Unburnt combustibles escaping a furnace – Key determinants: The fraction of combustibles leaving a furnace unconsumed primarily depends on which of the following factors?

Introduction / Context:
Incomplete combustion wastes fuel and increases CO/HC emissions, soot, and product variability. Understanding the drivers of unburnt loss helps optimize combustion tuning and thermal efficiency.

Given Data / Assumptions:

• Furnace firing with gaseous or liquid fuels (similar principles apply to pulverised coal).
• Steady operation with adjustable air/fuel ratio and burner settings.

Concept / Approach:
Completeness of combustion is a function of chemistry (air/fuel ratio, temperature) and mixing (turbulence, residence time). Air preheat influences ignition kinetics; flow patterns inside the furnace affect mixing and residence time distribution, while burner design dictates primary turbulence and jet penetration.

Step-by-Step Solution:
Air and temperature: too little air or too low temperature increases unburnt species.Burner design: poor atomisation/mixing or stratified jets cause pockets of unreacted fuel.Air preheat and gas motion: higher preheat improves ignition; induced mixing and velocity changes enhance reaction completion.All listed factors contribute materially; select the comprehensive option.

Verification / Alternative check:
Stack analysers show CO and unburnt hydrocarbons fall after tuning excess air, swirl, and staging while maintaining sufficient temperature and residence time.

Why Other Options Are Wrong:
Each single factor alone is insufficient; real furnaces require all three to be tuned for minimal loss on ignition.

Common Pitfalls:
Over-firing with excessive excess air to mask poor mixing (causing heat loss); ignoring burner maintenance leading to mal-atomisation.

Final Answer:
All (a), (b) and (c)