Pollution Control System
Non-recovery coke oven is a way of producing metallurgical coke without chemically purifying the distillation gases evolved during the carbonization process. The ovens are unique in nature such that no external heat source is required. All the energies for carbonization are supplied by burning the gaseous volatile products, released from the coal charge during carbonization. The combustion air (oxygen) is drawn into each chamber through primary air flue and secondary air flue. The heat generated by the combustion of the volatile product is in high temperature zone in the free space above the coal charges. This heat is radiated back into the coal charges so inducing downward carbonization. The product of combustion is drawn through gas exit flues down into a series of sole flues located below the oven floor. As the hot combustion products pass through sole flue, they give up some of their heat, which results in carbonization occurring in an upward direction. The gases leave the sole flues through connecting flues and pass along common gas flue to a discharge stack. Hence, carbonization results from application of heat both above and below the charge. This results in reducing the coking cycle and the better coke is produced from the optimum portion of coal charge.
POLLUTION CONTROL :
As the heat is generated due to the combustion of hydrocarbon evolved during carbonization and no coal particle is burnt directly, the pollution created by the system is different from the conventional coal burning where coal is used as source of fuel. The system operates at high temperature (1300°C). Hence, any external apparatus cannot be used for pollution control. The system for pollution control is integrated with the construction of the ovens. Moreover, the complete coke oven works at negative pressure. Hence there is no leakage of hot gases from the oven. To minimize the pollution, following measures have been proposed in the design:
PRINCIPLE :
There are two types of pollutants present in the combustion system:
- 1. Hot Gases like CO2, CO, NOx, SOx etc.
- 2. Suspended particulate Matter (SPM)
HOT GASES :
These are generated due to combustion of hydrocarbons released during carbonization. When hydrocarbons come in contact with oxygen, it breaks into carbon dioxide, carbon mono oxide, nitrous oxide and other variants. A system of recuperator has been incorporated with the design to generate hot air from the waste heat. This hot air is injected as primary air inside the oven, as secondary air inside various points of sole flue, waste heat tunnel and main tunnel for almost complete combustion of gases generated. Throughout the system, fuel : air ratio is maintained at 1 : 5 as per the norms for complete combustion. As a result, only traces of NOx, SOx are recorded from the chimney.
SPM :
These are generated in very small quantity during the process. It should be well understood that coal as such is not being burnt as fuel and no ash is being generated. However, method of expansion and contraction is adopted to separate SPM from the waste gases at various stages. The hot gas generated in the oven is brought down to sole flue through a relative small duct under high pressure. As soon as it enters into sole flue, the cross section of the flue is increased by at least three times to induce capillary action as in refrigeration. Due to sudden increase in volume, the pressure of the gas drops and it releases temperature and SPM. This SPM get settled in the bottom of the flue, which is cleaned periodically. Same principle is applied in the waste heat tunnel and main tunnel. In addition to this, baffle wall is constructed inside the flue channels to trap any SMP present in the waste gas. All these measures control the pollution as required. Hence the chimney emission is restricted around 150 mg/NM3.
CHIMNEY :
It is a very important and complex part of the coke oven system. The chimney is responsible to produce sufficient draught in the oven system to operate the recuperator and other pollution control equipment as these are integral part of the system. To ensure the efficiency, the height of the chimney is increased to 145 feet. Provision for hot air injection through tertiary air ducts around the chimney has also been provided at various heights so that combustion of residual unburnt hydrocarbon takes place and harmful carbon monoxide is converted into carbon dioxide. This also help for sedimentation of the SPM to settle down in the bottom of the chimney. As the chimney creates natural draught due to high temperature at the bottom of the chimney, Electrostatic precipitator or wet scrubber can not be used. As such installation of ESP is not required as there is no generation of ash in the system.
ENVIRONMENTAL PLAN :
An elaborate action plan has been incorporated to reduce the CO2 content and suspended particle content due to coal storage area around the plant. It is proposed to develop thick green belt around the boundary wall which is capable of fixing 120 gm Mol of atmospheric CO2 per min. calculated on the basis of annual dry matter accumulation.
All cares and precautions have been taken to create pollution free environment and to obtain the optimum efficiency in this technology.
It must be kept in mind that pollution in terms of suspended particles are due to COAL STORAGE AREA and not due to ovens itself. Periodic water sprinkling on coal is only solution.
The non-recovery coke ovens as per above modifications are capable of producing a coke, which is large, strong and abrasion resistant. The coarse mosaic microstructure, low porosity, large mean cell wall thickness, minimal temperature strength and stability and superior post reaction strength.
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PLANT AND EQUIPMENT FOR POLLUTION CONTROL:
- 1. Recuperator at the bottom of ovens
- 2. Recuperator in the waste heat tunnel
- 3. Recuperator in the main tunnel
- 4. Tertiary Air ducts around the chimney
WATER POLLUTION :
Water is used to quench the hot coke in a quenching tower. Water is circulated in closed circuit and hence not discharged out. At the time of quenching only steam comes out. There is no water pollution in this project.
