Supercritical Water Could Provide Greener Pathway to Corn Ethanol

High temperatures yield remarkable changes in water’s chemical reactivity
By G. Graham Allan, John R. Di Iorio and Milo U. Zorzino | February 22, 2012

The utilization of supercritical water in the ethanol process could dramatically change both the corn starch and cellulosic processes, potentially simplifying the process, eliminating inputs and reducing energy inputs, even while using extremely high-temperature water.

The conversion of corn to ethanol presently involves 13 distinct steps starting with step 1, the complex planting and fertilized growing of the crop that is carried out by the farmer. The mechanical engineering activity of harvesting and kernel separation constitute step 2.  The total harvested biomass amounts to roughly 9 tons per acre.  Of this, only about one-third (7,110 pounds, at $7 per bushel or 13 cents per pound) of the corn moves forward to the ethanol facility as a source of starch for chemical processing. Therefore, about 6 tons of the low-value stalks, cobs and leaves produced by the farmer currently plays no role.

Step 3 of the manufacturing process is reducing the physical size of the kernels for subsequent hydrolysis in the slurry tank, step 4; the jet cooker, step 5; and enzymatic liquefaction, step 6. After fermentation of the sugars generated from the kernels in step 7, the resultant acqueous ethanol produced is recovered from the beer in step 8 by distillation. The water in this distillate is removed with molecular sieves in step 9 and the anhydrous ethanol obtained is then denatured with gasoline in step 10. The economically mandatory processing of the stillage residues from the distillation system to yield dried distillers grain with solubles (DDGS, valued at about $220 per ton, or 11 cents per pound)  is accomplished  by sequential centrifugation, step 11, evaporation, step 12, and drying, step 13.

Comparison of the energy requirements of each of these steps relative to the energy content of the total ethanol yield shows where attention should be focused if the overall process efficiency is to be significantly improved. A 2008 review by A.A. Peterson and colleagues in the journal Energy Environmental Science suggested that about 60 percent of total energy input is consumed by the liquefaction, distillation and DDGS preparation steps.

Supercritical Changes
As an alternative to this traditional system, a new pathway based on the use of supercritical water, offers the promise of achieving reduction of this large energy input by virtue of the remarkable changes in the chemical reactivity of water observed when it is heated to above a temperature of 374 Celsius (705 degrees Fahrenheit), discussed in a 1999 paper by P.E. Savage published in Chemical Reviews. For example, Japanese scientists S. Saka and T. Ueno showed in research published in 1999 that cellulose is transformed into glucose in yields that are almost quantitative in a few seconds, without the need for acid catalysis.  Other polysaccharides, of course, will behave similarly. Waxy and oily esters will also be rapidly cleaved so that the kernels may not need to be milled at all and the associated equipment and energy costs thus avoided.

An additional energy-saving consequence of the exposure of the whole kernels to supercritical water will be that the slurry tank with enzyme addition, the subsequent jet cooking and liquefaction  will all become superfluous. The subsequent ancient fermentation of step 7 with its inherent inefficiencies should remain substantially unchanged, except for the fact that the amount of the incoming sugars will be augmented since the precursor starch will have been completely hydrolyzed by the supercritical water so that none of the starch is wasted in the generation of nonfermentable oligomers.

These changes will reduce the energy input associated with the diminished amount of material going through the centrifugation, evaporation  and  drum drying procedures needed to obtain DDGS solids that have a market value that is actually less than the original feedstock corn kernels.

Biomass Application
In addition to these energy-saving advantages, the aid of supercritical water can also be invoked to utilize the 6 tons per acre of biomass grown at the same time as the corn kernels. The polysaccharide content of the stover will be hydrolyzed to sugars available for fermentation, but what about the lignin content? 

Lignin can be visualized as a mass of individual aromatic rings joined by nonaromatic linkages, as described by J. Urquhart in a paper in Chemistry World 2011. At short reaction exposures, supercritical water does not affect aromatic rings but does cleave the inter-ring linkages. As a result, the giant lignin macromolecule is converted into a mixture of low-molecular weight phenols, as was shown in research published in 2002 by K. Ehara, S. Saka and H. Kawamoto in the Journal of Wood Science. With the current price of oil-derived phenol at about 86 cents per pound, there is an attractive economic opportunity for lignin-derived phenols as a replacement for some of the petroleum-based phenol now used in large-volume wood adhesives. Analogously, all of the structurally complex mycotoxins potentially present in corn will be cleaved into smaller and simpler nontoxic molecules that are unlikely to affect adversely the nutritional quality of any DDGS produced.   

From all of the foregoing, it is clear that there are excellent reasons why chemical-free supercritical water offers a greener prospect of significantly improving the economics of obtaining ethanol and chemicals from entire corn plants. The maximum benefits will be secured when the treatment with supercritical water is carried out continuously in an extruder, as recently disclosed in U.S. Patent No. 7,955,508 entitled “Supercritical Fluid Biomass Conversion Systems.”  This patent teaches that a specialized extruder can convey selected biomass materials from an upstream hopper to a downstream supercritical fluid reaction zone, while increasing the pressure from atmospheric to greater than 3,200 psi. The supercritical fluid reaction zone further heats the flowing and pressurized biomass materials to supercritical conditions by means of a circumferentially positioned, high efficiency, alternating current induction coil. The liquefied reaction products are then expelled through a specially configured and proprietary spear and tube reactor that ensures complete biomass depolymerization and liquefaction. The liquefied reaction products are next expelled into an innovative expansion/separation chamber that contains liquid water and a hydrocarbon solvent to facilitate liquid-liquid extraction and phase separation of the resulting products.

A detailed economic analysis of all of the labor, energy and equipment costs including depreciation associated with the system has shown that undried biomass can be transformed for less than a nickel per pound in the Pacific Northwest.

Authors: G. Graham Allan
Professor of Chemical Engineering,
University of Washington, Seattle
(425) 486-1649

John R. Di Iorio
Chemical Engineering Student
Milo U. Zorzino
Chemistry Student
University of Washington