Dry Quenching of Hot Coke
Dry Quenching of Hot Coke
Coke dry quenching (CDQ) is an energy saving process used during the production of coke in the coke oven battery. A CDQ plant is also called coke dry cooling plant (CDCP). In the traditional CWQ (coke wet quenching) process, the red-hot coke which is pushed from the coke oven is cooled by spraying water on the hot coke. The water used for cooling is vapourized and released into the atmosphere. An issue with this conventional system is the energy loss when the thermal energy of the red-hot coke is converted into the steam which is vapourized and released unused. Another drawback is that the CWQ process also produces airborne coke dust, and hence, the CWQ process is associated with high CO2 emissions and thermal energy loss.
During the CWQ process for cooling of the run of oven coke, sensible heat of the hot coke is dissipated into the atmosphere and is lost. In addition, there are air borne emissions (0.5 ton of steam per ton of coke, which is laden with phenol, cyanide, sulphide and dust) and a large quantity of water (around 0.6 cum per ton of coke) is needed for wet quenching. The contaminants in water are also discharged in the environment. Tab 1 shows the comparison of typical properties of coke produced by the CWQ process and the CDQ process.
|Tab 1 Comparison of typical properties of coke produced by CWQ and CDQ processes|
|Sl. No.||Parameters||Units||CWQ process||CDQ process|
|4||Average particle size||mm||65||55|
|5||Coke breeze rate (after cut at -15 mm)||%||10||13|
|8||Coke strength after reaction (CSR)||%||58||60|
In the CDQ process, the red-hot coke is cooled by gas circulating in an enclosed system, thereby preventing the release of airborne coke dust. The thermal energy of the red-hot coke, which is lost in the conventional CWQ process, is collected and reused as steam in the CDQ system. This technology uses less fossil fuel and results in lower CO2 emissions, thereby contributing to the prevention of global warming. Nowadays, CDQ plants have gathered a lot of attention from the world due to its efficient energy recovery and the characteristic of reducing the environment pollution. They are being regarded as an essential facility for the counter-measure against environmental problems like global warming by CO2 and air pollution. In a study, in which the energy saving calculation has been carried out based on the operation of an existing CDQ process, it has been shown that 85 % of the waste heat generated during coal carbonization is being recovered by the CDQ process.
The CDQ technique was introduced in Switzerland by the Sulzer brothers in the 1920s. A few decades later, an improved conception of the CDQ process for continuous operation was produced by the Giprokoks Institute in Russia. After pilot and pilot/commercial trials, the first full scale CDQ plant installation was commissioned in 1965 at the Cherepovets Iron and Steel Works in the then USSR. By 1978 around 50 CDCP modules of 56 tons per hour were in operation in the then USSR. Over the last two decades, the CDQ process has been gradually accepted, although the CWQ process is still popular. However, a large numbers of CDQ plants are operating in many countries. Fig 1 shows a CDQ plant module of Giprokoks design is having a capacity of 56 tons per hour.
Fig 1 CDQ plant module of Giprokoks design is having a capacity of 56 tons per hour
Japan purchased license from USSR in 1975 and three Japanese installations were commissioned in 1976 – 77. Since production capacity of coke ovens is increasing in recent years, CDQ capacity also have been demanded to adapt to this changing trend. In order to follow this tendency and satisfy various customer requirements, the development of large-scale CDQ plant comprising single chamber (single-chamber CDQ) for quenching a large quantity of coke has taken place in Japan. For single-chamber CDQ, availability is very important factor because there are no spare facilities. In order to achieve high availability, stable operation is essential. For inexperienced operator, however, it is difficult to achieve the stable operation by adjusting various parameters. In recent years, a new automation technology for CDQ plants has also been developed. Progressive increase in the capacity of CDQ chambers since 1975 is shown in Fig 2.
Fig 2 Progressive increase in the capacity of CDCP chamber since 1975
Hot coke after its pushing is brought from the coke oven battery to the CDQ plant in bottom opening bucket kept on the quenching car. This bucket is lifted at the CDQ plant by a hoisting/charging device to the top of the CDQ chamber and red hot coke is discharged into the pre-chamber by the charging device. Hot coke (temperature around 1000 deg C to 1100 deg C) is cooled in the chamber by the circulating gas. In the chamber the circulating gas moves counter-current to the coke movement, i.e. the circulating gas moves upwards while the coke moves downward by the gravity.
The circulating gas in a continuous running CDCP plant is a mixed gas which consists of mainly nitrogen (N2) along with small amounts of carbon di-oxide (CO2), carbon mono-oxide (CO), and hydrogen (H2). The typical composition of the circulating gas is N2 – 70 % to 75 %, CO2 – 10 % to 15 %, CO – 8 % to 10 %, and H2 – 2 % to 3 %. The hot coke, while descending in the chamber, is cooled to a temperature which is less than 200 deg C by the circulating gas blown from the lower zone of the cooling chamber and is discharged from the discharging facility at the bottom of the chamber. The passage time of the coke through the chamber is around 5 hours to 6 hours. The typical operating parameters for a CDQ plant of 56 tons/hour capacity are given in Tab 2.
