Following the train of components of Figure 2.1 and given the potential options for gasification, gas cleaning and conditioning, synthesis and separation, many routes to produce methanol from biomass can be imagined. The authors have previously analyzed the techno-economic performance of methanol from wood through 6 concepts, which will be recapitulated here. At the end of the section, results will be placed into broader perspective with other literature and with fossil gasoline and diesel.

Selection of Concepts

Some concepts chosen resemble conventional production of methanol from nat­ural gas, making use of wet gas cleaning, steam reforming, shift, and a solid-bed methanol reactor. Similar concepts have previously been analyzed by Katofsky (1993). Advanced components could offer direct or indirect energy benefits (liquid phase-methanol synthesis, hot gas cleaning), or economic benefits (autothermal reforming). Available process units are logically combined so the supplied gas composition of a unit matches the demands of the subsequent unit, and heat leaps are restricted if possible. The following considerations play a role in selecting concepts:

1. The IGT direct oxygen fired pressurized gasifier, in the normal and maximized H2 option, and the Battelle indirect atmospheric gasifier are considered for synthesis gas production because they deliver a medium calorific nitrogen undiluted gas stream and cover a broad range of gas compositions.

2. Hot gas cleaning is only sensible if followed by hot process units like reforming or (intermediate temperature) shifting. Hot gas cleaning is not applied after atmospheric gasification since the subsequent pres­surization of the synthesis gas necessitates cooling anyway.

3. For reforming fuel gas produced via an IGT gasifier, an autothermal reformer is chosen, because of higher efficiency and lower costs. The high hydrogen yield possible with steam reforming is less important here since the H2:CO ratio of the gas is already high. The BCL gasifier, however, is followed by steam reforming to yield more hydrogen.

4. Preceding liquid-phase methanol synthesis, shifting the synthesis gas composition is not necessary since the reaction is flexible toward the gas composition. When steam is added, a shift reaction takes place in the reactor itself. Before gas-phase methanol production the composi­tion is partially shifted and because the reactor is sensitive to CO2 excess, part of the CO2 is removed.

5. After the methanol passes through once, the gas still contains a large part of the energy and is expected to suit gas turbine specifications. The same holds for unreformed BCL and IGT gases, which contain energy in the form of C2+ fractions. When the heating value of the gas stream does not allow stable combustion in a gas turbine, it is fired in a boiler to raise process steam. The chemical energy of IGT+ gas is entirely in hydrogen and carbon monoxide. After once through meth­anol production, the gas still contains enough chemical energy for combustion in a gas turbine.

6. Heat supply and demand within plants are to be matched to optimize the overall plant efficiency.

7. Oxygen is used as oxidant for the IGT gasifier and the autothermal reformer. The use of air would enlarge downstream equipment size by a factor 4. Alternatively, oxygen-enriched air could be used. This would probably give an optimum between small equipment and low air sep­aration investment costs.

These considerations led to a selection of 6 conversion concepts (see Table 2.3). The six concepts selected potentially have low-cost and/or high-energy efficiency. The concepts are composed making use of both existing commercially available technologies, as well as (promising) new technologies.