Non recovery Coke Oven Battery
Non recovery Coke Oven Battery
Metallurgical coke is a hard carbon material produced in the process of the ‘destructive distillation’ of various blends of bituminous coal. It is produced by carbonization of coal at high temperatures (around 1100 deg C) in an oxygen deficient atmosphere in a coke oven.
The manufacture of coke by heating coal in deficient of air has its origins at the start of industrial revolution when Abraham Darby used it in the smelting of iron ores in 1709 in England. The method of coke production was initially the same as for the production of charcoal, stockpiling coal in round heaps, igniting the piles, and then covering sides with clay. This laid the foundation for beehive coke making. Gradual advances led to the development of beehive, reverberatory and byproducts ovens, culminating into regenerative coke ovens with recovery of the byproducts around a century ago.
The technology of non recovery coke ovens has arisen from the classic beehive ovens which supplied since the eighteenth century the coke for the industrial revolution. The beehive ovens were manually operated, with small heat recovery, just for heating the oven. Now, non-recovery ovens are modern construction, with highly mechanized operation, and automated to a certain degree. Gases generated by the combustion of the volatile matter are sent through down comers and further burnt to heat the oven bottom and sides.
There are three proven processes for the manufacture of metallurgical coke. These are (i) beehive coke ovens, (ii) by-product coke ovens, and (iii) non recovery coke ovens. When the heat energy of flue gases is recovered in the form of steam then the non recovery coke ovens are known as heat recovery or energy recovery coke ovens. In all the three processes, ovens are usually built in rows, one oven beside another with common walls between neighboring ovens. Such a row of ovens is termed a battery. A battery usually consists of many ovens, sometimes hundreds, in a row.
A beehive oven is a simple firebrick chamber built with an arched roof so that the shape inside is that of an old-fashioned beehive. Beehive coking is now an obsolete process because of the small quantity it manufactured and the very large amount of pollution it produced. However it is still being used at certain places.
In the by-product coke oven battery, coal is carbonized in absence of air since these batteries are operated with positive pressure in the ovens. The coke making process in these coke oven batteries is called byproduct coke making since the off gas is collected and sent to the by-product plant where different by-products are recovered.
In the process of coke making in the non recovery ovens, volatiles evolved during coal carbonization are not recovered as by-products but are combusted in the oven itself in the presence of controlled quantity of air and the heat of the volatiles of evolving gases is utilized for coking of the coal mass into coke and thus no external heating is required. The heat is generated by the combustion of volatile matter which is then penetrated into the coal mass through radiation from the oven top and also by conduction. The higher level of heat importantly is used to break up the potentially polluting hydro-carbons into the constituent combustible compounds and to burn them thus avoiding the potentially hazardous pollution. The heat consequent to combustion is only partially utilized during the process and the balance heat in the waste flue gas is recovered for energy generation.
The flue gas coming out of the coke oven carries a significant quantity of sensible heat in addition to some combustibles. As nothing other than coke is recovered from the coke ovens incorporating this technology, the coke ovens are called non-recovery coke ovens. When the combustibles present in the waste gas are burned and the generated heat along with the sensible heat of the flue gases is used for the production of steam and generation of power, the coke ovens are called heat recovery coke ovens or energy recovery coke ovens.
The basis for the non recovery coke ovens with heat recovery is the so called ‘Jewell-Thompson oven’. These ovens were developed in 1960 when three test ovens were successfully built at Vansant, VA. In 1972, 16 large Jewell Thompson ovens built. The first non recovery coke plant with heat recovery was commissioned at Indiana USA in March 1998. The plant had 268 ovens with a capacity of 1.3 million tons per annum and a heat recovery power plant rated at 100 MW. Jewell-Thomson oven is shaped with a rectangular ground area. The oven brick lining is composed of silica refractory material.
The key elements of the non-recovery coke oven technology are (i) coke is produced by heating coal, in a controlled atmosphere, thus liberating volatile matter (gas and moisture), (ii) the gas is combusted in an environmentally ‘smart’ way so as to produce the heat to make the coke, (iii) excess heat which is produced in the process is used to generate electricity, (iv) the process does not rely on the combustion of coal, only the gas liberated from the coal, (v) the greenhouse gas emissions of the process are typical of a simple gas fired power generator, that is, one which raises steam that passes through a turbine.
Non recovery coke ovens produce a quality coke for blast furnaces, cupolas, and ferro-alloy furnaces etc. These ovens are useful to obtain high quality coke for blast furnace operation with high pulverized coal injection, where better properties of coke are needed, or to obtain standard quality based on blends with some proportion of non-coking coals.
Comparison with by-product ovens
The comparison of coke making in the by-product coke oven and that in non-recovery coke oven is shown in Fig 1.
