WARMING-UP AND SWITCHING OF COKE OVEN BATTERIES ON TO PERMANENT HEATING REGIME WITH PROPANE-BUTANE

On completion of the construction, prior to commissioning, refractory brickwork of coke oven battery should be dried and then heated up to the operating temperature 1050-1100°C, at which it can be charged with coal blend for production of its own coke and coke oven gas.

Drying and heating of coke oven batteries is implemented mostly under a special technology previously developed by All-Union Coke-Chemical Station. According to this technology the battery can be warmed up using different kinds of fuels: coke oven gas, blast furnace gas, natural gas, propane-butane and also with solid fuel (coal). Drying and warming of coke oven batteries with the abovementioned fuels were tested in practice. The easiest and the cheapest way of coke oven battery warming up is it’s warming up with coke oven gas at operating coke & by-product plant. The battery is switched on to the permanent heating regime with the same coke oven gas when the temperature of brickwork reaches 750-800°C. The process of coke oven batteries warming-up with coke oven and natural gas and switching them on to the permanent regime with the same have been successfully developed by All-Union Coke-Chemical Station and today the procedure of coke oven battery heating-up is still performed according to these instructions.

Issue of choosing the fuel for heating up of the battery arises for newly constructed coke oven plant in case there is no coke oven gas available. In case the blast furnace gas is chosen, there should be not less than two blast furnaces in operation (to ensure stable supplying of gas for the battery heating). If there is no possibility of warming up the first battery with blast furnace or natural gas it could be warmed up with coal, as an option. Experience has shown that the process of warming up with coal is very time-consuming and complicated, as well as it has a the following significant disadvantages:

- it is very difficult to achieve sustainable heating of refractory brickwork;

- it is very difficult to provide smooth daily temperature rise;

- warming up process is labor consuming process, envisaged involvement of great number of the personnel;

- capabilities of coke machines testing and running along the battery are very limited at the final stage of warming up before the battery start up, (due to cluttered service platforms and the territory adjacent to the battery with the temporary equipment and necessary facilities for the coal supply and slag removal from the sites), taking into consideration that the reliable work of machines is one of the most important factors in successful start-up of coke oven battery;

- limited possibility of raising and maintaining the minimal temperature necessary for coking (in the range of 900-950°C) at the final stage of warming, reducing the intensity of gas emission from the first charged ovens, complicating the start of blowers and battery switching on to the permanent heating scheme with its own gas.

Warming-up of the battery with gases, unlike warming-up with coal, allows to achieve uniform heating of refractory brickwork along the length of the battery and accurate maintenance of daily temperature rise schedule, thus providing high quality of brickwork after heating; reaching higher temperatures at the final stage heating (all this is achieved with a relatively lower labor costs and less number of the personnel) giving the best possibilities for doing a good preparatory work for start up and testing machines.

In the early 70s of the last century Soviet Union specialists under the direction of N.A. Chemarda developed the technology of heating of coke oven batteries with propane-butane gas, which was first successfully applied during warming-up of Coke Oven Battery №1 of Iskenderun Steel Plant, Turkey in 1975. This made ​​it possible to apply this technology for warming of the first (head) coke oven batteries with propane-butane gas at many coke plants:

- Altai By-Product Coke Plant, USSR, (Coke Oven Battery №1 - 1981);

-  Vishakhapatnam Steel Plant, Andhra Pradesh, India, (COB №1- 1989);

- Metallurgical Plant of Neelachal Ispat Nigam Limited, (earlier Konark Met Coke Ltd.), Kalinganagar Industrial Complex, Duburi, Orissa, India, COB №1 - 2004);

- Coke Plant of Global Ispat Koksna Industrija doo Lukavac (GIKIL) in Lukavac, Tuzla Canton, Bosnia and Herzegovina (COB №4 - 2004);

- Integrated Iron & Steel Plant of Bhushan Steel Limited at Meramandali, Odisha, India (COB №1 and 2 - 2011-2012) ;

- Integrated Iron & Steel Plant of Jindal Stainless Limited, Kalinganagar Industrial Complex, Orissa, India (Coke Oven Battery №1 - 2010-2011, warmed up and switched on to the permanent heating regime with propane- butane gas).

Warming up of the first (head) coke oven batteries with propane-butane gas at newly built coke oven plants became popular for the following reasons.

