Introduction – Background
Beside economic and production matters, coke oven managers have also to consider the environmental impact of the coke production. Recently many coke plants in Europe and USA have been prosecuted (fines and closures) for pollutions and bad health condition due to the emission of fine particles at the stacks (CO, dust, gases). Also the China’s State Council has recently released its new Action Plan for Air Pollution Prevention and Control (Action Plan). FIB-Services International S.A. has developed a range of technologies for a simple and efficient treatment of the air emissions.
Origin of the Air Emission of a Coke Oven Battery
Air emissions have different origins (see illustration 1, source “Best Available Techniques Reference Document on the Production of Iron and Steel, European Commission, IPPC, December 2001”), among others :
- Emissions due to a bad sealing of the chambers (doors, charging holes, stand pipes)
- Emissions due to a bad coking (during the pushing)
- Emissions due to the quenching operation (quench car, quench tower)
- Emissions due to a bad regulation of the burners (mix gas/air)
All the above sources of emissions can be easily solved by improving the production process. But most visible emissions at the stack are due to the leakages between the coking chambers and heating flues. The holes, medium and small cracks in the heating walls allow the coking gases and coal dust to access the heating flues (to the stack), instead of the main collector (to the by-products).
Those troubles are more difficult to solve because small cracks are naturally present in the coke oven chamber, and cannot be sealed individually.
Since 1987, FIB-Services International has been providing the industry – among others the coke making industry – with self-developed technologies for the repair, the preventive maintenance and the reduction of air emissions.
For the reduction of air emissions of coke ovens due to big cracks and holes, FIB-Services International can propose a high quality ceramic welding process (called RPR-SC*). This ceramic welding process is applied for more than 20 years in coke plants all over the world, in the most important steelmaking group such as ArcelorMittal, JFE, China Steel, etc.
Unfortunately, ceramic welding is only suitable for the treatment of medium and big cracks, or holes (ceramic welding together with S-Bricks).
After many years of R&D, FIB-Services International has been able to introduce a brand new process for the treatment of microcracks : RPR-SL.
The RPR-SL Treatment
The “RPR-SL” product and its application process (patented) have been developed by FIB-services International with the aim to clog by one global operation the numerous microcracks occurring on the heating walls and the ceiling of a coke oven chamber. The result is a substantial dust emission reduction at the stack of the battery, allowing to meet the environmental regulations.
The “SL” process consists in the projection, by pressurised air through the charging hole cap, of a liquid mixture, which will decompose and burn in the hot atmosphere of the chamber. This combustion will produce a large quantity of gases (CO2 and water vapour) and a silica mist, which will spread in the whole volume of the chamber and infiltrate the microcracks owing to the overpressure due to the evolved combustion gases. In fact , as the microcracks are progressively clogged, the internal pressure increases more and more up to a maximum (up to 200 mm of water column has been recorded), enhanced by the absence of big leaks in the refractory construction or at the other outlets (the main repair operations like the ceramic welding being previously performed).
When the very fine silica particles, forming like a mist, enter the microcracks (pushed by the pressure difference between chamber and flue), a part of these particles will be trapped and adhere on the internal surface of the microcracks, forming like a Silica Layer (from which the brand name RPR-SL). This mechanism of silica particles deposition will progressively form a ceramic sponge, stopping, like a cork, the gas flow from the chamber toward the flue.
At service temperature, the fine silica particles will become interconnected owing to a global sintering by solid state diffusion.
A porous, but cohesive ceramic material is so created into the microcracks. Owing to this inter-particles ceramic bonding, the microstructure of this ceramic sponge becomes stable, and, as a consequence, the gas-tight character of the whole chamber is reached and will be kept for a long time.
Advantages of the process:
- Quick operation: about 5 minutes
- Only 1 operator
- Much more efficient than other techniques (dusting)
- Only 2 L of material per oven
Different industrial situations, which need an “RPR-SL” treatment are met:
- Most often, after the main repair operations on the chamber (ceramic welding, new door seals…), the microcracks need to be treated to diminish the opacity level measured at the stack to the minimum;
- After a long idling period in hot conditions, the whole battery has to be treated before starting back the production (when all the carbon deposit has burned, all the cracks are open). Result of the RPR SL treatment : after the first charging of the battery, no figure exceeds the opacity limit;
- After burning out all the carbon deposits on the chamber walls to avoid excessive pushing forces, important gas leakages are expected, an “RPR-SL” injection can solve the problem;
- Oven with a new battery, after the first heating up, numerous microcracks are developed due to thermal expansion stresses in the brick masonry. It induces a serious risk of exaggerated dust emission during the first charges. A preventive “RPR-SL” treatment will avoid this trouble.
