Biofuels and Fossil Fuels Pros and Cons
Comparison of Fuels
1. How much carbon dioxide is produced for a given amount of fuel mass burned? For a given mass of fuel burned, coal produces the greatest amount of carbon dioxide. Methane produces slightly less than gasoline (represented as octane), and ethanol produces the least.
2. How much carbon dioxide is produced for a given amount of energy released? This may be a better metric to characterize how “green” a fuel is. It compares how much carbon dioxide is released to generate a given amount of heat energy. Coal, which powers the bulk of the world’s electricity-generating plants, produces the most carbon dioxide for a given amount of energy released during combustion. Replacing coal with any other fuel on the list would cut carbon dioxide emissions for a given amount of heat produced. Ethanol, which is a candidate to replace some of the gasoline used in cars, produces just about the same amount of carbon dioxide as gasoline for a comparable amount of energy produced. This means that ethanol has no inherent benefit as a fuel other than the carbon dioxide that it might pull out of the air during its production. This carbon dioxide is returned right back to the atmosphere when the ethanol is burned.
3. How much energy can we get for a given mass of the fuel burned, or energy density? This metric is useful for fuels used for transportation. Using fuel energy to move fuel mass builds in extra inefficiency into the transportation system. On the basis of mass, hydrogen has a very high energy density. This makes hydrogen an attractive potential candidate for transportation systems. Hydrogen, unfortunately in gaseous form has a very low volume density making it less convenient than other fuels to power vehicles. Being a gas, hydrogen would need to be highly compressed or otherwise stored (such as through a hydride) in a small enough volume to keep the vehicle’s fuel tank a reasonable size. Ethanol is slightly better than gasoline in terms of mass. However, on the basis of volume, gasoline has a higher energy density. This means that gasoline requires a smaller fuel tank to go a given number of miles than mixes that include ethanol.
Coal is the lowest in this group, which helps to explain why coal (with the exception of its use to power locomotives and steamboats), is most widely used as a stationary source of energy heat of combustion evaluated here go into designing a fuel to operate in a real engine. It is equally important to keep in mind the overall life cycle of the fuel. This includes the energy that goes into producing and distributing the fuel and the carbon dioxide released during those processes.
Hydrogen is a unique fuel that does not produce carbon dioxide when it is burned.
As indicated the combustion products of various fuels, hydrogen generates only water. Finding an effective way to produce hydrogen would give it a more prominent role in the future energy mix. One particularly appealing idea is to use electricity generated by renewable sources to produce hydrogen by electrolysis.
PRODUCTION OF HYDROGEN
By electrolysis of water: Hydrogen can be produced by decomposing water using electric current or very high temperatures:
2H2O → 2H2 + O2
This reduces carbon dioxide emissions only to the extent that the electricity used for the electrolysis is not derived from burning fossil fuels.
From methane: Hydrogen can be produced by reacting methane (CH4) with water (H2O) in the form of steam. This consumes methane, requires energy, and produces carbon monoxide as a by-product:
CH4 + H2O → CO + 3H2
The Effect of Increasing Concentration
The impact of increasing the amount of a greenhouse gas in the atmosphere on global warming depends on how much of that greenhouse gas is present to begin with. Additional carbon dioxide will absorb only the infrared radiation that has not been absorbed by the carbon dioxide already present in the atmosphere. Adding a few molecules of a trace gas such as CFCl3 will have a proportionally greater impact because those molecules will not be competing with others of its type for the wavelengths that the gas most strongly absorbs. Consistent with their relatively low concentration, CFCs have the highest absorption fraction. Absorption nearly doubles for a doubling of concentration. Absorption of methane or nitrous oxide is a little more than 50 percent. Increasing their concentration has a smaller impact.
Carbon dioxide present in the atmosphere in comparatively high concentrations already absorbs close to 80 percent of the wavelengths it can absorb. Doubling the level of carbon dioxide in the atmosphere will not double its absorption but will promote a more modest increase. This is a simplified example of the types of scientific relationships among climate variables that are built into climate models.
Models help to predict the future climate changes.
This is one example of a negative feedback in which nature reduces the overall impact of human-generated greenhouse gases as their concentration builds up.
Isotopes-Determining How Old Something Is
Researchers use isotopes in climate studies to measure the age of a sample layer in an ice core, a tree ring, or the sediment from a borehole. Isotopes also can provide insight as to whether the source of carbon dioxide emissions is human or natural.
An isotope is a form of an element that has a different atomic structure from other isotopes of that element. What gives any element its chemical identity is the number of protons in its nucleus. For instance, carbon is carbon because-no matter what-it has six protons. Most carbon atoms also have six neutrons in the nucleus.
This form of carbon, called carbon-12 or 12C, is the most common form found in nature. (So we have 6 protons plus 6 neutrons that gives a total of 12 particles in the nucleus, or carbon-12). Carbon-12 is stable. It is not radioactive, and it does not decay.
If we add 2 extra neutrons to the nucleus, there are 14 particles in the nucleus, giving us carbon-14. Carbon-14 is not stable. It decays, taking 5730 years for half of it to decay. If we have a sample of carbon-14 and half of it is gone, the sample is 5730 years old. If only one-quarter of the sample is left, we know the sample is 11,460 years old (two times the half-life of 5730 years). If only one-eighth of the sample is left, we would know that the sample is 17,190 years old (three half-lives).
