Tuesday, May 3, 2011
Green liquor pretreatment, an existing technology used worldwide in hundreds of kraft pulp mills, is proposed as a potential pretreatment pathway for conversion of lignocellulosic biomass to ethanol. Three biomasses, natural mixed southern hardwood, Eucalyptus, and Loblolly Pine, were evaluated in process and financial simulations under two investment scenarios: green field and repurposing. A complete process model was developed in WinGEMS which provided material and energy balances necessary for economic analysis. The repurposing concept is centered on converting closed kraft pulp mills into cellulosic ethanol production facilities. Green liquor (Na2S/Na2CO3), named for its color, is produced in the kraft recovery process and is a pretreatment option compatible with existing equipment at a kraft pulp mill. Several advantages come with this concept: i) proven technology; ii) chemical recovery in place, iii) existing fiber supply chain and experienced labor force. Based upon experimental data, ethanol yields are higher for natural hardwood and Eucalyptus (67-68 gallons per dry short ton of biomass) compared to Loblolly Pine. Natural hardwood and Eucalyptus in the repurpose scenario are the most profitable scenarios with IRR ~19% as a result of low CAPEX, low enzyme costs (compared to Loblolly Pine) and higher ethanol yield. Ethanol revenue was assumed to be $2.00/gal with an additional $1.01/gal subsidy. The hardwood/Eucalyptus repurpose scenarios estimate production cost of $2.51 per gallon of ethanol, cash cost of $2.14/gallon and CAPEX of $3.15/gallon. Due to high CAPEX, a greenfield green liquor pretreatment scenario does not produce financeable returns under the current assumptions.