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Alternative FuelsAlternative Fuels
Larry Caretto
Mechanical Engineering 496ALT
Alternative EnergyAlternative Energy
March 19, 2009
Assignments:
Reading for tonight: Chapter 7 on fuels.
Reading for next Tuesday, Chapter 10 on biofuels.
Next Thursday, March 26 will be a presentation from Bob Litwin of Rocketdyne on design of a solar thermal electric power generatin plant.
The next midterm exam will be on Thursday, April 2, covers up to and including wind power.
Wind energy homework due next Tuesday, March 24.
2
Outline
• What are alternative fuels?
• How do we do fuel conversions?
- Chemical reactions and chemical energies
- Reactor types
- Production of liquid and gaseous products
• Policies on fuel conversion research
and development
• Integrated gasification/electric power
This lecture will cover the general topics of making nonconventional fuels such as manufactured gas and liquid fuels from coal.
The following lecture will cover biomass fuels, including fuel ethanol, biodiesel, refuse derived fuels (RDF), and direct combustion of biomass fuels.
4
Fuel Conversion Reactions
- C + O 2 → CO 2 (ΔH (^) R = -394 MJ)
- C + CO 2 → 2CO (ΔH (^) R = 171 MJ)
- C + H 2 O → CO + H 2 (ΔH (^) R = 130 MJ)
- C + 2H 2 O → CO 2 + 2H 2 (ΔH (^) R = 87 MJ)
- CO + H 2 O → CO 2 + H 2 (ΔH (^) R = -41 MJ)
- C + 2H 2 → CH 4 (ΔH (^) R = -75 MJ)
- CO + 3H 2 → CH 4 + H 2 O (ΔH (^) R = -206 MJ)
A negative heat of reaction means that energy is given off; a positive heat of reaction means that energy has to be added to carry out the reaction. The terms exothermic and endothermic are used to refer to reactions that , respectively, give off heat and require a heat input.
In addition to the simple effect of producing or releasing heat, the equilibrium of reactions, even if they produce heat, may require the production of high temperatures to make the reactions possible.
The basic reaction in converting coal to liquid and gaseous fuels in the reaction C + H 2 O → CO + H 2. Because this reaction is endothermic, there is a net energy input to make it go. The significance of the energies associated with the various reactions here can be seen by comparing them with the first reaction for the combustion of carbon.
The reaction, CO + H 2 O → CO 2 + H 2 (ΔH (^) R = -41 MJ), known as the water- gas shift reaction, is used in the production of hydrogen.
5
Synthetic Gases from Coal
• Variety of names
- Goal Gas, Town Gas, Producer Gas,
Illuminating Gas, Blue Gas Domestic Gas,
Water Gas, Carbureted Water Gas,
Manufactured Gas
• Classified by heating values
- Low Btu (50 to 200–250 Btu/scf)
- Medium Btu (about 500 Btu/scf?)
- High Btu (>900 Btu/scf)
Reference: http://www.zetatalk.com/energy/tengy11a.htm The most complete conversion of coal or coke to gas that is feasible was achieved by reacting coal continuously in a vertical retort with air and steam. The gas obtained in this manner, called producer gas, has a relatively low thermal content per unit volume of gas (100-150 Btu/cu ft). The development of a cyclic steam-air process in 1873 made possible the production of a gas of higher thermal content (300-350 Btu/cu ft), composed chiefly of carbon monoxide and hydrogen, and known as water gas. By adding oil to the reactor, the thermal content of gas was increased to 500-550 Btu/cu ft; this became the standard for gas distributed to residences and industry. Since 1940, processes have been developed to produce continuously a gas equivalent to water gas; this involves the use of steam and essentially pure oxygen as a reactant. A more recently developed process reacts coal with pure oxygen and steam at an elevated pressure of 3.09 Newtons per sq m (450 psi) to produce a gas that may be converted to synthetic natural gas. The most common modern process uses lump coal in a vertical retort. The coal is fed at the top with air, and steam is introduced at the bottom. The gas, air, and steam rising up the retort heat the coal in its downward flow and react with the coal to convert it to gas. Ash is removed at the bottom of the retort. Using air and steam as reacting gases results in a producer gas; using oxygen and steam results in a water gas. Increasing operating pressure increases the productivity. Two other processes currently in commercial use react finely powdered coal with steam and oxygen. One of these, the Winkler process, uses a fluidized bed in which the powdered coal is agitated with the reactant gases. The other, called the Koppers-Totzek process, operates at a much higher temperature, and the powdered coal is reacted while it is entrained in the gases passing through the reactor. The ash is removed as a molten slag at the bottom of the reactor. Both of these processes are being used for fuel gas production and in the generation of gases for chemical and fertilizer production. Producer gas is a mixture of approximately 25% carbon monoxide, 55% nitrogen, 13% hydrogen and 7% other gases. It is obtained by burning coal or coke in the generators with a restricted supply of air, or by passing air and steam through a bed of red hot fuel. Producer gas is cheap and used as a fuel mainly in glass furnaces and metallurgical furnaces. It also serves as a fuel in gas engines to operate tractors, motor cars and truckso. It is also used as a source of nitrogen for the preparation of ammonia.
