Бурение грунтовых зондов, установка энергетических колодцев

Ashes can cause a variety of problems particularly in up or downdraught gasifiers. Slagging or clinker formation in the reactor, caused by melting and agglomeration of ashes, at the best will greatly add to the amount of labour required to operate the gasifier If no special measures are taken, slagging can lead to excessive tar formation and/or complete blocking of the reactor. A worst case is the possibility of air-channelling which can lead to a risk of explosion, especially in updraught gasifiers.

Whether or not slagging occurs depends on the ash content of the fuel, the melting characteristics of the ash, and the temperature pattern in the gasifier. Local high temperatures in voids in the fuel bed in the oxidation zone, caused by bridging in the bed, may cause slagging even using fuels with a high ash melting temperature.

In general, no slagging is observed with fuels having ash contents below 5-6 percent. Severe slagging can be expected for fuels having ash contents of 12 percent and above. For fuels with ash contents between 6 and 12 percent, the slagging behaviour depends to a large extent on the ash melting temperature, which is influenced by the presence of trace elements giving rise to the formation of low melting point eutectic mixtures.

For gasification purposes the melting behaviour of the fuel ash should be determined in both oxidating and reducing atmospheres.

As far as ash content is concerned, raw wood and wood charcoals seldom present problems, the ash content being normally from 0.75 to 2.5 percent. However, in a number of tropical woods (22) charcoal ash contents may be much higher and those charcoal types are unsuitable for gasification purposes. Table 2.5 lists agricultural residues which have been tested with respect to their slagging properties in a small downdraught laboratory gas producer (19).

Up and downdraught gasifiers are able to operate with slagging fuels if specially modified (continuously moving grates and/or external pyrolysis gas combustion). Cross draught gasifiers, which work at very high temperatures of 1500° C and above, need special safeguards with respect to the ash content of the fuel. Fluidized bed reactors, because of their inherent capacity to control the operating temperature, suffer less from ash melting and fusion problems.

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