ST_UpperLeftX — Returns the upper left X coordinate of raster in projected spatial ref.
float8 ST_UpperLeftX(raster  rast);
Returns the upper left X coordinate of raster in projected spatial ref.
SELECt rid, ST_UpperLeftX(rast) As ulx
FROM dummy_rast;
 rid |    ulx
-----+------------
   1 |        0.5
   2 | 3427927.75