Energy optimization of the production of cellulosic ethanol from southern pine

Abstract

On the forefront of the recent expansion in biofuels research is the production of cellulosic ethanol, or ethanol produced from a cellulose containing feedstock. Cellulose is a six-carbon polysaccharide found in most plant life and is one of the most abundant organic compounds on the planet. While the first generation of ethanol facilities uses sugar and starch based (corn kernels) plants as their feedstock, the next generation will use cellulosic sources such as wood chips, switchgrass, and forest residues. These cellulosic sources require far less energy and resources to grow and harvest, and are also much more abundant.

A cellulosic source widely available in Georgia and much of the southeastern US is southern pine. This study involves the modeling of a complete 2000 dry ton per day pine to ethanol production facility with the AspenTech3 software Aspen Plus, which outputs a mass and energy balance as well as the capital cost of the equipment. A key parameter which affects the competitiveness of cellulosic ethanol is the internal processing energy required to convert the pine to ethanol. As a result, the heat and electrical load of every component within the facility is modeled and then quantified through the Aspen Plus simulation. After this base case energy analysis is developed, various alternate plant configurations are integrated in an attempt to reduce this process energy requirement.

The material that is not fermented into ethanol is burned on-site to provide steam and electricity to the plant, as well as excess electricity to be sold to the grid as a byproduct. As the facility processing energy requirement is decreased, more excess electricity is available for sale. The implementation of the alternate distillation scenarios effectively reduce the internal processing energy in a manner as to increase the amount of excess electricity sold to the grid by 13.5%. The additional equipment required in this alternate scenario returns a simple payback period of 1.1 years through the additional revenue of the increased electricity sale.