7.1 INTRODUCTION

In the mid-1970s Sanjay Amin, a graduate student working at the Massachu­setts Institute of Technology (MIT), was studying the decomposition of organic compounds in hot water (steam reforming):

While conducting an experiment in subcritical water, he observed that in addi­tion to producing hydrogen and carbon dioxide, the reaction was producing much char and tars. Herguido et al. (1992) also made similar observations in the steam gasification of biomass at atmospheric pressure.

Sanjay interestingly noted that when he raised the water above its “critical state,” the tar that formed in the subcritical state disappeared entirely (Amin et al., 1975). This important finding kick-started research and development on supercritical water oxidation (SCWO) for disposal of organic waste materials (Tester et al., 1993), which has now become a commercial option for disposal of highly contaminated organic wastes (Shaw and Dahmen, 2000).

Biomass in general contains substantially more moisture than do fossil fuels like coal. Some aquatic species, such as water hyacinth, or waste products, such as raw sewage, can have water contents exceeding 90%. Thermal gasifica­tion, where air, oxygen, or subcritical steam is the gasification medium, is very effective for dry biomass, but it becomes very inefficient for a high — moisture biomass because the moisture must be substantially driven away before thermal gasification can begin; in addition, a large amount of the extra energy (~2260 kJ/kg moisture) is consumed in its evaporation. For example, Yoshida et al. (2003) saw the efficiency of their thermal gasification system reduce from 61 to 27% while the water content of the feed increased from 5 to 75%. So, for gasification of very wet biomass, some other means such as anaerobic digestion (see Section 2.2) and hydrothermal gasification in high — pressure hot water are preferable because the water in these processes is not a

liability as it is in thermal gasification. Instead it serves as a reaction medium and a reactant.

The efficiencies of these processes do not decrease with moisture content. For anaerobic digestion and supercritical gasification, Yoshida et al. (2003) found the gasification efficiency to remain nearly unchanged, at 31% and 51%, respectively, even when the moisture in the biomass increased from 5 to 75%.

A major limitation of anaerobic digestion is that it is very slow, with a rela­tively low efficiency and, most important, it produces methane only, no hydro­gen. If hydrogen is the desired product, as is often the case, an additional step of steam reforming the methane (CH4 + H2O = CO + 3H2) must be added to the anaerobic digestion process.

Hydrothermal gasification involves gasification in an aqueous medium at very a high temperature and pressure exceeding or close to its critical value. While subcritical water has been used effectively for hydrothermal reaction, supercritical water has attracted more attention owing to its unique features. Supercritical water offers rapid hydrolysis of biomass, high solubility of inter­mediate reaction products, including gases, and a high ion product near (but below) the critical point that helps ionic reaction. These features make super­critical water an excellent reaction medium for gasification, oxidation, and synthesis.

This chapter deals primarily with hydrothermal gasification of biomass in supercritical water. It explains the properties of supercritical water and the biomass conversion process in it. The effects of different parameters on SCW gasification and design considerations for the SCW gasification plants are also presented.