducing its temperature.
In another aspect of the invention the present disclosure relates to the tail gas process of the Claus sulphur plant. The Claus sulphur plant is the desulfurization 10 process recovering sulphur from gaseous H2S. The reaction involves two steps 2 H2S +3 O2 → 2 SO2 + 2 H2O 4 H2S +2 SO2 → 3 S2 + 4 H2O
The tail gas from the Claus process is still containing combustible components 15 and sulfur compounds such as H2S, H2 and CO. These are the unconverted feed components and the intermediates which are not converted into products. The tail gas cannot be let out directly into the atmosphere as the H2S and CO are poisonous. The combustible tail gas is incinerated in an incinerator to produce a flue gas. The incineration is a combustion reaction in which the H2S, H2 and CO 20 are converted into the less harmful gases. The hot flue gas coming out from the incinerator comprises H2O, N2, O2, CO2, SO2, H2S, NO2, NO , SO3 and sometimes CO.
The temperature of the flue gas coming from the incinerator is usually in the range of 600oC to 1200oC. The flue gas is passed through a waste heat boiler to 25 recover the heat from the flue gas. The waste heat boiler is a device which converts the heat energy recovered from flue gas into electricity. This is done by generating the steam and passing the steam through a turbine which converts the
11
heat energy into mechanical energy. The mechanical energy is in turn converted to electrical energy.
The temperature of the flue gas coming out of the WHB is around 300 oC to 400 oC. The flue gas is then passed to the the stack or the chimney which passes the flue gas into the atmosphere. The stack is designed for the temperature range of 5 300 oC to 400 oC. In the event of failure of the waste heat boiler the whole Claus sulphur plant and the incinerator has to be stopped. The present invention provides a bypass duct that bypasses the WHB, in case of such failure. The bypass duct has air nozzles, which mixes air with the flue gas and the temperature of the flue gas is reduced. 10
The invention provides better operation of the Claus sulphur plant when the incinerator waste heat boiler encounters tube leakage problems. This is achieved by proper mixing of the air with the hot flue gas in advance to entry of air mixed flue gas to the stack. A mixing device dedicated for mixing air with the flue gas in a short distance ensures better control of the temperature of the flue gas to the 15 stack.
In an embodiment the invention relates to a system for continuous operation of the tail gas source unit and the incinerator comprising
a tail gas source unit (103, 104) which generates the tail gas;
an incinerator burner (112) provided with an incinerator air blower (114) 20 for incinerating or combusting the tail gas to produce a flue gas at a temperature in the range of 600oC to 1200oC;
a waste heat boiler (116) which recovers the heat from the flue gas and reduce the temperature of the flue gas to 200oC to 325oC;
a stack (116) which passes the flue gas into the atmosphere; 25
a bypass duct(117) which bypasses the waste heat boiler(WHB) (115) and connects the flue gas stream from the incinerator directly to the stack, wherein the waste heat boiler bypass is operated by diverting the
12
hot flue gas from the incinerator waste heat boiler encountering tube leak problem through switching valves in the hot flue gas flow
a shutdown valve(122) in flue gas line to divert the flue gas;
characterized in that the bypass duct has nozzles placed in the circumferential surface of the duct and the nozzles passthe air into the 5 duct to reduce the temperature of the flue gas from 600oC - 1200oC to 200oC - 325oC.
In another embodiment of the invention the tail gas generation unit is Claus sulphur plant(103, 104) and the tail gas is H2S, COS, CS2 and some entrained 10 sulphur.
In another embodiment of the invention the stack(116) is a chimney and it is designed to operate at a temperature of up to 350oC.
15
In another embodiment of the invention the bypass duct(117) is a mixing duct, which mixes the air and the flue gas.
In another embodiment of the invention the bypass duct consists of three circumferentially placed nozzles, directing towards the center of the main 20 flue gas flow and oriented at an angle preferably at 120o apart for the complete mixing of flue gas with air.
In another embodiment of the invention the bypass duct completely mixes the hot flue gas and air within an overall length equal to two to eight times 25 of the hot flue gas pipe diameter from the point of injection of air into the hot flue gas.
In yet another embodiment of the invention the length of the bypass duct to reduce the temperature to the desired range, is 14 meters. 30
13
In yet another embodiment of the invention the ratio of mass flow rate of the hot flue gas to the mass flow rate of the air injected to the bypass duct ranges from 0.25 to 0.8.
In yet another embodiment of the invention the bypass duct has a pressure 5 drop in the range of 1.0 to 1.4 mbar.
