J. M.N. VAN KASTEREN, Eindhoven University of Technology, The Netherlands

Abstract: This chapter discusses developments and possibilities in the field of alcohol production via synthesis gas based on biomass feedstocks. The most promising technologies based on biomass gasification are believed to be the catalytic and the biocatalytic routes. Biobased synthesis gas fermentation processes to methanol, ethanol and even butanol are being developed. The main bottleneck for these fermentation based processes are still the relatively low concentrations of alcohol in water (<5 wt%), which can be reached with bacteria. New alcohol-water separation processes are needed to make these processes become feasible.

Key words: gasification, bioalcohol, biocatalyst, fermentation, synthesis gas.

17.1 Introduction

Alcohol production via gasification is already a very well established process for methanol production. In fact most of the methanol produced nowadays is based on the catalytical conversion of synthesis gas (Ullmann’s Encyclopedia of Industrial Chemistry, 2001). The synthesis gas is being produced from fossil fuels, e. g. natural gas. For the higher alcohols these routes are not so common. Ethanol being the second largest alcohol produced is mainly produced via fermentation of sugars and for a smaller part via direct hydrolysis of ethylene. Research and developments are focused on production of ethanol directly from synthesis gas via a (bio)catalytical route making the synthesis gas route more interesting. Besides methanol and ethanol the most important alcohols are 1-propanol, 1-butanol, 2-methyl-1-propanol (isobutyl alcohol), the plasticiser alcohols (C6 — C11), and the fatty alcohols (C12 — C18), used for detergents. They are prepared mainly from olefins via the oxo synthesis, or by the Ziegler process (Ullmann’s Encyclopedia of Industrial Chemistry, 2001).

The aim of this chapter is to enlighten and discuss more the developments and possibilities in the field of alcohol production via synthesis gas based on biomass feedstocks. Environmental effects (greenhouse gas emissions), demand for independencies on fossil fuels and rising costs of fossil fuels have set an urge to diversify feedstocks and use biomass also as a chemical resource.

Since the 1970s the interest for the use of biomass as feedstock for chemicals and fuels has risen and resulted in an increase in the fermentation processes for ethanol especially as a fuel for automotive purposes. Also the increase in the use of biodiesel which contains 10 wt% methanol has increased the demand for biobased methanol.

Disadvantage of the fermentation routes of ethanol is that they can only convert sugar into ethanol, limiting the biomass feedstock from an economical and efficiency point of view to high yield sugar containing crops like sugar cane and sugar rich waste streams.

Gasification of biomass and subsequent conversion of the synthesis gas produced to alcohols would overcome these disadvantages. The problem with the present factories for methanol production is that they are based on a very large scale input (mainly natural gas) which means that very large amounts of biomass will have to be transported to one location. This is not economically attractive. Many options have been suggested such as to convert the biomass via digestion into methane which can be transported via pipelines to the factory. This means building up a pipe line infrastructure with subsequent high investment costs. Another route tried at a large methanol plant in Delfzijl in The Netherlands is to convert the glycerol byproduct from biodiesel production facilities into methanol via gasification (BioMCN, 2009). Disadvantage for this route is that there is a hydrogen shortage which has to be added from other sources.

The most promising technologies based on biomass gasification are believed to be the catalytic and the biocatalytic routes. The idea is in both case the same: convert synthesis gas via a (bio)catalyst into alcohols. Methanol is very difficult to achieve with biocatalyst because of its more toxic nature. For ethanol, this seems more promising and making smaller scale plants interesting (more fitting to the decentralised character of the biomass production process). Fermentation of the gasification product gas, however, is a rather new development.

Datar et al. (2004) have been working on the fermentation of producer gas, and have successfully produced ethanol. Figure 17.1 shows the schematic of the

biomass to ethanol process. The idea is to gasify the biomass to synthesis gas (CO + H2) and subsequently ferment this biosynthesis gas to ethanol via a direct fermentation process. The first step is to gasify the biomass input to synthesis gas.

The gasification/fermentation pathway is a very interesting alternative way of producing bioethanol. Via traditional fermentation processes, lignin, an important component of biomass cannot be fermented. Gasification and subsequent fermentation of the produced gas enables fermentation of all carbon and hydrogen containing material and also non biodegradable materials like plastics. The resulting higher feedstock efficiency should make the biobased, smaller scale processes economically feasible (Van Schijndel and Van Kasteren, 2004).