Sentence:  The large supply of natural gas at Chicago permitted <WSD>underfiring</WSD> the batteries of the Chicago By-Product Coke Oven Plant with a mixture of producer gas and natural gas. ... The mixing of by-product coke oven gas with blast furnace gas to correct the varying thermal deficiencies of the latter when used to <WSD>underfire</WSD> coke ovens is patented by Becker. 
Meaning of the word marked with <WSD> tags:  To intentionally operate a boiler, furnace, oven, "etc.", at a low level.