Oxidative Hydrothermal Dissolution (OHD): An efficient,

environmentally friendly process for the dissolution of coal and

biomass in aqueous media, for the production of fuels and chemicals.

Ken B. Anderson1,2,, John C. Crelling1,2 , William W. Huggett2, Derek Perry1,2, Tom Fullinghim2, Patrick

McGill2, Paul Kaelin2,

1. Thermaquatica Inc. Carbondale, IL 62901,

2. Department of Geology, Southern Illinois University Carbondale, Carbondale, IL 62901

Abstract:

Oxidative Hydrothermal Dissolution (OHD) is a novel coal conversion technology developed

with support from the Illinois Clean Coal Institute. OHD works by reaction of coal and/or

biomass with small amounts of oxygen in high temperature, high pressure, liquid water. This

breaks up the coal’s structure, resulting in the generation of low molecular weight, water

soluble products. Complete conversion of the coal is readily achievable with 70-90% recovery of

the original carbon as water soluble products. Most silicate minerals present in the coal pass

through the process essentially unaltered. Raw OHD product is an aqueous solution (not a

colloid or suspension) consisting of a mixture of low-medium molecular weight aromatic and

aliphatic acids and related derivatives that could potentially supplement or replace some

petroleum–derived products as chemical feed stocks. Raw OHD product can be pumped and

refined using conventional liquid processing technology.

Since the process uses only water and oxygen, it is inherently environmentally friendly. No

exotic solvents or expensive catalysts are required. It produces little CO2, and no NOx, SOx or

other toxic emissions. Harmful elements like mercury, arsenic etc., are not released to the

environment but either remain associated with their parent minerals or are retained in the

product solution and can be processed and captured by conventional waste water treatment

strategies.

Author to whom correspondence should be directed at kanderson@thermaquatica.com

Illinois Coal

Introduction

Coal is an enormously abundant resource Coal is a macromolecular organic solid. Its structural

characteristics vary with rank and maceral composition, but it can be generally described as consisting of

aromatic clusters (consisting of variable numbers of aromatic rings) linked together with aliphatic and

ether bridges, within which is occluded variable amounts of low MW materials. In most cases coal also

includes variable amounts of inorganic materials present as either discrete mineral phases or as

exchanged cations. Because of its nature it cannot easily be refined and is primarily used as a solid fuel

for energy production. The value of coal as a resource could be considerably increased if methods to

disrupt its macromolecular structure could be developed.

Various strategies, including: pyrolysis, gasification and direct liquefaction, have been attempted with

the goal of breaking up the macromolecular structure of coal to produce low MW products that can be

refined into various types of higher value products, including liquid fuels and chemicals. Oxidative

Hydrothermal Dissolution (OHD) is a novel, and highly effective, approach to achieving this goal1. OHD

works by reaction of the coal, or other macromolecular organic solid, with small amounts of O2, in liquid

water at elevated temperatures and pressures. This results in oxidative cleavage of reactive structures

in the coal, producing a suite of low MW organic products that are soluble in water. Conversion is

readily taken to completion in reasonable reaction times with 70-90% recovery of the original carbon as

water soluble products that can be refined into a variety of useful low MW products.

Experimental

Figure 1 illustrates a schematic of the experimental system used for micro-scale testing of OHD. This

reactor system is a semi-continuous design in which a fixed charge of coal (typically 10-100mg, 20-60

mesh) can be subject to OHD under precisely controlled conditions. An additional fully continuous OHD

reactor, illustrated in Figure 2 has also been constructed and successfully demonstrated.

For reasons of experimental convenience, O2 in these reactions is typically produced in situ by thermal

decomposition of H2O2. Testing has also be done using dissolved O2 with identical results but at the

scale used for these experiments, reproducible generation of solutions of dissolved O2 is experimentally

cumbersome. The reaction is typically carried out at temperatures of 220-350oC with oxidant loading of

0.002 – 0.01 M O2. Flow rates are varied according to experimental requirements. Reaction products

can be monitored by photodiode array (PDA) detection and/or can be collected for off-line analysis by

GC-MS and other techniques.

Figure 1. (overleaf) Schematic of semi-continuous micro reactor system used for testing and evaluation

of OHD of coal and other organic solids.

Figure 2. (overleaf) Schematic of continuous OHD micro reactor system built and operated at SIU.

Figure 1.

Figure 2.

Results and Discussion

The raw product (liquor) derived from OHD of bituminous coal is a clear solution of dissolved organic

products. In most cases colloidal solids are absent, as illustrated in Figure 3, below which shows both

raw OHD liquor derived from dissolution of Illinois coal and a diluted product prepared by addition of

water to the same raw liquor. The diluted product is a clear solution and does not contain suspended

solids

Figure 3. Raw and diluted OHD product derived from Illinois coal

Formation of OHD product is not the result of simple hydrolysis. Figure 4 illustrates PDA response as a

function of time for an experiment in which oxidant is discontinuously delivered as six equal pulses of

oxidant to a fixed charge (~10 mg) of Illinois No. 6 coal. Detector response is ~0 prior to delivery of

oxidant indicating that simple hydrolysis of the coal is insignificant at these conditions over the period of

the experiment. Response increases rapidly at the beginning of each pulse and decreases rapidly when

oxidant is discontinued, indicating that production of dissolved product is directly related to the delivery

of O2 and that response to delivery of oxidant is rapid. No soluble or insoluble catalyst is used and no

co-solvent is present.

