Will I be granted visa without any invitation? My purpose is more of sight seeing.

Sorry, I am afraid that you won't. It seems that Chinese embassy doesn't issue L type to Nigerians. You need to apply for a M or F type one. Both requires invitation letter and confirmation letter of invitation (both original and copy) issued by units authorized by Foreign Affairs Ministry of PRC. Besides, your company's business license is required for M type. Recent six months bank balance is needed for M and F types if you come to PRC for the first time. Your account should have 4 million Nigerian dollars.