LOCATION OF PLASTIC LAYER IN HORIZONTAL NON/HEAT RECOVERY OVENS

Introduction:

During carbonisation coal passes through a softening phase when the coal particles swell and become soft. This soft mass is termed as meta-plast and said to be plastic in nature. During this transformational phase coal releases most of its hydrocarbons, heavy oils and tar; the coal particles coalesce and are bound together by the plastic mass. On further heating the plastic mass resolidifies into a porous mass called semi-coke. On further heating the semi-coke contracts and becomes a structurally stable product which we know as metallurgical coke or simply coke. In any form of carbonisation – conventional recovery slot ovens or non/heat recovery ovens – two primary plastic layers are formed which travel from the heating surfaces towards the centre of the coal mass. The end of coking time is signalled when the two plastic layers join at the centre of the coal mass. Technically it is the net coking time: add to it some soaking time and the coke is ready.

The plastic layers are formed parallel to the heating surface. In slot ovens and vertical non/heat recovery ovens, the one layer each is formed parallel to the side walls. In horizontal non/heat recovery ovens one layer is formed at the top of the coal mass and other at the bottom.

In slot ovens, whether recovery or non/heat recovery, the mode of heat transfer is primarily by conduction. In horizontal non/heat recovery ovens the heat transfer from top (oven arch) is primarily radiation and that from the bottom (Oven sole) is by conduction.

Note: Conduction is the most efficient form of heat transfer and radiation is the least efficient

If the hydraulic regulation and air regulation, which in turn order the thermal gradients in non- recovery ovens, are proper, the two plastic layers meet right at the central axis of the coal mass. Else the location of the meeting point of the plastic layers is skewed. In the author’s experience often the meeting point is found to be nearer to the oven sole indicating imbalance in progression of coking.

Criticality of the location of plastic layer:

The central location of the plastic layer is important as this implies that –

- Coking rate has been equal from top and bottom

- Coking time is optimized for a given level of production

- Soaking time of coke is similar on both sides of the plastic layer

All three are important for better uniformity of coke inside the oven.

Influence of hydraulic regulation:

To overcome skewedness in location of plastic layer, the hydraulic regime and air regulation need to be calibrated so that the final meeting of the two plastic layers is equidistant from top and bottom. It is presumed that the designer has provided adequate passage for the partly combusted flue gas to pass from the oven chamber to the sole flue without significant pressure drop and that the passageways are clear. The design of the uptake passageway must be adequate to ensure insignificant pressure drop.

The temperature profile at the top and bottom of the oven is achieved by partial combustion of the coke oven gas emanating from the coal mass in the top part of the oven and burning rest of the combustibles below the sole. Target is also to burn all tarry material inside the oven chamber. Target is also to avoid majority of combustion below the sole as this may damage the oven sole and bottom arch side walls due to resultant high temperatures.

The differential combustion between the oven chamber and sole flue is achieved by three elements provided in the horizontal ovens – regulating primary air flow thru’ the primary air openings provided in the oven doors and/or oven roof, regulating secondary air thru the secondary opening provided in sole flue area and regulating outflow of flue gas flow from the sole flue area to the common duct by the uptake damper which therefore controls the draft inside the sole flue and the oven chamber.

The regulation of hydraulic regime is also important to minimize the back-travel of the oxidizing gases – CO2, H2O and O2 – from the free space above coal mass towards the coal mass. This phenomenon is known as convective mass transfer. When coke oven gas combusts, CO2 and H2O compounds are formed. O2 is available from the air that comes in thru the primary and secondary air windows. These oxidizing compounds burn the carbon in the coal mass causing yield loss.

Note: Convective mass transfer is the flow of material between a boundary surface and moving fluid

Gas evolution rate with coking time:

The suggested methodology works well in stamp charged Heat Recovery ovens taking a coal charge between 45t and 48t with bulk density in the range of 1.08 to 1.10 t per m3 and coal cake height of 1050 to 1100 mm. The primary air in one design could only be given through oven doors whereas in the other design it could be given through both door windows and through openings in oven roof. Secondary air ports were almost similar in both designs. One design had two waste gas ducts, one each running along pusher side and coke side whereas the other had only one duct running centrally over the top of the battery. The coking time in the two designs is 66 hrs and 70 hrs. The assessment of trend in gas evolution was done by measuring the suction in the uptakes every 2 hours during the first 30 hours, every 4 hours during the balance period of the coking time.

The methodology of hydraulic regulation is based on the principle that volume of coke oven gas and therefore flue gas varies as the coking time progresses. This is true irrespective of the design of oven.

The conclusion from the study was as follows:

- Phase A: gas evolution rate reaches its peak between 12 to 15 hours.

- Phase B: substantial gas evolution continues, although gradually declining, till 21 to 24 hours

- Phase C: gas evolution rate declines gradually till 32 to 34 hours after a which a plateau is formed till almost 63 to 65 hours (Phase D)

- Phase E: after this phase gas evolution subsides to very minimum and finally stops

All the phases are shown schematically in Figure below:

Phase E is the soaking period which is so essential for driving off the residual volatile matter entrapped within the plastic layer and stabilising the structure of coke near the plastic layer. The beginning of Phase E indicates the merging of the two plastic layers.

It must be mentioned here that the phenomenon occurs in all designs, but there may be slight differences in the onset period of various phases depending on the coking rate, bulk density of coal cake etc. It is for the operator to do meticulous and copious measurements to arrive at the times when one phase transitions to the next.

The trend of gas evolution rate emphasizes on the need to regulate the air window openings and the uptake dampers for reasons mentioned earlier.

 

A typical hydraulic regulation:

 

It is evident from the trend of gas evolution that after the peak is reached it is necessary to throttle both primary and secondary air and then uptake dampers for uniformity of carbonisation. The stepwise course of action required is outlined below:

Tight sealing of oven doors is very important in all phases specially in Phase E to minimize burn loss or carbon burn.

It is also seen that the sole flue temperature follows a trend similar to the trend of gas evolution rate. Ideally the sole flue temperature should be higher than the oven arch as the heat from the bottom must pass thru a 250 to 300 mm thick sole layer. However, this is seldom achieved. It is seen that after that starting somewhere in Phase C the oven arch temperature surpasses the sole temperature and continue that way. The better efficiency in heat transfer by conduction compensates for this phenomenon and still ensures that the plastic layer is located centrally.

Conclusion:

Compared to a recovery slot oven, the operation of a non-recovery oven is much simpler. The essentiality of a proper hydraulic regime is very important for getting the best coke from a given blend, optimum productivity and useful life of a coke oven battery.

 

The regulation steps discussed above are not universally applicable exactly as outlined. The regulation intervals may vary from plant to plant, but marginally. Every plant has its typicality from coal cake preparation to status of hardware provided by the designer to amenability of the technical staff in adapting to practices based on measurements. Irrespective of everything the inherent principle holds true. Every coke oven operator must understand that a well-balanced thermal profile brings the best out of a coal blend, optimizes power generation and productivity.