Hi there!

I've been growing this citrus tree for many years. Me and my grandmother planted it from seed when I was about 2 years old and it grew into a very nice tree since then. However, I wasn't able to identify for sure which kind of citrus tree it is exactly. I always thought it is a lemon tree, since the seed was from a lemon. But my friend told me there is no way that it is a lemon tree, according to its specific leaves. The tree hasn't been fruiting so far, but I already grafted him, and the budwood was accepted by the tree, so I hope I will see its fruit in the near future. :-)
However, could you please help me to identify this citrus tree? It has very large thorns/spikes I haven't seen on any citrus trees before. The longest one I found was about 5 cm long. According to my friend, its leaves shows that it could be either grapefruit or pomelo.

I will be glad for any answers, thank you! :-)

Here are links to the pictures of the tree:

http://s24.postimg.org/hevwawksl/MG_8550.jpg
http://s24.postimg.org/qwr1x7h39/MG_8544.jpg
http://s24.postimg.org/wfy6s63id/MG_8555.jpg
http://s24.postimg.org/8q8r3h551/MG_8556.jpg