What is safe limit of FFT(free flow temp) of coal ash in top charged recovery type coke oven battery?
Dear Nilu! In practice, the coking process in coke oven batteries , this problem does not occur. End of coking determines the temperature of 1050 oC and the highest temperature in the heating chanels of the order of 1350 oC . Under these conditions oxides are formed as result of the thermal dissociation of native mineral substance .
best regards, Andrzej
Dear Nilu Kumar,
Ash fusion temperatures give an indication of the softening and melting behavior of fuel ash. Ash fusion temperatures give an indication of the softening and melting behavior of fuel ash. These temperatures are widely cited in fuel specifications for boilers despite a relatively poor record of correlating with slagging or fouling behavior. Reasons for the poor predictive behavior include:
1. Fusion temperatures are based on fuel ash whereas deposits commonly are enriched and depleted in several elements relative to the fuel.
2. Fusion temperatures are measured over short time periods while heating ash at a rate of 8 +/- 3 ºC (15 +/- 5 ºF) per minute whereas ash deposits typically accumulate for hours and are formed during cooling relative to the bulk gas temperature.
3. Fusion temperatures do not account for either boiler design or boiler operation, both of which strongly influence slagging and fouling behavior. Fusion behavior changes when samples are allowed to stand at a given temperature.
4. Fusion temperatures generally significantly decrease if the samples equilibrate at a given temperature for an hour or so.
Fusion temperatures at one time were also quite subjectively measured, but this criticism has been addressed by the development of automated techniques for performing the measurements that require no intervention by the operator. Despite the shortcomings, fusion temperatures are valuable guides to the high-temperature behavior of the fuel inorganic material.
Generally, a temperature under reducing conditions should be equal to or lower than the corresponding temperature under oxidizing conditions. The difference in these temperatures generally increases with increasing iron content in the ash.
The coke making process involves carbonization of coal to high temperatures (1100°C) in an oxygen deficient atmosphere in order to concentrate the carbon. The commercial coke making process can be broken down into two categories: a) By-product Coke making and b) Non-Recovery/Heat Recovery Coke making. A brief description of each coking process is presented here. The majority of coke produced in the United States comes from wet-charge, by-product coke oven batteries attached Figure. The entire coke making operation is comprised of the following steps: Before carbonization, the selected coals from specific mines are blended, pulverized, and oiled for proper bulk density control. The blended coal is charged into a number of slot type ovens wherein each oven shares a common heating flue with the adjacent oven. Coal is carbonized in a reducing atmosphere and the off-gas is collected and sent to the by-product plant where various by-products are recovered. Hence, this process is called by-product coke making.
A good quality coke is generally made from carbonization of good quality coking coals. Coking coals are defined as those coals that on carbonization pass through softening, swelling, and re solidification to coke.
Regards,
Prem Baboo
Dear Andrzej Sir,
Thanks for your response.As you rightly said in coke oven operation this type of problem generally do not occur.But if a coal(coking) having FFT close to operating temperature of heating channels/wall made of refractory in the order of 1350-1370 deg.centigrade will not affect battery heating wall and operation due to fusion of coal ash?
with regards,
Nilu Kumar
Dear Nilu and Prem, obviously, the impact of mineral matter in coke (silicon and aluminium oxides, and other thermally stable compounds) and the active alkali compounds cause destruction of the ceramic brickwork. Mainly in reaction SiO2 + C (or H2) = SiO (gaseous) + CO2, and the silica may occur from feedstock and brickwork. The resulting gaseous SiO is very reactive and leads to destruction of brickwork. I would point out that the figure shown by Prem is association coking and blast furnace process, where the temperature at the bottom reaches a temperature of liquid pig-iron and slag, approx. 2000°C.
If the ash fusion temperature is low say 1000 degree celsius it may be helpful for binding of coal and coke. Higher ash fusion temperature also should help in binding whish is arequirement for coke making.
Dear Ramesh Sir,
As you rightly said lower fusion temperature would be helpful for agglomeration during carbonization but may be detrimental for heating walls made of refractory bricks in coke oven battery.This may lead to operational problem like hard pushing of coke ,sticker oven etc.
It has been reported that due to more severe reducing conditions inside coking chamber coal ash fusion temperature of high volatile coal is lower than standard ash fusion conditions .This may lead to reaction of coal ash with coke oven brick and may damage the refractory wall
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