Furnace Technology
Higher furnace temperature can not be achieved by use of a lean fuel gas in the furnace by

Increasing the draft in the furnace
Preheating the fuel gas
Oxygen enrichment of combustion air
Preheating the combustion air

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Furnace Technology
In a furnace employing forced draught as compared to induced draught,

None of these
Air is sucked in, so air leaks are more and hence the furnace efficiency is reduced
The fan operates hot and hence blades are liable to corrosion and erosion
Positive pressure exists in the furnace

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