National Public Radio: They call it “black liquor,” but the dark, chemical brew is not a beverage. It’s the sludge left over from the cooking process used to turn wood pulp into paper. Paper companies burn it to generate steam and make electricity. And a little over a year ago, they found that just by adding one-tenth of 1 percent of diesel fuel to the black liquor mix, they could qualify for the “alternative fuel mixture credit.” “In the short term, this has provided to us some extra cash flow and income …