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CO2 Emissions: LNG vs. Other Fuels Comparison

CO2 Emissions: LNG vs. Other Fuels Comparison

1. The critical need for fuel emission comparisons

In the context of global climate change, mitigating Greenhouse Gas (GHG) emissions has become a top priority. Carbon dioxide (CO2) from the combustion of fossil fuels is a primary contributor to these emissions. (1) Human activities, particularly the burning of fossil fuels for power generation, heating, and transportation, represent the largest sources of GHG emissions. (2) To make informed decisions regarding energy policy, industrial planning, and environmental impact assessments, a clear understanding and comparison of CO2 emissions from various fuel types are essential.

This report aims to provide a detailed, data-driven comparison of specific CO2 emissions from the combustion of Liquefied Natural Gas (LNG) versus other common fuels. The fuels under consideration include coal, charcoal, diesel oil (DO), fuel oil (FO), Liquefied Petroleum Gas (LPG), and Compressed Natural Gas (CNG). This report focuses on direct CO2 emissions from combustion, while also critically examining the overall greenhouse gas footprint of natural gas-derived fuels due to the presence of non-CO2 gases like methane. Information is compiled from reliable sources such as the U.S. Environmental Protection Agency (EPA), the U.S. Energy Information Administration (EIA), and the Intergovernmental Panel on Climate Change (IPCC). (1)

Mức Độ Phát Thải CO2 Khi Đốt Cháy. Nguồn: tham khảo internet
CO2 Emission Levels from Fuel Combustion (internet)

2. Understanding Emission Factors and Greenhouse Gas Accounting

To accurately assess the environmental impact of various fuels, a clear understanding of emission factors is fundamental. An emission factor quantifies the amount of greenhouse gas released per unit of activity, for example, per unit of fuel combusted. (11) Employing consistent units, such as kg CO2/MMBtu, kg CO2/GJ, or kg CO2/unit mass/volume, is crucial to ensure accuracy in comparisons. (5)

International and national organizations play a vital role in establishing standards and methodologies for greenhouse gas (GHG) accounting. The IPCC (Intergovernmental Panel on Climate Change) provides internationally recognized guidelines and default emission factors for national GHG inventories. (1)

The basic equation for calculating the carbon released from fuel combustion (Cr) is:

(Cr) là: Cr = Q • NCV • EF • (1-Sf) • F

Where EF is the specific carbon content (tonnes C/TJ). (1) The IPCC’s 2019 Refinement to the 2006 IPCC Guidelines updates and supplements existing data, including emission factors, to incorporate new scientific knowledge. (10)

Similarly, national agencies like the EPA (Environmental Protection Agency) and EIA (Energy Information Administration) also compile and publish emission factors for organizational and national GHG reporting, often referencing IPCC methodologies. (3)

Assessing the climatic impact of fuels extends beyond just CO2. Greenhouse gases are often measured in CO2 equivalent (CO2e), a unit that accounts for the different Global Warming Potentials (GWP) of various gases. (2)

GWP is an index that compares the impact of a gas on the atmosphere relative to CO2 over a specific timeframe (e.g., 20-year or 100-year GWP). (2) This is particularly important when evaluating natural gas-derived fuels, such as LNG and CNG.

3. Fuel Profiles and CO2 Emissions from Combustion

To enable a direct and accurate comparison, we will examine the key characteristics of each fuel, including their typical composition, energy density (how much energy is contained per unit of mass or volume), and the direct CO2 emission factor during combustion. We will primarily use kg CO2 per Gigajoule (GJ) as the consistent unit for emission factors, as it’s a common metric in energy and environmental reporting.

LNG (Liquefied Natural Gas)

Liquefied Natural Gas (LNG) is natural gas cooled to liquid form, primarily comprising methane (95%), with minor amounts of nitrogen, ethane, CO2, and propane. (13) LNG is favored for long-distance transport due to its reduced volume and high energy density. (13) LNG has an energy density comparable to diesel oil (13), with a mass energy density of 53.6 MJ/kg and a volumetric energy density of 22 MJ/L. (14)

The direct CO2 emission factor from natural gas combustion is 53.06 kg CO2 per MMBtu (5) or 0.0564 kg CO2 per MJ. (11) Natural gas, the main component of LNG, has a lower carbon-to-hydrogen ratio compared to coal or oil. When combusted, this results in the production of less CO2 per unit of energy. Data from (5) and (11) consistently show that natural gas has a lower direct CO2 emission factor per unit of energy than coal or fuel oil.

