Improving solid-fuel combustion efficiency: Which practice will <em>not</em> help achieve higher overall efficiency in firing solid fuels?

Introduction / Context:
Combustion efficiency reflects how much of the fuel’s chemical energy is converted into useful heat. In boilers and furnaces burning solid fuels, operational choices strongly influence stack losses, unburned carbon losses, and radiation/convection losses. Identifying counterproductive practices helps avoid energy waste.

Given Data / Assumptions:

• Standard solid-fuel firing in boilers/furnaces.
• Efficiency losses include flue-gas sensible heat, incomplete combustion, and moisture/ash effects.
• Heat-recovery equipment (economizers, air preheaters) is available.

Concept / Approach:

High flue-gas exit temperature means more sensible heat leaves the stack unrecovered, reducing efficiency. Conversely, optimizing particle size/moisture, employing efficient burners or stokers, and maintaining proper excess air improve mixing, burnout, and heat transfer—thus increasing efficiency. Lowering the stack temperature within dew point limitations is desirable.

Step-by-Step Solution:

Verification / Alternative check:

Boiler efficiency audits show strong sensitivity to exit-gas temperature; adding economizers/air preheaters lowers T_stack and recovers heat, demonstrating improved efficiency.

Why Other Options Are Wrong:

Fuel preparation, correct air supply, efficient firing, and heat recovery all contribute positively to efficiency by reducing losses and improving combustion completeness.

Common Pitfalls:

Reducing stack temperature below acid dew point can cause corrosion; air control should avoid both deficiency (CO/unburned) and excess (excess air loss). Balance is key.

Final Answer:

Keeping the flue-gas exhaust temperature very high