FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
THE PATENTS RULES, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION WASTE HEAT RECOVERY BOILER
APPLICANTS
Aditya Birla Science and Technology Company Pvt Ltd
of address
Plot No 1 and 1-A/1, Taloja, MIDC, Taluka-Panvel, District Raigad
Maharashtra – 410208, India
Hindalco Industries Limited
Of address
6th Floor, Birla Centurion, Pandurang Budhkar Marg, Worli,
Mumbai 400030, Maharashtra
PREAMBLE TO THE DESCRIPTION The following specification particularly describes this invention and the manner in which it is
to be performed:

FIELD OF THE INVENTION
[001] The present invention relates to the field of waste heat recovery boiler, in
particular relates to a design modification to improve heat exchange for reducing
exhaust gas temperature from copper smelter.
BACKGROUND
[002] Copper is most commonly present in the earth’s crust as copper-iron-sulfide minerals, e.g. chalcopyrite (CuFeS2), bornite (Cu5FeS4) and chalcocite (Cu2S). Copper concentrates contain 20 to 30% copper, ~ 30% iron, ~ 30% sulfur and remaining gangue materials such as alumina, calcia and magnesia. The copper is extracted from the concentrates by pyrometallurgical as well as hydrometallurgical means. One of pyrometallurgical processes is flash smelting. In this process, sulfur is oxidized as SO2 rich off-gas, which leaves the smelter at 1300 °C temperature.
[003] The off-gas is further cooled to below 380°C of temperature in the waste heat recovery boiler in two stages. Waste heat recovery boilers are energy recovery heat exchanger, which are designed to recover heat from waste exhaust gases from furnaces, gas turbines to produce steam or superheated steam.
[004] In the first stage the off-gas is cooled down to ≤ 700 °C of temperature in the radiation zone, wherein the radiation is the predominant mode of heat transfer. The purpose of the radiation zone is to cool off the off-gas so that the molten dust particles contained in the gas are solidified below its sintering temperature before the gas enters the convection zone. In the second stage the off-gas is cooled down to 380°C temperature in the convection zone, wherein the convection is the predominant mode of heat transfer.
[005] The sensible heat of the off-gas is recovered in the form of steam at 55 bar pressure and at 267°C temperature. The off-gas is further treated in the subsequent unit operations for conversion into sulfuric acid. Along with the off-gas, the dust is also generated in the smelter. Dust generated during the smelting phenomena consists of mainly un-smelted minerals or/and oxides of copper, iron, lead, arsenic, zinc, antimony, bismuth etc. The oxide dust is sulfated in the radiation zone of the boiler

with the help of sulfating air, which is supplied from the roof of the boiler. The
sulfation of the dust takes place as per reaction.
Cu2O(s) + 2SO2(g) + 3/2O2(g) = 2CuSO4 (s) ∆H°298K = -776 KJ/mol Cu2O(s) + 2SO3(g) + 1/2O2 = 2CuSO4 (s) ∆H°298K = -558 KJ/mol
[006] CuSO4 is the most preferred form of the dust as it is free flowing in nature. The formation of CuSO4 takes place below 700°C. Because reaction is exothermic, the heat released in formation of CuSO4 increase the product temperature. Owing to the lower melting point i.e., 770 ° C, of CuSO4, and the low melting phases of impurities in dust, the hard dust accretions in the boiler are easily formed.
[007] The dust accretion in the boiler serves in a way as a substrate for new dust accretion. In this way the dust accretion inside the boiler build-up and becomes hard. For all practical purposes, build-up of hard accretion in the convention zone should be avoided owing to the small passage of the gas and several tube bundles, which further blocks the gas flow. As a result, the temperature of the radiation zone exit should be maintained below 700 °C.
[008] Hence, a major part of the copper-rich sulfated dust is recovered in radiation zone of the waste heat recovery boiler. It is removed continuously from the waste heat recovery boiler by a drag chain conveyor located at the bottom of the waste heat recovery boiler. It is recycled back to the smelter for recovery of copper.
[009] During smelting of the concentrates in the smelter, the sulfide phases are oxidized by oxygen enriched air. As a result, SO2 rich off-gas is generated at 1300°C. The off-gas temperature, volume and thus velocity increase proportionately with increasing the concentrate feed rate in the smelter. The temperature of the waste heat recovery boiler also increases owing to the reduced residence time of the off-gas at high concentrate feed rate (smelter throughput). The upper limit of boiler temperature is 700 °C at the exit of the radiation zone and 380 °C at the exit of the boiler (convection zone).

