So set up command blocks like the picture above. Before you do any commands put this in (as a command not command block)  /scoreboard objectives add grenade dummy 
Then in the bottom command block put /setblock ~ ~1 ~ redstone_block 
and in the top command block put this command block /setblock ~ ~-1 ~ air 
Then the command block on the left (the one without the comparator) put /scoreboard players add @e[type=Item] grenade 1 {Item:{tag:{display:{Name:"Grenade"}}}} 

Then on the right put /execute @e[score_grenade_min=40] ~ ~ ~ execute @e[r=10,type=!Item] ~ ~ ~ summon PrimedTnt

Then on the command block connected to the comparator /kill @e[type=Item] {Item:{tag:{display:{Name:"Grenade"}}}} 

After you have the commands setup  make a 2 repeater clock with no extra ticks connect and activate the 1st command block. After that goes place another command block anywhere and a button or lever and put this command in it /give @p diamond 64 0 {display:{Name:"Grenade"}}  This gives you a stack of grenades. If you don't want the grenades to be diamonds change diamond to another item. For example exploding carrots, or exploding pufferfish. 
So to activate the grenade press q and run away it should take 10-12 seconds to blow up giving you time to run away. It blows up every single thing in a 10 block radius. If you want it blow everything up in a 20 block radius chang r=10 to r=20.
Also what you could do is get a texture pack that makes a piece of charcoal look like a real grenade if it was put charcoal insead of diamond.