TECHNO-ECONOMIC ANALYSIS OF THE OPTIMUM SOFTWOOD LIGNIN CONTENT FOR THE PRODUCTION OF BIOETHANOL IN A REPURPOSED KRAFT MILL

Most pretreatments for softwood are not capable of producing bioethanol from softwood economically due to the high amount of enzymes necessary for the conversion of the carbohydrates to monomeric sugars. Kraft pulping is one possible pretreatment that may be used for softwood since it is capable of decreasing the lignin content to levels that are amenable to efficient enzymatic hydrolysis using low dosages of enzymes. This paper evaluates the techno-economic potential of using the haft process for producing bioethanol from softwoods in a repurposed haft mill. Loblolly pine was pretreated using kraft pulping to Kappa numbers ranging from 120 to 30. The pretreated substrates were enzymatically hydrolyzed at low enzyme dosages of 5 and 10 FPU/gm of substrate. Pretreated residue with a kappa number of 80 had the highest sugar recovery of 28% and 39% at 5 and 10FPU/gm respectively. This level of sugar recovery would not be economically feasible. The pretreated residues were oxygen delignified and refined to improve the overall sugar yield at the low enzyme doses. In all cases oxygen delignification improved sugar recovery while refining was mostly effective for pulps with high lignin content. At 5 FPU/gm the sugar recovery for the 40 and 80 kappa kraft pulps was about 68% with oxygen delignification and refining. Increasing the enzyme dosage to 10 FPU/gm increased the sugar recovery for both these pulp to about 72%. These levels of sugar recovery enable for a competitive process for the production of ethanol from softwoods in a repurposed haft mill. Economic analysis for the pulps with the different initial lignin content showed that kraft pulps with an initial kappa number of 40 had an ethanol yield of 285 liter/ODt wood and the lowest total production cost of $0.55/liter. Pulps with initial kappa number of 120 had a total production cost of $0.64/liter and the ethanol yield was 264 liters/ODt wood.