1. LPG arrives in railroad tank cars or road tankers. Regasification plant that converts liquid propane-butane into the gaseous state at a pressure of 3-5 kg​​/ cm ² (0,3-0,5 MPa) is arranged to use liquid propane-butane. Gaseous propane-butane through the reducing station supplied to the battery under pressure of 500-600 mm of w.c. (5.6 kPa). It's quite mobile and reliable system of propane-butane gas delivery and usage allowing its application at plants remote from suppliers without the high cost of construction the gas line. Re-gasification plant (station) could be used again during the process of heating-up at another plant or applied at the plant for other purposes.

2. Simplicity of gas supply and combustion in temporary stoves as with using the other gases and special burner of relatively simple structure (Fig.1), developed by the engineer N.A. Chemarda, provides substantially complete combustion of pure propane-butane gas.

3. Net calorific value of propane-butane gas combustion, depending on the ratio of propane and butane may fluctuate within the limits of 21795 kcal/m³ (or 11,000 kcal/kg) with propane content about 100% to 28,340 kcal / m ³ (or 10900 kcal / kg). Using this gas for the battery warming up allows to raise the temperature of brickwork up to 1000-1050°C at the final stage. That is ensure a sufficient temperature reserve for arranging the procedure of charging of the first 16-20 ovens  and receiving of the required amount of own gas for blowing the gas pipeline, blowers start up and switching the battery on to the permanent heating with  return coke oven gas.

4. Unlike coke oven gas, propane-butane is supplied to the battery without inclusions (naphthalene, tar, ammonia, etc.) and water vapor (which is rather considerable for cold regions) that makes it possible to ensure its uniform distribution throughout all battery stoves and uniform ovens heating respectively. Unlike coke oven and blast furnace gases, propane-butane has no toxic gas components, such as carbon monoxide (CO).

Using propane-butane gas is important to organize its combustion in the stoves properly. It must be born in mind that combustion of propane-butane gas requires considerably more air (in 6.5-7 more volume) than it is used for Combustion of coke oven gas. Therefore, the inlet in the stove for gas and air should be possibly increased, providing greater suction and hence the required amount of air for combustion. At the initial stage of heating (drying) up to 100°C consumption of gas is very small and gas combustion at the burners may be unstable. There are cases of burners blanking with air and combustion damping. Therefore stainless steel meshes with a cell 1×1 or 2×2 mm (proposed by the engineer A.A. Levchenko) should be used to provide stable combustion in burners. At further increase of gas flow at brickwork temperature ~ 150°C and more the meshes are to be removed, as they inhibit gas-air flow at the increase of flow rate that can disrupt the intensity of gas-air mixing and cause smoke curing of the burner as a consequence.

To stabilize the combustion flame at the entrance to the stove it is recommended to set brick neck (small external stove) with thickness of one brick (230-250 mm), 8-9 courses in height (570-650 mm), with a window in neck roofing 100×100 mm size for "extra" air supply (Fig.2). The burner is mounted inside the neck in such a manner that its front portion (front elevation of the burner) is aligned with the line of the "face wall" facade so that at the temperature of 400-450°C the burner completely got into the opening in the "face wall" and did not sink inside the oven (on condition that temporary pipeline is permanently fixed with respect to brickwork moving during its expansion). This is necessary due to the following reasons.

1. At the initial stage "additional" air (passing through the top window) will reduce the speed of the main horizontal airflow and thus stabilize the combustion flame (especially if the mesh is installed on the burner). Later, at stabilization of the combustion flame the upper windows should be closed.

2. At the temperature of brickwork 400-450°C, the burner will go into the opening in the "face wall" (as a result of the brickwork growth it will come closer to the burner), the neck should be removed and plates for air supply regulation (movable flap) should be moved to the front of the "face wall". The gas-air flow rates and swirling patterns in the opening of the "face wall" will increase and it will improve gas mixing with air preventing the smoking of the torch. This burning arrangement was performed at heating of Coke Oven Battery №1 of Jindal Stainless Limited Steel Plant in India and proved itself. Moreover, such burner installation towards the front of the "face wall" promotes better warming of extreme heating flues.

The burners were preferably made of heat-resistant steel as per the design proposed by the engineer N.A. Chemarda, but practice shows the possibility of applying ordinary structural steel (with corresponding appropriate level of control over the heating process modes). The distance between the axes of vertical burner tubes is important. Using two types of burners is reasonable. At the first stage the burners should have lesser inter axial distance that will contribute to the stability of combustion flame. At temperature of brickwork round ~ 450° C the burners with a larger spacing between the vertical tubes are to be installed.

