6.1 Design details

For a better understanding of the specific advantages of the ferrocement design, it is useful to distinguish criteria of constructive design (which can be applied in a metal construction as well) from material-specific criteria (which are typical for the application of ferrocement). It will become obvious that the ferrocement charcoal gasifier, as realized in its present form, is possibly just the first step in a different way of building gasifiers.

The gasifier of Robert Reines is a down draft gasifier with a straight cylindrical reaction zone-this is common to all conventional small charcoal gasifiers. Furthermore, it is of an ,,open core" type. That means, primary air for the reaction zone is not entering through nozzles (wall nozzles, central nozzle), but has a more or less unlimited access to the combustion zone. The open core principle has been applied so far mainly in rice husk gasifiers in down draft operation, where the flow of primary air is uniform over the complete cross section of the fuel bed. This is slightly different in the ferrocement charcoal gasifier discussed here, where the fuel bunker, closed by a top cover in a water seal, is sitting on the combustion zone. The primary air enters through a circular slit between cylinder wall and bunker rim (see fig.9). The application of this open core principle has an important impact on the technical reliability of the gasifier: The problem of disturbed bunker flow, often reported from nozzle gasifiers, is definitely reduced, as no nozzles disturb the downward flow of the fuel column. In addition to this, the uniform temperature distribution over the cross section of the combustion zone improves the flow characteristics. As an additional effect, no nozzles can melt or flake, an effect which requires subsequent repair work in nozzle gasifiers.

The reactor cylinder in the gasifier is easily accessible by removing the bunker. This is an advantage to the refractory lining in closed metal vessels of most conventional gasifiers as it is easier to replace the refractory cylinder by another one of different diameter in order to adapt it to the engine size. In order to avoid thermal ruptures of the refractory walls, the cylinder is preferably made of three rings of three blocks each. The thermal insulation to the outer wall is made of compacted rice husk ash. According to Reines, the ash begins to melt near the refractory cylinder surface and thus seals the slits between the bricks.

Table 1: Typical performance data of the ferroment gasifier (Mean values, ar = as received) Reaction cylinder: 300 mm diameter

The configuration of the reaction zone of the Reines gasifier is not determined by the application of ferrocement as main building material; it can be done in the same way in a metal gasifier [12]. It has been said already that the open core principle is not completely new in gasifier construction. It is, however, not common in small charcoal gasifiers, and it is not necessarily guaranteed that it is effective in that case. Therefore, as a first step it had to be tested wether this design was as good-or even better-than a nozzle gasifier in terms of effective conversion of solid fuel to combustible gas.

6.2 Performance data

In table 1, data are listed which were measured at the operation of a standardized gasifier in ferrocement construction, designed for a power range of 2-10 kW. These data are derived from test series, carried out in the laboratory of FLEUS (Bremen University). More detailled data sets can be derived from the project report for BMFT [ 12].

FIGURE

The performance data largely depend on the mechanical resp. electrical load, connected to the engine. The values in table I resulted when a 6-cylinder engine with 2.0 cylinder volume and a speed of 2,500 rpm was connected, linked to a 3-phase alternator powering an electrical load (stove plates).

Diagr. 1 and 2 show very clear that the "nominal operation" of the gasifier has to be defined clearly. In diagr. 1 the specific fuel consumption of the ferrocement gasifier is plotted for three load situations (2, 6 and 10 kW). At 10 kW (which is a bit below the maximum power of 12 kW, which can be reached with he 2 liter engine) the fuel consumption per kWh is much less than at the load 2 kW. This is reflected as well in the overall efficiency of the system (the relation of usable electric energy to the energy content of the fuel): The efficiency is much better on 10 kW than on 2 kW.

FIGURE

This tendency is obviously common to all types of gasifiers and has to be recognized:

A gasifier has a narrow range of optimum performance. A deviation from the optimum working condition results in less efficient use of fuel, less operational stability, and less economic viability.