BACKGROUND OF THE INVENTION Hydrogen has enormous potential as a fuel source, due to its high energy value and cleanliness, if it can be produced economically. Hydrogen is also a very useful chemical reactant.

Conventional methods for generating hydrogen include steam reforming of natural gas and electrolysis of water. Improvement of these conventional methods is desirable to avoid continued reliance on uncertain and expensive supplies of natural gas, increase efficiency of hydrogen production and reduce energy consumption. Another potential source of hydrogen under study is biomass. Generally, biomass exists in the form of terrestrial or marine plants as well as some agricultural, industrial or municipal wastes. More specifically, it has been contemplated to derive hydrogen and other gaseous products through thermochemical conversion of biomass. For example, it is known that hydrogen and other gases are produced from steam pyrolysis of cellulose at atmospheric pressure. However, hydrogen yields have been too low to make such a conversion process commercially practical. In addition, conversion of carbon in the biomass feedstock to carbon in the gaseous product is relatively inefficient because, at conventional operating conditions, refractory tar and char are formed. These byproducts are also undesirable since they must be removed from processing equipment periodically.

Also, operating conditions in conventional thermochemical conversion techniques such as

oxidation or pyrolysis have limited the hydrogen generated to an amount that is substantially less than that predicted from thermodynamic equilibrium considerations. Moreover, reaction rates have been too slow for commercial exploitation as a means for hydrogen production.

To date, wet biomass, that is, plants with a high moisture content, such as water hyacinth, banana tree, cattails, green alga, and kelp, though plentiful, relatively inexpensive, and renewable, has not been considered useful feedstock for such thermochemical conversion techniques. In large part, the disadvantage of wet biomass is the high cost of water removal often required before thermochemical conversion can be carried out. Also, the variety of carbohydrates and other chemical species in these plants and the number of mechanisms through which they can react have obscured understanding of the means by which wet biomass may be most efficiently decomposed to form the desired gaseous products without tar or char byproducts.

Efforts to increase the efficiency of hydrogen production and reduce generation of undesirable residues have continued. Particular attention has been given to the many carbohydrates that compose biomass using the reaction chemistry of glucose as a simple model. For example, glucose (about 0.1 M) has been completely gasified without a catalyst to mostly hydrogen and carbon dioxide in about 30 seconds at 600°C and 34.5 MPa. Higher concentrations of glucose (up to about 22% by weight) have evidenced lower conversion to hydrogen, carbon dioxide and hydrocarbons without a catalys .

Studies from work in destruction of hazardous organic wastes have shown that carbon dioxide is produced from carbon monoxide at 500°C via the water

gas shift reaction in supercritical water. While this reaction plays a role in the production of hydrogen and carbon dioxide from glucose pyrolysis in supercritical water, much remains unknown about the pyrolysis of other carbohydrates in supercritical water since, at the high temperatures used for hazardous waste destruction, carbohydrates undergo many kinds of reactions, including dehydration and fragmentation, to form numerous species with a variety of reactivities.

Dairy waste has been converted in a supercritical water medium, at temperatures between 400°C and 450°C and pressures as high as 34.5 MPa, over catalysts containing alkali carbonates and nickel into a methane-rich gas.

In downdraft biomass gasification reactors, air containing tars and oils is directed downwardly past a fixed bed of carbon in order to crack out the tars and oils. Conventionally, for these techniques, the temperature is about 1000°C and pressure may be subatmospheric to about 4 MPa.

Accordingly, it is an object of the present invention to provide a method for converting organic matter such as biomass or organic waste to desirable products without the need for drying prior to treatment. It is another object of the present invention to provide a method for gasifying biomass or other organic matter that achieves a high efficiency of conversion of the organic matter to desirable gaseous products, and particularly hydrogen, at economical rates. It is yet another object of the present invention to achieve a high degree of gasification using an inexpensive, stable catalyst. An additional object of the present invention is to gasify concentrated organic matter feedstocks to hydrogen, methane and other simple

hydrocarbons. A further object of the present invention is to destroy organic waste efficiently and completely.

SUMMARY OF THE INVENTION The present invention achieves these and other objects by catalytically decomposing organic matter, such as wet biomass and organic waste, in a reaction medium, producing a hydrogen-enriched product and substantially no tar or char residues. In one aspect, the present invention provides a method that includes contacting a feedstock in a reaction medium with a carbon-containing catalyst for a time sufficient to gasify the feedstock, wherein the feedstock includes organic matter and the medium includes water at a pressure of 22.1 MPa or greater and a temperature of 374 °C or greater. The catalyst may be an activated carbon or a charcoal.