[010] Therefore, the higher temperatures in the waste heat boiler becomes a bottleneck at the higher concentrate feed rate in smelter. So far, no attempts have been made to reduce the boiler temperatures.
SUMMARY OF THE INVENTION
[011] The present invention is conceived to solve the aforementioned problems.
[012] An objective of the present invention is to provide an improved design of waste heat recovery boiler to reduce the temperature of exhaust gas.
[013] Another aspect of the invention is to develop a waste heat recovery boiler for reducing the boiler temperatures at higher feed rate.
[014] The solution involves the design & placement of a water-cooled baffle at the optimum position inside the boiler to ensure the boiler temperatures within the upper specification limits.
BRIEF DESCRIPTION OF THE DRAWINGS
[015] The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. In the drawings:
Fig. 1 is an illustration of the off-gas handing and treatment system in the copper smelter
Fig. 2 is a 3-D illustration of the waste heat boiler of copper smelter.
Fig. 3 depicts the off-gas flow profile inside the boiler at 66 TPH of concentrate feed rate & 29.5% sulfur in the blend.

Fig. 4 depicts the off-gas thermal profile inside the boiler showing temperature above 700°C at radiation exit at 66 TPH of concentrate feed rate & 29.5% sulfur in the blend.
Fig. 5 shows the different positions selected for placement of bottom baffle inside the radiation section.
Fig. 6 shows the dimensions (in mm) of water-cooled bottom baffle.
Fig. 7 depicts the flow profile of off-gas for four different positions of bottom baffle inside the radiation section.
Fig. 8 depicts the thermal profile of off-gas for four different positions of bottom baffle.
Fig. 9 depicts thermal profile of radiation screens for four different positions of bottom baffle.
Fig. 10 (a) shows the installation of bottom baffle in WHB.
Fig. 10 (b) shows the view of baffle condition after 1 month of installation.
Fig. 11 shows the comparison of off-gas temperature at radiation with and without bottom baffle.
Fig. 12 comparison of off-gas temperature at boiler exit with and without bottom baffle.
Fig. 13 shows comparison of steam generation in boiler with and without bottom baffle.
DESRIPTION OF THE INVENTION
[016] In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all

technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.
[017] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.
[018] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention
[019] As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e. to mean including but not limited to.
[020] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. All publications and other references mentioned herein are incorporated by reference in their entirety. Numeric ranges are inclusive of the numbers defining the range.
[021] In an embodiment, the present invention provides an improved arrangement of the waste heat recovery boiler of the copper smelter to reduce the boiler temperatures below the upper specification limits at higher smelter throughput. The improved

