THROOP, Pa.— The highest prices in years have made producing natural gas profitable just about anywhere. But with companies trimming their emissions and new tax breaks for waste-to-energy projects, some of the most lucrative places to extract gas are garbage dumps. Down the western slope of Keystone Sanitary Landfill, a knot of pipes, membranes and compressors draws fumes from within the mountain of rotting trash, separates the methane from other gases, and pumps it into northeast Pennsylvania’s natural-gas grid.

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