Difference Between Wobbe Index and Calorific Value

The key difference between Wobbe index and Calorific Value is that the Wobbe index indicates the interchangeability of fuel gases whereas the Calorific Value indicates the total amount of heat energy generated when we burn a unit mass of fuel.

The Wobbe index gives the relative ability of fuel gases to generate heat energy. The fuel gases that we talk about here are natural gas, liquefied petroleum gas, producer gas, etc. Calorific value, on the other hand, refers to the total energy generation of fuel. We usually give it for a unit mass of the fuel. However, according to the definitions of each term, they are two different values of fuels.

CONTENTS

1. Overview and Key Difference
2. What is Wobbe Index
3. What is Calorific Value
4. Side by Side Comparison – Wobbe Index vs Calorific Value in Tabular Form
5. Summary

What is Wobbe Index?

Wobbe index is an indicator of the interchangeability of fuel gases. The fuel gases include natural gas, liquefied petroleum gas (LPG) and town gas. On the other hand, this value measures the relative ability of fuel gases to deliver energy. For example, this value indicates whether a turbine will run on an alternative fuel source without any tuning or physical alteration.

Figure 01: We can use Wobbe index to compare the energy outputs of different fuel gases.

We can calculate the Wobbe index using the higher heating value (Vc) or higher calorific value and the specific gravity (Gs). The chemical symbol for Wobbe index is Iw. Then we can give the relationship between these three parameters in an equation as follows:

Iw    =   Vc / √Gs

This means the Wobbe index equals to the division of higher heating value from the square root of the specific gravity of the fuel. We can use this equation to compare the combustion energy outputs of different fuels gases with varying compositions that we use for the same appliance. The unit of measurement for this parameter is MJ/Nm3. For example, the wobbe index for natural gas is around 39 MJ/Nm³.

What is Calorific Value?

Calorific value is the total amount of heat energy that a unit mass of fuel can produce when we burn it completely. Therefore we can define it as the efficiency of fuel. We can use different units to measure this value. Ex: calories, kilocalorie, British thermal unit (BTU) and centigrade heat unit (CHU). When we consider the fuels containing hydrogen atoms, there are two different calorific values;

• Higher or gross calorific value
• Lower or net calorific value

The gross calorific value includes two values; when we completely burn a fuel containing hydrogen, it converts into steam, then if we cool it down to room temperature, the steam condenses into water. Therefore, gross calorific value includes both the heat liberated when we burn the fuel and the latent heat of condensation of steam.

However, under normal conditions, steam does not condense. It escapes along with hot gases produced there. Therefore. there is a lesser amount of heat available. This is what we call lower calorific value or the net calorific value.

What is the Difference Between Wobbe Index and Calorific Value?

Wobbe index is an indicator of the interchangeability of fuel gases whereas Calorific value is the total amount of heat energy that a unit mass of fuel can produce when we burn it completely. Therefore, this is the key difference between wobbe index and calorific value. When considering the units of measurement of each value, we can measure the Wobbe index using MJ/Nm3 while we measure the calorific value using calories, kilocalorie, British thermal unit (BTU) and centigrade heat unit (CHU).

The below infographic presents the difference between wobbe index and calorific value in tabular form for quick reference.

Summary – Wobbe Index vs Calorific Value

Both the Wobbe index and calorific value relate to different fuels. The difference between wobbe index and calorific value is that wobbe index indicates the interchangeability of fuel gases whereas calorific value indicates the total amount of heat energy generates when we burn a unit mass of fuel.

Reference:

1. “Summary.” Wobbe Index – Neutrium. Available here 
2. I am Proud to be a Programmer Follow. “Calorific Value.” LinkedIn SlideShare, 16 Oct. 2015. Available here  

Image Courtesy:

1.”Light my fire (2152952690)”By Michal Osmenda from Brussels, Belgium – Light my fire, (CC BY-SA 2.0) via Commons Wikimedia 

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