According to Sichuan Airlines, generally each passenger is entitled to a free baggage allowance: 40 kg for first class, 30 kg for business class, 20 kg for economy class. Besides, the passenger holding economy class ticket can only carry one piece of baggage. The dimension of each baggage can not exceed 20x40x55 cm. The total weight can not be larger than 5 kg. For more details, you can consult the airlines directly.

will Sichuan Airlines allow us to check in a wheelchair with our luggage on a domestic flight from Xian to Chengdu? Is this additional to our luggage allowance of 20kg? will we have to pay for the wheelchair as excess baggage or is it free. I have checked their website but cannot find the answer. Thanks.

Most airline don't allow passengers check-in with a wheelchair. You'd better to check your baggage and the wheelchair. There are some other information you can get in this website:
https://www.travelchinaguide.com/essential/baggage.htm