Field of the invention
The proposed invention refers to the improvement of the coke CSR produced in the
recovery type slot ovens by the technique of zero heating during soaking. The invention
also offers reduction in energy consumption of the coke oven and corresponding
reduction in the emission to air.
Description of the prior art
Blast furnace coke governs the permeability of the burden and also provides it with the
necessary physical support. The coke quality, hence, is very critical from a blast furnace
operator's point of view. Coke strength after reaction (CSR) is an accepted coke quality
parameter around which the performance of the blast furnace is measured. Coke
makers around the globe have been researching on techniques to improve the coke
CSR. Nakamura et. al (1977) have reported that the coal blend properties play a
dominant role (70%) as compared to the carbonization conditions in governing the CSR
of the resultant coke produced. Hence much of the work focuses on coal blend
optimization for CSR improvement. However significant amount of research has also
been carried out for exploring the effect of carbonization conditions or operating
parameters (like Bulk density, crushing fineness, heating rate etc) on coke CSR.

Coke oven heating is one such operating parameter which plays an important role in
governing the types of reactions taking place during carbonization phenomenon and
hence the nature of bonds formed in the resultant coke produced. The Japan Iron and
Steel Federation have developed a method for improving the coke strength by rapid
preheating and classification of coal blends with both caking and non or slightly coking
coals. The method, as described in US patent # 7,645,362 by Kato et. al (2010) states
that Method for pre-treating and improving coking coal quality for producing BF coke
comprise of (a) rapid-heating the coal charge in a fluidized-bed to a temperature range
between not lower than 300 °C. and not higher than the temperature at which the coal
charge begins to soften, at a specified heating rate, (b) classifying the rapid-heated coal
charge to fine- and coarse-size coal, and then (c-1) briquetting the fine-size coal or (c-2)
rapid-heating the fine- and coarse-size coal individually in a pneumatic preheater to a
temperature range between not lower than 300 °C and not higher than the temperature
at which the coal charge begins to soften, at a specified heating rate, and (d) forming
the fine-size coal. Hence, the method essentially refers to the pretreatment of coals or in
other words the coke oven heating at the start of the cycle. The method essentially
targets improvement in the coking qualities of the coals to produce high strength coke
for blast furnace.
Beck et. al in 1976 have developed a process (US patent # 3,970,523) for producing
cokes of large lump size and improved strength. The process essentially states the

coking rates in inch per hour based on the coke oven width and time of coking
operation. Also, it states the rate of temperature increase of the coal in the plastic range
for producing a good quality coke. The values for coking rate and rate of temperature
increase are based on the coking chamber of the high-efficiency oven with a width of at
least 500 millimeters (19.7 inches). The invention targets the rate of temperature
changes in the plastic range to improve the strength and lump size of the coke
produced. The inventors claim that to obtain lumps of satisfactory strength and large
size, the rate of temperature increase during the heating in the plastic range must be
decreased (reduced coking rates). They have also described the measures that needed
to be taken for achieving these reduced coking rates.
Soaking of coke is an important part of the coal carbonization cycle. Soaking time, as
stated by Roger Loison in Coke quality and production, is the extension of coking time
beyond what might appear strictly necessary. The coke is said to be soaked when it
remains in the hot oven during this soaking time. Soaking is carried out by holding the
carbonized coke inside the hot oven for the desired soaking time. R Sharma et. al
(2004) have found that soaking improves the coke quality, primarily due to the stabilized
pore structure.
Most of the work done in the area of coke oven heating pertains to the heating in initial
stages of carbonization. Specifically, heating at temperatures prior to the plastic range

or in the plastic range have been explored to obtain a coke of improved strength and/or
lump size. Also, there is a good amount of research done in the optimization of soaking
time for producing a high strength blast furnace coke. However, the heating rates during
soaking have not been explored in terms of its impact on the coke quality.
Hence, the main objective of the present invention is to provide method of heating
during the soaking time. This method increases the coke strength after reaction (CSR)
of the blast furnace coke produced from the laboratory scale recovery slot ovens with
both top charge and stamp charge methods of charging in the slot ovens.
Brief description of the invention
The soaking phenomena in the coke oven carbonization cycle is very important with
respect to the rearrangement of coke molecules to produce a micro-structure and
texture favorable for high value of Coke strength after reaction (CSR). As important the
soaking of coke is, the question still remains about the optimum heating during the
soaking time for obtaining the best quality coke. The present invention aims to answer
this question. The invention proposes a method of heating during the soaking time to
produce a coke with improved CSR than the one produced from the existing practice of
normal/or conventional heating.

