Compressed Air Best Practices, May 2017, by Don van Ormer.
http://www.airbestpractices.com/industries/oil-gas/making-sense-compressed-air-huge-chemical-plant

"A chemical plant spends an estimated $587,000 annually on electrical energy to operate their compressed air system. In addition, the plant has an expenditure on rental air compressors of equal or greater size - but this will not be covered in this article. The plant was built in the 1940s and modernized in the 1970s. The plant generates its own power and serves many processes. The average cost per kWh is $0.0359."