NEW YORK, April 17 (Reuters) - Utilities that face dwindling demand for their electricity are balking at high contract prices for coal, and some are looking at ways to renegotiate with the mining companies, industry experts said on Friday. In another blow to coal miners, who have seen spot prices plummet 25 percent this year, many power plants are instead burning natural gas, which is at its lowest price in more than six years. As a result, miners are being forced to cut production of coal they would have sold to the utilities at higher prices locked-in to contracts negotiated...