During the soaking of the coke mass inside the oven, the heat input has to be
completely stopped for obtaining an improvement in the coke CSR. The coke mass
during soaking hence stabilizes into a compact product as observed from the improved
avg. cell wall thickness and reduced avg. pore diameter. Also, the coke end
temperature drops, depending on the heat losses corresponding to the dimensions of
the oven. Thereby, reducing the quenching temperature of the coke and hence causing
lesser generation of micro fissures as compared to the coke quenched from higher
temperatures. Also, the activation energy (refer the Arrhenius law) for the solution loss
reaction increases for the coke and hence the rate of reaction decreases. This is also
one of the reasons for improved CSR, as CSR and reactivity have an inverse
relationship. Since the heating is stopped during soaking time, equivalent amount of
energy savings (for both electrically heated and gas fired ovens) and reduction in
emissions to air (for gas fired ovens) are also obtained.
Brief description of the drawings
Figure 1 illustrates a schematic for the 7 kg carbolite oven coal charge
Figure 2 illustrates an arrangement of probes inside the coal charge
Figure 3 illustrates the temperature profile inside WB coal charge in the 7 kg carbolite
oven coal charge
Figure 4 illustrates the microstructure analysis of coke.

Detailed Description of the invention
The coke strength after reaction (CSR) of any metallurgical coke, as many have
studied, mainly depends on the caking properties of the coal or coal blend and the rate
at which it is heated, among other factors. This fact has been reported by Nakamura et
al (1970). They have actually quantified the effect of coal blend properties (70%) on the
resultant coke quality (CSR) compared to that of the operating parameters (30%) like
bulk density, crushing fineness, soaking time, pre heating etc. The present invention
focuses on the soaking aspect of the coal carbonization. It has been widely accepted
that soaking time is very important for producing a good quality coke. Soaking allows
the thermal rearrangement of coke molecules to form a coke with good micro structure
(high cell wall thickness and low pore diameter).
The present invention suggests that if the heat input during the soaking time is reduced
or stopped completely, there is an improvement in the thermal rearrangement of the
coke molecules. Hence, an improvement in the CSR of coke is found, as compared to
the coke produced from the process where the coke mass is heated during soaking
time. The novelty of the invention lies in the modification of the operating procedure of
coke ovens for example, the recovery type coke ovens to reduce or completely stop the
heat input to the coal charge during the soaking time. This improves the CSR of the
resultant coke and is less energy intensive (since the heating is reduced/ stopped).
Also, for gas fired ovens the technique offers reduced environmental

emissions from gas under-firing corresponding to the time for which the oven heating
was reduced/ stopped. The invention works for any coal blend composition, coal
crushing fineness, bulk density, heating rate, use of any binder for producing
metallurgical coke. This is because the invention is about modification in the heating
procedure during soaking, prior to which the entire coke mass is already produced by
the virtue of these coal blend properties and the operating parameters.
Experiments were conducted to identify the temperature profile at different positions
inside the 7 kg carbolite oven. Figure 1 shows the schematic of the electrically heated 7
kg oven with the 9 cm wide coal charge inside. Coal charge from coal source 1 with
crushing fineness of 90% below 3.2mm at a bulk density of 1150 kg/m3 containing 10%
moisture was prepared in a cardboard box mould. The heating rate in the oven was kept
at 3°C/min. Three temperature probes were put in the coal charge for achieving the
aforementioned objective. One probe was put at the centre and the other two at the
surface of coal charge near the walls. The arrangement is shown in figure 2.
Temperature was noted at regular intervals. The coal charge was carbonized in a total
cycle time (carbonizing time + soaking time) in the oven for 5 hours. The coke was
quenched with service water and was tested for CSR and CRI (Coke Reactivity Index)
by the NSC method (Nippon steel). The temperature profile from the above experiment
in the 7 kg Carbolite oven is shown in figure 3. It shows two profiles; one at the centre

