Comparison of By-product Coke Ovens and Heat Recovery Coke Ovens
Comparison of By-product Coke Ovens and Heat Recovery Coke Ovens
There are three proven processes for the carbonization of coal for the production of metallurgical coke. These are (i) beehive oven process, (ii) the by-product coke oven process, and (ii) the heat recovery coke oven process. The heat recovery coke oven process is also known as non-recovery or energy recovery coke oven process. It is a modification of the beehive oven process and, and hence, it has largely phased out the beehive oven process. Thus, for the carbonization of the metallurgical coal the only two cokemaking technologies which are being used are (i) the by-product coke oven technology, and (ii) heat recovery (HR) coke oven technology. Both of these cokemaking technologies offer opportunities to produce high quality coke and to develop the energy balance while achieving the lowest possible operating cost.
The by-product coke ovens are prone to high level of pollution because of the positive pressure maintained in them. The stringent pollution laws and the high cost involved in the installation of pollution control equipment led to the revival of interest in the non-recovery coke oven technology during 1980s and 1990s. These non-recovery coke ovens complied with the stringent pollution control regulations. These non-recovery coke ovens are the heat recovery ovens since they are used not only for the production of coke, but also for the generation of power by means of waste gas heat recovery. Hence, the heat recovery type of coke ovens is energy efficient and environment friendly.
The selection of the appropriate technology for a particular situation needs a careful study since many different factors can affect the decision, including, for example, availability of land, plant energy balance and available energy sources and their costs, steel plant configuration and energy consumers, environmental considerations and economics consisting of the capital and operational cost. There is the distinct difference in the overall plant energy balance for each technology. The HR coke oven technology generates a large amount of electric power but does not produce any fuel gas, while the by-product coke oven technology produces large quantity of a rich fuel gas besides saleable by-products. Hence, this factor has a pronounced effect on the gas balance of the steel plant.
In both the processes, the coal preparation method is almost similar. In both the processes, selected coals are screened, crushed to less than 3 mm and blended based on their petrography for the production of a high-quality coke while using the most cost effective input coals. The coal blend is charged into the coke oven, and coke is formed by the destructive distillation of coal at temperatures of around 1,100 deg C and higher. At the end of the coking cycle, the hot coke is pushed from the oven into a quench car, which transports the hot coke for its quenching and stabilizing. Quenching is performed with either water (wet quenching) or nitrogen (dry quenching), after which the product coke is transported to the blast furnace (BF) or stockpile. Fig 1 shows a simplified cokemaking flow sheet.
Fig 1 Simplified cokemaking flow sheet
In the by-product coke technology, coal is carbonized in by-product ovens involving indirect heating of the coal mass. In this method coal is carbonized in the absence of air and heat is supplied to the coal mass by burning fuel gas in the combustion chamber adjacent to the coking chamber. The volatile matter comes out of the coal mass after attaining the required temperature which subsequently cooled at different stages for the recovery of the by-products in the by-product plant. The coke ovens which are based on this technology are known as by-product coke ovens.
In the heat recovery coke technology, 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 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 first type of the non-recovery coke ovens were the beehive ovens. Later the beehive ovens technology was upgraded to present non-recovery type of coke ovens.
In coke production from the heat recovery coke oven as well as from the byproduct coke oven ovens, the volatile matter of the coal is driven off in a reducing atmosphere. Coke is essentially the remaining carbon and ash. With byproduct ovens, the volatiles and combustion products are collected downstream of the oven chamber and refined in a chemical plant to produce coke oven gas and other products such as tar, ammonia, and benzol. In heat recovery ovens, all the coal volatiles are oxidized within the ovens. Main differences between by-product process and heat recovery process are shown in Fig 2.
Fig 2 Main differences between by-product process and heat recovery process
The two processes are described below.
By-product cokemaking — By-product cokemaking is so called since the volatile matter evolved during the process of carbonization is collected and treated to recover the by-product chemicals. The carbonization process is performed in narrow, tall ovens often called as slot ovens. These ovens operate under a non-oxidizing atmosphere. A positive pressure within the oven prevents air ingress and subsequent combustion of the volatile matter.
By-product coking plants are comprised of single oven chambers, being 12 m to 20 m long, 4 m to
8 m high, and 0.4 m to 0.6 m wide, in which the input coal is heated up indirectly. Several ovens are grouped to form one battery. A single battery may consist of upto 85 ovens. The front-end sides of the individual ovens are sealed with doors. The ovens are charged through charging holes in the oven top. As an alternative, the oven can also be charged from the side via one opened door after the input coal was stamped before in order to build a formed cake (stamp charging). Subsequently to 15 hours to 25 hours coking time the doors are opened and the produced hot coke is pushed by the coke pusher machine out of the oven into a coke quench car. Then the coke is quenched in a dry or wet quenching facility. The oven chamber is sealed again, initiating a new carbonization cycle.
