The Kohlenplatz was the coal storage yard storing the fuel that powered the Auschwitz-Birkenau crematoriums. The large deposits of coal were stored in a large area just south of the (DAW) Deutsche Ausrüstungswerke grounds, and next to the rail spur to the east than ran from the Bahnhof West to the Tobacco Monopoly building (the first prison of Auschwitz). Large quantities of coal were also stored at the crematoriums that were used throughout the use of the furnaces, quantities that were taken from the main supply from the Kohlenplatz. The coal was accessible to Birkenau from the rail spur and also on road, most probably through the use of horse and cart.
Coal was an important part of the Auschwitz industry, generally speaking. IG Farben, who worked closely with the Auschwitz III administration bought land and coal mines around the towns of Oświęcim to supply fuel for the large industrial plants they managed, with the use of Auschwitz slave labour. The large housing estate by the IG Farben complex near Monowitz, familiar as the modern day Osiedle Chemików was also powered with coal extracted from mines where Auschwitz prisoners worked. Coal mines were established into sub-camps of the Auschwitz III administration, such as Fürstengrube, Günthergrube and Janinagrube, all owned and extracted by IG Farbenindustrie, as well as other coal mines under various ownership using labour from the sub-camps of Charlottegrube, Jawischowitz and Neu-Dachs.
Information about sources we used while researching the industrial zones of Auschwitz you can find here.