In another aspect, the present invention provides a method in which a carbon-containing catalyst is provided in a reaction zone in a reactor, preheating and pressurizing the reaction zone, introducing a water-containing feedstock into the preheated and pressurized reaction zone, in which zone, water in the feedstock has a temperature of 374°C or greater and a pressure of 22.1 MPa or greater and wherein the feedstock includes organic matter and contacting the feedstock with the catalyst in the reaction zone to yield a product containing water, hydrogen, methane, carbon dioxide or mixtures thereof.

Suitable organic matter feedstocks for use in methods according to the present invention include biomass, organic waste and mixtures thereof. Suitable reaction media include water, whether in pure form or in a mixture with other substances, at

supercritical conditions that preferably are substantially free of oxygen.

The present invention achieves numerous advantages. Numerous types of feedstocks may be utilized, including organic wastes and biomass having a high moisture content. Hydrogen yields and conversion of carbon in the feedstock to gas are high even at high feedstock flow rates and concentrations. Virtually complete gasification is achieved in relatively short periods of time. The reaction product may be substantially free of tar or char, even when the feedstock has a high carbon concentration. The carbon-based catalyst is relatively inexpensive compared to other catalysts, is stable in the environment of a supercritical water medium and maintains activity over long periods of time.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood by reference to the appended figures wherein:

Figure 1 is a schematic diagram for a method according to the present invention for gasifying biomass in a supercritical water-containing medium; and Figure 2 depicts tar yield from catalytic gasification of glucose in supercritical water at increasing reaction temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The term "organic matter" as used herein will denote biomass, such as plant species, sewage sludge, agricultural or forestry waste, and organic waste such as organic solvents, organic acids and the like. Exemplary plant species include those having relatively high moisture contents such as water

hyacinth, banana tree, cattails, green alga, kelp or other aquatic species.

The phrase "critical point" as used herein refers to the thermodynamic state of a substance in which its liquid and gaseous phases co-exist in equilibrium at the highest possible temperature. For water, the critical point is a temperature of 647K (374°C) and a pressure of 221 bar (22.1 MPa). The term "supercritical" refers to a thermodynamic state at conditions beyond those recognized as the "critical point" of a substance. Thus, pure water at the supercritical condition has a temperature greater than 374°C and a pressure greater than 22.1 MPa. At or above the critical conditions, the density of water is a function of both temperature and pressure. Supercritical water acts as a dense gas with the solvation characteristics of a nonpolar organic. Thus, organic compounds and gases are completely miscible with it, but inorganic substances such as salts are immiscible.

The term "nonoxidizing" as used herein refers to the condition of being substantially free of oxygen or air.

A method according to the present invention generally involves contacting organic matter in a supercritical water-containing medium with a catalyst to produce hydrogen, carbon dioxide and other desired gaseous species. According to the present invention, organic waste or biomass having a high water content may be utilized as is for a feedstock or may be put in a form, e.g., a slurry with water, that facilitates adequate flow and contact with the catalyst. The feedstock is contacted with the catalyst at suitable temperature and pressure conditions so that the water is at supercritical conditions. Preferably, the temperature is about

600°C and the pressure is 22.1 MPa or greater. Under these conditions, the organic matter in the feedstock is rapidly and virtually completely gasified to produce hydrogen and other gases such as methane or other hydrocarbons, carbon monoxide and carbon dioxide, e.g., from thermal decomposition of the organic matter. Typically, less than 1 wt% of the reaction product is in the form of tars or chars. The gas phase of the reaction product may be separated from the rest of the reaction product by conventional gas-liquid separation techniques. To reduce any hazards of explosion, the gases may be cooled.

As will be appreciated by those of skill in the art, several thermodynamic paths may be followed in order to achieve supercritical conditions for water. For example, in a constant volume process, a feedstock containing a biomass and water mixture may be heated until supercritical conditions are reached. Alternatively, a method according to the present invention can be carried out in a constant pressure reactor scheme such as illustrated in Figure 1. Generally, the reactor scheme includes a feedstock supply and a heated flow-type reactor housing a catalyst and capable of maintaining conditions associated with a supercritical reaction medium.