It is desirable to supply liquid propane-butane with a greater content of propane (C₃H₈) to warm up the battery, i.e. with lower carbon content and calorific value (ideally desirable to deliver pure propane, with net calorific value 21795 kcal/ m³). This will improve gas combustion, reduce the flame smoking and carbon formation on the burners.

At Steel Plant of Jindal Stainless Limited (India) liquid propane-butane with the ratio of 60% propane, 40% butane (24,400 kcal/m³) was supplied for heating of Coke Oven Battery №1. After reaching 900°C brickwork temperature (upon switching the battery on to the permanent heating scheme with propane-butane) propane portion increased to 96.3 % (22040 kcal/m³). In comparison to propane-butane ratio of 25% propane, 75% butane (26700 kcal/m³) supplied for heating of Coke Oven Battery №1, Konark Met Coke Ltd. (India). There was some flame smoking and deposition of carbonaceous graphite in coking chambers and burners, though without after-effects (except for intensive cleaning of burners at the final stage of the battery warming-up). Liquid propane-butane in railroad tank car with ratio of 30 % propane, 70 % butane was supplied in autumn and 50 to 50 % in the cold time (respectively 26375 and 25060 kcal/m³) for heating of Coke Oven Battery №1 at Altai Coke Plant. At the last stage of heating there was also some flame smoking and deposition of carbonaceous graphite in coking chambers and burners. Konark Met Coke Ltd (India) warmed up with propane-butane gas two CDCP’s along with the battery itself. At first warming up was held with design burners supplied to the plant for heating with coke oven gas (Giprokoks design) that proved impossible. After modification of the existing burner in place (as per the proposal of L.Z. Bugay and V.A Kurmaev) two CDCP chambers were successfully warmed up and boilers alkaline extraction was done for heating with pure propane-butane gas. One feature of operating mode of regasification plant should be noted. Liquid propane-butane must be constantly pumped in from the gas storage ("tanks") and passed through the evaporators for regasification for constant composition of propane and butane mixture in the vapor phase. Long operation of the plant in the mode of gas aspiration (gas-phase) from "tanks" by-passing pumps and evaporators, stopping the work of the latter (this is possible for warm (tropical) climate conditions) should be avoided. Operation of the plant in the mode of gas-phase by-passing evaporators leads to gradual increase of butane share (heavier hydrocarbons) and reduction of propane percentage share in "tank".

Start up of the first (head) coke oven battery at the plant after warming up without switching it on to the permanent heating regime is rather complicated due to overlapping (in time) of the last stage of warming with testing of coke machines and equipment and completion of a large amount of prestart activities. A number of critical operations are executed simultaneously within a short period of time, namely: breaking of face walls and dismantling of temporary stoves, installation of coke oven doors, installation of plugs into the firing holes, charging of ovens with blend, purging of gas line facilities with switching of the blower, switching the battery on to the permanent heating regime with a small and unstable amount of reverse coke oven gas.

All these operations are performed at warming up of unloaded ovens on a temporary basis (until the end of their switching on to the permanent heating regime). At the same time the first 14-16 ovens prepared for the first charging are not included into the heating and charged with coal and heated only after receiving of return gas (after switching on to the permanent heating regime). Part of these ovens’ cumulative heat is lost at dismantling of temporary stoves and installation of doors prior to their charging with coal, which significantly reduces the temperature of brickwork at charging time, reduces the intensity of gas emission from the first charged ovens, complicating start up of blowers and switching the battery on to the permanent heating regime.

Failures, breakdowns of coke machines and equipment, problems with blower (electric blowers without speed adjustment can go into surge due to the small amount of gas) and gas path at start up operations can lead even critical situations due to the lack of gas for pipelines purging and switching on to the permanent heating regime.

Such start up works as filling the flash plates with mortar, lining of jamb bricks over the flash plates, calking of stand pipes goose necks, start up of valve boxes spraying, lining of tie rods with bricks, straightening and final fixing of coke machines tracks and other works should be carried out at a temperature in heating flues not below 800 - 850°C (normal temperature for execution of this work 950-1050°C). Heat transfer from the upper to the lower zone of the brickwork with its accumulation in the lower zone and overheating of the latter takes place at such temperatures (especially at starting 950-1050°C) in condition of heating under the temporary scheme. Temperature in the sole channels achieve critical for the integrity of the slab value ​​of 450°C and it is rather difficult to suppress its further growth.

Working conditions at the gas-valves intensively deteriorate due to this reason (and under the top base plate on the underjet battery), servicing of reversing mechanism, gas supply valves at gas distribution pipelines, switching of the battery on to the permanent heating regime becomes very complicated. During switching the battery on to the permanent heating regime prior to ovens charging with coal all these difficulties are not considerable or minimal.

