Home > WEEK 9 FOSSIL FUELS AND ENERGY Word Doc
WEEK 9 FOSSIL FUELS AND ENERGY Word Doc
Dimensions of matter and energy
The universe consists of materials (matter) and energy. Physical and chemical phenomena can be represented by four basic dimensions: mass (M), length (L; distance), time (t), and temperature (T). For example, the dimensions of velocity are Lt-1 and of volume L3. The dimensions of force are MLt-2 (mass*acceleration) and of work/energy ML2t-2 (force*distance). Since power is defined as the rate of doing work (or generating energy), its dimensions are ML2t-3. All properties of matter can be expressed in terms of these dimensions (e.g., density: ML-3; specific heat: L2t-2 T-1). For convenience, in heat generation and transfer problems, the composite dimension of Q is used to represent thermal energy (e.g., thermal conductivity: QL-1t-1T-1).
Units of matter and energy
In the universal system of units (SI: systeme universal), mass is measured in kilograms (kg), where 1 kg = 1000 grams, and 1000 kg = 1 ton. Distance is measured in meters (m) where 1m = 100 centimeters and 1000 m = 1kilometer (km). Time is measured in seconds and temperature in degrees Kelvin; one degree Kelvin has the same magnitude as one degree Celcius but 0 K (i.e., the absolute zero temperature) is 273o lower than 0oC; therefore in converting temperature from degrees Celcius to degrees Kelvin, we must add 273 (i.e., 100oC = 373 K). The unit of force is the Newton (1 kg m s-2), of energy/work the Joule (1 J =1 Newton m =1 kg m2s-2) and of power the Watt (1W = 1 J s-1). In the SI system, bigger amounts, e.g. of grams, are expressed as follows:
1 kilogram (1kg) = 103 grams; 1 megagram (Mg)=106 grams; 1 gigagram (Gg)=109 grams 1 teragram (Tg) = 1012 grams; 1 petagram (Pg) = 1015 grams
Similarly smaller amounts, e.g. of grams are expressed by
1 milligram (mg)= 10-3 grams; 1 microgram (g)=10-6 grams; 1 nanogram (ng)=10-9 grams; 1 picogram (pg) = 10-12 grams
Unfortunately for U.S. students, the U.S. is the last nation on Earth to continue using the so-called British units for measuring mass (1 lb. =454 grams; 2000 lb. = 1 short ton; 1.1 short tons = 1 ton) and distance (1 ft. =0.3048 m). The U.S. measure of thermal energy is the BTU (“British Thermal Unit”) where 1BTU = 1055 J = 1.055kJ. For other conversions between the S.I. and the U.S. system, etc., click here (linkto unitconversion file) and download a complete tabulation (N.J. Themelis, “Transport and Chemical Rate Phenomena”, 1995).
Fossil fuels: Coal, oil and gas
Most of the energy used presently by humanity is obtained by mining fossil fuels stored in the Earth and combusting them. These non-renewable fuels consist of three principal classes: Coals (containing mostly solid carbon plus some hydrocarbons), oils, and natural gas. The composition of U.S. coals (ultimate analysis) ranges from 61 to 84% carbon and 3.1-5.6% hydrogen; the balance consists of oxygen, nitrogen, sulfur and ash. Considering the atomic weights of C (12) and hydrogen (H), a coal containing 80%C and 5%H, would have the approximate hydrocarbon formula C1.3H. Generally, it can be assumed that coals are represented by the formula CH, oils by CH2 and natural gas by CH4.
Thermal energy is generated as the fossil fuel is combusted with oxygen to form either carbon dioxide or water vapor. If we neglect the relatively small amount of energy in the C-H bonds, the principal combustion reactions can be represented simply as:
C + O2 = CO 2 + 32.8 MJ per kilogram of carbon
H2 + 0.5O2 = H 2O + 123.5 MJ per kilogram of hydrogen
It can be seen that a higher fraction of hydrogen in the fuel results in a greater amount of energy released per unit mass of the fuel. Also, the combustion of carbon produces carbon dioxide, a greenhouse gas. For these reasons, the use of methane is preferable to fuel oil, and fuel oil to coal. In addition, coals contain larger amounts of metallic contaminants that are volatilized during combustion, such as mercury (0.1-0.3 parts per million) and cadmium (0.5-10 p.p.m.)
Fossil Fuel Reserves and Resources
As discussed in Grand Cycles, humanity at the present time generates about 6.5 billion tons of carbon per year. Nearly half of this amount remains in the atmosphere and continually increases its CO2 concentration. In this lecture we will consider for how many years humans can continue using this non-renewable resource. First, we need to differentiate between reserves and resources of fossil fuels and all other minerals that lie beneath the surface of the Earth. In mining, reserves are proven deposits that can be brought to surface economically at prevailing costs of operation and price of the usable mineral. Resources are fuels and minerals that are estimated to exist with a certain degree of accuracy but cannot be used at present time either because of technical or economic problems, or both.
