Hardware render buffer is only used if you want to render particles. Luminance option is best used for alpha instead of hardware alpha.
In your case since you only have animated text all you have to do is as mentioned above.Make sure the alpha is ticked in render globals.As a precaution also go into your primary camera attributes and make sure output settings>RGB and Mask are ticked on....Depth...if you want to output the zdepth along with it.