|Tab 2 Typical parameters for CDQ plant of 56 tons/hour capacity|
|2||Coke charge temperature||deg C||1,000 – 1,050|
|3||Coke output temperature||deg C||200|
|4||Gas inlet temperature||deg C||170|
|5||Gas outlet temperature||deg C||800 -850|
|7||Steam pressure||kg/sq cm||40|
|8||Steam temperature||deg C||440|
|9||Total gas volume||N cm/hour||84,000|
The high-temperature circulation gas (at around 800 deg C to 850 deg C) after a heat exchange process in the cooling chamber passes through the primary dust catcher and is supplied to the boiler. The circulation gas after a heat exchange process in the boiler is cooled down to around 180 deg C. The steam generated in the boiler is used either as a general-purpose process steam, or converted into super heated high pressure steam for the generation of electric power through a turbine generator.
The circulating gas passes through the secondary dust catcher into the gas circulation blower, by which its pressure is boosted, and its composition is corrected by addition of N2 gas and then the circulating gas is injected at the bottom of the CDQ chamber. If necessary, a sub-economizer is installed to decrease the temperature of circulation gas to around 130 deg C, improving the cooling efficiency of the cooling chamber.
There are some auxiliary facilities which include cut off device for cutting out the coke from the chamber, dust removing system for removing the dust in the circulating inert gas, and the secondary dust catcher installed before the circulation blower. The process flow of CDQ process is shown in Fig 3.
Fig 3 Process flow of the CDQ process
Benefits of CDQ process
CDQ process has many advantages in comparison with conventional CWQ process. Major advantages are described below.
Utilization of the sensible heat of hot coke – The electric power can be generated by dry quenching of coke without the consumption of the fossil fuels. This leads to the reduction of the CO2 emissions. As an example (Fig 4), CDQ with 200 tons per hour (t/h) capacity can generate around 36 MW of electric power. For the generation of the same electric power with a heavy oil fired boiler, 12 t/h of heavy oil is needed and this results into 36 t/h of CO2 emission to the atmosphere. Hence, a CDQ plant provides both the economical as well as the environmental advantages. Since it recovers sensible heat of the coke, there is net energy saving. Energy saving is around 0.25 Gcal/ton of coke. CDQ process thus enables effective utilization of energy which is dispersed into the atmosphere in the CWQ process.
Protection of environment – In CDQ, there is no white smoke which is normally seen coming out of the quenching tower in case of CWQ process. This white smoke has a high content of dust. The absence of white smoke in case of CDQ process is because all the processes are totally enclosed. As a result, the working environment around CDQ equipments is improved.
In general, with the CWQ process, large quantity of coke dust (around 300 g/t of coke to 400 g/t of coke) is emitted which is contained in the steam emitted to the environment. The recently developed process of ‘coke stabilizing quenching (CSQ) reduces the dust emission volume. But the CDQ process is still more efficient in prevention of dust emission. The dust emission volume from CDQ is less than 3 g/t of coke (Fig 4). This value has improved further with the continuous improvements of the CDQ process which is taking place.
The CDQ process is also environment friendly with respect to water pollution and water conservation. This is since no water is used in the CDQ process as against CWQ process where the quenching medium is water.
Fig 4 Benefits of the CDQ process
Contribution of CDQ process in productivity improvement at BF – The improved quality of coke produced by the CDQ process leads to the productivity improvement at the blast furnace (BF). As regards the coke produced by the CDQ process, it has got two excellent features in comparison to the quality of coke produced by the CWQ process. These features are (i) higher mechanical strength of the coke, and (ii) very low moisture content in the coke (almost zero). These features provide certain benefits at the BF which are described below.
The fuel consumption in BF is reduced by a few percent since the extra heat energy is not needed for the evaporation of moisture contained in the coke. It also contributes to the CO2 reduction at BF, and an improvement of power generation at TRT (top pressure recovery turbine) due to the increasing of the temperature at the top of BF.
By using the coke quenched with CDQ process, the permeability in BF is improved because of the high coke strength. Higher permeability in the BF improves the efficiency of reduction reaction in the BF, and this improves the hot metal productivity of the BF. BFs using coke produced by the CDQ process has comparatively more stable operation. Further, as the coke strength is increased, interior of BF can maintain the sufficient permeability and hence higher rates of pulverized coal injection can be achieved at the BF.
Improvement of coke quality – The quality of coke is improved by the CDQ process compared to the coke produced by the CWQ process. The quality of coke is more stable because of the low standard deviation (usually around 1.25 %). A uniform quality of coke helps in the BF operation. This is because the coke is cooled gradually by the circulating gas in the CDQ chamber instead of fast quenching by the spaying of water with water jets in case of the CWQ process. Also, the water-gas reaction is avoided. The improvement expected in the mechanical strength (drum index) is around 1.5 % and improvement in the coke strength after the reaction (CSR) is around 2.5 %. This improvement is since the hot coke which is cooled gradually by the circulating gas, is free from surface pores and internal cracks which are generally present in the wet quenched coke.
It is generally observed that the increase rate of coke strength is relatively less, in case there is high percentage of high-grade coking coal in the coal blend used for coal carbonization. The higher use of low coking coal in the coal blend provides additional cost advantage by the CDQ process.