Losses with Exiting Furnace Gases

Total flue gas loss, with excess air loss = (5.2) 

Evaluation of radiation loss through furnace cracks and other leaks is very difficult. The best policy is to deal with them by constant surveillance combined with immediate repair. Operators and maintenance persons must understand that they can only get worse, and will do so at accelerating rates. 

Sensible heat carried out of the furnace by the furnace gases (poc) is often the largest loss from high-temperature furnaces and kilns. It is evaluated by the available heat charts mentioned in section 5.1 100% — %available heat = %heat carried out through the flue. It can be reduced by careful air/fuel ratio control, use of oxy-fuel firing, and good furnace pressure control. 

Air/fuel ratio control also prevents excessive lean burning, which results in extra unused air passing through the furnace, absorbing heat, and carrying that heat out the flue, unabsorbed by the loads. Chapter 7 of reference 52 describes how a variety of air/fuel ratio control systems work and how to evaluate the savings from their use. 

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Equipment. All of the gasification experiments were conducted with the same apparatus employed in the earlier oxidation work and has been described in detail elsewhere ). The technique involved simultaneous measurements of mass loss (T6A) and exit gas compositions (gas chromatograph) in a vessel which behaved as an ideal back-mix reactor. All experiments were run under isothermal conditions. As before, powdered shale samples (200 mesh) of previously retorted oil shale from the Parachute Creek member in Colorado were suspended from an electrobalance and placed in a furnace. In this way continuous gravimetric readings were available to monitor the consumption of the char. The off-gases were analyzed on a Carle gas chromatograph equipped with a Carbosieve B column. 

Example 7.1 Given A car furnace (batch) 10 x 20 x 9 high inside is to heat 40 tons of steel loads from 60 F to 2250 F at a rate of 250°F per hour. Specific heat of steel, fromp. 275 of reference 52 is 0.165 Btu/lb F. Average flue gas exit temperature will be 2200 F. The fuel will be natural gas with 10% excess air. Average losses, in Btu/ft hr are roof 900, walls 500, door 1100, and car 600. 

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