In yet another embodiment of the invention the temperature of the flue gas is controlled by the airflow rate in the nozzles.
In another embodiment, the invention relates to a process for continuous 10 operation of the tail gas source unit and the incinerator comprising,
generating the tail gas from the tail gas generation unit(103, 104);
incinerating or combusting the tail gas to produce the flue gas at a temperature in the range of 600oC to 1200oC;
recovering heat from the flue gas by a waste heat boiler (116) to reduce 15 the temperature of flue gas to 200oC to 325oC;
passing the flue gas into the atmosphere by a stack or chimney;
characterized in that the flue gas is passed through a bypass duct by a shutdown valve, whereby the bypass duct bypasses the WHB and connects the flue gas directly to the stack, and 20
the bypass duct has nozzles placed in the circumferential surface of the duct for injection of air and mixing the flue gas with air which reduces the temperature of the flue gas to be passed to the stack.
In another aspect of the invention the present disclosure relates to the incinerator bypass system, demonstrated in a commercial Claus type sulphur plant (SRU) as 25 shown in Figure 1. The plant comprises main air blower (100), main burner (101), main combustion chamber (102), two Claus Converters (103, 104), heat recovery facilities like waste heat boiler (105) and three condensers (106,107,108), and re-heaters (109,110) and Tail gas coalescer (111), incinerator
14
burner (112), incinerator chamber (113), incinerator blower (114) incinerator waste heat boiler (115) and stack (116).
H2S rich gas also known as acid gas is fed to the main burner (101). Acid gas is partially burnt in the main burner with air from the air blower (100) to SO2. The gas containing H2S , SO2, N2 , H2O, COS, CS2 and sulphur passes through the 5 waste heat boiler(105) , condenser( 106), re-heater(109),converter (103) ,condenser (107), re-heater(110) , converter(104) , condenser(108) . Finally, the gas is burnt in the incinerator burner (112) using fuel gas from a source. Incinerator air blower (114) is provided to blow air to the incinerator burner (112) for combustion reactions. 10
The hot flue gas having temperature ranging from 600oC to 1200oC is either routed to the incinerator waste heat boiler (115) or to the incinerator bypass duct (117). Normal operation is done through the incinerator waste heat boiler which cools the flue gas to the desired temperature and generates steam. A shutdown valve (122) is installed in the flue gas pipe to facilitate the flow when the 15 incinerator waste heat boiler is in operation.
In another aspect of the disclosure the present disclosure relates to the process for controlling the hot flue gas temperature from the incinerator of the Claus sulphur plant. Control system for the temperature control is depicted in Figure 2. The mixing duct (117) comprises flue gas pipe (118) for the flow of hot flue gas from 20 the incinerator. The said manifold is kept in place with the flue gas pipe through a body flange. The duct has air manifold three nozzles for uniform distribution of air in the hot flue gas passing through the duct. The nozzles are oriented at an angle of 120oC along the circumference of the mixing duct.
The ratio of the flue gas to air is maintained in the range of 0.25 to 0.8 for 25 controlling the flue gas temperature at the exit of the mixing duct. For controlling the air flow to the mixing duct, one air flow controller (119) is installed in the main air line. The main air flow to the mixing duct is controlled by the flow and the temperature of the hot flue gas. Another air flow controller (120) is installed
15
in the trim air flow pipe. The trim air flow is controlled by the temperature of the flue gas at the exit recorded by the temperature indicator (121) at the stack (116). The results of the flue gas temperature and the ratio of flue gas to air mass flow is shown in Figure 3.
The incinerator waste heat boiler bypass consists of mixing duct (117). The 5 mixing duct is installed in the hot flue gas bypass line to stack. Injection of air from the air blower (114) is done into the mixing duct. The top view and front view of the duct are shown in Figure 4.
Based on computational fluid dynamics study for design of the mixing duct, estimated distance equivalent for complete mixing of air with the hot flue gas is 10 around 2 to 8.0 times of the flue gas pipe diameter and it depends on the flue gas temperature. With increase in the flue gas temperature, the distance increases. Figures 5 & 6 provide the contours of the velocity magnitude and temperature in the flue gas flow. Figure 5 and Figure 6 describe temperature profile evolution along the flow path for a system having three nozzles. It shows steady 15 temperature after a length of 10-14 meter from the air injection point. It also confirms complete mixing of air with hot flue gas.
