Combustion

Combustion

Concepts

Enthalpy

Chemical compounds have an energy value that we can use to determine the energy released by a chemical reaction.

Efficiency

The ratio of useful energy delivered to energy input.

Carbon Intensity

The ratio of carbon released to the amount of energy delivered.

Energy Density

The amount of heat energy released per mass of material combusted.

High Heating Value

This is the amount of heat delivered per unit of combusted material that includes the energy from condensation of the water vapor.

Low Heating Value

This is the amount of heat delivered per unit of combusted material when no condensation energy is captured.

Heat Rate

The amount of chemical energy input to produce an amount of electrical energy.

Combustion in Buildings

Combustion of natural gas is a widespread method of delivering heat to buildings. For the purposes of climate change, we want to provide human comfort with the lowest possible emissions of greenhouse gases.

Methane Combustion

• Methane + Oxygen -> Carbon Dioxide + Water + Energy

• CH~4~ + 2O~2~ -> CO~2~ + 2H~2~O

Moles

• One mole is 6.02 $\times 10^{23}$ molecules (or atoms) of a substance

Molar Weights

Material	Mass of one Mole (grams)
Carbon	12
Oxygen	16
Hydrogen	1

Mole Conversion

• One mole of methane burned equals one mole of CO~2~ released

• Conversion factor:

$\frac{\textrm{1 mole carbon dioxide released}}{\textrm{1 mole methane burned}}$

Mass Conversion

• We need to know the masses of these to make a conversion factor for

  mass of carbon dioxide released to mass of methane burned

• What is the mass of one mole of methane?

• 12 grams per oxygen atom + 4 mole hydrogen * 1 gram per hydrogen mole = 16 grams

• What is the mass of carbon dioxide?

• 12 + 2 * 16 = 44 grams per mole

Mass Conversion

• 16g CH~4~ + 64g 2O~2~ -> 44g CO~2~ + 36g 2H~2~O

Enthalpy of Formation

Substance	Enthalpy (kJ/mol)
Oxygen gas	0
Liquid water	-285.8
Water Vapor	-241.8
Methane	-74.9
Carbon Dioxide	-393.5

Energy Released

• Change in enthalpy is the sum of products minus the sum of reactants

• -393.5 + 2 -241.8 - (-74.9 + 2 0) = -802.2 kJ/mol CH~4~ burned

• This yields 50.1 kJ per gram

• Compare to the published value

Carbon Tax

To create a market signal, you have to assign a monetary value to carbon. The social cost of carbon calculations attempt to sum the costs created by carbon emissions. Carbon taxes or cap and trade programs add a cost to transactions involving carbon.

One question we ask is how much does a carbon tax change the cost of services being delivered.

If a carbon tax is currently about 13 USD/tonne CO2 equivalent, how much would this change the cost of one therm of natural gas? How about your monthly gas bill during the winter?

Caveat

You need to be careful when you encounter numbers on carbon emissions.

• Some carbon dioxide emissions are listed as the mass of carbon

• Others are listed as the mass of carbon dioxide

• You can always convert between the two using the molecular weight

  ratio (44/12)

Assumptions

• 100 cubic feet per therm

• 0.8 kg per cubic meter density of natural gas

• about $1 per therm

Carbon Tax Calculation

100 ft^3 * (1 m/3.3 ft)^3
         * 0.8 kg CH4/m^3
         * 44 kg CO2 / 16 kg CH4
         * 0.012 USD/kg CO2 => $0.0735

Electricity Generation

Electricity is generated through the combustion of coal, natural gas, oil, or diesel. Coal and natural gas combustion typically heats water into steam to turn a turbine connected to an electromagnetic generator. Diesel electricity generation occurs in an internal combustion engine where the pistons turn a crankshaft that turns the generator.

For the purposes of cost and carbon calculations, we are interested in how much fuel is used to create a unit of electrical energy. Generally, energy sources that have low cost and low carbon per unit of electricity are desirable.