The amount of heat a compost barrel will provide is a drop in the ole bucket, a big bucket. Unless your greenhouse is awfully small and the barrel awfully big it won't make much difference. Also you'd need to refill several times during winter.

What is the best way to heat a 10 by 10 greenhouse and will my plants survive of I don't have heat ans just keep them away from frost

It totally depends on what kinds of plants you are talking about. If you can give more details you'll have a better chance of getting a helpful answer.