After switching the battery on to the permanent heating regime prior to ovens charging with coal the following work should be performed:

- operational temperature 1050-1100 ° C is easily achieved, while the temperature in the sole channels is on the operating level, within 280-300° C, not above 300°C;

- all preparatory work is performed without stress;

- all internal stoves are removed, plugs are installed in the firing holes;

- all doors are installed simultaneously with adjustment of coke machines mechanisms;

- all ovens are charged within a short period of time.

Complexity of the battery heating on the permanent heating scheme with high carbon gases (propane- butane) is caused by its decomposition and deposition of carbonaceous graphite in the burners leading to clogging of channels at heating of the red-hot gas areas (temperature ≥ 800° C): gun-flues and upper gas rizer channels. To use propane-butane gas for the constant heating, it must be diluted with air up to 3500-4000 calories kcal/m³, which corresponds to the content of 13-15% in a mixture of volume of propane- butane gas and 87-85 % of the air (the upper limit of intolerable concentration of propane-butane in the gas mixture with air - 8.65%). Mixing of propane-butane with air directly at the entrance to each gas rizer channel is a variant of such dilution at underjet battery and at the inlet to gun-flue zone.

A group of Russian specialists, LLC "Ogneuporkoksservis" (the head of the group Kurmaev V.A.) was involved for carrying out preparatory work and switch coke oven battery on to the permanent heating regime with propane-butane gas after heating at Steel Plant of Jindal Stainless Limited (India).

Coke Oven Battery №1, Jindal Stainless Limited, PRC design, comprising of 64 ovens, intended for stamp charging (with a useful volume of the oven’s chamber 26.6 m³, height 4.3m, average width of oven 500mm). The battery production capacity is 430 thousand tons of coke per annum. Heating system: PVR with bottom supply of coke oven gas (non-hybrid heating), heating wall consists of 28 channels (14 pairs) with high burners (300 mm).

Submission of heating gas is carried out through one distribution gas pipeline on both sides of coke oven. Gas is supplied to each heating wall through a pair of under pier buttress collectors-distributors: gas is supplied through one manifold to the even heating gas channels through the other - to the odd. Each under pier buttress collector has its own pair of cocks - stopcock and reversing cock and its dosing diaphragm. 130 stopcocks and 130 reversing cocks are installed on the gas distribution pipeline. Chimney height 120 m, with a side outlet of combustion products (main flue is located in the end of the battery).

Technology of the battery heating with propane-butane with the coking gas supply for permanent heating scheme proposed by LLC "OKOS" is the following:

- distribution coke oven gas pipeline is temporarily isolated from reverse coke oven gas supply pipeline (from the gas heater also);

- connection pipes with flow-regulating valves for supply of propane-butane gas are cut into the distribution pipeline (Fig.3);

- holes of the correct size for the air supply from the bottom of the tunnel to the crossing piece (under natural suction) for mixing with propane-butane gas, entering the crossing piece through the calibrated cross section of heating walls gas manifolds are drilled in the lower plugs of each crossing piece on gas supply in each heating channel (in each gas rizer channel);

- calibrated sections for supplying of propane- butane gas into the heating walls gas manifolds and of these reservoirs into the crossing pieces of gas rizer channels, as well as holes for air supply in the plugs of crossing pieces are selected by means of calculation;

- appropriate hydraulic mode in the heating system ensures the necessary suction in the crosspiece of gas rizer channel for air suction from the tunnel for mixing with propane-butane gas.

Estimated gas mixture obtained in the cross piece should consist of 15-16% of propane- butane gas and 84-85% of the air and have a calorific capacity of ~ 4000 kcal/m³ (lowest calorific capacity). Guniting of all gas rizer channels with silica mortar was carried out at preparation to switching the battery on to the permanent heating regime. Tubes for blanking of sliding joints in the regenerators’ zone were not installed (not stipulated by the design).

It should be noted that at the change of reversing mechanism there is a 22-25 sec. pause in gas supply for heating, when there is no gas flow and gas pressure in the distribution manifold dramatically increases. At this point it is necessary to provide gas discharge from the gas pipeline with the aim of gas pressure lowering to a value ≤ 400 - 450 mm of w.c. ( 4.5 kPa). Otherwise, the emergency shut-off valve on reducing station will respond with complete shut-off of propane-butane gas supply to the battery, resulting in emergency stops of heating (up to the valve unlocking, which takes some time).

