Sentence:  <WSD>Flare</WSD> stacks are used in gas plants, petroleum refineries, and petrochemical plants to combust surplus hydrocarbons to produce combustion products that are neither toxic nor combustible. <WSD>Flares</WSD> frequently incorporate a liquid-gas separator at the base of the stack and steam assisted burner nozzles at the top of the stack to aid complete combustion. 
Meaning of the word marked with <WSD> tags:  A flame produced by a burn-off of waste gas (flare gas) from a flare tower (or flare stack), typically at an oil refinery.