Biogasification in Supercritical Water

Renewable sources of energy have been proposed as an alternative that may decrease the dependence of society on fossil fuels and contribute to avoid emissions of greenhouse gases such as CO2. In some cases, it also may be a form of residue disposal. Biomass, usually in the form of either energy crops (willow tree, wood chips) or agricultural wastes (sugarcane bagasse, rice husk) has the potential to generate fuel gases such as hydrogen, carbon monoxide and methane, through the thermal treatment known as gasification. Gasification, though, has been facing several technical difficulties that prevent its utilization in large scale. Among these difficulties, one can mention the efficiency loss due to the need of evaporating moisture of wet biomass, and the significant amount of particulates and tar formed in the process, which are carried by the gas.

Water at supercritical conditions has been shown to have the ability to dissolve organic compounds. In Supercritical Water Gasification, water at temperature and pressure above the critical point is used as solvent for biomass. The advantages of this approach are:

Since the solvent is water, thermal efficiency is not affected by biomass humidity;

A hydrogen rich gas can be produced by driving the water gas-shift reaction (CO + H2O ® CO2 + H2);

Reaction proceeds at an homogeneous medium, what tends to inhibit tar formation;

The product is compressed to about 30 MPa, avoiding additional work for compression of gases;

Hydrogen flammability is drastically reduced by water.

The main goal of my research is to evaluate the products and reaction kinetics of gasification, which is a fundamental tool for reactor design. The main challenge to reach this goal is to overcome the difficulties impose by the complex structure of biomass, which is a mixture of cellulose, hemicellulose and lignin. The chemical structures for these components are shown below.

Chemical structure of glucose (monomer for cellulose)

Chemical structures of the main components of hemicellulose

Chemical structure of the main components of lignin

We're currently performing experiments in batch reactors, using cellulose and lignin as model compounds in order to obtain a picture of how initial concentration, temperature and pressure affect product distribution and reaction kinetics. The gas product is analyzed in a GC with a thermal conductivity detector (TCD).

Future work planned for this project includes:

Evaluation of liquid product composition by High Pressure Liquid Chromatography (HPLC), leading to a better insight of the gasification process;

Experiments with biomass such as corn stover, waste wood and wheat straw;

Development of a slurry feeding system in order to perform gasification in flow reactors

Evaluation of catalyst performance (dissolved in the water, present in reactor walls and packed)

Development of a kinetic model able to describe SCWG