Acid hydrolysis of waste biomass containing appreciable percentages of cellulose and hemmi-cellulose has been under development over the past several years. Conversion is to two step process: conversion of cellulosic materials to sugars followed by fermentation of the sugars to ethanol. As is the case with the fermentation of syngas, acid hydrolysis is much more forgiving in terms of biomass feed composition than cellulase-based techniques because of the aggressiveness of the acid toward hydrolysis as opposed to problems with enzyme specificity. Problems do persist with costly high acid loss and heterogeneous introduction of the acid into the physical matrix of the waste biomass.

In 1982, the Tennessee Valley Authority (TVA) biomass development program renewed research previously carried out at the USDA Peoria Laboratory on converting agricultural waste into useful products by using an acid hydrolysis extraction/conversion process. TVA, with assistance from Mississippi State University (MSU), built a 4-ton-per-day concentrated acid hydrolysis experimental facility at Muscle Shoals, Alabama in 1985. In this concentrated acid process, cellulose and hemicellulose from hardwoods were reduced by sulfuric acid to xylose and glucose sugars. Unused acid was removed from the sugars by lime precipitation. The sugars were then fermented to ethanol. Process concepts were demonstrated and high hemicellulose and cellulose conversion efficiencies were achieved. However, process economics were poor as a result of two major problems: (1) the acid was continuously consumed, elevating operating costs, and (2) equipment costs were too high for a favorable investment return.

Between 1993 and 1998, The University of Southern Mississippi (USM) and TVA researchers continued to improve the concentrated acid process. A twin screw extruder reactor was utilized for low temperature continuous impregnation of hardwood sawdust with sulfuric acid. The shearing action of the extruder’s co-rotating screws increased acid exposure to the biomass structure by continually removing acid treated biomass surface; thereby, exposing new surfaces to the acid. A tube reactor was used to convert the hydrolyzate stream from the twin screw extruder into glucose and xylose sugar. Continuous ion exclusion techniques were perfected so that sulfuric acid could be separated from the sugars formed in the tube reactor. The acid stream from the separation step was concentrated and thereafter reused. The sugar stream from the separation step is then fermented into ethanol. Much of the USM/TVA work has been patented.

Analysis of cotton wastes collected from four ginning operations and the residues from a cotton seed acid delinting plant indicate that the cellulose content for cotton trash is higher than that found in hardwoods. Preliminary laboratory analysis has shown that cellulose-to-glucose conversions by acid hydrolysis averages about 85%; a value 20% greater than for typical hardwoods. In comparison to hardwood sawdust, cotton waste has a larger surface area and a cellulose crystalline structure that is more accessible to acid decrystallization and hydrolysis. Acid impregnation of hardwood sawdust requires expensive twin screw extruders and the use of 70% sulfuric acid at a rate of one pound of acid per pound of sawdust. However, because of the structural characteristics of cotton waste, it is expected that acid impregnation may be accomplished with less expensive in-line mixers while using less acid at a lower concentration. The use of less acid at a lower concentration would significantly reduce acid recovery cost. Thus, the potential for efficient cotton waste conversion appears favorable and process yields will probably exceed that projected for hardwood sawdust, i.e., greater than 55 gallons of ethanol per ton of cotton waste. In addition, operating cost and equipment investment will be less for cotton waste conversion than that expected for hardwood sawdust conversion.