and the other near the wall. The temperature profile for the two probes near the wall
shown in figure 2 indicates the same trend. Hence only one curve near the wall is
shown in the figure. The profiles indicate that, initially, the temperature near the walls
rises rapidly and that at the centre increases very slowly. The centre of the charge heats
up at a higher rate during the later stages owing to the heat it receives from both the
sides. This is concurrent to what has been reported in the literature (Osinski et al.
1993). It can be seen from figure 3, the coal charge/coke mass comes to a uniform flue
temperature of approximately 1010 °C after 160 mins. Industrially the flue temperature
ranges from 1000 to 1400 °C. The coke mass is being soaked beyond this point i.e.
after 160 minutes up to 300 mins (for 140 mins) by the heat supplied from the oven.
In the proposed method of zero heating during the soaking period the heat input to the
coke mass has to be stopped during the soaking period. A few set of experiments were
carried out to validate the method in the same 7 kg Carbolite oven. The carbonization
time was fixed at 180 minutes and the corresponding soaking time was 120 minutes. In
industrial ovens, the cycle time ranges from 4 to 96 hours. The oven heating was
switched off for 2 hours during the soaking time for all the validation tests. Normally in
industrial ovens, the heating switching off time ranges from 5% to 30% of the total cycle
time at said oven. Once the heat input to the oven stops, the coke oven walls start to
lose heat and cool down, a temperature gradient is developed inside the oven for the
heat flow from the coke mass towards the wall. Hence, the coke mass starts to lose

heat corresponding to the gradient between the oven walls and the coke mass. During
the time for which the oven heating was stopped, the temperature of the coke mass
drops to approximately 750-800 °C. Hence the micro-fissure generation during
quenching will be less as compared to the coke quenched from a higher temperature.
The following results were obtained:
1. Significant improvement in the CSR, measured by the NSC method, of the
resultant coke produced when compared to the conventional operation. The
CSR was found to improve by minimum 3 points for the coke produced from
the 7 kg oven.
2. Corresponding reduction in the CRI, measured by the NSC method, of the
coke produced.
3. The lateral and vertical expansion to the coke mass was not different than the
conventional method of 7 kg Carbolite oven operation
4. The push force was same for the coal carbonized under conventional heating
and zero heating during soaking.
5. From the coke micro-structure as seen in figure 4, the avg. cell wall thickness
increased and the avg. pore diameter decreased for the coke produced from
the proposed method.

6. The activation energy for the solution loss reaction (C + CO2 = 2 CO) has
increased for the coke produced from the proposed method leading to the
reduction in CRI and increase in CSR.
7. More fine and coarse mosaic structure was observed in the coke micro-
texture of the coke produced from the proposed method.
8. Energy savings corresponding to the time (soaking time) for which the oven
heating was stopped will be obtained along with reduction in emissions to air
for gas fired ovens


Table 2: Microstructure analysis and Activation energies for the solution loss
reaction of coke produced in the two cases.


WE CLAIM:
1. A method of producing coke, the method comprising the steps of
i) preparing a coal charge;
ii) carbonizing the coal charge in the oven over a total cycle duration at a flue
temperature in the range of 1000 to 1400 degrees Centigrade;
iii) allowing said carbonized coal to soak; and
iv) stopping the heat input to the oven after a predetermined time.
2. The method of producing coke as claimed in claim 2, wherein the total cycle
duration ranges from 4 to 96 hours.
3. The method of producing coke as claimed in claim 2, wherein said predetermined
time ranges from 5% to 30% of the total cycle duration at said oven.
4. The method of producing coke as claimed in claim 2, wherein the heating rate of
oven ranges from 2 to 4 degrees Centigrade per minute.
5. The method of producing coke as claimed in claim 2, wherein said coal charge
has a crushing fineness of 80-95% below 3.5 mm.

6. A method of producing coke with improved CSR (Coke Strength after Reaction)
by stopping the heat input during soaking of coke.

ABSTRACT

The invention relates to a method of producing coke, the method comprising the steps
of preparing a coal charge; carbonizing the coal charge in the oven over a total cycle
duration at a flue temperature in the range of 1000 to 1400 degrees Centigrade;
allowing said carbonized coal to soak; and stopping the heat input to the oven after a
predetermined time.