As U.S. natural gas prices jumped to a three-year high, coal has become a cheaper power-generating fuel for utilities, which are set to run coal-fired generators harder this winter. U.S. benchmark natural gas prices at Henry Hub have jumped from $4.23 per million British thermal units (MMBtu) at the start of November to above $5 per MMBtu by early December. Early on Friday, the front-month futures price was $5.084 per MMBtu. That’s the highest price in three years, as a polar vortex with freezing temperatures and snowstorms gripped most…