Because of the two extra neutrons in the carbon-14 atoms, they are heavier. This allows them to be physically separated from other isotopes. Hydrogen can be hydrogen-1, hydrogen-2, and hydrogen-3. Oxygen also has common isotopes, namely, oxygen-16, oxygen-17, and oxygen-18.
Isotopes are useful in determining temperature. Oxygen-18 (18O), having two extra neutrons in its nucleus, is slightly heavier. During warmer conditions, more of this form of oxygen gets incorporated into ice. Figure 5-11 shows the type of relationship between the mix of isotopes and temperature.
• The atmosphere consists of the following natural gases: nitrogen (78 percent), oxygen (21 percent), and others (1 percent).
• All fuels except hydrogen have carbon and will produce carbon dioxide when burned (completely).
• Molecules of gases in the atmosphere absorb light from various parts of the electromagnetic spectrum by vibrating in ways that depends on their chemical structure.
• Nitrogen and oxygen are simple two-atom molecules. They do not absorb much of the sunlight that comes into the earth’s atmosphere or the infrared light radiated from the earth’s surface.
• Water (H2O) can bend in more ways that other substances and can absorb specific wavelengths of energy from both the visible and infrared spectra.
• The greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) do not absorb visible light well but are “tuned” to the infrared energy the earth radiates when heated.
• Methane is less abundant but absorbs energy 20 times more strongly than carbon dioxide.
• Ozone in the troposphere absorbs infrared energy, but because of its low abundance, it plays a relatively minor role in global warming.
• Ozone in the stratosphere absorbs ultraviolet light that has a minor cooling effect on the earth’s temperature because it captures energy before it can reach the lower atmosphere.
• Halocarbons are very strongly absorbing, long-lived greenhouse gases found in the atmosphere in low concentrations.
• Industrial processes such as cement manufacturing add carbon dioxide to the atmosphere.
• Nature removes much of the new carbon dioxide that is released by human activities by absorption in the oceans. This is causing the pH of the oceans to decrease, which makes the oceans more acidic.
• Isotopes are varieties of natural elements that differ in the number of neutrons in their nuclei. Isotopes are used to determine the age of layers in ice-core samples and other fossil samples. Isotopes also can identify the origin of gases released into the atmosphere during various periods of history.
• The relative impact of greenhouse gases decreases as their concentration in the atmosphere increases.
• The integrated gasification combined cycle (IGCC) makes it easier to capture carbon from coal-fired electricity-generating plants.
Questions
1. What is it about methane that enables it to absorb several wavelengths of infrared light as strongly as it does?
(a) It has strong chemical bonds.
(b) It is a small molecule.
(c) It can vibrate in several ways that absorb infrared wavelengths.
(d) It reacts with oxygen to form carbon dioxide.
2. Which of the following most strongly absorbs light coming directly through the atmosphere from the sun?
(a) Nitrogen
(b) Oxygen
(c) Carbon dioxide
(d) Water vapor3. Which of the following would you expect, just looking at its chemical structure, to most strongly absorb light?
(a) CO (carbon monoxide)
(b) Cl2 (chlorine gas)
(c) CCl4 (carbon tetrachloride)
(d) O2 (oxygen)
4. What is a by-product of making calcium oxide from calcium carbonate in the production of cement?
(a) Nitrogen
(b) Carbon dioxide
(c) Water vapor
(d) Hydrogen
5. Which of the following may store carbon dioxide in a possible carbon sequestration plan?
(a) Methane
(b) Calcium bicarbonate
(c) Calcium chloride
(d) Ozone
6. What role does ozone in the troposphere play in global warming?
(a) Minor greenhouse gas
(b) None
(c) Chemically attacks carbon dioxide
(d) Absorbs ultraviolet wavelengths
7. Which of the following produces the most carbon dioxide for a given amount of energy provided during combustion?
(a) Coal
(b) Oil
(c) Natural gas
(d) Ethanol
8. Which of the following does not produce carbon dioxide when it burns?
(a) Ethanol
(b) Biodiesel fuel(c) Hydrogen
(d) Coal
9. If the concentration of each of the following were to double, which would cause the amount of infrared light absorbed to also double?
(a) A chlorinated, fluorinated hydrocarbon such as CF2Cl2
(b) Methane
(c) Nitrous oxide
(d) Carbon dioxide
10. When greenhouse gases absorb light, where does that energy go?
(a) It tightens up the chemical bonds.
(b) It is stored in the electrons of the atoms.
(c) It is trapped inside the nucleus.
(d) It causes the molecules of the gas to move.
11. Which of the following is a basic characteristic of a greenhouse gas?
(a) Absorbs visible light coming from the sun
(b) Absorbs infrared light radiated from the earth
(c) Absorbs ultraviolet light in the stratosphere
(d) Is transparent to light of all wavelengths
12. How do climatologists use isotopes?
(a) To break up ozone into elemental oxygen
(b) To store carbon dioxide
(c) To measure the concentration of carbon dioxide in an air sample
(d) To determine the age of ice-core layers