7
Hydrogen Production
- Possible uses
- Ammonia manufacture
- Petroleum refining
- Fuel for fuel cells
- Produced by initial gasification and water-gas
shift reaction
– C + H 2 O → CO + H 2 (ΔH R = 130 MJ)
– CO + H 2 O → CO 2 + H 2 (ΔHR = -41 MJ)
- Temperature behavior
- H 2 production favored by low temperatures
- Need 300 C < T < 700 C for reaction rate
Reference: National Research Council, Coal Energy for the Future , National Academy Press, 1995.
Acidic gases such as H 2 S, CO 2 , and HCl are catalyst poisons. They must be removed from the gas stream prior to the water-gas shift reaction to maintain catalyst activity.
The production of hydrogen is an important step in moving to the use of fuel- cells which, in general, require hydrogen as a fuel. We will discuss fuel cells as a separate topic later in the course.
Hydrogen can also be produced by the electrolysis of water, but this is an expensive process and it basically takes electricity which has been generated with whatever efficiency losses are considered for particular processes and converts the electricity back into fuel.
People have talked about the “hydrogen economy” for many years now. In the original discussions of that concept, hydrogen would be produced by electrolysis where the electric power would come from fusion power plants. As we discussed earlier the practical generation of electricity from fusion power is many years off.
Certainly, from the standpoint of global warming, hydrogen is the only fuel that can be burned without producing CO 2 , but conventional methods of hydrogen production can produce CO 2.
8
Gasification Reactors
• Entrained flow process – commercial
and development
• Fluidized bed process – development
and demonstration
• Moving fixed bed process – one
commercial, others development
• Notes pages have list of various
reactors and their state of development
Reference: National Research Council, Coal Energy for the Future , National Academy Press, 1995. Entrained flow Process Texaco (US) Commercial 1,260 – 1,480 C Shell (Europe/US) Commercial 1,370 – 1,540 C Destec (US) Commercial 1,040 C Prenflo (Europe) Commercial/demonstration 1,370 – 1,540 C Koppers Totzek (Europe) Commercial 1,480 C ABB/Combustion Engr Development 1,040 C IGC (Japan) Development 1,260 C HYCOL (Japan) Development 1,480 – 1,260 C VEW (Germany) Development
Fluidized-bed Process KRW(US/Europe) Demonstration/development 1,010 – 1,040 C Winkler/Lurgi (Europe) Demonstration/development 950 C Tampella/UGas (Finland/US) Development 980 – 1,040 C MCT Demonstration/development 1,090 – 1,260 C Moving Fixed-bed Process Lurgi (Europe) Commercial
British Gas/Lurgi (BG/L) Demonstration
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Entrained Flow Reactors
• Powdered coal gasified with a mixture
of steam and oxygen (or air)
• Reaction zone is where main part of
molten slag is collected
• High temperature products require
cooling prior to cleanup
• Little methane, compact, short reaction
times, insensitive to coal properties
Reference: National Research Council, Coal Energy for the Future , National Academy Press, 1995.
Entrained flow reactors are characterized by high exit temperatures. This leads to short reaction times because of the fast kinetics. The high temperatures make the process work regardless of the properties of the coal, so long as the coal can be pulverized below 200 mesh (44 micrometer) size. The high exit temperatures produce a gasifier that has less efficiency than other types. The gas produced is relatively free of tars, hydrocarbons heavier than methane and nitrogen compounds.
11
Texaco
Entrained
Flow
Gasification
Reactor
http://www.netl.doe.gov/coalpower/gasification/pubs/images/Tr6-8-1.jpg
Texaco entrained flow gasification reactor
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Fluidized-bed Reactors
• Operate at 760 C to 1,050 C, depending
on coal properties
• Have potential for greater efficiencies
due to lower temperatures
• Higher coal throughput rates compared
to moving fixed bed
• Less inert ash due to low temperatures
may cause more disposal problems
Reference: National Research Council, Coal Energy for the Future , National Academy Press, 1995.
The operating temperature depends on the coal reactivity and the ash softening temperature. The greater efficiency is because the outlet temperatures are better suited to gas cleaning processes so that little or no heat removal is required.
No high-pressure systems are commercially available but one atmospheric pressure one is.
The Tampella/U-Gas and the KRW gasifiers have a special ash agglomeration section which can reduce potential problems of the less inert ash.