Along, these lines, Rogner (1997) carried out a detailed analysis of all known fossil fuel deposits and classified them in the following eight classes:
Category I: actually measured reserves
Category II: oil deposits with high geologiacl probability of occurence
Category III: more speculative (lower probability of recovery) of occurrence
The above three categories represent oil and natural gas reserves that can be recovered with existing technology/practice.
Category IV: deposits with potential for “enhanced” recovery. At present, on the average, only 34% of identified oil and 70% of natural gas are recovered by means of the naturally-occurring primary pressure in the reservoir and by creating secondary pressure (e.g. water/steam injection). In the future, the % recovery in such deposits may be increased by injection of chemicals , surfactants , powders and other method of increasing permeability.
Categories V-VIII: non-conventional
oil and gas that presently cannot be brought to surface with conventional
methods, either for technical or economic reasons: Oil shales (mixtures
of sand and oil), heavy oils, CH4 in hydrate form, etc.
For example, by far the largest reserve
of fules are in the form of methane hydrates, weak-bonded combinations
of CH4 and water molecules, existing at subzero temperatures
at the bottom of the ocean. However, the technology for recovering this
fuel has not been developed and it is possible that the amount of energy
required to recover it may be close to their energy content.
The attached PowerPoint presentation shows the findings of the Rogner (1997) study with regard to:
Fossil fuel producing regions of the world
Estimates of conventional oil reserves, in gigatons of oil equivalent
Estimates of unconventional natural gas resources by type, in gigatons of oil equivalent
Estimates of coal reserves and resources, in Gtoe.
Effect of price of oil (per barrel, in 1990$) on converting fossil fuel resources to reserves
The McKelvey box showing the cost
vs quantity relationship of hydrocarbon resource recovery.
Potential for Increasing Energy Efficiency
The amount of production or services provided per unit of energy represents the intensity of energy use. As shown in the last slide of the PPt presentation, the amount of energy used per unit of production decreases with time in several cases studied. One of the reasons is that as people learn to operate a process, they become better at it. The other reason is that technological advances usually result in increased energy efficiency. However, we still have a long way to go in reducing energy usage, for the same amount of service and comfort provided. For example, the book “Factor Four” (E.V. Weizsacker, A.B. Lovins, L.H. Hunter, Earthscan, 1997) provides numerous examples as to how it is possible to reduce energy usage by 75% without sacrificing the quality of service provided.
As an example, most of the electricity in the U.S. is produced at a conversion efficiency (fuel energy to electric energy) of about 32%. Yet, co-generation of electricity and heat (to be used in heating buildings in the winter and cooling them in the summer –by means of “heat pumps”) can double the conversion efficiency to about 65% or even higher. How come such advanced technologies are not adopted widely? Part of the answer is that fuel costs are low relatively to the capital costs that would be required. The other part is that even when the capital charges over the life of a efficient-technolgy project are significantly less than the energy savings, it is difficult for most people to invest at present for benefits that will accrue over a long period of time. Even Biosphere 2 some years back, when there was an opportunity to change their conventional power generation plant to co-generation, they could not do it and opted for another conventional plant. The example below shows that even when the long term economic and environmental advantages are clear, people have a tendency to forego long-term benefits for short-term savings.
Sharpen your IE skills: An energy-efficient 25-W Marathon bulb made by Philips (to be exhibited in class) has a lifetime of 10,000 hours and costs $6. On the other hand an ordinary 100-W filament bulb costs only $0.75 and has a life of 1000 hours. Both bulbs deliver 1750 of lumens light output. If you use this light 8 hours a day 365 days a year, the Philips bulb will last 3.5 years and will consume 250 kWh. At $0/10 per kW (NYC), the capital and operating cost of the Philips bulb will be $31. On the other hand, the filament bulb will need to be changed at least 5 times and will use up 1000 kWh; therefore, its capital and operating cost will be about $104 dollars, or over three times the cost of the Philips bulb. How many people do you know who use the new energy efficient bulbs? Do you?
Example of energy efficiency:
Oil volume is still measured in “barrels”: 1 barrel = 42 gallons = 0.159 m3
Gross heating value of 1 gallon of oil (average): 143,000 BTU
Therefore 1 barrel oil= 6 million BTU
= 6,300 million J = 6,300 MJ = 6.3 GJ
Let us now consider the Essex county, NJ Waste-to-Energy plant that generates 60 MW by burning Municipal Solid Wastes (MSW). The amount of electric energy, in Joules produced in one hour, is: 60MW*3600s=216000 MJ
Considering that the usual efficiency of converting thermal to electrical energy is about 30%, how much fuel oil is conserved by that incinerator?
(216000 MJ/6,300 MJ/b)/0.30 = 114 barrels of oil/h
Readings: Class notes; Rogner, 1997, “An assessment of world hydrocarbon resources”, Annual Reviews of Energy and Environment 22:217-62
The following figures are from USDOE report (www.eia.doe.gov/oiaf/aeo/)
For information of current research
activities at Environmental Energy Technologies Division of Lawrence
Berkeley National Laboratory, please go to
http://eande.lbl.gov/
All Rights Reserved Powered by Free Document Search and Download
Copyright © 2011