Particles of ash
Explanation:
Sulfur is present in the coal and enters while the coal is formation under the crust in variety of forms like sulfate, sulfides, etc.
The burning of coal in the air turns this sulfur to oxidize to form sulfur dioxide. The reaction is shown below.
This gas when aided by the UV light and the particles of ash and dust which are released from burning pf coal to form sulfur trioxide gas.
This reaction of sulfur dioixde, with the fly ash involves series of the reactions which lead for formation of sulfur trioxide,
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