CHEMISTRY 253

Spring, 2015 - Dixon

Group Assignment #10 - Solutions

1.  Various fossil fuel sources (natural gas, oil, or coal) can be used for combustion in domestic heating and for electricity generation.  (8 pts)

a)  Which of these fuels is not desirable for domestic heating? and for what reason is it not desirable?

Coal.  Methane is very clean burning, oil is moderately clean burning, and coal is very dirty when burning without emission controls.  For powerplants, addition of emissions controls is not cost prohibitive, while it is cost prohibited for home use.  Coal is not burned in homes in most advanced/densely populated regions due to disadvantages from localized pollution.

b)  Considering cost, energy release from combustion and generation of carbon dioxide, what measure (give example units) should one compare if looking at whether substitution of one fuel for another will be beneficial (lead to a decrease in CO₂ emissions if energy production is constant)?  Don’t consider secondary effects such as emission of other pollutants or variations in combustion efficiency.

The best measure would be the ratio of energy produced per mol of CO₂ as waste (or kJ energy per mol CO₂ and the reciprocal measure is basically the same).

2.  Two environmental chemists at a barbecue are arguing over whether it is better to use charcoal or propane for grilling.  Rather than arguing over which leads to tastier food, they are arguing over which heating method releases more greenhouse gases to the atmosphere.  Charcoal typically is derived from heating wood under low oxygen conditions to produce mostly elemental carbon (essentially the same as coal although with fewer trace metals and sulfur) along with some CO, particulate matter, and water emissions.  Propane is removed from natural gas sources at low temperatures (requiring energy for refrigeration cooling needed for the separation) where it remains a liquid for transport to customers.  Propane’s lifetime is too short for it to be a significant greenhouse gas. 

List three factors that the chemists should think about in determining which cooking method is “greener”.  Explain how each factor could result in an increase (or decrease) in greenhouse gases.  (9 pts)

The following factors would be important in determining which cooking fuel is greener:

1) The degree to which the wood is renewable (is the same type of tree being regrown following wood removal?).  100% renewable wood will be greener, all other factors being similar.

2) The amount of heat released per mole of CO₂ generated for each fuel source.  Propane, being between methane and fuel oil in H to C ratio would be better than charcoal (similar to coal).

3) The amount of energy required for fuel preparation and source collection.  This is the collection and preparation of wood as well as its heating to transform it to charcoal.  Fossil fuels would probably be used in timber harvest and heating.  In propane production, refrigeration would be needed for the separation technique. 

4)  The degree to which other pollutants could contribute to the non-CO₂ greenhouse gases should be considered.  Carbon monoxide resulting from charcoal production and in charcoal burning could lead to more ozone production.  Propane, being a very volatile fuel is prone to leaks leading to ozone production.

5) Another factor that could matter is how these resources would be used if not for barbecue fuel.  For example, it could matter if the wood was from forest removal vs. scrap wood that would otherwise rot producing CO₂ and perhaps CH₄.  If propane needs to be removed from natural gas, it would make it can be considered surplus (particularly if it is vented without the barbecue fuel demand).

6)  Finally, charcoal does not self ignite, while propane only requires a spark.  Addition of lighter fluid could change the balance as lighter fluid is petroleum based and emits volatiles.

3.  Several pollution processes are linked to climate change where a reduction of pollution could limit near term warming of the Earth.  Which of the following reductions would lead to a near term reduction in warming?  Only one reduction will work.  Explain your answer.  (3 points)

a) reduction in sulfur emissions in burning coal

b) reduction of soot (light-absorbing particulates) from cleaner diesel engines

c) reduction of methyl bromine (a molecule only important for stratospheric ozone destruction).

b) is the only choice in which the removal of a pollutant only reduces substances leading to global warming.  Soot absorbs visible light, generally decreasing albedo and increasing the Earth’s temperature.  Sulfur emissions lead to greater sulfate aerosols, which cause cooling.  Methyl bromide reduces stratospheric ozone.  Stratospheric ozone retains heat as a greenhouse gas.