Before the feedstock is introduced, substantially deaerated water having little or no oxygen therein is fed into a reactor and then the pressure is increased to a desired level. A feedstock 12, e.g., a slurry of biomass and water, can then be supplied to the reactor scheme from a feeding system 14. Flow rate of the feedstock out of the feeding system 14 and to a reactor 16 is controlled, e.g., by a pump 18. Reactor 16, which may be a flow-type reactor, contains a heating zone

20, containing catalyst bed 22 through which the feedstock flows. A suitable form for such a reactor is a metal tube (0.375 inch OD, 0.187 inch ID and 18.9 inches long) made of a material such as INCONEL 625. Catalyst bed 22 may occupy a substantial portion of the reactor, e.g., about two-thirds of the heating zone of the reactor. On either end of the catalyst bed 22, i.e., at the front and back ends of a tube reactor, an inert material such as alumina beads may be used to confine the catalyst to the heating zone and to minimize action of the carbon- containing catalyst as an adsorbent.

Conditions within the reactor as the conversion proceeds may be monitored, such as via thermocouples, not shown, located along the length of and inside the reactor, and by other conventional means, such as pressure transducer 23.

Upon entry into heating zone 20, the feedstock is heated rapidly. For example, heating may be achieved by coils or resistance heater 24, or both, on the outside of the reactor. After the water in the feedstock reaches supercritical conditions, pyrolysis products of the feedstock contact catalyst bed 22 in the heating zone and decompose to gases. The decomposition products then pass out of the reactor via line 26 and the pressure of the reactor effluent may be reduced from the supercritical condition to atmospheric, such as through a back pressure regulator 27. Separation of the reactor effluent into gaseous and liquid phases may be achieved by a conventional separator 28, if desired. Such a separator also provides a convenient point for sampling.

Hydrogen is produced at high pressure, avoiding considerable expense of compression needed for commercial use.

The transformation of feedstocks containing organic matter and water in a method according to the present invention involves a catalytic thermal decomposition mechanism such as pyrolysis, rather than oxidation, using a carbon-based catalyst. As a result, samples of the effluent are substantially free of tar or char, that is, has less than about 1 weight per cent of such substances, as measured by conventional analytical techniques. Because of the pyrolytic reaction mechanism, the reaction medium is desirably substantially free of oxygen.

Concentration ranges for the organic matter contacting the catalyst may be adjusted by altering any or all of the following: the feedstock composition, its flow rate into the reactor, or the amount of catalyst in the reactor. For example, for glucose, a concentration up to about 22 wt% in water may be utilized. Also, a given type of biomass, as a whole or only certain portions, may be fed to the reactor, as collected or may be ground into a slurry before introduction into the reactor. In addition, the feedstock may contain other components other than water that become part of the reaction medium but do not interfere with the decomposition at supercritical conditions.

Catalysts useful in the present invention are carbon-based materials such as charcoals and activated carbons having high surface areas. Desirably, surface areas are about 1000 m/g. An exemplary charcoal is one based on wood or macadamia shell. Coconut shell or coal activated carbons are other useful types of carbon-based catalysts. The catalyst may be powdery or granular. A preferred coconut shell activated carbon is designated Type PE, a granular (12 x 30 mesh) activated carbon commercially available from Barnebey & Sutcliffe Co.

of Columbus, Ohio. Table I presents comparative data exemplifying the performance of these types of carbon catalysts in a method according to the present invention.

The time of exposure of the feedstock to the catalyst is generally that which is sufficient to effect virtually complete gasification. However, the time over which contact with the catalyst is carried out may be adjusted to take into account variations in the composition of the feedstock, and pressure and temperature of the supercritical water medium. For example, the dimensions of the conversion zone can be altered, e.g, by fabricating the reactor of tubes of relatively larger

or smaller diameters, in order to reduce or extend residence times.

Factors influencing efficient conversion of organic matter, such as biomass and organic waste, in a feedstock and production of desirably high gas yields include reaction temperature, residence time and feedstock concentration. Tables II through IX set forth the results of a number of tests run to provide guidelines useful in carrying out the present invention. Since glucose mimics the reaction chemistry of many carbohydrates that, together with lignin, compose biomass, it serves as a suitable model for the study of thermochemical conversion of carbohydrates, as presented in several of the following Tables.

Table II presents the results of a study of glucose gasification, with and without an activated carbon catalyst. As can be seen from this table, for the same glucose concentration, the conversion of carbon in the feedstock to gaseous product increases considerably by carrying out the gasification reaction over a carbon-containing catalyst such as activated carbon according to the present invention.

