Clean Coal – Coal takes millions of years to form. Comes from swamps.
Heating value = energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions.
Higher heating value = determined by bringing all the products of combustion back to the original pre-combustion temperature
Lower heating value = determined by subtracting the heat of vaporization of the water vapor from the higher heating value.
Higher heating values
- Hydrogen: 142 MJ/kg
- Gasoline: 47 MJ/kg
- Ethanol: 30 MJ/kg
- Coal – 15-27 MJ/kg.
4 different types of coal
- Contains 86-97% of carbon.
- Contains highest heating value
- Not common in the US
- Bituminous coal
- Contains 45-86% carbon
- Most common coal found in US
- Used to generate electricity, and to make steel.
- Subbutuminous coal
- Contains 35-45% coal
- 36% of coal in the US is of this type
- Contains 25-35% carbon
- Lowest rank of coal with lowest energy content
- Has high moisture content
- 7% of US coal is of this type.
Coal emits the most CO2 per kWh.
Coal is extracted through underground mining (1000 feet deep), and surface mining (less than 200 feet deep). 2/3 of coal production is surface mining.
Large Bucket Wheel Excavator – cost $100 million, weighs 14,000 tons, took 5 years to assemble. Can extract 240,000 m3 of coal per day.
Once coal is extracted, the coal is then processed and cleaned. Rocks and dirt, ash, and sulfur are removed to increase the heating value.
The US has 406 coal fired power plants, each of about 500 MW.
3000 miners die in China every year.
Miners get asthma, pneumoconiosis (lung disease), and heart attacks.
Coal also has air pollution problems: emits CO2, Sox, Nox, and particulate matter.
Surface mining causes contamination of water.
Clean Coal –
- Hypercoal – use higher quality coal without ashes
- 45% thermal efficiency
- 13% reduction in CO2.
- Carbon capture and sequestration
- CO2 from coal power plant is processed and stored underground.
- Coal gasification
- Burn coal, add oxygen, then CO2 is injected in a well. Purified syngas (hydrogen) goes through the gas turbine (Brayton cycle).
- Coal liquefaction
- Use the Fischer-Tropsch process to convert CO+H2 into oil. Needs a catalyst, so it can be expensive.
Solubility of CO2 as a function of temperature: solubility increases as temperature decreases.