Coke Making 231 Process Description

Coke is a residue obtained after heating coal to very high temperatures (1650 to 2200°F) in the absence of oxygen, and removing all its volatile components.14 It is further used as a reductant for

FIGURE 2.1 Iron and steel manufacturing process overview. (From U.S. EPA, Development Document for the Proposed Effluent Limitations Guidelines and Standards for the Iron and Steel Manufacturing Point Source Category, EPA-821-B-00-011, U.S. EPA, Washington, DC, 2000.)

blast furnace iron making, having good permeability that allows the free flow of gases within the furnace shaft. Nearly 1.3 to 1.35 t of bituminous (coking) coal is required for the production of 1 t of coke.1 The U.S. integrated iron and steel industry uses the byproduct process for the manufacture of almost all coke. Byproduct coke ovens allow the collection of volatile material emitted during the coking process. Coking is carried out in brick ovens called batteries, which consist of coking chambers, heating flues, and regenerative chambers. The coking chambers are located alternatively with heating chambers, with the regenerative chambers located underneath. Pulverized coal is charged into the oven through the openings provided at the top. The necessary heat for distillation of the volatile components is supplied by the external combustion of recovered coke oven gas, blast furnace gas, and natural gas through flues located between the ovens.

Finally, the coke produced is removed through doors on either end of the oven and pushed out into a quenching car to be transported to a quenching tower, where it is sprayed onto the coke mass to cool it. The coke is sized and sent to the blast furnace or for storage. The "foul" gas obtained during the coking operation is processed to recover byproducts such as tar, light oils, ammonia, and naphthalene. Foul gas cleaning involves the spraying of weak ammonia, which condenses some tar and ammonia from the gas. The remaining gas is cooled by passing through a condenser and is then compressed by an exhauster. Any remaining coal tar is removed by a tar extractor, either by impingement against a metal surface or collection by an electrostatic precipitator.

Ammonia is removed by passing the gas through a saturator, where ammonia reacts with sulfuric acid to form ammonium sulfate, which is crystallized and removed. In the Phosam process ammonia is scrubbed directly from coke oven gas with phosphoric acid and then stripped.19 The gas is further cooled to condense naphthalene. The light oils are removed in an absorption tower and subsequently refined. The last cleaning step is the removal of hydrogen sulfide in a scrubbing tower. The purified gas may be used as fuel for the coke ovens or in other plant combustion processes. However, the nonrecovery process for byproduct gas may also be used, in which the unpurified gas is burned within the process rather than being recovered. The energy recovered in the form of heat from the waste gases is passed through a waste heat boiler to generate steam for electricity production or process use. Figure 2.2 gives a flow diagram of the coke-making process and Table 2.1 provides the inputs and outputs of the coke-making process. Table 2.2 gives an overview of the key environmental and energy facts of coke making.

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