Fig 1 Comparison of non recovery coke oven with by product coke oven
The comparison of various parameters of coke making by the by-product coke oven and that by the non-recovery coke oven is is given in Tab 1.
|Tab 1 Comparison of by-product ovens and non recovery ovens|
|Sl. No.||Parameter||By-product oven||Non recovery oven|
|1||Oven pressure||Slight positive||Slight negative|
|2||Oven height||Tall ovens||Horizontal wide ovens|
|3||Brick shapes||Large numbers||Few numbers|
|4||Heat supply||External through fuel gas||Internal by partial combustion of volatiles|
|5||Heat transfer||indirect, horizontal, conduction||Direct, vertical, radiation and convection from top and indirect conduction from bottom|
|6||Pollution||Environmental unfriendly||Practically no pollution|
|8||Energy||Higher energy requirement||Low energy requirement|
|12||Coking coal||Quality restrictions||Flexibility available|
|13||By-products||Large numbers||No by-product|
|15||Wall pressure||susceptible||Non susceptible|
|19||Manpower||skilled||Not so skilled|
|20||Maintenance||Hot maintenance||Ovens can be cooled down for maintenance|
The comparison of Sankey diagrams of the by-product coke oven plant and the heat recovery coke oven plant configurations is given in Fig 2.
Fig 2 Sankey diagrams
Design and construction features of non recovery coke oven
There are several designs available for the non recovery coke ovens. The main features of some of the designs are described below.
The Jewell-Thompson coke ovens –Typical dimensions of the chambers of the coke ovens are 14 m length, 3.5 m to 3.7 m width, and 2.4 m to 2.8 m height. 40 tons to 50 tons of coal is charged per oven. Typical charging height is 1000 mm. The ovens are built with 23 refractory brick shapes. Coal blend is charged through one side, by means of a so called -pusher charger machine’ (PCM) moving over rails close to the ovens. Immediately after charging, the coal blends absorbs the heat from the refractories and the combustion of volatile matter starts.
Below the oven roof, partial combustion of volatile matter takes place, on top of the coal mass. Soon afterwards, gases are suctioned to the oven hearth, where more air is introduced to complete the combustion. The coking front advances from the bottom and from the top, joining somewhere in the middle. There is no pressure buildup occurs as in by-product coke ovens, so low volatile coals can be blended.
Temperature, pressure and inner combustion are controlled in the ovens. The time of coking is of the order of 48 hours. Coke withdrawal is carried out with the same PCM used for charging. Coke is quenched with water. All water used for coke quenching is recovered, with the exception of evaporation loss. A cross-section of Jewell-Thompson coke oven is shown in Fig 3.
Fig 3 Typical cross section of a non recovery oven
The Chinese designed non recovery ovens – The Chinese have built also, besides standard horizontal heat-recovery coke ovens, a vertical-type non-recovery oven. As regards the horizontal ovens, the oven roof is a 120 deg arch structure. Adjustable primary air inlets are evenly installed in the arch, forming a waste-gas-protecting layer between the coal and burning zone in the oven roof. Four linked arches are used at the oven bottom. On the base of the arches, adjustable secondary air inlets are installed to distribute the air in the flue, for further combustion of the exit gas to heat the oven bottom. Flues inside wall and bottom can be coordinated. A ventilation layer between the foundation of the oven and the sole prevents the base plate from overheating. Main wall is equipped with suction-adjusting facilities.
Oven door is divided into two sections, the upper one is fixed and the lower movable, in order to prevent soot leakage. They are made in cast iron and lined with ceramic fiber.
The dimensions of carbonization chamber of the horizontal heat-recovery coke oven battery of one of the design consists of 13,340 mm length, 3,596 mm width, 2,758 mm height with 4,292 mm of centre to centre distance of the chambers. The effective dimension of the coal cake is 1,300 mm length, 3,400 mm width, and 1,100 mm height. The bulk density of the charge coal is 1.0 tons/cum to 1.5 tons/cum. The oven has a coal charge capacity of around 50 tons and has 72 hours coking time.
There are two more designs available. The dimensions of carbonization chamber in these two designs consist of 13,334 mm and 15,440 mm length, 3,598 mm and 3,700 mm width, 2,888 mm and 2,693 mm height with 4,530 mm and 4,300 mm of centre to centre distance of the chambers respectively. The effective dimension of the coal cake is 12,750 mm and 14,850 mm length, 3,500 mm and 3,600 mm width, and 1,050 mm and 1,000 mm height respectively. The ovens have a coal charge capacities (on dry basis) of 42.393 tons and 51.856 tons and has coking time of 72 hours and 70 hours respectively.
The first vertical coke oven heat recovery battery has been built in 2002 in China. In comparison with horizontal-type, these coke ovens require less space and 20 % to 30 % less construction cost. More important, it is said that the separation between coking chamber and combustion chamber avoids the burning of the coke which may occur in horizontal-type oven. Heat comes only through refractories, as in conventional batteries. There are two layers of air cooling channels at the bottom of the batteries. The temperature of the foundation is between 100 deg C to 150 deg C, preventing failure. Main dimensions and features of the vertical ovens are listed in Tab 2.