Significantly, even when WHSV was increased six-fold from 3.7 to 22.2 (g/hr./g, complete carbon conversion was obtained, confirming the activity and efficiency of the carbon-containing catalysts in the present invention.

Table II

Glucose Gasification Over Activated Carbon Catalyst

In Supercritical Water at 600°C, 34.5 MPa

1.2 M glucose 1.2 M glucose 1.0 glucose (no catalyst) (with 0.60 g (with 2.77 g

Gas Product catalyst) catalyst) Yield* res . time = 34 s WHSV = 22.2 WHSV = 3.7 (g/hr) /g (g/hr)/g

H. 0.56 2.24 1.73

CO 3.18 0.79 1.32

CO. 0.29 3.09 2.54

CH. 0.84 1.23 1.10

C ₂ H ₄ 0.03 0.0 0.001

C ₂ H _β 0.20 0.35 0.33

C ₃ H _β 0.00 0.0 0.005

C ₃ H _β 0.00 0.13 0.19

Carbon Conversion" 80% 103% 103%

WHSV = weight hourly space velocity in g glucose/hr-g catalyst Catalyst = coconut shell activated carbon ^■ gas yield = mole of gas/ mole of reactant carbon conversion = mole of carbon in the gas/ mole of carbon in the reactant

Table III shows the effect of the ratio of the amount of glucose feedstock and that of the catalyst on gaseous product yield. Increasing the flow rate of feedstock relative to the catalyst, i.e., WHSV, hydrogen and carbon monoxide yields increase. Carbon conversion was very high.

Table III

Effect of WHSV on Gasi fication of 1.0 M Glucose Reactant

Over Activated Carbon Catalyst in

Supercritical Water at 600°C, 34. 5 MPa

Figure 2 illustrates that tar yield from the gasification of 1.0 M glucose is also a function of temperature. The lowest tar yield was obtained at 600°C while tar yield at 500°C was more than ten times higher. Generally, process conditions should be such that the water in the reaction medium is at supercritical conditions. Table IV shows the effect of reaction temperature on glucose conversion. All of the temperatures were in the supercritical range. Pressure and WHSV were held constant. The highest carbon conversion and hydrogen yield were obtained at 600°C and tar yields were very low for each run at the different supercritical temperatures tested. More significantly, carbon conversion increased from 51% to 98% and hydrogen yields increased dramatically as the temperature was increased from 550°C to 600°C.

Table IV Effect of Temperature on Gasification of 1.0 M Glucose Reactant Over Activated Carbon Catalyst in Supercritical Water ,WHSV=13.5 (g/hr) /g)

600°C 550°C 500°C

Gas Product Yield* 34.5 MPa 34.5 MPa 34.5 MPa

H_ 1.97 0.62 0.46

CO 2.57 1.67 1.57

CO. 1.54 0.73 0.85

CH. 0.90 0.37 0.25

C ₂ H ₄ 0.008 0.01 0.016

C ₂ H ₆ 0.25 0.10 0.07

C_H _β 0.009 0.03 0.04

C,H _β 0.11 0.05 0.036

Tar yield" 0.013 0.009 0.001

Carbon Conversion ^1* 98% 54% 51%

WHSV = weight hourly space velocity in g glucose/hr-g catalyst Catalyst = coconut shell activated carbon * gas yield = mole of gas/ mole of reactant ^b Tar yield = gram of dried residue/gram of reactant ^c carbon conversion = mole of carbon in the gas/ mole of carbon in reactant

Table V shows the effect of reaction pressure on glucose conversion. As can be seen, the highest carbon conversion and hydrogen yield were obtained at 34.5 MPa.

Table V Effect of Pressure on Gasification of 1.0 M Glucose Over Activated Carbon Catalyst in Supercritical Water at 600°C

WHSV=3.7 (g/hr) /g WHSV=9.3 (g/hr)/g

Gas Product Yield* 34.5 MPa 25.5 MPa

H. 1.73 1.18

CO 1.32 2.47 co ₂ 2.54 1.30

CH ₄ 1.10 0.86

C ₂ H ₄ 0.001 0.0081

C ₂ H _S 0.33 0.25

C ₃ H ₆ 0.005 0.014

C ₃ H _β 0.19 0.012

Carbon Conversion" 103% 92%

WHSV = weight hourly space velocity in g glucose/hr-g catalyst Catalyst = coconut shell activated carbon * gas yield = mole of gas/ mole of reactant ^b carbon conversion = mole of carbon in the gas/ mole of carbon in the reactant

Table VI sets forth results of the gasification of cellobiose, a dimer of glucose. Superior hydrogen yield and carbon conversion were obtained during the first three hours of operation, after which some catalyst deactivation was observed.

