Question

Calculate the amount of carbon dioxide (in kilograms) emitted when four alternative fuels are burned to provide the same amount of energy as \(10.0\) gallons of gasoline. Compare the carbon dioxide emissions from alternative fuels to gasoline (Worked Example 3.12). (a) Ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) Although \(\mathrm{E} 85\) is a blend of ethanol and gasoline, let's use pure ethanol in our calculation for simplification. A gallon of ethanol contains \(68 \%\) of the energy of a gallon of gas, so \(14.7\) gallons of ethanol provides the same amount of energy as \(10.0\) gallons of gasoline. The density of ethanol is \(0.79\) \(\mathrm{kg} / \mathrm{L}\) and \(1 \mathrm{~L}=0.2642\) gal. (b) Liquefied Petroleum Gas/Propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right) \quad\) A gallon of propane contains \(73 \%\) of the energy of a gallon of gas, so \(13.7\) gallons of propane provides the same amount of energy as \(10.0\) gallons of gasoline. The density of liquefied propane is \(0.49 \mathrm{~kg} / \mathrm{L}\) and \(1 \mathrm{~L}=0.2642\) gal. (c) Compressed Natural Gas \(\left(\mathrm{CH}_{4}\right) \quad\) It takes \(25.7 \mathrm{~kg}\) of natural gas, methane \(\left(\mathrm{CH}_{4}\right)\), to provide the same amount of energy as \(10.0\) gallons of gasoline. (d) Electricity from a Coal-Burning Power Plant An electric power plant using bituminous coal produces \(0.94 \mathrm{~kg} \mathrm{CO}_{2} / \mathrm{kWh}\), and an electric vehicle uses \(35 \mathrm{kWh}\) per 100 miles. (A kilowatthour, symbolized \(\mathrm{kWh}\), is a common unit of electrical energy equivalent to \(3.6\) megajoules.) (e) Electricity from a Natural Gas-Burning Power Plant An electric power plant using natural gas produces \(0.55 \mathrm{~kg} \mathrm{CO}_{2} / \mathrm{kWh}\) and an electric vehicle uses \(35 \mathrm{kWh}\) per 100 miles. (f) Which fuel produces the least amount of \(\mathrm{CO}_{2}\) when burned to provide energy for a car: gasoline, ethanol, propane, CNG, electricity from coal, or electricity from natural gas?

Short answer

Electricity from a natural gas-burning power plant produces the least CO2, with 19.25 kg CO2 emissions.

Step-by-step solution

Calculate CO2 Emissions from Gasoline

First, we'll calculate the CO2 emissions from burning 10.0 gallons of gasoline. The emission factor for gasoline is approximately 8.887 kg CO2 per gallon. Therefore, for 10.0 gallons: \[ 10.0 \text{ gallons} \times 8.887 \text{ kg CO2/gallon} = 88.87 \text{ kg CO2} \]

Calculate CO2 Emissions from Ethanol

We know ethanol provides 68% energy of gasoline and requires 14.7 gallons to match 10 gallons of gasoline energy. Converting gallons to liters:\[ 14.7 \text{ gallons} \times 3.785 \text{ L/gallon} = 55.6845 \text{ L} \]The density is 0.79 kg/L, so:\[ 55.6845 \text{ L} \times 0.79 \text{ kg/L} = 43.99 \text{ kg ethanol} \]Assuming complete combustion, ethanol's carbon content can be calculated from stoichiometry of the combustion process, yielding \( \approx 1.91 \text{ kg CO2 per kg ethanol} \), hence:\[ 43.99 \text{ kg ethanol} \times 1.91 \text{ kg CO2/kg ethanol} = 84.02 \text{ kg CO2} \]

Calculate CO2 Emissions from Propane

Propane provides 73% energy of gasoline and requires 13.7 gallons. Convert gallons to liters and find weight:\[ 13.7 \text{ gallons} \times 3.785 \text{ L/gallon} = 51.9045 \text{ L} \]Its density is 0.49 kg/L, so:\[ 51.9045 \text{ L} \times 0.49 \text{ kg/L} = 25.433 \text{ kg propane} \]The CO2 emission for propane is calculated with 3.00 kg CO2/kg propane. Therefore:\[ 25.433 \text{ kg propane} \times 3.00 \text{ kg CO2/kg propane} = 76.30 \text{ kg CO2} \]

Calculate CO2 Emissions from Natural Gas (CNG)

It requires 25.7 kg of methane (CH\(_4\)) to provide the same energy. Using the emission factor for methane, 2.75 kg CO2 per kg of CH\(_4\), we calculate:\[ 25.7 \text{ kg CH}_4 \times 2.75 \text{ kg CO2/kg CH}_4 = 70.68 \text{ kg CO2} \]

Calculate CO2 Emissions from Coal-Burning Electricity

Calculate kWh for 10.0 gallons of gasoline equivalent energy, considering EVs use 35 kWh/100 miles. For simplicity, assume equal energy provision. \[ 35 \text{ kWh} \times \frac{100}{100} = 35 \text{ kWh} \]Carry out the conversion assuming full distance (assuming simplification as units aren't directly convertible in stated method):\[ 35 \text{ kWh} \times 0.94 \text{ kg CO2/kWh} = 32.90 \text{ kg CO2} \]

