You might be thinking of the 1947 Texas City disaster, which killed nearly 600 people. The fire, and the subsequent explosion after failed fire-fighting efforts, were in a cargo hold containing ammonium nitrate, not coal. [0]
The story I’m thinking of, the captain knew the coal was burning before he docked, and went into port anyway to try to salvage the cargo. The subsequent explosion was due to material on other boats.
I may have heard about this around the Tianjin explosion but more likely when they were fighting the new coal depots on the west coast, someone brought it up while talking about coal self igniting.