Application of this process to a wide range of coal, biomass, and similar macromolecular organic solids

(including various types of lignocellulosic biomass, lignite, bituminous coal, anthracite and wood

charcoal) has been evaluated. In all cases, complete conversion of organic materials to soluble products

is readily achieved, although rates of reaction vary considerably. Petrographic analyses demonstrate

that dissolution of coal proceeds by etching of particle surface, as illustrated in Figure 5, which illustrates

photomicrographs of Anthracite before and after being subject to OHD conditions (In this case

dissolution was stoped before being taken to completion)

Figure 4. PDA response for OHD of IL #6 coal, 6 pulses of oxidant delivered. Time range illustrated = 65

minutes. 10 min start delay for baseline followed by 6X 1 min oxidant pulse with 6 min between pulses.

Figure 5. Petrographic analysis comparing (A) raw Anthracite and (B) residue after partial dissolution by

OHD.

As expected,Reaction rate is dependent on particle size, reaction temperature, oxidant loading and flow

rate/contact time, as well as varying with initial substrate, but is typically of the order of minutes for

complete dissolution for -20 +60-mesh bituminous coal. In general, low rank materials react faster than

high rank materials, (presumably due to the more poly-condensed nature of the high rank materials),

and macerals react in order liptinite>vitrinite> inertinite.

These data suggest that the process works by oxidative cleavage of labile structures, resulting in

disruption of the overall macromolecular structure. As low MW products are produced they dissolve

into the reaction medium (water), which at hydrothermal conditions is an excellent solvent for most

organics, and are separated from residual solid, thereby exposing fresh surface for subsequent reaction

with additional oxidant. Rapid removal of the water and separation of the produced organic solute or

quenching prevents over-oxidation of the dissolved product.

For most raw solids, 70-90% of the initial carbon is recovered as solublized products at optimal reaction

conditions. Minor amounts of gaseous products (CO and CO2) are also generated, with CO typically

dominating. No gaseous N or S oxides are generated. Inorganic N and S are retained in the aqueous

phase as sulfate and nitrate respectively. Organic S is at least partially retained as soluble organo-sulfur

compounds in the product liquor.

Characterization of the solublized products indicates that these consist of moderately complex mixtures

of low MW organics. For bituminous coal, these consist predominantly of:

(i) aliphatic carboxylic acids and diacids from C1 to ~ C20 (in many cases acetic acid is the single

most abundant product obtained and may account for up to 5~% of the raw product, depending

on the initial feedstock)

(ii) mono aromatic carboxylic acids, polyacids and phenols, including methoxylated analogues.

An example product distribution derived from Illinois coal is illustrated in Figure 6. The exact distribution

of products obtained is dependent on the nature of the starting material used. For humic coals, lignites

tend to give products dominated by simple aliphatic acids and diacids and monocarboxylic aromatic

acids/phenols whereas OHD products derived from higher rank coals are generally dominated by

aromatic products including di- and poly- carboxylic acids and related analogues.

As expected, biomass derived OHD products tend to be simpler than those derived from coals. the

distribution of compounds observed in the OHD product derived from soft wood lignin is illustrated in

Figure 7. This product is dominated by three related compounds all derived from oxidative cleavage of

guaiacyl lignin. Two of these are partial oxidation products and the major product, m-methoxy, p-

hydroxy benzoic acid (observed in these analysis as its fully methylated analogue for analytical

convienience) reflect complete oxidation of the C3 side chain of the original lignin.

Figure 6. Distribution of products identified in OHD product derived from Illinois #6 coal. Individual

products are listed in the table below:

ID Assignment

B 1,4-butanedioic acid

P 3-methoxy benzoic acid

V 1,2-benzene dicarboxylic acid

Z thiophene-2,5-dicarboxylic acid

AA 3,5-dimethoxy benzoic acid

BB 3,4-dimethoxy benzoic acid

CC methoxy benzene dicarboxylic acid (isomer undetermined)

GG 1,2,3-benzene tricarboxylic acid

HH 1,2,4-benzene tricarboxylic acid

II dimethoxy benzene dicarboxylic acid (isomer undetermined)

LL Unknown

MM Benzene tetracarboxylic acid (isomer undetermined)

Mass spectrometric and chromatographic analyses both indicate that high (>~500 amu) MW products

are absent in freshly prepared OHD products. If allowed to stand for extended periods, some

precipitation does occur, presumably due to oxidative coupling (e.g. phenolic coupling) of the primary

products. Raw liquor is also subject to growth of biomass if not kept completely sterile.

Silicate minerals are generally unaffected by OHD conditions and are retained unaltered in the reaction

residue. Pyrite undergoes rapid oxidation (analogous to pyrite weathering related to acid mine

drainage) with S released as aqueous sulfate and Fe re-precipitated as a mixed Fe(O)(OH) phase

functionally equivalent to Goethite. Most inorganic elements are retained in the solid phases with

which they are initially associated in the raw coal and the remainder is retained in the aqueous phase

with the solublized organic product.

Figure 7. OHD product of soft wood (conifer) lignin. . Inset upper trace is an enlargement of time 40-57

minutes of the lower chromatogram, included for clarity. Structures of major products are indicated.

Conclusions

Oxidative Hydrothermal Dissolution is a novel conversion strategy for the efficient conversion of coal

and other macromolecular solid organic materials to low MW water soluble organic products by

reaction with small amounts of molecular oxygen in subcritical (liquid) water at temperatures of ~ 200-

370oC . The process is simple and does not require use of exotic catalysts or solvents other than water.

Complete dissolution of the initial coal or other macromolecular organic solid with recovery of 70-90%

of the initial carbon as dissolved products is readily achievable in most cases. The process is robust and

widely applicable to a broad range of substrates.

Acknowledgement. The author gratefully acknowledges the Illinois Clean Coal Institute (ICCI) for their

generous support of the work described herein.

1. Ken B. Anderson, J.C. Crelling and W.W. Huggett

PCT/US2010/023886 Process for the dissolution of coal, biomass and other organic solids in superheated

water. 2010