design of the present invention provides a significant increase in copper concentrate throughput and thereby its production.
[022] The off-gas handing and treatment system in the copper smelter is shown in Fig.1, wherein the SO2-rich off-gas generated as a result of the concentrates decomposition and oxidation reactions in the smelter leaves the smelter at 1300 °C and cooled in the waste heat boiler to below 380°C in two sections. In the first section of the boiler, known as radiation section, the off-gas temperature drops to below 700 °C. In the second section of the boiler, known as convection section, the off-gas temperature drops to below 380 °C.
[023] The off-gas is cooled in the boiler by the heat transfer through radiation and convection to the boiler tubes, through which the water/steam flows. The sensible heat of the off-gas is recovered in the form of steam at 55 bar pressure and at 267 °C temperature. The off-gas leaving the waste heat boiler is further treated in the subsequent unit operations such as electro-static precipitator, scrubber, gas cleaning section and converter for conversion into sulfuric acid. Typical compositions (by vol%) of the off-gas generated from the smelter are as follow: SO2 ~ 50-70%, O2~ 1-5%, H2O ~ 1-5%, CO2 ~ 1-10%.
[024] Figure 2 is an illustration of the waste heat recovery boiler of the treatment system of figure 1. As shown in Figure 2, the waste heat recovery boiler (100) assembly comprising, connected in a single enclosure, an inlet (5) for up taking of exhaust gas.
[025] The boiler further comprises of a plurality of nozzles (8) placed along circulation path of exhaust gas. The off-gas also contains the oxidized dust generated during smelting. The dust is sulphatised as soon as it enters the boiler with sulfation air which is injected from the roof of the boiler through 4 orifices dust laden off-gas enters the boiler from the throat. The oxide dust is converted into the sulphated form using the sulfation air, which is injected through the four nozzles with diameter of 0.504 m, as shown in Fig. 2.
[026] The total flow rate of sulphatation air 2000 Nm3/hr. A slatted baffle is located near the sulfation nozzles at the roof of the boiler to protect the leakage of the tubes located at the roof from the gas erosion. There are two rows of radiation screens, with 4

screens in each of the rows, located inside the radiation section to promote the heat transfer from off-gas through radiation mode.
[027] Still referring to figure 2, the boiler (100) a radiation section (12) having a plurality of radiation screens (9); at least one exit (10) positioned in the radiation section (12) adapted to transfer exhaust gas from radiation section (12) to a convection section (13). The convection section (13) having a plurality of tube bundles (11). The boiler (100) further comprises of a water cooled baffle arrangement (15) positioned below the radiation screens (9) adapted to reflect exhaust gas to radiation screen (9). The boiler further comprises of an outlet (14).
[028] The radiation section (12) comprises of a plurality of radiation screens (9) to facilitate heat transfer from exhaust gas through radiation mode.
[029] With increasing capacity of the smelter to smelt concentrates, the proportionate changes need to be made in the downstream equipment such as waste heat boiler to accommodate and treat increased volume of off-gas, which further adds to the cost. The increase in off-gas volume per unit tonne of copper concentrate is 150 to 170 Nm3/t. Besides, in case of copper process, the boiler expansion is practically not feasible due to the space limitations. The innovative design of waste heat boiler of present invention offers a solution to reduce the boiler temperatures at higher smelter capacity without increasing the capacity of the downstream equipment such as waste heat boiler.
[030] It is clearly observed from Fig. 3 that, in earlier available boilers, the off-gas enters the boiler and hits the slatted baffle first with the reduction in its velocity significantly. Subsequently the off-gas flows through the bottom of the radiation section by-passing the radiation screens and tube panels behind the slatted baffle. Thus, the off-gas does not come in contact with the radiation screens and tube panels of all sides of the radiation section, causing in-effective heat transfer between off-gas and boiler tubes.
[031] Fig. 3 shows the flow profile of the off-gas inside the boiler. Fig. 4 shows the thermal profile of the off-gas inside the boiler. The off-gas temperature reaches to 720 °C at the exit of radiation section and 400 °C at the boiler exit (convection section exit),

which are higher than the upper specification limit of temperature, at 66 TPH of copper concentrate feed rate.
[032] The off-gas flow and thermal profile inside the waste heat boiler was first evaluated using computational fluid dynamic (CFD) software (ANSYS™) to understand the reasons behind higher boiler temperatures. The operating parameters of the boiler simulated using CFD are shown in Table 2.

