soot booger
n.— «In informal industry parlance, a soot booger is a crusty buildup of stuff that is in coal, but didn’t burn up in the boiler and is too heavy to float out of the smokestack—the soot. Big globs of it—the boogers—accumulate on the inside walls of the smokestack and have to be scraped off from time to time. Soot boogers are an undesirable by-product of burning coal.» —“Joe Lucas and ACCCE: Wiping away coal’s soot boogers” by Pete Altman NRDC Dec. 12, 2008. (source: Double-Tongued Dictionary)
Further reading
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