14-02: Green liquor pretreatment of mixed hardwood for the production of ethanol in a repurposed kraft mill

The pulp and paper industry is in decline due to falling demand for paper and board products and in some cases, loss of market pulp share to lower cost countries.  As a result, it would be very attractive to repurpose a kraft pulp mill for the production of ethanol.  Various pretreatment methods including water prehydrolysis, sodium carbonate, green liquor, and prehydrolysis followed by green liquor pretreatment were studied for hardwoods that would be compatible in a repurposed mill.  These results show that a pretreatment based on green liquor (sodium carbonate and sodium sulfide) is the most effective pretreatment, which produces highly digestible substrate with minimum carbohydrate loss.  The use of green liquor ensures that the chemicals used during the process can be recovered efficiently using proven technology in a kraft pulp mill.  In addition, repurposing a kraft mill can reduce capital investment up to 80% compared to any other Greenfield projects.  Based on our techno-economic analysis, it leads to production cash costs of $2.37 - $2.55 per gallon of ethanol and more than 80% of cash cost for fiber and enzyme costs.  Investment returns (% IRR) of 19 and 26% are realized for the small and large scale repurposed kraft mills, respectively, based on total ethanol revenues of $2.00 per gallon market price plus $1.01 subsidy.  We will also demonstrate the process simplifications so that this process based on green liquor pretreatment can be attractive in Greenfield application.  This work has been performed with the support from an industry-sponsored consortium.