Problem with using SCR to control power for an electric grill
The main reason to include opto-isolated drivers is to ensure there is no connection between the mains AC side and any other supplies, especially those you might get to touch. I'd suggest the following:


Get rid of the 12 V supply
Stay with the MOC3011 you have, they work quite well
If you are stuck using SCR's then you need two in antiparallel to switch both half cycles of the AC mains. The other way to do it is to use a single SCR and a Diode bridge, though the thermal losses are higher in this configuration.
Note: Your current circuit provides only a half wave supply to your heating element, since you only have a single SCR.  
The TRIAC output structure in the MOC3011 is not great at switching close to the zero crossing (it may have an on voltage drop of close to 3 V), so it's best when using them to wait until the voltage is high enough to ensure good turn on current for the power SCR(s). If you get an interrupt from zero crossings you can wait a fixed time (or sync your saw tooth generator) to ensure you have enough voltage to provide adequate gate current pulse amplitude. 


Here's the same information in schematic form:







simulate this circuit &ndash; Schematic created using CircuitLab

As for the element remaining powered all the time, you have a fault that cannot be ascertained from your current information since it's incomplete. Providing a full schematic may help.