(374av) Supercritical Water Gasification of Switchgrass Biochar

Biochar produced via hydrothermal carbonization process is a high density (coal-like) powder. It is viewed as an attractive feedstock for biomass utilization in gasification process due to almost uniform C, H, and O composition, fine particle size and reduced moisture retention capacity (i.e. hydrophobicity). Switchgrass, a major energy crop, was used in this study for producing biochar. Oxygen to carbon (O/C) ratio in hydrothermally produced biochar from switchgrass at 300°C is typically 0.32-0.33 and can be represented by a general formula (CH1.2O0.2)n. In this work, biochar is gasified in supercritical water.

Gasification of carbonaceous matters into fuel gases (synthesis gas, producer gas) followed by FT synthesis is a promising route to produce renewable fuels. The gasification is commonly accomplished via partial oxidation of the feedstock using sub-stoichiometric (insufficient) air or oxygen or by indirect heating (with or without steam). Typically, gasification is performed using relatively dry feedstock (moisture < 10 wt%) at temperatures higher than 750°C under atmospheric pressure condition.

Supercritical water (above 374°C and 22 MPa) gasification can utilize wet feedstock and have high gasification efficiency at comparatively low temperature (400-700°C). The use of water in supercritical condition has several advantages over the atmospheric pressure air / steam gasification. Supercritical water works both as reactant and as reaction medium. Density and dielectric constant of the water medium play major role in solubilizing organic compounds. Dielectric constant of water decreases from 78.5 at 25°C to 5 in the near critical region, which enhances the solubility of organic compounds.1 The homogeneous reaction medium with a minimal mass transfer resistance favors decomposition of organic compounds into gases, decreasing formation of tar and char.2 Furthermore, the fuel gas is produced at high pressure directly, which means a smaller reactor volume and a lower energy to pressurize the gas in a storage tank. The inorganic constituents of biochar which are not volatile and are expected to remain in the aqueous solution. This makes the resulting syngas gas cleaner and less corrosive compared to the conventional dry processes with salt-rich biomass.

In this study, supercritical water gasification of biochar into syngas was investigated in a batch and semi-continuous reactor in the temperature range of 400-600°C. The initial study at 500°C in supercritical water showed nearly 50-60 wt% of carbon in biochar converted to gaseous products containing mainly CO, CO2, CH4 and hydrogen gas. The goal is to develop a process to produce high heating-value syngas with minimum solid residue from high molecular weight carbonaceous material. Alkali salts are used in this study to understand its catalytic effect on gasification and water-gas shift reaction.3 The experimental study is focused on understanding the chemistry of biochar gasification and determining the influence of temperature and alkali salt on the gas yields, gas composition and carbon conversion efficiency to lay the foundation for engineering application.

References:

1. Kumar, S.; Gupta, R. B., Hydrolysis of Microcrystalline Cellulose in Subcritical and Supercritical Water in a Continuous Flow Reactor. Industrial and Engineering Chemistry Research 2008, 47, (23), 9321-9329.

2. Calzavara, Y.; Joussot-Dubien, C.; Boissonnet, G.; Sarrade, S., Evaluation of biomass gasification in supercritical water process for hydrogen production. Energy Conversion and Management 2005, 46, 615-631.

3. Yip, K.; Tian, F.; Hayashi, J.-i.; Wu, H., Effect of Alkali and Alkaline Earth Metallic Species on Biochar Reactivity and Syngas Compositions during Steam Gasification. Energy & Fuels 2010, 24, 173-181.