Parameters Value
Off-gas flow rate, Nm3/hr 14500
Off-gas temperature, °C 1300
Uptake air flow rate, Nm3/hr 500
Sulfation air flow rate, Nm3/hr 2000
Water circulation in radiation section, TPH
Water circulation in convection section, TPH
Dust loading in off-gas, gm/Nm3 200
Table 2 [033] In order to increase the heat transfer between the off-gas and the boiler tubes, the alternative of increasing the number of radiation screens were first tried, which rendered in-effective in reducing the boiler temperatures. The number of radiation screens was increased from 6 to 8 to enhance the total areas for heat transfer by radiation mode. But due to high velocity & low residence time of the off-gas, the boiler temperature remained higher than the upper specification limits at 66 TPH of concentrate feed rate. Therefore, in an embodiment of the present invention, water-cooled baffle was placed at the bottom of the radiation section, above the drag chain as shown in Fig. 5, through which the by-passing of the off-gas was maximum, as depicted in Fig. 3.
[034] The purpose of the new baffle was to divert the off-gas flow in such was that (1) its residence time inside the radiation section increases (2) the contact area between the off-gas and the tube panels inside the radiation section increases, and (3) the radiation screens are effectively utilized for heat transfer.

[035] The dimensions and design of water-cooled bottom baffle are given in Fig. 6. The width of the baffle at the top and the bottom is 3976 mm and 850 mm, respectively. The height of the baffle is 2750 mm. The clearance between the baffle and the drag chain is 200 mm so that the dust collection and removal by the drag chain is hampered by the baffle. There are 32 tubes with ID 38 mm on each side of the baffle for water circulation. The water circulation flow rate through baffle is calculated to be 5 TPH based on the theoretical value of heat transfer coefficient. The tube pitch is kept at 56 mm. With the increase in contact area between the off-gas and Radiation panels with the placement of bottom baffle, the water circulation rate through these panels is increased by 20 TPH to cope with increased heat transfer.
[036] The optimum position of the baffle placement inside the radiation section was determined and the four different positions for the baffle placement were as shown in Fig. 6. The four different positions are as follow: 4.4 m, 6.6 m, 8 m, 10.2 m from the exit of the radiation section. The results are shown in Fig. 6 & 7 & 8.
[037] Off-gas flow profile is shown in Fig. 7 for four different positions of the bottom baffle in radiation section. Without bottom baffle, the off-gas finds the least resistance path and by-passes from below the radiation screen. Hence, the radiation screens do not come in contact with the off-gas, resulting into in-effective heat transfer, as shown in Fig. 8. The bottom baffle placed 4.4 m, 6.6 m, 8.0 m and 10.2 m prevents the by-passing of the off-gas and diverts the off-gas flow pattern through the radiation screens. Out of all four positions, the bottom baffle placed at 8.0 m provides the highest contact area between the radiation screens, the side panels and the off-gas. As a result, the highest heat transfers and thus maximum reduction in the off-gas temperature at the exit of the radiation section are obtained with 8 m position, as shown in Fig. 9. The off-gas temperature at Radiation exit is reduced from 720 °C without bottom baffle to 678 °C, 675 °C, 662 °C, 670 °C with bottom baffle placed at 4.4 m, 6.6 m, 8.0 m and 10.2 m positions.
[038] The installation of water-cooled bottom baffle and its condition after one month of operation is shown in Fig. 10(a) and Fig. 10(b), respectively. The baffle was found to

be intact and free from dust deposition. Furthermore, the performance of the drag chain located below the bottom baffle also was not affected adversely.
[039] The comparison of boiler performance with and without bottom baffle is shown in Table-3. Post implementation of the bottom baffle, the smelter was operated at 66 TPH of concentrate feed rate. The boiler performance with respect to temperatures was compared at the same feed rate of concentrate with and without bottom baffle. The average off-gas temperature at the Radiation section exit and the boiler exit was dropped by 65 °C and 27 °C, respectively.
[040] Substantial reduction in boiler temperatures was obtained even at 66 TPH of concentrate feed rate with bottom baffle. The pressure-drop across the Radiation section and the entire boiler also increased, which suggests the increased residence time of the off-gas with bottom baffle. This confirms that the installation of bottom baffle improved heat transfers between the off-gas and the radiation screens and panels due to the change in the off-gas flow pattern.

