Energy Favorability

Carbon dioxide and water are carbon and hydrogen that has been oxidized. That means it is in a low-energy state. These molecules cannot release energy when undergoing chemical reactions.

Hydrocarbons are molecules containing carbon, hydrogen, and sometimes oxygen. These molecules are in a higher energy state and can release energy in combustion as they become lower-energy state molecules like water and carbon dioxide.

Photosynthesis

Plants use energy from the sun to take low-energy-state molecules (water and CO₂) and turn them into hydrocarbons.

CO₂ + H₂O + Sunlight (Radiation Energy) \to C_XH_YO_Z + O₂

Combustion

Combustion combines hydrocarbons and oxygen and releases energy as it forms low-energy-state molecules of water and CO₂.

C_XH_Y + O₂ \to CO₂ + H₂O + Heat Energy

Carbon and Carbon Dioxide

The atomic mass of carbon is 12 AMU. The atomic mass of oxygen is 16 AMU. So the mass of a carbon dioxide molecule is 12 + 16 x 2 = 44.

This leads to our proportion or conversion factor that says 12 kg of carbon becomes 44 kg of carbon dioxide.

Real combustion

The atmosphere is not purely oxygen, it also has nitrogen and other elements
Fossil fuels are not pure carbon and hydrogen, they contain impurities like sulfur and mercury
When all these chemicals participate in combustion, they produce sulfur oxides (SO_X), nitrous oxides (NO_X), and other chemicals
These chemicals are the cause of acid rain and other environmental effects

Green House Gases

Carbon dioxide, the gas resulting from combustion, is the main component of global warming.

Carbon Intensity

Carbon intensity is defined as the carbon released divided by the amount of services provided. This could be mass of CO2 per mile driven, or mass of CO2 per unit of electricity generated.

These are averages for the carbon intensity of electricity for some power plants.

Fuel Source	Carbon Intensity gram CO2/kWh	Carbon Intensity lb/kWh
Coal	1001	2.249
Natural Gas	469	1.135
Solar PV	46
Geothermal	45
Nuclear	16
Wind	12
Hydroelectric	4
PG&E	238	0.524
SCP CleanStart	58	0.128
SCP Evergreen	24	0.053

Some data from IPCC 2011 Annex II.

Mercury

The USGS measured mercury content in coal ranging from 6.5 – 27.0 lb Hg/10^{12} BTU.

Estimate Mercury per kWh

Assume a coal plant

35% efficient
Uses coal with 15.0 lb Hg per 10^{12} BTU

What is the mass of mercury generated per kWh?

\frac{15.0\;\textrm{lb Hg}}{10^{12}\; \textrm{BTU coal heat energy}} \frac{1\; \textrm{BTU coal heat energy}}{0.35\; \textrm{BTU electricity}} = 4.29 \cdot 10^{-11} \textrm{lb Hg per BTU electricity}

This is the correct dimensions (mass Hg divided by electrical energy) but we need to convert to mg per kWh.

\frac{4.29 \cdot 10^{-11}\; \textrm{lb Hg}}{\textrm{BTU electricity}} \frac{454\cdot 10^3\;mg}{lb} \frac{3412\;BTU}{kWh} = 0.066 \textrm{mg Hg per kWh}