Supercritical Water Gasification of Biomass

Grant: 99-01
Principal Investigator: Gary A. Aurand
Organization: The University of Iowa
Technical Area: Renewable Energy

Background and Significance:
Biomass feedstocks are a tremendous potential resource for providing energy and value-added products, especially in agricultural areas where waste biomass is abundant or where dedicated energy crops can be produced cheaply and efficiently. However, increased competitiveness will require advances in technologies for converting Iowa biomass to value-added products. This project is an investigation into the use of supercritical fluids, particularly supercritical water, to efficiently produce valuable products from biomass.

A supercritical fluid is defined as a substance that is at conditions of temperature and pressure that are above its vapor-liquid critical point. At supercritical conditions, a fluid does not meet the definition of a liquid because it can’t be made to boil by decreasing the pressure at constant temperature. Also it is not a vapor because cooling at constant pressure won’t cause it to condense. Water is a supercritical fluid above 374°C and 22 MPa, 706°F and 3191 psi. Supercritical fluids in general possess unique solvating and transport properties compared to liquids or gases. Supercritical fluids can have liquid-like densities, gas-like diffusivities, and compressibilities that deviate greatly from ideal gas behavior. Under supercritical conditions, solid solubility often is enhanced greatly with respect to solubility in the gas or liquid solvent. Supercritical water in particular has the ability to dissolve materials not normally soluble in liquid water or steam and also seems to promote some types of chemical reactions. These properties make supercritical water a very promising reaction medium for the conversion of biomass to value-added products.

Organic compounds, including lignocellulosic material such as solid biomass will readily dissolve in supercritical water. Once dissolved, supercritical water will efficiently break cellulose bonds. The reactions generally are not selective, resulting in the rapid formation of gaseous products. This type of biomass gasification can be used to produce hydrocarbon fuels for use in an efficient combustion device or to produce hydrogen for use in a fuel cell. In the latter case, hydrogen yield can be much higher than the hydrogen content of the biomass due to steam reforming where water is a hydrogen-providing participant in the overall reaction.

Many organic chemicals that typically do not react in water without the presence of strong acid or base catalysts will readily react under hydrothermal conditions. This fact can be exploited to convert biomass to valuable chemical products. Hydrolysis is a type of reaction that commonly occurs under hydrothermal conditions, and glucose is an intermediate hydrolysis product in the hydrothermal decomposition of cellulose.

Typically, the severe reaction conditions of supercritical water result in subsequent decomposition of hydrolysis intermediates to form primarily gaseous products. However, at slightly milder near-critical temperatures water can be used to convert biomass to valuable chemicals. For example, in principle, glucose from cellulose decomposition can be further converted to a variety of water-soluble liquid products such as fructose and erythrose. Erythrose is a high-value chemical with a catalog price of about $50/gram for 60% D-erythrose syrup. The development of such processes to convert biomass feedstocks to valuable fuels and chemicals is necessary for expanding a carbohydrate-based economy that will benefit agricultural states such as Iowa.

Project Objectives:
Specially constructed hydrothermal reactors will be used to study reactions of organic matter in supercritical water mixtures. Initially, purified biomass components such as cellulose and starch will be used as feedstocks, and the products will be identified and measured. These and subsequent investigations will increase understanding of the fundamental chemical reactions occurring during biomass conversion in supercritical or near-critical water. Using knowledge gained from earlier experiments, reactions using whole biomass will be conducted to find conditions favorable for the production of valuable products. The ultimate goal is to develop a practical process for the conversion of whole biomass such as corn stover into valuable chemicals or fuels.

Work to Date (Technical Report – March 2007)
Soybean oil esterification :
A larger capacity flow type reactor was designed and built to increase conversion of reactants. The new reactor has a volume of 70 ml. Technical grade linoleic acid purchased from Sigma was reacted with methanol at 270°C and 2500 psi. The solvent in the reaction products was removed under reduced pressure, then analyzed using NMR. The NMR spectrum of the esterification product does not indicate the presence of any unreacted linoleic acid, only methyl esters. Thus, the reaction appears to have proceeded to 100% conversion.

Additionally hydrolysis and transesterification reactions were performed to test the new system. Both reactions were performed at 270°C. The hydrolysis reaction was conducted at 1100 psi while the transesterification reaction was at 2500 psi. A full chemical analysis of the products of the two reactions has not been determined yet. However, visual observation of the products indicated that some conversion was attained.

Future experiments will be designed to adjust the reaction conversion for the purpose of obtaining reaction kinetics data for the relevant reactions.

Cellulose hydrolysis :
Reaction kinetic parameters were obtained for several reactions involved in the hydrothermal decomposition of cellulose. Specifically, activation energies and pre-exponential factors were obtained for the hydrothermal conversion of glucose, fructose, cellobiose, and cellotriose. For comparison, parameters were obtained for the conversion of maltose, and maltotriose. Kinetic constants were calculated for the formation of 5-HMF, glyceraldehyde, levoglucosan, and erythrose during decomposition of monosaccharides, and for the isomerization reaction between glucose and fructose. First-order reaction kinetics were assumed in each case.

Additionally, development of methods to measure the degree of polymerization for polysaccharides was begun. This will allow the quantification of the degree of hydrolysis for partially hydrolyzed cellulose and starch.

Organic acid esterfication:
Work was started on setting up a reactor for organic acid esterfication in supercritical alcohols and on developing protocols for chemical analysis.