14
KRW
Fluidized Bed
Reactor
http://www.netl.doe.gov/coalpower/gasification/pubs/images/29309_101.jpg
KRW Fluidized bed reactor
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Fischer-Tropsch Reaction
• nCO + 2nH 2 → (-CH 2 -) n + nH 2 O
• Uses synthesis gas over catalyst
• Patented in 1925 in Germany
• Basis for modern synthetic liquid fuels
• Interest waned after large discoveries of
oil in Middle East during the 1950s
• Current interest in gas to liquid fuels
The Fischer-Tropsch reaction is one path to liquid fuels from coal. It uses a synthesis gas from goal gasification. The synthesis gas can be cleaned to remove sulfur compounds. In fact, this step is generally required to avoid degradation of the catalysts used in the Fischer-Tropsch process.
An alternative to the Fischer-Tropsch process is the direct liquefaction of coal. That will be discussed subsequently.
German gasoline production during World War II and production of synthetic crude oils in South Africa during Apartheid was done by the Fischer-Tropsch process.
The web site, http://www.fischer-tropsch.org/,contains a large amount of present and historical information on the Fischer-Tropsch process. The site sponsored by Syntroleum Corporation in cooperation with Dr. Anthony Stranges, a professor of history at Texas A&M University, whose area of research is the history of alternative fuels processes. This site has several old documents, converted from printed to electronic form by scanners, dating back to the 1920s. It even has records of interviews of German scientists that were obtained after World War II to learn about the progress that they had made on the Fischer-Tropsch process during the War.
17
Anderson-Schulz-Flory
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
1.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. Chain Growth Probability
M ass Fraction
(^) C C2-C C5-C C12-C C19-C C41+
The product yield from Fischer-Tropsch reactions can be characterized by a polymerization distribution equation known as the Anderson-Schulz-Flory distribution. This distribution is given by the following equation, W (^) n = n αn-1^ (
- α) 2. In this equation, n is the number of carbon atoms in the resulting molecule, Wn is the mass fraction of a hydrocarbon with n carbon atoms, and α is a factor known as the chain growth probability.
This growth probability factor allows a general picture of the FT process. The design of a process with a particular catalyst and a given set of pressures and temperatures can then be interpreted by it’s effective chain growth probability.
This parameter is actually determined by measuring the weight fraction distribution and rewriting the distribution equation as follows: log(W (^) n /n) = [log(α)] n + log[(1 – α) 2 /α]. This equation says that a plot of log(Wn /n) versus n should be a straight line with a slope of [log(α)] and an intercept of log[(1 – α) 2 /α]. Thus, measurements of the product distribution, Wn , as a function of n can be plotted in this manner and the value of α can be determined.
The range of C 5 to C 11 compounds is typical of those found in gasoline and the range from C 12 to C 18 is typical of those found in Diesel fuel.
This distribution of actual refinery products is illustrated in the next slide.
19
Fischer-Tropsch Reactors
• Significant heat transfer problem due to
heat of reaction ~25,000 Btu/lbmole of
synthesis gas reacted
• Fixed bed reactors
• Fluidized bed reactors
• Slurry reactors
The Fischer-Tropsch reaction from the last slide is written as follows:
nCO + 2nH 2 → (CH 2 ) (^) n + nH 2 O
We can use the standard heats of formation for CO, H 2 , and H 2 O (gas) of - 47,518 Btu/lbmole, 0 Btu/lbmole, and –103,696 Btu/lbmole, respectively. The average heat of formation of the liquid fuel product, (CH 2 ) (^) n is -8,500n Btu/lbmole. The total moles of synthesis gas reacted in the reaction are 3n (combined total of CO and H 2 .)
The heat of reaction is n(-103,696) – 8,500n – [n(-47518) + 2n(0)] =
Dividing this by the 3n moles participating in the reaction gives the approximate energy release of 25,000 Btu/lbmole of synthesis gas shown in the chart. As usual, the negative heat of reaction indicates an energy release.
Additional information on the reactor types is presented on the following charts and note pages.
20
Shell Gas-to-
Liquids Process
Reference: http://www.shell.com/home/Framework?siteId=shellgasandpower- en&FC2=/shellgasandpower-en/html/iwgen/products_and_services/ what_is_gtl/ gas_to_liquid/zzz_lhn.html&FC3=/shellgasandpower- en/html/iwgen/products_and_services/what_is_gtl/gas_to_liquid/whatisgtl_ 12_1532.html
Yes that is really the URL!
Gas-to-liquid (GTL) conversions are used when there is no ready market for gas due to a lack of pipelines. In this case the gas is usually flared (burned) or reinjected for later use.
Gas can be transported if it is converted to a liquid. There are two ways to do this. One is to produce liquified natural gas which can be transported to a pipeline location and vaporized there. Several LNG plants have been proposed for the West Coast of the US, but many of these are controversial and may not be built.
Shell has an operating plant in Malaysia now producing liquid fuels from natural gas using the schematic shown above. They are also constructing a plant in Qatar, in conjunction with Qatar Petroleum that is scheduled for completion in two phases with projected dates of 2010 and 2011. When completed the plant will produce 140,000 barrels per day (bpd) of gas-to- liquid (GTL) products as well as approximately 120,000 bpd of associated condensate and liquefied petroleum gas
