Complex sentence: The mined coal was supplied exclusively to the adjacent coal-fired Centralia Power Plant, which is operated by TransAlta Centralia Generation LLC and the coal mine was operated by TransAlta Centralia Mining.
Split into simpler sentences: The mined coal was supplied exclusively to the adjacent coal-fired Centralia Power Plant. The power plant is operated by TransAlta Centralia Generation LLC and the coal mine was operated by TransAlta Centralia Mining.