How to Calculate Carbon Content of liquid fuels

Calculating the carbon content of liquid fuel is essential for estimating greenhouse gas emissions from the combustion of liquid fuel. The Alberta TIER quantification document does not provide methodologies for calculating the carbon content of liquid fuels. This blog post introduces a method for calculating the carbon content of liquid fuels.

Differences in Calculating Carbon Content for Liquid and Gaseous Fuels

For gaseous fuels (e.g., natural gas), the molar volume can be converted to mass using a fixed value of 23.645 m³/kmole:

$$V = \frac{nRT}{P} = 23.645\ \text{m}^3/\text{kmol}$$

where:

• $n = 1$ kmole
• $R = 8.314$ m³ Pa/(K kmole)
• $T = 288.15$ K
• $P = 101325$ Pa

At $15^{\circ}\text{C}$ and 1 atmosphere pressure, $V$ is 23.645 m³/kmole. This is not applicable for liquid fuels.

Liquid Carbon Content Calculation

The carbon content of a liquid fuel mixture is a weighted average of the carbon content of its individual components. First, calculate the weight percent (wt%) of carbon ($Wt\%C$) of each component in the fuel. This is done by multiplying the molecular weight of carbon by the number of carbon atoms in the compound and dividing by the compound's molecular weight.

$$Wt\%C_{\text{Cj}} = \frac{12.01 \times X}{MW_{\text{Cj}}}$$

where:

• $Wt\%C_{\text{Cj}}$ is the carbon content of the hydrocarbon compound on a mass percent basis (e.g., $Wt\%C_{\text{C2H6}}$, $Wt\%C_{\text{C3H8}}$)
• $12.01$ is the molecular weight of carbon
• $X$ is the number of carbon atoms in the compound (e.g., 2 for ethane C₂H₆, 3 for propane C₃H₈)
• $MW_{\text{Cj}}$ is the molecular weight of the individual hydrocarbon compound (e.g., 30.069 t/t-mol for ethane C₂H₆, 44.0956 t/t-mol for propane C₃H₈)

The total carbon content of the fuel mixture is then calculated as:

$$Wt\%C_{\text{mixture}} = \frac{1}{100} \sum_{i=1}^{n} (Wt\%_i \times Wt\%C_{\text{i}})$$

where:

• $Wt\%C_{\text{mixture}}$ is the carbon content of the fuel mixture on a mass percent basis
• $Wt\%_i$ is the weight percent of the individual fuel component
• $Wt\%C_{\text{i}}$ is the carbon content of the individual fuel component $i$, calculated using the formula above (e.g., $Wt\%C_{\text{C2H6}}$, $Wt\%C_{\text{C3H8}}$)

CO₂ Emissions from Combustion of Liquid Fuels

Carbon dioxide emissions from the combustion of liquid fuels can be calculated using:

$$E_{\text{CO2}} = FC \times D \times Wt\%C_{\text{mixture}} \times \frac{44}{12}$$

where:

• $E_{\text{CO2}}$ is the CO₂ emissions in mass (e.g., kg)
• $FC$ is the fuel consumption in volume (e.g., gal, m³)
• $D$ is the density of the fuel in mass/volume (e.g., lb/gal, kg/m³)

Calculation Example

A calculation example can be downloaded from here.

Reference

The calculation is simplified for easier understanding based on Equations 4-9 to 4-13 from API 2021, Compendium of Greenhouse Gas Emissions Methodologies for the Oil and Gas Industry.