Overview

Methanol is produced by a catalytic reaction of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2). These gases, together called synthesis gas, are generally produced from natural gas. One can also produce synthesis gas from other organic substances, such as biomass. A train of processes to convert biomass to required gas specifications precedes the methanol reactor. These processes include pretreatment, gasification, gas cleaning, gas conditioning, and methanol synthesis, as are depicted in Figure 2.1 and discussed in Sections 2.2-2.6.

Pretreatment

Chipping or comminution is generally the first step in biomass preparation. The fuel size necessary for fluidized bed gasification is between 0 and 50 mm (Pierik et al. 1995). Total energy requirements for chipping woody biomass are approx­imately 100 kJe/kg of wet biomass (Katofsky 1993) down to 240 kWe for 25-50 tonne/h to 3 x 3 cm in a hammermill, which gives 17-35 kJe/kg wet biomass (Pierik et al. 1995).

The fuel should be dried to 10-15% depending on the type of gasifier. This consumes roughly 10% of the energy content of the feedstock. Drying can be

FIGURE 2.1 Key components in the conversion of biomass to methanol.

done by means of hot flue gas (in a rotary drum dryer) or steam (direct/indirect), a choice that among others depends on other steam demands within the process and the extent of electricity coproduction. Flue gas drying gives a higher flexibility toward gasification of a large variety of fuels. In the case of electricity generation from biomass, the integration in the total system is simpler than that of steam drying, resulting in lower total investment costs. The net electrical system effi­ciency can be somewhat higher (van Ree et al. 1995). On the other hand, flue gas drying holds the risk of spontaneous combustion and corrosion (Consonni et al. 1994). For methanol production, steam is required throughout the entire process, thus requiring an elaborate steam cycle anyway. It is not a priori clear whether flue gas or steam drying is a better option in methanol production. A flue gas dryer for drying from 50% moisture content to 15% or 10% would have a specific energy use of 2.4-3.0 MJ/ton water evaporated (twe) and a specific electricity consumption of 40-100 kWh^twe (Pierik et al. 1995). A steam dryer consumes 12 bar, 200°C (process) steam; the specific heat consumption is 2.8 MJ/twe. Electricity use is 40 kWh/twe (Pierik et al. 1995).