This direct combustion advantage is often cited as a reason to transition from coal to natural gas, as LNG immediately reduces CO2 emissions at the point of combustion for the same energy output.

Coal: The Highest CO2 Emitter

Coal is a solid fossil fuel characterized by its high energy density and widespread availability. (15) It is primarily composed of carbon, with varying amounts of hydrogen, oxygen, nitrogen, and sulfur. The main types of coal include Anthracite, Bituminous, Sub-bituminous, and Lignite. (5)

The general energy density of coal is approximately 24 MJ/kg. (16) Specific types exhibit differences: Anthracite is around 29 GJ/tonne, Bituminous ranges from 24-29 GJ/tonne, and Lignite from 7-17 GJ/tonne. (9) These variations in carbon content and energy density directly contribute to coal’s position as the fuel with the highest CO2 emissions per unit of energy when combusted.

Direct CO2 Emission Factors from Coal Combustion by Type

Type CO2 Emission
Anthracite 103.69 kg CO2/ MMBtu or 2,602 kg CO2/ tonne
Bituminous 93.28 kg CO2 /MMBtu or 2,325 kg CO2/ tonne
Sub-bituminous 97.17 kg CO2 / MMBtu or 1,676 kg CO2/tonne
Lignite: 97.72 kg CO2 / MMBtu o 1,389 kg CO2/ tonne

 

  • The direct CO2 emission factor from general coal combustion is 0.096 kg CO2 per MJ (11), while Lignite specifically has a factor of 0.1012 kg CO2 per MJ (11).

Coal, particularly bituminous and anthracite coal, possesses a high carbon content and a relatively low hydrogen content compared to natural gas or petroleum. This chemical composition directly leads to a higher CO2 emission factor per unit of energy upon combustion, as there’s more carbon available to oxidize into CO2. Consistent data (5) indicates that coal exhibits the highest CO2 emissions per unit of energy among the fossil fuels listed. Coal remains a significant contributor to global CO2 emissions (1), and its high carbon content underscores the environmental benefits of transitioning away from coal for power and heat generation (15).

Charcoal

Charcoal is a product of the pyrolysis of wood or other biomass materials. While it originates from biomass and can be considered carbon-neutral within a sustainable cycle, the combustion of charcoal still directly releases CO2. The estimated direct CO2 emission factor from charcoal is approximately 112 kg CO2e/GJ (equivalent to 118.17 kg CO2/MMBtu).

Dầu Diesel (DO)

Diesel oil is a distillate fuel primarily used in diesel engines. (16) It’s a complex mixture of hydrocarbons (C10-C16), mainly alkanes, cycloalkanes, and aromatic hydrocarbons. (18) The energy density of diesel oil is 45 MJ/kg. (16) Generally, fuel oils (including diesel) have an energy density ranging from 42–44 MJ/kg. (15)

Direct CO2 Emission Factors from Diesel Oil Combustion:

  • Diesel oil (Mobile Combustion): 10.21 kg CO2 per gallon. (5)
  • Diesel oil: 0.0743 kg CO2 per MJ. (11)
  • Diesel oil (kWh): 0.30 kg CO2eq/kWh. (19)
  • No. 2 Distillate Fuel Oil: 73.96 kg CO2 per MMBtu, or 10.21 kg CO2 per gallon. (5)

Diesel oil is a petroleum-based fuel, and its combustion is a significant source of CO2 emissions, particularly in the transportation sector. (2) Its emission factors typically show it to be lower than coal but higher than natural gas on an energy basis. Its high energy density (16) makes it efficient for heavy-duty applications, but this also means a substantial amount of CO2 is released per unit of volume. Decarbonizing the transportation sector necessitates a transition away from diesel, highlighting the importance of alternative fuels like CNG/LNG or electrification.

