Temperature has an important effect on the conversion, the product distribution, and the energy efficiency of an SCW gasifier, which typically operates at a maximum temperature of nearly 600 °C. The overall carbon conversion increases with temperature; at higher temperatures hydrogen yield is higher while methane yield is lower. Figure 7.7 shows the temperature dependence of gasification efficiency and product distribution in a reactor operated at 28 MPa (30-s residence, 0.6-M glucose) (Lee et al., 2002). We see that the hydrogen yield increases exponentially above 600 °C, while the CO yield, which rises gently with temperature, begins to drop above 600 °C owing to the start of the shift reaction (Eq. 5.52).

Gasification efficiency is measured in terms of hydrogen or carbon in the gaseous phase as a fraction of that in the original biomass. Carbon conversion efficiency increases continually with temperature, reaching close to 100% above 700 °C. Hydrogen conversion efficiency (the fraction of hydrogen in glucose converted into gas) also increases with temperature. It appears strange that at 740 °C, the hydrogen conversion efficiency exceeds 100%, reaching 158%. This clearly demonstrates that the extra hydrogen comes from the water, confirming that water is indeed a reactant in the SCWG process as well as a reaction medium.

Hydrothermal gasification of biomass has been divided into three broad temperature categories: high, medium, and low with their desired products (Peterson et al., 2008). Table 7.2 shows that the first group targets production of hydrogen at a relatively high temperature (>500 °C); the second targets production of methane at just above the critical temperature (~374.29 °C) but below 500 °C; and the third gasifies at subcritical temperature, using only simple organic compounds as its feedstock. The last two groups, because of their low-temperature operation, need catalysts for reactions.

Temperature (°C)

Hydrogen — — ■ Methane

— Carbon dioxide ——— Carbon monoxide

(a)

450 500 550 600 650 700 750 800

Temperature (°C)

Hydrogen —— Oxygen —— Carbon

(b)

FIGURE 7.7 Effect of temperature on gas yield (a), and effect of temperature on gasification efficiency (b). (Source: Adapted from Lee et al, 2002.)