Figure 7 shows pressure profiles along flow path. Pressure remain or near to steady condition from the injection point. It indicates insignificant pressure drop for this system. 20
The process disclosed a quick change over of the flue gas. For tube leaks or any other operating problem, the flue gas is diverted to the bypass line by using the shutdown valve (122) in the flue gas bypass line. The interchange of the flue gas flow path is carried out through interlocks.
The process disclosed in the present invention provides a trouble-free operation 25 of the Claus type sulphur plant. Air injection to the hot flue gas makes the flue gas temperature suitable for operation the stack (116). The pressure drop in the mixing duct is in the range of 0.1 to 0.14 mbar which is much less than the
16
pressure drop for routing the flue gas through the incinerator waste heat boiler. This provides better operation of the incinerator (112) and the stack (116).
The prior art plants with incinerator waste heat boiler are prone to emit all the toxic gases like SOx, NOx and CO at higher concentration resulting in fumes at the top of the stack. In the present invention, air injected is used as diluent and it 5 reduces the concentrations of all the above toxic gasses ensuring safe discharge of the flue gas.
Example:
The present invention is now described by way of the following non-limited 10 example:
An existing commercial Claus plant of capacity 340 tons of sulphur production per day is revamped for testing the process and the incinerator waste heat boiler bypass system. The mixing duct is designed to inject air from the air blower into the hot flue gas. The air is injected through the three nozzles placed at 120o in the 15 circumference of the mixing duct.
The acid gas and air were fed to the plant for conversion of H2S to sulphur. Table 1 shows the acid gas flow, air flow and the tail gas flow from the plant to the incinerator.
Table 1 : Acid Gas ,Air & Tail Gas to Incinerator 20
Acid gas from ARU flow (kg/hr)
12043
Air flow (kg/hr)
27729
Tail gas (kg/hr)
22401
17
H2S concentration in the acid gas was in the range of 90 %(v) and the plant recovered 96% of H2S in the feed as elementary sulfur. Table 2 shows flow and composition of the tail gas from the plant
Table 2 : Flow and Composition of the Tail Gas
5
Component
kmol/hr
mole%
H2S
789.52
90.00
CO2
8.6
0.98
C1
0
0.00
C2
2.54
0.29
H2
0.14
0.02
CO
0
0.00
H2O
75.99
8.66
O2
0
0.00
N2
0
0.00
NH3
0.43
0.05
Table 3 shows design conditions of the incinerator for incineration of the above said tail gas. For design of the incinerator waste heat boiler bypass system, different operating conditions have been calculated. The results show an increase 10 in the flue gas temperature with increase in the flue gas flow.
18
Table 3 : Design conditions of Incinerator
S.No
Tail gas flow
kg/hr
Fuel gas flow
kg/hr
Air to Incinerator
kg/hr
Flue gas
kg/hr
Temperature
oC
1
22400
199
8607
38283
652
2
22400
276
10055
39808
703
3
22400
353
11222
41053
754
4
22400
444
13261
43182
800
5
22400
541
14906
44924
852
6
22400
663
17086
47226
909
7
22400
773
18887
49137
958
8
22400
905
21450
51832
1006
9
22400
1038
23931
54446
1050
10
22400
1215
27265
57957
1102
11
22400
1391
30688
61556
1146
12
22400
1601
34146
65224
1201
Table 4 shows design of the mixing device at various operating conditions of the hot flue gas. Computational fluid dynamics study for design of the mixing duct was carried out to optimize the number of nozzles and their orientation in the 5 mixing duct. Figures 5, 6 and 7 show the evolution of temperature and pressure along the mixing duct equipped with nozzles for air injection. The mixing duct designed consisted of three nozzles at angle of 120o apart from each other. Flue gas to air flow ratio was maintained from 0.2 to 0.8 for achieving around 310oC at the exit of the mixing duct. The complete mixing of the hot flue gas and air 10 was estimated at distance equivalent to 2 to 8.0 of the hot flue gas pipe diameter from the air inlet point to the mixing duct.
The results of the computation fluid dynamics study of the bypass duct are represented in Figures 5, 6 and 7. From figures 5 and 6, it is seen that the
19
distance needed for the flue gas to achieve the temperature of 323oC is 14 meters. Figure 7 shows the static pressure of the flue gas stream while passing through the duct. It is seen that the pressure of the flue gas throughout duct is in the range of 120 to 140 pascal with very less pressure drop.
