Ash melting is a complex process where shrinkage, sintering and swelling can occur. The test method specified in the standards provides information about fusion and melting behaviour of the composite inorganic constituents of the fuel ash at high temperatures.

The test method is empirical. The ash used for the test is a homogeneous material, prepared from the fuel and the determination is performed at a controlled rate of heating in a controlled atmosphere. In contrast, under full-scale conditions, the complex process of combustion and fusion involve heterogeneous mixtures of particles, variable heating rates and gas compositions.

The terms ash fusibility and ash softening are synonymous with ash melting.

The published standards require that the temperature is measured and recorded at 4 points: the initial rounding of the test piece, when the test piece has softened, when melted to a hemisphere and finally when it flows into a puddle.

The Furnace

The CAF furnace is designed to heat ash samples up to a maximum temperature of 1600oC in a controlled atmosphere and visually record the fusion of the samples for analysis. At these temperatures, there is good contrast between the sample and its background.

Biomass and combined (biomass/mineral) fuels frequently exhibit much lower fusion temperatures. The biomass option for the CAF digital overcomes this by the addition of an illumination system to provide improved image clarity at lower temperatures. For applications using the smaller cylindrical sample, the test piece mould will be supplied as standard from April 2010.

The furnace work tube is sealed and has a 'fail safe' gas control system for safety when testing with toxic and flammable gases. A sliding and rotating door mechanism provides unrestricted access to the work tube, allowing easy loading and unloading of the test samples using the sample loading tool provided. The large diameter work tube can accept up to 12 samples at any one time. The use of lightweight insulation allows the furnace to cool quickly, permitting multiple tests to be completed during the day.

An optional zoom lens (additional cost) is recommended for viewing up to 3 of the 3-5mm high samples of the following standards:

  DD CEN/TS 15404:2006 Solid recovered fuels - Methods for the determination of ash melting behaviour by using characteristic temperatures
  Final Draft prCEN/TS 15370-1 October 2005 Solid biofuels - Methods for the determination of ash melting behaviour - Part 1: Characteristic temperatures method

The start and finish temperatures can be pre-set and the rate of temperature rise is adjustable within the parameters of the relevant published standards. Independent overtemperature protection is provided by a separate temperature controller using its own thermocouple and contactor to override the heating circuit.

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