5.1 What makes a gasifier expensive?

For a small farmer in a developing country, the investment risk of installing a gasifier is normally too high when compared to the expected savings. Apart from the mere investment costs, a gasifier causes operational costs which are mainly dependent on the fuel price: Even if the basic material is free, the general costs of preparing the raw fuel in a way that makes it suitable for gasifiers have to be taken into account. Thus it is even more important that the mere investment costs for the gasifier remain as low as possible.

The system costs for a gasifier depend on two aspects: The simplicity or complexity of the design, but also to a considerable extent on the materials used. Recent considerations have shown that the latter offers the key to an economic success.

The core of a gas generator is the reaction chamber, in which the thermochemical conversion of solid fuel into usable gas takes place. In the combustion zone, temperatures of around 1200 °C are reached, and in the adjacent reduction zone temperatures between 850 and 650 °C have to be expected. These combustion and reduction zones have to be surrounded by high-temperature resistant vessel walls, that is, special steels or (most commonly) fire-resistant refractory bricks.

The rest of a gasifier unit mainly consists of vessels and pipes, that is, containers for fuel and ashes, as well as containers and pipes for gas cleaning and cooling. These vessels are traditionally made of metal. Their manufacture requires sheet forming tools and welding equipment, drilling machines, thread cutting equipment and so on, as well as skilled workshop personnel. The resulting price is oriented on the regionally common costs for labour at a metal shop. Therefore a metal system is always relatively expensive. Even the fact that in India for instance, manufacturing can be done for a fraction of the costs (converted into DM) that are involved in Germany, does not change the situation that the average income of an Indian farmer is also only a fraction of the income of his German counterpart. Thus, specific investment costs of 200 DM per installed kW for a gasifier in India may correspond to 2000 DM/kW in Germany-both values are too high for economic attractiveness.

Within the last few years only one breakthrough in manufacturing costs for gasifiers has been outlined, and this is based on turning one's back to metal constructions. As the vessels and pipes do not have to withstand any heavy mechanical strain, metal constructions-derived from the manufacture of heating systems-do not constitute a necessity. It has long been known that gasifiers can be built from brick work, but this has often posed a problem, when trying to achieve gastightness. At the AIT Bangkok Robert Reines [9] took another road when adopting the ferrocement technique, known from water tank construction and ship building technology. With this technique (which mainly implies the construction of wire armatures and wire-mesh forms which are then plastered with mortar, see fig. 5-9) it is possible to produce cylinders, tubes, lids, and so on, in any desired shape. The costs for material are lowered drastically, as merely wire, wiremesh, cement, and sand are needed. The problem of temperature strain is solved in two ways: the reaction zone is fenced in by a heat-resistant brick cylinder, and a general cooling tub surrounds all vessels.

Based on the impressive presentation of the AIT ferrocement gasifier, given by Robert Reines on the First International Workshop on Small Scale Producer Gas Systems in Bremen, 1989, the project CHAR focussed its activities on this approach. The steps of work were

- to gather experience with the technique of ferrocement construction

- to evaluate the system costs under varying local conditions -to test the performance of the ferrocement gasifier as well as the performance of competitive designs -to develop criteria and recommendations for practical application.