How to find integer inside a file with STRNCMP
Maybe make it as:

char id[] = "18";


This is if you want to find "18" in your file. That's what I understood from your question.

EDIT: I guess you want this http://playground.arduino.cc/Code/PrintingNumbers:

int num = 18;
char buffer[100]; // this will hold your number
itoa(num, buffer, 10);


Function description here. Also it's not to hard to make this your self for integers and it's a good exercise.