Coke is burned under specially designed boilers in carborundum lined furnace. Combustion is controlled by drafts, so that flue gases contain 17-18% carbon dioxide. At the same time steam is generated in the boilers to furnish power for the pumps and compressors and heat for the lye boiler. The hot flue gases, containing oxygen, carbon monoxide, nitrogen, dust and some sulphur and organic compounds, in addition to 18% carbon dioxide are passed through a heat exchanger and economizer. Here the temperature of the gases is reduced and the excess heat is taken up by the counter currently flowing strong lye. The gases are scrubbed in limestone packed towers with water from the coolers to remove sulphur dioxide and dust and to reduce the temperature to about 38oC. The dilute carbon dioxide is then fed into the bottom of the coke filled absorption towers where it passes counter current to an aqueous solution of sodium carbonate called weak lye. After absorbing the carbon dioxide, the solution of bicarbonate, known as strong lye, is pumped through heat exchangers, where the solution is heated before entry into the lye boiler. The absorbers remove all but about 9% of the carbon dioxide in flue gas, which is then released to the atmosphere.
The preheated solution of sodium bicarbonate is heated with exhaust steam (from the compressors) in the lye boilers. Here, at a temperature of about 118oC, the bicarbonate is decomposed into sodium carbonate (weak lye), which is returned through heat exchanger to the absorbers for reuse. The liberated gas, consisting of 99.8% carbon dioxide, escapes through the rectifying section of the lye boiler, where it warms the entering strong lye solution. The gas pass through a water cooler, where the temperature is further lowered and the moisture in the saturated gas is condensed and returned to the weak lye. The cooled gas is collected in a gas holder. In place of sodium carbonate, other absorbents, such as aqueous potassium carbonate, mono ethanolamine (10 to 20%) and triethanolamine may be used.
Before or during liquefaction, the concentrated gas is purified by removal of organic impurities that would affect its taste and odour, and the gas is dried. Various methods or so-called cycles of liquefaction are employed, such as pre-cooling, binary, ternary, bleeder, or flash cooling cycles. These operate on the raw gas or on a combination of raw and revert gases using two or three stage compression and ammonia refrigeration, carbon dioxide flash cooling, or a combination of both.
In the ternary cycle, the carbon dioxide from the gas holder is cooled to 4oC and raised to a pressure of 75 psi absolute in the first stage of the primary compressor. The compressors, usually two sets, are driven by the live steam from the boiler and the exhaust steam from them is used in the lye boiler.
From the first stage, the gas is passed through a purification system consisting of an oil separator and a scrubber, containing potassium permanganate solution or potassium dichromate solution, which oxidizes organic impurities. The gas from the scrubbers, raised in temperature to about 10oC, enters the second compression stage and is discharged at a pressure of about 350 psi absolute. The temperature is lowered by water coolers to 4oC, at which point some of the water and vaporized lubricating oils (such as glycerin) are condensed. The gas passes through a desiccant drying tower (calcium chloride), where sufficient water is removed to prevent freezing of the valves. The gas is compressed to 970 psi absolute in the third stage, passed through a cooler, and liquefied in a condenser at 26.7oC (critical temperature is 31.35oC). Non-condensable gas is purged, and the 99.9% liquid carbon dioxide, containing less than 0.1% moisture and free from organic impurities, is either fed to flash coolers or charged into cylinders.
All the materials to be used in the production of CO2 are environmentally friendly and the combustion products are parts of the atmosphere and do not need any treatment before releasing.
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