5
Table 4: Design Conditions of Mixing Device at different temperature and air flow
S.No
Flue gas
kg/hr
Incinerator
Temperature
oC
Quench
Air injection
Ton/hr
Stack temperature
oC
Flue gas/Air injection
1
38283
652
54
321
0.709
2
39808
703
65
322
0.612
3
41053
754
77
321
0.533
4
43182
800
90
321
0.480
5
44924
852
105
319
0.428
6
47226
909
122
321
0.387
7
49137
958
138
321
0.356
8
51832
1006
158
320
0.328
9
54446
1050
178
320
0.306
10
57957
1102
205
319
0.283
11
61556
1146
230
320
0.268
12
65224
1201
260
321
0.251
10
Advantages:
The process and apparatus disclosed in the present invention provide the following advantages:
20
• Trouble Free operation: Air injection to the hot flue gas makes the operation of the Claus type sulphur plant smooth and trouble free by reducing the temperature of the flue gas before its discharge to atmosphere through stack.
• Quick change over from the incinerator waste heat boiler steam 5 generation mode to incinerator waste heat boiler bypass mode operation. Continuation of operation is done by immediate switching over the flue gas flow through two shutdown valves in the flue gas flow paths. Any malfunction of the waste heat boiler forces stoppage of the flue gas flow through the incinerator waste heat boiler and allows the flue gas to flow 10 through the bypass system.
• Stable operation of incinerator: Bypass system causes minimum pressure drop.

We Claim:
1. A system for continuous operation of the tail gas source unit and the incinerator comprising
a tail gas source unit (103, 104) which generates the tail gas;
an incinerator burner (112) provided with an incinerator air blower (114) 5 for incinerating or combusting the tail gas to produce a flue gas at a temperature in the range of 600oC to 1200oC;
a waste heat boiler (115) which recovers the heat from the flue gas and reduce the temperature of the flue gas to 300oC to 400oC;
a stack (116) which passes the flue gas into the atmosphere; 10
a bypass duct (117) which bypasses the waste heat boiler(WHB) and connects the flue gas stream from the incinerator directly to the stack, wherein the waste heat boiler bypass is operated by diverting the hot flue gas from the incinerator waste heat boiler encountering tube leak problem through switching valves in the hot flue gas flow 15
a shutdown valve (122) in flue gas line to divert the flue gas;
characterized in that the bypass duct has nozzles placed in the circumferential surface of the duct and the nozzles passes the air into the duct to reduce the temperature of the flue gas from 600oC - 1200oC to 200oC - 325oC. 20
2. The system as claimed in claim 1, wherein the the tail gas generation unit is Claus sulphur plant(103, 104) and the tail gas consists of H2S, COS, CS2 and some entrained sulphur.
25
3. The system as claimed in claim 1, wherein the stack (116) is a chimney and it is designed to operate at a temperature of up to 350oC.
4. The system as claimed in claim 1, wherein the bypass duct (117) is a mixing duct, which mixes the air and the flue gas. 30
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5. The system as claimed in claim 1, bypass duct consists of three circumferentially placed nozzles, directed towards the center of the main flue gas flow and oriented at an angle preferably at 120o apart for the complete mixing of flue gas with air. 5
6. The system as claimed in claim 1, wherein the bypass duct completely mixes the hot flue gas and air within an overall length equal to two to eight times of the diameter of the hot flue gas pipe from the point of injection of air into the hot flue gas. 10
7. The system as claimed in claim 6, wherein the length of the bypass duct to reduce the temperature to the desired range, is 14 meters.
8. The system as claimed in claim 1, wherein the ratio of mass flow rate of 15 the hot flue gas to the mass flow rate of the air injected to the bypass duct ranges from 0.2 to 0.8.
9. The system as claimed in claim 1, wherein the bypass duct has a pressure drop in the range of 1.0 to 1.4 mbar.
10. The system as claimed in claims 1 to 8, wherein the temperature of the 20 flue gas is controlled by the airflow rate in the nozzles.
11. A process for continuous operation of the tail gas source unit and the incinerator comprising,
generating the tail gas from the tail gas generation unit (103, 104); 25
incinerating or combusting the tail gas to produce the flue gas at a temperature in the range of 600oC to 1200oC;
recovering heat from the flue gas by waste heat boiler (116) to reduce the temperature of flue gas to 200oC - 325oC;
passing the flue gas into the atmosphere by a stack or chimney; 30
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characterized in that the flue gas is passed through a bypass duct by a shutdown valve, the bypass duct bypasses the waste heat boiler (WHB) and connects the flue gas directly to the stack, and
the bypass duct has nozzles placed in the circumferential surface of the duct for injection of air and mixing the flue gas with airthereby reducing 5 the temperature of the flue gas to be passed to the stack.