Table VI

Gasification of Cellobiose Over Activated Carbon Catalyst

In Supercritical Water at 600°C, 34.5 MPa

0.235M cellobiose with 2.975 g activated carbon catalyst

WHSV=1.92 <g/hr)/g

Time on Stream (minutes) 53 192 237

Product Gas Yield*

H ₂ 5.22 2.88 2.47

10 CO 1.59 1.83 1.49 co ₂ 7.30 6.62 5.53

CH ₄ 1.87 1.96 1.40

C ₂ H ₄ 0.001 0.003 0.002

C ₂ H _β 0.47 0.48 0.34

15 C ₃ H ₆ 0 0 0

C ₃ H _β 0.19 0.20 0.12

Carbon Conversion"

102% 100% 79%

WHSV = weight hourly space velocity in g glucose/hr-g catalyst Catalyst = coconut shell activated carbon

20 * gas yield = mole of gas/ mole of reactant

" carbon conversion = mole of carbon in the gas/ mole of carbon in the reactant

Table VII sets forth results of gasification of other biomass feedstocks, depithed bagasse liquid extract and water 25. hyacinth. Nearly a third of the product from the bagasse extract was hydrogen and carbon conversion was high.

Table VII

Gasification of Depithed Bagasse Liquid Extract and of Water Hyacinth

Over Activated Carbon Catalyst in Supercritical Water at 600°C, 34.5 MPa

Table VIII sets forth results of gasification of sewage sludge. As can be seen, carbon catalysts can be effective in the thermal decomposition of such materials.

Table VIII

Gasification of Sewage Sludge Over Activated Carbon Catalyst

In Supercritical Water at 600*0, 34.5 MPa

28 g/1 Sewage Sludge With 2.96 g Catalyst WHSV = 0.5 (g/hr)/g

Product Gas Yield" Mole Fraction ^b

H ₂ 2.7 33

CO 3.5 2.9

C0 ₂ 66.2 36

CH ₄ 16.3 24

C ₂ H ₄ 0.05 0.04

C_H ₆ 7.7 5.7

C ₃ H ₆ 0.3 0.15

C ₃ H _β 1.7 0.89

Total Gas Yield ⁰ 98.4% --

WHSV = weight hourly space velocity in g glucose/hr-g catalyst

Catalyst = coconut shell activated carbon

* gas yield = gram of gas/ gram of reactant ^b mole fraction = mole of gas/total mole of gas in effluent ^c Total Gas Yield = grams of all gases/gram of reactant

Table IX shows that carbon-containing catalysts as used in a method according to the present invention effectively treat organic wastes such as methanol, methyl ethyl ketone, ethylene glycol and acetic acid. Hydrogen yields and carbon conversion were very high.

Table IX Catalytic Destruction of Representative Military Wastes over 2.824g Activated Carbon Catalyst in Supercritical Water at 600°C, 34.5 MPa

1.0M 0.1M Methyl 0.1M Ethylene 0.1M Acetic

Methanol Ethyl Ketone Glycol Acid

WHSV 0.76 0.17 0.16 0.14 (g/hr) /g)

Gas Product Yield*

H ₂ 1.61 2.97 1.27 1.08

CO 0.23 0.53 0.29 0.19 co ₂ 0.42 0.61 0.78 0.96

CH ₄ 0.22 1.10 0.61 1.02

C ₂ H ₄ 0.0 0.0 0.0 0.0

C ₂ H _β 0.0 0.28 0.14 0.01

C ₃ H ₆ 0.0 0.0 0.0 0.0

C ₃ H _β 0.0 0.01 0.0 0.0

Carbon 88% 71% 98% 111% Conversion"

Liquid Product

Unconverted 6.8% 2.2% 0 0 Reactant

WHSV = weight hourly space velocity in g waste/hr-g catalyst Catalyst = coconut shell activated carbon * gas yield = mole of gas/mole reactant ^b carbon conversion = mole of carbon in the gas/mole of carbon in the reactant

While the present invention is disclosed by reference to the preferred embodiments and examples set forth above, it is to be understood that these examples are intended in an illustrative rather than a limiting sense. It is contemplated that modifications will readily occur to those skilled in the

art, which modifications will be within the spirit of the invention and within the scope of the appended claims.