I think that the coke retort is what's otherwise known as a by-products plant. The gases driven off during the coking process are collected, then refined in various ways to separate the many components, which include benzene, ammonia, and light oil. In many plants, the coke gas is also used as a fuel in other mills, in re-heating furnaces.
Here's a
LINK to a thread on this topic. The area in question starts on page 5, although there's a lot of useful information throughout the thread.
You can model the coke ovens and/or the retort (I think that there are additional ovens included in the retort kit), although you'll need the coke oven kit in order to get the charging larry, quench car, and other ancillary equipment.
Wayne