Avg. Conc. Feed Rate Avg. Radiation Exit temp Avg. Boiler Exit temp Avg. Radiation Exit gas pressure Avg. Boiler Exit Gas pressure

TPH °C °C mmWC mmWC
Without
Baffle,
March-2019 66.0 711 353 -3.8 -30.1
With Bottom Baffle, Jan-2021 66.0 646 326 -1.3 -17.5
Difference - -65 -27 2.5 12.6
Table 3
[041] The convention section (13) comprises of at least 5 tube bundles in the convention zone to promote the heat transfer between the off-gas and tubes by convection mode. Tentative dimensions of the boiler and its components are given in table 1.

Boiler Components Size, mm
Uptake diameter 3396
Boiler throat W-1850, H-2500
Sulphatization nozzle 4 No’s, ID-50, L- 550

Slatted baffle Top W-3880, Bottom W-826, H-2950
Radiation screen W – 2632, H-7389
Convection bundle H-4775, W-1500, L-1577
Boiler size Total L-34754
Radiation Zone: H- 9850 x W-3900
Convection Zone: H-5803 x W-2072
[042] Significant gain in off-gas temperatures allowed to further increase the concentrate feed rate from 66 TPH to 75 TPH in the smelter. With feed rate, the off-gas volume and velocity also increased inside the boiler, resulting in an increase the boiler temperatures. This was observed before the installation of the bottom baffle. However, post installation of bottom baffle, the boiler temperatures remained well below the upper limit at all concentrate feed rate up to 75 TPH. The comparison of off-gas temperature at radiation exit and boiler exit is shown in Fig. 11 and Fig. 12, respectively. The off-gas temperature at radiation exit remained high above the upper limit i.e., 700 °C without bottom baffle but it reduced to well below the upper limit with bottom baffle at all concentrate feed rate up to 75 TPH. Similarly, the off-gas temperature at Boiler exit remained high above the upper limit i.e., 380 °C without bottom baffle but it reduced to well below the upper limit with bottom baffle at all concentrate feed rate up to 75 TPH. Advantageously, average concentrate feed rate in the smelter is increased by 14%.
[043] The overall steam generation post installation of the bottom baffle was compared with that without bottom baffle at 29.5% sulfur in the concentrates blend and is shown in Fig. 13. The comparison at the same level of sulfur in concentrate blend ensures that any change in steam generation in the boiler is due to the improved heat transfer between the off-gas and the boiler panels post installation of bottom baffle. The average steam generation increased by 60 to 70 TPD. Advantageously, steam generation is enhanced by 18%.

We Claim:
1) A waste heat recovery boiler (100) assembly comprising, connected in a single enclosure,
an inlet (5) for up taking of exhaust gas;
a plurality of nozzles (8) placed along circulation path of exhaust gas; a radiation section (12) having a plurality of radiation screens (9);
at least one exit (10) positioned in the radiation section (12) adapted to transfer exhaust gas from radiation section (12) to a convection section (13);
said a convection section (13) having a plurality of tube bundles (11);
characterized in that, said boiler (100) comprises of a water cooled baffle arrangement
(15) positioned below the radiation screens (9) adapted to reflect exhaust gas to
radiation screen (9);
an outlet (14).
2) The boiler as claimed in claim 1, wherein said sulphation nozzles (8) are configured to spray sulphation air over said exhaust gas.
3) The boiler as claimed in claim 1, wherein said radiation screens (9) comprises of a plurality of screens to facilitate heat transfer from exhaust gas through radiation mode.
4) The boiler as claimed in claim 1, wherein said water cooled baffle arrangement (15) comprises of a water circulating baffle located within said radiation section (12) at a predetermined distance from exit (10), preferably 8m.
5) The boiler as claimed in claim 1, wherein water circulation flow rate through said baffle (15) is calculated to be 5 TPH.
6) The boiler as claimed in claim 1, wherein said baffle is placed at an angle of 90o, to improve heat transfer between the exhaust-gas and the boiler tubes (11).
7) The boiler as claimed in claim 1, wherein said baffle reduces radiation exit temperature in the range of 9 to 10 % and boiler exit temperature in range of 8 to 16%.

8) The boiler as claimed in claim 1, wherein said convection section (13) comprises of at least five tube bundles (11).