Calculate CO2 Emissions from Natural Gas-Burning Electricity

Again calculate the kWh as before for 35 kWh. The emission factor is 0.55 kg CO2/kWh:\[ 35 \text{ kWh} \times 0.55 \text{ kg CO2/kWh} = 19.25 \text{ kg CO2} \]

Compare All CO2 Emissions

Now compare the calculated emissions: - Gasoline: 88.87 kg CO2 - Ethanol: 84.02 kg CO2 - Propane: 76.30 kg CO2 - CNG: 70.68 kg CO2 - Electricity from Coal: 32.90 kg CO2 - Electricity from Natural Gas: 19.25 kg CO2

Key concepts

Alternative Fuels

Alternative fuels serve as replacements for traditional gasoline and aim to reduce carbon emissions. They include ethanol, propane, compressed natural gas (CNG), and electricity from various power sources. Each of these alternatives provides the same energy as gasoline differently, requiring different amounts of fuel due to varying energy densities.

• Ethanol: Derived from plant materials, ethanol provides about 68% of the energy of gasoline, necessitating a greater volume to match energy output.
• Propane: Also known as liquefied petroleum gas, propane delivers around 73% of gasoline’s energy.
• Compressed Natural Gas (CNG): Mainly composed of methane, it is used in vehicles adapted for it and requires approximately 70% of gasoline's energy.

Each of these alternative fuels has unique properties, affecting both their energy efficiency and carbon emissions.

Energy Comparison

Energy comparison between fuels is critical to understanding CO2 emissions. The amount of energy produced per unit of fuel affects how much of it is needed to match gasoline.

• Gasoline: Considered a high energy-density fuel, providing plenty of energy with smaller volumes.
• Ethanol: Only provides 68% of the energy compared to the same volume of gasoline, requiring more fuel volume.
• Propane: Packs 73% of gasoline’s energy in the same volume.
• CNG: Less energy-dense, needing a larger quantity to match gasoline’s energy. It's a viable option due to cleaner combustion.

By comparing these energy outputs, we can evaluate which fuels might suit different needs or preferences based on energy efficiency and usage.

Fuel Combustion

Understanding fuel combustion helps explain how CO2 is produced. Combustion is the chemical reaction occurring when a fuel burns in the presence of oxygen. This process releases energy but also emits CO2, contributing to atmospheric CO2 levels.
Complete combustion of different fuels results in varying carbon production levels:

• Gasoline: Burns to primarily produce CO2 and water, releasing significant energy.
• Ethanol: Produces less CO2 than gasoline. When completely combusted, it transforms into carbon dioxide and water.
• Propane: Yields CO2 and water, similar to gasoline, but slightly less CO2.
• CNG: Produces CO2 and water, with less CO2 due to its high hydrogen content.

The differences in combustion by-products make alternative fuels a consideration for reducing environmental impacts.

Stoichiometry

Stoichiometry plays a vital role in calculating the amounts of reactants and products in chemical reactions, such as fuel combustion. By understanding the stoichiometry of a fuel's combustion reaction, we can estimate how much CO2 will be produced.
The combustion reactions can be represented as balanced chemical equations:

• Gasoline: Contains many hydrocarbons; a simplified version is \[ ext{C}_8 ext{H}_{18} + ext{O}_2 \rightarrow ext{CO}_2 + ext{H}_2 ext{O}\]
• Ethanol: \[ ext{C}_2 ext{H}_5 ext{OH} + ext{O}_2 \rightarrow ext{CO}_2 + ext{H}_2 ext{O}\]
• Propane: \[ ext{C}_3 ext{H}_8 + ext{O}_2 \rightarrow ext{CO}_2 + ext{H}_2 ext{O}\]
• CNG: \[ ext{CH}_4 + ext{O}_2 \rightarrow ext{CO}_2 + ext{H}_2 ext{O}\]

Understanding these equations helps predict emissions based on the fuel type and quantity.

Environmental Impact

The environmental impact of different fuels is primarily assessed by the amount of CO2 they emit. The emissions are influenced by how cleanly and efficiently each fuel burns and the quantity of fuel required to match gasoline's energy output.

• Gasoline: Traditional fuel with high emissions; contributes significantly to global CO2 levels.
• Ethanol: Lower emissions than gasoline but still a concern due to agricultural practices and energy used in production.
• Propane: Emits less CO2 than gasoline, making it an attractive alternative.
• CNG: Produces even less CO2 than propane, beneficial for both air quality and climate change mitigation.
• Electricity: Greatest potential for low impact when sourced from renewables. Coal-based electricity has high emissions, while natural-gas-powered electricity reduces those emissions.

Evaluating the full lifecycle and emissions of each fuel type shows the potential reductions in environmental impact when switching from gasoline to alternative options.