Khí thiên nhiên có hệ số phát thải CO2 trực tiếp khoảng 53.06 kg CO2 trên mỗi MMBtu. Điều này làm cho LNG trở thành một lựa chọn hấp dẫn để giảm lượng khí thải CO2 tại các nhà máy điện và cơ sở công nghiệp khi chuyển đổi từ các nhiên liệu nặng hơn.
Natural gas has a direct CO2 emission factor of approximately 53.06 kg CO2 per MMBtu. This makes LNG an attractive option for reducing CO2 emissions in power plants and industrial facilities when transitioning from heavier fuels.

Fuel Oil (FO) / Mazut

Fuel oil is a general term for petroleum products used as fuel for furnaces or boilers. It includes various types (e.g., Residual Fuel Oil No. 5, No. 6) and is a complex mixture of aromatic, aliphatic, and naphthenic hydrocarbons (C7 to >C50), asphaltenes, and heterocyclic compounds. (18) The energy density of fuel oil is 42–44 MJ/kg. (15) Crude oil (the origin of fuel oil) has an energy density of 44 MJ/kg. (16)

Direct CO2 Emission Factors from Fuel Oil Combustion:

  • Residual Fuel Oil No. 5: 72.93 kg CO2 per MMBtu, or 10.21 kg CO2 per gallon. (5)
  • Residual Fuel Oil No. 6: 75.10 kg CO2 per MMBtu, or 11.27 kg CO2 per gallon. (5)
  • Heavy Fuel Oil: 0.0774 kg CO2 per MJ. (11)
  • Fuel Oil (kWh): 0.29 kg CO2eq/kWh. (19)

Fuel oil, especially the heavier grades, has a higher carbon content than lighter petroleum products or natural gas. This results in higher CO2 emissions per unit of energy compared to natural gas. (5)

LPG (Liquefied Petroleum Gas): Closer to Natural Gas

LPG is a byproduct of crude oil extraction, existing as a mixture of propane and butane. (13) It’s stored in liquid form under moderate pressure. (21) The energy content of LPG is 25 MJ/L or 91,547 BTU/Gal. (22) Propane, a primary component, has an energy density of approximately 49.02 MJ/kg (calculated from 25 MJ/L and 0.51 kg/L). (22)

Direct CO2 Emission Factors from LPG Combustion:

  • LPG (Stationary Combustion): 61.71 kg CO2 per MMBtu, or 5.68 kg CO2 per gallon. (5)
  • LPG (Mobile Combustion): 5.68 kg CO2 per gallon. (5)
  • LPG: 0.0667 kg CO2 per MJ. (11)
  • LPG (liter): 1.69 kg CO2eq/liter. (19)

LPG, primarily composed of propane and butane, has a lower carbon content than diesel or fuel oil, resulting in lower CO2 emissions per unit of energy. (5) LPG is also noted for emitting fewer hydrocarbons and extending engine life. (13) The gaseous nature of LPG allows for cleaner and more complete combustion compared to liquid fuels. LPG serves as a cleaner alternative to gasoline and diesel in certain applications, contributing to reduced local air pollution and lower direct CO2 emissions compared to older, heavier fossil fuels.

CNG (Compressed Natural Gas): Equivalent to LNG

CNG is natural gas (primarily methane, 93.05%) compressed to high pressure (200-250 bar). (13) It is an environmentally friendly, clean alternative fuel. (13) The mass energy density of CNG is 53.6 MJ/kg, similar to LNG. Its volumetric energy density is 9 MJ/L. (14)

Direct CO2 Emission Factors from CNG Combustion:

  • CNG (Mobile Combustion): 0.05444 kg CO2 per scf. (5)
  • CNG (kWh): 0.18 kg CO2eq/kWh. (19)
  • Natural Gas (from which CNG is derived): 53.06 kg CO2 per MMBtu (5); 0.0564 kg CO2 per MJ. (11)

Like LNG, the primary component of CNG is methane, leading to lower CO2 emission factors per unit of energy compared to coal and petroleum products. (5) CNG is highlighted for its safety (lighter than air, disperses rapidly) and lower production costs. (13) CNG offers a direct pathway to reduce combustion-related CO2 emissions in sectors like transportation and industrial use, especially where natural gas pipeline infrastructure is available, providing a cleaner alternative to gasoline and diesel.

