Dear Bingley: I hadn't realized that coke was made so early. Searching for "Abraham Darby coke iron" brought up a site with a bit more information:scroll down 1/3
http://www.cottontimes.co.uk/darbyo.htm

I'm surprised to learn that they were able to make the equipment needed to make coke. I wonder if they were also able to use the gas produced in the process.