- More widely RPR-SL is also used as an indicator (to have a diagnostic of the oven) – During application, if the pressure is not increasing in the chamber, it is a clue that big leakages have not been managed (holes in the wall, door leakages etc). Also the white deposit of the RPR-SL is a good indicator of the location of the damages.
Industrial evidence of the success of the treatment can be ascertained by different ways:
a) when the heating flues are pressurized, gas leakage locations are indicated by candle flames, which are visible through the open door of the chamber; their disappearance after the “RPR-SL” treatment is an evidence of the stopping of the microcracks;
b) at the stack, by the measurement of the CO content and the opacity (dust emission level), after a coal charging operation.
A record of the opacity level made by the coke oven team (above figures) shows the advantage of the “SL” process in comparison with the classical “silica dusting” technique, particularly after the first and second charging operation: after the “SL” treatment, the first dust peak is considerably diminished and shortened and the second one is no more observed, while, after the silica dusting operation, the first and also the second dust peaks are well present.
Industrial Results : differents cases :
Case 1 : ArcelorMittal Krakow
Battery was on hot idle for a long period. The management decided to start the production again, but in order to avoid air emissions during the operation, and a degradation of the battery, a preventive maintenance programm including RPR SL application have been established.
2 years after restarting the production, the Polish Institute ICHPW issued a technical report on the condition of the battery : they stated that “the tightness of the wall is 1.25 %, which is similar to a new coke oven battery”
Case 2 : Comparison between silica dusting and RPR SL methods
The traditional way to close the microcracks is the silica dusting of coke oven. It consists in adding crystalline silica inside the coke oven chamber. This method has different inconvenient:
- The silica dust is “flying” during the application and can be inhaled by the operators (silica disease);
- Silica will stick in the regenerators, and it will lead to a bad regulation of the heating
- It takes a lot of time
The advantages of the RPR SL treatment vs. silica are :
- The result of the combustion of the liquid mixture is amorphous silica (no problem for the health)
- Fast application (only 5 minutes)
- Product will not fall in the regenerators
- Product is more efficient that the silica dust because the pressure in the oven is higher during the application and because the product will sinter.
In a Belgian coke plant, a comparison between both methods has been hold:
The results clearly show that air emissions at the stack are much more controlled with RPR SL than with silica dusting (emission peak is shorter).
Case 3 :
An Italian coke plant close to the city and the sea (tourism) decided to have a complete treatment of its battery by RPR-SL Material.
Befor the treatment of the microcracks by RPR SL, the bigger damages have been fixed with gunning, or ceramic welding.
The following figure shows the maximum emissions (y) per oven (x) before the injection (blue), after the first charge (pink), after the second charge (yellow), after the third charge (light blue), and after the fourth charge (purple).
Clearly the air emissions have been controlled.
Case 4: Before and After implementing RPR SL (Brazilian coke plant)
In July, this Brazilian coke plant has decided to implement the RPR-SL in its maintenance program as form end of July.
Case 5: Treatment after of 1 oven after the maintenance operation (US coke plant)
After the maintenance operation of a coke oven chamber (ceramic welding), the opacity at the stack during the charging was +/- 20 % during 12 minutes. After the RPR SL Treatment, the maximum recorded opacity was 0.12 %.
Case 6 : Treatment of a battery of 15 ovens after a long period of maintenance.
A complete battery of 15 ovens has been in maintenance (change of metallic bracing) during 2 weeks. During this period, the ovens were on hot idle. As a consequence, all the carbon inside the small cracks has burned. In order to avoid high air emissions at the stack during the first charging, the management has decided to proceed with a RPR SL Treatement of the ovens before starting up the production. To complete the operation (15 ovens), including safety and logistics, only 2 technicians during 3 hours, and 30 Litres have been necessary.
The emissions during the first charging operations are really low, close to the level of the emissions when the oven were empty.
- By applying the RPR technologies : ceramic welding, and especially the “SL” injection process, it is possible to reduce in a significant way CO and dust emissions at the stack of a coke oven battery in order to satisfy the environmental regulations;
- The choice of the method to apply this technology depends on local actual conditions and has to be discussed with the coke plant team;
- Most often, preliminary repair operations (ceramic welding on major and medium cracks, new door seals,….) are necessary to concentrate the effect of the “SL” treatment on the microcracks, which may only be treated by a global operation;
- To control on a safety manner the dust emission level of the whole battery, a “SL” treatment is recommended each 6 months.