INDEGINEOUSLY DEVELOPPED AFT FURNACE
FOR TEST OF ASH FUSION TEMPERATURE OF COAL
When Coal is used as a fuel in Boilers, ash formed after combustion of coal is required to be removed as powder residue. But the characteristic behaviours of the Coal Ash at high temperature region inside the boiler is very critical since the Ash, which melts at below the temperature prevailing in the boiler undergoes glass transitions and fuses to form slag on the wall and bottom of the boiler. It is therefore of vital importance to determine the Ash Fusion Temperature of the coal for determination of its suitability and efficiency of the boiler.
The Ash Fusion Temperature (commonly known as AFT) involves observation of four temperatures by viewing various stages of:
Initial Deformation Temperature (IDT):
This is reached when the tip of the mould first become rounded. It is accepted as the temperature at which the ash.
This is reached when the top of the mould takes on a spherical shape (As per ASTM designation – Height is equal to the base).
This is reached when the entire mould takes on a hemisphere shape (As per ASTM designation – Height is half of the base).
This is reached when the molten ash collapses to a flattened button on the furnace floor (fluid state).
MSK have developed the facility at its Central laboratory at Kolkata for carrying out the test of Ash Fusion based on ASTM D 1857-04. The equipment was developed indigenously and tests are carried out under oxidizing atmosphere on imported coal.
Electrically heated Tube Furnace (Muffle and Refractory Tube) capable of rising the temperature under control upto 1450°C for Imported Coal Ash, Mould made of brass (19mm height, width of the base 6.4mm), Supporting plate made of refractory material for supporting the cone, Digital PID controller, Eye piece for observation of the Cone deformation at various stages.
Preparation of Cone:
Ash is first prepared from the Coal Ash from 60 Mesh size coal at 800 – 9000C. About 3-5 gm ash is required for preparation of cone. Ash is powdered to 200 mesh by grinding. The same is further dried at 8200C for 1.5 hrs to ensure complete oxidation. The ignited ash is moistened with a solution of dextrin. Then it is pressed into cone mold to form triangular pyramid.
Observation in furnace:
Cone is placed inside Tube Furnace on supporting base and temperature is raised till it reaches about 11500C. The temperature is controlled at this stage to obtain a gradual rise of temperature and the shape of the cone is monitored through the sight glass. Physical Observation of deformation of cone with respect to temperature is recorded at an interval of 100C. The Ash undergoes Initial Deformation(IDT) of its tips followed by Softening Temperature(ST), Hemispherical Temperature(HT) and reaches Fluid Temperature(FT). The four stages of the cone under high temperature condition are recorded.
The Temperature recorded for tests conducted for some Imported Coal Ash were observed to have attained IDT upto FT between the range of 1220 °C and 1290°C.
The Ash Fusion characteristic of coal is directly related to its composition primarily SiO2, Al2O3, CaO, Fe2O3. Under oxidizing condition, the three component system of SiO2-Al2O3-CaO and SiO2-Al2O3-Fe2O3. The alkalis present in the form of oxides as K2O, Na2O also contribute to the fusion temperature of the ash as base component in separate three component system, when present in considerable amount. It is also believed by that under reducing conditions (CO/CO2), the base component of Iron exist as FeO. Apart from that other reductions in the system also occur reducing the AFT to a lower value.