17.3.1 Introduction

This section describes the design of a small-scale mixed waste (biomass/plastics) gasifier with a synthesis gas fermentation-unit, which produces ethanol. Part of the synthesis gas will be used for heating up the gasifier and for making electricity (see Fig. 17.2).

Research has been carried out and a design has been made by Van Kasteren et al. (2005). This report shows an analysis of the fermentation of synthesis gas to ethanol process with the aid of bacteria. Before fermentation can take place first the biomass has to be gasified and the synthesis gas cleaned. The next section describes the choice of gasifier.

17.3.2 Choice of gasifier

A wide range of biomass fuels such as wood, charcoal, wood waste as well as agricultural residues — maize cobs, coconut shells, cereal straws, rice husks can be

used as fuel for biomass gasification. Theoretically, almost all kinds of biomass with moisture content of 5-30% can be gasified; however, not every biomass fuel leads to the successful gasification. Most of the development work is carried out with common fuels such as coal, charcoal and wood. Key to a successful design of a gasifier is to understand the properties and the thermal behaviour of fuel as fed to the gasifier. Gasification systems: descriptions are found in Chapter 16 and elsewhere (Reed and Gaur, 2000). Attention has to be given on tar (also known as creostate, is a sticky, condensable vapour whose main constituents are benzene, toluene, indene, naphthalene and phenol) minimisation during gasification. Its formation is affected by temperature, type of feedstock used and run time. Many gasifier designs produce so much tar that the gas clean up equipment cost is several times the gasifier cost. For the synthesis gas to be used in ethanol production, the level of tar has to be reduced to <50 ppm.

It is known that tar components have an influence on bacteria. A study of Ahmed et al. (2006) shows that the presence of tar inhibits the growth of the bacteria Clostridium carboxidivorans P7T during synthesis gas fermentation. However, the bacteria are not killed. After an adaptation period the bacteria start growing again. It appears that the bacteria are going to produce more ethanol and less acetic acid. So in this respect tar is even beneficial for the process. The work shows that certain amounts of tar do not cause problems for the synthesis gas fermentation process as long as enough adaptation time is taken into account. Still more research is needed to determine which tar concentrations are acceptable. Conclusion is that a gas cleaning system for removal of tars remains necessary although for synthesis gas fermentation processes this needs not to be so elaborate as for catalytic or for combustion applications.

The choice of the gasifier is mainly based on the gasifier requirements as stated before. The most important requirement is the great variety in feed the gasifier must be able to deal with. In the case of a gasification of bio waste the presence of impurities has to be taken into account. Another important issue in the feed — requirements of the gasifier is the size of the feeding material for the gasifier. For an entrained flow and/or a fixed bed gasifier, a relatively small feed size is necessary. These gasifiers need more elaborate grinding than the other type of gasifiers, although grinding is necessary for all gasifiers.

The gasifier configuration must be as simple as possible, in order to keep maintenance at a low level. Besides that it is desirable to use a technique, which is already proven and in which some experience is required. The gasifier has to be robust in order to be able to work with waste streams which differ in composition in time. The entrained flow and the fixed bed gasifiers are most sensitive for this change in feed compositions. Also quick stop and/or start up is more complicated with entrained flow and fixed bed systems.

Another demand set for the gasifiers is the synthesis gas quality. Because the produced synthesis gas will go to the fermentation unit in order to obtain ethanol, the quality and exact composition of the CO-H2 is not very important, although it is desirable to obtain a continuous composition of the synthesis gas.

Reviewing all the advantages and disadvantages of the different types of gasifiers, a Circulating Fluidised Bed Gasifier (CFB) seems the best choice:

• CFB has a simple design and requires low maintenance.

• CFB is able to tackle a large range of feed without requiring a strict feed-size.

• CFB can be scaled up to deal with larger amounts of feed.

• CFB has high efficiency because of the circulating ash.

• CFB is relatively easy to stop and start.

• CFB produces synthesis gas of reasonable quality.

• CFB is commercially available and already in use in several plants worldwide.