|Tab 2 Technical parameters of vertical heat recovery coke ovens|
|2||Charge weight||tons||Around 24|
|3||Cake bulk density||t/cum||1.0-1.1|
|4||Time of coking||hours||38|
|5||Number of ovens||numbers||4×35|
|6||Partition wall thickness||mm||100|
|7||Thickness of the oven sole||mm||1,182|
|9||Centre temperature||deg C||1,000 +/- 50|
|10||Pushing coke weight||tons||Around 18|
|11||Exit gas||N cum/h||350,000|
|12||Exit gas temperature||deg C||950 +/- 50|
The coke ovens design of Sesa Goa –The coke ovens are narrower than the Jewell-Thompson coke ovens. This makes possible to use Roman arch for the roof. The ovens are built with aluminous refractories, and this implies a smaller width. The oven is having 10,760 mm length and 2,745 mm width. 21 numbers of ovens are connected to a stack. The ovens are with top charging. The use of aluminous refractories instead of silica refractories is due to their better behaviour under oxidizing atmosphere, better resistance to thermal shock and less volume changes upon cooling, when there is some delay in recharging the oven. The ovens are being operated with compacted coal charge.
The Uhde design – In the present Uhde design, the coke ovens are stamped charged. But the charging machine does not enter the oven. The dimensions of the ovens are having 3.8 m width and 15 m length. The lining is of silica bricks. The tunnel for exit gas runs laterally below the oven floor level, instead of over the ovens.
SJ 96 coke ovens -This oven has been developed in-house by Shanxi Sanjia and is characterized by the exceptional weight of the coal charge which 120 tons. Hence, there is a need for long coking time of ten days in comparison with the 48 hours to 72 hours of typical coking time of the other processes. Coal mass height is 1.8 m and leveling is done at 90 cm and 180 cm levels. Both charge and discharge are manual and with the oven cooled. Temperatures are of the order of 1,200 deg C in the upper coal layer and 1,150 deg C in the lower coal layer. Gas is burnt completely in the under flues below the oven, and its temperature is used to produce steam for power generation.
Coke making process in non recovery ovens
In non recovery coke ovens, all of the volatiles in the coal are burned within the oven to provide the heat required for the coke making process. The oven operates under negative pressure. Primary combustion air introduced though ports in the oven doors, partially burns the volatile matter in the oven chamber. Secondary air is introduced into the sole flues, which run in a serpentine fashion under the coal mass. The design of the flues and the control of the air flow the coking rate to be equalized both at the top and bottom of the coal mass.
In contrast to by-product coking process, in which the coke is heated indirectly by combustion of gas within the heating flues outside the oven chamber, exclusively, during non-recovery coking the necessary heat is transferred both directly and indirectly into the oven chamber.
Coal is charged onto the oven floor at the beginning of the cycle. The carbonization process is started by the heat which exists from the previous carbonization cycle. Coal charging of the ovens is accomplished through the open pusher side door. Very often the coal is stamped before, and then the coal is charged into the hot oven chamber. Typical charging levels lie at 1,000 mm.
The released raw coke oven gas is partially combusted in the crown by the addition of ambient air through the oven doors and the gases pass through the down-comers into the heating flues situated in the oven sole. This flue system is beneath the oven floor and here by way of a further supply of the ambient air, the complete combustion of the raw gas takes place at temperature ranging from 1,200 deg C to 1,400 deg C. The gases then pass into an afterburner tunnel where any remaining not combusted gases are oxidized. The afterburner tunnel system routes the hot gases to the heat recovery steam generators.
In all the recent installation, the hot waste gas is utilized to generate energy, and subsequently is subjected to desulphurization before being exhausted into the atmosphere. The time of coking varies with the types of ovens and ranges from around 48 hours to 72 hours depending upon the design of the coke ovens. After the completion of the coking time, the coke is pushed out and normally wet quenched.
Due to the negative pressure, under which the process of coking is running, emissions from leaks at the doors are avoided in principle. Dust emissions occurring during coke pushing are exhausted via a coke side shed. Very often suction devices are installed at the pusher side, too, in order to capture emissions caused during charging.
The techniques for emission control during charging, pushing and quenching are similar to those applied to the by-product ovens.
Due to the temperatures generated, all of the toxic hydro-carbons and by-products of the volatile matter are incinerated within the oven. Hot gases pass in a waste gas tunnel to heat-recovery steam generators, where high pressure steam is produced for either heating purposes or power generation. The cool waste gas is cleaned in a flue gas desulphurization plant prior to being discharged to the atmosphere. The process flow diagram is shown in Fig 4.
Fig 4 Process flow diagram for non recovery coke oven
Benefits of non recovery coke ovens
The various benefits of the non recovery ovens include (i) no waste water treatment plant is needed, (ii) no net waste water discharge since all the waste water is used in coke quenching, (iii) higher flexibility is available in the coal blend selection because of the elimination of wall pressure constraints, (iv) improved coke strength is attributed to slow heating at higher temperatures and longer soak time causing consistent crystal growth, (v) need lesser space when compared with coke oven and by-product plant, (vi) plant can be constructed with modular construction, (vii) since non recovery ovens work under negative pressure, it results into ambient air being pulled into coke oven at any available intake point and thus eliminates any fugitive emissions, (viii) hazardous air pollutants are destroyed in the oven by thermal oxidation, and (ix) there are lesser numbers of process control points.