End gas bleeder 200 mm width with hydraulic seal Н_min= 300 mm of w. c. (3 kPa), providing gas discharge at reversing (see Fig. 3) was envisaged by the design at gas distribution pipeline of Coke Oven Battery №1, Jindal Stainless Limited. It is necessary to provide a gas bleeder with water seal at head coke oven batteries where gas bleeder not provided by the design.

On the 22^nd of December, 2010 Coke Oven Battery №1 was switched on to the permanent heating regime with propane-butane gas at a temperature of 880-900°C after preparatory work and testing of reversing mechanism. Heating at a permanent heating regime with propane-butane lasted for 16 days (up to the battery switching on to the permanent heating regime with its own coke oven gas) without significant problems. Within the first start up days (after 2-3 days of the switching) loose carbonaceous graphite began to appear in some burners and gas rizer channels. After adjustment work on air supply for gas mixture in crossing pieces and performing other activities the situation with formation of graphite was stabilized, though surveillance over graphite formation and cleaning of burners and gas riser channels continued, avoiding graphite clogging.

Cleaning was performed regularly, but without tension, gas combustion in heating channels was normal, without soot. On the chimney soot was not observed.

Temperature in control heating flues was maintained at the level of 1040-1060°C, in the end heating flues at the level of 950-960°C; in the sole channels - 270-280°C. Heating gas pressure in the pipeline was at the level of 40-60 mm of water. c. (0.4-0.6 kPa).

The main disadvantages that prevent normal heating on permanent basis and lead to carbon formation at some burners and gas riser channels are the following.

1. Installation of plugs into the firing holes under the upper holes took much time - a defining moment. It is very difficult (if not - impossible) to maintain necessary hydraulic regime in the heating system and to provide uniform heating throughout the length of the battery and heating wall.

2. Significant leakages were observed in sleeve joints (non-threaded joints) at the top of the vertical steel tubes through which the gas is supplied to each heating flue. Acceptance of erection work of fittings connecting to heating flues was done by the Indian party before the arrival of LLC "OKOS" specialists. Depression in cross-pieces necessary for normal gas mixing with air, as well as gas inflammation in separate gas rizer channels, which should not take place at metered suction of air through holes in the plugs of cross-pieces occurred due to supplementary air suction.

3. Immediately after switching the battery on to the permanent heating regime reversing mechanism was operating with 30 min interval between the reversings (under the Chinese design). Such operating mode contributes to carbon formation in the top part of the flues. In practice, it was confirmed that the smaller time interval between the reversing, reduce the carbon formation of the burners and flues appropriately. The operating mode of reversing mechanism was set to 20 minutes between the reversings three days after the battery switching on to the permanent heating regime.

Thereafter, upon further heating carbon formation sharply decreased. After series of activities carried out in the course of the first 7-8 days after switching the battery on to the permanent heating regime, such as:

-  completion of the plugs installation into the firing holes under the holes;

- switching of reversing mechanism on to the mode of 20 minutes pause between the reversings;

- sealing of sleeve joints of vertical steel tubes;

- adjustment of air and gas metering in the crossings, battery heating with propane-butane gas under the permanent heating scheme got stabilized and did not cause any concern in the process of further heating under this scheme.

All preparatory works for the battery start up were executed at the battery heating with propane-butane gas under the permanent heating regime before charging with coal. Internal stoves and all doors on ovens were removed, 19 ovens were charged with coal, gas lines was purged with coke oven gas, gas blower was successfully started, coal blend in two oven chambers (103, 111) was coked and the first coke was pushed with good condition. After performing of all these works and obtaining of stable amount of own gas, the battery was switched on to heating with its own coke oven gas, on the 7^th of January, 2011.

Heating of coke oven batteries with propane-butane gas was tested by the experience and proved itself as a reliable way of preparation for the start up of the first (head) batteries on the newly constructed Coke plants. In addition, practice has shown that propane-butane gas can be used for heating under a permanent heating regime, at least for the underjet battery, after performing some additional work, relatively small and not expansive. Specialists of LLC "OKOS" developed idea / approach allows heating with propane- butane gas at a permanent heating regime for gun-flue type coke ovens.

Of course, technology of heating with propane- butane gas needs to be improved. A better mix of propane-butane gas and air at burners and stoves for complete combustion of gas should be provided. Currently these minor drawbacks can not influence the choice of heating technology with propane-butane gas of the first (head) coke oven batteries, which is confirmed by the experience of LLC "OKOS" in India and Bosnia & Herzegovina.