The coke ovens normally have the complex twin-flue construction which is essential for maintaining high and constant temperature profiles throughout the battery. The main emission sources from the ovens and the coke is exposed to the atmosphere. Taller ovens allow greater amounts of coke to be produced per oven, thus minimizing the number of charges and pushes and related emissions to make the needed tonnage.
The gas evolving on coal carbonization leaves the oven chamber through a standpipe (offtake) and is passed on via a common gas collecting main to the gas treatment facilities and to the by-product recovery plant. The ovens are run at a slightly positive pressure of 10 to 15 mm water column.
The oven chambers are heated through heating flues, located between the chambers, in which cleaned coke oven gas or blast furnace gas is burnt. The temperature in the heating flues lies normally between 1150 deg C to 1350 deg C. Battery operation, i.e. charging and pushing is carried out by large machines which very often are running with high level of automation.
Coke oven gas (COG) as generated during the process of coking is not suited for use as under firing gas for the coke oven batteries and for other applications, because of technical, and of environmental related reasons in particular. The necessary cleaning is carried out in the by-product plant which comprises a complex chemical plant.
Volatile matter which is driven off during the carbonizing process passes through a collector main to the byproduct chemical plant. Tars are condensed by cooling the crude gas with flushing liquor and then in a primary cooler. An electrostatic precipitator removes the remaining tars. The gas is further treated, producing additional byproducts, including light oil, naphthalene, and ammonium sulphate etc. The cleaned gas, known as coke oven gas (COG), is normally stored in a gas holder and boosted in pressure for use around the steel plant as a heating fuel or as reducing gas for blast furnace injection.
Heat recovery cokemaking – In heat-recovery coke ovens, all of the volatiles in the coal are burned within the oven to provide the heat required for the cokemaking process. The oven has normally a horizontal design and 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 conventional coking by 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.
The basis for modern non-recovery plants is the so-called Jewell-Thomson oven, several ovens of which are grouped together to form one battery. The ovens are characterized by a tunnel-like shape with a rectangular ground area and an arched top. The dimensions of the chambers of modern plants run up to 14 m x 3.5 m to 3.7 m x 2.4 m to 2.8 m (length x width x height). Coal charging (upto 50 tons) 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 1000 mm.
The carbonization process is started by the heat still existing from the preceding carbonization cycle. The released coke oven gas is partly burnt by addition of ambient air through the doors and passed through so-called down comers into the heating flues situated in the oven sole. By way of a further supply of air, the complete combustion of raw gas is affected here at temperatures between 1200 deg C and 1400 deg C.
With plants according the state of the art, the hot waste gas is utilized to generate energy, and subsequently is subjected to desulphurization before exited into the atmosphere. The time of coking in Jewell-Thomson ovens amounts to around 48 hours. After that time, the coke is pushed out and quenched in wet mode, normally.
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 hydrocarbons and byproducts 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.
Figure 3 shows a cross-section of the by- product oven and heat-recovery oven.
Fig 3 Cross-section of the by- product oven and heat-recovery oven
Comparison regarding coke quality
Although exhibiting lower overall CSR compared to the heat-recovery oven coke, the by-product oven produces a more uniform coke quality along the coke cake width, because of the narrow width and bilateral heating of the oven. The top-section coke of the heat-recovery oven displayed unique structural characteristics ascribed to the availability of free space atop this coke and the use of radiant heat to drive its coking process.
The heat-recovery oven bottom centre as well as the by-product oven coke display low surface area and total porosity, indicating restrictive coke cake expansion. Limited swelling is attributed to the coal charge weight overlying the heat-recovery oven bottom coke and the constrained nature of the by-product oven chamber. However, the shorter coking time used in the by-product oven disadvantageously deprives the by-product oven coke of sufficient time to fully develop its carbon structure. Hence, the heat-recovery oven coke demonstrates better carbon structural development. For the by-product oven coke, carbon structural development seems to have a stronger impact on CSR than surface area does, while for the heat-recovery oven top centre coke, surface area appears to be the parameter that most affects CSR.
Tab 1 provides the comparison between the two types of cokemaking technology.
Tab 1 Comparison of byproduct ovens and heat recovery ovens
|Parameter||By-product oven||Heat recovery oven|
|Oven pressure||Slight positive||Slight negative|
|Oven height||Tall ovens||Horizontal wide ovens|
|Brick shapes||Large numbers||Few numbers|
|Heat supply||External through fuel gas||Internal by partial combustion of volatiles|
|Heat transfer||Indirect, horizontal, conduction||Direct, vertical, radiation and convection from top and indirect conduction from bottom|
|Pollution||Environmental unfriendly||Practically no pollution|
|Energy||Higher energy requirement||Low energy requirement|
|Coking coal||Quality restrictions||Flexibility available|
|Byproducts||Large numbers||No byproduct|
|Co product||CO gas||Steam/Power|
|Wall pressure||susceptible||Non susceptible|
|Manpower||Skilled||Not so skilled|
|Maintenance||Hot maintenance||Ovens can be cooled down for maintenance|