TURKEY, N.C.—The Align RNG biogas processing facility here is so small, you would miss it if you weren’t looking for it. Just four small silver mounds beside a massive 100-foot grain silo under which trucks drive day-in, day-out loading up with hog feed. In a landscape dominated by the infrastructure of hog and poultry farming—slaughterhouses, barns, slat manufacturers and truck washers—the mounds could be mistaken for backup generators or small storage tanks from a distance. But they’re actually part of a boom in turning hog waste into renewable natural gas (RNG) that’s building out hundreds of miles of underground pipelines,…