Fuel preheating practices in furnace operation: In standard industry practice, which of the following fuels are generally <em>not</em> preheated before entry to the furnace (air may be preheated separately)?

Introduction / Context:
Preheating combustion air is a cornerstone of furnace efficiency. However, preheating the fuel itself depends on fuel phase, safety, and practicality. Understanding which fuels are commonly preheated and which are not helps avoid unsafe practices and focus on the largest efficiency gains.

Given Data / Assumptions:

• “Standard practice” refers to typical industrial furnaces.
• Air preheat via recuperators/regenerators is common.
• Heavy fuel oils are often warmed for viscosity control, but gaseous and solid fuels are generally not preheated before the burner/grate.

Concept / Approach:
Liquid heavy oils require heating to reduce viscosity and enable proper atomization—this is fuel preheat. In contrast, gaseous hydrocarbon fuels are usually supplied at ambient temperature for safety and equipment simplicity; heating gas fuels poses explosion risks and adds limited benefit. Solid fuels (e.g., coal) are not practically preheated inline; instead, their drying or devolatilization occurs in the combustion zone or upstream processing, not as a controlled preheat into the furnace chamber.

Step-by-Step Solution:
Identify fuels typically preheated: heavy oils for atomization.Identify fuels typically not preheated: solids and gaseous fuels.Therefore, choose “Both (a) and (b)”.

Verification / Alternative check:
Burner and furnace manuals specify oil heaters, but gas trains omit fuel preheaters; solid fuel systems address drying/volatiles via combustion air distribution and bed design.

Why Other Options Are Wrong:
(a) only or (b) only: incomplete; both categories are generally not preheated.Neither (a) nor (b): incorrect, as industry practice avoids routine preheating of both solid and gaseous fuels.“Only heavy fuel oils”: this is the category that is preheated, not “not preheated.”

Common Pitfalls:
Confusing air preheat with fuel preheat. Air is widely preheated; fuel preheat is limited mainly to viscous liquid fuels for atomization, not to gas or solid fuels.

Final Answer:
Both (a) and (b)