Table 1: Comparative CO2 Emission Factors by Fuel Type (Combustion)

This table provides a direct overview of the CO2 intensity from the combustion of each fuel type. Presenting the CO2 emission factors in a unified table allows for direct comparison, making it easy to discern the differences in CO2 released per unit of energy. This forms a crucial basis for a better understanding of the direct environmental impact of each fuel.

Fuel Type Primary Composition (Summary) Energy Density (MJ/kg or MJ/L) CO2 Emission Factor (kg CO2/MMBtu) CO2 Emission Factor (Mass/Volume Unit) Source
LNG (Liquefied Natural Gas) Methane 53.6 MJ/kg; 22 MJ/L 53.06 (from Natural Gas) 0.05444 kg CO2/scf (CNG) 5
Coal (Bituminous) Carbon 24 MJ/kg 93.28 2,325 kg CO2/short ton 5
Charcoal Carbon Not directly available (Wood: 16 MJ/kg) 118.17 (calculated from 112 kg CO2e/GJ) 112 kg CO2e/GJ 16
Diesel Oil (DO) Hydrocarbons (C10-C16) 45 MJ/kg 73.96 (No. 2 Distillate Oil) 10.21 kg CO2/gallon 5
Fuel Oil (FO) Heavy Hydrocarbons (C7->C50) 42-44 MJ/kg 72.93 – 75.10 (Residual Oil No. 5-6) 10.21 – 11.27 kg CO2/gallon 5
LPG (Liquefied Petroleum Gas) Propane, Butane 49.02 MJ/kg; 25 MJ/L 61.71 5.68 kg CO2/gallon 5
CNG (Compressed Natural Gas) Methane 53.6 MJ/kg; 9 MJ/L 53.06 (from Natural Gas) 0.05444 kg CO2/scf 5

 

Note: The kg CO2/MMBtu values for Charcoal are calculated from kg CO2e/GJ. The values for LNG and CNG utilize natural gas emission factors as they are derived from natural gas. Values are rounded and may vary slightly depending on the specific data source and exact fuel composition.

LNG and Its Role in Reducing Direct CO2 Emissions

Through this analysis, it is evident that Liquefied Natural Gas (LNG) and Compressed Natural Gas (CNG) exhibit the lowest direct CO2 emissions from combustion when compared to coal, diesel oil, and fuel oil. Coal stands out as the fuel with the highest direct CO2 emissions.

How do CO2 emissions from LNG combustion compare to those from other fuels?
How do CO2 emissions from LNG combustion compare to those from other fuels?

The transition from high-carbon intensity fuels such as coal and heavy fuel oils to LNG can significantly contribute to the reduction of direct CO2 emissions at power plants and industrial facilities. This represents a crucial step in our efforts to minimize our carbon footprint and move towards a more sustainable energy future.

References

  • EPA. (n.d.). GHG Emission Factors Hub. (3)
  • EIA. (2024). Carbon Dioxide Emissions Coefficients. (4)
  • IPCC. (2022). Climate Change 2022: Mitigation of Climate Change. (1)
  • EPA. (n.d.). Sources of Greenhouse Gas Emissions. (2)
  • EPA. (2024). Emission Factors for Greenhouse Gas Inventories. (5)
  • Climate Accountability Institute. (2023). Oil Emission Factor Calc. (6)
  • NSSGA. (2021). NSSGA Users Guide. (7)
  • IPCC. (2006). 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 2: Energy, Chapter 2: Stationary Combustion. (8)
  • UTI. (n.d.). What is CNG gas? (13)
  • Energy Education. (n.d.). Energy density. (16)
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  • Climatiq. (2023). Charcoal Emission Factor. (17)
  • Winnipeg. (n.d.). Emission factors. (19)
  • The Climate Registry. (2025). 2025 Default Emission Factors. (26)
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  • CLU-IN. (n.d.). DNAPL-heavy-oil-details. (20)
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  • Wikipedia. (n.d.). Compressed natural gas. (14)
  • Unitrove. (n.d.). What is CNG? (23)

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